The Physical and Metallurgical

To attempt to optimize the anomalous phenomena that today go under the label "cold fusion" the experimentalist should be aware of the many aspects of the behavior of hydrogen in metals and of its entry into and egress from metals. This paper discusses the equilibrium characteristics of the isotopes of hydrogen in metals. The first section discusses the thermodynamics of the terminal solutions of metalhydrogen systems including the enthalpies of solutions, H-H interactions, effect of third elements, distribution of isotopes between the phases, site occupation, and the molar volume of hydrogen in metallic solutions. The mobility of hydrogen in a metal lattice is a very large subject. This discussion is restricted to the kinetics of hydrogen diffusion, at and above room temperature, with respect to the variation with temperature, hydrogen concentration, isotopic mass and concentration of third elements. A distinction is made between the effects on the mobility and the effects associated with the non-ideality of the solution. The decrease of the diffusivity due to attractive interactions with lattice defects such as those generated by cold work are discussed in terms of trapping theory. Brief consideration is given to diffusion of hydrogen along grain boundaries and along dislocation cores as well as to diffusion motivated by gradients of electrical potential, of temperature and of mechanical stress. When hydrogen is absorbed from the molecular gas at fixed pressure and temperature, the overall driving force can be expressed in terms of thermodynamic parameters; the kinetic impediments to the ingress of hydrogen control the rate of entry and these are discussed. When hydrogen is presented to the metal by electrochemical means or by partially dissociated hydrogen gas the driving force for entry into the metal cannot be expressed thermodynamically, although the concept of input fugacity is often used. This concept is discussed and incorrect inferences sometimes made from it are pointed out. The entry and the egress of hydrogen produces mechanical stresses in the metal which modify the thermodynamics of metal-hydrogen systems. They necessitate a distinction to be made between coherent and incoherent phase diagrams, and change the driving force for the exchange of hydrogen between the metal and the environing gas phase. More importantly, the generated stresses can relax by producing dislocations, grain rotation, cracks and microvoids. Examples of these phenomena are discussed. The generation of such lattice defects interacts in complicated ways with the intrinsic decohesioning effect of dissolved hydrogen to seriously affect the mechanical properties of metals. Some implications of these considerations for cold fusion research are pointed out.