Preparation And Optimization Of Glassy Narrow Band Gap Te-based Thermoelectric Materials

Due to the accelerated growth of the world's population and industrial production,the environmental impact of global climate change and energy shortage is becoming increasingly alarming.Seeking alternative clean energy sources has become one of the main themes around the world in the 21 st century.Thermoelectric(TE)materials have attracted significant interest due to the effective conversion between heat and electrical energy.And the thermoelectric generators are solid-state devices with no moving parts,they are reliable,scalable and silent,making them ideal for power generation and waste heat recovery.The preparation of nanoscaled thermoelectric materials has showed a remarkable enhancement of thermoelectric properties by effectively suppressing the thermal conductivity.This implies that amorphous states with structural disordering would provide an alternative strategy for the development of novel thermoelectric materials with remarkable properties.Recently,studies on glassy thermoelectric materials have been focused on telluride glasses,which have high Seebeck coefficient and moderate electrical conductivity and extremely low thermal conductivity due to their highly disordered structure and the quite low phonon energy resulting from large atomic weight of Te.Among these telluride glasses,the main components for the glass formation are As2Te3 and GeTe4,however,such materials generally have poor electrical conductivity and high toxicity,which largely limiting the developments and applications as thermoelectric materials.Therefore,an effective way to promote the thermoelectric efficiency is expected by preparing the glassy narrow band-gap tellurides.Unfortunately,such tellurides are not good glass formers.Although considerable attempts have been made to vitrify the narrow-band-gap tellurides,only amorphous thin films were prepared,and complete glassy alloys generated by the liquid-solid transitions have been rarely reported due to the high critical cooling rate for the glass formation in the narrow band-gap tellurium-based semiconductors.How to realize the vitrification of the narrow band-gap tellurium-basedsemiconductors is the major scientific problem,which restricting the development of this study.This paper will focus on the experimental studies the glass transition behaviors and the thermoelectric properties of narrow-band-gap Te-based semiconductors in order to exploit new routes in preparing glassy Te-based thermoelectric materials.We attempted to vitrify the narrow band-gap tellurides by jointly considering the thermodynamic and kinetic factors.Earlier studies showed that the thermodynamic parameter,entropy of fusion,is directly related to the glass formation of materialsand thus narrow band-gap tellurides with low entropy of fusion are focused here as starting materials to prepare the thermoelectric glasses.Kinetically,formulating binary eutectic or eutectic-like systems using the telluride thermoelectric materials can help the suppression of atomic diffusion and enhance the viscosity of the compositions near eutectics with hardly damaging the thermoelectric properties,which have been proven to be advantageous for glass formation.Based on the design concept,novel Ga2Te3-SnTe glasses are successfully synthesized by melt spinning.Glass transition thermodynamic and kinetic parameters are characterized to interpret the glass transition features and the glass forming ability of Te-based semiconductors.The glass-forming region and the optimized glass-forming composition are also explored.The newly prepared Ga2Te3-SnTe glassy system broadens the research scope of Te-based semiconductor glasses and provides more choices for the development of thermoelectric materials.Bulk(Ga2Te3)34(SnTe)66 glass was successfully synthesized by combing melt-spinning with spark plasma sintering technique,enabling the detailed analyses of the structure and thermoelectric properties.The crystallization kinetics of the newly developed(Ga2Te3)x(SnTe)100-x(x=32,34,36mol%)chalcogenide glasses were investigated by differential scanning calorimetry(DSC)under non-isothermal conditions.The kinetic parameters such as activation energies and Avrami exponents were determined by using different methods.The thermal stability of these glasses was evaluated by various criterions and the crystallization phases was analysed by X-ray diffraction.The dependence of the thermoelectric properties for glassy,as-annealed and as-cast(Ga2Te3)34(SnTe)66 samples will be intensively investigated and with different degree of crystallization The dependence of the s thermoelectric properties on the relaxation and crystallization will be intensively investigated.The temperature dependence of the thermoelectric properties for glassy,as-annealed and as-cast(Ga2Te3)34(SnTe)66 samples will be also analysed.The studies will provide experimental and theoretical references for the understanding of the glass transition of narrow-band-gap Te-based alloys together with the exploitation of new glassy thermoelectrical materials.