Inelastic response of alumina

This paper investigates the effects of microcracking, plasticity, and strain rate dependent pore closure on the inelastic deformation wave profiles of a low density (3.55 mg/m³) and a high density (3.88 mg/m³) aluminas. This is accomplished by means of numerical simulations of the measured plane shock wave profiles in these aluminas. The wave profiles were generated over a wide range of impact velocities (80 m/s to 2200 m/s). An internal-state-variable based ceramic model was used in the simulations to describe the inelastic strains due to microcracking, microplasticity, and pore collapsing. The microcrack size, number of microflaws, and limiting speed of the crack growth controlled the shape of the inelastic wave portion of the velocity profile at low velocity impact. The porosity content and the strain rate sensitivity parameter did not significantly influence the shapes of the low velocity profiles. However, these two parameters greatly influenced the profile shapes when the ceramic was shocked at high velocities well above the Hugoniot elastic limit. The simulations of high velocity experiments clearly demonstrated the need for describing the pore collapse process in order to match the measured wave profiles.     

Role of magnesia and silica in alumina microstructure evolution

The effects of MgO and SiO₂ additive distributions on alumina grain morphology have been characterized using high-resolution imaging secondary ion mass spectrometry (HRI-SIMS). In alumina samples singly-doped with MgO, the concentration of Mg segregated to grain boundaries is independent of grain boundary length for a majority of grain boundaries studied. Mg segregant therefore redistributes from grain boundaries to microstructural sinks, such as pores and/or second phases, during grain coarsening. In samples singly-doped with SiO₂, abnormal grain growth develops and the concentration of Si at grain boundaries is also independent of grain boundary length. Redistribution of segregants is again necessary in this case to maintain constant grain boundary composition. Codoping with Mg/Si > 1 suppresses abnormal grain growth as a result of increased mutual solid solubility of both ions and an associated decrease in grain boundary segregation. Grain growth kinetics for doped aluminas are reconsidered in light of these observations.     

Role of alumina in flexure of glass/epoxy composites

The influence of particulate additions of alumina on the flexural properties of glass-fabric/epoxy composites was studied. The additions improved translaminar flexural strength, while decreasing interlaminar strength. The translaminar bending modulus showed an increasing trend whereas its interlaminar value showed a decrease, up to additions of 3 vol%. The mechanisms of deformation and the fracture features have been discussed with the aid of scanning electron microscopy.     

Characterization of nanoporosity in polymer-derived ceramic fibres by X-ray scattering techniques

Ceramic fibres with Si-C-O and Si-N-C-O compositions, prepared by pyrolysis of polymer precursors, generally have densities lower than those calculated from a volume additivity rule. However, techniques often used to detect porosity such as electron microscopy methods, surface area and porosimetry measurements show that little surface-connected porosity is present. X-ray scattering measurements, both wide-angle (WAXS) and small-angle (SAXS), show considerable scattering in the range 1° < 2 < 10° (CuK). Treatment of the scattering data by the classical Guinier (low angle limit) and Porod (high angle limit) methods indicate that closed, globular, nanometre-scale porosity (1 to 3 nm diameter) is present in all ceramic fibres examined. X-ray scattering power correlates quantitatively with the volume fraction porosity, as expected if porosity is the dominant facto affecting X-ray scattering. Nano-particles of excess carbon and of-SiC nanocrystallites, though present, are minor contributors to the scattering of X-rays in these ceramic fibres. Fibres are three-dimensional, not of fractal dimension, and are not oriented. As density increases with increasing pyrolysis temperature, average pore size increases and pore volume fraction decreases. This results from a thermodynamically favourable reduction of surface free energy and apparently occurs by a viscous flow process.     

Controlled release of small molecules from silica xerogel with limited nanoporosity

Conventional sol–gel processing requires several distinct steps involving hydrolysis, condensation and drying to obtain a highly porous, glassy solid material. With the goal of achieving controlled release of small molecules, herein we focus on the acceleration of the condensation and drying steps by casting the hydrolyzed sol on a large open surface to achieve a denser 100 % silica xerogel structure. Thus, cast xerogel with a more limited porosity was prepared. The effect of synthesis parameters during sol–gel synthesis on the release kinetics of bupivacaine, vancomycin and cephalexin was investigated. The release kinetics fitted well with the Higuchi model, suggesting a diffusional release mechanism. Combining the release and nanostructure data, the formation mechanism of cast xerogel is described. Without introducing additional precursors or additives into sol–gel systems, sol–gel casting is an easy technique that further expands the applicability of sol–gel materials as excellent carriers for the controlled release of a variety of drugs.     

Effect of counterpressure during equal-channel angular pressing on nanoporosity formation in ultrafine-grained copper

Results of small-angle X-ray scattering and high-precision density measurements showed that the application of counterpressure during the equal-channel angular pressing (ECAP) of ultrafine-grained copper leads to a decrease in nanoporosity and an increase in mechanical properties of the ECAP-processed metal.     

Optical and nuclear characterization of Xe-induced nanoporosity in SiO2

We performed RBS, infrared (IR) and C-V measurements in order to follow the evolution of Xe, bubbles/cavities and other defects (with a focus on NBOHC: non-bridging oxygen hole center) and dielectric constant (k), in high dose Xe implantation in SiO₂. As-implanted sample provides the lowest value of k which increases with post thermal annealing. In the meantime, the concentration of negatively charged defects decreases with annealing while Xe out-diffuses after annealing at 1100 °C leaving Xe free cavities in the sample. By combining these results one can determine the contribution of nanoporosity in dielectric constant evolution.     

Development of titanium-dioxide-based aerogel catalyst with tunable nanoporosity and photocatalytic activity

Nanostructured highly porous TiO(2)/WO(3)/Fe(3+) aerogel composite photocatalysts are prepared, characterized and tested for model photocatalytic reactions. The catalyst structure is tailored to capture environmental pollutants and enable their decomposition in situ under both ultraviolet (UV) and visible light through oxidation to smaller benign molecules. A novel and green method is utilized to prepare the unique surfactant-templated aerogel composite photocatalyst that has a highly accessible porous nanostructure with high surface area and tailored pore size distribution. The sol-gel process is combined with supercritical extraction and drying. Supercritical drying with heat treatment results in titanium dioxide with anatase crystal form. Templates used further enable retention and tuning of the nanopore structure and surface properties. The synthesized catalysts were characterized using SEM, FIB, XRD and porosimetry prior to post-evaluation in model reactions. The bandgap of the catalyst particles was also determined using diffuse reflectance. The resulting aerogel TiO(2)/WO(3)/Fe(3+) has similar photocatalytic capability compared to highly optimized commercial Degussa P25 under UV exposure and offers much superior photocatalytic capability under visible light exposure. The model reaction utilized employed methylene blue (MB) photooxidation under visible and UV light.     

Multifunctional substrates of thin porous alumina for cell biosensors

We have fabricated anodic porous alumina from thin films (100/500 nm) of aluminium deposited on technological substrates of silicon/glass, and investigated the feasibility of this material as a surface for the development of analytical biosensors aiming to assess the status of living cells. To this goal, porous alumina surfaces with fixed pitch and variable pore size were analyzed for various functionalities. Gold coated (about 25 nm) alumina revealed surface enhanced Raman scattering increasing with the decrease in wall thickness, with factor up to values of approximately 10⁴ with respect to the flat gold surface. Bare porous alumina was employed for micro-patterning and observation via fluorescence images of dye molecules, which demonstrated the surface capability for a drug-loading device. NIH-3T3 fibroblast cells were cultured in vitro and examined after 2 days since seeding, and no significant (P > 0.05) differences in their proliferation were observed on porous and non-porous materials. The effect on cell cultures of pore size in the range of 50–130 nm—with pore pitch of about 250 nm—showed no significant differences in cell viability and similar levels in all cases as on a control substrate. Future work will address combination of all above capabilities into a single device.     

离心成型结合模板法制备孔径均匀的多孔氧化铝陶瓷

将等直径的发泡聚苯乙烯(EPS)小球排列成有序的模板,通过在模板内离心成型制备孔径均匀的多孔氧化铝陶瓷.研究了多孔陶瓷孔结构的调整方法,分析了离心参数对孔壁生坯密度的影响,借助了TG-DTG曲线确定了焙烧工艺,用扫描电镜表征了最终产物的显微结构.结果表明,多孔陶瓷的孔结构可以通过改变小球的直径和所承受的附加载荷来调整.当氧化铝浆料的固相含量超过50%(体积分数),离心成型的物质分离现象被抑制,孔壁具有较高的生坯密度63.4%和烧结密度98.8%,当烧结产物的孔隙率从75.6%增加到83.2%,压缩强度由3.2MPa降到1.78MPa.     

Bonding of alumina ceramics with nanoscaled alumina powders

Nanoscaled Al₂O₃-powders can be employed for diffusion bonding of alumina ceramics. In order to accomplish bonding of the ceramics, Al₂O₃-nanopowder with a median particle size of 14 nm in diameter is sandwiched between two commercial microcrystalline corund discs, followed by uniaxially hot compressing of the assembly in vacuum at 80 MPa and 1100 °C for 2 h. Scanning electron microscopic investigations reveal a nanocrystalline structure of the joint with a mean grain size of about 50 nm in diameter and extensive consolidation of the powder without substantial shrinkage void formation. Microhardness measurements across the interface yield a value of 200 HV. In order to achieve complete densification and strength enhancement of the joint material, the sample is subsequently sintered at 1500 °C and 1600 °C for several hours in air. It was found that the hardness of the joint depends strongly on the porosity content and/or grain size and that a hardness of 1700 HV is obtained when both a mean particle size of about 1 μm and complete densification of the joint is achieved. The results show for the first time that Al₂O₃-nanopowders are suitable for diffusion bonding of alumina ceramics. Possible mechanisms are discussed.     

In vitro evaluation of osteoconductivity and cellular response of zirconia and alumina based ceramics

Developed ceria/yttria stabilized zirconia and ceria/yttria stabilized zirconia toughened alumina supported formation of apatite layer when immersed in simulated body fluid without any prior surface treatment. The formed mineral layer was confirmed as hydroxyapatite through X-ray diffraction patterns. The calcium/phosphate atomic ratio obtained from energy dispersive X-ray spectroscopy was found to be little less (Ca/P = 1.5) than that of pure hydroxyapatite (Ca/P = 1.7) which indicates the probability of mixed type calcium-phosphate compound formation. The achieved thickness of apatite layer was estimated through a surface profilometer and as high as ~ 17 μm thickness was found after 28 days of soaking. The biocompatibility of the developed materials was ensured through in vitro human osteoblast like cell (MG63) culture on ceramic discs. The morphology of attached cells was characterized through scanning electron microscopy and fluorescent microscopy which show multilayered interconnected cell growth within 8 days of culture period. Moreover, differentiation of MG63 cells was evaluated through MTT assay, total protein content and alkaline phosphatase activity.     

Fracture of hot-pressed alumina and SiC-whisker-reinforced alumina composite

The flexural strength of hot-pressed alumina and SiC-whisker-reinforced alumina composite were evaluated as a function of temperature (20 to 1400° C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 800° C, in which the failure mode was a mixture of transgranular and intergranular crack propagation. In this region, the alumina composite showed significantly higher fracture strength and toughness compared to polycrystalline alumina. Above 800° C, both materials (alumina and alumina composite) displayed a decreasing fracture strength due to the presence of subcritical or slow crack growth which occurred intergranularly. Flexural stress rupture evaluation in the temperature range 600 to 1200° C has identified the stress levels for time-dependent and time-independent failures.     

Generalized free volume diagrams from positron annihilation data for the study of nanoporosity in silica gel specimens

Generalized positronium annihilation diagrams are proposed in this paper. In particular, we point out how the free volume plots arising from a very simple model can be utilized to determine the electronic layer thickness ΔR in which the ortho-positronium (o-Ps) annihilation events occur. We comment on the relevant role played by the Positronium Annihilation Lifetime Technique (PALT) as a powerful tool for studying diverse properties of materials and discuss the range of pore size distributions detectable by this technique. These considerations have been applied to the study of different silica gel specimens, prepared by the sol-gel technique. We found that the free volume curves fit well to the experimental data provided the thickness parameter ranges between ΔR_min= 0.71 nm and ΔR_max= 2.3 nm and also observed a bimodal pore size distribution which corresponds to the existence of two types of nanopores present in the silica gel samples. The variation of the mean free volume radius R with the chemical composition and the correlation between the positronium annihilation parameters and the potential catalysis properties of the modified silica gels, are also presented. Received: 23 June 2001 / Accepted: 23 July 2001     

The role of starting powder size on the compressive response of stand-alone plasma-sprayed alumina coatings

Cylindrical stand-alone tubes of plasma-sprayed alumina were tested in uniaxial compression at room temperature, using strain gages to monitor axial strains. The effect of lamella size on the mechanical response was investigated by employing different starting powders to fabricate samples. The average powder sizes investigated included 9 m, 19 m and 32 m alumina; the resulting effective lamella diameters were 10 m, 28 m, and 55 m, respectively. Similar stress-strain hysteresis was observed on unloading in all tubes, independent of lamella size. A strong correlation between the failure stress and the cumulative strain at failure was also observed for tubes fabricated from the three powders. For samples with approximately constant densities, tubes plasma sprayed with the 9 m powder exhibited greater moduli than tubes sprayed from either 19 or 32 m powders. This difference was attributed to the greater percentage of unmelted -Al₂O₃ in the coating.     

Bone cell–materials interaction on alumina ceramics with different grain sizes

The objective of this work was to study adhesion, proliferation and differentiation of osteoblast cells (OPC1) on alumina ceramic, a bio-inert material. Alumina ceramic with different average grain sizes, 1 μm and 12 μm, respectively, were used in as-prepared condition without any grinding and polishing to understand the influence of grain size on cell–material interactions. Scanning electron microscopy and confocal imaging were used to study attachment, adhesion and differentiation of OPC1 cells. Cells attached, proliferated and differentiated well on both the substrates. Adhesion of cells, as assessed by observing the production of vinculin, was found to be a consistent phenomenon on both the substrates. On day 5 of cell culture, significant cell-attachment was observed and vinculin was detected throughout cytoplasm. MTT assay showed that proliferation of OPC1 cells was consistently higher in the case of 12 μm-alumina. Cells of different morphology, nodular, plate-like as well as elongated, were found to get anchored at grains, grain boundaries as well as pores. On day 16, there were clear signs of mineralization as well. Over all, alumina with average grain size of 12 μm showed better cell-attachment, growth and differentiation compared to 1 μm grain size samples.     

Base alumina ceramics with dispersoids: mechanical behaviour and tissue response afterin vivo implantation

Base alumina ceramics with dispersoids (BAC) are a new class of ceramics with improved mechanical properties as compared to pure alumina. They are obtained by dispersion of powder within an alumina matrix. Of the three new ceramics studied here, A20Z possesses the best mechanical properties as well as tribological properties superior to those of pure alumina whether it is used in ceramic-ceramic or ceramic-polyethylene combination.Mechanical behaviour and tissue response afterin vivo implantation were studied. Small four-point flexion bars obtained by pressureless sintering were implanted subcutaneously in the rat. The mechanical properties were unmodified even after one year from implantation.The tissue response was studied up to one year on cylinders implanted in the paravertebral muscles of Wistar rats, and was evaluated by qualitative examination of the encapsulating membrane and measurement of its thickness in relation to pure alumina. The tissue responses were comparable for both alumina and the three tested ceramics.