Calcining and its Effects on Sintering and Properties of Lead-Zirconate Ceramics.

The research sequence of PZT formation during the calcination of mixed oxides has been studied using dilatometry, quantitative x-ray diffraction analysis, and microstructural characterization. The differences in the sequences proposed earlier (e.g., the formation of the PbO(,ss) phase) have been traced to the differences in the bulk density of the powder compacts. Interesting changes in the microstructure accompanied by expansion and contraction of the compacts corresponding to the different stages of reaction occur during calcining. The effect of the calcining conditions on the sintering and properties of PZT ceramics has been investigated. The role of (i) the different intermediate phases obtained by varying the temperature and periods of calcining, and (ii) the compact density during calcining on the nature of densification is explained. The isothermal and the dilatometric studies of sintering indicate the following two distinct phenomena of densification. (1) Inhomogeneous densification due to the density changes and chemical inhomogeneities in the partially calcined samples: The expansion occurring during the PZT formation results in a chemically inhomogeneous ceramic with large pores, which densifies on further heating inhomogeneously within the ceramic regions, leaving large pores behind. Fired ceramics from the uncalcined and partially calcined mixes have lower densities (60%-75% theoretical) and hence poor electro-mechanical properties. (2) Reactive sintering during the final stages of PZT formation during calcining: The rapid shrinkage above 800(DEGREES)C implies enhanced densification due to chemical reaction and phase transformations. This phenomenon has been used to reactively sinter PZT ceramics at temperatures ((TURN)1150(DEGREES)C) much lower than the conventional conditions (1300(DEGREES)C) after partially calcining the oxide mixture to the maximum expansion point (840(DEGREES)C). The electromechanical properties of the resulting dense fine grained ceramics are comparable to the conventional sintered samples. The enhanced densification is attributed to the chemical reaction and phase and morphological transformations occurring during the early stages of sintering. The compact density during calcining does not seem to play a very dominant role in the densification and properties of PZT at the temperatures and periods of calcining studied.