Microstructures And Properties Of β-Zn₄Sb₃ Prepared By Rapid Solidification Method

P-typeβ-Zn4Sb3 compound has become one of the most promising thermoelectric (TE) materials because it has exceptional thermoelectric properties in the intermediate temperature range. The maximum ZT value reaches 1.3 at 670K because of its extremely low thermal conductivity originated from its complex crystal structure. While the fragility and the microcracks result from the phase transition greatly decrease the mechanical property and process ability, which limits its commercial application. Therefore, to fabricateβ-Zn4Sb3 bulk material with not only high thermoelectric performance but also high mechanical durability is of vital significance.In this research, we focus on p-typeβ-Zn4Sb3 compound. The thermoelectric properties and mechanical properties are expected to be improved by obtaining low-dimensional structure combining doping or producing second phase material methods. We explore the feasibility of preparing the nanostructuredβ-Zn4Sb3 bulk material by combining melt-spinning (MS) with spark plasma sintering (SPS) technique, study the phase transform and the microstructure formation during the MS processes, and the influences of MS process on TE properties and mechanical properties are investigated. Base on the above research works, we study the influences of second phase and doping element on TE properties and mechanical properties for MS-SPS materials. The main obtained results are as follows:We developed a novel synthesis technique that is MS-SPS method to quick prepare nano-structured p-typeβ-Zn4Sb3 bulk material. The ingots prepared by melting method are used as starting materials. After MS treatment, the single phase ingot transforms into multi-phase ribbons containing not only Zn4Sb3 but also Zn3Sb2 and ZnSb. The process parameter of MS process (linear speed of the spinning cooper wheel) has great influences on the microstructure of MS ribbons. With higher linear speed of the spinning cooper wheel, we can get ribbons with smaller grain size. After SPS treatment, the multi-phase ribbons can be transformed to single-phaseβ-Zn4Sb3 in a very short time, and the nanostructure induced by melt spinning technique can be preserved after SPS processing. Compared with the sample prepared by the traditional melting method (M-ingot), the Seebeck coefficient of the MS-SPS samples increases significantly and the thermal conductivity decreases remarkably, which leads to a great improvement in the thermoelectric figure of merit (ZT). Moreover, the mechanical strength of the MS-SPS samples has great improvement compared with M-ingot sample.By adjusting the amount of Zn, we studied the influences of Zn and ZnSb second phase on TE properties and mechanical properties of MS-SPSβ-Zn4+xSb3 compounds. With a little Zn excess (x<0.08), we can get (3-Zn4Sb3 bulk material containing ZnSb second phase because of Zn volatilization during preparation. And ZnSb phase has bad influence on the TE performance ofβ-Zn4Sb3 material. While moderately superfluous Zn improves the electrical transport properties significantly. Excess Zn may lead to nano-scalled second Zn phase dispersed on the boundary which optimizes the electrical and thermal transport properties, and leads to an improvement on the ZT value. The ZT value of Zn4.32Sb3 sample reaches 1.13 at 700K. Moreover, the Zn second phase has good influence on the mechanical properties. With the increase of Zn content, the mechanical properties of the MS-SPS samples increase greatly.A range of Zn4-xCdxSb3 compounds are synthesized using a traditional melting method and MS-SPS method. The results show that we can get single phase Zn4Sb3 material when x<0.15, and we can find Cd peaks in XRD patterns of Zn3.85Cd0.05Sb3. It means that the solid solubility limit of Cd in the Zn-Sb system is x<0.15. With increasing Cd content x, the room-temperature carrier concentration of Zn4-xCdxSb3 compounds decreases, while the carrier mobility is nearly unchanged. For Zn4-xCdxSb3 compounds (x< 0.15), the electrical conductivity and thermal conductivity decrease with increasing x, while the Seebeck coefficient raises largely. So this leads to a great improvement in the ZT value, the maximum ZT value of 1.05 is obtained at 700K for Zn3.90Cd0.10Sb3 sample prepared by melting method. Because of high thermal conductivity, the ZT values of the Zn4-xCdxSb3 samples prepared by melting method are rather low. So we adopt MS-SPS method to prepare the Zn3.95Cd0.05Sb3 sample with lower thermal conductivity. The results show that the MS-SPS Zn3.95Cd0.05Sb3 sample has fine nanostructure and so has very low thermal conductivity. Compared with the sample prepared by melting method, the ZT value of MS-SPS Zn3.95Cd0.05Sb3 sample is largely improved, reaching 1.20 at 700K.