HPHT Synthesis And Properties Of CoSb₃-based Thermoelectric Materials

Thermoelectric material is one of kind of function materials that can directly convert electricity and heat. Thermoelectric devices based on thermoelectric materials could convert heat to electricity, transferred through or pumped by the charge carriers in the thermoelectric materials, and rejected at the heat sink. The advantages of solid-state thermoelectric devices are lightweight, small, portable, inexpensive, quiet performance and the ability for localized'spot'cooling. Thus, there is now a renewed interest in research on improved for thermoelectric applications.The use of TE device is limited by their low efficiencies. A prerequisite for an efficient energy conversion in thermoelectric devices is indexed by the dimensionless figure of merit, ZT=TS2σ/κ, where T is the absolute temperature, S is the Seebeck coefficient,σis the electrical conductivity, andκis the thermal conductivity. Because of the low energy converting efficiency (<10%), several categories of thermoelectric materials used currently have some limitations in applications. Therefore, the development of novel thermoelectric materials with high thermoelectric properties has been the principal concerns of the related researches.The recent interest in semiconducting skutterudite materials is due to their potential as effective thermoelectric materials. The structure of binary skutterudite compound belongs to the body-centred cubic space group Im3 and has two relatively large voids at positions in the crystal lattice. Filling these voids with lanthanide, actinide, and alkaline-earth ions to form filled skutterudite had been proved an effective method to reduce the lattice thermal conductivity. The void-filling atoms"rattle"in their voids and substantially affect the phonon propagation through the lattice. In this paper, we present data on the thermoelectric properties of Sm filled skutterudites to discuss the dimensionless figure of merit ZT.Up to date, the hot pressing (HP), spark plasma sintering (SPS), mechanical alloying (MA) and high temperature and high pressure (HTHP) have been used to synthesized the thermoelectric materials.Recently, high-pressure technology as a potentially tool to increase the rate of discovery of useful materials has spread all over the world. Comparing to other preparation methods for thermoelectric materials, the method of high-pressure and high-temperature (HPHT) has many advantages, including the ability to tune rapidly and cleanly, restraining the disorder phase separation and other complicating factor during the preparation of materials.In this work, we tries to study the performance of CoSb₃ based skutterudite compounds. Firstly, we synthesized the pure CoSb₃ skutterudite compounds by HPHT technique and studied the electrical properties of CoSb₃. The results indicated that the electrical resistivity increased with an increase of synthetic pressure. The calculation of the band structure studies show that the band gap increases with an increase of pressure, which may be source of decreasing of carrier concentration.Skutterudites compound CoSb_2.750Ge_0.250-xTex (x=0.125, 0.150,0.175, 0.200) were studied by HPHT. The electrical resistivity of CoSb_2.750Ge_0.250-xTex decreases with increasing Te concentration. Because of the compensating effect of Ge. Seebeck coefficient measurements revealed that all the samples were n-type.The measurement of transport properties show that the Seebeck coefficient and resistivity increase with an increase of synthetic pressure. The largest power factor at room temperature reaches to 9.0μW cm-1 k2 for the CoSb_2.750 Ge0.100Te0.150 at 2.0 GPa.We synthesized LaxCo4Sb12 samples with different La filling content by HPHT method, at 1.5-4.5 GPa, and 900 K. all the samples are single phase with Im3 structure from the test of XRD. With increasing of La filling fraction, the lattice spacing increase linearly.We test the electrical resistivity and the Seebeck coefficient of LaxCo4Sb12 samples at room temperature. The results indicate that the LaxCo4Sb12 skutterudite compounds show p-type conduction. The filling La atoms may contribute some electron to the conduction band. The value of power factor 5.16μW·cm-1·K-2 was obtained at x= 0.5.The skutterudite compounds of SmxCo4Sb12 were synthesized by HPHT technique. The XRD patterns indicate that the major phase is CoSb₃ phase, which is crystallized in a cubic CoAs3-type structure with Im3 space group. In addition, compared with the samples prepared by a traditional method, the processing time of HPHT method is reduced from a few days to less than an hour, and the Sm filling fraction limit increases. The magnitude of Seebeck coefficients of these n-type compounds increases with increasing temperature, the values that are dependent on the doping level. The electrical resistivity of SmxCo4Sb12 samples increases and then decreases with an increasing Sm filling fraction. For the samples with filling fraction x=0.5, electrical resistivity increases with increasing temperature monotonously, indicating the character of a degenerated or heavily doped semiconductor. The power factor increases significantly and the maximum of power factor shifts toward higher temperature with the increasing the filling fraction of Sm. A maximum value of 25.1μW cm-1K-2 is obtained in Sm0.5Co4Sb12 sample at 665K, which is much higher than pure CoSb₃ prepared by the traditional method at normal pressure.The thermal conductivity of the Sm-filled skutterudites is lower as compared to that of CoSb₃. ZT increases almost linearly with increasing temperature for SmxCo4Sb12 compounds. The maximum ZT value is 0.81 for Sm0.5Co4Sb12 at 723K.In conclusion, the results obtained in our work show that HPHT is an effective technique for synthesizing the CoSb₃-based skutterudite TE materials. The performance for the TE materials with the structure of CoAs3 could be improved by high pressure according to the results by now.