Study On Thermoelectric Properties Of ZnO Composite And Ba-la-nb Co-doped SrTiO₃

Thermoelectric materials can directly convert heat and electric energy to each other,can realize waste heat power generation and refrigeration,and have important application value in energy saving and emission reduction.Sr Ti O₃ has the advantages of high Seebeck coefficient,good stability,low cost and non-toxicity.It is a kind of n-type oxide thermoelectric material with great potential.However,the high thermal conductivity leads to a low dimensionless thermoelectric figure of merit(ZT).How to reduce thermal conductivity while maintaining high conductivity research focus Sr Ti O₃ thermoelectric materials.The doped modified ZnO has excellent electrical properties,and the band gap is similar to STO.The combination of the two is expected to reduce the thermal conductivity through the interface effect while maintaining high electrical properties;Compared with Sr,Ba has a larger relative atomic mass and ion radius.If Sr is successfully replaced,it can enhance the lattice scattering effect,reduce the thermal conductivity of the material,and increase the ZT value.Therefore,this thesis is based on La and Nb double-doped Sr Ti O₃,and systematically studies the effects of ZnO composite La-Nb-Sr Ti O₃(ZnO/STO)and Ba,La,Nb multi-element doping on the microstructure and thermoelectric properties of Sr Ti O₃,and the following conclusions are drawn:(1)Ga_xZn_(1-x)O(x=0.02%mol),n Ga_xZn_(1-x)O/(1-n)La10Nb10(n=0.1%mol,0.87%mol),Bax La10Nb10(x=0.5%mol-20%mol)and Bax La(10-x)Nb10(x=1%mol-9%mol)powders are prepared by hydrothermal method.There have no impurity phase,and the doping elements are all doped into the crystal lattice.All powder materials are sintered in a vacuum hot pressing furnace to obtain bulk materials.The Ga_xZn_(1-x)O(x=0.02%mol)bulk sintered at 1000°C has the highest conductivity.The conductivity is 1123 S/cm at 1023 K,and the power factor is 0.81 m W/m K²;(2)Using La_0.1Sr_0.9Nb_0.1Ti_0.9O₃(La10Nb10)as the matrix and Ga_xZn_(1-x)O(x=0.02%mol)as the composite phase,n Ga_xZn_(1-x)O/(1-n)La10Nb10(n=0.1%mol,0.87%mol)material.Studies have shown that the 10%mol ZnO composite material and87%mol ZnO material sintered at 1300°C undergo a reduction reaction with C,and Zn is precipitated.The grain size of Ga_xZn_(1-x)O is larger than that of STO,about 1-3?m,which is distributed at the trigeminal grain boundary of La10Nb10 grains,which causes the electrical conductivity of the bulk to decrease and the thermal conductivity to increase,which seriously reduces the electrical and thermal properties of the composite bulks.The10%mol ZnO composite sample has a power factor is 0.69 m W/m K² at 1023 K and a ZT value of 0.22 which was sintered at 1300°C;(3)In the Bax La10Nb10(x=0.5%mol-20%mol)system,Ba,La,and Nb elements were successfully incorporated into the crystal lattice.With the increase of Ba doping concentration,the bulk STO grains grow up and the second phase morphology changes.The second phase includes Nb Doped with Ti_nO_2n-1,Ti_nO_2n-1 and elemental Nb.The complex microscopic obviously changes have a severe impact on samples with high doping concentration,but it is conducive to the improvement of the thermoelectric performance of samples with low doping concentration.The ZT values of Ba1La10Nb10and Ba3La10Nb10 are higher than La10Nb10,and the power factor of Ba1La10Nb10sample is 0.93 m W/m K² at 1023 K,and the ZT value is 0.31;(4)In the Bax La(10-x)Nb10(x=1%mol-9%mol)system,Ba6La4 and Ba9La1,which are doped with more Ba,appear as second phase Ti_nO_2n-1;because Ba doping does not provide additional electrons,the decrease in the La doping amount leads to a decrease in the carrier concentration,which greatly reduces the electrical performance.At the same time,the ratio of La to Ba also affects the thermal properties,resulting in the lower thermal conductivity of the Bax La(10-x)Nb10 series samples,resulting in the Ba1La9Nb10samples at 1023 K with a power factor of 0.73 m W/m K² and a ZT value of 0.24.