Preparation And Properties Of NTC Thermosensitive Ceramic Materials Used For Low Temperature

The transition metal elements-based perovskite oxides La-Mn-O with a general formula ABO₃ have the potential to be a significant candidate for producing high precision temperature sensor,due to their negative temperature coefficient（NTC）effects and low resistivity which makes them suitable for low temperature measurements.Therefore,this thesis had selected them as basic materials and tuned their electrical properties by doping other ions at its A-and/or B-sites.For A-sites,the Ca ion was chosen as the doping ion because its radius is close to that of La ion,and for B-sites,the Co ion was selected as the doping ion due to its valence state is variable.The influence of different ions doping on the electrical properties of the basic materials had been investigated.Furthermore,the solid-state reaction and sol-gel method were both used to prepare the powders and their influences on the properties of materials were contrasted.In addition,the reasons for the difference and the action mechanism had been discussed.The TG-DSC,XRD,SEM,EDS,and other analysis methods had been used to investigate the complex properties of the prepared materials.The effects of doped ions at different sites on the stability of the prepared materials and their action mechanism were also analyzed.The specific research contents and conclusions are as follows:（1）Comparing the La_1-xCa_xMnO₃materials prepared by the conventional solid-state reaction（0≤x≤0.25）and sol-gel method（0≤x≤0.3）,the powders prepared by the solid-state reaction method had larger particle size,smaller specific surface area,and required higher calcination temperature to form the perovskite structure.The ceramics sintered at 1300℃had high density,but large grain size,varying between 4.987μm and 7.060μm.As for the powders samples prepared by the sol-gel method,the average particle size of the precursor powder was only 77 nm,the specific surface area was increased by 66.57%,and the sintering temperature reduced by 100℃.The grain size of ceramic sample sintered at 1200℃was below 2μm,smaller grain size leading to an increase of the grain boundary,which caused the increase of resistivity.Moreover,the stability of ceramic materials also was improved by the Ca doping.（2）A series of NTC thermistor ceramics based on La_1-xCa_xMnO₃ system were investigated,it was found that the main XRD peaks shifted to the higher 2θvalue with the increase of Ca content,which means that the crystal structure of ceramics changed from hexagonal structure to orthorhombic structure with the decrease of the crystal symmetry and the interplanar spacing.Meanwhile,with increasing Ca concentration,the SEM results showed that the grain size decreased and also some content of Mn₃O₄ phase appeared at the grain boundary.The resistivity and the thermal constant（B）were also decreased,which may result from the increase of the Mn³⁺/Mn⁴⁺carrier caused by the Ca doping.The addition of Ca increased the stability of the ceramic materials as well with the resistance drift as low as 0.38%（x=0.2）after aging 1000 h at 125℃.（3）A series of NTC thermistor ceramics based on La Mn_1-yCo_yO₃（0≤y≤0.3）system were investigated.With increasing the content of Co substitution,the XRD peaks shifted to the low-angled direction,and the crystal structure changed from the cubic structure（y=0.0）to the hexagonal perovskite（y=0.1,0.2）and finally formed the orthorhombic structure（y=0.3）with an increasing interplanar spacing.With the increase of Co content,the grain size increased first and then decreased with a certain change of the microstructure of the grains.The resistivity at25°C and B value of the whole system first decreased and then increased,and their variation range were 1.528-7.553Ω·cm and 988-2078 K,respectively.The resistance drifts of this system increased with Co content increasing,and the resistance drift at y=0.3 was 5.6 times of y=0.0after aging 1000 h at 125℃.