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排序方式: 共有679条查询结果,搜索用时 15 毫秒
631.
The modelling of metal forming processes in recent years has brought the development of different analytical and/or numerical techniques. The SERR technique is eminently suitable for analyzing drawing/extrusion of sections having re-entrant corners. However, in its present form, it cannot be used to analyse forming processes where the deformation zone has curved boundaries, since the elementary regions will no longer remain rigid. The present study is an endeavor to remodel this technique so that it can handle round billets. The circular cross section of the round billet is approximated by a regular polygon of equal area and the number of sides of the polygon is progressively increased until convergence of the drawing stress is achieved. As a test, the drawing of hexagon section bars from round billets through the straightly converging dies is analyzed 相似文献
632.
The crystal structure of Pb3BiV3O12 was solved using single-crystal X-ray diffraction technique. The compound crystallizes in the cubic system (No. 220) with eulytite structure with a = 10.7490(7) Å, V = 1241.95(14) Å3 and Z = 4. The final R1 value of 0.0198 (wR2=0.0384) was achieved for 359 independent reflections during the structure refinement. The Pb2+ and Bi3+ cations occupy the special position (16c) while the oxygen anions occupy the general position (48e) in the crystal structure. Unlike many other eulytite compounds, all the crystallographic positions are fully occupied. The structure consists of edge-shared Pb/Bi octahedra linked at the corners to independent [VO4]3− tetrahedra units, generating a eulytite-type network in the crystal lattice. 相似文献
633.
Polycrystalline Ba4SrSmTi3V7O30 sample was prepared using high-temperature solid-state reaction technique. Preliminary X-ray structural analysis of the compound shows the formation of a single phase compound (orthorhombic crystal system) at room temperature. Microstructures of the compound exhibit uniform distribution of grains over the surface of the sample. Detailed studies of dielectric and electrical properties as a function of frequency (1 kHz to 1 MHz) and temperature (31 °C to 475 °C) show that, the compound exhibits a diffuse ferroelectric phase transition. Measurements of electrical conductivity (ac) as a function of temperature suggest that the compound has semiconducting properties much above the room temperature with a negative temperature coefficient of resistance behavior. The existence of ferroelectricity was confirmed from polarization studies. 相似文献
634.
Mallick G Griep M Lastella S Sahoo S Hirsch S Ajayan PM Karna SP 《Journal of nanoscience and nanotechnology》2010,10(9):6062-6066
Current rectification property of as-grown single-walled carbon nanotubes (SWNTs) is investigated. The SWNTs are grown by chemical vapor deposition (CVD) process. The process allowed to grow long strands of SWNT bundles, which are then used to fabricate multiple arrays of switching devices with the channel length of 3, 5, 7 and 10 microm on a 15 mm x 15 mm SiO2 on Si substrate. Regardless of the channel length, a majority of the fabricated devices show current rectification characteristics, with high throughput of current (I) in the forward bias (V) giving the forward and reverse current ratio (Ifor/Irev) of approximately 10(6). Atomic force microscopic (AFM) analysis of the device structure and surface topology of SWNT suggest the observed rectification of current to possibly result from (a) cross-tube junctions, (b) a mixture of metallic and semiconducting tubes in the SWNT bundles, and/or (c) chirality change along a single tube. The exact mechanism underlying the observed rectification could not be conclusively established. However, the analyses of the experimental results strongly suggest the observed rectification to result from Schottky-type diode properties of SWNTs with mixed chirality along the tube. 相似文献
635.
Collaborative investigations on thoron and radon in some rural communities of Balkans 总被引:1,自引:0,他引:1
Zunić ZS Celiković I Tokonami S Ishikawa T Ujić P Onischenko A Zhukovsky M Milić G Jakupi B Cuknić O Veselinović N Fujimoto K Sahoo SK Yarmoshenko I 《Radiation protection dosimetry》2010,141(4):346-350
This paper deals with the results of the first-field use in the Balkans, i.e. Serbia and Republic of Srpska (Bosnia and Hercegovina), of a passive polycarbonate Mark II type and poliallyldiglycol carbonate (Cr-39) alpha track detectors sensitive to thoron as well as to radon. Both types of solid state nuclear track detectors were designed and supplied by National Institute of Radiological Sciences (NIRS), Chiba, Japan. The commercial names for these detectors which all have been field tested in Balkan rural communities are known as: UFO and RADUET passive discriminative radon/thoron detectors. No database of thoron and thoron progeny concentrations in dwellings in Serbia or Balkans region exist, and as a result, the level of exposure of the Serbian population to thoron and its progeny is unknown so far. 相似文献
636.
S B Sulaiman N Sahoo Sigrid Markert J Stein T P Das K Nagamine 《Bulletin of Materials Science》1991,14(2):149-156
Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa2Cu3O7 and YBa2Cu3O6 systems. The results of these investigations provide satisfactory explanations of available63Cu and17O nuclear quadrupole interaction data. The electron distributions obtained rule out the presence of Cu3+ ions and are supportive of the presence of Cu2+, Cu1+, O1− ions in the O7 system and Cu2+, Cu1+ and O2− in the O6 system with actual charges departing significantly from the formal charges, especially in the O7 system, indicating the importance of covalency effects. Suggestions will be made regarding possible sources that can bridge
the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters. 相似文献
637.
Y.?R.?RaoEmail author K.?S.?Jena D.?Sahoo P.?K.?Rout Shakir?Ali 《Journal of the American Oil Chemists' Society》2005,82(10):749-752
The seeds of ambrette (Abelmoschus moschatus Linn), after selective extraction of fragrance from the seed coat, are flaked and extracted with hexane to obtain a fatty
oil. The FA composition and iodine value of the oil indicate it possesses saturated, monounsaturated, and polyunsaturated
FA in ratios close to the recommendations of the United Nations World Health Organization; these characteristics make it suitable
as an edible oil. Acute oral toxicity and safety evaluation in a 13-wk feeding trial on albino rats showed the oil is comparable
to groundnut oil in all the parameters and is suitable for edible use. 相似文献
638.
Inadequate control of weld metal composition due to vaporization of volatile alloying elements is a serious problem in the
welding of many important engineering alloys. Effectiveness of surface active elements such as oxygen or sulfur in blocking
vaporization sites on the weld pool surface was investigated. Several iron samples doped with oxygen or sulfur were exposed
to a carbon dioxide laser beam in pulsed mode. The time average metal vaporization rates and the emission spectra were compared
with those obtained from ultra pure iron samples. Since the weld pool surface area and temperature distribution are affected
by oxygen and sulfur, the true effects of these elements on metal vaporization rates cannot be easily evaluated from welding
data. Therefore, rates of isothermal vaporization of iron and copper drops doped with oxygen or sulfur were determined both
in the presence and the absence of low pressure argon plasma. These rates were compared with the rates of vaporization of
ultrapure metal drops. Presence of sulfur or oxygen in metals always resulted in increased metal vaporization rates. The results
are analyzed on the basis of interfacial phenomena. 相似文献
639.
Recently, commercial Zn-Al foundry alloys such as ZA-27 have found increasing use for many applications and have competed effectively against copper, aluminium and iron-based foundry alloys. However, the elevated temperature (> 100°C) properties of zinc-aluminium alloys are unsatisfactory and restrict their use in some applications. One viable approach to improving the elevated temperature properties is to reinforce the zinc-aluminium alloys with alumina fibres. In this investigation, the mechanical properties of a Zn-Al alloy reinforced with alumina fibres were evaluated. Tensile, compression and impact properties were determined at 25, 100 and 150°C. Lubricated wear tests were also performed on the unreinforced alloy and composites. It was found that although fibre reinforcement did result in some improvement of tensile and compression properties at elevated temperatures, the composites had poor toughness and ductility. The presence of a brittle SiO2 layer at the fibre/matrix interfaces resulted in fibre/matrix decohesion under tensile loading, impairing the performance of the reinforced materials. Some improvement in wear resistance was noted for the composite materials but fibre reinforcement did not yield significant improvement in fatigue resistance. 相似文献
640.
The paper describes a theoretical study of normal impact between solids with small-scale surface asperities. An elastic-plastic impact model is used to study the normal impact between rough surfaces in presence of surface forces. The well-established elastic and plastic adhesion indices are used to consider the different conditions arising out of varying impact velocity and material parameters. The study shows that the coefficient of restitution is affected by material-properties, surface-topography and impact velocity. Results clearly indicate that high impact velocity is desirable in order to achieve low coefficient of restitution. However, there are certain combinations of material properties and surface conditions, given by the choice of adhesion indices, where coefficient of restitution falls to a very low value. 相似文献