Fine Alumina Powder Research Articles

Oxidation of fine aluminum particles: thermally induced transformations in particle shells and kinetics of oxide nucleation.

This paper investigates the process of oxidation of fine aluminum powder, consisting of spherical Al particles of 'metal core/oxide shell' type, when heated in air at temperatures below 550 °C. The highly dispersed aluminum powder 'Alex' used in this work (particle size: 0.05-1.5 μm and number average particle diameter (DN): 0.11 μm) was produced by electric explosion of a thin Al wire in argon with subsequent passivation in an oxygen-containing atmosphere. For the first time, the influence of the particle size on the oxidation process has been identified. During the reaction, individual γ-Al2O3 oxide nuclei grow at the surface of Al particles with diameters less than 300 nm without laterally overlapping to form a protective passivating layer, as typically occurs during the oxidation of micron-sized particles or bulk metal. The localization of γ-Al2O3 nuclei is determined by the regions where peeling of the primary amorphous oxide film from the surface of the metal core of an Al particle occurs due to the thermal decomposition of aluminum hydroxides (T ≈ 350 °C) present within the particle shells. A significant increase in the contributions of the following factors leads to a high oxidation rate: the diffusion within the disordered structure of a particle core and the surface diffusion of cations (Ea = 80.6 ± 5.3 kJ mol-1, 400-450 °C), the surface and grain boundary diffusion of oxygen during the growth of γ-Al2O3 crystallites (Ea = 108.3 ± 11.2 kJ mol-1, 470-500 °C) and the surface and grain boundary transport of anions through an amorphous and underlying nanocrystalline oxide layer with a constant thickness (Ea = 205.1 ± 8.6 kJ mol-1, 520-550 °C). The results obtained make it possible to expand the theoretical understanding of the manifestations of the size effect in solid-phase reactions and take into account its influence in practice.

Drilling of GFRP with Liquid Silicon Rubber Reinforced with fine Aluminium Powder on Hole Surface Quality and Tool Wear using DOE

Drilling of GFRP with Liquid Silicon Rubber Reinforced with fine Aluminium Powder on Hole Surface Quality and Tool Wear using DOE

Production of Reactive Alumina from Domestic Sources and Its Use in Alumina-Based Self-Flowing Castable Refractories

High-quality alumina raw materials allow corundum-based brick and monolithics to perform to the full potential of alumina as a refractory material. Dense packing of the matrix to a submicron range and reduction of water demand of castables can be facilitated by a new family of multimodal reactive alumina. Dispersing aluminas ensure uniform mixing of dispersion and setting adjustment additives. In this study, fine reactive alumina powder production was investigated using Bayer gibbsite as a starting material. Experimental studies consist of two steps; in the first step, the soda content was reduced by means of boric acid and distilled water and then the powders obtained optimum conditions were ground in an attritor ball mill using distilled water for 8 hours. In the second step, physical characteristics such as rheological behaviours, bulk density, open porosity and thermal shock resistance, bending and compressive strength and fracture toughness in accordance with ASTM E399 standard were determined. Finally, KIC values of the refractory samples which was prepared with the reactive alumina produced from Seydişehir gibbsite were higher than that of the refractory mixture containing commercial reactive alumina obtained from a refractory company.

Исследование влияния добавок LiCl и LiF на кинетику электроимпульсного плазменного спекания мелкозернистого оксида алюминия

The effect of the lithium chloride and lithium fluoride additives (2 wt. %) on the kinetics of Spark Plasma Sintering of industrial fine alumina powder was investigated. The Al2O3 + 2 % LiF powder was obtained by mixing the α-Al2O3 fine powders with the aqueous solution of LiF. The Al2O3 + 2 % LiCl fine powder was obtained by joint grinding of the components in a planetary mill. The sintering of the powders was performed with the heating rates of 10 and 50 °C/min up to the temperature corresponding to the end of the shrinkage. The ceramics sintered with the heating rate of 10 °C/min had the relative density of 97.4 – 98.7 %. The addition of lithium fluoride into the alumina powder was found to allow reducing the temperature of the beginning of the intensive powder shrinkage from 1400 – 1500 °C down to 1255 – 1335 °C. LiCl was shown to evaporate at low heating temperatures and not to affect the compaction intensity of the Al2O3 powder. The presence of the overheated LiF melt (not having enough time to evaporate from the specimen volume completely) in the ceramics leads to the appearance of the residual porosity and to the reduction of the hardness of the ceramics. Using the Young – Cutler equation, the powder compaction mechanisms were determined for pure alumina α-Al2O3 and for the powder compositions with the LiCl and LiF additives in the rapid heating conditions. The sintering activation energy was shown to be close to the one of the grain boundary diffusion. The LiF melt was found to promote the sliding of the Al2O3 fine particles during the low-temperature compaction stage.

High-Strength and Heat-Insulating Cellular Building Concrete Based on Calcined Gypsum

A cellular concrete with a fine porous structure was experimentally made using the corrosion technique for aluminum powder as an expanding agent in an aqueous solution of Ca(OH)2. The originality of this paper was the use of our own production method for the fine aluminum powder through atomizing the recycled molten waste of this metal using concentrated jets of nitrogen. Additionally, the waste melting technique involved our own microwave heating method. A high weight proportion of calcined gypsum (maximum 82.3%) represented the main concrete binder. Using moderate contents of coal fly ash (3.6-11.1%) together with perlite (4.6-6.4%) to reduce the pore size and silica fume (0.3-1.2%) with pozzolanic properties, the aim was to obtain a macrostructure characterized by a very low pore size and to increase the compressive strength (by up to 4.1 MPa), despite the relatively low density (below 641 kg/m3). An industrial method of increasing the mechanical strength by steam curing fresh concrete was applied.

Studying the adhesion strength of plasma powder metal coatings on the surface of graphite

Coatings applied onto the surface of specimens made of UHP electrode graphite are studied. Coatings are applied by thermal spraying in two stages: spraying of a sublayer (layer I) of the powder materials being tested; spraying of the main layer of the plasma coating, consisting of a mechanical mixture of fine spherical aluminum (Al) and copper (CuCr1) powders in an equal volume ratio. The adhesion strength between the coating and the graphite surface is studied as dependent on the following factors: sublayer thickness; the type of preliminary surface preparation; the chemical composition of the powder material of the sublayer. It has been found that the highest adhesion strength between the coating and the graphite base is achieved when the sublayer thickness is 80 µm, when a thread with a depth of 1.0 mm is cut on the base surface, and when Al powder is used as a sublayer.

Natural pozzolan–based green geopolymer foam for thermal insulation

The current study investigates the possibility of volcanic Tuff of Earth of Datça (ED) in Turkey to be used as an aluminosilicate source in producing a geopolymer foam for thermal insulation. An extensive evaluation of the effects of fine sand–to–pozzolan and Al powder–to–pozzolan ratios on the physical, mechanical, and thermal properties and morphology (porosity, average and maximum pore diameter, pore size distribution) of the pores were carried out. The sodium silicate and potassium hydroxide (12.5 M) solutions with an activator ratio of 2.5 were used as alkali activators, and Al powder was used as a foaming agent. Research results reveal that Earth of Datça is a suitable precursor for producing a geopolymer foam. Fine sand and aluminum powder contents are critical to the optimum foam structure. The addition of finely ground silica sand ensured the volumetric stability of the binder and prevented the collapse after swelling of the binder. The optimum Al powder–to–pozzolan ratio was determined as 0.5% because it gives higher physical, mechanical, and thermal properties due to the more homogenous microstructure with finer pore size and narrower pore size distribution lower degree of interconnectivity between the pores. Research results also show that the natural volcanic Tuff of Datça Peninsula as an aluminosilicate source gives promising results in the field of producing highly porous geopolymers with low thermal conductivity (0.087–0.134 W/mK), high porosity (72.3–82.6%) and an adequate compressive strength (0.40–2.09 MPa). This study contributes to the literature that Earth of Datça–based geopolymer foam may function well as an insulation material for building enclosures.

Natural pozzolan-based green geopolymer foam for thermal insulation

The purpose of the current study is to investigate the possibility of volcanic Tuff of Earth of Datça (ED) in Turkey to be used as an aluminosilicate source in the production of a geopolymer foam for thermal insulation. An extensive evaluation of the effects of fine sand-to-pozzolan and Al powder-to-pozzolan ratios on the physical, mechanical, and thermal properties and morphology (porosity, average and maximum pore diameter, pore size distribution) of the pores were carried out. The sodium silicate and potassium hydroxide (12.5 M) solutions with an activator ratio of 2.5 were used as alkali activators and Al powder was used as a foaming agent. Research results reveal that Earth of Datça is a suitable precursor for the production of a geopolymer foam. Fine sand and aluminum powder contents are key factors on optimum foam structure. Addition of finely ground silica sand ensured the volumetric stability of the binder and prevented the collapse after swelling of the binder. The optimum Al powder-to-pozzolan ratio was determined as 0.5 % because it gives higher physical, mechanical and thermal properties due to the more homogenous microstructure with finer pore size, narrower pore size distribution and lower degree of interconnectivity between the pores. Research results also show that natural volcanic Tuff of Datça Peninsula as aluminosilicate source gives promising results in the field of producing highly porous geopolymers with low thermal conductivity (0.087-0.134 W/mK), high porosity (72.3-82.6 %) and an acceptable compressive strength (0.40-2.09 MPa). This study contributes to the literature that Earth of Datça-based geopolymer foam may function well as an insulation material for building enclosure.

McIntyre Powder and its potential contributions to cardiovascular disease risk: A literature review through the McIntyre Powder historical lens.

McIntyre Powder (MP) is a fine aluminum powder that was developed to prevent silicosis in gold and uranium mine workers in Ontario, Canada, and was administered to miners there from 1943 to 1979. Mine workers were exposed to high concentrations (35.6 mg/m3) of MP for approximately 10 min before every work shift. Contemporary physical and chemical characterizations of this powder have revealed that 12% of the powder is in the ultrafine particle size‐range (nanoparticles); and the remaining 88%, in the fine particulate size range (below 2.5 µm in diameter). The confluence of ultrafine particulate (UFP) composition and high airborne concentration of MP would be expected to overwhelm the defense mechanisms of the lung and increase the lung dust burden of the mine worker exposed to respirable dust in the mine. Published studies revealing associations between air pollution particulates and increased risk for cardiovascular disease (CVD) shown a dose–response relationship with ambient PM2.5 and UFP and suggest that miners exposed to MP may also be at increased risk of CVD. The historical perspective of the use of MP in northern Ontario hard‐rock mines and its potential implications for CVD in exposed mine workers are discussed.

Effect of grain boundary state and grain size on the microstructure and mechanical properties of alumina obtained by SPS: A case of the amorphous layer on particle surface

The work was aimed at the investigation of kinetics of Spark Plasma Sintering (SPS) of the α-Al2O3 particles with amorphous surface layers and investigation of the effect of the amorphous layers on the grain growth and on the mechanical properties of alumina. The objects of investigations comprised:(i) submicron α-Al2O3 powder, (ii) submicron α-Al2O3 powder with the amorphous layers on the particles' surfaces, and (iii) the fine-grained α-Al2O3 powder. The submicron powders (i) and (ii) were used to analyze the effect of the amorphous layers on the sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle sizes on the shrinkage kinetics. The effect of the temperature regime and of the rate (Vh) on the shrinkage kinetics of the submicron and fine alumina powders has been studied. The shrinkage curves were analyzed using the Young–Cutler and Coble models. The sintering kinetics was shown to be determined by the intensity of grain boundary diffusion for the submicron powders and by simultaneous lattice diffusion and grain boundary one for the fine powders. The amorphous layers on the surfaces of the submicron α-Al2O3 particles were found to affect the grain boundary migration rate and the Coble equation parameters at the final stages of SPS. The abnormal characteristics of the alumina ceramics sintered from the submicron powder with the amorphous layers on the particles’ surfaces were suggested to originate from the increased concentration of the defects and of the excess free volume at the grain boundaries formed during crystallization of the amorphous layers.

Influence of powder characteristics on the sintering behaviour of reactive alumina powders

Fine reactive alumina powders have a significant impact on the properties of refractories at high temperatures. Except for the hydration of cement and in situ reactions, the performance of castables is also correlated with the sintering of reactive alumina. The characteristics of fine alumina powders produced by various manufacturers are quite different. This work aims to investigate the comprehensive influence of powder characteristics on the sintering behaviour of reactive alumina powders. Three types of commercial reactive alumina powders were selected for experiments. The linear shrinkage, bulk density, apparent porosity and microstructure of alumina compacts calcined at different temperatures were analysed. It was demonstrated that large specific surface area and higher SiO2 content had a positive effect on the densification of alumina compacts. The calculation based on the hypothetical composition of alumina grain boundary indicated that SiO2 mainly participated in the formation of liquid. Therefore, more liquid was generated at the grain boundary of alumina grains containing higher SiO2 content, which resulted in better sinterability of reactive alumina powders.

Effect of particle size on mechanical properties of alumina ceramic processed by photosensitive binder jetting with powder spattering technique

As an Additive Manufacturing technique, Binder Jetting enables the fabrication of customized and complex ceramic parts. However, the insufficiency of powder packing in green parts restricts the final products’ densifications and strengths. To form a layer of ceramic green parts, Photosensitive Binder Jetting with powder spattering technique provides entrapment of ceramic powder by printing of photo-curable resin and recoating by releasing the powder through a vibrating mesh. This recoating technique enables the processing of fine alumina powders, the average particle sizes of which are 3 μm, 1.65 μm, and their mixtures. Apparent densities, porosities, mechanical properties, and microstructures of the sintered parts were investigated. The alumina sample with the apparent density of 3.70 g/cm3, the compressive strength of 94.87 MPa, the biaxial strength of 50.06 MPa, and the porosity of 41.01 % was attained by the mixture with 70:30 wt.% of the 3 μm and 1.65 μm powders respectively.

Low thermal emissivity coating based on aluminium / acrylic composite coatings

Low emissivity coatings were synthesized by using fine flake Aluminium (Al) powder as a filler within acrylic resin to from the desired composite coatings. In the current work, ball milling was applied to prepare Al with different shapes and sizes. The technical parameters of prepared Al powders with respect to the initial raw Al are characterized by scanning electron microscope (SEM), X-Ray diffraction (XRD), and Energy Dispersive X-Ray Analysis (EDX). Moreover, parameters that affect the emissivity of the coating was investigated; such as coating thickness, particle size, spin coating, ball milling time and the content of coated Al powder. The thermal signature is highly affected by the variation in Al content (5%, 20%, 25%, 35%, 40% and 60%) at different temperatures (50°C, 70°C, 90°C). The results indicate that the perfect percentage for the filler (Al) in the matrix within the range (35 wt. - 40 wt. %) and Al fine flake powder particle, which gives the lowest infrared emissivity of 0.385 μm and 0.412 μm for (3-5) μm, (8-12) μm, respectively.

3D printing of fine alumina powders by binder jetting

Additive manufacturing of ceramics is still at an early-development stage; however, the huge interest in custom production of these materials has led to the development of different techniques that could provide highly performing devices. In this work, alumina (α-Al2O3) components were produced by binder jetting 3D printing (BJ), a powder-based technique that enables the ex-situ thermal treatment of the printed parts. The employment of fine particles has led to high green relative density values (>60 %), as predicted by Lubachevsky-Stillinger algorithm and DEM modelling. Then, extended sintering has been observed on samples treated at 1750 °C that have reached a final density of 75.4 %. Finally, the mechanical properties of the sintered material have been assessed through bending test for flexural resistance and micro-indentation for Vickers hardness evaluation.

Rapid Solidification and Surfaces Alloying Using Continuous CO<sub>2</sub> Laser to Generate Ni-Aluminide on Steel

Rapidly solidified thin micro-alloyed surface layers are generated by laser melting of plasma thermal sprayed steel surfaces. Samples of carbon steel are plasma sprayed with fine nickel and aluminum powders. Laser surface melting generated a thin localized molten pool of metal with steep horizontal thermal gradient. The latter triggered intense vortex formation in the molten pool which thoroughly mixed the nickel and aluminum powders within the molten pool in a fraction of a second. As the sample is moved away with a predefined velocity, the cold substrate quenched the melt pool, generating rapidly solidified micro-alloyed surfaces. A 2.5 kilowatts continuous carbon dioxide laser was used for surface melting; laser power was maintained at 800 watts while the samples were moved with respect to the laser beam at linear velocities in the range of 100-200 mm/min. The technique generated metallurgical bonded novel surfaces. Depth of the laser modified layer was achieved in the range of 0.2-0.4 mm. Refined microstructures of pre-austenite grain size in the range of 4±2 µm were generated. Micro-hardness measurements of the modified layer indicate an almost three times enhancement in the hardness values; the latter are, in general indicative of mechanical strength of the material. The shape of the solid/liquid interface of the advancing molten pool determines the orientation of the growing dendrites; at higher velocities of sample translation with respect to the laser beam, these are almost parallel to the sample surface. The orientation of the dendrites, the uniformity in surface alloying (within fraction of a second) and the resulting hardness values are explained with the help of the modeled shape of the liquid metal pool. The laser processed material proved to be a flexible technique to synthesize novel surfaces for surface sensitive applications.

Processing of Local Bauxite to Obtain Highly Purified and Fine Alumina Powder

A study on local bauxite was conducted at Mineral Research Centre, Ipoh to produce highly purified fine alumina (Al2O3) powder by using Bayer process as a basis. The study was conducted on a local bauxite from Johor. The raw material being classified having the specific gravity of 2.42 and the major constituent of mineral was identified as gibbsite in which indicates of having 58.7% Al2O3. Moreover, this raw material being classified as high silica content of bauxite ore because of the presence amount of SiO2 was slightly more than 7.0%. Four (4) major processes were performed at Mineral Research Centre such as sample preparation, characterization of raw material, precipitation and characterization of product. In order to produce highly purified and fine alumina, some alternative on leaching and precipitation processes were studied. These due to leaching process tends to pre-determine the rate of recovery and the purity of alumina whereas a very effective precipitation process can be applied to produce fine size of alumina powder. This paper presents some results of leaching and precipitation processes and probably help to show how various processing parameters will affect the effectiveness of alumina recovery, purity as well as its sizes.

Visualization of flow past square cylinders with corner modification

This article presents the results for flow past a square cylinder and two square cylinders of the same and different sizes with corner modifications by varying the spacing ratio. Here, experimental work is conducted in a recirculatory channel filled with water. A set of aluminum discs made to rotate to create the flow in the test section. The motor is used to vary the speed of the water. Fine aluminum powder is used as a tracer medium. It is observed that vortex shedding frequency decreases by placing the second cylinder in the downstream of the first cylinder. For similar size cylinders, the width of the eddy in the middle of the cylinders increases with an increase in spacing ratio. With the increase of spacing ratio to 6, the flow past each cylinder behaves like a single square cylinder. If the upstream square cylinder size is smaller than the downstream square cylinder, the eddy size is reduced in between the cylinder compared to the downstream of the second cylinder. If the upstream square cylinder size is bigger than the downstream square cylinder, the eddy size is larger in between the cylinder compared to the downstream of the second cylinder.

Studies on the Low-Temperature Synthesis of Fine α-Al2O3 Powder by Precipitation Route

Fine alpha-alumina (α-Al2O3) powder was prepared by simple precipitation technique using aluminium salt and ammonia as a precipitating agent. In this method, an aqueous solution of aluminium nitrate and ammonia were mixed with continuous stirring both in the presence and in the absence of polyethylene glycol (PEG). A controlled pH range of 7.5–8.5 was maintained throughout the synthesis process. A comparative study between the alumina powder prepared in the presence and absence of PEG was drawn into attention. The washing of the precipitate was deliberately avoided in one case to study the effect of washing on phase formation temperature. The synthesized powders were calcined at different temperatures and characterized by X-ray diffraction study, thermal analysis, infrared analysis, microstructural study, and particle size analysis. The results showed less agglomerated fine alumina powder formation in its pure α-Al2O3 phase at 1050°C in the presence of PEG.

Studies on activated carbons towards the performance improvement of cryosorption pump

Cryosorption pumps are the only possible solution to pump helium gas in Tokamak. In cryosorption pump, due to weak van der Walls forces the adsorbate gas molecules are held by the adsorbent well above the boiling point of the adsorbate gas. Therefore selection of best adsorbent is of utmost importance for the optimum performance of cryosorption pump towards the pumping of helium gas. In this paper, we report the pore size distribution and surface area of different activated carbon adsorbents to arrive at the best activated carbon for the development of cryosorption panel. Also probably for the first time, an attempt has been made to estimate the thermal conductivity of liquid helium based small size prototype cryopanel. The thermal conductivity of the panel has been improved by mixing the fine aluminium powder as filler to the epoxy adhesive. From the experimental studies it is found that KCC/IIS01 is the best adsorbent towards the development of cryosorption pump. Mixing of 35% fine aluminium powder as a filler to the epoxy adhesive improves the thermal conductivity of the cryopanel by ~14% and in turn enhances the pumping speed by ~2.35 times as reported in our earlier studies.

Microstructure and mechanical properties of Al–B4C composite at elevated temperature strengthened with in situ Al2O3 network

This study evaluated the mechanical properties and thermal properties of Al–12 vol%B4C composite at elevated temperature strengthened with in situ Al2O3 network. The composite was fabricated using powder metallurgy (PM) with raw materials of fine atomized aluminum powders, and the associated microstructures were observed. At 350 °C, the composite had ultimate tensile strength of UTS = 137 MPa, yield strength of YS0.2 = 118 MPa, and elongation of e = 4%. Besides, the mechanical properties of the composite remained unchanged at 350 °C after the long holding periods up to 1000 h. The excellent mechanical properties and thermal stability at 350 °C were secured by in situ am-Al2O3 network that strengthened the grain boundaries. The interfacial de-bonding and brittle cracking of B4C particles were the main fracture mechanisms of the composite. In addition, the influence of sintering temperature and rolling deformation on the microstructures and mechanical properties was studied.