Synthesis, structural and magnetic characterization of nanocrystalline nickel ferrite—NiFe2O4 obtained by reactive milling

Nanocrystalline nickel ferrite (NiFe₂O₄) has been synthesized from a stoichiometric mixture of oxides NiO and α-Fe₂O₃ in a high energy planetary mill. An annealing at 350 °C, after milling, was used to improve the solid state reaction. The obtained powders were investigated by X-ray diffraction, magnetic measurements, scanning electron microscopy, X-ray microanalysis and differential scanning calorimetry. The particles size distribution was analyzed using a laser particle size analyser. The nickel ferrite begins to form after 4 h of milling and continuously form up to 16 h of milling. The obtained nickel ferrite has many inhomogeneities and a distorted spinel structure. The mean crystallites size at the final time of milling is 9 ± 2 nm and the lattice parameter increases with increase the milling time. DSC measurements revealed a large exothermic peak associated with cations reordering in the crystalline structure. The magnetization of the obtained powder depends on the milling time and annealing. After the complete reaction between the starting oxides the milling reduces the magnetization of the samples. The magnetization increases after annealing, due to the reorganization of the cations into the spinel structure.     

Influence of wet milling conditions on the structural and magnetic properties of Ni3Fe nanocrystalline intermetallic compound

Nanocrystalline Ni₃Fe compound is obtained by dry and wet milling under argon atmosphere, using benzene as surfactant. X-ray diffraction patterns show that the Ni₃Fe phase is formed after 2 h of milling followed by annealing (350 °C for 4 h). The mean size of the crystallites is 12 ± 2 nm for 40 h of wet milling. Iron and chromium contamination was evidenced by X-ray microanalysis. The increase in benzene quantity shifts the powder particle size distribution towards smaller particle size ranges. DSC analysis revealed the presence of benzene in the wet milled powders. Spontaneous magnetization decreases with increasing milling time and benzene quantity. Annealing at 350 °C for 4 h leads to a substantial increase of the spontaneous magnetization.     

Synthesis of α-Mo-Mo5SiB2-Mo3Si nanocomposite powders by two-step mechanical alloying and subsequent heat treatment

A two-step mechanical alloying process followed by heat treatment was developed as a novel approach for fabrication of Mo-12.5 mol%Si-25 mol%B nanocomposite powders. In this regard, a Si-43.62 wt.% B powder mixture was milled for 20 h. Then, Mo was added to the mechanically alloyed Si-B powders in order to achieve Mo-12.5 mol%Si-25 mol%B powder. This powder mixture was further milled for 2,5,10 and 20 h. All of the milled powders were annealed at 1100 °C for 1 h. After first step of milling, a nanocomposite structure composed of boron particles embedded in Si matrix was formed. On the other hand, an α-Mo/MoSi₂ nanocomposite was produced after second step while no ternary phases between Mo, Si and B were formed. At this stage, the subsequent annealing led to formation of α-Mo and Mo₅SiB₂ as major phases. The phase evolutions during heat treatment of powders can be affected by milling conditions. It should be mentioned that the desirable intermetallic phases were not formed during heat treatment of unmilled powders. On the other hand, α-Mo-Mo₅SiB₂-Mo₃Si nanocomposites were formed after annealing of powders milled for 22 h. With increasing milling time (at the second step), the formation of Mo₃Si during subsequent heat treatment was disturbed. Here, an α-Mo-Mo₅SiB₂-MoSi₂ nanocomposite was formed after annealing of 30 and 40 h milled powders.     

Microstructural evolution of nanostructured Ti0.9Al0.1N prepared by reactive ball-milling

Nanocrystalline stoichiometric Ti_0.9Al_0.1N powder has been prepared by ball-milling the α-Ti (hcp) and aluminum (fcc) powders under N₂ at room temperature. Initially, α-Ti phase partially transformed to the transient cubic β-Ti phase and Ti_0.9Al_0.1N (fcc) phase is noticed to form after 3 h of milling. Nanocrystalline stoichiometric Ti_0.9Al_0.1N phase is formed after 7 h of milling. After 1 h of milling, all Al atoms are diffused into the α-Ti matrix. The transient β-Ti phase is noticed to form after 1 h of milling and disappears completely after 7 h of milling. Microstructure characterization of unmilled and ball-milled powders by analyzing XRD patterns employing the Rietveld structure refinement reveals the inclusion of Al and nitrogen atoms into the Ti lattice on the way to formation of Ti_0.9Al_0.1N phase. Microstructure of ball-milled samples is also characterized by HRTEM. The particle size of Ti_0.9Al_0.1N phase, as obtained from XRD method, is ∼5 nm which is very close to that obtained from HRTEM.     

Study of manganese ferrite powders prepared by a soft mechanochemical route

Manganese ferrite, MnFe₂O₄ have been prepared by a soft mechanochemical route from mixture of (a) Mn(OH)₂ and α-Fe₂O₃ and (b) Mn(OH)₂ and Fe(OH)₃ powders in a planetary ball mill. The mixture was activated for varying duration. Soft mechanochemical reaction leading to formation of the MnFe₂O₄ spinel phase was followed by X-ray diffraction, Raman spectroscopy, scanning and transmission microscopy and magnetization measurements. The spinel phase formation was first observed after 12 h of milling and its formation was completed after 25 h in both cases. The synthesized MnFe₂O₄ ferrite has a nanocrystalline structure with a crystallite size of about 40 and 50 nm respectively for cases (a) and (b). There are five Raman active modes. Measurements after 25 h of milling show magnetization values of 70.4 emu/g and 71.1 emu/g respectively for cases (a) and (b). In order to understand better the whole process of phase formation, Mössbauer measurements were done.     

Structural and magnetic properties of nanostructured Fe50(Co50)-6.5 wt% Si powder prepared by high energy ball milling

This work investigates the effects of 6.5 wt% Si addition and milling times on the structural and magnetic properties of Fe₅₀Co₅₀ powders. For this purpose, at first the elemental Fe and Co powders were milled for 10 h to produce Fe₅₀Co₅₀ alloy and then Si was added and the new product was milled again for different times. The microstructural and magnetic properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). The results show that the minimum crystallite size of the as-milled powders (∼12 nm) has been achieved after introducing Si and milled for 8 h (total milling time of 18 h). Also an amount of 188 emu/g has been achieved for M_s. This amount of M_s is higher than most of those which have been already reported for M_s of different Fe-Si systems.     

Structural, microstructural and magnetic properties of amorphous/nanocrystalline Ni63Fe13Mo4Nb20 powders prepared by mechanical alloying

This paper focuses on the magnetic, structural and microstructural studies of amorphous/nanocrystalline Ni₆₃Fe₁₃Mo₄Nb₂₀ powders prepared by mechanical alloying. The ball-milling of Ni, Fe, Mo and Nb powders leads to alloying the element powders, the nanocrystalline and an amorphization matrix with Mo element up to 120 h followed by the strain and thermal-induced nucleation of a single nanocrystalline Ni-based phase from the amorphous matrix at 190 h. The results showed that the saturation magnetization decreases as a result of the electronic interactions between magnetic and non-magnetic elements and finally increases by the partial crystallization of the amorphous matrix. The coercive force increases as the milling time increases and finally decreases due to sub-grains formation.     

XRD and HREM studies from the decomposition of icosahedral AlCuFe single-phase by high-energy ball milling

In this investigation the Al₆₄Cu₂₄Fe₁₂ alloy was melted in an induction furnace and solidified under normal casting conditions. In order to obtain the icosahedral phase (i-phase) in a single-phase region, the as-cast sample was subject to a heat treatment at 700 °C under argon atmosphere. Subsequently, the i-phase was milled for different times in order to evaluate phase stability under heavy deformation. X-ray diffraction (XRD) and high-resolution electron microscopy (HREM) analysis were conducted to the structural characterization of ball-milled powders. XRD results indicated a reduction in quasicrystal size during mechanical ball milling to about 30 h. HREM analysis revealed the presence of aperiodic nano-domains, for example, with apparent fivefold symmetry axis. Therefore, the i-phase remains stable over the first 30 h of ball-milling time. However, among 30-50 h of mechanical milling the i-phase transforms progressively into β-cubic phase.     

Phase transition of Ni-Mn-Ga alloy powders prepared by vibration ball milling

This study investigated the phase transformation of the flaky shaped Ni-Mn-Ga powder particles with thickness around 1 μm prepared by vibration ball milling and post-annealing. The SEM, XRD, DSC and ac magnetic susceptibility measurement techniques were used to characterize the Ni-Mn-Ga powders. The structural transition of Heusler → disordered fcc occurred in the powders prepared by vibration ball milling (high milling energy) for 4 h, which was different from the structural transition of Heusler → disordered fct of the powders fabricated by planetary ball milling (low milling energy) for 4 h. The two different structures after ball milling should be due to the larger lattice distortion occurred in the vibration ball milling process than in the planetary ball milling process. The structural transition of disordered fcc → disordered bcc took place at ∼320 °C during heating the as-milled Ni-Mn-Ga powders, which was attributed to the elimination of lattice distortion caused by ball milling. The activation energy for this transition was 209 ± 8 kJ/mol. The Ni-Mn-Ga powder annealed at 800 °C mainly contained Heusler austenite phase at room temperature and showed a low volume of martensitic transformation upon cooling. The inhibition of martensitic transformation might be attributed to the reduction of grain size in the annealed Ni-Mn-Ga particles.     

Microstructural and mechanical properties of Al-Mg/Al2O3 nanocomposite prepared by mechanical alloying

The effect of milling time on the microstructure and mechanical properties of Al and Al-10 wt.% Mg matrix nanocomposites reinforced with 5 wt.% Al₂O₃ during mechanical alloying was investigated. Steady-state situation was occurred in Al-10Mg/5Al₂O₃ nanocomposite after 20 h, due to solution of Mg into Al matrix, while the situation was not observed in Al/5Al₂O₃ nanocomposite at the same time. For the binary Al-Mg matrix, after 10 h, the predominant phase was an Al-Mg solid solution with an average crystallite size 34 nm. Up to 10 h, the lattice strain increased to about 0.4 and 0.66% for Al and Al-Mg matrix, respectively. The increasing of lattice parameter due to dissolution of Mg atom into Al lattice during milling was significant. By milling for 10 h the dramatic increase in microhardness (155 HV) for Al-Mg matrix nanocomposite was caused by grain refinement and solid solution formation. From 10 to 20 h, slower rate of increasing in microhardness may be attributed to the completion of alloying process, and dynamic and static recovery of powders.     

Synthesis behavior of nanocrystalline Al-Al2O3 composite during low time mechanical milling process

In this work, four different volume fractions of Al₂O₃ (10, 20, 30 and 40 vol.%) were mixed with the fine Al powder and the powder blends were milled for 5 h. Scanning electron microscopy analysis, particle size analysis and bulk density measurements were used to investigate the morphological changes and achieving the steady state conditions. The results showed that increasing the Al₂O₃ content can provide the steady state particle size in 5 h milling process. It was found that increasing the volume fraction of Al₂O₃ leads to increasing the uniformity of Al₂O₃. Standard deviations of microhardness measurements confirmed this result. The XRD pattern and XRF investigations depicted that increasing the Al₂O₃ content causes an increase in the crystal defects, micro-strain and Fe contamination during 5 h milling process of nanocrystalline composite powders while the grain size is decreased. To investigate the effect of milling time, Al-30 vol.% Al₂O₃ (which achieved steady state during 5 h milling process) was milled for 1-4 h. The results depicted that the milling time lower than 5 h, do not achieve to steady state conditions.     

A new method of synthesizing ultrafine vanadium carbide by dielectric barrier discharge plasma assisted milling

A new high efficiency method of synthesis of ultrafine vanadium carbide (VC) at a low carburization temperature has been developed. Firstly, a mixture of V₂O₅ and graphite powders is milled using dielectric barrier discharge plasma assisted milling (denoted as DBDP milling) for 4 h, and then the milled powders are carburized at 1200 °C, causing the V₂O₅ to react completely with graphite to form ultrafine VC. The formation temperature of VC is much lower than that needed in the conventional milling and heating process. This is because of the greatly enhanced reaction between V₂O₅ and graphite arising from the unique lump-like morphology and large number of clean surface contacts and greater surface area induced by DBDP milling.     

Mechanochemical synthesis of nanocrystalline ZnWO4 at room temperature

Single nanocrystalline ZnWO₄ powders were successfully synthesized by ball milling at room temperature. A stoichiometric mixture of ZnO and WO₃ in a 1:1 molar ratio was subjected to intense mechanical treatment in air using a planetary ball mill (Fritsch - Premium line - Pulversette No. 7) for a period varying from 5 to 300 min. The influence of the four different milling conditions was investigated on the formation of ZnWO₄. The products obtained were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmer-Teller (BET) surface area, infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The synthesis of ZnWO₄ powder started after 5 min milling time and finished after 30 min milling time at a higher speed (1000 rpm). The mechanical treatment up to 300 min did not lead to phase and structure change of ZnWO₄. The product obtained contained nanoparticles with a size of about 50 nm. The photocatalytic activity of the ZnWO₄ powders obtained was investigated by degradation of a model aqueous solution of Malachite Green (MG) upon UV-light irradiation.     

Fabrication of W-20wt.%Ti alloy by pressureless sintering at low temperature

A powder metallurgical technology of low temperature and pressureless is used to fabricate a W-20wt.%Ti alloy using milled TiH₂ powders and micro-sized W powders. The microstructure of the milled TiH₂ powders and the bulk W–Ti alloy were studied. It is indicated that TiH₂ nanoparticles with the size of 8 to 15 nm were obtained after milling for 48 h and the decomposition temperature decreased from 520.2 °C to 395.5 °C. The W-20wt.%Ti alloy prepared at 1200 °C for 80 min had a relative density of 97.8% which was composed of α-Ti, W and β(W/Ti) solid solution. A preparation mechanism of the W–Ti alloy is also proposed based on the experimental results.     

Development of nano-structure Cu-Zr alloys by the mechanical alloying process

Cu-Zr alloys have many applications in electrical and welding industries for their high strength and high electrical and thermal conductivities. These alloys are among age-hardenable alloys with capability of having nano-structure with high solute contents obtainable by the mechanical alloying process. In the present work, Cu-Zr alloys have been developed by the mechanical alloying process. Pure copper powders with different amounts of 1, 3 and 6 wt% of commercial pure zirconium powders were mixed. The powder mixtures were milled in a planetary ball mill for different milling times of 4, 12, 48 and 96 h. Ball mill velocity was 250 rpm and ball to powder weight ratio was 10:1. Ethanol was used as process control agent (PCA). The milling atmosphere was protected by argon gas to prevent the oxidation of powders. The milled powders were analysed by XRD technique and were also investigated by SEM observations. Lattice parameters, crystal sizes and internal strains were calculated using XRD data and Williamson-Hall equation. Results showed that the lattice parameter of copper increased with increasing milling time. The microstructure of milled powder particles became finer at longer milling time towards nano-scale structure. SEM observations showed that powder particles took plate-like shapes. Their average size increased initially and reached a maximum value then it decreased at longer milling times. Different zirconium contents had interesting effects on the behavior of powder mixtures during milling.     

Formation of titanium nitride produced from nanocrystalline titanium powder under nitrogen atmosphere

The initially globular-shaped Ti powder particles were flattened to ‘pan-cake’ like shape after 12, 16, and thin flakes after 20 h of mechanical milling. Although no change peak positions of HCP Ti crystal structure, the increase in peak intensity with milling time was evident. It is found that the greater surface to volume ratio of the milled Ti powders accelerated the N₂ uptake and subsequent formation of TiN at lower temperatures (884, 856 and 833 °C for 12, 16 and 20 h, respectively) than in the unmilled powder (∼ 1100 °C). Higher nitrogen content of 41–44 at.% by EDS analysis confirmed the high rate of dissolution on the milled powders.     

Mechanochemical solid state synthesis of (Cd0.8Zn0.2)S quantum dots: Microstructure and optical characterizations

(Cd_0.8Zn_0.2)S quantum dots with a mixture of both cubic (Zinc-blende) and hexagonal (Wurtzite) phases have been prepared within 75 min by mechanical alloying the stoichiometric mixture of Cd, Zn and S powders at room temperature in a planetary ball mill under Ar. The Rietveld analysis of X-ray powder diffraction data reveals relative phase abundances of both cubic and hexagonal phases and several microstructure parameters like lattice parameters, particle sizes, lattice strains, concentrations of different kinds of stacking faults, etc. in both the phases. At the time of formation, hexagonal phase dominates over the cubic phase (molar ratio ∼0.6:0.4), but in course of milling up to 15 h, the hexagonal phase partially transforms to cubic phase and the molar ratio becomes ∼0.4:0.6. Particle sizes of hexagonal and cubic phases reduce to ∼4.5 nm and 12.5 nm, respectively, after 15 h of milling. The hexagonal phase contains a significant amount of lattice strain in comparison to cubic phase. The presence of different kinds of stacking faults is revealed clearly from the high resolution transmission electron microscope (HRTEM) images.     

Spark plasma sintering consolidation of nanostructured TiC prepared by mechanical alloying

Spark plasma sintering technique was used for the consolidation of nanostructured titanium carbide synthesized by mechanical alloying in order to avoid any important grain growth of the compact materials. The TiC phase was obtained after about 2 h of mechanical alloying. Towards the end of the milling process (20 h), the nanocrystalline powders reached a critical size value of less than 5 nm. Some physical and mechanical properties of the consolidated carbide were reported as a function of the starting grain size powders obtained after different mechanical alloying durations. The crystalline grain size of the bulk samples was found to be increased to a maximum of 120 nm and 91 nm for carbides mechanically alloyed for 2 h and 20 h respectively. The Vickers hardness showed to be improved to about 2700 Hv for a maximum density of 95.1% of the bulk material.     

Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

Phase transformation and exchange bias effects in mechanically alloyed Fe/magnetite powders

Nanostructured powders processed by ball milling of a mixture of Fe and Fe₃O₄ at room temperature are shown to undergo an incomplete redox reaction with formation of FeO during the milling process. This reaction is favored by the high energy introduced during the mechano-alloying process. Concurrent effects of milling such as grain refinement down to the nanometre scale lead at the end of the milling processes to a mixed multiphase powder of nanograins, with Fe and Fe oxide grains inter-dispersed. We show that in the as-milled Fe/Fe₃O₄ powder, during milling process, wüstite (FeO) is formed as a consequence of the redox reaction. Moreover, with increasing temperature, the system undergoes an inverse phase transformation towards the initial Fe and Fe₃O₄ phases until about 450 °C. Above this temperature the reduction reaction Fe + Fe₃O₄ = 4FeO is reinitiated, resulting in sharp decrease of Fe and Fe₃O₄ content from about 550 °C and almost complete disappearance of these phases at about 900 °C. This transformation was investigated via an energy-dispersive in situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray patterns after few minutes exposure, at selected temperatures, ranging between 20 °C and 1000 °C. The structural and magnetic characterizations of the nanograin powders, as-milled and annealed at several temperatures, are studied using XRD, SEM and magnetic measurements. Such ferromagnetic-antiferromagnetic composites are extensively studied as they exhibit exchange bias effect, with a large impact in technological applications. The magnetic behaviour and intrinsic mechanisms leading to the occurrence of exchange bias effects are discussed and related to the samples microstructural features. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanograin powder.