Vapor-Phase Fundamentals

A discussion of vapor-phase fundamentals begins with the basic gas laws, which apply to any vapor-phase deposition technique. These techniques employ gases at low pressure (less than 1 atm) and therefore are well described by basic laws such as the ideal gas law and the kinetic gas theory, which are presented in undergraduate physical chemistry. For the purposes of vapor deposition, the critical gas parameters include (1) concentration, (2) velocity distribution, (3) flux, and (4) mean free path. The concentration of gas particles in a low-pressure gas, less than 1 atm, is given by the ideal gas law, 

Inorganic Materials Synthesis and Fabrication, By John N. Lalena, David. A. Cleary, Everett E. Carpenter, and Nancy F. Dean Copyright 2008 John Wiley Sons, Inc. 

Gas particles, at a given temperature and pressure, do not all have the same velocities. Instead, the velocities are described by the famous Maxwell velocity distribution, g(vx), 

The mean free path of a gas molecule is the average distance it will travel before colliding with another gas-phase molecule. Conceptually, this will be equal to the velocity divided by the frequency of collisions. The frequency of collisions between like molecules, Z, is given as 

Having defined the mean free path, we next define the Knudsen number, Kn. The Knudsen number is defined as the ratio of the mean free path to a characteristic distance  

---

The combined translational, rotational and vibrational contributions to the molar heat capacity, heat content, free energy and entropy for 1,3,4-thiadiazoles are available between 50 and 2000 K. They are derived from the principal moments of inertia and the vapor-phase fundamental vibration frequencies (68SA(A)36l). 

Typical experimental values of HETP for a random packing such as 50-mm PaH rings, and a stmctured packing, such as Intalox 2T of Norton Co., under the same system conditions, are shown in Figure 25. Many designers of packed columns prefer the use of HETP instead of but the latter is more fundamental and discrirninates between Hquid- and vapor-phase resistances. It should be noted that terms such as H and N are based on... 

Grolmes, M. A. and Fauske, H. K., An Evaluation of Incomplete Vapor-Phase Separation in Freon-12 Top Vented Depre-ssurization Experiments, Multi-Phase Flow and Heat Transfer III. Part A Fundamentals. Proceedings of the Third Multi-Phase Flotv and Heat Transfer Symposium-Workshop, Miami Beach, FL, April 18-20, 1983. 

A fundamental functional property of a neutralizing amine (vapor-phase amine) is its volatility. Derived from this function is relative volatility and the DR. 

The fundamental idea of this procedure is as follows For a system of two fluid phases containing N components, we are concerned with N — 1 independent mole fractions in each phase, as well as with two other intensive variables, temperature T and total pressure P. Let us suppose that the two phases (vapor and liquid) are at equilibrium, and that we are given the total pressure P and the mole fractions of the liquid phase x, x2,. .., xN. We wish to find the equilibrium temperature T and the mole fractions of the vapor phase yu y2,. .., yN-i- The total number of unknowns is N + 2 there are N — 1 unknown mole fractions, one unknown temperature, and two unknown densities corresponding to the two limits of integration in Eq. (6), one for the liquid phase and the other for the vapor phase. To solve for these N +2 unknowns, we require N + 2 equations of equilibrium. For each component i we have an equation of the form... 

Chemical vapor deposition is a synthesis process in which the chemical constituents react in the vapor phase near or on a heated substrate to form a solid deposit. The CVD technology combines several scientific and engineering disciplines including thermodynamics, plasma physics, kinetics, fluid dynamics, and of course chemistry. In this chapter, the fundamental aspects of these disciplines and their relationship will be examined as they relate to CVD. 

Korbach and Stewart [Ind. Eng. Chem. Fundamentals, 3 (24), 1964] have studied the vapor phase hydrogenation of benzene in a batch recycle reactor. 

The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid (usually expressed as amount per unit mass or per unit surface area of solid) and the thermodynamic quantity, water activity (aw), at constant temperature and pressure. At equilibrium the chemical potential of water sorbed to the solid must equal the chemical potential of water in the vapor phase. Water activity in the vapor phase is related to chemical potential by... 

The six fundamental vibrational frequencies for SeF and TeFe are given in Table XII (21,37,38,103). Force constants for SeFs, calculated with the frequencies from vapor-phase Raman spectra (21) and using isotope shifts and Coriolis coupling constants as additional data (103), are listed in Table XIII in comparison to TeFg (1,24,104,125,139). 

Volatilization (also referred to as vaporization or evaporation) is the conversion of a chemical from the sohd or hquid phase to a gas or vapor phase. The partitioning of a volatile compound in the subsurface environment comprises two distinct patterns volatilization of contaminant molecules (from the liquid, sohd, or adsorbed phase) and dispersion of the resulting vapors in the subsurface gas phase or the overlying atmosphere by diffusive and turbulent mixing. Even though the two processes are fundamentally different and controlled by different chemical and environmental factors, they are not wholly independent under natural conditions only by integrating their effects can volatilization be characterized. 

Rather exotic methods like vapor phase, multicomponent, and template syntheses are considered to be important in cases where the other ways failed. Generally, the main purpose of their application was to prove fundamental principles. 

Stringfellow GB (2001) Fundamental aspects of organometallic vapor phase epitaxy. Materials Science and Engineering B-Solid State Materials for Advanced Technology 87(2), 97-116... 

There are numerous materials, both metallic and ceramic, that are produced via CVD processes, including some exciting new applications such as CVD diamond, but they all involve deposition on some substrate, making them fundamentally composite materials. There are equally numerous modifications to the basic CVD processes, leading to such exotic-sounding processes as vapor-phase epitaxy (VPE), atomic-layer epitaxy (ALE), chemical-beam epitaxy (CBE), plasma-enhanced CVD (PECVD), laser-assisted CVD (LACVD), and metal-organic compound CVD (MOCVD). We will discuss the specifics of CVD processing equipment and more CVD materials in Chapter 7. 

A.1 Fundamentals of Vapor Phase Synthesis. In this section we will concentrate on the vapor phase synthesis of some structural ceramics, such as carbides and nitrides. The principles described here apply equally well to the production of oxide ceramics, but we reserve some of this description for later sections, particularly with respect to the formation of optical fibers. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

The fundamental cause of boiling-point elevation and freezing-point depression in solutions is the same as the cause of vapor-pressure lowering (Section 11.6) the entropy difference between the pure solvent and the solvent in a solution. Let s take boiling-point elevations first. We know that liquid and vapor phases are in equilibrium at the boiling point (Tb) and that the free-energy difference between the two phases (AGvap) is therefore zero (Section 8.14). 

Physical property data for many of the key components used in the simulation for the ethanol-from-lignocellulose process are not available in the standard ASPEN-Plus property databases (11). Indeed, many of the properties necessary to successfully simulate this process are not available in the standard biomass literature. The physical properties required by ASPEN-Plus are calculated from fundamental properties such as liquid, vapor, and solid enthalpies and density. In general, because of the need to distill ethanol and to handle dissolved gases, the standard nonrandom two-liquid (NRTL) or renon route is used. This route, which includes the NRTL liquid activity coefficient model, Henry s law for the dissolved gases, and Redlich-Kwong-Soave equation of state for the vapor phase, is used to calculate properties for components in the liquid and vapor phases. It also uses the ideal gas at 25°C as the standard reference state, thus requiring the heat of formation at these conditions. 

---