Effect of PbO evaporation on the composition and dielectric properties of PbO-MgO-Nb 2 O 5 based dielectrics
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M. F. Yan and W.W. Rhodes AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 15 September 1989; accepted 7 November 1989) 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: Nb 2 O 5 phase diagram near the stoichiometric composition of Pb(Mgi/3Nb2/3)O3 (PMN). While a ternary system of PbTiO3-Pb(Coi/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 CC 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 12 000 and 18 500, 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.
I. INTRODUCTION Lead magnesium niobate has been widely investigated in both the single crystal1"5 and ceramic6"15 forms. The ceramic PMN generally had a smaller dielectric constant than the single crystal value of 20000 to 25000 measured at 1 kHz. It is generally assumed that a nonferroelectric pyrochlore phase is responsible for this decrease. A two-step process was developed1617 to successfully prepare pyrochlore-free ceramic PMN. While the dielectric constant was increased to 18 000 by this method, the sintering temperature of the ceramic PMN remained in excess of 1100 °C. For PMN dielectric to be used for low firing and high dielectric constant capacitor applications, modified PMN compositions and/or processes have been developed to decrease the sintering temperature to between 900 and 1000 °C and to tailor the properties to specifications. Many modified PMN compositions through solid solutioning with other lead-based relaxors have been discussed.18 Excess PbO and excess MgO are also used to assist in densification and/or to eliminate the formation of pyrochlore phase. These results were reviewed in Ref. 18. Alternatively, it has also been reported that small particle size coupled with fast firing resulted in 97% theoretical density at sintering temperatures of 1000 °C.19 J. Mater. Res., Vol. 5, No. 3, Mar 1990
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In two previous papers,20'21 we have described the preparation and dielectric properties of ceramic ternary compositions in the PbTiO3-Pb(Co1/3Nb2/3)O3-PMN system. The compositions were written with
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