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 (N...

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.

Complex impedance analysis of polycrystalline Pb2Bi3LaTi5O18, prepared by a high-temperature solid-state reaction technique has been carried out. XRD analysis indicated the formation of a single-phase orthorhombic structure. Impedance plots were used as a tool to analyse the behaviour of the sample as a function of frequency and temperature. The bulk resistance has been observed to decrease with rise in temperature showing a typical negative temperature coefficient of resistance (NTCR) type behaviour like that of semiconductors. The ac impedance studies revealed the presence of grain boundary effect at 450°C and showed polydispersive non Debye-type dielectric relaxation. The frequency dependent ac conductivity at different temperatures indicated that the conduction process is thermally activated process. The activation energy for bulk (0.67 eV) and grain boundary (0.73 eV) was estimated from the temperature variation of respective conductivities.

Complex impedance studies on tungsten-bronze electroceramic: Pb2Bi3LaTi5O18

We review the current status of the photorefractive tungsten bronze family crystals in terms of their growth problems and applications, with special emphasis on the current results for the Sr1−xBaxNb2O6 solid-solution system. Ferroelectric morphotropic phase-boundary materials are discussed as an appropriate goal for future development.

Progress in photorefractive tungsten bronze crystals

Strontium barium niobate single crystals, growth and ferroelectric properties

We review the chronological development of ferroelectric tungsten bronze Sr1-x Ba x Nb2O6 (SBN) single crystals in terms of crystal growth and applications ranging from pyroelectric to surface acoustic wave (SAW) to nonlinear optics, with specific emphasis on the compositions SBN:60, SBN:75 and SBN:50. In addition to a number of bulk crystal applications, these crystals are also now being used as substrates for the growth of various lattice-matched thin films, including ferroelectric and superconducting oxide materials.

Tungsten bronze Sr1−xBaxNb2O6: a case history of versatility

Ferroelectric SBN single crystals Sr61Ba39Nb2O6 approximately one inch in diameter have successfully been grown Piezoelectric, high frequency dielectric and surface acoustic wave properties of the SBN crystals have been examined and are reported

Tungsten bronze family crystals for acoustical and dielectric application

Single crystals of Sr2KNb5O15 (SKN), approximately 5-8 mm in diameter and 10-20 mm long, have successfully been grown by the Czochralski technique. The crystal has a ferroelectric tetragonal tungsten bronze structure with a Curie temperature of about 150°C. The dielectric and electromechanical coupling constants for this crystal have been established: the polar axis dielectric constant, K33, exceeds 20,000 at the Curie temperature while the coupling constants are K33 = 0.52 and K31 = 0.19, respectively. High frequency dielectric properties of SKN samples were also determined between 90-100 GHz. These measurements indicate that the bulk of the polar axis permittivity has relaxed below 90 GHz, implying that the low frequency dielectric response is not soft-mode controlled.

Low and high frequency dielectric properties of ferroelectric tungsten bronze Sr2KNb5O15 crystals

Microwave measurements on Sr.61Ba.39Nb2O6 single crystal samles show a large nonlinear susceptibility at 58 GHz and a large refractive index (∼ 18 to 20 along the polar axis) at 35 GHz. For propagation along the direction of an applied electric field in a sample cut at 45° to the polar axis, the change in index with applied field is found to be at least 1.2 × 10−6 m/V, corresponding to a half-wave voltage of 2000 V.

Microwave dielectric properties of Sr.61Ba.3Nb2O6 single crystals at 35 and 58 GHz

Permittivity and dielectric loss have been measured for a number of single crystal samples of barium strontium potassium sodium niobate (BSKNN) in three frequency ranges: 0–1 MHz, 30–40 GHz, and 90–100 GHz. This work is part of an ongoing study of millimeter wave properties of ferroelectrics with potential applications in active components such as phase shifters, modulators, and switches. BSKNN has been selected as typifying one extreme in the tungsten bronze family, where all 15- and 12-fold coordinated cation sites are occupied. Measurements on these crystals show significantly lower millimeter wave absorption than is found in strontium barium niobate (SBN) crystals grown by the same techniques. The permittivities along the principal crystal axes showed a substantial decrease with increasing frequencies over the range of observations, which may be indicative of a relaxation mechanism occurring in the GHz frequency region.

Dielectric properties of ferroelectric tungsten bronze Ba2-xSrxK1-yNayNb5O15 crystals at RF and millimeter wave frequencies

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