High-energy Milled Research Articles

Mechanochemical synthesis of MgTa 2O 6 ceramic

MgTa 2O 6 powders were prepared by mechanochemical synthesis from MgO and Ta 2O 5 in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa 2O 6 after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa 2O 6 from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.

Dispersion and field emission properties of multi-walled carbon nanotubes by high-energy milling

Multi-walled carbon nanotubes (MWNTs) were high-energy milled for 2 h in texanol after a polycarboxylic acid polymeric dispersant had been added in order to enhance the dispersion. The degree of MWNT dispersion was significantly enhanced by high-energy milling compared to the intact sample, which increased the density of surface-exposed MWNTs with a screen-printed paste. However, the emission properties of high-energy milled MWNTs did not show such a high emission current density relative to their increased surface-exposed density. Further investigation using Raman spectroscopy and X-ray diffraction evidenced the milling-induced MWNT damage, which explained the relatively lower emission current density of high-energy milled MWNTs.

Hot-pressed Sm2Fe17Nx∕Fe–Co composites: Factors controlling densification and in situ nitrogenization of Sm2Fe17 phase

The effects of powder preparation and conditions of hot pressing on the structure and density of consolidated Sm2Fe17Nx and Sm2Fe17Nx∕Fe–Co magnets have been studied. Densities achieved in the case of Sm2Fe17Nx powders prepared through a low-energy milling were higher than after a high-energy milling. The difference is mostly caused by the different particle size, and it can be eliminated by an additional low-energy milling of the coarse high-energy milled powder. The presence of a ductile soft magnetic phase greatly facilitates densification, leading to considerably higher absolute and relative densities in hot-pressed Sm2Fe17Nx∕Fe0.65Co0.35 composites. We have found that during hot pressing, nitrogenization of the Sm2Fe17 phase may occur in situ if pressed together with Fe–Co–N powders. Because Fe–Co–N releases nitrogen below the decomposition temperature of Sm2Fe17Nx, we were able to fabricate the Sm2Fe17Nx∕Fe0.65Co0.35 composite with the density up to 7.6g∕cm3 by hot-pressing mixtures of Sm2Fe17 and (Fe0.65Co0.35)89N11 powders.

Effect of milling methods on performance of Ni–Y 2O 3-stabilized ZrO 2 anode for solid oxide fuel cell

A Ni–YSZ (Y 2O 3-stabilized ZrO 2) composite is commonly used as a solid oxide fuel cell anode. The composite powders are usually synthesized by mixing NiO and YSZ powders. The particle size and distribution of the two phases generally determine the performance of the anode. Two different milling methods are used to prepare the composite anode powders, namely, high-energy milling and ball-milling that reduce the particle size. The particle size and the Ni distribution of the two composite powders are examined. The effects of milling on the performance are evaluated by using both an electrolyte-supported, symmetric Ni–YSZ/YSZ/Ni–YSZ cell and an anode-supported, asymmetric cell. The performance is examined at 800 °C by impedance analysis and current-voltage measurements. Pellets made by using high-energy milled NiO–YSZ powders have much smaller particle sizes and a more uniform distribution of Ni particles than pellets made from ball-milled powder, and thus the polarization resistance of the electrode is also smaller. The maximum power density of the anode-supported cell prepared by using the high-energy milled powder is ∼850 mW cm −2 at 800 °C compared with ∼500 mW cm −2 for the cell with ball-milled powder. Thus, high-energy milling is found to be more effective in reducing particle size and obtaining a uniform distribution of Ni particles.

Thermal syntheses of magnesium borate compounds from high-energy milled MgO–B 2O 3 and MgO–B(OH) 3 mixtures

The comparative effects of high-energy milling on solid-state thermal syntheses of different magnesium borate compounds from MgO–B 2O 3 and MgO–B(OH) 3 mixtures were investigated for initial MgO:B 2O 3 mole ratios of 1:0.125, 1:0.25, 1:0.5, 1:1, 1:2 at temperatures between 500 and 900 °C. It was observed that magnesium oxide–boric acid mixtures react only by high-energy milling to form hydrous magnesium borates. Following high-energy milling of the initial solid mixtures, anhydrous magnesium borate compounds started to form at temperatures as low as 500 °C, and purer and more crystalline phases were obtained at higher temperatures compared to unmilled compositions.

Ultrasmall-angle X-ray scattering (USAXS) studies of morphological trends in high energy milled NaAlH 4 powders

Transition metal dopants added to complex metal hydrides, specifically to sodium aluminum hydride (NaAlH 4), by high energy ball milling enhances dehydrogenation kinetics and induces dehydrogenation reaction reversibility. This study uses the power-law scattering parameter, p, gained from ultrasmall-angle X-ray scattering (USAXS) data to elucidate differences in NaAlH 4 particle morphology as dopant type and mill time is varied. Four dopant types were used. Two dopant types were used to represent the best kinetic enhancements having high desorption rates (e.g. TiCl 2, TiCl 3) and were compared with two dopant types which do not perform as well (e.g. ZrCl 3 and VCl 3). USAXS data for the doped hydrides were compared with undoped and milled NaAlH 4 powders. Mill times used were 0 min (blended), 1, 5, and 25 min. As indicated by the USAXS power-law scattering data, the undoped NaAlH 4 powders are comprised of primary particles each having a high surface area. The high particle surface area in the undoped NaAlH 4 particle system persists as mill time increases—with only the 25 min sample undergoing a marked a decrease in primary particle surface area. Alternatively, the doped powders milled for 1, 5, and 25 min show uniformly decreasing hydride particle surface area. These decreases in particle surface area may be explained by either the colloidal particles increasing in surface smoothness or decreasing internal void space. TiCl 3-doped NaAlH 4 powders show the trend of maintaining particles having a morphology comprised of higher particle surface area during the high energy milling stage of powder processing compared with other dopants.

The properties of high-energy milled pre-alloyed copper powders containing 1 wt.% Al

The microstructural and morphological changes of inert gas atomized pre-alloyed Cu-1 wt.% Al powders subjected to hith-energy milling were studied. The microhardness of hot-pressed compacts was measured as a function of milling time. The thermal stability during exposure at 800 ?C and the electrical conductivity of compacts were also examined. During the high-energy milling, severe deformation led to refinement of the powder particle grain size (from 550 nm to about 55 nm) and a decrease in the lattice parameter (0.10 %), indicating precipitation of aluminium from the copper matrix. The microhardness of compacts obtained from 5 h-milled powders was 2160 MPa. After exposure at 800?C for 5 h, these compacts still exhibited a high microhardness value (1325 MPa), indicating good thermal stability. The increase of microhardness and good thermal stability is attributed to the small grain size (270 and 390 nm before and after high temperature exposure, respectively). The room temperature electrical conductivity of compacts processed from 5 h-milled powder was 79% IACS. .

Changes in hydrogen storage properties of binary mixtures of intermetallic compounds submitted to mechanical milling

Electrochemical loading of hydrogen in single and binary mixtures of the intermetallic compounds, TiNi, TiFe, Zr 30Ni 70 and Zr 70Ni 30 have been investigated after energetic mechanical milling. Enhanced hydrogen storage of the 50/50 (by weight) TiNi/ZrNi binary mixtures are only found after high energy milling (450 rpm for 12 h). The high energy milled 50/50 and 30/70 TiFe/Zr 30Ni 70 binary mixtures exhibited good electrochemical loading (∼98 mA h g −1) at 25 °C but this capacity was decreased by ∼12% on increasing the temperature to 55 °C. On the other hand, the 70/30 TiFe/Zr 30Ni 70 binary mixtures showed a 65% increase in the hydrogen capacity with temperature over the same range. High energy milling of the TiFe with carbon materials again led to a substantial increase in the storage capacity but here, this was attributed to a partial reduction of the surface oxides by the graphite/activated carbon. Thermal analysis of the samples using high pressure differential scanning calorimetry on the milled Zr 70Ni 30 sample showed a rapid deterioration of the hydrogen absorption/desorption features with thermal cycling over the range 100–310 °C due to sintering of the samples.

All solid state Li-ion secondary battery with FeS anode

We investigated the effect of the particle size on the rate capability and the cycle performance by comparing those of a non-milled FeS (NM-FeS) and a high energy milled FeS (HM-FeS) anode. The particle size of FeS powder decreased from several 10 μm to ca. 300 nm by the high energy milling. The HM-FeS showed better high-rate capability and cycle performance owing to the small particle size.

Magnetic properties of high-energy milled Fe 78Si 13B 9 nanocrystalline powders and powder-based nanocomposites

The paper deals with the combined influence of the thermal nanocrystallization temperature and time and the milling time on the properties of the nanocrystallization process and the structure of powders based on METGLAS (metallic glass). Thermal treatment of Fe 78Si 13B 9 metallic glass was carried out for 1 h at the temperature range of 100–600 °C with 50 °C temperature steps. The heat treated material was then processed by high-energy milling. It was shown, that the use of a combination of thermal nanocrystallization and high-energy milling produced metallic glass-based nanocrystalline powders containing Fe-α, Fe 3B and Fe 2B crystallites in the amorphous matrix. Then, the nanocrystalline powders were used to construct composite toroidal cores.

Microstructures and properties of nanostructured thermal sprayed coatings using high-energy milled cermet powders

This study demonstrates the potential of high-energy milling to use nanostructured cermet powders for thermal spraying utilizing a TiC–Ni-based composite as model material. The microstructure of coatings processed by VPS and HVOF spraying of nanostructured composite powders is characterized and compared to the initial microstructure of the feedstock. Thus, the effect of different microstructures, which can be produced by high-energy milling, on the microstructural evolution during spraying is evaluated. Results show that partial dissolution and reprecipitation of hard phase material as well coarsening of the binder phase crystallite size occur during the spraying process. However, a homogeneously dispersed hard phase distribution similar to that of the nanostructured precursor powder with hard phase sizes in the range of 100 nm is formed. Additionally, hard phase particles bigger than of 300 nm are retained during spraying. First results concerning hardness and wear resistance of respective coatings are shown and discussed.

Structural disorder in the high-energy milled magnesium ferrite

The structural and magnetic evolution in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by Mössbauer spectroscopy. It is found that the nanostructural state of the milled MgFe2O4 is characterized by a mechanically induced cation redistribution between tetrahedral (A) and octahedral [B] sites. The reduced concentration of iron ions at (A) sites in the mechanically treated samples leads to the variation in the number of magnetic and nonmagnetic (A)-site ions as nearest neighbors of the Fe3+[B] ions. This results in a broad distribution of magnetic hyperfine fields at the [B] sites. In addition to the local magnetic fields B(6), B(5), and B(4) characteristic of nonactivated ferrite and corresponding to Fe3+[B] ions with n=6, 5, and 4 nearest (A)-site iron neighbors, respectively, the distribution curves of mechanically treated samples show additional components at smaller magnetic fields. The weight of the B(6) field decreases with increasing milling time, and the B(5) field becomes the most probable hyperfine field component in the distribution curve of the mechanically activated samples. The degree of inversion in MgFe2O4 is calculated from the probabilities of the different [B]-site surroundings as well as from the Mössbauer subspectral areas. Excellent agreement is obtained in the two independent procedures for the determination of the cation distribution. This enables us to separate from the [B]-site magnetic field distribution profile the contribution arising from the mechanically induced “new” nearest-neighbor (A)-site configuration with n=3 nearest (A)-site iron neighbors. Taking into account the nanoscale nature of the mechanically activated MgFe2O4, the observed spin canting, which increases with increasing milling time, is attributed to the noncollinear spin structure of the near-surface atoms. In strongly activated ferrite, the magnetic hyperfine splitting breaks down totally and the Mössbauer spectrum is dominated by a superparamagnetic relaxation effect.

Alcohol sensor based on a non-equilibrium nanostructured xZrO 2–(1− x)α-Fe 2O 3 solid solution system

Non-equilibrium nanostructured solid solution xZrO 2–(1− x)α-Fe 2O 3 powders have been prepared using the high-energy ball milling technique, and their particle size, structural properties and alcohol gas properties have been systematically characterized using X-ray diffraction (XRD) and gas sensing measurements. Experimental results show that particle size of the powder is drastically milled down to less than 10 nm after 20 h of high-energy ball milling. Our modified structural model, □ 1/3+ZrO 2+ yO 2→Zr 1/3 4++2(1− y)O s 2−+4 yO s −; with □ 1/3 denoting the 1/3 available octahedral sites of the corundum structure; for these non-equilibrium nanostructured solid solution xZrO 2–(1− x)α-Fe 2O 3 materials can explain both the lattice expansion of these high energy milled sample as well as the charge neutrality in terms of additional oxygen dangling bonds at the particle surfaces. The screen-printed thick film gas sensors made from such mechanically alloyed materials demonstrate a very high gas sensitivity value of 1097 at 1000 ppm of alcohol gas in air. It is believed that the high gas sensitivity value is related to the enormous oxygen-dangling bonds at the particle surfaces.

Characterization of high-energy milled alumina powders

The utilization of reactive high-energy milling has been reported for the synthesis of ceramic powders namely, metal oxides, carbides, borides, nitrides or mixtures of ceramics or ceramic and metal compounds. In this work, high-energy milling was used for reduction of alumina powders to nanometric particle size. The ceramic characteristics of the powders were analyzed in terms of the behavior during deagglomeration, compaction curves, sintering and microstructure characterization. It was observed that the high energy milling has strong effect in producing agglomeration of the nanosized powders. This effect is explained by the high-energy impact of the balls, which may fracture particles or just cause the particles compacting. In this case, strong agglomerates are produced. As the powder surface area increases, stronger agglomerates are produced.