научная статья по теме NEW HYDROGEN ABSORBING ALLOYS OF LAVES PHASE TYPE Комплексное изучение отдельных стран и регионов

Текст научной статьи на тему «NEW HYDROGEN ABSORBING ALLOYS OF LAVES PHASE TYPE»

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T.N.Bezuglaya, S.V.Mitrokhin and V.N.Verbetsky

Chemistry Department, Lomonosov Moscow State University, Moscow 119899, RussianFederation

NEW HYDROGEN ABSORBING ALLOYS OF LAVES PHASE TYPE

1. INTRODUCTION

Intensive development of metal hydride chemistry in the recent two decades is stipulated by the basic scientific reasons as well as by perspectives of their application in different fields of technique, particularly in metal hydride technology for storage transport and purification of hydrogen.

The progress in research of the intermetallic hydrides of CaCu5 and Laves phase types allows even now to choose unique compositions with preset properties for peculiar tasks of metal-hydride technology. Still this problem is topical up to now.

This work is the part of a series of studies on research and development of new highly effective "hydrogen accumulating alloys". TiMn2 intermetallic compound of hexagonal C14 structure type is characterised by a rather wide homogeneity region (64-70 at.% Mn [1, 2]), And is a perspective hydrogen absorbing material. Most complete investigation of absorption properties of Ti-Mn-H2 system was conducted in [3]. Of all studied alloys TiMn1.5 was found to be the best for application because of large amount of absorbed hydrogen and well determined plateau. Further studies [4] showed that substitution of titanium and manganese

by other light transition metals significantly change the character of their interaction with hydrogen. Still no systematic investigation of transition metals substitution on the properties of TiMn2 in the homogeneity region was carried out.

The Ti-Mn-V system is of particular interest for practical applications since all three component metals are light 3d-transition elements. However, there are no references on the state diagram of this system, though some alloys were studied as hydrogen storage materials [3, 4].

In the binary Zr-Mn system there exists ZrMn2 compound isostructural to TiMn2 and also possessing a wide homogeneity region (25.5-34.4 at.% Mn) [5].

From this point of view one can expect rather high solubility of zirconium and vanadium on the TiMn2 Laves phase.

The aim of present work is to determine the phase boundaries of Laves phase in (Ti,Zr)-Mn-V systems and to study the influence of zirconium and vanadium on hydrogen absorption properties of Ti-Mn alloys.

2. EXPERIMENTAL PART

Starting metals were iodide titanium (99.99%) and zirconium (99.99%), electrolytic vanadium (99.99%)

and remelted electrolytic manganese (99.9%). The alloys were melted from starting metals in the arc furnace with tungsten nonconsumable electrode on the copper water-cooled hearth in argon atmosphere (1.5 atm). The samples were remelted 3-4 time to insure homogeneity. Manganese was taken in excess (~4%) to compensate the losses while melting. The alloys were annealed at 1100-1150 K for 240 h in quartz ampoules under argon atmosphere (0.1 Pa). After annealing the samples were cooled to room temperature at 0.5 O/min. they were investigated by X-ray analysis, electronic microscopy probe microanalysis and PC-isotherm method.

PC-isotherms were measured in conventional Sieverts type device. The hydrogen used was obtained from LaNi5-hydride phase.

X-ray analysis was carried out in DRON-2 diffractometer with CuKa (Ni-filter). Silicon was used as an inner standard. Cell parameter determination error is ± 0.1%. The samples were crushed in the agate mortar to fine powders and were inserted in quartz holder by means of acetate glue.

Electron probe microanalysis was carried out in JXA-733 X-ray analyser coupled with microanalytical LINK-2 system.

Qualitative and quantitative composition of phases was obtained basing on results of electron microscopy and electron probe microanalysis.

3. RESULTS AND DISCUSSION

3.1. Metallic (Ti.Zr)-Mn-V systems

70 ternary and 60 quaternary alloys were prepared to study the phase compositions of metallic Ti-Mn-V amd (Ti0.9Z0.i)-Mn-V systems. According to X-ray and electron probe analysis the phase boundaries of ^-phase were determined as 25-42 at.% Ti, 38-63 at.% Mn, 0-25 at.% V for ternary system and 22-42 at.% Ti, 36-65 at.% Mn, 0-26 at.% V. The phase neighbouring the Laves phase were stated (Fig.1,2). One should pay attention to rather high solubility of vanadium in TiMn2 compound (up 25 at.%, Fig.3a)).

Alloys with titanium concentration more than 42 % also contain 9-phase (~50 % Ti) with tetragonal

structure (a = 8.12 A, c = 12.87 A) and P-phase with bcc structure (Fig.3b). In manganese-rich alloys (>65 at.%) besides Laves phase there can be found also a-Mn (bcc) and c-phase with tetragonal structure. On the vanadium-rich side ^-phase neighbours two-phase region with VMn cubic phase (a = 2.95-3.01 A). Phase composition of alloys studied in [3, 4] agrees in all with our results.

Cell parameters of single phase alloys gradually increase with increasing titanium and vanadium content almost according to Vegard rule.

It should be noted that introduction of some zirconium in quaternary system leads to an insignificant extension of Laves phase region boundaries.

3.2. Hydrogen interaction with alloys of (Ti,Zr)-Mn-V systems

The influence of chemical composition of ^-alloys on hydride formation was studied at different temperatures by PC-isotherm method. The results are summarised in Table 1, 2. The initial hydriding reaction takes place in rather mild conditions - pressure up to 50 atm and at room temperature. Fig. 4,5 show the desorption isotherms for studied alloys at room temperature, where also the TiMn1.5 isotherm is pictured for comparison. Studied alloys show good absorption capacity (1.8-2.0 mass.% hydrogen) higher than that of TiMn15. As follows from Fig.4 there is a gradual decrease of dissociation pressure values for ternary alloys with increasing titanium and decreasing manganese content. Comparing isotherms for alloys 2 and 8 (Table 1) one can conclude that with decreasing manganese concentration and increase of vanadium content by 6 % plateau pressure decreases by 5 atm. Obviously, this is explained by the fact of substitution of manganese by vanadium and titanium leading to increase of elementary cell volume. In fact the metallic radii of titanium (1.47 A) and vanadium (1.36 A) are both greater than that of manganese (1.12 A). Ternary alloys with manganese content greater that 60 at.% do not react with hydrogen up to the pressure of 100 atm.

The quaternary sample 11 (Table 2) with lowest titanium (27 %) and zirconium (3 %) content and high manganese concentration (65 %) also did not uptake hydrogen at experimental conditions. However, sample 1 (Table 2) with the same Ti/Zr ratio but smaller content of manganese and greater vanadium concentration reacts with hydrogen in the same conditions forming hydride phase with 1.98 mass.% of hydrogen. This alloy is characterised by the highest dissociation pressure of 11 atm. The same tendency can be found for alloys 3 and 12 (Table 2). As can be expected for alloys with the same concentration of titanium and zirconium the substitution of manganese for vanadium leads to decrease of plateau pressure value (samples 5, 8; Fig.5). The lowest plateau pressure (0.1 atm) is obtained for the sample 10 (Table 2) with maximum titanium (40 %) and zirconium (6 %) content.

The influence of alloy composition on the dissociation pressure value can be seen in Fig.6, 7 showing the dependence of plateau pressure on vanadium and manganese concentration at constant titanium content (35 %) for both ternary and quaternary alloys. Vanadium contri

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