Formation Mechanisms of Perovskite Pb(Ni 1/3 Nb 2/3 )O 3 in Reaction-sintering Process
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Formation Mechanisms of Perovskite Pb(Ni1/3Nb2/3)O3 in Reaction-sintering Process Yi-Cheng Liou, Yi-Chen Huang and Chi-Ting Wu Department of Electronic Engineering, Kun-Shan University of Technology, Tainan Hsien 710, Taiwan, R.O.C.
ABSTRACT Formation mechanisms of perovskite Pb(Ni1/3Nb2/3)O3 (PNN) in the reaction-sintering process were investigated. Only the reflections of raw materials were detected at 400oC. For pellets heated to 500oC, pyrochlore phase Pb2Nb2O7 (P2N2) are found. Cubic Pb3Nb4O13 (P3N4) pyrochlore phase formed and became the main phase in pellets heated to 600oC. After being heated to 800oC, Pb3Nb2O8 (P3N2) pyrochlore phase formed in the pellets. As the pellets were heated to 900oC, the peak of NiNb2O6 columbite phase appeared. In pellets heated to 950oC, the weak peaks of PNN perovskite phase appeared. This means the perovskite phase formed at a temperature between 900oC and 950oC. As the pellets were heated to 1000oC, a large amount of PNN was formed and became dominant. The PbO, NiNb2O6 and Pb3Nb2O8 phases disappeared. The P3N4 content increased with temperature in pellets heated to 1050-1200oC. For pellets heated to 1250oC, the P3N4 content decreased significantly.
INTRODUCTION Lead nickel niobate Pb(Ni1/3Nb2/3)O3 (PNN) is a member of lead-based complex perovskite ceramics. It exhibits a broad maximum in the dielectric constant. PNN single-crystals showed typical relaxor ferroelectric behavior with dielectric constant ~2500 at room temperature and ~4000 at –120oC under 1 kHz [1]. As lead titanate (PbTiO3 or PT) is added, the Curie temperature of the solid solution PNN-PT shifts to a higher temperature. The maximum dielectric constant also increases to values of 10000-20000 [2]. Monophasic perovskite PNN is very difficult to obtain by the conventional mixed-oxide method due to the appearance of stable pyrochlore phases (e.g., Pb3Nb4O13 and Pb2Nb2O7) during calcination. [3]. Veitch reported pure phase perovskite PNN ceramic could be prepared via the columbite precursor method [4]. This method was first proposed by Swartz and Shrout to prepare single-phase perovskite Pb(Mg1/3Nb2/3)O3 (PMN) [5]. Two calcination stages were involved, MgNb2O6 columbite formed first and followed by the formation of PMN perovskite. Liou and Wu proposed a simplified columbite route to produce single-phase perovskite PMN ceramics [6]. The mixture of MgNb2O6 and PbO was pressed into pellets and sintered directly. The second calcination and pulverization steps in the columbite routes were bypassed. Reaction-sintering process is a simple and effective route to synthesize ceramics. The calcination step is bypassed and the mixture of the raw materials is sintered directly. Kong and Ma proposed PZT ceramics prepared by sintering the oxide mixture directly [7]. Almost at the same time, Liou et al. proposed reaction-sintering process in preparing PMN and Pb(Fe0.5Nb0.5)O3 (PFN) ceramics [8-12]. Without any calcination stage involved, the mixture of PbO, Mg(NO3)2 (or Fe(NO3)3) and Nb2O5 was pressed and sintered directl
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