Characteristics of the Ti 1 . 27 Fe + 11 wt . % Ni Composite Obtained by ArcMelting and Ball Milling

1 Facultad de Química, Universidad Autónoma del Estado de México, Avenida Colón Esquina con Paseo Tollocan s/n, Toluca, 50120 Méx, Mexico Departamento de Química, Instituto Nacional de Investigaciones Nucleares, Carretera México-Toluca s/n, La Marquesa, 52750 Ocoyoacac, Méx, Mexico Departamento de Física, Instituto Nacional de Investigaciones Nucleares, Carretera México-Toluca s/n, La Marquesa, 52750 Ocoyoacac, Méx, Mexico


Introduction
e intermetallic compound TiFe is one of the most wellknown alloys as a hydrogen storage material, because of abundance and low cost of raw material and moderate conditions for hydrogenation/dehydrogenation processes.However, it requires some activation treatments before hydrogen reaction, for example, heating over 400-450 ∘ C in vacuum and subsequent annealing in hydrogen at pressures of 0.7-1.0MPa, followed by cooling to room temperature and exposure to hydrogen at pressures of 3.0-6.5MPa.e activation processes of hydrogenation, charging-discharging, have to be repeated several times to obtain reproducible pressure-concentration isotherms [1][2][3][4][5].To improve hydrogenation/dehydrogenation processes, several treatment have been reported with respect to the TiFe alloy composition, by substituting Fe in FeTi alloy for transition elements such as Ni, Mn, Zr, and others or by modifying the surface of the alloy via the induction of an speci�c oxide, such as NbO, Cr 2 O 3 [6][7][8][9][10][11].Currently, new development and research are focusing on obtaining metal matrix nanocomposites with outstanding microstructural properties, such as speci�c area, size grain, dislocations, and diameter pore.Recently, mechanical alloying (MA) and mechanical grinding (MG) have extensively been used to synthesize various nonequilibrium alloys, nanocomposites, amorphous and nanocrystalline materials.As it is known that modi�ed nanocomposites are used as hydrogen storage materials, these alloys were also applied for the Ni-MH batteries.A large number of works on hydrogen absorption and activation properties for TiFe alloys produced by MA or MG have been reported [12][13][14][15][16][17][18].
e present work is related to the characteristics of the Ti 1.27 Fe + 11 wt.%Ni composite obtained by arc melting and ball milling process.e addition of nickel on TiFe alloy, improved the hydrogen absorption-desorption process, the difference in the hydrogen storage characteristics for this composite was correlated with its composition and microstructure.

Experimental Procedure
For the synthesis of TiFe alloy, a mixture (atom ratio 1.27 : 1) of Ti (98.0%, <325 mesh) and Fe (99.5%, <200 m) was melting in an arc melting device under an argon atmosphere on a water-cooled Cu hearth.A button shape alloy was obtained, crushed and pulverized in a mortar of stainless steel, and ground into powder <115 mesh to obtain particles size less than 125 m in diameter.To embrittle the alloy powder was immediately loaded into the reactor and thermaly treated at 350 ∘ C for 3 h under hydrogen pressure of 4 MPa.Ti 1.27 Fe + 11 wt.%Ni was then mechanically milled into a tungsten carbide vial together with stainless steel balls under argon atmosphere.e ball-to-powder weight ratio was 4 : 1. Process control agent of methanol was added to the powder mixture to prevent agglomeration and reaction with Ti. e milling was carried out for 5 h in argon atmosphere by using a high energy ball mill type Spex 8000 designed at Instituto Nacional de Investigaciones Nucleares [19].
e hydrogen absorption property of Ti 1.27 Fe + 11 wt.%Ni composite was evaluated in a 50 mL capacity stainless steel reactor, by exposing the powder sample to gaseous hydrogen (99.999% nominal purity).Prior to hydriding reaction, the ball-milled powder was vacuum-heated up to 100 ∘ C during 3 h.e hydrogen absorption of composite was carried out at temperature of 100 ∘ C under H 2 pressures between 0.2 and 1.4 MPa during 30 minutes.
Samples characterization, before and aer hydrogen reaction, were carried out by X-ray diffraction (XRD) analysis on a Siemens D5000 diffractometer with Cu K radiation, scanning electron microscopy (SEM; Phillips XL30), and transmission electron microscopy (TEM; Jeol 2010 HT).Energy dispersive X-ray (EDS) analysis was used for elemental analysis of the selected microarea and inductively coupled plasma-optical emission spectroscopy (ICP) for elemental quantitative analysis.e surface area was determined from the nitrogen adsorption isotherm by the BET method and the pore size distribution from the branch desorption by the BJH method.Nitrogen adsorption of the milled samples was measured at −196 ∘ C with an equipment of physisorption, Belsorp Max Japan INC. Prior to the measurement, the samples were degassed at 150 ∘ C for 3 hours in nitrogen atmosphere.To evaluate the hydrogen content in the composite, it was analyzed by simultaneous differential technique (SDT), analyzer (DSC-TGA) before and aer the hydrogenation process using Calorimeter TA Instruments-Waters model SDT Q600, previously calibrated.e XRD patterns of Ti-Fe/Ni mixture at various stages of the process are presented in Figure 1.X-ray diffraction pattern of initial powder mixture shows only Bragg re�ections from elemental Ti and Fe, Figure 1(a).Figure 1(b) shows characteristics re�ections of the formation of the phase TiFe during melting.Other peaks of small intensity were also identi�ed that correspond to the TiFeO phase.Figure 1(c being added and milling with the Ti 1.27 Fe alloy.Aer milling in an inert atmosphere for 5 h, the peaks of the TiFe pattern are broadened indicating the formation of a nanocrystalline cubic structure.Broadening of all the diffraction re�ections and decrease in intensity of single TiFe phase with CsCl type structure suggest the existing of microstrains and/or small crystallite sizes in the existing crystalline phases.e  lattice parameter of Ti 1.27 Fe alloy aer milling was 0.2972 nm for TiFe phase.is lattice parameter value is slightly small, with respect to the reference TiFe value of 0.2976 nm from JCPDS (Card no.19-0636) [20].e crystallite size of TiFe phase was estimated to be about 11 nm.It was calculated from the width of its XRD peaks, by using the Sherrer formula.Also re�ections of nickel and TiFeO were observed as it is shown in Figure 1(d).As it can be noticed, pure Ni diffraction can be seen aer 5 h milling, implying that nickel is only dispersed in the Ti 1.27 Fe alloy.

Results and Discussion
e nanocrystalline structure of the milled sample was veri�ed by TEM observations.Figure 2(a) shows a dark-�eld TEM image from a powder particle of Ti 1.27 Fe + 11 wt.%Ni composite prepared by ball milling for 5 h.From the dark-�eld image, the crystallites of small size, about 12 nm, are clearly displayed in the dark-�eld imaging mode.Crystallite sizes are consistent with the X-ray diffraction estimation from broadening of the peaks.Selected area electron diffraction (SAD) pattern of composite powder aer ball milling process is shown in Figure 2(b).SAD pattern exhibited diffraction rings for the (110), (200), and (211) this con�rms that the composite has crystalline cubic structure that corresponds to the TiFe phase and this was according to XRD result.A dendritic structure and homogeneous morphology were observed for the melted alloy that is typical for powder of brittle metallic materials.Aer 5 h of milling of Ti 1.27 Fe + Ni mixture, Figure 3(c), a great reduction in particle size less than 4 m is observed.us the fracture and fragmentation of Ti 1.27 Fe/Ni during ball milling decrease drastically the particle size.Furthermore SEM showed that nickel is completely dispersed in Ti 1.27 Fe alloy.e dispersion of nickel in the Ti 1.27 Fe alloy during the milling could accelerate the hydrogen absorption kinetics.e image of Figure 3(d) corresponds to the hydrogenated composite, heated at 100 ∘ C under hydrogen pressure of 0.8 MPa.It is observed that morphology of the particles is sponged and tends to disaggregate into �ne particles due to the interaction with hydrogen.
ICP-OES and EDS chemical composition of Ti 1.27 Fe + 11 wt.%Ni composite with 5 h of milling is shown in Table 1.Results show that the deviation of Ti-Fe-Ni ratio from nominal composition was negligible.It must be said that oxygen is not detected by ICP-OES; however, oxygen was found in the composite by EDS, which corresponds to the TiFeO as mentioned in XRD Figures 1(b) and 1(d).
According to the BET method, the Ti 1.27 Fe + 11 wt.%Ni composite has a surface area of 3.2073 m 2 /g.us the fracture and fragmentation of composite powders during ball milling increase drastically the surface area.e pore size distribution of the mesoporous composite was calculated from adsorption branch of the isotherm by the BJH method.An average pore radium of 12.1 � has found total speci�c pore volume (P/P  ) of 0.022912 cm 3 /g.a pressure of 0.8 MPa; however, this capacity decreased at higher pressures, which means that the system has reached its saturation pressure starting to release hydrogen by pressure effect.

Conclusions
In this work, we have successfully obtained the Ti

F 1 :
) corresponds to the XRD pattern of nickel powder, before XRD pro�les of Ti-Fe/Ni mixtures at different stages of processing: (a) elementary powders mixtures, (b) arc melting Ti 1.27 Fe, (c) Ni powder, and (d) Ti 1.27 Fe + 11 wt.%Ni aer 5 h of ball milling.

T 1 : 7 T 2 :a
Composition Ti 1.27 Fe + 11 wt.%Ni composite analyzed by ICP-OES and EDS.Sample Nominal composition (at.%) ICP-OES composition (at.%) EDS composition (at.%) Ball milling Hydrogen absorption/desorption capacity for the Ti 1.27 Fe + 11 wt.%Ni composite as a function of the pressure.Hydrogenation conditions: T = 100 ∘ C, t = 30 min.e SEM images of Ti 1.27 Fe alloy, Ti 1.27 Fe + 11 wt.%Ni composite and hydrogenated composite, are shown in Figures 3(a)-3(d).e average particle size of Ti 1.27 Fe alloy obtained by arc melting crushed and pulverized in a mortar of stainless steel is about 125 m in diameter as it is observed in Figure 3(a).Morphology of the same sample is shown in Figure 3(b).

F 2 :
TEM images of Ti 1.27 Fe + 11 wt.%Ni composite aer 5 h of ball milling: (a) dark-�eld image and (b) selected-area electron diffraction patterns.

3. 2 .F 3 :F 4 :
Hydrogenation Properties.Hydrogenation properties for the Ti 1.27 Fe + 11 wt.%Ni composite obtained for 5 h milling at 100 ∘ C are in Figure4and Table2.Hydrogenation process for 0.5 h at 100 ∘ C and pressures of 0.6-1.8MPa and only one cycle were tested.Composite sample was activated by heat treatment at 100 ∘ C in vacuuming and 1.33 Pa in hydrogen atmosphere for 3 h prior to hydrogenation process.As can be noted the maximum capacity absorption/desorption of hydrogen for the composite was 2.107 wt.% at 100 ∘ C and SEM micrographs of Ti 1.27 Fe/Ni at different stages of processing: (a) Ti 1.27 Fe alloy-as melted, (b) homogenized, (c) TiFe + 11 wt.%Ni aer 5 h of ball milling, and (d) hydrogenated composite.T�A pro�le (�) and its derivative (---) of hydrogenated Ti 1.27 Fe + 11 wt.%Ni composite indicating wt.% H 2 released as function of the temperature.
1.27 Fe + 11 wt.%Ni composite by arc melting and ball milling in a short period of time.e process of embrittlement at 350 ∘ C and 4 MPa in hydrogen atmosphere of the Ti 1.27 Fe alloy obtained by arc melting facilitates the obtaining of composite of Ti 1.27 Fe + 11 wt.%Ni by ball milling in a time of 5 h.A nanocrystalline TiFe single phase of CsCl-type structure has been observed aer 5 h of milling.e obtained composite is enough close with its theoretical composition.e produced composite by ball milling is able to absorb hydrogen at 100 ∘ C and low pressures in 0.5 h.e surface area and crystallite size improve the capacity of hydrogen absorption.e presence of Ni in the Ti 1.27 Fe alloy enhances its catalytic activity for the hydrogenation process without prior activation.Future research is aimed to investigate the entire process of hydrogenation/dehydrogenation of this composite.