F.-J. Liu

Properties and characteristics of fluorinated hydriding alloys

Abstract An “F-treatment technique” based on the chemical reaction between F − ions in an aqueous solution with metallic components in hydriding alloys has been developed in this laboratory (KUCEL) since 1990 to improve surface properties and characteristics of hydriding alloys. This is a broad approach to provide a highly reactive and protective fluorinated surface for hydrogen uptake on several hydriding families. In this paper, the F-treatment effects on hydrogen absorption behaviors in gas-solid and liquid-solid reactions and the fluorinated surface properties based on the surface and crystal-structure analyses performed by X-ray photoelectron spectroscopy, electron probe microanalysis, and X-ray diffraction and microscope are discussed. The merits and application potentials of the proposed technique are summarized.

HYDRIDING BEHAVIOR OF F-TREATED MG2NI AT MODERATE CONDITIONS

Abstract The F-treatment effects on the initial activation characteristics and on the cyclic hydriding-dehydriding behavior of the Mg2Ni alloy as a function of temperature, pressure and particle size have been investigated. Metallographic studies have also been performed. It was found that with the formation of magnesium fluoride on the particle surface, some microchannels were generated along the interfaces of the Mg-rich Mg + Mg2Ni eutectic structure. These are believed to contribute to the rapid initial activation at low temperatures, such as 30°C, and to the only partial hydriding ability, which increases slightly with increasing temperature, pressure and number of cycles. Small-size powders are effective in improving the initial activation performance, but not the hydriding cyclic behavior. A growth process of the hydride phase from the surface to the bulk associated to cycling and resulting from the cracking growth of the bulk was suggested.

Activation characteristics of chemically treated LaNi4.7Al0.3

Abstract The H2 activation characteristics of the hydriding AB5 intermetallic compound LaNi4.7Al0.3 were determined before and after chemical surface treatment in an F-containing aqueous solution (F-treatment). The F-treatment results in an Ni-rich surface which is overlaid by a net-like LaF3 layer and results in a pronounced decrease in the time required for activation, as much as two orders of magnitude. The ease of activation is attributed mostly to the Ni-rich sublayer, although the LaF3 overlayer may contribute to long term protection of the sublayers from deleterious gases such as air and H2O vapor. Increasing the ease of activation will allow the use of thinner wall, lower cost containers for practical hydriding applications.

A method for improving the long-term storability of hydriding alloys by air/water exposure

Abstract The F-treatment effects on long-term storability of several conventional hydriding alloy systems including AB 5 and A 2 B as well as La-substituted AB 2 -types were systematically investigated as functions of surface condition, particle size and air or water exposure. It was found that the fluoride coating formed through this proposed F-treatment technique could provide substantial protection from surface contamination resulting from long-time exposure to air and ever more so to water. The F-treatment was particularly effective in protecting surface reactivity from both air and water exposure for small particles which were ground by hydriding-dehydriding cycles and had a large specific surface area.

Surface properties of the fluorinated La-incorporated Ti/Zr-based AB2 laves phase alloys

Abstract A new class of fluorinated Ti/Zr-based AB 2 laves phase alloys with an extremely high reactivity to hydrogen was demonstrated by incorporating a small amount of rare earth metals such as La. Great benefits on improving the activation characteristics both in gas–solid and liquid–solid reactions were demonstrated. This is known to contribute to the formation of the La-fluoride layer on those La-enriched secondary phases homogeneously distributed at the grain boundaries of the AB 2 matrix. This research is based on a fluorination procedure invented in our laboratory in 1991 by using an aqueous F − -ion containing solution (F-solution) to form a fluorinated layer on conventional hydriding alloy surfaces. The aim of this paper is to elucidate the fluorination effects of La-incorporated Ti/Zr-based AB 2 laves phase alloys on the surface properties regarding their activation characteristics in gas–solid and electrochemical reactions through ESCA, EPMA, XRD studies and microscope observation. The results are to be correlated with former studies on the fluorinated AB 5 alloys, i.e. LaNi 4.7 Al 0.3 . The roles of Ni on the surface of both the AB 2 phase and La-enriched second phase before and after the fluorination will be described in this paper.

F-treatment effect on the hydriding properties of the La-substituted AB2 compound (Ti, Zr)(Mn, Cr, Ni)2

Abstract The La-substitution effects of AB2 hydriding compounds (Laves phase) Ti0.5−x/2Zr0.5−x/2LaxMn0.8Cr0.8Ni0.4 (x = 0.1-0.1) on the hydriding properties including initial activation characteristics, pressure-composition-temperature property and long storability in air and water have been investigated and compared with those of F-treated alloys. It was found that a small La substitution could dramatically improve the initial activation and surface protection against air or water exposure, especially after F-treatment. La could not be substituted into the AB2 phase but formed La-rich particles which served as catalytic sites for hydrogen uptake and protected against the normally damaging of F-treatment on the AB2 alloy surface. No significant deterioration of the surface reactivity was observed for F-treated 0.05La alloy even after 6 months exposure to air and water. The optimum La content should be in the range of 0.03–0.05.

Effects of Ni-substitution and F-treatment on the hydriding behaviors and microstructures of AB2-compound (Ti,Zr)(Mn,Cr)2

Abstract The effects of Ni-content and F-treatment of AB 2 compounds Ti 0.5 Zr 0.5 (Mn 0.5 Cr 0.5 ) 2− x Ni x (ifx = 0.0 to about 1.4) on the initial activation characteristics, P-C-T property, and microstructure have been investigated. It was found that Ni can substitute into this AB 2 family of compounds which have the essentially single C14 Laves structure between x = 0.0 and about 1.0 Ni and become the C15 Laves structure between x = 1.0 and about 1.4 Ni. All the compositions were easily activated at 40°C and 1.0 MPa H 2 pressure. This family of compounds exhibited high hydrogen storage capacity and low hysteresis. The plateau pressures could be raised over two orders of magnitude by the increase of Ni-content. The F-treatment, which has been successfully used on AB 5 compounds, such as LaNi 4.7 Al 0.3 (1,2), did not show beneficial effects on this AB 2 family of compounds.

SURFACE TREATMENT EFFECT ON THE LOW TEMPERATURE HYDRIDING BEHAVIOR OF MG2NI

The effects on the hydriding behavior, long term storability in air and microstructure of Mg2Ni alloy resulting from surface treatments using (1) HCl acid (HCl-treatment), (2) F-containing solution after HCl etching (HCl + F-treatment) were determined and compared with those due to (3) F-treatment alone. Both the treatments (1) and (2) could clean up the originally contaminated surface as F-treatment did. HCl leached some pure Mg phase from the eutectic structure of the Mg-rich Mg2Ni alloy. This resulted in an increase of specific surface area and led Mg2Ni to partially hydride under very moderate temperature and pressure conditions. HCl-treatment alone could not form a fluorinated surface structure as F-treatment did. Instead of this, some magnesium hydroxide was formed. The fluorinated surface formed by F-treatment provided strong protection from surface contamination by long term air exposure.

Surface and metallographic microstructure of the La-added AB2 compound (Ti, Zr)(Mn, Cr, Ni)2

Abstract The surface properties and metallographic microstructures of AB 2 -hydriding compounds Ti 0.5− x /2 Zr 0.5− x /2 La x Mn 0.8 Cr 0.8 Ni 0.4 (x = 0.0–0.1) were investigated for the purpose of understanding dramatically improved hydriding properties by small amounts of La addition. The results suggest that the discrete La Ni particles decorated on the crushed powder surfaces serve as both catalytic sites, such as a window, during initial activation and a protective coating against surface contamination from long-time air/water exposure. Simultaneously, these La Ni particles could protect the AB 2 alloy surface against the normally damaging F-treatment by forming a highly reactive LaF 3 compound to further accelerate the hydrogen absorption as it did in the LaNi 4.7 Al 0.3 alloy.

F-treatment effect on the initial activation characteristics of MgLaNi amorphous alloys

Abstract Treatment of MgLa 1− x Ni x (x = 0.4–0.7) amorphous alloys with a fluorine-containing aqueous solution leads to a material easily activated under moderate conditions. The maximum H-capacity of the F-treated MgLa 0.3 Ni 0.7 during 10 h activation at 75°C reached about 1.3 in H/M ratio or 2.4 wt.%, three times that of the untreated one. The amorphous structure and the ribbon shape were maintained after hydrogenation. With increasing temperature, the non-crystalline structure of the amorphous alloy tended to become crystalline, resulting in a decrease of H-capacity. XRD analysis suggested that the initial hydriding of the amorphous alloys caused an irreversible decomposition of the alloys with the formation of LaH 3 and of Mg and Mg 2 Ni hydrides after crystallization. A La Mg-fluorinated surface with a net-like structure was formed after F-treatment which is believed to play an important role in the rapid hydrogen absorption under such moderate conditions.