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Digital phase diagram and thermophysical properties of KNO3-NaNO3-Ca(NO3)2 ternary system for solar energy storage
Accepted Manuscript Digital phase diagram and thermophysical properties of KNO3-NaNO3-Ca(NO3)2 ternary system for solar energy storage Mengmeng Chen, Yuesong Shen, Shemin Zhu, Peiwen Li PII:
S0042-207X(17)30961-2
DOI:
10.1016/j.vacuum.2017.09.003
Reference:
VAC 7573
To appear in:
Vacuum
Received Date: 18 July 2017 Revised Date:
3 August 2017
Accepted Date: 4 September 2017
Please cite this article as: Chen M, Shen Y, Zhu S, Li P, Digital phase diagram and thermophysical properties of KNO3-NaNO3-Ca(NO3)2 ternary system for solar energy storage, Vacuum (2017), doi: 10.1016/j.vacuum.2017.09.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Digital phase diagram and thermophysical properties of
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KNO3-NaNO3-Ca(NO3)2 ternary system
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for solar energy storage
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Mengmeng Chen1, Yuesong Shen1*, Shemin Zhu1*, Peiwen Li2
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1 ： Jiangsu Collaborative Innovation Center for Advanced Inorganic Function
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Composites, Jiangsu National Synergetic Innovation Center for Advanced Materials
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(SICAM), College of Materials Science and Engineering, Nanjing Tech University,
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Nanjing 210009, China
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2: Department of Aerospace and Mechanical Engineering, University of Arizona,
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Tucson, AZ 85721, United States
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*Corresponding author: Prof. Yuesong Shen.
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State Key Laboratory of Materials-Oriented Chemical Engineering, College of
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Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P.R.
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China
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Address: No.5 Xinmofan Road, Nanjing Tech University, College of Materials
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Science and Engineering, 210009, Nanjing, China
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E-mails: [email protected] (Y.S. Shen)
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Tel: +86 25 83587927
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Fax: +86 25 83582195
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ACCEPTED MANUSCRIPT Abstract: The molten nitrate salt has become one of the most promising heat storage
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and transfer medium for solar energy. A series of molten salt systems containing
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KNO3-NaNO3, Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2
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were designed and prepared to study the liquidus surface, and the thermal stability of
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the KNO3-NaNO3-Ca(NO3)2 ternary system was mainly studied. A 3D stable molten
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temperature diagram was developed to predict melting points and thermal
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decomposition points in the KNO3-NaNO3-Ca(NO3)2 ternary system, as well as its
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eutectic temperature and composition. The predicted eutectic composition is 42%
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KNO3-17% NaNO3-41% Ca(NO3)2, which has a low predicted melting point of
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129.1 °C and a high predicted decomposition temperature of 597.9 °C. Moreover, a
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series of melting points and thermal decomposition points obtained from the 3D stable
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molten temperature diagram were respectively verified experimentally using
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thermo-gravimetric (TG) and differential scanning calorimetry (DSC) methods. The
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experimental results were in excellent agreement with that of obtained values from the
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3D stable molten temperature diagram.
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Key words: Molten salt; KNO3-NaNO3-Ca(NO3)2; Thermal stability; 3D stable
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molten temperature diagram
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1. Introduction
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Energy crisis and environmental pollution are two major problems in the world,
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as a clean and renewable energy, the solar energy has clean pollution-free and
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inexhaustible supply outstanding merits and becomes one of the most promising new
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energies [1]. However, solar energy is affected by the weather and the shift of day and 2
ACCEPTED MANUSCRIPT night, and has lower energy density. How to utilize solar energy high-effectively has
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become a research focus of new energy source field. The present status of solar energy
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utilization includes solar energy photo-thermal utilization, photovoltaic utilization and
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photochemical utilization, among them the photo-thermal utilization requires the
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lowest cost and obtains the highest efficiency, also it has become the most promising
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solar energy utilization technology [2]. Meanwhile, heat storage and transfer media is
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one of the critical techniques of solar thermal conversion. Molten salts with merits of
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high thermal stability, large specific heat, moderate density, low viscosity, low vapor
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pressure and low cost have become a research hotspot and development direction of
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heat storage and transfer media [3].
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Common molten salts including nitrates, sulfates, carbonates, chlorides and
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fluorides used as solar energy heat storage and transfer media have been
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systematically studied until now [4], particularly the molten nitrates have been widely
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applied to large scale experimental and commercial solar thermal power plants based
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on their lower cost and better thermal properties synthetically. Solar salt [5] (60%
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NaNO3-40% KNO3), Hitec [6] (7% NaNO3-53% KNO3-40% NaNO2) and Hitec XL
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[7] (45% KNO3-7% NaNO3-48% Ca(NO3)2), as current well-known heat storage
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media, have been applied in large scale experimental and commercial solar thermal
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power plants. However, the highest operating temperature of most mixed nitrates is
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about 600 °C, once the upper limit temperature is exceeded, the mixed nitrates begin
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to decompose. The quaternary and penta-basic mixed nitrates also have been reported
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in recent research in order to widen phase change temperature and improve the latent
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ACCEPTED MANUSCRIPT heat. Ren et al. exploited a quaternary mixed nitrates system of KNO3-NaNO3-LiNO3-
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Ca(NO3)2·4H2O, and found that their melting point could be down to below 90 °C
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while their decomposition temperature could be above 600 °C [8]. Raade et al. tested
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over 5000 unique mixtures of inorganic salts during the screening process, resulting in
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a low melting point of 65 °C and a thermal stability limit over 500 °C [9]. Overall, it
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was a big challenge to complete the large number of possible combinations of salt
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mixtures.
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At present, it is an excellent way to study solidification, solid-state reaction,
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phase transformation, and oxidation of molten salts by phase diagrams. Phase
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diagrams are the foundation in performing basic materials research, it serves as a road
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map for materials design and process optimization [10]. In order to show the complete
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phase diagram, 3D space coordinate must be adopted, namely 3D phase diagram
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which is determined by the concentration plane and the corresponding vertical
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temperature coordinate. Moreover, a 3D phase diagram is intuitive and informative.
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The presented work investigates the melting point of the Hitec XL is the lowest
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among the Solar salt, Hitec and Hitec XL. Hitec XL has a composition of 45%
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KNO3-7% NaNO3-48% Ca(NO3)2, it contains 59% water [11], and the composition of
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the mixture is eutectic after the dehydration, with a content of 43% KNO3-15%
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NaNO3-42% Ca(NO3)2 [12]. Until now, the ternary system has been reported by many
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researches. Menzies et al. [12] developed ternary phase diagram of the system and
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determined that the 43% KNO3-15% NaNO3-42% Ca(NO3)2 composition had the
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lowest melting point of 175 °C for the first time. Then Jänecke [13] and Bergman et al.
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KNO3-10% NaNO3-63% Ca(NO3)2 (melting point of 130 °C) and 30% KNO3-9%
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NaNO3-61% Ca(NO3)2 (melting point of 133 °C), respectively. Moreover, the most
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widely accepted phase diagram of the system was developed by Bergman et al. In
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1982, a patent reported a mixture of 44% KNO3-12% NaNO3-44% Ca(NO3)2, which
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had a melting point of 130 °C[15]. Bradshaw et al. [16] and Kearney et al. [17]
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investigated (43-50%) KNO3-(7-34%) NaNO3-(16-48%) Ca(NO3)2 mixtures, their
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melting points ranged from 120 °C to 190 °C. The investigations shows that the
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eutectic temperature and composition of the KNO3-NaNO3-Ca(NO3)2 ternary system
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are not consistent, or even a great difference. In addition, there have been no
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systematic reports about thermal decomposition points of the KNO3-NaNO3-
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Ca(NO3)2 ternary system so far. Therefore, it is of great practical significance to
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construct the 3D phase diagram of the KNO3-NaNO3-Ca(NO3)2 ternary system.
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Current phase diagram drawing softwares include MS Excel [18] and Delphi [19],
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and some professional phase calculation softwares, such as Thermo-calc [20] and
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FACT [21], but these softwares can only construct the phase planes. In order to realize
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the visualization of the KNO3-NaNO3-Ca(NO3)2 ternary phase diagram, Matlab is
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adopted to study it. A 3D phase diagram can be rotated freely and viewed from
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different positions in the Matlab environment, meanwhile, the eutectic point of the
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ternary system and text labels can be marked by programming with Matlab language.
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At present, Matlab has become a common senior data analysis and drawing tool for
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researchers and engineers based on its actual simple, open-source and credible.
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KNO3-NaNO3-Ca(NO3)2 ternary system based on the different systems of KNO3-
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NaNO3, Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2. Also as
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part of this effort, we predicted the eutectic composition and its temperature span.
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This will provide valuable data and basic theory support for its industrialization.
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Moreover, this study suggests that this method can be successfully extended to predict
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the eutectic points in other molten salt systems.
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2. Experimental scheme
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In order to obtain a 3D stable molten temperature diagram of the KNO3-NaNO3-
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Ca(NO3)2 ternary system, a series of molten salt systems containing KNO3-NaNO3,
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Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2 were designed and
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prepared. Thermophysical properties were measured, including melting point and
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thermal stability.
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(1) Preparation of mixed molten salts. The nitrates in the present study were KNO3,
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NaNO3 and Ca(NO3)2·4H2O with a purity of 99 %, and the
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chemical
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design principle of the average distribution, the systems of KNO3-NaNO3, Ca(NO3)2-
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NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2 were prepared in different
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ratios. Then, the pre-dried pure chemicals were melted inside an alumina crucible in
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the furnace and fused at 350 °C for at least 24 h with periodic mixing in order to
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obtain sufficient homogenization. Finally, the mixture was taken out, ground, dried
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and stored for experiments.
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summary
of
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Table 1 Chemical sample information for experiment Chemical Name
Source
Chemical Specifications
Initial Mass Fraction Purity
Preprocessing Method
KNO3 NaNO3 Ca(NO3)2·4H2O
Xi-long Chemical Xi-long Chemical Xi-long Chemical
AR AR AR
≥99.0 ≥99.0 ≥99.0
vacuum drying vacuum drying thermal decomposition
(2) Melting point measurement. Melting points of molten salts were measured by an
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infrared thermometer (HT-8963, HCJYET, Inc.), under the atmosphere protection of
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nitrogen. The accuracy of temperature collection in the instrument is 0.1 K. The
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prepared molten salt sample was heated and maintained at 300 °C for 10 h to remove
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the absorbed water, and then a small sub-sample was loaded into an alumina crucible.
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The crucible was placed on a resistance furnace, the temperature was slowly raised.
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When the sample was melted into liquid completely, the temperature value was tested
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immediately by an infrared thermometer. The experiment was repeated several times
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to verify the recurrence of the melting temperature in the system.
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(3) Thermal stability measurement. Thermal stability was measured by the
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experimental set-up as shown in Fig.1. Temperature could be recorded in the
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instrument with an accuracy of 0.1 K. A small amount of the sample was loaded into a
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Al2O3 crucible, then the crucible is placed in the heating pit. The experiments were
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carried-out from room temperature up to 800 °C at a heating-rate of 10 °C·min-1 in
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purified nitrogen atmosphere. The cooling rate was also set at 10 °C·min-1. This
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process is repeated for at least six times to ensure reproducibility of the results. For
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each measurement, we all use the new corresponding sample. The same heat flow
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behavior with temperature was observed with the salt in several repeated experiments.
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Hence it can be concluded that the inflection point observed in the TG curve refers to
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Fig.1The experimental set-up for thermal stability
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(4) Component analysis. The powder X-ray diffraction measurements were carried out
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using X-ray diffractometer (Rigaku DMAX-RB) with a radiation of Cu Kα
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(λ=1.5406Å). The 2θ scans cover the range 10-85° with a step size of 0.02° and a scan
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rate of 5° min-1. JSM-5900 scanning electron microscopy (SEM) was used to observe
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the morphology of the sample, and the chemical composition was analyzed
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quantitatively by EX-250 energy dispersive X-ray spectroscopy (EDX).
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(5) The verification of melting points and thermal stability in the 3D stable molten
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temperature diagram. Melting point and thermal stability were verified by DSC/TG
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(DSC-PC200, NETZSCH, Inc.). Endothermic heat flow and temperature could be
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recorded in the instrument with an accuracy of 0.0001 m·W-1 and 0.01 K respectively.
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A small sub-sample from the solidified salt was used for simultaneous DSC/TG
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experimentation. The experiments were carried-out from room temperature up to
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800 °C at a heating-rate of 10 °C·min-1 in purified nitrogen atmosphere with a flow
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rate of 20 ml·min-1. The cooling rate was also set at 10 °C·min-1. This cycle is
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repeated for at least six times to ensure reproducibility of the results. For each
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TG curves, the decomposition temperature of sample was obtained, and the melting
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temperature could be obtained from the DSC curves.
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3. Results and discussion
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Different mass proportions of molten salt systems containing KNO3-NaNO3,
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Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2 have been tested.
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The liquidus surface is studied and a 3D stable molten temperature diagram is
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developed and discussed in the following section.
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3.1 Thermophysical properties and the 3D phase diagram
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3.1.1 Melting points of binary nitrates
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The melting points of binary nitrates (KNO3-NaNO3, Ca(NO3)2-KNO3 and
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NaNO3-Ca(NO3)2) were measured through a number of experiments(mass fraction),
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as shown in Table 2.
Table 2 Melting points (Tm) of binary nitrates (KNO3-NaNO3, Ca(NO3)2 -KNO3 and NaNO3-Ca(NO3)2) at the pressure p = 0.1 MPa KNO3/wt.% 0
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KNO3-NaNO3
Ca(NO3)2 -KNO3
NaNO3-Ca(NO3)2
Tm/°C
Ca(NO3)2 /wt.%
Tm/°C
NaNO3/wt.%
Tm/°C
306.8
0
334.0
0
561.0[22]
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10
291.1
10
299.1
10
--
20
277.1
20
265.9
20
427.4
30
251.8
30
198.6
30
347.7
40
234.7
40
150.4
40
302.0
50
227.1
50
160.2
50
238.8
60
228.7
60
245.6
60
225.8
70
263.0
70
344.1
70
248.6
80
284.8
80
414.1
80
269.8
90
313.6
90
--
90
290.0
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334.0
100
561.0[22]
100
306.8
Standard uncertainties u are u(Tm) = 0.1 °C, and u(p) = 0.005 MPa.
3.1.2 Melting points of the KNO3-NaNO3-Ca(NO3)2 ternary system The melting points of the KNO3-NaNO3-Ca(NO3)2 ternary system with different
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ratios are listed in Table 3. And the values are mostly around 200 °C. With the
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adjustment of the mass fraction of KNO3, Ca(NO3)2 and NaNO3, the melting point
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could be reduced to below 150 °C, and the mixture of 40% KNO3-20% NaNO3-40%
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Ca(NO3)2 has a melting temperature of 133.4 °C.
Table 3 Melting points (Tm) of the KNO3-NaNO3-Ca(NO3)2 ternary system at pressure p = 0.1 MPa Component/ wt.% No.
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Component/ wt.%
Tm/°C
NaNO3
Ca(NO3)2
1
10
10
80
398.2
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
10 10 10 10 10 10 10 20 20 20 20 20 20 20 30 30 30
20 30 40 50 60 70 80 10 20 30 40 50 60 70 10 20 30
70 60 50 40 30 20 10 70 60 50 40 30 20 10 60 50 40
347.3 282.5 205.7 199.6 224.5 237.1 250.3 340.5 270.9 181.5 176.8 204.1 215.2 237.3 252.0 166.8 153.4
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KNO3
No.
NaNO3
Ca(NO3)2
19
30
40
30
182.4
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
30 30 40 40 40 40 40 50 50 50 50 60 60 60 70 70 80
50 60 10 20 30 40 50 10 20 30 40 10 20 30 10 20 10
20 10 50 40 30 20 10 40 30 20 10 30 20 10 20 10 10
196.9 211.5 148.3 133.4 150.0 176.0 219.5 141.1 157.8 197.1 206.8 166.5 192.7 218.3 209.6 223.5 262.4
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Standard uncertainties u are u(Tm) = 0.1°C, and u(p) = 0.005 MPa.
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3.1.3 Coordinate transformation of raw data
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Tm/°C
KNO3
It is necessary to write a program in M file in the Matlab environment for 10
ACCEPTED MANUSCRIPT realizing the visualization of the ternary phase diagram. First, the triangular
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coordinates should be converted to rectangular coordinates. Due to constant pressure,
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only the mass fraction coordinates should be transformed into rectangular coordinates.
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Fig.2 presents the schematic of the KNO3-NaNO3-Ca(NO3)2 ternary system, the
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system makes NaNO3 as coordinate origin. Triangular coordinates, a, b and c are the
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mass fraction of KNO3, Ca(NO3)2 and NaNO3, respectively. The corresponding
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relations between the rectangular coordinates F (x, y) and triangular coordinates F (a,
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b) are:
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c =1 – b – a
(1)
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x =a + b/2
(2)
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y= √3b/2
(3)
composition points would be plotted in the triangular coordinates.
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All the conversion relations of mass fraction were completed in Matlab, and
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Fig.2 Relationship between the triangular coordinates and rectangular coordinates
3.1.4 The construction of the 3D phase diagram 11
ACCEPTED MANUSCRIPT A 3D phase diagram can be constructed after coordinate transformation.
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However, experimental data is not enough to draw a smooth liquid surface. Therefore,
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it is necessary to obtain valuable temperature data points between isotherms. More
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data could be added by “meshgrid” and “griddata” function in Matlab based on the
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experimental data. “meshgrid” function is used for dividing finer mesh within the
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scope of the selected data, and “griddata” function is used for obtaining temperature
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values of every grid point by the interpolation method based on the selected data. The
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style of invoking “griddata” function is:
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zg = griddata(x, y, Z, xg, yg, 'cubic')
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(4)
and x，y，z are the x-coordinate matrix, y-coordinate matrix and z-coordinate matrix of
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known data points, respectively; xg and yg are the xg-coordinate matrix and
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yg-coordinate matrix of the interpolation points, respectively. zg is the Z-coordinate
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matrix of the returning interpolation points. The “cubic” represents that the chosen
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interpolation method is cubic polynomial interpolation. Then a 3D phase diagram of
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the KNO3-NaNO3-Ca(NO3)2 ternary system could be constructed by the interpolation,
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as shown in Fig.3.
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Fig.3 Phase diagram of KNO3-NaNO3-Ca(NO3)2 ternary system: (a) composition profiles of molten salts in the experiment, (b) isotherm diagram, (c) liquidus projection and (d) liquidus surface of 3D phase diagram of the KNO3-NaNO3-Ca(NO3)2 system
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Fig.3 (a) is the composition profiles of molten salts in the experiment, which
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consists of a series of experimental composition points. It can be seen that the
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experiment is designed based on the principle of average distribution. Fig.3 (b) and (c)
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present the isotherm diagram and liquidus projection of the KNO3-NaNO3-Ca(NO3)2
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ternary system, respectively. Furthermore, the liquidus surface of 3D phase diagram
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of the KNO3-NaNO3- Ca(NO3)2 ternary system is shown in Fig.3 (d), which illustrates
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that the liquidus surface is smooth, and there is no sharp rise or fall on the surface.
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The whole process of the interpolation in Matlab environment is relatively simple.
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Meanwhile, the eutectic temperature and composition of the KNO3-NaNO3-Ca(NO3)2
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ternary system can be predicted by programming in Matlab, namely the predicted
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eutectic temperature is 129.1 °C and the composition is 42% KNO3-17% NaNO3-41%
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performed to describe the eutectic point (eutectic temperature and composition) of the
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KNO3- NaNO3-Ca(NO3)2 ternary system. However, all the results are not completely
248
consistent. The predicted eutectic composition obtained in this work is closer to the
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results reported by Menzies et al. [12] and Michel [15], but which still has a
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difference of up to ± 3 %. In addition, the predicted eutectic temperature is in good
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agreement with the result of Michel [15], and the two values differ in this case by less
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than 1 °C.
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Fig.4 illustrates the liquidus surface of the 3D phase diagrams of the
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KNO3-NaNO3-Ca(NO3)2 from the different angle of view. Temperature value of any
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point on the liquidus surface of the KNO3-NaNO3-Ca(NO3)2 ternary system can be
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obtained. Meanwhile, the liquidus surface of KNO3-NaNO3-Ca(NO3)2 ternary system
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could be rotated freely and viewed from different positions in Matlab. Matlab has a
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complete application development and data processing environments, but the program
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could not be run out of the Matlab environment. For beginners, there are still some
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difficulties.
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3.1.5 Thermal stability and the 3D stable molten temperature diagram The thermal decomposition point temperatures of the KNO3-NaNO3-Ca(NO3)2 ternary system with different ratios are listed in Table 4.
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Fig.4 The liquidus surface of 3D phase diagrams of KNO3-NaNO3-Ca(NO3)2 from the different angle of view
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Table 4 Thermal decomposition point temperatures (Td) of the KNO3-NaNO3-Ca(NO3)2 ternary system at pressure p = 0.1 MPa Component/ wt.% No.
Ca(NO3)2
1
0
0
100
545
2
0
100
0
650
3
100
0
0
600
4
50
0
50
5
0
50
50
6
50
50
0
7
0
80
20
8
80
0
9
80
20
10
0
20
11
40
12
40
13
40
14
20
15
20
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NaNO3
580
450
600
550
540
0
600
80
560
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50
10
600
20
40
600
60
0
600
20
60
530
60
20
600
Standard uncertainties u are u(Td) = 0.1°C, and u(p) = 0.005 MPa.
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Td/°C
KNO3
A 3D stable molten temperature diagram of the KNO3-NaNO3-Ca(NO3)2
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ternary system is constructed based on the different systems of KNO3-NaNO3,
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Ca(NO3)2- NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2, as shown in
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Fig.8. It can be seen that the 3D stable molten temperature diagram consists of the
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liquidus surface (the lower surface) and the thermal decomposition surface (the upper surface). The coordinate values of the lower surface (x, y, T1) and the upper surface (x, y, T2) could be output by running “find” function based on Matlab. T1
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and T2, respectively, represent the melting point temperature and the thermal
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decomposition point temperature of the corresponding composition, x and y are
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corresponding to the mass fraction of NaNO3 and Ca(NO3)2. As shown in Fig.5(a) 16
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eutectic point are (0.17, 0.41, 129.1) and (0.17, 0.41, 597.9), respectively. It
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indicates that the predicted melting temperature and thermal decomposition
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temperature of the 42% KNO3-17% NaNO3-41% Ca(NO3)2 ternary system are T1
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= 129.1 °C and T2 = 597.9 °C.
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Fig.5 3D stable molten temperature diagram of KNO3-NaNO3-Ca(NO3)2 ternary system and thermodynamic properties of the predicted eutectic mixture: (a) and (b) are the 3D stable molten temperature diagrams of KNO3-NaNO3-Ca(NO3)2 ternary system in different point of view(T1 and T2 are the melting point temperature and the thermal decomposition point temperature of the predicted eutectic composition ), (c) and (d) are DSC curve and TG curve of the predicted eutectic mixture
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In addition, in the 3D stable molten temperature diagram, some melting points
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are higher than upper limit temperatures, therefore, the upper surface intersects with
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the lower surface near the apex of Ca(NO3)2. The values of melting points and thermal 17
ACCEPTED MANUSCRIPT decomposition points of the KNO3-NaNO3-Ca(NO3)2 ternary system all can be
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obtained from the 3D stable molten temperature diagram. Moreover, thermal stability
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of the predicted eutectic mixture verified experimentally by TG and DSC methods is
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shown in Fig.5(c) and (d). That indicates thermal decomposition point of the predicted
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eutectic mixture is at temperatures up to 595.8 °C, DSC curve shows melting point of
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this mixture is about 125.3 °C. There are two independent endothermic peaks existing
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in the DSC curve. Prior to melting, the endothermic peak of DSC curve at 50 °C is
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due to a solid-solid type phase transition on the salt. Besides, the melting temperature
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ranges of the predicted eutectic mixture with typical melting peaks are narrow, around
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20 °C, which is appropriate and displays that the salt can melt in a relatively short
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range of temperature and period of time.
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The predicted melting temperature (T1 = 129.1 °C) of the eutectic mixture is
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about 3.8 °C higher than the experimentally determined value (125.3 °C) and the
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predicted thermal decomposition point (T2 = 597.9 °C) is about 2.1 °C higher than the
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corresponding experimental value (595.8 °C). Possibly, there exists a systematical
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error in temperature measurement. On the whole, the tendency of the experimental
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results is in excellent agreement with the predicted phase diagram, within the range of
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error permitting.
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3.1.6 Component analysis
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To examine the composition of the eutectic salt (42% KNO3-17% NaNO3-41%
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Ca(NO3)2), X-ray diffraction and energy dispersive X-ray spectroscopy (EDX) were
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utilized to study the phases. Fig.6 (a) showed the XRD patterns of NaNO3, KNO3 and 18
ACCEPTED MANUSCRIPT Ca(NO3)2, by comparing the patterns with the standard card, all of the corresponding
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peaks were confirmed to arise from NaNO3, KNO3 and Ca(NO3)2. The XRD pattern
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of the eutectic salt was shown in Fig.6 (b), it could be seen that the intensity as well as
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the degree of KNO3 and NaNO3 did not change, as for the corresponding pure nitrate,
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which demonstrated the excellent phase stability of this component. However, there
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was no diffraction peak of Ca(NO3)2. Binding the literatures [23] and [24], it could be
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preliminarily inferred that there may be some double salts (KNO3·Ca(NO3)2 or
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4KNO3·Ca(NO3)2) existing in the system. By comparing the patterns with the
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standard card, some of the peaks were confirmed to arise from KNO3·Ca(NO3)2 and
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4KNO3·Ca(NO3)2, and the existence of them led to a lower melting point of this
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component. Therefore, it can be seen that the composition of the eutectic salt contains
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KNO3, NaNO3, KNO3·Ca(NO3)2 and 4KNO3·Ca(NO3)2, and the phases were marked
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in the corresponding degree in Fig.6 (b).
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Fig. 6 XRD patterns: (a) NaNO3/KNO3/Ca(NO3)2; (b) the eutectic salt
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Fig.7 (a) showed the SEM image of eutectic salt, Fig.7 (b) was the energy
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dispersive X-ray spectroscopy (EDX) of the Spectrum 1 area corresponding in Fig.7
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(a). From Fig.7 (b), it can be seen that the eutectic salt was composed of the element 19
ACCEPTED MANUSCRIPT K, Ca, Na, O and N. This further validated that the formation of double salts led to no
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diffraction peak of Ca(NO3)2 in the XRD pattern of the eutectic salt (Fig.6 (b)). In
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addition, the quantitative results of the relevant elements for EDX were shown in
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Table 5. Through computational verification, the experimental element content was
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basically consistent with the theoretical value, within the range of error permitting.
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Fig. 7 Spectrum analysis of the eutectic salt: (a) SEM image; (b) EDX
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Table 5 The quantitative results for EDX Weight %
Atom %
Formula
Compnd %
NK
13.87
18.71
N
13.87
55.71
65.75
O
55.71
2.89
2.37
Na
2.89
14.91
7.21
K
14.91
12.62
5.96
Ca
12.62
100.00
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Na K KK
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Ca K
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Total
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100.00
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3.1.7 The verification of thermophysical properties in the 3D stable molten
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temperature diagram
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Any three compositions were selected to verify the thermodynamic properties in
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the 3D stable molten temperature diagram. The samples were prepared for differential 20
ACCEPTED MANUSCRIPT thermal gravimetric analysis (TG and DSC), as shown in Fig.8. The comparison of the
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experimental values and the obtained values from the 3D stable molten temperature
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diagram are presented in Table 6.
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Fig.8 Thermodynamic properties of the KNO3-NaNO3-Ca(NO3)2 ternary system in different
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ratios:(a) and (b) are DSC curves and TG curves of the samples, (c) obtained values from 3D stable molten temperature diagram of the KNO3-NaNO3-Ca(NO3)2 ternary system
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Table 6 Comparison of predicted and experimental temperatures of the KNO3-NaNO3-Ca(NO3)2 ternary system at pressure p = 0.1 MPa
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40
Tm/°C
Td /°C
NaNO3
Ca(NO3)2
predicted
experimental
predicted
50
10
219.5
217.4
597.7
599.0
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KNO3
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Component/wt.% No.
experimental
2
40
52
8
225.4
221.5
597.2
595.5
3
38
57
5
230.7
227.1
599.9
601.1
Standard uncertainties u are u(T) = 0.1°C, and u(p) = 0.005 MPa.
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Fig.8 (a) presents that the typical DSC thermograms of the selected samples, the
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DSC curves reveal a corresponding prominent endothermic peak at 217.4 °C,
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221.5 °C and 227.1 °C, respectively, indicating the melting temperatures of the
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samples. From Fig.8 (b), the TG curves have a slight downward trend when the 21
ACCEPTED MANUSCRIPT temperature rises to 100 °C. Obviously, the reason is that the samples still have some
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residual moisture, which may be water-absorbing effect of the molten salts or residual
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crystal water of calcium nitrate. The corresponding upper limit temperatures are about
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599.0 °C, 595.5°C and 601.1 °C, respectively. When the corresponding upper limit
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temperatures are exceeded, the curves show a clear downward trend, indicating the
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thermal decomposition of the molten salts.
Fig.8 (c) illustrates that the predicted melting points and thermal decomposition
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points of samples 1-3 are about T1
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225.4 °C and T2
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By comparing the predicted and the experimentally determined values in Table 4, it is
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found that the absolute difference value is less than 4 °C. The predicted values of the
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samples and that determined experimentally are in excellent agreement. Therefore, it
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is believed reasonably that the 3D stable molten temperature diagram of the
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KNO3-NaNO3-Ca(NO3)2 ternary system is reliable. Finally, all these result would be
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helpful to further study on the KNO3-NaNO3-Ca(NO3)2 ternary system for solar
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energy storage.
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4. Conclusions
= 597.7 °C, T1
(2)
=
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(3)
= 230.7 °C and T2
(3)
= 599.9 °C, respectively.
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= 219.5 °C and T2
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(2)
(1)
The visualization of the phase diagram of the KNO3-NaNO3-Ca(NO3)2 ternary
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system is achieved by Matlab. A 3D stable molten temperature diagram provides
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convenience for predicting the temperature span of the KNO3-NaNO3-Ca(NO3)2
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ternary system as a heat transfer medium.
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A 3D stable molten temperature diagram was developed to predict melting points 22
ACCEPTED MANUSCRIPT and thermal decomposition points of the KNO3-NaNO3-Ca(NO3)2 ternary system
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for the first time, as well as its eutectic temperature and composition. It is helpful to
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further analyze and study the ternary system.
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The composition, melting temperature and upper limit temperature of the eutectic
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mixture are predicted, namely 42% KNO3-17% NaNO3-41% Ca(NO3)2, 129.1 °C
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and 597.9 °C. Moreover, the predicted melting point of the eutectic mixture is
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consistent with that determined experimentally.
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The predicted melting and thermal decomposition temperatures of the selected
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compositions were respectively verified experimentally using DSC and TG methods.
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And the experimental results were in excellent agreement with predicted values
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from the 3D stable molten temperature diagram.
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Furthermore, isotherm diagram, liquidus projection and liquidus surface of the 3D
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phase diagram of the KNO3-NaNO3-Ca(NO3)2 ternary system can be constructed by
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interpolation method in Matlab. However, the disadvantage is that crystallization
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lines between two-phase regions of the 3D phase diagram cannot be displayed,
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which should be further improved.
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Acknowledgements This work was supported by the National Key Research and Development
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Program of China (No. 2016YFC0205500), National Natural Science Foundation of
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China (No. 51272105), Jiangsu Provincial Science and Technology Supporting
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ACCEPTED MANUSCRIPT Program (BE2013718), Project Funded by the Priority Academic Program
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Development of Jiangsu Higher Education Institutions (PAPD).
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References
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[1] X. Wei, Q. Peng, J. Ding, X. Yang, J. Yang, B. Long, Theoretical study on thermal stability of molten salt for solar thermal power, Appl. Therm. Eng. 54 (2013) 140-144. [2] Y. Tian, C Y. Zhao, A review of solar collectors and thermal energy storage in solar thermal applications, Appl. Energ. 104 (2013) 538-553. [3] Q. Peng, X. Yang, J. Ding, X. Wei, J. Yang, Design of new molten salt thermal energy storage material for solar thermal power plant, Appl. Energ.112 (2013) 682-689. [4] L. Ye, C. Tang, Y. Chen, S. Yang, M. Tang, The thermal physical properties and stability of the eutectic composition in a Na2CO3–NaCl binary system. Thermochim. Acta 596 (2014) 14-20. [5] K. Vignarooban, X. Xu, A. Arvay, K. Hsu, A. M. Kannan, and J. Yan, Heat transfer fluids for concentrating solar power systems–A review, Appl. Energ.146 (2015) 383-396. [6] J. C. Gomez, N. Calvet, A. K. Starace, G. C. Glatzmaie, Ca(NO3)2-NaNO3-KNO3 Molten Salt Mixtures for Direct Thermal Energy Storage Systems in Parabolic Trough Plants, J. Sol. Energy Eng. 135 (2013) 420-431. [7] A. Gil, M. Medrano, I. Martorell, A. Lazaro, P. Dolado, B. Zalba, L. F. Cabeza, State of the art on high temperature thermal energy storage for power generation.Part1-Concepts, materials and modellization, Renew. Sust. Energ. Rev. 14 (2010) 31-55. [8] N. Ren, Y. T. Wu, C. F. Ma, L. X. Sang, Preparation and thermal properties of quaternary mixed nitrate with low melting point, Sol. Energ. Mat. Sol. C. 127 (2014) 6-13. [9] J. W. Raade, D. Padowitz, Development of molten salt heat transfer fluid with low melting point and high thermal stability, J. Sol. Energy Eng.133 ( 2011) 91-96. [10] Y. A. Chang, S. Chen, F. Zhang, X. Yan, F. Xie, Phase diagram calculation: past, present and future, Prog. Mater Sci. 49 (2004) 313-345. [11] J. E. Pacheco, S. K. Showalter, W. J. Kolb, Development of a molten-salt thermocline thermal storage system for parabolic trough plants, J. Sol. Energy Eng. 124 (2002) 153-159. [12] A. W. C. Menzies, N. N. Dutt, The Liquidus Surface of the Ternary System Composed of the Nitrates of Potassium, Sodium, and Calcium, J. Am. Chem. Soc. 33 (1911) 1366-1375. [13] E. Jänecke, The Quaternary System Na, K, Ca, Mg//NO3 and Its Subsystems, Z. Elektrochem. Angew. Phys. Chem. 48 (1942) 453-467. [14] A. G. Bergman, I. S. Rassonskaya, N. E. Shmidt, Izvest. Sektora. Fiz.-Khim. Anal., Inst. Obshch. Neorg. Khim. Tr. Fiz. Inst. Akad. Nauk SSSR, 26 (1955)156-163. [15] D. I. Michel, Use of a ternary mixture of salts as a heat transmitting medium and/or as a heat storage medium, EP0049761.1982. [16] R. W. Bradshaw, D.E. Meeker, High-temperature stability of ternary nitrate molten salts for solar thermal energy systems, Sol. Energ. Mater. 21 (1990) 51-60. [17] D. Kearney, U. Herrmann, P. Nava, B. Kelly, R. Mahoney, J. Pacheco, R. Cable, N. Potrovitza, D. Blake, H. Price, Assessment of a molten salt heat transfer fluid in a parabolic trough solar field, J. Sol. Energy Eng.125 (2003) 170-176. [18] S. Akıska, DisChart: a new application for building and drawing discrimination diagrams
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using digitize method, Earth Sci. Inform. 8 (2015) 799-807. [19] I. Olejarczyk-Wożeńska, H. Adrian, B. Mrzygłód, Mathematical Model of the Processoof Pearlite Austenitization, Arch. Metall. Mater. 59 (2014) 981-986. [20] B. Sundman, B. Jansson, J. O. Andersson. The thermo-calc databank system, Calphad 9 (1985) 153-190. [21] R. I. Olivares, W. Edwards, LiNO3–NaNO3–KNO3 salt for thermal energy storage: thermal stability evaluation in different atmospheres, Thermochim. Acta 560 (2013) 34-42. [22] P. I. Protsenko, A. G. Bergman, Ternary system of fused nitrates of calcium, potassium, and sodium, Russ. J. Inorg. Chem. 20 (1950) 1365-1375. [23] J. C. Gomez, N. Calvet, A. K. Starace, G. C. Glatzmaier, Ca(NO3)2-NaNO3-KNO3 molten malt mixtures for direct thermal energy storage systems in parabolic trough plants, J. Sol. Energy Eng. 135 (2013) 420-431. [24] I. I. IIyasov, T. I. Dunaeva, Ternary system of potassium, cesium, and calcium nitratres, Russ. J. Inorg. Chem. 22 (1977) 265-267.
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Highlights A 3D stable molten temperature diagram of the system (K, Na, Ca/NO3) is developed.
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The eutectic composition and temperature span of the system were predicted.
The predicted eutectic composition is 42% KNO3-17% NaNO3-41% Ca(NO3)2.
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The predicted eutectic mixture has a wide temperature span from 129.1 to
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S0042-207X(17)30961-2
DOI:
10.1016/j.vacuum.2017.09.003
Reference:
VAC 7573
To appear in:
Vacuum
Received Date: 18 July 2017 Revised Date:
3 August 2017
Accepted Date: 4 September 2017
Please cite this article as: Chen M, Shen Y, Zhu S, Li P, Digital phase diagram and thermophysical properties of KNO3-NaNO3-Ca(NO3)2 ternary system for solar energy storage, Vacuum (2017), doi: 10.1016/j.vacuum.2017.09.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Digital phase diagram and thermophysical properties of
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KNO3-NaNO3-Ca(NO3)2 ternary system
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for solar energy storage
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Mengmeng Chen1, Yuesong Shen1*, Shemin Zhu1*, Peiwen Li2
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1 ： Jiangsu Collaborative Innovation Center for Advanced Inorganic Function
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Composites, Jiangsu National Synergetic Innovation Center for Advanced Materials
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(SICAM), College of Materials Science and Engineering, Nanjing Tech University,
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Nanjing 210009, China
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2: Department of Aerospace and Mechanical Engineering, University of Arizona,
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Tucson, AZ 85721, United States
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*Corresponding author: Prof. Yuesong Shen.
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State Key Laboratory of Materials-Oriented Chemical Engineering, College of
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Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P.R.
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China
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Address: No.5 Xinmofan Road, Nanjing Tech University, College of Materials
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Science and Engineering, 210009, Nanjing, China
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E-mails: [email protected] (Y.S. Shen)
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Tel: +86 25 83587927
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Fax: +86 25 83582195
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ACCEPTED MANUSCRIPT Abstract: The molten nitrate salt has become one of the most promising heat storage
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and transfer medium for solar energy. A series of molten salt systems containing
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KNO3-NaNO3, Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2
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were designed and prepared to study the liquidus surface, and the thermal stability of
27
the KNO3-NaNO3-Ca(NO3)2 ternary system was mainly studied. A 3D stable molten
28
temperature diagram was developed to predict melting points and thermal
29
decomposition points in the KNO3-NaNO3-Ca(NO3)2 ternary system, as well as its
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eutectic temperature and composition. The predicted eutectic composition is 42%
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KNO3-17% NaNO3-41% Ca(NO3)2, which has a low predicted melting point of
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129.1 °C and a high predicted decomposition temperature of 597.9 °C. Moreover, a
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series of melting points and thermal decomposition points obtained from the 3D stable
34
molten temperature diagram were respectively verified experimentally using
35
thermo-gravimetric (TG) and differential scanning calorimetry (DSC) methods. The
36
experimental results were in excellent agreement with that of obtained values from the
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3D stable molten temperature diagram.
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Key words: Molten salt; KNO3-NaNO3-Ca(NO3)2; Thermal stability; 3D stable
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molten temperature diagram
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1. Introduction
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Energy crisis and environmental pollution are two major problems in the world,
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as a clean and renewable energy, the solar energy has clean pollution-free and
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inexhaustible supply outstanding merits and becomes one of the most promising new
44
energies [1]. However, solar energy is affected by the weather and the shift of day and 2
ACCEPTED MANUSCRIPT night, and has lower energy density. How to utilize solar energy high-effectively has
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become a research focus of new energy source field. The present status of solar energy
47
utilization includes solar energy photo-thermal utilization, photovoltaic utilization and
48
photochemical utilization, among them the photo-thermal utilization requires the
49
lowest cost and obtains the highest efficiency, also it has become the most promising
50
solar energy utilization technology [2]. Meanwhile, heat storage and transfer media is
51
one of the critical techniques of solar thermal conversion. Molten salts with merits of
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high thermal stability, large specific heat, moderate density, low viscosity, low vapor
53
pressure and low cost have become a research hotspot and development direction of
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heat storage and transfer media [3].
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Common molten salts including nitrates, sulfates, carbonates, chlorides and
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fluorides used as solar energy heat storage and transfer media have been
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systematically studied until now [4], particularly the molten nitrates have been widely
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applied to large scale experimental and commercial solar thermal power plants based
59
on their lower cost and better thermal properties synthetically. Solar salt [5] (60%
60
NaNO3-40% KNO3), Hitec [6] (7% NaNO3-53% KNO3-40% NaNO2) and Hitec XL
61
[7] (45% KNO3-7% NaNO3-48% Ca(NO3)2), as current well-known heat storage
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media, have been applied in large scale experimental and commercial solar thermal
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power plants. However, the highest operating temperature of most mixed nitrates is
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about 600 °C, once the upper limit temperature is exceeded, the mixed nitrates begin
65
to decompose. The quaternary and penta-basic mixed nitrates also have been reported
66
in recent research in order to widen phase change temperature and improve the latent
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ACCEPTED MANUSCRIPT heat. Ren et al. exploited a quaternary mixed nitrates system of KNO3-NaNO3-LiNO3-
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Ca(NO3)2·4H2O, and found that their melting point could be down to below 90 °C
69
while their decomposition temperature could be above 600 °C [8]. Raade et al. tested
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over 5000 unique mixtures of inorganic salts during the screening process, resulting in
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a low melting point of 65 °C and a thermal stability limit over 500 °C [9]. Overall, it
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was a big challenge to complete the large number of possible combinations of salt
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mixtures.
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At present, it is an excellent way to study solidification, solid-state reaction,
75
phase transformation, and oxidation of molten salts by phase diagrams. Phase
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diagrams are the foundation in performing basic materials research, it serves as a road
77
map for materials design and process optimization [10]. In order to show the complete
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phase diagram, 3D space coordinate must be adopted, namely 3D phase diagram
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which is determined by the concentration plane and the corresponding vertical
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temperature coordinate. Moreover, a 3D phase diagram is intuitive and informative.
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The presented work investigates the melting point of the Hitec XL is the lowest
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among the Solar salt, Hitec and Hitec XL. Hitec XL has a composition of 45%
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KNO3-7% NaNO3-48% Ca(NO3)2, it contains 59% water [11], and the composition of
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the mixture is eutectic after the dehydration, with a content of 43% KNO3-15%
85
NaNO3-42% Ca(NO3)2 [12]. Until now, the ternary system has been reported by many
86
researches. Menzies et al. [12] developed ternary phase diagram of the system and
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determined that the 43% KNO3-15% NaNO3-42% Ca(NO3)2 composition had the
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lowest melting point of 175 °C for the first time. Then Jänecke [13] and Bergman et al.
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ACCEPTED MANUSCRIPT [14] also published ternary phase diagrams, the eutectic compositions were 27%
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KNO3-10% NaNO3-63% Ca(NO3)2 (melting point of 130 °C) and 30% KNO3-9%
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NaNO3-61% Ca(NO3)2 (melting point of 133 °C), respectively. Moreover, the most
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widely accepted phase diagram of the system was developed by Bergman et al. In
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1982, a patent reported a mixture of 44% KNO3-12% NaNO3-44% Ca(NO3)2, which
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had a melting point of 130 °C[15]. Bradshaw et al. [16] and Kearney et al. [17]
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investigated (43-50%) KNO3-(7-34%) NaNO3-(16-48%) Ca(NO3)2 mixtures, their
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melting points ranged from 120 °C to 190 °C. The investigations shows that the
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eutectic temperature and composition of the KNO3-NaNO3-Ca(NO3)2 ternary system
98
are not consistent, or even a great difference. In addition, there have been no
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systematic reports about thermal decomposition points of the KNO3-NaNO3-
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Ca(NO3)2 ternary system so far. Therefore, it is of great practical significance to
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construct the 3D phase diagram of the KNO3-NaNO3-Ca(NO3)2 ternary system.
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Current phase diagram drawing softwares include MS Excel [18] and Delphi [19],
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and some professional phase calculation softwares, such as Thermo-calc [20] and
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FACT [21], but these softwares can only construct the phase planes. In order to realize
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the visualization of the KNO3-NaNO3-Ca(NO3)2 ternary phase diagram, Matlab is
106
adopted to study it. A 3D phase diagram can be rotated freely and viewed from
107
different positions in the Matlab environment, meanwhile, the eutectic point of the
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ternary system and text labels can be marked by programming with Matlab language.
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At present, Matlab has become a common senior data analysis and drawing tool for
110
researchers and engineers based on its actual simple, open-source and credible.
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ACCEPTED MANUSCRIPT In this work, we mainly studied 3D stable molten temperature diagram of the
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KNO3-NaNO3-Ca(NO3)2 ternary system based on the different systems of KNO3-
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NaNO3, Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2. Also as
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part of this effort, we predicted the eutectic composition and its temperature span.
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This will provide valuable data and basic theory support for its industrialization.
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Moreover, this study suggests that this method can be successfully extended to predict
117
the eutectic points in other molten salt systems.
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2. Experimental scheme
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In order to obtain a 3D stable molten temperature diagram of the KNO3-NaNO3-
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Ca(NO3)2 ternary system, a series of molten salt systems containing KNO3-NaNO3,
121
Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2 were designed and
122
prepared. Thermophysical properties were measured, including melting point and
123
thermal stability.
124
(1) Preparation of mixed molten salts. The nitrates in the present study were KNO3,
125
NaNO3 and Ca(NO3)2·4H2O with a purity of 99 %, and the
126
chemical
127
design principle of the average distribution, the systems of KNO3-NaNO3, Ca(NO3)2-
128
NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2 were prepared in different
129
ratios. Then, the pre-dried pure chemicals were melted inside an alumina crucible in
130
the furnace and fused at 350 °C for at least 24 h with periodic mixing in order to
131
obtain sufficient homogenization. Finally, the mixture was taken out, ground, dried
132
and stored for experiments.
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summary
of
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6
ACCEPTED MANUSCRIPT 133
Table 1 Chemical sample information for experiment Chemical Name
Source
Chemical Specifications
Initial Mass Fraction Purity
Preprocessing Method
KNO3 NaNO3 Ca(NO3)2·4H2O
Xi-long Chemical Xi-long Chemical Xi-long Chemical
AR AR AR
≥99.0 ≥99.0 ≥99.0
vacuum drying vacuum drying thermal decomposition
(2) Melting point measurement. Melting points of molten salts were measured by an
135
infrared thermometer (HT-8963, HCJYET, Inc.), under the atmosphere protection of
136
nitrogen. The accuracy of temperature collection in the instrument is 0.1 K. The
137
prepared molten salt sample was heated and maintained at 300 °C for 10 h to remove
138
the absorbed water, and then a small sub-sample was loaded into an alumina crucible.
139
The crucible was placed on a resistance furnace, the temperature was slowly raised.
140
When the sample was melted into liquid completely, the temperature value was tested
141
immediately by an infrared thermometer. The experiment was repeated several times
142
to verify the recurrence of the melting temperature in the system.
143
(3) Thermal stability measurement. Thermal stability was measured by the
144
experimental set-up as shown in Fig.1. Temperature could be recorded in the
145
instrument with an accuracy of 0.1 K. A small amount of the sample was loaded into a
146
Al2O3 crucible, then the crucible is placed in the heating pit. The experiments were
147
carried-out from room temperature up to 800 °C at a heating-rate of 10 °C·min-1 in
148
purified nitrogen atmosphere. The cooling rate was also set at 10 °C·min-1. This
149
process is repeated for at least six times to ensure reproducibility of the results. For
150
each measurement, we all use the new corresponding sample. The same heat flow
151
behavior with temperature was observed with the salt in several repeated experiments.
152
Hence it can be concluded that the inflection point observed in the TG curve refers to
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134
7
ACCEPTED MANUSCRIPT the decomposition temperature of the sample.
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153
SC
154
Fig.1The experimental set-up for thermal stability
156
(4) Component analysis. The powder X-ray diffraction measurements were carried out
157
using X-ray diffractometer (Rigaku DMAX-RB) with a radiation of Cu Kα
158
(λ=1.5406Å). The 2θ scans cover the range 10-85° with a step size of 0.02° and a scan
159
rate of 5° min-1. JSM-5900 scanning electron microscopy (SEM) was used to observe
160
the morphology of the sample, and the chemical composition was analyzed
161
quantitatively by EX-250 energy dispersive X-ray spectroscopy (EDX).
162
(5) The verification of melting points and thermal stability in the 3D stable molten
163
temperature diagram. Melting point and thermal stability were verified by DSC/TG
164
(DSC-PC200, NETZSCH, Inc.). Endothermic heat flow and temperature could be
165
recorded in the instrument with an accuracy of 0.0001 m·W-1 and 0.01 K respectively.
166
A small sub-sample from the solidified salt was used for simultaneous DSC/TG
167
experimentation. The experiments were carried-out from room temperature up to
168
800 °C at a heating-rate of 10 °C·min-1 in purified nitrogen atmosphere with a flow
169
rate of 20 ml·min-1. The cooling rate was also set at 10 °C·min-1. This cycle is
170
repeated for at least six times to ensure reproducibility of the results. For each
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8
ACCEPTED MANUSCRIPT measurement, we all use the new corresponding sample. Then through analyzing the
172
TG curves, the decomposition temperature of sample was obtained, and the melting
173
temperature could be obtained from the DSC curves.
174
3. Results and discussion
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171
Different mass proportions of molten salt systems containing KNO3-NaNO3,
176
Ca(NO3)2-NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2 have been tested.
177
The liquidus surface is studied and a 3D stable molten temperature diagram is
178
developed and discussed in the following section.
179
3.1 Thermophysical properties and the 3D phase diagram
180
3.1.1 Melting points of binary nitrates
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The melting points of binary nitrates (KNO3-NaNO3, Ca(NO3)2-KNO3 and
182
NaNO3-Ca(NO3)2) were measured through a number of experiments(mass fraction),
183
as shown in Table 2.
Table 2 Melting points (Tm) of binary nitrates (KNO3-NaNO3, Ca(NO3)2 -KNO3 and NaNO3-Ca(NO3)2) at the pressure p = 0.1 MPa KNO3/wt.% 0
EP
KNO3-NaNO3
Ca(NO3)2 -KNO3
NaNO3-Ca(NO3)2
Tm/°C
Ca(NO3)2 /wt.%
Tm/°C
NaNO3/wt.%
Tm/°C
306.8
0
334.0
0
561.0[22]
AC C
184 185
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181
10
291.1
10
299.1
10
--
20
277.1
20
265.9
20
427.4
30
251.8
30
198.6
30
347.7
40
234.7
40
150.4
40
302.0
50
227.1
50
160.2
50
238.8
60
228.7
60
245.6
60
225.8
70
263.0
70
344.1
70
248.6
80
284.8
80
414.1
80
269.8
90
313.6
90
--
90
290.0
9
ACCEPTED MANUSCRIPT 100 186 187
334.0
100
561.0[22]
100
306.8
Standard uncertainties u are u(Tm) = 0.1 °C, and u(p) = 0.005 MPa.
3.1.2 Melting points of the KNO3-NaNO3-Ca(NO3)2 ternary system The melting points of the KNO3-NaNO3-Ca(NO3)2 ternary system with different
189
ratios are listed in Table 3. And the values are mostly around 200 °C. With the
190
adjustment of the mass fraction of KNO3, Ca(NO3)2 and NaNO3, the melting point
191
could be reduced to below 150 °C, and the mixture of 40% KNO3-20% NaNO3-40%
192
Ca(NO3)2 has a melting temperature of 133.4 °C.
Table 3 Melting points (Tm) of the KNO3-NaNO3-Ca(NO3)2 ternary system at pressure p = 0.1 MPa Component/ wt.% No.
M AN U
193 194
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188
Component/ wt.%
Tm/°C
NaNO3
Ca(NO3)2
1
10
10
80
398.2
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
10 10 10 10 10 10 10 20 20 20 20 20 20 20 30 30 30
20 30 40 50 60 70 80 10 20 30 40 50 60 70 10 20 30
70 60 50 40 30 20 10 70 60 50 40 30 20 10 60 50 40
347.3 282.5 205.7 199.6 224.5 237.1 250.3 340.5 270.9 181.5 176.8 204.1 215.2 237.3 252.0 166.8 153.4
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KNO3
No.
NaNO3
Ca(NO3)2
19
30
40
30
182.4
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
30 30 40 40 40 40 40 50 50 50 50 60 60 60 70 70 80
50 60 10 20 30 40 50 10 20 30 40 10 20 30 10 20 10
20 10 50 40 30 20 10 40 30 20 10 30 20 10 20 10 10
196.9 211.5 148.3 133.4 150.0 176.0 219.5 141.1 157.8 197.1 206.8 166.5 192.7 218.3 209.6 223.5 262.4
195 196
Standard uncertainties u are u(Tm) = 0.1°C, and u(p) = 0.005 MPa.
197
3.1.3 Coordinate transformation of raw data
198
Tm/°C
KNO3
It is necessary to write a program in M file in the Matlab environment for 10
ACCEPTED MANUSCRIPT realizing the visualization of the ternary phase diagram. First, the triangular
200
coordinates should be converted to rectangular coordinates. Due to constant pressure,
201
only the mass fraction coordinates should be transformed into rectangular coordinates.
202
Fig.2 presents the schematic of the KNO3-NaNO3-Ca(NO3)2 ternary system, the
203
system makes NaNO3 as coordinate origin. Triangular coordinates, a, b and c are the
204
mass fraction of KNO3, Ca(NO3)2 and NaNO3, respectively. The corresponding
205
relations between the rectangular coordinates F (x, y) and triangular coordinates F (a,
206
b) are:
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199
207
c =1 – b – a
(1)
208
x =a + b/2
(2)
209
y= √3b/2
(3)
composition points would be plotted in the triangular coordinates.
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All the conversion relations of mass fraction were completed in Matlab, and
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212 213 214
Fig.2 Relationship between the triangular coordinates and rectangular coordinates
3.1.4 The construction of the 3D phase diagram 11
ACCEPTED MANUSCRIPT A 3D phase diagram can be constructed after coordinate transformation.
216
However, experimental data is not enough to draw a smooth liquid surface. Therefore,
217
it is necessary to obtain valuable temperature data points between isotherms. More
218
data could be added by “meshgrid” and “griddata” function in Matlab based on the
219
experimental data. “meshgrid” function is used for dividing finer mesh within the
220
scope of the selected data, and “griddata” function is used for obtaining temperature
221
values of every grid point by the interpolation method based on the selected data. The
222
style of invoking “griddata” function is:
M AN U
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215
zg = griddata(x, y, Z, xg, yg, 'cubic')
223
(4)
and x，y，z are the x-coordinate matrix, y-coordinate matrix and z-coordinate matrix of
225
known data points, respectively; xg and yg are the xg-coordinate matrix and
226
yg-coordinate matrix of the interpolation points, respectively. zg is the Z-coordinate
227
matrix of the returning interpolation points. The “cubic” represents that the chosen
228
interpolation method is cubic polynomial interpolation. Then a 3D phase diagram of
229
the KNO3-NaNO3-Ca(NO3)2 ternary system could be constructed by the interpolation,
230
as shown in Fig.3.
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12
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Fig.3 Phase diagram of KNO3-NaNO3-Ca(NO3)2 ternary system: (a) composition profiles of molten salts in the experiment, (b) isotherm diagram, (c) liquidus projection and (d) liquidus surface of 3D phase diagram of the KNO3-NaNO3-Ca(NO3)2 system
234
Fig.3 (a) is the composition profiles of molten salts in the experiment, which
235
consists of a series of experimental composition points. It can be seen that the
236
experiment is designed based on the principle of average distribution. Fig.3 (b) and (c)
237
present the isotherm diagram and liquidus projection of the KNO3-NaNO3-Ca(NO3)2
238
ternary system, respectively. Furthermore, the liquidus surface of 3D phase diagram
239
of the KNO3-NaNO3- Ca(NO3)2 ternary system is shown in Fig.3 (d), which illustrates
240
that the liquidus surface is smooth, and there is no sharp rise or fall on the surface.
241
The whole process of the interpolation in Matlab environment is relatively simple.
242
Meanwhile, the eutectic temperature and composition of the KNO3-NaNO3-Ca(NO3)2
243
ternary system can be predicted by programming in Matlab, namely the predicted
244
eutectic temperature is 129.1 °C and the composition is 42% KNO3-17% NaNO3-41%
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231 232 233
13
ACCEPTED MANUSCRIPT Ca(NO3)2. As summarized in the literatures [11-17], several measurements have been
246
performed to describe the eutectic point (eutectic temperature and composition) of the
247
KNO3- NaNO3-Ca(NO3)2 ternary system. However, all the results are not completely
248
consistent. The predicted eutectic composition obtained in this work is closer to the
249
results reported by Menzies et al. [12] and Michel [15], but which still has a
250
difference of up to ± 3 %. In addition, the predicted eutectic temperature is in good
251
agreement with the result of Michel [15], and the two values differ in this case by less
252
than 1 °C.
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Fig.4 illustrates the liquidus surface of the 3D phase diagrams of the
254
KNO3-NaNO3-Ca(NO3)2 from the different angle of view. Temperature value of any
255
point on the liquidus surface of the KNO3-NaNO3-Ca(NO3)2 ternary system can be
256
obtained. Meanwhile, the liquidus surface of KNO3-NaNO3-Ca(NO3)2 ternary system
257
could be rotated freely and viewed from different positions in Matlab. Matlab has a
258
complete application development and data processing environments, but the program
259
could not be run out of the Matlab environment. For beginners, there are still some
260
difficulties.
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14
266 267 268 269 270
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3.1.5 Thermal stability and the 3D stable molten temperature diagram The thermal decomposition point temperatures of the KNO3-NaNO3-Ca(NO3)2 ternary system with different ratios are listed in Table 4.
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Fig.4 The liquidus surface of 3D phase diagrams of KNO3-NaNO3-Ca(NO3)2 from the different angle of view
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261 262 263
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271 272 273 274 15
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Table 4 Thermal decomposition point temperatures (Td) of the KNO3-NaNO3-Ca(NO3)2 ternary system at pressure p = 0.1 MPa Component/ wt.% No.
Ca(NO3)2
1
0
0
100
545
2
0
100
0
650
3
100
0
0
600
4
50
0
50
5
0
50
50
6
50
50
0
7
0
80
20
8
80
0
9
80
20
10
0
20
11
40
12
40
13
40
14
20
15
20
SC
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NaNO3
580
450
600
550
540
0
600
80
560
M AN U
20
50
10
600
20
40
600
60
0
600
20
60
530
60
20
600
Standard uncertainties u are u(Td) = 0.1°C, and u(p) = 0.005 MPa.
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277
Td/°C
KNO3
A 3D stable molten temperature diagram of the KNO3-NaNO3-Ca(NO3)2
279
ternary system is constructed based on the different systems of KNO3-NaNO3,
280
Ca(NO3)2- NaNO3, KNO3-Ca(NO3)2 and KNO3-NaNO3-Ca(NO3)2, as shown in
281
Fig.8. It can be seen that the 3D stable molten temperature diagram consists of the
283 284
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282
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liquidus surface (the lower surface) and the thermal decomposition surface (the upper surface). The coordinate values of the lower surface (x, y, T1) and the upper surface (x, y, T2) could be output by running “find” function based on Matlab. T1
285
and T2, respectively, represent the melting point temperature and the thermal
286
decomposition point temperature of the corresponding composition, x and y are
287
corresponding to the mass fraction of NaNO3 and Ca(NO3)2. As shown in Fig.5(a) 16
ACCEPTED MANUSCRIPT and (b), the coordinate values of the lower surface and upper surface of the
289
eutectic point are (0.17, 0.41, 129.1) and (0.17, 0.41, 597.9), respectively. It
290
indicates that the predicted melting temperature and thermal decomposition
291
temperature of the 42% KNO3-17% NaNO3-41% Ca(NO3)2 ternary system are T1
292
= 129.1 °C and T2 = 597.9 °C.
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288
293 294 295 296 297 298
Fig.5 3D stable molten temperature diagram of KNO3-NaNO3-Ca(NO3)2 ternary system and thermodynamic properties of the predicted eutectic mixture: (a) and (b) are the 3D stable molten temperature diagrams of KNO3-NaNO3-Ca(NO3)2 ternary system in different point of view(T1 and T2 are the melting point temperature and the thermal decomposition point temperature of the predicted eutectic composition ), (c) and (d) are DSC curve and TG curve of the predicted eutectic mixture
299
In addition, in the 3D stable molten temperature diagram, some melting points
300
are higher than upper limit temperatures, therefore, the upper surface intersects with
301
the lower surface near the apex of Ca(NO3)2. The values of melting points and thermal 17
ACCEPTED MANUSCRIPT decomposition points of the KNO3-NaNO3-Ca(NO3)2 ternary system all can be
303
obtained from the 3D stable molten temperature diagram. Moreover, thermal stability
304
of the predicted eutectic mixture verified experimentally by TG and DSC methods is
305
shown in Fig.5(c) and (d). That indicates thermal decomposition point of the predicted
306
eutectic mixture is at temperatures up to 595.8 °C, DSC curve shows melting point of
307
this mixture is about 125.3 °C. There are two independent endothermic peaks existing
308
in the DSC curve. Prior to melting, the endothermic peak of DSC curve at 50 °C is
309
due to a solid-solid type phase transition on the salt. Besides, the melting temperature
310
ranges of the predicted eutectic mixture with typical melting peaks are narrow, around
311
20 °C, which is appropriate and displays that the salt can melt in a relatively short
312
range of temperature and period of time.
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302
The predicted melting temperature (T1 = 129.1 °C) of the eutectic mixture is
314
about 3.8 °C higher than the experimentally determined value (125.3 °C) and the
315
predicted thermal decomposition point (T2 = 597.9 °C) is about 2.1 °C higher than the
316
corresponding experimental value (595.8 °C). Possibly, there exists a systematical
317
error in temperature measurement. On the whole, the tendency of the experimental
318
results is in excellent agreement with the predicted phase diagram, within the range of
319
error permitting.
320
3.1.6 Component analysis
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313
321
To examine the composition of the eutectic salt (42% KNO3-17% NaNO3-41%
322
Ca(NO3)2), X-ray diffraction and energy dispersive X-ray spectroscopy (EDX) were
323
utilized to study the phases. Fig.6 (a) showed the XRD patterns of NaNO3, KNO3 and 18
ACCEPTED MANUSCRIPT Ca(NO3)2, by comparing the patterns with the standard card, all of the corresponding
325
peaks were confirmed to arise from NaNO3, KNO3 and Ca(NO3)2. The XRD pattern
326
of the eutectic salt was shown in Fig.6 (b), it could be seen that the intensity as well as
327
the degree of KNO3 and NaNO3 did not change, as for the corresponding pure nitrate,
328
which demonstrated the excellent phase stability of this component. However, there
329
was no diffraction peak of Ca(NO3)2. Binding the literatures [23] and [24], it could be
330
preliminarily inferred that there may be some double salts (KNO3·Ca(NO3)2 or
331
4KNO3·Ca(NO3)2) existing in the system. By comparing the patterns with the
332
standard card, some of the peaks were confirmed to arise from KNO3·Ca(NO3)2 and
333
4KNO3·Ca(NO3)2, and the existence of them led to a lower melting point of this
334
component. Therefore, it can be seen that the composition of the eutectic salt contains
335
KNO3, NaNO3, KNO3·Ca(NO3)2 and 4KNO3·Ca(NO3)2, and the phases were marked
336
in the corresponding degree in Fig.6 (b).
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324
337
Fig. 6 XRD patterns: (a) NaNO3/KNO3/Ca(NO3)2; (b) the eutectic salt
338
Fig.7 (a) showed the SEM image of eutectic salt, Fig.7 (b) was the energy
339
dispersive X-ray spectroscopy (EDX) of the Spectrum 1 area corresponding in Fig.7
340
(a). From Fig.7 (b), it can be seen that the eutectic salt was composed of the element 19
ACCEPTED MANUSCRIPT K, Ca, Na, O and N. This further validated that the formation of double salts led to no
342
diffraction peak of Ca(NO3)2 in the XRD pattern of the eutectic salt (Fig.6 (b)). In
343
addition, the quantitative results of the relevant elements for EDX were shown in
344
Table 5. Through computational verification, the experimental element content was
345
basically consistent with the theoretical value, within the range of error permitting.
M AN U
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341
346
Fig. 7 Spectrum analysis of the eutectic salt: (a) SEM image; (b) EDX
347
Table 5 The quantitative results for EDX Weight %
Atom %
Formula
Compnd %
NK
13.87
18.71
N
13.87
55.71
65.75
O
55.71
2.89
2.37
Na
2.89
14.91
7.21
K
14.91
12.62
5.96
Ca
12.62
100.00
100.00
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Element Line
Na K KK
AC C
Ca K
EP
OK
Total
348
100.00
349
3.1.7 The verification of thermophysical properties in the 3D stable molten
350
temperature diagram
351
Any three compositions were selected to verify the thermodynamic properties in
352
the 3D stable molten temperature diagram. The samples were prepared for differential 20
ACCEPTED MANUSCRIPT thermal gravimetric analysis (TG and DSC), as shown in Fig.8. The comparison of the
354
experimental values and the obtained values from the 3D stable molten temperature
355
diagram are presented in Table 6.
356
Fig.8 Thermodynamic properties of the KNO3-NaNO3-Ca(NO3)2 ternary system in different
357 358 359 360 361
ratios:(a) and (b) are DSC curves and TG curves of the samples, (c) obtained values from 3D stable molten temperature diagram of the KNO3-NaNO3-Ca(NO3)2 ternary system
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353
Table 6 Comparison of predicted and experimental temperatures of the KNO3-NaNO3-Ca(NO3)2 ternary system at pressure p = 0.1 MPa
362
40
Tm/°C
Td /°C
NaNO3
Ca(NO3)2
predicted
experimental
predicted
50
10
219.5
217.4
597.7
599.0
AC C
1
KNO3
EP
Component/wt.% No.
experimental
2
40
52
8
225.4
221.5
597.2
595.5
3
38
57
5
230.7
227.1
599.9
601.1
Standard uncertainties u are u(T) = 0.1°C, and u(p) = 0.005 MPa.
363
Fig.8 (a) presents that the typical DSC thermograms of the selected samples, the
364
DSC curves reveal a corresponding prominent endothermic peak at 217.4 °C,
365
221.5 °C and 227.1 °C, respectively, indicating the melting temperatures of the
366
samples. From Fig.8 (b), the TG curves have a slight downward trend when the 21
ACCEPTED MANUSCRIPT temperature rises to 100 °C. Obviously, the reason is that the samples still have some
368
residual moisture, which may be water-absorbing effect of the molten salts or residual
369
crystal water of calcium nitrate. The corresponding upper limit temperatures are about
370
599.0 °C, 595.5°C and 601.1 °C, respectively. When the corresponding upper limit
371
temperatures are exceeded, the curves show a clear downward trend, indicating the
372
thermal decomposition of the molten salts.
Fig.8 (c) illustrates that the predicted melting points and thermal decomposition
SC
373
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367
374
points of samples 1-3 are about T1
375
225.4 °C and T2
376
By comparing the predicted and the experimentally determined values in Table 4, it is
377
found that the absolute difference value is less than 4 °C. The predicted values of the
378
samples and that determined experimentally are in excellent agreement. Therefore, it
379
is believed reasonably that the 3D stable molten temperature diagram of the
380
KNO3-NaNO3-Ca(NO3)2 ternary system is reliable. Finally, all these result would be
381
helpful to further study on the KNO3-NaNO3-Ca(NO3)2 ternary system for solar
382
energy storage.
383
4. Conclusions
= 597.7 °C, T1
(2)
=
M AN U
(1)
(3)
= 230.7 °C and T2
(3)
= 599.9 °C, respectively.
EP
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= 597.2 °C, T1
= 219.5 °C and T2
AC C
384
(2)
(1)
The visualization of the phase diagram of the KNO3-NaNO3-Ca(NO3)2 ternary
385
system is achieved by Matlab. A 3D stable molten temperature diagram provides
386
convenience for predicting the temperature span of the KNO3-NaNO3-Ca(NO3)2
387
ternary system as a heat transfer medium.
388
A 3D stable molten temperature diagram was developed to predict melting points 22
ACCEPTED MANUSCRIPT and thermal decomposition points of the KNO3-NaNO3-Ca(NO3)2 ternary system
390
for the first time, as well as its eutectic temperature and composition. It is helpful to
391
further analyze and study the ternary system.
392
The composition, melting temperature and upper limit temperature of the eutectic
393
mixture are predicted, namely 42% KNO3-17% NaNO3-41% Ca(NO3)2, 129.1 °C
394
and 597.9 °C. Moreover, the predicted melting point of the eutectic mixture is
395
consistent with that determined experimentally.
396
The predicted melting and thermal decomposition temperatures of the selected
397
compositions were respectively verified experimentally using DSC and TG methods.
398
And the experimental results were in excellent agreement with predicted values
399
from the 3D stable molten temperature diagram.
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Furthermore, isotherm diagram, liquidus projection and liquidus surface of the 3D
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phase diagram of the KNO3-NaNO3-Ca(NO3)2 ternary system can be constructed by
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interpolation method in Matlab. However, the disadvantage is that crystallization
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lines between two-phase regions of the 3D phase diagram cannot be displayed,
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which should be further improved.
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Acknowledgements This work was supported by the National Key Research and Development
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Program of China (No. 2016YFC0205500), National Natural Science Foundation of
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China (No. 51272105), Jiangsu Provincial Science and Technology Supporting
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ACCEPTED MANUSCRIPT Program (BE2013718), Project Funded by the Priority Academic Program
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Development of Jiangsu Higher Education Institutions (PAPD).
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Highlights A 3D stable molten temperature diagram of the system (K, Na, Ca/NO3) is developed.
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The eutectic composition and temperature span of the system were predicted.
The predicted eutectic composition is 42% KNO3-17% NaNO3-41% Ca(NO3)2.
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The predicted eutectic mixture has a wide temperature span from 129.1 to
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597.9 °C.