Browse Topic: Silicon alloys
In working with various medical equipment such as needles, syringes, trocars, cannulas, guide-wires, catheters, and valves, medical device designers must account for friction in the form of insertion, drag, and break-loose forces. A biocompatible silicone lubricant can significantly reduce friction at interfaces between components and between components and human tissue.
Anodizing is applied to improve the durability and the corrosion resistance of aluminum alloy parts of engines and car bodies. Generally, anodic oxide film is formed using direct current anodizing (DCA). However, in the case of anodizing high silicon aluminum alloy cast parts, it is difficult to derive uniform film thickness distribution. Furthermore, it takes a long treatment time which causes low productivity. In this study, the authors have developed an anodizing method by using high-frequency switching anodizing (HSA) to solve these problems. The growth process of anodic oxide film is susceptible to the metallographic structure. Thus, the typical DCA application to the high silicon aluminum alloy produces a non-uniform film thickness, while HSA has the potential to form uniform film without being affected by metallographic structure. Moreover, the current density of HSA is higher than that of DCA which reduces treatment time to 1/5 as the film formation enhances. Our investigation is to apply HSA to the mass produced engine pistons that require both high durability and low cost.
An advanced technique of deep reactive-ion etching (DRIE) has been developed for fabricating smooth vertical walls in silicon wafers. These walls are suitable for use as bounding surfaces of optical waveguides in photonic and optoelectronic devices. The roughness of a typical 8-µm-high vertical wall surface of a waveguide made by this technique is <20 nm; by keeping the roughness at such a low level, one helps to ensure that the waveguide is capable of low-loss optical transmission.
Two reports present additional details about the method described in "Reaction-Forming Method for Joining SiC-Based Ceramic Parts" (LEW-16661), NASA Tech Briefs, Vol. 23, No. 3 (March 1999), page 50. To recapitulate: A carbonaceous mixture (typically a paste) is applied to a joint between parts. The parts are clamped together and heated to a temperature of 115±5 °C for 10 to 20 minutes; this action partly cures the mixture, gluing the parts together with just enough strength that one need not clamp the parts during subsequent processing. Silicon or a silicon alloy in tape, paste, or slurry form is applied to the joint region. The parts are heated to a temperature between 1,250 and 1,425 °C for 5 to 10 minutes, causing the silicon or silicon alloy to melt, infiltrate the joint, and react with carbon. The finished joint, which is typically as strong as the parent material, contains silicon carbide with silicon and other phases. The amounts of the phases can be adjusted, by choice of the compositions of the reactants, to obtain joints with tailorable microstructures and thus tailorable thermomechanical properties.
The figure depicts a proposed miniature, electrically actuated, one-time-opening isolation valve that would be made mostly of silicon, by use of micromachining techniques. Isolation valves are needed in systems in which fluids must be stored for long times until use, with no leakage or contamination prior to release. Miniature isolation valves like this one could serve to control the release of propellant liquids or gases in microspacecraft, or of stored chemical reagents in portable in situ chemical-analysis apparatuses. Eventually, such apparatuses may include one-time-use biochemical-analysis chips.
The MacWafer™ code computes gravitational and thermal stresses in silicon wafers and uses these results to determine the maximum allowable temperature variation across a wafer, maximum processing temperatures, and maximum allowable heating and cooling rates. This information is of particular interest in the case of processing 300-mm wafers coupled with fast ramp technologies. The program runs interactively on Apple Macintosh, IBM PC, and PC clones, and workstation computers as well. Execution time is typically about 20 seconds on a Motorola 68040 processor operating at 33 MHz.
When a silicon wafer is cut from an ingot, it is essentially impossible to align the cut perfectly with the crystal structure. Therefore the surface contour of the wafer will be a flat plane on which terraces consisting of additional atomic layers will be scattered. An atomic step will be found on the surface where each additional layer is encountered. At elevated temperatures, these atomic steps will migrate, but they cannot be eliminated. At the current level of device technology, the effects of steps on the wafer surface can be largely ignored. In future generations of integrated circuits, however, the sizes of these steps will become comparable to some device feature sizes and will affect circuit performance and yields.
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