Vapor-Phase Chemical Reactions

High-quality powders of refractory carbides and nitrides can be obtained by the more recent processing techniques of chemical-vapor deposition (CVD), RF-plasma torch, combustion synthesis, and sol-gel.l H l 

The CVD process is reviewed in more detail in Ch. 15. In a CVD reaction, if the temperature and supersaturation are sufficiently high, the product is primarily powder precipitated from the gas phase.1 1 Such powders have few impurities, small diameter, and great uniformity. In addition, the temperatures required to sinter CVD powders are generally lower than those for conventional powders. 

How much of an impact CVD carbide and nitride powders will have is not clear at this time since applications have yet to reach the commercial stage in any significant manner. Still, the success of a number of development programs strongly suggests that CVD may soon become a major powder-production technology. This is already evident in Japan which is a recognized leader, particularly in refractory nitrides. 

---

Effect of molecular diffusion and vapor-phase chemical reactions Liquid metal vapors consist of molecules and gaseous atoms. Working with alkali metals, Ewing et al. (1967) found that the molecules are principally dimers and tetramers. The... 

The rates of homogeneous vapor phase chemical reactions are usually faster than heterogeneous chemical reactions of particulate PAHs with sunlight and oxidants in the atmosphere, particularly due to light shielding and stabilizing (toward both oxidation and photolysis) effects in the adsorbed state (Behymer and Hites 1988). 

Many known self-assembly processes - quenching, solidification, crystallization, solution- and vapor-phase chemical reactions, and polymerization - occur in specific solution environments. For a self-assembly processes in an aqueous environment - as for most biological processes - a mix of appropriate conditions is required, for example, a narrow temperature range or reagent concentrations. It is therefore likely that environmental and experimental conditions will play an important role in synthesizing new nanomaterials and nanodevices with desired properties and functionalities. 

Vapor phase chemical reactions can be classified under various categories. The simplest is the thermal decomposition of a precursor which is often a chloride... 

Hua et al. (1995) proposed a supercritical water region in addition to two reaction regions such as the gas phase in the center of a collapsing cavitation bubble and a thin shell of superheated liquid surrounding the vapor phase. Chemical transformations are initiated predominantly by pyrolysis at the bubble interface or in the gas phase and attack by hydroxyl radicals generated from the decomposition of water. Depending on its physical properties, a molecule can simultaneously or sequentially react in both the gas and interfacial liquid regions. 

Many types of reactions exist. This results in chemical reactors with a wide variety of configurations, operating conditions, and sizes. We encounter reactions that occur in solely the liquid or the vapor phase. Many reactions require catalysts (homogeneous if... 

Ion mobility spectrometry (IMS) is an instrumental method where sample vapors are ionized and gaseous ions derived from a sample are characterized for speed of movement as a swarm in an electric field [1], The steps for both ion formation and ion characterization occur in most analytical mobility spectrometers at ambient pressure in a purified air atmosphere, and one attraction of this method is the simplicity of instrumentation without vacuum systems as found in mass spectrometers. Another attraction with this method is the chemical information gleaned from an IMS measurement including quantitative information, often with low limits of detection [2 1], and structural information or classification by chemical family [5,6], Much of the value with a mobility spectrometer is the selectivity of response that is associated with gas-phase chemical reactions in air at ambient pressure where substance can be preferentially ionized and detected while matrix interferences can be eliminated or suppressed. In 2004, over 20000 IMS-based analyzers such as those shown in Fig. 1 are placed at airports and other sensitive locations worldwide as commercially available instruments for the determination of explosives at trace concentration [7],... 

A quantitative understanding of certain primary combustion phenomena, e.g., liquid fuel-droplet vaporization and burning, gas phase chemical reaction kinetics, radiation heat transfer from combustion products, and mixing of reactants and combustion products, is required to develop a rational approach for the effective utilization of synfuels in industrial boiler/furnace systems. Those processes are defined by the interaction of a number of mechanisms which are conveniently described in terms of physical and chemical related processes. The physical processes are ... 

Since we are not concerned presently with ignition and extinction phenomena (23,24, 25,26,27) caused by slow chemical heat release rates, the gas-phase chemical reactions can be assumed to occur at rates much faster than the gas-phase heat and mass transfer rates. This implies that the gas-phase consists of two (one in the case of pure vaporization) convective-diffusive regions separated by a fiame of infinitesimal thickness, at which the outwardly diffusing fuel vapor reacts stoichiometrically and completely with the inwardly diffusing oxidizer gas. 

In the chemical vapor deposition (CVD) process, heat is supplied through resistive heating, infrared heating, laser beam or plasma to effect a gas-phase chemical reaction involving a metal complex. The metal produced from the reaction deposits by nucleation and growth on the hot substrate which is placed in the CVD reactor. Effective reactants... 

There are five unknowns in these equations the extent of reaction 2f and the number of moles of each species in the vapor phase. There are also five equations to be solved the four phase equilibrium relations (Eqs. 7, 8, 9, and 11) and one vapor-phase chemical equilibrium relation (Eq. 12), It is possible, with some difficulty, to solve these equations for the five unknowns. 

Measurements of the urban aerosol mass distribution have shown that two distinct modes often exist in the 0.1 to 1.0 pm diameter range (Hering and Friedlander 1982 McMurry and Wilson 1983 Wall et al. 1988 John et al. 1990). These are referred to as the condensation mode (approximate aerodynamic diameter 0.2 pm) and the droplet mode (aerodynamic diameter around 0.7 pm). These two submicrometer mass distribution modes have also been observed in nonurban continental locations (McMurry and Wilson 1983 Hobbs et al. 1985 Radke et al. 1989). Hering and Friedlander (1982) and John et al. (1990) proposed that the larger mode could be the result of aqueous-phase chemical reactions. Meng and Seinfeld (1994) showed that growth of condensation mode particles by accretion of water vapor or by gas-phase or aerosol-phase sulfate production cannot explain existence of the droplet mode. Activation of condensation mode particles, formation of cloud/fog drops, followed by aqueous-phase chemistry, and droplet evaporation were shown to be a plausible mechanism for formation of the aerosol droplet mode. 

Chemical Equilibrium Applied. The major use of the principles of this chapter is to calculate the equilibrium composition of a system. This application in vapor-phase hoihogeneous reactions has been discussed above. 

The dimensionless time has once more been defined relative to the characteristic time for vapor deposition, o is the ratio of the mass transfer rate to the gas-phase deposition rate, oj is the ratio of the initial aerosol concentration of AB to the corresponding concentration of the rest of the aerosol species, o is the ratio of the deposition velocities of the two gas-phase species, 04 is the ratio of the aerosol deposition velocity to the deposition velocity of A(g), o is the ratio of the emission (or gas-phase chemical reactions) of A to its initial deposition rate, 6 is the ratio of the emission rates of A and B, (77 is the ratio of the initial gas-phase concentrations of A and B, and finally o is the ratio of the initial concentrations of gas species A and aerosol AB. 

To calculate residue curve maps for the synthesis of TAME one has to proceed in the same manner as the MTBE example and calculate phase equilibria bet veen liquid and vapor phases, chemical equilibrium constants in the liquid phase, and kinetics of the chemical reactions. 

The process involves a gas-phase chemical reaction in which a solid material is deposited on the substrate. Nanostructure ceramics and composites are the most common types of nanomaterials produced by CVD, which uses carrier gas vapors on the substrate to keep the metal ion at the zero valent state and then depositing it on a hot-wall reactor to form a solid material on cooling. It is a relatively slow process with a good control over the chemical composition and size, along with a large area of flexibility and relatively low reproducibility. 

The character of the association is that of a chemical reaction in the vapor phase. Each reaction... 

Temperature strongly influences the yield distribution and the physical/chemical properties of the products such as carbon content, pH, viscosity, and oxygen content of the bio-oil. The addition of catalysts to the reaction system has been shown to lower the reaction temperature (Mertinkat et al., 1999) and as the thermal degradation reactions of biomass and waste material during pyrolysis are endothermic this can lead to reduced energy demand and operating costs. Reaction temperature is a critical parameter that affects vapor phase catalyzed reactions (Morris et al., 2011). 

Once a stable particle is formed, it can grow or shrink owing to mass transfer processes between the gas and the particle phase. These processes are governed mainly by the actual particle size, by the ratio of mean free path and particle diameter (Knudsen number), by the molecular diffusion coefficient, and most importantly, by the difference between the gas phase and the particle surface equilibrium vapor pressures of the transferred chemical species. Vapor pressures in the gas phase that are higher than the equilibrium vapor pressure at the particle surface result in a net mass flux toward the particle surface (i.e., the particle gains mass and growth takes place). Gas-phase vapor pressures that are lower than the equilibrium vapor pressure at the particle surface cause a net mass flux directed away from the particle (i.e., the particle loses mass and shrinks). The most important mechanisms that influence the equilibrium vapor pressure at the particle surface are the Kelvin effect, the effect of nonvolatile solute, aqueous-phase chemical reactions, and latent heat release. 

Absorption, Dissociation, and Aqueous-Phase Chemical Reactions The diffusive penetration of gases or gas mixtures into a condensed phase (e g., droplet) is called absorption. In equilibrium, the absorbed gas is dissolved at a certain concentration inside the droplet and the equilibrium vapor pressure over the droplet surface is proportional to the concentration at the droplet surface (Henry s law). The concentration inside the droplet itself can be influenced by dissociation or chemical reactions (sulfur production by oxidation of dissolved SO2 to SOt ). If these processes represent a sink for the solute, the concentration inside the droplet and, consequently, the vapor pressure at the droplet surface is decreased (i.e., mass transfer is enhanced). Typical gases that dissolve into atmospheric water droplets are CO2, SO2, NH3, H2O2, and O3. 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

---