A study on the effects of silica particle size and milling time on synthesis of silicon carbide nanoparticles by carbothermic reduction

Silicon carbide nanoparticles were produced by a carbothermic reduction of nano and micro size silica with graphite at 1450 °C for 1 h. The SiC nanoparticles were characterized by XRD, SEM and TEM. The results showed that in the case of nano silica, milling up to 20 h could develop SiC particles of 5–25 nm with some residual SiO₂ particles. By extending milling time to 40 h, more energy was provided and produced Fe contamination, which could act as catalyst and increased SiC yield as well as Fe₂Si phase formation after heat treatment. Coarser particles of micro silica caused higher Fe erosion, more SiC formation with 20–70 nm size and presence of Fe₂Si phase at shorter milling times after heat treatment. Leaching treatment could purify SiC nanoparticles. Increase of milling from 20 to 40 h changed the morphology from polygonal shape to spherical with some reduction in the particle size.     

A facile synthesis of TiB2 nano-particles via mechano-thermal route

A mixture of TiO₂, B₂O₃ and Si powders was milled up to 50 h using a high energy planetary ball mill. The milling products were heat treated at 850, 1200 and 1300 °C. Effects of Si content and addition of catalyst on the phase evolutions and morphology of the products were investigated. XRD results showed that phase composition of the 50 h milled sample after heat treatment at 1300 °C consists of nano-crystalline TiB₂ with mean crystallite size of 50 nm together with some Ti₂O₃, Si and SiO₂ phases. Addition of 2 mol of NaCl in the same sample facilitates the process and resulted in the single phase TiB₂with a mean crystallite size of 30 nm. SEM and TEM micrographs exhibited the nano-sized particles of TiB₂.     

Preparation of WC nanoparticles by twice ball milling

In order to improve the ball milling efficiency of WC powders and thus to fabricate nano-grained WC–Co cemented carbides with high mechanical properties, WC nanoparticles were prepared by twice ball milling in nylon vessels. The best technology to disperse WC powders in alcohol was investigated at first. Based on the dispersion results, 2 wt.% PEG was used with La₂O₃ as additive to improve ball milling efficiency. The particle size, crystal structure, surface morphology and surface properties were tested by a laser particle sizer, XRD, FE-SEM and FT-IR, respectively. During the first ball milling, sample d achieved the best milling performance, including average particle size (168 nm) and grain size (27.2 nm) among samples a (pure WC), b (with PEG), c (with La₂O₃) and d (with PEG and La₂O₃). La₂O₃ could greatly decrease particle size and grain size while PEG could narrow particle size distribution. During the second milling, the particle size and grain size of sample d reached 89 nm and 13.2 nm at 96 h, respectively. The results indicated that twice ball milling can greatly improve particle size and grain size compared with the first ball milling, and further narrow the size distribution. In conclusion, multiple ball milling can reduce the particle size of certain powders with suitable milling technology.     

Preparation of Mo(Si, Al)2–ZrO2 nanocomposite powders by mechanical alloying

Phase transformations and the final formation of Mo(Si, Al)₂–ZrO₂ nanocomposite during high-energy ball milling of a series of Mo–Si–Al–ZrO₂ powders were investigated. Mechanical alloying led to phase transformations from the initial Mo–Si–Al powders mixture to Mo_ss (2 h) → C40 Mo(Si, Al)₂ (4, 8 h) → Mo_ss (12 h) phases. The phase transformations studied by XRD are discussed considering the alloying and second phase effects. Finally, the Mo_ss matrix reinforced with ZrO₂ particles nanocomposite structure was studied by means of TEM. The Mo_ss matrix phase formed was revealed to be strongly inhomogeneous even after 12 h of mechanical alloying and Mo-, Si- and Al-enriched regions were observed. The ZrO₂ nanostructured phase, evenly distributed in the Mo_ss matrix, had grain size of about 5–20 nm.     

The formation and evolution of oxide particles in oxide-dispersion-strengthened ferritic steels during processing

The fabrication of oxide-dispersion-strengthened (ODS) steels is a multi-stage process involving powder ball milling, degassing and consolidation by hot isostatic pressing. Y is introduced by mechanical alloying (MA) with either Y₂O₃ or Fe₂Y so a high density of Y–Ti–O-based oxide nanoparticles is formed. The evolution of ～2 nm oxide nanoparticles and larger >5 nm grain boundary oxides has been studied at each step of the processing. The nanoparticle dispersions produced in material MA with Fe₂Y were comparable to those produced by alloying with Y₂O₃. Hence the majority of oxygen which forms the nanoparticles must be incorporated from the atmosphere during MA, rather than being introduced via the alloying additions. During mechanical alloying, a high density of subnanometer particles are formed (2.5 ± 0.5 × 10²⁴ m^?3). The oxygen content of the nanoparticles decreases slightly on annealing, and then the composition of the nanoparticles remains constant throughout subsequent processing stages. The nanoparticle size increases during processing up to ～2 nm radius, while the number density decreases to 4 ± 0.5 × 10²³ m^?3. Grain boundary oxides (>5 nm) have a Ti–Cr–O-rich shell, and contain no Y before consolidation, but have similar core composition to the matrix nanoparticles after consolidation. The formation of the larger grain boundary oxides is shown to take place during the degassing and consolidation stages, and this occurs at the expense of the nanoparticles in the matrix. This work provides a mechanistic understanding of the importance of controlling the oxygen content in the powder during MA, and the resulting impact on the formation of the ODS microstructure.     

Synthesis and characterization of MoSi2-Mo5Si3 nanocomposite by mechanical alloying and heat treatment

The nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by mechanical alloying from Mo and Si powder mixture at room temperature. The phase evaluation of powder after various milling durations and heat treatments were assessed via X-ray diffraction (XRD) and a differential thermal analysis (DTA). Morphology and microstructure of powder particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that nanocomposite of MoSi₂-Mo₅Si₃ powder was synthesized by combustion reaction of Mo and Si powder using ball milling. In the early stages of ball milling β-MoSi₂ was produced. However with continued milling for 48 h α-MoSi₂ and Mo₅Si₃ phases were formed. DTA results of 24 h and 48 h as milled mechanical alloyed specimens showed a well-defined peak at 852 °C and 920 °C relating to the formation of α-MoSi₂. The activation energy for 24 h and 48 h milled specimens were –128.6 KJ/mol and –121.4 KJ/mol respectively. Annealing the milled specimens at 1000 °C for 2 h revealed the phase transformation of β-MoSi₂ to α-MoSi₂ and the formation of Mo₅Si₃. The crystallite size of α-MoSi₂ and Mo₅Si₃ were about 9 nm and 12 nm after 48 h mechanical alloying. These values increased slightly to 18 nm and 14 nm after annealing at 1000 °C.     

Synthesis of nanocrystalline Cd–Zn ferrite by ball milling and its stability at elevated temperatures

High energy ball milling of stoichiometric (0.5:0.5:1 mole fraction) mixture of CdO, ZnO and α-Fe₂O₃ powders in air at room temperature results in formation of a non-stoichiometric Zn-rich (Zn,Cd)Fe₂O₄ phase with normal spinel structure having tetrahedral vacancies. The ferrite phase is initiated at 1 h of milling and after 25 h milling, 0.96 mole fraction of ferrite is formed and 0.04 mole fraction of CdO phase remained unreacted. The phase stability study of nanocrystalline non-stoichiometric (Zn,Cd)Fe₂O₄ powder annealed at elevated temperatures reveals that the Zn-rich ferrite phase remained stable up to 973 K and then slowly transformed towards Cd-rich (Cd,Zn)Fe₂O₄ phase following the release of divalent cations from ferrite lattice of normal spinel structure. The non-stoichiometric ferrite phase with almost similar composition has also been obtained by conventional ceramic route by sintering the same stoichiometric mixture at 973 K for 1 h. Microstructure characterization in terms of several lattice imperfections, relative phase abundances, cation distribution, and phase stability studies of unmilled, ball-milled and annealed samples is made by employing the Rietveld's structure refinement methodology using X-ray powder diffraction data. The analysis reveals that the particle size of ferrite phase reduces to ～7 nm after 25 h of milling and after annealing at 1273 K for 1 h it grows up to ～700 nm. However, in case of ferrite prepared by ceramic route it grows up to ～250 nm which is quite less than the annealed ball-milled samples.     

Maghemite polymer nanocomposites with modulated magnetic properties

A method is presented for the production of maghemite polymer nanocomposites with modulated magnetic properties. Magnetic nanocomposites prepared using this method show regular variation in the magnetic blocking temperature from 2 K to 300 K, and variation in the saturation magnetization from 0 to 50 emu g⁻¹ (Fe₂O₃). The method is based on the in situ formation of maghemite nanoparticles in nitrogen-base polymer matrixes. The particle size can be varied regularly from 1.5 nm to 16 nm by changing the ratio of iron loading in the polymer and/or the Fe(II)/Fe(III) ratios. The particles are isolated and uniformly distributed within the matrix. The materials were characterized by electron microscopy, electron energy loss spectroscopy, Mössbauer spectroscopy, infrared spectroscopy, small angle X-ray scattering, wide angle X-ray scattering and magnetic measurements. The nanocomposites obtained are useful model material for the study of the magnetic behavior of magnetic nanoparticles, as well as for use in many industrial and biomedical applications.     

Nanocrystalline Fe–Nb–(B,Ge) alloys from ball milling: Microstructure,thermal stability and magnetic properties

Fe₇₅B₂₀Nb₅, Fe₇₅Ge₁₀B₁₀Nb₅ and Fe₇₅Ge₂₀Nb₅ alloys were prepared by ball milling from pure powders and their microstructure and magnetic properties were studied. A nanocrystalline solid solution of α-Fe type is the main phase formed, although traces of some intermetallics were found in the Fe–B–Nb alloy. The local arrangements of Fe atoms in Ge containing alloys continuously evolve with milling time. The obtained powders are thermally stable even heating up to 773 K. After heating up to 1073 K, intermetallic compounds are detected. The best soft magnetic properties are achieved after heating up to 773 K, due to stress relaxation of the nanocrystalline microstructure (for Fe–Ge–Nb alloy, coercivity ∼ 600 A/m).     

Influence of thermal treatment on structure development and mechanical properties of amorphous Fe73.5Cu1Nb3Si15.5B7 ribbon

Ribbon-shaped amorphous samples with the stoichiometric composition Fe_73.5Cu₁Nb₃Si_15.5B₇ prepared by the melt spinning process were annealed at temperatures ranging from 693 K to 1123 K for 1 h under vacuum. In the early annealing stage, the alloy undergoes a specific nucleation process where Cu clusters precipitate from an amorphous matrix. Further heating initiates the partial crystallization of alloy forming the α-Fe–Si nanocrystallites. Subsequent Vickers hardness tests showed high values depending on the annealing temperature. It was found that the hardening process includes two stages. This behavior correlates well with results of density dislocation calculations. A crystallite size of 10 nm for the α-Fe–Si particles correlated very well with a maximum hardness of the material.     

Investigation of planetary milling for nano-silicon carbide reinforced aluminium metal matrix composites

High-energy planetary milling was used for mixing aluminium powders with 1 vol.% of silicon carbide (SiC) nanoparticles. A number of milling parameters were modified for constituting the relationship between the energy input from the balls and the hardness of the bulk nanocomposite materials. It was shown that mixing characteristics and reaction kinetics with stearic acid as process control agent can be estimated by normalised input energy from the milling bodies. For this, the additional parameter characterising the vial filling was determined experimentally. Depending on the ball size, a local minimum in filling parameter was found, laying at 25 or 42% filling of the vial volume for the balls with diameter of 10 and 20 mm, respectively. These regions should be avoided to achieve the highest milling efficiency.After a hot compaction, fourfold difference of hardness for different milling conditions was detected. Therewith the hardness of the Al–1 vol.% nanoSiC composite could be increased from 47 HV_0.5 of pure aluminium to 163 HV_0.5 when milling at the highest input energy levels.     

Microstructural evolution of mechanically alloyed Mo–Si–B–Zr–Y powders

Elemental powder mixtures with compositions of Mo–13.8Si, Mo–20B and Mo–12Si–10B–3Zr–0.3Y (at.%) were respectively milled in a high energy planetary ball mill at a speed of 500 rpm. Microstructural evolution of powder particles during milling processes was evaluated. The results show that B can hardly be dissolved into Mo under present milling conditions and the additions of B and Si both accelerate the refining rate of Mo crystallites. For Mo–12Si–10B–3Zr–0.3Y system, the morphology and internal structure of powder particles change significantly with milling time. After 40 h of milling, an almost strain-free super-saturated molybdenum solid solution with a grain size of about 6.5 nm forms. The grain refinement mechanism and dissolution kinetics of solute atoms are highlighted. Both thermodynamic calculation and experimental results reveal that for the present alloy composition it is more favorable to form solid solution than amorphous phase.     

Evolution of Nd2Fe14B nanoparticles magnetism during surfactant-assisted ball-milling

The aim of this work was the fabrication of Nd₂Fe₁₄B nanoparticles, through a “top-down” technique like surfactant-assisted high-energy ball-milling, and the monitoring of the milling-process stages with respect to the structure and magnetism. The ball-milling of a 40 μm powder under Ar atmosphere, in an organic solution of oleic acid/oleylamine, resulted in 15 nm isolated nanoparticles after 20 h, presenting improved magnetocrystalline anisotropy compared to the initial material. By extending the mechanical process up to 100 h a gradual formation of elongated nanoparticles followed by structural amorphization was observed.     

Crystal structure,magnetocaloric effect and magnetovolume anomalies in nanostructured Pr2Fe17

Using high-energy ball milling, nanostructured Pr₂Fe₁₇ powders can be obtained from their arc-melted bulk alloys. High-resolution X-ray and neutron powder diffraction experiments reveal that the Th₂Zn₁₇-type rhombohedral crystal structure is maintained, after milling for 10 h, with almost unchanged values for both crystalline lattice parameters (Δa; Δc < 0.05%) and vanishing mechanically induced microstrain (<0.1%). Although the mean crystalline size decreases down to 20 ± 3 nm, magnetovolume anomalies observed in pure Pr₂Fe₁₇ are still present with a significant volume decrease on heating from 5 up to 320 K. After the milling, a significant increase in the magnetic anisotropy, due to the drastic reduction in crystalline size, is observed, while the value of the magnetic moment seems to be increased slightly (5%). In addition, the magnetocaloric effect of bulk and nanostructured Pr₂Fe₁₇ has been investigated. The magnetic entropy change, |ΔS_M|, decreases from 6.3 to 4.5 J kg^?1 K^?1 under an applied magnetic field μ₀H = 5 T after the milling process. However, the width of the |ΔS_M|(T) curve is substantially enlarged and hence the refrigerant capacity is enhanced. These findings make the iron-based nanostructured Pr₂Fe₁₇ powders interesting for applications in magnetic refrigeration at around room temperature.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.     

Synthesis and characterization of pure nanocrystalline magnesium aluminate spinel powder

Synthesis of nanocrystalline magnesium aluminate spinel (MgAl₂O₄) by mechanical activation of a powder mixture containing Al₂O₃ and MgCO₃ with subsequent annealing was investigated. Simultaneous thermal analysis (STA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques were utilized to characterize the as-milled and annealed samples. Results showed that pure nanocrystalline spinel could be fabricated completely by 5 h of mechanical activation with subsequent annealing at 1200 °C for 1 h with a crystallite size of about 45 nm. Further milling had no significant effects on structure or phase composition of spinel phase after subsequent annealing. The nanocrystalline spinel powder obtained after 60 h of milling and subsequent annealing at 1200 °C for 1 h had a crystallite size of about 25 nm according to Williamson–Hall approach and particle sizes smaller than 200 nm. Enhanced mechanical properties were observed in samples prepared from the powder mixture and milled for a longer period.     

First-principles assessment of hydrogen absorption into FeAl and Fe3Si: Towards prevention of steel embrittlement

We characterize the ability of two potential surface alloys, FeAl and Fe₃Si, to prevent H incorporation into steel, with a view toward inhibiting steel embrittlement. Periodic density functional theory calculations within the generalized gradient approximation are used to evaluate H dissolution energetics and the kinetics of H diffusion into and through FeAl and Fe₃Si. We predict increased dissolution endothermicities and diffusion barriers in both alloys compared to bulk Fe. Fe₃Si is predicted to be the most effective at inhibiting H incorporation, with a 1.91 eV [0.97 eV] surface-to-subsurface diffusion barrier on the (1 1 0) surface [(1 0 0 surface)] and a 0.79 eV endothermicity to bulk dissolution, compared to a 1.02 eV [0.38 eV] barrier and 0.20 eV dissolution energy in pure Fe [37]. We therefore propose that a thin layer of Fe₃Si may provide protection against H embrittlement of the underlying steel.     

Structural evolution and grain growth kinetics of the Fe–28Al elemental powder during mechanical alloying and annealing

The structural evolution and grain growth kinetics of the Fe–28Al (28 at.%) elemental powder during mechanical alloying and annealing were studied. Moreover, the alloying mechanism during milling the powder was also discussed. During mechanical alloying the Fe–28Al elemental powder, the solid state solution named Fe(Al) was formed. The lattice parameter of Fe(Al) increases and the grain size of Fe(Al) decreases with increasing milling time. The Fe and Al particles were first deformed, and then, the composite particles of the concentric circle-like layers were generated. Finally, the composite particles were substituted by the homogeneous Fe(Al) particles. The continuous diffusion mixing mechanism is followed, mainly by the diffusion of Al atoms into Fe. During annealing the milled Fe–28Al powder, the order transformation from Fe(Al) to DO₃-Fe₃Al and the grain growth of DO₃-Fe₃Al occurred. The grain growth kinetic constant, K = 1.58 × 10⁻⁹ exp(−540.48 × 10³/RT) m²/s.     

Synthesis of (Mo1−x–Crx)Si2 nanostructured powders via mechanical alloying and following heat treatment

MoSi₂–CrSi₂ nanocomposite powder was successfully synthesized by ball milling of Mo, Si and Cr elemental powders. Effects of the Cr content, milling time and annealing temperature were studied. X-ray diffraction (XRD) was used to characterize the milled and annealed powders. The morphological and microstructural evolutions were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). High temperature polymorph (HTP) of MoSi₂ begins to form after 50 h of milling and completes after 70 h of milling. MoSi₂–CrSi₂ composite powder was also prepared with a combination of short milling time (50 h) and low temperature annealing (850 °C). Annealing led to the HTP to low temperature polymorph (LTP) transformation of MoSi₂. MoSi₂–CrSi₂ nanocomposite powder with the mean grain size less than 50 nm was obtained at the end of milling. This composite maintained its nanocrystalline nature after annealing. A spherical morphology was procured for 50 h milled powder with 0.25 mole Cr.     

Electrospinning of carbon nanofiber supported Fe/Co/Ni ternary alloy nanoparticles

Polyacrylonitrile (PAN)-based carbon nanofiber supported Fe/Co/Ni ternary alloy nanoparticles were prepared by using the electrospinning technique for potential fuel cell applications. The solution was prepared by adding pre-solved catalytic precursor into PAN/DMF solution. The effect of PAN and catalyst precursor concentration on solution properties (viscosity and conductivity) and heat stabilization temperature has been investigated. Electrospun nanofibers were characterized by field emission scanning electron microscope, transmission electron microscope, energy dispersive spectrometer and X-ray diffractometer. It has been found that ternary nanoparticle size is in the range of 5–115 nm (average: 20 nm) and is a crystal alloy of Fe, Co and Ni. Also, TEM results demonstrate that in some regions metal nanoparticles tend to agglomerate into larger particles mainly due to the non-uniform distribution of nanoparticles in as-spun condition. PAN-derived carbon nanofiber mean diameter was measured as 200 nm by varying from 40 nm to 420 nm.