High Temperature NTC Properties' Investigation Of LaMnO₃ And High-resistance Phase Oxide Bilayer Structure Creamics

High temperature thermistors are widely used in many fields,such as aerospace,automobile industry,household appliances and so on.However,its applied to harsh environment,and its temperature is more than 300?,which also requires good stability of materials.At present,few materials can be applied in practice,so it is very important to study new high temperature?Negative temperature coefficient,NTC?thermistors.LaMnO₃ is p-type electric conductor that conducts electricity by transiting cations,it has been widely studied for its good conductivity and stability at high temperature.High resistance phase oxides have the characteristics of high melting point,high resistance and stable properties.It is expected that through the design of the bilayers structure which could contain the high temperature NTC characteristics and high stability,in order to work stably under high temperature.In this paper,LaMnO₃-Y₂O₃ composite ceramic materials with different proportions were firstly prepared to study the influence of conventional sintering and vacuum hot-pressing sintering methods on its microstructure and temperature resistance characteristics.LaMnO₃/Y₂O₃ bilayers NTC thermistors were prepared,but according to its SEM images,Y₂O₃ did not reach the sintering temperature of the porcelain,and the mechanical properties of the sintered samples were poor,which brought inconvenience to subsequent tests.In order to solve this problem,pure phase Y₂O₃ layer is replaced with LaMnO₃-Y₂O₃ composite layer.Firstly,the LaMnO₃/?0.3LaMnO₃-0.7Y₂O₃?bilayers NTC thermistors were prepared.It was found that the cracking phenomenon of the sample was serious and hard to successfully preparing.By reducing the proportion of Y₂O₃ in the composite layer,the0.3LaMnO₃-0.7Y₂O₃ was adjusted to 0.7LaMnO₃-0.3Y₂O₃ and 0.8LaMnO₃-0.2Y₂O₃,and the cracking condition of the samples was improved.The main findings are as follows:?1?The samples were sintered through conventional sintering and vacuum hot-pressing sintering respectively.It can be obtained from the analysis results of SEM images that the samples prepared by vacuum hot-pressing sintering have fewer pores compared with the samples prepared by conventional sintering,the grain sizes of the two phases are more uniform,and the samples has a higher relative density.The resistivity of conventional sintered samples is significantly higher than that of vacuum sintering due to the existence of many pores,smaller grain size and more obvious scattering effect of grain boundary on carriers.?2?The effects of different molar ratios on the temperature-resistance characteristics and micro-morphology of?0.7LaMnO₃-0.3Y₂O₃?/Y₂O₃ bilayers thermistors were investigate.According to the section SEM images of the bilayers thermistors,it has no obvious interface,and the Y₂O₃ layer of unsintered porcelain has no obvious grain,so the mechanical properties of the samples are poor.LaMnO₃/?0.3LaMnO₃-0.7Y₂O₃?bilayers thermistors were investigate.The samples have a good linear relationship between resistivity and temperature from 400?to1000?.The resistivity and B_100/50000/500 of the?0.7LaMnO₃-0.3Y₂O₃?/Y₂O₃ bilayers thermistors were 2.20×10⁵-1.46×10⁸?cm and 6293-11218 K.But the bilayers samples appeared cracking and it hard to successfully preparation.With different mole ratio of LaMnO₃/?0.7LaMnO₃-0.3Y₂O₃?and LaMnO₃/?0.8LaMnO₃-0.2Y₂O₃?bilayers thermistors found that changing the proportion of compound layer can effectively improve the cracking of the sample,but the temperature measurement range is from 100?to 500?relatively lower than the former.?3?LaMn_0.6Cr_0.4O₃/Al₂O₃ bilayers thermistors were prepared by conventional sintering method.The sintering match of the bilayers thermistors was good.It can be seen from the SEM images that the two-phase interface was clearly visible without cracks or layering indicate that its sintering performance has significantly improved byaddingsinteringaids.TheresistivityandB_600/900ofthe?0.7LaMnO₃-0.3Y₂O₃?/Y₂O₃ bilayers thermistors were 1.38×10⁷-2.42×10⁸?cm and8442-10276 K.It can be seen that the bilayers structure thermal ceramics has good performance within the range of temperature measurement.