Submicron Alumina Particles Research Articles

Tribological characterisation of 6061 aluminium alloy reinforced with submicron alumina particle under solid lubrication at room temperature

The tribological behaviour of aluminium (Al) 6061 alloy reinforced with 20% submicron (0.7 µm) alumina (Al2O3) particles has been investigated at room temperature under solid lubricated conditions of graphite and molybdenum disulphide (MoS2). Pin-on-disc wear tests were carried out at various stresses, with the composite pin sliding against an AISI 4041 steel counterface disc. At room temperature and under low and moderate loads, graphite proved to be effective in reducing wear rate by a factor of 6 and coefficient of friction from 0.50 ± 0.01 to 0.20 ± 0.01. Under MoS2 lubrication, the wear rate and coefficient of friction were significantly reduced at all tested loads (5 to 110 N), indicating that MoS2 is an effective lubricant in reducing wear rate by a factor of 40 and coefficient of friction from 0.50 ± 0.01 to 0.06 ± 0.01 for the composite at all loads.

Formulation of a Ceramic Ink for 3D Inkjet Printing.

Due to its multi-material capabilities, 3D inkjet printing allows for the fabrication of components with functional elements which may significantly reduce the production steps. The potential to print electronics requires jettable polymer-ceramic composites for thermal management. In this study, a respective material was formulated by functionalizing submicron alumina particles by 3-(trimethoxysilyl)propylmethacrylate (MPS) and suspending them in a mixture of the oligourethane Genomer 4247 with two acrylate functionalities and a volatile solvent. Ink jetting tests were performed, as well as thermal conductance and mechanical property measurements. The material met the strict requirements of the printing technology, showing viscosities of around 16 mPa·s as a liquid. After solidification, it exhibited a ceramic content of 50 vol%, with a thermal conductance of 1 W/(m·K). The resulting values reflect the physical possibilities within the frame of the allowed tolerances set by the production method.

Highly oriented α-Al2O3 transparent ceramics shaped by shear force

Alumina platelets were arranged horizontally in submicron alumina particles by shear force in the flow of slurries during casting. The obtained alumina green bodies with platelets were pressureless-sintered in vacuum, producing ceramics with thoroughly oriented grains and high transmittance. The effects of sintering parameters on the densification, microstructure evolution, and orientation degree of alumina ceramics were investigated and discussed. The results showed that the densification, grain size, orientation degree, and in-line transmittance were increased with increasing sintering temperature. The enhancement of orientation degree was mainly coherent with grain growth. The grain-oriented samples exhibited a much higher in-line transmittance (at 600 nm) of 61 % than that of the grain random sample (29 %). Moreover, the transmission remained a high level in the ultraviolet range (<300 nm).

Electrophoretic deposition of plasma activated sub-micron alumina powder

The positive effect of plasma mediated treatment of powder fraction in water-based dispersion is a rather well documented fact. Despite its importance for ceramic processing, the applicability of plasma treatment of ceramic particles in a non-aqueous dispersions remains unexplored. In the presented article, a specific planar type of dielectric barrier discharge - Diffuse Coplanar Surface Barrier Discharge (DCSBD), operated in atmospheric pressure air was used for the surface activation of sub-micron alumina particles. These were consequently dispersed in organic solvent of 2-propanol and evaluated for the performance during the process of electrophoretic deposition (EPD). The surface of ceramic particles and the dispersion behaviour were investigated using thermogravimetric analysis, XPS, measurement of zeta potential and particle size distribution. Plasma treatment of ceramic particles had a substantial effect on the deposition rate, which caused a significant reduction of surface roughness and slight improvement of the density and mean grain size of the fabricated coatings. Moreover, the plasma treatment resulted in a simultaneous anodic and cathodic EPD.

A high temperature seeding technique for particle image velocimetry

Non-intrusive measurements of gas velocities via particle image velocimetry (PIV) or laser Doppler velocimetry (LDV) requires entraining particles into the flow field. There are many techniques and materials available for seeding gas phase flows. However, when the flow temperatures exceed 200 °C, the available options for seed materials becomes limited. In high temperature applications refractory seed materials are required. The established technique for seeding flows with metal oxide powders is via fluidized beds by themselves or in combination with cyclone separators. These systems are fraught with problems which limit their ability to provide consistent, uniform flow seeding. In this work, we describe a technique for reliably introducing metal oxide particles into high temperature flows. The employment of pH stabilization techniques typically used to obtain stable dispersions in ceramic processing can provide a source of seed material for high temperature air flows. By pH stabilizing submicron alumina particles in ethanol, a stable dispersion is obtained which when atomized, produces a high quality aerosol. Commercial grade alumina is used with a moderate size distribution. The technique is not limited to alumina/ethanol and is also demonstrated with an alumina/H2O system. Other ceramic powders in other polar solvents can also be used once their point of zero charge (pHpzc) of the powder in the solvent has been determined. We present an example of the pH stabilized dispersions applied to a very challenging high temperature supersonic flow and a particle dynamics analysis across a shock.

Effect of the interfacial reaction of aluminum and yttrium oxide on aging behavior of aluminum–magnesium–silicon alloy reinforced with yttrium oxide modified sub-micron alumina

Age-strengthening behavior of Al–Mg–Si alloy composites reinforced with sub-micron alumina particles modified by yttrium oxide under the temperature of 130°C, 160°C and 190°C was investigated. The mechanical performances of unmodified and modified composites after the aging treatment were evaluated with the tensile test. The results indicate that the hardness level of modified composites is obviously increased compared to unmodified composites at different aging states, and the process of aging precipitation of the matrix alloy in modified composites is accelerated by the interfacial reaction of aluminum and yttrium oxide. The aging microstructures of modified composites have a significant difference compared to non-modified composites, and present a large amount of dislocations and precipitates in the matrix alloy. The modified composites treated by the aging exhibit excellent tensile properties. It is suggested that the interfacial reaction resulted from the surface modification of particles with the rare-earth yttrium oxide is beneficial in promoting the process of aging precipitates and strengthening the aging microstructures and properties of Al alloy matrix in modified composites.

Alternative coating technologies for metal–ceramic nanocomposite films: potential application for solar thermal absorber

Metal–ceramic nanocomposite films for solar absorbers have been obtained by electro-deposition. This coating technique uses a mixed solution of nickel nanoparticles and sub-micron alumina particles. The relative ratio of Ni-to-Al in these films is varied by tuning the concentration of nickel and alumina particles in the starting solution. Three films with different Ni/Al ratio have been prepared and characterized with X-ray photo-electron spectroscopy and scanning electron microscope. Even in absence of any heat treatment, significant amount of nickel in metallic state has been found in all three samples.

Preparation and Characterization of Meso-Porous Alumina Spheres

Mesoporous alumina is usually used as adsorption material, catalyst carrier or separation media due to its high adsorption and good thermal stability. However, the nanosized powder trends to aggregation except that its size is in sub-micron scale. The best choice is that the sub-micron alumina particles with mesoporous structure are prepared. In the present work, the mesoporous alumina spheres were prepared by micro emulsion method using aluminium isopropoxide as raw material and water/dimethylbenzene emulsion as reaction media. The obtained powder was characterized by XRD, TEM and BET. The results show that the particles are global. The particles size is in the range of 100 ~ 200 nm to several micrometers. The particle owns the worm-like structure with the pore size of 5 nm. The BET surface area of the mesoporous alumina is 81.8 m2/g. The obtained powder is suit to be used as the packing material in chromatograph.

Bimetallic low thermal-expansion panels of Co-base and silicide-coated Nb-base alloys for high-temperature structural applications

The fabrication and high temperature performance of low thermal expansion bimetallic lattices composed of Co-base and Nb-base alloys have been investigated. A 2D sheet lattice with a coefficient of thermal expansion (CTE) lower than the constituent materials of construction was designed for thermal cycling to 1000 °C with the use of elastic–plastic finite element analyses. The low CTE lattice consisted of a continuous network of the Nb-base alloy C-103 with inserts of high CTE Co-base alloy Haynes 188. A new coating approach wherein submicron alumina particles were incorporated into (Nb, Cr, Fe) silicide coatings was employed for oxidation protection of the Nb-base alloy. Thermal gravimetric analysis results indicate that the addition of submicron alumina particles reduced the oxidative mass gain by a factor of four during thermal cycling, increasing lifetime. Bimetallic cells with net expansion of 6 × 10 −6/°C and 1 × 10 −6/°C at 1000 °C were demonstrated and their measured thermal expansion characteristics were consistent with analytical models and finite element analysis predictions.

Particle contamination in turbulent flow within a Hard Disk Drive enclosure

A proposed aerosol model, that uses an Eulerian frame of reference, is used in this paper to study particle flow and interactions in transient flow conditions within the Hard Disk Drive (HDD) enclosure in view of the implications of sub-micron airborne contamination inside these enclosures. The effects of particle collision due to Brownian motion and turbulence on particle size and concentration are studied in this paper for sub-micron alumina particles. The evolution of particle transport and concentration in sensitive regions inside the enclosure are also investigated to understand the impact of localised changes in the airflow characteristics on the particles.

Synthesis and mechanical and tribological characterization of alumina–yttria stabilized zirconia (YSZ) nanocomposites with YSZ synthesized by means of an aqueous solution–gel method or a hydrothermal route

In the present study, yttria stabilized zirconia (YSZ) nanoparticles, prepared by means of an aqueous solution–gel method or a hydrothermal route, are incorporated in a matrix of submicron alumina particles by wet mechanical milling. The microstructural characteristics and the mechanical and tribological properties of the obtained alumina–YSZ nanocomposites are evaluated as a function of different processing conditions like milling time, YSZ amount, sintering procedure and synthesis method of YSZ. It is noticed that the synthesis procedure and the agglomeration degree of the YSZ nanoparticles seriously affect the densification process of the alumina–YSZ nanocomposites and also their mechanical and tribological properties. The most probable cause for the difference is that the hydrothermally prepared YSZ nanoparticles are not as homogeneously distributed in the alumina matrix as the solution–gel prepared nanoparticles. Moreover the former nanoparticles have surface groups which release undesired gases during sintering. Thus to obtain a dense sample, nanocomposites with these nanoparticles require a higher sintering temperature and this has a negative effect on the mechanical and tribological properties of these materials.

Ink-jet delivery of particle suspensions by piezoelectric droplet ejectors

The volume and velocity of droplets ejected from a piezoelectric droplet generator, as used in ink-jet printing, have been studied for a range of concentrated suspensions of submicron alumina particles as a function of driving signal voltage, frequency, and peak shape. Drop velocity and volume are found to show a linear relation with driving voltage, but show a more complicated and periodic behavior with changing frequency and peak width. This periodic dependence is shown to be a function of the acoustic properties of the fluid-filled chamber in the droplet generator. However, a simple model considering propagation of pressure waves along the tubular actuator is not consistent with experimental data if the ends are modeled as step changes in acoustic impedance. By comparing the data with that in the literature, it is proposed that the acoustic boundary condition (impedance change) at the orifice where drops are ejected is a function of orifice size and extrapolates to a closed boundary condition, as the orifice diameter tends to zero. At equivalent driving signal frequencies, the drop volume ejected, normalized by actuator volume displacement, is shown to be a function of the Ohnesorge number of the orifice through which the drops are ejected.

Effect of temperature and strain rate on the compressive flow of aluminum composites containing submicron alumina particles

Uniaxial compression tests were conducted on aluminum composites containing 34 and 37 vol pct submicron alumina particles, to study the effect of temperature and strain rate on their flow stress. For temperatures between 25 °C and 600 °C and strain rates between 10−3 and 1 s−1, the flow stress of the composites is significantly higher than that of unreinforced aluminum tested under similar conditions. This can be attributed to direct strengthening of the composites due to load sharing between the particles and the matrix, and an enhanced in-situ matrix flow stress resulting from a particle-induced increase in dislocation density. The composites, however, exhibit the same stress dependence on temperature and strain rate as unreinforced aluminum, indicating a common hardening mechanism, i.e., forest dislocation interactions. The forest hardening present under the explored testing conditions masks the effects of direct dispersion strengthening operative at lower deformation rates in these materials.

Effect of aggregate size on sediment bed rheological properties

Three different types of aggregates of submicron alumina particles were produced utilising either polymer, high salt (1.0 M), or low salt (0.075 M) aggregation conditions. All three types of aggregates had similar structural properties (mass fractal dimension = 2.0). The typical size (d[4,3]) of the polymer aggregates was 125 microns, the high salt aggregates 12 microns and the low salt aggregates 4 microns. It was found that smaller aggregates produced higher gel points and higher apparent maximum packing fractions. Larger aggregates produced higher shear and compressive yield stresses at all volume fractions. The relative effect that aggregate size and inter-particle attraction has on the sediment yield stresses was investigated in the salt system. The salt concentration was adjusted so as to produce different size aggregates with the same final salt concentration and thus same level of inter-particle attraction. The size of the aggregates and the magnitude of the inter-particle attraction were found to have similar contributions to the compressive yield stress over the range of size and attraction investigated.

Effect of reinforcement connectivity on the elasto-plastic behavior of aluminum composites containing sub-micron alumina particles

The mechanical properties of composites consisting of an aluminum matrix with 34 and 37 vol.% sub-micron Al 2O 3 particles were studied in compression for two reinforcement architectures: interconnected and discontinuous. Both the elastic and plastic behaviors of these composites are successfully modeled using a self-consistent approach: the classical self-consistent and the three-phase self-consistent models for the interconnected and discontinuous architectures, respectively. At ambient temperature, an interconnected architecture offers only a modest increase in stiffness and strength over a discontinuous architecture of equal volume fraction. At elevated temperatures (250, 500 and 600 °C), the interconnected reinforcement becomes increasingly more effective at strengthening the composites. However, the relative increase in strength due to interconnectivity can only be exploited at small strains (1–5%) due to the early development of compressive flow instabilities in the interconnected composites. While microstructural damage controls the instability strain of the interconnected composites at ambient temperature, their low strain-hardening coefficient is the main contribution to flow instabilities at elevated temperature.

Collection efficiency of sintered ceramic filters made of submicron spheres

The performance of filters made of sintered submicron alumina particles was evaluated. The filter has a high collection efficiency and high pressure drop, requiring the development of a special measuring system for its evaluation. The system consists of a polydisperse NaCl particle generator, a differential mobility analyzer (DMA), an ejector to supply aerosols for testing filters with high pressure drop, and a mixing-type condensation nucleus counter (CNC) capable of obtaining a stable reading of very low concentration particles. Penetrations as low as 10 −9 can be measured in the particle diameter range of 0.02–0.14 μm. Two filters made by sintering 0.60 and 0.84 μm alumina particles were evaluated. The experimental data collected served as the basis of theoretical development. Following the single fibre theory, the filter penetration is calculated by using a single sphere as the element. Both the diffusion and interception collection mechanisms were taken into account. The resulting equation gives a general trend of efficiency curves as a function of the parameters involved, e.g. the test aerosol size, packed particle size and filtration velocity. However, it is not sufficiently accurate for providing quantitative performance results.

Experimental Study of Alumina Particle Removal From a Plane surface

Micron and submicron alumina particles are often used for the mechanical polishing of the GaAs wafers processed in the microelectronic industry. A better understanding of the adhesion mechanisms is the key factor for the particle removal and for the optimisation of the industrial chemical cleaning. However, the nature and the strength of the complex interactions occurring between asymmetrical alumina particles and the surface remain unclear. Thus, an experimental study of the detachment of asymmetrical alumina particles in adhesive contact with a glass plate was done using a specially designed shear stress flow chamber. A series of experiments was performed to measure the shear stress necessary to remove individual alumina particles (of 3 and 0.3 µm nominal size) under various chemical solutions (diluted ammonia, surfactant and glycerol). Then the effects of the particle size, the resting time, the pH and the nature of the chemical solutions used for the removal of the alumina particles was characterised in terms of percentage of alumina particles detached. Results have shown that the longer the resting time, the more adherent the particles are. Moreover, it was found that the ammonia solution gives the best particle removal rate (80%) because of the strong repulsive electrostatic interactions between the alumina particles and the glass surface, both being charged negatively in a basic solution.

Synthesis of Titania coated Alumina Particles by a Hybrid Sol-gel Method

ABSTRACTModifying their surface with a coating of another ceramic material can dramatically alter the properties of ceramic particles. In the present work we have demonstrated that the Al2O3 particles can be successfully coated by TiO2 using a novel sol-gel technique. The nature of these coatings was predicted on the basis of scanning electron microscopy imaging in conjunction with the micro-Raman scattering measurements. The surface morphology of these particles shows that either individual or group of sub-micron alumina particles are coated with the nano-crystalline titania particles. The thickness of the titania coating could be varied by changing the precursor sol concentration. Amorphous titania was converted to anatase phase at 400°C and upon further heating it started transforming to rutile phase, and both these phases coexisted in the coated particles that were heat treated up to 800°C. The mechanical strength of the titania coating was measured qualitatively by ultrasonicating the coated powders for longer duration to observe that titania coatings are strongly adhered with the alumina particles.

Comparison of a regenerative thermal oxidizer to a rotary concentrator

The positive effect of plasma mediated treatment of powder fraction in water-based dispersion is a rather well documented fact. Despite its importance for ceramic processing, the applicability of plasma treatment of ceramic particles in a non-aqueous dispersions remains unexplored. In the presented article, a specific planar type of dielectric barrier discharge - Diffuse Coplanar Surface Barrier Discharge (DCSBD), operated in atmospheric pressure air was used for the surface activation of sub-micron alumina particles. These were consequently dispersed in organic solvent of 2-propanol and evaluated for the performance during the process of electrophoretic deposition (EPD). The surface of ceramic particles and the dispersion behaviour were investigated using thermogravimetric analysis, XPS, measurement of zeta potential and particle size distribution. Plasma treatment of ceramic particles had a substantial effect on the deposition rate, which caused a significant reduction of surface roughness and slight improvement of the density and mean grain size of the fabricated coatings. Moreover, the plasma treatment resulted in a simultaneous anodic and cathodic EPD.

Characterization of submicron alumina dispersions with poly(methacrylic acid) polyelectrolyte

Colloidal dispersion of two submicron alumina powders (A-16SG and APA-0.2) and their mixtures were investigated in an aqueous suspension with and without dispersant (ammonium salt of poly(methacrylic acid), PMAAN). The ζ-potential and isoelectric point (iep) of the powders in aqueous solution were determined in the presence of the dispersant and electrolyte (NaCl). The adsorption of dissociated PMAA − on alumina powders was determined via the titration technique. The dispersant was observed to exert a strong influence on the surface potential, flowing behaviors and packing densities of the aluminas. The results showed that the dispersion conditions were strongly affected by factors including the solid concentration of submicron alumina particles, the pH value and the concentration of PMAAN in the suspensions, and that these three factors were cross-influencing. A reaction model at the interface of alumina—solution was developed, based on the dissociation of PMAAN and the adsorption of surface hydrocarboxyl groups on alumina surface.