Structure study on rapidly solidified hydrogen storage alloy Zr(NiM) 2.1

Transcription

1 Journl of Alloys nd Compounds (1999) L Structure study on rpidly solidified hydrogen storge lloy Zr(NiM) 2.1 G.L. Lu *, K.Y. Shu, L.S. Chen, X.Y. Song, X.G. Yng, Y.Q. Lei, Q.D. Wng, b b b b b Centrl Lbortory, Zhejing University, Hngzhou , Chin b Deprtment of Mter. Sci. nd Eng., Zhejing University, Hngzhou , Chin Abstrct Molten lloy Zr(NiM) 2.1 (MMn, Cr, V) is rpidly solidified by melt spinning method with cooling rtes of lloy ribbons between Ks nd Ks. The structure is nlyzed by mens of XRD nd TEM. The dt from XRD re further nlyzed by the Rietveld method. The results show tht ll the rpidly solidified lloys with different cooling rtes re composed only of micro-crystlline C14 Lves nd C1 Lves phses, different from the ppernce of the non-lves phse constituents Zr7M10 nd Zr9Ni11 together with the C14 nd C1 Lves phse in the slowly cooled mster lloy. The C14 Lves phse in rpidly solidified lloys ssumes lminr structure, in which the dimensions of C14 Lves micro-crystllites re lmost constnt, 1 nm long the c direction nd 3 4 nm long the nd b directions. The bundnce of C14 Lves phse increses in rpidly solidified lloys s the cooling rte increses, nd on the contrry, the bundnce of C1 Lves phse decreses. When the cooling rte increses from Ks to Ks, the bundnce of C1 Lves phse decreses from wt.% to 43.3 wt.%, nd the crystllite size of C1 phse decreses from 4.1 nm to 17.8 nm Elsevier Science S.A. All rights reserved. Keywords: Rpidly solidified hydrogen storge lloys; Lves phse; Rietveld nlysis 1. Introduction [4]. There thus exists complex reltionship between electrochemicl properties nd phse composition. It is well known tht nneled nd rpidly solidified In this pper, we studied the reltionship between phse lloys would be more homogeneous in composition, with composition nd electrochemicl properties of Zr-bsed improved microstructures nd improved physicl nd AB2 type pseudo-binry lloy by mens of rpid chemicl properties. Generlly fter nneling or rpid solidifiction method melt-spinning with single roller solidifiction, the microstructures of mny single phsed t different cooling rtes. metl hydrogen storge lloys re chnged, nd their electrochemicl properties improved, yet they still mintin the single phse structure, for exmple still of CCu 2. Experimentl type [1]. Cst Zr-bsed pseudo-binry hydrogen storge lloys re multiphse lloys tht contin AB2 type Lves 2.1. Smple preprtion phses nd AB22x type non-lves phses. Electrochemicl properties of such lloys re closely relted to the phse The mster lloy consists of Zr, Ni, Cr, Mn nd V composition which in turn re relted to the components elements with purity ll bove 99.9%. The formul of the nd combintions in B-side [2]. Nickel is the indispensble lloy is Zr(MNi) 2.1 (MMn, Cr, V). Alloys were melted B component of ll Zr-bsed AB2 type electrode lloys, repetedly in vcuum rc furnce for four times on nd is lso the element for forming non-lves phses [3]. copper crucible cooled by wter. The cooling rte ws Phse composition is chnged by nneling tretment, below 200 Ks. The mster lloy ws re-melted in which generlly worsens the electrochemicl properties *Corresponding uthor. qurtz crucible nd rpidly solidified on rotting copper roller, the melt spinning method. The cooling rte of lloy ribbons ws between Ks nd Ks. The ribbons were ground into powder mechniclly nd used s experimentl smples / 99/ $ see front mtter 1999 Elsevier Science S.A. All rights reserved. PII: S (99)

2 108 G.L. Lu et l. / Journl of Alloys nd Compounds (1999) XRD nd TEM nlysis pplied ccording to the symmetry properties of the crystl []. XRD dt for Rietveld nlysis were obtined t 28C Rietveld refinements of ll smples were performed on on horizontl Rigku D/ mx-iiib powder diffrctometer, 8 PC computer with LS1 softwre [7]. using CuK rdition nd power of 40 kv330 ma, with diffrcting bem monochrometer over the rnge u with step intervl of u nd count time of s per step. 3. Results nd discussion BF 2, which ws nneled t 008C for 2 hours in ir tmosphere, ws used s non-intrinsic brodening smple 3.1. Phse composition to mesure the instrument function nd to extrct the lloy s micro-structure dt []. Fig. 1 shows the XRD ptterns of lloys. The cooling The morphology nd electron diffrcted ptterns for rte of smples decreses from the bottom to the top of the rpidly solidified lloy with cooling rte of figure. There re obvious differences in XRD ptterns Ks were observed nd nlyzed using Philips CM12 between slowly nd rpidly solidified lloys. The ptterns TEM. of rpidly solidified lloys contin two kinds of profile. One is dispersion micro-crystllite profile, nother is shrp pek belong to C1 Lves phse. But the diffrction 2.3. Extrction of micro-structurl prmeters intensity of micro-crystllite nd the mount of lloy trnsformed into C1 Lves phse decresed s the cooling The micro-structurl prmeters of smples were de- rte decresed. There re lot of shrp peks in slowly termined by mens of Rietveld nlysis combined with solidified lloy XRD ptterns. Fig. 2 shows the Rietveld Fourier nlysis for brodening peks. The phenomeno- nlysis pttern of slowly solidified lloy smples, which logicl reltionship, describing the trend of profile width is composed of C1 nd C14 Lves phse, Zr7M10 nd nd shpe s function of diffrction ngle s populr Zr9M11 non-lves phses nd monoclinic ZrO 2, typicl Rietveld refinement routines, ws replced by fitting composition of Zr-bsed pseudo-binry hydrogen storge prmeters of crystllite size, shpe nd micro-strin. The lloy. fitting prmeters were trnsformed to the prmeters of The pek positions nd reltive intensity for microthe pseudo-voigt (pv) function by inverting the Wrren crystllite indicte tht C14 Lves phse is the predomi- Averbch procedure for single pek. Then the ex- nnt phse. Fig. 3 shows electron diffrction ptterns of perimentl dt were fitted. The nisotropy of size nd rpidly solidified lloy with cooling rte of strin tht results in the profile brodening in different Ks in two selected res. The distribution of diffrcted mnner with different Miller indices ws lso considered dots indictes hexgonl nd cubic symmetry respectively. by introducing two size strin prmeter tensors into the Clculted results show tht 0.0 nm, c0.81 nm for model. Prmeter tensors here re similr to the method ptterns of hexgonl symmetry, nd 0.71 nm for cubic used in the temperture fctor. Some constrint ws symmetry. Fig. 1. XRD ptterns of mster lloy nd rpidly solidified lloys. ) mster lloy, cooling rte below 200 Ks ; b) cooling rte Ks ; c) cooling rte Ks ; d) cooling rte 2310 Ks ; e) cooling rte Ks ; f) cooling rte Ks.

3 G.L. Lu et l. / Journl of Alloys nd Compounds (1999) Fig. 2. The observed, clculted, nd difference of CuK X-ry powder diffrction ptterns for slowly solidified lloy composed of five phses. The tick mrks below the ptterns represent the positions of ll possible Brgg reflections. They re C1, C14, Zr9M 11, Zr7M 10, ZrO2 from the top to the bottom respectively Phse bundnce nd chrcteristics of of C14 nd C1 Lves phse were dopted on the bsis of microstructure their different symmetries. An nisotropic microstructure model ws dopted for C14 Lves phse nd n isotropic Fig. 4 shows XRD Rietveld nlysis ptterns of rpidly microstructure model for C1. Tble 1 lists the numericl solidified lloys with the cooling rte of results of fitting criteri for ll smples. The vlue of fitting Ks nd Ks, respectively. The clculted index is in the scope of generl Rietveld nlysis [8]. pttern mtches the experimentl pttern very well in the These results indicte the model nd results re relible. entire rnge, indicting the selected structure model for the The results of phse bundnce, microstructure prmerefinement is correct. In this refinement, structure models ters nd cell prmeters for mster lloy nd rpidly Fig. 3. The selected re electron diffrcted ptterns for rpidly solidified lloy. ) Hexgonl symmetry, oriented long the [0113] direction. b) Cubic symmetry, oriented long the [011] direction.

4 110 G.L. Lu et l. / Journl of Alloys nd Compounds (1999) Fig. 4. Observed (dots), clculted (line), nd difference (line) results of CuK X-ry powder diffrction ptterns for C14, C1 Lves. ) cooling rte below 200 Ks ; b) cooling rte Ks. solidified lloys of different cooling rtes re listed in Tble 1 Tbles 2 nd 3. It is clerly shown tht rpid solidific- The fitting index of Rietveld nlysis for mster lloy nd rpidly tion process leds to the formtion of metstble micro solidified lloys crystllite C14 Lves phse. The bundnce of microcrystllite C14 phse decreses nd C14 trnsforms into Cooling rte R R S R wp p B C1 Lves phse s the cooling rte decreses. But the Ks C14 C1 microstructure chrcteristics re lmost invrible. Fig., shows the TEM imge of lloy t cooling rte of Ks. It is clerly shown tht TEM imge hs two different morphologies. One is of lmellr structure, nd nother is of homogeneous ppernce corresponding to micro-crystllite C14 Lves nd C1 Lves phse, Note: R weighted pttern fctor; R pttern fctor; R Brgg respectively. A prticulrly interesting lmellr chrcteriswp p B fctor; SGoodness of fit. tic of micro-crystllite is consistent with the results of

5 G.L. Lu et l. / Journl of Alloys nd Compounds (1999) Tble 2 The phse bundnce nd cell prmeters for mster lloy nd rpidly solidified lloys Cooling rte Abundnce (wt.%) Cell prmeters (nm) Ks C14 C1 C1 C14 c, (1.3) 72.77(1.71) (1) 0.007(3) () (2.1) 88.29(1.8) (1) (192) (71) (2.2) 1.0(2.42) (2) 0.04(74) (34) (1.) 1.90(1.99) (2) (43) (22) (1.90) 4.33(1.79) (2) (44) (22) (1.98) 43.3(1.7) 0.703(2) (4) (22) Tble 3 The crystllite size nd micro-strin for rpidly solidified lloys Cooling rte C1 C14 2 1/2 2 1/2 2 1/2 Ks M nn (nm) ke lnn M nn (nm) ke lnn M nn (nm) ke lnn, (1) (4) 17.2() (3) (4) (1) 3.3() (8) (10) (2) 3.0(1) (2) (1) (3) 4.2(4) (2) () (3) 3.7(2) (2) () (2) 3.(2) (2) Note: subscript nn for C1, long, b, c xis directions respectively. nn1122 for C14, long, b xis directions, 33 long c xis direction. Rietveld nlysis presented in Tble 3. The crystllite is only 1 nm in the c-xis direction nd 3 4 nminthe- nd b-xis directions. All these results indicte tht the flkelike crystllites of rpidly solidified ribbons re of lmellr structure. It should be noted tht C14 Lves phse of rpidly solidified lloys is different from tht of its mster lloy either in formtion mechnism or structure chrcteristics. The micro-crystllite C14 Lves phse is metstble phse. It coexists with C1 Lves phse within certin cooling rte rnge. We believe tht the lloy is entirely micro-crystlline bove certin cooling rte. On the contrry, it is only C1 Lves phse s the cooling rte is lowered into certin rnge. But when the cooling rte is lowered still more, component segregtion will occur, with the ppernce of non-lves phse nd C14 Lves gin, s hppened in the slowly cooled mster lloy Electrochemicl properties Fig.. TEM imges of rpidly solidified lloys showing the presence of two different morphologicl regions, lmellr nd homogeneous. The electrochemicl properties were tested in stndrd tri-electrode cells. The results indicte tht electrochemicl properties re closely relted to the phse composition nd microstructure of the rpidly solidified lloy. The experimentl results lso show tht the dischrge cpcity of multiphse s-cst lloy is 300 mahg, lower thn tht of the rpidly solidified lloy cooled t bout Ks contining minly C1 Lves phse. But when the cooling rte of rpidly solidified lloy reches.3310 Ks, it contins only 43.3 wt.% C1 Lves phse, nd its dischrge cpcity drops to 20 mahg. The cycle life of the rpidly solidified lloy is much longer thn tht of the s-cst lloy. And the higher the cooling rte of rpidly solidified lloy, the longer the cycle life is due to the increse of micro-crystllite C14 Lves

6 112 G.L. Lu et l. / Journl of Alloys nd Compounds (1999) phse. The bove results indicte tht there exists definite 97101, 90100, 94100) nd the Ntionl Advnced reltionship between the rpid solidifiction process nd Mteril Committee of Chin (No ). electrochemicl properties. Detils re reported elsewhere [9]. References 4. Conclusions Rpidly solidified (melt-spun) pseudo-binry lloy [1] W. Tng, G. Sun, J. Alloys Comp. 203 (1994) 19. [2] S.R. Ovshinsy, M.A. Fetcenko, Russ J. Sci. 17 (1993) 20. Zr(NiM) is composed of micro-crystllite C14 Lves [3] X.R. Hu, J.M. Gu, L.S. Chen, G.L. Lu, W.K. Zhng, Y.Q. Lei, Q.D. 2.1 Wng, Chinese Journl of Mteril Reserch, to be submitted. phse nd C1 Lves phse. The bundnce of micro- [4] W.K. Zhng, Disserttion Submitted to Zhejing University for the crystllite C14 Lves phse decreses s the cooling rte of Degree of Doctor of Philosophy, 1997, pp rpidly solidified lloy is lowered, nd t the sme time, [] D. Louer, ˆ J.I. Lngford, J. Appl. Cryst. (1988) 430. the bundnce of C1 Lves phse increses. The micro- [] L. Lutterotti, R. Scrdi, J. Appl. Cryst. 23 (1990) 24. crystllite C14 Lves ssumes flke-like shpe. The [7] L. Lutterotti, R. Scrdi, J. Appl. Cryst. 2 (1992) 49. [8] E. Prince, The Rietveld Method, in: R.A. Young (Ed.), Oxford existence nd bundnce of micro-crystllite C14 Lves University Press, Oxford, hve gret effects on the lloy s electrochemicl prop- [9] K.Y. Shu, Y.Q. Lei, X.G. Yong, G.F. Lin, Q.D. Wng, G.L. Lu, L.S. erties. Chen, Effect of Rpidly Solidified Process on the Alloy Structure nd Electrochemicl Performnces of Zr(NiMnM) 2.1, Interntionl Symposium on Metl Hydrogen Systems, Fundmentl nd Appliction, Acknowledgements Oct. 4 9, 1998, Hngzhou, Chin. This work ws supported by Ntionl nd Zhejing Provincil Nturl Science Foundtion of Chin (No.