Aqueous slip casting of transparent yttrium aluminum garnet (YAG) ceramics

Transparent YAG ceramics were prepared by slip casting an aqueous dispersed mixture of commercial Al₂O₃ and Y₂O₃ powders. The powders were co-dispersed with poly(acrylic acid) and citric acid. Polyethylene glycol of 0.5 wt.% (PEG 4000) and 0.5 wt.% tetraethyl orthosilicate were added as binder and a sintering aid, respectively. Dried samples were vacuum sintered at 1800 °C for 16 h. In general, YAG ceramics cast from Newtonian suspensions were optically transparent and had optical transmittances >80% from 340 to 840 nm. Slightly flocculated dispersions, as evidenced by higher viscosity and non-Newtonian rheology, resulted in translucent samples with large pores and lower optical transmittances.     

Co-precipitation synthesis route to yttrium aluminum garnet (YAG) transparent ceramics

Yttrium aluminum garnet (YAG) precursor was synthesized via a coprecipitation method with aluminum nitrate and yttrium nitrate as raw materials, using ammonium hydrogen carbonate (AHC) as the precipitant. Fine and low-agglomerated YAG powder was obtained by calcining the precursor at 1200 °C. The primary crystallites were measured to be ～120 nm in size and weakly agglomerated to a particle size of ～500 nm, indicating a high degree of sinterability. With 0.5 wt% tetraethyl orthosilicate (TEOS) and 0.1 wt% magnesia as sintering aids, transparent YAG ceramics were fabricated by vacuum sintering at 1730–1790 °C for various hours. The influences of sintering temperature and holding time on the microstructure and transmittance of YAG ceramics were discussed.     

Structure formation in yttrium aluminum garnet (YAG) fibers

Polycrystalline yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) fibers were prepared from aqueous solutions of aluminum chlorohydrate and yttrium chloride. Fiber processing was accomplished via dry spinning. Poly(vinylpyrrolidone) (PVP) was used as spinning aid. Polycrystalline YAG fibers were obtained by pyrolysis of the green fibers followed by sintering at defined temperatures in air. Ceramic fibers were 9–16 μm in diameter. Differential scanning calorimetry/thermogravimetric analysis coupled with mass spectrometry (DSC/TGA-MS) showed an exothermic peak at 920 °C assigned to the crystallization of YAG and an overall ceramic yield of 38% at 1400 °C. X-ray diffraction (XRD) analysis showed that phase-pure YAG can be obtained at 1600 °C after intermediate formation of Y₂O₃ and monoclinic yttrium aluminum oxide (YAM, Y₄Al₂O₉) phases.     

Transparent yttrium aluminum garnet (YAG) ceramics by spark plasma sintering

Commercial nanocrystalline yttrium aluminum garnet (nc-YAG) powders were used for fabrication of dense and transparent YAG by spark plasma sintering (SPS). Spherical 34 nm size particles were densified by SPS between 1200 and 1500 °C using 50 and 100 MPa pressures for 3, 6, and 9 min durations. Fully dense and transparent polycrystalline cubic YAG with micrometer grain size were fabricated at very moderate SPS conditions, i.e. 1375 °C, 100 MPa for 3 min. Increase in the SPS duration and pressure significantly increased the density especially at the lower temperature range. The observed microstructure is in agreement with densification by nano-grain rotation and sliding at lower densities, followed by curvature driven grain boundary migration and normal grain growth at higher densities. Residual nanosize pores at the grain boundary junctions are an inherent microstructure feature due to the SPS process.     

Prospects for creating new optically transparent materials with yttrium oxide and yttrium aluminum garnet (review)

The properties of optically transparent ceramic materials made of yttrium oxide and yttrium — aluminum garnet were analyzed. The promise of preparing a transparent ceramic that combines both of these compounds was demonstrated. Methods of synthesizing yttrium oxide and yttrium — aluminum garnet were examined. Selection of the method of synthesis was substantiated. The possibility of creating such materials without modifying additives was confirmed.     

Fabrication of transparent neodymium-doped yttrium aluminum garnet ceramics by high solid loading suspensions

Dense neodymium-doped yttrium aluminum garnet (Nd:YAG) transparent ceramic was obtained by slip casting and solid-state reaction. The colloidal behavior of the aqueous suspensions of neodymia, yttria, and alumina mixed powders using Dispex A as dispersant was investigated. The variation in zeta potential due to pH alteration was studied. The isoelectric point (IEP) was at pH 4.5 and 4 for the specimens with and without Dispex A, respectively. The optimal dispersion conditions were achieved for the suspensions at pH 9.6 with 0.4 wt% Dispex A. The green body prepared by slip casting was vacuum sintered from 1200 °C to 1750 °C. The grain size of the sintered body increased, and the pore size decreased with increasing sintering temperature. Pore-free Nd:YAG transparent ceramic with a grain size of 5–10 μm was obtained by sintering at 1750 °C for 10 h. The in-line transmittance of the annealed specimen reached 80.8% at 1064 nm.     

Co-precipitation synthesis and sintering of yttrium aluminum garnet (YAG) powders: the effect of precipitant

YAG precursors were co-precipitated from a mixed solution of aluminum and yttrium nitrates using ammonia water and ammonium hydrogen carbonate as precipitants, respectively. Phase evolution of the precursors during calcination and sinterability of the resultant YAG powders were compared between the two methods. The use of ammonia water produced a hydroxide precursor with an approximate composition of Al(OH)₃·0.3[Y₂(OH)₅(NO₃)·3H₂O] which transformed to pure YAG at about 1000°C via YAlO₃ phase. Severe agglomeration caused poor sinterability of the resultant YAG powders. The use of ammonium hydrogen carbonate produced a carbonate precursor with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.8H₂O. The precursor directly converted to pure YAG at about 900°C. The precursor was loosely agglomerated and the resultant YAG powders showed good dispersity and excellent sinterability. For the same calcination temperature of 1100°C, YAG powders from the hydroxide precursor and the carbonate precursor densified to ∼81.2 and ∼99.8% of the theoretical, respectively, by vacuum sintering at 1500°C for 2 h.     

The effect of precipitant concentration on the formation procedure of yttrium aluminum garnet (YAG) phase

In this paper, the effect of precipitant concentration on the formation procedure of YAG phase, which is actually a solid reaction controlled by the diffusion of aluminum ions into the yttria particles, is investigated. Under higher precipitant concentration, the size of yttria particles formed is in micron level and alumina particle is in the form of clusters of small particles. Therefore the element diffusion distance is lengthened and higher temperature (≥1350 °C) is essential for the completion of phase transitions. Under lower precipitant concentration, the sizes of yttria particles and alumina particles are both smaller than 100 nm and two kinds of oxides are mixed uniformly, so the phase transition procedure can complete at lower temperatures (≤1200 °C).     

Synthesis of nanopowders of yttrium aluminum garnet doped by cerium(III)

New methods for synthesizing yttrium aluminum garnet doped by cerium have been developed. Nanopowders of precursors are prepared by the combustion of Al, Y, and Ce nitrates in glycerin and in a solution of fructose with microwave activation of the process. The synthesized structures are investigated using IR spectroscopy, X-ray powder diffraction, and a combined thermal and dilatometric analysis. It is shown that the formation of the garnet phase is completed when precursors are heated to 1000°C. It is established that the mechanisms of crystallization of the garnets prepared in glycerin and in fructose are different.     

Synthesis of yttrium aluminum garnet (YAG) powder by homogeneous precipitation combined with supercritical carbon dioxide or ethanol fluid drying

YAG precursors were synthesized by the urea method in aqueous solution using supercritical carbon dioxide and ethanol fluid drying technique, respectively. The composition of the precursors, the phase formation process and the properties of the calcined powders were investigated by means of XRD, IR, TG/DSC, BET, TEM and SEM. Compared with the classically prepared powders at room temperature in air, the amorphous precursor dried by supercritical CO₂ fluid was loosely agglomerated and directly converted to pure YAG at about 900 °C. The resultant YAG powders showed good dispersity with an average crystallite size about 20 nm and specific surface area of 52 m² g⁻¹. However, the precursor dried by supercritical ethanol fluid was crystalline. Extensive phase segregation occurred during the drying process and resulted in the formation of separate phases such as monoclinic Y(OH)₃ and pseudoboehmite. YAM and YAP phases appeared in the calcination process and phase pure were not detected until 1200 °C.     

Luminescent and magnetic properties of cerium-doped yttrium aluminum garnet and yttrium iron garnet composites

Functional materials exhibiting magnetic and luminescent properties have been recognized as an emerging class of materials with great potential in advanced applications. Herein, properties of multifunctional ceramic composites consisting of two garnets, luminescent cerium-doped Y₃Al₅O₁₂ (Ce:YAG) and magnetic Y₃Fe₅O₁₂ (YIG), are reported. On increasing the sintering temperature, both the photoluminescence and saturation magnetization of the Ce:YAG-YIG composites decreased gradually because of the interdiffusion of trivalent ions such as Al³⁺ and Fe³⁺. At a constant sintering temperature of 1100?°C, the YIG contents in the composites increased, thereby causing their luminescent properties to degrade and the saturation magnetizations to increase. For application to electronics, Ce:YAG-YIG composite thin films were integrated on quartz substrates by sputtering the ceramic target. The composite thin films exhibited both magnetic and luminescent properties after annealing. These techniques facilitate the incorporation of multifunctional nanocomposites into various devices.     

Densification of SiC by colloidal processing and SPS without sintering additives

Abstract

Silicon carbide is one of the most important ceramics used as structural and functional materials in a wide variety of applications. Many studies have reported the densification of SiC using oxide and nonoxide additives such as the Al₂O₃, B₄C and Al–B–C system. However, it is difficult to densify SiC at temperatures below 2000°C without sintering additives even if spark plasma sintering (SPS) is used. The authors attempted to densify SiC using colloidal processing and SPS without sintering additives. A commercially available SiC powder with the average particle size of 0·55 μm was used as the starting material. The densities of the green body prepared by slip casting and the sintered body by SPS were 65·5 and 98·7% respectively.     

Effect of sodium on the creep resistance of yttrium aluminium garnet (YAG) fibres

Aligned YAG fine fibres had been made previously from an aqueous sol–gel process, but it was suspected that sodium contamination in one of the starting materials lessened the creep resistance of the final product. Therefore, sodium-free gel precursor fibres were made using the same process, and upon firing in air at 900 °C these formed pure phase YAG fibres, of 4 μm diameter. When steamed over 200–500 °C for 3 h, the amorphous gel fibre formed single phase nanocrystalline YAG between 400 and 500 °C, an extraordinarily low temperature for this material to crystallise. Upon postfiring up to 1550 °C, grains averaging 0.5 μm and pores of 0.17 μm had formed, but despite this porosity and smaller grain size, the sodium-free fibre exhibited superior creep resistance than the sodium contaminated fibres reported previously by the authors, typically creeping at temperatures 50 °C higher. This demonstrates that even small levels of sodium contamination are harmful to creep resistance in YAG.     

Pressureless reaction sintering of yttrium aluminium garnet (YAG) from powder precursor in the hydroxyhydrogel form

Pressureless reaction sintering of YAG was carried out by using a precursor powder in the hydroxyhydrogel form synthesized by flash polycondensation technique. Reactive silica additive was used to modify Al-O-Y network where Si⁴⁺ was incorporated in the solvated form. The sintering experiments were carried out with the processed powder up to 1650 °C under ambient ambience. Densification indicated effective sintering with well-formed microstructure. Phase identification by XRD indicated only YAG in the sintered specimen.     

The effect of powders homogenisation conditions on the synthesis of yttrium aluminium garnet (YAG) by a solid-state reaction

The work presents results of research on the influence of the state of homogenisation of reagent powders i.e. aluminium oxide and yttrium oxide on the synthesis of yttrium aluminium garnet. Fine powders of commercial aluminium oxide and yttrium oxide were mixed in the same manner in a laboratory ball mill but under four different conditions. Three batches of powders were prepared in water of various pH values, selected on the basis of the zeta potential measurements, and one batch was prepared in propanol. They were consolidated by filter pressing and then sintered in air at a temperature from 900 °C to 1700 °C for 1 h, with heating rate of 5 °C/min. Samples were subjected to dilatometric studies, phase composition determination, measurement of apparent density and pore size distribution, as well as a microscopic examination. Mixing environment affected, to a given extent, almost all the features of green, and partially sintered materials as well as course changes of linear dimensions. As a result, the relative densities of samples sintered at a maximum temperature ranged from 58.31% to 98.25%. The highest density was achieved for the material originating from the suspension having a pH of 9.5, in which heterocoagulation occurred.     

Formation of nanostructured particles of cerium-activated yttrium aluminum garnet by the combustion method

The synthesis of ultradispersed powders of the yttrium aluminum garnet (activated by ions of cerium, lithium, silicon, and manganese) by the combustion method in the presence of hexamethylenetetramine has been optimized. The structural, physicochemical, and spectral-luminescence properties of synthesized finely dispersed samples have been studied. It has been found that exclusively the phase of garnet Y₃Al₅O₁₂ of the cubic modification is identified in the products of the synthesis performed under optimum condition.     

Densification of plasma sprayed YSZ electrolytes by spark plasma sintering (SPS)

Solid oxide fuel cells (SOFC) are promising candidates for alternative power generation systems due to their high-energy conversion efficiencies, and low emissions of environmentally hazardous by-products. Plasma spray (PS) is an effective, and relatively inexpensive process for fabricating high performance yttria stabilized zirconia (YSZ) electrolyte for SOFC. Yet, because of the numerous inter-granular defects introduced to the electrolyte by the plasma spray process, the electrolyte is not gas tight and consequently, the energy efficiency of the cell is severely curtailed. In order to improve the performance of the SOFC, spark plasma sintering (SPS) is introduced as a post-spray treatment to enhance the density of the PS YSZ electrolyte rapidly, and effectively. In this study, spark plasma sintering (SPS) was performed at 1200, 1400 and 1500 °C. Each sintering cycle had a holding time of 3 min. Single and multiple SPS cycles (3 min at preset temperature per cycle) were used to treat the plasma sprayed yttria stabilized zirconia (PS YSZ) electrolytes. The microstructure of as-received and SPS treated electrolytes as examined by scanning electron microscopy (SEM) demonstrated a microstructure transition above 1200 °C, where the typical plasma sprayed lamella structure transformed to a granular-type structure. The porosity of as-received and SPS post-treated electrolytes, which were determined by a mercury intrusion porosimeter (MIP) revealed a significant reduction in pores at 1500 °C. Average pore size reduced from 0.2 to 0.08 μm. The ionic conductivity of the electrolytes is evaluated by AC impedance spectroscopy to characterize the effect of SPS on enhancing the ionic conductivity of the electrolytes.     

Processing and properties of porous SiC ceramics prepared by yttrium aluminum garnet infiltration

Oxide bonded porous SiC ceramics were synthesized by infiltrating a liquid precursor of yttrium aluminum garnet into porous powder compact of SiC followed by sintering at 1300‐1500°C in air. Infiltration rate was estimated using weight gain by the liquid precursor sol into porous SiC powder compact as a function of time and was explained by Darcy's and Ficks's laws. The effects of SiC particle sizes and sintering temperatures on the formation of bonding phases, microstructure, SiC oxidation degree, flexural strength, porosity, and pore size distribution of porous SiC ceramics were studied. Various crystalline oxide phases were detected by XRD analysis. Depending on the starting SiC powder sizes and sintering temperatures, the porosity of the final ceramics varied nearly in the range of ~29‐41 vol. % with the variation of average pore diameter between ~5 and 30 μm. Flexural strength varied from 41 to 8 MPa depending on porosity. The effect of corrosion on oxide bond phases was investigated in strong acidic and basic medium at 90°C. The ceramics showed better corrosion resistance in acidic medium compared to basic medium. In basic medium, significant reduction in flexural strength (~42%) was arisen.