Highly oriented α-Al2O3 transparent ceramics shaped by shear force

Alumina platelets were arranged horizontally in submicron alumina particles by shear force in the flow of slurries during casting. The obtained alumina green bodies with platelets were pressureless-sintered in vacuum, producing ceramics with thoroughly oriented grains and high transmittance. The effects of sintering parameters on the densification, microstructure evolution, and orientation degree of alumina ceramics were investigated and discussed. The results showed that the densification, grain size, orientation degree, and in-line transmittance were increased with increasing sintering temperature. The enhancement of orientation degree was mainly coherent with grain growth. The grain-oriented samples exhibited a much higher in-line transmittance (at 600 nm) of 61 % than that of the grain random sample (29 %). Moreover, the transmission remained a high level in the ultraviolet range (<300 nm).     

Effect of second phase addition of zirconia on the mechanical response of textured alumina ceramics

The effect of second phase addition of zirconia on the mechanical response of textured alumina was analysed. Highly textured monolithic tape-casted alumina was obtained through templated grain growth. Compositions containing 1, 2, 5 and 10 vol% of (i) non-stabilised and (ii) 3 mol% yttria-stabilised zirconia, respectively, were investigated. XRD analyses revealed that the texture degree decreased with increasing second phase content. Microstructural analysis showed zirconia grains inside the textured alumina grains for contents ≤ 5 vol%, affecting the mode of fracture. Fracture toughness of textured alumina significantly decreased with the addition of a second phase. In the case of non-stabilised zirconia, the constraint of the alumina matrix and the small grain size led to a lower fracture toughness in comparison to monolithic textured alumina (K_Ic = 5.1 MPa m^1/2). The fracture toughness of textured alumina with 3 mol% yttria-stabilised zirconia was comparable to equiaxed alumina, independent of the content ratio (K_Ic = 3.5 MPa m^1/2).     

Nanoscale crystal imperfection-induced characterization changes of manganite nanolayers with various crystallographic textures

(La,Sr)MnO₃ (LSMO) nanolayers with various crystallographic textures were grown on the sapphire substrate with and without In₂O₃ epitaxial buffering. The LSMO nanolayer with In₂O₃ epitaxial buffering has a (110) preferred orientation. However, the nanolayer without buffering shows a highly (100)-oriented texture. Detailed microstructure analyses show that the LSMO nanolayer with In₂O₃ epitaxial buffering has a high degree of nanoscale disordered regions (such as subgrain boundaries and incoherent heterointerfaces) in the film. These structural inhomogeneities caused a low degree of ferromagnetic ordering in LSMO with In₂O₃ epitaxial buffering, which leads to a lower saturation magnetization value and Curie temperature, and higher coercivity and resistivity.     

Mechanical properties of 1 mol% CeO2‐10 mol% Sc2O3 co‐doped and stabilized ZrO2 synthesized through hydro/solve‐thermal method

Ultra‐fine 1 mol% CeO₂‐10 mol% Sc₂O₃ co‐doped and stabilized ZrO₂ (1Ce10ScSZ) powders with average grain size less than 10 nm in diameter were prepared by hydro/solve‐thermal method using either deionized water, ethanol, or methanol as solvent. As‐synthesized powders were characterized in terms of phase structure, particle morphology, and chemical composition by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high‐resolution transmission electron microscopy (HRTEM), and inductively coupled plasma‐optical emission spectroscopy (ICP‐OES), respectively. Sintering studying was conducted on pellets of 15 mm in diameter and 3 mm in thickness under uniaxial compaction using 25 MPa at either 600, 800, 1000, 1100, 1200, 1400, or 1500°C for 1 hour. Phase transitions and grain morphologies of those sintered samples were characterized by XRD and field emission scanning electron microscopy (FESEM). Mechanical properties were characterized on dense pellets sintered at 1500°C by nanoindentation. Experimental results showed that ethanol was more effective to synthesize agglomerate‐free 1Ce10ScSZ powders as compared with deionized water and methanol. Choice of solvent affected the environment of hydro/solve‐thermal solution, which led to variation of chemical compositions of powders and porosities of sintered pellets, and therefore, influenced their mechanical performance. Our study showed that solvent was important to make dense, thin, and mechanically robust 1Ce10ScSZ electrolyte for potential applications in electrochemical devices. Absolute values of hardness (H) and Young's modulus (E) measured from our samples are much higher and more consistence than those results obtained from commercial vendors reported in literatures.     

Predicting the Grain-Size Distributions in High-Density, High-Purity Alumina Ceramics

This paper discusses the image-analyzer-based grain-size distributions (GSDs) of fully densified ceramics obtained from pressure casting a high-purity alumina powder, develops an algorithm for predicting the GSDs as a function of sintering time and temperature, and compares of the GSDs thus predicted with those experimentally observed. The GSD data for all sintered specimens, when nondimensionalized in terms of the median grain size, reduced to a single self-preserving GSD curve. The median grain was predicted as a function of sintering time and temperature using the classical kinetics equation. The GSDs predicted using the algorithm developed tallied well with those that were experimentally obtained.     

Suppressed grain growth in highly porous barium titanate foams by two‐step sintering

Porous barium titanate has gained significant attention in recent years for their potential use in applications such as scaffolds for bone tissue engineering, stress sensors, gas sensors, and many others. However, there is very little control over the grain size of the material during the sintering processes specially to achieve little or no growth of the starting powders. Here, using the two‐step sintering method barium titanate foams were shown to be synthesized with controlled grain size of the struts without significant differences in the pore structure of the materials. In order to evaluate the applicability of two‐step sintering for a variety of processing methods, highly porous (>80% porosity) foams synthesized through the direct polyurethane foaming method were used to create conditions furthest from bulk where two‐step sintering has shown success. Two‐step sintering parameters were identified and the processing conditions were confirmed to not alter the mechanical properties of the samples due to expected residual stresses or thermal shock resulting from the rapid heating and cooling rates employed.     

Magnetic slip casting for dense and textured ceramics: A review of current achievements and issues

Ceramic materials are ubiquitous in technologies operating under high mechanical, thermal or chemical constrains. Research in ceramic processing aims at creating ceramics with properties that are still challenging to obtain, such as toughness, transparency, conductivity, among others. Magnetic slip casting is a process where an external magnetic field is used to createcontrolled texture in ceramics. Over the past 20 years of research on magnetic slip casting, dense and textured ceramics of multiple chemistry were found to exhibit enhanced properties. This paper reviews the progress in the field of magnetic slip casting, details the processing parameters and the textures obtained for a diverse range of compositions. The structural and functional properties of the magnetically textured slip casted and sintered ceramics are presented. This overview of the magnetic slip casting process allows to identify critical directions for future advancement in advanced technical ceramics.     

Tape Casting, Pressureless Sintering, and Grain Growth in Ti3SiC2 Compacts

In this work we demonstrate that fine Ti₃SiC₂ powders can be tape-cast and/or cold-pressed and pressureless-sintered in Ar- or Si-rich atmospheres to produce fully dense, oriented microstructures in which the basal planes are parallel to the surfaces. Carbon- and/or Si-rich environments suppress grain growth. In the case of the tape casting, the C-residue from binder burnout results in small core grains relative to the surface grains that can grow significantly. When sintering in high Si activities, titanium silicide phases form at the grain boundaries that slow grain growth. Annealing the latter in Ar at 1600°C, for extended periods (30 h), rids the samples of these grain-boundary phases, which in turn results in grain growth. The advantage of the latter process is that the final grain size distribution is more uniform from surface to bulk.     

NaNbO3 templates-induced phase evolution and enhancement of electromechanical properties in <00l> grain oriented lead-free BNT-based piezoelectric materials

Recently developed Bi_0.5Na_0.5TiO₃(BNT)-based piezoceramics face two urgent obstacles: high driving field required to trigger large strain and poor temperature stability. Highly oriented (1-x)(0.83Bi_0.5Na_0.5TiO₃-0.17Bi_0.5K_0.5TiO₃)-xNaNbO₃ (BNT-BKT-xNN) piezoceramics were synthesized using NN templates to resolve both obstacles. Measurements of polarization and strain hysteresis loops as well as phase transition temperature revealed a phase evolution from ergodic relaxor to ferroelectric phases, generating a high strain of 0.43% and large S_max/E_max = 720pm/V for textured BNT-BKT-4NN ceramics. The field-dependent strain was largely depended on the degree of texturing. Most intriguingly, grain-oriented specimens provided excellent actuating performance characterized by both large S_max/E_max = 693 pm/V at a low driving field of 45 kV/cm and enhanced temperature stability with S_max/E_max = 537pm/V at 120 °C. This was basically ascribed to the facilitated switching between ergodic relaxor and ferroelectric phases owing to the grain-oriented structure. As a consequence, design of <00l> oriented microstructure opens the possibility to produce efficient BNT-based piezoceramics for transferral into real-world applications.     

碳化硅木质陶瓷的显微结构及力学性能

以汉麻秆芯碳化后的碳粉为原料,分别采用注浆和干压成型工艺制备素坯,通过反应烧结制备出碳化硅木质陶瓷.研究了注浆成型工艺中悬浮稳定剂的种类和添加量对浆料性能的影响.采用激光共聚焦显微镜、扫描电子显微镜和X射线衍射仪等分析了碳化硅木质陶瓷的显微结构、物相组成及力学性能.结果表明:采用注浆成型制备的碳化硅木质陶瓷力学性能优异,实测的游离硅含量同理论计算结果一致,说明渗硅过程中硅碳反应充分,烧结体显微硬度、弯曲强度、弹性模量和断裂韧性分别为22.3 GPa、397 MPa、290 GPa和3.0 MPa·m1/2.     

Anisotropic growth of α-Fe2O3 nanostructures

In this work, we report on the anisotropic growth of $\alpha$-Fe₂O₃ nanoslabs which are produced by co-precipitation method with the addition of sucrose. In our previous work, we have argued that such behavior can be related with the chelating agent. Experiments of X-ray diffraction (XRD), high-resolution transmission electronic microscopy (HRTEM) and magnetic measurements as a function of temperature and an applied magnetic field are used to characterize the samples. The HRTEM image of the sample prepared with 10 mmol/l of sucrose consists of faceted-like nanoslabs while that prepared without sucrose exhibits particles with a non-uniform shape. In this way, we can state that both the HRTEM images and the analysis of the XRD patterns show clearly a preferential growth of the [110] crystallographic direction. To strengthen our supposition, besides T- and field-dependence of magnetization are consistent with a superparamagnetic behavior the fit of the (Zero Field Cooling and Field Cooling) ZFC-FC curve for sample grown with 10 mmol/l of sucrose presents a strong increase of the effective anisotropy constant, ${\mathrm{K}}_{\mathit{eff}}$, which can be related with the increasing of the shape magnetic anisotropy.     

Metal Reference Line Technique for Obtaining Dihedral Angles from Surface Thermal Grooves

A metal reference line (MRL) technique is described for the measurement of surface–grain-boundary dihedral angles, Ψ_s, from thermal grooves at a sample surface using scanning electron microscopy (SEM). Metal lines deposited onto a thermally grooved surface using photolithography conform to the contours of the grain-boundary groove and provide a high-contrast reference line for measuring Ψ_s by SEM. Measurements of Ψ_s from optical interferometry and calculated from groove dimensions using surface diffusion models of thermal grooving are compared with the metal reference line measurements from the same thermally grooved surface of MgO-doped Al₂O₃. Distributions of Ψ_s are found to shift to lower angles and approach the true Ψ_s value as the resolution of the technique increases, with the MRL technique having the highest resolution, a median angle of 113°± 1° and a distribution of angles from 90°± 5° to 139°± 3°.