Development of reversible and durable thermochromic phase-change microcapsules for real-time indication of thermal energy storage and management

Applied Energy 264 (2020) 114729

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Development of reversible and durable thermochromic phase-change microcapsules for real-time indication of thermal energy storage and management

T

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Ya Zhang1, Huan Liu1, Jinfei Niu, Xiaodong Wang , Dezhen Wu State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

designed the thermochromic • We phase-change microcapsules with a sandwich shell structure.

thermochromic layer was well iso• Alated by the silica base shell and polymeric outer shell.

system exhibits a highly re• This versible and durable thermochromic response capability.

system has a high latent heat• This storage capacity with real-time indication by color.

study offers a new insight for • This design of thermochromic heat-storage systems.

A R T I C LE I N FO

A B S T R A C T

Keywords: Phase-change microcapsules Sandwich-structured configuration Reversible thermochromic behavior Thermal energy storage Reliability and durability

We reported a design of novel thermochromic phase-change microcapsules (TCMs) with a sandwich-structured shell for reversible and durable indication of thermal energy storage and management in real-time. Two types of TCMs with red and blue color indicators were successfully constructed by fabricating a silica base shell onto the n-docosane core, followed by formation of a thermochromic indication layer and a polymeric protective layer, and their multilayered configuration and well-defined core-shell structure were confirmed by microstructural investigation and chemical composition analysis. These two types of TCMs not only showed an outstanding latent heat-storage/release capability with a high capacity over 150 J/g, but also exhibited a good shape stability, high thermal stability and excellent phase-change reversibility and durability. The optimum operation conditions for thermal energy charge/discharge were also determined by nonisothermal and isothermal differential scanning calorimetric analyses. Most of all, the two types of TCMs presented an entirely reversible thermochromic behavior individually with high-contrast red and blue color indications for the phase-change state of n-docosane core. Both of them exhibited high reversibility and long cycle life in thermochromic indication, which meets the design requirements for durable indication of latent heat storage and thermal management in real-time. In the light of an innovative configuration of sandwich-structured shell and a smart

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Corresponding author. E-mail addresses: [email protected], [email protected] (X. Wang). 1 The two authors made equal contributions to this work. https://doi.org/10.1016/j.apenergy.2020.114729 Received 6 July 2019; Received in revised form 12 February 2020; Accepted 22 February 2020 0306-2619/ © 2020 Elsevier Ltd. All rights reserved.

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combination of latent heat-storage and thermochromic functions, the TCMs designed by this study has a great potential for applications in smart fibers and textiles, wearable electric devices, energy-saving buildings, temperature-sensitive medical system, safety clothing, smart windows, aerospace engineering and many more.

1. Introduction

Thermochromic materials are a class of temperature-sensitive materials containing discoloration compounds and other auxiliary components, and their color can vary with a variation of temperature in a certain range due to the mechanism that the absorption spectrum of discoloration substances can be transformed during the heating and cooling processes [22]. The unique thermochromic transition for thermochromic dyes with excellent reversibility often accompanies with various photochromic reactions. It has been broadly accepted that the introduction of thermochromic dyes can greatly enrich the functionality and intelligence of a material [23]. For example, a phase-change microcapsule system along with a reversible thermochromic indicator is able to show the phase-change state of PCM core inside the microcapsules by indicating a color variation, and therefore such a combination can effectively improve the efficiency of latent heat storage and thermal management and significantly enhance the thermal energystorage efficiency and operation facilitation [24]. Moreover, the application of thermochromic materials can contribute to an improvement of energy efficiency by controlling energy balance in energy-saving buildings [25]. The study of reversible thermochromic phase-change microcapsules (TCMs) for thermal energy storage and temperature indication has gained a great deal of attention from many scientists in recent three years. As reported in the latest literature, there have been two methods developed for preparation of TCMs by now. One is the addition of thermochromic pigments as functional additives into the shell materials when fabricating the TCMs and the other is the incorporation of thermochromic dyes into the PCM core of microcapsules. Wang et al. [25] successfully assembled various thermochromic pigments and PMMA shell on the surface of n-octadecane core through suspension-like polymerization and found that the resultant microcapsules had good thermal energy storage-release performance and high thermal stability. However, these thermochromic systems seem not to show a reversible thermochromic indication due to the environmental interference toward thermochromic pigments. In this case, the TCMs were normally fabricated by embedding thermochromic dyes into the PCM core as reported in many references [26]. As one of the main classifications of thermochromic dyes, the thermochromic leuco dyes are a type of organic color former that needs to be compounded with a color developer and a nonvolatile solvent when used for a thermochromic indicator [27]. The solvent can perform a phase transition from solid to liquid when the ambient temperature reaches its melting point, and then the color developer and former are dissolved in the solvent, thus resulting in a color change. Oppositely, the dyes can revert to their initial colors due to the solidification of solvent at a temperature below the freezing point. Evidently, such a reversible thermochromic behavior for thermochromic compound (TC-compound) involves the solid–liquid phase transitions, which can effectively absorb and release the thermal energy from environment as well as can monitor the state of solvent by color variation [28]. Thanks to this mechanism, the utilization of TC-compounds has been recognized as a promising pathway to achieve a reversible thermochromic indication for the phase-change state of TCMs. Tözüm et al. [29] prepared two types of TCMs based on PMMA and poly(methyl methacrylate-comethacrylic acid) shells by using crystal violet lactone (CVL) as a color former, bisphenol A (BPA) as a color developer and 1-tetradecanol as a solvent, and they observed a reversible thermochromic behavior from these two TCMs. Wu et al. [30] synthesized a type of reversible TCMs with 1-hexadecanol as a PCM and co-solvent, spirolactone derivative as a color former and phenolic hydroxyl compound as a color developer by complex coacervation with modified gelatin, and they also observed a stable and reversible color response to temperature variation. Geng

Microencapsulation of solid-liquid phase change materials (PCMs) is considered as a promising technology to effectively solve the leakage problem of PCMs during phase transitions and has attracted extensive attention over 30 years [1]. By encapsulating the PCM core with a variety of tight shells in a well-defined core-shell microstructure, this technology not only can provide an adequate protection from the surrounding materials and environments for encapsulated PCMs, but also offers a greater heat transfer area for them to enhance the thermal response and thermal energy-storage efficiency [2]. A great number of studies indicated that a variety of organic polymers, inorganic substances, and organic–inorganic composites could be employed as shell materials to encapsulate solid–liquid PCMs [3,4]. As expected, these phase-change microcapsule systems all presented satisfactory phasechange performance for latent heat storage and thermal management in a stable form or shape [5]. Although most of researchers focused on various microencapsulation techniques to fabricate a tight shell surrounding the PCM core, the introduction of shell materials into the PCM-based thermal energy-storage systems obviously reduced their latent heat-storage capacity and resulted in a decline in thermal energystorage density accordingly due to the inert shell materials without any phase-change behaviors [6]. With relatively low phase-change enthalpies, the signal functional phase-change microcapsules are mainly developed for traditional applications in thermal energy storage and management such as energy-saving buildings, latent functional thermal fluids, heating/cooling exchange systems, fibers and textiles, food industry and solar thermal energy storage systems [7,8]. With consideration of functional diversity of inorganic and organic materials, the design and construction of inorganic or organic functional shells for novel phase-change microcapsule systems has attracted a great deal of interest in recent years [9]. These novel microcapsule systems not only exhibited an acceptable thermal energy-storage capacity and excellent long-term durability without leakage of liquid PCMs during the melting process, but also achieved a diversity of additional functions from the specially designed shells. These attractive additional functions included an effective magnetic response from the PUA/Fe3O4 [10] and SiO2/ Fe3O4 hybrid shells [11], fluorescent indication from the PUF/Nile red hybrid shell [12], photocatalysis from the TiO2 shell [13], antibiosis from the ZnO shell [14], electrochemical properties from the flowerlike SnO2/SiO2 double-layered shell [15], superhydrophobic performance from the PUF shell [16] and flower-like ZnO/SiO2 doublelayered shells [17], flame retardancy from the amino-modified PMF shell [18], selective absorption capability from the molecularly imprinted polymer/SiO2 double-layered shell [19], and enhanced biocatalytic activity from the SiO2/Fe3O4 [20] and TiO2/Fe3O4 composite shells [21]. In these bi- or multifunctional phase-change microcapsule systems, the addition functions are derived either from the intrinsic physicochemical nature of the inorganic shells or from the functional fillers added in the shell materials. Moreover, the fabrication of a functional layer onto the base shell can also endow the phase-change microcapsules with an expected function. The design and construction of these specific functional shells not only can compensate the loss of latent heat-storage capacity for the resultant phase-change microcapsules, but also can expand their application range from traditional fields to modern hi-tech areas such as health-care clothing systems, solar photodegradation and detoxification for the water containing organic pollutants, biomedical thermal therapy, cancer therapy and dual modal bioimaging, drug delivery and control release, self-cleaning and antifouling coating, supercapacitors, and Li-ions battery cells [9]. 2

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Moreover, He et al. [31] reported a type of reversible thermochromic functional membrane with TCMs as a temperature regulator and polyvinyl alcohol/water-soluble polyurethane as a polymer matrix. This type of thermochromic flexible membrane was found to exhibit a bifunctional feature of effective temperature regulation and reversible thermochromic indication and therefore could be potentially applied for wearable temperature sensors. In addition, there were some reports about the preparation and applications of thermochromic phase-change composites by mixing thermochromic indicators with supporting materials in the melt state [32,33], and these composites hardly gained

et al. [26] designed and fabricated a series of TCMs for the state indication of latent heat storage/release by using CVL as a color former, BPA as a developer and 1-tetradecanol served as a PCM and solvent, and they found that their reversible thermochromic behavior could present a visual evidence for thermal energy storage and release performance. Geng and co-workers [28] further modified these reversible TCMs by silver nanoparticles and found that the resultant TCMs not only achieved excellent thermal properties and reversible thermochromic performance, but also exhibited antibacterial activity and enhanced heat transfer after the introduction of silver nanoparticles.

Fig. 1. Schematic representation of synthetic strategy and formation mechanism for TCMs. 3

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synthetic fibers and textiles in the textile engineering field, and these thermoregulatory fibers and textiles can improve the thermal comfort of human body with a temperature-color alarming response when applied in the protective garments specially under extreme conditions [28]. In addition to these major applications, the TCMs also exhibit the broad and promising application perspectives in flexible thermochromic polymeric membranes for wearable temperature sensors, the cold chain transport for temperature-sensitive pharmaceuticals with cooing and thermochromic indication, the thermal and colorful stealth for anti-counterfeiting or protective targets, etc. Moreover, this work will offer a new insight for the design of highly reversible and durable TCMs with reliable and real-time thermochromic indication for an imperceptible change in temperature during the thermal management of PCMs and also make sufficient contributions to an innovative strategy for the development of high-performance TCMs for potential hi-tech applications in smart fibers and textiles, wearable electric devices, energy-saving bulildings, temperature-sensitive medical system, safety clothing, smart windows, aerospace engineering and many more.

reliable and durable thermochromic performance without any protection and isolation. Although the above researchers have all mentioned the reversible thermochromic performance achieved by incorporating thermochromic leuco dyes and relevant color developers into the shell, there were few characterizations indicating a reliable and durable thermochromic behavior for the resultant TCMs. On the other hand, according to the published work, only the co-solvents could be used as PCMs for the TCMs due to the requirement for coordination with thermochromic leuco dye, and the PCM core inside the TCMs was compelled to rely on a restricted range of co-solvents. This greatly limited the design for the TCMs with a desired phase-change temperature and latent heat-storage capacity. Moreover, all of the TCMs currently reported are unexceptional to employ organic polymers as shell materials, and consequently the thermal response and heat transfer of the system are retarded due to the low thermal conductivity of organic polymers. These drawbacks are actually disadvantageous for the practical applications of TCMs. Nowadays, microencapsulated PCMs have been widely accepted by industrial communities for thermal management and energy-saving applications in construction engineering and textile engineering as two major engineering areas [34–36]. To effectively control the balance between thermal energy availability and demand for enhancing the energy efficiency of microencapsulated PCMs when applied in these two areas, there is an urgent requirement for highly reversible and durable TCMs to provide a real-time and reliable indication for the state of thermal energy storage and management so as to adjust thermal energy supply in respect to the practical demand. However, the current techniques and corresponding TCM products reported by literature cannot meet such a requirement. With this motivation in mind, we attempted to design and construct a phase-change microcapsule system with a layer-by-layer structure for highly reversible and durable thermochromic indication. This design for shell configuration is not a simple hybridization or combination of the base shell with an expected functional additive or layer as reported in our previous studies but an innovative sandwich-structured shell configuration. This shell configuration is composed of a silica base shell surrounding the PCM core, a thermochromic compound intermediate layer, and a PMMA outer layer. With the successful construction of such a sandwich-structured shell configuration, it is expected that the silica inner shell can provide an effective protection and barrier for the PCM core along with high thermal conduction, the TC-compound intermediate layer offers a thermochromic indicator for the phase-change state of PCM core, and the PMMA outer layer acts as a protective shell with high transparence for the thermochromic indication layer. Owing to the physical isolation between the PCM core and thermochromic indication layer by a silica base shell, the mutual interference from these two parts can be completely avoided. This superiority may greatly expand the selective range for PCMs and thermochromic leuco dyes when designing a thermochromic phase-change system. It is believed that this unique sandwichstructured shell configuration not only can ensure a high reliability and long-term durability of reversible thermochromic performance for the resultant TCMs but also can help the TCMs to gain a satisfactory latent heat-storage capacity, stable phase-change behavior, high thermal stability and rapid thermal response and heat transfer. The research findings obtained from this study can provide technology solutions from materials design to synthetic technique for the applications of TCMs in construction engineering and textile engineering. For example, the TCMs can be applied for energy-saving interior building materials like delignified wood-based composites and plasterboards, in which they not only can perform temperature-displayable thermal energy storage and management but also can remind users to regulate air-conditioners or electric heaters in time by a realtime color indication. As a result of this application, both energy efficiency and energy-saving effectiveness are greatly improved in the construction engineering field [25,32]. The TCMs can also be incorporated into polymer fiber matrix to prepare the thermochromic

2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS) and n-docosane were commercially supplied by Acros Organics USA and used as a silicon source and a paraffin-type PCM, respectively. CVL and 6-(diethylamino)-fluoran (DEAF) and were kindly provided by Changshu Dyesuffs & Chemicals Co., Ltd, China and used as blue and red thermochromic color formers, respectively. Methacrylate (MMA), 2,2′-azobis(2-methylpropionitrile) (AIBN), (3-mercaptopropyl)trimethoxysilane (MPTMS), cetyltrimethyl ammonium bromide (CTAB), Span-80, Tween-80, BPA, cetanol, ethanol and HCl aqueous solution (37.5 wt%) were commercially received from Tianjin Fuchen Chemical Reagent Co., Ltd., China. MMA were distilled to remove the initiator prior to use, and the other chemicals and reagents were used as received without further purification. 2.2. Synthesis of thermochromic phase-change microcapsules Two types of TCMs with red and blue thermochromic indicators were prepared through a two-stage synthetic strategy as schematically depicted Fig. 1, in which the synthetic mechanism was also described in detail. In the first synthetic stage, a type of microencapsulated n-docosane with a thiol-functionalized silica shell was prepared as the base phase-change microcapsules according to the methodology described in our previous work [17]. The based microcapsules were synthesized through interfacial polymerization in an oil-in-water (O/W) emulsion templating system using n-docosane as a paraffin-type PCM, TEOS as a silica source, CTAB as a cationic emulsifier and formamide as a dispersion medium. The optimum formulation for core/shell ratio was adopted to maintain a reasonable balance between the heat-storage capacity and structural stability according to our previous experimental results [11,17]. In a typical procedure: 3 g of TEOS, 0.7 g of MPTMS, 3 g of n-docosane and 50 ml of formamide were mixed in a three-neck round-bottom flask with magnetic agitation at 55 °C for 30 min, and then 0.6 g of CTAB as a cationic emulsifier was added into the flask with stirring to obtain a homogeneous emulsion. Afterward, 6 ml of HCl aqueous solution was added slowly into the flask to initiate the hydrolysis and polycondensation of TEOS under a moderately acidic condition, and then the resultant mixture was stirred continuously for 5.5 h, followed by aging at the same temperature for 24 h. The resulting suspension was filtered, washed, and dried at room temperature to obtain the base microcapsules. The red and blue TC-compounds were prepared by a melt-blending method respectively using DEAF as a red color former, CVL as a blue color former, BPA as a color developer and cetanol as a solvent. The optimum mass ratio of color former/color developer/solvent was 4

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temperature (Tc) and melting temperature (Tm) could be directly obtained from DSC thermograms, and the fusion heat (ΔHm) and solidification heat (ΔHc) were calculated by imposing a definite integral toward DSC thermograms in the melting and crystallization temperature regions, respectively. Moreover, based on the thermal analysis results from DSC measurements, the encapsulation efficiency (Een), thermal energy-storage efficiency (Ees) and thermal energy-storage capability (Ces) of base microcapsules and TCMs could be predicted using three simple equations as below [19]:

determined as 1/3/50, and the blend temperature was set as 90 °C. With the formation of base microcapsules in the first synthetic stage, the red and blue TC-compounds were respectively assembled onto the microcapsule surface through hydrogen bonding, followed by fabrication of a PMMA protective layer through surfactant-assisted polymerization as described in Fig. 1. In a typical procedure for the synthesis of TCMs: 2.0 g of base microcapsules, 0.5 g of TC-compound and 100 ml of deionized water were mixed in a three-neck roundbottom flask under magnetic agitation at 60 °C for 2 h, and then 0.1 g of Span-80 and 0.1 g of Tween-80 were added into the flask with vigorous agitation for 3 h to obtain a surfactant-assisted templating system. In succession, 2.0 ml of MAA and 0.02 g of AIBN were added into this system with stirring for 30 min, and then the reaction system was heated to 85 °C to conduct free radical polymerization for 6 h under a nitrogen atmosphere. After the polymerization was completed, the resultant product was filtered, washed and dried at room temperature over 72 h to obtain the TCMs.

Een (%) =

ΔHm,MicroPCM × 100% ΔHm,PCM

(1)

Ees (%) =

ΔHm,MicroPCM + ΔHc,MicroPCM × 100% ΔHm,PCM + ΔHc,PCM

(2)

Ces =

(ΔHm,MicroPCM + ΔHc,MicroPCM)·ΔHm,PCM × 100% (ΔHm,PCM + ΔHc,PCM)·ΔHm,MicroPCM

(3)

where ΔHc,PCM and ΔHm,PCM are the solidification and fusion heat values of pure n-docosane, respectively, and ΔHc,MicroPCM and ΔHm,MicroPCM are the solidification and fusion heat values of microcapsule samples, respectively. The isothermal and nonisothermal latent heat-storage/release behaviors of TCMs were analyzed by programmed dynamic DSC scans at different operation temperatures and scanning rates, respectively, and the heat history were eliminated prior to formal measurements. The thermal conductivity of microcapsule samples was measured by an HS-DR-5 thermal conductivity tester. Thermogravimetric analysis (TGA) was carried out at a heating rate of 10 °C/min on a TA Instruments Q50 thermogravimetric analyzer with a nitrogen flushing gas at a flow rate of 50 ml/min. Infrared thermography was performed with a Testo™ 875–1i infrared thermal imaging camera to evaluate the practical performance of TCMs in thermal energy storage and management. The surface temperature evolutions as a function of time were derived from infrared thermographic analysis in the heating and cooling stages.

2.3. Characterizations The microstructure and morphology of microcapsule samples were examined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with a Zeiss SUPRA™ 55 scanning electron microscope and a Hitachi JEM-3010 transmission electron microscope. The size distributions and average diameter of base microcapsules and TCMs were measured by image analysis toward the SEM micrographs with the Nano Measure™ software, and more than 250 microcapsules were captured from each of the SEM micrographs for image analysis. Fourier-transform infrared (FTIR) spectroscopy was performed on a Nicolet iS5 infrared spectrometer to collect the vibration modes of functional groups in microcapsule samples in the wavenumber range of 400–4500 cm−1. Energy-dispersive X-ray (EDX) spectroscopy was conducted with an Oxford INCA EDX spectrometer to analyze the surface elemental compositions of microcapsule samples. X–ray photoelectron (XPS) spectra of microcapsule samples were recorded on a Thermo Fisher Scientific EscaLab 250Xi photoelectron spectrometer with focused monochromatized Al–Kα radiation. Differential scanning calorimetry (DSC) was carried out on a TA Instruments Q20 differential scanning calorimeter at a scanning rate of 10 °C/min to investigate the phase-change behaviors and associated thermal parameters of microcapsule samples. The crystallization

3. Results and discussion 3.1. Structural design and morphological characterization To ensure a highly reversible, reliable and durable thermochromic

Fig. 2. Schematic representation of thermochromic mechanisms for red and blue TCMs. 5

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base microcapsules. Fig. 3a shows the DSC thermograms of base microcapsules and two TC-compounds. It is important to note that both the melt and crystallization temperature ranges of two TC-compounds show good agreement with those of encapsulated n-docosane inside the base microcapsules. Moreover, the two TC-compounds are found to present a perfect reversible discoloration–coloration behavior during the heating and cooling processes as observed in Fig. 3b, indicating that these two TC-compounds can meet the design requirement for thermochromic indication of the phase-change state of TCMs with expected colors in real-time. The two TC-compounds were afterwards coated onto the surface of the base microcapsules through hydrogen-bonding self-assembly between the thiol and hydroxyl groups, followed by fabrication of a rigid and transparent PMMA layer through surfactantassisted self-assembly polymerization. This finally leads to the successful construction of a sandwich-structured shell onto the n-docosane core. As an expected result, the formation of PMMA outer layer not only can provide effective protection for the TC-compounds on the surface of base microcapsules but also can support a clear display of the color variation of TCMs due to a transparent nature of PMMA. The morphology and microstructure of base microcapsules and two TCMs were investigated by SEM, and the resulting micrographs are shown in Fig. 4, in which the size distribution curves obtained from image analysis of SEM micrographs were also presented. It is noticeable that the base microcapsules reveal a regularly spherical morphology as well as a compact and smooth surface without any defects (see Fig. 4a and b). Furthermore, a well-defined core-structure along with a reasonable wall thickness can be observed in Fig. 4c and d. Such a regularly spherical core–shell microstructure was confirmed by the TEM micrographs shown in Fig. 5a and b. The base microcapsules are also found to show a relatively narrow size distribution in the range of 2.0–4.5 μm according to the observation in Fig. 4g, and their average diameter is determined as 3.31 μm. It is interestingly observed from the SEM micrographs in Fig. 4e and f that the microcapsule surface looks slightly rough in the presence of TC-compound intermediate layer and PMMA outer layer on the base microcapsules. The image analysis results show that the size distributions of two TCMs seem to become slightly broader than that of the base microcapsules as seen in Fig. 4h and i. The average diameter is also found to increase by 0.31 μm for the red TCMs and by 0.34 μm for the blue ones due to the fabrication of TCcompounds and PMMA layers. In addition, the TEM micrographs shown in Fig. 5c–f clearly demonstrated a spherical core–shell structure with a rough outer layer for the two TCMs. These micrographic observation results identify that a multilayered microcapsule structure has been successfully constructed for the red and blue TCMs in accordance with

effect as well as a high thermal energy-storage capacity and good structural stability for the TCMs, we designed a sandwich-structured configuration for the shell of phase-change microcapsules and used ndocosane as a paraffin-type PCM core. Then, we successfully synthesized two types of TCMs with red and blue thermochromic indicators according to the synthetic strategy schematically described in Fig. 1. Such a sandwich-structured configuration was constructed by a silica inner shell, a TC-compound intermediate layer and a PMMA outer layer. With the formation of this characteristic sandwich-structured shell onto the n-docosane core, the silica base shell provides an effective protection and barrier for the n-docosane core, the TC-compound intermediate layer acts as a thermochromic indicator for the phasechange state of n-docosane core and the PMMA outer layer plays a protective role in thermochromic indication of TC-compound layer. As illustrated in Fig. 1, the microencapsulated n-docosane with a thiolfunctionalized silica shell was synthesized as the base phase-change microcapsules in the first synthetic stage. The fabrication of thiolfunctionalized silica shell not only can provide a tight and high thermally conductive inorganic base shell for the n-docosane core, but also can adsorb the TC-compounds onto the surface of base microcapsules through hydrogen bonding. In the second stage of the experiment, DEAF and CVL were adopted as thermochromic leuco dyes to provide a real-time thermochromic indication for the TCMs by red and blue colors, respectively. Nevertheless, these two thermochromic leuco dyes are only used as color formers to indicate red and blue colors with the aid of BPA as a color developer. Therefore, two types of TC-compounds were prepared respectively by melt blending DEAF and CVL with BPA and a solvent, and then their thermochromic effectiveness was achieved in accordance with the intermolecular electron transfer mechanism as schematically described in Fig. 2. As color formers, both DEAF and CVL are typical electron donors. On the other hand, BPA is a representative electron acceptor and therefore can act as a color developer. When DEAF and CVL are compounded with BPA, the electron transfer occurring between the electron donor and acceptor can lead to opening and closing of lactonic ring in the color formers. Such a change in molecular structure is able to cause the coloration and discoloration of TC-compounds [37]. However, the electron donating–accepting process only takes place in the melting state. Therefore, the thermochromic temperatures of TC-compounds should be consistent with the phase-change temperatures of encapsulated n-docosane so that the phase-change state can be well indicated by the TCMs. For this reason, we adopted cetanol as solvent to adjust the melt temperatures of two TC-compounds to make them a good match for the encapsulated n-docosane inside the

Fig. 3. (a) DSC thermograms of base microcapsules and the compounds containing DEAF and CVL. (b) Digital photographs of reversible thermochromic behaviors of the compounds containing DEAF and CVL. 6

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Fig. 4. SEM micrographs of (a–d) base microcapsules, (e) red TCMs and (f) blue TCMs. Particle size distribution curves of (g) base microcapsules, (h) red TCMs and (i) blue TCMs.

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Fig. 5. TEM micrographs of (a, b) base microcapsules, (c, d) red TCMs and (e, f) blue TCMs.

vibration of terminal silanol groups. The existence of abundance silanol groups on the silica shell may facilitate further surface self-assembly for the TC compounds. In the case of the two TCMs, their FTIR spectra not only show all of the characteristic peaks of base microcapsules, but also exhibit two absorption peaks at 1071 and 931 cm−1 due to the CeO stretching vibration as well as two peaks at 1762 and 1607 cm−1 corresponding to the C]O stretching vibration. Moreover, a set of weak adsorption bands appearing within 500–700 cm−1 is associated with the bending vibration of methoxyl group. These characteristic spectral bands identify the presence of PMMA layer. On the other hand, an adsorption peak for the CeN stretching vibration can be clearly distinguished at 1364 cm−1, which is attributed to the tertiary amine groups in the molecules of DEAF and CVL. A set of very weak adsorption peaks can indeed be noted in the wavenumber range of 710–830 cm−1 due to the CeH in-plane bending vibration of phenyl rings, although this range seems to overlap with the SieO stretching vibration. In addition, a blue shift of infrared band corresponding to the OeH stretching vibration is clearly identified in the infrared spectra of two TCMs. This may be ascribed to the presence of phenolic hydroxyl groups of BPA. All of the results suggest the presence of TC-compounds

our design scheme. 3.2. Chemical structure and composition analysis The component compositions and chemical structures of base microcapsules and two TCMs were first examined by FTIR spectroscopy to prove the presence of n-docosane core and the formation of multilayered shell composed of silica, TC-compounds and PMMA. Fig. 6a displays the corresponding spectra obtained from the FTIR measurement. The infrared spectrum of base microcapsules clearly shows a series of absorption peaks corresponding to the characteristic infrared absorption peaks of n-docosane as marked in Fig. 6a, which is in good agreement with the characteristic bands of pure n-docosane as a control. Furthermore, there are three absorption peaks observed at 1071, 802 and 466 cm−1 corresponding to the SieOeSi and SieO stretching vibrations. These infrared bands could be easily identified in both the silica and n-docosane species and therefore proved the encapsulation of n-docosane with a silica shell. It is noteworthy that a broad peak centered at 3433 cm−1 and a weak peak at 944 cm−1 appear in the infrared spectrum of base microcapsules due to the OeH stretching 8

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Fig. 6. (a) FTIR spectra of (1) pure n-docosane, (2) base microcapsules, (3) red TCMs and (4) blue TCMs. EDX spectra of (b) base microcapsules, (c) red TCMs and (d) blue TCMs.

from the deconvoluted Si 2p and O 1s peaks in the local regions of binding energy, which provide the chemical bonding information of the SieO and OeH bonds for the base microcapsules. On the other hand, the XPS survey spectra of two TCMs not only display the weakened Si 2s and 2p signals for silica, but also exhibit new signals for N and S atoms. By curve fitting for the deconvoluted O 1s, C1s and N 1s peaks in the local regions of binding energy, the chemical bonding information of the CeC, C]C, C]O, CeO, CeOH, O]CeO, CeN bonds for the two TCMs can be explicitly achieved as illustrated by the high-resolution XPS spectra in Fig. 7d–i. Moreover, the CeOeC bond can be identified from the fitting curves in the high-resolution XPS spectra of red TCMs (see Fig. 7g and h), indicating the presence of DEAF in this type TCMs. Based on the results obtained from EDX and XPS analyses together with the FTIR characterization, it can be clearly confirmed that the silica shell has been well fabricated onto the n-docosane core with success, followed by successful formation of the TC-compound and PMMA layers on the surface of silica shell.

and PMMA layers on the surface of base microcapsules. Considering of a relatively low identification level of infrared spectroscopy for complicated organic compounds, EDX and XPS characterizations were conducted to further confirm the layer-by-layered shell conformation of two TCMs, the corresponding spectra are shown in Figs. 6b–d and 7. As seen in Fig. 6b, the Si and O elements associated with silica can be well identified by two intensive characteristic signals, while a weak signal of C element is noted due to the detection of n-docosane under the silica shell. The EDX data mentioned in the insert of Fig. 6b supported the presence of silica shell with an abundance of silanol groups. It is important to note in Fig. 6c and d that there are not only two characteristic signals corresponding to Si and O elements, but also an enhanced signal for C element and two distinct signals for N and S elements in the EDX spectra of two TCMs. The formation of PMMA layer can be proved by an enhanced carbon signal as well as an increase of carbon atomic percentage shown in the EDX data. The presence of N and S elemental signals indicates that the TC-compounds have been assembled onto the surface of base microcapsules through hydrogen bonding with thiol groups. These results indicate that the follow-up presence of TC-compounds and PMMA layers on the base microcapsules. As a powerful chemical analysis tool, XPS spectroscopy could accurately detect the surface elemental distribution and coordination of a material and therefore was utilized to further examine the chemical compositions and molecular structures of base microcapsules and two TCMs. As observed in Fig. 7, the base microcapsules show two clear signals corresponding to Si and O atoms in their XPS survey spectrum (see Fig. 7a). Furthermore, the associated high-resolution XPS spectra in Fig. 7b and c reveal a series of well-resolved fitting curves obtained

3.3. Phase-change behavior and thermal performance The phase-change behaviors of base microcapsules and two TCMs were characterized by DSC, and their thermal performance was analyzed by use of the relevant DSC data. Fig. 8 shows the obtained DSC thermograms and associated thermal parameters of microcapsule samples as well as pure n-docosane as a control. As seen in Fig. 8a, a characteristic bimodal crystallization behavior is observed for both pure n-docosane and three microcapsule samples in the DSC cooling thermograms. The presence of two exothermic peaks is due to the occurrence of metastable rotator phase before pure or encapsulated n9

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Fig. 7. (a) XPS survey spectra of base microcapsules and two TCMs. High-resolution XPS spectra of (b, c) base microcapsules, (d–f) red TCMs and (g–i) blue TCMs.

DSC characterizations can also provide important thermal energystorage/release data for PCMs and the associated microcapsules. It is found that pure n-docosane exhibits an outstanding thermal energystorage capability with a ΔHc of 242.1 J/g and a ΔHm of 242.9 J/g according to the date in Fig. 8d. Nevertheless, the ΔHc and ΔHm values of base microcapsules are found to decrease by approximately 50 J/g as a result of the presence of inert silica shell. It is noteworthy that the ΔHc and ΔHm were further reduced after fabricating the TC-compound and PMMA layers onto the base microcapsules. However, the remaining latent heat of fusion or solidification for the resultant two TCMs is still higher than 150 J/g. Therefore, the latent heat capacities of two TCMs are considerably satisfactory for the application in thermal energy storage and management. In addition, three characteristic parameters of Een, Ees and Ces could be used to describe the latent heat-storage performance and efficiency of base microcapsules and two TCMs. According to the characteristic parameters listed in Fig. 8f, the base microcapsules are observed to achieve an Een value of 71.1%, and the Een value seems to only decline to 63.2% for the red TCMs and 64.3% for the blue ones due to follow-up encapsulation with the TC-compound and PMMA layers. The Ees is normally used to describe the phase transition efficiency from liquid to solid and vice versa for microencapsulated n-docosane and therefore should be determined by the total ΔHc and ΔHm values of both pure n-docosane and the microencapsulated one. It is noteworthy that the Ees values for both the base

docosane crystallizes fully. Such a metastable rotator phase can easily be assembled during the phase transition from liquid to solid at a temperature higher than the normal crystallization temperature, and therefore it broadly exists in paraffin waxes as reported by a number of references [38,39]. Although pure n-docosane shows a single endothermic peak for melting in the DSC heating thermograms, it is interesting to note a shoulder at the temperature lower than its Tm as shown in Fig. 8b. This phenomenon may be ascribed to the melting of metastable rotator phase. However, it is found that this shoulder almost disappears in the DSC thermograms of base microcapsules and two TCMs due to a rapid heat transfer and fast thermal diffusion. The data of thermal conductivity listed in Fig. 8e could support this deduction. There is no doubt that the encapsulation of n-docosane with a highly thermally conductive inorganic silica shell can effectively improve the thermal conductance of resultant microcapsules, leading to a prompt thermal response accordingly. It can be concluded that the microencapsulation seems not to influence the phase-change behavior of encapsulated n-docosane. However, there is a considerable reduction in Tc for both the base microcapsules and two TCMs (see Fig. 8c). The decrease of Tc may be due to the crystallization confinement caused by a small inner space inside the microcapsules. Moreover, it seems that the Tc’s of TCMs are slightly lower than that of base microcapsules due to the presence of low thermally conductive polymeric layer, which results in a slow heat transfer accordingly. 10

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Fig. 8. (a, b) DSC thermograms, (c) phase-change temperatures, (d) phase-change enthalpies (e) thermal conductivities and (f) encapsulation parameters of (S0) pure n-docosane, (S1) base microcapsules, (S2) red TCMs and (S3) blue TCMs.

microcapsules and two TCMs are always lower than the Een ones slightly, suggesting that the multilayered encapsulation of n-docosane seems to have no influence on the latent heat-storage/release efficiency of resultant TCMs. Moreover, it is important to note that the two TCMs achieved a very high value in Ces, indicating that most of the n-docosane inside the TCMs is functionally valid and can well perform reversible phase transitions to store and release latent heat.

behaviors and thermal parameters of two TCMs were investigated by means of a programmed DSC scanning methodology at different operation temperatures. The development of relative degree of fusion/ solidification (Xt) during the operation process for latent heat charge/ discharge can be expressed by Eq. (4) and obtained from the isothermal DSC scans:

3.4. Isothermal and nonisothermal heat-charging/discharging behaviors

Xt =

The latent heat-charging/discharging performance of PCMs under the isothermal and nonisothermal operation conditions plays a vital role in their practical applications, and the relevant study can provide some optimum operation parameters for thermal energy storage and management of PCMs. The isothermal heat-charging/discharging

where t is the arbitrary solidification or fusion time, and dH/dt is the heat flow during the solidification or fusion process. Furthermore, the mean heat-charging/discharging speed (Rh) during the isothermal fusion/solidification process could be calculated by Eq. (5). 11

t ) dt ∫0 ( dH dt ∞ ) dt ∫0 ( dH dt

× 100% (4)

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Fig. 9. Isothermal 3D heat-flow charts of red TCMs during (a) the heating and (b) cooling processes. Isothermal 3D heat-flow charts of blue TCMs during (c) the heating and (d) cooling processes.

Rh =

ΔH tend − tonset

respectively. Meanwhile, the amounts of latent heat charged and mean heat-charging speed are both observed to exhibit an ascending trend with an improvement of heat-charging temperature. It is understandable that a higher heat-charging temperature can facilitate a full melting phase transition for the encapsulated n-docosane inside the TCMs, and therefore the increase of operation temperature can effectively improve the latent heat-storage capacity and heat-charging speed of the TCMs. In addition, it is noticeable that the charged latent heat at the given temperatures could be discharged equably at a fixed cooling rate, thus resulting in almost identical exothermic peaks for the two TCMs during the heat-discharging process as observed in Figs. 9a, 9c, 10a and 12a. This result suggests that there is no effect of heat-charging temperature on the amounts of latent heat charged and heat-charging speed under the isothermal operation condition. Similarly to the phenomena observed in the isothermal heat-charging process, an enlarging trend in width and amplitude of exothermic peak is also observed with an increase of operation temperature for latent heat discharge (see in Fig. 9b and d) and confirmed by the regular programmed DSC thermograms in Figs. 11a, 11b, 13a and 13b. This may be attributed to the variation of Gibbs free energy of thermal energy system, leading to a change in exothermic behavior for crystallization of encapsulated n-docosane. It is noteworthy that the two TCMs exhibit a very similar development of relative degree of solidification at different operation temperatures for latent heat discharge as observed in Figs. 11c and 13c. It seems that the heat-discharging temperature of 34 °C results in a slower development of relative degree

(5)

where tonset and tend are the onset and end time of crystallization/ melting phase transition, respectively, and ΔH is the associated fusion or solidification heat. Fig. 9 illustrates the 3D heat-flow charts of two TCMs during the isothermal heat-charging/discharging processes, and the associated DSC thermograms and the plots of thermal behaviors obtained from programmed DSC scans are shown in Figs. 10–13. As observed in Fig. 9a and c, the heat flows for the two TCMs seem to be enhanced by improving the operation temperature for heat latent charge, and their endothermic peaks are found to exhibit an enlarging trend in width and amplitude with an increase of heat-charging temperature. A clearer variation trend can be found in Figs. 10a, 10b, 12a and 12b, which could be explained by the fact that a heat-charging temperature may cause higher Gibbs free energy for a thermal energystorage system and therefore thermodynamically promotes the melting phase transition of encapsulated n-docosane inside the TCMs. This means that the isothermal heat-charging process can be completed in a shorter period. This deduction can be further confirmed by the development of relative degree of fusion at different operation temperatures during the heat-charging process as seen in Figs. 10c and 12c. According to these two characteristic plots, the two TCMs are found to show the shortest period for latent heat charge at 52 °C but the longest one at 42 °C. It can also be observed in Figs. 10d and 12d that both types of TCMs are able to store the smallest and the largest amounts of latent heat at the heat-charging temperatures of 52 and 42 °C, 12

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Fig. 10. (a, b) Programmed DSC thermograms, (c) development of relative degree of fusion, and (d) fusion heat and mean heat-charging speed of red TCMs under the isothermal heat-charging condition.

Fig. 11. (a, b) Programmed DSC thermograms, (c) development of relative degree of solidification, and (d) solidification heat and mean heat-discharging speed of red TCMs under the isothermal heat-discharging condition.

13

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Fig. 12. (a, b) Programmed DSC thermograms, (c) development of relative degree of fusion, and (d) fusion heat and mean heat-charging speed of blue TCMs under the isothermal heat-charging condition.

Fig. 13. (a, b) Programmed DSC thermograms, (c) development of relative degree of solidification, and (d) solidification heat and mean heat-discharging speed of blue TCMs under the isothermal heat-discharging condition.

14

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34.5 °C. In this case, a high latent heat-release capacity and rapid heatdischarging speed could be achieved for the TCMs as long as the latent heat discharge is carried out at an operation temperature considerably lower than 34.5 °C. These results obtained from isothermal thermal analysis clearly indicate the correlation between the heat-charging/ discharging temperature and the total amounts of latent heat charged/ discharged and can provide a real-time indication for determining the optimum operation condition for latent heat storage and thermal management of TCMs. The nonisothermal latent heat-charging/discharging behaviors and thermal parameters of two TCMs were also investigated by programmed cyclic DSC scans at different scanning rates. Fig. 14 shows the

of solidification compared to the other operation temperatures, indicating that an overhigh heat-discharging temperature is disadvantageous to the nucleation and crystal growth of encapsulated ndocosane inside the TCMs. Such an explanation can also be supported by the data listed in Figs. 11d and 13d, in which the amounts of latent heat discharged and mean heat-discharging speed are both found to show a descending trend with increasing the heat-discharging temperatures. It is accepted that the crystallization process of encapsulated n-docosane could be thermodynamically promoted at a heat-discharging temperature much lower than the crystallization peak temperature. According to the aforementioned thermal analysis results, the two TCMs both present a crystallization peak temperature of approximately

Fig. 14. Nonisothermal 3D heat-flow charts of (a) red TCMs and (b) blue TCMs at a scanning rate of 10 °C/min. 15

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Fig. 15. Nonisothermal programmed DSC thermograms of (a) red TCMs and (b) blue TCMs at different scanning rates.

scanning rate. This suggests that the high heating and cooling rates are disadvantageous to the latent heat charge and discharge by the TCMs, respectively. It is evident that a short time scale can lead to insufficient crystallization and melting of encapsulated n-docosane and consequently reduces the total amounts of latent heat charged and discharged, respectively. It is also found in Fig. 16 that the mean heatcharging and discharging speeds increase with an increase of scanning rates under the nonisothermal condition, indicating that a high heating and cooling rate can accelerate the crystallization and melting processes of encapsulated n-docosane and therefore effectively improves the latent heat-charging and discharging speeds of the TCMs, respectively. Therefore, to achieve the optimum effectiveness in latent heat storage and thermal management, a balance between the total amounts of latent heat charged and discharged and the scanning rate should be carefully considered when determining the nonisothermal operation condition for the TCMs.

representative 3D heat-flow charts for the two TCMs at a scanning rate of 10 °C/min, and the regular DSC thermograms and associated thermal parameters obtained from nonisothermal DSC scans are presented in Figs. 15 and 16, respectively. It is interesting to observe that the two TCMs reveal a uniform heat-charging/discharging behavior during the cyclic DSC scanning process under the nonisothermal condition as seen in Fig. 14, and the latent heat charge and discharge are highly concentrated within a short period. Furthermore, a highly homologous DSC profile with the same peak amplitude and rhythm can be observed in the DSC thermograms at the same scanning rates as seen in Fig. 15. This indicates a highly stabile solid–liquid phase transition achieved for the two TCMs under the nonisothermal operation condition. It is important to note that the time required for full latent heat charge and discharge is reduced apparently by improving the heating and cooling rates, respectively. This phenomenon can be explained by the fact that a faster scanning rate normally results in a smaller time scale for melting and crystallization phase transitions. This means that the encapsulated ndocosane inside the TCMs is bound to complete latent heat charge and discharge in a shorter period. The thermal analysis data shown in Fig. 16 disclose that there is an ascending trend in the amounts of latent heat charged and discharged for the two TCMs with a decrease of

3.5. Practical thermal energy-storage and thermochromic performance The practical latent heat-storage and release performance of two TCMs was examined by means of an infrared thermographic method. 16

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Fig. 17 illustrates some representative thermographic images for the two TCMs as well as pure silica shell as a control during the heating and cooling processes. The real-time evolution of surface temperatures as a function of time for these three samples could be clearly known by monitoring their color variation. It is interesting to observe that compared to pure silica shell the two TCMs show a deeper blue color as an indication of slightly lower surface temperature at the initial heating stage. This may be due to a higher thermal conductivity of pure silica shell, resulting in a faster thermal response accordingly. The surface temperature of pure silica shell is found to rise up rapidly with an increase of heating time and promptly become close to the background temperature. It is noteworthy that the two TCMs display a blue color for a long while and then tend to show a green color at the later heating stage as observed in Fig. 17, implicating that a distinct hysteresis effect takes place in surface temperature. Such temperature hysteresis is attributed to the occurrence of latent heat absorption resulting from the melting phase change of encapsulated n-docosane, which effectively prevents the temperature rise of the TCMs under the heating condition. These results suggest that the temperature variation of TCMs is dominated by the latent heat absorbed from encapsulated n-docosane at the initial heating stage, whereas the temperature rise of pure silica shell is completely dependent on the sensible heat from a heating effect. Similarly to the phenomena observed in the heating process, the two TCMs also present a much slower color variation than pure silica shell during the cooling process, indicating a slower decline in surface temperature. This phenomenon can be explained by the fact that the latent heat released by crystallization of encapsulated n-docosane offsets the cooling effect and therefore delays the temperature descent of the TCMs. The temperature evolutions of two TCMs as a function of heating and cooling time were obtained from infrared thermographic analysis, and the resulting plots are presented in Fig. 18, which can provide a quantitative temperature variation for the two TCMs and pure silica shell. It is important to observe that there is a temperature hysteresis region for the two TCMs in the temperature range of 33–45 °C during the heating process and another one in the range of 30–47 °C in the

Fig. 16. Plots of fusion/solidification heat and mean heat-charging/discharging speeds of (a) red TCMs and (b) blue TCMs under the nonisothermal condition.

Fig. 17. Infrared thermographic images of (S0) pure silica shell, (S1) red TCMs and (S2) blue TCMs during the heating and cooling processes. 17

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plots of pure silica shell only show a fast rise during the heating process and a continuous decrease without any time lags in the cooling stage. There is no temperature hysteresis region appearing in both the heating and cooling stages, implicating no additional heat absorption and release happens in these two stages. These results clearly confirm high efficient temperature regulation and thermal management for the two TCMs through the effective absorption and release of latent heat by their PCM core. To understand the reversible thermochromic behavior in a visible way, the color variations of red and blue TCMs were recorded with a digital camera when the isothermal heating and cooling operations were carried out. Fig. 19 shows a series of real-time digital images of two TCMs during the isothermal heating and cooling processes. It is noticeable that the red and blue TCMs display high-contrast red and blue colors at 25 °C, respectively. A gradual discoloration phenomenon could be clearly observed when an isothermal heating operation was conducted for the two TCMs at 54 °C. With the specimen temperature rising up to 54 °C, the initial colors are found to almost fade away. Furthermore, a gradual recovery for two colors could also be seen under the isothermal cooling at 25 °C. It is important to note that the color recovery is so perfect for the two TCMs as to be identical to their initial colors, indicating excellent thermochromic reversibility. Such a reversible thermochromic behavior can also be observed in a real-time video as shown by Video 1S, which vividly displays a prompt, sensitive and reversible thermochromic response to temperature variation for the two TCMs. As aforementioned in Fig. 2, the discoloration and color recovery of the TCMs is derived from the reversible electron transfer between the color formers and color developer. However, such an electron transfer behavior is easily disturbed by the surrounding environment and contact materials, thus resulting in a deterioration in color indication efficiency and thermochromic reversibility. To overcome this drawback, we designed and constructed a sandwich-structured shell for the TCMs. The thermochromic performance of two TCMs is contributed by their TC-compound intermediate layer as a thermochromic indicator. The PMMA outer layer not only can offer an effective protection to the TCcompound intermediate layer but also can provide a good optical transmission for thermochromic indicator. In this case, the TC-compound layer can well maintain its reversible electron transfer effectiveness when suffering the environmental interference and thermal impact. Such a design makes the TCMs a clear and high-contrast color indication and outstanding thermochromic reversibility. It has been demonstrated in Fig. 19 and Video 1S that the two TCMs exhibit a prompt and reversible thermochromic response to temperature

Fig. 18. Plots of temperature evolution as a function of time obtained from infrared thermographic analysis for two TCMs and pure silica shell during (a) the heating and (b) cooling processes.

cooling stage. These two temperature hysteresis regions are found to be in accordance with the melting and crystallization temperature regions of encapsulated n-docosane. By contrast, the temperature evolution

Fig. 19. Digital photographs for reversible thermochromic behaviors of two TCMs during the isothermal heating and cooling processes. 18

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silica at the end of thermal degradation of base microcapsules. This value is approximately consistent with the aforementioned encapsulation ratio. There is a noticeable change in thermal degradation behavior for the two TCMs, and both of them are found to exhibit a two-step degradation behavior due to the evaporation of n-docosane and the pyrolysis of TC-compound and PMMA layers. It is noteworthy that the pyrolysis of TC-compound and PMMA layers occurs in the temperature range of 390–415 °C, which is much higher that the temperature region for n-docosane evaporation. In this case, the construction of additional two layers seems to lead to a slight improvement in the Tmax corresponding to the evaporation of n-docosane core. These results indicate the thermal stability of TCMs can be further enhanced by fabrication of sandwich-structured shell compared to normal microencapsulated ndocosane with a single silica shell. An isothermal heat impact experiment was carried out in a rapid heating process to examine the heat resistance and shape stability of two TCMs. Fig. 20c show some representative optical photographs taken during the heat impact test for the two TCMs, base microcapsules and pure n-docosane. It is clearly observed from these photographs that the base microcapsules and two TCMs always present an unchanged aspect with a stable shape until the temperature rises up to 100 °C. It seems that no leakage of n-docosane core from the microcapsules was found at the end of heat impact test, indicating a good stability in shape and form for the two TCMs as well as the base microcapsules. By contrast, the specimen of pure n-docosane is observed to show a great change in shape and form during the rapid heating process as seen in Fig. 20c. This is ascribed to a flowable nature of liquid n-docosane at a temperature higher than its melting temperature. Moreover, it is important to note that the two TCMs exhibit an excellent color recovery when suffering a heat impact at 100 °C. Although both of them undergo discoloration in the heating stage, their initial colors can be completely recovered when cooling to room temperature, indicating good heat resistance and operation reliability for the two TC-compounds used in the TCMs. These results can determine an upper working temperature of 100 °C for the two TCMs without any leakage, physical deformation

variation in the range of 25–54 °C. This temperature range shows perfect agreement with the crystallization and melting temperature regions of encapsulated n-docosane inside the TCMs as seen in Fig. 8a and b. This means that the two TCMs are able to give a real-time indication for the phase-change state of n-docosane core by red and blue color variations according to the working mechanisms described in Fig. 2. Therefore, it is believed that, with a highly efficient, reliable and reversible thermochromic performance, these two types of TCMs can provide an accurate and clear real-time indication about the phasechange state of PCM core for practical thermal energy-storage applications. Nevertheless, a slightly uncompleted color-fading phenomenon is noted at temperatures near 54 °C in Fig. 19. This may due to the poor fluidity of TC- compounds formed as a thin layer confined between the silica base shell and PMMA outer shell, resulting in a uncompleted electron transfer in the TC-compounds and leading to their slightly uncompleted discoloration accordingly. 3.6. Thermal stability and heat-impact resistance TGA was conducted to evaluate the thermal stability of base microcapsules and two TCMs as well as pure n-docosane as a control, and the resulting thermograms are illustrated in Fig. 20. Pure n-docosane is observed to show no mass loss until 150 °C in its TGA thermogram, followed by a rapid weight loss in the temperature range of 150–261 °C. The evaporation of n-docosane leads to a typical one-step thermal degradation behavior and makes almost no residual char remained. Furthermore, the characteristic temperature (Tmax) at a maximum decomposition rate can act as an important indicator for the thermal stability of n-docosane, and it is determined as 258.6 °C in accordance to Fig. 20b. In the case of the base microcapsules, a similar one-step thermal degradation behavior can be observed in their TGA thermogram, and however their Tmax seems to shift to a higher temperature, suggesting that the encapsulation with an inorganic silica shell has a good sealing effect to prevent the evaporation of n-docosane core. Moreover, a char yield of 21.56 wt% was also obtained as undegradable

Fig. 20. (a) TGA and (b) DTG thermograms of two TCMs, base microcapsules and pure n-docosane. (c) Digital photographs of (S0) pure n-docosane, (S1) base microcapsules, (S2) red TCMs and (S3) blue TCMs under a heat impact. 19

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the isothermal heating and cooling processes for the two TCMs. The whole discoloration–coloration behavior was also recorded by a video camera to give a real-time display of reversible thermochromic behavior as shown in Video 1S. It is surprising to observe that these two types of TCMs can still keep a prompt and sensitive thermochromic response to temperature variation even if they suffer the environmental impact of 500 thermal cycles. Both of them not only show an almost perfect real-time discoloration behavior in the isothermal heating stage but also exhibit a complete recovery to their initial red and blue colors. There is no deterioration found in discoloration and color recovery for the two TCMs. These results clearly demonstrate that the reversible thermochromic performance of two TCMs is in an excellent state of preservation after 500 thermal cycles. FTIR spectroscopy and TGA were performed to examine the chemical and thermal stabilities of two specimens of TCMs after 500 thermal cycles, and the resulting infrared spectra and TGA thermograms are given in Fig. 23. As seen in Fig. 23a and c, the infrared spectra are found to present almost the same location and intensity for each characteristic peak of two TCMs before and after the thermal cycle test, indicating that there is no change in the chemical structures and compositions of two TCMs after 500 thermal cycles. Meanwhile, it is notable in Fig. 23b and d that their TGA thermograms almost coincide with each other before and after the thermal cycling experiment, implying that the two TCMs have good thermal and chemical stabilities to undertake multicycle heating and cooling alternating impacts. These analysis results indubitably confirm an excellent reliability and durability of both phase-change and thermochromic behaviors as a result of the reasonable design of sandwich-structured shell for the two TCMs. With excellent phase-change and thermochromic reversibility, these two types of TCMs achieved a high reliability and long-term durability

and thermochromic invalidation.

3.7. Reliability and durability To assess the reversibility, reliability, and durability of phasechange and thermochromic behaviors, a consecutive thermal cycling experiment was performed by conducting a heating–cooling alternating treatment toward the two TCMs with 500 thermal cycles. Then, DSC measurements were carried out for the specimens of TCMs after every 50 thermal cycles to examine the reversibility and reliability of phasechange behavior. Fig. 21 displays the resulting DSC thermograms and associated phase-change thermal parameters, and the FTIR spectra and TGA thermograms of two TCMs before and after the thermal cycling experiment. It is noticeably observed in Fig. 21a and c that these DSC thermograms reveal a good coincidence in crystallization and melting behaviors with a variation of cycle number, and there is almost no shift found from the positions of melting and crystallization peaks. It is also observed in Fig. 21b and d that the ΔTc and ΔTm of two TCMs only exhibit a very small fluctuation within about ± 0.25 °C with an increase of cycle number. Furthermore, their ΔHc and ΔHm are also observed to preserve relatively stable values with a variation of cycle number. The red TCMs are found to show a slight fluctuation by 0.37 J/ g in ΔHc and by 0.49 J/g in ΔHm before and after the thermal cycling experiment. As for the blue TCMs, their ΔHc and ΔHm fluctuate by 0.52 and 0.34 J/g, respectively, before and after the thermal cycling experiment. The investigation of thermochromic performance was carried out by monitoring the real-time discoloration–coloration behaviors of two TCMs after the thermal cycling experiment with a digital camera. Fig. 22 displays a series of representative photographs captured during

Fig. 21. (a) DSC thermograms and (b) plots of phase change temperature and enthalpy as a function of cycle number for red TCMs after every 50 thermal cycles. (c) DSC thermograms and (d) plots of phase change temperature and enthalpy as a function of cycle number for blue TCMs after every 50 thermal cycles. 20

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Fig. 22. Digital photographs for reversible thermochromic behaviors of two TCMs after 500 thermal cycles during the isothermal heating and cooling processes.

Fig. 23. (a) FTIR spectra and (b) TGA thermograms of red TCMs before and after 500 thermal cycles. (c) FTIR spectra and (d) TGA thermograms of blue TCMs before and after 500 thermal cycles.

4. Conclusions

for the application in thermal energy storage and thermochromic indicator for the phase-change state. It is believed that the TCMs developed by this work have a great potential for hi-tech applications in smart fibers and textiles, wearable electric devices, energy-saving buildings, temperature-sensitive medical system, safety clothing, smart windows, aerospace engineering and many more.

We successfully synthesized two types of TCMs with red and blue color thermochromic indicators by fabricating a silica base shell onto the n-docosane core and then self-assembling a TC-compound layer onto the surface of base microcapsules through hydrogen bonding, 21

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followed by formation of a PMMA protective layer through surfactantassisted self-assembly polymerization. The well-defined core–shell microstructure was confirmed for the synthesized TCMs by SEM and TEM characterizations, and the sandwich-structured shell configuration was also identified with reference to the formation of silica base shell and the follow-up presence of thermochromic intermediate layer and PMMA outer layer. These two types of TCMs not only showed an outstanding latent heat-storage/release capability with a high capacity over 150 J/g, but also exhibited a good shape stability, high thermal stability and excellent phase-change reliability and durability. The optimum operation conditions for thermal energy storage and release were also determined by nonisothermal and isothermal DSC thermal analyses. Most of all, the two type of TCMs presented an entirely reversible thermochromic behavior individually with high-contrast red and blue color indications in real-time for the phase-change state of PCM core. The high reliability and long-term durability in thermochromic indication were confirmed for the TCMs by thermal cycle test results. In the light of an innovative configuration of sandwich-structured shell and a smart combination of latent heat-storage and thermochromic indicator, the TCMs developed by this study exhibit a great potential for hi-tech applications in smart fibers and textiles, wearable electric devices, energy-saving buildings, temperature-sensitive medical system, safety clothing, smart windows, aerospace engineering and many more.

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CRediT authorship contribution statement Ya Zhang: Investigation, Formal analysis, Methodology, Software. Huan Liu: Visualization, Data curation. Jinfei Niu: Methodology, Software. Xiaodong Wang: Conceptualization, Validation, Writing review & editing, Funding acquisition. Dezhen Wu: Supervision, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work is supported by the National Natural Science Foundation of China (Grant Nos.: 51673018, 51873010 and 51903010). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2020.114729. References [1] Pielichowska K, Pielichowski K. Phase change materials for thermal energy storage. Prog Mater Sci 2014;65:67–123. [2] Qureshi ZA, Ali HM, Khushnood S. Recent advances on thermal conductivity enhancement of phase change materials for energy storage system: A review. Int J Heat Mass Transfer 2018;127:838–56. [3] Su WG, Darkwa J, Kokogiannakis G. Review of solid–liquid phase change materials and their encapsulation technologies. Renew Sustain Energy Rev 2015;48:373–91. [4] Yataganbaba A, Ozkahraman B, Kurtbas I. Worldwide trends on encapsulation of phase change materials: A bibliometric analysis (1990–2015). Appl Energy 2017;185:720–31. [5] Umair MM, Zhang Y, Iqbal K, Zhang SF, Tang BT. Novel strategies and supporting materials applied to shape-stabilize organic phase change materials for thermal energy storage – A review. Appl Energy 2019;235:846–73. [6] Castell A, Solé C. An overview on design methodologies for liquid–solid PCM storage systems. Renew Sustain Energy Rev 2015;52:289–307. [7] Zhang N, Yuan YP, Cao XL, Du YX, Zhang ZL, Gui YW. Latent heat thermal energy storage systems with solid-liquid phase change materials: A review. Adv Eng Mater 2018;20:1700753. [8] Huang X, Zhu CQ, Lin YX, Fang GY. Thermal properties and applications of microencapsulated PCM for thermal energy storage: A review. Appl Therm Eng 2019;147:841–55.

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