Fabrication and Electrical Properties of Textured Sr0.53Ba0.47Nb2O6 Ceramics by Templated Grain Growth

Textured Sr_0.53Ba_0.47Nb₂O₆ ceramics with a relative density of >95% were fabricated using templated grain growth (TGG). Acicular KSr₂Nb₅O₁₅ template particles synthesized via a molten salt process were aligned by tape casting in a mixture of solid-state-synthesized SrNb₂O₆ and BaNb₂O₆ powders. The resulting ceramics possessed strong fiber texture along the polar axis ([001]) of the strontium barium niobate. Samples with 15.4 wt% templates attained a textured fraction of 0.82 after sintering at a temperature of 1450°C for 4 h. These materials showed peak dielectric constants of 7550 at 1 kHz, remanent polarizations of 13.2 μC/cm², saturation polarizations of 21 μC/cm² (60%–85% of the single-crystal value), piezoelectric strain coefficients of 78 pC/N (70%–85% of the single-crystal value), and room-temperature pyroelectric coefficients of 2.9 × 10⁻²μC·(cm²·°C)⁻¹ (52% of the single-crystal value). These results show that TGG is a viable option for accessing single-crystal properties in polycrystalline ceramics.     

Templated Grain Growth of <001> Textured PMN-28PT Using SrTiO3 Templates

Highly textured PMN-28PT (0.72Pb(Mg_1/3Nb_2/3)O₃–0.28PbTiO₃) ceramics were produced by templated grain growth on <001> oriented platelet-shaped SrTiO₃ template particles with an aspect ratio of 10–15. The templates were aligned in PMN-28PT matrix powder via tape casting and fired in an O₂–PbO atmosphere at 1150°C for up to 15 h. This resulted in textured ceramics with a 40 micrometer grain size and without residual templates. The volume fraction of textured material (  f  ) and the orientation parameter ( r ) were quantified by fitting X-ray diffraction rocking curve data to the March–Dollase equation. Processing conditions were optimized to achieve the best possible values of f and r for the chosen templates and matrix powder. A texture fraction of at least 81 vol% and an orientation parameter of 0.2 were achieved when all random matrix grains were consumed (a perfect textured ceramic would show a texture fraction of 100 vol% and an orientation parameter of 0).     

Preparation of 〈110〉-Textured BaTiO3 Ceramics by the Reactive-Templated Grain Growth Method Using Needlelike TiO2 Particles

Dense, highly 〈110〉-textured BaTiO₃ ceramics were prepared by the reactive-templated grain growth method. Needlelike TiO₂ (rutile) particles with their needle axis parallel to 〈001〉 were used as reactive template particles. Slurry containing an equimolar mixture of TiO₂ and BaCO₃ was tape cast to form a green compact, in which TiO₂ particles were aligned with their needle axis parallel to the casting direction. Calcination of the green compact changed TiO₂ particles into BaTiO₃ grains with their 〈110〉 direction parallel to the casting direction, for which the topotaxial relation of was responsible. Sintering yielded a dense, highly textured BaTiO₃ compact.     

Particle Orientation During Tape Casting in the Fabrication of Grain-Oriented Bismuth Titanate

Slurries containing platelike Bi₄Ti₃O₁₂ particles have been tape cast to prepare green sheets with aligned particles. The slurries contain well-dispersed particles and show nearly Newtonian flow behavior. The effect of slurry composition and casting conditions on the particle orientation has been examined. The particle orientation in the green sheet is determined mainly by powder content; other parameters, such as binder content, casting speed, and blade opening, have little effect. The interaction between particles is a main cause for particle alingnment. The slurry with a large powder content is favorable for preparing dense grain-oriented ceramics.     

Fabrication of Na0.5Bi0.5TiO3–BaTiO3-Textured Ceramics Templated by Plate-Like Na0.5Bi0.5TiO3 Particles

Textured 0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃ (NBT–6BT) ceramics were fabricated by templated grain growth (TGG) using anisotropically shaped Na_0.5Bi_0.5TiO₃ (NBT) templates. Platelet NBT was synthesized by the topochemical technique, using precursor Na_0.5Bi_4.5Ti₄O₁₅ (NBIT). The NBT particles have an average length of 10–15 μm and a thickness of 1 μm, which are suitable templates for obtaining textured ceramics (especially NBT-based ceramics) by the TGG process. This study revealed that the NBT templates are effective in inducing grain orientation in NBT–6BT ceramics. For NBT–6BT ceramics textured with 5 vol% NBT templates, a Lotgering factor of 0.87 and a d ₃₃ of 299 pC/N are given.     

High Thermal Conductivity in Silicon Nitride with Anisotropie Microstructure

Silicon nitride was fabricated by tape casting of α-Si₃N₄ powder with 5 wt% Y₂O₃ and 5 vol% rodlike β-Si₃N₄ seed particles, followed by tape stacking, hot pressing under 40 MPa, and annealing at 1850°C for 2-66 h under a nitrogen pressure of 0.9 MPa. Silicon nitrides fabricated by this procedure exhibited a highly anisotropic microstructure with large elongated grains (developed from seed particles) uniaxially oriented parallel to the casting direction. Thermal conductivities parallel to the grain alignment were much higher than those measured in other directions and exhibited high values of up to 120 W/(m.K). The anisotropic thermal conductivity of the specimen could be explained by the rule of mixture, considering that large elongated grains developed from seeds have higher thermal conductivity than a small-grained matrix.     

Tailoring particle alignment and grain orientation during tape casting and templated grain growth

The quality of crystallographic alignment in textured ceramics produced by tape casting and templated grain growth (TGG) has been little studied despite its demonstrated impact on magnetic, piezoelectric, and optical properties. Physical and crystallographic alignment of anisotropic template particles is shown to be directly linked to the casting rate, gap height, and casting viscosity during tape casting. These parameters are shown to affect the shape and magnitude of the shear rate profile under the doctor blade during casting which in turn causes a gradient in the torque acting on anisotropic particles. The magnitude of the torque, the time the slurry is exposed to torque during casting, and the ratio of casting height to template diameter are demonstrated to enable the particle alignment process to be tailored to produce well-aligned template particles. Crystallographic alignment of the textured ceramic was quantified by grain misalignment angle (full width at half maximum, FWHM) and degree of orientation (r) and is directly correlated with the degree of torque during casting. High-quality alignment (FWHM = 4.5°; r = 0.13) was demonstrated in the model TGG system consisting of submicrometer alumina and 5 vol% 11 μm diameter template platelet particles.     

Textured PMN–PT and PMN–PZT

A promising way to improve the performance of piezoelectric ceramics is grain orientation by templated grain growth. In this work lead-based piezoelectric ceramics Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃ (PMN–32PT) and Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃ (PMN–37PT–21PZ) ceramics were textured via templated grain growth process. For texturization (001)-oriented BaTiO₃ (BT) platelets (approximately 10 μm × 10 μm × 2 μm) were utilized as templates. The texturized ceramics were accomplished by aligning the templates by tape casting. The template growth into the matrix resulted in textured ceramics with Lotgering factors between 0.94 and 0.99 for both compositions. Consequences of the texture are enhanced dielectric and piezoelectric properties. Unipolar strain-field measurements of textured ceramics showed 0.25% strain s ₃₃ at 3 kV/mm. Large signal d ₃₃^* of up to 878 pm/V were determined directly from strain measurements. Compared with randomly oriented ceramics in texturized samples unipolar strain s ₃₃ and large signal d ₃₃^* was enhanced by a factor of up to 1.8.     

Effect of Particle Sizes of Starting Materials on Microstructure Development in Textured Bi0.5(Na0.5K0.5)0.5TiO3

Microstructure development in Bi_0.5(Na_0.5K_0.5)_0.5TiO₃ prepared by a reactive-templated grain growth process was dependent on the sizes of platelike Bi₄Ti₃O₁₂ (BiT) and equiaxed TiO₂ particles used as starting materials. Calcined compacts were composed of large, platelike template grains and small, equiaxed matrix grains, the sizes of which were determined by those of the BiT and TiO₂ particles, respectively. Texture was developed by the growth of template grains at the expense of matrix grains during sintering, and a new mechanism of grain growth was proposed on the basis of microstructure observation. The grain growth rate was determined by the template and matrix grain sizes, and a dense ceramic with extensive texture was obtained using small BiT and TiO₂ particles.     

Silicon Carbide Platelet/Alumina Composites: I, Effect of Forming Technique on Platelet Orientation

SiC-platelet-reinforced Al₂O₃-matrix composites were made by three different forming techniques, i.e., slip casting, tape casting, and dry compaction of a granulated powder. All samples were densified with hot pressing at 1650°C and 25 MPa for 0.5 h. The orientation of SiC platelets in the composites was studied before and after hot pressing using optical microscopy and a pole figure X-ray device. X-ray diffraction of the (0006) plane of silicon carbide (6H) was used to analyze the degree of preferred orientation. It was found that both tape casting and die pressing could give rise to preferred orientation in green bodies with the faces of SiC platelets parallel to the tape faces or perpendicular to the pressing direction, respectively. The preferred orientation in die-pressed samples also showed an increase with the increase of the compaction stress; however, this reached a saturation level at about 70 MPa in a similar way to the green density. Samples formed by slip casting gave a platelet orientation close to a random one in the green body. After hot pressing, preferred orientation was observed in both slip-cast and tape-cast samples with the faces of SiC platelets perpendicular to the direction of hot pressing. The effect of platelet size on the orientation was also investigated. The preferred orientation in platelet composites was found to yield higher toughness than the random state.     

Dielectric and Electromechanical Properties of Textured Niobium-Doped Bismuth Titanate Ceramics

Textured Nb-doped bismuth titanate ceramics (Bi₄Ti_{3− x /5}- Nb _{x /5}O₁₂, where x = 0.02) were fabricated by templated grain growth. It was found that the use of a fine precursor powder led to enhanced densification of the ceramic, while Nb doping reduced electrical conduction and dielectric loss, which enabled poling at high temperatures and high electric fields. Sintered tapes showed anisotropic dielectric and piezoelectric properties when measured parallel and perpendicular to the casting plane (e.g., the remanent polarization differed by more than a factor of 15 in the two directions). The piezoelectric constant parallel to the casting plane of the tape was ∼30 pC/N, or ∼77% of the single-crystal value. Thermal depoling studies demonstrated that high-temperature piezoelectric applications are possible up to ∼450°C in textured, doped bismuth titanate.     

Processing Strategy for Producing Highly Anisotropic Silicon Nitride

Silicon nitride with a preferred orientation of large elongated grains was obtained by tape casting of raw powder slurry seeded with rodlike β-Si₃N₄ particles, followed by a gas pressure sintering under 1 MPa nitrogen pressure. The large elongated grains developed from seeds lay in planes parallel to the casting direction in a two-dimensional distribution. Increased fracture toughness (11.1 MPa·m^1/2) and bending strength (1100 MPa) were achieved in the direction perpendicular to the grains alignment compared to specimens with a random distribution of elongated grains. Morover, the specimens exhibited a high Weibull modulus of 46 due to the uniform distribution of large grains.     

Tape Casting of Fine Alumina/Zirconia Powders for Composite Fabrication

Ceramic films, containing AI₂O₃, with up to 40 vol% ZrO₂, have been fabricated using the tape casting process. Finer powders (average mean diameter of 250–300 nm) than have generally been reported for tape casting were used in this study. The optimum formulation for tape casting is affected substantially by decreasing particle size. For example, the amount of dispersant needed is increased. Moreover, the amount of plasticizer/binder must be increased so as to maintain the solids content in the dried tapes below a critical level (about 55 vol% in this case), which decreases with particle size. Rheological studies on the effectiveness of menhaden fish oil and phosphate ester as dispersants show that phosphate ester can be used in lower concentrations, for the preparation of higher solids loading slurries, and was therefore selected for further study. The amount of dispersant required to obtain minimum slurry viscosity was found to be primarily dependent upon the effective particle surface area, defined as that available to the dispersant molecules. In the case of particles composed of agglomerated crystallites (such as the ZrO₂, powder used here), this may be considerably less than that measured by nitrogen absorption. Moreover, the porous internal structure of such powders is filled with solvent, which increases the effective solids loading of the slurry, and thus its viscosity. Particle morphology also influences the packing efficiency; i.e., the green density decreases as ZrO₂, is added.     

Texture Development and Microstructure Evolution in Liquid-Phase-Sintered α-Alumina Ceramics Prepared by Templated Grain Growth

Texture development in alumina that contains calcia and silica and has been templated with platelet-shaped α-Al₂O₃ particles has been evaluated. The texture fraction is shown to be related directly to template growth. Texture quality is controlled by the template concentration, decreasing at template concentrations of >10%, as a result of template–template interactions during tape casting. Almost fully textured alumina has been obtained at template concentrations of ≥20%. The growth of template grains is much more rapid in the radial direction and is shown to be inversely related to the thickness of the grain-boundary liquid. The activation energy for growth (376 kJ/mol) and the inverse relation with the grain-boundary thickness indicate that template growth in the radial direction is controlled by Al³⁺ diffusion.     

Fabrication of Highly Textured Fine‐Grained α‐Alumina by Templated Grain Growth of Nanoscale Precursors

[0001] textured alumina ceramics with a fine grain size were fabricated between 1400°C and 1600°C via templated grain growth (TGG) using fine alumina platelets (~0.6 and ~3 μm diameter) aligned by tape casting in either a 50 nm α‐Al₂O₃ matrix powder, or in a seeded boehmite sol. The 3 μm templates could be readily aligned by tape casting in both matrices (orientation parameters r = 0.27 and 0.18, respectively), whereas 0.6 μm diameter templates were well aligned in the seeded boehmite sol only (r = 0.29). Improved alignment in boehmite sols is attributed to inorganic gelation, resulting in a strongly pseudo‐plastic rheology that preserves template alignment against the influence of Brownian motion. The in situ formation of fine α‐Al₂O₃ matrix after transformation in the seeded boehmite system results in a higher driving force for TGG and improves texture development. The combination of 3 μm templates with a seeded boehmite matrix results in extremely high texture qualities (texture fraction f = 0.97–0.99, r = 0.17) while maintaining a relatively fine grain size (5–10 μm in diameter and 1.5–3 μm in thickness). Although undoped samples can be fully textured at 1600°C, adding as little as ~0.25 wt% CaO/SiO₂ dopant improves TGG kinetics and yields full texture at 1400°C.     

Tape-Casting Performance of Ethanol Slurries for the Processing of Textured PMN–PT Ceramics from Nanocrystalline Powder

The rheological behavior and tape-casting performance of ethanol-based Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) slurries is studied. A nanocrystalline powder obtained by mechanochemical activation was used. Suspensions were prepared up to solid contents of 32 vol%, significantly higher than those usually reported for nonaqueous systems. The feasibility of replacing toxic solvents commonly used for tape casting of functional ferroelectric materials, often toluene, with ethanol is demonstrated. The density, phases and microstructure of the green tapes and sintered ceramics are presented. Cube-shaped templates were successfully aligned during the tape casting for the processing of 〈001〉-textured PMN–PT ceramics by templated grain growth. An effect of the lamination of tape stacks on the template orientation is described.     

Highly Textured BaTiO3 via Templated Grain Growth and Resulting Polarization Reversal Dynamics

Samples of bulk textured polycrystalline BaTiO₃ ceramics were fabricated using a templated grain growth (TGG) approach in order to investigate effects of polycrystallinity and texture related to ferroelectric domain reversal under high‐power drive conditions. Barium titanate platelets were formed via two‐step topochemical conversion of bismuth titanate platelets grown via molten salt synthesis, then aligned via tape casting within a matrix of fine BaTiO₃ powder. The coarse‐grained parts showed a high degree of crystallographic texture after sintering. Combined with ceramics of similar density and polycrystallinity, but random orientation and commercial single‐crystal specimens, this sample set enabled direct isolation of crystallographic texture and polycrystallinity as the primary variables for high‐power polarization reversal studies. These studies have also demonstrated a link between grain size and polarization reversal time that strongly suggests that grain boundaries serve effectively as nucleation sites during the ferroelectric switching process.     

Topochemical Synthesis of Plate-Like Na0.5Bi0.5TiO3 from Aurivillius Precursor

Plate-like Na_0.5Bi_0.5TiO₃ (NBT) particles with perovskite structure were synthesized by topochemical microcrystal conversion from plate-like particles of layer-structured Na_0.5Bi_4.5Ti₄O₁₅ (NBIT) at 950°C in NaCl molten salt. As the precursors of NBT, plate-like NBIT particles were first synthesized by molten salt process by the reaction of Bi₄Ti₃O₁₂, Na₂CO₃, and TiO₂. After the topochemical reactions, layer-structured NBIT particles were transformed to the perovskite NBT platelets. NBT particles with a thickness of approximately 0.5 μm and a length of 10–15 μm retained the morphology feature of the precursor. High-aspect-ratio NBT platelets are suitable templates to obtain textured ceramics (especially NBT-based ceramics) by (reactive) template grain growth process.     

Synthesis of High Aspect Ratio Platelet SrTiO3

A method for synthesis of high aspect ratio platelet seeds by growth of SrTiO₃ on Sr₃Ti₂O₇ core particles is reported. The aim of this study was to identify and control the morphology and size of SrTiO₃ particles via molten salt synthesis. Platelet and tabular morphologies with rectangular faces were obtained using rutile and anatase, respectively. Platelet SrTiO₃ particles with an edge length of 10–40 μm and a thickness of 1–4 μm were obtained. High aspect ratios (edge length to thickness) of 7–10 were measured for platelet particles as opposed to lower aspect ratios of 2–4 for tabular particles. Highly anisotropic platelets are suitable template candidates to achieve textured ceramics.     

Mechanical Properties and Fracture Toughness of α-Al2O3-Platelet-Reinforced Y-PSZ Composites at Room and High Temperatures

YPSZ/Al₂O₃-platelet composites were fabricated by conventional and tape-casting techniques followed by sintering and HIPing. The room-temperature fracture toughness increased, from 4.9 MPa·m^1/2 for YPSZ, to 7.9 MPa·m^1/2 (by the ISB method) for 25 mol% Al₂O₃ platelets with aspect ratio = 12. The room-temperature fiexural strength decreased 21% and 30% (from 935 MPa for YPSZ) for platelet contents of 25 vol% and 40 vol%, respectively. Al₂O₃ platelets improved the high-temperature strength (by 110% over YPSZ with 25 vol% platelets at 800°C and by 40% with 40 vol% platelets at 1300°C) and fracture toughness (by 90% at 800°C and 61% at 1300°C with 40 vol% platelets). An amorphous phase at the Al₂O₃-platelet/YPSZ interface limited mechanical property improvement at 1300°C. The influence of platelet alignment was examined by tape casting and laminating the composites. Platelet alignment improved the sintered density by >1% d _th , high-temperature strength by 11% at 800°C and 16% at 1300°C, and fracture toughness by 33% at 1300°C, over random platelet orientation.