Mechanical activation on aluminothermic reduction and magnetic properties of NiO powders

We report the mechanically activated aluminothermic reduction of NiO [NiO–Al(x wt.%) with x  =  0, 20, 40] into NiO–Ni–Al2O3 nanocomposites using high-energy planetary ball milling under dry milling and the resulting structural and magnetic properties. Structural studies reveal that both NiO and NiO–Al powders exhibit a face centered cubic structure with large crystal size reduction. However, the NiO–Al milled powders unveil the process of aluminothermic reaction kinetics, which changes from gradual reaction as a function of milling time for x  =  20 powders to self-propagating combustion reaction for x  =  40. This allows us to achieve a maximum NiO reduction of 40% and 90% for x  =  20 and 40, respectively. The process of NiO reduction by Al is further confirmed through thermal studies. Pure NiO shows an antiferromagnetic (AFM) nature, which transforms into a ferromagnetic (FM) one with the moderate magnetization of about 1 emu g−1 with decreasing crystal size. The formation of FM Ni from AFM NiO matrix in milled NiO–Al powders could be precisely monitored by the change in the magnetization, which increases up to 4 emu g−1 and 28 emu g−1 for the gradual and combustion reactions, respectively. This results in a considerable exchange bias and its magnitude strongly depends on the relative fractions of NiO and Ni phases. Thermomagnetization data confirm the presence of mixed magnetic phases and the component of induced FM phase fades out due to the formation of Ni from the reduction of NiO. The changes in the structural and magnetic properties of milled NiO–Al powders are discussed on the basis of milling time-dependent mechanically activated reduction reaction of NiO into NiO–Ni–Al2O3 nanocomposites. The process of mechanical activation on the aluminothermic reduction allows for a controlled reduction of NiO; thus, it is suitable for the applications in catalysis and the ore reduction process.

Keywords: transition metal oxides, mechanical activation, aluminothermic reduction, nanocomposites, exchange bias, thermomagnetization (Some figures may appear in colour only in the online journal)

Introduction
Recently, the nanostructured transition metal (TM) based oxides have been extensively studied from both applied and fundamental points of view [1]. In particular, the possibility of tuning the properties of nanostructured NiO by a partial reduction process has opened up novel paths for widespread research activities in the nanoscale science. The partial reduction of TM oxides forms composites, which consist of TM nanoparticles embedded in a TM oxide matrix and find applications in various advanced fields such as sensor, supercapacitor, solar cell, microwave, battery, spintronics, solid oxide fuel cell, electrochromic, resistive random access memory, etc [2][3][4][5][6]. Although different synthesis methods such as chemical precipitation [7], electro-deposition [8], solid state [9], micro-emulsion [10], spray pyrolysis [11], sol-gel [12], hydrothermal [13], etc have been employed, the high-energy ball milling [14][15][16] has been proven as the simplest and inexpensive method for the synthesis of nanosized NiO powders with controlled magnetic properties. The ball milling process is a solid state method producing final materials in powder form with nanosized crystals, which can be tailored in various shapes and dimensions by compacting and giving proper heat treatment for availing commercially. This method is also called as mechanochemical synthesis or reactive milling, as it activates solid-solid and solid-liquid chemical reactions by utilizing the mechanical energy supplied during milling.
The reduction reaction of bulk NiO was first reported by Benton and Emmett in 1924 under hydrogen (H 2 ) atmosphere [17]. Sharma et al [18] showed the complete reduction of NiO using graphite as a reducing agent in both a closed system and in vacuo. Similar reduction reaction was reported by Budarin et al [19] using carbon monoxide as a reductive gas. Kim et al demonstrated the reduction of NiO to Ni using two different reducing gases (N 2 and H 2 ) along with carbon and obtained 75% and 100% reduction of NiO, respectively [20]. Recently, Rashidi et al [21] used methane for reducing NiO and Jeangros et al [22] reported that the reduction of NiO by H 2 in an environ mental transmission electron microscope produces Ni nucleation on NiO either by epitaxial or by the formation of randomly oriented grains. However, the first investigation on the solid-state reaction of NiO-Al using high-energy mechanical alloying to prepare Ni/Al 2 O 3 nanocomposite was reported by Matteazzi and LeCaer [23] using dry milling condition under argon (Ar) atmosphere. Subsequently, the mechanically activated carbothermal reduction of NiO by graphite was reported by Yang et al using the mechanical alloying process [24]. Later on, Doppiu et al reported that the mechanochemical reduction of NiO in H 2 atmosphere is a progressive one, which results in the formation of Ni-NiO nanocomposite through the partial reduction of NiO into Ni [25]. They also reported that a threshold energy is necessary to instigate the reduction. Oleszak [26] and Udhayabanu et al [27] utilized the mechanical activation of aluminothermic reduction of NiO for the development of NiAl/Al 2 O 3 and Ni/Al 2 O 3 nanocomposites under different process controlling agents, respectively. A careful review of literature reveals that (i) the study of maximum NiO reduction and the ability to control the properties of subsequent Ni are the major challenges in the fields of ore reduction, resistive random access memory, catalysis and solid oxide fuel cells [28][29][30] and (ii) though the process of reduction in NiO has been reported using ball milling, the detailed analyses of the dynamics of the reduction process and the systematic evolutions of magnetic properties as a course of reduction are still missing. Therefore, in this article, we report the systematic investigations on the reduction of NiO by Al in a high-energy planetary ball mill under dry milling in Ar atmosphere as a function of milling period (t m ) and Al content and the resulting structural and magnetic properties of novel nanocomposites. In order to compare the results, we have also carried out milling for pure NiO without Al under the similar milling conditions and characterized.

Experimental details
Weighed quantities of high purity (>99.9%) NiO and Al powders corresponding to the mixture of NiO-Al (x wt.%) with x = 0, 20 and 40 were taken in a high-energy planetary ball mill filled with high purity Ar gas. The milling process of NiO-Al powders was carried out for different t m (=0-30 h) in a hardened steel vial together with 8 mm diameter hardened steel balls in the mill operated at 500 rotations per minute with a ball-to-powder weight ratio of 10:1. The optimization of milling speed was done mainly by monitoring the variation in the structural and magnetic properties of the resulting nanocomposites in the milled powders. To avoid any excess heat generated during dry milling, the mill was programmed to halt for 15 min after every 15 min of operation.
To understand the evolution of nanostructure in NiO-Al powders, the powders were milled at different t m and characterized. The phase and structural evolutions were analyzed using x-ray diffraction (XRD) obtained through high-power x-ray diffractometer (Rigaku TTRAX III 18 kW) using Cu-K α radiation (λ = 1.540 56 Å). XRD data were collected at a slow scan rate of 0.005° s −1 for quantitative analysis of structural parameters. The microstructural properties of the pure NiO and milled NiO-Al powders were analyzed using transmission electron microscope (TEM, Technai TF20). Differential scanning calorimetry (DSC) was carried out for the as-mixed and milled powders in an Ar atmosphere using LABSYS evo, SETARAM Instrumentation (Caluire, France) and NETZSCH STA 449 F3A00. Magnetic properties were characterized using vibrating sample magnetometer (VSM, LakeShore Model 7410) by performing (i) room temperature initial magnetization (IM) curves and magnetic hysteresis (M-H) loops and (ii) high-temperature thermomagnetization (M-T) measurements from 300 K to 1200 K performed at 4 °C min −1 heating rate under the applied magnetic field of 2 kOe. No additional peaks corresponding to any other phases or compounds were observed within the resolution of high-power x-ray diffractometer. However, the broadness of the NiO peaks increases considerably and the peak position shifts to lower angles with increasing t m . While the first one can well be attributed to effective size reduction, the latter one is mainly due to the change in the lattice parameters in the milled powders. As a result, the color of pure un-milled NiO powder changes from pale-green into dark green after milling. This could be attributed to the existence of non-stoichiometry in NiO caused by the defects, size reduction [15], oxidization of Ni 2+ to Ni 3+ [31] due to breaking of Ni 2+ -O 2− -Ni 2+ super-exchange interaction as evident from Raman spectra [16,32]. On the other hand, the as-mixed NiO-Al powder shows individual Bragg reflections corresponding to fcc structure of NiO and Al. However, the XRD patterns of the milled NiO-Al powders strongly depend on Al content and t m , and hence the process of milling is described separately.

Results and discussion
It is observed for x = 20 powders that (i) after 5 h of milling, the intensity of the Al peaks is decreased largely along with the considerable broadening in NiO peaks. In addition, the formation of new peaks at 2θ = 44.5° and 51.8° is observed. This indicates the commencement of NiO reduction reaction, which results in the materialization of Ni. (ii) With increasing t m > 5, the intensity of Al peaks decreases further and then disappears eventually for t m > 10. This could be attributed to its smaller quantity after its utilization for the NiO reduction and its reduced crystal size. (iii) A progressive peak broadening of NiO peak along with a significant shift in the peak position is observed till 30 h. While the peak broadening confirms the refinement of NiO crystals, the peak shift is originated due to atomic disorder induced by dissolution of Al in NiO matrix and the reduction. Furthermore, the intensity of the Ni peaks increases gradually with increasing t m up to 30 h. This results in substantial changes in the color of the milled NiO-Al powders as follows: The pale-green color of the as-received NiO powder changes into dark green after 5 h of milling mainly due to the non-stoichiometry in NiO and weak reduction of NiO. On further increasing t m , the dark green color transforms into black one due to the formation of Ni. (iv) Nevertheless, no additional Bragg peaks corre sponding to any other phases or compounds were observed. In order to evaluate the percentage of NiO reduction with t m , we have utilized the change in the integrated intensity of NiO(2 0 0) peak using the relation [33] , where A is the integrated intensity of NiO(2 0 0) peak before milling, B is the integrated intensity of NiO(2 0 0) peak in milled NiO-Al powder at a given t m and C is the percentage of reduction of NiO. The percentage of NiO reduction increases gradually with increasing t m and reaches a maximum of about 40% at the end of 30 h milling. This process also decreases the average size of NiO crystals largely into nanoscale regime.
On the other hand, the XRD patterns of the NiO-Al (40 wt.%) powders reveal that (i) the peak intensities of NiO and Al decrease suggestively for 0.5 h of milling. (ii) Interestingly, the process of aluminothermic reduction of NiO starts even by 3 h of milling, which results in the formation of distinct Ni peaks at 2θ = 44.5°, 51.8° and 76.3°. In addition, the development of additional peaks at 2θ = 45.6°, 60.6° and 66.6° corresponding to Al 2 O 3 is observed. (iii) With increasing t m > 3, the NiO(2 0 0) and NiO(3 1 1) peaks at 2θ = 43.35° and 75.46°, diminish progressively and merge into Ni(1 1 1) and Ni(2 2 0) peaks at 2θ = 44.5° and 76.3°, respectively at the end of 30 h of milling. While the NiO(2 2 0) and NiO(2 2 2) peaks disappear completely, the existence of highly strained NiO(1 1 1) peak is still observed after 30 h of milling. (iv) A close observation of Ni(2 0 0) peak at 2θ = 51.8° unveils a considerable peak broadening with increasing t m from 3 to 30 h. This may be correlated to the refinement of Ni crystals after its formation from NiO reduction. These results suggest that the reduction process for x = 40 powders is quite instantaneous and rapid with the maximum reduction of about 90%. This confirms that the reaction process changes from gradual reaction for x = 20 to self-propagating combustion reaction for x = 40, which needs a typical critical t m of 3 h for the combustion reaction to be ignited. Therefore, the reduction process clearly relies on t m and Al content under the present milling conditions.
In a conventional self-propagating high temperature synthesis process, the temperature of the reaction has to be raised at the temperature range of 800-1100 °C under the reactive gas atmosphere [17,19,21,22]. However, the reduction of NiO strongly depends on the types of the reducing gas and the processing conditions. The NiO reduction was also carried out by mixing the NiO with C powders followed by consecutive heating process under different gases [20]. However, the temperature needed for the NiO reduction is still quite high. On the other hand, the reactive milling promotes mechanically activated solid state chemical reactions between oxides and reducing agents such as Na, Mg, Ca and Al for the reduction process, as the milling process creates large number of defects, grain boundaries and sub-grain boundaries, which favors mass transfer and diffusion path length with reduced activation energy [34]. To understand the process of aluminothermic reaction kinetics for the presently investigated samples in correlation with XRD results, we have carried out thermal analysis of the un-milled and milled NiO-Al powders using DSC and the curves are depicted in figure 2. The curves show (i) sharp endothermic peaks at around 650 °C corresponding to Al melting and (i) one exothermic peak at about 1040 °C for x = 20 powder corresponding to NiO reduction and two exothermic peaks at 1000 °C and 1070 °C corresponding to two-stage reduction of NiO [24]. On the other hand, the milled NiO-Al powders exhibit a broad exothermic peak at 440 °C indicating the partial reduction of unreacted NiO present in the as-milled powders. However, we observed a significant endothermic Al melting peak at 5 h milled x = 20 powder and 0.5 h milled x = 40 powder indicating the presence of unreacted Al in the as-milled powders. Similarly, the absence of Al melting peak in DSC curves of other milled samples confirms the solid state reduction of NiO in these milled samples. These results are in good agreement with the XRD patterns (see figures 1(b) and (c)) and strongly support the nature of NiO reduction reaction occurring with different Al content at different t m . Therefore, the observed self-propagating combustion reaction for x = 40 powders follows the reaction 3NiO + 2Al → 3 Ni + Al 2 O 3 since the reduction of NiO by Al is highly exothermic in nature and in agreement with the earlier reports [35,36]. On the other hand, for x = 20 powders, the amount of Al available for the ignition of the above combustion reaction is considerably low and hence promotes only the gradual reduction process. However, a detailed thermal analysis and heat treatment at high temperatures of the as-milled samples at different t m would elucidate the nature of the reactions more in details.
To study the structural refinement, the lattice constant (a NiO ) and crystallite size of NiO (D NiO ) were calculated after eliminating the contribution from instrumental broadening.   To understand the evolution of nanocomposites with different Al content, the pure NiO and milled NiO-Al powders were characterized using TEM technique. Figure 4      To understand the changes in the magnetic properties of NiO and NiO-Al milled powders, we correlate both the structural and magnetic properties. In pure un-milled NiO (pale green colored) powder, the spins within AFM coupled (1 1 1) planes are compensated and hence do not contribute to net magnetic moment [37]. Hence, the M-H loop of un-milled NiO powder displays a weak response to the applied magnetic field. On the other hand, the milled NiO powders undergo large size reduction and change in the color due to non-stoichiometry [31,38], which result in a breaking of Ni 2+ -O 2− -Ni 2+ super-exchange interaction [16,31,32,39] and increase in the number of uncompensated spins on the surfaces with respect to particle core [40] upon increasing t m . This leads to an alignment of particles' net moment in a relatively low field and enhances net moments along with high H C . Also, the exchange coupling between the induced FM and AFM core instigates the exchange bias effect, which decreases progressively with the size reduction of NiO. Furthermore, the lattice expansion observed in finer crystallites plays a crucial role in controlling the magnetic exchange interaction between the uncompensated surface spins and particle core spins. This is in agreement with the earlier report by Li et al that room temperature magnetic crossover of NiO is due to the lattice expansion [41]. Del Bianco et al [42] reported that saturation magnetization of NiO increases at a rate of 0.62 emu g −1 per Ni wt.% in hydrogenated NiO. A quantitative comparison with the presently investigated samples indicates that the as-milled NiO powders have about 1.7 wt.% Ni enriched spatial regions.
In the case of NiO-Al milled powders, the variation of M 12 kOe strongly depends on the amount of NiO reduction. Considering the AFM nature of NiO and ascribing the increase of the magnetization in the nanocomposite is mainly due to the formation of ferromagnetic (FM) Ni from NiO reduction, the percentage of Ni was estimated by relating the values of M 12 kOe with respect to saturation magnetization of bulk Ni (~55 emu g −1 ) [43]. Figure 7 depicts the variations of NiO reduction determined from XRD analysis and percentage of Ni obtained from magnetic measurements as a function of t m for x = 20 and 40 powders. It can be observed that the percent age of Ni in the nanocomposite increases progressively up to a maximum of 7.5% with increasing t m to 30 h. This is in good correlation with respect to the gradual reduction of NiO by Al for x = 20 powders. As a result, H E increases up to 30 h supporting the gradual formation of Ni in NiO matrix, which induces the exchange interaction between FM Ni and AFM NiO. On the other hand, for x = 40 powders, the intensity of NiO and Al XRD peaks is reduced up to t m = 1 due to the dissolution of Al in NiO without any ignition of NiO reduction. As a result, the formation of Ni is not observed. With increasing t m ⩾ 3 h, the self-propagating combustion reaction is ignited suddenly, which reduces 90% of NiO into NiO-Ni-Al 2 O 3 nanocomposite and provides about 52% of Ni after 5 h of milling. On further increasing t m > 5 h, a considerable refinement of Ni nanocrystals occurs, which increases H C progressively and decreases M 12 kOe . Since larger fraction of NiO is reduced into Ni, H E turns out to be low as compared to x = 20 powders. It may be noted that the milled NiO-Al powders exhibit RTFM despite having the average size of D Ni in the range below 25 nm, which is significantly below the critical size (~34 nm) of spherical Ni particles for single domain behavior at RT [44]. Hence, Ni nanoparticles will be of single domain but prone to thermal fluctuations [45,46]. Therefore, the obtained RTFM in the presently investigated samples can be correlated to one or more of the following several origins: (i) The milled powder exhibits quite irregular morphology (see figure 4) and hence the shape anisotropy could play a key role. (ii) Since the milled powders are subjected to severe fracture and cold welding during milling process, the strain anisotropy could contribute to total anisotropy of the nanoparticles. (iii) Surface anisotropy of the nanoparticles may also play an important role [45,47]. (iv) When the FM nanoparticles are embedded in an AFM matrix, an additional uniaxial type anisotropy is also introduced to the nanoparticles [46,48]. In order to analyze the effective magn etic anisotropy in the milled NiO-Al powders, we have fitted IM curves using law of approach of the magnetization to satur ation (LAS) as defined in equation (1) where M(H) is the magnetization in an applied magnetic field H, M S is the saturation magnetization, χ is high-field susceptibility, and, a and b are constant coefficients [49]. The constant coefficient b is related to the effective magnetic anisotropy of the cubic crystalline materials as given in equation (2), The determined value of K eff as a function of t m for NiO-Al milled powders are shown in figure 8. For the sake of compariso n, K eff of bulk Ni is also shown in the figure. The obtained values of K eff for the milled powders are found to be larger as compared to K eff of the bulk Ni. This could be attributed to the contribution from various anisotropies such as magnetocrystalline anisotropy, shape anisotropy, strain anisotropy and exchange anisotropy for these fine nanosized Ni in nanocomposites prepared by ball milling process. Zhang et al [46] also reported size dependent enhanced K eff of the Ni nanoparticles in the range between 1.2 × 10 4 J m −3 and 6 × 10 4 J m −3 . High values of K eff have also been reported in other nanocrystalline system [50] prepared by mechanical alloying process. However, in practice, it is impossible to separate these effects individually. Thus, the larger K eff plausibly shifts the blocking temperature of such fine nanoparticles above room temper ature and provides thermal stability at room temperature. In order to study the stability of FM in the milled NiO-Al powders, high-temperature M-T measurements were performed under the applied field of 2 kOe. Figure 9 depicts the normalized M-T curves for un-milled and milled powders with x = 0, 20 and 40 at different t m . The insets display the same data in logarithmic scale for magnetization to demonstrate the relative variation of magnetization close to zero at higher temperatures. The magnetization of the pure un-milled NiO powder increases gradually with increasing temperature up to 525 K and then decreases above 525 K. Since the pure NiO powder exhibits AFM nature at room temperature, the  Néel temperature (T N ) is determined from the peak in M-T curve and found to be about 528 K. This is in good agreement with the earlier reports [51,52]. On the other hand, the NiO powders milled for more than 1 h show a continuous decrease in magnetization with increasing temperature and the temperature at which the magnetization becomes zero shifts to higher temperature with increasing t m up to 30 h. Although the high temperature magnetic phase transition (T C ) should be associated with Ni due to the formation of uncompensated surface spin, T C is considerably large as compared to its bulk counterpart (~630 K). This can be attributed to the stress induced during the ball milling process or strains due to the Ni and NiO lattice-constant mismatch arising at the interface and competing exchange interaction between the induced FM and AFM core [53,54]. The existence of stress is evident from the non-smooth decrease of magnetization in M-T curves, which acts more like hydrostatic one and hence increases T C [54]. Such high T C has also been reported in Ni/NiO system [55]. In contrast, the magnetization of the milled NiO-Al powders decreases with increasing temperature, but exhibits two different magnetic phase transitions: (i) a large magnetization drop at about 640 K and (ii) a gradual decrease of magnetization up to 900 K. While the first one is due to Ni having magnetic phase transition of 630 K, the latter one is due to the magnetic phase transition of induced FM in NiO [15,55]. The increased amount of drop in magnetization at around 640 K suggests the existence of higher Ni content with increasing t m , as evidenced from the structural studies. On the other hand, the nature of magnetic phase transition of Ni in x = 40 powders is quite sharp due to the existence of large fraction of Ni as compared to x = 20 powders. These results show good correlations between the structural, thermal, magnetic and thermomagnetic properties of milled NiO-Al powders. Furthermore, the process of mechanical activation on the aluminothermic reduction allows for a controlled partial reduction of NiO and hence optimized amount of Ni produced.

Conclusions
We have systematically studied the effect of mechanical activation on the aluminothermic reduction reaction of NiO by subjecting the NiO and Al powders with different Al content to high-energy planetary ball milling under dry milling conditions and characterized structural and magnetic properties as a function of milling time and Al content. The milling process in pure NiO powder reduced the crystallite size down to 11 nm without changing the fcc structure. Therefore, the antiferromagnetic (AFM) nature of bulk NiO transformed into FM with a maximum magnetization of about 1 emu g −1 for the powder milled at 30 h. However, in the NiO-Al powders, the reduction of NiO occurred progressively up to a maximum of 40% for NiO-Al (20%) powders with increasing milling time and hence the magnetization increased gradually from 0.12 emu g −1 to 4 emu g −1 due to the increase in Ni content to 7.5%. On the other hand, the increase of Al content [NiO-Al (40%)] changed the reduction process to self-propagated combustion reaction type, which needed a critical time of 3 h to ignite the reaction process. Therefore, the magnetization changed drastically to 28 emu g −1 with a maximum of 90% NiO reduction with the yield of nearly 52% Ni. The microstructural studies revealed that the formation of FM Ni in AFM NiO matrix led to exchange bias effect. However, the magnitude of exchange bias depended on the relative fractions of Ni and NiO phases. High-temperature thermomagnetization data confirmed the presence of mixed magnetic phases in the milled NiO-Al powders and the nature of magnetic phase transition strongly depended on the amount of NiO reduction. The observed results showed a good correlation between structural, thermal and magnetic properties of milled NiO-Al powders. The controlled reduction of NiO in NiO-Al powder mixtures under present milling conditions finds suitable applications in the fields of catalysis and ore reduction.