Fine Alumina Particles Research Articles

Microstructure-based discrete simulations of the compaction of refractory powder composites

This work describes an original methodology to simulate the compaction behavior of refractories by taking explicitly into account the microstructure of each composite phase. The Discrete Element Method (DEM) is used to model the composite as a mixture of fine and coarse alumina particles and graphite flakes. The binder phase is accounted for by a specific contact law which adjusts the contact rigidity according to the indentation between the coated particles. Small indentations lead to a soft response while above a critical indentation, the contact becomes stiffer. This simple model incorporates the granular behavior of the composite as well as the continuous matrix of the binder. Composites with various compositions are numerically submitted to closed-die compaction. The densification behavior during compaction depends primarily on the binder phase. The contribution of each phase to the total axial stress is investigated, thus providing new insights into the complex behavior of these composites.

Phase composition and concentration of yttriumin films during the deposition of aluminumand aluminum alloys from the gas phase

The work is devoted to the study of the stabilizing effect of yttrium additions during the deposition of thin aluminum films, which are used for the manufacture of elements of micro- and nanoelectronic devices. The surfaces of Al films doped with aluminum oxide were investigated using a scanning electron microscope before and after annealing for 300420 s at a temperature of 500 C. It is shown that fine alumina particles are uniformly distributed on the surface of the films during thermal evaporation of a wire made of an Al Al2O3 alloy. By the method of quantitative metallography, the content of the AlxOy phase in the Al films was determined: when spraying wire from the Al Al2O3 alloy, its content was 1012% of the mass fraction; when spraying wire made of Al Al2O3 alloy and Al wire in a ratio of 50:50 1% mass fraction.

The role of thermophoresis on aluminum oxide lobe formation

This work studies the influence of phoretic motion (thermophoresis and diffusiophoresis) of fine alumina particles (“smoke”) produced during the combustion of aluminum. Direct numerical simulations on a single aluminum droplet burning in a quiescent environment suggest that thermophoresis is the main mechanism driving smoke back to the aluminum surface, hence a major contributor to the oxide lobe development. The presence of this lobe is found to distort the flowfield, which favors hot and smoke-rich regions closer to the lobe, thereby enhancing thermophoresis. This combination of aerodynamic and thermophoretic effects leads to a mass rate of deposited smoke which is consistent with experimental data. A simplified model, deduced from simulation results, is able to predict the size of the final oxide residue in good agreement with measurements. This study supports that aluminum oxide present on the burning aluminum particle is largely due to material formed in the flame, subsequently desposited by thermophoresis.

A simple repair method of fatigue cracks using stop-holes reinforced with wedge members: applicability to reinitiated cracks and effects of an anti-fatigue smart paste

ABSTRACT A simple method of repairing fatigue cracks using stop-holes reinforced with wedge members, that was previously proposed by the author, has been examined for the case of reinitiated cracks. By using this method, the stress intensity factor range around a reinitiated crack tip is expected to be reduced by the wedge load effect of the wedge members. The chief advantages of this method are that the repair work can be easily performed from only one side of a cracked structure, and that the wedge member can be set so adaptive as to maintain the wedge load automatically and effectively as the reinitiated crack grows. Specifically, slope-type wedge members have been adopted, and an adaptive mechanism of the wedge member has been devised using a pulley and a wire-type displacement metre. Fatigue tests were performed on a steel plate specimen with a drill hole and a notch, and validity of the above repair method was experimentally examined using both of simple and adaptive wedge members. In addition, an anti-fatigue smart paste, which consists of fine alumina particles and silicone grease, was applied to the periphery of the drill hole, and its effects on automatic restraint and visual detection of crack growth were investigated. FE analyses using contact elements were also carried out for a comparative study. As a result, it was found that the strain range on the specimen side is reduced to 44.0 % and the fatigue life is prolonged by 12.4 times by application of the adaptive wedge member as compared with the case of the conventional stop-hole. As for the anti-fatigue smart paste, it was found that the fatigue life is further prolonged by about three times as compared with the case of the adaptive wedge member and the paste has an evident function to facilitate visual detection of cracks.

A Simple Repair Method of Fatigue Cracks Using Stop-holes Reinforced with Wedge Members

A simple method of repairing fatigue cracks using stop-holes reinforced with wedge members, that was previously proposed by the author, has been examined for the case of reinitiated cracks. By using this method, the stress intensity factor range around a reinitiated crack tip is expected to be reduced by the wedge load effect of the wedge members. The chief advantages of this method are that the repair work can be easily performed from only one side of a cracked structure, and that the wedge member can be set so adaptive as to maintain the wedge load automatically and effectively as the reinitiated crack grows. Specifically, slope-type wedge members have been adopted, and an adaptive mechanism of the wedge member has been devised using a pulley and a wire-type displacement meter. Fatigue tests were performed on a steel plate specimen with a drill hole and a notch, and validity of the above repair method was experimentally examined using both of simple and adaptive wedge members. In addition, an anti-fatigue smart paste, which consists of fine alumina particles and silicone grease, was applied to the periphery of the drill hole, and its effects on automatic restraint and visual detection of crack growth were investigated. FE analyses using contact elements were also carried out for a comparative study. As a result, it was found that the strain range on the specimen side is reduced to 44.0% and the fatigue life is prolonged by 12.4 times by application of the adaptive wedge member as compared with the case of the conventional stop-hole. As for the anti-fatigue smart paste, it was found that the fatigue life is further prolonged by about three times that in the case of the adaptive wedge member and the paste has an evident function to support visual detection of cracks.

Effect of Surface Modification on In-Depth Transformations and Flexural Strength of Zirconia Ceramics.

Chairside surface adjustments of zirconia dental restorations enhance the toughening stress-induced tetragonal-to-monoclinic phase transformation and domain reorientation by ferro-elastic domain switching (FDS), but also trigger subsurface damage, which could compromise long-term clinical performance. The purpose of this study was to assess the depth of phase transformation, associated FDS, and flexural strength of dental zirconia (BruxZir HT 2.0), after chairside surface treatments. Square specimens were sectioned from CAD/CAM blocks and sintered according to manufacturer's recommendations (n = 30). They were left as-sintered (AS; control), air abraded with fine (AAF) or coarse (AAC) alumina particles, ground (G) or ground and polished (GP). Roughness was measured by profilometry. Crystalline phases were investigated by grazing incidence X-ray diffraction (GIXRD) (n = 3). GIXRD data were fit using semi-log regression protocols to assess transformation depth and extent of FDS. The mean biaxial flexural strength was measured according to ISO 6872. Subsurface damage was assessed from SEM images using a bonded polished interface configuration. Flaw distribution was assessed by Weibull analysis. Results were analyzed by Kruskal-Wallis with Tukey's adjustment for multiple comparisons (p < 0.05). Air-abraded and ground groups exhibited higher mean surface roughness than control. AAF group exhibited the highest flexural strength (1662.6 ± 202.6 MPa) with flaw size (5.9 ± 1.8 μm) smaller than transformation (14.5 ± 1.2 μm) or FDS depth (19.3 ± 1.1 μm), followed by GP group (1567.2 ± 209.7 MPa) with smallest FDS depth (9.3 ± 2.0 μm) and flaw size (2.6 ± 1.8 μm), but without m-phase. AAC group (1371.4 ± 147.6 MPa) had the largest flaw size (40.3 ± 20.3 μm), transformation depth (47.2 ± 3.0 μm) and FDS depth (41.2 ± 2.2 μm). G group (1357.0 ± 196.7 MPa) had the smallest transformation depth (8.6 ± 1.5 μm), and mean FDS depth (19.8 ± 3.7 μm) and flaw size (18.6 ± 3.1 μm). AAC and AAF exhibited the highest Weibull modulus (11.2 ± 0.4 and 9.8 ± 0.3 μm, respectively). Variations in mean biaxial flexural strength were explained by the balance between the depth of toughening mechanisms (phase transformation and FDS) and subsurface damage. AAF and GP groups were the most efficient surface adjustments in promoting the highest mean biaxial flexural strength.

Automatic restraint and visual detection of fatigue crack growth when applying an anti-fatigue smart paste to bolt holes

ABSTRACTAn anti-fatigue smart paste, which consists of fine alumina particles and silicone grease with low viscosity, was applied to a bolt hole (and its periphery in some cases) in a steel plate specimen, and the effects of the smart paste on automatic restraint and visual detection of fatigue crack growth were experimentally investigated through fatigue tests. Fractographic observations using a scanning electron microscope were also carried out. As a result, approximately 20∼410% increase in failure life was produced by the wedge effect of the alumina particles in the smart paste. When the smart paste was applied, remarkable black colour developed in the white paste along the paths of crack growth, exceedingly facilitating the visual detection of the crack growth.

Nanocarbon containing Al2O3 – C continuous casting refractories: Effect of graphite content

Alumina-carbon refractories are broadly used in the continuous casting area in the production of steel, mainly in the devices used to control flow, because of their thermal and mechanical properties. Continuous casting refractories contain approximately 30% of residual carbon after coking to impart and improve certain refractory properties. But this leads to the threat of carbon pick up by the molten steel for high-quality steel production. In the present work, nanocarbon is used in combination with graphite to reduce the total carbon content. The changes in physical and mechanical properties like bulk density and strength with the phase and microstructural development were studied. The density and strength values are decreasing with increase in the amount of graphite in the composition. Formation of aluminum carbide in nanocarbon containing batches enhances the strength development. High surface area nanocarbon possess high reactivity than the graphite and helps in carbide phase formation and it can easily disperse in the gaps between coarse, medium and fine alumina particles, thereby reduces the porosity, and enhances density and strength development.

Spray freeze granulation of submicrometre alpha-alumina using ultrasonication

Granulation is a key factor towards improvement of the flowability of fine ceramic powders to make them suitable for industrial dry pressing. Controlled granulation of fine alumina particles with a primary particle size of ∼ 150 nm was carried out using spray-freeze drying, which led to the production of flowable granules with high crushability. The fracture surface of uniaxially die-pressed green bodies made from granules with density values of ≥ 50%of theoretical showed a uniform microstructure. Sintering experiments were performed using conventional single- and two-stage radiant heating methods followed by density and grain size measurement and characterisation of the final dense compacts to study the efficiency of two-stage sintering in grain growth elimination. The results have been compared with those of alumina bodies prepared using similar suspension by the slip-casting route.

Novel growth method of carbon nanotubes using catalyst-support layer developed by alumina grit blasting

We propose an efficient method of growing carbon nanotube (CNT) arrays on a variety of metals, alloys, and carbon materials using chemical vapor deposition (CVD) assisted by a simple surface treatment of the materials. The main feature of this method is the application of grit blasting with fine alumina particles to the development of a catalyst-support layer required for the growth of CNTs on various conductive materials, including ultra-hard metals such as tungsten. Auger electron spectroscopy shows that grit blasting can form a non-continuous layer where alumina nanoparticles are embedded as residues in the blasting media left on the treated surfaces. This work reveals that such a non-continuous alumina layer can behave as the catalyst-support layer, which is generally prepared by sputter or a vacuum evaporation coating process that considerably restricts the practical applications of CNTs. We have attempted to grow CNTs on grit-blasted substrates of eighteen conventionally used conductive materials using CVD together with a floating iron catalyst. The proposed method was successful in growing multi-walled CNT arrays on the grit-blasted surfaces of all the examined materials, demonstrating its versatility. Furthermore, we found that the group IV metal oxide films thermally grown on the as-received substrates can support the catalytic activity of iron nanoparticles in the CVD process just as well as the alumina film developed by grit blasting. Spectral emissivity of the CNT arrays in the visible and infrared wavelength ranges has been determined to assess the applicability of the CNT arrays as a black coating media.

High temperature mechanical behavior of alumina dispersion strengthened copper alloy with high content of alumina

The microstructure and its effects on the high temperature mechanical behavior of Cu–2.7%Al2O3 (volume fraction) dispersion strengthened copper (ADSC) alloy were investigated. The results indicate that fine alumina particles are uniformly distributed in the copper matrix, while a few coarse ones are distributed on the grain boundaries. Tensile tests results show that Hall-Petch mechanism is the main contribution to the yield strength of ADSC alloy at room temperature. Its high temperature strength is attributed to the strong pinning effects of alumina particles on the grain and sub-grain boundaries with dislocations. The ultimate tensile strength can reach 237 MPa and the corresponding yield strength reaches 226 MPa at 700 °C. Tensile fracture morphology indicates that the ADSC alloy shows brittleness at elevated temperatures. Creep tests results demonstrate that the steady state creep rates at 400 °C are lower than those at 700 °C. The stress exponents at 400 °C and 700 °C are 7 and 5, respectively, and the creep strain rates of the ADSC alloy are controlled by dislocation core diffusion and lattice diffusion.

Experimental study of the aerosolization of fine alumina particles from bulk by a vortex shaker

Abstract The use of a vortex shaker for particle aerosolization from pseudo-bidisperse micron-sized alumina bulk powders has been studied in view of its application as a dustiness characterization method. A parametric study has been carried out with respect to the bulk mass as well as the vortex speed. A minimum sample size and lower and upper limits for the rotational speed have been identified for this powder. Within these boundaries, the aerosolized particles’ mass concentration is found to be proportional to the sample’s kinetic energy multiplied by its mass. At too high rotational speeds, the particle size distribution is changed.

Fabrication of a low temperature co-fired ceramic package using powder blasting technology

This paper presents the application of the powder blasting technique for the fabrication of a low temperature co-fired ceramic package that is to host an optical sensor device. The package consists of several layers of green tape ceramic featuring aperture sizes ranging from 3 to 0.1 mm in diameter. These layers have been patterned using fine alumina particles of 9 μm in diameter in combination with an electroplated nickel mask coated with a protective layer. The latter is based on a photosensitive material laid down on top of the metallic surface, exposed and developed to free the metal apertures from the coating. The results show that the wide range of apertures size present in the mask can be used to machine the green ceramic. The quality of the structures is smooth and vias shapes are perfectly circular. These results were obtained at 50 psi with a flow rate of 0.20 g/s for a distance of 20 mm between the tip of the nozzle and the substrate.

Microstructure and Mechanical Properties of Aluminum Alloy Composites Strengthened with Alumina Particles

The mechanical properties and microstructures of aluminum-matrix composites fabricated by the dispersion of fine alumina particles less than <TEX>$20{\mu}m$</TEX> in size into 6061 aluminum alloys are investigated in this study. In the as-quenched state, the yield stress of the composite is 40~85 MPa higher than that of the 6061 alloy. This difference is attributed to the high density of dislocations within the matrix introduced due to the difference in the thermal expansion coefficients between the matrix and the reinforcement. The difference in the yield stress between the composite and the 6061 alloy decreases with the aging time and the age-hardening curves of both materials show a similar trend. At room temperature, the strain-hardening rate of the composite is higher than that of the 6061 alloy, most likely because the distribution of reinforcements enhances the dislocation density during deformation. Both the yield stress and the strain-hardening rate of the T6-treated composite decrease as the testing temperature increases, and the rate of decrease is faster in the composite than in the 6061 alloy. Under creep conditions, the stress exponents of the T6-treated composite vary from 8.3 at 473 K to 4.8 at 623 K. These exponents are larger than those of the 6061 matrix alloy.

Study on the Properties of Al-Al2O3 Composite Using Synthesized Alumina from Sol Gel Technique

Alumina reinforced aluminum is one of metal matrix composite (MMC) systems that can maintain its properties although at high temperature. Among various processes to produce MMC, powder metallurgy is the best method because of its efficiency dispersion of fine alumina particles. In this study, alumina powders were synthesized through sol gel method which is known as one of the ideal method to produce good properties of powders. The performances of synthesized alumina powders were then observed through the fabrication of composite. The percentage of alumina (0wt%, 10wt%, 20wt% and 30wt %) and sintering temperature (500°C and 550°C) were varied in order to observed their effects on the produced composite. Result shows that by increasing the percentage of alumina and sintering temperature, mechanical properties were increased where 30wt% alumina and 550°C sintering temperature give the optimum results. Comparison study by using 30wt% alumina and 550°C sintering temperature on the composite using synthesize and commercial alumina shows similar/comparable properties of composite.

Optimization of Fine Alumina Gelcasting UsingIn SituDynamic Rheology

In situdynamic rheology was used to investigate the polymerization process of gelcasting suspensions based on fine alumina and water solution of methacrylamide and methylene bisacrylamide monomers. Small‐strain oscillatory shear measurements were able to detect the progress of the whole gelation process from the liquid suspension to the elastic gel. The parameters describing the gelation kinetics and final gel properties such as idle time, total gelation time, and equilibrium complex viscosity were correlated with the initiator/accelerator concentration, linear to cross‐linking monomer ratio, polymerization temperature, and alumina loading. A strong catalytic effect of fine alumina particles on the decomposition of ammonium persulfate into free radicals was established. The catalytic activity of alumina powder was controlled by ions adsorbed on the particle surface. The catalytic effect of alumina in the presence of the Dolapix CE 64 dispersant made the polymerization process independent of the accelerator concentration and enabled the gelation of concentrated ceramic suspensions at and below room temperature without any accelerator. The variations of the polymerization process in the range of investigated gel compositions did not affect the particle packing in green bodies and the densities of sintered ceramic bodies were similar.

Effect of particle size distribution of alumina on strength of glass-infiltrated alumina

Glass-infiltrated alumina composites were prepared by infiltrating glass into a pre-sintered alumina. Three different alumina preforms were obtained from various combinations of fine and coarse alumina particles. After infiltration of glass into the porous alumina preforms, their microstructure and strength were studied. The highest bending strength of 510MPa was observed when the composite was made by mixing coarse and fine alumina powders at a ratio of 6:4. The infiltrated glass corroded the alumina preform, and the dissolved aluminum ions reprecipitated on the alumina grains during the heat-treatment for infiltration.

Tribological Behavior of Bronze Composite Coatings Obtained by Plasma Thermal Spraying

Bronze aluminum composite coatings containing different amounts of alumina were fabricated by plasma spray process and their tribological properties were investigated using ball-on-disk (BOD) and rubber wheel (RW) tests at room temperature. Main wear mechanisms in pure bronze coatings during the ball-on-disk friction test were abrasion and intersplat delamination. The addition of alumina in bronze coatings clearly enhances their wear resistance. To explain this behavior, this article proposes an additional wear mechanism in the composite coatings that involves the rupture of the alumina lamellae located just below the wear track leading to a uniform distribution of fine alumina particles enveloped by the bronze matrix, which increase the surface hardness and hinder the wear. The deposition of debris on the wear track of composite coatings provokes an enhancement of the wear resistance as well. Bronze coatings show a low and stable friction coefficient of around μ = 0.3. Nevertheless, coatings with reinforcing particles of alumina show an abrupt transition in the friction coefficient from values around μ = 0.4–0.8, related to the modification of the surface contacts on the wear track due to the formation of a compacted debris layer deposited during the tribological test.

Modification of hot glass surface with alumina by combustion CVD

A combustion CVD process with aluminium acetylacetonate as a precursor was used to apply alumina on a float glass surface at a temperature of 600°C in order to improve its hydrolytic resistance. The generated surfaces were investigated by XPS, SEM, AFM, and leaching experiments. It was found that fine alumina particles were generated, which cover the surface, and that the percentage of alumina on the surface determined its hydrolytic resistance. While there was no diffusion of aluminium into the glass matrix detected, deposited alumina demonstrated a measurable influence on the sodium–silicium-ratio at the glass surface.

The effect of production parameters on microstructure and wear resistance of powder metallurgy Al–Al 2O 3 composite

Aluminum matrix composite is one of the most conventional types of metal matrix composites. This paper deals with the effect of production parameters on wear resistance of Al–Al 2O 3 composites. Alumina powder with a particle size of 12, 3 and 48 μ and pure aluminum powder with particle size of 30 μ were used. The amount of added alumina powder was up to 20%. Ball milling was utilized to blend the powders. The range of sintering temperature and time were 500, 550 and 600 °C and 30, 45, 60 and 90 min respectively. It was found that increasing sintering temperature results in increasing density, hardness and wear resistance and homogenization of the microstructure. However at certain sintering temperatures and time, considerable grain growth and reduction of hardness value occurred, leading to the degradation of wear resistance. The results showed that at high alumina content, relative density of the composite increases. However, after raising the particle size of alumina, relative density initially increases and then drops to lower values. Increasing weight percent of alumina powder leads to higher hardness and consequently improves the wear resistance of Al–Al 2O 3 composite. The use of fine alumina particles has a similar effect on hardness and the wear resistance. Finally, a finer grain size was observed, at high amount and low size of the reinforcement particle.