Papers by Keyword: Mechanosynthesis

Abstract: The optimal modes (temperature, time, pressure force) of spark plasma sintering (SPS) and hot pressing of boron carbide obtained by various methods are determined. The initial powders were obtained from soot and amorphous boron by the mechanochemical synthesis method, by the high-temperature synthesis (SHS) method and by the carbon reduction method. The structure and the properties of SPS sintered and hot-pressed boron carbide blanks were determined. The highest value of the relative density was achieved during SPS sintering of blanks from B₄C powders obtained by mechanosynthesis and SHS methods. It was found out the optimal conditions for sintering blanks from B₄C powder obtained by mechanosynthesis. The density value reaches 99.0 rel.% at 1500 °C/25 MPa and sintering time of 45 min. For powders obtained by the SHS method, the density of sintered blanks is 98.5 rel.%. at 1800 °C/30 MPa with sintering time of 45 min. The highest value of the relative density was achieved during the hot pressing of blanks from B₄C powders obtained by mechanosynthesis.

Abstract: In order to produce metal matrix composites (MMCs), aluminium powder was milled for a total time of 5 hours. Aluminium nitride was the ceramic reinforcement chosen to improve the mechanical behaviour of the aluminium matrix. In order to form it in situ, an ammonia gas flow was incorporated during a certain period of the milling process. Two different conditions of NH3 flow during milling were studied: short time (5 min) and long time (3 h). In both cases, milling started with a 2 h period of mechanical alloy in vacuum (5 Pa). Then, NH3 was incorporated during the stipulated time (5 min or 3 h), after which the milling process continued under vacuum to complete 5 hours. The powders were cold pressed and vacuum sintered to produce compacts. The results showed that compacts with better mechanical properties are obtained when short duration ammonia gas flow is used. The use of short flows provides good control of the amount of ceramic second phases formed. This allows the produced compacts to reach ultimate tensile strength higher than 400 MPa.

Abstract: Photocatalytic processes have been applied to treatment of organic effluents through the mineralization of these pollutants on a semiconductor surface. Obtaining nanosemiconductors is desirable for the increasing of particle surface area and improvement in photocatalytic efficiency. In this paper, it was evaluated the influence of High Energy Milling (HEM) as a technique to produce nanocrystalline zinc oxide. The photocatalytic activity of the milled powders to degrade Rhodamine-B dye when exposed to ultraviolet and visible radiation also was investigated. The powders were milled during 4 and 10 hours by dry media milling and 10 hours by wet media milling. The results indicated that there were no detectable powder contamination during the millings and the reduction of crystallite size was function of time and media of milling. All of the assessed samples demonstrate high degradation of the dye, which corroborates with the potentiality of this technique to photocatalysts production. The material milled during 10 hours by dry media milling showed the best results under the experimental conditions.

Abstract: Composite powder of Fe/Fe₂O₃ type was synthesized by mechanical milling using commercially Fe and Fe₂O₃ powders in mass ratio of 35/65. The milling process leads to the powder homogenization, powder activation and formation of some Fe/Fe₂O₃ composite particles. The Fe/Fe₂O₃ composite powder obtained by mechanical milling and the un-milled Fe/Fe₂O₃ mixture were subjected to the reactive sintering procedure in argon atmosphere at 1100 °C for 6 hours. The sintering procedure promotes the reaction of the Fe with the Fe₂O₃ and the result is a sintered composite compact of Fe/Fe₃O₄/FeO type. The microstructure of the Fe/Fe₃O₄/FeO sintered composite compacts presents iron clusters in an oxide matrix. A more homogeneous iron clusters size and distribution in oxide matrix is observed in the case of the sintered compact obtained from mechano-activated powder. The X-ray diffraction (XRD), laser particles size analysis (LPSA), optical (OM) and scanning electron (SEM) microscopies techniques were used for the investigations.

Abstract: A novel and green mechanochemical technique, the grinding-based method, was applied for synthesis of multi-metal cyanide (MMC) complexes. In order to discovery the effect of metal element on structure and catalyst efficiency, the obtained three MMCs were made from deferent three metal salts groups, which including ZnCl2 to NiCl2 to K3Fe(CN)6 molar ratio of 4:3:1 (MMC-1), ZnCl2 to NiCl2 to K3Co(CN)6 molar ratio of 4:4:1 (MMC-2), and ZnCl2 to NiCl2 to K4Fe(CN)6, respectively. And three MMCs were further used for copolymerization of CO2 and propylene oxide. FTIR and 1H NMR results shown, the alternating copolymerization obtained anticipated poly(propylene carbonate) with high catalytic activity. Comparing to the conventional solvent-based synthesis, the strikingly efficient and practically applicable grinding-based method reveals clear merits for syntheses of MMC complexes.

Abstract: Silicon nitride nanowires have been synthesized by mechanosynthesis method. The silicon powders were first milled for 160 hours under the presence of ammonia gas with 300kPa. And then they were heat-treated at 1250 for 2 hours in the pure nitrogen gas. The phase and the microstructure of the as-prepared product were detected by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results showed that the product consisted of α-Si3N4 nanowires. The nanowires had the length of tens of millimeters and the diameters of 50～100nm. The vapor-solid (VS) mechanism was employed to interpret the nanowires growth.

Abstract: The production of La0.95Sr0.05Ga0.90Mg0.10O3-δ powders was achieved at room temperature by a mechanosynthesis route in a high energy planetary ball mill starting from a mixture of lanthanum, strontium, gallium and magnesium oxides. The milling was carried out in nylon containers, using zirconia balls and a balls:powder mass ratio of 10:1. The planetary rotation was kept constant at 650 rotations per minute (rpm), and the container at 1300 rpm, in the opposite direction. The formation of the perovskite phase was detected from the early milling stages and nearly completed after milling for 360 min, as shown by powder X-ray diffraction. Transmission electron microscopy results revealed that powders consist of agglomerates of homogeneous, crystalline particles with an average equivalent diameter of about 16-17 nm, in excellent agreement with average crystallite size estimates obtained from X-ray diffraction.

Abstract: Proposed advanced mechanosynthetic systems [1] require molecular tools able to bind and transfer reactive moieties with high reliability at 300 K (failure rates << 10–10 per transfer operation). Screening of a large number of candidate tools at the AM1 level enabled the identification of a structure, DC10c, that is calculated (at the B3LYP/6- 31G(d,p) level) to meet these stringent requirements when used to transfer carbon dimers to any of a target class of graphene-, nanotube-, and diamond-like structures [2]. The favorable energy of transfer (exoergic by a mean energy ≥ 0.261 aJ per dimer) results from avoidance of the generation of high-energy radical sites during dimer release by means of π-delocalization to form a strained aromatic ring on the binding face of the empty structure. These energies are compatible with transfer-failure rates ~ 10–24 per operation at 300 K, and overall failure rates << 10–10.

Abstract: Ceramics of morphotropic phase boundary (MPB) Pb(Mg1/3Nb2/3)O3-PbTiO3 have been processed from powders synthesized by mechanochemical activation of oxides. Their properties have been characterised and are compared with those of a MPB Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramic material by the columbite route. Current experiments on texturing by templated grain growth from the powders synthesised by mechanochemical activation are briefly described. MPB Pb(Zn1/3Nb2/3)O3-PbTiO3 has also been mechanosynthesised, though perovskite is not stable under heating and ceramics cannot be processed. Pb(Fe1/2Nb1/2)O3 is being investigated as a means of stabilising the perovskite, and MPB ceramic materials of the ternary system have been successfully prepared.