Whisker handling

Sihcon carbide has been described as a mild inhalation irritant (143). The threshold limit value for siUcon carbide in the atmosphere is 5 mg/m. Because of increased interest in SiC whiskers as a reinforcement for composites, the ASTM has estabUshed Subcommittee E34.70 on Single-Crystal Ceramic Whiskers to write standards for handling this form of SiC (144). 

Elemental carbon is usually handled in three forms graphite, diamond, and amorphous carbon. Graphite and amorphous carbon have been extensively used in electrochemistry because of their high electrical conductivity, chemical stability, versatility, and low cost. For electrochemical applications, such materials can be manufactured in bars, powders, and fibers or can even form conducting composites when appropriate binders are used. A number of carbon-based materials, such as pyrolytic carbon, carbon blacks, activated carbons, graphite fibers, whiskers, glassy carbon, etc., have been used in electrochemistry for decades (Yoshimura and Chang, 1998). 

Still stronger and stiffer forms of fibrous materials are the unidirectional crystals called whiskers. Under favorable conditions, crystal-forming materials will crystallize as extremely fine filamentous single crystals a few microns in diameter and virtually fi-ee of the imperfections found in ordinary crystals. Whiskers are far stronger and stiffer than the same material in bulk form. To date their use is limited principally due to special handling requirement during fabrication into RPs and cost. 

Conclusive experimental studies on the effects of respirable fibrous SiC dusts (whiskers) are not yet available [255]. In the United States ASTM has published recommendations for safe handling procedures related to all ceramic fibers, including silicon carbide whiskers [256]. 

ASTM, Standard Practice for Handling SiC whiskers, E 1437-91, Subcommittee E 34.70, American Society for Testing and Materials, Philadelphia, USA, 1991. 

Other forms ofvery fine oxide fibers exist or are being developed. Single crystal whiskers which pose health risks and also present difficulties for handling, because of their very small dimensions, have not found wide use. Nano-oxide fibers are being developed which may be fine enough for liquid crystal technology to be used so as to handle them but as yet remain a possibility for the future. 

As with all materials that have been recently developed, the health risks associated with SiC whiskers are not well known however, because their sizes and shapes are similar to that of asbestos, therefore they are considered hazardous. Airborne dispersion and subsequent inhalation are the most serious health hazards. However, with proper worl lace handling requirements and procedures, large quantities of SiC whiskers can be safely processed. No release of whiskers has been observed from dense ceramic matrix composites during fracture or wear processes. Material Safety Data Sheets (MSDS) are packaged with all products containing loose whiskers and the directions in the data sheets should be followed. At the present time, the American Society for Testing of Materials (ASTM) has developed procedures and handling practice standards for SiC whiskers [14]. 

Standard Practice for Handling Silicon Carbide Whiskers, ASTM Designation E 1437-91 (Philadelphia ASTM 1991). 

PTH Hole Fill Exposed Copper after Soldering Post-Assembly Corrosion of Exposed Surfaces Handling Defects Solder Bridging Plugged Holes Equipment Safety Paste misprints Black-Pad, Black-Line Nickel Brittle Fracture Solder Mask Attack RF Signal Loss Post-Assembly Corrosion of Exposed Surfaces Tin Whiskers Ionic cleaning Failures Solder Mask Attack Post-Assembly Corrosion of Exposed Surfaces Post-Assembly Corrosion of Exposed Surfaces Tarnish Solderjoint Microvoids Electrochemical Migration... 

It should be obvious that care should be exercised when handling or working with whiskers. Too little is known about most whiskers to predict their long-range impact on health. There is small doubt they should be handled with care, because most are very small and extremely refractory. 

Other elements of the feed system. Further, steel whiskers on the tire chips can cause significant bridging in material handling systems -bridging that is difficult to break up due to the interweaving of fuel particles. 

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