Gary L. Messing

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.

Piezoelectric properties of zirconium-doped barium titanate single crystals grown by templated grain growth

Single crystals of Ba(ZrxTi1−x)O3 were grown by templated grain growth (TGG). Millimeter size single crystals of Ba(ZrxTi1−x)O3 were produced by heating a BaTiO3 crystal in contact with a sintered polycrystalline matrix of 4.5, 5.0, or 8.5 mol % Zr-doped barium titanate for 30 h at 1350 °C. To facilitate boundary migration, the ceramic compact was made 3 mol % TiO2 excess. The 4.5 and 5.0 mol % Zr-doped crystals were orthorhombic at room temperature, and for a pseudocubic (001) orientation, they showed remanent polarizations of 13 μC/cm2 and a high field d33 of 340–355 pC/N. The 8.5 mol % Zr-doped crystal [again oriented along the pseudocubic (001)] was rhombohedral at room temperature with a remanent polarization of 10 μC/cm2. A k33 value of 0.74 from resonance measurements was observed for the 4.5 mol % Zr-doped crystal.

Piezoelectric properties of 〈001〉 textured Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics

The piezoelectric properties of (1−x) Pb(Mg1/3Nb2/3)O3–xPbTiO3 (x=0.3–0.35), ceramics with a high degree of 〈001〉 fiber texture were investigated for possible actuator applications. Piezoelectric coefficients (d33) in excess of 1200 pC/N associated with strain levels up to >0.3% were observed in samples prepared by a reactive templated grain growth process. No excess PbO was used in the starting composition. A high degree of fiber texture was achieved using 〈001〉 oriented BaTiO3 template particles in a fine-grained precursor for the PMN–32PT matrix. High densities together with texture resulted in a significant increase in strain levels and d33 values compared to their polycrystalline counterparts. Peak dielectric constants on the order of 22 000 with losses of ∼2% and well-saturated hysteresis loops with a Pr∼27 μC/cm2 were recorded on the textured samples. These domain engineered, textured ceramics have tremendous potential for high-performance actuators.

Modeling of Solid Particle Formation During Solution Aerosol Thermolysis: The Evaporation Stage

The evaporation state of solution aerosol thermolysis (SAT) was modeled to study the effect of various parameters on solid particle formation by solute precipitation. A comparison of the characteristic time constants for various processes demonstrated that droplet shrinkage and solute diffusion are the slowest processes, and that the fast processes, i.e., vapor diffusion and heat conduction in the gas phase and the liquid phase, can be assumed to have reached steady state. Differential equations for these faster processes were thus simplified and were solved numerically along with a modified solute diffusion equation, using an explicit first-order finite difference scheme. The computations were done until the solute concentration at the droplet surface reached the critical supersaturation. Then, if the solute concentration at the droplet center is higher than the equilibrium saturation, volume precipitation is proposed to occur. Solutes with a large difference between critical supersaturation and equilibr...

(Reactive) Templated Grain Growth of Textured Sodium Bismuth Titanate (Na1/2Bi1/2TiO3-BaTiO3) Ceramics—I Processing

Textured (Na1/2Bi1/2)TiO3-BaTiO3 (5.5 mol% BaTiO3) ceramics with <100>pc (where pc denotes the pseudocubic perovskite cell) orientation were fabricated by Templated Grain Growth (TGG) and Reactive Templated Grain Growth (RTGG) using anisotropically shaped template particles. In the case of TGG, molten salt synthesized SrTiO3 platelets were tape cast with a (Na1/2Bi1/2)TiO3-5.5 mol%BaTiO3 powder and sintered at 1200°C for up to 12 h. In the RTGG approach, Bi4Ti3O12 (BiT) platelets were tape cast with a Na2CO3, Bi2O3, TiO2, and BaCO3 powder mixture and reactively sintered. The TGG approach using SrTiO3 templates resulted in >90% texture along [001] whereas the RTGG approach using BiT templates resulted in 80% texture. The grain orientation distribution along the textured direction, as measured by X-ray rocking curve, showed a full width at half maximum of ∼8° and a texture fraction of 80%.

(Reactive) Templated Grain Growth of Textured Sodium Bismuth Titanate (Na1/2Bi1/2TiO3-BaTiO3) Ceramics—II Dielectric and Piezoelectric Properties

Abstract(Na1/2Bi1/2)TiO3-BaTiO3 (<6.5% BaTiO3) ceramics with <001>pc orientation were fabricated by Templated Grain Growth (TGG) or Reactive Templated Grain Growth (RTGG) using tabular SrTiO3 template particles. The maximum electrically-induced strain was 0.26% at 70 kV/cm. d33 coefficients over 500 pC/N were obtained for highly textured samples (f∼ 90%) when driven at high electric fields. Under these conditions, the materials show considerable hysteresis in the strain—field response, even after poling. Berlincourt piezoelectric coefficients for the same samples gave d33 of 200 pC/N.

Alpha alumina transformation in seeded boehmite gels

Abstract The kinetics and surface area changes during the γ to α-Al 2 O 3 transformation sequence of an α-Al 2 O 3 seeded boehmite sol-gel are compared to an unseeded boehmite sol gel. The seeded gels exhibited increased kinetics, however the activation energy for the γ to α-Al 3 O 3 transformation was lowered only 17% indicating the α-Al 2 O 3 transformation is growth controlled. Transmission electron microscopy showed that each α-Al 2 O 3 seed acts as a multiple nucleation site for the θ to α-Al 2 O 3 transformation. The higher surface area of the seeded boehmite results from the prevention of vermicular microstructure development. Thus, by seeding boehmite sol gels with α-Al 2 O 3 the growth process during transformation to α-Al 2 O 3 is controlled resulting in a fine, uniform microstructure which sinters at significantly lower temperatures.

MATERIALS SCIENCE : Toward Pore-Free Ceramics

Efforts are under way to create perfectly dense ceramics for use in applications ranging from lasers to health care.

Dielectric and piezoelectric properties of 〈001〉 fiber-textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 ceramics

The 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 (PMN–32.5PT) ceramic composition (with 1 wt. % excess PbO) was fiber textured in the 〈001〉 direction by the templated grain growth process using 5 vol % oriented {001}-BaTiO3 platelet crystals as the templates. The templated ceramics annealed at 1150 °C for 5 h attained texture fractions as high as 0.9. The fiber-textured samples showed an increase in the piezoelectric, electromechanical coupling, and compliance coefficients when poled and measured in the 〈001〉-textured direction. The low drive field (<5 kV/cm) d33 coefficients in the 〈001〉, measured directly from unipolar strain-field measurements, were ∼1150 pC/N. This d33 coefficient is 1.2–1.5 times greater than randomly oriented samples. The poled emax and ert for a 0.9-textured PMN–32.5PT ceramic were 21 500 and 2450, respectively. Factors limiting further property improvements are discussed.

⟨001⟩ textured (K0.5Na0.5)(Nb0.97Sb0.03)O3 piezoelectric ceramics with high electromechanical coupling over a broad temperature range

⟨001⟩-oriented (K0.5Na0.5)(Nb0.97Sb0.03)O3 (KNNS) ceramics with a narrow orientation distribution (full width at half maximum=7.0°) were produced by templated grain growth using NaNbO3 templates. Excellent electromechanical properties were obtained from −70 °C to the polymorphic phase transition (PPT) at 160 °C. Textured KNNS ceramics show very high electromechanical coupling factors kp=0.64 and k31=0.37, high piezoelectric constants d33=208–218 pC/N and d31=−82 pC/N, and modest strain hysteresis (6.3%) at room temperature. These properties are superior to those of randomly oriented KNN-based ceramics with similar PPT temperatures.