Hairui Sun

Effect of HPHT processing on the structure, and thermoelectric properties of Co4Sb12 co-doped with Te and Sn

Te–Sn co-doped Co4Sb12 bulk polycrystalline materials Co4Sb11.7−xTexSn0.3 have been prepared using a high pressure and high temperature method and then characterized using X-ray diffraction. The aim was to use the disorder of the lattice orientation generated by both high pressure and doping with Te and Sn to reduce the thermal conductivity. The thermoelectric properties were measured at room temperature. As expected, as the synthesis pressure increased, the Seebeck coefficient and electrical resistivity increased, but the thermal conductivity decreased greatly. A minimum thermal conductivity of 2.41 W m−1 K−1 was obtained at room temperature for Co4Sb11Te0.7Sn0.3 synthesized at 3 GPa.

Thermoelectric transport properties and crystal growth of BiSbTe3 bulk materials produced by a unique high-pressure synthesis

We report a significant enhancement in the figure of merit ZT of bulk BiSbTe3 fabricated by high pressure sintering technique under variable pressures. High pressure intervention during synthesis processes can effectively adjust the thermoelectric transport properties and crystal structures of the reaction products. The novel vibration modes produced by high pressure in Raman spectra can induce a positive role in the reduction of thermal conductivity. Meanwhile, the electrical resistivity can also obtain an efficient decrease due to the effect of high pressure on the texture and the preferred orientation. Because of these positive effects, the maximum ZT has reached 1.4 at 432 K from synthesized BiSbTe3 at 2.5 GPa in a short period. Therefore, we present that the appropriate high-pressure has a positive effect on the thermoelectric transport properties to produce the significant enhancement in thermoelectric properties of BiSbTe3.

The enhanced photoelectrochemical performance of CdS quantum dots sensitized TiO2 nanotube/nanowire/nanoparticle arrays hybrid nanostructures

This study aims at improving the photoelectrochemical performance mainly from two aspects: the increased number of heterojunctions and the advantages of nanotube/nanowire/nanoparticle arrays hybrid nanostructures. TiO2 nanoparticles porous layer was sensitized by CdS quantum dots (QDs), and coated on the top of the material, hydrogen-treated TiO2 nanotubes with nanowires, which was also sensitized by CdS QDs (This nanostructure is defined as CdS/H:TTWP). CdS/H:TTWP was successfully fabricated by the methods of electrochemical anodization, successive ionic layer adsorption and reaction (SILAR) and sol–gel. The data from the photoelectrochemical (PEC) performance test shows that the photocurrent density of CdS/H:TTWP is 7.6 mA cm−2 at −0.43 V vs. Ag–AgCl under AM 1.5 G illuminations.

Suppressing adverse intrinsic conduction of Bi2Te3 thermoelectric bulks by Sb and Cu co-substitutions via HPHT synthesis

Chemical substitution combined with high-pressure tuning (high pressure and high temperature, HPHT) was applied to synthesize Bi2Te3 polycrystalline bulks. The synthesis time was sharply shortened to about half an hour, which is a distinct advantage over conventional synthesis methods for mass production. Acceptor-like antisite and substitutional defects owing to Sb and Cu substituting Bi sites were responsible for the increase in carrier concentration. Pressure-induced multiple textures and microstructures contributed to phonon scattering, including grain boundaries, lattice distortions, and dislocations. A maximum ZT value of 1.20 was achieved for Cu0.005Bi0.5Sb1.495Te3 at 473 K. Moreover, an available ZT value of 1.17 was obtained in a wide temperature range of 423–523 K. The increases in carrier concentration and band gap effectively suppressed the adverse intrinsic conduction and delayed the onset of the bipolar effect. Cu-substituted samples exhibit new vibrational modes in Raman spectroscopy, which implies that the substitutions induced significant changes in lattice vibrations.

Performance improvement by using ammonia water-synthesized TiO2 nanotubes with nanowire porous film mixed nanostructures

In this work, the morphology of TiO2 nanotubes with small nanowires directly formed on top (designated as TTW) was treated by the method of wet-chemical reaction (WCR) with an alkaline solution. This method was designed to change the surface morphology and chemical activity of the material, thus improving its UV-visible (UV-vis) light absorption and photoelectrochemical (PEC) performance. We explored different alkaline reagents and different processing times to treat the TTW. After treatment by the WCR method with ammonia water (NH3·H2O), it was found that a hybrid nanostructure of a thinner wall TiO2 nanotube array at the bottom and a nanowire porous film on the top (designated as TTWPF), was obtained. The wires are formed by chemical dissolution of the oxide on the top, and there is no grain boundary between the tubes and wires in TTWPF. So, it can make full use of the advantages of nanotubes and nanowires. For one thing, the nanowire porous film on the top can fully utilize the characteristics of both a large specific surface and a high activity in the generation of electron–hole pairs, and the thinner wall nanotube array at the bottom can provide more direct pathways for photogenerated electron transfer. Also, the nanowire porous film can recycle the light scattered by the three-dimensional topography of the highly ordered nanotube array. Meanwhile, the TTWPF exposes more {001} facets of high energy, which have substantial effects on the surface separation and the transportation of photogenerated electron–hole pairs. The data of the PEC performance indicate that the photocurrent density of the bare TTW when treated by the method of WCR with NH3·H2O can be increased by up to 238%.

One-step synthesis of type-I silicon clathrate Ba8Si46 under high pressure and high temperature

In this paper, the silicon clathrate Ba8Si46 was successfully synthesized using low-cost antioxidative azide Ba(N3)2 and Si as precursors by means of high pressure and high temperature (HPHT). The clathrate phase was one-step synthesized by high pressure chemical method within a short time. The reaction temperature and pressure were optimized to achieve good-quality crystalline products with a composition of Ba7.7Si46 and its transition temperature (Tc) is about 8.3 K. The new preparation route presented in this paper provides an alternative to the multistep HPHT synthesis applied so far. One-step synthesis of type-I silicon clathrate Ba8Si46 by high pressure chemical method can shorten the synthesis period of time greatly.

Rapid synthesis and thermoelectric properties of clathrate Ba8Cu6Ge15Si25 by high temperature and high pressure

Ba8Cu6Ge15Si25 were successfully synthesized by a simple high pressure and high temperature (HPHT) method to investigate the microstructures and thermoelectric (TE) properties. After high pressure synthesis, a highly dense bulk material with lots of fine-layered structure, lattice defects and disorders has been obtained. As expected, the thermal conductivity decreased greatly and the ZT value has been improved significantly, which reaches up to 0.43 at around 773 K. Comparing with other methods, HPHT could shorten the synthesis time from several days to half an hour. It reveals that the HPHT method will become an effective approach for optimizing the TE performance of these materials.

HPHT synthesis, structure and electrical properties of type-I clathrates Ba{sub 8}Al{sub x}Si{sub 46−x}

Abstract Clathrate compounds Ba8AlxSi46−x were successfully synthesized using the method of high-pressure and high-temperature (HPHT). In this process, we used BaSi2 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.