Alumina Particles (alumina + particle)

Distribution by Scientific Domains


Selected Abstracts


Development, Analysis, and Application of a Glass,Alumina-Based Self-Constrained Sintering Low-Temperature Cofired Ceramic

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 5 2007
Takahiro Takada
The effects of an inner constraint layer and alumina particles on the microstructure, strength, and shrinkage of the laminated low-temperature cofired ceramic (LTCC) green sheet were investigated. Alumina particles of several sizes were used in the inner-constraint layer in order to strengthen the LTCC substrate. Smaller alumina particles in the inner-constraint layer produced a substrate with a high bending strength. Sintering shrinkage in the x,y direction of the LTCC is related to the bending strength of the debinded alumina particle layer used for an inner-constraint layer. A larger pore size in the inner-constraint layer was found to increase the distance of the glass penetration from the glass,alumina layer into the inner-constraint layer. The total thickness of the constraint layer changes the shrinkage in the x,y direction and the bending strength. [source]


Voltammetric Sizing of Inert Particles

CHEMPHYSCHEM, Issue 7 2005
Trevor J. Davies
Abstract The average size of inert particles is determined using a simple electrochemical procedure. Alumina particles are deposited on an edge-plane graphite electrode, and a cyclic voltammogram is recorded. The scan rate employed varies between 0.2 and 2 V,s,1. At these scan rates the diffusion layer thickness is greater than the size of the alumina particles, minimizing the influence of the particles, height on the observed voltammetry. The average size of the particles is determined via comparison of the experimental voltammograms with simulations. [source]


Development, Analysis, and Application of a Glass,Alumina-Based Self-Constrained Sintering Low-Temperature Cofired Ceramic

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 5 2007
Takahiro Takada
The effects of an inner constraint layer and alumina particles on the microstructure, strength, and shrinkage of the laminated low-temperature cofired ceramic (LTCC) green sheet were investigated. Alumina particles of several sizes were used in the inner-constraint layer in order to strengthen the LTCC substrate. Smaller alumina particles in the inner-constraint layer produced a substrate with a high bending strength. Sintering shrinkage in the x,y direction of the LTCC is related to the bending strength of the debinded alumina particle layer used for an inner-constraint layer. A larger pore size in the inner-constraint layer was found to increase the distance of the glass penetration from the glass,alumina layer into the inner-constraint layer. The total thickness of the constraint layer changes the shrinkage in the x,y direction and the bending strength. [source]


Kinetic Studies of Mullite Synthesis from Alumina Nanoparticles and a Preceramic Polymer

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2008
Flavio Griggio
The crystallization kinetics of mullite formation in a diphasic precursor consisting of a silicone resin filled with commercial ,-alumina nanoparticles (15 nm mean particle size, specific surface area of 100 m2/g), heated in air from 1250 to 1350C, was studied by X-ray diffraction. Transitional ,-alumina and amorphous silica from the pyrolysis of the preceramic polymer exhibited a remarkable reactivity, as demonstrated by a very low incubation time (from 500 s at 1250C to 20 s at 1350C), a high mullite yield (about 80 vol%, after 100 s at 1350C), and a low activation energy for nucleation (67760 kJ/mol). The activation energy values found were lower than those reported previously for other diphasic systems, including sol,gel precursors. Besides the high specific surface of nanosized ,-alumina particles, the low energy barrier could be attributed to the highly reactive silica deriving from the oxidation of Si,CH3 bonds in the silicone and to the homogeneous dispersion of the nanosized filler inside the preceramic polymer. Furthermore, the possibility of applying plastic shaping processing methods to the mixture of a preceramic polymer and nanosized filler makes this approach particularly valuable, in comparison, for instance, with sol,gel based alternatives. [source]


Influence of Impurities on Dispersion Properties of Bayer Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004
J. Kiennemann
Among the different impurities contained in Bayer alumina (Ca, Fe, Na, Si, Mg), calcium was found to greatly influence the dispersion of alumina particles in water. Up to 90% of calcium is dissolved at acidic pH, whereas calcium remains on the alumina surface in the basic pH range and screens the negative Al,O, charges. The presence of calcium, through reducing repulsive interactions between particles, has a negative effect on the dispersion of alumina. The adsorption of the Na+ salt of poly(acrylic acid) (PAA-Na) is strongly influenced by Ca2+/PAA-Na interactions in suspension with an increase of the maximum adsorbed amount of PAA-Na in the presence of calcium. The amount of PAA-Na needed to reach a high electrostatic repulsion and a minimum of viscosity is 2 times higher in the presence of 400 pm calcium than for a low calcium content (,80 ppm). Finally, with an appropriate amount of PAA-Na, a similar state of dispersion can be reached with or without the presence of calcium. [source]


Chemically Bonded Phosphate Ceramics: II, Warm-Temperature Process for Alumina Ceramics

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2003
Arun S. Wagh
This is the second of three papers on a dissolution model that describes the formation of chemically bonded phosphate ceramics. In this paper, we discuss the kinetics of formation of aluminum phosphate ceramics between 100 and 150C. Using basic thermodynamic formulations, we calculated the temperatures of maximum solubility of alumina and its hydrated phases and predicted the temperatures of formation of ceramics. Differential thermal and X-ray diffraction analyses on samples made in the laboratory confirm these temperatures. The resulting ceramics of alumina bonded with aluminum phosphate (berlinite) show a high compressive strength of 16 000 psi. We have concluded that rapid evaporation of excess water in the slurry generates porosity in the ceramics, and that better processing methods are needed. A consolidation model is presented that describes the microstructure of the ceramic. It predicts that a very small amount of alumina must be converted to form the bonding phase; hence, the product is mostly alumina with a thin coating of berlinite on the surface of alumina particles. [source]


Investigation on the oxidation behaviour of gamma titanium aluminides coated with thermal barrier coatings

MATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 7 2008
R. Braun
Abstract In the present study, the applicability of thermal barrier coatings (TBCs) on ,-TiAl alloys was investigated. Two alloys with the chemical compositions of Ti-45Al-8Nb-0.2B-0.15C and Ti-45Al-1Cr-6Nb-0.4W-0.2B-0.5C-0.2Si were used. Before TBC deposition, the specimens were pre-oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron-beam physical vapour deposition (EB-PVD). The oxidation behaviour of the ,-TiAl specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900,C. Post-oxidation analysis of the coating systems was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). No spallation of the TBC was observed for pre-oxidised specimens of both alloys when exposed to air at 850,C for 1100 cycles of 1,h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO2/Al2O3 protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900,C, the oxide scales on the alloy Ti-45Al-8Nb-0.2B-0.15C pre-oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900,C. [source]


Photocatalytic Coatings for Environmental Applications,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2005
Norman S. Allen
ABSTRACT A series of nano- and micronparticle-grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photocatalytic activities have been determined in a series of paint films by FTIR, chalking, color, gloss change and weight loss after artificial weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of chargecarrier dynamics in the pigments. Pigment particle size, surface area and properties clearly play an important role in dispersion and any polymer-pigment interactions. Photooxidation studies on several types of paint films show a clear demarcation between nanoparticle- and pigmentary-grade titanium dioxide, with the former being more active because of their greater degree of catalytic surface activity. The photosensitivity of titanium dioxide is considered to arise from localized sites on the crystal surface (i.e. acidic OH), and occupation of these sites by surface treatments inhibits photoreduction of the pigment by ultraviolet radiation; hence, the destructive oxidation of the binder is inhibited. Coatings containing 2,5% by weight alumina or alumina and silica are satisfactory for generalpurpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7,10% weight. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminum phosphates of other metals. For example, the presence of hydrous alumina particles lowers van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminum oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilization in various water- and oilbased paint media in comparison with conventional organic stabilizers. Hindered piperidine stabilizers are shown to provide no additional benefits in this regard, often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated in this study. In the former case, surface erosion is shown to be controlled by varying the ratio of admixture of durable pigmentary-grade rutile (heavily coated) and a catalytic-grade anatase nanoparticle. For environmental applications in the development of coatings for destroying atmospheric pollutants such as nitrogen oxide gases (NOX), stable substrates are developed with photocatalytic nanoparticle-grade anatase. In this study, porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NOx gases. For the development of microbiological substrates for the destruction of harmful bacteria, effective nanoparticle anatase titania is shown to be important, with hydrated high surface area particles giving the greatest activity. [source]


Mechanical properties of Al2O3/polymethylmethacrylate nanocomposites

POLYMER COMPOSITES, Issue 6 2002
Benjamin J. Ash
Alumina/polymethylmethacrylate (PMMA) nanocomposites were produced by incorporating alumina nanoparticles, synthesized using the forced gas condensation method, into methylmethacrylate. The particles were dispersed using sonication and the composites were polymerized using free radical polymerization. At an optimum weight percent, the resulting nanocomposites showed, on average, a 600% increase in the strain-to-failure and the appearance of a well-defined yield point when tested in uniaxial tension. Concurrently, the glass transition temperature (Tg) of the nanocomposites dropped by as much as 25C, while the ultimate strength and the Young's modulus decreased by 20% and 15%, respectively. For comparison, composites containing micron size alumina particles were synthesized and displayed neither phenomenon. Solid-state deuterium NMR results showed enhanced chain mobility at room temperature in the nanocomposites and corroborate the observed Tg depression indicating considerable main chain motion at temperatures well below those observed in the neat polymer. A hypothesis is presented to relate the thermal and mechanical behavior observed in the composites to the higher chain mobility and Tg depression seen in recent ultrathin polymer film research. [source]


Production of Highly Loaded Nanocomposites by Dispersing Nanoparticles in Epoxy Resin

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 9 2010
H. Nolte
Abstract The objective of this study was the investigation of techniques for dispersing alumina nanoparticles with different surface modifications in epoxy resin. In order to prepare the matrix suspension, high contents of fillers (up to 50,wt,%) were dispersed by conducting shear mixing techniques in a high performance laboratory kneader. The intention was to attain solutions that were stable against re-agglomeration, while the mass fraction and the product fineness were maintained as high as possible. Therefore, both the formulations and the dispersion parameters were varied systematically. An epoxy resin was used as a carrier fluid and a corresponding amine hardener system was chosen. Tests were performed using alumina particles and surface modified alumina particles at different particle concentrations. Furthermore, the effect of diluting the colloidal suspensions and the resulting long term stability were also examined. The matrix suspension was examined with respect to viscosity, stability and particle size distribution. [source]


In-Situ Preparation of Polymer-Coated Alumina Nanopowders by Chemical Vapor Synthesis

CHEMICAL VAPOR DEPOSITION, Issue 1 2003
M. Schallehn
Abstract Nanocrystalline alumina particles coated with polyethylene have been prepared by a two-step chemical vapor synthesis (CVS) process using a hot-wall reactor to synthesize the nanocrystalline alumina core, and a RF plasma reactor for the subsequent polymer coating. The particle radius is about 4,nm, with the radius of the ceramic core being about 2.5,nm and the coating thickness about 1.5,nm. The powders have been characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer,Emmett,Teller (BET), small-angle neutron scattering (SANS), and high-resolution transmission electron microscopy (HRTEM). [source]


Voltammetric Sizing of Inert Particles

CHEMPHYSCHEM, Issue 7 2005
Trevor J. Davies
Abstract The average size of inert particles is determined using a simple electrochemical procedure. Alumina particles are deposited on an edge-plane graphite electrode, and a cyclic voltammogram is recorded. The scan rate employed varies between 0.2 and 2 V,s,1. At these scan rates the diffusion layer thickness is greater than the size of the alumina particles, minimizing the influence of the particles, height on the observed voltammetry. The average size of the particles is determined via comparison of the experimental voltammograms with simulations. [source]