Fundamental Considerations Relating to the Insertion of Hydrogen Isotopes into Mixed Conductors at High Activities
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FUNDAMENTAL CONSIDERATIONS RELATING TO THE INSERTION OF HYDROGEN ISOTOPES INTO MIXED CONDUCTORS AT HIGH ACTIVITIES ROBERT A. HUGGINS Department of Materials Science & Engineering Stanford University Stanford, CA 94305
ABSTRACT Some features of the electrochemical insertion of hydrogen isotopes into mixed conductors under dynamic electrolytic conditions are discussed. Unusual magnitudes of thermodynamic parameters in both the surface region and the bulk can be obtained under some conditions. These can vary greatly, depending upon the surface composition, as well as experimental conditions. It is thus possible to obtain species distributions and related phenomena inside solids during electrolysis at atmospheric pressure that are equivalent to those that would be present if there were a very high external pressure of one of the components. The transport of hydrogen isotope interstitial species is very microstructure - dependent. Readily visible microstructural changes occur upon insertion and deletion of these species. This leads to the very sporadic nature of structure - dependent phenomena often observed. These considerations may play an important role in the apparent irreproducibility of experimental observations of the "cold fusion" phenomenon. INTRODUCTION The relatively easy absorption of large amounts of hydrogen into metallic mixed conductors was first reported by Graham over a hundred years ago [1]. He also showed that hydrogen could be introduced into such materials by electrochemical charging [2, 3]. Since that time there has been a large amount of work done on various aspects of hydrogen in mixed conductors, both metals and non-metals. This has provided much fundamental knowledge about the thermodynamic, kinetic and structural aspects of interstitial solid solutions, as well as the development of a number of important experimental techniques. Hydrogen and its isotopes are rapidly transported in many electronically conducting solids, and large concentrations can often be readily absorbed under experimental conditions that are rather easily obtained. There is a considerable amount of practical interest in the use of hydrogen - permeable metallic membranes for the separation of gases, and much research effort has gone into the exploration of the use of hydrogen absorption in mixed conductors for the storage of hydrogen, and more recently, for use as rechargeable electrodes in battery systems. In addition to these essentially positive matters, it has been found that hydrogen can have very significant, and generally deleterious, effects upon the behavior of solids in technological applications. One of the most important practical areas involves the influence of hydrogen on various mechanical properties, and hydrogen embrittlement and hydrogen - related stress corrosion cracking of high strength steels are very important practical problems. On the other hand, the reversible insertion of hydrogen, and the accompanying large volume changes that can cause decrepitation, are sometimes used as a method for the produ
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