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7STRESS-STRAIN INFLUENCES ON HYDROGEN DIFFUSION EFFECTS IN PALLADIUM ALLOYS
A. Kufudakis, J. Cermak, F. A. Lewis*
Member of International Editorial Board
Institute of Physics, Czech Acad. Sci. Praha 8, Czech Republic
* School of Chemistry, The Queen's University of Belfast BT9 5AG, Northern Ireland, U.K.
Initial diffusion effect observations were made with Pd81Pt19 membranes in the forms of tubes (Fig. 1) with both internal and external surfaces coated with electrochemical depositions of palladium black, as part of an original research objective of development of a form of hydrogen storage electrode [1].
The Pd81Pt19 alloy composition, has advantages of a relatively high value of hydrogen diffusion coefficient at 25 °C in combination with retained elastic reversibility of gross dimensional changes, together with accurate available data concerning pressure hydrogen (p) content and relative electrical resistance (n), R/Ro— n relationships, involved in cycles of absorption (Fig. 1a) and desorption (Fig. 1b) of hydrogen, have proved valuable central factors in regard to their suitability for the preliminary observations and interpretations of phenomena which have become generally ref erred to as "Uphill Hydrogen Diffusion Effects".
Quite fortunately, from standpoints of rather straightforward further experimental amplifications and extensions.
As represented in Fig. 1, such a "tube geometry" allowed subsequent alternative adaptive usage for purposes of measurements of internal hydrogen gas pressures [3], surface electrode potentials [2] and internal tube volume [2].
Internal press. (p)
Po*
Time
а
Time b
Fig. 1. Diagramatic illustrations of Uphill Effect Changes of hydrogen pressure within tubular membranes initiating from quasi steady state pressures (po) after in: (a) resumption of cathodisation of outer surface at point X and cessations at points Y and, in (b) resumptions of anodisation of outer surfaces at point X* and cessations at points Y*, and following introduction (c) and removal (d) of hydrogen at outer wall surfaces. Hydrogen contents n represent the atomic ratios H/Pd and H/(Pd + Pt)
In the initial series of Uphill Effect observations, hydrogen had already been introduced elec-trolytically into the Pd81Pt19 tube outer surface. After temporary cessation of electrolysis, as closely constant as possible steady state internal tube pressure with reference to conditions of desorption, were then allowed to be established.
Following interruptions of cathodic discharge, quasi steady-state pressures of hydrogen were developed within the tubes, as, for example, is represented by po, in Fig. 1a.
Observations following resumed cathodisations, as for example is represented by X in Fig. 1a and 1b have been found to produce initial decreases of hydrogen pressure within the tube.
Qualitatively represented in Fig. 2 are indications of elastic "bending" of outer surfaces experimentally observed in studies of membrane volume expansion recorded, as part of an extended series of studies with Pd81Pt19 alloys [2].
Simultaneously with changes in the hydrogen distrubution in the membrane wall as indicated in Fig. 2.
Fig. 2. Representation of non uniformity of hydrogen interstitial distribution in walls of tubular membranes in courses of Uphill Effect operation with electrolyti-cally generated hydrogen entering outwardly bending outer surfaces — with hydrogen gas dissociated atoms entering the inner surface to thermodynamically compensate for Uphill Effect depletion
References
1. Gorsky W. S. Phys. Zh. Sowjetunion 1935; 8: 457.
2. Lewis F. A. Polish J. Chem. 1997; 71: 1661.
3. Cermak J., Kufudakis A., Lewis F. A. Z. Phys. Chem. 1993;181:233.
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International Scientific Journal for Alternative Energy and Ecology ISJAEE № 9(29) 2005 Международный научный журнал «Альтернативная энергетика и экология» АЭЭ № 9(29) 2005
