Papers by Keyword: Hydrogen Reduction

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Authors: G.P. Martins, Tawiwan Kangsadan, Grant Scott, Christopher Wagner, Jeff Van Hoose
Abstract: High purity molybdenum metal powder is produced commercially from hexavalent molybdenum precursors, viz.: ammonium dimolybdate (ADM) or molybdenum trioxide. One conventional process incorporates first-stage and second-stage flowsheet components, with hydrogen gas serving as reductant. This two-stage strategy is employed in order to minimize the formation of volatile molybdenum species that would otherwise be generated at the high temperature required to obtain molybdenum (Mo) in a single stage conversion of the molybdenum precursor. Although molybdenum powder has been produced commercially for over a century, a comprehensive understanding of the kinetic mechanisms and powder characteristics, e.g. oxygen content and particle morphology, is far from being definitive. In fact, it might be argued that the “art” and engineering, in a commercial context, has advanced ahead of the fine-detail science-derived metallurgical process-engineering. Theoretical contributions presented in this paper are focused primarily on the fundamentals of the conversion process associated with second-stage reduction process – MoO2 to Mo and the factors that contribute to the oxygen content of the molybdenum powder product (1000 to 100 ppm(w) O, range). Thus, equilibrium-configuration details concerning both solid and gas phases are addressed, including the volatile hexavalent molybdenum vapor complexes as well as solubility of oxygen in molybdenum. In regard to the role of a chemical vapor-transport mechanism on powder morphology in second-stage conversion of MoO2 to Mo, it is shown that the partial pressure of the prominent molybdenum hydroxide vapor-complex (MoO2(OH)2) is far too low to support such a mechanism. This contention has been corroborated by employing helium to control the partial pressures of hydrogen and water in the gas phase. Secondarily, a limited assessment of the intrinsic rate-controlling mechanisms that can contribute to the residual oxygen-content of the Mo powder product is also provided. Powder morphology, and its concomitant influence on specific surface-area of the Mo powder product, is found to correlate with the oxygen-content determination of the powder produced during second-stage reduction, and according to the processing strategy employed. Consequently, it has been found cogent to “partition” second-stage reduction into: i) a relatively high-rate Primary Reduction Sequence, and ii) a lower rate Deoxidation Sequence.
Authors: Yang Zhang, Yun Wang, Yong Sun, Jin Sheng Jia, Bing Qiang Zhang
Abstract: As a key material for microchannel plate, the lead silicate glasses doped with different content of Bi2O3 was prepared by melt-quenching method. The effect of Bi2O3 content on thermal properties, chemical stability, and resistivity before and after hydrogen reduction of the prepared glasses were studied. The results show that the coefficient of thermal expansion increases with the increasing of Bi2O3 content, while the transition temperature and softening temperature, the acid and alkali resistance, the volume and surface resistivity decrease gradually. Glass with 10% Bi2O3 and without PbO was used to prepare MCP, which has a resistance of 70MΩ and gain of 4,200 at 900V.
Authors: Gil Su Kim, Dae Gun Kim, Sung Tag Oh, Myung Jin Suk, Young Do Kim
Abstract: The effect of Cu on the hydrogen reduction of molybdenum oxide powders was investigated by measuring the humidity change during a non-isothermal process of hydrogen reduction. The presence of Cu induced a shift in the reduction temperature and strongly affected the reduction processes of MoO3→Mo4O11→MoO2, which comprised the contained chemical vapor transport of MoOx(OH)2. This study suggests that the surface of the Cu grains acts as a nucleation site for the reduction of MoOx(OH)2 to MoO2 particles from MoO3 or Mo4O11 phases. Such an activated reduction process results in the deposition of metallic Mo and MoO2 particles on the surface of the Cu grains.
Authors: Xiao Dong Fan, Na Tian, Cai Yin You
Abstract: Electroless plating was used to coat Fe layers on the hard magnetic Nd-Fe-B powders to fabricate Nd-Fe-B/α-Fe heterostructured magnetic powders. The heat treatment was performed to study the property evolution of the heterostructured magnetic powders. The results show that Fe coating was oxidized to Fe2O3 while drying; through the hydrogen reduction annealing, Fe2O3 was reduced to α-Fe. The coercivity of the heterostructured magnetic powders increased from 111.3 kA/m (1.4 kOe) to 524.7 kA/m (6.6 kOe) after annealing at 650°C. However, the demagnetization curve of powders presents a kink due to un-ideal coupling between hard and soft magnetic phases because of the aggregation of α-Fe. The magnetization processes of the heterostructured powders transferred from the dominant nucleation mechanism to domain wall pinning mechanism after the heat treatment.
Authors: Jung Yeul Yun, Shun Myung Shin, Dong Won Lee, Jong Nam Kim, Jei Pil Wang
Abstract: Ni-based superalloys are used extensively in the hot section of gas turbine engines owing to their inherent elevated temperature strength and creep resistance. As such, aircraft engine manufactures are continually striving to push the envelope of the capabilities of such high temperature structure materials in order to increase both engine performance and efficiency [1,2].
Authors: Young Jung Lee, Baek Hee Lee, Gil Su Kim, Kyu Hwan Lee, Young Do Kim
Abstract: Magnetic properties of nanostructured materials are affected by the microstructures such as grain size (or particle size), internal strain and crystal structure. Thus, it is necessary to study the synthesis of nanostructured materials to make significant improvements in their magnetic properties. In this study, nanostructured Fe-20at.%Co and Fe-50at.%Co alloy powders were prepared by hydrogen reduction from the two oxide powder mixtures, Fe2O3 and Co3O4. Furthermore, the effect of microstructure on the magnetic properties of hydrogen reduced Fe-Co alloy powders was examined using XRD, SEM, TEM, and VSM.
Authors: Xiao Ming Fu
Abstract: Fine tungsten powder is prepared with blue tungsten oxide (BTO) through the hydrogen reduction. The samples were characterized with the scanning electron microscope (SEM), fisher sub-sieve sizer (FSSS) and the particulate size description analyzer (PSDA). Fine tungsten powder is easily obtained when the reduction temperature is low. With the increasement of the reduction temperature, the grain size of tungsten powder becomes coarse. The increase of the weight of BTO in the ceramic boat leads to the increasement of the thickness of its bed. Therefore, the weight of BTO in the ceramic boat ought to reduce if fine tungsten powder is prepared. Fine tungsten powder can be obtained when the hydrogen flow increases.
Authors: Sung Tag Oh, Se Joong Yoon
Abstract: The reduction behavior of Al2O3/CuO powder mixtures, prepared from Al2O3/CuO or Al2O3/Cu-nitrate, was investigated by using thermogravimetry and hygrometry. Also, the influence of powder characteristics on the microstructure and properties of hot-pressed composites is analyzed. It was found that the hydrogen reduction process of nano-sized oxide particles and properties of sintered composites strongly depends on their distribution and size in composite powder. The formation mechanisms of nano-sized Cu dispersions are discussed based on the powder characteristics and reduction kinetics. In addition, the dependence of the microstructure and mechanical properties of hot-pressed composites on powder characteristics is discussed in terms of the initial size and distribution of Cu particles.
Authors: Nurul Syazwina binti Che Ibrahim, Sivakumar Ramakrishan, Sheikh Abdul Rezan, Norlia binti Baharun, Reza Alizadeh, Parham Roohi
Abstract: Reduction of iron oxide by hydrogen is important in the production of direct reduced iron. This method of iron production is gaining increasing significance as an alternative route to the blast furnace technology with the many difficult issues facing the latter, the most important being the problem related to environmental. In order to reduce the emission of greenhouse gases CO2, particularly for iron making, the production of Direct Reduced Iron (DRI) using hydrogen as the reducing gas instead of carbon monoxide is being considered. Reduction of pure hematite by hydrogen was studied at the laboratory scale, varying the experimental conditions like temperature (700oC and 800oC) and porosity (20% and 40%). Then, a Kinetic Modelling was conducted using Matlab software based on independently measured physical and thermodynamic properties of the reaction system and experimentally measured properties of the reactant solid (Fe2O3), gas phase (H2) and reactant product (Fe). There is a gap that occurs between the predicted result and the experimental result although the model explicated the trend and the behaviour of the reduction rate of Ferric Oxide and indicated a good homogeneity to the experimental conditions used in this research.
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