Chemical vapour transport and condensation

An Au-catalysed chemical vapour transport and condensation (CVTC) process was used to produce ZnO nanorods and nanowires on Si02 and Si substrates [58], ZnO nanorods with a wide band gap (3.37eV) are regarded as promising candidates for the fabrication of nanoelectronic devices. In this work, EDX spectra of the tip and the body of ZnO nanorods were captured which indicates that Au-Zn alloyed droplets were present at the tips of the fabricated nanorods pointing to a nanorod growth via a vapor-liquid-solid (VLS) mechanism. 

Many compounds and salts are sensitive to the presence of water vapour or moisture. When compounds interact with moisture, they retain the water by either bulk or surface adsorption, capillary condensation, chemical reaction and, in extreme cases, a solution (deliquescence). Deliquescence is where a solid dissolves and saturates a thin film of water on its surface. It has been shown that when moisture is absorbed to the extent that deliquescence takes place at a certain critical relative humidity, the liquid film surrounding the solid is saturated. This process is dictated by vapour diffusion and heat transport rates (Kontny et al. 1987). 

The formation of amorphous volatile products during carbothermal reduction processes can cause problems by condensing in the cooler parts of the furnace and altering the composition of the original mixture. Al NMR shows that these amorphous phases all contain Al solely in tetrahedral coordination, with chemical shifts consistent with the potassium feldspars microcline and sanidine (54-58 ppm). Thermodynamic analysis confirmed the feasibility of vapour-phase transport of both Al and alkali metal impurities from the clay which can then condense to form short-range feldspar-like units (Ekstrom et al. 1996). 

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