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International Journal of Hydrogen Energy 28 (2003) 395–401

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In&uence of annealing treatment on Laves phase compoundcontaining a V-based BCC solid solution phase—Part II:

Electrochemical properties

Yunfeng Zhu, Hongge Pan∗, Mingxia Gao, Yongfeng Liu, QidongWangDepartment of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

Received 22 March 2002; accepted 25 June 2002

Abstract

As the second part, the e<ects of annealing treatment on the electrochemical properties of two multi-component Lavesphase hydrogen storage alloys containing BCC solid solution are reported in this paper. It is found that the discharge capacity,the activation property, the cycle life and the high rate dischargeability of both alloys are improved after annealing treatment.However, the exchange current density I0 and the limiting current density IL decrease. The changes in electrochemical propertieshave been discussed related to the changes in crystal structures as have been reported in the >rst part. It is concluded that theoverall electrochemical properties of the hydrogen storage alloys can be improved greatly by an appropriate annealing treatment.? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Hydrogen storage alloy; C14 Laves phase; V-based solid solution; Annealing treatment; Electrochemical property

1. Introduction

In part I [1], the e<ects of annealing treatment on thecrystal structures of the alloys Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2and Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 were investigated. Theresults showed that both alloys were mainly composed ofthe C14 Laves phase matrix and the dendritic V-based solidsolution phase, with a small amount of TiNi-based thirdphase precipitated in them in addition owing to the com-positional segregation during solidi>cation. The content ofthe TiNi-based precipitated phase was reduced greatly by anappropriate annealing treatment. In this part, the e<ects ofannealing treatment on the electrochemical properties willbe reported.

The multi-component, multi-phase hydrogen storagealloys with improved performances are regarded as the newapproach for the negative electrode materials of the nickel–metal hydride (Ni–MH) secondary batteries due to the

∗ Corresponding author. Tel.: +86-571-8795-2576; fax: +86-571-8795-1152.

E-mail address: honggepan@zjuem.zju.edu.cn (H. Pan).

particular and/or synergistic e<ect of di<erent phases inthe alloys [2–6]. Notten [2] formulated a new class ofhighly electro-catalytic materials with two phases: onebeing the LaNi5-based bulk phase responsible for hydro-gen storage, and the other being the precipitated phase,such as MoCo3, responsible for the electro-catalytic activa-tion of the electrochemical hydrogen reaction of the bulkphase. Ovshinsky et al. [6] developed some multi-element,multi-phase disordered hydrogen storage alloys and pointedout that the disordered state at the metal/electrolyte inter-face had led to the formation of some surface oxides withinwhich very >ne metallic nickel alloy inclusions (less than70 JA in diameter) were suspended to provide the electrodesurface with exceptionally good electrochemical catalyticactivity.

In this work, we also prepared two multi-componentLaves phase hydrogen storage alloys containing BCC solidsolution and the e<ects of annealing treatment on the elec-trochemical properties, including the discharge capacity,the activation, the cycle life, the high rate dischargeability,the electrochemical impedance spectroscopy (EIS), the ex-change current density I0 and the limiting current densityIL were studied systematically.

0360-3199/02/$ 30.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.PII: S0360 -3199(02)00151 -9

396 Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 395–401

2. Experimental

The preparation of the as-cast and the henceforth an-nealing treatment of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 andTi0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloys have been reportedin detail in Part I [1]. Part of the as-cast and the annealedalloy samples were mechanically crushed and ground to thepowder of 300 mesh size for electrochemical measurements.

All test electrodes were prepared by >rst mixing thor-oughly 0:1 g alloy powder with 0:4 g carbonyl nickelpowder and then cold-pressing the mixture under a pres-sure of 25 MPa into a pellet of 10 mm diameter and about1 mm in thickness. The electrochemical measurementswere performed in a half-cell consisting of a working elec-trode (MH electrode) of the alloy to be tested, a sinteredNi(OH)2/NiOOH counter electrode with excess capacityand a Hg/HgO reference electrode. The electrolyte was a6 M KOH solution, controlled at 30 ± 1◦C. In order toreduce the IR drop during the electrochemical impedancespectroscopy measurements, the Hg/HgO reference elec-trode was equipped with a Luggin tube. The dischargecapacity and the cycle life of the test electrode were de-termined by the galvanostatic method. Each electrode wascharged at 100 mA=g for 5 h followed by 10 min restand then discharged at 60 mA=g to the cut-o< potential of−0:6 V vs. the Hg/HgO reference electrode. For the investi-gation of high rate dischargeability, the discharge capacitiesat several di<erent discharge current densities were tested.Electrochemical impedance spectroscopy (EIS) measure-ments were conducted at 50% depth of discharge (DOD)at 298 K using Solartron SI1287 Electrochemical Interfacewith a 1255B Frequency Response Analyzer. Before EISmeasurements, the test electrodes were completely acti-vated by cyclings. The EIS spectra of the electrodes wereobtained in the frequency range of 100 kHz to 5 mHz withan ac amplitude of 5 mV under open-circuit conditions.The linear polarization curves and the anode polarizationcurves of the test electrodes were measured by scanningthe electrode potentials (Solartron SI1287 potentionstat) at50% DOD at the scanning rate of 0:1 mV=s from −5 to5 mV (vs. open circuit potential) and 5 mV=s from 0 to500 mV (vs. open-circuit potential) at 298 K, respectively.

3. Results and discussion

3.1. Discharge capacities and cycle life

Fig. 1(a) and (b) show the cycle life curves of theTi0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy and the Ti0:8Zr0:2V3:733

Mn0:747Cr1:12Ni1:4 alloy, respectively. Both alloys were an-nealed for 5–11 h at 1273 K. The maximum dischargecapacities are presented in Table 1. It is obvious fromthe table that for both alloys, the annealing treatmentleads to the increase in maximum discharge capacity.Besides, with the prolongation of the holding time, the

Fig. 1. Discharge capacity vs. cycle number for the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy (a) and the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy (b) at 303 K.

maximum discharge capacity is further increased. For ex-ample, the maximum discharge capacity of the as-castTi0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy is 320:3 mA h=g andit is increased to 353:4 mA h=g for the alloy annealedat 1273 K for 11h, and the maximum discharge capac-ity of the as-cast Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy is306:0 mA h=g, while that of the alloy annealed at 1273 Kfor 11 h is increased to 339:8 mA h=g.

It can also be seen from Table 1 that the number ofactivation cycles for fully activating the alloy electrodesdecreases after annealing treatment. For both as-cast al-loys, it needs 15 and 35 activation cycles, respectively.However, it only needs 6 and 7 activation cycles for bothalloys annealed at 1273 K for 11 h, respectively. So itis concluded that the annealing treatment makes the al-loy electrodes to be activated more easily, and the longerthe holding time, the easier of the alloy electrode to beactivated.

For investigation of the e<ects of annealing treatmenton the cycle life, the capacity decay rate Dr;c at a certain

Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 395–401 397

Table 1The electrochemical properties of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy and the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy

Alloys Samples Cmax (mAh/g) Naa D60;c

b (mAh/g cycle) R600c (%)

Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 As-cast 320.3 15 0.57 67.79

1273 K × 5 h 338.7 9 0.58 71.151273 K × 8 h 348.7 8 0.51 72.971273 K × 11 h 353.4 6 0.44 75.37

Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 As-cast 306.0 35 0.35 72.92

1273 K × 5 h 310.8 12 0.28 74.991273 K × 8 h 327.8 10 0.34 76.711273 K × 11 h 339.8 7 0.28 78.81

aThe cycle numbers needed to activate the electrodes.bThe capacity decay rate at the discharge current density Id = 60 mA=g.cThe high rate dischargeability with discharge current density Id = 600 mA=g.

discharge rate r can be expressed as [7]

Dr;c =Cr;max − Cr;NN − Nmax

; (1)

wherein Dr;c is the capacity decay rate at a certain dischargerate r, Cr;max the maximum capacity at the discharge rater, Cr;N the capacity at cycle number N at the dischargerate r and Nmax the cycle number when the maximumcapacity reaches. The capacity decay rate D60;c of eachelectrodes at the discharge rate 60 mA=g after 150 charge–discharge cycles is calculated and presented in Table 1.It can be seen that the annealing treatment results in thedecrease in capacity decay rate for both alloys, whichmeans that the cycle life increases. For example, the capac-ity decay rate of the as-cast Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2alloy is 0:57 mA h=g cycle and it is decreased to0:44 mA h=g cycle for the alloy annealed at 1273 Kfor 11 h, and the capacity decay rate of the as-castTi0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy is 0:35 mA h=gcycle, while that of the alloy annealed at 1273 K for 11 his decreased to 0:28 mA h=g cycle. However, the capacitydecay rate of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy an-nealed at 1273 K for 5 h increases slightly. But in any case,the cycle life can be improved by an appropriate annealingtreatment, such as 1273 K × 11 h annealing.

3.2. High rate dischargeability

Fig. 2(a) and (b) show the high rate dischargeabilityHRD of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy and theTi0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy, respectively. TheHRD is calculated from the following formula:

HRD =Cd

Cd + C60× 100%; (2)

Fig. 2. High rate dischargeability HRD of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy (a) and the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy (b) at 303 K.

398 Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 395–401

wherein Cd is the discharge capacity with cut-o< potentialof −0:6 V vs. Hg/HgO reference electrode at the dischargecurrent density Id, C60 is the residual discharge capacity withcut-o< potential of −0:6 V vs. Hg/HgO reference electrodeat the discharge current density I =60 mA=g after the alloyelectrode has been fully discharged at the large dischargecurrent density under investigation. The HRDwith dischargecurrent density Id = 600 mA=g are listed in Table 1. It canbe found that the HRD of both alloy electrodes increasesafter annealing treatment. Besides, with the prolongation ofthe holding time, the HRD is increased continuously andreaches the maximum at 1273 K × 11 h.

3.3. AC impedance and polarization

Fig. 3(a) and (b) show the electrochemical impedancespectra (EIS) of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloyelectrode and the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloyelectrode at 50% DOD, respectively. For all the alloysamples, it can be seen that the EIS spectra consists oftwo semicircles at the high-frequency region followed bya straight line at the low-frequency region. The depressedshape of the two semicircles, especially the smaller semi-circle in the high-frequency region, which looks like astraight line, may be due to the porosity e<ects of thealloy electrode surface. According to the analysis modelproposed by Kuriyama et al. [8], the smaller semicirclein the high-frequency region is attributed to the contactresistance between the alloy powder and the conductivematerial, while the larger semicircle in the low-frequencyregion is attributed to the charge transfer resistance on thealloy surface. For the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy,the radius of the larger semicircle in the low-frequencyregion increases after annealing treatment, especially forthe alloy annealed at 1273 K for 11 h, which indicates thatthe charge transfer resistance of the electrode surface is in-creased. However, for the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4alloy, the annealing treatment does not a<ect the chargetransfer resistance of the electrode surface much and it isincreased slightly with the prolongation of the annealingtime.

Fig. 4(a) and (b) show the linear polarization curvesof the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy electrode andthe Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy electrode at 50%DOD, respectively. The exchange current density I0; whichcan be calculated by the following formula [2]:

I0 =IRTF�

(3)

are listed in Table 2, where R; T; F; � denote the gas con-stant, the absolute temperature, the Faraday constant andthe overpotential, respectively. As can be seen in the table,all the annealed alloy samples have a lower I0 than that ofthe as-cast alloy samples, and with the prolongation of theholding time, the I0 decreases continuously. The variationof the I0 is associated with the polarization resistance on

Fig. 3. Electrochemical impedance spectra of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy (a) and the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy (b) measured at the 50% DOD and 298 K.

the electrode surface. So the decrease in I0 means that thepolarization resistance increases after annealing treatment.

Fig. 5(a) and (b) show the anode polarization curvesof the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy electrode andthe Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy electrode at 50%DOD, respectively. The limiting current densities IL ob-tained from the >gures are listed in Table 2. It can be seenthat the IL decreases for both alloys after annealing treat-ment, and the variation is consistent with that of the I0 withthe prolongation of the holding time.

3.4. Discussion

In part I [1], it has been concluded that three phases,namely the C14 Laves phase, the V-based solid solutionphase and the TiNi-based phase coexist in the as-cast alloys.The TiNi-based phase is found precipitated in both the C14Laves phase and the V-based solid solution phase, which canbe ascribed to the compositional segregation during solidi>-cation. However, the content of the TiNi-based precipitated

Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 395–401 399

Fig. 4. Linear polarization curves for the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy (a) and the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy (b) with a scan rate of 0:1 mV=s measured at the50% depth of discharge (DOD) and 298 K.

phase is reduced greatly by an appropriate annealing treat-ment, and thus the alloys are mainly composed of theC14 Laves phase and the V-based solid solution phase. It

Table 2The exchange current density I0 and the limiting current density IL of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy and theTi0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy

Alloys Samples I0 (mA/g) IL (mA/g)

Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 As-cast 158.2 1112.8

1273 K × 5 h 140.6 1102.91273 K × 8 h 122.3 1058.91273 K × 11 h 62.4 818.7

Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 As-cast 145.3 1294.7

1273 K × 5 h 137.7 1129.61273 K × 8 h 124.4 1055.91273 K × 11 h 118.0 1045.1

was investigated by Tsukahara et al. [3,4] that the V-basedsolid solution phase alone had very little electrochemicaldischarge capacity in the KOH electrolyte due to the lackof electro-catalytic activity. However, it could be activatedto absorb and desorb a large amount of hydrogen with thepresence of a secondary phase, such as the C14 Laves phaseor the TiNi phase, which was considered to act both asa micro-current collector and as an electro-catalyst. In thecurrent study, it is believed that all the three phases, withthe V-based solid solution phase playing the chief role, arethe hydrogen storage phases and the C14 Laves phase andthe TiNi-based phase working mainly as the electro-catalystand the micro-current collector.

The discharge capacities of both alloys studied in thispaper are improved greatly by annealing treatment. Somereasons are considered as follows: >rstly, as have been con-cluded in part I [1], the unit cell volumes of both the C14Laves phase and the V-based solid solution phase increaseafter annealing treatment. Nakano et al. [9] showed that theincrease in unit cell volume of the hydrogen storage phaseresulted in the increase in the −RH value and thus in-creased the hydrogen desorption capacity, where −RH isthe partial molar enthalpy for the hydride formation. So theincrease in unit cell volumes of both the C14 Laves phaseand the V-based solid solution phase here is believed to bethe >rst reason for the increase in discharge capacities ofthe annealed alloys. Secondly, Kuriyama et al. [10] stud-ied the V–Ti–Ni-based multi-phase hydrogen storage elec-trode alloy by in situ scanning tunneling microscopy (STM)and showed that the TiNi-based secondary phase can absorband desorb hydrogen reversibly in a 6 M KOH aqueous so-lution, and the hydrogen atoms in the second phase wereconsidered to di<use into the V–Ti main phase after theTiNi-based secondary phase is saturated with hydrogen. Soit is believed the TiNi-based phase precipitated in the as-castalloy mainly plays the role of providing the electro-catalyticactivity for the hydrogen absorption of the alloy and alsoparticipate in the hydrogen storage. However, as the C14Laves phase is electrochemically active so that the presence

400 Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 395–401

Fig. 5. Anode polarization curves for the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy (a) and the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy (b) with a scan rate of 5 mV=s measured at the50% depth of discharge (DOD) and 298 K.

of the TiNi-based phase is not a prerequisite and the TiNiphase has a lower discharge capacity than the C14 Lavesphase or the V-based solid solution phase. So the decreasein content of the TiNi-based phase after annealing treatmentin the current study is believed to be another reason for theincrease in discharge capacity of the alloy. Thirdly, it wasfound in part I [1] that the content of the V-based solidsolution phase in the alloy and the content of the V elementin the V-based solid solution phase all increased after anneal-ing treatment. In addition, the V-based solid solution phaseis know to be the structure of hydrogen storage alloy withvery large discharge capacity even higher than 500 mA h=g[11]. Therefore, the increase in content of the V-based solidsolution phase in the alloy and the increase in content of theV element in the V-based solid solution phase after anneal-ing treatment here are also responsible for the increase indischarge capacity.

It is believed that the capacity degradation of the hydro-gen storage alloys is mainly caused by two reasons: one isthe intrinsic capacity decay owing to the pulverization ofthe alloys during charge–discharge cycles, and the other is

the oxidation and the corrosion of the alloy components inthe alkaline electrolyte. In the current study, the cycle lifeof both alloys is improved by annealing treatment. As theannealing treatment reduces the content of the TiNi-basedprecipitated phase and makes the alloy more homogenous,which results in a higher pulverization resistance of the al-loy and thus improves the cyclic durability of the alloyelectrode. Besides, it has been reported by Tsukahara et al.[4] that the cyclic durability of the V-based solid solutionalloy increased with an appropriate heat treatment owing tothe decrease in V content in the secondary phase (the C14Laves phase or the TiNi-based phase), as the V element wasa corrosive constituent in the secondary phase. In the >rstpart [1], the content of the V element was found to decreasein the C14 Laves phase, which makes us believe that thecorrosion and dissolution of the V element decreases duringcycles after annealing treatment and thereby improves thecycle life. Therefore, the improve in cycle life of both al-loys is attributed to the increase in pulverization resistanceof the alloys and the decrease in V content in the C14 Lavesphase after annealing treatment.

As stated above, the charge transfer resistance of the elec-trode surface increases, and the exchange current densityI0 and the limiting current density IL of the alloy electrodedecrease after annealing treatment. As a result, one maythink that the high rate dischargeability HRD of the alloyelectrode will be worsened. However, it is not the case inthe present study and the HRD increases for both alloysafter annealing treatment. Notten et al. [2,12,13] reportedthat the kinetics of the hydrogen storage alloy electrodewas always determined both by the combined e<ect of thecatalytic activity and the speci>c surface area of the elec-trode. Kim et al. [14] also found that the reaction surface areaplayed an important role in the HRD of the alloy electrode.They proved it with the alloys with high Mn content, whichhad constantly a large reaction surface area and thus a higherHRD. Accordingly, we believe that the reaction surface areaof the alloy electrode is increased after annealing treatmentand its e<ect on the HRD surpasses the decrease in speci>csurface catalytic activity and hence improves the rate capa-bility of the alloy electrode. Besides, the increased reactionsurface area also leads to a fast activation of the electrodeduring the initial charge–discharge cycles, and brings betteractivation properties for the annealed alloy samples.

4. Conclusions

The e<ects of annealing treatment on the elec-trochemical properties and the correlation betweenthe electrochemical properties and the crystal struc-tures of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy and theTi0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy were investigatedin this paper. It is found that the annealing treatment cansimultaneously improve the discharge capacity, the activa-tion property, the cycle life and the high rate dischargeability

Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 395–401 401

of both alloys. However, the charge transfer resistance ofthe electrode surface increases, and the exchange currentdensity I0 and the limiting current density IL of the alloyelectrode decrease after annealing treatment. Even so, westill believe that the overall electrochemical properties ofthe hydrogen storage alloys can be improved greatly by anappropriate annealing treatment.

References

[1] Yunfeng Zhu, Hongge Pan, Mingxia Gao, YongfengLiu, Qidong Wang, Int J Hydrogen Energy, S0360-3199(02)00078-2.

[2] Notten PHL, Hokkeling P. J. Electrochem Soc 1991;138:1877.[3] Tsukahara M, Takahashi K, Mishima T, Isomura A, Sakai T.

J Alloys Compounds 1996;236:151.[4] Tsukahara M, Takahashi K, Mishima T, Isomura A, Sakai T.

J Alloys Compounds 1996;243:133.

[5] Akiba E, Iba H. Intermetallics 1998;6:461.[6] Ovshinsky SR, Fetcenko MA. Appl Phys A 2001;72:239.[7] Li Chuan-Jian, Wang Xin-Lin. J Alloys Compounds

1998;270:242.[8] Kuriyama N, Sakai T, Miyamura H, Uehara I, Ishikawa H,

Iwasaki T. J Alloys Compounds 1993;202:183.[9] Nakano H, Wakao S, Shimizu T. J Alloys Compounds

1997;253,254:609.[10] Kuriyama N, Chartouni D, Tsukahara M, Takahashi K,

Takeshita HT, Tanaka H, Schlapbach L, Sakai T, Uehara I.Electrochem Solid-State Lett 1998;1:37.

[11] Iwakura C, Choi W, Miyauchi R, Inoue H. J Electrochem Soc2000;147:2503.

[12] Notten PHL, Einerhand REF, Daams JLC. J AlloysCompounds 1994;210:221.

[13] Senoh H, Morimoto K, Inoue H, Iwakura C, Notten PHL. JElectrochem Soc 2000;147:2451.

[14] Kim DM, Lee SM, Jang KJ, Lee JY. J Alloys Compounds1998;268:241.