Synthesis And Preparation Of Bi_1.5ZnNb_1.5O₇ Ceramics

Microwave dielectric ceramics based Bi₂O₃-ZnO-Nb₂O₅(BZN) system have been widely noted as a promising low temperature cofiring ceramics system since they were explored. In the course of previous study on the cubic pyrochlore phase Bi_1.5ZnNb_1.5O₇, the powders were prepared by the conventional solid state method (CS). The solid state reaction lead to agglomeration and impurity easily, and the calcining tmperature of the powders prepared by CS is high (about 800℃). The capability of powders will affect on the bulk density and dielectric behaviors of ceramics. To overcome the shortcomings of CS, the Bi_1.5ZnNb_1.5O₇ powders was synthesized by hydrothermal method(HTM) and molten salt synthesis(MSS) in this paper, and the systemic investigations were focused on the effects of different HTM parameters on the phase and microstructure of the powders and the bulk density and dielectric behaviors of ceramics.The single-phase Bi_1.5ZnNb_1.5O₇ nanopowder was successfully synthesized by HTM from the starting materials: Bi(NO₃)₃·5H₂O, ZnO, Nb₂O₅ and the mineralizer: KOH. The results show that the hydrothermal conditions have obvious effects on the phase and the grain sizes of Bi_1.5ZnNb_1.5O₇ nanopowders, but not obvious on the morphology. TEM photographs reveal that the powders present the regularly granular shape and its sizes are about 30⁵0nm. With the increase of KOH concentrations and reaction times, the sizes of Bi_1.5ZnNb_1.5O₇ nanopowders initially became small, and then big, but the tendency of the specific surface areas was reverse when the molar ratio of Nb to Bi was two. With the increase of synthesis temperature, the sizes of the powders also became increasingly small, and the specific surface areas became big, but the tendency was not very obvious when the synthesis temperature is above 180℃. The maximal specific surface area of the nanopowders is 28.8 m²Â·g^-1, the minimal powders'size is 51 nm and the minimal grain size calculated by Scherrer equation is 43nm when the Bi_1.5ZnNb_1.5O₇ powder is synthesized under the hydrothermal conditions with a synthesis temperature of 220℃, a reaction time of 24 h and a KOH concentration of about 1.8 mol·L^-1.The phase of Bi_1.5ZnNb_1.5O₇ samples did not change when the ceramics was sintered using the compound of powders synthesized by HTM and CS. With the increase of X (wt%), the mass of nanopowders and sintering temperature, the dielectric constantsεinitially increased, and then decreased, and the change of dielectric losses tanδwas even reverse. The grain sizes of the ceramics samples became fined when the nanopowders synthesized by HTM was mixed with the powders synthesized by CS, but the bulk density decreased. However, when the X was 20 or 30, a few of grains grew unusually, and the dielectric behaviors decreased. As a whole, it can be obtained that the maximal dielectric constant is 148 and the minimal dielectric loss is 3.365×10-4 when X is 10.The phases ofαandβcould be obtained when the powders were synthesized by KCl molten salt synthesis at 800⁹50℃using the component oxides as raw materials. The phase ofαcould be obtained at 950¹000℃. The effect of synthesizing temperature on the morphology and sizes of the powders is obvious but the mass ratio of raw materials to salt and reaction times are not. With the increase of synthesis temperature, the powders appear to be grained shape and its sizes increase. The size of the powders synthesized by MSS at 1000℃for 2h is about 2⁵μm when the weigh ratio of the raw materials to salts is one. Comparing with CS, the powders synthesized by MSS does not appear the tendency of agglomerate, but its sizes were bigger and synthesis temperature were high than those by CS. The bulk density of ceramics samples synthesized by MSS corresponds to those by CS, however, the sintering temperature lightly increased. The dielectric constant is about 155 and the dielectric loss is about 3.1×10-3.