Element Doping And Nanocomposite Regulate Thermoelectric Transport Characteristics Of N-type PbTe Alloys

The advancement of human industrialization leads to energy consumption and environmental pollution,so it is urgent to seek a kind of clean and sustainable energy.Thermoelectric materials,as a kind of material which acts as a bridge connecting heat and electric energy,are gradually widely studied.Among high performance thermoelectric materials,lead telluride(PbTe)has attracted much attention due to its excellent crystal symmetry,intricate electron band structure,good phonon anharmonicity and great thermoelectric properties in the middle temperature region.p-type and n-type thermoelectric materials are essential components of thermoelectric devices,whose properties need to be matched to be used at the same time.The thermoelectric devices made of the same materials can reduce the module cracking caused by thermal stress and improve the service life of the devices because of the same thermal expansion coefficient.At present,the thermoelectric value(z7)of p-type PbTe has been improved to more than 2.0,but the thermoelectric performance of n-type PbTe is relatively low.Therefore,it is very important to further optimize the thermoelectric properties of n-type PbTe.Among the many thermoelectric performance optimization strategies,element doping and composition are the most widely used.Since the resistivity,Seebeck coefficient and thermal conductivity are coupled with each other through the carrier concentration,it is the most basic strategy to optimize the thermoelectric performance to regulate the carrier concentration through element doping.By doping deep level elements,the carrier concentration of PbTe alloy can be optimized in a wide temperature region,and the point defect scattering can be enhanced to reduce the thermal conductivity.Composition is an important way to increase the performance of thermoelectric materials.On the one hand,it can increase grain boundary density and introduce phase boundary,enhance phonon scattering and reduce thermal conductivity.On the other hand,the absolute value of Seebeck coefficient of the sample can be improved through the energy filtering effect at the heterogeneous interface,so as to realize the synergistic optimization of electrical and thermal performance.Therefore,in this paper,on the basis of iodine(I)doping,further choose deep level elements gallium(Ga)or indium(In)doping;In addition,a small amount of tourmaline with intrinsic low thermal conductivity and magnetic oxide CaMnO3 were synthesized in n-type PbTe alloy doped with I,and the regulation rule between the amount of recombination and the thermoelectric properties was studied.The results of this paper are as follows:(1)n-type Pb1-xGaxTe0.995I0.005(x=0%,2%,3%,5%)alloys was successfully prepared by solid phase melting method.With the increase of Ga content,the carrier concentration increased and stabilized at 1019cm-3,close to the optimal carrier concentration.Ga doping forms deep impurity levels in the electron band structure through Ga 4s orbitals corresponding to Ga1+valence states,while Ga3+states are shallow impurity levels.In contrast to Ga3+impurity,which are already ionized at low temperatures,the deep impurity level of Ga1+ ionizes at temperatures higher than 473 K,contributing additional electrons and regulating the temperature-dependent carrier concentration at medium-high temperatures,The high carrier concentration makes Pb0.95Ga0.05Te0.995I0.005 gets the 20.9 μW/K2 cm at 668 K.In addition,gallium doping enhanced phonon scattering defects,decrease the thermal conductivity of lattice,Pb0.95Ga0.05Te0.995I0.005 alloy won the highest zT value 0.94 at 767 K.Similarly,n-type Pb1-xInxTe0.995I0.005(x=0.0%,0.2%,0.3%,0.5%)alloys was successfully synthesized.When the content of I was determined,the content of In was changed,and an electron was released instead of the Te,and the deep level In was introduced near the bottom of the conduction band.At low temperature,the carrier concentration decreases when electrons are captured.At high temperature,the captured electrons are released into the conduction band,and the power factor is optimized in the whole temperature region.At 570 K,the power factor reaches 18 μW/K2cm,especially at room temperature,the power factor is above 10 μW/K2 cm.To minimize the Pb0.995In0.003Te0.995I0.005 lattice thermal conductivity is 1.15 W/mK.Through the thermoelectric performance of the synergistic optimization,finally Pb0.995In0.002Te0.995I0.005 has the largest zT value 0.76.(2)The effects of different compound contents of tourmaline(x=0.0%,0.1%,0.2%,0.3%)on the thermoelectric properties of PbTe0.995I0.005 were studied.The Al3+ in tourmaline enter PbTe0.995I0.005 lattice instead of Pb2+,resulting in increased carrier concentration.At the same time,additional ionized impurities are scattered in the material,resulting in a significant decrease in mobility.After the composition of tourmaline,the grain boundary density of the sample increases,the phonon scattering is enhanced,and the lattice thermal conductivity decreases significantly.The minimum lattice thermal conductivity of the x=0.2%sample is 1.02 W/mK at 669 K.Due to the significant decrease of lattice thermal conductivity,zT value increases significantly in the medium-high temperature region(520-770 K),and the maximum zT value of x=0.2%sample at 669 K is 0.88.(3)The effects of different contents of CaMnO3 on the phase structure,microstructure and thermoelectric properties of PbTe0.995I0.005 alloy was investigated.CaMnO3 is embedded in the matrix grain as the second phase and does not cause any change in the crystal structure of PbTe.CaMnO3 inhibited the growth of matrix grains during sintering,the grain size decreased and the grain boundary density increased.Due to energy filtering effect,the x=0.1%CaMnO3 composite samples of the absolute value of Seebeck coefficient is increased,under the 768 K for a maximum of |S|=246 μV/K.