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.     

Grain-orientation-engineered PMN-10PT ceramics for electrocaloric applications

The electrical properties of ferroelectric materials depend on their crystal orientations. The 〈001〉 and 〈111〉 textured 0.90Pb(Mg_1/3Nb_2/3)O₃–0.10PbTiO₃ (PMN-10PT) ceramics were synthesized using a template grain growth method and conventional solid sintering process. The Lotgering factors of the 〈001〉 and 〈111〉 textured PMN-10PT ceramics with 5% anisotropic BaTiO₃ (BT) precursors were 95% and 87%, respectively. The influence of grain orientation direction on dielectric properties was investigated, revealing that the permittivity of textured PMN-10PT ceramics (ε_r〈001〉 = 12200, ε_r〈111〉 = 11800) was lower than that of random ceramics (ε_r = 18700). Electrocaloric (EC) responses of the random and textured samples were analyzed using an indirect polarization-deduced method based on Maxwell's equations. When the applied field was 50 kV cm⁻¹, the 〈111〉 textured ceramics exhibited an enhanced adiabatic temperature change of 1.30 K, which was 20% higher than that of the random samples. In addition, the EC performance of the 〈111〉 oriented samples was significantly improved compared to that of the 〈001〉 oriented ones. This work characterizes the grain-orientation-dependent EC responses in the PMN-PT system, which would be a promising approach to EC response improvement in ferroelectric ceramics.     

Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors

Ca₃Co₄O₉ ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca₃Co₄O₉ ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)₂ as a template and CaCO₃ as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m⁻¹ K⁻².     

Etching resistance and etching behavior of h-BN textured ceramics under Xe plasma condition

The h-BN ceramic with two-dimensional laminar structure is an important constituent material of Hall thruster channel, and the differences in its chemical bonding structures lead to anisotropy of BN grains in different orientations. In this paper, h-BN matrix textured ceramics are prepared by hot-press sintering. Xe plasma is used to etch the textured ceramics with different preparation processes and orientations. The effects of microstructure and h-BN grain orientation on plasma etching resistance of the materials are investigated. The results show that the intergranular phase has superior plasma etching resistance compared to the h-BN material. During the etching, the sample surface maintains dynamic equilibrium on the macroscopic scale, while preferential etching exists on the microscopic scale. The h-BN grains exhibit point etching, line etching and composite etching behavior during the etching process. The etching resistance of textured ceramics is related to the intrinsic plasma etching resistance of the matrix material, but less correlation to the preparation process, microstructure, and orientation of the BN grains of the matrix material, moreover, there is no anisotropic etching behavior.     

Translucent Yttria‐ and Silica‐Doped Mullite Ceramics with Anisotropic Grains Produced by Spark Plasma Sintering

Translucent, high‐performance, mullite ceramics with anisotropic grains were prepared by the spark plasma sintering (SPS) of a powder mixture consisting of commercial mullite powder, which already contained small amounts of alumina (θ and α) and silica (cristobalite) (≤3 wt% in total), to which 2 and 1 wt% of yttria and amorphous silica was admixed, respectively. The combination of low‐viscosity Y₂O₃–Al₂O₃–SiO₂ transient liquid formation and SPS sintering provided enhanced densification, also provoking anisotropic grain growth (which became exaggerated after 20 min of SPS dwell time), at a relatively low sintering temperature of 1370°C. In this way, it was possible to meet the conflicting demands for obtaining a dense mullite ceramic with anisotropic grains, ensuring good mechanical properties, while preserving a noticeable light transmittance. In terms of mechanical and optical properties, the best results were obtained when SPS dwell times of 5 and 10 min were employed. The as‐sintered samples possessed densities in the range 3.16–3.18 g/cm³, anisotropic grains with an aspect ratio (AR) of 7 and a grain thickness of approximately 0.45 μm, a flexural strength between 350 and 420 MPa, a Vickers indentation toughness and a hardness of approximately 2.45 MPa·m^1/2 and 15 GPa, respectively, and an optical transmittance of between 30% and almost 50% in the IR range.     

Texture in silicon carbide via aqueous suspension material extrusion and seeded grain growth

Textured SiC with anisotropic crystallographic texture is proposed as a material with improved mechanical properties. Textured SiC was created via alignment of platelet seed particles during aqueous suspension material extrusion, also known as direct ink writing, and subsequent pressureless liquid phase sintering and annealing. The microstructure and texture of the SiC fabricated with and without 5 vol% platelet seeds, and with and without annealing at 2050 °C and 2150 °C was explored via SEM, XRD, and EBSD. All samples fabricated had over 95% theoretical density. Annealing leads to the development of large, high aspect ratio plate-shaped grains among a matrix of many finer, low aspect ratio grains. Higher annealing temperatures and addition of platelet seeds both increased the size of the large grains. Samples were found to be textured regardless of having platelet seeds. Via XRD and EBSD, unseeded SiC was found to have texture where the crystallographic direction [0001] had a preferred orientation perpendicular to the normal direction. This occurred for both direct ink written and cast SiC, so the texture development must have occurred during sintering, though the mechanism is unknown. For seeded SiC, platelet seeds aligned in direct ink writing seeded the grain growth to develop crystallographic texture. The texture was mainly influenced by the alignment of platelet seed particles via shear stresses in the print nozzle, causing a one-dimensional texture where [0001] is perpendicular to the printing direction. However, it was found that the texture was not the expected concentric alignment of platelet particles in direct ink writing, so the shear stresses in the nozzle are not solely responsible for the texture developed.     

Dispersion and rheology for direct writing lead-based piezoelectric ceramic pastes with anisotropic template particles

Controlling the rheology of direct writing pastes is essential for producing high quality printed ceramics. Ceramic pastes were formulated to explore the relation between surface chemistry and rheology of complex pastes of Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT) powder, large BaTiO₃ (BT) platelet particles, and a commercial poly(acrylic) acid-based binder system. Zeta potential of the ceramic powder, the conformation of the poly(acrylic) acid, and the effect of these factors on rheology were evaluated as a function of suspension pH. Effective dispersion and amenable rheology for direct writing were achieved at mixing pH 5. Additions of 0.3 to 2.6 vol% BT tabular particles dramatically altered the rheology of the pastes due to the shear alignment of the BT particles. Powder-organic interactions and the size and concentration of BT platelet particles can be tailored to direct write either space-filling filaments to form dense ceramics or non-flowing filaments to form spanning ceramic structures.     

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.     

Giant electro-strain in textured Li+-doped 0.852BNT–0.11BKT–0.038BT ternary lead-free piezoelectric ceramics

Texturing is an effective approach to improving the piezoelectricity of piezoelectric ceramics. In this work, <001> textured Li⁺-doped 0.852Bi_0.5Na_0.5TiO₃–0.11Bi_0.5K_0.5TiO₃–0.038BaTiO₃ ternary lead-free piezoelectric ceramics are prepared by the reactive templates grain growth (RTGG) method. X-ray diffraction (XRD) results demonstrate a high orientation degree of 77% along the <001> direction. Outstanding electro-strain response, which is higher than most of reported BNT-based textured ceramics, is achieved due to the contribution of oriented-grains along the <001> direction. A large electro-strain of 0.55% with a relatively low hysteresis is obtained at 6.5 kV/mm with corresponding large signal piezoelectric coefficient () of 846 pm/V in the textured ceramics, which is 49% higher than that of the random ceramics. Besides, the electro-strain could reach as high as 0.52%@5.5 kV/mm ( = 945 pm/V) at 100°C. These results indicate that the RTGG is an effective way to design high performance lead-free piezoelectric materials.     

Effect of initial particle size on grain microstructure of textured ferroelectric ceramics: A phase-field method and brush technique

A phase-field model was employed to simulate the grain microstructure evolution of the textured ceramics. Textured KSr₂Nb₅O₁₅ (KSN) ceramics were chosen as the research object and prepared by using acicular KSN particles as raw materials. A method combining the brush technique with the rolling process was proposed for the directional arrangement of the acicular particles. The effects of the initial particle length distribution and diameter on the grain growth behavior and electrical properties were investigated. It was found the influence of the initial particle length distribution on the grain growth rate mainly depended on its diameter. The use of coarse particles was beneficial to obtain a microstructure with a strongly anisotropic morphology and homogeneous grain size. The obtained KSN ceramics exhibited a high piezoelectric constant d₃₃ of 68 pC/N and Curie temperature of 120 ℃, which was closely related to the grain microstructure.     

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.     

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.     

Anisotropic effects of oxygen vacancy defects on the electrical properties of highly oriented bismuth titanate ferroelectric ceramics

Bismuth titanate (Bi₄Ti₃O₁₂, BIT) exhibits a high Curie temperature and anisotropic electrical performance owing to its layered perovskite structure, and hence, it is an important ferroelectric material for high-temperature piezoelectric applications. It is crucial to understand the effects of the anisotropy in BIT-based ferroelectrics for developing novel high-temperature piezoelectric materials. In this study, a highly textured BIT ceramic was fabricated using the tape-casting technique from highly grain-oriented BIT platelets prepared by the molten salt method. The textured BIT ceramic showed a dense microstructure and high grain orientation along the (00l) plane with a texturing degree F_00l = 0.86. It exhibited significant anisotropy in the electrical properties along the directions parallel and perpendicular to the axis of the tape-casting plane. Double ferroelectric hysteresis P–E loops and normal ferroelectric P–E loops were observed in the parallel and perpendicular samples, respectively. In addition to the layered crystal structure and domains, the anisotropy in the arrangement of the oxygen vacancy defects and their transport in the structure led to a significant anisotropy in the ferroelectric properties of the textured BIT ceramics. This work demonstrates the anisotropic arrangement of the oxygen vacancy defects and its effect on the electrical properties of high-temperature bismuth layer-structured ferroelectrics.     

Grain-orientation-engineered multilayer NaSr2Nb5O15 ferroelectric ceramics via templated grain growth

It is difficult to take both texture degree and density into account by the conventional method, especially when one-dimensional particles are used as templates to prepare textured ferroelectric ceramics. Here, we proposed a strategy to improve the texture degree and density of ceramics simultaneously by a grain-orientation-engineered technique. To solve the problem of directional arrangement of template particles, template layers without mixed matrix were prepared by the brushing technique. Dense textured NaSr₂Nb₅O₁₅ ceramics with grain size gradient distribution were designed by the lamination of template and matrix layers. An evaluation method of texture degree was developed. Ferroelastic and ferroelectric domains were observed simultaneously in the oriented grains, which was further confirmed that the ferroelectric-ferroelastic phase transition occurred for the NaSr₂Nb₅O₁₅ ceramics at the low temperature. The obtained textured NaSr₂Nb₅O₁₅ ceramics exhibited the high piezoelectric constant (d₃₃ =134 pC/N). The present research offers a route for designing grain-oriented ferroelectric ceramics.     

Achieving large piezoelectric response and ultrahigh electrostriction in [001] textured BiScO3-PbTiO3 high-temperature piezoelectric ceramics

[001] textured 0.40BiScO₃-0.60PbTiO₃-0.125 mol%Nb⁵⁺ (BS-60PT-0.125Nb) high-temperature piezoelectric ceramics were synthesized using templated grain growth process. A high texture degree F₀₀₁ of 99% was obtained using 2 vol% BaTiO₃ (BT) templates. The piezoelectric charge constant d₃₃ and the unipolar strain under 40 kV cm⁻¹ at room temperature for the textured ceramics are 646 pC N⁻¹ and 0.36%, respectively, which is over two times as those for untextured ceramics (∼243 pC N⁻¹ and 0.17%). The electrostriction Q₃₃ value of the textured sample remarkably increased from 0.034 m⁴ C⁻² to 0.068 m⁴ C⁻² under 30 kV cm⁻¹, showing a twice higher than untextured. Compared with random ceramics, the improvement piezoelectric response of the textured ceramics is primarily attributed to the increase of the dielectric constant ε_r and electrostriction coefficient Q₃₃ along [001] orientation, which is originating from the anisotropy of piezoelectricity. The BS-60PT-0.125Nb textured ceramics have large piezoelectric response and ultrahigh electrostriction with high temperature stability (high depolarization temperature T_d of ∼360°C and high Curie temperature T_c of 421°C), showing great potential for the piezoelectric applications at high temperatures.     

Fabrication of micro-scale textured grooves on green ZrO2 ceramics by pulsed laser ablation

The green ZrO₂ ceramics were fabricated by cold isostatic pressing. Pulsed laser ablation with a wavelength of 1064 nm was performed to fabricate micro-scale textured grooves on the surface of green ZrO₂ ceramics. The influence of laser parameters on surface quality was studied. The heat-affected zone around the machined grooves and micromorphology of laser-irradiated surface were investigated. Results showed that micro-scale textured grooves with a width of 30–50 µm and a depth of 15–50 µm on the green ZrO₂ ceramic surfaces were successfully fabricated by pulsed laser ablation. The laser parameters had a profound influence on the surface quality of micro-scale textured grooves. Better surface quality could be obtained with frequency below 40 Hz, power below 6 W, and scanning velocity above 200 mm/s. A sintering layer was found on the laser-irradiated surfaces when frequency was above 60 Hz, power was above 10 W, and scanning velocity was below 150 mm/s. Analysis of this sintering layer revealed clear melting and resolidification of ZrO₂ particles.     

Novel strategy for the enhancement of anti-counterfeiting ability of photochromic ceramics: Sm3+ doped KSr2Nb5O15 textured ceramics with anisotropic luminescence modulation behavior

Sm³⁺ doped KSr₂Nb₅O₁₅ (KSN-Sm) textured ceramics with anisotropic photochromic and luminescence modulation behaviors provided a new strategy for the enhancement of anti-counterfeiting ability. The KSN-Sm textured ceramics were fabricated by the tape casting technology, which exhibited obvious grain-orientation, with Lotgering factor f_(00l) of 0.62. The textured sample possessed evident difference of reflectivity, photochromic, luminescent and luminescence modulation properties among various grain-orientated directions. The difference of luminescent emission intensity was over than 30 % and the luminescence modulation ratios △R_t are 75.3 % and 63.3 % along paralleled and vertical [00l] orientations, respectively. These optical anisotropies were attributed to the different refractive indexes, distributions of photochromic centers and energy transfer rates at various orientations. This work is hopeful to achieve the multidirectional data recording and enhancement of anti-counterfeiting ability of photochromic ceramics by the anisotropic properties of textured ceramics.     

Texture development and dielectric relaxor behavior of 0.80Na0.5Bi0.5TiO3–0.20K0.5Bi0.5TiO3 ceramics templated by plate-like NaNbO3 particles

Plate-like NaNbO₃ particles were used as templates to fabricate grain-oriented 0.96(0.8Na_0.5Bi_0.5TiO₃–0.2 K_0.5Bi_0.5TiO₃)–0.04NaNbO₃ (NKBT) ceramics. The effects of the sintering temperature and the soaking time on the grain orientation and the microstructure of the textured NKBT ceramics were investigated, and the dielectric relaxor behavior is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.8. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane. The degree of grain orientation increases initially, then decreases with increasing sintering temperature, and increases continuously with increasing soaking time. The textured NKBT ceramics shows obvious dielectric relaxor characteristics which can be well explained by microdomain–macrodomain transition theory with calculating criterion K. The results show that formation of texture is beneficial to microdomain–macrodomain transition, which lead to weaken relaxor behavior and raise the dielectric constant at T_tr.     

Optimisation of matrix composition for texturing of morphotropic phase boundary Pb(Mg1/3Nb2/3)O3–PbZrO3–PbTiO3 piezoelectric ceramics using BaTiO3 template

Texturing is an innovative method for fabricating high-performance piezoelectric ceramics. However, the optimal conditions for successful texturing must be evaluated to obtain high performance. Herein, the texturing behaviour of Pb(Mg_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMN–PZ–PT) piezoelectric ceramics using plate-like BaTiO₃ crystals as a template was found to be strongly dependent on the chemical composition of the matrix. Given that the solubility of BaTiO₃ in the matrix increased with the PZ content, an excessive amount (>20 mol%) of the BaTiO₃ template was necessary for texturing a 0.2PMN–0.38PZ–0.42PT, while 3 mol% of the BaTiO₃ template could texture a 0.4PMN–0.22PZ–0.38PT. Owing to well-aligned textured grains and the complete consumption of randomly oriented matrix particles, a textured 0.4PMN–0.22PZ–0.38PT sample prepared by tape-casting under optimised conditions yielded a Lotgering factor of 97 % and an induced strain of 0.2 % with an electric field of 2 kV/mm.     

Effect of different templates on microstructure of textured Na0.5Bi0.5TiO3–BaTiO3 ceramics with RTGG method

One-dimensional needle-like KSr₂Nb₅O₁₅ (KSN) and two-dimensional plate-like Bi_2.5Na_3.5Nb₅O₁₈ (BNN) particles were used as templates to fabricate grain-oriented Na_0.5Bi_0.5TiO₃–BaTiO₃ (NBTBT) ceramics by reactive-templated grain growth. The effects of the template concentration on the microstructure and orientation of NBTBT ceramics were investigated, and the mechanism of grain growth was discussed. The results show that NBTBT textured ceramics cannot be obtained with KSN template, since the needle-like KSN particles were aligned randomly along the tape casting direction. Ceramics contain perovskite NBTBT and tungsten–bronze-type KSN phase. Some cucurbit-like KSN grains resulted from the defects of needle-like KSN templates were detected. Textured ceramics with orientation factor more than 60% were obtained successfully when the plate-like BNN was used as templates. The results show that the texture fraction of NBTBT textured ceramics increase with increasing the content of BNN. The textured ceramics exhibited {h 0 0} preferred orientation have brick wall microstructure with strip-like grains aligning in the same plane as the casting plane. The reaction and crystal formation of textured NBTBT ceramics with plate-like BNN templates are by diffusion–recrystallization process. Finally the selecting rules of suitable templates are proposed.