Commercial Alumina Powder Research Articles

Co-continuous silica–aluminum composite

In this work, a metal matrix composite Al–5SiO 2 was produced using fossil silica fibers spikes as reinforcement. After being heat-treated at 600 °C, the original fiber morphology was retained but its microstructure has changed from solid silica to the interconnected Al–Al 2O 3 interlaced phases known as co-continuous structure. Powder metallurgy, using commercial aluminum powder and the silica fibers as starting materials, followed by hot extrusion were used to produce the composite. The co-continuous microstructure was obtained partially or totally on the fibers as the result of the reduction reaction, which occurs during the heat treatment, by solid diffusion and by the liquid Al–Si alloy, in local equilibrium, produced by the silicon released during the reaction. The internal structure of the fibers was characterized on polished and fractured samples using SEM and optical microscopy. The heat-treated samples were submitted to uniaxial tensile tests and the results were compared with results obtained from annealed and original work-hardened conditions. Microstructure analyses and tensile tests showed a strong interfacial bonding between the fibers and the matrix in the heat-treated extruded samples, justifying the higher UTS obtained, compared with work-hardened and annealed conditions.

Characterization of dispersion strengthened copper with 3wt%Al2O3 by mechanical alloying

The copper matrix has been dispersion strengthened with 3wt.%Al2O3 by mechanical alloying. Commercial alumina powder with an average particle size of 0.75mm was used for alloying. The mechanical alloying process was performed in a planetary ball mill up to 20h in air. After milling all powders were treated in H2 at 4000C for 1h, and finally hot pressing was used for compaction (800oC, 3h, Ar). Structure observations revealed a lamellar structure (Al2O3 particles largely restricted to interlamellar planes between adjacent copper lamellae) accompanied also by structure refinement. These structural changes were mostly completed in the early stage of milling, and retained after compaction. Micro hardness was found to progressively increase with milling time. So, after 5h of milling the micro hardness of the Cu+3twt%Al2O3 compact was 1540MPa, i.e. 2.5 times greater than for the as-received electrolytic copper powder (638MPa) compacted under identical conditions, while after 20h of milling it was 2370 MPa. However after exposing the tested compact at 800oC up to 5h, the achieved hardening effect vanished.

市販高純度α‐アルミナ粉末表面上における水和物，吸着物層の分析

The surface of six different grades of commercial sub-micron high purity alpha alumina powders produced by two different processes, in-situ chemical vapor deposition (AA powders) and hydrolysis of aluminum alkoxide (AKP powders) methods are evaluated by Temperature Programmed Desorption Mass Spectrometry (TPDMS). All AA powders show almost the same quantities of H2O and CO2 desorbed molecules. On the other hand, the different grades of AKP powders show different quantities of H2O and CO2 desorbed molecules. The AKP-3000 powder quantity of desorbed H2O molecules is similar to that of the AA powders. However, the AKP-3000 powder quantity of desorbed CO2 molecules is about seven times larger than that of the AA powders. The maximum of the CO2 desorption peak is the range 230-270°C for all powders. The desorbed CO2 peak is considered to evolve from the adsorbed CO2 molecules forming hydrogen carbonyl groups by interaction with AlIV-OH groups on the α-alumina surfaces. The amount of hydrogen carbonyl groups on surface of the AKP powders is larger than for the AA powders, indicating that on AKP powder surfaces the amount of AlIV-OH groups is larger. These results show that on the surface of commercial high purity alpha alumina powder a complex hydrated layer exists by interaction between surface hydroxyl groups and CO2 adsorbed molecules, unlike commonly accepted stable alpha alumina structure.

Synthesis of Al 2O 3–NbC by reactive milling and production of nanocomposites

Reactive high-energy milling can lead to self-sustaining reactions in the synthesis of a variety of systems, with the reaction being observed after an induction or ignition time which produces a temperature increase in the reactants. The products of such reactions are usually very strong agglomerates which, would be not very useful for further processing; to obtain fully dense sintered samples, it would be necessary an additional and difficult desagglomeration procedure. Therefore, the knowledge and control of the mechanism during reactive high-energy milling may be important in preventing or reducing this agglomeration and improve the physical properties of the powder reaction products. In the present work, high-energy ball milling of the powder mixture Nb 2O 5–Al–C was performed in a SPEX 8000 shaker/mill. The reaction was monitored by a thermocouple fixed in the external surface of the vial. With the condition of ball to mass ratio of 4:1 ignition occurs after 190 min milling. Powder mixtures were characterized after different milling times before and after the reaction. Also, alumina powder was added as diluent to the reactant mixture aiming to decrease the reaction temperature. The powders were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The reaction products after 1 h were desagglomerated and mixed with commercial ultra-fine alumina powder, producing alumina matrix nanocomposites with 5 wt.% of NbC.

Chemical inhomogeneity in commercial alumina powders and its effect on abnormal grain growth during sintering

Chemical inhomogeneity in four commercial alumina powders and its effect on the abnormal grain growth (AGG) during sintering have been investigated. Classification and chemical analyses revealed that relatively small portions of coarse particles in all the powders contained a significantly higher concentration of impurities than their corresponding large portions of fine particles. Hot pressing showed that AGG occurred for all the coarse portions whereas there was no significant indication of AGG for the fine portions. AGG also occurred for all unclassified powders during sintering at the interfaces of bilayer specimens fabricated by repeated centrifugal casting, which could also be attributed to the coarse particles mass-segregated during centrifugal casting. Results of this investigation indicate that coarse particles in commercial purity alumina powder generally contain high concentration of impurities and thus can cause AGG during sintering. Particle aggregates were attributed to the high concentration of impurities in the coarse portions.

Microstructural pathways for the densification of slip cast alumina

The effect of slip casting condition and hence particle packing on densification and microstructural evolution was evaluated during isothermal sintering of a commercial alumina powder at 1350 °C. While there was significant effect of flocculation on solid volume fraction at short sintering times there appeared to be no inherent barrier to the eventual attainment of the same limiting solid volume fraction. Microstructural pathways based on average microstructural parameters and microstructural populations revealed no effect of casting condition on grain-scale pathways at this temperature. It is thought that microstructural pathways at higher length scales based on grain clusters will reveal the effect of slip casting independent of grain coarsening behavior.

Microstructural Evolution of Alumina Ultrafine Powders Obtained by High-Energy Milling

The use of ultrafine powders in dense ceramics production, minimizing the grain size, has shown an increasing interest due the improvement upon several properties. A peculiarity of these powders is the mechanisms acting during initial stages of sintering. In this paper, high-energy milling was used to prepare nanometric particle size alumina from commercial alumina powder. The powders were characterized by determination of XRD crystallite size, specific surface area and high resolution SEM. The microstructural evolution during sintering was analyzed using measurements of density, BET surface area and grain size distribution, determined by high resolution SEM. The milling process results in lower temperatures of beginning of densification and the beginning of grain growth at higher densities during sintering, however the final results at maximum densities are similar. The high specific surface area obtained by milling is completely eliminated with a heat treatment at 900°C due a low temperature process of coarsening of the crystallites and the densification of the crystallite.

Densification of Reactive Milled Al<sub>2</sub>O<sub>3</sub>-TiC Composite Powders

The synthesis of Al 2 O 3 - TiC composite powder can be achieved by reactive milling of powder mixtures of TiO 2 , Al and C. In this case the reactive milling occurs through a sudden highly exothermic self-sustaining reaction started after a certain milling time. Due to the high temperatures reached during the reaction, the products are formed as strong agglomerates. For the synthesis of the Al 2 O 3 + TiC, probably due the high melting point of the reaction products, particularly of the TiC, these agglomerates are formed by crystallites in the nanometric range. In the present work, Al 2 O 3 - TiC composite was synthesized by reactive milling in a shaker/mill apparatus, followed by a deagglomeration processing. The products of reactive milling were mixed with a commercial ultra-fine alumina powder to produce alumina matrix composites with 5 wt% of TiC. The powder densification was performed by hot pressing and by pressureless sintering between 1400 and 1600 °C. We report on the microstructure characterization of the dense TiC+Al 2 O 3 composites by SEM and TEM. It was observed that TiC is dispersed in the alumina matrix microstructure as both agglomerated inclusions and also dispersed nanometric inclusions. Vickers Hardness measurements of the sintered composites indicated values of about 13.0GPa.

Characterization of coarse particles in alumina powders by a wet sieving method

Abstract The wet-sieving method was applied to determine the content of coarse particles in commercial alumina powders by four independent research organizations participated in the Round-Robin Testing. The results showed that those powders clearly contain 700 to several thousands ppm of coarse particles with the size >25 μm, whereas they could not be detected with the conventional particle size analyzer because of their low concentration. SEM and polarized light microscope observations revealed that the coarse particles contained several types of aggregates which survived during the production processes of alumina by the Bayer process.

Aluminium Matrix Composites Reinforced with Co-continuous Interlaced Phases Aluminium-alumina Needles

An Al-5SiO2 (5 wt% of SiO2) aluminium matrix fiber composite was produced where the reinforcement consists of fossil silica fibers needles. After being heat-treated at 600 °C, the original fiber morphology was retained but its microstructure changed from solid silica to an interconnected (Al-Si)/Al2O3 interlaced structure named co-continuous composite. A technique of powder metallurgy, using commercial aluminium powder and the silica fibers as starting materials, followed by hot extrusion, was used to produce the composite. The co-continuous microstructure was obtained partially or totally on the fibers as a result of the reaction, which occurs during the heat treatment, first by solid diffusion and finally by the liquid Al-Si in local equilibrium, formed with the silicon released by reaction. The internal structure of the fibers was characterized using field emission electron microscope (FEG-SEM) and optical microscopy on polished and fractured samples.

Effect of alumina particle size and distribution on infiltration rate and fracture toughness of alumina–glass composites prepared by melt infiltration

Four commercial alumina powders having different particle sizes (0.44, 2.85, 7.08 and 39.81 μm) were presintered for 2 h at 1120 °C and then lanthanum–aluminosilicate glass was infiltrated for up to 4 h at 1100 °C to prepare the densified alumina–glass composites. Alumina having a bimodal and broader particle size distribution was the most effective to the packing. The penetration rate constant increased with raising the alumina particle size having a parabolic dependence of infiltration distance on time described by the Washburn equation. However, the optimum mechanical properties were observed for the composite containing the 2.85 μm alumina. Although the higher fracture toughness of the composites may be achieved by increasing the alumina particle size probably due to the dispersion toughening, the larger alumina particles reduce the strength.

Ignition and Combustion of Aluminum{Air Suspensions in a Reactor for High-Temperature Synthesis of Alumina Powder

A method for flame stabilization in a reactor for synthesis of metal oxides is developed by analyzing combustion of dispersions of metal powders. An experimental reactor designed to implement this method is described. Ignition and combustion of commercial aluminum powders (ASD-1 and ASD-4) in air were studied, and their reliable ignition and stable combustion in the reactor combustion chamber were confirmed. It is shown that efficient combustion of an aluminum–air mixture depends on powder dispersity, flow mixing conditions, and combustion chamber pressure. Key words: aluminum, powder, suspension, flame, stabilization, combustion, synthesis, oxide.

Combustion of Mixtures of Commercial Aluminum Powders and Ultrafine Aluminum Powders and Aluminum Oxide in Air

The paper studies the combustion of mixtures of commercial aluminum powders (ASD-1 and ASD-4) and ultrafine powders of Al and γ-Al2O3 in air. It is shown that the combustion of coarsely dispersed commercial powders is accompanied by binding of air nitrogen with formation of AlN and AlON. The combustion of mixtures proceeds in two stages with the possible formation of intermediate gaseous and liquid products. The processes of sintering and incomplete combustion play an important role in the combustion of mixtures of commercial powders and ultrafine powders of aluminum.

Processing of mesoporous alumina from a powder precursor

Oxides with significant mesopore volume can be produced from gels through ambient pressure drying (APD) if the oxide network within the gel is sufficiently strong. To obtain a strong alumina gel, a commercial alumina powder (100 nm diameter) was combined with different amounts of ethanol, water, and acid. Significant mesoporosity was observed in dried and calcined gels prepared from this precursor and ethanol alone, particularly when “poor wettability” was established in the gel through azeotropic distillation in toluene. When water (or water and nitric acid) was also utilized the pore volume decreased dramatically; this observation may be attributed to the creation of amorphous alumina from the crystalline precursor that is displaced into the interparticle pores, and in which mesopores are poorly preserved during APD.

Improving the attrition resistance of slurry phase heterogeneous catalysts

Slurry phase heterogeneous catalysts for processes such as Fischer–Tropsch (F–T) synthesis must exhibit a high degree of attrition resistance. The precipitated Fe–Cu catalyst used for F–T synthesis is quite weak in its as-prepared state. Spray-drying yields spherical particles which show some improvement in attrition resistance. However, the formation of fines (<5 μm) in this powder shows that it is not suitable as a slurry phase catalyst. In this paper, we report on the use of a silica binder to improve the strength of spray-dried agglomerates. The attrition resistance was measured using ultrasonic fragmentation followed by sedigraph particle size analysis. The attrition strength of the iron oxide catalyst agglomerates was compared to that of a commercial alumina powder, which was used as a reference material. The role of calcination (before or after spray-drying) and the method of silica binder addition (before or after spray-drying) was investigated.

Surface finish and mechanical strength of dense alumina

Disks of commercial alumina powder were fabricated by slip casting and sintering (1600°C, 2 h). Two different surface treatments were performed: a coarse machining, using a 70 grit diamond wheel (C) and a fine one with SiC paper of 120 and 320 grit (F). In order to study the mechanisms of material deformation and removal and the final superficial features, worn surfaces were analyzed by scanning electron microscopy, profilometry, and residual stress measurements by X-ray diffraction. The fracture strength of the treated specimens was evaluated in biaxial flexure, including a statistical analysis of the data. The results were analyzed, keeping in mind the characteristics of the different surface finishes.

Alumina disks with different surface finish: thermal shock behavior

Disks of commercial alumina powder were fabricated by slip casting and sintering (1600°C, 2 h). Two different surface treatments were performed: a coarse wear, using a 70 grit diamond wheel (CAW) and a fine one with SiC paper of 120 and 320 grit (FAW). The thermal shock resistance of the worn specimens was evaluated testing the disks by sudden cooling with a high-velocity air jet. The critical temperature differential ( ΔT C) was determined increasing the initial temperature of the sample in 10°C until the crack propagation was detected. The values of the mean ΔT C were 940°C for FAW specimens and 765°C for CAW ones. The statistical distributions of the results were analyzed in function of the fracture strength of the specimens measured in biaxial flexion and the characteristics of the different surface finish determined by scanning electron microscopy, profilometry, and residual stresses measured by X-ray diffraction.

Surface Hydration States of High Purity .ALPHA.-Al2O3 Powders.

Surface conditions of three types of commercial high purity alpha alumina powders produced by three different processes, in-situ chemical vapo deposition (A powders), hydrolysis of aluminum alkoxide (B powders), and a pioneer chemical vapor deposition (no longer in the market) (C powder) methods are evaluated by FT-IR (Fourier transform infrared) transmission and Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy. The spectroscopy results suggest that the surface characteristics of as-received high purity alpha alumina powders are not unique. In some of the B powders with small size, a mixture of aluminum trihydroxides like bands is observed. A specific band at 3472cm-1 observed on the B and C powders does not belong to band spectra of the aluminum hydroxide references. Relating to the predicted band on infrared spectra of hydrated alpha alumina surface obtained from reported molecular dynamics calculation, this band may be associated with hydrogen bonded hydroxyl groups of a fully hydroxylated alpha alumina surface.

Evaluation of the heat transfer coefficient in thermal shock of alumina disks

Disks of a high-purity commercial alumina powder were fabricated by slip casting, precalcined, sintered and machined with SiC paper (120 and 320 grit). The specimens were tested in thermal shock conditions from several temperatures (Ti) between 870 and 980°C using a high-velocity air jet at room temperature (T0). The temperature differential between the disk and the air jet was incremented in 10°C until crack propagation was detected. During the air impinging, the temperature was recorded on the lower specimen surface at the central point and at a peripheral one. The coefficient governing the convective heat transfer on the specimen surface, h, was estimated by fitting the calculated temperature profiles with those measured during the test. Three alternative models were proposed for the temperature calculations using a finite element analysis.

ESA measurement for electrophoretic deposition of ceramic materials

The aim of this work is the preparation of a ceramic material consisting of two solid components by electrophoretic deposition (EPD) from aqueous suspensions. This method is based on the motion of charged particles in the electric field. Conditions for homogeneous and for graded deposition should be detected. For this purpose the electrokinetic properties of commercial alumina and zirconia powders were investigated by measuring the electrokinetic sonic amplitude (ESA) using the ESA 8000 system (Matec). The conditions for EPD were chosen from the electrokinetic properties. The amount of polyelectrolyte addition influences the rate of deposition and the homogeneity of the deposited layer. If the mobilities of the components differ in their magnitude and if there are no interactions a concentration gradient across the thickness of the layer can be adjusted.