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High-thermoelectric performance of TiO2-x fabricated under high pressure at high temperatures
Accepted Manuscript High-thermoelectric performance of TiO2-x fabricated under high pressure at high temperatures Haiqiang Liu, Hongan Ma, Taichao Su, Yuewen Zhang, Bing Sun, Binwu Liu, Lingjiao Kong, Baomin Liu, Xiaopeng Jia PII:
S2352-8478(17)30030-8
DOI:
10.1016/j.jmat.2017.06.002
Reference:
JMAT 97
To appear in:
Journal of Materiomics
Received Date: 23 April 2017 Revised Date:
17 June 2017
Accepted Date: 22 June 2017
Please cite this article as: Liu H, Ma H, Su T, Zhang Y, Sun B, Liu B, Kong L, Liu B, Jia X, Highthermoelectric performance of TiO2-x fabricated under high pressure at high temperatures, Journal of Materiomics (2017), doi: 10.1016/j.jmat.2017.06.002. 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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ACCEPTED MANUSCRIPT High-Thermoelectric Performance of TiO2-x Fabricated under High Pressure at High Temperatures Haiqiang Liu a, Hongan Ma a, *, Taichao Su b, Yuewen Zhang a, Bing Sun a, Binwu Liu , Lingjiao Kong a, Baomin Liu a, Xiaopeng Jia a, *.
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a
a. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
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b. School of Materials Science and Engineering, Henan Polytechnic University,
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Jiaozuo 454000, China E-mail: [email protected]; [email protected] ABSTRACT
We present the work about the initiative fabrication of multi-scale hierarchical TiO2-x
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by our strategy, combining high pressure and high temperature (HPHT) reactive sintering with appropriate ratio of coarse Ti to nanosized TiO2. Ubiquitous lattice defects engineering has also been achieved in our samples by HPHT. The
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thermoelectric performance was significantly enhanced, and rather low thermal
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conductivity (1.60 W m-1 K-1) for titanium oxide was reported here for TiO1.76. Correspondingly, a high dimensionless figure of merit (zT) up to 0.33 at 700 ℃ was realized in it. As far as we know, this value is an enhancement of 43% of the ever best result about nonstoichiometric TiO2 and the result is also exciting for oxide thermoelectric materials. The moderate power factor, the significantly reduced thermal conductivity and the remarkable synergy between electrical properties and thermal conductivity are responsible for the excellent thermoelectric performance. We
ACCEPTED MANUSCRIPT develop a facile strategy for preparing multi-scale hierarchical TiO2-x and its superior ability to optimize thermoelectric performance has been demonstrated here. KEYWORDS: High pressure; High temperature; Multi-scale; TiO2-x; Thermoelectric;
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Integrated phonon scattering 1. Introduction
With an enormous amount of heat being lost as waste heat ， the need for
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high-performance thermoelectric materials is becoming compelling. Thermoelectric
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materials can convert heat into electricity directly and vice versa. For a thermoelectric material to be competitive the high efficiency is required, which is evaluated by its dimensionless figure of merit, zT= (S2σ/κ)T, where S, σ, κ and T are Seebeck coefficient, electrical conductivity, thermal conductivity and temperature in Kelvin,
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respectively. The excellent thermoelectric performance of a material requires high power factor (S2σ) and low thermal conductivity (κ) [1-3]. However, the interdependent characteristic of S, σ and κ impedes the improvement of zT value. The
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increment of electrical conductivity usually leads to a decrease of Seebeck coefficient.
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Similarly, reducing thermal conductivity often brings about a decrease in electrical conductivity. Therefore, increasing electrical properties but decreasing thermal conductivity are of great importance for thermoelectric materials. During the past decades, efforts have been persisted for achieving the synergy
between electrical properties and thermal conductivity and several successful strategies have been developed. Thus far, the most effective approach for enhancing zT has been to minimize the thermal conductivity, particularly through
ACCEPTED MANUSCRIPT nanostructuring [4-6]. Nanostructuring itself in bulk thermoelectrics, however, scatters phonons with short and medium mean free paths (~3–100 nm), but rendering phonons with longer mean free paths largely unaffected. Based on this, multi-scale
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hierarchical thermoelectrics has been proposed and employed for further improving zT value [7, 8]. While when it comes to conventional alloy-based thermoelectrics, such as Bi2Te3 and PbTe, high cost, high toxicity, and poor thermal stability restrict
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them [9]. In contrast, transition metal oxides are free of these disadvantages and their
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prominent stability at high temperature makes them promising candidates for thermoelectric applications. Titanium dioxide is among the intensively studied and the most widely used transition metal oxides, which takes advantage of its versatility [10-12].
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Anatase, rutile, and brookite are three most common polymorphs of TiO2. Of these, rutile is a stable phase under the ambient conditions, while both brookite and anatase are metastable [13]. Due to the tunable semiconducting properties and the large
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Seebeck coefficient, titanium dioxide displays tantalizing thermoelectric behavior. For
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titanium dioxide, the change of stoichiometry is viable in a relatively wide range, ranging from TiO to TiO2 [14]. TinO2n-1, which is known as Magnèli phase, has been explored as a potential thermoelectric material. Substoichiometric TinO2n-1 is formed when reducing happens to titanium dioxide, and the so-called crystallographic shear structures, in which dense planar defects are regularly introduced in the mother rutile structure, is evolved [15]. Over the past decades, some groups have reported their efforts in improving zT value of titanium oxide and great progress has been made. A
ACCEPTED MANUSCRIPT high zT value of 1.64 obtained in TiO1.1 has been reported [16], but verifications for it are needed [17]. He et al. fabricated nonstoichiometric TiO2-x by oxidizing TiO and the following direct current induced hot press, with the highest zT being about 0.2–
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0.23 [17]. And through the solid phase reaction of TiO2 and TiN, zT value of the resultant sample mainly consisting of Ti7O13 reached 0.4 (at 1150 K) [18]. Many other efforts have also been focused on exploiting the thermoelectric property of TiO2-x. The
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precursory studies show that the thermoelectric performance of titanium oxide
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strongly depends on the preparation conditions, particle size and microstructure. The low electrical conductivity and the high thermal conductivity are two chief obstacles to the good thermoelectric performance of titanium oxide. Current methods of preparing thermoelectrics involve tedious steps, leading to the cost and complexity,
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compromising the reproducibility of the materials.
Inspired by the above mentioned, we propose introduce lattice deformations and dislocations to titanium dioxide by high pressure and high temperature (HPHT) when
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reducing happens, which may be beneficial for both carrier concentration and thermal
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conductivity, in addition to dense planar defects. And integrated phonon scattering from atomic-scale lattice defects to nanoscale and mesoscale grain boundaries –can be harnessed for further lowering thermal conductivity through multi-scale hierarchical TiO2-x.
Here, we present the initiative work about the successful fabrication of nonstoichiometric TiO2-x by HPHT. And combining the use of coarse (about 48 µm) Ti and fine (about 25 nm) TiO2 with appropriate HPHT reactive sintering parameters,
ACCEPTED MANUSCRIPT multi-scale hierarchical TiO2-x was achieved in just two hours, in addition to ubiquitous lattice defects brought about by HPHT, such as dislocations and lattice deformations. As a result for titanium oxide prepared by HPHT, an encouraging zT
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value of up to 0.33 at 700 ℃ was reported here. The reasons why thermoelectric performance can be substantially enhanced by our innovative strategy are discussed.
Our method, HPHT, on the other hand, is simple, timesaving, cost effective, and
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has been employed for improving performance of other thermoelectrics and the
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excellent properties under high pressure can be preserved to ordinary pressure [19-21]. Furthermore, it was demonstrated here that HPHT is also valid for TiO2-based thermoelectrics and improved control of size distribution is the case for the solid phase reaction under HPHT. Our results further enlighten us that the strategy used
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here is meaningful for the fabrication of superior thermoelectric materials. 2. Results and discussion 2.1 Sample synthesis
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Ti powder (99.99% metals basis, 300 mesh) and TiO2 powder (99.8% metals basis, 25
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nm, anatase) were purchased from Aladdin and used as starting materials without further purification. The two powders were weighted according to the stoichiometries of TiO2-x (x = 0.24, 0.20, 0.16), followed by mixing them using an agate mortar for about one hour. Then the mixture was cold pressed into a cylinder with a thickness of about 7 mm and 10.5 mm in diameter. The obtained initial bulk disk was wrapped up in molybdenum foil to prevent probable contamination and then subjected to HPHT reactive sintering, which was conducted by a China-type large volume cubic
ACCEPTED MANUSCRIPT high-pressure apparatus (CHPA) (SPD - 6 × 1200). The parameters used here for HPHT reactive sintering were all 1080 ℃, 3 GPa, and 2 hours. The heating and pressure working on the sample chamber were produced by an energized graphite
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crucible and a hydraulic pressed multianvil technique, respectively. A thermocouple was fixed at the surface of the sample chamber to measure the temperature and the pressure was indicated by the change in the resistance of standard materials. The
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sample chamber was “quenched” from sintering temperature to room temperature
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quickly before the pressure was released. Samples of TiO2-x (x = 0.24, 0.20, 0.16) were successfully fabricated using these processes, and the HPHT reactive sintering
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procedure is shown in Fig. 1.
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Fig. 1. Schematic of HPHT reactive sintering for TiO2-x. 2.2 Characterization
The phase structures of all the samples were examined by X-ray diffraction using a Cu Kα radiation (D/MAX-RA). The studies of morphology and microstructure of the samples were carried out using field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2200FS), respectively. Parallelepiped specimens (about 3.0 mm by 3.0
ACCEPTED MANUSCRIPT mm by 8 mm) cut from the disc-shaped samples were used for simultaneous measurements of electrical resistivity and Seebeck coefficient from 50 ℃ to 700 ℃ by ZEM-3 (Ulvac-Riko Co., Japan). Room temperature Hall measurement was
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applied to reveal the carrier concentration and mobility. The thermal conductivity was calculated using κ = λCpD, where λ is the thermal diffusivity coefficient obtained by a laser flash method (Netzsch LFA 457) from 50 ℃ to 700 ℃, Cp is the heat capacity
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determined by Dulong-Petit law, which says Cp to be temperature independent, and D
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refers to the bulk density of the sample measured by the Archimedes method. 2.3 Phase Composition and Microstructure
Fig. 2 presents the XRD patterns of our samples obtained by HPHT. It is shown that the whole intensity of the diffraction peaks is rather low and some diffraction peaks
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are broadened, bringing about difficulty of precisely identifying peaks. These might be related to abundant lattice deformations, dislocations, regions with low crystallinity and the presence of multi-scale structure, which will be discussed later. As such,
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sample TiO1.84 shows relatively strong diffraction lines, which match well with those
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of the rutile TiO2 (PDF#77-442). And the comparisons of the peaks between our samples indicate that samples TiO1.80 and TiO1.76 should consist of rutile TiO2 (PDF#77-442) and Magnèli phases (Ti7O13 (PDF#85-1059) and Ti4O7 (PDF#71-574) being the most suitable in our opinion), with rather weak peaks, being within the detection limits of the measurement. The XRD results demonstrate that the phases are in agreement with the Ti-O phase diagram [22]. For samples TiO1.80 and TiO1.76, Magnèli phases are the primary phases and a small amount of rutile TiO2 was retained.
ACCEPTED MANUSCRIPT Furthermore, the comparison between TiO1.80 and TiO1.76 shows that the later possesses more oxygen deficiency than the former. All of these, together with the change in color of the samples into black blue, suggest the happen of reducing to TiO2.
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And the more Ti powder was added, the darker the sample is, indicating greater reducing happening. Herein, the change of color is due to the formation of oxygen vacancies along with reduction reaction, relating to crystallographic shear structures
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of Magnèli phases [15, 23]. The strong reducing environment was brought about by
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extrusion of high pressure and the great ability of Ti to capture oxygen under high temperature. Additionally, HPHT has advantages of accelerating reactive sintering progress and lowering difficulty of sintering, for high pressure is highly favorable for
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contact of the raw materials, and nanoparticles possess high reactivity.
Fig. 2. XRD patterns of samples TiO2-x (x = 0.24, 0.20, 0.16) fabricated by HPHT reactive sintering.
The SEM images in Fig. 3 show cross-sectional morphologies of the samples prepared by HPHT. Fig. 3a presents to us a distinct image that irregularly shaped particles with varied length scales, ranging from several nanometers to micrometers or
ACCEPTED MANUSCRIPT larger, randomly distributed in TiO1.76. Confined by visual range, larger grains were absent from Fig. 3a. In contrast, narrow particle size distributions are evident for the other two samples, as shown in Fig. 3b and c. Here, we can find that TiO1.84 was
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compacted mostly. The details of morphologies of the three samples revealed by the second row of Fig. 3 highlight differences between them. We can observe coexistence of pores and precipitates with varied length scales in TiO1.76, as indicated in Fig. 3d.
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However, TiO1.80 and TiO1.84 are free of these (Fig. 3e and f). A clear scenario has
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appeared to us that the compactness of the sample has a negative relationship with addition of Ti power, being consistent with the measured bulk density listed in Table 1, and appropriate ratio of coarse Ti to fine TiO2 assists the formation of multi-scale structure. In general, if starting materials with great difference in size were used, one
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could expect low compactness of the resultant sample. Furthermore, HPHT has the ability of suppress the growth of particles [24-26]. Besides, we speculate from the differences between our samples that Ti powders added might act as an obstacle to the
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sintering progress. Collectively, all of above mentioned, combining with appropriate
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HPHT reactive sintering conditions, leaded to the existence of multi-scale structure in sample TiO1.76.
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Fig. 3. Low magnification SEM images of samples TiO2-x prepared by HPHT: (a) TiO1.76, (b) TiO1.80, (c) TiO1.84; High magnification SEM images for our samples: (d) TiO1.76, (e) TiO1.80, (f) TiO1.84.
Table 1. Hall coefficient (RH), carrier concentration (n), carrier mobility (µ), Seebeck
temperature. Samples
RH
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coefficient (S), electrical resistivity (ρ) and bulk density (D) for our samples at room
n
µ
S
ρ
D
-0.20
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TiO1.76
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[cm3 C-1] [1019 cm-3] [cm2 V-1 S-1] [µV K-1] [Ω cm] [g cm-3] 3.12
18.52
96
0.01
4.20
TiO1.80
-1.29
0.48
25.91
123
0.05
4.24
TiO1.84
-4.39
0.14
36.30
296
0.12
4.27
Transmission electron microscopy (TEM) observations were conducted on the
representative sample TiO1.76 fabricated by HPHT to investigate the detailed microstructure of it, as shown in Fig. 4. In Fig. 4a, a localized wavy pattern with a scale of about 30 nm can be seen. Meanwhile, Fig. 4b exhibits that sample TiO1.76 is
ACCEPTED MANUSCRIPT rich in nanosized particles, circled by solid lines, leading to abundant interfaces. These nanoparticles are random in shape, size and orientation (the orientation labeled by double sided arrows). The occurrence of these nanoparticles may be the joint result
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of the use of nanosized TiO2 and appropriate HPHT reactive sintering conditions, for particle growth can be restrained by high pressure [24-26]. Further investigation of Fig. 4b reveals that perfect lattice plane is very rare for TiO1.76, but a considerable
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number of lattice defects, such as lattice deformations and dislocations (marked by
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lines), are common for it. What is more, some regions with low crystallinity are evident in Fig. 4b (pointed by dashed lines). We attribute these regions to being defect enrichment regions. Fig. 4c and its corresponding inverse fast Fourier transform (IFFT) image in Fig. 4d display a bunch of dislocation, which is also widespread in
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TiO1.76. The mechanism by which these abundant lattice defects were developed can be ascribed to strain induced by high pressure. All of these discussed microstructures further confirmed the above XRD results. It is needed to point out for Fig. 4 that the
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interfaces are found to be fuzzy and all particles are coherent with their surroundings.
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In addition to these, plentiful oxygen vacancies, titanium interstitials and titanium vacancies are definitely included in TiO2-x [27-29]. It is therefore apparent that multi-scale hierarchical TiO2-x has been fabricated successfully by our innovative approach.
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Fig. 4. HRTEM images showing the detailed microstructures of the representative sample TiO1.76: (a) A localized wavy pattern; (b) Nanoparticles, lattice defects, random orientation, and regions with low crystallinity; (c) Bunch of dislocation; (d)
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The IFFT image corresponding to (c).
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2.4 Thermoelectric Properties
Fig. 5. The temperature dependence of (a) electrical resistivity, (b) Seebeck coefficient, and (c) power factor for TiO2-x (x = 0.24, 0.20, 0.16). The temperature dependences of electrical resistivity, Seebeck coefficient and power factor for our samples TiO2-x (x = 0.24, 0.20, 0.16) are plotted in Fig. 5. As shown in Fig. 5a, a semiconducting behavior is exhibited for all samples, with the electrical
ACCEPTED MANUSCRIPT resistivity decreasing with the measured temperature. The electrical resistivity is lower with increasing oxygen nonstoichiometry over the entire measured temperature range. Specifically, the electrical resistivity at 50 ℃ significantly decreases from
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0.10 Ω cm for TiO1.84 to 0.008 Ω cm for TiO1.76. The decrease of the electrical resistivity of our samples should be the result of the increased density of carriers. The carrier concentration increases from 0.14 × 1019 cm-3 for TiO1.84 to 3.12 × 1019
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cm-3 for TiO1.76 at room temperature, as shown in Table 1. This significant increment
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in carrier concentration is attributed to the increase in the oxygen deficiency [15, 30]. Correspondingly, the carrier mobility decreases from 36.30 cm2 V-1 S-1 for TiO1.84 to 18.52 cm2 V-1 S-1 for TiO1.76. The variation of the carrier mobility of the samples from TiO1.84 to TiO1.76 may have certain relations with increment of planar shear defects
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and the formation of multi-scale structure along with more and more Ti powder added. Lattice defects and abundant interfaces can diminish the carrier mobility by enhancing scattering. Therefore, the decrease of electrical resistivity of samples from TiO1.84 to
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TiO1.76 provided that the increment of carrier concentration is sufficient to compensate
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for the decrease of carrier mobility. It is mentionable that the coherent interfaces between particles, being discussed above, could weaken the hampering effect from nanoparticles against carrier transport [31]. Another information conveyed by Fig. 5a is that the reduction rate of electrical resistivity with measured temperature is reduced by the increase of oxygen deficiency. Namely, the electrical resistivity of TiO1.84 is more significantly lowered with temperature than the ones of the other two samples. This is supposed to be due to that oxygen defects introduced additional electronic
ACCEPTED MANUSCRIPT states into the band gap of the mother rutile TiO2 [18]. Fig. 5b presents the temperature dependence of Seebeck coefficient for our samples TiO2-x (x = 0.24, 0.20, 0.16). The Seebeck coefficients are negative for all samples
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over the entire measured temperature range, indicative of n-type electrical transport property. The absolute value of Seebeck coefficient for TiO1.84 decreases with temperature up to 200 ℃ and then turn to increase. While the ones for the other two
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samples increase monotonously with the measured temperature. The absolute values
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of the Seebeck coefficient are 250, 130 and 103 µV K-1 at 50 ℃ for TiO1.84, TiO1.80 and TiO1.76, respectively. This change is in agreement with that of electrical resistivity. It is generally known that the absolute value of Seebeck coefficient has a negative relationship with the carrier concentration. Note that both the electrical conductivity
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and the absolute value of Seebeck coefficient for TiO1.76 and TiO1.80 depend positively on measured temperature. A similar phenomenon can be found in previous works [30, 32]. The power factor values for all three samples were calculated according to the
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above mentioned values of electrical resistivity and Seebeck coefficient, and the
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results are illustrated in Fig. 5c. It clearly shows that all the power factor values response positively to the increase of measured temperature, and samples of TiO1.76 and TiO1.80 show better electrical performance when compared with TiO1.84. On the other hand, all the values are not saturated until up to 700 ℃. For TiO1.76 and TiO1.80, they are comparable to each other in electrical performance. The highest power factors for TiO1.76 and TiO1.80 are 5.43 and 5.63 µW cm-1 K-2 at 700 ℃, respectively, which are larger than that of these previous reports about thermoelectric titanium
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oxides [23, 32-36].
Fig. 6. The temperature dependence of (a) total thermal conductivity, (b) lattice
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conductivity for TiO2-x (x = 0.24, 0.20, 0.16).
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thermal conductivity, and (c) the ratio of lattice thermal conductivity to total thermal
Fig. 6a shows the total thermal conductivities (κtot) as a function of temperature for our samples. The total thermal conductivity essentially includes electronic thermal conductivity (κele) and lattice thermal conductivity (κlat). Hence, the estimation of κlat
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can be made by subtracting κele from κtot, κlat = κtot - κele. Electronic thermal conductivity is estimated from Wiedemann-Franz’s law, κele = LT/ρ, where L is known as the Lorenz number (2.45 × 10-8 V2 K-2 was used here), T is the temperature in
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Kelvin, and ρ denotes electrical resistivity [30]. The temperature dependences of the
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lattice thermal conductivity (κlat) and the ratio of κlat to κtot for our samples are shown in Fig. 6b and c, respectively. As depicted in Fig. 6a, the total thermal conductivities (κtot) of our samples show a significant reduction with increasing oxygen nonstoichiometry. For samples of TiO1.84 and TiO1.76, their total thermal conductivities decrease up to 700 ℃, by and large. While a weak temperature dependence of thermal conductivity is the case for TiO1.80. These same trends are observed in lattice thermal conductivity as well, Fig. 6b. Such complicated thermal behavior as
ACCEPTED MANUSCRIPT mentioned above is also common in previous works about titanium oxide [17, 18, 30, 32, 34]. As pointed out above, the electrical resistivities for all samples decrease with measured temperature, which will definitely result in the increment of electronic
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thermal conductivity. Thus, the decrease of total thermal conductivity can mainly be ascribed to the reduction of lattice contribution caused by the enhanced phonon scattering. Fig. 6c shows us the ratio of κlat to κtot. Judging from it, decrease in the
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ratio of κlat to κtot with temperature is observed for all samples. In particular, this ratio
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for TiO1.76 is reduced from 0.95 at 50 ℃ to 0.68 at 700 ℃, and the one for TiO1.80 ranges from 0.98 to 0.83 at the same temperature range. This implies that phonon scattering is intensified with temperature.
As far as we know, the κtot of TiO1.80, ranging from 2.31 to 2.51 W m-1 K-1, is
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comparable to or lower than the most previously reported values [15, 17, 34, 37]. More importantly, the κtot obtained in TiO1.76 is rather low, ranging from 1.88 to 1.60 W m-1 K-1 [18, 32, 33, 38]. It was well established that high densities of planar
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crystallographic shear defects in Magnèli phases act as important phonon scattering
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centers to reduce thermal conductivity [15, 18, 37]. Given the fact that ubiquitous lattice deformations, lattice dislocations, and regions with low crystallinity have been successfully introduced into our samples by HPHT, which is an additional scattering mechanism. Apart from these, we have achieved multi-scale hierarchical TiO1.76 as discussed above, creating phonon scattering agents at various length scales. On the other hand, at high temperature, high frequency phonons with short wave length (＜ 100 nm) would dominate heat flow in lattice, being scattered considerably by
ACCEPTED MANUSCRIPT nanoscales, point defects or even atomic-scale defects. Simultaneously, umklapp scattering is increased at elevated temperature, playing an important role in further reducing lattice thermal conductivity. Both high frequency phonon scattering and
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umklapp scattering would be more and more highlighted with elevated temperature. This might be responsible for the negative temperature dependence exhibited in Fig. 6. Collectively, heat-carrying phonons with all-length scales are scattered effectively and
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efficiently, resulting in significantly reduced thermal conductivity [8]. All the
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mechanisms of phonon scattering for our sample TiO1.76 are illustrated in Fig. 7.
Fig. 7. Mechanisms of phonon scattering for our sample TiO1.76.
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The calculated dimensionless figure of merit (zT) for all three samples is illustrated in Fig. 8a. The results show that TiO2-x fabricated by HPHT can greatly enhance the
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thermoelectric performance at high temperature because of the moderate power factor, significantly reduced thermal conductivity and the remarkable synergy between electrical properties and thermal conductivity, corresponding to a maximum zT value of 0.33 for multi-scale hierarchical TiO1.76 at 700 ℃, which is an inspiring and relatively high result for n-type thermoelectric oxides. To the best of our knowledge, it is the highest result and an enhancement of 43% of the ever best one for nonstoichiometric TiO2-x, Fig. 8b.
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comparison between different works.
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3. Conclusion
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Fig. 8. (a) The temperature dependence of zT value for our samples and (b) The
In conclusion, multi-scale hierarchical nonstoichiometric TiO1.76 has been successfully fabricated via HPHT reactive sintering in assistance with the use of coarse Ti powder and nanosized TiO2 powder to enhance their thermoelectric
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performance. Ubiquitous lattice defects engineering in our samples is another achievement. The power factors of TiO1.76 and TiO1.80 reach up to 5.43 and 5.63 µW
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cm-1 K-2 at 700 ℃, respectively; furthermore, the thermal conductivity of our samples is reduced to 1.60 W m-1 K-1 at 700 ℃ for TiO1.76, and a high dimensionless figure of
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merit up to 0.33 is obtained at 700 ℃ in TiO1.76. It was demonstrated here that HPHT is also valid for the fabrication of titanium dioxide based thermoelectric materials. Through the combination of the appropriate ratio of coarse Ti to fine TiO2 and suitable HPHT conditions, we can easily obtain the desirable microstructure to achieve the promotion of thermoelectric performance. Thus, the strategy of reducing the thermal conductivity via rational engineering of multi-scale hierarchical structure and lattice defects, being conducted by HPHT, has great potential to achieve high-performance
ACCEPTED MANUSCRIPT oxide thermoelectric materials. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant
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No. 51171070), the Project of Jilin Science and Technology Development Plan (20170101045JC), and Graduate Innovation Fund of Jilin University (Project No. 2016065).
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Physical Properties of the Four Polymorphs of TiO2. Mol Simul 1991;6;239-44. [14] S. Andersson, B. Collen, U. Kuylenstierna, A. Magnéli. Phase analysis studies on the titanium-oxygen system. Acta chem scand 1957;11;1641-52. [15] S. Harada, K. Tanaka, H. Inui. Thermoelectric properties and crystallographic shear structures in titanium oxides of the Magnèli phases. J Appl Phys 2010;108;083703. [16] N. Okinaka, T. Akiyama. Latent Property of Defect-Controlled Metal Oxide: Nonstoichiometric Titanium Oxides as Prospective Material for High-Temperature Thermoelectric Conversion. Jpn J Appl Phys 2006;45;7009-10. [17] Q. He, Q. Hao, G. Chen, B. Poudel, X. Wang, D. Wang, Z. Ren. Thermoelectric property studies on bulk TiOx with x from 1 to 2. Appl Phys Lett 2007;91;052505. [18] M. Mikami, K. Ozaki. Thermoelectric properties of nitrogen-doped TiO2-x compounds. J Phys : Conf Ser 2012; 379;012006. [19] D. Polvani, J. Meng, N. Chandra Shekar, J. Sharp, J. Badding. Large improvement in thermoelectric properties in pressure-tuned p-type Sb1. 5Bi0. 5Te3. Chem Mater 2001;13;2068-71. [20] S.V. Ovsyannikov, V.V. Shchennikov, G.V. Vorontsov, A.Y. Manakov, A.Y. Likhacheva, V.A. Kulbachinskii. Giant improvement of thermoelectric power factor of Bi2Te3 under pressure. J Appl Phys 2008;104;053713. [21] H. Sun, X. Jia, L. Deng, P. Lv, X. Guo, Y. Zhang, B. Sun, B. Liu, H. Ma. Effect of HPHT processing on the structure, and thermoelectric properties of Co4Sb12 co-doped with Te and Sn. J Mater Chem A 2015;3;4637-41. [22] J.L. Murray, H.A. Wriedt. The O−Ti (Oxygen-Titanium) system. J Phase Equilib 1987;8;148-65. [23] Y. Lu, M. Hirohashi, K. Sato. Thermoelectric Properties of Non-Stoichiometric Titanium Dioxide TiO2-x Fabricated by Reduction Treatment Using Carbon Powder. Mater Trans 2006;47;1449-52. [24] P.W. Zhu, X. Jia, H.Y. Chen, W.L. Guo, L.X. Chen, D.M. Li, H.A. Ma, G.Z. Ren, G.T. Zou. A new method of synthesis for thermoelectric materials: HPHT. Solid State Commun 2002;123;43-7. [25] B. Sun, X. Jia, D. Huo, H. Sun, Y. Zhang, B. Liu, H. Liu, L. Kong, B. Liu, H. Ma. Effect of High-Temperature and High-Pressure Processing on the Structure and Thermoelectric Properties of Clathrate Ba8Ga16Ge30. J Phys Chem C 2016;120;10104-10. [26] Y. Zhang, X. Jia, H. Sun, B. Sun, B. Liu, H. Liu, L. Kong, H. Ma. Enhanced thermoelectric performance of nanostructured CNTs/BiSbTe bulk composite from rapid pressure-quenching induced multi-scale microstructure. Journal of Materiomics 2016; 2;316-23. [27] M.K. Nowotny, A. T. Bak, J. Nowotny. Electrical Properties and Defect Chemistry of TiO2 Single Crystal. I. Electrical Conductivity†. J Phys Chem B 2006; 110;16270-82. [28] M.K. Nowotny, A. T. Bak, J. Nowotny. Electrical Properties and Defect Chemistry of TiO2 Single Crystal. II. Thermoelectric Power†. J Phys Chem B 2006;110;16283-91.
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Highlights HPHT was employed for the fabrication of thermoelectric titanium oxide for the first time. Multi-scale hierarchical TiO2-x was achieved by our strategy. Ubiquitous lattice defect engineering was realized in our samples by HPHT. A rather low thermal conductivity (1.60 W m-1 K-1) for titanium oxide was reported here for TiO1.76. The highest zT value of 0.33 for nonstoichiometric TiO2 was obtained in our sample TiO1.76.
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Haiqiang Liu is a graduate student in State Key Lab of Superhard Materials, Jilin University. He focuses his researches on the synthesis and performance optimization of thermoelectric titanium oxide by high pressure and high temperature method.
S2352-8478(17)30030-8
DOI:
10.1016/j.jmat.2017.06.002
Reference:
JMAT 97
To appear in:
Journal of Materiomics
Received Date: 23 April 2017 Revised Date:
17 June 2017
Accepted Date: 22 June 2017
Please cite this article as: Liu H, Ma H, Su T, Zhang Y, Sun B, Liu B, Kong L, Liu B, Jia X, Highthermoelectric performance of TiO2-x fabricated under high pressure at high temperatures, Journal of Materiomics (2017), doi: 10.1016/j.jmat.2017.06.002. 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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ACCEPTED MANUSCRIPT High-Thermoelectric Performance of TiO2-x Fabricated under High Pressure at High Temperatures Haiqiang Liu a, Hongan Ma a, *, Taichao Su b, Yuewen Zhang a, Bing Sun a, Binwu Liu , Lingjiao Kong a, Baomin Liu a, Xiaopeng Jia a, *.
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a
a. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
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b. School of Materials Science and Engineering, Henan Polytechnic University,
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Jiaozuo 454000, China E-mail: [email protected]; [email protected] ABSTRACT
We present the work about the initiative fabrication of multi-scale hierarchical TiO2-x
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by our strategy, combining high pressure and high temperature (HPHT) reactive sintering with appropriate ratio of coarse Ti to nanosized TiO2. Ubiquitous lattice defects engineering has also been achieved in our samples by HPHT. The
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thermoelectric performance was significantly enhanced, and rather low thermal
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conductivity (1.60 W m-1 K-1) for titanium oxide was reported here for TiO1.76. Correspondingly, a high dimensionless figure of merit (zT) up to 0.33 at 700 ℃ was realized in it. As far as we know, this value is an enhancement of 43% of the ever best result about nonstoichiometric TiO2 and the result is also exciting for oxide thermoelectric materials. The moderate power factor, the significantly reduced thermal conductivity and the remarkable synergy between electrical properties and thermal conductivity are responsible for the excellent thermoelectric performance. We
ACCEPTED MANUSCRIPT develop a facile strategy for preparing multi-scale hierarchical TiO2-x and its superior ability to optimize thermoelectric performance has been demonstrated here. KEYWORDS: High pressure; High temperature; Multi-scale; TiO2-x; Thermoelectric;
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Integrated phonon scattering 1. Introduction
With an enormous amount of heat being lost as waste heat ， the need for
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high-performance thermoelectric materials is becoming compelling. Thermoelectric
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materials can convert heat into electricity directly and vice versa. For a thermoelectric material to be competitive the high efficiency is required, which is evaluated by its dimensionless figure of merit, zT= (S2σ/κ)T, where S, σ, κ and T are Seebeck coefficient, electrical conductivity, thermal conductivity and temperature in Kelvin,
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respectively. The excellent thermoelectric performance of a material requires high power factor (S2σ) and low thermal conductivity (κ) [1-3]. However, the interdependent characteristic of S, σ and κ impedes the improvement of zT value. The
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increment of electrical conductivity usually leads to a decrease of Seebeck coefficient.
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Similarly, reducing thermal conductivity often brings about a decrease in electrical conductivity. Therefore, increasing electrical properties but decreasing thermal conductivity are of great importance for thermoelectric materials. During the past decades, efforts have been persisted for achieving the synergy
between electrical properties and thermal conductivity and several successful strategies have been developed. Thus far, the most effective approach for enhancing zT has been to minimize the thermal conductivity, particularly through
ACCEPTED MANUSCRIPT nanostructuring [4-6]. Nanostructuring itself in bulk thermoelectrics, however, scatters phonons with short and medium mean free paths (~3–100 nm), but rendering phonons with longer mean free paths largely unaffected. Based on this, multi-scale
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hierarchical thermoelectrics has been proposed and employed for further improving zT value [7, 8]. While when it comes to conventional alloy-based thermoelectrics, such as Bi2Te3 and PbTe, high cost, high toxicity, and poor thermal stability restrict
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them [9]. In contrast, transition metal oxides are free of these disadvantages and their
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prominent stability at high temperature makes them promising candidates for thermoelectric applications. Titanium dioxide is among the intensively studied and the most widely used transition metal oxides, which takes advantage of its versatility [10-12].
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Anatase, rutile, and brookite are three most common polymorphs of TiO2. Of these, rutile is a stable phase under the ambient conditions, while both brookite and anatase are metastable [13]. Due to the tunable semiconducting properties and the large
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Seebeck coefficient, titanium dioxide displays tantalizing thermoelectric behavior. For
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titanium dioxide, the change of stoichiometry is viable in a relatively wide range, ranging from TiO to TiO2 [14]. TinO2n-1, which is known as Magnèli phase, has been explored as a potential thermoelectric material. Substoichiometric TinO2n-1 is formed when reducing happens to titanium dioxide, and the so-called crystallographic shear structures, in which dense planar defects are regularly introduced in the mother rutile structure, is evolved [15]. Over the past decades, some groups have reported their efforts in improving zT value of titanium oxide and great progress has been made. A
ACCEPTED MANUSCRIPT high zT value of 1.64 obtained in TiO1.1 has been reported [16], but verifications for it are needed [17]. He et al. fabricated nonstoichiometric TiO2-x by oxidizing TiO and the following direct current induced hot press, with the highest zT being about 0.2–
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0.23 [17]. And through the solid phase reaction of TiO2 and TiN, zT value of the resultant sample mainly consisting of Ti7O13 reached 0.4 (at 1150 K) [18]. Many other efforts have also been focused on exploiting the thermoelectric property of TiO2-x. The
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precursory studies show that the thermoelectric performance of titanium oxide
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strongly depends on the preparation conditions, particle size and microstructure. The low electrical conductivity and the high thermal conductivity are two chief obstacles to the good thermoelectric performance of titanium oxide. Current methods of preparing thermoelectrics involve tedious steps, leading to the cost and complexity,
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compromising the reproducibility of the materials.
Inspired by the above mentioned, we propose introduce lattice deformations and dislocations to titanium dioxide by high pressure and high temperature (HPHT) when
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reducing happens, which may be beneficial for both carrier concentration and thermal
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conductivity, in addition to dense planar defects. And integrated phonon scattering from atomic-scale lattice defects to nanoscale and mesoscale grain boundaries –can be harnessed for further lowering thermal conductivity through multi-scale hierarchical TiO2-x.
Here, we present the initiative work about the successful fabrication of nonstoichiometric TiO2-x by HPHT. And combining the use of coarse (about 48 µm) Ti and fine (about 25 nm) TiO2 with appropriate HPHT reactive sintering parameters,
ACCEPTED MANUSCRIPT multi-scale hierarchical TiO2-x was achieved in just two hours, in addition to ubiquitous lattice defects brought about by HPHT, such as dislocations and lattice deformations. As a result for titanium oxide prepared by HPHT, an encouraging zT
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value of up to 0.33 at 700 ℃ was reported here. The reasons why thermoelectric performance can be substantially enhanced by our innovative strategy are discussed.
Our method, HPHT, on the other hand, is simple, timesaving, cost effective, and
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has been employed for improving performance of other thermoelectrics and the
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excellent properties under high pressure can be preserved to ordinary pressure [19-21]. Furthermore, it was demonstrated here that HPHT is also valid for TiO2-based thermoelectrics and improved control of size distribution is the case for the solid phase reaction under HPHT. Our results further enlighten us that the strategy used
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here is meaningful for the fabrication of superior thermoelectric materials. 2. Results and discussion 2.1 Sample synthesis
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Ti powder (99.99% metals basis, 300 mesh) and TiO2 powder (99.8% metals basis, 25
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nm, anatase) were purchased from Aladdin and used as starting materials without further purification. The two powders were weighted according to the stoichiometries of TiO2-x (x = 0.24, 0.20, 0.16), followed by mixing them using an agate mortar for about one hour. Then the mixture was cold pressed into a cylinder with a thickness of about 7 mm and 10.5 mm in diameter. The obtained initial bulk disk was wrapped up in molybdenum foil to prevent probable contamination and then subjected to HPHT reactive sintering, which was conducted by a China-type large volume cubic
ACCEPTED MANUSCRIPT high-pressure apparatus (CHPA) (SPD - 6 × 1200). The parameters used here for HPHT reactive sintering were all 1080 ℃, 3 GPa, and 2 hours. The heating and pressure working on the sample chamber were produced by an energized graphite
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crucible and a hydraulic pressed multianvil technique, respectively. A thermocouple was fixed at the surface of the sample chamber to measure the temperature and the pressure was indicated by the change in the resistance of standard materials. The
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sample chamber was “quenched” from sintering temperature to room temperature
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quickly before the pressure was released. Samples of TiO2-x (x = 0.24, 0.20, 0.16) were successfully fabricated using these processes, and the HPHT reactive sintering
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procedure is shown in Fig. 1.
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Fig. 1. Schematic of HPHT reactive sintering for TiO2-x. 2.2 Characterization
The phase structures of all the samples were examined by X-ray diffraction using a Cu Kα radiation (D/MAX-RA). The studies of morphology and microstructure of the samples were carried out using field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2200FS), respectively. Parallelepiped specimens (about 3.0 mm by 3.0
ACCEPTED MANUSCRIPT mm by 8 mm) cut from the disc-shaped samples were used for simultaneous measurements of electrical resistivity and Seebeck coefficient from 50 ℃ to 700 ℃ by ZEM-3 (Ulvac-Riko Co., Japan). Room temperature Hall measurement was
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applied to reveal the carrier concentration and mobility. The thermal conductivity was calculated using κ = λCpD, where λ is the thermal diffusivity coefficient obtained by a laser flash method (Netzsch LFA 457) from 50 ℃ to 700 ℃, Cp is the heat capacity
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determined by Dulong-Petit law, which says Cp to be temperature independent, and D
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refers to the bulk density of the sample measured by the Archimedes method. 2.3 Phase Composition and Microstructure
Fig. 2 presents the XRD patterns of our samples obtained by HPHT. It is shown that the whole intensity of the diffraction peaks is rather low and some diffraction peaks
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are broadened, bringing about difficulty of precisely identifying peaks. These might be related to abundant lattice deformations, dislocations, regions with low crystallinity and the presence of multi-scale structure, which will be discussed later. As such,
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sample TiO1.84 shows relatively strong diffraction lines, which match well with those
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of the rutile TiO2 (PDF#77-442). And the comparisons of the peaks between our samples indicate that samples TiO1.80 and TiO1.76 should consist of rutile TiO2 (PDF#77-442) and Magnèli phases (Ti7O13 (PDF#85-1059) and Ti4O7 (PDF#71-574) being the most suitable in our opinion), with rather weak peaks, being within the detection limits of the measurement. The XRD results demonstrate that the phases are in agreement with the Ti-O phase diagram [22]. For samples TiO1.80 and TiO1.76, Magnèli phases are the primary phases and a small amount of rutile TiO2 was retained.
ACCEPTED MANUSCRIPT Furthermore, the comparison between TiO1.80 and TiO1.76 shows that the later possesses more oxygen deficiency than the former. All of these, together with the change in color of the samples into black blue, suggest the happen of reducing to TiO2.
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And the more Ti powder was added, the darker the sample is, indicating greater reducing happening. Herein, the change of color is due to the formation of oxygen vacancies along with reduction reaction, relating to crystallographic shear structures
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of Magnèli phases [15, 23]. The strong reducing environment was brought about by
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extrusion of high pressure and the great ability of Ti to capture oxygen under high temperature. Additionally, HPHT has advantages of accelerating reactive sintering progress and lowering difficulty of sintering, for high pressure is highly favorable for
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contact of the raw materials, and nanoparticles possess high reactivity.
Fig. 2. XRD patterns of samples TiO2-x (x = 0.24, 0.20, 0.16) fabricated by HPHT reactive sintering.
The SEM images in Fig. 3 show cross-sectional morphologies of the samples prepared by HPHT. Fig. 3a presents to us a distinct image that irregularly shaped particles with varied length scales, ranging from several nanometers to micrometers or
ACCEPTED MANUSCRIPT larger, randomly distributed in TiO1.76. Confined by visual range, larger grains were absent from Fig. 3a. In contrast, narrow particle size distributions are evident for the other two samples, as shown in Fig. 3b and c. Here, we can find that TiO1.84 was
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compacted mostly. The details of morphologies of the three samples revealed by the second row of Fig. 3 highlight differences between them. We can observe coexistence of pores and precipitates with varied length scales in TiO1.76, as indicated in Fig. 3d.
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However, TiO1.80 and TiO1.84 are free of these (Fig. 3e and f). A clear scenario has
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appeared to us that the compactness of the sample has a negative relationship with addition of Ti power, being consistent with the measured bulk density listed in Table 1, and appropriate ratio of coarse Ti to fine TiO2 assists the formation of multi-scale structure. In general, if starting materials with great difference in size were used, one
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could expect low compactness of the resultant sample. Furthermore, HPHT has the ability of suppress the growth of particles [24-26]. Besides, we speculate from the differences between our samples that Ti powders added might act as an obstacle to the
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sintering progress. Collectively, all of above mentioned, combining with appropriate
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HPHT reactive sintering conditions, leaded to the existence of multi-scale structure in sample TiO1.76.
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Fig. 3. Low magnification SEM images of samples TiO2-x prepared by HPHT: (a) TiO1.76, (b) TiO1.80, (c) TiO1.84; High magnification SEM images for our samples: (d) TiO1.76, (e) TiO1.80, (f) TiO1.84.
Table 1. Hall coefficient (RH), carrier concentration (n), carrier mobility (µ), Seebeck
temperature. Samples
RH
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coefficient (S), electrical resistivity (ρ) and bulk density (D) for our samples at room
n
µ
S
ρ
D
-0.20
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TiO1.76
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[cm3 C-1] [1019 cm-3] [cm2 V-1 S-1] [µV K-1] [Ω cm] [g cm-3] 3.12
18.52
96
0.01
4.20
TiO1.80
-1.29
0.48
25.91
123
0.05
4.24
TiO1.84
-4.39
0.14
36.30
296
0.12
4.27
Transmission electron microscopy (TEM) observations were conducted on the
representative sample TiO1.76 fabricated by HPHT to investigate the detailed microstructure of it, as shown in Fig. 4. In Fig. 4a, a localized wavy pattern with a scale of about 30 nm can be seen. Meanwhile, Fig. 4b exhibits that sample TiO1.76 is
ACCEPTED MANUSCRIPT rich in nanosized particles, circled by solid lines, leading to abundant interfaces. These nanoparticles are random in shape, size and orientation (the orientation labeled by double sided arrows). The occurrence of these nanoparticles may be the joint result
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of the use of nanosized TiO2 and appropriate HPHT reactive sintering conditions, for particle growth can be restrained by high pressure [24-26]. Further investigation of Fig. 4b reveals that perfect lattice plane is very rare for TiO1.76, but a considerable
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number of lattice defects, such as lattice deformations and dislocations (marked by
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lines), are common for it. What is more, some regions with low crystallinity are evident in Fig. 4b (pointed by dashed lines). We attribute these regions to being defect enrichment regions. Fig. 4c and its corresponding inverse fast Fourier transform (IFFT) image in Fig. 4d display a bunch of dislocation, which is also widespread in
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TiO1.76. The mechanism by which these abundant lattice defects were developed can be ascribed to strain induced by high pressure. All of these discussed microstructures further confirmed the above XRD results. It is needed to point out for Fig. 4 that the
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interfaces are found to be fuzzy and all particles are coherent with their surroundings.
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In addition to these, plentiful oxygen vacancies, titanium interstitials and titanium vacancies are definitely included in TiO2-x [27-29]. It is therefore apparent that multi-scale hierarchical TiO2-x has been fabricated successfully by our innovative approach.
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Fig. 4. HRTEM images showing the detailed microstructures of the representative sample TiO1.76: (a) A localized wavy pattern; (b) Nanoparticles, lattice defects, random orientation, and regions with low crystallinity; (c) Bunch of dislocation; (d)
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The IFFT image corresponding to (c).
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2.4 Thermoelectric Properties
Fig. 5. The temperature dependence of (a) electrical resistivity, (b) Seebeck coefficient, and (c) power factor for TiO2-x (x = 0.24, 0.20, 0.16). The temperature dependences of electrical resistivity, Seebeck coefficient and power factor for our samples TiO2-x (x = 0.24, 0.20, 0.16) are plotted in Fig. 5. As shown in Fig. 5a, a semiconducting behavior is exhibited for all samples, with the electrical
ACCEPTED MANUSCRIPT resistivity decreasing with the measured temperature. The electrical resistivity is lower with increasing oxygen nonstoichiometry over the entire measured temperature range. Specifically, the electrical resistivity at 50 ℃ significantly decreases from
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0.10 Ω cm for TiO1.84 to 0.008 Ω cm for TiO1.76. The decrease of the electrical resistivity of our samples should be the result of the increased density of carriers. The carrier concentration increases from 0.14 × 1019 cm-3 for TiO1.84 to 3.12 × 1019
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cm-3 for TiO1.76 at room temperature, as shown in Table 1. This significant increment
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in carrier concentration is attributed to the increase in the oxygen deficiency [15, 30]. Correspondingly, the carrier mobility decreases from 36.30 cm2 V-1 S-1 for TiO1.84 to 18.52 cm2 V-1 S-1 for TiO1.76. The variation of the carrier mobility of the samples from TiO1.84 to TiO1.76 may have certain relations with increment of planar shear defects
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and the formation of multi-scale structure along with more and more Ti powder added. Lattice defects and abundant interfaces can diminish the carrier mobility by enhancing scattering. Therefore, the decrease of electrical resistivity of samples from TiO1.84 to
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TiO1.76 provided that the increment of carrier concentration is sufficient to compensate
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for the decrease of carrier mobility. It is mentionable that the coherent interfaces between particles, being discussed above, could weaken the hampering effect from nanoparticles against carrier transport [31]. Another information conveyed by Fig. 5a is that the reduction rate of electrical resistivity with measured temperature is reduced by the increase of oxygen deficiency. Namely, the electrical resistivity of TiO1.84 is more significantly lowered with temperature than the ones of the other two samples. This is supposed to be due to that oxygen defects introduced additional electronic
ACCEPTED MANUSCRIPT states into the band gap of the mother rutile TiO2 [18]. Fig. 5b presents the temperature dependence of Seebeck coefficient for our samples TiO2-x (x = 0.24, 0.20, 0.16). The Seebeck coefficients are negative for all samples
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over the entire measured temperature range, indicative of n-type electrical transport property. The absolute value of Seebeck coefficient for TiO1.84 decreases with temperature up to 200 ℃ and then turn to increase. While the ones for the other two
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samples increase monotonously with the measured temperature. The absolute values
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of the Seebeck coefficient are 250, 130 and 103 µV K-1 at 50 ℃ for TiO1.84, TiO1.80 and TiO1.76, respectively. This change is in agreement with that of electrical resistivity. It is generally known that the absolute value of Seebeck coefficient has a negative relationship with the carrier concentration. Note that both the electrical conductivity
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and the absolute value of Seebeck coefficient for TiO1.76 and TiO1.80 depend positively on measured temperature. A similar phenomenon can be found in previous works [30, 32]. The power factor values for all three samples were calculated according to the
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above mentioned values of electrical resistivity and Seebeck coefficient, and the
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results are illustrated in Fig. 5c. It clearly shows that all the power factor values response positively to the increase of measured temperature, and samples of TiO1.76 and TiO1.80 show better electrical performance when compared with TiO1.84. On the other hand, all the values are not saturated until up to 700 ℃. For TiO1.76 and TiO1.80, they are comparable to each other in electrical performance. The highest power factors for TiO1.76 and TiO1.80 are 5.43 and 5.63 µW cm-1 K-2 at 700 ℃, respectively, which are larger than that of these previous reports about thermoelectric titanium
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oxides [23, 32-36].
Fig. 6. The temperature dependence of (a) total thermal conductivity, (b) lattice
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conductivity for TiO2-x (x = 0.24, 0.20, 0.16).
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thermal conductivity, and (c) the ratio of lattice thermal conductivity to total thermal
Fig. 6a shows the total thermal conductivities (κtot) as a function of temperature for our samples. The total thermal conductivity essentially includes electronic thermal conductivity (κele) and lattice thermal conductivity (κlat). Hence, the estimation of κlat
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can be made by subtracting κele from κtot, κlat = κtot - κele. Electronic thermal conductivity is estimated from Wiedemann-Franz’s law, κele = LT/ρ, where L is known as the Lorenz number (2.45 × 10-8 V2 K-2 was used here), T is the temperature in
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Kelvin, and ρ denotes electrical resistivity [30]. The temperature dependences of the
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lattice thermal conductivity (κlat) and the ratio of κlat to κtot for our samples are shown in Fig. 6b and c, respectively. As depicted in Fig. 6a, the total thermal conductivities (κtot) of our samples show a significant reduction with increasing oxygen nonstoichiometry. For samples of TiO1.84 and TiO1.76, their total thermal conductivities decrease up to 700 ℃, by and large. While a weak temperature dependence of thermal conductivity is the case for TiO1.80. These same trends are observed in lattice thermal conductivity as well, Fig. 6b. Such complicated thermal behavior as
ACCEPTED MANUSCRIPT mentioned above is also common in previous works about titanium oxide [17, 18, 30, 32, 34]. As pointed out above, the electrical resistivities for all samples decrease with measured temperature, which will definitely result in the increment of electronic
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thermal conductivity. Thus, the decrease of total thermal conductivity can mainly be ascribed to the reduction of lattice contribution caused by the enhanced phonon scattering. Fig. 6c shows us the ratio of κlat to κtot. Judging from it, decrease in the
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ratio of κlat to κtot with temperature is observed for all samples. In particular, this ratio
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for TiO1.76 is reduced from 0.95 at 50 ℃ to 0.68 at 700 ℃, and the one for TiO1.80 ranges from 0.98 to 0.83 at the same temperature range. This implies that phonon scattering is intensified with temperature.
As far as we know, the κtot of TiO1.80, ranging from 2.31 to 2.51 W m-1 K-1, is
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comparable to or lower than the most previously reported values [15, 17, 34, 37]. More importantly, the κtot obtained in TiO1.76 is rather low, ranging from 1.88 to 1.60 W m-1 K-1 [18, 32, 33, 38]. It was well established that high densities of planar
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crystallographic shear defects in Magnèli phases act as important phonon scattering
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centers to reduce thermal conductivity [15, 18, 37]. Given the fact that ubiquitous lattice deformations, lattice dislocations, and regions with low crystallinity have been successfully introduced into our samples by HPHT, which is an additional scattering mechanism. Apart from these, we have achieved multi-scale hierarchical TiO1.76 as discussed above, creating phonon scattering agents at various length scales. On the other hand, at high temperature, high frequency phonons with short wave length (＜ 100 nm) would dominate heat flow in lattice, being scattered considerably by
ACCEPTED MANUSCRIPT nanoscales, point defects or even atomic-scale defects. Simultaneously, umklapp scattering is increased at elevated temperature, playing an important role in further reducing lattice thermal conductivity. Both high frequency phonon scattering and
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umklapp scattering would be more and more highlighted with elevated temperature. This might be responsible for the negative temperature dependence exhibited in Fig. 6. Collectively, heat-carrying phonons with all-length scales are scattered effectively and
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efficiently, resulting in significantly reduced thermal conductivity [8]. All the
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mechanisms of phonon scattering for our sample TiO1.76 are illustrated in Fig. 7.
Fig. 7. Mechanisms of phonon scattering for our sample TiO1.76.
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The calculated dimensionless figure of merit (zT) for all three samples is illustrated in Fig. 8a. The results show that TiO2-x fabricated by HPHT can greatly enhance the
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thermoelectric performance at high temperature because of the moderate power factor, significantly reduced thermal conductivity and the remarkable synergy between electrical properties and thermal conductivity, corresponding to a maximum zT value of 0.33 for multi-scale hierarchical TiO1.76 at 700 ℃, which is an inspiring and relatively high result for n-type thermoelectric oxides. To the best of our knowledge, it is the highest result and an enhancement of 43% of the ever best one for nonstoichiometric TiO2-x, Fig. 8b.
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comparison between different works.
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3. Conclusion
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Fig. 8. (a) The temperature dependence of zT value for our samples and (b) The
In conclusion, multi-scale hierarchical nonstoichiometric TiO1.76 has been successfully fabricated via HPHT reactive sintering in assistance with the use of coarse Ti powder and nanosized TiO2 powder to enhance their thermoelectric
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performance. Ubiquitous lattice defects engineering in our samples is another achievement. The power factors of TiO1.76 and TiO1.80 reach up to 5.43 and 5.63 µW
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cm-1 K-2 at 700 ℃, respectively; furthermore, the thermal conductivity of our samples is reduced to 1.60 W m-1 K-1 at 700 ℃ for TiO1.76, and a high dimensionless figure of
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merit up to 0.33 is obtained at 700 ℃ in TiO1.76. It was demonstrated here that HPHT is also valid for the fabrication of titanium dioxide based thermoelectric materials. Through the combination of the appropriate ratio of coarse Ti to fine TiO2 and suitable HPHT conditions, we can easily obtain the desirable microstructure to achieve the promotion of thermoelectric performance. Thus, the strategy of reducing the thermal conductivity via rational engineering of multi-scale hierarchical structure and lattice defects, being conducted by HPHT, has great potential to achieve high-performance
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No. 51171070), the Project of Jilin Science and Technology Development Plan (20170101045JC), and Graduate Innovation Fund of Jilin University (Project No. 2016065).
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Highlights HPHT was employed for the fabrication of thermoelectric titanium oxide for the first time. Multi-scale hierarchical TiO2-x was achieved by our strategy. Ubiquitous lattice defect engineering was realized in our samples by HPHT. A rather low thermal conductivity (1.60 W m-1 K-1) for titanium oxide was reported here for TiO1.76. The highest zT value of 0.33 for nonstoichiometric TiO2 was obtained in our sample TiO1.76.
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Haiqiang Liu is a graduate student in State Key Lab of Superhard Materials, Jilin University. He focuses his researches on the synthesis and performance optimization of thermoelectric titanium oxide by high pressure and high temperature method.