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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Effects of Pressure on the Microstructure and Simultaneous Optimization of the Electrical and Thermal Transport Properties of Yb0.5Ba7.5Ga16Ge30 Bing Sun,† Xiaopeng Jia,‡ Jiaqiang Zhao,*,† Yingde Li,† Haiqiang Liu,‡ and Hongan Ma*,‡ †
Department of Physics and Electronic Science, Weifang University, Weifang 261061, People’s Republic of China National Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China
‡
ABSTRACT: The thermoelectric (TE) properties of n-type polycrystalline Yb0.5Ba7.5Ga16Ge30 bulks can be optimized by high-pressure and high-temperature (HPHT) sintering. After HPHT sintering, abundant nanograins are randomly distributed in the sample. Grains are refined by HPHT, with the grains being smaller with higher pressure. In comparison with the arcmelted sample, the samples obtained by quenching under high pressure possess a great number of nanograins and lattice structural disorders. Lower thermal conductivity is benefited by our deliberately engineered microstructures via HPHT, and the minimum thermal conductivity is 0.86 W m−1 K−1 at 773 K. The thermal conductivity and electrical properties are optimized simultaneously by raising the reactive sintering pressure. In comparison with the arc-melted sample (0.56), a maximum zT value of 1.13 at 773 K is obtained for the Yb0.5Ba7.5Ga16Ge30 sample fabricated at 5 GPa. This demonstrates that HPHT provides an effective strategy to improve TE performance through simultaneously enhancing electrical and thermal transport properties and should be applicable to other thermoelectric materials.
1. INTRODUCTION Thermoelectric (TE) effects can convert thermal energy into electrical energy directly. They also provide a practical method for power generation from waste heat. TE materials based on TE effects have been considered to be a green and sustainable solution for overcoming the global energy dilemma.1 A highefficiency TE generator should possess a high dimensionless figure of merit, zT = S2σT/(κe + κl), where T, S, σ, κe, and κl are the absolute temperature, the Seebeck coefficient, the electrical conductivity, the electronic thermal conductivity, and the lattice thermal conductivity,2 respectively. The development of TE materials with higher TE efficiency (zT) have become the main thrust of research. The good performance of a TE material requires a higher power factor, PF = S2σ, and a lower thermal conductivity (κ).3−5 Recently, two main approaches have been used for enhancing zT: one is to reduce κ by phonon engineering1,6,7 and the other is to boost PF = S2σ, by using the quantum confinement effect8,9 in low-dimensional nanostructured materials. However, it is difficult to achieve synergistic optimization of the electrical properties and thermal conductivity because they counter each other. An increase in the electrical conductivity usually results in a decrease in the Seebeck coefficient. Similarly, a decrease in the thermal conductivity often brings about a decrease in the electrical conductivity. Therefore, it is difficult yet important to optimize the electrical properties and thermal properties for a TE material simultaneously. © XXXX American Chemical Society
Among potential thermoelectric materials, type I clathrate materials are recognized as some of the most promising highperformance TE materials ever since the “phonon glass electron crystal” (PGEC) concept was proposed.10−12 Interestingly, we find that an type I clathrate, which has a very stable and complex crystal structure, exhibits an intrinsically ultralow thermal conductivity. A type I clathrate possesses an open framework structure. A well-crystallized host framework structure encapsulates a large number of the guest atoms that effect the interaction of the heat-carrying phonons by “rattling”. The rattling of the guest atoms can reduce the lattice thermal conductivity (κl).13,14 Meanwhile, the well-crystallized framework can ensure good electrical properties. These characteristics and unique electrical-/phonon-transport properties ensure that the TE properties of this system can be upgraded substantially. During the past decades, efforts in upgrading the zT value of type I clathrate compounds have been reported, and some progresses have been achieved. A zT value of up to 1.35 has been reported in Ba8Ga16Ge30, but it is a single-crystal sample. In addition, Tang et al.15 synthesized YbxBa 8‑xGa 16Ge30 compounds by a spark plasma sintering (SPS) method, and the maximum zT value reached was 1.1 at 950 K. Zhang et al.16 prepared Ba7.7Yb0.3Ni0.1Zn0.54Ga13.8Ge31.56 compounds by a Received: January 11, 2018
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DOI: 10.1021/acs.inorgchem.8b00061 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry high-pressure technique, and the maximum zT value reached 0.91 at 900 K. As we know, pressure can enhance the mechanical stability of the samples.17 As expected, the zT value of n-type polycrystalline clathrates Ba8Ga16Ge30 reached 1.14 at 773 K by the HPHT method.18 In comparison with the conventional approaches, the HPHT method 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 performance.19 It is identified as an excellent substitutive method, which can effectively improve the TE performance of a large proportion of TE materials, such as oxide,19 PbTe,20 skutterudites,21−23 and Bi2Te3.24 These studies indicate that multitudinous nanograins, grain boundaries, lattice structural disorders, and lattice defects are ubiquitous inside their samples by high-pressure treatment. These specific structures play a very important role in the I-type clathrate compounds as well. Therefore, we have tried to inquire into the effects of HPHT sintering on the TE properties and structures of the Yb-filled clathrate Yb0.5Ba7.5Ga16Ge30 alloys. In this paper, we developed suitable HPHT conditions for Yb0.5Ba7.5Ga16Ge30 at various pressures (3, 4, and 5 GPa), aiming at systematically studying the effect exerted by HPHT sintering on the microstructures and TE properties of Yb0.5Ba7.5Ga16Ge30 and achieving improvement in its thermoelectric performance. As expected, the microstructure was strongly tuned by HPHT. Enhanced electrical conductivity, upgraded PF, and suppressed thermal conductivity were brought about synchronously by elevated high pressure. As a result, a higher zT value of 1.13 was reached at 773 K for the samples fabricated at 5 GPa, which is obviously higher than the results of arc melting. Therefore, the HPHT treatment can enhance the TE properties of the I-type clathrate compounds effectively and can be applied to the exploration of superior TE materials.
Figure 1. Schematic of our HPHT reactive sintering for Yb0.5Ba7.5Ga16Ge30.
technique (LFA 457 Micro Flash, Netzsch), and the specific heat capacity (Cp) was measured by Linseis STA PT-1750 equipment with a sapphire revision. The κ value was derived using the relationship κ = DtCpD, D being the bulk density of samples determined by the Archimedes method.
3. RESULTS AND DISCUSSION 3.1. XRD. The XRD patterns of as-sintered Yb0.5Ba7.5Ga16Ge30 samples are exhibited in Figure 2. We can see from Figure 2a that all XRD patterns match well with type I clathrate phases. In addition, we confirmed the crystal structures and the lattice parameters (a) of the sintered clathrate Yb0.5Ba7.5Ga16Ge30 samples by cell refinement. The sintered Yb0.5Ba7.5Ga16Ge30 samples still belong to a cubic crystal in the space group Pm3̅n (No. 223). The lattice parameters (a) of the sintered Yb0.5Ba7.5Ga16Ge30 display a slight decrease from 10.7681(1) Å (2 GPa) to 10.6431(2) Å (5 GPa). This could be the result of higher-pressure sintering. The possible reason for the decrease in the lattice parameters (a) is that high pressure can cause decreases in the interatomic and interplanar distances and lattice constants, leading to a shift of the peaks to higher angles, as indicated in Figure 2b. 3.2. SEM Micrographs and Chemical Composition. The cross-sectional morphologies of our samples prepared at various pressures were observed by SEM, as shown in Figure 3. Figure 3a shows the SEM micrograph of Yb0.5Ba8Ga16Ge30 alloy, which was prepared by arc melting. From Figure 3a, we do not find any clear grains/precipitates. Figure 3b−d shows low-magnification SEM morphologies, clearly presenting to us the effect posed by HPHT on the grain size of our samples. From these pictures, we can see that grains/precipitates are refined by HPHT, with the grains/precipitates being smaller with higher pressure. This has to do with the capability of HPHT to refine grains/precipitates and restrain growth.25−27 The morphological details of our samples revealed by Figure 3e−g, high-magnification SEM morphologies, give us an indepth understanding of our samples. An increase in the sintering pressure results in a increase in these grains/ precipitates. The above differences of structures may again arise from the effect of high pressure, retarding grain/ precipitate growth and facilitating reactive sintering. In comparison to the sample prepared by arc melting, a large number of grains/precipitates, which have a rich surface area and grain boundary, are randomly arranged in the high-pressure sintered samples. All of these microstructures could effectively
2. EXPERIMENTAL SECTION 2.1. Experimental Procedure. Elemental barium (Ba, 99.5%, metal basis), gallium (Ga, 99.999%, metal basis), germanium (Ge, 99.999%, metal basis), and ytterbium (Yb, 99%, metal basis), purchased from either Alfa Aesar or Aldrich, were used as raw and processed materials according to the stoichiometry. Arc melting Yb, Ba, Ga, and Ge gave a Yb0.5Ba7.5Ga16Ge30 ingot, and then the Yb0.5Ba7.5Ga16Ge30 ingot was ground to a fine powder using a Retsch MM400 mixer mill under argon protection. The HPHT reactive sintering was processed on a China-type large volume cubic highpressure apparatus (CHPA) (SPD-6 × 1200). The conditions used here for HPHT reactive sintering were as follows, respectively: 3 GPa, 1083 K; 4 GPa, 1093 K; 5 GPa, 1103 K. In addition, the HPHT reactive sintering time was 1 h. Before the pressure was released, highpressure quenching was undertaken on the sample chamber. Samples of Yb0.5Ba7.5Ga16Ge30 were successfully sintered using the above steps, and Figure 1 diagrammatically shows our HPHT reactive sintering. 2.2. 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). We used a least-squares technique with the program package GSAS to determine the unit cell dimensions. The morphologies, elemental compositions, and microstructures of the samples were examined via field-emission scanning electron microscopy (SEM, JEOL JSM-6700F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F). A bar shape (2 × 2 × 8 mm) and a disc shape (diameter 10 mm, thickness 2 mm) were cut from our obtained samples to measure the Seebeck coefficient (S), the electrical resistivity (ρ), and the thermal diffusion coefficient (Dt). S and ρ were measured simultaneously with ZEM-3 equipment (ULVACRIKO), Dt was measured using the laser flash B
DOI: 10.1021/acs.inorgchem.8b00061 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. (a) XRD patterns of sintered Yb0.5Ba7.5Ga16Ge30 at different pressures. (b) Expanded view showing a systematic shift of the (321) peak (the same as in (a)) with an increase in pressure (the pressures refer to the sintering pressure).
elements of Yb, Ba, Ga, and Ge are uniformly distributed in the Yb0.5Ba8Ga16Ge30 samples. This certifies that these samples by HPHT sintering are homogeneous. Table 1 shows the Archimedes density, the actual compositions, and the XRD crystal density. As shown in Table 1, the Archimedes density and XRD crystal density of the Yb0.5Ba7.5Ga16Ge30 compounds increase with an increase in sintering pressure. 3.3. HRTEM. We obtained more detailed information about the microstructures through HRTEM measurements. The common features of HPHT sintered samples are the largescale nanograins and lattice disorders (Figure 5a,b). Nanograins Figure 3. SEM images of the fractured surface for clathrate Yb0.5Ba7.5Ga16Ge30 prepared by arc melting (a) and sintering at 3 GPa (b, e), 4 GPa (c, f), and 5 GPa (d, g).
enhance phonons of different frequency scattering, which can significantly decrease the lattice thermal conductivity. The chemical homogeneity of our samples was studied via EDS elemental mapping. We can see from Figure 4 that the
Figure 5. HRTEM images of multiple microstructures: nanocrystals and lattice distortions of 3 GPa (a, c−e) and 5 GPa (b). The microstructures such as amorphous region, fringes, and grain boundaries can be seen. Figure 4. SEM image and EDS elemental mapping of Ba, Ga, Ge, and Yb elements for Yb0.5Ba7.5Ga16Ge30..
with a size of several nanometers are seen in the HRTEM images. The quenching under high pressure is the main reason that the nanosized features were generated. A large number of
Table 1. Synthesis Pressure, Actual Composition Obtained by EDS Analyses, and Density EDS analysis (%) synthesis pressure (GPa)
Yb
Ba
Ga
Ge
actual composition
Archimedes density (g cm−3)
XRD crystal density (g cm−3)
arc melting 3 4 5
0.83 0.79 0.77 0.72
13.98 14.02 14.04 14.09
28.77 28.18 27.98 27.65
56.76 57.01 57.21 57.54
Yb0.45Ba7.55Ga15.35Ge30.65 Yb0.43Ba7.57Ga15.22Ge30.78 Yb0.42Ba7.58Ga15.11Ge30.89 Yb0.39Ba7.61Ga14.93Ge31.07
5.745(3) 5.826(3) 5.843(1) 5.844(5)
5.76(2) 5.84(1) 5.86(1) 5.87(2)
C
DOI: 10.1021/acs.inorgchem.8b00061 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry representative lattice defects are present, including an amorphous region, fringes, and grain boundaries (Figure 5c− e), which are retained under ambient conditions. Specially, the amorphous region and fringes are marked by rectangles in Figure 5c,d. The quenching process generated a great number of these microstructures, and these microstructures are ubiquitous in the HPHT sintered samples. In a word, the nanograins, lattice disorders, amorphous region, fringes, and grain boundaries will hinder phonon propagation and reduce the lattice thermal conductivity. 3.4. Thermoelectric Properties. The relationship between the electrical resistivity and temperature for Yb0.5Ba7.5Ga16Ge30 compounds using HPHT sintering are shown in Figure 6. All of the samples show metal-like behavior, and the electrical resistivity increase with an increase in temperature over the whole working temperature range, a trend similar to that in the literature.28−30 In the range of the testing temperature, the sample sintered at 5 GPa has the minimum electrical resistivity value, which reaches 0.96 mΩ cm at room temperature and 2.24 mΩ cm at around 773 K. We can observe from Figure 6a that the higher the pressure, the lower the electrical resistivity. Specifically, the electrical resistivity at room temperature significantly decreases from 1.69 mΩ cm for the sample prepared by arc melting to 0.96 mΩ cm for the sample sintered at 5 GPa. Clearly, higher pressure is favorable for improvement of the electrical conductivity of Yb0.5Ba7.5Ga16Ge30 compounds. This means that the charge carriers of the samples are affected by the microstructural changes, and the band gap of the clathrate n-type Yb0.5Ba7.5Ga16Ge30 compounds may partially be decreased by the high pressure. Figure 6b shows the variation of Seebeck coefficient with temperature for Yb0.5Ba7.5Ga16Ge30 compounds using HPHT sintering. The Seebeck coefficients are negative, and they indicate that the majority carriers of Yb0.5Ba7.5Ga16Ge30 samples are electrons. With an increase in temperature, the absolute Seebeck coefficients of all samples increases, and the sample obtained by arc melting reached a maximum value of 176.5 μV K−1 at about 773 K. In the range of the testing temperatures between 300 and 773 K, with an increase in pressure, the absolute Seebeck coefficient of all samples slightly decreases. Figure 6c shows the power factors of our samples as a function of the temperature. An increase in the reactive sintering pressure can enhance the power factor of n-type Yb0.5Ba7.5Ga16Ge30 samples and gives the maximum PF value of 12.72 μW cm−1 K−2 at 773 K. The present results are proof that the introduction of higher pressure via HPHT can advance the electrical properties of n-type Yb0.5Ba7.5Ga16Ge30 samples. Figure 7a explicitly reveals the total thermal conductivities (κ) as a function of the temperature of the samples derived from different pressures. The lattice thermal conductivity (κl), as shown in Figure 7b, is calculated by subtracting the electronic component (κ e ) from κ. According to the Wiedemann−Franz law (κe = LσT), κe is calculated to have a Lorenz31 number of L = 2.44 × 10−8 W Ω K−2. In comparison with arc melting, HPHT sintering makes a great difference in both the κ and κl values of our samples, suppressing them. The κ and κl values decrease simultaneously with an increase in pressure. This implies that the Yb0.5Ba7.5Ga16Ge30 sample sintered at 5 GPa demonstrates the lowest κ value of 0.86 W m−1 K−1 and the lowest κl value of 0.02W m−1 K−1 at 773 K. Notably, a scenario has been evidenced that a higher reactive sintering pressure of HPHT is beneficial for lowering κ and κl. The reason may be ascribed to the HPHT sintering, whicn has
Figure 6. Temperature dependence of (a) electrical resistivity, (b) Seebeck coefficient, and (c) power factor for Yb0.5Ba7.5Ga16Ge30 clathrate compounds derived from different pressures.
an significant effect on the microstructures of n-type Yb0.5Ba7.5Ga16Ge30 samples (Figures 3 and 5). These microstructures, such as nanograins/precipitates and lattice disorders, can effectively scatter the phonons of different frequencies and reduce κl. Figure 7c shows the temperature dependence of zT for ntype Yb0.5Ba7.5Ga16Ge30 derived from different pressures. With an increase in the temperature, the zT values of all the samples D
DOI: 10.1021/acs.inorgchem.8b00061 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
1.13 is obtained at 773 K in the sample derived from 5 GPa. In comparison with the arc-melting sample, it increases by twofold at the same temperature. Furthermore, improvements in TE properties indicate that the HPHT sintering has a significant effect on the microstructural changes of n-type Yb0.5Ba7.5Ga16Ge30 samples.
4. CONCLUSION The HPHT quenching method has a synergistic role in optimizing the TE properties of n-type polycrystalline Yb0.5Ba7.5Ga16Ge30 nanostructured bulk material with nanocrystals and multiple microstructures. By this approach, both the electrical properties and thermal conductivities are simultaneously optimized. The multiscale interfaces and lattice disorders, including an amorphous region, fringes, and grain boundaries, could distinctly decrease the thermal conductivity by full-spectrum phonon scattering. As a consequence, the combination of both enhanced PF and reduced κ leads to a relatively high zT value of 1.13 at 773 K in the sample derived from 5 GPa. The improvement in the zT value was ascribed to the plentiful microstructures arising from quenching from a higher sintering pressure. We could expect to further improve the zT value by optimizing the dimension distribution of microstructures corresponding to various phonon modes.
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AUTHOR INFORMATION
Corresponding Authors
*J.Z.: tel, +86-13864603227; fax, 86-0536-8785608; e-mail,
[email protected]. *H.M.: tel, +86-13504451109; fax, 86-0431-85168858; e-mail,
[email protected]. ORCID
Hongan Ma: 0000-0002-5867-840X 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 Natural Science Foundation of Shandong Province (Grant No. ZR2017MF040).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b00061 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b00061 Inorg. Chem. XXXX, XXX, XXX−XXX
