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1.
With an improved die, a method for manufacturing gears by means of tube extrusion has been developed to a stage where, without noticeable wear of the die, 40,000 workpieces can be manufactured. The paper presents a numerical analysis of the stresses and deflection of the die, offering also a comparison between the modified and the conventional symmetric orifice. Based on the results obtained and using recent work on crack prediction for extrusion dies, the area most prone to rupture is determined. Moreover, a concept for the precorrection of the die is offered to improve the tooth quality of the gears. 相似文献
2.
The purpose of this study was to investigate the mediating role of self-efficacy in the development of students' intentions to become entrepreneurs. The authors used structural equation modeling with a sample of 265 master of business administration students across 5 universities to test their hypotheses. The results showed that the effects of perceived learning from entrepreneurship-related courses, previous entrepreneurial experience, and risk propensity on entrepreneurial intentions were fully mediated by entrepreneurial self-efficacy. Contrary to expectations, gender was not mediated by self-efficacy but had a direct effect such that women reported lower entrepreneurial career intentions. The authors discuss practical implications and directions for future research. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
3.
Experimental studies were performed with a 4.0 in. (10.2 cm.) diameter extraction column containing ten dualflow type trays and operated in a controlled cycling mode. The systems methyl isobutyl ketone/acetic acid/water and toluene/acetone/water were used. The overall stage efficiency was found to be a function of the ratio of the volume of a phase transferred during a cycle to the volume of that phase held up on the tray, of the feed-to-solvent ratio, and of the phase throughput rates. A non-equilibrium model was developed to represent the experimental data. For the HIBK/acetic acid/water system, overall stage efficiencies were in the range of 30 to 38 percent, corresponding to HETS values of 0.46 to 0.55 meters. For the toluene/acetone/water system, overall stage efficiencies were in the range of 20 to 31 percent, corresponding to HETS values of 0.55 to 0.95 meters. 相似文献
4.
Boris F. Belokopytov Kestutis S. Laurinavichius Tatyana V. Laurinavichene Maria L. Ghirardi Michael Seibert Anatoly A. Tsygankov 《International Journal of Hydrogen Energy》2009
Eight natural microbial consortia collected from different sites were tested for dark, hydrogen production during starch degradation. The most active consortium was from silo pit liquid under mesophilic (37 °C) conditions. The fermentation medium for this consortium was optimized (Fe, NH4+, phosphates, peptone, and starch content) for both dark fermentation and for subsequent purple photosynthetic bacterial H2 photoproduction [Laurinavichene TV, Tekucheva DN, Laurinavichius KS, Ghirardi ML, Seibert M, Tsygankov AA. Towards the integration of dark and photo fermentative waste treatment. 1. Hydrogen photoproduction by purple bacterium Rhodobacter capsulatus using potential products of starch fermentation. Int J Hydrogen Energy 2008;33(23):7020–26], in the presence of the spent dark, fermentation effluent. The addition of Zn (10 mg L−1), as a methanogenesis inhibitor that does not inhibit purple bacteria at this concentration, also did not inhibit dark, fermentative H2 production. The influence of various fermentation end products at different concentrations (up to 30 g L−1) on dark, H2 production was also examined. Added lactate stimulated, but added isobutyrate and butanol strongly inhibited gas production. Under optimal conditions the fermentation of starch (30 g L−1) resulted in 5.7 L H2 L−1 of culture (1.6 mol H2 per mole of hexose) with the co-production mainly of butyrate and acetate. 相似文献
5.
6.
Kuczewski B Marquardt CM Seibert A Geckeis H Kratz JV Trautmann N 《Analytical chemistry》2003,75(24):6769-6774
Capillary electrophoresis (CE) was coupled to ICPMS in order to combine the good performance of this separation technique with the high sensitivity of the ICPMS for the analysis of plutonium and neptunium oxidation states. The combination of a fused-silica capillary with a MicroMist AR 30-I-FM02 nebulizer and a Cinnabar small-volume cyclonic spray chamber yielded the best separation results. With this setup, it was possible to separate a model element mixture containing neptunium (NpO2(+)), uranium (UO2(2+)), lanthanum (La3+), and thorium (Th4+) in 1 M acetic acid. The same conditions were also suitable for the separation of various oxidation states of plutonium and neptunium in different aqueous samples. All separations were obtained within less than 15 min. A detection limit of 50 ppb identical with 2 x 10(-7) M (3-fold standard deviation of a blank) was achieved. To prove the negligible disturbance of the plutonium and neptunium redox equilibria during the CE separations, plutonium and neptunium speciation by CE-ICPMS in acidic solutions was compared with the results of UV/visible absorption spectroscopy and was found to be in good agreement. The CE-ICPMS system was also applied to study the reduction of Pu(VI) in a humic acid-containing groundwater at different pH values. 相似文献
7.
Tatyana V. Laurinavichene Darya N. Tekucheva Kestutis S. Laurinavichius Maria L. Ghirardi Michael Seibert Anatoly A. Tsygankov 《International Journal of Hydrogen Energy》2008
In this study, the H2-photoproduction capacity of Rhodobacter capsulatus B10 was measured as a function of variations in the nature and concentration of volatile fatty acids (VFAs) and other products of dark fermentation. When an equimolar mixture of VFAs was provided, C4 substrates (butyrate and isobutyrate) were not consumed until the C2–C3 substrates (acetate, propionate, and lactate) became unavailable, but in order for the cells to produce H2 at high rates they could not be exposed to severe growth substrate depletion. Among other possible fermentation products, the highest inhibition was observed by the addition of butanol (50% inhibition at 50 mM). The influence of high concentrations of VFAs, phosphate (used to stabilize the pH during dark fermentation) and some heavy metals (known inhibitors of methanogenesis) was also shown. Based on the results, the conditions of fermentation can be manipulated to avoid the inhibition of subsequent H2 photoproduction by photosynthetic bacteria. 相似文献
8.
CJ Smith Y Zhang CM Koboldt J Muhammad BS Zweifel A Shaffer JJ Talley JL Masferrer K Seibert PC Isakson 《Canadian Metallurgical Quarterly》1998,95(22):13313-13318
The enzymes cyclooxygenase-1 and cyclooxygenase-2 (COX-1 and COX-2) catalyze the conversion of arachidonic acid to prostaglandin (PG) H2, the precursor of PGs and thromboxane. These lipid mediators play important roles in inflammation and pain and in normal physiological functions. While there are abundant data indicating that the inducible isoform, COX-2, is important in inflammation and pain, the constitutively expressed isoform, COX-1, has also been suggested to play a role in inflammatory processes. To address the latter question pharmacologically, we used a highly selective COX-1 inhibitor, SC-560 (COX-1 IC50 = 0.009 microM; COX-2 IC50 = 6.3 microM). SC-560 inhibited COX-1-derived platelet thromboxane B2, gastric PGE2, and dermal PGE2 production, indicating that it was orally active, but did not inhibit COX-2-derived PGs in the lipopolysaccharide-induced rat air pouch. Therapeutic or prophylactic administration of SC-560 in the rat carrageenan footpad model did not affect acute inflammation or hyperalgesia at doses that markedly inhibited in vivo COX-1 activity. By contrast, celecoxib, a selective COX-2 inhibitor, was anti-inflammatory and analgesic in this model. Paradoxically, both SC-560 and celecoxib reduced paw PGs to equivalent levels. Increased levels of PGs were found in the cerebrospinal fluid after carrageenan injection and were markedly reduced by celecoxib, but were not affected by SC-560. These results suggest that, in addition to the role of peripherally produced PGs, there is a critical, centrally mediated neurological component to inflammatory pain that is mediated at least in part by COX-2. 相似文献
9.
AIMS: To determine the prevalence of lymphoid follicles in Helicobacter pylori positive and negative gastritis in antral and body type gastric mucosa in patients with non-ulcer dyspepsia (NUD), duodenal ulcer, or gastric ulcer; to correlate follicle presence with patient age; to evaluate the correlation between the prevalence of lymphoid follicles and active and inactive gastritis and its severity; and to assess the positive predictive value of lymphoid follicle prevalence with respect to H pylori infection. METHODS: Gastric biopsy specimens, graded according to the Sydney system, from 337 patients were studied. RESULTS: Lymphoid follicles occurred more often in antral mucosa (78%) than in body type mucosa (41%) and were observed in 85% of patients with H pylori positive gastritis. There was no significant difference between NUD and gastric and duodenal ulcer disease with regard to the presence of lymphoid follicles. The positive predictive value of the presence of lymphoid follicles in H pylori infection was 96%. Lymphoid follicles were more commonly observed in patients aged between 10 and 29 years. Lymphoid follicles were more frequently found in pangastritis of all subtypes than in antral gastritis and also in active gastritis than in inactive gastritis. The presence of lymphoid follicles correlated strongly with the degree and severity of gastritis. CONCLUSION: Lymphoid follicles are a constant morphological feature of H pylori associated gastritis. 相似文献
10.
The use of a diffraction spectrometer developed by Deslattes for the determination of mammographic kV is extended to the measurement of accurate, relative x-ray spectra. Raw x-ray spectra (photon fluence versus energy) are determined by passing an x-ray beam through a bent quartz diffraction crystal, and the diffracted x-rays are detected by an x-ray intensifying screen coupled to a charge coupled device. Two nonlinear correction procedures, one operating on the energy axis and the other operating on the fluence axis, are described and performed on measured x-ray spectra. The corrected x-ray spectra are compared against tabulated x-ray spectra measured under nearly identical conditions. Results indicate that the current device is capable of producing accurate relative x-ray spectral measurements in the energy region from 12 keV to 40 keV, which represents most of the screen-film mammography energy range. Twelve keV is the low-energy cut-off, due to the design geometry of the device. The spectrometer was also used to determine the energy-dependent x-ray mass attenuation coefficients for aluminium, with excellent results in the 12-30 keV range. Additional utility of the device for accurately determining the attenuation characteristics of various normal and abnormal breast tissues and phantom substitutes is anticipated. 相似文献