E. Roncari

A microstructural study of porous piezoelectric ceramics obtained by different methods

Piezoelectric porous lead zirconate titanate (PZT) ceramics prepared by different methods have been examined from a microstructural point of view. The effect of polymer volume and properties and sintering temperature on microstructure of samples and on their physical properties has been investigated. The wide range of pore volumes and pore size distributions was obtained by three different procedures: (a) reduction of the sintering temperature of the die pressed PZT samples; (b) use of a different forming technique like tape casting, starting from slurries with high volume of organic content; (c) addition of different volume concentrations of organic polymer to the PZT powder. It has been found that varying process parameters like quantity of polymer and sintering temperature changes both final porosity as well as pore size distribution, introducing dispersion in physical property values. The differences between the microstructural aspect of samples prepared by different methods and their influence on the physical properties of the samples are presented and discussed.

Processing and characterization of high Qm ferroelectric ceramics

Abstract Use of ferroelectric ceramics in applications like piezoelectric transformers was made possible by the development of new materials with high electromechanical coupling coefficients and high mechanical quality factor. “Hard’’ ferroelectric ceramics of complex composition based on lead zirconate titanate with Nb, Mg, Mn and Li additives have been prepared. The perovskitic phase was produced by solid phase reaction of the oxides. The crucial role played by the intermediate mixing and grinding procedures in the assessment of the final properties of the material was investigated. Densification up to approximately the theoretical density value was achieved. The polarization was obtained by subjecting the samples at 30xa0kVxa0cm−1 poling electric field, in a silicon oil bath heated at 120°C. Their structural and morphological properties were checked by X-ray diffraction analysis and scanning electron microscopy. Ferroelectric and piezoelectric properties were determined in agreement with IEEE measurement standards. The optimized samples presented very high quality and electromechanical coupling factors, together with small dielectric loss.

Pyrochlore phase and microstructure development in lead magnesium niobate materials

Abstract Microstructure characteristics of powder and sintered samples of lead magnesium niobate (PMN) have been compared and related to the presence or not of pyrochlore phase. Two kinds of samples obtained from two different batches of powders, batch A and B, containing 1 and 0 vol.% of pyrochlore phase respectively, were analysed. The two powders show the same morphology. The sintered samples, with pyrochlore content around 13 vol.% for samples from batch A and 0 vol.% for samples from batch B, exhibit totally different microstructures The grain size increases drastically from about 1 μm for samples with pyrochlore to 4–5 μm for pyrochlore free samples although starting from powders with same granulometry. The maximum dielectric constant of samples as fired, ground and annealed were measured and related to microstructures and amount of pyrochlore. On the basis of these results and data reported in literature, mostly relating dielectric properties to stoichiometry of the starting powder, it has been hypothesised that different sintering mechanisms come into play and govern the microstructure. Consequently the dielectric properties obtained were correlated to the sintering mechanism occurred rather than to the absolute value of pyrochlore phase present, as its commonly accepted.

Porous piezoelectric ceramic hydrophone

The construction and evaluation of a hydrophone based on porous piezoelectric ceramics with high dhgh figure of merit (FOM) is described. It has been shown that, in order to improve the hydrophone signal-to-noise ratio, a piezoelectric material with a high FOM should be employed. A porous piezoelectric material has been prepared by mixing calcined lead zirconate titanate (PZT) powder with fine particle starch powder. Square plate samples have been cold pressed from this material, which were then heated to eliminate the organic component, sintered, electroded, and poled in a high electric field. An optimum pore volume fraction of approximately 40% has been selected in order to obtain materials with high piezoelectric coefficients and reasonably good mechanical resistance. For this composition a hydrostatic figure of merit of approximately 10−11u200am2/N has been obtained that is a few orders of magnitude higher than traditional piezoceramics. Square plate elements were assembled in a planar hydrophone which wa...

PZT-based suspensions for tape casting

Abstract A PZT powder of composition Pb 0.988 (Zr 0.52 Ti 0.48 ) 0.976 Nb 0.024 O 3 was prepared by conventional mechanical mixing of the raw materials in water, freeze-drying and calcination. Sedimentation tests of four deflocculants in an azeotropic mixture of methyl ethyl ketone/ethanol were carried out to check the dispersing effect. Attention was focused on a phosphate ester based deflocculant. Electrical conductivity and electrophoretic mobility were investigated. Slurries for tape casting were prepared with commercial binder and plasticizer and different volumes of solid. The rheological behaviour was recorded as yield stress values and flow curves. The slurries were tape-cast and technological results compared with previous characterization results.

Direct synthesis of PMN samples by spray-drying

Abstract The processing and characterisation of Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) materials, obtained either by spray-drying the solution of the precursors or by the conventional “columbite” method, were investigated and the morphological and micro-structural characteristics were compared. The acid solution of ammonium-peroxo-niobium complex, magnesium and lead nitrates was spray-dried and the precursor powder obtained was calcined at different temperatures ranging from 350 to 900xa0°C. The morphologies and the XRD patterns of the powders were compared. The calcined powders exhibited a pyrochlore phase above 400xa0°C converting into an almost pure perovskite phase at 800xa0°C. The powder calcined at 350, 500 and 800xa0°C were sintered at different temperatures, ranging from 950 to 1150xa0°C, always resulting in a pure perovskite PMN material. The XRD patterns of as-fired surfaces of samples sintered at 950 and 1050xa0°C showed an unwanted PbO phase together with the main PMN, nevertheless this secondary phase is not present in the ground surfaces. The high reactivity of sprayed powder is reflected in the formation and densification of pure perovskite PMN material with a faster process as regards the conventional one; in particular samples of about 96% theoretical density were obtained starting from the amorphous powder calcined at low temperature (350xa0°C) through a reaction sintering process. Furthermore, due to the better flowability of the spray-dried powder, the cold consolidation process is highly improved and no binder addition to powder is necessary.

Processing of a multilayer bender type actuator

Abstract A PZT material with high d 31 was developed and the processing of a bending type multilayer actuator by stacking and co-firing platinum electroded tape cast layers was investigated. The thermocompression step, as well as the binder burnout heating profile, proved to be critical and a tempering process was introduced that prevents the microcracks formation during densification. All the processing steps were optimized and prototype actuators made of 11 layers, each layer being 50 μm thick, were fabricated. The final microstructure was studied as well as the no-load deflection at increasing driving voltage.

Tape Casting of Functionally Graded AlN-SiC-MoSi2 Composites

In the view to develop a novel class of electroconductive ceramics where the conductive phase is molybdenum disilicide, composites with AlN-SiC-MoSi2 at 55/15/30 and 60/20/20 vol% concentration respectively were produced by tape casting and sintering. Two compositions were designed in order to tailor the electrical resistivity of functionally graded components, which are constituted by two or more portions of the studied composites. In order to optimise the tape casting process, the non-aqueous ceramic suspensions were investigated controlling binder and plasticizer content and the binder to plasticizer ratio. Compositions of the slurries were related to the green and final density and to the microstructural features of mono and multilayers samples. Controlled debonding and sintering tests were carried out under nitrogen atmosphere at temperatures of 1850°C, with appropriate heating gradients, protecting the samples in a powder bed. Density and microstructure of the sintered samples were examined and correlated to the processing conditions.