The Effect Of The High Pressure Sintering Process On The Microstructure And Thermoelectric Properties Of CoSb₃

CoSb₃ is one of the promising thermoelectric materals in the moderate temperature range, and it becomes a popular reseach spot in the thermoelectric materals. In this thesis CoSb₃ compound powders are prepared by solid-state reaction method. And nano-structured powders are produced using the high-energe ball-milling method. Bulk CoSb₃ materials are sintered by high pressure method under the pressure of 1.5～6.0GPa. CoSb₃ materials of different scales are prepared using the micron, nano and mic-nano structured powder as the starting materiels.The effect of the high pressure sintering process on the grain-size, microstructure, phase and the thermoelectrical properties is emphatically investigated.The experimental results show that for the micron-structured materials the major phase of skutterudite and traces of metal impurities of Sb and CoSb₂ phase coexist in some of the samples, and that the grain sizes of all the samples increase after sintering. At the range of 3.0 to 6.0GPa, the grain size increases with the increasing pressure. At the range of 1.5 to 3.0GPa, the grain size also increases, but with a descending growth rate. All the materials show p-type transport behaviors, and the sample sintered under 2.0GPa shows higher electrical conductivity than the 5.7GPa sample which may be due to the impurities. Samples with little cracks are produced under 3.0～6.0GPa, it shows that the electric properties of these samples are better than the 2.0 sample and are close to the 5.7GPa sample. The 3.0GPa sample has higher power factor than the others.Nano-structured powders are produced using the high-energe ball-milling method. The grain sizes and phases of the powders are measured. It is found that the grain-sizes decrease as the milling time increases. After 150h of milling the grain-sizes are below 1μm, the average grain size is about 300nm. The powders of 80nm are seperated by centrifugal apparatus and they are used as the the starting materiels to sinter the bulk CoSb₃ by HP method. The sintered samples show that the impurities exist in all the samples which are produced under 2,3,4GPa. The impurities increase as the pressure increase. The sintered samples have many pore spaces inside and are not dence. The grain sizes grow at the pressure of 2 and 4GPa, but increase little at 3GPa.Micron-Nano mixed powders are sintered under 3GPa.It is found that unbroken samples are produced when the sample contains nano-structured powder less than 15%. The impurities and cracks exist in these samples as well. The Micron-Nano sample shows low thermoelectric properties.