Preparation Of La_1-x(Ca, Sr)_xMnO₃ By Co-precipitation Method And Its Electrical Transport Properties

In this paper,the general rules and experimental conditions of LCMO ceramics producing by co-precipitation method were first discussed,and the contents of Mn(NO3)2,Ca(NO3)2 and other drugs required by the experiment were determined.On this basis,the optimum calcination temperature,calcination time and sintering temperature for preparation of LCMO ceramics by co-precipitation method were analyzed,which made the electrical performance of the prepared LCMO ceramics much improved when compared with the previous studies and close to the experimental results of sol-gel method.Due to its characteristics of simple principle,convenient preparation and short experimental period,co-precipitation method has certain advantages over sol-gel method,and the experimental results also prove that co-precipitation method is a feasible and potentially useful method for preparing LCMO ceramics.After that,we also discussed the La1-xCaxMnO3(x=0.28～0.33)and La0.72(Ca1-xSrx)0.28MnO3(x=0～0.04)by co-precipitation method,and obtained a series of polycrystalline ceramic targets with good electrical performance.Finally,the samples were qualitatively analyzed by means of XRD,SEM and standard four-probe method.Test results showed that when Mn(NO3)2 and Ca(NO3)2 were calculated under the content of 49.5%and 101.5%,respectively,the electrical properties of the LCMO ceramic preparing by sol-gel method and co-precipitation,respectively,are the best.The target prepared by sol-gel method display the lowest room temperature resistivity of 0.023Q·cm and the highest TCR(43.4%·K-1);The target prepared by co-precipitation method display the lowest room temperature resistivity of 0.021Ω·cm and the highest TCR(32.4%·K-1).The electrical properties of the two are very similar,so we can explore the process of preparing LCMO by co-precipitation with reference to the existing experimental experience of sol-gel method.Compared to two kinds of methods,you can see that although there is slightly higher TCR in the samples of the sol-gel method,its resistance is bigger than the samples of co-precipitation.Since the smaller resistance of the target material samples by co-precipitation method may have better conductivity,it is proved that the co-precipitation is likely to be an effective method in the preparation of LCMO.In terms of mixing method,the total drop method has a great advantage over the positive drop method and the reverse drop method.Compared with the experimental results obtained by using various precipitating agent and their different concentrations,the 2 times molar amount(NH4)2CO3 is a better precipitating agent in the experiment.The optimum pH in Solution is 9.5.The combined solid phase,calcinating 8h at 500℃ and sintering 12h at 1450℃ are a better method.The resistivity of La1-xCaxMnO3(x=0.28-0.33)samples decreases gradually with the increase of Ca doping amount,Tp moves towards higher temperature and TCR also increases.With the increase of Sr doping,the resistivity of La0.72(Ca1-xSrx)0.28MnO3(x=0-0.04)decreases and Tp moves towards higher temperature,which can be explained by the combination of the Jahn-Teller distortion and the Double Exchange mechanism.