Design And Electro-thermal Transport Characterization Of Novel High Conductivity Tellurium-based Glasses

Tellurium-based glasses,due to their large atomic weight and small chemical bonding constants compared to traditional inorganic glasses,bulk metallic glasses,and semiconductor glasses such as sulfur-based and selenium-based glasses,which can significantly reduce the phonon energy and transmission rate,exhibiting low thermal conductivities and high Seebeck coefficients,making tellurium-based glasses a novel class of thermoelectric materials with relatively high research value for mitigating the energy crisis and improving the recovery and utilization of waste heat.However,the currently reported Ge Te-based and As Te-based glass systems have limited the practical application of such materials on account of their low conductivity,poor glass transition temperature and high toxicity.In contrast,the chalcogenide glassy thermoelectric materials with higher application prospects would belong to the narrow bandgap tellurium-based semiconductor materials（e.g.Bi₂Te₃-based,Sb₂Te₃-based,SnTe-based or Pb Te-based）.However,pure tellurium possessed very poor glass-forming ability and was unable to form stable glassy states and the higher the conductivity of the telluride,the greater of the difficulty in forming glass.Therefore,how to realize the synergistic regulation of the glass formation ability and high electrical conductivity of tellurium-based materials,grasp the glass formation mechanism of such narrow band-gap tellurium-based materials,and design novel tellurium-based glasses with high electrical conductivity and excellent thermoelectric transport properties have emerged as the main scientific problems and challenges in this field.In this paper,the design and thermoelectric properties of novel high conductivity tellurium-based glasses were systematically investigated to address the above problems.First,the glass transition characteristics of narrow bandgap tellurium-based semiconductor materials were elucidated based on the thermodynamic and kinetic characterization parameters of the materials:the SnTe narrow band-gap semiconductor with low entropy of melting and green environment was proposed as the object of study from the thermodynamic aspect to avoid the blindness of group element selection.The pseudo-binary Ga₂Te₃-SnTe system that could form deep eutectic with SnTe was constructed kinetically to enhance the glass formation ability of the system.Then,self-doping introduces Sn monomers to improve the conductivity of the glass samples while maintaining the ability to form Ga₂Te₃-SnTe parent glass,which is further improved by selecting more conductive and highly coordinated metal Ag monomers for doping.The Sn-Ga₂Te₃-SnTe and Ag-Ga₂Te₃-SnTe high conductivity tellurium-based glass systems were realized by melt-quench-cooling in combination with vacuum melt spin-dumping technique.The glass formation region for the Sn_x[（Ga₂Te₃）₃₄（SnTe）₆₆]_100-xsystem was x=0～10 mol%and x=0～15 mol%for the Ag_x[（Ga₂Te₃）₃₄（SnTe）₆₆]_100-xsystem.With the increasing amount of Sn and Ag doping,the glass transition temperatures of the systems were gradually reduced and the widths of the super-cooled liquid-phase regions were gradually decreased,indicating that the stability of the glass systems was weakened.Non-isothermal crystallization kinetics of Sn-Ga₂Te₃-SnTe and Ag-Ga₂Te₃-SnTe glass systems have been carried out by employing various kinetic equation models.The critical parameters such as the characteristic temperature,activation energy,and Avrami index of the samples were determined to reveal the intrinsic correlation between the crystallization mechanism and the precipitated phases.The crystallization processes of both groups of glassy systems were investigated to contain two phases,the first step of crystallization corresponding to the precipitation of the SnTe phase and the second stage of precipitation of the Ga₆SnTe₁₀crystallization phase,whereby the final crystallization products were mechanical mixtures of the SnTe phase and the Ga₆SnTe₁₀phase,providing the theoretical basis for the preparation and practical application of glassy bulk materials.The Spark plasma sintering technology technique was used to fabricate high-density bulk samples and to illustrate the mapping relationship between the sintering process parameters and the density and micro-structure of the samples,in order to determine the optimal sintering process parameters.Highly densified（>96%）completely glassy bulk Sn₈[（Ga₂Te₃）₃₄（SnTe）₆₆]₉₂sample with a density of 5.5917 g/cm³was successfully prepared at 460 K,using a 5-min dwell time and 450 MPa pressure.Highly densified Ag₈[（Ga₂Te₃）₃₄（SnTe）₆₆]₉₂and Ag₁₀[（Ga₂Te₃）₃₄（SnTe）₆₆]₉₀bulk glasses with densities of5.5281 g/cm³and 5.5595 g/cm³were prepared under the sintering temperature of 450 K,sintering pressure of 450 MPa and dwell time of 5 min,respectively.To quantify the effects of different temperatures and component fractions on electrical transport parameters as well as thermal transport behaviors of the samples were systematically analyzed,revealing the structure-dependent relationship between electrical and thermal transport properties.Thus,the prospects for the application of high conductivity tellurium-based bulk glass as a thermoelectric material and the optimization and improvement strategies were discussed.Glass transition of tellurium-based semiconductors was able to dramatically reduce the thermal conductivity of the materials,the Sn₈[（Ga₂Te₃）₃₄（SnTe）₆₆]₉₂and Ag₁₀[（Ga₂Te₃）₃₄（SnTe）₆₆]₉₀glasses exhibited extremely low thermal conductivities as well as relatively high glass-transition temperatures.As compared to the typical Ge-Te-based,As-Te-based,and As-Se-Te-based chalcogenide glass systems,the Sn-Ga₂Te₃-SnTe and Ag-Ga₂Te₃-SnTe glass systems presented higher room-temperature electrical conductivity as well as larger glass-transition temperatures.In particular,Ag₁₀[（Ga₂Te₃）₃₄（SnTe）₆₆]₉₀glass displayed a large room-temperature electrical conductivity of 46 S?m^-1,which was enhanced by a factor of～130 compared to the undoped（Ga₂Te₃）₃₄（SnTe）₆₆glass parent,and increased by almost an order of magnitude compared to the Sn-doped glassy samples.The research in this paper will contribute to the aim of solving the technical challenge of the difficult glass transition of narrow band-gap tellurium-based semiconductors,which will open up new ideas for the design of new glassy materials;establish new models for the investigation of tellurium-based glass transition characteristics and formation mechanisms;and provide new options for the development of high conductivity tellurium-based semiconductor glasses,the development of micro-crystalline glass composites,and the exploration of new thermoelectric materials.The preparation of new high conductivity Sn-Ga₂Te₃-SnTe and Ag-Ga₂Te₃-SnTe tellurium-based semiconductor glass systems will also offer excellent scientific significance and application value for the development of the fields of far-infrared detection,bio-sensing,and 5G communication.