Fundamentals

63

where A and B are constants. The driving force for diffusion was reported to be mainly an electric field across the thin film (100–200nm thick) in addition to the concentration gradient.

2.3.2 Vacuum

It is generally believed that all materials vaporize; however, several modes of vaporization are possible. Some materials will vaporize congruently to a gas of the same composition as the solid, which is also called sublimation. Others will vaporize incongruently to a gas and a different condensed phase. It is also possible for more than one stable gas molecule to form. Decomposition to the elements may also occur, which is called direct vaporization. In multicomponent materials where the various components exhibit greatly different heats of vaporization, selective vaporization may occur.

The deterioration of ceramics in a vacuum in many cases is the equilibration of the material with a low partial pressure of oxygen. In such a case, a lower oxide of the metal may form along with some oxygen represented by the following equation:

(2.38)

Sublimation of solid spheres controlled by gaseous diffusion through a boundary layer was first suggested by Langmuir [2.98] in 1918. The reduction in size was given by the equation:

		(2.39)
where:
ro	=	initial radius
r	= radius at time t
K	=	geometrical constant (~2)
D	= diffusion coefficient of gas through boundary layer
Vo	= molar volume of evaporating species
P	= equilibrium partial pressure of gas
R	=	gas constant

Copyright © 2004 by Marcel Dekker, Inc.

64		Chapter 2
T	=	temperature
t	=	time

2.3.3 Glasses

The corrosion of glasses by atmospheric conditions, referred to as weathering, is essentially attack by water vapor. Weathering occurs by one of two mechanisms. In both types, condensation occurs on the glass surface; however, in one type, it evaporates, whereas in the other, it collects to the point where it flows from the surface, carrying any reaction products with it. The latter type is very similar to corrosion by aqueous solutions. The former type is characterized by the formation of soda-rich films, according to Tichane and Carrier [2.99]. This soda-rich film has been shown to react with atmospheric gases such as CO2 to form Na2CO3, according to the work of Simpson [2.100] and Tichane [2.101].

The electronics industry is one area where vapor attack of glasses may be of importance. Sealing glasses and glass envelopes have been developed that resist attack by alkali vapors and mercury vapors. In their study of some CaOand Al 2O3-containing glasses, Burggraaf and van Velzen [2.102] reported that alkali vapor attack increased greatly above a temperature that coincided approximately with the transformation range* (Tg) of the glass, indicating that one should use a glass with the highest possible Tg.

In the manufacture of flat glass by the Pilkington or PPG processes, glass is floated onto a bed of molten tin in a chamber containing a reducing atmosphere (N2+~10%H2). The hydrogen present in the atmosphere above the glass can act upon the top surface of the glass causing reduction of the most reducible species present. All commercial flat glass contains

* The transformation range of a glass is the range of temperatures where the glass transforms from a viscous liquid to an elastic solid upon cooling. The actual temperature of this range depends upon the cooling rate.

Copyright © 2004 by Marcel Dekker, Inc.