Thermodynamic deposition

Deposition in the thoracic region is the sum of aerodynamic and thermodynamic deposition of particulate material. Aerodynamic deposition depends on aerodynamic particle size, total volumetric flow rate, anatomical dead space, tidal volume, functional residual capacity (FRC) (combined residual and expiratory reserve volume or the amount of air remaining in the lungs after a tidal expiration) and diameter of the airways. Thermodynamic deposition depends on anatomical and physical characteristics, such as tidal volume, anatomical dead space, functional residual capacity and the transit time of air within each region. Thermodynamic particle size, which is derived from the diffusion coefficient, particle shape factor and the particles mass density, influence thermodynamic deposition. 

Pocza J F, Barna A and Barna P B 1969 Formation processes of vacuum deposited indium films and thermodynamical properties of submicroscopic particles observed by in situ electron microscopy J. Vac. Sc/. Techno . 6 472... 

Aqueous potassium permanganate solutions are not perfectiy thermodynamically stable at 25°C, because Mn02, not MnO is the thermodynamically stable form of manganese in water. Thus permanganate tends to oxidize water with the evolution of oxygen and the deposition of manganese dioxide, which acts to further catalyze the reaction. 

If the gas has the correct composition, the carbon content at the surface increases to the saturation value, ie, the solubiUty limit of carbon in austenite (Fig. 2), which is a function of temperature. Continued addition of carbon to the surface increases the carbon content curve. The surface content is maintained at this saturation value (9) (Fig. 5). The gas carburizing process is controlled by three factors (/) the thermodynamics of the gas reactions which determine the equiUbrium carbon content at the surface (2) the kinetics of the chemical reactions which deposit the carbon and (J) the diffusion of carbon into the austenite. 

In the attempt at diamond synthesis (4), much unsuccesshil effort was devoted to processes that deposited carbon at low, graphite-stable pressures. Many chemical reactions Hberating free carbon were studied at pressures then available. New high pressure apparatus was painstakingly buHt, tested, analy2ed, rebuilt, and sometimes discarded. It was generally beheved that diamond would be more likely to form at thermodynamically stable pressures. 

Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electro deposition, and other phenomena in aqueous solutions.Erom the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

The deposition of metals directly from these halides would require high temperatures to be efficient, but because of the thermodynamic stabilities of the hydrogen halides, direct reduction can readily be carried out with hydrogen at lower temperamres. The general reaction... 

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

In the electrochemical series of elements, copper is near the noble end and will not normally displace hydrogen, even from acid solutions. Indeed, if hydrogen is bubbled through a solution of copper salts, copper is slowly deposited (more rapidly if the process is carried out under pressure). (See Section 1.2 for thermodynamic considerations.)... 

Free-energy-concentration diagrams have been used in the study of the thermodynamic influence on the non-stoichiometry of the solid titanium carbide deposited from H2-CH4-TiCl4 gas mixtures at 1 900 K. The authors show how, from the partial pressure measurements of Ti vapour over a range of... 

A simple calculation based on the solubility product of ferrous hydroxide and assuming an interfacial pH of 9 (due to the alkalisation of the cathodic surface by reaction ) shows that, according to the Nernst equation, at -0-85 V (vs. CU/CUSO4) the ferrous ion concentration then present is sufficient to permit deposition hydroxide ion. It appears that the ferrous hydroxide formed may be protective and that the practical protection potential ( —0-85 V), as opposed to the theoretical protection potential (E, = -0-93 V), is governed by the thermodynamics of precipitation and not those of dissolution. 

Ramsdellite is thermodynamically unstable toward a transformation into the stable ft -modification. Hence, it is rarely found in natural deposits. Natural ramsdellite has a stoichiometry close to the composition of Mn02 and can be considered another true modification of manganese dioxide. Attempts to synthesize ramsdellite in the laboratory usually lead to materials of questionable composition and structural classification. It is very likely that synthetic ramsdellite materials are more or less well-crystallized samples of the y-modification that will be described in more detail below. 

Lithium metal is chemically very active and reacts thermodynamically with any organic electrolyte. However, in practice, lithium metal can be dissolved and deposited electrochemically in some organic electrolytes [5]. It is generally believed that a protective film is formed on the lithium anode which prevents further reaction [6, 7]. This film strongly affects the lithium cycling efficiency. 

In the region of pure CH4, the equilibrium is governed by Equation 4. For this reaction, the equilibrium constant increases with temperature so that at high enough temperatures there will be appreciable dissociation of CH4 to H2 and graphite. In the lower temperature range considered here, the thermodynamic equilibrium indicates only a very small amount of dissociation so the intersection of the graphite deposition curve with the H2-CH4 line occurs at almost pure CH4. As the temperature increases, the point of intersection will move toward pure H2 on the H2-CH4 line. 

The general theory was worked out by Roozeboom (Zeitschr. physik. Chem., 1899) from the standpoint of the theory of thermodynamic potential. The equations (2a, h), (3a, h) of the preceding section apply equally well to the present case, and details need not be given here. The liquid solidifies at a constant temperature when it has the same composition as the solid deposited— the so-called eutectic point. 

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. 

Chemical thermodynamics is concerned with the interrelation of various forms of energy and the transfer of energy from one chemical system to another in accordance with the first and second laws of thermodynamics. In the case of CVD, this transfer occurs when the gaseous compounds, introduced in the deposition chamber, react to form the solid deposit and by-products gases. 

In many cases, a more complete understanding of CVD reactions and a better prediction of the results are needed and a more thorough thermodynamic and kinetic investigation is necessary. This is accomplished by the calculation of the thermodynamic equilibrium of a CVD system, which will provide useful information on the characteristics and behavior of the reaction, including the optimum range of deposition conditions. 

As shown above, a thermodynamic analysis indicates what to expect from the reactants as they reach the deposition surface at a given temperature. The question now is, how do these reactants reach that deposition surface In other words, what is the mass-transport mechanism The answer to this question is important since the phenomena involved determines the reaction rate and the design and optimization of the CVD reactor. 

In the previous sections, it was shown how thermodynamic and kinetic considerations govern a CVD reaction. In this section, the nature of the deposit, i.e., its microstructure and how it is controlled by the deposition conditions, is examined. 

---