Long-term Milling Research Articles

Tailoring a low-energy ball milled MnCo2O4 spinel catalyst to boost oxygen evolution reaction performance

The development of cost-efficient oxygen evolution reaction (OER) catalysts is one of the most important tasks facing modern techniques for hydrogen production. In this work, for the first time, a low-energy ball milling process of MnCo2O4 (MCO) spinel powders, with a mechanical modification time exceeding 1 day was used. After 6 days of ball-milling, the obtained overpotential of the electrocatalyst reached the value of 375 mV at 10 mA cm−2, which is a relatively low value obtained for this type of compound. The studies showed how the mechanical (low-energy long-term milling process) and chemical modification of the fragmented spinel powder nanoparticle surfaces affects the increase of the electrocatalytic properties. The addition of the appropriate amount of conductive carbon black (cCB) and Nafion ionomer to the ink of the MCO spinel also has a significant effect on the improvement of the catalytic performance of the manganese-cobalt oxide during the milling process. By reducing the amount of Nafion to 10 % of its initial value, the overpotential dropped to 352 mV at 10 mA cm−2 after 30 days of ball-milling. This shows that catalyst ink and layer composition are important factors influencing the catalyst’s efficiency in the OER.

Surface modification of carbonyl iron with organosilicon block copolymer performed by wet ball milling

The paper shows the possibility of producing plate-like carbonyl iron particles covered with a surface layer modified with a poly(dimethylsiloxane) – poly(phenylsilsesquioxane) block copolymer by means of wet ball milling. At the initial stages, the milling is accompanied with the formation of an intermediate layer enriched with rigid poly(phenylsilsesquioxane) blocks followed by the copolymer precipitation. Long-term milling leads to the complete decomposition of the polymer on the surface with the formation of silicon oxycarbide and silicon carbide, with the shape of the particles being changed from plate-like to stone-like. Mechanochemical treatment goes also with dehydrogenation/hydrogenation of the milling medium followed by the formation of alkyl derivatives in the liquid and carbides in the particles as well as oxidation of the medium with the formation of ester groups. Other additives of various types (sodium dodecyl sulfate, stearic acid, N-phenylanthranilic acid) introduced into the block copolymer solution under milling facilitates the formation of multicomponent modifying layers. The most of modifying organic layers formed under wet ball milling have been shown to exhibit anticorrosive properties. The particles covered with as-modified layers may be used as magnetic fillers for microwave materials.

Molecular transformations in interfaces and liquid media under wet ball milling of iron with N-phenylanthranilic acid

This paper provides FTIR and XPS studies on molecular transformations in interfaces and liquid media under wet ball milling of iron with N-phenylanthranilic acid (NPhA) used as a chelator and corrosion inhibitor. The initial stages of ball milling are accompanied with the formation of a variety of compounds, including complex compounds of iron, polymeric NPhA, ester groups, and non-volatile alkyl derivatives. The NPhA derivatives adsorbed on the surface stabilize the flake-like shape of particles and significantly increase their corrosion resistance in both acidic and neutral media. It has been shown that chelators/corrosion inhibitors may be successfully used as effective process controlling agents for wet ball milling of transition metals of low corrosion resistance to synthesize corrosion-resistant flake-shaped particles. Long-term milling has been shown to make towards the destruction of oxygen-containing groups, including the dehydration of NPhA itself to acridone, a polycyclic compound. The destruction of the modifying NPhA-based surface layer leads to the agglomeration of particles. The intensive destruction of the milling medium ingredients taking place during long-term milling facilitates the accumulation of oxides and carbides.

The Quality of Fe14Cr ODS Powder Alloys During Milling and Upon Heating and Its Impact on the Mechanical Properties of Consolidated Steels

Oxide dispersion-strengthened ferritic steels (ODSFSs) are promising structural materials for applications in fusion and fission power reactors, but further improvement in their (high-temperature) mechanical properties and ferrite phase stability is required. This work demonstrates that an approach to produce Fe14Cr ODSFSs with a stable ferrite phase and improved strength could involve grain size strengthening by long-term milling with a tiny amount of nitrogen. Fe-14Cr-3W-0.4Ti-0.25Y2O3 powders were ball-milled up to 170 hours under an argon atmosphere. In addition to X-ray diffraction, the change in product quality during milling and upon heating was thoroughly investigated by more sensitive magnetic and thermal analysis by measuring the saturation magnetization σs, coercivity Hc, Curie temperature Tc, and temperature of ferrite-austenite (α → γ) transition Tα→γ. A pronounced modification of magnetic and microstructure parameters was observed when milling over 70 hours and upon heating above 800 °C and was found to be generated by long-term milling with a tiny amount of nitrogen. Upon heating, the nitrogen, incorporated during milling, developed a α → γ transition region, with the decomposition of nitrides precipitated at the earlier stage of heating followed by austenite decomposition, nitrogen degassing, and microstructure refinement to a grain size of a few tenths of a nm (e.g., 28 nm by heating at 910 °C of 100-hour milled powder). The resulting ferrite phase with refined grains is highly stable to (further) heating. The powders milled for 70 and 100 hours containing 0.175 and 0.500 wt pct nitrogen, respectively, were consolidated at 1100 °C with subsequent annealing at 1050 °C and subjected to Vickers hardness and 3-point bending tests. The steel produced from the powder milled for 70 hours shows lower hardness, higher density (close to the theoretical value of 7.8 g/cm3), and fracture strain. The ductility of this ODS alloy (0.075 fracture strain) is comparable with Eurofer97 (0.075 fracture strain), whereas its strength (2070 MPa ultimate stress) is significantly higher than that of Eurofer97 (1222 MPa ultimate stress). This improvement was attributed to grain size strengthening—the refined grains (promoted by milling with nitrogen) could be effectively pinned by Y-Ti-O dispersoids.

Thermochemical properties of nanometer CL-20 and PETN fabricated using a mechanical milling method

2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and pentaerythritol tetranitrate (PETN), with mean sizes of 73.8 nm and 267.7 nm, respectively, were fabricated on a high-energy ball-mill. Scanning electron microscope (SEM) analysis was used to image the micron-scale morphology of nano-explosives, and the particle size distribution was calculated using the statistics of individual particle sizes obtained from the SEM images. Analyses, such as X-ray diffractometer (XRD), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS), were also used to confirm whether the crystal phase, molecular structure, and surface elements changed after a long-term milling process. The results were as expected. Thermal analysis was performed at different heating rates. Parameters, such as the activation energy (ES), activation enthalpy (ΔH≠), activation free energy (ΔG≠), activation entropy (ΔS≠), and critical temperature of thermal explosion (Tb), were calculated to determine the decomposition courses of the explosives. Moreover, the thermal decomposition mechanisms of nano CL-20 and nano PETN were investigated using thermal-infrared spectrometry online (DSC-IR) analysis, by which their gas products were also detected. The results indicated that nano CL-20 decomposed to CO2 and N2O and that nano PETN decayed to NO2, which implied a remarkable difference between the decomposition mechanisms of the two explosives. In addition, the mechanical sensitivities of CL-20 and PETN were tested, and the results revealed that nano-explosives were more insensitive than raw ones, and the possible mechanism for this was discussed. Thermal sensitivity was also investigated with a 5 s bursting point test, from which the 5 s bursting point (T5s) and the activation of the deflagration were obtained.

Variability in Migration Routes Influences Early Marine Survival of Juvenile Salmon Smolts.

Variability in animal migratory behavior is expected to influence fitness, but few empirical examples demonstrating this relationship exist. The initial marine phase in the migration of juvenile salmon smolts has been identified as a potentially critical life history stage to overall population productivity, yet how fine-scale migration routes may influence survival are unknown. Large-scale acoustic telemetry studies have estimated survival rates of outmigrant Pacific salmon smolts through the Strait of Georgia (SOG) along the British Columbian coastline to the Pacific Ocean, but these data have not been used to identify and characterize fine-scale movements. Data collected on over 850 sockeye salmon (Oncorhynchus nerka) and steelhead (Oncorhynchus mykiss) smolts detected at an array in the Strait of Georgia in 2004–2008 and 2010–2013 were analyzed to characterize migration routes and link movements to subsequent survival at an array 250 km further along the marine migration pathway. Both species exhibited disproportionate use of the most eastern route in the Strait of Georgia (Malaspina Strait). While many smolts moved across the northern Strait of Georgia acoustic array with no indication of long-term milling or large-scale east-to-west movements, large proportions (20–40% of sockeye and 30–50% of steelhead) exhibited a different behavior, apparently moving in a westward or counterclockwise pattern. Variability in migratory behavior for both species was linked to subsequent survival through the Strait of Georgia. Survival for both species was influenced by initial east-to-west location, and sockeye were further influenced by migration timing and duration of time spent near the northern Strait of Georgia array. Westward movements result in a net transport of smolts from Malaspina Strait to the Strait of Georgia, particularly for steelhead. Counterclockwise movements may be due to the currents in this area during the time of outmigration, and the higher proportion of steelhead smolts exhibiting this counterclockwise behavior may reflect a greater exposure to wind-altered currents for the more surface-oriented steelhead. Our results provide an empirical example of how movements can affect migration survival, for which examples remain rare in movement ecology, confirming that variability in movements themselves are an important part of the migratory process.

Optimization of drying process for PB1121 variety of paddy

Rough rice is typically harvested at moisture contents ranging from 20% to 24%, and subsequently dried to approximately 12% for safe long-term storage and milling. Harvesting paddy at high moisture content normally results in high yields and less damage and prevents field losses due to dropping and shattering. Drying of grains creates moisture and temperature gradient within the kernel resulting in the development of tensile stresses at the surface and compressive stress at the interior of the rice kernel. These stresses may result in kernel fissuring and subsequent breakage during milling. Thus, drying is considered to be one of the most critical operations in post-harvest processing of paddy which has great influence on milling yield. Basmati rice possesses unique cooking, eating and digestive qualities. India accounts for more than 70% of the world basmati rice production. Among basmati, PB1121 is widely grown in India and contributes a major share of rice exports from the country. Therefore drying of PB1121 was carried out with the objective to optimize the drying variables for minimum drying time and maximum head yield. Drying was carried out in 2-passes using a cross-flow dryer. The drying experiment in thin layer was conducted as per a central composite rotatable design in response surface methodology with 3 factors (drying air temperature, tempering time and moisture reduction in first drying pass) at 5 levels. The variables chosen for the experiment were drying air temperature (40–70 °C), tempering time (60–240 min) and moisture reduction percentage (3–10%) in the first pass of drying. Optimization of drying variables was done through the use of a desirability function by combining all responses into one measurement. The desirability functions were minimum drying time, maximum brown rice yield and head rice yield. Total drying time ranged from 35 to 132 min in two stages of drying, while in continuous drying it varied from 41 to 204 min. Head rice yield ranged from 39.77 to 71.56%. Optimum conditions for drying of paddy grains were found to be temperature of 58.3 °C, 7.13% moisture reduction and 148 min tempering time. At optimized conditions, total drying time and head rice yield were calculated at 53.1 min and 54.53%, respectively.

Effect of Transition-Metal Additives on Thermoelectric Properties of YB22C2N

The higher boride compound YB22C2N has been reported as a promising n-type high-temperature thermoelectric material and possible counterpart to boron carbide. To investigate the influence of transition-metal additives on the thermoelectric properties of YB22C2N, a series of Rh, Co, Cu, and Ni samples were prepared. The resistivity and Seebeck coefficient of the samples were measured in the temperature range of 323 K to 1073 K. Samples with Rh and Co additives showed a considerable reduction of resistivity in comparison with pure YB22C2N and maintained their semiconducting properties at high temperatures. A sample with Co, obtained using long-term ball milling, showed the highest absolute value of Seebeck coefficient among all previously studied YB22C2N-based materials. Analyses of the influence of transition-metal additives and processing methods such as ball milling on the thermoelectric properties of YB22C2N are presented.

Influence of process parameters on breakage kinetics and grinding limit at the nanoscale

In long-term milling experiments, in a stirred media mill, a grinding limit where no further particle breakage occurs was identified. During mechanical stressing of the particles, defects are generated in the crystalline lattice, which allows real fracture of nanoparticles. Below a critical size, defects cannot be stored or generated in the crystallites and the overall limit of grinding is reached. This limit is strongly influenced by material properties and hardly affected by most of the process conditions. However, the breakage kinetics strongly depend on the process parameters and suspension conditions as long as the grinding limit is not reached. Based on these findings, two mechanisms of nanoparticle breakage are proposed. Proper choice of process parameters saves not only up to 90% of the energy input to reach the grinding limit but also leads to a higher product quality in terms of crystallinity and less milling bead wear. © 2010 American Institute of Chemical Engineers AIChE J, 2011

In situ synthesized TiC/Ti 5Si 3 nanocomposites by high-energy mechanical alloying: Microstructural development and its mechanism

In situ synthesis of TiC/Ti 5Si 3 nanocomposites powder was performed by high-energy ball milling of a Ti–SiC powder mixture having an 8:3 molar ratio. The 25 h milled final product was featured by the nanocrystalline Ti 5Si 3 matrix reinforced with the uniformly dispersed TiC nanoparticles. The crystalline size of the in situ formed Ti 5Si 3 and TiC constituents was ∼14 and ∼8 nm, respectively. The evolutions of constitution phases and particle structures of the milled powders were studied and the predominant mechanisms behind the microstructural developments were elucidated. It showed that the disappearance of X-ray diffraction peaks of SiC in the milling system was significantly behind the disappearance of Ti peaks. The SiC constituent decomposed gradually within 25 h of milling, while the Ti constituent reacted speedily after a relatively short time of 10 h. The structures of the milled TiC/Ti 5Si 3 nanocomposites powders experienced a successive change: pre-refining – coarsening – re-refining on increasing the applied milling time, due to the competitive action between the mechanisms of cold-welding and fracturing. The plastic deformation and cold-welding occurred at the initial milling stage where the ductile Ti element existed. Only fracturing of the particles was operative in a long-term milling system (≥15 h), due to the formation of brittle intermetallic compound Ti 5Si 3 and ceramic compound TiC. The refinement of the milled powder particles was based on a layered fracturing mechanism.

Mechanochemical synthesis of stoichiometric MgFe2O4 spinel

The influence of long-term milling of a mixture of (1) MgO and α-Fe2O3, (2) MgCO3, and α-Fe2O3, and (3) Mg(OH)2 and α-Fe2O3 powders in a planetary ball mill on the reaction synthesis of nanosized MgFe2O4 ferrites was studied. Mechanochemical reaction leading to formation of the MgFe2O4 spinel phase was followed by electron microscopy, (SEM and TEM), X-ray diffraction and magnetization measurements. The spinel phase was observed first in cases (1) and (2) after 5 h of milling, and its formation was observed in all cases after 10 h. The synthesized MgFe2O4 ferrite has a nanocrystalline structure with a crystallite size of about 11, 10, and 12 nm, respectively for cases (1)–(3). Magnetic measurements after 10 h of milling show magnetization values of 19.8 J/(Tkg), 23.5 J/(Tkg) and 13.8 J/(Tkg), respectively for the cases (1)–(3).

Tailoring deformation-induced effects in Co powders by milling them with α–Al2O3

The evolution of the structural and magnetic properties of metal-ceramic, cermet, nanocomposite powders, consisting of Co and α–Al2O3in different proportions, prepared by ball milling has been investigated. The overall microstructure of the system, after long-term milling, is found to be very sensitive to the amount of α–Al2O3, yielding a less refined and faulted hexagonal-close-packed (hcp)-Co structure for the sample with larger α–Al2O3percentage. The increased presence of the ceramic counterpart also causes a delay of the face-centered-cubic (fcc) to hcp-Co stress-induced transformation during ball milling. The results seem to indicate an evolution of the role of α–Al2O3, from increasing locally the strain rate of the mechanical work for small amounts of ceramic to absorbing milling energy for large amounts of α–Al2O3. The magnetic properties correlate with the obtained microstructure, where the amount of hcp-Co and stacking faults and the isolation of the Co particles by the α–Al2O3control the coercivity.

Kinetics of the sigma-to-alpha phase transformation caused by ball milling in near equiatomicFe−Cralloys

Kinetics of the sigma-phase decomposition in three series of $\ensuremath{\sigma}\text{\ensuremath{-}}\mathrm{FeCr}$ intermetallic compounds caused by ball milling in a vibratory mill was followed by M\"ossbauer spectroscopy. It can be described by an exponential decrease with milling time. The decomposition leads to the formation of the $\ensuremath{\alpha}$-phase as well as that of an amorphous phase. The former is expected from the phase diagram, but the latter is not. The amount of the $\ensuremath{\alpha}$-phase increases with milling time, reaches a maximum characteristic of the milling conditions, and, finally, after a slow decrease, remains constant. The decrease can be explained in terms of the $\ensuremath{\alpha}$-phase partial amorphization. The amount of the amorphous phase stays rather constant with a small tendency of increasing for long-term milling times. Its structure seems to evolve with milling time.

Nb2O5 “Pathway Effect” on Hydrogen Sorption in Mg

In the present work we investigate the hydrogen sorption mechanism in a MgH(2)/Nb(2)O(5) composite and analyze why Nb(2)O(5) could strongly improve hydrogen sorption kinetics in magnesium. Hereby we make use of the fact that Nb(2)O(5) nanoparticles are able to reduce the milling time significantly with the achievement of excellent sorption kinetics, and can so exclude effects occurring at long-term milling that make difficult the study of the mechanism. On the basis of extensive chemical, crystalline, and microstructural characterization of the MgH(2)/Nb(2)O(5) nanopowder system, a "pathway model" is proposed, which explains the kinetic hydrogen sorption improvement by a formation of pathways of niobium oxide species with lower oxidation state that facilitate the hydrogen transport into the sample. This mechanism is shown to be supported by additional oxidation experiments, which indicate increased oxygen diffusion through these pathways.

NiAl-Al2O3intermetallic matrix composite prepared by reactive milling and consolidation of powders

Reactive milling of nickel oxide and aluminium powders corresponding to the stoichiometric reaction 3NiO + 5Al resulted in the formation of intermetallic matrix composite NiAl-Al2O3, with 28 wt% of alumina. Prolongation of the milling process allowed obtaining the microstructure with nanosize range of crystallites of both phases, as showed XRD measurements and TEM observations. The refinement of microstructure was accompanied with an increase of lattice strain as a result of ball milling. The particles size and morphology changed from several tens of micrometers and polyhedron shape observed immediately after the reaction took place, to several micrometers and spherulitic shape after long-term milling.

Local structure and magnetic properties of mechanical alloyed Co–C compositions

Formation of nanocrystalline Co particles embedded in an amorphous C matrix produced by the mechanical alloying process was studied. The formation of the Co5C95 alloy was examined by the x-ray diffraction (XRD) and extended x-ray absorption fine structure methods. XRD analysis displayed the decrease in the Co grains size, the transformation from the face-center-cubic (fcc)+hexagonal close-packed (hcp) phases to the hcp phase and an amorphization of carbon matrix with milling time. At the early stage of milling, the coercivity, HC, increased because of the higher hcp phase HC than that of the fcc phase. Partial HC increase could be also caused by the decrease of Co particles size. The subsequent powder milling led to the decrease in the HC due to thermal process and/or due to the transformation of hcp-Co to a random close-packed phase under a long-term milling. Magnetization of the compositions decreased gradually with milling. The size of single domain particles after the 60 h of milling was about 10 nm. The Co–C and Co–Co bond distances, deduced by pair distribution function analysis, were equal to 1.92±0.02 Å and 2.48±0.02 Å, respectively.

Correlation between stacking fault formation, allotropic phase transformations and magnetic properties of ball-milled cobalt

A correlation between microstructure, allotropic phase transformations and magnetic properties of cobalt subjected to ball milling is presented. The starting cobalt, which is a mixture of hexagonal close-packed (hcp) and face-centered cubic (fcc) phases, is found to develop into an almost pure hcp phase after short-term milling. At the same time, the coercivity, HC, and the uniaxial magnetic anisotropy, Ku, increases. For longer milling times, plastic deformation mechanisms introduce large amounts of stacking faults in the hcp structure and, consequently, fcc-Co is in part recovered, even though this phase is usually metastable at room temperature. After long-term milling, the high degree of structural disorder and the partial transformation from hcp to fcc-Co significantly reduce both HC and Ku. The effect of the milling intensity, i.e. increase of the ball-to-powder weight ratio, is mainly to accelerate the structural and magnetic changes induced in Co. © 2003 Elsevier B.V. All rights reserved.

Microstructural effects and large microhardness in cobalt processed by high pressure torsion consolidation of ball milled powders

The effects of plastic deformation on Co microstructure after a high pressure torsion (HPT) process on unmilled and ball milled powders have been studied and compared to those induced simply by ball milling (BM). X-ray diffraction analyses reveal that both processing routes generate large amounts of stacking faults in Co, which result in a mixture of hcp and fcc phases. However, significant differences in the amount and type of faults accumulated are encountered between BM and HPT. The combination of both processing routes induces a heavily distorted microstructure in Co. In particular, BM+HPT brings about a drastic reduction in crystallite size. Remarkably, high values of microhardness (in excess of H V=7.3 GPa) are obtained in foils prepared by long-term milling followed by HPT. It is proposed that, in addition to the crystallite size refinement, the large amounts of twin faults, together with the two-phase mixture, also contribute to enhance the microhardness.

The effect of MgNi 2 intermetallic compound on nanostructurization and amorphization of Mg–Ni alloys processed by controlled mechanical milling

Two Mg–Ni alloys with 27.9±11.1 and 57.5±0.8 at% Ni, fabricated by ingot metallurgy (IM) and containing ∼9 and ∼79 vol% of the MgNi 2 phase, respectively, were ball (mechanically) milled in a magnetic Uni-Ball-Mill 5 under controlled shearing mode for 10, 30, 70 and 100 h. The evolution of the microstructure of milled powders is presented. It is observed that the Mg 2Ni phase undergoes a partial amorphization in the Mg–Ni alloys containing ∼79 vol% of the MgNi 2 phase while no amorphization of Mg 2Ni is observed in the alloy containing only ∼9 vol% of MgNi 2. The results are rationalized in terms of the enthalpy effects based on the application of Miedema’s semi-empirical model to the phase changes in ball-milled intermetallics and the critical nanograin size required to be formed in the Mg 2Ni phase before triggering its amorphization, which is enhanced by the presence of hard MgNi 2 phase during ball milling. The milled powders of 27.9±11.1 at% Ni alloy, after long-term milling for 100 h, did not absorb hydrogen.

Microstructural aspects of the hcp-fcc allotropic phase transformation induced in cobalt by ball milling

A detailed correlation between microstructure evolution and allotropic phase transformations occurring in Co when subjected to ball milling has been carried out. After short-term milling, the starting mixture of hcp + fcc Co develops into an almost pure hcp phase. However, for longer milling times, plastic deformation introduces large amounts of stacking faults, especially of twin type, in the hcp structure. As a consequence, some of the hcp Co is converted back into fcc and the hcp unit cell is progressively anisotropically distorted. After long-term milling, a steady 'pseudo-equilibrium' state is observed, where all microstructural parameters, including the fcc percentage, tend to level off. However, the milling intensity can still be adjusted to increase further the stacking-fault probability and, consequently, the amount of fcc Co in the milled powders. The results imply that the stacking-fault formation, rather than the local temperature rise or crystallite size reduction associated with the milling process, is the main mechanism governing the hcp-fcc transformation.