Additions of excess PbO to the perovskite Pb ((Mg,irNbz,s)o.u.Tio tn]Or solid solution enhanced the formation of a liquid phase at 840"C, which served us a densification aid for the ceramics. The liquid phase allowed elimination of pores and promoted grain growth during sintering. With additions of 1 to 2 wt% excess PbO, densities in excess of 97% of theoretical were obtained at a sintering temperature of' 950°C. The peak dielectric constants of the resulting ceramics were over 18 000 at 30 "C and dissipation factors less than I % . Additions of PbO in excess of 2 wt% resulted in inferior dielectric properties due mainly to the dilution of the ferroelectric phase. RLAXORS based on the perovskitePb(Mg,,,Nb,,,)0,-PbTi03 solid solutions exhibit excellent dielectric properties and are of current interest for multilayer capacitors. Previous studies'.' in this system have been primarily concerned with processing characteristics and dielectric properties of the solid solutions in which it has been demonstrated that both the starting materials and sintering conditions greatly influence the densification process and dielectric properties of the resulting ceramics. However, it has been pointed out' that the densification behavior at elevated temperatures is mostly controlled by solidstate processes in which rapid elimination of pores and sintering into dense ceramics are difficult to achieve. More recently, it has been r e p~r t e d~.~ that excess PbO can be used as a sintering aid for the processing of Pb(Mgl ~,NbZ,,)0,-PbTi03-based capacitors, Compositions containing as high as 6 wt% excess PbO were sintered at teniperatures between 900" and 1000°C. The resulting ceramics showed high densities and improved dielectric properties. However. the data published in these studies could not explain clearly the effect of excess PbO on the sintering behavior and dielectric properties of Pb(Mg liiNbZ,3)03-based ceramics.T h e purpose of the present investigation was to examine the role of excess PbO on the densification behavior of Pb(Mgl,,Nb,,,)Oi-PbTi03-based solid solutions with particular interest in liquid formation in the system. The effect of the liquid phase on the microstructure devel-

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