ADVANCED ENGINEERING MATERIALS, Issue 12 2009
Sandip P. Harimkar
Abstract The study reports on the formation of regular multi-faceted surface grains during laser surface modification of alumina ceramics. The formation of such faceted morphology of the grains can be strongly correlated with the formation of crystallographic texture. Such evolution of crystallographically-textured, multi-faceted grains opens up new avenues for laser surface modification, where regular three dimensional surface features are formed through solidification processing. [source]

Strength recovery of machined Al2O3/SiC composite ceramics by crack healing

FATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 12 2007
M. ONO
ABSTRACT Alumina is used in various fields as a machine component. However, it has a low fracture toughness, which is a weakness. Thus, countless cracks may be initiated randomly by machining, and these cracks decrease the component's mechanical properties and reliability. To overcome this problem, a crack-healing ability could be a very useful technology. In this study, Al2O3/SiC composite was sintered. This alumina exhibits excellent crack-healing ability. Small specimens for a bending test were made from the Al2O3/SiC. A semicircular groove was machined using a diamond ball-drill. The machining reduced the local fracture stress from approximately 820,300 MPa. The machined specimens were crack-healed under various conditions. The fracture stress of these specimens after crack healing was evaluated systematically from room temperature (RT) to 1573 K. It was found that the local fracture stress of the machined specimen recovered almost completely after crack healing. Therefore, it was concluded that crack healing could be an effective method for improving the structural integrity of machined alumina and reducing machining costs. [source]

Controlled Fabrication of Multitiered Three-Dimensional Nanostructures in Porous Alumina,

ADVANCED FUNCTIONAL MATERIALS, Issue 14 2008
Audrey Yoke Yee Ho
Abstract We present the fabrication of multitiered branched porous anodic alumina (PAA) substrates consisting of an array of pores branching into smaller pores in succeeding tiers. The tiered three-dimensional structure is realized by sequentially stepping down the anodization potential while etching of the barrier layer is performed after each step. We establish the key processing parameters that define the tiered porous structure through systematically designed experiments. The characterization of the branched PAA structures reveals that, owing to constriction, the ratio of interpore distance to the anodization potential is smaller than that for pristine films. This ratio varies from 1.8 to 1.3,nm,V,1 depending on the size of the preceding pores and the succeeding tier anodization potential. Contact angle measurements show that the multitiered branched PAA structures exhibit a marked increased in hydrophilicity over two-dimensional PAA films. [source]

Nanoparticle Coating for Advanced Optical, Mechanical and Rheological Properties,

ADVANCED FUNCTIONAL MATERIALS, Issue 16 2007
F. Hakim
Abstract Primary titania nanoparticles were coated with ultrathin alumina films using Atomic Layer Deposition (ALD). The deposited films were highly uniform and conformal with an average growth rate of 0.2,nm per coating cycle. The alumina films eliminated the surface photocatalytic activity of titania nanoparticles, while maintained their original extinction efficiency of ultraviolet light. Deposited films provided a physical barrier that effectively prevented the titania surface from oxidizing organic material whereas conserving its bulk optical properties. Parts fabricated from coated powders by pressureless sintering had a 13,% increase in surface hardness over parts similarly fabricated from uncoated particles. Owing to its homogeneous distribution, the secondary alumina phase suppressed excessive grain growth. Alumina films completely reacted during sintering to form aluminum titanate composites, as verified by XRD. Coated particles showed a pseudoplastic behavior at low shear rates due to modified colloidal forces. This behavior became similar to the Newtonian flow of uncoated nanoparticle slurries as the shear rate increased. Suspensions of coated particles also showed a decreased viscosity relative to the viscosity of uncoated particle suspensions. [source]

Extremely High Silver Ionic Conductivity in Composites of Silver Halide (AgBr, AgI) and Mesoporous Alumina,

ADVANCED FUNCTIONAL MATERIALS, Issue 4 2006
H. Yamada
Abstract The silver ionic conductivity in heterogeneous systems of AgBr:Al2O3 and AgI:Al2O3 is highly enhanced by utilizing mesoporous Al2O3 as the insulating phase. The highest Ag+ conductivity of 3.1,×,10,3,,,1,cm,1 (at 25,°C) has been obtained for the AgI:Al2O3 composite with an Al2O3 volume fraction of 0.3. For AgBr:Al2O3, the enhancement of the conductivity is satisfactorily explained in the framework of the ideal space-charge model, while in the case of AgI:Al2O3 stacking disorder is also considered to contribute to the ionic conductivity. [source]

Surfactant-Assisted Synthesis of Alumina with Hierarchical Nanopores

ADVANCED FUNCTIONAL MATERIALS, Issue 1 2003
W. Deng
Abstract Using surfactant-assisted synthesis, aluminas with hierarchical nanopores are produced. The hierarchical structures are composed of mesopores of 4 nm diameter, and macropores with diameters of about 300 nm. The structures were found to be stable to the thermal removal of the surfactant. Synthesis factors affecting the appearance of the hierarchically structured alumina material are presented. A potential mechanism for the formation of the uniquely structured aluminas is proposed. [source]

Highly Ordered Nanoporous Alumina on Conducting Substrates with Adhesion Enhanced by Surface Modification: Universal Templates for Ultrahigh-Density Arrays of Nanorods

ADVANCED MATERIALS, Issue 18 2010
Jinseok Byun
A highly ordered nanoporous anodized aluminum oxide template with excellent adhesion is fabricated on various conducting substrates by surface modification of the substrates. This template can be universally utilized to fabricate laterally long-range-ordered and hexagonally packed arrays of freestanding and vertically aligned metal, semiconductor, and conducting polymer nanorods on various substrates, including flexible substrates (see image). [source]

Homogeneously Dispersed Ceria Nanocatalyst Stabilized with Ordered Mesoporous Alumina

ADVANCED MATERIALS, Issue 13 2010
Quan Yuan
Homogeneously dispersed ceria nanocatalyst can be synthesized and stabilized with ordered mesoporous alumina by a one-step sol,gel method. This novel material possesses high thermal stability and exhibits 100% conversion at room temperature for CO oxidation when loaded with Au. This established approach can be expanded to the controlled synthesis and stabilization of other nanocatalysts with wide applications. [source]

A Screening Design Approach for the Understanding of Spark Plasma Sintering Parameters: A Case of Translucent Polycrystalline Undoped Alumina

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 5 2010
Yann Aman
An experimental screening design was used to evaluate the effects of spark plasma sintering (SPS) parameters such as heating rate, sintering temperature, dwell duration, and green-shaping processing on the relative density, grain size, and the optical properties of polycrystalline alumina (PCA). It is shown that heating rate and sintering temperature are the most critical factors for the densification of PCA during SPS. Green-shaping processing could prevent grain growth at low SPS sintering temperatures. No predominant SPS parameters are observed on the optical properties. Hence, the optical properties of PCA are controlled by microstructural evolution during the SPS process. [source]

Cell Proliferation of Human Fibroblasts on Alumina and Hydroxyapatite-Based Ceramics with Different Surface Treatments,

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2010
Juliana Marchi
Biocompatibility is an important characteristic of dental implant material, and in vitro tests are required to elucidate the interaction between these materials and human tissues. Cell proliferation assays were done with fibroblasts plated on the surface of alumina and hydroxyapatite sintered samples, each with a different surface treatment (sintered, rectified, or polished). After 1, 2, and three days, the samples were prepared for scanning electron microscopy observations. The data were compared by analysis of variance followed by Tukey's test. It was concluded that neither the hydroxyapatite or alumina substrate is cytotoxic, and hydroxyapatite is more biocompatible than alumina. [source]

THIS ARTICLE HAS BEEN RETRACTED Effect of Silica Sol on the Properties of Alumina-Based Duplex Ceramic Cores

INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 1 2008
Yexia Qin
A series of alumina-based ceramic cores sintered at 1300°C, 1400°C, and 1500°C for 5 h were prepared, and the phases and microstructures were characterized by X-ray diffraction and scanning electron microscopy. The effect of colloidal silica sols on the properties of ceramic core was discussed. The properties of these materials were determined. The results indicated that the microstructure of the core is characterized by the presence of substantially unreacted Al2O3 particles having a polycrystalline composition consisting essentially of in situ synthesized 3Al2O3·2SiO2 on the surface of the Al2O3 particles. The colloidal silica sol contents do not have an appreciable effect on the densification and shrinkage of the alumina ceramic core. The ceramic cores of 5 wt% colloidal silica sol contents sintered at 1500°C for 5 h showed the smallest creep deformation in the present research. [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]

Selective Barrier Perforation in Porous Alumina Anodized on Substrates,

ADVANCED MATERIALS, Issue 7 2008
Jihun Oh
A new method for perforating the barrier oxide at the base of pores in alumina, which does not involve etching of the alumina, is reported. Anodization of Al layers on W leads to formation of WO3 "plugs" that can be selectively etched without widening the as-anodized pores. We demonstrate this technique, used with templated pore formation, by creating Ni nanoelectrode arrays with fixed electrode spacings (200,nm) but varied electrode diameters. [source]

Fabrication of Metal Nanohole Arrays with High Aspect Ratios Using Two-Step Replication of Anodic Porous Alumina

ADVANCED MATERIALS, Issue 18 2005
T. Yanagishita
Metal nanohole arrays with straight holes of high aspect ratio have been fabricated using a new replication process based on the preparation of polymer pillars, both sides of which are supported and can therefore be maintained upright. These nanopillar arrays are demonstrated as negatives for the preparation of metal (Ni) nanohole arrays (see Figure). [source]

Pore network model for catalytic dehydration of methanol at particle level

AICHE JOURNAL, Issue 2 2009
Hossein Beigi
Abstract ,-Alumina is used as a catalyst for converting methanol to dimethyl ether. The process takes place in a packed or fluidized bed reactor consisting of microporous particles with distributed pore sizes and interconnectivities. The efficiency of the process is, however, significantly affected by the pore space structure of the particles. All the previous attempts for modeling this phenomenon have used continuum formulation of the problem based on classical equations of mass transport and reaction, without any regards for the effect of pore space morphology. In this article, we study the catalyst's performance by developing a network model for the pore space, with distributed pore sizes and interconnectivities. The network model is used to study the effect of several parameters such as pore space morphology, concentration, and temperature on catalyst's effectiveness factor. The results will be used for reactor simulations. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source]

Fracture strength of copy-milled and conventional In-Ceram crowns

JOURNAL OF ORAL REHABILITATION, Issue 7 2001
Jung-Won Hwang
The purpose of this study was to compare the fracture resistance of copy-milled and conventional In-Ceram crowns. Four groups of 10 uniform sized all-ceramic anterior crowns were fabricated for this test: (1) In-Ceram Spinell (2) In-Ceram Alumina (3) Celay In-Ceram Spinell, and (4) Celay In-Ceram Alumina crowns. All specimens were cemented on stainless steel master die with resin cement and stored in 37 °C water for one day prior to loading into a universal testing machine. Using a steel ball at a crosshead speed of 0·5 mm min,1, the crowns were loaded at 30 °C angle until catastrophic failure occurred. Mean fracture strength was analysed and compared. Under the conditions of this study and the materials used, the following conclusions were drawn: 1. The strength of Celay In-Ceram anterior crowns had a slightly higher fracture strength than conventional In-Ceram crowns. 2. In-Ceram Alumina crowns had a significantly higher fracture strength than In-Ceram Spinell crowns in both conventional and copy milling methods. [source]

Effect of Applied Stress on IR transmission of Spark Plasma-Sintered Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2010
Dibyendu Chakravarty
The effect of applied stress on IR transmittance of nanocrystalline alumina prepared by spark plasma sintering was evaluated. Transparent alumina with maximum transmittance >80% was obtained over the entire mid-IR wavelength range of 3,5 ,m by applying a high stress of 275 MPa at 1150°C using specially designed high-strength compound dies. The transmittance observed was similar to previous reports at identical wavelengths, but at a lower sintering temperature. The transparent samples have an average grain size of 0.3 ,m and a hardness of 23 GPa. At lower stresses and sintering temperatures, transmittance reduced drastically due to remnant pores in the matrix as observed from the microstructural analysis. The effect of porosity was found to be critical in developing transparency as even a marginal decrease in porosity led to substantial increase in transmittance. [source]

Hydrothermal Synthesis of Alpha Alumina (,-Al2O3) Powders: Study of the Processing Variables and Growth Mechanisms

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2010
Wojciech L. Suchanek
Alpha alumina (,-Al2O3) powders and ,-Al2O3/boehmite (,-AlOOH) mixtures with controlled ,-AlOOH contents were synthesized hydrothermally under alkaline or acidic conditions at 380°,435°C for 1,10 days, under 6.9,14.5 MPa pressure, from concentrated precursors without stirring. The precursors were formed by mixing different types of aluminum hydroxides with water, and optionally with ,-Al2O3 seeds, hydrogen peroxide, sulfuric acid, dopants (i.e., KMnO4), and/or other additives. The experiments were performed on industrial scale in large production autoclaves. The synthesized ,-Al2O3 powders exhibited up to 100% phase purity, 99.98% chemical purity, equiaxed morphology, low aggregation levels, narrow crystallite size distributions with primary particle sizes ranging between 100 nm and 40 ,m, and high reproducibility. Precursor types, seeds, chemical additives, and temperature/time of the hydrothermal synthesis were found to govern properties of the powders. Different growth mechanisms for nanosized and rough powders are discussed. Results of this study enable the use of hydrothermal ,-Al2O3 powders in a multitude of applications, and make their hydrothermal production a commercial reality. [source]

Biomimetic Preparation and Characterization of Bioactive Coatings on Alumina and Zirconia Ceramics

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2010
Irena Pribo
For the preparation of bioactive coatings on alumina and zirconia ceramic surfaces a fast biomimetic method using a supersaturated solution containing Na+, Ca2+, Cl,, HCO3,, and PO43, ions was used. The coatings were analysed with the use of an X-ray diffraction spectrometer and a transmission electron microscope equipped with an energy-dispersive spectroscopy detector. After the precipitation both coatings were composed of poorly crystallized, nanosized, plate-like particles with the octacalcium phosphate (OCP) crystal structure. The adhesion of the coatings was improved by a heat treatment at 1050°C for 1 h. During this heat treatment the calcium phosphate layer, deposited from a supersaturated solution onto the surface of the substrates, was sintered to form a dense coating. At the same time the OCP crystal structure was transformed into that of hydroxyl apatite, the coating's crystallinity was increased, and the particles grew isotropically up to 300 nm in size. The bioactivity of the coated ceramic was confirmed before and after the heat treatment using a simple simulated body fluid test. [source]

Mechanisms and Mechanics Governing the Indentation of Polycrystalline Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2008
Zhensong Wei
A constitutive model for the inelastic deformation of polycrystalline alumina that accounts for both microcrack growth and plastic slip is implemented into a commercial finite element code. To establish its applicability, the code is used to simulate the deformation that occurs upon spherical and conical indentation. Inelastic zones and indentation pressures are predicted over a range of grain size and compared with measurements. The model replicates the mechanism transition from plasticity control at small grain sizes to micro-crack-control at large grain sizes. It also predicts an extensive micro-crack-dominated inelastic zone at large grain size that reduces the indentation pressures. [source]

Dense Alumina,Zirconia Coatings Using the Solution Precursor Plasma Spray Process

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2008
Dianying Chen
For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al2O3,40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of ,-Al2O3 and tetragonal ZrO2 phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1,5 ,m) and unmelted spherical particles (<0.5 ,m). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al2O3,40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°,1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity. [source]

Neutron Diffraction of Zirconia-Dispersed Alumina with Increasing Stress and Temperature

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2008
Geoffrey A. Carter
Neutron and X-ray diffraction were used to investigate changes in the phase composition and lattice parameters with temperature and pressure of a 20-wt% zirconia-dispersed alumina composite. Neutron diffraction was used to determine the variation of tetragonal/monoclinic zirconia phase abundance for the bulk with stress. The bulk tetragonal/monoclinic ratio decreased by 40% for a stress of 550 MPa. A difference in the tetragonal/monoclinic ratio between the near-surface and the bulk of the processed samples is believed to contribute to observed variations in nonuniform strain in the matrix, which has the effect of prestressing the sample. Placing a stress on the sample initially releases this prestressing. [source]

Synthesis, Densification, and Phase Evolution Studies of Al2O3,Al2TiO5,TiO2 Nanocomposites and Measurement of Their Electrical Properties

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2007
Vikas Somani
Alumina,aluminum titanate,titania (Al2O3,Al2TiO5,TiO2) nanocomposites were synthesized using alkoxide precursor solutions. Thermal analysis provided information on phase evolution from the as-synthesized gel with an increase in temperature. Calcination at 700°C led to the formation of an Al2O3,TiO2 nanocomposite, while at a higher temperature (1300°C) an Al2O3,Al2TiO5,TiO2 nanocomposite was formed. The nanocomposites were uniaxially compacted and sintered in a pressureless environment in air to study the densification behavior, grain growth, and phase evolution. The effects of nanosize particles on the crystal structure and densification of the nanocomposite have been discussed. The sintered nanocomposite structures were also characterized for dielectric properties. [source]

Microwave Dielectric Properties of Sintered Alumina Using Nano-Scaled Powders of , Alumina and TiO2

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2007
Cheng-Liang Huang
The microstructure and the microwave dielectric properties of nano-scaled , alumina (,-Al2O3) ceramics with various added amounts of nano-scaled TiO2 have been investigated. The sintering temperature of nano-scaled , alumina can be effectively lowered by increasing the TiO2 content. The Q×f values of nano-scaled , alumina could be tremendously boosted by adding an appropriate amount of TiO2. However, introducing excessive TiO2 into the alumina ceramics would instead lead to a decrease in the Q×f values. The phases of TiO2 and Al2TiO5 co-existed at 1350°C, and the maximum Q×f value appeared right after the eradication of TiO2 phase at 1400°C. Consequently, increasing the TiO2 content to 0.5 wt% yielded a Q×f value of 680 000 GHz (measured at 14 GHz) for nano-scaled , alumina prepared at 1400°C for duration of 4 h. In addition, a very low loss tangent (tan ,) of 2 × 10,5 was also obtained at 14 GHz. The ,f value is strongly correlated to the compositions and can be controlled through the existing phases. In fact, ,f could be adjusted to near zero by adding 8 wt% TiO2 to , alumina ceramics. A dielectric constant (,r) of 10.81, a high Q×f value of 338 000 GHz (measured at 14 GHz), and a temperature coefficient of resonant frequency (,f) of 1.3 ppm/°C were obtained for nano-scaled , alumina with 8 wt% TiO2 sintered at 1350°C for 4 h. Sintered ceramic samples were also characterized by X-ray diffraction and scanning electron microscopy. [source]

Grain-Boundary Diffusion of Cr in Pure and Y-Doped Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2007
K. Bedu-Amissah
The diffusive transport of chromium in both pure and Y-doped fine-grained alumina has been investigated over the temperature range 1250°,1650°C. From a quantitative assessment of the chromium diffusion profile in alumina, as obtained from electron microprobe analysis, it was found that yttrium doping retards cation diffusion in the grain-boundary regime by over an order of magnitude. The Arrhenius equations for the undoped and Y-doped samples were determined to be: ,Db=(4.77±0.24) × 10,7 exp (,264.78±47.68 (kJ/mol)/RT)(cm3/s) and ,Db=(6.87±0.18) × 10,8 exp (,284.91±42.57 (kJ/mol)/RT)(cm3/s), respectively. Finally, to elucidate the mechanism for this retardation, the impact of yttrium doping on diffusion activation energies and prefactors was examined. [source]

Two-Stage Sintering of Alumina with Submicrometer Grain Size

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2007
Katarína Bodi
This work verifies the applicability of two-stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high-temperature (1400°,1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine-grained alumina with a relative density of 98.8% and a grain size of 0.9 ,m was prepared by two-stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 ,m. [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]

Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline Alumina

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004
Guo-Dong Zhan
The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of ,-Al2O3 nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiCw/,-Al2O3 nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (,118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of ,-Al2O3 to ,-Al2O3 of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiCw/HEBM,-Al2O3 nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m1/2. [source]

Critical Factors Affecting the Wettability of ,-Alumina by Molten Aluminum

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2004
Ping Shen
The wetting behaviors of ,-Al2O3 single crystals with three different faces,R(0112), A(01120), and C(0001),and polycrystals (PC) by molten aluminum were studied over a wide temperature range using both a conventional and an improved sessile-drop method. The critical factors affecting the wettability, such as temperature, atmosphere, substrate surface roughness, and crystallographic orientation, and the influence from the experimental technique, were thoroughly investigated. The results show that the aluminum surface oxidation and the thickness of the oxide film have a pronounced effect on the wettability, especially at low temperatures. To eliminate this effect, the experimental temperature must be over a critical value. Vacuum favors lowering this value compared with atmosphere, and the improved sessile-drop method, particularly using an impingement-dropping mode (I-mode), helps to weaken this effect by mechanical disruption and removal of the oxide film. However, the dropping distance and the dropping force must be controlled to prevent an overspreading of the drop. The effects of the substrate surface roughness and temperature are not significant in the case of a clean aluminum surface and a fine-prepared alumina surface. On the other hand, the effect of the alumina surface crystallographic orientation is noticeable and the wettability is in the order of R > A > PC > C. The intrinsic contact angles of the Al/,-Al2O3 system in the temperature range of 1000°,1500°C were estimated to be 76°,85° for the R and A faces, 88°,100° for the C face, and 77°,90° for the polycrystal, depending on the temperature. [source]

Effect of Oligosaccharide Alcohol Addition to Alumina Slurry and Translucent Alumina Produced by Slip Casting

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2003
Yuji Hotta
A slurry used to produce dense green compacts by slip casting should exhibit low viscosity, high solids content, and good dispersion. Slurries with good characteristics were produced in the present study by adding oligosaccharide alcohol to an Al2O3 slurry with an NH4+ salt of poly(methacrylic acid) (NH4+ -PMA). The role of NH4+ -PMA and oligosaccharide alcohol in the Al2O3 slurry was examined by DTA, ,-potential measurement, high-pressure liquid chromatography, and viscometry. The viscosity of the slurry with NH4+ -PMA and oligosaccharide alcohol was lower than that of the slurry with NH4+ -PMA at a high solids content. Oligosaccharide alcohol did not interact with the Al2O3 surface. However, the Al2O3 slurry with NH4+ -PMA was influenced by the addition of oligosaccharide alcohol. We found that the dispersibility of the slurry was greatly improved by adding oligosaccharide alcohol. The transmittance of the Al2O3 ceramics produced by slip casting using the slurry with both NH4+ -PMA and oligosaccharide alcohol was higher than that of ceramics produced by slip casting using the slurry with NH4+ -PMA alone. The increased optical property resulted from low viscosity, which was attributed to the addition of oligosaccharide alcohol, at a high solids content. [source]