The Near Elimination of Hysteresis in Pd/Cr2O3 Composites

Although its value varies with the microstructure, hysteresis always accompanies hydride formation and decomposition in Pd(pure)-H, e.g., it is present even in nano-crystalline and thin films. Internal oxidation of Pd0.93Cr0.07 alloys leads to a pure Pd matrix containing nano-crystalline chromia precipitates. The characteristic hysteresis of Pd-H almost disappears in this form of Pd. The reasons for this are discussed. Introduction Hysteresis is an almost universal feature of first order solid state transitions [1]. Its presence means that when such a transition is reversed, it follows a different path than that taken in the forward direction leading to a loss of useful work. Hysteresis is, of course, a reflection of the irreversibility of the transition. Hysteresis For metal-hydrogen systems which form hydride phases in the region of co-existence of the dilute and hydride phases the phase rule predicts an invariant pH2, the plateau pressure. The formation of the hydride phase can be written as 1⁄2 H2(g) + (b-a)MHa = (b-a)MHb (1) where a and b are the H-to-metal atom ratios in the co-existing dilute and hydride phases, respectively. The reverse of this reaction is hydride decomposition. The most common manifestation of hysteresis in metal-H systems is that the minimum plateau pH2 needed for hydride formation, pf, i.e., the forward reaction of equation (1), is greater than that for hydride decomposition, , the reverse of (1). Hysteresis is best evaluated quantitatively as either RTln(p d p f/pd) J/ mol /2H2 or else as RT§lnpdr J/ mol metal where r=(H-to-Pd) atom ratio. Hysteresis should be a minimum for applications of metal-hydrogen systems because it represents a loss of efficiency. Hysteresis always accompanies hydride formation/decomposition in Pd-H. There have been reports of its absence but these are believed to be spurious [2]. It is clear, however, that the extent of hysteresis depends upon the microstructure of the Pd. Compared to