La _0.67 (Ca _1-x Sr _X ) _0.33 MnO ₃ Polycrystalline Study On Preparation And Electrical Properties Of Materials

The perovskite manganite oxide Lao.67(Ca1-xSrx)0.33MnO3(LCSMO) has attracted much attention in the last decades due to its metal-insulator transition and colossal magnetoresistance effect near the Curie temperature. Owing to its application potential in the magnetic storage device, magnetic detector and infrared detector, it has become another research hotpot among the strong correlation materials after the high temperature superconductors. Since the infrared detector should be very sensitive to the change of temperature, the material must have a high temperature coefficient of resistance (TCR). The research of this thesis mainly focuses on the enhancement of TCR value of the material.The sol-gel method has been used to prepare the La0.67Ca0.33MnO3 (LCMO) polycrystalline. The sintering conditions were optimized and the influence of chelating agent on the properties of polycrystalline was investigated. Three different chelating agents, which were EDTA, GA and CA, were used to prepare the samples at the same conditions. We studied the electric transport behaviors and found that the polycrystalline prepared by CA has the best properties.For the system using CA as the chelating agent, the change of Mn ions concentration at B site can largely affect the TCR value. We also studied the effects of the small vibration of the Mn content on the metal-insulator temperature, TCR, resistance and the polycrystalline structure. Finally, the TCR value as high as 42.58% is obtained, which is close to the value of single crystal sample. What’s more, the method we used is much simple and efficiency compared with the preparation of single crystal.On the bases of the former results, Sr was doped in LCMO at the optimum Mn concentration to further enhance the Tp to room temperature and maintain a relatively high TCR at the same time. The experiment results show that the LCMO polycrystalline target with a Tp of 310 K and TCR as high as 9% was finally obtained, which is larger than the previous reports.In addition, the Tp can be easily adjusted between 260 K and 310 K.With the high quality polycrystalline, we used the pulsed laser deposition technique to prepare the LCSMO thin film. The crystalline quality was improved after optimizing the preparation conditions and the sample with a TCR of 5% and TP of 310 K was acquired. These results make the LCSMO has a potential to be used as the room temperature thermal radiation detector.