X.-L. Wang

Surface study of chemically treated LaNi4.7Al0.3 alloy

Abstract Hydride-forming alloys with highly reactive surfaces have been developed by treating with a fluorine-containing aqueous solution. They exhibit excellent reactivity for hydrogen uptake. The surface properties of the fluorine-treated LaNi 4.7 Al 0.3 alloy were studied by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy. Surface compositions were analysed and compared with that of the untreated sample. After fluorine treatment, fine net-like crystallites of less than 1 μm are formed and segregation of the alloy components takes place on the surface.

A study of the surface composition and structure of fluorinated Mg-based alloys

It has been observed that hydriding alloys may provide a highly reactive surface for hydrogen uptake and a protective nature against impurities by treatment with a F−-containing aqueous solution (also containing K+). In this study, the surface composition and structure of fluorinated Mg, Mg2Ni alloy, MgMg2Ni eutectic alloy and amorphous Mg50(La30Ni70)50 were studied by means of X-ray photoemission spectroscopy, electron probe microanalysis and X-ray diffractometry. The samples were treated under various conditions by changing the treatment time and the ratio of sample weight to solution volume (w/v ratio). The surface morphology, composition and chemical valence state after F-treatment were analyzed and compared with those of untreated samples. It was observed that a rose petal-shaped structure was formed and both MgF2 and KMgF3 were formed on the surfaces of the samples after F-treatment. The formation of these compounds was independent of the initial surface state of particles, i.e. regardless of oxidized or hydroxidized or clean surface states, but dependent on treatment conditions. When the treatment time is longer or the w/v ratio is smaller, KMgF3 forms on the surface and co-exists with MgF2. Contrary to this, only MgF2 forms on the surface when using a shorter treatment time or larger w/v ratio.

Surface characteristics of fluorinated hydriding alloys

Abstract To overcome the shortcomings of hydriding alloys such as poor initial activation characteristics, high sensitivity to impurities and pyrophoricity, a new surface treatment method has been developed by employing an F − -containing aqueous solution. It has been successfully applied to AB, AB 2 , A 2 B and AB 5 type alloys. Distinguished characteristics have been obtained by fluorinating alloy surfaces. In this work, the surface structures and compositions of the fluorinated AB, AB 2 , A 2 B and AB 5 type alloys were determined by means of SEM, EPMA and XPS. The reactivity and stability of the fluorinated surface were examined after exposing fluorinated alloys to water and air for various periods, and after repeating hydriding/dehydriding (H/D) cycles with pure and impure hydrogen containing CO. It is the main purpose of this paper to reveal the surface structures and compositions of the fluorinated fine powder alloys, and to reveal the relationship between fluorinated surface structures and improved reaction characteristics in both gas-solid and liquid-solid systems.

Consistent determination of the intrinsic kinetic properties between hydrogen and hydriding alloys

Abstract In the study of reaction kinetics between hydrogen and hydriding alloys, what is the most important is to obtain intrinsic kinetic data. Any results obtained under inconsistent conditions cast serious doubts on the data interpretations. At present, although many kinetic studies have been performed, there is little agreement either in experimental results or in theoretical interpretations. In this paper the experimental errors causing deviations of kinetic results are analyzed and classified as follows: (1) thermal effects caused by the heat of reaction, (2) relationship between the gas holder volume and the sample mass, (3) history and number of hydriding/dehydriding cycles, (4) experimental conditions, (5) surface state of particles, (6) size of particles. The above error sources, which still remain in most of the present studies, are discussed in detail, and proposals are made for consistent and precise kinetic determinations.

Reaction kinetics of the mmni4.5al0.5-h system

The hydriding and dehydriding reaction kinetics of the aluminum-substituted mischmetal-nickel hydride were studied. The experiments were carried out by a step-wise method in the α and α+β phase regions in the temperature range from 303.2 to 333.2 K. The effects of the phase change on reaction characteristics were examined. The reaction was found to proceed differently between different phase regions for both the hydriding and dehydriding processes. The reaction order, rate constant, and apparent activation energy change with phase regions under isothermal conditions. However, those parameters are kept constant within a given phase region.

Stability and tolerance to impurities of the fluorinated surface of hydrogen-absorbing alloys

Abstract The surface structure of a fluorinated LaNi 4.7 Al 0.3 alloy that was ground by hydriding-dehydriding (HD) cycling was determined by means of scanning electron microscopy, electron probe microanalysis and X-ray photoelectron spectroscopy. The change and the stability of the fluorinated surface were studied after exposure to water and air for various periods, and after further HD cycles. It was found that an LaF 3 layer was formed on the surfaces of the particles and on the surfaces of the cracks that were formed during HD cycling, after treating the sample with an F − -containing aqueous solution. Excellent hydriding characteristics were observed for the fluorinated alloy. After exposure of the fluorinated sample to air and water for a long period, and after repeating the reaction cycles with pure hydrogen gas or H 2 gas containing CO, a stable LaF 3 layer still remained on the surface. Excellent reactivity was still maintained, correspondingly. The degradation of the hydriding reaction rate and of the hydrogen capacity of the fluorinated alloy were found to be very small, even when using a H 2 + 300 ppm CO gas mixture at lower temperatures. On the contrary, this gas mixture deactivates the untreated alloy severely under the same conditions. These observations indicate that the overlying fluoride layer can effectively protect the sublayer from impurities.

Hydrogen purification using fluorinated LaNi4.7Al0.3 alloy

Abstract Hydriding characteristics of fluorinated LaNi4.7Al0.3 in hydrogen containing CH4, CO2 and CO were investigated by cyclic absorption-desorption tests and the purification efficiency at temperatures ⩽353.2 K were examined. Fluorinated alloys showed excellent tolerance to impurities, especially to the extremely poisonous CO. The alloy without treatment was completely deactivated after only a few hydriding/dehydriding cycles in hydrogen containing 300 ppm or 1000 ppm CO. However, the degradation of the hydriding reaction rate and hydrogen capacity of the fluorinated alloy was small. The gas chromatography measurements on hydrogen released from fluorinated alloys showed that the purity was higher than 99.9999% after a few percent release of hydrogen even for a high concentration of CO at low temperature. Based on these results, it is believed that practical hydride-based hydrogen purification from gas mixtures may be realizable by using fluorinated alloys.

Effects of carbon monoxide on the hydriding reactions of the untreated and fluorinated LaNi4.7Al0.3 alloys

Abstract A method for the surface treatment of the hydriding alloys has been developed to improve the hydriding properties. After F-treatment, LaF 3 was formed on the outer surface and metallic Ni was precipitated in a subsurface for the LaNi 4.7 Al 0.3 alloy. By utilizing the unique surface characteristics, the effects of carbon monoxide on hydriding reaction rates and hydrogen absorbing capacities were investigated by cyclic hydriding/dehydriding (H/D) tests. The degradation of hydriding reaction rate and hydrogen capacity of the fluorinated alloy was found considerably small even under the existence of the extremely poisonous impurity CO (H 2 + 1000 ppm CO) at lower temperatures. In contrast, the gas mixture deactivates the untreated alloy severely under the same conditions. It was also found that even if the fluorinated alloy was deactivated after repeating many H/D cycles in the gas mixture, the reactivity can be easily recovered by only a cycle of H/D reaction in pure hydrogen. This offers the possibility to move toward practical metal hydride-based applications, especially under the existence of impurities.

Hydriding-dehydriding reactions of LaNi4.7Al0.3H system under quasi-isothermal conditions

Abstract Using a highly heat-sensitive reactor, the average temperature deviations were kept within ±0.1 °C of a set temperature during hydriding and dehydriding reactions of metal hydrides. The reaction kinetics of the LaNi 4.7 Al 0.3 -H system were measured in this reactor by a stepwise method in the temperature range 313.2–443.2 K. Experimental data obtained under quasi-isothermal conditions are presented.

Surface properties of the fluorinated calcium-based AB5 alloys

Abstract The F-treatment of CaNi 5 and Ca 1− x La x Ni 4.7 Al 0.3 ( x = 0.2−0.8) alloys was performed under various conditions. Surface compositions and structures of the fluorinated alloys were studied by means of X-ray photoemission spectroscopy, electron probe microanalysis and scanning electron microscopy. CaF 2 was found on the surface of CaNi 5 and CaF 2 + LaF 3 on the surface of Ca 1−x La x Ni 4.7 Al 0.3 after F-treatment. Metallic Ni precipitated in a sublayer after F-treatment. Also, in the case of CaNi 5 , Ni-rich and Ca-rich phases were formed on the surface by means of F-treatment. These fluoride surface layers provide an effective protection from impurities and high reactivity with hydrogen owing to the catalytic effects of segregated Ni which works catalytically to decompose H 2 molecules to atoms. This chemical treatment method offers a great possibility for wide applications of metal hydrides, especially under the existence of various impurities.