[en] Polycrystalline SnSe bulks were synthesized by a simple and rapid High Pressure and High Temperature (HPHT) method in pressure range of 1–5 GPa, and the thermoelectric performances were assessed after high pressure was released. HPHT can not only sharply shorten synthetic time to 25 min, but also tune thermoelectric properties in a broad range. More importantly, the beneficial thermoelectric properties under high pressure are effectively retained to ambient conditions via “quenching” procedure. The intrinsically high electrical resistivity of SnSe is remarkably reduced by HPHT, which is ascribed to pressure-induced band gap narrowing. A minimum electrical resistivity of 0.1 Ω cm at 5 GPa and maximum power factor of 1 × 10"−"4 Wm"−"1K"−"2 at 3 GPa for SnSe_0_._9_8Te_0_._0_2 are achieved at ambient conditions. Besides, the first principle calculations reveal that high pressure can fundamentally shrink interatomic distances and lattice parameters, which thus lead to a decreased band gap. The pressure coefficient of band gap dE_g/dP = −0.074 eV/GPa is obtained. The variations of electronic structure under high pressure are in accordance with the trend in measured thermoelectric properties. - Highlights: • HPHT a simple and rapid synthetic method (from elements to bulk in 25 min). • HPHT can distinctly reduce the intrinsically high electrical resistivity of SnSe. • The high pressure effects are retained to ambient conditions. • The calculations verify the interatomic distance shrinkage and band gap narrowing.

[en] Highlights: • The synthesized time was sharply diminished to half an hour via the HPHT method. • The microstructures for Ba_0.3Ni_0.15Co_3.85Sb₁₂ samples were analyzed. • With increasing pressure, the change of electrical properties was researched. • The maximum zT 0.91 of the Ba_0.3Ni_0.15Co_3.85Sb₁₂ was obtained at 700 K. In this letter, Ni substitution for Co and Ba filling into the voids were simultaneously applied to the CoSb₃ bulks by the high pressure and high temperature (HPHT) method. As the results, the polycrystalline Ba_0.3Ni_0.15Co_3.85Sb₁₂ skutterudite bulks were successfully synthesized and the systematic thermoelectric properties were investigated. In our experiments, the synthesized time was sharply shortened to 30 min, which is markedly an advantage for mass production. The multiple textures and microstructures were revealed, including vortices-like structures, defects, and lattice dislocations. Remarkably, pressure tuning led to the increase of the Seebeck coefficient and electrical resistivity, yielding an optimized power factor of 11.7 × 10⁻⁴ Wm⁻¹K⁻² in the Ba_0.3Ni_0.15Co_3.85Sb₁₂ sample prepared at 3 GPa at room temperature, which yielded a 10-fold improvement in power factor and showed distinct reduction of thermal conductivity compared with pure CoSb₃ sample. The maximum zT value 0.91 was obtained at 700 K for Ba_0.3Ni_0.15Co_3.85Sb₁₂ sample. The experimental results suggest that the effective elements doping combining with HPHT synthesis could optimize electrical and thermal transport properties in skutterudites.

[en] Clathrate compounds Ba_8Al_xSi_4_6_−_x were successfully synthesized using the method of high-pressure and high-temperature (HPHT). In this process, we used BaSi_2 as one of the starting materials in place of Ba metals, which reduces the complexity of the program caused by the extremely high chemical reactivity. By using this method, the processing time was reduced from few days to an hour. X-ray diffraction and structural refinement indicated this composition crystallized in type-I clathrate phase. Bond length analysis showed the Ba atoms in small dodecahedron had spherical thermal ellipsoids while those in large tetrakaidecahedron displayed anisotropic thermal ellipsoids. The negative Seebeck coefficient indicated transport processes were dominated by electrons as carriers, and increased with the increasing temperature. The electrical properties, including Seebeck coefficient and Power factor, were greatly enhanced by Al substitution. - Graphical abstract: Left: The cavity structure diagram of a China-type large volume cubic high-pressure apparatus, and the Type-I clathrate structure of sample synthesized using HPHT. Middle: X-ray Rietveld refinement profile for Ba_8Si_4_6 and element mapping for Ba_8Al_1_6Si_3_0. Right: Temperature dependence of Seebeck coefficient for Ba_8Al_xSi_4_6_−_x prepared by HPHT. - Highlights: • HPHT is a simple and rapid synthetic approach. • We use BaSi_2 as one of the starting materials replacing Ba metals. • The processing time reduces from few days to an hour. • Structure determination is refined by Rietveld analysis of XRD data. • Variable temperature electrical properties are characterized.

[en] Polycrystalline bulk materials Ba_8Cu_xGe_ySi_4_6_-_x_-_y (x = 0, 6; y = 0, 20) were prepared by high-pressure and high-temperature (HPHT) method and characterized using X-ray diffraction and Rietveld refinement. Results of EDS analysis showed that the solubility limit of copper composition in this clathrate is about 5. Electrical properties measurements between 300 K and 720 K revealed the behavior of a degenerate semiconductor for all samples. As the framework elements increased, the Seebeck coefficient and electrical resistivity increased, but the thermal conductivity decreased greatly, which leads to significant enhancement on thermoelectric properties of the clathrates. The lattice defects of samples generated by high pressure efficiently reduced the lattice thermal conductivity. - Highlights: • HPHT is a simple and rapid synthetic approach (an hour). • We used BaSi_2 as one of starting materials replacing Ba metals. • We discussed the microstructures and their formation mechanism. • The enhancement in thermoelectric performance was obtained via substitution.

[en] In_0_._1Co_4sb_1_1Te_0_._8Ge_0_._2 skutterudite alloys were synthesized by high temperature and high pressure (HTHP) method, and the effect of the In filling and Te, Ge co-doping atoms on thermoelectric properties was investigated under different pressures. The synthetic time was sharply reduced from a few days to 30 min. A fairly good ZT value of 1.12 at 773 K was obtained due to both the remarkably enhanced power factor and the low thermal conductivity