Alpha-alumina Particles Research Articles

Adsorption and transformation of tetracyclines on alpha alumina particles with surface modification by anionic surfactant

The adsorption and transformation of tetracyclines (TCs) antibiotics, including oxytetracycline (OTC), chlortetracycline (CTC), and tetracycline (TC), on the sodium dodecyl sulfate (SDS) surfactant-modified α-Al2O3 particles were comprehensively investigated in this study. The TCs adsorption was significantly enhanced by using the modified adsorbents compared with the use of the unmodified adsorbents. The experimental conditions were systematically optimized and found to be pH 4, NaCl 1 mM, the contact time of 180 min, and the adsorbent dosage of 25 mg. mL−1. The high maximum adsorption capacities were approximately 320, 85, and 91 mg. g−1 for TC, OTC, and CTC, respectively. Meanwhile, the great removal efficiencies of the three antibiotics TC, OTC, and CTC were correspondingly 91.85, 88.4, and 98.3%. The TCs adsorption isotherm and kinetics on the SDS-modified α-Al2O3 particles mainly governed by the electrostatic and hydrophobic interactions were clarified by a suitable two-step model, the Fourier transform infrared spectroscopy (FT-IR) and zeta potential measurements. Meanwhile, the TCs structural transformation determined by the liquid chromatography with tandem mass spectrometry (LC-MS/MS) measurement was promoted through the adsorption on the α-Al2O3 surface. The TCs transformation rates strongly affected by the TCs adsorption were in the order of CTC > TC > OTC. The found results are promised that the SDS-modified α-Al2O3 particles might behave as high-performance adsorbents to remove the TCs from aqueous solutions.

The effect of C-coated alumina particles on properties of MgO–C refractories

In this study, carbon-coated α-Al2O3 powder was prepared by two routes: a physical method and a chemical method. The obtained carbon-coated α-Al2O3 powders were adopted to prepare MgO–C refractory materials for the purpose of regulating the in-situ synthesis of MgAl2O4. The effect of the carbon-coating α-Al2O3 powder addition on the service behavior of MgO–C refractories was studied. The results show that both the physical and chemical methods for coating alpha-alumina particles could regulate the solid-phase formation of spinel in the magnesia carbon material, which weakened the solid-phase reaction of the spinel in the magnesia carbon material, thus reducing the volume expansion. Samples containing coated alpha Al2O3 micropowder, prepared by the chemical method, presented the largest bulk density, the minimum apparent porosity, the largest cold and hot modulus of rupture, and the best thermal shock and corrosion resistance.

Simultaneous adsorption of anionic alkyl sulfate surfactants onto alpha alumina particles: Experimental consideration and modeling

Simultaneous adsorption of four anionic alkyl sulfate (AS) surfactants, including sodium octyl sulfate, sodium decyl sulfate, sodium dodecyl sulfate and sodium tetradecyl sulfate named as C8, C10, C12 and C14, onto alpha alumina (α-Al2O3) particles were systematically investigated. Concentrations of all mixtures of anionic AS surfactants in aqueous solution were quantified by capillary electrophoresis coupled with contactless conductivity detection (CE-C4D). Adsorption isotherms of the four anionic AS surfactant homologs increased with either the decrease of pH from 6.0 to 4.0 due to enhancement of electrostatic attractions between the positively charged α-Al2O3 surface and the anionic AS surfactants or the increase of carbon chain length due to enhanced hydrophobic interactions. At different salt concentrations, a common intersection point (CIP) obtained for adsorption isotherms demonstrated that the contributions of electrostatic and hydrophobic interactions were reversed. The ionic strength effect on anionic AS surfactant adsorption isotherms onto α-Al2O3 particles was thoroughly studied. Simultaneous adsorption of the four AS surfactants onto the α-Al2O3was controlled by both electrostatic and hydrophobic interactions that were in accordance with Langmuir and two-step models.

Size-dependent quasi Brittle–Ductile transition of single crystalline alpha-alumina particles during microcompression tests

Dense ceramic coatings can be fabricated through the kinetic impaction of fine solid particles at room temperature without any heat assistance or binder, and such a coating process is known as aerosol deposition (AD). In this process, solid ceramic particles that are generally fragile exhibit plastic deformation when they are refined; this is the key to the deposition and densification of ceramic coatings in AD. However, previous work has failed to address the transition criteria related to the size of ceramic particles during plastic deformation; the size is significantly associated with their fracture strength and strain. To eliminate the grain boundary effect, single crystalline alpha-alumina particles were selected and their strength and strain before fracture were systematically measured in a quasi-static compression test. The fracture strength of the alumina particles increased when the particle size decreased, and the particles exhibited quasi-plastic deformation when the fracture strength reached the compressive strength of alumina. This size-dependent brittle–ductile transition of ceramic materials in compression implies that the intragrain plastic deformation of ceramic particles can be attributed to the solid particle deposition. Thus, appropriately designing the intra grain architecture in ceramic particles may allow highly efficient deposition and further densification in AD.

Fabrication of highly permeable polyamide membranes with large “leaf-like” surface nanostructures on inorganic supports for organic solvent nanofiltration

Organic solvent nanofiltration (OSN) has shown great potential in molecular separation and solvent purification. Polyamide (PA) thin-film composite (TFC) membranes on polymer supports have been widely used in OSN, but the development of solvent resist supports is still a big challenge. Inorganic supports have good solvent resistance, but their applications were hindered by their high price and low solvent permeability. How to enhance the membrane permeability while maintain a high solute rejection is another challenge. Here we report the use of an inexpensive inorganic support made of large alpha-alumina particles. The surface was smoothed by multilayer coatings first with smaller alpha-alumina particles followed by carbon nanotubes. The hierarchical inorganic support is 20 times more permeable than conventional polymer supports. A standard interfacial polymerization process was then used to make a PA TFC membrane on the inorganic supports. As the surface roughness and average pore size decreased with multilayer coatings, the PA layer showed increased size of the “leaf-like” surface structure and thus increased surface area for transport due to the superhydrophilicity of the inorganic support and the ultrafine porous structure of the carbon nanotube layer. The enlarged surface area combined with the highly permeable inorganic support resulted a significant improvement in membrane flux. A high ethanol permeance of 10.6 ± 0.3 L m-2 h-1 bar-1 and a high rhodamine B rejection of 99.8% ± 0.2% were achieved. The membrane permeance is approximately 4–13 times higher than that of conventional PA TFC membranes fabricated on polymer supports. Our results indicate that the potential benefits of inorganic supports are not limited to their good solvent resistance, strong mechanical strength, and high thermal and chemical stabilities and thus desires more attention in future studies.

Particle-stabilized gel-core microcapsules: Synthesis and properties

We propose a new and versatile synthesis route for the formation of particle-stabilized gel-core microcapsules. With this route, microcapsules can be produced in a large size range of a few to hundreds of micrometres. Due to the high mechanical stability developed upon gelation, the capsules can be harvested as dry powders. Implementing in-situ hydrophobization to modify the wetting properties of particles allows preparing capsule shells from almost any kind of particulate materials. In the present work, we demonstrate the formation of microcapsules with shells from 60nm gamma alumina and 200nm alpha alumina particles. These particle shells are combined with sodium acrylate and poly(ethylene glycol) based gel-cores. Shape, gel-core microstructure, and swelling of the capsules in deionized water are characterized. Modelling the release of a low molecular weight dye from the capsules reveals Fickian kinetics for 16 and polymer relaxation dominated kinetics for 280μm sized capsules, respectively. Given the flexibility in materials selection, our processing route could lead to the design of new microcapsule systems for sensing and release applications.

Effect of time in plasma electrolytic oxidation process on titanium substrate with addition nano alpha alumina powder in electrolyte and investigate wear behavior of coating

Titanium and its alloys have wide applications in industry and recently researches have been expanded in this area. The main reasons are unique features of titanium such as biological behavior, corrosion resistance and high strength to weight ratio. The fundamental problem of titanium is low wear resistance that leads to be worn in industrial applications. Plasma electrolytic oxidation (PEO) is a novel method to create ceramic coatings on metals and improve wear resistance. Base of this method is similar to electrochemical coating methods with difference the high voltage discharge phenomenon. In this research hard nano alpha-alumina particles (hardness of 9 Mohs) were added to the electrolyte for improving wear properties of titanium.

Corrosion- and wear-resistant properties of Ni–Al2O3 composite coatings containing various forms of alumina

This article describes the effect of the addition of different phases of alumina particles on the properties of electrodeposited Ni–Al2O3 composite coatings. The corrosion- and wear-resistant properties of Ni–Al2O3 composite coatings electrodeposited from a nickel sulfamate bath containing (i) alpha-alumina particles (Ni–Al2O3-1), (ii) gamma-alumina particles (Ni–Al2O3-2), and (iii) mixture of alpha, gamma, and delta alumina particles (Ni–Al2O3-3) have been studied. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies showed superior corrosion resistance of Ni–Al2O3-2 composite coatings compared with other two coatings. The SEM images and EDAX spectra also corroborated well with the observed corrosion results. The pin-on-disk wear studies showed improved wear resistance of Ni–Al2O3-1 composite coating containing alpha alumina compared with other two coatings. The transfer of material from the pin onto the disk was evident from the optical microscopy images of the wear tracks and Raman spectra of the wear track. This study shows that the addition of pure gamma-alumina particles enhances the corrosion resistance, and that pure alpha-alumina particles enhance the wear resistance of Ni composite coatings to a greater extent.

Self-Assembly of Monolayer-Thick Alumina Particle−Epoxy Composite Films

Monolayer-thick composite films composed of alpha-alumina and Spurr's epoxy were prepared via a self-assembly process known as fluid forming. The process makes use of a high-spreading-tension fluid composed of volatile and nonvolatile components to propel particles across the air-water interface within a water bath. Continuous addition of the particle suspension builds a 2D particle film at the air-water interface. The spreading fluid compresses the film into a densely packed array against a submerged substrate. The assembled monolayer is deposited onto the substrate by removing the substrate from the bath. A dispersion containing a narrow size distribution, 10 microm alpha-alumina particles, light mineral oil, and 2-propanol was spread at the air-water interface and the alumina particles were assembled into densely packed arrays with an aerial packing fraction (APF) of 0.88. However, when mineral oil was replaced by Spurr's epoxy nonuniform films with low packing density resulted. It was found that replacing 2-propanol with a mixture of 2-propanol and 1-butanol with a volume ratio of 4:1 produced uniform, densely packed alumina/epoxy composite films. The role of the solvent mixture will be discussed.

Fluidizable reforming catalyst development for conditioning biomass-derived syngas

Abstract A multi-stage catalyst development approach is used to evaluate and optimize fluidizable tar reforming catalysts for conditioning syngas from biomass gasification. Previous work showed that catalyst fluidization is required to efficiently reform biomass-derived pyrolysis oil and its fractions to syngas as fluidization optimizes contact between catalyst particles and feedstocks while reducing coke formation. Biomass-derived tars like pyrolysis oils are also complex, largely aromatic feedstocks that require fluidization to improve reforming efficiency. Because industrial reforming catalysts are designed for fixed-bed operations and not for fluidized processing, attrition resistant supports and catalysts required development. We identified and tested particulate aluminas for attrition resistance under fluidized steam reforming conditions and prepared nickel-based catalysts from the strongest supports. The performance of these catalysts was tested in a microactivity test system (MATS), which used 1 g catalyst quantities, had high throughput, and measured oxidation, reduction and model tar compound steam reforming. Promising candidates from MATS screening were next evaluated in a laboratory-scale fluidized bed reactor with 250 g of catalyst using more realistic flow conditions and input streams. A 60 kg batch of the best catalyst identified in the laboratory fluid bed was then prepared and evaluated for biomass-derived syngas conditioning in a pilot-scale reformer. Fresh and used catalysts were characterized with scanning electron, energy dispersive X-ray, and inductively coupled plasma spectroscopies. The most active and attrition resistant catalyst identified through multi-stage testing contains nickel, magnesium and potassium on 90% alpha alumina particles of 100–400 μm size.

Studies on the onset velocity of turbulent fluidization for alpha-alumina particles

Transition from bubbling to turbulent fluidization regime for alpha-alumina particles was experimentally studied in the 0.15 m i.d. cold flow model. Experiments were carried out at 1 atm and ambient temperature. Both standard deviation and amplitude spectra analysis were used to determine the transition. Transition velocity of 0.8 m/s was obtained for the particles under investigation.

High-resolution electron microscope observation of interface microstructure of a cast Al-Mg-Si-Bi-Pb(6262)/Al2O3p composite.

High-resolution electron microscopy was employed to characterize the interface structure of a cast Al-Mg-Si-Bi-Pb aluminium(6262)-based composite reinforced by alpha alumina particles with a trace of beta alumina in order to investigate the behaviour of alloying elements in cast composites. Except for a few primary Mg2Si particles, few reaction products were detected at the interface of Al/alpha-Al2O3 due to the unfavourable reaction kinetics during the squeeze-casting process. The Mg2Si particle has an orientation relationship with alpha-Al2O3 of [011]Mg2Si//[1210]alpha-Al2O3 (111)Mg2Si//(0006)alpha-Al2O3. A significant amount of MgAl2O4 was found on the surface of the beta-Al2O3 particles, which is in contrast to the small degree of reaction found on alpha-Al2O3 particles. MgAl2O4 and beta-Al2O3 particles have the following orientation relationship: [011]MgAl2O4//[1210]beta-Al2O3 (111) MgAl2O4//(0006)beta-Al2O3. The similar crystal structure of beta-Al2O3 to MgAl2O4 favours MgAl2O4 nucleation and growth on the surface of beta-Al2O3. Interfacial energy minimization dominates the atomic structure of the interface with the result that close packed planes and directions in the Al2O3 reinforcement and reaction products are parallel to the interfaces. Bi and Pb were found in the form of metallic nanometre particles between Al2O3 particles, or between the MgAl2O4 and Al2O3 particles, or in the open channels of beta-Al2O3 filled by the Al matrix.

Effects of MA 956 superalloy and alpha-alumina particles on some markers of human osteoblastic cells in primary culture.

One of the problems associated with the modern biomaterials used in prostheses is osteolysis, which, although its exact origin is unknown, has been associated with wear particles. Osteoblasts seem to participate directly in this phenomenon. This paper investigates in vitro cellular response to the wear particles from the metal substrate and ceramic covering (alpha-alumina) of a new titanium yttrium aluminum alloy, MA 956, that has been proposed as a biomaterial because of its exceptional mechanical and electrochemical properties. The effect of different sizes (10 and 80 microm) of MA 956 and alpha-alumina particles on osteoblast function was studied in primary human bone cell cultures. Cells were harvested from trabecular bone fragments obtained during knee arthroplasty. Osteoblastic cell response to the particles was measured by assaying C-terminal type I procollagen (PICP), alkaline phosphatase, and osteocalcin secretion, with and without 1.25(OH)(2)D(3) stimulation, in the cell-conditioned medium. Both sizes of MA 956 and alpha-alumina particles decreased PICP secretion in nonstimulated osteoblastic cells, but this secretion was not affected in the cultures stimulated with 1.25(OH)(2)D(3). Only the 10 microm alpha-alumina particles inhibited alkaline phosphatase activity in 1.25(OH)(2)D(3)-stimulated and nonstimulated cultures. The rise in osteocalcin levels after 1.25(OH)(2)D(3) stimulation was lower in the presence of the 10 microm MA 956 particles than in the presence of alpha-alumina particles. Although both materials seem to have directly affected in vitro osteoblastic cell function, the increase in osteocalcin levels after 1.25(OH)(2)D(3) stimulation was lower after exposure to MA 956 particles than the increase observed after exposure to alpha-alumina particles. Therefore, it does not seem that osteocalcin stimulated bone resorption, suggesting that MA 956 would be less likely to provoke osteolysis.

Growth characteristics of boehmite-derived ultrafine theta and alpha-alumina particles during phase transformation

Growth characteristics of the crystallite and the agglomerate during θ→α-A1 2O 3 phase transformation of ultrafine (nano-sized) alumina powders were investigated, using X-ray powder diffraction analysis, BET techniques, differential thermal analysis (DTA), and transmission electron microscope observation of the quenched samples. The DTA profile of phase transformation is composed of two portions, nucleation and crystallite growth, which correspond directly to the formation of α-A1 2O 3. The area of each portion and the nucleation temperature were correlated to the measured size of θ-A1 2O 3 crystallite (Scherrer formula). The maximum crystallite size in the powder system of θ-A1 2O 3 before transformation is approximately 20 nm. The nucleation can be achieved with the transformation of one θ-A1 2O 3 crystallite to one α-A1 2O 3 crystallite, at about 17 nm. These α-A1 2O 3 crystallites then grew drastically to a size of 45–55 nm. This can be considered as the primary crystallite size, from which point the agglomerates or polycrystals α-A1 2O 3 are formed with sizes ranging from several tens to larger than 200 nm (BET technique). It is noted that the growth of the ultrafine α-A1 2O 3 crystallites is possibly carried out through the coalescence of the newly formed α-A1 2O 3 nuclei.

Improved structural properties of SiO2 sol-gel films

Silicon oxide films with a small amount of alpha alumina particles embedded in the glass matrix were prepared using the sol-gel method. It is found that the structural properties of these samples are superior to those observed in similarly prepared films, but without the alumina particles. This conclusion was deduced from analysis of the oxygen diffusion measurements through the SiO2 layer, using a novel method, and from the measured dielectric breakdown field. It is found that the oxygen diffusion coefficient is much lower and the dielectric breakdown field is larger in samples with the alumina particles.

Constrained densification in boehmite-alumina mixtures for the fabrication of porous alumina ceramics

It is demonstrated that inherent constrained densification characteristics of the bimodal size distributed powders can be utilized to fabricate porous alpha alumina. Seeded boehmite was mixed with coarse alpha alumina particles to produce various compositions of bimodal mixtures. The densification behaviour of the mixtures was correlated with shrinkage and microstructural evolution. The mixtures sintered at 1200 °C contained more than 30% porosity and the mechanical strength of the mixtures was increased by 2–4 fold relative to the coarse particles alone with similar porosity (<5 MPa). © 1998 Chapman & Hall

Measurement of Interaction Forces between Silica and α-Alumina by Atomic Force Microscopy

Interaction forces between a silica sphere and an α-alumina substrate at various pH values were measured by atomic force microscopy (AFM). As expected, at pH values of 10.8 and 10.2 when the surfaces are similarly charged, a repulsive force was observed. On the other hand, at pH values of 5.5, 6.4, and 8.6 when the surfaces are oppositely charged, attractive forces were observed. Experimental force vs separation distance curves were found to be in good agreement with theoretical predictions based on electrostatic and van der Waals interactions. Interestingly, when the force/radius values at a particular separation distance were plotted against pH, the transition from an attractive to a repulsive force occurred at pH 9.3, which is very close to the point of zero charge (pzc) of α-alumina as determined from electrophoresis experiments. These results suggest that AFM force measurements can be used to estimate the pzc of materials. This method may be of particular significance for soluble salt minerals where conventional electrophoretic measurements are not possible at high ionic strengths. Finally, results from transmittance studies further confirmed the interaction between silica and alpha alumina particles in suspensions at various pH values as would be expected based on the results from atomic force microscopy measurements.

Morphological Forms of α‐Alumina Particles Synthesized in 1,4‐Butanediol Solution

Phase‐pure, monodispersed, hexagonal plates of single‐crystal α‐alumina (∼2 μm wide and ∼0.5 μm thick) have been prepared via precipitation by treating an aluminum hydrous oxide precursor in 1,4‐butanediol at 300°C under autogenous vapor pressure. Present work shows that KOH is the only reagent that precipitates an aluminum hydrous oxide precursor suitable to synthesize α‐alumina in 1,4‐butanediol solution. In contrast, the use of NaOH or NH4OH as the precipitating reagent for the precursor material does not yield the alpha phase. The solution pH at which the precursor materials are precipitated is also a critical factor for the formation of α‐Al2O3. Phase‐pure α‐alumina powders were also only synthesized from the aluminum hydrous oxide precursors precipitated in the pH range from 10 to 10.5. The results of X‐ray diffraction and scanning electron microscopy indicate that longer reaction times promote the phase transformation from the intermediate boehmite phase to α‐alumina. The complete transformation from boehmite to α‐alumina requires reaction times of about 12 h.

Effect of Hydroxyapatite Microcrystals on Macrophage Activity

Hydroxyapatite (HAp) microcrystals were synthesized by a neutralization reaction of Ca(OH)2 suspension and H3PO4 solution using an ultrasonic homogenizer. The in vitro interaction of HAp microcrystals with rat peritoneal macrophages was investigated by measuring the viability, acid phosphatase (ACP) activity, lactate dehydrogenase (LDH) activity and intracellular calcium content. HAp calcined at 800 degrees C and alpha-alumina particles (alumina) were used as comparative materials. Macrophages actively phagocytosed HAp microcrystals by dissolving them. However, no damage in macrophages exposed to HAp microcrystals was observed by transmission electron microscopy. Macrophages in the presence of HAp microcrystals showed less ACP and LDH activity and higher intracellular calcium content than those in the presence of calcined HAp and alumina. HAp microcrystals had excellent biocompatibility to macrophages as well as sintered HAp.

Forming of Alumina Insulation Layers by Double Step Electrophoretic Deposition

Electrophoretic coating was performed to form alumina insulation layers for the tungsten double helical filaments of a CRT heater. Alpha alumina particles an average diameter of about 4μm were dispersed in ethanol-water suspension containing with dehydrated Al(NO3)3 or dehydrated Al(NO3)3+Mg(NO3)2⋅6H2O. Positively charged alumina particles in the suspension (43wt% solid) were deposited on tungsten double helical filaments by passing a direct current at 100V. Thickness of the alumina layer formed by dehydrated Al(NO3)3 suspension did not reach 30μm even for a long time current flow, and alumina particles were tightly packed within the spaces between the coils of the double helical filaments. On the other hand, that formed by the suspension with dehydrated Al(NO3)3+Mg(NO3)2⋅6H2O increased with deposition time, but there was insufficient packing of alumina particles between the coils. A new electrophoretic deposition process for uniform packing of Al23O3 particles was proposed. The inner layer with closely packed Al2O3 particles was formed first by deposition from the dehydrated Al(NO3)3 suspension, and the outer layer was deposited successively from the suspension of mixed electrolytes of dehydrated Al(NO3)3 and Mg(NO3)2⋅6H2O. The new deposition process (double step method) enables to pack Al2O3 particles closely, and the area ratio occupied by Al2O3 particles becomes higher than 50%.