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HYDROGEN CONTENTS AND ELECTRICAL RESISTIVITIES OF PALLADIUM - SILVER ALLOYS
R. A. McNicholl, F. A. Lewis
School of Chemistry, The Queen's University of Belfast Belfast, BT9 5AG, UK E-mail: i.gibson@qub.ac.uk
Companion co-ordinating measurements of electrical resistance have provided a valuable additional experimental parameter guide to alterations with hydrogen contents of other properties of the palladium - hydrogen and palladium - silver - hydrogen systems.
Introduction
Conjoint measurements of electrical resistance have proved important experimental monitors of changes of hydrogen content for many experimental studies within the fields of research of hydrogen-metal systems [1-11].
Relationships between Relative Electrical
Resistance (R/R0 where R0 shows initial
Hydrogen free Resistance) and Hydrogen Content (H-Pd, H-Me, n)
Initial reports of the influence of absorbed hydrogen on electrical resistivities of palladium and palladium alloys were included in the original stud-
ies of Graham [1] and Sieverts with Hagen [2]. Almost simultaneously a palladium - silver comprehensive alloy composition-dependent study was reported by Rosenhall [3], in conjunction with complementary X-ray measurements.
As indicated in figs. 1 and 2, these results of Rosenhall [3] were in good general agreement with results in the later study of Carson [4,5] in which the electrical resistance measurements had been co-ordinated with recordings of electrode potential and derived values of hydrogen pressure p made with the same specimen, typically at 25 °C. Corresponding [4-6] changes of hydrogen content, n = H/(Pd + Ag) (atomic ratio) have primarily been governed by diffusive transport of dis-
Atm. % Ag
in Pd-Ag Alloys
a 54.80
b 49.26
c 44.75
d 39.07
e 34.75
f 29.22
g 25.84
h 19.10
i 10.10
Fig. 1. Plots of both absorption (open) and desorption (full), symbols for R/R0-n relationships for Pd and Pd-Ag alloys at 25 °C — indicative of decreasing hys-teretic differences with increasing Ag content
Fig. 2. More detailed forms of relationships between hydrogen content, n and R/R0 obtained from electrode potential studies [4] and where measurements denoted by circles have been obtained in cases of gas studies from [2]
International Scientific Journal for Alternative Energy and Ecology
ISJAEE № 3(11) (2004)
R. A. McNicholl, F. A. Lewis Hydrogen contents and electrical resistivities at palladium - silver alloys
solved hydrogen molecules through the interfacial solution diffusion layer according to the relation An = k0k0 (P — p)dt where the rate constant k0 had typical experimental values of ~1016 molecules • cm-2 • sec-1 equivalent to current densities of ~3 mA • cm-2 and k0 was a constant for adjustment of units. Analogous measurements [4, 5] were also employed for determinations of allied desorption relationships, and also in subsequent related studies with Pd-B-Ag alloys [6].
Experimental
Results of further measurements are reported here of analogous hydrogen absorption and desorption p-n and R/R0 - n relationships similarly derived [4, 5] for the Pd77Ag23Hn system [4, 5] with Pd77Ag23 alloy specimens in the form of thin strips formed from the same Pd77Ag23 composition billet, that had been employed previously for strain gradient estimations involved in hydrogen diffusion studies [7].
Results and Discussion
Derived results [4] of hydrogen content (n) dependences of hydrogen pressure (p) and relative electrical resistance (R/R0) at 323 K are plotted in fig. 3. (Quantitively similar forms and features have been derived at 298 K and 348 K).
Over the initial range of hydrogen content (n), the gradually decreasing extent of incremental increase of R/R0 with n seems diagnostic [4-8] of an a-phase range of hydrogen content. The following S-shaped regions of still gradually increasing (p) with increasing (n) have similarities to supercritical forms (with respect to a and P-phase regions of co-existence) of the Pd-H system [5]. Complementarily, however, there are then decreases of R/R0 with increases of n to closer to its original values at n = 0 before a final region of steep increase of R/R0 with n (also illustrated by the dashed line in fig. 2). This latter part of the derivation is consistent with a region of purely P-phase and an overall R/R0 - n placement position between curves g and h in fig. 2.
The extents of the gradual alterations of forms of R/R0 - n relationships with increasing Ag alloying content in figs. 1 and 2 have also been supported by results of studies of R/R0 relations with p at higher ranges of hydrogen pressure p as reported by Szafranski and Baranowski [8-9].
It has to be noted here, that these results with the Pd77Ag23Hn system seem broadly in keeping with results of electrochemical measurements of electrical resistivity changes reported in a recent study of the Pd80Ag20Hn system [10] but in which, unfortunately, insufficient comparative attention seems to have been drawn to relevant results [1-6] of earlier studies.
Fig. 3. Experimental plot of absorption (open) and desorption (closed) p-n and R/R0 - n relations at 323 K similarily derived to those for ref. 4
Acknowledgments
Grateful thanks are due for support of this area of research by the Royal Society of London and the Czech Academy of Sciences. Thanks are also due to Ted B. Flanagan for informative discussions and to Johnston Matthey Co. for production of the Pd77 Ag23 alloy.
References
1. Graham T. // Proc. Royal. Soc. London. 1869. 17; 500.
2. Hagen H., Sieverts A. Z. // Phys. Chem. 1933. 165A; 1; Sieverts A., Hagen H. Z. // Phys. Chem. 1935. 174A; 247.
3. Rosenhall G. // Anal. Phys. 1933, 1935. 18; 24; 150; 207.
4. Carson A. W., Lewis F. A., Schurter W. H. // Trans. Faraday Sec. 1967. 63; 1443.
5. Lewis F. A. The Palladium Hydrogen System. Academic Press, 1967. P. 92.
6. Burch R., Lewis F. A. Trans. Faraday Sec., 1970. 66; 727.
7. Lewis F. A., Bucur R. V., Tong X.Q., Sakamoto Y., Kandasamy K. // Hydrogen Power; Theoretical and Engineering Solutions; Grimstand 1997; ed. T.O. Saetre, Kluwer, Dordrecht 1998. P. 615 / See also: Cermäk J., Kufudakis A., Lewis F.A. // ISJAEE. 2002. No.4.
8. Szafranski A. W., Baranowski B. // Phys. Stat. Sol., A. 1972. 9; 435.
9. Szafranski A. W. Ibid. 1973. 19; 459; Szafranski A. W. // Polish J. Chem. 1981. 55; 2137; 2143; Z. Phys. Chem. 1993. 179; 373.
10. Toth J., Garaguly J., Peter L., Tompa J. // J. Alloys Compds. 2000. 312; 117.
11. Lewis F. A., Kandasamy K., Tong X. Q. Hydrogen in Metals Systems II, Solid State Phenomena, 2001. 73-75; 207.
International Scientific Journal for Alternative Energy and Ecology ISJAEE № 3(11) (2004)
