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Bulk ceramics made with PMN-PT nanopowders

Piezoelectric (1-x) Pb (Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) bulk ceramics with x=0.2, 0.3, 0.35 and 0.4 have been processed from perovskite phase nanopowders synthesised by direct mechanochemical activation of precursors. This procedure, unlike solid state reactions at high temperature, allows the one step synthesis of the perovskite phase from the constituent oxides, and provides high chemical homogeneity as compared with the widely used columbite route (two successive solid state reactions). Systematic studies of their sintering behaviour have been carried out at temperatures between 900 and 1250 degrees Celsius. Conventional sintering and hot pressing were explored. A PbO atmosphere was found to be necessary to limit lead oxide volatilisation and to avoid the formation of second phases at the surface when temperatures above 1000oC were used. PbZrO3 (PZ) and PMN-PT packing were tested for creating such atmosphere. Ceramics sintered at 1200 degrees Celsius with PZ packing presented densities of ~90% and a grain size of ~4mm. Decrease of the sintering temperature from 1200 to 1150oC resulted in a reduction of density and of grain size (from 4.0+/-1.0 to 2.6+/-1.0 microns for x=0.35). Increase to 1250 degrees Celsius caused a drastic decrease of density and the development of large cubic grains (15±5 microns, also for x=0.35). Use of PMN-PT instead of PZ for packing increased density at 1200 degrees Celsius up to ~95%, but decreased grain size. Hot pressing at 900 degrees Celsius and 25 MPa provided ceramics with hardly porosity and submicron (0.1-0.5 microns) grain size, and increase of the temperature to 1000oC resulted in the appearance of porosity and caused limited grain growth. Differences with x (composition) were not significant. Therefore, processing of series of PMN-PT ceramics with increasing x and similar microstructure, porosity and grain size, on which the effect of structure (composition) on properties can be isolated, from these powders was feasible and was achieved. Processing of series of PMN-PT ceramics with a given x and increasing grain size, on which the effect of crystal size on properties can be isolated, was also done.

The electrical, mechanical and electromechanical properties of these two series have been described by using a number of techniques. These were the dielectric permittivity and losses as a function of temperature and frequency by conventional impedance analysis, ferroelectric hysteresis loops as a function of frequency, the low frequency Young´s modulus and mechanical losses as a function of temperature, amplitude and frequency by dynamical mechanical analysis in three points bending, the s11 and s12 mechanical compliances and d31 piezoelectric coefficient in complex form, and so losses, as a function of temperature by analysis of piezoelectric radial resonances, and direct measurement of deformation under high fields by a linear variable differential transducer. Main features of these ceramics are their high compositional homogeneity and crystallographic quality. They are then very suitable for fundamental studies in the PMN-PT system, such as the relaxor to ferroelectric phase transition and its dependence on composition and crystal size, the ferroelectric phases present across the solid solution and their relationship to the macroscopic electric, mechanical and electromechanical properties, and the behaviour of the mechanical and piezoelectric losses, which are associated with the dynamics of domains. As a matter of fact, these bulk ceramics from powders synthesised by mechanochemical activation presented better defined features associated with these phenomena than columbite samples reported in the literature.

An article on this result was prepared and has been recently published (M. Algueró, C. Alemany, B. Jiménez, J. Holc, M. Kosec and L. Pardo, “Piezoelectric PMN-PT ceramics from mechanochemically activated precursors”. Journal of the European Ceramic Society 24:6 937-940 (2004)). A second article is in preparation, initially thought for Journal of the American Society (“Processing and properties of piezoelectric Pb (Mg1/3Nb2/3)O3-PbTiO3 ceramics from powders synthesised by mechanochemical activation”), and at least two more manuscripts, specifically dealing with properties, will be prepared in the next months.

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Instituto de Ciencia de Materiales de Madrid - CSIC
28049 Madrid
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