Research On Micro/Nano Semiconductive Ceramics And Their Sensitive Components

Semiconductive ceramics are generally inorganic non-metal polycrystalline materials with semindonductive properties.Their sensitive components have found applications in many high-tech fields such as microelectronic,photoelectronic and laser technology. Recently,the rapid development of microelectronic technology has registered increasing research interests in micro/nano semincounductive ceramics.This disseration was to develop micro/nano seminconductive ceramics and their sensitive components by using nanophase semiconductive ceramic powders,where the key fabrication technics were systematically investigated.The conduction mechanism of micro/nano semiconductinve ceramics was also discussed in order to provide both experimental and theoretical basis for the development of novel sensitive components.Firstly,attempts were made to synthesizing seminconductive ceramic nanopowders controllablly.The basic principle and characteristics of wet-chemcial methods such as sol-gel method,polymer network method and hydrothermal synthesis were briefly reviewed.A lot of experiments were conducted to optimize the synthesis process,resulting in uniform tin oxide nanopowders(even low as 4.1 nm) and barium titanate nanopowders (below 40 nm) with slight agglormerization and good thermal stablility,which were much desirable for micro/nano semincoundutive ceramics.After a careful study of the fabrication techniques such as slurry preparing, screen-printing,drying and sintering processes,we prepared nanocrystalline tin oxide thick-film sensors based on the as-synthesized nanopowders,the sensing performances of which toward hydrogen sulfide gas were much better considering the sensitivity,selectivity, response properties and operation temperature(around 150℃).A power law between the sensitivity and the partial pressure of hydrogen sulfide has been obtained.At same time, most favorable sensing behaviors were observed for the sensor samples based on tin oxide nanopowders synthesized by polymer network mehod,which might be attributed to the loose and porous microstructure with good uniformity.Meanwhile,the technical approaches to the realization of fine-grained mircostrucure were explored by studying the semincounctive characteristics and conduction mechanism of barium titanate ceramics.After optimizing the material composition and fabrication conditions in the water-based tape casting and sintering process,chip thermistors with fine-grained microstructre were developed based on barium titanate nanopowders synthesized by the sol-gel method,where adequate concentrations of yttrium element was introduced in the presintering process as the donor,while manganese element was introcueced in the sintering process as the acceptor,separately.Results indicated that twice doping was more appropriate for the inhibition of grain growth and the enhancement of electrical properties.Also,increasing the presintering temperature properly was helpful to to the reliazation of fine-grained microstucture when sintered at relatively tow temperature. The typical samples sinterd at 1280℃have smaller grain size of about 3.0μm,with room-temperature resistivity of about 360Ω·cm and the resistance jump of above 10~4.Finally,theoretical analysis was presented concerning the influence of grain size on the conduction properties of fine-grained barium titanate seminconductive ceramics.After the discussion of the microstructue and conduction model of polycrystalline barium titanate ceramics,an inverse relation between the resistance and the grain size of barium titante ceramics was calculated by using a tetrakaidecahedron model.Then.the properties of grain boundaries were discussed as far as the fine-grained barium titanate ceramics were concerned.With the decrease of the grain size,the barrier layer induced by the diffusion of Ba-vacancies from the boundary into the bulk of the grain becomes so narrow.As a result, the barrier effect may lead to the increase of the resitivity of the grains.Besides,the tunneling current caused by the quantum tunneling effect may lead to the decrease of the reistance jump and withstand voltage.