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排序方式: 共有443条查询结果,搜索用时 31 毫秒
11.
W. Theisen 《Materialwissenschaft und Werkstofftechnik》2004,35(6):377-384
Wear Resistant Fe‐Base Alloys with Niobium Carbide Martensitic Fe‐base alloys from the system Fe‐Cr‐C are widely used as chilled cast irons and tool steels. Because of the low hardness of their FeCr‐carbides this paper reports about new alloys with primarily solidified harder niobium carbides. It focuses on a secondary hardenable welding alloy, a coating material for composite castings, a chilled casting and a corrosion resistant cold work tool steel, which are investigated with respect to their process related microstructure and abrasive wear behaviour. 相似文献
12.
HEM SHANKER RAY 《Mineral Processing and Extractive Metallurgy Review》2013,34(1):139-153
Iron ore fines and coal fines can be made to react to produce direct reduced iron (DRI) fines. There are three ways to achieve the reactions, namely, (a) using homogeneous mixtures, (b) arranging the ore and coal fines in separate but adjacent layers and (c) by fluidizing a mixture of iron ore fines and coal fines by air. This paper mainly discusses the work done by the author and his coworkers on the kinetic aspects of reactions in such systems. 相似文献
13.
Z. PanF. Xu S.N. MathaudhuL.J. Kecskes W.H. YinX.Y. Zhang K.T. HartwigQ. Wei 《Acta Materialia》2012,60(5):2310-2323
We have systematically investigated the microstructural evolution of niobium (Nb) subjected to severe plastic deformation via equal channel angular extrusion (ECAE) up to 24 passes. The starting Nb billet material consists of a centimeter-scale grain size with a columnar structure. We have found that the grain size reduction of the Nb is almost saturated at ∼300 nm after eight passes of ECAE. However, the population of high-angle grain boundaries continues to increase with further ECAE, and no saturation appears to have been reached at 24 passes. We have evaluated the mechanical properties of the samples with different number of ECAE passes over a wide range of strain rates, from quasi-static to high strain rates. We have used strain-rate jump tests to examine the strain-rate sensitivity (SRS) of the processed samples and found that the SRS of the ECAE-processed Nb is ∼0.012, which is a factor of three smaller than that of the coarse-grained counterpart. The activation volume derived for plastic deformation indicates that the double-kink formation of screw dislocations is still the predominant deformation mechanism in the ECAE-processed Nb. Quasi-static true stress-strain curves exhibit elastic-nearly perfectly plastic behavior. The quasi-static yield strength is also nearly saturated after eight passes of ECAE. High-strain-rate compressive true stress-strain curves show uniform flow softening. However, the dynamic peak stress keeps rising with an increased number of ECAE passes, suggesting a strong grain boundary contribution to dynamic strengthening. Scanning electron microscopy of post-loaded surfaces displays a morphology of diffuse shear bands accompanying highly compressed grains. In our report, we demonstrate that grain boundaries of severely deformed metals play different roles at low, quasi-static vs. high-strain rates of mechanical loading. The difference is primarily determined by the strength of grain boundaries acting as dislocation barriers at different loading rates. This discovery is significant for the understanding of the effect of the microstructure as a function of the applied loading rate. 相似文献
14.
In this research, the effect of NbF5 as an additive on the hydrogen desorption kinetics of MgH2 was investigated and compared to TiH2, Mg2Ni and Nb2O5 catalysts. The kinetics measurements were done using a method in which the ratio of the equilibrium plateau pressure to the opposing pressure was the same for all the reactions. The data showed NbF5 to be vastly superior to the other catalysts for improving the desorption kinetics of MgH2. The rates of desorption were found to be in the order NbF5 ? Nb2O5 > Mg2Ni > TiH2 > Pure MgH2. Kinetic modeling measurements showed that chemical reaction at the phase boundary to be the likely process controlling the reaction rates. TPD analyses showed the mixture with NbF5 has the lowest desorption temperature although it was accompanied with some weight penalty. 相似文献
15.
It is difficult to economically recover rare earths (RE) and niobium (Nb) from Bayan Obo tailings by the existing metallurgical processes. In this study, a novel hydrometallurgy process was employed for separating and recovering RE and Nb from Bayan Obo tailings. Firstly, by sulfating roasting at 250 °C and subsequent leaching at 60 °C, the RE and Nb present in the polymetallic minerals can be efficiently extracted into the leach solution. Secondly, after the reduction of Ti4+ and Fe3+ ions (to Ti3+ and Fe2+ ions) with iron powders followed by hydrolysis at pH 2.01, the Nb can be efficiently precipitated from the leach solution. The impurities present in the precipitated product can then be removed by treating with NH3⋅H2O–H2C2O4 system at pH 4.50. Thirdly, the RE can be efficiently precipitated at pH 7.15 from the filtrate of above hydrolysis reaction mixture. Finally, the impurities present in the crude RE can be removed by oxalate co-precipitation method. The yield of RE and Nb in this novel process reaches up to 90% and 78%, respectively. Both the Nb (60.67 wt% Nb2O5) and RE products (>88.65 wt% RExOy) have high application value. 相似文献
16.
In this work, effects of Niobium (Nb) defects on TiO2 surface using density functional theory (DFT) are investigated. Based on formation energy of the defects, their occurrences in two different extreme conditions, O-rich and O-poor conditions, are evaluated. Effects of Nb defects on surface and its electronic structure are studied and it is demonstrated that Nb doping widens valence band in deep energy level leaving the band gap without any change and it also lowers oxygen vacancy defect concentration due to the stronger bonding of Nbsub defect with oxygen atoms specially bridging oxygen (most probable defect site for Oxygen vacancy). Higher density of Nb substitutional defects (Nbsub) are examined and it is shown that higher density doping of TiO2 surface leads to uniform distribution of defects over the anatase structure as a result of interaction of Nb defects when they are close and this fact prevents segregation of Nb atoms in Nb-doped TiO2. 相似文献
17.
《Ceramics International》2016,42(11):13024-13034
Core-shell NbC/C nano composite has been synthesized by reacting niobium pentaoxide (Nb2O5) with metallic magnesium and acetone (C3H6O). The ensued powder samples are characterized by X-ray diffraction (XRD), differential scanning calorimetry/thermal gravimetric analysis (DSC/TGA), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis. The synthesis temperature, holding time and heating rate has been optimized to facilitate the in-situ reduction-carburization of Nb2O5. The broadening of XRD diffraction peaks corresponding to NbC is analyzed using Williamson-Hall (W-H) analysis by calculating various parameters such as strain, stress, strain energy density for all the samples. The mean particle size of nano NbC is estimated by Scherrer's criterion which is highly correlated with W-H analysis in less stressed system. The electron microscopy analysis showed that particles have a partial faceted morphology along with carbon layer and having wide particle size distribution. BET analysis depicted large surface area having a combination of micropores and mesopores. All the results of various characterizations have been used to predict the mechanism of formation of core-shell NbC/C nano composite. 相似文献
18.
样品溶液经CL-5209萃淋树脂在2mol/L硝酸介质中分离铀后,用催化极谱法测定铌。铌在0.8%H2SO4-0.05mol/L酒石酸-2%KSCN-0.08%抗坏血酸-0.0004%聚乙烯醇底液中,于-1.3V处呈现络合催化氢波(vs.SCE)可用二次导数测定。铌峰电流与浓度(0.005~0.03μg/mL)有线性关系。称样0.1g时,铌的测定下限为0.5μg/gU、6次测定相对标准偏差<15%,加入回收率为91%~119%。 相似文献
19.
F. M. T. Mendes C. A. Perez R. R. Soares F. B. Noronha M. Schmal 《Catalysis Today》2003,78(1-4):449-458
Ammonium oxalate complex of niobium was investigated as an aqueous precursor for the preparation of x% Nb2O5/Al2O3 (x=5, 10, 20 and 30 wt.%) samples. Catalysts with the same Nb2O5 contents were also prepared from the traditional niobium oxalate/oxalic acid aqueous solution. The catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), temperature-programmed reduction (TPR), infrared spectroscopy of chemisorbed pyridine and X-ray photoelectron spectroscopy (XPS). A comparison with the preparation method using the niobium oxalate salt was performed. The results showed that the niobium precursor influence the species growing leading to phases with different reducibility. The XPS revealed the presence of multilayers of niobium oxide on the Nb2O5/Al2O3 samples prepared by using niobium ammonium oxalate complex, while the ones obtained from niobium oxalate led to Nb2O5 particles islands. The addition of niobium oxide decreased the fraction of Lewis acid sites and increased the fraction of Brønsted acid sites, independent of the precursor salt. However, the creation of BAS was more pronounced on the Nb2O5/Al2O3 samples prepared from niobium oxalate. 相似文献
20.
The experimental results of niobium enrichment in laboratory and in blast furnace in Baotou Iron andSteel Corporation were reported.The effects of temperature,constituents of slag and silicon content in hotmetal on niobium recovery were studied and the results have been verified by blast furnace operation.Opticaland electronic microscopy and EPMA was observed that at the metal-slag interface there is NbC deposit layerobstructed Nb entering into the metal,By raising the temperature of hearth and agitating the bath,the reduc-tion of niobium oxide in slag could be speeded up,the solubility of Nb in hot metal could increase and thedeposited NbC layer became thinner and even disappeared,in result,Nb recovery was increased.A compre-hensive extraction process combining ore dressing with pyro-hydro-metallurgy was suggested for Baotouniobium resource which was difficult to be concentrated formerly.The methods of increasing niobium recoverywere also suggested. 相似文献