Michael Ege

Hydrogen permeability measurement through Pd, Ni and Fe membranes

Abstract An ultrahigh vacuum apparatus for the measurement of hydrogen gas permeation through metal membranes has been developed to perform studies in the ranges of low pressure and medium temperature. The sealing of the hydrogen gas at the specimen is done by gold O-rings which yield vacuum tightness. The hydrogen permeation is measured for pure Pd, Ni and Fe membranes over a pressure range of 0.1–100 mbar and a temperature range of 50–440°C. The results are compared with data in the literature. It shows that the permeation is exceedingly affected by the surface contamination on the specimens in the case of Pd. The contaminants are taken off from the Pd surface by short aging around 200–300°C in air. After cleaning the surface, the permeation amount increases and agrees well with the literature data. The activation enthalpies for permeation are (0.144±0.003), (0.600±0.003) and (0.382±0.004) eV for Pd, Ni and Fe, respectively, in the present investigation. For Ni and Fe, such a special treatment to clean the surface is not necessary.

Surface adsorbed atoms suppressing hydrogen permeation of Pd membranes

Surface analysis of Pd thin foil is done by Auger electron spectroscopy after various treatments. The results are related to hydrogen gas permeation through Pd membrane on the ranges of low pressure and medium temperature.

Mechanical relaxation processes of hydrogen in dilute palladium-rare earth alloys

Abstract After charging with hydrogen, dilute Pd100 − xREx alloys (RE=Y, Ce, Gd) (x = 1 … 9) show three thermally activated relaxation maxima of the internal friction between 10 and 300 K. Two of these maxima are identified as a Zener maximum of hydrogen pairs in the β-phase and as a Snoek-Koster maximum, which are both also observed in H-charged pure Pd. The third maximum appears only in the binary alloys. The activation parameters of this relaxation are τ0 = 3·10−12±0.5 s for me pre-exponential factor and the activation enthalpy lies between Ha = 0.18 eV and Ha = 0.26 eV. Ha increases with increasing RE content whereas it decreases with increasing hydrogen content. The relaxation strength is independent of the RE content and depends linearly on the hydrogen concentration. These features are characteristic for the formation and reorientation of HRE (RE=Y, Ce, Gd) complexes. Furthermore, the shape factor of the strain ellipsoid of the HRE dipoles, which characterizes the anistropy and strength of the local atomic displacements, was determined as |λ1–λ2|=0.036.

Isotope effects of the mechanical relaxation of H and D in dilute PdY and PdGd alloys

Abstract The internal friction spectra of dilute PdY and PdGd alloys charged with H or D show three relaxation maxima. The maxima attributed to the stress induced ordering of H or D pairs (Zener maximum) and to the reorientation of H–Y or H–Gd pairs (D–Y or D–Gd pairs, respectively) (reorientation maximum) show clear isotope effects concerning their temperature position as well as their activation parameters. In case of the reorientation maximum (activation enthalpy H a,H M2 =0.18–0.26 eV, H a,D M2 =0.20–0.26 eV, depending on H or D as well as Y or Gd concentration) which occurs at temperatures slightly above 100 K the pre-exponential factor of the jump frequency is of the same order of magnitude as the Debye frequency of the host lattice ( ν Deb =5.7·10 12 s −1 ) for D, whereas it is one order of magnitude smaller for H. In case of the Zener maximum ( H a,H M1 =0.11 eV, H a,D M1 =0.15 eV) which occurs at temperatures of about 80 K the pre-exponential factors for D and H are lower by two and four orders of magnitude, respectively, as compared to the Debye frequency. These results are interpreted in terms of thermally activated tunnelling jump processes which occur either adiabatically for higher temperatures or non-adiabatically for lower temperatures.

Reorientation of diatomic HY complexes in dilute PdY alloys

Abstract The reorientation of diatomic HY complexes in hydrogen-doped dilute Pd 100− x Y x alloys ( x = 1, 2, 4.5, 8) has been studied by means of internal friction (IF) in an inverted torsion pendulum and in a vibrating reed apparatus. As reported earlier, the internal friction of hydrogen-doped dilute Pd 100−x Y x alloys shows three maxima that are identified as the Zener peak, the reorientation peak of diatomic HY complexes and the Snoek—Koster peak. The width of the reorientation peak exceeds that of a single Debye maximum, and increases with the Y content as well as with the hydrogen content of the sample. For a fixed hydrogen content, the maximum shifts to higher temperatures if the Y content of the alloys increases; in contrast, for a fixed Y content, there is a shift to lower temperatures caused by an increasing hydrogen content. The measured reorientation peak can be fitted by a superposition of Debye maxima using a constant pre-exponential factor and spectra of activation enthalpies for the relaxation times. The analysis of the hydrogen content dependence of the relaxation strength yields a value of | λ 1 – λ 2 | =0.036 for the elastic anisotropy of the diatomic HY complexes. The activation parameters for the relaxation time of the reorientation of an isolated complex are 0.18 eV and 3×10 −12 s. The properties of the activation enthalpy spectra can be interpreted in terms of a model which takes into account the elastic long-range interaction between an HY complex and the Y atoms in the vicinity of this complex.

Mechanical and magnetic relaxation of H and D in dilute PdGdFe

Abstract The relaxation of H and D in Pd 90 Gd 5 Fe 5 was investigated over a wide temperature range by means of internal friction (IF) using a vibrating reed apparatus (frequencies 450 Hz…2100 Hz) and an inverted torsion pendulum (1 Hz…12 Hz) and by means of magnetic after-effect (MAE) (relaxation times corresponding to frequencies of 0.005 Hz…0.17 Hz). After charging with H, Pd 90 Gd 5 Fe 5 H 5.65 shows a relaxation maximum in the MAE at 68 K ( f >=0.005 Hz) and a maximum of the IF at 108 K ( f =4.7 Hz). The activation parameters were determined independently by both methods to τ 0 H =4·10 −12±0.5 s and H a H =(0.19±0.02) eV. With decreasing H or D content this maximum shifts to higher temperatures, similar to the shift in the binary alloy Pd 100− x Gd x . Owing to this concentration dependence, together with the similar activation parameters, the relaxation maximum can be attributed to the reorientation of H–Gd complexes. For identical concentrations, the IF relaxation maximum shows an isotope shift of about 5 K to lower temperatures for deuterium with respect to hydrogen. For a concentration of 3.85 at.%, activation parameters of τ 0 H =6·10 −12±0.5 s, H a H =(0.21±0.01) eV and τ 0 D =6·10 −13±0.5 s, H a D =(0.23±0.01) eV were obtained.