Preparation And Properties Of Titaniun Based Oxide Thermoelectric Materials

Energy has always been a hot issue of attention.Fossil fuel is non-renewable and brings serious environmental pollution after used.Therefore,developing new clean energy technology is imminent.Thermoelectric conversion is a technology that can directly transform energy and electric energy into one another.It mainly includes two fields:thermoelectric generation and thermoelectric refrigeration.The thermoelectric refrigerator,which is prepared by thermoelectric materials,does not need refrigerant?Freon etc.?and does not cause environmental pollution because of the leakage of refrigerant.The thermoelectric generator,prepared by thermoelectric materials,can realize the recycling of waste heat and convert the low grade heat source into electric energy,thus achieving the purpose of energy saving and energy reuse.Therefore,the thermoelectric conversion technology has a broad prospect,and the thermoelectric material,as the core problem of the thermoelectric conversion technology,has become one of the hot topics in the current international research.Traditional thermoelectric materials?Bi₂Te₃,PbTe and SiGe?are difficult to prepare and are prone to oxidation and decomposition,thus reducing the life and performance of thermoelectric devices.Oxide thermoelectric materials have become a new research direction in the field of thermoelectric materials,because of their advantages of simple preparation,non-toxic property,strong antioxidant capacity and long time work at high temperature.In temperature difference generators and thermoelectric refrigerators,p and n type semiconductor thermoelectric materials are required to form a closed loop according to a certain arrangement,and their thermoelectric value?ZT?are similar,so it can be applied in practice.At present,the development of p type oxide thermoelectric materials is much faster than that of n type oxide thermoelectric materials.Therefore,developing high performance n type oxide thermoelectric materials is a top priority.The goal of this paper is to prepare high performance n type oxide thermoelectric materials.The research show that the titanium based oxides?SrTiO₃ and TiO_x?have the intrinsic high Seebeck coefficient and are the potential n type oxide thermoelectric materials,but their electrical conductivity is low and the thermal conductivity is high,which affect the further enhancement of the thermoelectric properties.Therefore,the research ideas of this paper are to explore a simple preparation method to obtain low density bulk materials and reduce their thermal conductivity.The electrical conductivity is enhanced by the method of compound graphene in SrTiO₃ or the preparation of anoxic TiO_x,and the effective enhancement of thermoelectric properties is achieved.The main contents of this paper are as follows:first,nanometer oxide powder is prepared by hydrothermal method,and then solid paraffin is added as binder,formed by pressing machine,and then sintered at a high temperature.The thermoelectric properties of the samples are tested at room temperature,and the thermoelectric properties of the material are evaluated.?1?The effects of different binder?alcohol,liquid paraffin and solid paraffin?and temperature on the formation of titanium based oxide bulk materials were studied.It was found that the best results were obtained when solid paraffin wax was used as binder?binder?10wt.%?and sintered at 1250?for 6 hours.Because the titanium based oxides have very high melting points?the melting point of SrTiO₃ and TiO₂ are 2060?and1850?,respectively?,it is necessary to ensure that there are enough sintering temperature and time to produce recrystallization.Meanwhile,the higher content of the binder can effectively avoid the large deformation of the sample in the sintering process,keeping the sample shape basically unchanged.?2?The preparation and room temperature thermoelectric properties of SrTiO₃/graphene composites were studied.The nano strontium titanate powders were added with different mass fraction of graphene,and 10wt.%solid paraffin were used as binder.The SrTiO₃/graphene composite thermoelectric materials were prepared under the condition of 6 hours heat preservation at 1250?.The highest Seebeck coefficient(-156.4?V·K^-1),electrical conductivity(201.1 S·cm^-1)and ZT value?0.0492?were obtained in the SrTiO₃ composite containing 2wt.%graphene.The lowest thermal conductivity(2.596 W·m^-1K^-1)was obtained in SrTiO₃ composites containing 3wt.%graphene.The addition of graphene,which can be used to encapsulate SrTiO₃ particles in graphene to a conductive channel effectively improved the conductivity and carrier concentration of SrTiO₃,but reduced the Seebeck coefficient of SrTiO₃ matrix.The decrease of the thermal conductivity of SrTiO₃ was mainly due to the decrease of density,result from decomposing and evaporating of the binder?solid paraffin?during the sintering process,and forming a large number of pores inside the matrix.?3?Thermoelectric properties of titanium oxide?TiO_x?materials with multiple components were studied.The solid paraffin with different mass fraction was added to the nano TiO₂ powder as a binder,and the TiO_x was prepared under the condition of1250?heat preservation for 6 hours.It is found that when the amount of solid paraffin is added to 15wt.%,the sample with the highest Seebeck coefficient(-98.3?V·K^-1),power factor(115.9?W·m^-1K^-2)and the highest ZT value?0.0078?are obtained,which was content of TiO,Ti₃O₅ and TiO₂ in TiO_x materials are 51.6wt%,41.0wt.%and 7.4wt.%.When the amount of solid paraffin was 20wt.%,the highest conductivity(226.4 S·cm^-1)and the lowest thermal conductivity(0.92 W·m^-1K^-1)were obtained,respectively,when the contents of solid paraffin were 20wt.%and 25wt.%.Due to the decomposition and evaporation of solid paraffin in the sintering process,a large number of pore structures were formed in the TiO_x matrix,while the carbon produced by the decomposition of solid paraffin reacted with partial oxygen in TiO₂ and the samples with non-stoichiometric TiO_x were prepared.The more solid paraffin was added,the content of the TiO in the sample was higher,leading to low the density of the samples,which was beneficial to the reduction of thermal conductivity.It is also found that the three kinds of Seebeck coefficient,conductivity and thermal conductivity are difficult to improve at the same time.The improvement of ZT value can not simply increase a certain factor,but need to find the best value between the three factors.