Transparent alumina doped with rare‐earth or transition metal oxides represents a group of materials suitable for optical applications. Because of the birefringence of alumina, their preparation is ...demanding on the quality of starting materials, their advanced shaping, and pressure‐assisted sintering. Spark plasma sintering and hot isostatic pressing have proven at achieving pore‐free fine microstructures exhibiting high in‐line transmission and a sufficient intensity of emitted light. The present review aims to summarize the results published on this topic, to compare them and on this base to propose other possible and/or appropriate approaches to future.
A 2 M 3 O 12-type materials are widely studied due to their outstanding chemical flexibility, tunable low positive, negative, to near-zero coefficients of thermal expansion, and vast potential ...applicability from cooker hobs to housing for optical devices. Among the mechanisms for tuning their coefficient of thermal expansion (CTE) are the substitution of the A3+ cation, the insertion of bulky molecules inside the nanochannels of their crystal structure, and, possibly, the formation of extrinsic oxygen vacancies. Although the formation of oxygen vacancies in already crystalline structures is widely studied for various oxides, their evolution during crystallization from the amorphous precursor is scarcely reported. In this study, the amorphous powder was synthesized by co-precipitation, and different concentrations of extrinsic oxygen vacancies were formed through crystallization of the amorphous material into orthorhombic Al2W3O12 in air atmosphere, within the temperature interval between 500 and 620 °C. Amorphous powder and crystalline Al2W3O12 were characterized by differential scanning calorimetry and thermogravimetry, X-ray powder diffraction, transmission electron microscopy, diffuse reflectance spectroscopy, electron paramagnetic resonance, Raman, and X-ray photoelectron spectroscopies to investigate the influence of the calcination temperature on the formation of extrinsic oxygen vacancies, and to understand the oxygen vacancies formation during crystallization. It was found that the unit-cell volume increased up to 0.36% with the introduction of oxygen vacancies, in comparison to the almost extrinsic vacancy-free phase, owing to the weakening of W–O bonds in WO4 tetrahedra. The formation of orthorhombic Al2W3O12 occurs via a two-stage process. The oxygen-deficient amorphous structure is first formed upon calcination at 500 °C of the as-synthesized amorphous powder by water and hydroxyl group release. The increase in calcination temperature in air causes structural organization, while at temperatures as high as 620 °C, an almost extrinsic-point-defect-free counterpart phase is formed. Therefore, it is possible to crystallize orthorhombic Al2W3O12 with largely different concentrations of oxygen vacancies by varying calcination temperatures in air.
The paper describes the preparation of laminate piezo-ceramic composite consisting of Al2O3, ZrO2 and BaTiO3 layers and proves the idea of residual stresses utilization for crack deflection and ...handling with the brittleness of BaTiO3. The laminate was prepared by alternate electrophoretic deposition. Although the laminate was sintered at 1300 °C and consisted of layers having a density between 57 % (ZrO2) and 73 % (BaTiO3), the hardness and elastic modulus of layers corresponded to those of free sintered monolithic ceramics at a comparable level of porosity. The crack deflection at the interface between individual layers was observed having the same effect and magnitude as deflection observed in the case of fully dense Al2O3/ZrO2 laminates. An interlayer developed on the interface between Al2O3 and BaTiO3 had no negative impact on crack propagation.
•The crack path formation variety at the surface and in the interior of ceramic laminates was described for the first time.•The deflection of cracks formed at the surface reflects mostly elastic ...properties and not the level of internal stresses.•The behaviour similar to the tunnelling crack formation was observed during indentation.•The direction of crack propagation (from the surface or interior) is an important factor for the resulting crack path.•The depth at which internal stresses become significant is comparable to the thickness of laminate layers.
Laminates with alternating layers are well known from nature. The strongly bonded alumina/zirconia (Al2O3/ZrO2) layers can combine high fracture resistance with high strength and stiffness when properly tailored. The presence of compressive residual stresses formed in Al2O3 layers can suppress and deflect cracks propagating through the layers. The crack path is governed by both the elastic properties and the internal stress field of individual layers. The laminates with various layer-thickness ratios ranging from 0.1 to 3 were used to investigate the effect of residual stresses and influence of crack formation pattern on the crack path development. The indentation surface cracks observed in various alumina-zirconia laminates exhibit the same crack deflection independently on the level of internal stresses. The crack deflection observed on the fracture surfaces of bending specimens was related to the indentations cracks. The complicated crack path was explained experimentally by 3D reconstruction with the support of numerical simulations.
Hydroxyapatite (HA) and bioglass (BG) ceramics have become of prime importance in bone tissue engineering. Besides the appropriate composition, the microstructure of bone replacement plays a crucial ...role. In the present work, particle composites and functionally graded material (FGM) based on HA and BG prepared by electrophoretic deposition were thoroughly characterised in terms of the preparation method, sintering process, phase composition and microstructure. The sintering was monitored by high-temperature dilatometry in two directions, the sintering rates were calculated, and the overall sintering process was discussed. The SEM showed the continuous change in the microstructure of FGM with gradual interconnected porosity favourable for bio-applications. The fundamental fractographic analysis proved the crack development in FGM related to the sintering process, and the recommendations for the reduction of the crack development were given. The phase transformations during thermal treatment were analysed using X-ray diffraction analysis and deeply discussed.
Ferroelectric ceramics, especially piezoceramics, are widely used in the field of sensors, micromanipulators, and capacitors. Barium titanate and its derivates are the first choices between lead-free ...materials. The main aim of the paper is to clarify fundamental processes taking place in the vicinity of Curie temperature with a focus on the fracture behaviour of pure polycrystalline barium titanate. The explanation of observed changes in the mechanical behaviour of this material is based on the experimental approach supported by numerical simulations utilising features of the real microstructure on the grain level. Several model materials with various grain microstructures were manufactured from the submicron barium titanate powder sintered at various temperatures. Two resulting materials with a suitable distribution of grains were selected for further investigation. The grain size influenced not only the exact position of the temperature of the Curie point but also the kinetics of the lattice transformation, elastic, and fracture properties. The significant drop observed in the fracture resistance was attributed to the development of localised internal thermal stresses, which was supported by the results of the performed numerical simulations. The coincidence of the volume change of neighbouring grains due to lattice transformation together with a significant variation in elastic properties can lead up to a 20% decrease in the measured fracture toughness. Understanding this behaviour is essential for the processing and correct application of lead-free barium titanate materials.
Highly transparent Dy3+ and Dy3+/Cr3+ polycrystalline alumina ceramics were prepared with the real in-line transmittance (RIT) up to 55% (at λ = 632 nm), one of the highest values reported for ...luminescent rare-earth elements doped alumina. The RIT of doped alumina decreased more sharply with increasing mean grain size than predicted by the models for pure alumina. Co-doping with Dy3+ and Cr3+ resulted in a moderate increase of the RIT, which was independent on grain size. EDX analysis revealed that dysprosium segregated at grain boundaries and chromium was dispersed throughout the alumina matrix. A thermodynamic model for diffusion of multiple species in systems with multiple sorts of traps was proposed. The photoluminescence excitation and emission spectra of Dy3+-doped alumina showed peaks characteristic for Dy3+-doped materials. In co-doped Dy3+/Cr3+ alumina, Dy3+ emitted in the blue and yellow regions and Cr3+ in the red region, creating a suitable combination for obtaining white light.
We report on successful preparation of Er3+ doped transparent alumina (0.1–0.17 at.%) exhibiting visible light photoluminescence using wet shaping method and hot isostatic pressing. The effects of ...dopant amount, type of doping powder and powder pre-treatment on final microstructure, real in-line transmittance and photoluminescence characteristics were studied.
The real in-line transmittance ranged between 28 and 56%, depending on processing parameters. The transparency decreased with increased amount of dopant. The decrease is dependent on the type of doping powder and its pre-treatment.
The photoluminescence spectra measured in both visible and NIR region showed typical emission bands due to the presence of Er3+ ions. The decay profiles of the 4S3/2→4I15/2 transition were fitted with a 2-exponential function, with faster component in the range of 360–700ns and slower component around 1.6-2.4μs. The intensity of emissions and lifetime of the 4S3/2 level decrease significantly with increasing concentration of Er3+ ions.
The preparation of a transparent ultra-fine-grained doped ceramics requires a
homogeneous dopant distribution in a matrix. In the present work, two
thermodynamical phenomena allowing the preparation ...of such ceramics (the
dissolution of the dopant and the formation of undesirable secondary phases)
were experimentally and theoretically studied. A general
thermodynamic-kinetic model was developed for dopant dissolution, which was
verified for the experimental conditions used in this work. The model and
experiment showed that Mn3O4 dopant with overall concentration of 1 at.% and
particle size of 50 nm is dissolved and homogenized in a fine-grained
alumina matrix within less than one hour at a temperature of 1220 ?C. For
the purposes of the study of the formation of undesired secondary phase, the
phase diagram of the Al2O3-Mn3O4 system was calculated using the CALPHAD
approach. Detailed STEM observations combined with EDX and EELS chemical
analyses showed that the data used for the calculation of the phase diagram
need some modifications because they overestimate the solubility of Mn in
the alumina and underestimate the solubility of Mn in the MnAl2O4 spinel.
The positive effect of silica on microstructural, mechanical and biological properties of calcium phosphate scaffolds was investigated in this study. Scaffolds containing 3D interconnected spherical ...macropores with diameters in the range of 300-770 µm were prepared by the polymer replica technique. Reinforcement was achieved by incorporating 5 to 20 wt % of colloidal silica into the initial hydroxyapatite (HA) powder. The HA was fully decomposed into alpha and beta-tricalcium phosphate, and silica was transformed into cristobalite at 1200°C. Silica reinforced scaffolds exhibited compressive strength in the range of 0.3 to 30 MPa at the total porosity of 98-40%. At a nominal porosity of 75%, the compressive strength was doubled compared to scaffolds without silica. When immersed into a cultivation medium, the formation of an apatite layer on the surfaces of scaffolds indicated their bioactivity. The supportive effect of the silicon enriched scaffolds was examined using three different types of cells (human adipose-derived stromal cells, L929, and ARPE-19 cells). The cells firmly adhered to the surfaces of composite scaffolds with no sign of induced cell death. Scaffolds were non-cytotoxic and had good biocompatibility in vitro. They are promising candidates for therapeutic applications in regenerative medicine.
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