Phonon Heat Transport Regulation Of Titanate-based Materials And Their Thermoelectric Properties

Thermoelectric materials can interchange heat and electricity directly,and as green energy materials,they have drawn more and more interests.Currently,typical thermoelectric materials include Bi2Te3,PbTe,etc.However,toxicity,high cost and low thermal stability restrict their large-scale applications.Meanwhile,the potential of oxide as thermoelectric material was reported,and benefitting from their environment-friendliness,low cost and stable thermochemical properties,oxide has soon become one of the hot topics in thermoelectric community,and is expected to play an important role in the large-scale recovery of waste heat.Thermoelectric performance of materials is usually evaluated by dimensionless figure of merit ZT=(S2 ?T)/?,where S,?,? and T represent Seebeck coefficient,electrical conductivity,thermal conductivity and absolute temperature,respectively.Moreover,S2? is also called as power factor.Thermoelectric materials with high performance require high electrical conductivity,high Seebeck coefficient,and low thermal conductivity.Compared with current alloy thermoelectric materials,oxide' ZTs are still low,and the main reason lies at high phonon heat transport.As a result,reducing phonon heat transport and the corresponding thermal conductivity have become the main work on the oxide thermoelectric materials for thermoelectric researchers.This paper focuses on Nd2/3-xLi3xTiO3(briefly as NLTO)and SrTiO3(briefly as STO).Approaches of preparing nano-scale superlattcie structure and constructing composites with low thermal conductivity were applied to the research on NLTO and Nb-STO respectively to decrease phonon heat transport and thermal conductivity,improve the electrical properties and advance the thermoelectric performance.For NLTO,it's for the first time to research its thermoelectric properties.Firstly,NLTO ceramics with nano-scale superlattice structure were prepared and their micro-strucuture,phonon heat transport and thermal conductivity were researched.Superlattice structure in the grain produces plenty of interfaces to scatter phonons,effectively reduces phonon heat transport,leading to a glass-like thermal conductivity of-2W/(m-K)which is much lower than that of most other titanates.Furthermore,electron-doping was achieved by cation-sites vacancy compensation.Electron carriers were produced and the electrical conductivity of NLTO was drastically increased,enabling NLTO polycrystalline ceramics change from lithium ionic conductor with low electrical conductivity into electronic conductor with high electrical conductivity.NLTO with vacancy compensation still holds the glass-like thermal conductivity of?2 W/(m·K)and processes the maximum dimensionless thermoelectric figure of merit ZT=0.019 at 500K.To further advance ZT values,the substitution of Nb for Ti in NLTO was also performed.After the substitution,superlattce structure and the corresponding glass-like thermal conductivity was also reserved.Moreover,compared with that of cation-vacancy compensation,thermoelectric performance was improved due to the advanced oxidation-resistant temperature and the maximum ZT=0.05 was achieved at 650K.For the optimization of the thermoelectric performance of SrTiO3(briefly as STO),taking Sr(Ti0.85Nb0.15)O3(briefly as Nb-STO)as the matrix,via the addition of second phase,including yttria-stabilized zirconia(briefly as YSZ),mesoporous silica(briefly as MS)and K2Ti6O13(briefly as KTO)nanowires,to form composites to investigate the variation of the microstructure,phonon heat transport,thermal conductivity and thermoelectric properties of Nb-STO.Composites were prepared by conventional pressureless sintering.Results show that the addition of second phase with low thermal conductivity can promote the growth of grains,strengthen the relative density,increase the carrier mobility and finally advance the electrical conductivity of Nb-STO.Meanwhile,thermal barrier was formed at grain boundaries of Nb-STO due to the addition,which dramatically reduce the phonon heat transport,lower the thermal conductivity of Nb-STO.C arrier concentration is almost unaffected as the addition,which leads to an almost unchanged Seebeck coefficient.In summary,the addition of any of YSZ,MS and KTO nanowire,dimensionless thermoelectric figure of merit ZT of Nb-STO was significantly improved and the maximum ZT=0.34 was achieved at 900K for the addition of KTO nanowire.The paper also compared the results of both NLTO and Nb-STO from oxidation-resistant temperature,thermal conductivity,electrical conductivity,Seebeck coefficient,power factor and dimensionless thermoelectric figure of merit.The remained problems for both titanates and the corresponding future work were also given.The work in this paper shows that Nd2/3-xLi3xTiO3 with nanometer superlattice structure is a promising new thermoelectric material.Meanwhile,for SrTiO3,forming composites by adding the second phase with low thermal conductivity can effectively advance its thermoelectric performance.Also,the two approaches of reducing phonon heat transport,decreasing thermal conductivity and advancing thermoelectric performance in the paper can be a reference to the research of other thermoelectric oxide or even non-oxides.