Whiskers-special shape

The modified diimide decomposition process uses gas mixtures of Si2Cl2, NH3, and H2. Amorphous whiskers are deposited on quartz or graphite substrates coated with a metal catalyst. The uniqueness of this method is the ability to form whiskers of a special shape, e.g., a coil. Whiskers with the best coil regularity have been obtained at 1200°C on a graphite substrate with Fe as the catalyst. All whiskers have a very smooth surface,... 

By controlling growth parameters, whiskers with a special shape can be prepared. Regular arrays of whiskers with conical tips are obtained when the silicon supply from the chemical reaction is stopped. The shaped whisker shown in Figure 15 has a large (2 jm) base diameter and the same sharp tip as that shown in Figure 14. 

The carrier is a monolith with a shape similar to a honeycomb. The mineral cordierite (Mg, Fe)2Al Si50jg is used as the carrier material. Because of its system of chaimels, the carrier has an active surface of 30 000 m. The channels of the monolith are covered by a wash-coat of y-Al Oj. An alternative wash-coat carrier is a special stainless steel with 5% A1 and micro-alloyed with yttrium and cerium. By a carefully controlled heat treatment in an oxidizing environment AljO, whiskers are formed. On the wash-coat a layer of platinum metals is applied, which is responsible for the catalytic function. Platinum with 20% rhodium dominated earlier but nowadays palladium-rich alloys or pure palladium are mainly used. In 2001, the automobile industry accounted for nearly 72% of the total palladium demand. 

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