Structure formation particle synthesis

Besides the already mentioned techniques, a low-temperature plasma has been adopted to enhance the reaction in CVC. Through the synthesis of AIN UFPs by an RF-plasma-enhanced CVC using trimethylaluminum [A1(CH3)3] and NH3 as reactants, the effect of experimental parameters on the rate of powder formation, particle size, and structure was examined (60). A high RF current was primarily connected to a high electron density, which activated the gas-phase reaction to promote the powder formation rate. The increase of both susceptor temperature and A1(CH3)3 concentration also increased the powder formation rate and enhanced the grain growth, where both mechanisms—coalescence by particle collision and vapor deposition on to particle surfaces—were believed to occur. 

As discussed earlier, analysis of temperature profiles obtained by microthermocouple measurements have elucidated the unique conditions associated with the combustion synthesis process. However, this approach does not directly identify the composition or microstructure of the phases formed. It is important to recognize that most published investigations in the field of combustion synthesis only address the final product structure. Considerably less has been reported about the structure formation processes leading to the final product. Most results that describe the evolution from the initial reactants to the final product are inferred by the effects of processing variables (e.g., density, dilution, particle size) on the final microstructure (see Section V). To date, only a few investigations have directly identified initial product structure. As discussed earlier, identification of this structure is important since the initial structure represents the starting point for all subsequent material structure formation processes. Thus, the focus of this section is on the initial stages of the structure formation mechanisms in combustion synthesis and novel methods developed especially for this purpose. 

The morphological pattern of the products of silicon vapor combustion in gaseous nitrogen at condensation synthesis with skeleton crystal formation as well as denchite growth of silicon nitride crystals in melted metal salts proves the existence of the nonequilibrium mechanism of structure formation in the case of SHS. The mechanism appears to be the basis of the conception of nanodispersed particle formation under the combustion mode [28]. 

Studies of the evolution of structure formation of the boron nitride in the SHS wave showed that synthesis is accompanied by a sharp change of the scale of heterogeneity of the medium, even if the maximum process temperature was below the melting point of boron [26]. The initial boron powder consisted of agglomerates (specific surface 16 m g ) of spherical shape particles with a typical size of 0.1 pm (Figure 2.8a). The final product, that is, h-BN, has the morphology of plate crystals with the characteristic size of 20 pm and a thickness of less than 100 nm (Figure 2.8b). 

How do the process and formulation parameters during particle synthesis affect the primary particle formation, aggregate structure, and solid bridge formation ... 

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