Synthesis Of Ordered Mesoporous In₂O₃ And The Study Of Thermoelectric Properties

Currently the annual global energy consumption is 4.1×10²⁰ J.By the end of this century,the predicted population and economic growth will be more than triple this global energy consumption rate.At present,fossil fuels are still the main energy sources used by people.However,fossil fuels are non-renewable resources,and there is not much energy left now.In addition,the use of fossil energy exacerbates global environmental problems.There is no doubt that the energy crisis and environmental degradation are the two major problems of the 21st century.Thermoelectric materials can directly convert thermal energy into electrical energy,which is a kind of green energy onversion technique,and will not cause pollution to the environment.It is expected to alleviate the global energy crisis.In addition,photoelectric conversion devices are only sensitive to the short-wave length part of solar energy.While thermoelectric conversion technology can not only convert the short-wave length part of solar energy into electric energy,but also increase the utilization efficiency of solar energy for the medium-long band.In recent years,thermoelectric conversion materials have attracted extensive attention.Thermoelectricity is based on three important transmission phenomena:the Seebeck effect,the Peltier effect and the Thomson effect.Thermoelectric?TE?conversion can convert temperature difference into electric potential by seebeck effect and vice versa by Peltier effect.The energy conversion efficiency of a TE material is evaluated by the dimensionless figure of merit?ZT?,which is determined by the Seebeck coefficient,electrical conductivity and thermal conductivity.Since the thermoelectric materials are semiconductors,the contribution of electronic thermal conductivity is relatively small.Therefore,the effective way to further increase ZT is to reduce the lattice thermal conductivity by enhancing the phonon scattering.In recent years,some experts have pointed out that the best way to improve the ZT of thermoelectric materials is to regulate their microstructure.Porous thermoelectric materials contain porous structure.The porous structure inside the material can greatly reduce the thermal conductivity.Accordingly,porous structure can improve its thermoelectric properties.In this paper,two kinds of In₂O₃ with different pore sizes were prepared by hardtemplate method.The preparation process was divided into two steps.The first step is prepare two mesoporous SiO₂?KIT-6?hard templates with different pore sizes.KIT-6was synthesized by hydrothermal method with P123?EO20-PO70-EO20?as the structure guide agent and TEOS as the silicon source.Pore size is controlled by hydrothermal temperature and in this thesis we chose two temperature 50?and100?.The samples were tested by high-resolution transmission electron microscopy?HRTEM?,small-angle X-ray diffraction?SAXRD?,nitrogen adsorption/desorption,and positron lifetime measurements.The TEM image and SAXRD pattern reveal that the KIT-6 template consists of a large number of ordered mesoporous with cubic Ia3d symmetry.From the TEM images it can be clearly seen that the pore ordering becomes better with increasing synthesis temperature from 50^?C to 100^?C.According to the nitrogen adsorption/desorption measurements,the single point BET?P/P₀=0.2?specific surface area of 50^?C-KIT-6 and 100^?C-KIT-6 is 683 m²g^-1and 723 m²g^-1,high pore volume reaching 0.4 cm³g^-1 and 0.8 cm³g^-1,respectively.Two mesoporous In₂O₃ were prepared with different pore size by using KIT-6template synthesized from the first step above?designated as 50^?C-In₂O₃ and 100^?C-In2O3?.Small angle X-ray scattering and high resolution transmission electron microscope measurements all confirm ordered pore structure in the synthesized In₂O₃.N₂ adsorption/desorption analysis indicates that the pore size in these two In₂O₃ is about11 nm and 9 nm,respectively.Positron lifetime measurements suggest existence of both micropores and mesopores in the porous In₂O₃ samples,and the mesopore size in100^?C-In₂O₃ is smaller than that in 50^?C-In₂O₃.The porosity of 50^?C-In₂O₃ and 100^?C-In₂O₃ is about 44%and 40%,respectively.In addition,commercial nanoscale In₂O₃powder?30 nm,purity>99.9%?was also selected for a comparative study.The powder was treated by a spark plasma sintering?SPS?process at 600^?C,800^?C and 900^?C,respectively,for 5min under pressure of 80 MPa.These samples are labeled as SPS-600,SPS-800 and SPS-900,respectively.Comparing with the bulk In₂O₃ obtained by SPS sintering of the commercial In₂O₃ nanopowders at high temperature of 900^?C,the thermal conductivity of porous In₂O₃ decreases by more than an order of magnitude.For the 100^?C-In₂O₃,the thermal conductivity is as low as 0.58 Wm^-1K^-1 at room temperature of 25^?C,which is below the amorphous limit.On the other hand,the electrical conductivity of the porous In2O3 is deteriorated by the pore structure,but it is partially compensated by the increase of Seebeck coefficient.The overall ZT factor increases to higher than 0.08 at 300^?C for 50^?C-In₂O₃,which is almost three times that of the bulk In₂O₃.Our results strongly suggest that construction of pore structure is a more effective way to reduce thermal conductivity and enhance the thermoelectric performance in In₂O₃.