Vapor-solid growth

Melt growth Vapor growth Hydrothermal synthesis Solid-state reactions... 

Weisberg M. K., Zolensky M. E., and Prinz M. (1997) Fayalitic olivine in matrix of the Krymka LL3.1 chondrite vapor-solid growth in the solar nebula. Meteorit. Planet. Sci. 32, 791-801. 

Vapor phase growth is commonly used to produce nanowires. Starting with the simple evaporation technique in an appropriate atmosphere to produce elemental or oxide nanowires, vapor-liquid-solid, vapor-solid and other processes are made use of. 

Practical growth of solid material phases, such as by vapor phase metal deposition, electrodeposition, etc., occurs away from the idealized thermodynamic models. 

Figure 1-1. Four regions of vapor reactivity within a CVD reactor. Only location 4 contributes to productive film growth. (1) In the vapor phase, homogenous, in the region of the substrate (2) in the vapor phase, homogenous, not in the region of the substrate (3) at a vapor/solid interface, heterogenous, not on the substrate however, on the reactor wall or other comparable surface (4) at a vapor/solid interface, heterogenous, on the substrate.

Recently Schwetz [169] gave an overview of the different mechanisms forming SiC whiskers. One of the processes technically used for SiC whisker production has been mentioned before in Sect. 4.1.2, the pyrolysis of rice husks. This process counts among the type VS (Vapor-Solid mechanism) since the dominating reaction responsible for the whisker growth is the second step of the reaction sequence ... 

Nanowires are seen as a solution with which to improve the sensitivity, selectivity, stability and response time of metal-oxide gas sensors. Meier et al (2007) grew Sn02 nanowires of 100 nm in diameter by the vapor-solid growth method. For testing, they were deposited onto micromachined hotplates with a focused ion beam scanning electron microscope (FIB-SEM), as shown in Fig. 6.19. Due to their diameter being similar to the Debye length, a completely depleted conduction channel can be obtained. Maximum response to CO and NH3 occurred at about 260°C. 

Figure 4.7 illustrates the VLS whisker growth sequence. At 1400 °C, the soUd catalyst melts and forms the liquid catalyst ball. Carbon and silicon from the gas phase are dissolved in the liquid alloy, which soon becomes supersaturated such that solid SiC precipitates at the interface with the carbon substrate. Continued dissolution of the gas species into the liquid catalyst allows the whisker to grow, lifting the catalyst ball from the substrate as additional SiC precipitates. These VLS whiskers are typically larger in diameter (4—6 pm) than those formed by the vapor solid process (VS-whiskers) in fact, they frequently grow to a length of tens of millimeters. 

Almost all short fibers, which are derived from the vapor phase, grow by a vapor-liquid-solid (VLS) phase transformation, including single crystal silicon whiskers and carbon nanotubes. Only rarely does the growth of short fibers or whiskers occur by a vapor-solid (VS) phase transformation, and the evidence for this type of phase transformations is often difficult to obtain experimentally. 

The highest internal order, that of a single crystal, is obtained when the tip temperature is above the liquidus of the substrate, yields a liquid tip, and proceeds by a vapor-liquid-solid phase transformation. The lowest internal order, that of an amorphous structure, is obtained when the tip temperature is below the glass transition temperature of the substrate. A solid tip yields a vapor solid phase transformation. Between the liquidus and the glass transition temperature of a substrate, intermediate internal order is that of a polycrystalline fiber. In this case, whisker growth is either governed by a VLS and/or by a VS phase transformation. 

Vapor-solid growth A direct vapor to solid phase transformation, responsible for the catalyzed growth of some fibers. 

Figure 8.14 Mechanism of fiber growth. (1) Solid catalyst particle. (2) Short filament having grown on a solid particle. (3) Short filament on the liquid particle. (4) Rapid lengthening. (5) Fiber. Source Reprinted with permission from Gadelle P, The growth of vapor-deposited carbon fibres, Cartoon Fibers Filaments and Composites, Figueiredo JL, Bernardo CA, Baker RTK, Hiittinger KJ eds., Kluwer, Dordrecht, 115, 1990. Copyright 1990, Springer.

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