N. Mommer

Influence of the microstructure on the desorption kinetics of single- and multiphase LaNiFe alloys

Abstract The influence of the microstructure on the desorption kinetics of deuterium was studied on single-phase La(Ni 0.7 Fe 0.3 ) 5 and multi-phase La(Ni 0.7 Fe 0.3 ) 5 –Ni 0.7 Fe 0.3 alloys. At low hydrogen concentrations [D]/[AB 5 ]≤0.5, the desorption kinetics of activated microcrystalline alloys are dominated by the influence of dislocations, which act as trapping sites for hydrogen atoms. At higher concentrations, where both the α and hydride phase are present, the phase transformation of the latter or diffusion in the β phase is the rate-controlling process with an activation enthalpy of 0.39±0.03 eV for deuterated samples. The deuterium concentration range of the α phase extends up to [D]/[AB 5 ]=1.2 for the nanocrystalline material and this range broadening allows the study of the deuterium diffusion in the α phase without the influence of traps. The measured activation enthalpy for deuterium diffusion in the α phase is Q =0.50±0.03 eV.

Hydrogen diffusion in multi-phase LaNiFe alloys

Abstract The dependence of the hydrogen diffusion on the composition and microstructure of multi-phase LaNiFe alloys was investigated by magnetic after-effect and thermal desorption spectrometry measurements. Heterogeneous alloys of the composition La x (Ni 0.7 Fe 0.3 ) 1− x in the range of x =0.08... 0.11 were produced by eutectic solidification. The volume fractions of the phases were varied by changing the initial composition between hypoeutectic and eutectic. In addition, the characteristic eutectic microstructure, as grain size and lamellar spacing was varied by different cooling rates. By applying the melt-spin technique a nanocrystalline grain structure with lamellar spacings of about 10 nm was achieved. The amount of hydrogen absorbed in the initial charging depends strongly on the grain size of the eutectic microstructure. The activation enthalpy of short-range diffusion depends on microstructure and hydrogen concentration.

Magnetic relaxation of hydrogen in La(Ni0.7Fe0.3)5

Abstract The short-range hydrogen diffusion in the α-phase of the intermetallic compound La(Ni0.7Fe0.3)5 has been studied by means of magnetic after-effect (MAE) measurements below the Curie temperature Tc=250 K. At a hydrogen concentration of cH=4.7 at.% the MAE spectrum shows one broad relaxation maximum at 125 K, with an activation enthalpy of Q=(0.32±0.02) eV and a pre-exponential factor of τ0=2×10−12±0.5 s. With lower hydrogen content this maximum shifts to higher temperatures corresponding to an increase of the activating enthalpy. At a concentration of cH=0.4 at.% it splits up into a double maximum at 140 and 160 K. The corresponding enthalpy spectrum shows two peaks with activation enthalpies of (0.36±0.03) and (0.43±0.03) eV. The results are discussed taking into account the crystal structure of the AB5 compound and hydrogen-induced internal stresses.

Magnetic relaxation of hydrogen in RE2Fe17 compounds (RE = Dy, Gd, Y, Nd or Sm)

Abstract The short-range diffusion of hydrogen in RE2Fe17 (RE = Dy, Gd, Y, Nd or Sin) intermetallic compounds was investigated by magnetic after-effect (MAE) measurements. After charging the powder samples in a high-purity H2 atmosphere (0.13 MPa; 473 K) for 2 days, the hydrogen concentrations for DY2Fe17H4.4, Gd2Fe17H3.8, Y2Fe17H3.0, Nd2Fe17H3.5 and Sm2Fe17H5.0 were determined by thermal desorption spectroscopy. Charged samples show several hydrogen-induced MAE relaxation maxima in the temperature range between 80 and 220 K. These relaxation maxima are assigned to jumps of hydrogen atoms between various interstitial sites and discussed with respect to possible diffusion paths. Numerical evaluation yielded activation enthalpies of Q = 0.46, 0.46, 0.49, 0.48 and 0.42eV for Gd2Fe17, Y2Fe17, Nd2Fe17, Sm2Fe17 and Dy2Fe17 respectively, for jumps of hydrogen between octahedral sites which are part of the long-range diffusion paths in these compounds as well.

Magnetic relaxation in Sm2Fe14Ga3 and Sm2Fe14Ga3C

Magnetic after-effect measurements of Sm2Fe14Ga3 and the carbide Sm2Fe14Ga3C were performed in the temperature range of 140 to 450 K. In the case of Sm2Fe14Ga3C, the relaxation spectrum is composed of two characteristic maxima at 220 and 320 K. The latter maximum is not present in Sm2Fe14Ga3 and therefore assigned to local jumps of interstitially dissolved carbon atoms while the former is attributed to hydrogen. Numerical evaluation yielded an activation enthalpy of (0.96 ± 0.02) eV and a pre-exponential factor τ0=3×10−14±0.5 s for the short-range diffusion of C atoms. The corresponding values for the short-range diffusion of hydrogen are (0.64±0.04) eV and τ0=3×10−14±1 s.

Hydrogen and Deuterium Diffusion in Nanocrystalline La(Ni0.7Fe0.3)0.5–Ni0.7Fe0.3 Alloys

Nanocrystalline hypo- to hypereutectic La(Ni 0.7 Fe 0.3 ) 5 -Ni 0.7 Fe 0.3 alloys were produced by melt-spinning with an ultra-fine eutectic microstructure. The short-range diffusion of hydrogen and deuterium in these alloys was studied by magnetic after-effect (MAE) measurements. The activation enthalpies Q decrease with increasing hydrogen content and show an isotope effect. Values for Q are 0.27 to 0.48 eV for hydrogen and Q = 0.32 to 0.50 eV for deuterium depending on concentration. The initial hydrogen uptake of the AB 5 storage phase of these heterogeneous alloys did not depend on their composition which strongly indicates that the ductile Ni 0.7 Fe 0.3 phase did not influence the absorption pressure.

Measurement of N and C diffusion in Sm2Fe17 by magnetic relaxation

Abstract Magnetic after-effect (MAE) measurements of nitrided and carburized Sm 2 Fe 17 compounds were performed in the temperature range of 140 K to 480 K. Both nitrided and carburized compounds show relaxation maxima at 285 and 300 K, respectively, which are absent in pure Sm 2 Fe 17 compounds. Therefore, these relaxation maxima are attributed to jumps of interstitially dissolved nitrogen or carbon atoms. Numerical evaluation yielded an activation enthalpy Q N (0.84±0.05) eV and a pre-exponential factor τ 0 N =3·10 −15±1 s for the short-range diffusion of N atoms. The corresponding values for the carbon diffusion are Q C =(0.91±0.05) eV and τ 0 C =1·10 −15±1 s. The carbon and nitrogen content of the samples was determined from the increase in mass during nitrogenation or carburization to Sm 2 Fe 17 N 1.2 and Sm 2 Fe 17 C 2.6 .