| Thermoelectric (TE) materials can realize the dreactly convertion of electricity and thermal by the physical effect of the material, which is either used for power generations grounding on Seebeck coefficient or for cooling by Peltier effect. As one of earliest found and most important thermoelectric material, Bismuth telluride (Bi2Te3) and its alloys are mainly used for thermoelectric cooling. Owning to its many advantages such as simple procedure, easy industrialization, no phase transition and prepared material with high TE's performance, zone melting technique has become the demonating technique to prepare commercial applied bismuth telluride material. Compared with tranditional mechanical refrigeration, the mouduls prepared by zone melted material still display much lower efficience because of lower TE's performance; at the same time, the bad mechnicle properties of zone melted material generate some difficulties in material process and device preparation.In this research, starting with the commercialized zone melted n-type Bi2Te3 based material, we employ melt spinning (MS) subsequently combined with spark plasma sintering (SPS) technique to obtain densified bulk material with nanostructures, and systematically explore the effect of MS and SPS technique parameters on the microstructures and TE's properties, which is in order to obtain optimal MS+SPS technique parameters.We studied the possibility to prepare high performance n-type Bi2Te3 based material with nanostructures and developed a novel synthesis technique that is MS+SPS to prepare high performance n-type Bi2Te3 based bulk material. The microstructures analysis of ribbon samples indicate that the contact surface exhibits unconspicuous crystalline and without distinct interfaces, whereas the free surface (which is opposite side of cintact surface) distributes dendritic structures having widths of several hundreds nanometers. The ribbons were hand ground into powder and then sintered into bulk pellet by SPS technique. Compared with zone melted (ZM) sample, the size of crystalline grains of sinterd samples are dramatically decreased after MS+SPS process, and the crystalline grains don't display preferred orientation, furthermore, lots of refined layer structures with the size of 10-100 nm are obseaved. The thermoelectric properties of ZM sample exist apparantlly preferred orentetion, and the ZTmax along the base plane can reach 0.72 at 420 K; afer hand grinding and SPS, the preferred orentetion of thermoelectric properties decreases but the ZT value has somewhat reduction; the thermoelectric properties of MS+SPS sample are anisotopic and maximum ZT value can reach 0.90 at 360 K, whose room temperature ZT increase 50% compared with ZM sample. The measurements of pressive strength for three samples prepared by different techniques indicate that:due to its great crystalline and slippage of base planes, ZM sample shows lowest strength and only 40 MPa; after SPS process, the strength can reach 110 MPa, but the strength of MS+SPS sample increases dramantically and reaches 200 MPa, which is about 400% improvement compared with ZM sample.On condition that the injection pressure and nozzle size are fixed, the effects of cooling rate which is altered by variation of rotating speed of copper roller on the microstructures and TE's properties of MS+SPS samples were investigated. With the increasing of cooling rate, the crystalline sizes of both surfaces for ribbons decrease, and some more refined microstructures are also obseaved. The HRTEM analysis of 40 m/s ribbon indicate there are lots of nanodots with the size of 10-30 nm in the contact surface and also amounts of amphous structures distribute around the nanodots. The FESEM photos of MS+SPS bulk samples show the grain size has the tendency of reduction with the cooling rate increasing, and lots of layer structure can also be observed which is consistent with previous results. From the HRTEM photos, lots of nanocrystals with the size of 5-20 nm can be observed which inherits very well the morphology of ribbons. The effects of cooling rate on thermoelectric transport properties were investigated:with the cooling rate increasing, electrical and thermal conductivity decrease gradually but Seebeck coefficient increase, and the ZTmax of 30 m/s sample can reaches 0.93 at 360 K, whose room temperature ZT is about 47% improvement compared with ZM sample.Based on the fixed MS technique conditions, the effects of sintering temperature and pressures of SPS on the microstructures and thermoelectric properties were systematically studied. The SPS temperature has great impacts on the microstructures and thermoelectric properties:there are heaps of ribbons randomly stacked throughout the SPS-300 and SPS-350 samples. The micro-morphology of the ribbons is largely preserved, as is evident in the SPS-400 sample. Significant changes in the micro-morphology occurred when the SPS temperature is increased to 450℃. No trace of a ribbon was found; conversely, lots of closely packed microsized crystals are observed. The crystalline size grows bigger with the sinter temperature increasing and the crystalline size of SPS-500 grows remarkably. The electrical conductivity and Seebeck coefficient increase but no distinct change is observed for thermal conductivity with the sinter temperature increasing and the resulted figure of merit ZT increase synchronously. Corrspondingly, the ZTmax of SPS-450 sample can reaches 0.96 at 360 K, whose room temperature ZT is about 53% improvement compared with ZM sample. Sample SPS-500 exhibits anomalousness may becacaue of element volatilization. Moreover, the effects of sinter pressure were also investigated when the sinter temperature was fixed in 450℃, and the research indicated that the sinter pressure has negligiable impacts on the microstructures and thermoelectric properties of MS+SPS samples.In conclusion, starting with commercial ZM ingots, the opitimal MS+SPS technique conditions are as follows:MS rotating speed is 3000 rpm and injected pressure is 0.02 MPa; SPS temperature is 450℃and pressure is 20 MPa.
|
PDF Full Text Request