Fast firing of lead magnesium niobate at low temperature
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Fast firing of lead magnesium niobate at low temperature Dipika Saha, A. Sen, and H. S. Maiti Central Glass & Ceramic Research Institute, Calcutta 700 032, India (Received 14 November 1994; accepted 21 July 1995)
A fast-firing technique for the sintering of lead magnesium niobate relaxor ceramics at relatively low temperature has been described. In this process, the samples containing excess PbO (up to 5 wt. %) are directly introduced into a furnace maintained at a temperature of 950 ±C and kept there for 15 –80 min, followed by a postsintering annealing treatment at 800 ±C for 10 h. The importance of fast heating as well as annealing treatment has been justified. The sintered samples are near-phase-pure perovskite materials showing high bulk densities (.94%), uniform and dense microstructure, and satisfactory dielectric properties (K max . 13,000). The technique is simple and economic, does not require any controlled atmosphere, and minimizes hazards from lead volatilization.
I. INTRODUCTION
Current demands of the multilayer ceramic capacitor (MLC) industries are oriented toward the development of components with increased volumetric efficiency of capacitance as well as reduced fabrication cost. Ferroelectric relaxor materials have received a great deal of interest because of the possible advantages1 they offer over traditional dielectric compositions such as modified barium titanate. The perovskite lead magnesium niobate Pb(Mg1/3 Nb2/3 )O3 (hereafter designated PMN), was first synthesized by Soviet workers in the late 1950s.2 However, its importance as a dielectric for MLC’s has been realized only very recently. PMN-based materials show high dielectric constant values, and the maximum of permittivity for polycrystalline PMN lies between 12,000 and 15,000.1 A limitation to the utilization of ceramic PMN in device applications has been the lack of a simple and reliable fabrication technique. First, the preparation of PMN is complicated by the formation of a lead niobate based pyrochlore phase (P3 N2 ) during the initial stages of reaction between the constituent oxides.3,4 The subsequent transformation of the pyrochlore phase to perovskite is sluggish and necessitates the use of repeated calcinations at relatively high temperatures for long times. This processing problem has been greatly reduced by using the “Columbite Precursor” method5–7 where MgO (from magnesium hydroxycarbonate, for example) and Nb2 O5 are prereacted to form MgNb2 O6 , followed by its reaction with PbO to get the perovskite PMN. It has also been reported5,8 that excess MgO (up to 5 mol %) totally eliminates the pyrochlore formation. In an earlier investigation,7 the present authors observed that complete conversion to perovskite phase was possible even without adding excess MgO, provided MgNb2 O6 prepared by calcining MgO and Nb2 O5 was 932
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J. Mater. Res., Vol. 11, No. 4, Apr 1996
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phase pure. This condition could be satisfied by calcining t
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