Whiskers from organic solvents

Until 1995, micrometer scale whiskers of the lll-IV semiconductors have been grown only by metal-catalyzed or metal-organic CVD processes at relatively high temperatures, e.g., SiSOO C. Lack of useful crystallization mechanisms for highly covalent non-molecular solids has so far prevented their growth at lower process temperatures. A recent breakthrough [18] 

The process is based on a novel reaction using the otherwise well-studied metal-organic CVD process under conditions which support low temperature growth, i.e., catalysis of the reaction by protic hydrocarbon solvents and catalysis of whisker growth by metallic indium flux particles (droplets). The flux particles, in turn were produced in-situ during the reaction as a partial decomposition product of t-BusIn (Equation 13). 

Amorphous and polycrystalline fibers and near-single crystal whiskers were obtained having diameters of 10-150 nanometers and lengths of several micrometers. Growth of whiskers and short fibers by this method is believed to proceed by a solution-liquid-solid (SLS) phase transformation, suggesting that similar synthesis routes may now also become available for other covalent short fibers and perhaps whiskers. 

Abbreviations (Table V Equation 13) t-Bu, tert-butyl Me, methyl Ph, phenyl Et, ethyl. 

The reaction between tertiary butyl indium and PHs, yielding InP fibers, is catalyzed by liquid indium metal particles, which are produced in-situ as a partial decomposition product of t-butyl indium. Since indium particles were found in the tips of InP fibers, it seems that liquid indium droplets perform the same function in this SLS process as liquid gold or other molten metal droplets in a VLS process. 

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Cellulose whiskers without surface modification display strong interactions and have been reported as difficult to disperse in water. Sulfate-functionalized tunicate whiskers were prepared by sulfuric acid hydrolysis of cellulose pulp derived from tunicates (StyelacldVa). These high aspect ratio nanoparticles formed colloidal suspensions in water (Figure 14.4). A stable suspension of tunicin whiskers was obtained in an organic solvent (Af,Af-dimethylformamide) without surfactant addition or chemical surface modification. Both the high value of the dielectric constant of DMF and the medium wettability of tunicin whiskers were supposed to control the stability of the suspension (Figure 14.5). 

Organic fillers such as cellulose whiskers still suffer from poor thermal stabihty, which inhibit their use in many thermoplastics whose processing temperatures are close or above their degradation temperature. These whiskers are still produced in aqueous dispersions, so even by applying solvent exchange and lyophili2ation, the volume of nanowhiskers produced is well below commercial viabihty. Optimized separation methods must be developed and scaled up. 

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