全文获取类型
收费全文 | 511篇 |
免费 | 5篇 |
专业分类
化学 | 342篇 |
晶体学 | 23篇 |
力学 | 19篇 |
数学 | 34篇 |
物理学 | 98篇 |
出版年
2020年 | 8篇 |
2018年 | 4篇 |
2017年 | 5篇 |
2016年 | 4篇 |
2014年 | 8篇 |
2013年 | 16篇 |
2012年 | 20篇 |
2011年 | 20篇 |
2010年 | 15篇 |
2009年 | 11篇 |
2008年 | 11篇 |
2007年 | 18篇 |
2006年 | 16篇 |
2005年 | 20篇 |
2004年 | 9篇 |
2003年 | 10篇 |
2002年 | 13篇 |
2001年 | 9篇 |
2000年 | 20篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 15篇 |
1992年 | 7篇 |
1991年 | 14篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 17篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1980年 | 3篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1976年 | 7篇 |
1975年 | 9篇 |
1974年 | 6篇 |
1973年 | 8篇 |
1972年 | 5篇 |
1971年 | 7篇 |
1970年 | 10篇 |
1969年 | 11篇 |
1968年 | 4篇 |
排序方式: 共有516条查询结果,搜索用时 15 毫秒
61.
Studies on Oxide Catalysts. XLIV. Modification of the Catalytic Behavior of Modified ZSM-5 Zeolites by Thermal Treatment The catalytic activity, the shape selectivity, and the deactivation behavior of ZSM-5 in paraffin cracking reactions can be regulated by a thermal pretreatment. A minimum of the deactivation rate as a function of zeolite pretreatment time at 600°C found in the hexane cracking could be verified in the methanol conversion reaction to gasoline. 相似文献
62.
Christiane Werner Frederik Wegener Stephan Unger Salvador Nogués Pierrick Priault 《Rapid communications in mass spectrometry : RCM》2009,23(16):2428-2438
Recent advances in understanding the metabolic origin and the temporal dynamics in δ13C of dark‐respired CO2 (δ13Cres) have led to an increasing awareness of the importance of plant isotopic fractionation in respiratory processes. Pronounced dynamics in δ13Cres have been observed in a number of species and three main hypotheses have been proposed: first, diurnal changes in δ13C of respiratory substrates; second, post‐photosynthetic discrimination in respiratory pathways; and third, dynamic decarboxylation of enriched carbon pools during the post‐illumination respiration period. Since different functional groups exhibit distinct diurnal patterns in δ13Cres (ranging from 0 to 10‰ diurnal increase), we explored these hypotheses for different ecotypes and environmental (i.e. growth light) conditions. Mass balance calculations revealed that the effect of respiratory substrates on diurnal changes in δ13Cres was negligible in all investigated species. Further, rapid post‐illumination changes in δ13Cres (30 min), which increased from 2.6‰ to 5‰ over the course of the day, were examined by positional 13C‐labelling to quantify changes in pyruvate dehydrogenase (PDH) and Krebs cycle (KC) activity. We investigated the origin of these dynamics with Rayleigh mass balance calculations based on theoretical assumptions on fractionation processes. Neither the estimated changes of PDH and KC, nor decarboxylation of a malate pool entirely explained the observed pattern in δ13Cres. However, a Rayleigh fractionation of 12C‐discriminating enzymes and/or a rapid decline in the decarboxylation rate of an enriched substrate pool may explain the post‐illumination peak in δ13Cres. These results are highly relevant since δ13Cres is used in large‐scale carbon cycle studies. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
63.
In this work, a parallel pore model (PPM) and a pore network model (PNM) are developed to provide a state-of-art method for the calculation of several characteristic pore structural parameters from inverse size-exclusion chromatography (ISEC) experiments. The proposed PPM and PNM could be applicable to both monoliths and columns packed with porous particles. The PPM and PNM proposed in this work are able to predict the existence of the second inflection point in the experimental exclusion curve that has been observed for monolithic materials by accounting for volume partitioning of the polymer standards in the macropores of the column. The appearance and prominence of the second inflection point in the exclusion curve is determined to depend strongly on the void fraction of the macropores (flow-through pores), (b) the nominal diameter of the macropores, and (c) the radius of gyration of the largest polymer standard employed in the determination of the experimental ISEC exclusion curve. The conditions that dictate the appearance and prominence of the second inflection point in the exclusion curve are presented. The proposed models are applied to experimentally measured ISEC exclusion curves of six silica monoliths having different macropore and mesopore diameters. The PPM and PNM proposed in this work are able to determine the void fractions of the macropores and silica skeleton, the pore connectivity of the mesopores, as well as the pore number distribution (PND) and pore volume distribution (PVD) of the mesopores. The results indicate that the mesoporous structure of all materials studied is well connected as evidenced by the similarities between the PVDs calculated with the PPM and the PNM, and by the high pore connectivity values obtained from the PNM. Due to the fact that the proposed models can predict the existence of the second inflection point in the exclusion curves, the proposed models could be more applicable than other models for ISEC characterization of chromatographic columns with small diameter macropores (interstitial pores) and/or large macropore (interstitial pore) void fractions. It should be noted that the PNM can always be applied without the use of the PPM, since the PPM is an idealization that considers an infinitely connected porous medium and for materials having a low (<6) pore connectivity the PPM would force the PVD to a lower average diameter and larger distribution width as opposed to properly accounting for the network effects present in the real porous medium. 相似文献
64.
Metal-halide perovskite semiconductors are certainly one of the hottest topic in solar energy conversion. Optimization of both the absorber material and device architecture has led to an astoundingly rapid increase in the reported device efficiencies. Initially developed in the context of dye-sensitized solar cell research, metal-halide perovskite devices now reach efficiency values and hence need to be compared to more conventional photovoltaic technologies such as silicon, copper indium gallium diselenide and cadmium telluride. Strong direct band gap absorption, long charge carrier diffusion length, ease and flexibility in processing at low temperatures and facile tunability makes these materials ideal for solar energy conversion applications. This review will both reflect on favorable properties of these hybrid and ionic semiconductors as well as reflecting on lead toxicity, material and device stability as the most critical issues that need to be solved in order for these materials to become technologically viable. 相似文献
65.
A number of plane stress numerical analyses of the mode I elastoplastic fracture mechanics problem have been performed in the past using the Huber–Mises yield criterion. This study employs instead the Tresca yield condition using an incremental theory of plasticity for a stationary crack. A commercial finite element program is used to solve the opening mode of fracture problem (mode I) for a square plate containing a central crack under generalized plane stress loading conditions. A biaxial uniform tensile traction is applied to the edges of a thin plate composed of a linear elastic non-work hardening material under small strain assumptions. The finite element results are compared with the analytical predictions of the Dugdale plastic strip model for a crack in an infinite plate subject to a biaxial uniform load at infinity. 相似文献
66.
David J. Unger 《Journal of Elasticity》2014,117(2):139-161
An analytical technique is developed that reduces the unknown elastic-plastic boundary of a linear elastic-perfectly plastic material containing an elliptical hole under tensile plane stress loading conditions into an equivalent mathematical problem with known boundaries. This mathematical transformation may facilitate this problem’s solution by either analytical or numerical means. Yield is assumed to occur in this analysis under the Tresca yield criterion. An example elastic-plastic problem illustrating this method is drawn from existing literature in the form of a perturbation solution for an elliptical hole derived by a series expansion about a circular boundary. 相似文献
67.
68.
69.
70.
MnBi films were prepared by annealing Bi-Mn composite films. The grain size of the MnBi films was strongly influenced by the
temperatureT
s
of the film substrate during evaporation of Bi and Mn. For 55-nm-thick films, the numberN of MnBi grains per mm2 decreased from 4×106 per mm2 forT
s
=+90°C to 1 per mm2 forT
s
=−40°C. The diameters of the largest grains observed were about 5 mm. For interpretation of this result, the formation process
of MnBi films was observed magneto-optically and electronmicroscopically. It was found thatN was determined by the structure of Bi in the Bi-Mn composite films and that this strongly depended onT
s
. 相似文献