Systematics of Solid State Chemical Processes

Let us first give a systematic bird s eye view of the different subjects which will be treated later. One may subdivide processes primarily according to the physicochemical nature of the systems involved (the chemist s approach), or according to the acting driving forces (the physicist s view). In the latter case, we can choose the corresponding forces according to such basic fields of physics as mechanics, electro- 

Chemical systems are commonly subdivided into homogeneous, inhomogeneous, and heterogeneous phases. We will therefore distinguish chemical processes in the following systems. 

The above classification of chemical processes was based on the system s physical chemistry. A similar classification can be applied to electronic processes if we consider the effectively charged structure elements and assume that we can determine extremely small component concentrations or deviations from the stoichiometric composition. The well-known p-n junction process can serve as an example since it is a transport process (including local relaxation) in a single phase, inhomogeneous system. 

The fundamental question in transport theory is Can one describe processes in nonequilibrium systems with the help of (local) thermodynamic functions of state (thermodynamic variables) This question can only be checked experimentally. On an atomic level, statistical mechanics is the appropriate theory. Since the entropy, 5, is the characteristic function for the formulation of equilibria (in a closed system), the deviation, SS, from the equilibrium value, S0, is the function which we need to use for the description of non-equilibria. Since we are interested in processes (i.e., changes in a system over time), the entropy production rate a = SS is the relevant function in irreversible thermodynamics. Irreversible processes involve linear reactions (rates 55) as well as nonlinear ones. We will be mainly concerned with processes that occur near equilibrium and so we can linearize the kinetic equations. The early development of this theory was mainly due to the Norwegian Lars Onsager. Let us regard the entropy S(a,/3,. ..) as a function of the (extensive) state variables a,/ ,. .. .which are either constant (fi,.. .) or can be controlled and measured (a). In terms of the entropy production rate, we have (9a/0f=a) 

Rewriting 5 G in terms of the, system s chemical potentials and concentrations yields 

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