Study On The Low-Temperature Sintering Characters And Electeic Properties Of Mn-Co-Ni-O Based NTC Thermistors

Mn-Co-Ni-O based ceramics with spinel structure are the most important materials for NTC thermistors. The technique of multilayer components is hotspot of NTC thermistors. To prepare the multilayer components, the low-temperature sintering is the pivotal technique.The Mn-Co-Ni-O based ceramics were used as main materials. The aim of our work is to decrease the sintering temperature. Synthesis of high active nano-powders and doping were used as main techniques. A series of works have been done in our study. We can explain the general rule about the formation of stable sol-gel. The reactive mechanism about the auto-combustion of dried gel was also studied. The effects of various additives on the microstructure, sintering characters and electric properties of Mn-Co-Ni-O based ceramics were shown in the paper. At last, the sintering temperature of Ni₁Co_0.2Mn_1.8O₄ baseed ceramics was decreased to 950℃, while it kept excellent electric properties.According to the traditional technique, the Mn-Co-Ni-O based ceramics were prepared. It can be concluded that single spinel structure is important factor for NTC materials to keep perfect electric properties. The compositions of Mn-Co-Ni based oxides with single spinel structure have been given as follows: A_1-1.2Mn_2-1.8O₄ (A: the others cations in the oxides), which were studied in whole paper. In the same time, the effects of Cu contents on the microstructure, electric properties and sintering temperature of Mn-Co-Ni-Cu-O based ceramics were also studied. It is conclude that Cu in the Mn-Co-Ni-Cu-O can decrease the sintering temperature to 1100℃, but also decrease the material constant B obviously. So, we tried to improve the electric properties of Mn-Co-Ni-Cu-O based ceramics.We have explored the technique about synthesis of nano-powder via sol-gel methods. The sol-gels were prepared from nitrate-citric acid-EG system and nitrate-oleic acid system respectively. The dried-gels were obtained, and had auto-combustion in air, then were calcined in oven at 800℃. Finally, the Ni₁Co_0.2Mn_1.8O₄ powders with 40 nm crystallite were prepared, which have high sintering activities and uniform compositions. The powders were sintered at 1100℃ to form compact ceramics with more then 95% relative densities. Meanwhile the sintering temperature of samples prepared via traditional technique is 1200-1250℃. The main factors which have effects on the stability of sol-gel have been given as follows: one is the formation of network between citric acid and EG, the other is the formation of chemical bond between citric acid and cations. The model of Non-Crystalloid Precursors has been set up. The model can tell us how to obtain the stable non-crystalloid precursors in nitrate-oleic acid systems, and it is also useful to discuss the reactive mechanism of gels auto-combustions. Using the DSC and XRD techniques, the reactive mechanism from dried gels to nano-powders have...