Journal of Magnetism and Magnetic Materials 419 (2016) 189–197

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Journal of Magnetism and Magnetic Materials

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Mechanochemical synthesis of nanocrystalline Fe and Fe–B magneticalloys

Majid Mohammadi, Ali Ghasemi n, Majid TavoosiDepartment of Materials Engineering, Malek-Ashtar University of Technology (MUT), Shahin-Shahr, Isfahan, Iran

a r t i c l e i n f o

Article history:Received 2 February 2016Received in revised form5 June 2016Accepted 15 June 2016Available online 16 June 2016

Keywords:Intermetallic compoundsPowder mtallurgyMagnetic propertiesNanostructures

t combination of Fe2O3, B2O3 and CaH2 powof B, Fe, Fe95B5 and Fe85B15 alloys.

Reaction product Weight pe

B2O3

B 35.55Fe 0Fe95B5 1.23Fe85B15 3.83

x.doi.org/10.1016/j.jmmm.2016.06.03753/& 2016 Elsevier B.V. All rights reserved.

esponding author.ail address: ali13912001@yahoo.com (A. Ghas

a b s t r a c t

Mechanochemical synthesis and magnetic characterization of nanocrystalline Fe and Fe–B magnetic al-loys was the goal of this study. In this regard, different Fe2O3–B2O3 powder mixtures with sufficientamount of CaH2 were milled in a planetary ball mill in order to produce nanocrystalline Fe, Fe95B5 andFe85B15 alloys. The produced samples were characterized using X-ray diffraction (XRD), differentialscanning calorimetry (DSC), scanning electron microscopy (SEM) and vibrating sample magnetometer(VSM). The results showed that, nanocrystalline Fe, Fe95B5 and Fe85B15 alloys can be successfully syn-thesized by the reduction reaction of Fe2O3 and B2O3 with CaH2 during mechanical alloying. Thestructure of produced Fe95B5 and Fe85B15 alloys was a combination of Fe and Fe2B phases with averagecrystallite sizes of about 15 and 10 nm, respectively. The produced nanocrystalline alloys exhibited softmagnetic properties with the coercivity and saturation of magnetization in the range of 170–240 Oe and9–28 emu/g, respectively. Increasing the boron content has a destructive effect on soft magnetic prop-erties of Fe–B alloys.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

Nanocrystalline materials are a class of materials with grainsizes smaller than 100 nm. Since these materials are characterizedby low crystalline size and high volume fraction of grain bound-aries (which may comprise as much as 10–50% of the total crystalvolume), their magnetic behavior may be quite different fromthose of their coarse grain counterparts. In fact, the nanocrystalinematerials display a very low magnetic anisotropy which is causedby the fact that the originated grains are significantly smaller than

Fig. 1. The XRD patterns of stoichiometric B2O3-CaH2 powder mixture after 10 and50 h of milling time.

der mixtures in order to for-

rcentage

Fe2O3 CaH2

0 64.4555.85 44.1553.9 44.8549.83 64.34

emi).

Fig. 4. The SEM morphological micrographs of B2O3–CaH2 powder mixture af

Fig.3. The XRD pattern of B2O3–CaH2 powder mixture after 50 h of milling andannealing at 420 °C for 1 h.

Fig. 2. The DSC curve of B2O3–CaH2 powder mixture after milling for 50 h.

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197190

the correlation length of the ferromagnetic exchange interactions[1–4].

Nanocrystalline ferromagnetic iron-based alloys are the most in-teresting magnetic materials. Due to their special structure and cor-responding novel properties, interest in these alloys has increasedrapidly. Among these alloys, Fe–B nanocrystlline alloys are wellknown due to soft magnetic properties. These alloys are usuallyproduced by physical or chemical methods such as melt-spinning,sol–gel and co-precipitation. For example, nanometer-size iron par-ticles consisting of some boron (Fe–B particles) are known to beeasily prepared through a boro-hydride reduction of correspondingmetal salt solutions [5]. These methods have several limitations in-cluding high energy consumption, the requirement of high purityraw materials and inhomogeneous microstructure [6,7]. In fact, si-multaneous reduction of Fe and boron oxides by means of solid-stateprocesses (such as reduction–diffusion and mechanochemical pro-cesses) is one of the simple methods to overcome these limitations.

It has been shown that solid state reduction of metal oxideswith calcium or calcium hydride is a successful method to prepare

ter 50 h of milling and annealing at 420 °C for 1 h in two magnifications.

Fig. 5. The XRD patterns of Fe2O3–CaH2 powder mixture before and after 5 h ofmilling time.

Fig. 6. The SEM micrographs of Fe2O3–CaH2 powder mixture after 5 h of milling time in two magnifications.

Fig. 7. The XRD pattern of Fe2O3–B2O3–CaH2 powder mixture in order to formationof Fe95B5 alloy after 5 h of milling.

Fig. 8. The XRD pattern of Fe2O3–B2O3–CaH2 powder mixture in order to formationof Fe85B15 alloy after 5 h of milling.

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197 191

of metals (Zr and Ti [8]) and alloys (NiTi [9] and Fe–B [10–15])nanoparticles. The undesirable CaO phase in this reaction can beeliminated by washing with water for several times to obtain thedesired nanoparticles [8–10].

The formation of Fe-based magnetic materials by means of hightemperature reduction–diffusion process was discussed in theprevious literatures [10–17]. However, according to our knowl-edge, the production of these magnetic materials during me-chanochemical process has not been properly investigated. Theprinciple of mechanochemical processing is based on conversionof mechanical energy into chemical energy. This phenomenonenhances the reactivity of solid. The advantages of this method aredirect use of oxides as raw materials, low cost and shorter pro-ducing period [3].

Therefore, the mechanochemical reduction of B2O3 and Fe2O3

powder mixture by CaH2 was the goal of this study. In this regard,the reduction reactions of B2O3–CaH2, Fe2O3–CaH2 and B2O3–

Fe2O3–CaH2 powder mixtures during milling were investigatedand the produced samples were characterized using XRD, FE-SEM,DSC and VSM.

2. Experimental procedures

Different powder mixtures of Fe2O3 (Merck, 499.8%), B2O3

(Merck, 499.8%) and CaH2 (Merck, 499.7%) (According to Ta-ble 1) were mechanically milled in a planetary ball mill at argonatmosphere (the rotation speed of 800 rpm and the ball to powderratio of 20:1). Milling media consisted of five 20mm diameterhardened chromium steel balls confined in a 120 ml volume har-dened chromium steel vial (maximum Fe contamination frommilling media was estimated about 0.03 wt%). It is important tonote that, before mechanochemical process in ternary systems, theFe2O3 and B2O3 powder mixture were milled for 10 h (at samecondition to above) and then CaH2 was added to as-milledpowders.

Fig.9. The SEM micrographs of Fe2O3–B2O3–CaH2 powder mixtures after 5 h of milling in order to formation of (a) Fe95B5 and (b) Fe85B15 alloys before washing process.

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197192

XRD technique using a diffractometer with Cu Ka radiation(λ¼0.15406 nm; 40 kV; Philips PW3710) was used to follow thestructural changes of the specimens (step size: 0.05°; time perstep: 1 s). The average crystallite sizes of the produced sampleswere estimated by analyzing broadening of XRD peaks usingScherer's formula [18]. The L81/1750 DTA differential thermalanalyzer was used to study the structural changes of milled sam-ples during annealing. The samples were placed in Al2O3 pans andheated in dynamic argon atmosphere up to 1000 °C at a heatingrate of 20 °C/min. Morphological characterization of the sampleswas carried out by scanning electron microscopy (VEGA-TESCAN-XMU) at an accelerating voltage of 20 kV. Magnetic properties(saturation magnetization and approximate coercivity) of pro-duced samples were monitored using a vibrating scanningmagnometer (VSM) under an applied field up to 10 KOe.

3. Results and discussion

3.1. The B2O3–CaH2 reaction during milling

In order to study the B2O3–CaH2 reaction during milling, apowder mixture of B2O3 and CaH2 (sample 1 in to Table 1) wasmechanically milled for different periods of time, and the pro-duced samples were examined using X-ray diffractometer. TheXRD patterns of B2O3–CaH2 powder mixture, after different millingtime, are presented in Fig. 1. As seen, the XRD pattern of initialpowder mixture consists of several peaks corresponding to B2O3

and CaH2 phases. In the early stage of milling, only the broadeningof B2O3 and CaH2 peaks accompanied by remarkable decrease intheir intensities occurred as a result of crystalline sizes refinementand increments in the lattice strains. The formation of a broadamorphous halo in 2θ range of about 25–35° is another occurrencein milled powders. The XRD pattern of milled powder up to 50 honly consists of B2O3 and CaH2 peaks. In fact, there is no evidenceof reaction products in this XRD pattern. This result indicates thatthe amorphization of B2O3–CaH2 powder mixture is the onlyconsiderable change that occurred in this powder system duringmilling.

The DSC curve of B2O3–CaH2 powder mixture after 50 h of

milling at a constant heating rate of 20 °C/min is shown in Fig. 2.As seen, this DSC curve consists of one exothermic and one en-dothermic peak. To analyze the reaction responsible for the exo-thermic peak (420 °C), the milled sample (for 50 h) was annealedin an argon atmosphere at 420 °C for 1 h. The XRD pattern ofannealed sample (Fig. 3) consists of B, CaO and Ca(OH)2 phases.Therefore, the exothermic peak in DSC curve (Fig. 2) should beattributed to the B2O3–CaH2 reaction:

B2O3þ3CaH2-2Bþ3CaOþ3H2↑ (1)

2B2O3þ3CaH2-4Bþ3Ca(OH)2 (2)

In fact, the endothermic peak at 450 °C can be related to thefollowing reactions:

CaH2-CaþH2 (3)

Ca(OH)2-CaOþH2O (4)

The SEM morphological micrographs of B2O3–CaH2 milledpowder mixture before and after annealing are presented in Fig. 4.As seen, the average particle size of annealed sample is about1 μm. In these micrograph, boron elemental particles are in a loosestate interspersed with other reaction products (CaO and Ca(OH)2)and are not distinct.

3.2. The Fe2O3–CaH2 reaction during milling

Same as above, the stoichiometric Fe2O3–CaH2 powder mixture,according to reaction 5, was mechanically milled for differentperiods of time and the produced samples were examined usingX-ray diffractometer.

Fe2O3þ3CaH2-2Feþ3CaOþ3H2↑ (5)

⎛⎝⎜

⎞⎠⎟ ( )∆ = − ∆ = −° °H G125.3

Kcalmole

, 118.7 Kcal/mole298 298

The XRD patterns of this powder mixture, before and after 5 h of

Fig. 10. The (a) Fe, (b) B, (c) Ca, and (d) O elemental maps of Fe2O3–B2O3–CaH2 powder mixture after 5 h of milling in order to formation of Fe95B5 alloy (before washingprocess).

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197 193

milling, are presented in Fig. 5. The diffraction pattern of initialpowder mixture shows several peaks corresponding to Fe2O3 andCaH2 phases. After 5 h of milling, the Fe2O3 and CaH2 peaks van-ished completely and several additional peaks corresponding to Fe(with the average crystallite size of about 15 nm) and CaO phasesdeveloped in XRD pattern. The lattice parameter of produced Fephase was calculated of about 2.86 Å which is so near to the latticeparameter of pure Fe. Thus, our data shows that the reductionreaction of Fe2O3 with CaH2 occurred during the milling.

The adiabatic temperature, Tad, for this reaction (1860 K) ishigher than the Schaffer critical value of 1600 K [19]. This resultillustrated that a self-propagation reaction takes place duringmilling process in this powder mixture. The SEM micrographs ofmilled powders after 5 h of milling, exactly after exothermic re-action, are presented in Fig. 6. As seen, these micrographs containuniform spherical shapes of partially melted particles (with theaverage particle size of about 2 μm) which can be related to anexothermic explosive reaction that occurring during milling.

3.3. The Fe2O3–B2O3–CaH2 reactions during milling

In order to preparation of nanocrystalline Fe95B5 and Fe85B15 alloys,two different powder mixtures of Fe2O3–B2O3 with adequate amountof CaH2 (according to Table 1) were mechanically milled for differentperiods of time. The XRD patterns of 53.9Fe2O3–1.23B2O3–44.85CaH2

and 49.83Fe2O3–3.83B2O3–46.34CaH2 powder mixtures after 5 h ofmilling are presented in Figs. 7 and 8, respectively. As seen, there is noevidence of initial powder peaks in these XRD patterns and thestructure of produced samples consist of Fe, Fe2B, CaO and Ca(OH)2(reaction products) peaks. This result confirms that during millingprocess, Fe2O3 and B2O3 phases were simultaneously reduced by CaH2.The Fe2O3–B2O3–CaH2 reaction during milling has done in two ways:

(1�x)Fe2O3þ xB2O3þ 3CaH2-2Fe1�xBxþ3CaOþ3H2↑ (6)

(1�x)Fe2O3þ xB2O3þ 3CaH2-2Fe1�xBxþ3Ca(OH)2 (7)

The SEM micrographs of 53.9Fe2O3–1.23B2O3–44.85CaH2 and49.83Fe2O3–3.83B2O3–46.34CaH2 powder mixtures after 5 h of

Fig.11. The SEM micrographs of milled Fe2O3–B2O3–CaH2 powder mixtures in order to formation of (a and b) Fe95B5 and (c) Fe85B15 alloys after washing process.

Fig. 12. The (a) Fe and (b) B elemental maps of prepared Fe95B5 alloy (after washing process).

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197194

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197 195

milling are presented in Fig. 9(a) and (b), respectively. As seen, theconsequent reaction products of Fe1�xBx alloys are in a loose stateinterspersed with fine particles of the by-product CaO and Ca(OH)2.The elemental analysis of these samples also confirms this point. Inthis regards, the elemental maps of 53.9Fe2O3–1.23B2O3–44.85CaH2

powder mixture after 5 h of milling are presented in Fig. 10. As seen,the F and B elements are uniformly distributed in CaO matrix. Byattention to this point, a necessary subsequent process serves tochemically remove the excess CaO and Ca(OH)2 phases. For thepurpose of cleanly removing the residual CaO and Ca(OH)2, washingwith dilute acetic acid and water was repeatedly used [14].

The SEM micrographs of these powder mixtures, after washingprocess, are presented in Fig. 11. As seen, the recovered Fe95B5 andFe85B15 particles have agglomerated morphology with the averageparticle size of about 300 nm (Fig. 11 (b)). According to the XRD

Table 2The hysteresis loops, the saturation magnetization and coercivity of produced Fe95B5 an

Alloy Hc (Oe) Ms (emu/g)

Fe95B5 Before 19173 2172

After 17975 2871

patterns, the formed particles are combination of Fe (with latticeparameter of about 2.86 Å) and Fe2B phases (in agreement withFe–B equilibrium phase diagram [20]) with average crystallite sizesof about 1474 and 1174 nm, respectively. In fact, the formedFe2B phase in produced alloys is uniformly distributed in Fe matrixas a result of proper mixing of Fe2O3–B2O3 powder mixture beforemechanochemical process. The elemental maps of B and Fe ele-ments in prepared Fe95B5 alloy, after washing process (Fig. 12), alsoconfirm this result.

3.4. Magnetic characterization of Fe95B5 and Fe85B15 nanocrystallinealloys

The hysteresis lopes, the saturation of magnetization (Ms) andcoercivity (Hc) of produced Fe95B5 and Fe85B15 nanocrystalline

d Fe85B15 alloys before and after washing process.

Table 2 (continued )

Alloy Hc (Oe) Ms (emu/g)

Fe85B15 Before 23774 9.572

After 20073 1474

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197196

alloys before and after washing process are shown in Table 2. Bypaying attention to these results, several points can be concludedas follows:

1. The produced Fe–B nanocrystalline alloys exhibit soft magneticproperties with the coercivity and saturation of magnetizationin the range of 170–240 Oe and 9–28 emu/g, respectively.

2. By increasing the B content, the coercivity and saturation ofmagnetization increases and decreases, respectively. Thischange may be attributed to some important factors that are

explained as follows: (a) it is clear that the percentage of Fe2Bphase increases by increasing the B content from 5 to 15. In fact,this phase acts as a pin and reduces the domian's wall move-ments and decreases the soft magnetic properties;(b) increasing the B content can increase the stress magneticanisotropy of the sample and decrease the soft magneticproperties [4,21,22].

3. After removal of CaO and Ca(OH)2 by washing, the saturation ofmagnetization and coercivity of produced samples increasesand decreases, respectively. The saturation of magnetization of

M. Mohammadi et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 189–197 197

milled samples is low due to the presence of non-magnetic CaOand Ca(OH)2 phases. It is clear that the saturation of magneti-zation can rise by reduction of the percentage of CaO and Ca(OH)2 phases from the produced samples (increasing theamount of magnetic phases per unit volume). Moreover, re-duction of coercivity can be related to the formation of a non-magnetic layer or chemical inhomogeneity (which is not de-tectable by XRD analysis) during washing process [10].

4. Conclusions

In this paper, mechanochemical synthesis of nanocrystalline Feand Fe–B magnetic alloys have been investigated. The resultsshowed that except the B element, nanocrystalline Fe, Fe95B5 andFe85B15 alloys can be successfully synthesized by simultaneity re-duction reaction of Fe2O3 and B2O3 oxides with CaH2 during me-chanical alloying. The structure of formed Fe95B5 and Fe85B15 al-loys was a combination of nanocrystalline Fe and Fe2B phases(with average crystallite sizes of about 14 nm). The produced na-nocrystalline alloys exhibited soft magnetic properties with thecoercivity and saturation of magnetization in the range of 170–240 Oe and 9–28 emu/g, respectively. Increasing the B content hasa distractive effect on soft magnetic properties of Fe–B alloys. Thehigher coercivity and the lower saturation of magnetization ap-peared in alloys with higher percentage of boron.

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