Study On NTC Characteristics Of The BaSnO₃ Ceramics

The BaSnO₃ ceramic, when being prepared in air, usually is an n-type semiconductor with low resistivity due to the large number of oxygen vacancies the material contained. Because of the high sintering temperature required, it was difficult to prepare BaSnO₃ with a dense crystalline structure, well-developed grains and clear grain boundaries by the conventional solid-state reaction method, and studies on this topic were rarely reported. In our laboratory, previously a process has been developed to fabricate BaSnO₃ ceramics with relative density of more than 95% and well-developed grains by the conventional solid-state-reaction method. BaCO₃ and SnO2, together with a few additives, were used as the raw material. Clear grain-boundary effect was observed in the electrical measurements when the material was doped with manganese and other acceptor impurities. These early studies provide the basis for the work that is to be described in this dissertation.The scope of this dissertation covers the study on the synthesis and NTC characteristics of the BaSnO₃ electrical ceramics:(1) BaSnO₃ ceramics with NTC characteristics were prepared by the tradition solid-state-reaction method. BaSnO₃ was identified as the main crystalline phase from the XRD spectrum. Morphological study on the samples using SEM showed dense crystalline structure, well-developed grains and clear grain boundaries.(2) The mechanism of the NTC characteristics of BaSnO₃ was studied and the Double-Schottky-Barrier model was used. The grains of BaSnO₃ are n-type semiconductors because of the presence of two kinds of defects: the oxygen vacancies and the tantalum substitutionals. On the other hand the acceptors segregate into the grain boundaries and give rise to acceptor interfacial states, so the Double-Schottky Barriers are formed near the grain boundaries. The NTC behavior of BaSnO₃ is the result of the thermal activation of the conduction-band carriers, which is different from the tradition NTC materials in which NTC characteristics derives from the polaron conduction.(3) The effect of Mn2+ doping on the NTC characteristics of BaSnO₃ electrical ceramics was investigated. It shows that with more manganese dopants, B gets bigger and the linearity of lnR-1/T relationship improved. But excessive manganese doping leads to high room-temperature resistivity. The optimal Mn-doping level is found to be 1.8mol% in order to achieve the best overall NTC characteristics: B=6049K with an excellent linearity in lnR-1/T, while the room temperature resistivity is 1.0×107??cm.(4) Instead of homogeneous doping of manganese impurities, BaSnO₃ electrical ceramics with good NTC characteristics were prepared by thermal-diffusion technique following the surface coating of ceramics with manganese-containing compounds. Both surfaces of the circular BaSnO₃ ceramic substrates were coated with Mn(NO₃)2. They were then processed at 1000℃, 1100℃and 1200℃for 2hrs, respectively. The test shows that samples processed at 1200℃have better NTC characteristics. In this paper, the thermal-diffusion process is analyzed in the framework of diffusion theory. The method was further studied by processing the coated samples at 1200℃for different periods of time. It showed that the quality of the samples was first improved by longer processing time (bigger B), but then the effect saturated and further processing resulted in a quick rise of room-temperature resistivity. The optimal thermal-treatment time was found to be around 90 minutes.(5) To achieve higher B value and lower room-temperature resistivity, the samples with single-side coating were studied. With the same process, the room-temperature resistivity of the samples with single-side coating is about half of the ones with both sides coating. But their B values and linearties are almost the same. In conclusion, experimental and theoretical studies on the NTC characteristics of n-type BaSnO₃ ceramics were performed. The samples with low room-temperature resistivities, high B values and good linearities were prepared by single-side Mn(NO₃)2 coating of BaSnO₃ substrates followed by thermal diffusion. This dissertation provides theoretical analysis and experimental evidence for the research on BaSnO₃ ceramics and new type of NTC ceramics.