α-Al2O3 Powders Research Articles

Effect of citric acid on the microstructure and optical properties of transparent YAG ceramics by reactive-SPS

Commercial α-Al2O3 and Y2O3 powders with 0.5 wt% TEOS were used to fabricate transparent polycrystalline YAG ceramics by reactive spark plasma sintering. The powder mixtures were dispersed with dispersants of PAA and citric acid in aqueous slurry. YAG specimens sintered at 1400 °C for 3 min were used to investigate the effect of processing parameters on the microstructure and the optical properties. It is found that a pretreatment of the powders mixture with 0.6 wt% citric acid can significantly improve the microstructure homogeneity of the as-sintered YAG ceramics. After post-annealing in wet air, the specimen sintered from the treated powders mixture was transparent, while the specimen from the untreated powders mixture was hazy. The effects of citric acid for this were investigated in this paper.

Effect of process factors on properties of high dispersion spherical α-Al2O3 particles prepared by hydrothermal method

Spherical α-Al2O3 powders were synthesized from a mixed solution consisting of Al2(SO4)3·18H2O, Al(NO3)3·9H2O and CH4N2O by hydrothermal method with muffle calcination and microwave calcination. Effects of raw material ratio, reaction temperature, and filling factor on properties of products were investigated. Properties of thermal behavior, functional groups, phase composition, microstructure and specific surface of precursors and products were characterized by DSC-TG, FT-IR, XRD, SEM and BET, respectively. Results show that SO42− plays an important role in the sphere formation of precursors and particle size can be changed by adding different proportion of NO3−. Precursors are amorphous spherical hydrated alumina containing SO42−, and the conversion of crystalline phase is from amorphous to γ-phase and then to α-phase at 1100 °C. Spherical morphology can be retained after crystallization to α-Al2O3. Compared with muffle furnace, Microwave calcination can increase calcining rate by three times and reduce insulation time by three quarters. When hydrothermal reaction temperature is 120 °C, the particles can be precipitated with good sphericity and narrow grain size distribution. If hydrothermal reaction temperature is too high or too low, spherical morphology can't be obtained. Size of the particles can be regulated by the different raw material ratio and filling factor from 800 nm to 3 μm, and specific surface area is in the range of 8 m2/g~15 m2/g.

Modification of matrix for magnesia material by in situ nitridation

Abstract Mg-α-SiAlON bonded periclase composites were prepared at 1550 °C in a nitrogen atmosphere using sintered magnesia particles, Al metal powder, Si powder, magnesia, and α-Al2O3 powder as raw materials and were bonded with a phenolic resin. The modification mechanism was investigated. Tabular-shaped Mg-α-SiAlON can be obtained through the diffusion-reaction-dissolution-precipitation mass transfer mechanism. It was found that modification of the matrix for the magnesia material led to a direct bonding mode and a card structure. The optimal percentage of powder material was determined to be 40 wt%, by which a well-developed card structure was obtained. The sintered Mg-α-SiAlON bonded periclase composites exhibited excellent cold modulus of rupture (12.5 MPa), and the residual strength retention rate was 3 times higher than that of a magnesia specimen after the third thermal shock. The enhanced thermal shock resistance was attributed to a lower thermal expansion coefficient and the “crack deflection” mechanism of tabular-shaped Mg-α-SiAlON.

Effect of boehmite sol on the performance of alumina microfiltration membranes

Abstract Precursor film method was used to prepare highly permeable ceramic microfiltration membranes in this work. The performance of alumina microfiltration membranes was improved by adding boehmite sol in the membrane precursor film. With increasing the boehmite sol content, the effective average pore size of the membrane was continuously decreased and the separation efficiency of the membrane was increased. These improvements were due to that the boehmite sol not only helped the dispersion of the α-Al2O3 powder in the membrane forming slurry, but also formed γ-Al2O3 in the gaps among the existed α-Al2O3 particles which was beneficial for membrane sintering and pore size decreasing. For the membrane prepared with 45 wt% boehmite sol, the effective average filtration pore diameter was 178 nm and the water permeance of the membrane reached 1691 Lm−2h−1bar−1, which was much better than the values reported before. Moreover, the reusability of the membrane was confirmed using a recycling test.

Effect of (Tb+Y)/Al ratio on Microstructure Evolution and Densification Process of (Tb0.6Y0.4)₃Al₅O12 Transparent Ceramics.

(Tb0.6Y0.4)3Al5O12 transparent ceramics were successfully fabricated by solid-state reactive sintering using Tb4O7, Y2O3, and α-Al2O3 powders as raw materials. The effect of (Tb+Y)/Al ratio on microstructure evolution and densification process was investigated in detailed. The results showed that the grain growth kinetics were significantly affected by (Tb+Y)/Al ratio. Al-rich and Tb-rich phases appeared in part of the samples of different ratios. Particularly, excess aluminum increased the diffusing process, leading to a higher densification rate, while samples with excess terbium ratios displayed a smaller grain size and lower relative density. The optical quality was highly related to the amount of the secondary phase produced by different (Tb+Y)/Al ratios. Finally, (Tb0.6Y0.4)3Al5O12 transparent ceramics have been fabricated through pre-sintering in vacuum, followed by hot isostatic sintering (HIP), and the best transmittance of sample with a 4 mm thickness was approximately 78% at 1064 nm.

Grain growth kinetics and growth mechanism of columnar Al2O3 crystals in xNb2O5–7.5La2O3-Al2O3 ceramic composites

xNb2O5–7.5La2O3-Al2O3 ceramic composites with in-situ-grown columnar Al2O3 crystals were successfully prepared by microwave sintering at 1450–1525 °C using α-Al2O3, Nb2O5, and La2O3 powders as raw materials. X-ray diffraction results indicated that the main phases were Al2O3, LaNbO4, and Nb2O5 in the prepared samples. A field emission scanning electron microscope (FESEM) showed that the Al2O3 crystals appeared as columnar in the structure. Moreover, the grain size of the columnar Al2O3 crystals increased with the Nb2O5 content. The ratio of the major axis to the minor axis of the crystals was largest when the Nb2O5 content was 15 vol%. Furthermore, the grain-growth kinetics index (n), growth activation energy (Q), and growth mechanism of the columnar Al2O3 crystals were studied. The results indicated that the Nb2O5 addition could promote formation and growth of columnar Al2O3 crystals, and the grain-growth activation energy indicated that the dissolution process controls the crystal growth. The growth mechanism of the columnar Al2O3 crystals was also studied. The present work demonstrated that Nb2O5 is a good additive for the preparation of Nb2O5–7.5La2O3-Al2O3 composite ceramics with columnar Al2O3 crystals.

Fabrication and properties of 10 at.% Yb:Y3Sc1.5Al3.5O12 transparent ceramics

10 at.% Yb:Y3Sc1.5Al3.5O12 (Yb:YSAG) transparent ceramics were fabricated by the solid state reactive of high-purity Y2O3, α-Al2O3, Yb2O3 and Sc2O3 powders, utilizing MgO and tetraethoxysilane (TEOS) as the sintering aids. The Yb:YSAG ceramic sample sintered at 1820 °C for 10 h has a dense and homogenous microstructure with average grain size of 26.9 μm, whose in-line transmittance reaches 83.2% at 1100 nm (2 mm thick). The absorption and emission cross sections are calculated to be 5.09 × 10−21 cm2 at 941 nm and 12.33 × 10−21 cm2 at 1031 nm, respectively. The full width at half maximum (FWHM) of the main emission peak centered at 1031 nm is 12.4 nm, which is about 1.3 times larger than that of the 10 at.% Yb:YAG ceramics. By using the three-mirror resonator, the Yb:YSAG ceramic laser output power of 1.67 W at 1031 nm and slope efficiency of 13.1% were obtained with an output coupler (OC) of T = 5%.

Novel iron-rich mullite solid solution synthesis using fused-silica and α-Al2O3 powders

An iron-rich mullite solid solution was synthesized by using α-Al2O3, fused-silica and Fe2O3 powders at elevated temperatures. The phase compositions and microstructures of the synthesized samples were characterized by X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and field emission scanning electron microscope (FESEM). The occurrence states of Fe2+ and Fe3+ at different temperatures were analysed by X-ray photoelectron spectroscopy (XPS). The influence and the reaction mechanism of Fe2O3 addition on the synthesis of the mullite solid solution were clarified. The results showed that with the addition of Fe2O3, a liquid-solid sintered state was formed at 1300–1400 °C, which leaded to a reduction in the mullitization temperature. After sintering at 1600 °C, the quantitative analysis showed that the mullite phase content was 100%. Meanwhile, the Fe3+/2+ ions entered completely into the mullite to form a stable solid solution, which exhibited the crystal morphology of a spherical shape and a short columnar shape with a low aspect ratio. The crystal grains were interwoven and squeezed each other, showing good structural stability. The refractoriness under load (RUL) property of the sample sintered at 1700 °C was slightly higher than that of the sample sintered at 1600 °C.

Magnetron sputtering fabrication of α-Al2O3:Cr powders and their thermoluminescence properties

For many years doped α-Al2O3 has attracted interest as a dosimeter for personal, environment monitoring and food control. The alumina single crystal growth is a difficult process; however, materials in form of powders, ceramics and coating are possible to obtain. In this study for the first time Cr doped α-Al2O3 powders were prepared by DC reactive magnetron sputtering followed by milling and oxidation. The morphology and phase analysis was performed; content of residual impurities was determined and thermostimulated glow curves were measured after different x-ray irradiation times (radiation doses). The prepared powder shows the dosimetry properties up to 20 kGy dose.

Microstructure and optical properties of alumina sintered from various phases

Self-synthesized and commercial alumina (boehmite, γ-Al2O3, α-Al2O3) powders were consolidated using an identical spark plasma sintering cycle, and optically translucent samples were obtained. The benefit of higher pressure is remarkable grain growth suppression. Additionally, the shorter dwell time at higher pressure advantageously leads to a reduction in grain size while conserving the optical transparency and without affecting the density.

Microstructure, optical and electrical characterizations of nanocrystalline ZnAl2O4 spinel synthesized by mechanical alloying: Effect of sintering on microstructure and properties

Nanocrystalline ZnAl2O4 spinel compound has been synthesized by mechanical alloying the stoichiometric mixture of ZnO and α-Al2O3 powders. Required phase is initiated within 1 h of milling and nanocrystalline ZnAl2O4 mixed spinel phase has been obtained after 4 h of milling. Phase transformation from mixed spinel to normal spinel structure occurs through redistribution of cations in tetrahedral and octahedral void spaces after 5 h of sintering at 1173 K. Structure and microstructure characterizations of all milled samples have been carried out by analyzing the respective XRD patterns employing Rietveld's refinement method and HRTEM images. The optical band gap of semiconducting ZnAl2O4 is blue shifted due to the sintering effect. All synthesized samples exhibit PL emissions primarily within the UV region. Both the dc and ac electrical conduction mechanisms have been explained respectively by the hopping transport phenomenon and frequency dependent conductivity behaviour, which obeys the Jonscher power law. A better optical property, enhanced electrical conductivity and low dielectric constant value make the synthesized semiconducting ZnAl2O4 compound suitable for optoelectronic applications.

Densification and Mechanical Properties of YAG Ceramics Fabricated by Air Pressureless Sintering

Yttrium aluminum garnet (Y3Al5O12, YAG) ceramics with a relative density of about 99.5% were fabricated by solid-state reaction and pressureless sintering using commercial α-Al2O3 and Y2O3 powders as raw materials in air. The effect of different sintering additives, e.g. tetraethyl orthosilicate (TEOS), MgO and TEOS + MgO, on the relative density and mechanical properties of the YAG ceramic samples were investigated. When the dopant content of TEOS exceeded 0.5 wt.%, a sudden grain growth appeared, as well as a residual second phase located in triangle grain boundary. MgO was more effective as a sintering additive than silica in promoting the densification of the YAG ceramics. The relative density of the ceramics gradually decreased with the increase of MgO doping content and fixed TEOS content. The mechanical properties of the YAG ceramics fabricated by pressureless sintering in air at 1600–1710°C were tested. The fracture surface morphology of the MgO-doped ceramic samples was mainly transgranular, whereas the TEOS-doped samples were mainly intergranular.

Preparation of high purity α-Al2O3 particles derived from industrial waste solution via microwave calcining and seeding technique

Anodic aluminium oxidation course is an important method for metal surface finishing, which produces large quantity of waste acid solutions containing and Al3+. In this study, high purity α-Al2O3 particles were synthesized from this waste solution via recrystallization and microwave calcination with seeds addition. Effects of pH value, recrystallization times, seeding technique and calcination process on the properties of products were investigated. Phase composition, microstructure, crystal grain size and purity of products were studied by XRD, TG-DSC, SEM, BET, laser particle size analysis and ICP-AES. The results show that when pH value is 3.0 and recrystallization time is twice, high purity NH4Al(SO4)2 · 12H2O (≥99.999%) precursor is produced. Addition of seeds and microwave technique can lower the calcination temperature and improve the performance of products. Well dispersed ultrafine α-Al2O3 powders (≥99.99% mass fraction), which have short rod-like shape with 150 nm in length and 100 nm in diameter, can be obtained at 1050 °C for 30 min, which is 150 °C lower than conventional calcination, through microwave heating the precursor with 5 wt% seeds content. Seeds can improve the phase transformation rate of α-Al2O3, and microwave can increase the rate of precursor decomposition.

Controlled Synthesis and Application of α-Al2O3 for Lu3Al5O12: Ce3+ Green Spherical Phosphors

α-Al2O3 powders with different morphologies, namely fibrous, sheet-like, and spherical, were prepared by the hydrothermal-thermolysis method. Subsequently, polycrystalline, transparent cerium doped lutetium aluminum garnet (Lu3Al5O12:Ce3+) green phosphors were synthesized by high temperature solid-state method using commercial lutetium(III) oxide, cerium(III) oxide, and as-prepared Al2O3 powders with different morphologies. The phases, morphologies, and photoluminescent properties of the prepared phosphors were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM), and photoluminescence spectroscopy(PL). Moreover, the influences of the morphologies of α-Al2O3 on the types of crystal structure, morphologies, and photoluminescent properties of LuAG:Ce3+ green phosphors were investigated. The results indicated that the morphologies and particle sizes of the α-Al2O3 powders could be controlled by the additives and parameters. Notably, the spherical α-Al2O3 powders with good dispersibility were found to be the excellent base materials of LuAG:Ce3+ green phosphors for white light emitting diodes.

Electric Conductivity of α-Al2O3 Suspensions in Carbonate and Carbonate-Chloride Melts

The temperature dependence of the specific electric conductivity of suspensions based on carbonate and carbonate-chloride melts immobilized with α-Al2O3 powder was studied. For carbonate-chloride melts immobilized with α-Al2O3, the decrease in the specific electric conductivity considerably exceeded the effect associated with a decrease in the volume fraction of charge carriers. The Raman spectroscopy and Xray diffraction analysis showed that there was no chemical interaction of α-Al2O3 with the carbonate and carbonate-chloride melt that could lead to the formation of additional nonconducting phases in these systems in the temperature range under study.

Influence of high-energy ball milling and additives on the formation of sphere-like α-Al2O3 powder by high-temperature calcination

Abstract In comparison with the typically worm-like α-Al2O3 powders formed from an unground Al(OH)3 precursor calcined at 1450°C, spherical α-Al2O3 powders with ~1 μm in size were prepared by the calcination of a ground Al(OH)3 precursor with 5 wt.% [NH4]+BF4 − under the same calcination conditions. The influence of a high-energy ball milling pretreatment as well as of the additives on the morphological evolution of α-Al2O3 powders was studied using the commercial precursor Al(OH)3 as raw material. The results indicate that the morphology of α-Al2O3 powders is closely related to the morphology of the Al(OH)3 precursor and to the introduction of different additives. The refinement of the Al(OH)3 precursor in aggregate size and of the primary crystal size by high-energy ball milling has effectively suppressed the neck growth of α-Al2O3 grains. In contrast to the findings made previously with the introduction of 5 wt.% [NH4]+Cl−, the morphology of the α-Al2O3 particles could be significantly improved from a ground Al(OH)3 precursor with the addition of 5 wt.% [NH4]+BF4 −, which resulted in the formation of spherical α-Al2O3 powders with 1 μm size at 1450°C.

Preparation of High-Quality Porous Calcium Hexa-Aluminate Ceramics using Salt as Both Reaction Medium and Pore-Former

Porous calcium hexa-aluminate (CA6) ceramics were synthesized by a molten salt-mediated technique using CaCO3 and α-Al2O3 powders as the main raw materials. The effects of salt type and content, along with firing temperature on phase formation, microstructural evolution, pore generation and size distribution, and mechanical strength of fired samples were investigated and compared. Among the three tested salts (KCl, NaCl, and CaCl2), KCl showed the best whereas CaCl2 showed the worst accelerating effect on CA6 formation. In the case of the former, phase-pure CA6 was formed after 3 h at 1500 °C, whereas in the case of the latter, CA2 was formed instead as a main phase. The salt not only acted as a reaction medium, accelerating CA6 formation, but also acted as a pore former, creating high levels of porosity in the fired samples. The apparent porosity of the fired samples clearly increased with an increase in KCl content. But the majority of pores were still micron sized and the pore size distribution was relatively narrow. This, along with the possible toughening effect from the well-developed plate-like CA6, conferred relatively high mechanical strength on the final highly porous CA6 ceramics.

Catalytic Decomposition of Nitric Oxide by LaCoO3 Nano-particles Prepared by Rotary CVD

Catalytic direct decomposition of NO by perovskite-type catalysts has attracted much attention for the various possible components and the unique structure. LaCoO3 nanoparticles were precipitated on α-Al2O3 micro powders by rotary chemical vapor deposition (rotary CVD) and its catalytic performance for the decomposition of NO was investigated. LaCoO3 nano-particles with 100 nm in average diameter and 1.5% in mass were uniformly dispersed on α-Al2O3 powder. The conversion of NO increased with increasing temperature from 400 to 950 °C, and reached 28.7% at 950 °C. The gas velocity of transformed NO on LaCoO3 nano-particles catalyst per mass unit was 7.7 mL/(g min), showing a good catalytic activity over the calculated results of pure catalysts. After five times of aging performance experiments, the NO conversion kept the same value, showing a good aging performance and thermal stability.

Weakly agglomerated α-Al2O3 nanopowders prepared by a novel spray precipitation method

In this paper, weakly agglomerated and well dispersed α-Al2O3 powders were synthesized by a novel spray precipitation method. It was demonstrated that the as-prepared powders exhibited better dispersity than powders from conventional precipitation due to the increased phase contact and reaction area during the precipitation process. The effects of different titration ways, calcination temperature and holding time on the morphology, phase composition and sintering behaviour of Al2O3 powders were systematically investigated. Weakly agglomerated and well crystallized α-Al2O3 powders were obtained when the as-prepared precursors were calcined at 1150 °C for 2 h in air. The average particle size of α-Al2O3 powders with higher sintering activity was approximately 68.6 nm, and the specific surface area was above 22.4 m2 g−1.

Application of rocking-scan method to detect the low-content diamonds in a complex mixture

A method for detecting large-size diamonds with low content in a complex mixture was described. Phase identification via conventional X-ray diffraction method is unsuitable because of the large-size and low-content characteristics of the diamonds. Thus, we proposed a convenient approach (i.e., rocking scan) for assessing the presence of low-content diamonds in a complex mixture. A thorough rocking scan of the (111), (220), and (311) planes revealed the presence of low-content diamonds prepared by reducing dense carbon dioxide with alkali metals. This procedure can be successfully applied to several standard mixture samples prepared by mixing α-SiO2, α-Al2O3, graphite, and pure diamond powders, which are commercially available. We estimated that the detection limit was at or below the 0.1 wt% level by using the proposed method under the current condition. This method is straightforward, routinized, and may be specifically developed to satisfy the requirements of public and private institutions for a rapid identification of other mixture phases, of which the large-size and low-content characteristics do not allow an instant phase identification by conventional X-ray diffraction methods.