Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Fabrication of Sn doped Ba(Mg1/3Ta2/3)O3 transparent ceramics by a solid state reaction method

Transparent disordered BMT ceramics were obtained by solid state reaction. Sn⁴⁺ ions were incorporated to make the B site of the perovskite structure disordered. The stoichiometric powder mixture with and without Sn doping was calcinated at 1300 °C, respectively and they were both characterized. After dry pressing, the pellets with Sn doping were sintered at 1600 °C in oxygen atmosphere for 4 h. The grain size of the transparent ceramics is around 12 μm. No pores were detected in or among the grains. The inline transmittance of the material is 66% at 580 nm. The refractive index is 2.09 at 1600 nm.     

Synthesis of nanocrystalline samarium-doped CeO2 (SDC) powders as a solid electrolyte by using a simple solvothermal route

Samarium-doped CeO₂ is a leading electrolyte for applications in solid oxide fuel cells (SOFCs), which requires a typical sintering temperature of 1400–1600 °C. In this work, fully dense CeO₂ ceramics doped with 10–20 at.% samarium have been fabricated by a simple polyol process. The XRD and SEM results show that a complete solid solution between CeO₂ and samarium was obtained at the sintering temperature of 1300 °C. And also the densification temperature is significantly lower than those (1400–1600 °C) reported for the SDC powders processed by modified sol–gel process and hydrothermal treatment. The resultant ceramics show the sizes of ultrafine grain are lower than 1 μm.     

Two-step sintering of fine alumina–zirconia ceramics

This work investigates the feasibility to the fabrication of high density of fine alumina–5 wt.% zirconia ceramics by two-step sintering process. First step is carried out by constant-heating-rate (CHR) sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 1400 and 1450 °C is effective for the first step sintering (T₁) due to its highest densification rate. The isothermal sintering is then carried out at 1350–1400 °C (T₂) for various hours in order to avoid the surface diffusion and improve the density at the same time. The content of zirconia provides a pinning effect to the grain growth of alumina. A high ceramic density over 99% with small alumina size controlled in submicron level (0.62–0.88 μm) is achieved.     

Effect of YF3 on the phase stability and sinterability of hydroxyapatite—Partially stabilized zirconia composites

Pure hydroxyapatite (HA), HA and partially stabilized zirconia composites (PSZ) with YF₃ and HA–PSZ composite containing 5 wt% PSZ without YF₃ were sintered in air at 900 °C, 1100 °C and 1300 °C for 1 h. The reactions and transformation of the phases in the composites were determined by X-ray diffraction. All the composites with or without YF₃ showed desirable thermal stability below 1300 °C and besides various amounts of CaZrO₃, any amount of tri-calcium phosphate (TCP) was not observed. Above 1100 °C, composites with YF₃ showed higher thermal stability than the composites without YF₃. On the other hand, pure HA started to decompose and TCP was observed at 1300 °C. Composites with YF₃ showed improved thermal stability than the composite containing 5 wt% PSZ without YF₃ and pure HA at lower sintering temperatures such as 900 °C and 1100 °C. However, it was observed that the increasing amount of YF₃ addition caused negative effect on the thermal stability of the composites. 5ZHA composites with YF₃ showed the highest relative density among all of the composites with or without YF₃.     

Densification ability of combustion-derived Al2O3 powders

Nanocrystalline Al₂O₃ powders containing different amounts of MgO (0.1–5.0 mol%) or added boehmite (AlOOH) have been synthesized by combustion synthesis from aluminium nitrate and magnesium nitrate, using urea or sucrose as fuels. The as synthesized alumina powders were deagglomerated, compacted by dry pressing and sintered at 1625 °C for 2 h. For comparison purposes, a commercial high purity α-Al₂O₃ powder (ACC) was also processed following the same route. The sintered materials were characterized for bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity, microstructure using SEM, and XRD phase composition. In comparison to boehmite, the MgO had a considerable effect on the densification behaviour of combustion-synthesized powder.     

Reactive synthesis for porous Ti3AlC2 ceramics through TiH2, Al and graphite powders

Porous Ti₃AlC₂ ceramics were fabricated by reactive synthesis. The process of fabrication involved five steps: (i) the pyrolysis of stearic acid at 450 °C; (ii) the decomposition of TiH₂ at 700 °C, which leads to the conversion of TiH₂ to Ti; (iii) the solid–liquid chemical reaction of solid Ti and molten Al at 800–1000 °C, which converts the mixture to Ti–Al compounds; (iv) the newly synthesized Ti–Al compounds that react with surplus Ti and graphite to form ternary carbides and TiC at 1100–1200 °C; and (v) reactive synthesized ternary carbides and TiC that yield porous Ti₃AlC₂ at 1300 °C.     

Mechanochemical synthesis and characterisation of BaTa2O6 ceramic powders

Mechanochemical synthesis was used to prepare BaTa₂O₆ powders from BaCO₃ and Ta₂O₅ precursors in a planetary ball mill. Effect of milling time and heat treatment temperature on the formation of BaTa₂O₆ and on the microstructure was investigated. Intensive milling of starting materials resulted in crystallization of BaTa₂O₆ even after 1 h of milling time and single phase BaTa₂O₆ was obtained after 10 h of milling under optimal conditions. The powder derived from 10 h of mechanical activation had crystallite size of 22 nm. But the increase in milling time did not decrease the crystallite size further. High energy milling activated the powders that although 1 h of milling led to formation of single phase BaTa₂O₆ at 1200 °C, this temperature decreased to 900 °C after 5 h of milling. No significant grain growth was observed when the milled powders were heat treated below 900 °C. However, annealing at 1100 and 1200 °C gave an average BaTa₂O₆ grain size of 180 and 650 nm, respectively. An unidentified phase started to form at 1100 °C increasing to high amounts at 1200 °C and they had different shapes and sizes than BaTa₂O₆ grains. These elongated large grains were thought to be due to liquid phase formation caused by iron contamination.     

Influence of glass phase on Al2O3 fiber-reinforced Al2O3 composites processed by slip casting

The present work describes the processing of alumina fiber reinforced alumina ceramic preforms consisting of chopped Al₂O₃ fibers (33 wt%) and Al₂O₃ (67 wt%) fine powders by slip casting. The preforms were pre-sintered in air at 1100 °C for 1 h. A lanthanum based glass was infiltrated into these preforms at 1250 °C for 90 min. Linear shrinkage (%) was studied before and after glass infiltration. Pre-sintered and infiltrated specimens were characterized by scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, porosimetry and flexural strength. The alumina preforms showed a narrow pore size distribution with an average pore size of ∼50 nm. It was observed that introducing Al₂O₃ fibers into Al₂O₃ particulate matrix produced warp free preforms with minor shrinkage during pre-sintering and glass infiltration. It was observed that the infiltration process fills up the pores and considerably improves the strength and reliability of alumina preform.     

Preparation of UO2/TiO2 composite ceramic fuel by sol gel-press forming method

This paper studies the preparation of UO₂/TiO₂ composite ceramic fuel through sol gel-press forming. The research shows that at 1200–1300 °C, UO₂/TiO₂ composite ceramic grains are small, uniform and compact. Among them, UO₂–0.5 wt%TiO₂ has the highest density and good mechanical strength at 1250 °C.     

Sintering of surfactant modified ZnO–Bi2O3 based varistor nanopowders

Surfactant modified nano-origin ZnO–Bi₂O₃ varistor powder was prepared in presence of cetyltrimethyl ammonium bromide (CTAB) surfactant through an aqueous reflux reaction at 100 °C. The compacted varistor discs made from the nano-origin powders were subjected to step-sintering, microwave sintering and solid-state sintering. The influences of CTAB in different sintering methods were analyzed from the densification characteristics, evolution of sintered microstructures and associated varistor properties (I–V). The conventional solid-state sintering produced 96% theoretical sintered dense samples at 1100 °C. The step and microwave sintered samples showed 93% and 99% sintered densities, respectively, with controlled microstructures having grain sizes in the range of 2–6 μm at the given conditions. The CTAB advantages were clearly seen in grain structuring and grain boundary properties, in addition to the enhanced densification and homogenous microstructures for obtaining high breakdown voltage and non-linearity coefficient.     

Synthesis and sintering of Y2O3-doped ZrO2 powders using two Pechini-type gel routes

Yttria-tetragonal zirconia polycrystal (ZrO₂ + 4.5 mol% Y₂O₃) nanocrystalline powder was synthesized by two Pechini-type gel routes, the in situ polymerized complex (IPC) method and the PEG/AF method. FTIR spectra confirmed coordination of metal ions with the polymer by different routes, depending on the method used. The crystallite size of the powder increased from 5 nm to 8 nm when the temperature was increased from 450 °C to 600 °C and calcination times increased from 2 h to 24 h. The morphology of the powders differed only when the organic impurities were not completely eliminated. After calcination, the morphology of the powders produced by the two methods showed porous agglomerates composed of smaller particles. All the resulting microstructures were very similar, regardless of the method employed to obtain the powder or the calcination times and temperatures.     

Effect of alumina addition on initial sintering of cubic ZrO2 (8YSZ)

The effect of 0–10 wt% alumina addition on the initial sintering of 8 mol% Y₂O₃ cubic ZrO₂ (8YSZ) was studied. Activation energy and initial stage of sintering mechanism were analyzed in order to understand the effect of the alumina in the sintering process. The analysis was carried out using the analytical method for constant rate heating (CRH). The activation energy decreased from 716 to 599 kJ/mol for undoped 8YSZ to 2.16 wt% of alumina–8YSZ, respectively. The mechanism for the initial stage of sintering for <2.16% Alumina–8YSZ changed from grain boundary diffusion (GBD) to volumetric diffusion (VD). With 10 wt% of alumina, the activation energy increased to 854 kJ/mol which was thought due to the change in the initial stage of sintering mechanism from VD to GBD.     

Recession behavior of Lu2SiO5 under a high speed steam jet at high temperatures

Study of recession behavior of Lu₂SiO₅ bulk was performed in high speed steam jet with a velocity of ∼50 m/s temperature range between 1300 and 1500 °C for 100 h. X-ray results showed that no phase change was observed for all samples after steam exposure. Detailed scanning electron microscopy examinations showed some cracks formation was observed on the bulk surface for the samples of 1400 and 1500 °C. Also, porous structure was formed on the bulk surface for the samples of 1300 and 1400 °C. As for 1500 °C sample, the porous structure disappeared after exposure test. The high magnification images of 1300 °C sample showed the depletion of grain boundary glassy phase. However, for 1400 °C sample, boundary phase was formed again, and the grain growth can be identified for the sample of 1500 °C. The recession mechanism can be explained by a mass transfer of evaporated species from the bulk surface and the weight loss rate measured can be expressed by Arrhenius plot.     

Phase and microstructural evolution of yttrium-doped nanocrystalline alumina: A contribution of advanced microscopy techniques

Well-dispersed nano-crystalline transition alumina suspensions were mixed with yttrium chloride aqueous solutions, with the aim of producing Al₂O₃-Y₃Al₅O₁₂ (YAG) composite powders. DTA analysis allowed to highlight the role of yttrium on the α-phase crystallization path. Systematic XRD and HRTEM analyses were carried out in parallel on powders calcined in a wide temperature range (600-1300 °C) in order to follow phase and microstructural evolution. A thin, homogeneous yttrium-rich layer was yielded on the alumina particles surface; yttrium diffusion into the alumina matrix was negligible up to 1150 °C whereas, starting from 1200 °C, aggregates of partially sintered alumina particles appeared, stuck together by yttrium-rich thin films. Moreover, in the yttrium-richer zones, such as alumina grain boundaries and triple joints, yttrium-aluminates precipitated at alumina particles surface. Finally, at 1300 °C, alumina-YAG composite powders were produced, in which YAG was homogenously distributed among the alumina grains.     

Boehmite coating on θ-Al2O3 particles via a sol–gel route

θ-Al₂O₃ powders in slurry form were coated by boehmite nanoparticles prepared from alumina sol that consists of aluminum tri-sec-butoxide (Al(OC₄H₉)₃) as a precursor. The θ-Al₂O₃ powders presented an iso-electrical point (IEP) at pH ∼ 9.2, and formed stable suspensions without notable gravity settling at acidic conditions (pH 3–4). Boehmite nanoparticles with a mean particle size ca. 68 nm were derived from the hydrolysis of Al(OC₄H₉)₃ sol. Well-dispersed boehmite/θ-Al₂O₃ aqueous mixtures consisting of 3 and 9 wt.% of boehmite were then prepared by addition of θ-Al₂O₃ powder into the Al(OC₄H₉)₃ sol followed then by vigorous agitation at 90 °C and in a pH range 3–4. IR and particle-size measurement both revealed possible coating of boehmite on the θ-Al₂O₃ particles. From TEM observation, “thickness” of the boehmite layer did not vary much with the boehmite concentration. Agglomeration of the particles became nonetheless apparent as the boehmite concentration was increased. The boehmite coating on θ-Al₂O₃ surface inhibited grain growth during the θ- to α-phase transformation upon heating. Mean grain size of the boehmite-coated alumina was substantially smaller than that of the un-coated one.     

Spinelisation and properties of Al2O3–MgAl2O4–C refractory: Effect of MgO and Al2O3 reactants

The effect of particle size of MgO and Al₂O₃ on the spinel formation associated with permanent linear change on reheating (PLCR) and microstructure of Al₂O₃–MgAl₂O₄–C refractory is investigated as a function of heating cycle at 1600 °C with 2 h holding at each cycle. It was found that rate of spinel formation and associated volume expansion is very much dependent on the reactivity and particle size of the reactant. When the reactants are very fine and reactive there is considerable amount of spinel formation, whereas coarser reactants with lower reactivity show negligible formation of spinel phase and associated expansion. Magnesia and alumina with moderate reactivity develops optimum PLCR of the refractory. It continuously increases with the number of heating cycles. The SEM photomicrographs show that in Al₂O₃–MgAl₂O₄–C refractory the spinel phase is formed in between the calcined bauxite grain and the EDX analysis indicates that the spinel phase formed is stoichiometric in nature.     

Mechanical properties of sintered La9.33Si2Ge4O26 oxyapatite materials for SOFC electrolytes

Mechanical properties of La_9.33Si₂Ge₄O₂₆ prepared by mechanical alloying and subsequent sintering at 1300–1400 °C for 1 h were evaluated. Hardness and Young's modulus values in the range 7.3–9.6 GPa and 106–135 GPa, respectively, were obtained from nanohardness tests. The fracture toughness values derived from the Palmqvist method varied between 3.5 and 3.9 MPa m^1/2 from classical microindentation test with an indentation load of 9.8 N. Yield stress (σ_y) was determined by inverse analysis from microhardness tests. The maximum value of σ_y (1829 MPa) was obtained for the sample sintered at 1400 °C showing the highest density (5.42 g/cm³).     

Co-precipitated ZnAl2O4 spinel precursor as potential sintering aid for pure alumina system

Ultra-fine ZnAl₂O₄ spinel hydrogel precursor synthesized from mixed salt solutions of Zn²⁺ and Al³⁺ ions using ammonium hydroxide–hexamethylenetetramine as basic media for co-precipitation was used as bonding material and sintering aid for pure alumina system. The hydrogel powder exhibited some well-defined ZnAl₂O₄ spinel phases at 800 °C. Alumina compacts were fabricated by incorporating small proportions of the precursor in alumina powder and firing at different temperatures (1350–1500 °C). The degree of densification was studied by measurement of fired shrinkage, apparent porosity, bulk density and cold crushing strength. Phase compositions and microstructural features of sintered samples were evaluated by XRD and SEM respectively. Addition of 0.2% hydrogel powder to alumina exhibited remarkable influence on development of high mechanical strength. The in situ formed ZnAl₂O₄ spinel dopant acted as a grain growth inhibitor in the alumina system.     

Synthesis of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 powders through the molten salt method

The Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ powder synthesis through molten salt method was investigated in the temperature range of 650–700 °C for 2–4 h. The XRD results indicated that the optimal synthesizing temperature for molten salt method was 700 °C, significantly lower than that for conventional processing route of solid state reaction method, where a calcining temperature of 850 °C was needed. The SEM results revealed better crystallization of the powders obtained through molten salt method, compared with those through the conventional processing route of solid state reaction method.