MECHANISMS OF ANODIC BONDING.

摘要

The mechanisms of formation of anodic bonds between glasses and metals are examined. Prior workers have suggested electrochemical, electrostatic, and thermal mechanisms for bond formation, but the dominant mechanism has not been clearly defined. The process is found to be an electrochemical analog to thermal glass-to-metal seals, where the metal surface is oxidized into the glass due to the development of large electric fields across the anodic depletion layer.;While Joule heating of the depletion layer has been suggested important, a detailed heat transfer model and the experimental data indicate that the temperature increase during bonding is insufficient to cause appreciable interfacial softening. Heat is generated both in the bulk glass and in the depletion layer.;The means of uniting irregular surfaces are discussed. Coverage of local defects is possible by densification of the depletion layer and dissolution of anodic protrusions. Elastic and plastic deformation are important for uniting smooth surfaces. A model for viscous flow is proposed, which indicates that viscous flow is less important than previously proposed.;The electrochemical model for anodic bonding is consistent with prior studies of bonding and glass polarization, the experimental and theoretical results of this study, and the interfacial structure requirements for thermal seals. Electrostatic attraction and thermal softening mechanisms are inconsistent with the aggregate data. The electrochemical situation is unusual due to the properties of glass as an electrolyte: transient polarization processes are important, the anodic reaction is irreversible, the mobilities of ions involved in the reactions are vastly different, and the depletion layer structure is quite different from that of the bulk material.;The transient current response at constant potential is examined experimentally and modelled with an equivalent circuit. The results indicate that many ions are moving within a charge-neutral depletion layer, the anodic process is the limiting electrode reaction, and the rate of polarization is governed by the glass resistance. Kinetic data from prior studies are analyzed and support a model where the bonding kinetics are governed by the glass resistivity.