全文获取类型
收费全文 | 72篇 |
免费 | 4篇 |
专业分类
化学 | 45篇 |
晶体学 | 1篇 |
力学 | 2篇 |
数学 | 18篇 |
物理学 | 10篇 |
出版年
2020年 | 1篇 |
2018年 | 1篇 |
2016年 | 2篇 |
2015年 | 2篇 |
2013年 | 2篇 |
2012年 | 1篇 |
2011年 | 4篇 |
2010年 | 2篇 |
2009年 | 2篇 |
2008年 | 2篇 |
2006年 | 1篇 |
2005年 | 3篇 |
2004年 | 3篇 |
2003年 | 2篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1991年 | 2篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1969年 | 1篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1952年 | 1篇 |
1885年 | 2篇 |
1882年 | 2篇 |
排序方式: 共有76条查询结果,搜索用时 31 毫秒
41.
The solution space of the travelling salesman problem under 2-opt moves has been characterized as having a big-valley structure, in which the evaluation of a tour is positively correlated to the distance of the tour from the global optimum. We examine the big-valley hypothesis more closely and show that while the big-valley structure does appear in much of the solution space, it breaks down around local optima that have solutions whose evaluation is very close to that of the global optimum; multiple funnels appear around local optima with evaluations close to the global optimum. The appearance of multiple funnels explains why certain iterated local search heuristics can quickly find high-quality solutions, but fail to consistently find the global optimum. We then investigate a novel search operator, which is demonstrated to have the ability to escape funnels at evaluations close to the global optimum. 相似文献
42.
Yukihara EG Whitley VH McKeever SW Akselrod AE Akselrod MS 《Radiation measurements》2004,38(3):317-330
This paper examines the effect of high-dose irradiation on the optically stimulated luminescence (OSL) of Al2O3:C, principally on the shape of the OSL decay curve and on the OSL sensitivity. The effect of the degree of deep trap filling on the OSL was also studied by monitoring the sensitivity changes after doses of beta irradiation and after step-annealing of samples previously irradiated with high doses. The OSL response to dose shows a linear-supralinear-saturation behavior, with a decrease in the response for doses higher than those required for saturation. This behavior correlates with the sensitivity changes observed in the samples annealed only to 773 K, which show sensitization for doses up to 20-50 Gy and desensitization for higher doses. Data from the step-annealing study leads to the suggestion that the sensitization is caused by the filling of deep electron traps, which become thermally unstable at 1100-1200 K, whereas the desensitization is caused by the filling of deep hole traps, which become thermally unstable at 800-875 K, along with a concomitant decrease in the concentration of recombination centers (F+ -centers). Changes in the shape of the OSL decay curves are also observed at high doses, the decay becoming faster as the dose increases. These changes in the OSL decay curves are discussed in terms of multiple overlapping components, each characterized by different photoionization cross-sections. However, using numerical solutions of the rate equations for a simple model consisting of a main trap and a recombination center, it is shown that the kinetics of OSL process may also be partially responsible for the changes in the OSL curves at high doses in Al2O3:C. Finally, the implication of these results for the dosimetry of heavy charged particles is discussed. 相似文献
43.
‘Deficiency catalysis’ is the concept applied to metalation chemistry that increments of TMEDA or an equivalent or two of an ether activate alkyllithiums in hydrocarbon media. Amplification of this concept to conflicting reports of α- and ortho-metalation of ortho-methylanisole (o-MA) is explored. Both regioselective metalations were realized in selectivity ratios of >7:1 for the desired isomers. Some rationale is provided for the various selectivities. These results are at variance with prior results and interpretations thereof found in the literature. 相似文献
44.
E. G. Yukihara V. H. Whitley J. C. Polf D. M. Klein S. W. S. McKeever A. E. Akselrod M. S. Akselrod 《Radiation measurements》2003,37(6):627-638
The influence of deep traps on the 450 K thermoluminescence (TL) peak of Al2O3:C is studied. Depending upon the sample and on the degree of deep trap filling, features such as the TL width, area and height can vary considerably. These effects are interpreted to be due to: (a) sensitivity changes introduced by competition mechanisms involving deep electron and hole traps, and (b) the multiple component nature of the 450 K TL peak. The influence of the deep traps on the TL was studied using different excitation sources (beta irradiation or UV illumination), and step annealing procedures. Optical absorption measurements were used to monitor the concentration of F- and F+-centers. The data lead to the suggestion that the competing deep traps which become unstable at 800–875 K are hole traps, and that the competing deep traps which become unstable at 1100–1200 K are electron traps. Both the dose response of the TL signal and the TL sensitivity are shown to be influenced by sensitization and desensitization processes caused by the filling of deep electron and hole traps, respectively. Changes in the TL peak at low doses were also shown to be connected to the degree of filling of deep traps, emphasizing the influence of deep trap concentration and dose history of each sample in determining the TL properties of the material. Implications of these results for the optically stimulated luminescence properties are also discussed. 相似文献
45.
Ohne ZusammenfassungMit 1 Abbildung und 1 Tabelle 相似文献
46.
A series of ion-exchange equilibrium experiments were completed and analyzed by a mass action model. The isotherms at various counter-ion concentrations are regressed to determine the average number of binding sites, Z, the mass action constant, K, and the effective capacity of the exchanger, Pm. The estimates for Pm are 1-10 mmol of protein adsorbed per liter of packed Sepharose exchanger, in agreement with values determined from fitting the Langmuir isotherms. A geometric model, developed to relate the charge distribution of the exchanger and the size of the protein to the protein capacity, also confirms the estimates of Pm. The Z values estimated (and confirmed with an independent slope method) range from 0.8 to 3.6, indicating that most of the ionized sites on the protein do not undergo ion exchange. Geometric model estimates of exchanger sites covered by a protein at maximum surface coverage show that only 1-32% of the sites actually undergo ion exchange. This result indicates that adsorbed protein molecules "block" many exchanger sites, preventing other molecules from ion exchanging at those sites. This high degree of blocking by proteins also indicates that there is an excess of exchanger sites, as evidenced by Z showing no dependence on charge separation distance over the range 6-10 A. Scatchard pots of the equilibrium data indicate that there is competition between at least two different binding forms of each protein studied. For the high-affinity systems, cooperative binding was observed at low exchanger loading; the character then changed to heterogeneous competitive binding as the exchanger loading increased. The effect on an isotherm plot is a subtle but systematic deviation from the Langmuir and mass action models. More important, the competition means that Z represents an average for competing binding forms and, as such, Z can be fractional. 相似文献
47.
Molecular simulations of the pressure, temperature, and chemical potential dependencies of clay swelling 总被引:1,自引:0,他引:1
Smith DE Wang Y Chaturvedi A Whitley HD 《The journal of physical chemistry. B》2006,110(40):20046-20054
A new method for the determination of clay swelling thermodynamics from computer simulation is discussed and evaluated. This method allows for the determination of temperature, pressure, and water chemical potential dependence of clay swelling from simulations at a single thermodynamic state point. The temperature dependence and pressure dependence of clay swelling are shown to be directly related to the composite system entropy and volume change, respectively, that accompany swelling. Expressions for the chemical potential dependence of clay swelling are used to determine constant pressure layer spacing and adsorption isotherms, quantities that are well suited for comparison with experimental measurements. This method is evaluated through grand isoshear ensemble simulations of Na-montmorillonite, a prototypical swelling clay. Approximations associated with all expressions are discussed with explicit calculations used to demonstrate their regimes of validity. 相似文献
48.
K. Randle R. Wellum J. E. Whitley 《Journal of Radioanalytical and Nuclear Chemistry》1973,16(1):205-214
Radiochemical separations are essential for the determination of trace elements in silver artefacts by neutron activation
analysis due to the high levels of both short and long-lived activities produced by the matrix, but the sensitivity of the
technique permits the examination of small samples. The noble metals are considered most significant from a diagnostic standpoint,
and radiochemical techniques have been developed for their determination in samples of a few milligrams. Methods have been
investigated for the determination of Pd, Rh, Ir and Pt in samples removed from museum specimens of known provenance. 相似文献
49.
J. E. Whitley A. B. Moyes P. Bowden 《Journal of Radioanalytical and Nuclear Chemistry》1979,48(1-2):147-158
The significance of rare earth distribution patterns in trace element geochemistry is briefly reviewed. Established methods
for their determination at the Reactor Centre by instrumental and radiochemical techniques are described, and examples of
recent applications discussed. 相似文献
50.