Two‐Dimensional Orientation in Bi4Ti3O12 Prepared Using Platelet Particles and a Magnetic Field

Tailoring the crystallographic orientation in piezoelectric ceramics is very useful for improving their properties. Orientation in ceramics can be controlled by templated grain growth, hot forging, etc. We have focused on using a strong magnetic field for the crystallographic orientation even in diamagnetic ceramics. In a previous study, although only a one‐directional orientation could be controlled by these methods, it was difficult to control the multi‐axis orientation in the ceramics. In this study, we demonstrated that alignments of the c‐axis and the <100> axis in Bi₄Ti₃O₁₂ piezoelectric ceramics were controlled by using a strong magnetic field as well as platelet particles. We also estimated the degree of orientation by an electron back scattering diffraction analysis. When the magnetic field was applied to the platelet particles, appropriately 80% of the grains were aligned with the tilt angle made by the c‐axis and the vertical direction less than 10° and 73% of grains were oriented with the angle between the <100> axis and the magnetic field less than 10°.     

Piezoelectric K0.5Na0.5NbO3 Ceramics Textured Using Needlelike K0.5Na0.5NbO3 Templates

Improved performance by texturing has become attractive in the field of lead‐free ferroelectrics, but the effect depends heavily on the degree of texture, type of preferred orientation, and whether the material is a rotator or extender ferroelectric. Here, we report on successful texturing of K_0.5Na_0.5NbO₃ (KNN) ceramics by alignment of needlelike KNN templates in a matrix of KNN powder using tape casting. Homotemplated grain growth of the needles was confirmed during sintering, resulting in a high degree of texture parallel to the tape casting direction (TCD) and the aligned needles. The texture significantly improved the piezoelectric response parallel to the tape cast direction, corresponding to the direction of the strongest <001>_pc orientation, while the response normal to the tape cast plane was lower than for a nontextured KNN. In situ X‐ray diffraction during electric field application revealed that non‐180° domain reorientation was enhanced by an order of magnitude in the TCD, compared to the direction normal to the tape cast plane and in the nontextured ceramic. The effect of texture in KNN is discussed with respect to possible rotator ferroelectric properties of KNN.     

Fabrication of c-axis–oriented AlN bulk ceramics using a rotating magnetic field

Controlling the crystallographic orientation in bulk ceramics is expected to improve their properties that depend on the axis of the crystal structure. The development of crystallographic orientation using a magnetic field has been reported in various types of bulk ceramics with anisotropic crystal structures. In this study, c-axis–oriented AlN bulk ceramics were prepared by slip casting in a rotating high magnetic field. The c-axes of 61% of the grains were aligned within 20°. Due to this orientation, the thermal conductivity of AlN was improved. A thermal conductivity difference of approximately 14% at room temperature was observed for the samples with different orientation axes, which is higher than its intrinsic anisotropy.     

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.     

Effect of sintering additive on crystallographic orientation in AlN prepared by slip casting in a strong magnetic field

The preparation of oriented AlN bulk ceramics with and without additives was achieved by slip casting in a high magnetic field. The a and b axes of the AlN were aligned parallel to the direction of the magnetic field. The degree of crystallographic orientation was controlled by the viscosity of the slurry and the grain growth during sintering attributed to the sintering additives. The mechanical properties of the textured AlN depended on the direction of the crystallographic orientation.     

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.     

The orientation mechanism of (Ca,Sr)Bi4Ti4O15 ceramics prepared by slip casting in high magnetic field and subsequent sintering

A high magnetic field of 10 T was introduced into a processing of slip casting for fabricating (Ca,Sr)Bi₄Ti₄O₁₅ (abbreviated as CSBT) ceramics. Feeble magnetic CSBT particles in green compacts were partially aligned through rotating a gypsum mold containing the CSBT slurry in the magnetic field. The green compacts were sintered at 1200 °C for different time without magnetic field. With increasing of the sintering time, the preferable orientation degree of CSBT ceramics rapidly went up at the initial stage, and then slowly increased at the medium and final stages. The mechanism of the orientation degree increasing during the sintering can be attributed to a processing in which large oriented particles coarsen small randomly oriented particle.     

Fabrication of textured bismuth titanate by templated grain growth using aqueous tape casting

Textured bismuth titanate (Bi₄Ti₃O₁₂) ceramics were fabricated by templated grain growth (TGG), using plate-like Bi₄Ti₃O₁₂ particles prepared by a molten salt method as the templates. The templates were aligned in the fine-grained matrix by aqueous tape casting with their major surface parallel to the casting plane. Effect of sintering conditions on the grain orientation in the material was investigated. It was found that the degree of grain orientation (Lotgering factor, f) increased with the increase in sintering temperature, soaking time and heating rate. High Lotgering factor (f⩾0.92) can be obtained through careful control of the sintering parameters. The textured Bi₄Ti₃O₁₂ ceramics showed a high anisotropy in its dielectric properties in the directions parallel and perpendicular to the casting plane.     

Effect of sintering conditions on microstructure orientation in α-SiC prepared by slip casting in a strong magnetic field

In general, the mechanical and physical properties of a crystal depend on the direction of the crystal axis. The controlled development of the crystallographic texture in ceramics is very useful for improvement of their properties. The preparation of the textured SiC polycrystal was achieved by slip casting in a strong magnetic field. The effects of the sintering conditions and sintering additives on the degree of orientation in the SiC were investigated. The pressing during the liquid phase sintering prevented the development of a texture in the SiC prepared by slip casting in a strong magnetic field.     

Effect of wet-jet milling on the c-axis orientation of hydroxyapatite ceramics fabricated using a strong magnetic field

Controlling the crystal orientation of hydroxyapatite, an inorganic material that is a major component of human hard tissues is important to fabricate better artificial bones and artificial tooth roots. To obtain highly oriented hydroxyapatite ceramics under a strong magnetic field, a good dispersion of the raw materials in the slurry must be obtained. This study investigates the effect of wet-jet milling of a slurry on the orientation of hydroxyapatite ceramics fabricated using a strong magnetic field. Although the prolonged ball milling with ZrO₂ balls of the raw powder fractures the primary particles of hydroxyapatite, wet-jet milling is used to successfully pulverize agglomerated hydroxyapatite raw powder without changing the morphology of the primary particles. Evidently, ceramics with a highly oriented c-axis of hydroxyapatite are obtained by molding the wet-jet milled slurry in a strong rotating magnetic field. They exhibit anisotropy in fracture toughness, and the fracture toughness calculated from the crack length perpendicular to the rotating axis is higher than that calculated from the crack length parallel to it. Furthermore, these values are higher than those of randomly oriented hydroxyapatite ceramics, which may be owing to the crystal orientation dependence of the fracture toughness of the hydroxyapatite grains and grain boundaries.     

Microstructure and Anisotropic Properties of Textured ZrB2 and ZrB2–MoSi2 Ceramics Prepared by Strong Magnetic Field Alignment

Dense, highly textured ZrB₂ and ZrB₂–MoSi₂ ceramics were fabricated via a strong magnetic field alignment method followed by spark plasma sintering. Unlike with the previous studies, which only focused on the alignment of single‐phase particles, both ZrB₂ and MoSi₂, which exhibited a magnetic anisotropy, have been aligned in this study. The alignment of MoSi₂ in the same direction of ZrB₂ enhanced the degree of orientation of ZrB₂, decreased the grain size, but increased the aspect ratio of the platelet ZrB₂ grains. The microstructure and anisotropic mechanical properties as well as the oxidation resistance in different directions were discussed.     

Fabrication of textured ferroelectric ceramics by magnetic alignment via gelcasting

Grain-oriented ferroelectric ceramics have attracted more interest recently because they may provide near single crystal properties. In the present study, a novel process combining magnetic alignment and gelcasting was explored to prepare grain-oriented ferroelectric ceramics with different crystal structures. In a strong magnetic field, ceramic particles in slurry were aligned by the magnetic force and then locked in situ by polymerization via a gelcasting technique. This process was found effective for ferroelectric ceramics with a bismuth layer structure (Bi₄Ti₃O₁₂) and tungsten bronze structure (Sr_0.5Ba_0.5Nb₂O₆). The sintered samples show highly anisotropic structure and enhanced physical properties. However for perovskite structured ferroelectric ceramics (BaTiO₃), the green compact shows grain orientation, while after sintering the sample become random again.Thus for certain materials using the conventional ceramic processes, i.e., using conventional starting powders, gelcasting under strong magnetic fields (10 T) and pressure-less sintering, the preparation of dense grain-oriented ceramic materials is possible.     

Fabrication and electrical properties of textured (Na,K)0.5Bi0.5TiO3 ceramics by reactive-templated grain growth

Textured (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) ceramics with a relative density of >94% were fabricated by reactive-templated grain growth. Plated-like Bi₄Ti₃O₁₂ template particles synthesized by the NaCl–KCl molten salt process were aligned by tape casting in a mixture of original oxide powders. The effect of sintering temperature on the grain orientation and electrical properties of textured NKBT ceramics were investigated. The results show that the textured ceramics have a microstructure with plated-like grains aligning in the direction parallel to the casting plane. The degree of grain orientation increased at increasing sintering temperature. The textured ceramics show anisotropic electrical properties in the directions parallel and perpendicular to the casting plane. The dielectric constant parallel to {h 0 0} plane is three times higher than that of the perpendicular direction in textured NKBT ceramics. The optimized sintering temperature is 1150 °C where the maximum dielectric constant is 2041, the remnant polarization is 68.7 μC/cm², the electromechanical coupling factor (k₃₁) and the piezoelectric constant (d₃₃) amount to 0.31 and 134 pC/N, respectively.     

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.     

Electron spin resonance of transparent alumina ceramics fabricated by spark plasma sintering

Transparent α‐alumina ceramics are fabricated using spark plasma sintering. Paramagnetic defects related to the optical properties of the ceramics have been investigated using electron spin resonance (ESR) analyses. An isotropic ESR signal at g = 2.003 (S = 1/2) with a linewidth of 0.5 mT is formed during sintering. The g = 2.003 signal intensity has a weak correlation with the absorbance in the visible region but does not correlate with the real in‐line transmission (RIT) at 650 nm. An ESR signal with a fine structure attributed to Fe³⁺ was detected in both the α‐Al₂O₃ starting powder and the sintered ceramic samples. The degree of c‐axis orientation of the grains has been determined using the Fe³⁺ signal intensity, which depends on the angle between the directions of the c‐axis and the applied magnetic field. The ESR analysis indicated that the c‐axis tends to be oriented in the direction of the sintering pressure. The degree of c‐axis orientation was found to correlate with the RIT in highly densified ceramics.     

Synthesis of rod-like ZrB2 powders

Rod-like ZrB₂ powders were synthesised at 1500°C in vacuum by boro/carbothermal reduction using ZrO₂, B₄C and graphite as the starting materials. During the heating process, the ZrB₂ grains primarily grow along the c axis to form a rod-like morphology without any heterogeneous catalyst. The final products are pure rod-like ZrB₂ particles, which are thought to be promising starting powders to prepare high performance ultrahigh temperature ceramics with unique microstructures such as textured one through tape casting process.     

Porous PZT Ceramics with Aligned Pore Channels for Energy Harvesting Applications

In this study, aligned porous lead zirconate titanate (PZT) ceramics with high pyroelectric figures‐of‐merit were successfully manufactured by freeze casting using water‐based suspensions. The introduction of aligned pores was demonstrated to have a strong influence on the resultant porous ceramics, in terms of mechanical, dielectric, and pyroelectric properties. As the level of porosity was increased, the relative permittivity decreased, whereas the Curie temperature and dielectric loss increased. The aligned porous structure exhibited improvement in the compressive strength ranging from 19 to 35 MPa, leading to easier handling, better processability and wider applications for such type of porous material. Both types of pyroelectric harvesting figures‐of‐merit (F_E and F′_E) of the PZT ceramics with a porosity level of 25–45 vol% increased in all porous ceramics, for example, from 11.41 to 12.43 pJ/m³/K² and 1.94 to 6.57 pm³/J, respectively, at 25°C, which were shown to be higher than the dense PZT counterpart.     

Hydrothermally synthesized BaTiO3 textured in a strong magnetic field

Hydrothermally synthesized BaTiO₃ was colloidally consolidated by slip casting and by electrophoretic deposition in a high strength magnetic field of 17.4 T. Textured BaTiO₃ was successfully prepared by both processing techniques, with the {001} plane aligning normal to the direction of the magnetic field. The initial alignment in the green powder compact was preserved and enhanced after sintering from 1400 °C to 1500 °C. The sintering temperature hardly influenced the degree of texture in slip cast samples but did influence the grain size of all BaTiO₃ ceramics and the degree of texture in the electrophoretically deposited samples. The BaTiO₃ ceramics processed in the strong magnetic field showed crystallographic texture but not morphological, so the microstructure did not show any anisotropy. The highest Lotgering factor (0.85) was measured for the ceramics made by electrophoretic deposition and sintered at 1500 °C.     

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.     

Enhanced ionic conductivity of aluminum tungstate by crystallographic orientation in a strong magnetic field

The ionic conduction of multiply charged ions, rather than singly charged ions, is beneficial for energy storage and sensor applications. The low mobility of multiply charged ions is one important obstacle to the implementation of these applications. Chemical methods, such as doping and solid solution formation, have been used to improve ionic conductivity. However, the apparent performance of ceramic electrolytes can be improved by the crystallographic alignment of anisotropic grains. In this study, crystal-oriented aluminum tungstate ceramics were processed by slip casting in a strong magnetic field. The b-axis- and c-axis-oriented aluminum tungstate ceramics can be produced by this technique. The orientation of grains along the b-axis could enhance ionic conductivity by at least 1.77 times compared to that of a randomly oriented sample and 2.13 times compared to that of the c-axis-oriented sample. The results of this study suggest that this method can improve the ionic conductivity of an anisotropic material using polycrystalline processing instead of difficult single-crystal synthesis techniques.