Microwave Preparation And Thermoelectric Properties Of TiNiSn(Bi,Te) Half-Heusler Thermoelectric Materials

As the environmentally friendly functional materials,thermoelectric materials can realize the direct solid-state conversion of thermal and electric energy and play a key role in power generation and waste heat recovery.Half-Heusler alloy has gradually become an emerging thermoelectric material in the middle and high temperature area due to its excellent electrical properties,mechanical properties and thermal stability,what's more,its components are environmentally friendly,non-toxic and abundant.In this paper,TiNiSn-based thermoelectric materials were prepared by microwave synthesis combined with microwave sintering.Furthermore,the effects of different microwave sintering time on the phase composition,microstructure and thermoelectric properties of TiNiSn bulk materials were explored.TiNiSn-based bulk thermoelectric materials with Bi and Te doped in Sn site were quickly prepared by the process of cold compression-microwave synthesis-ball milling-secondary warm pressing-microwave sintering.The phase and the composition of thermoelectric materials was analyzed by XRD and EDS,respectively.The morphology and size of the microstructure are analyzed using SEM,etc.,and thermoelectric transport parameters of the bulk materials including Seebeck coefficient,electrical conductivity and thermal conductivity were comprehensively evaluated.Finally,the effects of microwave sintering time and the amounts of Bi/Te doping on the microstructure and thermoelectric properties of TiNiSn bulk were compared.The single-phase TiNiSn-based Half-Heusler high-temperature thermoelectric materials with high density and uniform structure were successfully prepared by this method,and the preparation time was greatly shortened.The effects of Bi doping?x=0.001,0.003,0.005,0.01,0.02 and 0.03?and Te doping?x=0.01,0.03,0.05,0.1,0.2,0.25 and 0.3?on the composition,phase distribution and thermoelectric properties of TiNiSn alloys were studied.XRD results showed that the main phase of the sample is still TiNiSn alloy.When doping with different elements,lattice distortion occured and point defects increased due to the difference in atomic radius,so the diffraction peak shifted.The fluctuation scattering of stress field and mass field were introduced into the material due to the difference in atomic radius and mass difference,which inevitably affected the thermal conductivity of the material.In addition,the interface effect between the second phase of TiNi₂Sn and the matrix phase of TiNiSn also helps to reduce the lattice thermal conductivity of the material.The results of surface scanning showed that the distribution of each phase is relatively uniform,and there is no obvious phenomenon of component aggregation.However,the SEM images showed that the grain size in the sample is significantly reduced to below 6?m,and the presence of a large number of nano-precipitates and nano-pores in the microstructure increases the phonon scattering mechanism.The warm pressing process plays a key role in improving the density of the final samples.Overall,the TiNiSn_0.997Bi_0.003 and TiNiSn_0.99Te_0.01 samples significantly improved the thermoelectric performance of the matrix material.To further optimize the thermoelectric figure of merit of TiNiSn-based bulk thermoelectric materials prepared by this method,the effects of microwave sintering time on the microstructure and thermoelectric properties of TiNiSn alloy were studied.XRD results showed that the purity of the samples can be controlled by controlling the microwave sintering time.When the sintering time was too long to 10min,a large number of second phase of Heusler alloy could be precipitated.With the decrease of sintering time,the purity of the main phase was significantly improved.However,when the sintering time continued to decrease to 5 min,the impurity peak in the XRD spectrum increased.At the sintering time of10 minutes,the density of the sample is higher.With the decrease of sintering time,the increase of porosity leads to the decrease of density.Therefore,the overall resistivity increased and the thermal conductivity decreased significantly.However,different sintering time will affect the grain size and uniformity of the structure,so it has a slight effect on the thermal conductivity.In general,as the microwave sintering time decreases,the thermoelectric figure of merit has improved significantly.When the sintering time is 6min,the thermoelectric figure of merit is the highest,and then as the sintering time continues to decrease,the zT value begins to deteriorate.In this paper,the high-density Half-Heusler thermoelectric alloy was prepared by the process of microwave synthesis-warm pressing-microwave sintering.The preparation method has the advantages of simple process,short cycle and low energy consumption.The obtained bulk thermoelectric material has high density,fine grains,uniform structure and good thermoelectric performance,which it is easy to mass-produce and has a good prospect of industrial industrialization.