Study On High Temperature NTC Resistance

With the special R-T characteristic and the advantages of small size, relatively strong reliability, high sensitivity and low cost, NTCR has been widely applied in the process of the circuit protection, temperature measurement and control and temperature compensation, it has been the indispensable electronic component in the area of aerospace and electronic instruments.With advances in electronic technology, stringent requirements about the performance of some electronic components working in the harsh environment are put forward, in which high temperature the development of NTCR is included. Material with basic properties of high resistance value and low B value is required. Traditional NTC thermistors are made of spinel structure and it is not stable at high temperature. Some studies found rare earth element and manganese element oxide is a kind of material of perovskite structure and this oxide behaves NTC effect and high stability, but low room temperature resistivity. In this thesis, La Mn O₃ is the main conductive structure, complexed with the high temperature solution spinel phase Mg Al₂O₄. By adjusting the two phase and doping different elements, we hoped to achieve our goals. High-temperature solid phase method is used to prepare NTC thermistor.First we explored the sintering process of the two-phase powder samples. Samples were calcined at different temperature. After XRD diffraction analysis, we successfully prepared La Mn O₃ powder sample at 1200? and Mg Al₂O₄ powder sample at 1300?.We studied the impact of introducing Mg Al₂O₄ to La Mn O₃ phase after powder samples preparation and found that the introducing increased resistivity and B values of component. In order to get the suitable room temperature resistivity, we adjusted the ratio of the two phases and obtained the conclusion that the appropriate ratio of the two phases was 0.3:0.7. In addition, heat treatment of components in reducing atmosphere and heating up again in the air showed that resistivity of components significantly increased and then returned to the level before the heat treatment. We could describe the composite system is p-type conductivity.Then, we studied the effect of elements doping on room temperature resistivity, B value and working temperature expanding of components by doping in the B bit of perovskite structure with Al³⁺, Cr³⁺ and Ti⁴⁺ on the basis of the system 0.3 Mg Al₂O₄-0.7 La Mn O₃. The results showed that with the doping of Al³⁺ and Cr³⁺ in the B bit, the grains grew smaller, grain boundaries reduced and porosity increased, room temperature resistivity and B value of components increased, too. The working temperature extent of 0.3Mg Al₂O₄-0.7 La Mn_0.5Cr_0.5O₃ system was widened to 445?.High valence doping of Ti⁴⁺ in the B bit not only inhibited the growth of crystal grain boundaries and diluted the concentration of conductivity ionic. When the doping amount of Ti⁴⁺ exceeded 0.4, doping effect weakened and temperature resistivity of components increased significantly.