In Suit Prepared Perovskite/tungsten Bronze Biphase Composites And Their Dielectric And Pyroelectric Properties

The performance of IR imagers is dependent on the pyroelectric materials used in the uncooled focal plane arrays (UFPAs). As we known, the strongest responsivity of the pyroelectric materials is generated near the Curie point; so, the imagers request a curious operating temperature in order to obtain the strongest responsivity. Furthermore, a composite material having several dielectric peaks, i.e. Curie temperature points, around room temperature will possess both a stronger pyroelectric effect and a wider temperature range near the ambient through the "composite effect". Both higher performance and temperature stability of the IR imagers would be obtained without any temperature stabilizer. And it is significant for IR imaging.The aim in this paper is to prepare the composite materials with higher pyroelectric properties through the composite of Ba1-xSrxTiO3(BST) and SrxBa1-xNb2O6(SBN), and to provide the foundation for developing new IR imagers with higher performance. In the present study, the composite ceramics B0.7S0.3T1-yNy with both the perovskite and the tungsten bronze phases were successfully prepared in situ by controlling some excessivecontents of components and traditional ceramic process in 0.7BaO·0.3SrO·(1-y)TiO2·yNb2O5 composite system. The relationships between the dielectric/pyroelectric properties and the microstructures of the composites B0.7S0.3T1-yNy were investigated by XRD, SEM, EDXA, impedance analyzer and the testing system for measuring pyroelectric current. Some interesting results and conclusions obtained in the work are as flows:(1) The results, about the investigation on preparation and formation of the composites B0.7S0.3T1.yNy, show that the perovskite BST (Nb) system is not stable any longer and the second phase tungsten bronze SBN appears while the excessive content of Nb2O5 is above 6 mol%, similarly, the tungsten bronze SBN(Ti) system is not stable any longer either and the second phase perovskite BST appears while the excessive content of (BaO+SrO) is above 11mol%. In the composite ceramics B0.7S0.3T1-yNy in which the SBN phase and BST phase coexist, the tungsten bronze phase exists in the state of SBN (Ti) solid solution while the perovskite phase almost exists in the state of pure BST or BST (Nb) solid solution doped only with little Nb2O5.(2) The dielectric properties of the composite ceramics B0.7S0.3T1-yNy have both characteristics of the perovskite phase and the tungsten bronze phase. The r and tan of theB0.7S0.3T1.yNy depend on the relative content of the perovskite phase as well as the solubility of Nb5+ ions in it. The phase transition temperature of the tungsten bronze phase depends on the solubility of Ti4+ ions in the phase and the relative content of the perovskite phase. The lowest transition temperature is around 200°C and it is about 90°C lower than the pure tungsten bronze phase SBN.(3) The microstructure of the composite ceramics B0.7S0.3T1.yNy consisted of two kinds of grains. The smaller polygonal grains belonged to the BST phase, and the larger ones to the SBN phase. The coexistence of the two phases inhibited the growth of the BST crystal. The density of microstructure of the composite ceramic was higher than that of both the pure BST and SBN sintered at same temperature and for same time, due to the void among the larger SBN grains were filled with the smaller BST grains.(4) A new formula is firstlyput forward to calculate the dielectric constant of a composite material with two phases prepared in situ, according to the experimental results about the composite ceramics B0.7S0.3T1-yNy and considering the effects of solid solution between the two phases.(5) The formation mechanism of the composite ceramics B0.7S0.3T1-yNy is investigated. The results revealed that the phase transition of the tungsten bronze phase increased as the content or the grain size of the perovskite phase increased and decreased as the grain size of the tungsten bronze phase increased. The defects between the grain boundaries of different phase increased when the tw...