High Aspect Ratio Pores Research Articles

Modification of Porous Alumina Membranes Using Al2O3 Atomic Layer Controlled Deposition

Al2O3 films were deposited with atomic layer control inside the pores of Anodisc alumina membranes. To achieve this controlled deposition on a high aspect ratio structure, a binary reaction for Al2O3 chemical vapor deposition (2Al(CH3)3 + 3H2O → Al2O3 + 6CH4) was separated into two half-reactions: (A) AlOH* + Al(CH3)3 → Al-O-Al(CH3)2* + CH4 and (B) AlCH3* + H2O → AlOH* + CH4, where the asterisks designate the surface species. The trimethylaluminum [Al(CH3)3] (TMA) and H2O reactants were employed alternately in an ABAB... binary reaction sequence to deposit the Al2O3 film. Because each half-reaction is self-limiting, atomic layer controlled Al2O3 deposition was achieved on the surface of the high aspect ratio pores. To determine the necessary reaction conditions, surface species during each half-reaction were periodically monitored using in situ transmission FTIR spectroscopy. Ex situ gas flux and permporometry measurements were also performed to determine the effect of the binary reaction sequence on the...

Deposition of Tin Oxide into Porous Silicon by Atomic Layer Epitaxy

The deposition of conformal coatings into porous silicon layers was successfully demonstrated. Tin oxide films were formed from and precursors by atomic layer epitaxy. The influence of the porous substrate structure on the deposition parameters was analyzed from the viewpoint of formation mechanism, growth rate, and layer composition. The covered porous substrates were characterized by means of Rutherford backscattering, secondary ion mass spectrometry, cross‐sectional transmission electron microscopy, and ellipsometry. The mesoporous structure of the Si substrate uniquely determines the gas‐phase diffusion and physisorption of the precursors. The processing parameters favoring chemisorption are more critical for porous silicon than those for a flat surface. Even a small decrease in the deposition temperature results in a considerable increase in the growth rate through gas‐phase reactions, and the process becomes chemical vapor deposition‐like. Conformal step coverage was obtained on extremely high (140:1) aspect ratio pores if the deposition conditions were chosen such that chemisorption was the growth rate determining step in the process.

Fabrication of high aspect ratio structures for microchannel plates

Substrates for microchannel plates have been fabricated using Si micromachining techniques. High aspect ratio pores are constructed using reactive ion etching and streaming electron cyclotron resonance etching, and low-pressure chemical vapor deposition (LPCVD). In one process, 40 μm deep pores with 2 μm openings on 4 μm centers are directly etched in Si. Alternatively, pores with aspect ratios of 30:1 are constructed in a low-stress SiNx membrane using a sacrificial template process whereby pillars of Si are etched and then subsequently backfilled with a dielectric using LPCVD.

Application of Controlled Interfacial Pore Structures to Kinetic Studies in Alumina

The application of controlled-geometry interfacial pore structures to fundamental kinetic studies in alumina is described. Results from studies of the morphological stability of high aspect ratio pore channels, crack healing, pore coarsening and pore elimination in sapphire are presented.

DIFFUSION AND REACTION IN POROUS SOLIDS: A MODEL FOR TWO DIFFERENT HETEROGENEOUS REACTIONS

We have developed a model to simulate the concentration distribution of a reacting species within an isothermal, heterogeneous, porous structure. In our model we allow for two different reactions—one at the base of a cylindrical pore and another at the pore wall. Concentration distributions have been simulated for Damkohler numbers ranging over several orders of magnitude. Closed-form solutions were obtained for first-order reactions. Concentration variations in the radial direction were calculated from a two-dimensional model; the error involved in using the one-dimensional model was quantified for a representative case. The effect of neglecting the reaction at the base of the pore was calculated for several cases pertaining to high aspect-ratio pores

低温下の岩石の熱膨張について

Thermal expansion of several rocks down to 80K was measured with a dilatometer under dry and wet conditions at atmospheric pressure. According to the results of thermal expansion measurements, the rocks could be classified into two groups: granitic and non-granitic. For the non-granitic rocks in this study, i.e., tuff, sandstone, andesite, and basalt, the thermal expension coefficient increased in proportion to porosity but in inverse proportion to density, Young's modulus, and the strength. On the other hand, for granitic rocks (two granites and quartz-diorite), the thermal expansion coefficient increased in proportion to density and Young's modulus, but decreased with an increase in porosity. These differences are attributable to the pore and crack shape within the rocks. Low aspect ratio cracks are the main cause of granitic rock porosity, and they cancel the thermal expansion of the whole rock because they can provide space for the expansions of the surrounding grains. On the contrary, high aspect ratio pores, mainly within non-granitic rocks, only negligibly affect the thermal expansion of the whole rock.When the rock was saturated with water, the thermal expansion coefficient became higher than that under a dry condition. As the thermal expansion coefficient of ice is several times higher than those of rocks, the ice in pores and cracks contributes to the increase in the thermal expansion coefficient. And in the case of granitic rocks, this is partly because the ice existing in the crack increases its net aspect ratio and reduces the space available for the expansion of the mineral grains. The large and complicated hysteresis patterns of wet rocks seen upon thermal cycling are due to temperature differences between the freezing and melting of water in the rock pores and cracks.

Microcrack studies of basalts from the Iceland Research Drilling Project

Microcavities in basaltic core from the IRDP borehole in Reydarfjördur, Iceland, can be characterized by using direct observations with the scanning electron microscope (SEM) and inversion of velocity data. In the Iceland basalts, neither method provides an accurate, complete picture of the crack aspect ratio spectrum, but by using both SEM and inversion data a complete spectrum may be obtained. SEM observations provide information on high aspect ratio pores and cracks (>0.005) that cannot be separated by velocity inversion theory, while velocity inversion theory provides information on low aspect ratio cracks (< 0.005) that are too narrow to be seen with the SEM. Alteration rims along cracks, blunt ends, and aspect ratios which imply closure pressures greater than the vertical stress all provide evidence that cracks are open in situ. Cracks coincident with grain boundaries are abundant in the basalt flows and suggest an origin early in the rock's history, probably during cooling. Crack healing and alteration are observed to increase with depth, causing a shift in crack shape distribution toward higher aspect ratios.