Whisker growth

Lackey, W., Hanigofsky, J., and Freeman, G., Experimental Whisker Growth and Thermodynamic Study of the Hafnium-Carbon System for Chemical Vapor Deposition Applications, 7] Amer. Ceram. Soc., 73(6) 1593-98 (1990)... 

Futamoto, M., Yuito, I, andKawabe, U., Hafnium Carbide and Nitride Whisker Growth by Chemical Vapor Deposition, /. Cryst. Growth, 61(l) 69-74 (Jan./Feb. 1983)... 

The phenomenon of pseudopolymorphism is also observed, i.e., compounds can crystallize with one or more molecules of solvent in the crystal lattice. Conversion from solvated to nonsolvated, or hydrate to anhydrous, and vice versa, can lead to changes in solid-state properties. For example, a moisture-mediated phase transformation of carbamazepine to the dihydrate has been reported to be responsible for whisker growth on the surface of tablets. The effect can be retarded by the inclusion of Polyoxamer 184 in the tablet formulation [61]. 

Fig. 2.23 The VLS mechanism of unidirectional growth of a fiber. Vapor condenses on a substrate and continues to localize at the site, providing the necessary material for whisker growth.

Unfortunately, it has not been possible to suppress or control the whisker formation, to date. The whisker formation clearly results from the low thermodynamic stabihty of the adducts in the gas phase toward dissociation, which is the initial step of the whisker formation. Temperature-dependant deposition studies clearly showed an influence of the substrate temperature on the whisker growth at lower temperature the growth of single crystallites is preferred, whereas at higher temperatures, uncontrollable whisker growth takes place as can be seen from Fig. 10. 

Whisker growth of hexagonal /3-HfP, cubic o -ZrP, and hexagonal jS-TiP is possible via a nuxed-metal, impurity-activated chenucal vapor deposition (CVD) process from HfCLi/ZrCLi/TiCLi + PCI3 -L H2 -E Ar gas mixtures at 1050 °C. Si-EPd and Si-EPt were used as nuxed-metal impurity components. Such whiskers are stable in concentrated HCl. ... 

Active centers of whisker growth are thought to arise during reaction at relatively low temperatures [3,4]. This factor especially influences the dynamics of NiC CF studied during linear cooling. 

The reaction to form silicon carbide whiskers, as described in a later section, will occur at or near the surface of carbon-containing materials. The dependence of the reaction and its kinetics on the concentration of carbon is critical. Figure 4 depicts the phase relationship of silicon and carbon at very dilute concentrations of carbon, which favor whisker growth [126]. The solubility of the carbon in silicon is greatly affected by the concentration of carbon present. For example, the solubility of carbon at 2000°C is estimated to be about 10 wt %. 

Fig. 11. Illustration of the VLS process for SiC whisker growth. After Shalek and Parkinson [110]. Reproduced with permission of Materials Research Society, Pittsburgh.

Fig. 12. Identification chart for SiC whisker growth conditions. After Milewski et al. [79]. Reproduced with permission of Chapman Hall, London.

The mechanisms of whisker growth vary depending on the substrate used for the reaction. For example, the Si3N4 whiskers grown on mullite show axial defects, characteristic of crystallization by an axial-screw-dislo-cation mechanism [29], In the presence of a catalyst, whisker growth occurs by means of a VLS mechanism, involving crystallization of the nitride from drops of binary or ternary alloys of silicon with iron and aluminum. The iron and aluminum act as solvents for silicon. 

In addition, the vapour-solid (VS) mechanism was proposed to explain whisker growth for some materials. It should be pointed out that the morphology of the crystal growth has a very complex relationship with the catalyst and the processing conditions. 

In early VLS whisker growth studies, there was no attempt to control the liquid alloy droplet size. Consequently, whisker diameters were large, ranging from 100 nm to 0.2 mm. The smallest whisker diameters that could be produced were limited by the minimum stable liquid droplet diameter ( 100 nm). In 2000, researchers began to apply size-selected nanometer-size colloidal metal particles as seeds to promote semiconductor nanowire growth. 

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