Study On The Preparation And The Thermoelectric Properties Of ZnO And Bicuseo Oxide Materials

With the rapid development of society today,the demand for energy is increasing.However,traditional fossil energy,which is the main energy supply for daily and industrial use,is decreasing.This ever-increasing contradiction urges people to continuously speed up the exploration of new energy sources and related technologies.Thermoelectric materials have become a hot topic in recent years by virtue of their ability to directly convert electrical energy and thermal energy.After continuous development,the types of thermoelectric materials have become very rich.Oxide thermoelectric materials have been widely studied in recent years due to their characteristics of non-toxic,non-polluting,high-temperature stability,and environmental friendliness.As thermoelectric materials,oxide compounds have some significant drawbacks,low conductivity or high thermal conductivity,leading to lower thermoelectric conversion efficiency.Therefore,finding and exploring ways to increase the thermoelectric conversion efficiency of oxides can promote the wide range of applications of thermoelectric materials.ZnO can maintain good stability in high temperature environment,as well as good electrical properties,for example,high Seebeck coefficient and low resistivity.However,the high thermal conductivity of ZnO has become an obstacle to limit the improvement of its thermoelectric properties.Al、Bi and Sn as dopant to improve the thermoelectric properties of ZnO.Due to the limitation of the solubility of Bi in the matrix,Bi₂O₃ precipitates in ZnO.Precipitated Bi₂O₃ with large resistance makes the material very low in electrical conductivity.In addition,the precipitation of Bi₂O₃ accelerates the mass transport of ZnO matrix and increases the grain size,which leads to a higher thermal conductivity.Doping with Sn significantly increases the conductivity,but Seebeck coefficient is very small.Therefore,Sn is also not a preferred dopant source for ZnO matrix.Al-doped ZnO not only improves the conductivity of the material,but also maintains a small lattice thermal conductivity.Finally,the ZnO:Al sample has a 3-fold increase in ZT at760°C compared to the pristine phase.As an emerging medium-high temperature oxide thermoelectric material in recent years,BiCuSeO has lower thermal conductivity（<1 W/mK）,larger Seebeck coefficient（325 V/K at room temperature）,but lower electrical conductivity（112 S/m at room temperature）.Thermoelectric properties of BiCuSeO can be improved by optimizing electrical properties or thermal properties.BiFeO₃,In and FeO as dopant to improve the thermoelectric properties.We add nano-sized BiFeO₃ into BiCuSeO matrix and hope to manipulate the carrier mobility through its spontaneous polarization and finally improve the thermoelectric performance.Room temperature Hall test results show that addition of BiFeO₃ can manipulate the carrier mobility of materials.By testing the thermoelectric properties of the prepared samples,we found that the addition of BiFeO₃ nano-particles improves the thermoelectric properties of the material within the temperature range of 300K-550K.With the increase of temperature,the composite of BiFeO₃ has no effect on the thermoelectric properties of the matrix.The results of In as a dopant to improve the thermoelectric properties of BiCuSeO shows that In element can enhance carrier mobility without changing the carrier concentration in BiCuSeO matrix,which can increase the electrical conductivity of the material while maintain a large Seebeck coefficient.The power factor（PF）of the In element doped sample can reach 4.6μW/cmK² at 800K.At the same time,the thermal conductivity of all doped samples in the test temperature range is kept low（<1 W/mK）.Finally,we achieved a ZT value of 0.58 at 800K,which is a 24%improvement,compared to pristine phase.The substitution of Fe²⁺in FeO for Bi³⁺in BiCuSeO is expected to increase the hole concentration of the material and thus enhance the conductivity.However,the results show that FeO doping failed to improve conductivity,the thermoelectric properties of the matrix have not improved.