The Criteria for Interfacial Electro-Thermal Equilibrium

When the surface of a first material is brought into contact with the surface of a second material the contact region is called an interface. Since the time of James Clerk Maxwell it has been customary to treat a material electrically as having well-defined bulk properties and having surfaces of zero-thickness. In order to obtain a better understanding of the interface this paper reviews Maxwell's original argument to justify a zero-thickness-surface and reexamines the interface problem assuming electrical charges are actually particles having a finite thickness. Thermodynamics requires that in thermal equilibrium any movement of free charge cannot produce a net electrical current anywhere in the materials or across their interface. For materials in contact and in thermal equilibrium this reexamination gives a set of equations that can be called the interfacial electro-thermal equilibrium (IETE) criteria. A well-defined interfacial potential results from this criteria.