Ender Suvaci

Templated Grain Growth of Textured Piezoelectric Ceramics

ABSTRACT Crystallographic texturing of polycrystalline ferroelectric ceramics offers a means of achieving significant enhancements in the piezoelectric response. Templated grain growth (TGG) enables the fabrication of textured ceramics with single crystal-like properties, as well as single crystals. In TGG, nucleation and growth of the desired crystal on aligned single crystal template particles results in an increased fraction of oriented material with heating. To facilitate alignment during forming, template particles must be anisometric in shape. To serve as the preferred sites for epitaxy and subsequent oriented growth of the matrix, the template particles need to be single crystal and chemically stable up to the growth temperature. Besides templating the growth process, the template particles may also serve as seed sites for phase formation of a reactive matrix. This process, referred to as Reactive TGG (RTGG), has been used to obtain highly oriented Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6, and (Na1/2Bi1/2)TiO3-BaTiO3. Highly oriented Bi4Ti3O12, Sr2Nb2O7, CaBi4Ti4O15, Pb(Mg1/3Nb2/3)O3-PbTiO3, Sr0.53Ba0.47Nb2O6 and (Na1/2Bi1/2)TiO3-BaTiO3 ceramics have been produced by TGG. The resulting ceramics show texture levels up to 90%, and significant enhancements in the piezoelectric properties relative to randomly oriented ceramics with comparable densities. For example, piezoelectric coefficients of textured piezoelectrics are from 2 to 3 times higher than polycrystalline ceramics and as high as 90% of the single crystal values. In textured PMN-PT, a low field (< 5 kV/cm) piezoelectric coefficient (d 33) of ∼1600 pC/N was obtained with > 0.3% strain (at 50 kV/cm). The high field dielectric and electromechanical properties of textured perovskites are more hysteretic than those of single crystals, probably as a result of clamping by the residual template particles, residual random grains, the presence of non-ferroelectric second phases, and a wide orientation distribution. Lateral clamping of one grain by another may also be an important factor in fiber-textured samples. Means to further improve the quality of texture and thus properties of textured piezoelectric ceramics by TGG are presented.

Morphological control of particles

Abstract The objective of this review is to highlight the theoretical and practical aspects of particle morphological control. Materials with directional properties are opening new horizons in material science. Structural, optical, and electrical properties can be greatly augmented by the fabrication of composite materials with anisotropic microstructures or with anisotropic particles uniformly dispersed in an isotropic matrix. Examples include structural composites, magnetic and optical recording media, photographic film, and certain metal and ceramic alloys. The new applications and the need for model particles in scientific investigations are rapidly outdistancing the ability to synthesize anisotropic particles with specific chemistries and narrowly distributed physical characteristics (e.g. size distribution, shape, and aspect ratio). Anisotropic particles of many compositions have been produced but only a few (γ-Fe2O3 and AgI) are produced with any degree of chemical and physical control. These two examples are the result of literally decades of study. Unfortunately, the science and technology (mainly the technology) that have evolved are maintained as proprietary information. Thus, while we generally know what systems yield single crystal, anisotropic-shaped particles, we do not know how to make powders of these crystals with the desired control of shape uniformity, aspect ratio and phase composition. Particle shape control is a complex process requiring a fundamental understanding of the interactions between solid state chemistry, interfacial reactions and kinetics, and solution (or vapor) chemistry. During synthesis of other than a large single crystal the parameters controlling crystal growth must be balanced with the requirements for anisotropic powder nucleation and growth. Although there has been considerable progress in large single crystal growth and the synthesis of powders composed of monodispersed, spherical particles, these efforts have not often been transferred to the synthesis of anisotropic particles.

Reaction-based processing of textured alumina by templated grain growth

Highly textured, dense alumina ceramics were fabricated by a new processing route which utilizes a mixture of Al metal powder, alumina powder, alumina platelet (template) particles and a liquid phase former. The process involves dry forming the powder mixture (e.g. uniaxial pressing, and roll compaction) to align the plate-like template particles. The addition of a calcium aluminosilicate glass reduces constrained densification by the template particles and allows attainment of high density at ∼1450°C. The degree of orientation (i.e. r is 1 for a random sample and 0 for a perfectly textured material) and volume fraction of textured material, f, were measured by X-ray-based rocking curve technique and SEM-based stereological analysis, respectively. It has been shown that texture quality (the r parameter) is controlled by initial strain during forming, sintering time and temperature. In addition, alumina ceramics with the volume fraction of textured material ranging from 1 to ∼100% can be obtained.

Kinetics of template growth in alumina during the process of templated grain growth (TGG)

Kinetics of texture development in liquid-phase sintered alumina ceramics has been studied during templated grain growth (TGG); a technique for developing crystallographic texture in ceramic bodies via the grain growth of aligned anisometric particles in a dense and fine grain size matrix. It has been shown that significant template growth is observed only at high densities (∼91% TD). The template growth in the radial direction followed a cubic growth law just like the growth of a single crystal {1120} and {0112} planes during the initial stage of TGG, suggesting that diffusion is the rate-limiting step for radial growth.

Modeling Anisotropic Single Crystal Growth Kinetics in Liquid Phase Sintered α-Al2O3

AbstractSingle crystals of α-alumina with {0001}, {11

The Reaction‐Bonded Aluminum Oxide Process: I, The Effect of Attrition Milling on the Solid‐State Oxidation of Aluminum Powder

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The Reaction-Bonded Aluminum Oxide (RBAO) Process: II, The Solid-State Oxidation of RBAO Compacts

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