| Manganese oxides La1-xCaxMnO3(LCMO) with perovskite structure have rich physical phenomenon and magnetic phase diagram. Colossal magnetoresistance (CMR) materials which have strong laser induced voltage (LIV), high insulator to metal transition temperature (Tp) and high temperature coefficient of resistance (TCR) are the foundation of manufacturing high quality photo detector, infrared locator and bolometer, etc. Ca doping content has significant influence on the LIV, Tp and TCR of LCMO film. In this thesis, we studied the preparation process of La1-xCaxMn03(x=0.0-1.0) powder, target and thin film, which were prepared by sol-gel method and pulsed laser deposition technique, respectively. The structure, phase, morphology and electrical properties were investigated by thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), resistance-temperature (R-T) and LIV measurement. The goal is to optimize the process and improve Tp, TCR and LIV effect of LCMO materials, and find the relationship between Ca doping content and LIV effect.LCMO phase can be synthesised by powder prepared by sol-gel method at a temperature as low as500℃. XRD pattern shows that LCMO diffraction peaks can be detected for the powder calcined at550℃. The calcined powder has uniform size (-500nm) and spherical shape. The coprecipitation powder needs a higher calcination temperature to synthesis LCMO phase. The powder size is bigger (-2μm), the shape is irregular polygon, and the powder intend to agglomerate to bulk. This is due to the coprecipitation powder is bigger and the component is not as uniform as sol-gel powder.Reducing the calcination temperature will improve the shrinking percentage of target, which has big influence on electrical properties of coprecipitation target. The target with large shrinking ratio has normal Tp and larger TCR. The grain size of sol-gel target is uniform and there is less porosity. Although the coprecipitation targets were sintered under the same conditions, they have a smaller grain size and more porosity. When Ca doping content was in the range of0.175-0.5, in which the LCMO materials shows insulator to metal transition, the electrical transport properties of targets prepared by the two methods show evident difference. This maybe due to the sol-gel target has a more uniform phase and less grain boundary. In other doping range, they have the similar conduction behavior.Thin films prepared by the two kinds of targets have the similar electrical transport property as well as the surface morphology, but film deposited by sol-gel target has a more uniform grain size and stronger LIV single. Temperature and oxygen pressure heavily affect the structure and phase transition temperature. Tp can be improved by increasing oxygen pressure. The film grown on substrate with little tilt angle has better quality. But if the tilt angle is too large, the growth quality will be worse. LIV single increased with the tilt angle increasing. The mismatch of lattice constant between film and substrate has a significant influence on the properties of film. The film grown on LSAT which has very small mismatch shows higher TCR, on LAO substrate which induce compressive stress in La0.67Ca0.33Mn03shows higher Tp and larger Up, on STO which induce tensile stress in La0.67Ca0.33MnO3shows worse properties. When Ca doping content is in the range0.175-0.28, LCMO films show larger TCR.We demonstrated that the better LIV effect will appear when Ca doping content is around0.5. The largest ΔS, Up and short τ have been detected in film with Ca content of0.5. When Ca content is far away0.5from both the two sides, Up will decrease.τ decreases with increasing Ca content. The reason maybe that the Seebeck coefficient anisotropy AS reachs the largest in La0.5Ca0.5Mn03film. |
PDF Full Text Request