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8phase transformations in a composite
prepared by high-energy ball milling
of aluminium- and vanadium-based alloys in hydrogen
O.G. Ershova1, V.D. Dobrovolsky1, M.V. Lototsky12, V.A. Yartys2, B.P. Tarasov3
1 Institute for Problems of Materials Science of National Academy of Sciences of Ukraine, 3 Krzhyzhanovsky Str., 03142, Kiev, Ukraine phone.: +38-067-5739229, fax: +38-044-4242131, E-mail: [email protected]
2 Institute for Energy Technology, POB 40, N-2027, Kjeller, Norway phone: +47-63806453, fax: +47-63812905, E-mail: [email protected]
3 Institute of Problems of Chemical Physics of the Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia Phone/fax: +7-496-5221743, E-mail: [email protected]
The experimental studies aimed to finding the routes to provide the reversibility of the Al-AlH3 system were carried out. These activities included the experiments on high energy ball milling of Al alloy in hydrogen atmosphere, in the presence of a solid hydrogenation catalyst (easy-hydrogenated b.c.c. alloy on the basis of vanadium). It was shown that high energy ball milling of an Al alloy containing Mg and Li additives with the V75Ti10Zr7 5Cr7 5 catalytic additive in hydrogen results in the increase of the H absorption capacity of the final product, as compared to the hydrogenated V alloy. Such an increase can be explained only by assuming a contribution of the Al alloy to the hydrogen uptake. The absorbed hydrogen can be released by heating the composite to 400-500 oC, however, the heating results in sintering to form Al - V intermetallic phases which do not absorb hydrogen anymore at experimental conditions.
Introduction
Aluminium hydride is a very prospective hydrogen storage material combining high H storage capacity (10 wt.% H; 0,148 gH/cm3) with rather low (~150 oC) decomposition temperature [1-3]. However, it cannot be synthesised by direct interaction of the parent Al metal with hydrogen at reasonable conditions. The direct hydrogenation of aluminium takes place only at high (30-50 kBar) hydrogen pressures [4, 5], in the course of electrolysis of NaAlH4 [6], or by the bombardment of Al with high-energy beams of hydrogen / deuterium ions [7].
Mechanical alloying (MA) has been proven to be an efficient method of producing MH materials with the enhanced H sorption performances. The work of many researchers [8-10] has proved this as an efficient way to modify composition and structure of the parent material, thus improving its hydriding and dehydriding kinetics. During MA, powder particles are trapped between colliding balls or between ball and container walls. These high-energy impacts induce fracturing and cold welding of particles and define the ultimate structure of the powder. The product often exhibits unusual physico-chemical properties and
enhanced reactivity, in particular, with respect to hydrogen. The most efficient way for the synthesis of the enhanced MH materials is the MA in hydrogen atmosphere where an easily-hydrogenated alloy / intermetallide is added to the parent material [10].
In this study we made an attempt to hydrogenate aluminium by high-energy ball milling of an Al alloy in hydrogen atmosphere, in the presence of an easy-hydrogenated b.c.c. alloy on the basis of vanadium.
Experimental
Aluminium alloy doped by 5,1 wt.% Mg, 2,1 wt.% Li, 0,17 wt.% Sc, 0,08 wt.% Zr was prepared by induction melting. The easy-hydrogenated as cast alloy V75Ti10Zr75Cr75 [11, 12] used as hydrogenation catalyst was arc-melted in argon atmosphere (ingot ~50 g).
Fillings of the aluminium alloy and a piece of the vanadium alloy were loaded into steel ball-milling vial, together with steel balls (weight ratio «Al alloy / V alloy» = 2:1, total weight of the charge = 4 g, balls to charge ratio = 25:1). The vial was evacuated with a rotary pump and filled with
International Scientific Journal for Alternative Energy and Ecology № 2 (58) 2008
© Scientific Technical Centre «TATA», 2008
O.G.Ershova, VD.Dobrovolsky, M.V.Lototsky, V.A.Yartys, B.P.Tarasov
Phase transformations in a composite prepared by high-energy ball milling of aluminium- and vanadium-based alloys in hydrogen
hydrogen at starting pressure of 12 bar. A home-made planetary ball mill was used, the rotation speed was 1500 rpm. The HEBM procedure was carried out in 15 steps each of those included milling itself followed by cooling the vial to room temperature and refilling it with hydrogen at P=12 bar; the total milling time was 2 hours (Table 1).
Table 1
Ball milling procedure
Duration, minutes
Step # Milling Cooling to RT / Refilling with H2
1 1 15
2 3 15
3 5 15
4..15 10 60
After unloading in an argon glove box, the sample (dark-grey powder) was taken for the XRD analysis and for a series of thermal desorption spectra (TDS) measurements in argon or hydrogen atmospheres (P=1 bar), in temperature range 20 to 550 oC. A quartz reactor with the sample (m=0,2 g) was attached to a volumetric setup (Fig. 1)
equipped with a piston mechanism; displacements of the latter were measured using a sensor calibrated on changes of volume of gas in the system. The reactor was equipped with a K-type thermocouple and heated by electric furnace connected to a controller providing linear heating (ramp rate 5 deg / min). The thermocouple and the displacement sensor were connected to a data acquisition system (based on ICP DAS Interface Modules). After loading the sample, the system was evacuated by a rotary pump, filled with argon or hydrogen prior to starting the desorption experiments during the heating. After the maximum temperature of the TDS experiment was reached, the furnace was switched off, and the reactor was cooled to room temperature (in so doing, the logging of gas volume change and temperature was continued) followed by unloading the sample for the XRD analysis.
The XRD analysis of the sample and products of its thermal decomosition was carried out using DRON-3M instrument, Cu-Km radiation (A= 1,5405999 A); Bragg angle range 28=20...90°, resolution 0,1o. The XRD patterns were processed using GSAS and Powder Cell software.
Fig. 1. Schematics of a volumetric TDS setup
Международный научный журнал «Альтернативная энергетика и экология» № 2(58) 2008 © Научно-технический центр «TATA», 2008
Results and discussion
Typical spectrum of hydrogen thermo-desorption (Ar atmosphere) from the ball-milled sample is presented in Fig. 2. The desorption starts at T~100 oC; maximum rate of gas evolution is observed at T=250..300 oC; hydrogen release is completed at T~500 oC. TDS taken both in Ar and in H2 showed similar behaviour, and during cooling the sample in H2 the absorption was not observed. In all cases the total amount of the desorbed hydrogen (150 ccm/g at T =400 oC and 170 ccm/g at T =560 oC) exceeds the
max ° max '
value (133 ccm/g) calculated with the assumption that only V alloy has been completely hydrogenated during the ball-milling (Xmax~400 ccm/g for the individual V75Ti10Zr7 5Cr7 5 as cast alloy [12]).
Fig. 2. Integral (a) and difference (b) spectra of hydrogen thermodesorption from the sample ball-milled in hydrogen. The dashed line corresponds to maximum H sorption capacity of the V alloy present in the composite
The XRD analysis (Fig. 3A-D) shows that the starting Al alloy (A) consists of a single Al phase; the observed lattice period (a= 4.0680(2) A) is higher than for pure aluminium (a=4,0411 A [13]). The ball milled composite (B) contains the Al phase (a=4,0392(4) A), together with f.c.c. VH2 (a=4,2795(6) A); the pattern shows significant line broadening. The calculated amount of Al in the composite, 75,3(3) wt.%, is rather close to its content in the charge (66,7 wt.%). Although there was not clear indication of presence of AlH3, a trace peak observed at 28=27,8..28° could be ascribed to the [102] line of its a-modification.
As it is seen from Fig. 3 (C, D), during thermo-des-orption sintering of the composite takes place. Already at Tmax=400 oC fraction of Al (14(1) wt.%; a=4,047(1) A) is significantly reduced; the BCC phase which should be
Fig. 3. XRD pattern of starting alloy (A) and ball-milled composite before (B) and after (C, D) hydrogen thermo-desorption
formed after complete decomposition of VH2 has the lattice period a=3,133(3) A that is close to the value for V7Al3 reported in literature [14]). This measured value is significantly higher compared to those for the dehydrogenated V75Ti10Zr75Cr75 (a=3,0420(3) A for the as cast [12] and a=3,0349(7) A for the annealed alloy [11]). In addition, new phases identified as BCT-VAl3 [15] (a=3,796(8) A, c=8,44(4) A) and FCC-VAl10 [16] (a=14,596(8) A) are formed. Both aluminium and the BCC phase disappear at Tmax=560 oC, and VAl3 (a=3,7921(9) A, c=8,381(4) A), together with VAl10 (a=14,593(4) A), were observed in the sample at highest applied temperature.
Increased H storage capacity of the ball milled composite, compared to the hydrogenated V75Ti10Zr7 5Cr7 5, can be explained only by assuming a contribution of Al alloy to the hydrogen uptake. Since at ambient pressure-temperature conditions pure Al metal does not form a hydride, hydrogenation of its alloy may be related to the influence of the doping additions, Mg and Li. These elements form a solid solution with Al; this may either change the thermodynamics of the hydrogenation reaction to produce alane, or to participate in the hydrogenation individually, by forming corresponding binary hydrides, MgH2 and LiH.
The presented results do not allow to conclude that the ball milling of the Al alloy with the V75Ti10Zr7 5Cr7 5 catalytic additive in hydrogen results in the formation of AlH3. At the same time, it would be early to reject this hypothesis. some data, including the above-mentioned increased H storage capacity of the ball milled composite, presence of an extra peak close to the strongest line of a-AlH3 in the
International Scientific Journal for Alternative Energy and Ecology № 2(58) 2008
© Scientific Technical Centre «TATA», 2008
O.G.Ershova, V.D.Dobrovolsky, M.V.Lototsky, V.A.Yartys, B.P.Tarasov
Phase transformations in a composite prepared by high-energy ball milling of aluminium- and vanadium-based alloys in hydrogen
XRD pattern, unresolved peak of gas evolution on differential TDS pattern at 7~180 oC, as well as significant reduction of lattice period of Al phase during ball milling, still allow to suppose that such a formation takes place, and the residual content of AlH3 in the sample after its ball milling (estimated starting from the increase of the H capacity and quantitative analysis of the XRD pattern) is about 2-4 wt. %. Most probably, heating of the sample during ball milling results in the thermal decomposition of the unstable AlH3 which could be formed in the course of this process.
Conclusion
It was shown that high energy ball milling of an Al alloy containing Mg and Li additives with a V75Ti10Zr7 5Cr7 5 catalytic additive in hydrogen results in the increase of the H absorption capacity of the final product, as compared to the hydrogenated vanadium alloy. Such an increase can be explained only by assuming a contribution of the Al alloy to the hydrogen uptake. The absorbed hydrogen can be released by heating the composite to 400-500 oC, however, the heating results in sintering to form Al - V intermetallic phases which do not absorb hydrogen anymore at experimental conditions.
Acknowledgement
This work is carried out in the framework of the INTAS Project 05-1000005-7665 «NEW ALANE: Novel Reversible Hydrogen Storage Materials Based on the Alloys of Al».
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Международный научный журнал «Альтернативная энергетика и экология» № 2(58) 2008 © Научно-технический центр «TATA», 2008
