α-Al2O3 Powders Research Articles

製造工程の違いが及ぼすαアルミナ粉体の表面状態とその水和挙動への影響

In the case of high purity α-Al2O3 powder, the surface may contain an aluminum hydroxide phase, which may bring differences in surface property of α-Al2O3 powders. The aluminum hydroxide phase present on the surface sometimes leads to porous structure in sintered bodies. Therefore, the investigation for hydration ability of powder surfaces is very important for ceramic manufacture. In this paper, the effects of manufacturing process of commercial high purity α-Al2O3 powders on their surface characteristics are discussed.

Thermoluminescence and optically stimulated luminescence characteristics of α-Al2O3：C crystal

In this work，α-Al2O3：C crystal was grown by temperature gradient technique using α-Al2O3 and graphite powder as raw materials， and the thermoluminescence (TL) and optically stimulated luminescence (OSL) of the crystal were investigated by Ris TL/OSL-DA-15. The as-grown α-Al2O3：C crystal shows a single TL peak at 462K and blue emission peak at 410nm. The TL intensity increases as irradiation dose increases， but no shift is found in the position of the glow peak. The OSL decay curve of α-Al2O3：C crystal consists of two components， namely the fast component and the slow component. With the increasing of the dose，the decay rate of the fast component almost remains unaltered while that of theslow component increases. The TL response of the crystal shows excellent linear characteristic in the dose range of 5×10-6—10Gy， and saturation is observed at 30Gy. The OSL response shows excellent linear characteristic in the dose range of 5×10-6—60Gy， and reaches saturation at 100Gy. Compared with TL response， OSL response shows higher sensitivity and wider linear dose response.

Electrokinetic sonic amplitude measurement of concentrated alumina suspensions: Effect of electrosteric stabilization

This paper describes a convenient and rapid colloidal measuring process of electrokinetic (ζ-potential, ESA signal) properties of the relatively concentrated aqueous suspensions of a submicron α-Al2O3 powder with the anionic polyelectrolyte (Dolapix CE64) dispersant by applying of an advanced electrokinetic sonic amplitude "ESA" technique without dilution. In addition to ESA measurement, the following measurements were also applied: the viscosity, sedimentation, Bingham yield value and prticle size distribution. Very good agreement is observed from the relationship between the electrokinetic (ζ, ESA) values, the viscosity and its pH dependence for each concentrated α-Al2O3 suspension with different amounts of dispersant. The results indicate that the amount of 0.4 mass% Dolapix CE64 is the optimum amount of dispersant that can cause electrosteric effect and decreases mean (d50) particle size of different concentrated (2 vol% and 45.68 vol%) α-Al2O3 suspensions near to the original size. Finally, the study illustrates that Dolapix CE64 is an effective dispersant at all solid loading conditions.

Effect of anions on preparation of ultrafine α-Al2O3 powder

Ultrafine alumina power was obtained by calcining the precursor at 1 200 °C for 2 h, which was prepared by homogeneous precipitation method using aluminium salts and urea as raw materials. The effects of anions on the morphology, particle size, surface area and configuration of the precursors were studied. The results show that the reactions of urea with aluminium nitrate and aluminium chloride result in agglomerates gels with bad filtering performance, the morphology is fibrillar. Aluminium sulphate-urea reactions result in the direct formation of amorphous powders with good filtering performance, of which morphology are regular spherical particles with larger granularity and smaller surface area. The reaction of mutual compound of aluminium sulphate and aluminium nitrate with molar ratio of 40:60 with urea can produce precursor with good filtering performance, spherical morphology, and uniform granularity distribution in the particle size range of 2–3 μm.

Glycothermal synthesis of α-Al2O3 in 1,4-Butadional organic solvent

α-Al2O3 powders with different grain sizes and morphologies have been synthesized in the system of Al - 1, 4-butadional by glycothermal treatment. The effects of reaction temperature, and reaction time on synthesis of α-Al2O3 powders have been investigated. After the glycothermal treatment, the morphologies of α-Al2O3 powders are mainly hexagonal platelets, hexagonal prism, and icohedron. The crystal habits of α-Al2O3 powders have been also discussed.

Dielectric relaxation studies of nanocrystalline CuAlO2 using modulus formalism

Transparent semiconductor CuAlO2 in nanodimension was prepared by mechanical alloying of Cu2O and α-Al2O3 powders in toluene medium. The formation of single phase CuAlO2 with an average crystallite size of 45 nm was confirmed by x-ray diffraction analysis. The nanocrystalline nature of CuAlO2 was confirmed by atomic force microscopy. Frequency-dependent dielectric property measurements and the relaxation behavior were studied using impedance spectroscopy. Dielectric relaxation time estimated from the modulus curves was found to be decreasing with increase in temperature. The existence of stretched exponent parameter β<1 in the present nanocrystalline CuAlO2 implies non-Debye type relaxation behavior in this material.

A NOVEL CHEMICAL ROUTE FOR PREPARATION OF NANOCRYSTALLINE α-Al2O3 POWDER

A novel chemical method for preparing nanocrystalline α-Al2O3 powder has been developed in this study. The aluminum hydroxide precipitates were initially formed in a three-dimensional space network micro-area of the wet gel, and then the precursor was obtained after the wet gel was dried. The nanocrystalline α-Al2O3 powders were obtained after the precursor was calcined at 1100°C. The precursor and as-calcined powders were characterized using TGA-DTA, XRD, and TEM. The α-Al2O3 powders calcined at 1100°C for 2 h are of narrow distribution, little agglomeration, and small particle size of ∼51 nm. The reason for the synthesis of the high-quality powders is explained.

Characterization on Microstructure Homogeneity of θ-Al2O3 Powder Systems During Phase Transformation

The traditional differential thermal analysis (DTA) techniques are employed to evaluate the kinetic behavior induced by the inter-particle relationships in a nano-particle θ-Al2O3 powder system during θ- to α- phase transformation. Discrepancies in homogeneity and inter-particle distance of the powder system may result in the diagnostic DTA profiles which can be use to evaluate the characteristics of the powder system as well as the formed α-Al2O3 particles. Based on the acknowledgement, a quasi-homogeneous transformation shows that a duration of 90 seconds is needed for one θ-crystallite to transform into one α-nucleus with a heating rate of 10 oC min-1 and fabrication of the high phase-pure and mono-sized α-Al2O3 powders is obtained.

Artificial neural network analysis of preparation of nano α-Al2O3 powders by thermal decomposition of ammonium aluminium carbonate hydroxide

Nano α-Al2O3 powders were prepared by thermal decomposition of ammonium aluminium carbonate hydroxide (AACH). The effects of α-Al2O3 crystal seeds, reactant concentration, content of active agent (PEG), annealing temperature and ZnF2 addition on the preparation of nano α-Al2O3 powder were studied. The results showed that nano α-Al2O3 powder with apparent crystallite size ∼40 nm could be prepared, and the complete transformation temperature of α-Al2O3 could be reduced from 1250 to 1050°C as α-Al2O3 seeds and ZnF2 were added. The prepared nano α-Al2O3 powders were characterised by X-ray diffraction (XRD), differential thermal analysis and thermogravimetric (TG-DTA) and scanning electron microscopy (SEM). A back propagation (BP) artificial neural network (ANN) has been used to establish models between the relative content of the nano α-Al2O3 powders and the preparation parameters. The results of checking experiments were in accordance with the calculated values.

Initial Stages of Sintering of Alumina by Thermo-Optical Measurements

The behavior of nanostructured and submicrometer α-Al2O3 powders during the initial stages of field-assisted sintering technique (FAST), conventional, and microwave sintering was investigated using the laser-flash technique for thermo-optical measurements (TOM). An enhanced neck formation due to surface diffusion at very early stages of sintering was found in FAST samples. No significant difference due to heating rate has been found in these various samples.

Effects of drying method on preparation of nanometer α-Al2O3

Ammonium aluminum carbonate hydroxide (AACH) precursor was synthesized by the precipitation reaction of aluminum sulfate and ammonium carbonate. Then the precursor was dealt with five drying methods including ordinary drying, alcohol exchange, vacuum freeze-drying, glycol distillation, n-butanol azeotropic distillation respectively and calcined at 1 200 °C for 2 h to get α-Al2O3. The effects of drying methods on preparation of nanometer α-Al2O3 were discussed, and the optimal drying method was confirmed. The structural properties of powders were characterized by XRD, SEM and BET measurements. The results show that vacuum freeze-drying, glycol distillation and n-butanol azeotropic distillation can prevent the powders from aggregating, and among them the n-butanol azeotropic distillation is the best method. The nanometer α-Al2O3 powder with non-aggregation can be manufactured using n-butanol azeotropic distillation and the average particle size is about 40 nm.

Synthesis of Aluminum Titanate Solid Solution by Solid State Reaction of Fine Commercial Al<sub>2</sub>O<sub>3</sub>, TiO<sub>2</sub> and MgO Powders

The synthesis of aluminum titanate solid solution (Al2(1-x)MgxTi1+xO5, x=0, 0.1, 0.2) powders was realized by solid state reaction method using fine commercial α-Al2O3, TiO2 and MgO powders as raw materials. The effect of the introduced amount of MgO on the phase formation of aluminum titanate solid solution was investigated by XRD following the phase evolution. It was revealed that the introduction of MgO strongly affects the formation of Al2(1-x)MgxTi(1+x)O5 solid solution. The easily formed MgTi2O5 acts as the nucleus prompting the formation and lowering the synthesis temperature. The microstructure and properties of the solid solution ceramics were also studied by SEM and measuring the fracture strength and the thermal expansion coefficient.

Effects of Adding Form of Additives on the Structure and Properties of High-Purity Alumina Ceramics

High-purity alumina ceramics was prepared using high-purity α-Al2O3 powder as raw material, nitrates or oxides of magnesium, chromium and copper as additives by a wet ball milling with a later dry pressing forming and normal pressure sintering process. The influence of additives on the sintering temperature, microstructure and bending strength of the prepared alumina ceramics was studies. The results showed that the additive doped with nitrate can be dispersed uniformly in the body with molecule scale, and the oxides obtained by decomposing of nitrates have the higher reactivity. Thus, the nitrate additives have better capacity than oxide additives in reducing the sintering temperature and inhibiting the abnormal grain growth, and the alumina ceramics prepared by adding of nitrate additives have higher density and bending strength.

Sintering Kinetic of Fine α-Alumina Powder with ZrO<sub>2</sub> Impurity

Sintering kinetics of the α-Al2O3 powders with average particle size 0.1 μm and ca.1% ZrO2 impurity was investigated. Several two-step sintering methods were tried, and the results were compared with the conventional (one-step) process. The temperature (<1600oC), holding time (up to 20 h), heating rate (2 oC /min to 90 oC/min), initial grain size, and porosity were controlled. The optimal results of two-step sintering, a smaller grain size (0.57 μm) and a good density (>98% TD) were achieved by heating to 1450oC, then holding at 1350oC for 20 h. The sintering mechanism was determined, belonging to lattice diffusion in the temperature region of 1400 to 1600oC. The activation energy was 688 kJ/mol, which was interpreted as a volume diffusion of Al3+ ions in polycrystalline matrix.

The Preparation of Alumina Nanopowders from Solution Derived Precipitates through Polymer Gel Method

Non-aggregated nanoscale α-Al2O3 powders with average size ranging from 7 to 20 nm were prepared successfully at various conditions with the use of the hydrated aluminum sulfate extracted from kaolin by polyacrylamine (PAA) gel method. The calcination temperature was relatively lower about 100°C, compared to conventional solid-state reaction. It revealed that PAA worked as a chelating agent to disperse the Al3+ ions homogeneously, and the size of alumina nanoparticles could be controlled by varying the concentration of crosslinking agent.

Structural and spectroscopic characterization of Al2−x Cr x O3 powders obtained by polymeric precursor method

Al2O3 and Al2−xCrxO3 (x = 0.01, 0.02 and 0.04) powders have been synthesized by the polymeric precursors method. A study of the structural evolution of crystalline phases corresponding to the obtained powders was accomplished through X-Ray Diffraction and UV-vis spectroscopy (reflectance spectra and CIEL*a*b* color data). The obtained results allow to identify the γ-Al2O3 to α-Al2O3 phase transition. The single-phase α-Al2O3 powder was obtained after heat treatment at 1050 °C for 2 h. The results show that the green to red color transition and ruby luminescence lines observed for the powders of Al2−xCrxO3 are related to the γ to α-Al2O3 phase transition and the temperature and time range for such transition depends on the chromium content.

Microstructure Study of Alumina Ceramics from Powder Mixtures of Nano and Microsized Particles

In this work, was proposed to produce nanosized α-Al2O3 powder from precipitation reactions, in order to reduce the alumina ceramics sintering temperature. For this an addition of nanosized powder in the commercial microsized powders was done. The nano and microsized powder particle mixtures were obtained by precipitation of nanosized aluminum hydroxide in microsized Al2O3 in liquid suspension. Several compositions of nano and microsized powders were carried out. These powder compositions were based on data of simulated particle packing. The nano and microsized powder mixtures was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric and differential thermal analysis (TGA and DTG). The powder mixtures were compacted by uniaxial (75MPa) and isostatic (300MPa) pressing, producing cylindrical samples. The powder compacted samples were sintered at 1400oC. Sintered ceramics were characterized by XRD and SEM. The results showed that the higher density of sintered ceramic was obtained for 65% of microsized powder and 35% of nanosized powder mixture. This composition of size of particles of the powder was in agreement with the theoretical simulation.

Combustion Synthesis of α-Al<sub>2</sub>O<sub>3</sub> Powders

Among ceramic materials, the alumina has high importance because of its characteristics of resistance and refractory. The possibility to improve the final characteristics of this material, open possibilities for new applications. The aim of this work is to synthesize (α-Al2O3) alumina powders by combustion reaction and to evaluate the effect of the urea content in the final characteristics of the α-Al2O3 powders. Three compositions were studied : a) stoichiometric, b) with 10% of urea reduction and c) with 20% of urea reduction. The initial solution composition was based on the total valence of the reagents by using chemical concepts of the propellant. This powders were characterized by X-ray diffraction (XRD), granulometric determination by laser diffraction, nitrogen adsorption by BET, scanning electron microscopy (SEM), infrared spectroscopy (IR) and helium picnometer. The results showed that the urea reduction change the temperature of the reaction from 525 to 463oC , the characteristics of the powders, and principally the reduction of the particles size.

Study on Fracture Behavior of Alumina Ceramics Toughened by Combining In Situ Growth Abnormal Grains, 3Y-TZP and Cu

α-Al2O3 powders, a small amount of 3Y-TZP and Cu powders were used as principle materials in which adequate amount of TiO2-MgO-CAS system additives were added by wet grinding .The compacts were prepared by hot-pressed sintering. The results showed that the fracture toughness was significantly enhanced by synergistic toughening of in-situ growth abnormal grain, phase transformation and metal particles. Under the sinter condition of 30MPa and 1550 for 1h, the maximum fracture toughness reached to 7.90MPa·m1/2 and the bending strength is in advance of 600Mpa, which was one time higher than un-toughen alumina ceramics. The effects of multiphase synergistic toughening on the fracture toughness of alumina ceramic were investigated; meanwhile, the fracture behavior and the mechanisms of toughening were analyzed.

Synthesis of Nanocrystalline α-Al<sub>2</sub>O<sub>3</sub> Using a 2-Step Calcination Method

Nanocrystalline α-Al2O3 powders have been prepared by the pyrolysis of a resin compound of aluminum with polyester by a two-step calcination process. A polymeric precursor was prepared using a complexing agent to keep the metal ions in homogeneous solution, which gives sufficient flexibility for the system to exist homogeneously throughout the reaction without undergoing precipitation. The metal-ion-polyester resin forms the precursor material on complete polymerization reaction of aluminum nitrate, citric acid and ethylene glycol. A single-phase α-Al2O3 powder resulted after calcinations above 1150°C, but during heat treatment α-Al2O3 particles grow very fast by coalescence. So 2-step calcination was used, where the first step was done in a reducing atmosphere at above 1150°C and second calcinations were done in oxidizing atmosphere at the relatively low temperature of 1000°C. The precursors and the heat-treated final powders have been characterized by X-ray diffractometry, thermogravimetry , transmission electron microscopy(TEM), and BET surface area analysis. The nanocrystalline α-Al2O3 particles obtained by this 2-step calcinations method had an average specific surface area of >170m2/g, with an average particle size between 40 and 60nm.