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Effect of PbO evaporation on the composition and dielectric properties of PbO-MgO-Nb2O5 based dielectrics
Published online by Cambridge University Press: 31 January 2011
Abstract
We have studied the weight loss during sintering and the resulting phase constituents and dielectric properties of a series of compositions in the lead-rich region of the PbO:MgO:Nb2O5 phase diagram near the stoichiometric composition of Pb(Mg1/3Nb2/3)O3 (PMN). While a ternary system of PbTiO3–Pb(CO1/3Nb2/3)O3–PMN was used, only the PMN component was varied in composition. The observed weight loss was ascribed to PbO evaporation from the nonstoichiometric PMN component. It was found that the sintered PMN compositions at about 1000°C correspond to the stoichiometric composition with varying amounts of excess MgO. These sintered compositions can be calculated by taking the Nb ion as the controlling cation in forming the PMN unit cell, from which the excess MgO can be determined. The excess PbO in the starting compositions, above the amount needed to form the sintered compositions of Pb(Mg1/3Nb2/3)O3 plus excess MgO, serves as a densification aid during the sintering process. Subsequently, the excess PbO is lost through evaporation. The peak dielectric constant of these compositions varies between 12000 and 18500, with the highest value attained in the sintered composition corresponding to stoichiometric PMN plus 0.229 mole of excess MgO. The optimum processing conditions for this composition are also identified.
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