Role of excess PbO on the microstructure and dielectric properties of lead magnesium niobate
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Stoichiometric and 2 wt. % excess lead oxide containing lead magnesium niobate (PMN) ceramics have been prepared by partial oxalate route. Dielectric measurements with frequency showed a typical relaxor behavior for stoichiometric PMN, while PMN with excess PbO shows a scatter in the dielectric curves at all temperatures above Tc. Under the same processing conditions, the dielectric constant (Kmia) decreases drastically from 16,300 to 9500 at 1 KHz for PMN without and with excess PbO, respectively. Microstructure studies revealed a second phase (unreacted PbO) segregated in the grain boundaries for excess PbO samples. A careful analysis of the data on dielectric properties and phases present coupled with microstructural detail indicate that the second phase in the grain boundary has a pronounced effect on the dielectric properties and not the pyrochlore phase.
I. INTRODUCTION Lead-based relaxors are the newest and the most widely investigated electroceramics in the last decade for Multilayer Capacitors (MLC). Synthesis of this family of compounds has been found to be most difficult due to the pyrochlore phase formation, mainly from the loss of PbO during sintering, irrespective of the methods1"11 used. The perovskite relaxor ferroelectric lead magnesium niobate [Pb(Mgi/3Nb2/3)O3] exhibits unusually high dielectric constant, low sintering temperature, and broad dielectric maxima at Tc. The stable pyrochlore phase in PMN is also known to be detrimental for dielectric properties.12 Initial efforts were directed toward minimizing this pyrochlore phase. Present authors have recently observed13 that for a relaxor material (PMN) with a small amount of pyrochlore phase (2-4%), the dielectric constant increases from 4000 to 24,000 when the grain size increases from 0.5 /mm to 3.5 /im, indicating that grain size offsets any reduction in the dielectric constant caused by the pyrochlore phase. This implies that in the stoichiometric PMN for a given sintering temperature, grain size, and density, PbO loss will prove to be an important factor in deciding the dielectric properties. Addition of excess PbO has been the most widely used method to compensate PbO loss. However, none of these reports discusses the effect of excess PbO on the dielectric properties. For example, Lejeune and Boilet3 and Kang and Yoon14 have emphasized addition of excess PbO in their work, and both groups observed that excess PbO not only aids in the elimination of pyrochlore phase but also enhances density through liquid phase sintering. However, the excess PbO segregates in the grain boundary and leads to a reduction in dielectric constant. This reduction in dielectric constant was explained on the J. Mater. Res., Vol. 10, No. 4, Apr 1995
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basis of "grain volume effect" on the high permittivity of PMN grain.15 Further, the logarithmic law for a mixture of dielectrics also points out that the lower dielectric constant of PbO (=20) will have a higher influence on the permittivity of PMN than that of
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