Effects of Pressure on the Microstructure and Simultaneous Optimization of the Electrical and Thermal Transport Properties of Yb_0.5Ba_7.5Ga₁₆Ge₃₀

Abstract

The thermoelectric (TE) properties of n-type polycrystalline YbBaGaGe 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 arc-melted 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 K 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 YbBaGaGe 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.

Strategies and challenges of high-pressure methods applied to thermoelectric materials

“…59 These led to significant improvements in its power factor to ∼3.4 mW K −2 m −1 and its figure of merit to ZT ∼ 0.75 at 7 GPa. 59 A study of Yb 0.5 Ba 7.5 Ga 16 Ge 30 synthesized under HP-HT conditions showed that at a synthesis pressure of 5 GPa, this thermoelectric material possessed a maximum value of ZT ∼ 1.13 at 773 K. 161 Little attention has been paid to the high-pressure thermoelectric properties of other thermoelectric materials incorporating Group IV elements, for example, Si 1−x Ge x alloys. 162,163 However, experimental studies on high-quality single crystals of Ge-and Ge-doped Si showed that the pressure dependence of their Seebeck coefficients may be quite spectacular.…”

High pressure: a feasible tool for the synthesis of unprecedented inorganic compounds

“…78 Recently, it has been successfully demonstrated that the Ge-based clathrate compound Yb 0.5 Ba 7.5 Ga 16 Ge 30 synthesized under a pressure of 5 GPa and a temperature of 773 K has a higher ZT value of 1.13. 79 These results show that, compared with other materials, type I clathrates still have complex and diverse situations in terms of improving the thermoelectric performance.…”

Intermetallic borides: structures, synthesis and applications in electrocatalysis

“…8a) or arc-melting chamber. 47 ture of 1600-1800°C. 48 Some borides, including WB 3 , 49 MnB, 50 Mn 3 B 4 , 51 and MoB 4 , 52 were also synthesized successively with a high pressure and high temperature method.…”