5R7. Nonequilibrium Nondissipative Thermodynamics: With Application to Low-Pressure Diamond Synthesis. Springer Series in Chemical Physics, Vol 68. - Ji-Tao Wang (Dept of Microelectronics, Fudan Univ, Shanghai, 200433, China). Springer-Verlag, Berlin. 2002. 254 pp. ISBN 3-540-42802-X. $89.95.
Reviewed by JD Felske (Dept of Mech and Aero Eng, SUNY at Buffalo, 330 Jarvis Hall, Buffalo NY 14260-4400).
This monograph focuses on the phenomenon of reaction coupling and how, through a non-equilibrium chemical pump effect, a reaction may occur which otherwise would be thermodynamically impossible. In contrast to Prigogine’s consideration of such phenomena in dissipative biological systems, the present text treats a nondissipative, inanimate system-low-pressure CVD growth of diamond films.
The first two chapters introduce the relevant concepts from equilibrium and irreversible thermodynamics. In addition, a special thermodynamic category is defined–nonequilibrium, nondissipative–which applies to complex systems (especially open systems) wherein thermodynamic coupling occurs between simultaneous processes such that in a system of two coupled reactions, one reaction can exhibit a negative rate of entropy production while the other exhibits an equally large positive rate, thereby resulting in a zero overall rate of entropy production. As the author points out, this concept has met with some resistance in the scientific community.
Chapters 3 and 4 detail the interesting history of various attempts to produce diamond in the laboratory. Both empirical/anectdotal as well as scientific efforts are described. The first scientific efforts, based on the equilibrium phase diagram for carbon, are shown to have necessarily required very high pressures. Low-pressure pyrolysis techniques were subsequently investigated and, like the high-pressure approach, are shown to have met with limited success. As the author points out, however, the essential leap forward was made by the Russian group headed by Deryagin, who discovered the activated low-pressure diamond synthesis technique which enabled the production of diamond films at a rate several orders of magnitude faster than by pyrolysis. The theoretical questions regarding the thermodynamics and kinetics of this activated process are shown to have remained an enigma for decades. Several of the models, which were put forth, are discussed, and the shortcomings of each are carefully delineated. Much of the remainder of the text is then dedicated to the author’s resolution of the thermodynamic question. Also put forward is the author’s (reasonable) hypothesis for the atomic/molecular interactions occurring at the graphite and diamond surfaces.
Chapter 5 details the author’s nondissipative reaction coupling model for explaining the phenomenon of activated low-pressure diamond synthesis. This model is shown to be driven by a chemical pump that requires an input of energy. In the process considered (the transformation of graphite to diamond), atomic hydrogen is generated from molecular hydrogen by an input of energy at high temperatures (filament, plasma, microwave,…). Consequently, at the lower substrate temperatures, this atomic hydrogen is at superequilibrium. Such concentrations of atomic hydrogen are highly reactive with the unsaturated bonds of graphite but quite unreactive with the saturated bonds of diamond. The overall coupling process is thereby unidirectional: the hydrogen reacts with the graphite surface producing gaseous hydrocarbons which are then transported to the diamond surface and aid the growth of various diamond facets (ie promotes (111) growth; promotes (100) growth). Upon deposition, the carbon is associated into the diamond lattice and the hydrogen leaves the diamond surface in molecular form.
Thermodynamically, the interaction of the superequilibrium atomic hydrogen with the graphite is considered to produce an activated graphite surface whose free energy becomes greater than the free energy of the diamond surface. Hence, the superequilibrium atomic hydrogen acts as a chemical energy pump, which results in diamond being the favored growth phase. The overall chemical pump reaction is written as a sum of two reactions: where χ is an experimentally determined “pump parameter” which, if large enough, results in the necessary reduction in free energy to enable the overall reaction to occur.
The concept of a phase diagram for this complex system is then presented. Such diagrams, unlike the usual phase diagrams, represent a nonequilibrium system. In the present application, they are also for the special class of stationary nonequilibrium states (ie, time invariant). The basis for their calculation is Prigogine’s principle of minimum entropy production combined with the assumption that the processes is nondissipative. This combination requires a zero rate of entropy production for the overall process or, equivalently, a zero rate of free energy dissipation. The details of computing such phase diagrams are then carefully presented. The nonequilibrium aspect arises from there being two temperatures in the system (filament and substrate). Hydrogen atoms are generated from hydrogen molecules under equilibrium conditions at the filament temperature whereas the graphite and diamond surfaces are at the (lower) substrate temperature. The phase predictions (gas, graphite, diamond, “carbon”) of these nonequilibrium phase diagrams are demonstrated to be quantitatively consistent with data taken under a wide variety of conditions. Such agreement represents a strong endorsement of the nonequilibrium, nondissipative, and chemical pump model.
In chapters six and seven, nonequilibrium phase diagrams are developed for a number of other systems. In Chapter 6, the binary systems (C-H, C-O) are treated. In Chapter seven, several ternary systems (C-H-O, C-H-F, C-H-Cl) are considered. The eighth chapter presents some details regarding other debates associated with the concept of reaction coupling. It also presents a detailed discussion and correction of the “unified barrier” model. Chapter 9 presents some observations concerning a number of different systems: activated CVD of cBN, the Belousov-Zhabotinsky oscillating chemical reactions, Schroedinger’s “negative entropy,” and the similarities of reaction coupling between biological and inanimate systems. The author concludes with his overview of “modern thermodynamics” for which he defines the following divisions (subdivisions): Nondissipative Thermodynamics (equilibrium; nonequilibrium) and Dissipative Thermodynamics (linear; nonlinear).
I found Nonequilibrium Nondissipative Thermodynamics: With Applications to Low-Pressure Diamond Synthesis to be well written (although a bit repetitive). The historical aspects of the successes and failures to produce diamond from carbon were both interesting and enlightening. Also, the author’s presentation of his nondissipative, nonequilibrium, and chemical pump model was very clearly and convincingly made. Overall, this book should be of interest not only to those who work in diamond film production but to thermodynamicists and biochemists as well.