Article pubs.acs.org/IC
High-Pressure and High-Temperature Synthesis and PressureInduced Simultaneous Optimization of the Electrical and Thermal Transport Properties of Nonstoichiometric TiO1.80 Haiqiang Liu, Hongan Ma,* Yuewen Zhang, Bing Sun, Binwu Liu, Lingjiao Kong, Baomin Liu, and Xiaopeng Jia*
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State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China ABSTRACT: We developed suitable high-pressure and hightemperature (HPHT) conditions for improvement of the thermoelectric properties of nonstoichiometric TiO1.80. X-ray diffraction, scanning transmission microscopy, transmission electron microscopy, and ultraviolet spectral measurements demonstrate that the crystal structures and microstructures are strongly modulated by our HPHT. The electrical properties and thermal conductivity are improved simultaneously by raising the reactive sintering pressure. The band gap was narrowed, contributing to the increase of the electrical properties with the pressure. In addition, relatively low thermal conductivities were obtained here as a result of a full spectrum of phonon scattering, benefiting from our deliberately engineered microstructures via HPHT. As a consequence, a high dimensionless figure of merit (zT) of 0.36 was obtained at 700 °C in the sample fabricated at 5 GPa. As far as we know, this is higher than all of the results of nonstoichiometric titanium oxide by other means and an enhancement of 57% of the best ever result. HPHT offers us a promising alternative for optimization of the thermoelectric properties, and it is worthwhile to popularize it.
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INTRODUCTION With a great deal of heat being wasted, the demand for thermoelectric materials with high performance is becoming compelling. Thermoelectric material can realize direct conversion between electricity and heat. Also, its competitiveness is evaluated by its dimensionless figure of merit, defined as zT = S2σT/κ, where S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. The good performance of a thermoelectric material requires the combination of a high power factor (S2σ) and a low thermal conductivity (κ).1−3 However, it is difficult to achieve synergistic optimization of the electrical properties and thermal conductivity because they counter each other. Increasing the electrical conductivity often brings about a decrease of the Seebeck coefficient. Similarly, a decrease in the thermal conductivity usually results in a decrease in the electrical conductivity. Therefore, it is of great importance for a thermoelectric material to increase its electrical properties but decrease its thermal conductivity simultaneously. When speaking of conventional alloy-based thermoelectric materials, such as PbTe and Bi2Te3, we are confronted with high toxicity, high cost, and poor thermal stability.4 Transitionmetal oxides, fortunately, are free of these drawbacks, and their good resistance against high temperature makes them promising candidates as thermoelectric materials. Environmentally friendly TiO2-based thermoelectrics have great unexploited thermoelectric performances at the high temper© 2017 American Chemical Society
ature range, becoming typical representative of oxide thermoelectric materials. These characteristics, including regulable wide band gap, versatile crystal structures, electron-transport behaviors depending on the oxygen vacancy, and unique electrical/phonon-transport properties, ensure that the thermoelectric properties of this system can be upgraded substantially. During the past decades, efforts in upgrading the zT value of titanium oxide have been reported, and some progresses have been achieved. A zT value of up to 1.64 has been reported in TiO1.1,5 but its reliability needs to be verified.6 He et al. reported their highest zT values of about 0.2−0.23 in nonstoichiometric TiO2−x, which was prepared by oxidizing TiO and the subsequent direct-current-induced hot press.6 Harada et al. obtained his best zT of 0.12 at 773 K in the hotpressed specimen of TiO1.90.7 Lee et al. demonstrated a high zT value of 0.132 at 750 K for substoichiometric TiO 2−x synthesized by in situ plasma spray.8 Many other investigations have also been conducted for the thermoelectric performance of TiO2. So far, the most-reported zT values are below 0.2 for titanium oxide and this needs to be improved. The precursory results demonstrate that the thermoelectric performance of titanium oxide has a strong relationship with the synthetic conditions, microstructures, and grain sizes. The improvement of the thermoelectric performance of titanium oxide is mainly Received: July 3, 2017 Published: August 30, 2017 11275
DOI: 10.1021/acs.inorgchem.7b01677 Inorg. Chem. 2017, 56, 11275−11281
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large-volume cubic high-pressure apparatus (SPD-6x1200). The conditions used here for HPHT reactive sintering were as follows: 2 GPa, 1060 °C; 2.5 GPa, 1070 °C; 4 GPa, 1098 °C; 5 GPa, 1116 °C, respectively. Also, the HPHT reactive sintering time was 2 h. An energized graphite crucible was used to heat the sample chamber and the hydraulic-pressed multianvil technique for the pressure. The temperature of the sample chamber was indicated by the platinum− rhodium thermocouples fixed on its surface, while a change of the resistance of the standard materials indicates the pressure. Before the pressure was unloaded, high-pressure quenching was undertaken on the sample chamber. Samples of TiO1.80 were successfully prepared using the above steps, and Figure 1 diagrammatically shows our HPHT reactive sintering.
obstructed by its poor electrical conductivity and high thermal conductivity. Improving the electrical conductivity but lowering the thermal conductivity is imperative for TiO2 to be an excellent thermoelectric material. Thus far, minimizing the thermal conductivity through refining grains is among the most effective routes for upgrading zT.9−11 Nanostructuring is widely employed for the performance improvement of thermoelectric materials. However, the nanostructure is restricted by the fact that only phonons with short and medium mean-free paths (∼3−100 nm) can be scattered by it; thus, phonons with longer mean-free paths remain unaffected. In contrast, a multiscale hierarchical structure has the added advantage of scattering phonons with a broader range of wavelengths.12,13 Alternatively, chemical tuning combining both foreign atom doping and the formation of ample intrinsic defects is the most common method used to improve the electrical conductivity of oxides. On the other hand, chemical tuning is also useful for lowering the thermal conductivity. Current approaches of fabricating the thermoelectric materials are tedious in steps, resulting in the cost, complexity, and compromise of the reproducibility of the materials. Compared with the conventional approaches, the method of high-pressure tuning together with high-temperature tuning, namely, high pressure and high temperature (HPHT), offers a new degree of freedom beyond the temperature and chemical composition for optimization of the thermoelectric properties and broadens the breadth of opportunities of obtaining good thermoelectric performances. HPHT features rapid tuning and the capacity of adjusting the structural transition and band gap and characterizes engineering microstructures. What is more, the good properties achieved at high pressure can be preserved to ambient one. HPHT has shown particular advantages in preparing thermoelectric materials.14−17 To date, however, the employment of HPHT in the improvement of the thermoelectric properties of titanium oxide is an unexploited field. It is meaningful to develop suitable HPHT conditions for optimization of the thermoelectric properties of titanium oxide. Enlightened by that mentioned above, we developed suitable HPHT conditions for titanium oxide and, consequently, successfully fabricated nonstoichiometric TiO1.80 at various pressures (2, 2.5, 4, and 5 GPa), aimed at systematically studying the impact exerted by HPHT on the microstructure and thermoelectric properties of TiO1.80 and achieving improvement of its thermoelectric performance. As expected, the microstructure and band gap were strongly tuned by HPHT. Enhanced electrical conductivity, upgraded power factor, and suppressed thermal conductivity were brought about synchronously by elevated high pressure. As a result, a high zT of 0.36 was obtained at 700 °C for the sample fabricated at 5 GPa, which is higher than the results of other means. Our HPHT method can be applied for the exploration of superior thermoelectric materials.
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Figure 1. Schematic of our HPHT reactive sintering for TiO1.80.
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CHARACTERIZATION Studies of the phase structures of all of our samples were carried out by X-ray diffraction (XRD) using Cu Kα radiation (D/MAX-RA). The morphologies and microstructures of the samples were examined by employing field-emission scanning electron microscopy (SEM; JEOL JSM-6700F) and highresolution transmission electron microscopy (HRTEM; JEOL JEM-2200FS), respectively. A bar of 3 × 3 × 8 mm was cut from the obtained sample and used for simultaneous measurements of the Seebeck coefficient and electrical resistivity from 50 to 700 °C by a Namicro-III model thermoelectric measurement system. The optical spectra of the samples were monitored by a UV-3150 double-beam spectrophotometer to reveal the band gaps. Another disklike sample with properties identical with those of the bar-shaped sample was used to measure the thermal conductivity. The thermal conductivity was the product of κ = λCpD, where λ denotes the thermal diffusivity coefficient measured by a laser flash method (Netzsch LFA 457) from 100 to 700 °C, Cp refers to the heat capacity obtained by the Dulong−Petit law, which announces that Cp has no relationship with the temperature, and D is the bulk density of the sample gained by the Archimedes method.
EXPERIMENTAL SECTION
The commercial elemental powders of Ti (99.99% metals basis, 300 mesh) and TiO2 (99.8% metals basis, 25 nm, anatase) were used as raw materials without further purification. They were weighted according to the stoichiometry of TiO1.80 and mixed thoroughly using an agate mortar. Then the well-mixed powders were shaped into a cylinder with a diameter of about 10.5 mm and a thickness of 7 mm. The obtained cylinder was encased with molybdenum foil to prevent probable contamination, followed by the assembly for HPHT reactive sintering. HPHT reactive sintering was carried out on a China-type
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RESULTS AND DISCUSSION Figure 2A reveals the XRD results of our samples fabricated by various pressures and the standard patterns of rutile TiO2 (PDF 77-442) and Ti9O17 (PDF 85-1061). The first impression of it 11276
DOI: 10.1021/acs.inorgchem.7b01677 Inorg. Chem. 2017, 56, 11275−11281
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Figure 2. (A) XRD results for our nonstoichiometric TiO1.80 fabricated by various pressures. (B) Expanded view showing the shift of the peaks of our samples to higher angles.
Figure 3. (A−D) Low-magnification SEM morphologies of our samples prepared by various pressures. (E−H) High-magnification SEM morphologies of all of our samples.
and the ability of titanium to pillage oxygen under high temperature created the strong reducing environment. Additionally, it is the advantage of HPHT to accelerate reactive sintering progress and lower the difficulty of sintering because high pressure is beneficial for contacting starting materials. Therefore, nonstoichiometric TiO1.80 is successfully obtained by our time-saving and efficient HPHT method. The cross-sectional morphologies of our samples prepared by various pressures were observed by SEM, shown in Figure 3. Parts A−D of Figure 3 are low-magnification SEM morphologies, clearly presenting to us the effect posed by HPHT on the grain size of our samples. From them, we can see that grains are refined by HPHT, with the grains being smaller with higher pressure. This lies in the capability of HPHT to refine grains and restrain its growth.16−18 The morphological details of our samples revealed by Figure 3E−H, highmagnification SEM morphologies, give us an in-depth understanding of our samples. It is observed that both samples prepared by 2 and 2.5 GPa are decorated by pores and precipitates with varied length scales, as shown in Figure 3E,F. In comparison to these two samples, only precipitates with varied length scales can be readily detected in the samples fabricated by 4 and 5 GPa and ample precipitates are the case for the sample fabricated by 5 GPa (Figure 3G,H). Herein, it is evident that multiscale hierarchical structures have been formed, corresponding to our XRD analysis. The above
is that the peaks are rather weak and broadened, rendering precise identification of the peaks difficult. This might be ascribed mostly to the existence of multiscale hierarchical structures and the low crystallinity relating to pervasive lattice defects brought about by HPHT, which will be discussed later. Another possible reason responsible for the difficulty of peak identification is that high pressure can cause decreases in the interatomic and interplanar distances and lattice constants, leading to the shift of peaks to higher angles, as indicated in Figure 2B.18 This shift of the peaks means that the impact exerted by HPHT on the lattice structures was successfully preserved to ambient conditions, which might affect the band structures and thermoelectric properties. As such, the patterns of all samples can be indexed to rutile TiO2 (PDF 77-442) and Ti9O17 (PDF 85-1061), being within the detection limitation of the measurement. Given the fact that the chamber for HPHT reactive sintering was squeezed by high pressure, the residual oxygen in it is negligible. So, it is understandable that the actual oxygen content of our samples stays at the same level. Meanwhile, according to the Ti−O phase diagram, our TiO1.80 system is composed of multiple phases.6,19 All of these make it clear that the phase transition here is driven by high pressure and help us make the above judgements. It is needed to point out that all resultant samples are black-blue, indicating the formation of oxygen vacancies.7,20 Collectively, all of these manifest the reduction to TiO2. The high-pressure extrusion 11277
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Figure 4. (A) Low-magnification TEM image of our sample prepared at 5 GPa. (B and D) HRTEM images of the sample prepared at 5 GPa. (C) IFFT image corresponding to the area marked by the yellow rectangle in part B.
Figure 5. Temperature dependence of the (A) electrical resistivity, (B) Seebeck coefficient, and (C) power factor for our samples derived from different pressures.
differences and formation of multiscale hierarchical structures may again arise from the effect of high pressure, retarding grain growth and facilitating reactive sintering. On the basis of these results, the possibility of obtaining desirable morphologies via HPHT is declared. TEM was utilized to characterize the detailed microstructures of our representative sample prepared at 5 GPa, as illustrated in Figure 4. The low-magnification TEM image (Figure 4A) reveals that our sample consists of particles with random sizes.
Meanwhile, some black areas are also evident, hinting at precipitates on the matrix. Analysis of the microstructures observed in the HRTEM image in Figure 4B further confirms the existence of multiscale hierarchical particles. We have indicated some nanoparticles, orientations of the lattice fringe, and spaces between lattice planes, as marked in Figure 4B. As can be seen, the nanoparticles are of various shapes and both the lattice fringe orientation and interplanar distance are random. Combining with the above-mentioned XRD results, 11278
DOI: 10.1021/acs.inorgchem.7b01677 Inorg. Chem. 2017, 56, 11275−11281
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Figure 6. Optical absorption spectra and energy band gaps for our samples derived from different pressures.
150, 139, and 114 μV K−1 for samples derived from 2, 2.5, 4, and 5 GPa, respectively. This change is consistent with that of the electrical resistivity. The power factors of our samples as a function of the temperature are shown in Figure 5C. Enhancement of the power factor ignited by an increase in the reactive sintering pressure is presented here. The present results are proof that the introduction of higher pressure via HPHT can advance the electrical properties of titanium oxide. Figure 7A exhibits variation of the total thermal conductivities (κtot) of our samples derived from different pressures. The total thermal conductivity is the collection of the electronic thermal conductivity (κe) and lattice thermal conductivity (κlat), and κe can be estimated using Wiedemann− Franz’s law, κe = LT/ρ, where L is the well-known Lorenz number (2.45 × 10−8 V2 K−2 was adopted here), T is the absolute temperature, and ρ is the electrical resistivity.22 Therefore, κlat was calculated by subtracting κe from κtot, as plotted in Figure 7B. It can be readily seen that high pressure makes a great difference in both the total and lattice thermal conductivities of our samples, suppressing them. At 100 °C, the total (lattice) thermal conductivities are 2.24 (2.22), 2.04 (1.99), 2.08 (2.02), and 1.64 (1.52) W m−1 K−1 for our samples derived from 2, 2.5, 4, and 5 GPa, respectively. This indicates that phonons dominate the thermal conductivity in our samples. A scenario has been evidenced that a higher reactive sintering pressure of HPHT is beneficial for lowering the thermal conductivity. As has been established, κlat is on intimate terms with the phonon relaxation time τ:16
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THERMOELECTRIC PROPERTIES Figure 5 displays the temperature dependence of the electrical property of our samples derived from different pressures. For the electrical resistivities (ρ) shown in Figure 5A, except the sample prepared at 5 GPa, all samples demonstrate semiconducting behavior, with the electrical resistivity decreasing with the measured temperature. However, the electrical resistivity of the one fabricated at 5 GPa decreases with the temperature and then increases when the temperature is over 300 °C, showing a semiconducting-to-semimetallic transforming behavior at 300 °C. On the whole, during the entire test temperature range, the higher the pressure is, the lower the electrical resistivity gets. Specifically, the electrical resistivity at 50 °C significantly diminishes from 0.0923 Ω cm for the sample derived from 2 GPa to 0.0085 Ω cm for the one from 5 GPa. Clearly, higher pressure is favorable for improvement of the electrical conductivity of titanium oxide. The mechanism by which the electrical resistivity is lowered with increasing pressure may be mainly related to the structural changes induced by HPHT. In order to have further insight into this, we monitored the band gaps of all samples through the optical spectra, as illustrated in Figure 6. A negative relationship between the band gap of our sample and the pressure can be found, which might be one possible reason for the value of the electrical resistivity being opposite to that of the pressure. Figure 5B is the temperature dependence of the Seebeck coefficient (S) for our samples. The negative Seebeck coefficients of all samples indicate n-type electrical-transport properties. The absolute value of the Seebeck coefficient of the sample derived from different pressures shows different temperature-dependent trends. Typically, the absolute value of the sample derived from 5 GPa increases monotonously with the measured temperature, while the absolute value of the one derived from 2 GPa decreases in the first stage and then increases with the test temperature. The comparison between our samples fabricated by different pressures leads us to the theory that high pressure might partly be the cause. The absolute values of the Seebeck coefficients at 50 °C are 191,
κlat = 1/3c Vνl = 1/3c Vν 2τ
where cV, ν, and l are the constant-volume specific heat, phonon velocity, and phonon mean-free path, respectively. For a specific material, cV and ν vary slightly, whereas τ depends on the phonon scattering mechanisms. Considering that the heat flow in a material is carried by a full spectrum of phonons and phonon can only be effectively scattered by a medium with a comparable scale, creating scattering agents with multilength scales is meaningful. On the other hand, the phonon scattering relaxation time can be written as23 11279
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Figure 7. Temperature dependence of the (A) total thermal conductivity and (B) lattice thermal conductivity for our samples derived from different pressures.
Figure 8. (A) Temperature dependence of the zT value for our samples. (B) Comparison of zT between different works.
τ −1 = τp−1 + τn−1 + τme−1 + τmi−1 + τu−1
the test temperature. Therefore, not only is HPHT useful for improvement of the electrical properties, but HPHT is also feasible in lowering the thermal conductivity of titanium oxide. Taking advantage of the simultaneous optimization of the electrical properties and thermal conductivity brought about by HPHT, the dimensionless figure of merit (zT) of our sample has a positive relationship with the reactive sintering pressure and a relatively high zT of 0.36 is obtained at 700 °C in the sample derived from 5 GPa, as exhibited in Figure 8A. Additionally, no saturation occurs during the whole measured temperature. To the best of our knowledge, it is higher than all of the results of nonstoichiometric titanium oxide fabricated by other means. An enhancement of 57% of the best ever result by other means has been achieved via our HPHT method (Figure 8B).
Here τp, τn, τme, τmi, and τu are relaxation times corresponding to phonon scattering by point defects, nanoscale scattering agents, mesoscale scattering agents, micrometer-scale grain boundaries, and Umklapp scattering, respectively. On the basis of the above discussions and the thermal conductivities, all of these phonon scattering mechanisms should have been introduced into our samples through deliberately engineered microstructures via HPHT. It is all of these phonon scattering mechanisms that contribute to the relatively low thermal conductivities of our samples, in which phonon scatterings by nanoscale and mesoscale agents should be the primary reasons for the inverse relationship between the thermal conductivity and reactive sintering pressure. They are intensified with the further refinement of grains induced by higher pressures of HPHT. A further reduction in the thermal conductivity might originate from the engineered lattice defects by HPHT, which disturb the propagation of phonons with short wavelengths. The sample derived from 5 GPa is the biggest beneficiary of these, with the thermal conductivity ranging from 1.64 to 1.48 W m−1 K−1. Last but not the least, high-frequency phonons with short and medium wavelengths would dominate heat flow in a material at high temperature. Both high-frequency phonon scattering by point defects and nanoscale agents and Umklapp scattering will be highlighted by elevated test temperatures, accounting for the decrease of the thermal conductivity with
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CONCLUSION In this work, suitable HPHT conditions were developed and utilized for enhancing the thermoelectric properties of nonstoichiometric TiO1.80. By this approach, both the electrical properties and thermal conductivities are optimized. This simultaneous optimization is related to the structural change and the modulation of microstructures toward desirable ones, induced by HPHT. The thermal conductivity was greatly suppressed by a full spectrum of phonon scattering thanks to the HPHT-modulated microstructures. As a consequence, a high zT of 0.36 was obtained at 700 °C in the sample derived 11280
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from 5 GPa, higher than the results of other means. HPHT provides us a new avenue in the optimization of thermoelectric materials, and its popularization is worthwhile.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.M.). Telephone: +8613504451109. *E-mail:
[email protected] (X.J.). Telephone: +8613943190363. ORCID
Hongan Ma: 0000-0003-1479-617X Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51171070), the Project of Jilin Science and Technology Development Plan (Project 20170101045JC), and the Graduate Innovation Fund of Jilin University (Project 2016065).
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REFERENCES
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