La_1-x(Ca,Sr)_xMnO₃:Ag_0.2 Material Preparation And TCR Performance Research

The doped perovskite manganese oxide has a rich electronic phase diagram,a variety of different transport properties,and other physical properties,and has therefore been extensively studied.Additionally,these materials are typically characteristic by so called colossal magnetoresistance?CMR?effect which occurs at around the metal-insulator transition temperature?T_P?.Such materials can be used to make magnetic memories,magnetic detectors,infrared detectors,etc.The infrared detector is a device that is extremely sensitive to temperature changes,which requires the material to have a high temperature coefficient of resistance?TCR?,which dominated by the sharpness of the metal-insulator transition.In this dissertation,we mainly study the La_1-xCa_xMnO₃ system.Preparing the materials,which have the T_P near room temperature and high TCR,is the main purpose of this experiment.For La_0.7Ca_0.3MnO₃,Sr,Pb,and Ba were selected for doping experiments..In general,the samples doped with Sr have good crystallinity,high density,a certain improvement in electrical conductivity,a decrease in the resistivity,and a higher TCR value at the same time as T_P,so Sr can be used as a further study of the doping elements.Corresponding to the La_1-xCa_xMnO₃ system,with the increase of Ca doping,the resistivity of samples decreases,T_P increases,and TCR increases first and then decreases.At x=0.28,TCR has reached 78.8?4?·K^-1,the largest in the series,but its T_P is only 256.3 K,far below room temperature?300 K?,which greatly limits its further application.On this basis,La_0.72Ca_0.28MnO₃:Ag_0.2 was selected for Sr doping.With the increase of Sr doping amount,the T_P of polycrystalline samples moved to the high temperature direction.The Curie temperature?T_C?increased continuously,and the resistivity decreased continuously,and TCR decreased continuously.It is worth mentioning that when x=0.07,T_P reaches 301.8 K?near room temperature?,at this time the TCR value is 24.3?4?·K^-1,which is much higher than the other literature,and the performance indicators have reached the application standard.For the La_1-xCa_xMnO₃:Ag_0.2 series samples,a series of polycrystalline targets with excellent properties were selected for preparing thin films.Different series of unannealed films were prepared first,and then these films were processed with annealing.First,the La_0.72Ca_0.28MnO₃:Ag_0.2 film was post-annealed at different temperatures in air atmosphere.The result showed that the electrical properties of the films annealed in the air atmosphere were improved.Compared to the unannealed films,the crystallinity were enhanced,the surface was more flat,but T_P increased slightly and far below room temperature.It was found that at higher post-annealing temperature?1400??,the thin film was obviously evaporated,the samples became insulating up to 100K.Then,we post-annealed La_0.7Ca_0.3MnO₃:Ag_0.2 films at different temperatures in an oxygen atmosphere.The results showed that the samples annealed in oxygen had lower resistance and the T_P is relatively higher,all above220 K.Annealing at 1200°C in an oxygen atmosphere,the TCR of the corresponding film reached31.6?4?·K^-1,but the T_P was only 220.8 K.Finally,comparing the post-annealing conditions of the films at different temperatures,we chose 1200°C as the annealing temperature.The La_0.67Ca_0.33MnO₃:Ag_0.2 film was post-annealed at different times in an oxygen atmosphere.The overall crystallinity of films is high,the surface is relatively flat,and the resistance is small.It can be seen that under a longer annealing time,the resistance increases as the evaporation amount of the thin film increases.With the increase of annealing time,the T_P of the film sample showed a trend of increasing first and then decreasing.The T_P of the film sample was the highest at 285.7 K after annealing for 3 hours.The TCR of the film sample showed a similar trend with T_P.The TCR increased first and then decreased with the increase of annealing time.The TCR of the film after annealing for 3 hours was the highest,which was 20.3?4?·K^-1.In summary,the film samples annealed for 3 h have better electrical properties.The post-annealing treatment of the above thin film is preferred.Annealing at 1200°C.for 3 hours under oxygen atmosphere is a good choice for post-annealing.The performance of the sample under this annealing process is better.