The Research Of Preparation For Ni-Mn-O Negative Temperature Coefficient Thermistor By Coprecipitation And Sintering Process

Powders preparation is one of the key processes in negative temperature coefficient (NTC) thermistor production.In this paper,NTC thermistor ceramic powders were prepared by coprecipitation,the relation between precipitation conditions and properties of sintered ceramics were studied,and the sintering processes were also investigated.The main research contents include the following:1.The influence of different precipitators(NaOH,Na₂CO₃,C₂H₂O₄) on microstructure and electric properties of Ni-Mn-O NTC thermistors was investigated.The results show: the better crystalline powder were formed using oxalic acid as a coprecipitator,sintered ceramics had higher density and also with well homogeneity for resistivity and B value. Then it was investigated that the effect of pH value,the raw materials,additive order on the properties of NTC thermistor with oxalic acid as a coprecipitator.2.The preparation of the Ni-Mn-O NTC thermistor by homogeneous coprecipitation using diethyl oxalate as a coprecipitator and their eleltrical properties were investigated. The results showed:at appropriate conditions,particle size of precursor was uniformity, resistivity and B value of sintered samples had well consistency.The sintering process of Ni-Mn-O of NTC thermistor ceramics was also studied.It was found that the density and resistivity of the ceramics keeped an increase sintering at raised temperature,whereas B values were essentially unchanged;the resistivity of slow cooling rate samples was lower than that of rapid cooling rate samples.3.The sintering process of Fe-Ni-Mn-O NTC thermistor ceramics prepared by the coprecipitation was studied.The results suggested that the resistivity of samples with slow cooling rate was less than that of samples with rapid cooling rate,cooling rate had an important influence on resistivity especially for samples with higher Fe content.In addition,the resistivity of samples would decrease when undergoing a further heat treatment,heat treatment had a remarkable influence on resistivity especially for samples with higher Ni content.4.The properties of Mn-Cu-Ni-O system NTC thermistor and the ZrO₂-composited materials were studied.It was shown that with increasing Cu content,resistivity and B value decreased rapidly whereas aging increased sharply.The presence of ZrO₂ may increase resistivity and B value and restrain degree of aging.