Kinetic Processes in Composites

Due to the fact that industrial composites are made up of combinations of metals, polymers, and ceramics, the kinetic processes involved in the formation, transformation, and degradation of composites are often the same as those of the individual components. Most of the processes we have described to this point have involved condensed phases—liquids or solids—but there are two gas-phase processes, widely utilized for composite formation, that require some individualized attention. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) involve the reaction of gas phase species with a solid substrate to form a heterogeneous, solid-phase composite. Because this discussion must necessarily involve some of the concepts of transport phenomena, namely diffusion, you may wish to refresh your memory from your transport course, or refer to the specific topics in Chapter 4 as they come up in the course of this description. 

Chemical vapor deposition (CVD) involves the reaction of gaseous reactants to form solid products. There are two general types of CVD processes (a) the thermal decomposition of a homogeneous gas to form a solid and (b) the chemical reaction of two or more gaseous species to form a solid. Both types of CVD reactions are used industrially to form a variety of important elements and compounds for semiconductor, superconductor, and ceramic coating applications. 

An example of the first kind can be found in the decomposition of trichloromethyl-silane to give silicon carbide and hydrogen chloride  

The reaction of titanium tetrachloride with boron trichloride and hydrogen to give solid titanium diboride is an example of the second type of CVD reaction  

Reaction (3.106) illustrates the primary advantage of a CVD process. Solid titanium diboride, TiB2, melts at 3325°C, yet it can be produced via reaction (3.106) at 1027°C or lower on a suitable substrate by the CVD process. Additionally, the diffusion of gas 

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Giggenbach W.F (1997) Relative importance of thermodynamic and kinetic processes in governing the chemical and isotopic composition of carbon gases in high-heatflow sedimentary basins. Geochim. Cosmochim. Acta 61, 3763— 85. 

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

In this chapter, we restrict discussion to approaches based on conventional radical polymerization. Living polymerization processes offer greater scope for controlling polymerization kinetics and the composition and architecture of the resultant polymer. These processes are discussed in Chapter 9. 

CVI is a special CVD process in which the gaseous reactants penetrate (or infiltrate) a porous structure which acts as a substrate and which can be an inorganic open foam or a fibrous mat or weave. The deposition occurs on the fiber (or the foam) and the structure isgradually densified to form a composite.The chemistry and thermodynamics of CVT are essentially the same as CVD but the kinetics is different, since the reactants have to diffuse inward through the porous structure and the by-products have to diffuse out.f l Thus, maximum penetration and degree of densification are attained in the kinetically limited low-temperature regime. 

The desired product is P, while S is an unwanted by-product. The reaction is carried out in a solution for which the physical properties are independent of temperature and composition. Both reactions are of first-order kinetics with the parameters given in Table 5.3-2 the specific heat of the reaction mixture, c, is 4 kJ kg K , and the density, p, is 1000 kg m . The initial concentration of /I is cao = 1 mol litre and the initial temperature is To = 295 K. The coolant temperature is 345 K for the first period of 1 h, and then it is decreased to 295 K for the subsequent period of 0.5 h. Figs. 5.3-13 and 5.3-14 show temperature and conversion curves for the 63 and 6,300 litres batch reactors, which are typical sizes of pilot and full-scale plants. The overall heat-transfer coefficient was assumed to be 500 W m K. The two reactors behaved very different. The yield of P in a large-scale reactor is significantly lower than that in a pilot scale 1.2 mol % and 38.5 mol %, respectively. Because conversions were commensurate in both reactors, the selectivity of the process in the large reactor was also much lower. 

As this volume attests, a wide range of chemistry occurs at interfacial boundaries. Examples range from biological and medicinal interfacial problems, such as the chemistry of anesthesia, to solar energy conversion and electrode processes in batteries, to industrial-scale separations of metal ores across interfaces, to investigations into self-assembled monolayers and Langmuir-Blodgett films for nanoelectronics and nonlinear optical materials. These problems are based not only on structure and composition of the interface but also on kinetic processes that occur at interfaces. As such, there is considerable motivation to explore chemical dynamics at interfaces. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

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