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The reactions occurring during sintering of stoichiometric BaTi03 with small additions of LiF were studied at temperatures between 700" and 900°C. BaTi03 reacts with LiF to form a cubic solid solution and Li2Ti03. During sintering, the cubic solid solution coexists with Li2Ti03 and forms a liquid phase at 7 4 0 ' 5°C . The occurrence of a liquid phase at this temperature results in an enhancement of the sintering process and leads to the development of a highly dense microstructure.INTERING of BaTiO, using LiF as an adseveral years. The primary interest in this area arises from the fact that a small addition of L F allows the formation of a liquid phase significantly below the normal sintering temperature of BaTiO,, causing a substantial reduction of the processing temperature for BaTi03-based electronic ceramics. The beneficial effect of L F additions to the sintering of BaTi03 was first reported by Walker et al., who found that 0.5 to 3.0 wt% LiF aided densification of BaTiO, at a much lower temperature than could be achieved without additives. Amin et al. and Anderson et al. have sintered BaTiO, with minor additions of LiF at temperatures below 900°C using long firing periods and obtained BaTiO, capacitors with improved dielectric properties. In a more comprehensive study, Haussonne et ~1 .~have reported the effect of 1 to 2 wt% L F additions on the sintering characteristics and dielectric properties of BaTiO, with variable Ba/Ti ratio and observed that the densification behavior and dielectric properties of the ceramics depend largely on the presence of a pseudocubic perovskite phase, the formation of which requires an excess of Ba. A significant weight loss due mainly to the volatilization of large amounts of both Li and F was observed during sintering which caused a considerable shift of the Curie temperature of BaTiO,. The densification of LiF-fluxed BaTiO, with variable Ba/Ti ratio has also been studied by Tolino and B l~m ,~ who pointed out that the initial Ba/Ti ratio and LiF addition are independently important in determining the densification of BaTiO,. In a recent publication, Desgardin et al. have reported the use of BaLiF3 as a sintering aid which was found to provide a better control of the overall s ' ditive has received much attention for CONTRIBUTING EDITOR-R. M. SPRIGGS

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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