NTC Properties’ Investigation Of BiFeO₃ Ceramics

Negative temperature coefficient（NTC）thermistor is a kind of sensing material which resistance value decreases with the increase of temperature.It has the characteristics of small volume,low cost,fast response and high sensitivity.It is widely used in daily life and industrial production.With the development of aerospace,metal smelting,energy and other industries,researchers’demand for temperature detection in high temperature environment is greatly increased.The traditional spinel thermistor is unstable at high temperature,which can not meet the requirements of high temperature detection.Perovskite thermistor can keep stable phase structure at high temperature,which is considered to be a high temperature NTC thermistor.BiFeO₃is a kind of multiferrous material with perovskite structure,which can be used in intelligent sensors,new magnetic memory and other multifunctional integrated devices.It is considered that it can be used in intelligent sensor,new magnetic memory and other multi-functional integrated devices.It was found that BiFeO₃material has the characteristics of resistance decreasing with the increase of temperature,and can be developed into NTC thermistor.Compared with traditional spinel thermistor,BiFeO₃with perovskite structure has higher stability at high temperature and can be used in high temperature environment.However,due to the volatility of Bi and the variable valence of Fe,it is easy to produce heterophase during the preparation process,which affects the properties of ceramics and causes the leakage current phenomenon.In this thesis,BiFeO₃ceramics were modified by doping La and Ti at A and B positions respectively.The structure and electrical properties of BiFeO₃ceramics were studied,and the conductive mechanism was discussed.The main results are as follows:（1）La_xBi_1-xFe O₃（LBFO）materials of x=0.0,0.1,0.2 and 0.3 were synthesized by sol-gel method.LBFO ceramics were prepared by rapid liquid phase sintering and conventional sintering respectively.It was found that rapid liquid phase sintering can inhibit the volatilization of Bi element,due to the sintering time is shortened.The conventional sintering spends several hours,while the liquid phase sintering only takes ten minutes.In such a short sintering time,LBFO ceramics can inhibits the volatilization of Bi element,reduce the generation of oxygen vacancy and control the valence change of Fe element.X-ray diffraction（XRD）and Raman spectrum analysis show that with the increase of La doping,the twisted diamond R3c changes to the orthogonal Pbnm.Scanning electron microscopy（SEM）showed that the grain size of LBFO ceramics decreased from 10μm to 1μm.With the increase of La doping,the volatility of Bi decreased from 6.48%to 1.43%.The results show that LBFO ceramics have negative temperature coefficient characteristics,the resistivity（100°C）is 4.32×10⁶Ω·cm～2.71×10⁸Ω·cm,material constant(B_100/700)is 6000K～9000K,and temperature range of LBFO is 100℃～700℃.After aging at 300℃for 500 hours,the resistivity change of LBFO is less than 1%.（2）Bi Fe_1-xTi_xO₃（BFTO）materials of x=0.0,0.1,0.2 and 0.3 were synthesized by sol-gel method.BFTO ceramics were prepared by rapid liquid phase sintering.X-ray diffraction（XRD）shows that with the increase of La doping,the twisted diamond R3c changes to rhombic system.SEM showed that the grain size of BFTO ceramics decreased from 10μm to 1μm.The results showed that BFTO ceramics had negative temperature coefficient characteristic,the resistivity（100°C）was 2.71×10⁸Ω·cm～1.92×10¹¹Ω·cm,the material constant(B_100/700)was 8000K-10000K,and temperature range of BFTO was 100℃～700℃.When Ti ions are doped,Fe ions at the position of B-site are replaced,The electron jump decreases and the resistivity increases.