The Al2W3O12 ceramic with near-zero thermal expansion is a promising candidate for thermal shock resistance applications. However, the often-reported microcracks in the microstructure hinder the ...practical use of Al2W3O12. This work focuses on optimising the material processing—powder synthesis, calcination, milling, and sintering—to prepare dense polycrystalline Al2W3O12 with fine microcrack-free microstructure. The amorphous powder produced by the co-precipitation method had the highest specific surface area reported for this material, resulting in improved sinterability. Two rapid sintering methods were employed—Rapid Pressure-less Sintering and Spark Plasma Sintering. Both methods led to shorter processing times and lower sintering temperatures. However, Al2W3O12 samples produced by Spark Plasma Sintering were nearly fully dense with a density of ∼98% of theoretical density, the best value reported to date for this phase. The obtained fine crack-free microstructure of the samples led to a superior mechanical response compared to that achieved by Rapid Pressure-less Sintering.
Alumina ceramics found their utilisation in many applications which can be further extend by attaining functional properties; in our case the transparency obtained through precise processing and ...photoluminescence due to erbium (Er) doping. In order to examine the optical, mechanical and fractographic response of transparent alumina on Er doping, slip casted samples containing 0–0.15at.% of erbium nano-oxide were pre-sintered by two-step sintering regime and then hot isostatically pressed. Prepared samples exhibited fully dense submicron microstructure and corresponding high transparency (RIT up to 60%). Positive influence of doping on the Vickers hardness resulted in values up to 27GPa (at 10N load). Moreover, the comparison of the Vickers hardness determined at different loadings with literature data showed that the Er doped alumina is one of the hardest material in this category. The samples were characterised also in terms of fracture toughness and fractographic behaviour.
Transparent Cr2O3-doped alumina ceramics were prepared by slip casting, followed by pre-sintering in ambient atmosphere and hot isostatic pressing. The effect of dopant concentration on material ...properties, including microstructure and optical properties was evaluated. Real in-line transmittance in the range of 20–44 % was measured for the ceramics with the mean grain size <520 nm: the transmittance decreased with increasing grain size and Cr content. The excitation spectra consisted of two broad bands with maxima at 404 nm and 558 nm, corresponding to 4A2g → 4T1g and 4A2g → 4T2g transitions of Cr3+ ions in octahedral sites of α-Al2O3. The intensive deep red narrow emissions under violet/green light excitation, R-lines (2Eg → 4A2g transition), were observed at 692.5 nm and 693.8 nm, that are very close to ruby single crystal. The highest emission was achieved at the Cr3+ concentration of 0.4 at.%. The luminescence decay curves exhibited single-exponential behaviour with decay times of ∼3.6 ms.
Al2O3 and ZrO2 monoliths as well as layered Al2O3/ZrO2 composites with a varying layer thickness ratio were prepared by electrophoretic deposition. The sintering shrinkage of these materials in the ...transversal (perpendicular to the layers, i.e. in the direction of deposition) as well as in the longitudinal (parallel with layers interfaces) direction were monitored using high-temperature dilatometry. The sintering of layered composites exhibited anisotropic behaviour. The detailed study revealed that sintering shrinkage in the longitudinal direction was governed by alumina (material with a higher sintering temperature), whilst in the transversal direction it was accelerated by the directional sintering of zirconia layers. For interpretation of such anisotropic sintering kinetics, the Master Shrinkage Curve model was developed and applied. Crack propagation through laminates with a different alumina/zirconia thickness ratio was described with the help of scanning electron microscopy and confocal laser microscopy.
Information on the sintering activation energy is currently focused on evaluation of single-phase ceramic systems. This work shows the results of high-temperature dilatometry measurements of layered ...and particle composites based on alumina and zirconia. Layered composites with different layer thickness ratios and particle composites with variable composition in the entire concentration range were prepared by electrophoretic deposition allowing manufacturing composites with precious design and strongly bonded interfaces. The phenomena observed during the high-temperature dilatometry measurements are discussed, and the data were used to calculate the sintering activation energies of composites using the modified Master Sintering Curve concept. By covering a wide range of composite designs, it was possible to determine differences in activation energies and to show their dependence on the direction in the case of laminate composites given by the directionally dependent sintering behaviour. Sintering activation energies of layered composites were always higher than for monoliths due to constrained sintering showing maximum sintering activation energies at lower volumes of zirconia in the layers for longitudinal and transversal orientation of the samples. A similar trend was identified in particle composites due to slowed down alumina densification by the pinning effect. Additionally, mechanical properties represented by Vickers hardness and indentation elastic modulus were related to the microstructure developed during sintering. The effects of interconnectivity of phases present in the composites together with other parameters of the microstructure were described.
The Eu3+-doped transparent aluminas were prepared by wet shaping technique followed by pressure-less sintering and hot isostatic pressing. The effect of dopant amount on microstructure, real in-line ...transmission (RIT), photoluminescence (PL) properties, hardness and fracture behaviour was studied. The RIT decreased with increasing amount of the dopant. The PL emission spectra of Al2O3:Eu3+ ceramics exhibited predominant red light emission with the highest intensity (under 394nm excitation) for material containing 0.125at.% of Eu3+ and colour coordinates (0.645, 0.355) comparable with commercial red phosphors. The doping resulted in hardness increase from 26.1GPa for undoped alumina to 27.6GPa for Eu-doped sample. The study of fracture surfaces showed predominantly intergranular crack propagation micro-mechanism.
In the present work, the preparation of sintered lithium-doped tricalcium phosphates was studied, along with their physical, mechanical, and biological properties. Calcium phosphates were shaped via ...the use of electrophoretic deposition (EPD), using colloidally milled dispersions of hydroxyapatite (HAp) particles. The dispersions were stabilised with monochloroacetic acid. Lithium was incorporated into the structure via an addition of lithium chloride, which also served to optimise the deposition process. The dispersions were milled colloidally for periods of 0–48h. The colloidal milling resulted in two effects: i) disintegration of the commercial HAp powder (10µm) agglomerates, ii) unimodal distribution of the HAp particles (~ 170nm). The fine particles of the milled HAp dispersions accelerated the deposition rate, and increased the mass of the deposit. The reduced size of the initial particles, owed to the milling, led to the superior arrangement of the particles during deposition and to reduced porosity after sintering (1050–1250°C). The HAp decomposed into tricalcium phosphate phases during sintering. At a sintering temperature of 1250°C, grain growth occurred, which consequently resulted in a slight degradation of the mechanical properties (reduction in hardness and Young's modulus). In contrast, the hardness and Young's modulus increased as the dispersion milling time increased (smaller grain size after sintering); however, the fracture toughness did not change. The results of the biological testing confirmed the bioactivity of the material through the growth of the apatite layer in the simulated body fluid (SBF), and the biodegradation of the prepared materials in the Tris-HCl solution. With regard to the preparation of compact lithium-doped tricalcium phosphates, the best results were obtained in the case of the sample that utilised the dispersion that was milled for 48h, and was sintered at 1050°C.
In progressive particle or layered composites based on a combination of BaTiO3 and Al2O3, serving as e.g. ceramic harvesters, new phases are formed during heat treatment. The dominant one is ...BaTiAl6O12. This study provides information about the microstructural, mechanical and optical properties of the BaTiAl6O12 ceramics. The evolution of the phases during the solid-state reaction synthesis of the BaTiAl6O12 was monitored. The fully dense samples prepared by spark plasma sintering had indentation Vickers hardness and indentation elastic modulus within ranges of 10.1–13.7GPa and 132.0–187.0GPa, depending on loading force. The three-point bending tests of the BaTiAl6O12 samples resulted in flexural strength of 129.9MPa and fracture toughness of 1.8MPam1/2. The sample showed blue broad-band emission under UV excitation due to the charge-transfer transition of the Ti4+ and defect sites. The BaTiAl6O12 evinced low permittivity (ɛ′)=16 and dielectric loss (tanδ) <0.0003 at a frequency 1kHz.
En los compuestos de partículas o capas progresivas basados en una combinación de BaTiO3 y Al2O3, que sirven, por ejemplo, como capacitadores cerámicos, se forman fases nuevas durante el tratamiento térmico. La dominante es BaTiAl6O12. Este estudio proporciona información sobre las propiedades microestructurales, mecánicas y ópticas del cerámico BaTiAl6O12. Se monitorizó la evolución de las fases durante la síntesis por reacción en estado sólido de BaTiAl6O12. Las muestras totalmente densas preparadas mediante spark plasma sintering exhibieron una dureza Vickers por indentación y un módulo elástico por indentación entre 10,1-13,7GPa y 132,0-187,0GPa, respectivamente, dependiendo de la fuerza de carga aplicada. Los ensayos de flexión en tres puntos de las muestras de BaTiAl6O12 dieron como resultado una resistencia a la flexión de 129,9MPa y una tenacidad a la fractura de 1,8MPam1/2. La muestra presentó una emisión azul de banda ancha bajo excitación UV debido a la transición de transferencia de carga del Ti4+ y sitios defectuosos. El BaTiAl6O12 mostró una baja permitividad (ɛ′)=16 y una pérdida dieléctrica (tanδ) <0,0003 a una frecuencia de 1kHz.
The development and optimisation of piezoceramics are targeted usually to enhance their piezoelectric properties evaluated by both the direct or indirect measurement methods. The presented work aims ...to elaborate on the correlation of one direct (Berlincourt) and two indirect (convert and field-dependent) piezoelectric measurement methods on various material states. The role of the ceramic powder treatment by ball milling and electrophoretic deposition (EPD) technique on the determined electric properties as well as basic physical and mechanical properties of (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 ceramics (BCZT) was investigated. It was found that the EPD technologically supported by milling allows obtaining thick and dense deposits (>2 mm). After sintering, the BCZT ceramics with a relative density of >95%, hardness in the range of 2.3–2.9 GPa and piezoelectric coefficients of d33* = 940 pm/V, d33(E=0) = 427 pm/V and d33 = 364 pC/N can be achieved. Reported results also suggest that indirect (field-dependent) and direct (Berlincourt) measurements of the piezoelectric coefficients can be comparable at optimal poling conditions.
The stabilisation of hydroxyapatite (HA) particles in dispersions containing 2-propanol and monochloroacetic acid (MCAA) was studied. The surface of HA (Ca2+ cations) was stabilized by chemical ...adsorption of non-dissociated MCAA molecules via polar Cl atoms followed by dissociation (cleavage of the protons), which led to negative charging of the HA particles. Colloidal dispersions were milled for 0, 2 and 24h in a planetary ball mill before electrophoretic deposition (EPD). The electrical conductivity of dispersions was modified by adding an indifferent electrolyte (LiCl) to obtain crack-free deposits with a smooth surface. HA and HA/t-ZrO2 composites containing 2, 4, 8 and 16wt% zirconia microfibres were prepared by EPD. HA decomposed into α-TCP, β-TCP and partially substituted calcium phosphate by the action of Li+ cations during sintering. Elastic modulus and Vickers hardness of composites increased with milling time of the dispersions prior to EPD and decreased with the quantity of zirconia microfibres in the deposits, depending on the composite density. Fracture toughness of the composites was twice as high as that of pure HA due to the presence of orientated zirconia microfibres, finer microstructure, tougher β-TCP phase and pores. The two-week soaking in simulated body fluid revealed the bioactivity of all prepared materials.
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