Development of thermoelectric materials using high-pressure synthesis technique

Thermoelectric materials, which convert heat energy directly into electricity, play an important role in providing globally sustainable energy. In recent years, much effort has been exerted in improving thermoelectric efficiency using various modern synthesis methods. The high-pressure synthesis method has various advantages such as the possibility of synthesizing new materials with impossible structures and compositions at ambient pressure. In this review, we will focus on exploring new compounds using a high-pressure apparatus. The high-pressure synthesis and thermoelectric measurement technique will be discussed. In addition, in-situ X-ray diffraction experiments under high temperature and high pressure to determine the conditions for high-pressure synthesis will be discussed. Finally, the thermoelectric properties of several compounds (skutterudites and magnesium silicide) prepared under high pressure will be reviewed

High-pressure synthesis of new filled skutterudite compounds SrT4As12 (T = Fe, Ru, Os)

We have succeeded in synthesizing samples of new filled skutterudite compounds SrT 4As12 (T = Fe, Ru, and Os) using a high-pressure synthesis technique. These compounds have lattice constants of 8.351, 8.521, and 8.561 Å, respectively. The physical properties of the compounds are reported for the first time. The temperature dependences of the electrical resistivity for SrT 4As12 (T = Fe, Ru, and Os) exhibit metallic behaviors. The electrical resistivity and magnetic measurements indicates that SrOs4As12 is a new superconductor with a transition temperature of 4.8 K. A broad maximum of magnetic susceptibility at around 50 K and a large electronic specific heat coefficient of 58 mJ mol−1 K−2 suggest that SrFe4As12 is a nearly ferromagnetic metal with spin fluctuations of Fe 3d electrons

High-pressure Synthesis and Bulk Modulus of Non-centrosymmetric Superconductor Mo3Al2C

Mo3Al2C is a superconductor without inversion symmetry (the transition temperature T C ~ 9K). We have succeeded in preparing high-quality samples of Mo3Al2C using the high-pressure synthesis technique. The samples were characterized by powder x-ray diffraction (XRD) analysis. Furthermore, powder XRD patterns for the samples with synchrotron radiation have been studied under high pressures up to around 10 GPa and the volume versus pressure curve for the compound has been investigated. A bulk modulus was estimated to be 221 GPa

The structural, transport, and magnetic properties of Yb-filled skutterudites YbyFexCo4−xSb12 synthesized under high pressure

The effects of Fe-substitution on partially Yb filled skutterudites YbyFexCo4-xSb12 are presented from the viewpoint of crystal structure and thermoelectric, magnetic, and transport properties. A series of polycrystalline n-type YbyFexCo4-xSb12 (0.21 ≤ y ≤ 0.47, 0 ≤ x ≤ 0.76) samples were prepared using a high-pressure and high-temperature method. X-ray diffraction data suggest that all the compounds are high-purity skutterudites. For the YbyFexCo4-xSb12 with Yb content higher than 0.29 and Fe content lower than 1, the lattice constant shows a saturated behavior despite the change in the Yb/Fe content. Rietveld refinement based on the synchrotron radiation X-ray data implies that the rectangular Sb4 ring is transformed into square with increasing Yb content and/or Fe substitution content. The Yb valence gradually decreases as the Fe content increases from magnetic susceptibility analysis. According to the specific heat analysis, higher Yb filling benefits the lower Debye temperature while the Fe substitution leads to an increased Debye temperature. The Einstein temperature decreased with increasing Yb filling fraction, but Fe substitution for the Co site does not change the Einstein temperature further. Fe-substitution causes the reduction of total thermal conductivity κ, which mainly originates from the decrease of electron thermal conductivity contribution. The resistivity, Seebeck coefficient, thermal conductivity, and figure of merit (ZT) were effectively tuned due to the optimization of the carrier concentration. At the same carrier concentration, the hall mobility was decreased by Fe substitution. The proper Fe substitution content (0.2 in Yb0.25Fe0.2Co3.8Sb12) can result in a relatively high effective mass

High-pressure and high-temperature synthesis of heavy lanthanide sesquisulfides Ln2S3 ( Ln=Yb and Lu)

Detailed pressure-temperature phase diagrams of heavy lanthanide sesquisulfides Ln2S3 (Ln = Yb and Lu) have been investigated by in-situ x-ray diffraction experiments under high pressure and high temperature using synchrotron radiation and multi-anvil press. Based on the results of the in-situ observation, the single γ-phase (Th3P4-type structure, I3d) samples of Ln2S3 (Ln = Yb and Lu) have been synthesized under high pressure. The physical properties of the compounds were studied by electrical resistivity, specific heat, and magnetic susceptibility measurements between 2 K and 300 K

Suppression of ferromagnetic spin fluctuations in the filled skutterudite superconductor SrOs4As12 revealed by As-75 NMR-NQR measurements

Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe4As12 [Q.-P. Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out As-75 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in the newly discovered isostructural superconductor SrOs4As12 with a superconducting transition temperature of T-c similar to 4.8 K. Knight shift K determined by the NQR spectrum under a small magnetic field (<= 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/T1T, is nearly independent of temperature above similar to 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe4As12, we found that the values of vertical bar K vertical bar and 1/T1T in SrOs4As12 are smaller than those in SrFe4As12, indicating no obvious ferromagnetic spin correlations in SrOs4As12. From the temperature dependence of 1/T-1 in the superconducting state, an s-wave superconductivity is realized

Anomalous elastic softening of SmRu_{4}P_{12} under high pressure

The filled skutterudite compound SmRu_4P_{12} undergoes a complex evolution from a paramagnetic metal (phase I) to a probable multipolar ordering insulator (phase II) at T_{MI} = 16.5 K, then to a magnetically ordered phase (phase III) at T_{N} = 14 K. Elastic properties under hydrostatic pressures were investigated to study the nature of the ordering phases. We found that distinct elastic softening above T_{MI} is induced by pressure, giving evidence of quadrupole degeneracy of the ground state in the crystalline electric field. It also suggests that quadrupole moment may be one of the order parameters below T_{MI} under pressure. Strangely, the largest degree of softening is found in the transverse elastic constant C_{T} at around 0.5-0.6 GPa, presumably having relevancy to the competing and very different Gruneisen parameters \Omega of T_{MI} and T_{N}. Interplay between the two phase transitions is also verified by the rapid increase of T_{MI} under pressure with a considerably large \Omega of 9. Our results can be understood on the basis of the proposed octupole scenario for SmRu_4P_{12}.Comment: 7 pages, 7 figure

Enhanced thermoelectric performance of optimized Yb-filled and Fe-substituted skutterudite compounds Yb0.6FexCo4−xSb12

Optimized Yb-filled Fe-substituted skutterudites Yb0.6Fe x Co4? x Sb12 (x = 0, 0.5, and 1.0) were synthesized using a high-pressure technique. The samples were characterized by powder X-ray diffraction (XRD) analysis and electron probe microanalysis (EPMA). The thermoelectric transport properties of the samples such as Seebeck coefficient, electrical conductivity, carrier concentration, and thermal conductivity were studied in the temperature range of 2?300 K. With 1/8 Fe substitution for the Co site (x = 0.5), the Seebeck coefficient and thermal conductivity were obviously optimized. Carrier concentration analysis indicates that proper Fe substitution can effectively compensate for the excess electrons and optimize the electric transport properties. The reduction in the total thermal conductivity κ could be mainly caused by the reduction in electron contribution to κ. Compared with an only-Yb-filled compound, a 26% improved figure of merit ZT was achieved at 300 K at the Fe substitution ratio x = 0.5 (ZT = 0.11). This result confirms that Fe substitution on the Co site is an effective approach to tuning and optimizing the thermoelectric properties of CoSb3-based skutterudites

Filled skutterudite superconductor CaOs4P12 prepared by high-pressure synthesis

In this paper, we report the transport, thermodynamic, and superconducting properties of a new filled skutterudite CaOs4P12 synthesized under high pressure and high temperature. The electrical resistivity of 3.4–4.8 mΩcm, carrier concentration of 3.8–6.1 × 1020cm−3 , and positive Hall coefficient suggest that CaOs4P12 is a semimetal with hole carriers. An anomaly due to low-energy optical modes corresponding to an Einstein temperature of 150 K was observed in the specific heat. Resistivity, dc magnetic susceptibility, and specific heat measurements indicate bulk superconductivity below 2.5 K. The specific heat anomaly at Tc, ∆C/γTc ≈ 1.4, is in agreement with the BardeenCooper-Schrieffer (BCS) value of 1.43. The electron-phonon coupling constant λep is estimated to be 0.47. CaOs4P12 is classified as a BCS-type, weakly coupled type-II superconductor with an upper critical field of Hc2 ≈ 22 kOe and Ginzburg-Landau coherence length of ξ ≈ 12 nm

Thermoelectric properties of partially filled skutterudites RxCo4Sb12 (R = Ce and Nd) synthesized under high pressures

We report the thermoelectric properties of the partially Ce or Nd filled skutterudite compounds CexCo4Sb12 and NdxCo4Sb12 prepared under high pressures and temperatures. The samples were characterized by X-ray diffraction. The actual filling ratio x of Ce or Nd was estimated by scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDX). SEM-EDX results indicate that the maximum x values of Ce and Nd can be increased to 0.37 and 0.33, respectively. These values have been considered the highest for any CexCo4Sb12 and NdxCo4Sb12 reported thus far. The electrical resistivity, thermal conductivity, and Seebeck coefficient measurements of the compounds were performed from 5 to 760 K. Furthermore, the Hall coefficient and specific heat of the compounds were also measured below 300 K. The Seebeck and Hall coefficients of both Ce- and Nd-filled samples exhibited the n-type conductor behavior. The maximum dimensionless figure-of-merit (ZT) values of CexCo4Sb12 and NdxCo4Sb12 were determined to be 0.26 and 0.48 at 700K, respectively. (C) 2018 The Japan Society of Applied Physic