Porous Alumina (porous + alumina)

Distribution by Scientific Domains

Selected Abstracts

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

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]

Selective Barrier Perforation in Porous Alumina Anodized on Substrates,

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

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]

Three-Dimensional Printing of Complex-Shaped Alumina/Glass Composites,

Wei Zhang
Abstract Alumina/glass composites were fabricated by three-dimensional printing (3DP) and pressureless infiltration of lanthanum-alumino-silicate glass into sintered porous alumina preforms. The preforms were printed using an alumina/dextrin powder blend as a precursor material. They were sintered at 1600,C for 2,h prior to glass infiltration at 1100,C for 2,h. The influence of layer thickness and sample orientation within the building chamber of the 3D-printer on microstructure, porosity, and mechanical properties of the preforms and final composites was investigated. The increase of the layer thickness from 90 to 150,m resulted in an increase of the total porosity from ,19 to ,39,vol% and thus, in a decrease of the mechanical properties of the sintered preforms. Bending strength and elastic modulus of sintered preforms were found to attain significantly higher values for samples orientated along the Y -axis of the 3D-printer compared to those orientated along the X - or the Z -axis, respectively. Fabricated Al2O3/glass composites exhibit improved fracture toughness, bending strength, Young's modulus, and Vickers hardness up to 3.6,MPa m1/2, 175,MPa, 228,GPa, and 12,GPa, respectively. Prototypes were fabricated on the basis of computer tomography data and computer aided design data to show geometric capability of the process. [source]

Activation Mechanism and Infiltration Kinetic for Pressureless Melt Infiltration of Ti Activated Al2O3 Preforms by High Melting Alloy,

Srdan Vasi
The infiltration mechanism of X3CrNi13-4 in titanium activated porous alumina preforms has been studied. Investigations revealed isolated steel-covered titanium particles beyond the infiltration front. The only transport path possible for the steel to form such wetted islands is through the gas phase. Supersaturation due to the mixing of the steel gas phase with the titanium rich gas phase over the activator particle surfaces is proposed as condensation mechanism. Progressive condensation leads to the formation of a melt network, which serves as pathway for the original steel melt to infiltrate the preforms and to fill the remaining pore space in the non-wetting X3CrNi13-4/Ti-Al2O3 sytem. [source]

Augmentation of boiling heat transfer from horizontal cylinder to liquid by movable particles

Yoshihiro Iida
Abstract This paper presents a series of experimental results on a passive augmentation technique of boiling heat transfer by supplying solid particles in liquid. A cylindrical heater 0.88 mm in diameter is placed in saturated water, in which a lot of mobile particles exist, and the nucleate and film boiling heat transfer characteristics are measured. Particle materials used were alumina, glass, and porous alumina, and the diameter ranged from 0.3 mm to 2.5 mm. Particles are fluidized by the occurrence of boiling without any additive power, and the heat transfer is augmented. The maximum augmentation ratio obtained in this experiment reaches about ten times the heat transfer coefficient obtained in liquid alone. The augmentation ratio is mainly affected by the particle material, diameter, and the height of the particle bed set at no boiling condition. The augmentation mechanism is discussed on the basis of the experimental results. 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 28,41, 2002 [source]

Formation of Thick Porous Anodic Alumina Films and Nanowire Arrays on Silicon Wafers and Glass,

O. Rabin
Abstract A method for the fabrication of thick films of porous anodic alumina on rigid substrates is described. The anodic alumina film was generated by the anodization of an aluminum film evaporated on the substrate. The morphology of the barrier layer between the porous film and the substrate was different from that of anodic films grown on aluminum substrates. The removal of the barrier layer and the electrochemical growth of nanowires within the ordered pores were accomplished without the need to remove the anodic film from the substrate. We fabricated porous anodic alumina samples over large areas (up to 70 cm2), and deposited in them nanowire arrays of various materials. Long nanowires were obtained with lengths of at least 9 ,m and aspect ratios as high as 300. Due to their mechanical robustness and the built-in contact between the conducting substrate and the nanowires, the structures were useful for electrical transport measurements on the arrays. The method was also demonstrated on patterned and non-planar substrates, further expanding the range of applications of these porous alumina and nanowire assemblies. [source]

Subcritical crack growth behavior of Al2O3 -glass dental composites

Qingshan Zhu
Abstract The purpose of this study is to investigate the subcritical crack growth (SCG) behavior of alumina-glass dental composites. Alumina-glass composites were fabricated by infiltrating molten glass to porous alumina preforms. Rectangular bars of the composite were subject to dynamic loading in air, with stressing rates ranging from 0.01 MPa/s to 2 MPa/s. The SCG parameter n was determined to be 22.1 for the composite, which is substantially lower than those of high-purity dense alumina. Investigations showed that glass phases are responsible for the low n value as cracks propagate preferentially within glass phases or along the interface between glass phases and alumina phases, due to the fact that glasses are more vulnerable to chemical attacks by water molecules under stress corrosion conditions. The SCG behavior of the infiltration glass was also investigated and the SCG parameter n was determined to be 18.7. 2003 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 65B: 233,238, 2003 [source]

Wide-range length metrology by dual-imaging-unit atomic force microscope based on porous alumina

Dongxian Zhang
Abstract A new dual-imaging-unit atomic force microscope (DIU-AFM) was developed for wide-range length metrology. In the DIU-AFM, two AFM units were combined, one as a reference unit, and the other a test one. Their probes with Z piezo elements and tips were horizontally set in parallel at the same height to reduce errors due to geometric asymmetry. An XY scanner was attached to an XY block that was able to move in the X direction with a step of about 500 nm. A standard porous alumina film was employed as the reference sample. Both reference sample and test sample were installed at the center of the XY scanner on the same surface and were simultaneously imaged. The two images had the same lateral size, and thus the length of the test sample image could be accurately measured by counting the number of periodic features of the reference one. The XY block together with the XY scanner were next moved in the X direction for about 1.5 ,m and a second pair of reference and test images were obtained by activating the scanner. In this way, a series of pairs of images were acquired and could be spliced into two wide-range reference and test images, respectively. Again, the two spliced images were of the same size and the length of test image was measured based on the reference one. This article presents a discussion about the structure and control of the DIU-AFM system. Some experiments were carried out on the system to demonstrate the method of length calculation and measurement. Experiments show a satisfactory result of wide-range length metrology based on the hexagonal features of the porous alumina with a periodic length of several tens of nanometers. Using this method the DIU-AFM is capable of realizing nanometer-order accuracy length metrology when covering a wide range from micron to several hundreds of microns, or even up to millimeter order. Microsc. Res. Tech. 64:223,227, 2004. 2004 Wiley-Liss, Inc. [source]