HPHT Synthesis And Thermoelectric Performance Of Al Doped Non-stoichiometric Ratio Titanium Oxide

Thermoelectric materials have been realized or have potential application value in many fields such as refrigeration of electronic devices,components of spacecraft power supply equipment,waste heat recovery,etc.due to their direct conversion between thermal energy and electrical energy.Thermoelectric materials have their corresponding performance evaluation methods,which are mainly evaluated by a parameter zT value composed of electrical parameters and thermal parameters,where thermal parameters include thermal conductivity?,absolute temperature T,electrical parameters include:resistivity?,Seebeck coefficient S.Due to the correlation between several parameters?,?,and S that constitute the zT value,it is necessary to comprehensively consider several parameters when optimizing the thermoelectric performance of the material,resulting in the current performance of the material still cannot fulfill the large-scale commercial application standards.It is a huge challenge that researchers need to overcome.At present,Bi₂Te₃,PbTe and other materials used in the medium and low temperature systems have made rapid progress.Materials mainly used in high temperature areas still need to be further explored.TiO₂ is a thermoelectric material in high temperature areas with broad application prospects.The advantages of low cost and easy access,basically non-toxic and less environmental pollution are conducive to the large-scale development and utilization of materials,it is worthy of in-depth exploration and research.Reducing the lattice thermal conductivity is a common method to optimize the thermoelectric properties of materials.Using high temperature and high pressure to synthesize samples can form the microstructure that helps reduce the lattice thermal conductivity of the material.In addition,by adjusting the synthetic pressure of the sample,the forbidden band width of the sample can be adjusted to optimize the electrical properties of the material.In this paper,the effect of different synthesis pressures and different Al and Ti doping ratios on Al-doped non-stoichiometric titanium oxide-based thermoelectric materials was studied using high temperature and high pressure methods.The specific contents are as follows:1.The TiAl_0.02O_1.78 samples were prepared by the high temperature and high pressure method under the synthetic pressure of 3.0 GPa,4.0 GPa and 5.0 GPa respectively.The micro-morphology and thermoelectric properties of the samples after synthesis were tested,and the variation law and mechanism of the micro-morphology and thermoelectric properties of the samples in the synthetic pressure range of 3.0 GPa to 5.0GPa were studied:Scanning electron microscopy characterization results of the synthesized samples show that the TiAl_0.02O_1.78 samples synthesized in the synthetic pressure range of 3.0 GPa to 5.0 GPa gradually increase with the synthesis pressure of the samples:the average grain size of the samples gradually decreases,the reduction in grain size is beneficial to the formation of more grain boundaries in the TiAl_0.02O_1.78sample to enhance the scattering ability of the phonon and reduce the lattice thermal conductivity of the sample.The results of the UV-Vis spectrometer on the synthesized sample show that for TiAl_0.02O_1.78samples synthesized in the synthetic pressure range of 3.0 GPa to 5.0 GPa,the width of the forbidden band gradually decreases with the increase of the synthetic pressure of the sample.The reduction of the forbidden band width of the sample is helpful to reduce the resistivity of the sample.When the synthetic pressure is 5.0 GPa and the test temperature is 973 K,the best zT value is 0.20@973 K.2.Using the high temperature and high pressure method to explore the optimal Al and Ti doping ratio of Al-doped non-stoichiometric titanium oxide-based thermoelectric materials when the synthesis pressure is 5.0GPa.TiO₂ was doped with different proportions of Al powder and Ti powder at 5.0 GPa,and TiAl_xO_1.80-x?x=0.02,0.04,0.06?samples were prepared respectively.The effect and mechanism of Al-doped non-stoichiometric titanium oxide were studied.The microscopic morphology test results of the synthesized TiAl_xO_1.80-x samples by scanning electron microscopy and transmission electron microscope showed that TiAl_xO_1.80-x?x=0.02,0.04,0.06?samples all showed rich grain boundaries,lattice distortion and other typical characteristics of high-pressure synthesis samples,these micro-morphologies help to reduce the lattice thermal conductivity of TiAl_xO_1.80-x samples.The variable temperature electrical test results of the samples show that the power factor of the TiAl_0.04O_1.76 sample in the middle and high temperature region?the test temperature is higher than 573 K?is higher overall.When the test temperature is 973 K,the best zT value of TiAl_0.04O_1.76 sample is 0.22@973 K.