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排序方式: 共有105条查询结果,搜索用时 15 毫秒
1.
G Charach PD Rabinovich FM Konikoff I Grosskopf MS Weintraub T Gilat 《Canadian Metallurgical Quarterly》1998,29(3-4):125-136
In most patients with atherosclerosis, the underlying metabolic derangement remains undefined. Animal experiments have suggested that the ability to produce and excrete large amounts of bile acids may be an adaptation mechanism to cholesterol overload protecting against the atherogenic effects of cholesterol. However, there are very few data on bile acid excretion in human atherosclerosis. In the present study, we have investigated fecal bile acid secretion in subjects with and without coronary artery disease. The target group consisted of 30 patients with proven coronary artery disease and the control group consisted of 27 matched subjects without clinical or laboratory evidence of coronary atherosclerosis. Fecal bile acids were measured by gas-liquid chromatography from 24-hr stool collections under a controlled diet. The patients excreted significantly less bile acids than the controls (325+/-135 vs. 592+/-223 mg/day, respectively, p < 0.0001). The difference was primarily due to a reduced excretion of secondary bile acids. Less than 50% of deoxycholate was excreted by patients (180+/-81 mg/day) as compared to controls (367+/-168 mg/day, p < 0.0002), while lithocholic acid excretion was 111+/-62 mg/day in patients vs. 190 +/-70 mg/day in controls (p < 0.005). The fecal output of the two primary bile acids, cholic and chenodeoxycholic acid, did not differ significantly between patients and controls. The fecal output of total bile acids correlated with that of both secondary bile acids in patients as well as in controls. These findings suggest that patients with coronary heart disease are unable to excrete adequate amounts of bile acids to rid themselves of excess cholesterol, even if they are able to maintain a plasma cholesterol level comparable to that of healthy controls. 相似文献
2.
Matching with don't-cares and a small number of mismatches 总被引:1,自引:0,他引:1
In matching with don't-cares and k mismatches we are given a pattern of length m and a text of length n, both of which may contain don't-cares (a symbol that matches all symbols), and the goal is to find all locations in the text that match the pattern with at most k mismatches, where k is a parameter. We present new algorithms that solve this problem using a combination of convolutions and a dynamic programming procedure. We give randomized and deterministic solutions that run in time O(nk2logm) and O(nk3logm), respectively, and are faster than the most efficient extant methods for small values of k. Our deterministic algorithm is the first to obtain an O(polylog(k)⋅nlogm) running time. 相似文献
3.
R. Chaim 《Journal of Materials Science》2006,41(23):7862-7871
Spark plasma sintering (SPS) is a newly discovered old technique which recently has been used for superfast densification of ceramic powders. Simultaneous application of pulsed high dc current densities and load is the necessary condition for rapid and full densification of ceramic powders by SPS. Commercial nanocrystalline magnesium oxide (nc-MgO) and yttrium aluminum garnet (nc-YAG) powders were densified to optical transparency using spark plasma sintering at distinctly different homologous temperatures (0.3 T
m for nc-MgO and 0.7 T
m for nc-YAG). The observed microstructure, density and grain size evolutions versus the SPS temperature were analyzed. The enhanced densification of the nc-MgO powder at the present SPS conditions was related to plastic deformation followed by diffusion processes. Densification of nc-YAG powder was described by the formation of viscous layer at the particle surfaces and corresponding densification by grain rotation and diffusion through the liquid phase. Densification by normal grain growth takes place at higher relative densities, regardless of the material. 相似文献
4.
Dense nanocrystalline ZrO2-3 wt% Y2O3 ceramics with grain sizes ranging between 23 to 130 nm were tested by ultrasonic pulse echo and Vickers hardness. The elastic modulus and hardness results were corrected for the residual porosity and the phase content. The corrected elastic moduli exhibited continuous decrease with decrease in the grain size. In contrast, no correlation was found between the corrected hardness and grain size. The percolative composite model was used to describe the changes in the elastic moduli in terms of percolation of the elastic wave through the intercrystalline phase at the percolation threshold. The absence of correlation with the hardness results was explained due to the other energy absorbing mechanisms such as microcracking beneath the indenter. 相似文献
5.
R. Chaim 《Journal of Materials Science》2013,48(1):502-510
The effects of the applied electric field during the spark plasma sintering of ceramic nanoparticles were examined at various stages of the process. It was assumed that local intensification of the electric field arises due to the nanoscale structural features. Enhanced surface conductivity is expected in the nanoparticles during the heating, which otherwise are electrically non-conducting as a bulk. Percolation of the electric current at “optimal” electrical conductivity is obtained by fractal dimension. The defective nanoparticle surfaces experience charging–discharge cycles which lead to local breakdown and to plasma formation due to the ionized surface molecules. High local temperatures which evolved in a nonlinear fashion at the particle surfaces lead to enhanced sintering and densification kinetics, consistent with the flash sintering phenomenon. The contribution of the pondermotive force to the enhancement of the diffusion kinetics is discussed. Temperature windows for enhanced densification kinetics via plastic deformation or plasma-assisted processes are estimated for MgO, Al2O3, and YAG. 相似文献
6.
Rachman Chaim Amit Shlayer Claude Estournes 《Journal of the European Ceramic Society》2009,29(1):91-98
Nanocrystalline Y2O3 powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol?1 and associated diffusion coefficients higher by 103 than expected for Y3+ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature. 相似文献
7.
Shapiro Michael Laufer Gabriel Gutfinger Chaim 《Industry Applications, IEEE Transactions on》1986,(2):230-234
The separation of fine aerosol particles by a packed granular-bed filter, enhanced by external electrostatic fields, was studied experimentally and theoretically. The filtration efficiencies of charged and neutralized aerosols were measured for external fields aligned with the air flow, transverse to the flow, and opposite to the flow. Theoretical models of electrostatically enhanced granular-bed filtration of micrometer and submicrometer particles were developed. Experimental results which demonstrate the relative merit of each configuration were presented and compared with the theory. The parallel-field configuration yielded the best filtration efficiency followed by the transverse configuration. 相似文献
8.
A diffusionless cubic (c)→metastable tetragonal (t') phase transformation occurs in certain alloys in the ZrO2 -Y2 O3 system on quenching from elevated temperatures. Microstructural features due to this phase transformation, principally anti-phase domain boundaries (APB's) and mechanical accommodation twins, have been characterized using transmission electron microscopy. Certain differences between our interpretation and those of other workers are discussed. 相似文献
9.
Filtration of charged aerosols by granular bed filters enhanced by an externally applied electrostatic field was studied experimentally. The filtration efficiencies of latex aerosols by sand beds were measured for various aerosol and bed granule diameters. The results were compared with theoretical solutions. It was demonstrated that high filtration efficiencies of charged aerosols may be achieved when moderate electrostatic fields are applied to a filter. 相似文献
10.
Pure and dense nanocrystalline MgO with grain size ranging between 25 and 500 nm were prepared by hot-pressing. Vickers microhardness
was found to increase with decrease in the grain size down to 130 nm, following the Hall–Petch relation. Further decrease
in the grain size was followed by continuous decrease in microhardness. A composite model was used to describe the microhardness
behavior in terms of plastic yield of the nanocrystalline grains accompanied by strain accommodation and nanocracking at the
grain boundaries (gb’s). Good agreement between the experimental and the calculated values indicates that gb’s may have significant
effect on strengthening and ductility of nanocrystalline-MgO ceramics in the nanometer size range. Critical grain size exists
below which limited plastic deformation within the grains and nanocracking at gb’s enhance the brittleness of the ceramic. 相似文献