The Preparation And Properties Of Bulk, Thick Film And Thin Film NTC Thermistors

Manganite based transition metal oxide ceramics with AB₂O₄spinel structure are themost important materials for negative temperature coefficient （NTC） thermistors. How toprepare the NTC thick film and thin film thermistors with high performance is the hot spot inNTC research area. The studies on the electronic conduction mechanism in polycrystallineNTC film thermistors are the difficult and key points in exploring the intrinsic mechanism.The main research targets of this thesis are the manganite based NTC thermistors withspinel structure. The main objectives of our work are to prepare NTC film thermistors andmake clear the electronic conduction mechanism in polycrystalline NTC film thermistors.Based on a large number of experimental data on NTC bulk thermistors, a series of workswere conducted by employing screen printing method to prepare NTC thin film thermistors,spin-coating method to prepare NTC thin films, and temperature-dependent ac impedancespectra to analyze the conduction mechasnism. We systematically studied the effects oftechnological parameters of NTC bulk, thick film and thin film thermistors on their propertiesand intrinsic conduction mechanism by taking the small-polaron hopping conduction astheoretical basis.The influences of chemical composition, sintering temperature and powder preparationmethod on the microstructure and electrical properties were studied on the Mn-Ni-O binary,Mn-Co-Ni-O ternary and Mn-Co-Ni-Cu-O quaternion systems. The results showed that theNi_0.85Co_0.3Mn_1.85O₄（MCN） bulk thermistors fabricated with powder by sol-gel method andsintered at1180oC for3h had much better performances than those with powder bytraditional solid-state reaction method.Based on the experimental results of bulk thermistors, the MCN thick film thermistorswere prepared on Al₂O₃substrates by screen printing method. The effects of solid content（50%,67%and75%）, sintering temperature （1150,1180and1200oC） and the shape ofelectrodes （terminal and interdigital） on the microstructure and electrical properties werediscussed in detail. The results demonstrated that the best performance could be obtained forMCN thick film thermistors with67%solid content and sintered at1180oC. The R-T relationof MCN thick film thermistors with terminal electrodes seemed to follow an exponential law（R=R0exp（B/T））, however, the R-T relation of MCN thick film thermistors with interdigitatedelectrodes seemed to follow linear relation （R=R0+AT）. To further investigate the influence ofelectrode shape on the R-T relation, it was known that the exponential R-T relation forterminal electrodes was not affect by the breadth of terminal electrodes, and the linear R-T relation with interdigitated electrodes was not affect by the interval of fingers. Evidently, thelinear R-T relation should be attributed to the length of finger （a value）. To decrease the avalue, it was seen clearly that the R-T relation of MCN thick film thermistors evolvedgradually from linear to exponential. The electric potential and field distribution in MCNthick films with different shape electrodes were analyzed simply by using the software Ansoftmaxwell12.For MCN thin film thermistors prepared by spin-coating method on Al₂O₃substrates, theeffects of annealing temperature and thickness on the microstructure and electrical propertieswere studied. It was found that the MCN thin film thermistor annealed at750°C had lowerresistance (R₂₅=4.8MΩ), higher characteristic temperature （T₀=3720.6K）, and small agingcoefficient （ΔR/R=3.7%） after aging at150°C for360h. The T₀and aging coefficient wasnot affected by the thickness of the thin film. The R₂₅was linearly decreasing with theincreasing of thickness. In order to reduce the R₂₅, the Ni_0.85Co_0.3CuxMn_1.85-xO₄（MCCN，x=0，0.15，0.3and0.45） thin film thermistors were prepared on Al₂O₃substrates by sol-gel methodat700°C, and the effects of chemical composition on the microstructure and electricalproperties were studied. Ni0.85Co0.3Cu0.15Mn1.7O4seemed to be the optimum composition forthe MCCN thin films with R₂₅ of0.56MΩ, T₀of4107.0K and Δ R/R of5.7%.Through the analysis of temperature-dependent ac impedances of the NTC thick filmthermistors with different interdigitated electrode length （a value）, the R-T behaviors of grainboundary （GB） and grain were obtained. The resistance of GB (R_gb) was higher than thate ofgrain （R_g）; the hopping type of electrons in the grain is Variable-Range-Hopping （VRH）; theR_gand T₀gare decreasing with the increasing of the a value. With the decreasing of the a value,the R_gb-T relation will evolve gradually from linear to exponential, which is the main reasonfor the changes of R-T relation of MCN thick film thermistors.By the analysis of temperature-dependent ac impedances of MCN thin film thermistors,the grain size influence on the conductions of grain and GB was obtained. The conductionmechanisms of grain and GB both followed the small-polaron hopping model. In all samples,the R_gbwas greater than the R_g, indicating the GB was of dominated resistance. The T₀gwaslarger than T₀gbimplying that the grain was of dominated characteristic temperature. Thehopping type of GBs was probably by Nearest-Neighbor-Hopping （NNH）, and that of grainmight be a transition from VRH to NNH. We suggest that the samples are the systems withmixed NNH and VRH, and the electrical conductions of the samples are dominated by NNH.These studies on the technological parameters and the electronic conduction mechanisms of NTC thick and thin film thermistors will be beneficial for the component design andindustrial applications.