The Preparation And IS-based Electrical Properties Of High-performance NTC Ceramics And Thick Films

With the continuous expansion of thermistors in various applications, the requirement for the accuracy of temperature measurements, sensing and control is also increasing, which requires the development of higher temperature-sensitivity bulk thermistors. Simultaneously, as the rapid development of information technology, the miniaturization and integration of electronic elements has been the development mainstream of electronic technology and film of device is a key technology for the resolution of the problems. In this paper, a few of novel negative-temperature-coefficient (NTC) thermistor ceramics were studied based on tracking the development of the domestic and international thermistor materials. Lead-free thick film NTC thermistors were prepared using the novel thermistor materials as functional phase of the thick-film paste. It is the main objective to develop high-performance thermistor ceramics and lead-free NTC thick films. The phases, microstructures and electrical properties of the thick-film NTC thermistors and ceramics were systemically analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and resistance-temperature tester. To reveal the conduction mechanism and characteristics of the thermistors, impedance spectroscopy (IS) of thermistor materials was systematically analyzed by impedance analyzer. The contributed elements in conducting process were fitted by a least square method.The cubic perovskite-structure BaFexSn1-xO3NTC thermistor ceramics were systematically studied. The lattice parameter (a) of the ceramics increases with the rise of Fe content. The larger-size grains and clear grain boundaries are observed in the thermistor ceramics sintered at high temperatures. The BaFeexSn1-xO3thermistor ceramics show good NTC thermistor behavior. With increasing Fe content, the room-temperature resistivity (ρ25) of BaFexSn1-xO3ceramics decrease to the2kΩ·cm by the0.2MQ.cm, and the themistor constant (β25/85) also falls to the3900K by the4900K. At lower Fe content, the magnitude of ceramic resistance is mainly determined by grain and grain boundary, and the ceramic-electrode interface resistance is very small. At higher Fe content, the total resistance of thermistor ceramics is completely attributed to the grain and grain boundary. In the analysis on the variable-temperature impedance of BaFeo.5Sn0.5O3thermistor ceramic, it is found that the grain resistance and grain-boundary resistance with high activation energy show the excellent NTC thermistor behavior. Further, the introduction of Sr at Ba-site leads to the decrease of ρ25,β25/85and Ea.The Ba1-mBimFeo.9Sn0.103ceramics still keep the cubic perovskite structure after the introduction of Bi element at Ba-site of BaFexSn1-xO3.With the rise of Bi content, the lattice constant (a) and the sintered temperatures of the Ba1-mBimFe0.9Sn0.103ceramics decrease. At the same time, the increasing P25,β25/85and Ea of the ceramics are in the range of1k-748M Q·cm,4464-9314K and0.385-0.803eV, respectively. The aging rate of the ceramics annealed at the atmosphere of N2, O2and air as the following orders: the aging rate of ceramic annealed at O2> the aging rate of ceramic annealed in air> the aging rate of unannealed ceramic>the aging rate of ceramic annealed at N2.The electrical properties of Ba1-mBimFe0.9Sn0.1O3thermistor ceramics are mainly from the contribution of grain boundary, ferroelectric grain and grain shell. The three elements show the different magnitude of electrical resistance at various Bi\Ba rate. For the compositions with m<0.2, the resistance of ferroelectric grain is relatively large. For the compositions with0.2≤m≤0.5, the grain boundary shows higher resistance value. When m≥0.6the larger grain-shell resistance occurs. The Ba-site at Ba1-mBimFe0.9Sn0.103ceramics was substituted for the minor Y element. The P25>,β25/85and Ea of the substituted ceramics increase with the rise of Y content.The p25,β25/85and Ea of BaFeo.4Sn0.6O3/BaBiO3composite ceramics, decreasing with the rise of BaBiO3content, are in the range of0.1-183.8kΩ·cm,2832-5450K and0.25-0.47eV, respectively.In order to obtain a low-resistance sintering additive of thick films, a novel monoclinic perovskite-type BaCoⅡxCoⅢ2xBi1-3xO3thick film were investigated. With the added Co3O4, the room-temperature resistivity of BaBiO3thick films sharply decreases to2Ω·cm by0.22MΩ·cm, and the thermistor constant down to1100K by3914K. The thermistor characteristics of the composite BaCoⅡ0.02CoⅢ0.04Bi0.9403thick films were studied after the addition of the BaSb0.04Sn0.96O3, ZrO2, MnO2and BaFeo.9Sn0.1O3, respectively.The high-performance BaCoⅡ0.02CoⅢ0.04Bi0.9403/Ba0.7Bi0.3Fe0.9Sn0.103NTC composite thick films were systematically studied for the first time. For the higher BaCo0.02ⅡCo0.04ⅢBi0.94O3content, the large melt is found in the composite thick films. With the reduction of BaCo0.02ⅡCo0.04ⅢBi0.94O3content in the thick films, the even, fine grains and the grain chains occur. Simulateneously, the adhsive force is weakened, the values of ρ25,β25/85and Ea increase. The adhsive force,ρ25,β25/85and Ea are in the range of760-880kg/cm2,35.5Ω·cm-1.34MΩ·cm,2067-6139K and0.177-0.527eV, respectively. Furthermore, the film-forming and electrical properties are greatly affected by the sintered temperatures and the holding time in the heat-treatment process. For the70%Ba0.7Bi0.3Fe0.9Sn0.1O3+30%BaCo0.02ⅡCo0.04ⅢBi0.94O3thick films annealed in various atmospheres, the aging values of the thick films annealed in N2, O2, air and un-annealed, are1.2%,12.3%,6.7%and4.5%, respectively. For the lower Ba0.7Bi0.3Fe0.9Sn0.1O3content, the conduction contribution of Ba0.7Bi0.3Fe0.9Sn0.1O3/BaCo0.02ⅡCo0.04ⅢBi0.94O3thick films is mainly made of grain boundaries and grains similar to that of the ceramics. The composite thick films show the short-range conduction mechanism at the lower Ba0.7Bi0.3Fe0.9Sn0.1O3content. With the further increase of Ba0.7Bi0.3Fe0.9Sn0.1O3content, the grain chains of composite thick films are the main conduction contribution and the thick films show the long-range conduction mechanism. In the grain chains, when the thick-film grains are made from the high-resistance thermistor phases, the total resistance of thick film is relatively larger. Once the mid-and low-resistance thermistor phases as the grains of grain chains, however, the resistance value of thick film show smaller.Furthermore, each of the Ag2O and BaSb0.04Sn0.96O3were added into the Ba0.7Bi0.3Fe0.9Sn0.1O3/BaCo0.02ⅡCo0.04ⅢBi0.94O3composite thick films. The addition of Ag2O and BaSb0.04Sn0.96O3don’t have too much effect on the electrical properties and the conduction mechanism of the thick films is similar to the previous chapater.