Anisotropic Pores Research Articles

Application of USAXS analysis and non-interacting approximation to determine the influence of process parameters and ageing on the thermal conductivity of electron-beam physical vapor deposited thermal barrier coatings

Electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) display a lower thermal conductivity compared with the deposited bulk material. This effect is achieved due to the presence of pores within these films. The spatial and geometrical characteristics of the porosity influence directly the magnitude of the achieved reduction of the thermal conductivity. In this work, three EB-PVD coating containing different microstructures were manufactured by varying the manufacturing process parameters during the deposition process. Their corresponding thermal conductivities were measured via the laser flash analysis method (LFA) in both the as-coated state and after ageing (1100 °C/100 h). Analysis of the pore formation during processing was carried out by ultrasmall-angle X-ray scattering (USAXS). This technique is supported with a computer based modeling developed by researchers at Advanced Photon Source (APS) in ANL, USA, and in National Institute of Standards and Technology (NIST), USA. The model enables the characterization of the size, shape, volume and orientation of each of the pore populations in EB-PVD TBCs. The effect of these spatial and geometrical characteristics of the porosity on the thermal conductivity of the EB-PVD coatings were studied via a non-interacting approximation based on Maxwell's model. Results of LFA measurements and the applied approximation indicate an interrelation between the microstructure and the thermal properties of the analyzed EB-PVD coatings. Microstructures containing a higher volume fraction of fine anisotropic intra-columnar pores, and larger voids between feather-arms oriented at lower angles toward the substrate plane correspond to lower thermal conductivity values. Inter-columnar gaps do no significantly contribute to lowering the thermal conductivity due to their orientation parallel to the heat flux and their lower volume fraction compared with the volume occupied by the primary columns. On heat treatment, the deepest section of the gaps between feather-arms break-up into arrays of nano-sized low aspect ratio voids. The anisotropic, elongated intra-columnar pores evolve toward low aspect ratio shapes that are less effective in reducing the thermal conductivity.

Nondestructive Structural Analysis on Deformation Properties of Highly Porous Aluminum Material

The cell-structure of highly porous aluminum material prepared by melt foaming technology was investigated under deformation with fine-focus X-ray 3D-CT to make clear the development target porous material for automobile industries with improved reliability. It was confirmed that structures with more fine, more uniform and exclusion peculiar anisotropic pores would make improved mechanical properties of the material.

Gold-Silver Alloy Nanowires Electrochemically Grown in the Nanopores of Aluminum Anodic Oxidation Film from Cyanide-Free Bath and Their Color Properties

AuAg alloy nanowires in the geometrically anisotropic pores of the aluminum anodic oxidation films with a mean diameter of 10nm and depth of 10μm were successfully prepared by the electrodeposition of gold and silver under the bath voltage of 6V AC in a cyanide-free bath (Ag2SO4-HAuBr4-(NH4)2S2O3). The aspect ratio of the alloy nanowires was controlled by changing the electrodeposition time. The maximum position of absorption band due to the transverse surface plasmon resonances of AuAg alloy nanowires shifts to a longer wavelength with an increase of the composition of Au. The color and shading of the anodized aluminum deposited with alloy nanowires were simply tuned in a wide range.

Color properties of gold–silver alternate nanowires electrochemically grown in the pores of aluminum anodic oxidation film

Repeated electrodeposition of gold and silver on anodized aluminum plate yielded Au–Ag alternate nanowires in the geometrically anisotropic pores of the anodic oxidation films with mean diameter 10 nm and depth 10 μm, respectively. The absorption maximum wavelengths in the visible region resulting from the transverse surface plasmon resonances of Au and Ag nanorods are invariant with their growth. Consequently, the color of the anodized aluminum was simply tuned in a wide range, which is impossible to attain in each monometallic system.

Modification of cubic susceptibility tensor in birefringent porous silicon

AbstractPolarization‐sensitive third‐harmonic generation in birefringent (110) porous silicon layers reveals a modification in the cubic susceptibility tensor of silicon caused by strongly anisotropic pores. The ratio $ (\chi _{1111} ^{(3)} + \chi _{1122} ^{(3)} / \chi _{3333} ^{(3)}) $ of cubic susceptibility tensor components for the porous layers is shown to be 40% higher than this ratio for crystalline silicon. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropy in the wet-etching of semiconductors

The surface chemistry and electrochemistry of the anisotropic etching of semiconductors are reviewed. Recent insights into the anisotropic chemical etching of silicon in alkaline solution and of electrochemical etching of anisotropic pores in n-type semiconductors are described. The possible role of galvanic effects in open-circuit etching is emphasized.

Diffusive–convective and diffusive–electroconvective transport in non-uniform channels with application to macromolecular separations

The effects of various geometrical parameters on effective diffusive, convective and electroconvective transport coefficients are studied by using models of constricted and expanded pores. The analysis of these simple pores offers insight into the transport behavior of macromolecules in media consisting of anisotropic aligned pores. A number of cases are analyzed including hydrodynamic flow, hindered transport with hydrodynamic flow, and electrophoretic transport with and without hindrance. The effects of the geometrical parameters of the model pores on the effective transport parameters are determined. The implications of changes of these geometrical parameters, applied electrical fields, and bulk flow rates on separations are illustrated using the time required to separate a binary mixture of solutes for specified resolutions. The results show that by appropriately selecting the flow rate in the case of hydrodynamic flow or the electrical field strength for the case of electrophoresis an optimum or minimum time of separation can be determined for the separation of two comparably sized molecules with sizes close to those of the narrower part of the pore.

Strain Tolerant Porous Silicon Nitride

An approach to material strain tolerance, which basically makes it possible to lower the elastic modulus while retaining strength, was experimentally confirmed using as an example a porous silicon nitride composed of oriented anisotropic grains and pores. The porous structure consisting of tightly tangled rodlike grains and anisotropic pores was obtained by using β‐Si3N4 whiskers. This material exhibited a low Youngs modulus while retaining a relatively high fracture stress, even though it contained 14.4% porosity. Consequently, the strain to failure of silicon nitride was appreciably increased.

Acoustic and Elastic Properties of PZT Ceramics with Anisotropic Pores

It has been shown that anisotropic porosity in PZT reduces the d31/d33, ratio, raising the hydrostatic piezoelectric response of PZT significantly. In the present work, this behavior is related to Poisson's ratio by applying the Gibbs thermodynamic energy function, literature data for dense PZT ceramics and for materials with hexagonal crystal symmetry, and experimental results for PZT with anisotropic porosity. Acoustic as well as mechanical tests of PZT ceramics with anisotropic pores demonstrate the elastic anisotropies that cause the reduction in Poisson's ratio. The dependence of d31/d33 on Poisson's ratio is also detailed. In addition, a close relation between d31/d33 and the longitudinal acoustic velocity is demonstrated. Flexural strength tests indicate that anisotropic pores provide dhn‐enhanced PZT ceramics with significantly greater strength than isotropic pores.

X-ray scattering from coals

This paper describes the results of X-ray scattering experiments on a series of vitrain-coals of varying rank. The scattering at high angles is interpreted in terms of condensed aromatic layers; the diameters of the layers are determined and are found to increase with increasing rank; the percentage carbon in the form of layers is also estimated. Fourier transforms calculated for the scattering at lower angles show that the layers occur partly singly, and partly in groups of two, three or more stacked parallel to each other. The degree of local parallel stacking increases with increasing rank. There is a preferred orientation of the layers parallel to the bedding plane, which becomes more marked the higher the rank of the coal. The diffraction peak at ~20 Å, found for coals with 85 to 94% C, is considered to be a consequence of ‘liquid-type’ packing of the most frequently occurring groups containing two to three layers. The diameters of the layers determined from the transforms (~8 Å) are in good agreement with the results of intensity calculations, and the values obtained from the scattering at high angles. The scattering at very small angles is measured with a two-crystal spectrometer for spacings up to ~5000 Å. The scattering is related to porosity; the scattering curves do not support the existence of a close-packed arrangement of ‘micelles’ of fairly constant diameter less than ~5000 Å, but suggest that there are anisotropic cracks and pores of a wide range of sizes, some of which must exceed ~5000 Å. The total scattering indicates the existence of disk cracks preferentially orientated parallel to the bedding plane. In terms of a proposed structural model, coalification is a process of condensation, ordering of the layers, and flattening of the structure. A ‘liquid-type’ structure is formed, which is most perfect at ~89% C and accounts for the minimum of porosity. Anthracitization is probably accompanied by a clustering of the layers, which results in a rapid increase of layer diameter, in irregular packing, and in an increase of porosity. The properties of the proposed structure model of coal are discussed and compared with the known physical and chemical properties of coal, e. g. density, optical data, porosity and mechanical properties, and some of the problems still outstanding are indicated.