全文获取类型
收费全文 | 4033篇 |
免费 | 84篇 |
国内免费 | 35篇 |
专业分类
化学 | 3030篇 |
晶体学 | 17篇 |
力学 | 98篇 |
数学 | 471篇 |
物理学 | 536篇 |
出版年
2022年 | 31篇 |
2021年 | 41篇 |
2020年 | 48篇 |
2019年 | 47篇 |
2018年 | 32篇 |
2016年 | 58篇 |
2015年 | 60篇 |
2014年 | 109篇 |
2013年 | 172篇 |
2012年 | 202篇 |
2011年 | 237篇 |
2010年 | 127篇 |
2009年 | 110篇 |
2008年 | 172篇 |
2007年 | 199篇 |
2006年 | 218篇 |
2005年 | 185篇 |
2004年 | 164篇 |
2003年 | 150篇 |
2002年 | 168篇 |
2001年 | 75篇 |
2000年 | 78篇 |
1999年 | 45篇 |
1998年 | 36篇 |
1997年 | 48篇 |
1996年 | 62篇 |
1995年 | 36篇 |
1994年 | 41篇 |
1993年 | 41篇 |
1992年 | 39篇 |
1991年 | 32篇 |
1990年 | 34篇 |
1989年 | 27篇 |
1988年 | 23篇 |
1987年 | 24篇 |
1986年 | 32篇 |
1985年 | 60篇 |
1984年 | 51篇 |
1983年 | 41篇 |
1982年 | 50篇 |
1981年 | 44篇 |
1980年 | 53篇 |
1979年 | 67篇 |
1978年 | 51篇 |
1977年 | 39篇 |
1976年 | 44篇 |
1975年 | 26篇 |
1974年 | 30篇 |
1973年 | 29篇 |
1969年 | 21篇 |
排序方式: 共有4152条查询结果,搜索用时 11 毫秒
971.
An analytical interface reconstruction algorithm in the PLIC‐VOF method for 2D polygonal unstructured meshes 下载免费PDF全文
Piecewise linear interface calculation (PLIC) schemes have been extensively employed in the volume‐of‐fluid (VOF) method for interface capturing in numerical simulations of multiphase flows. Polygonal unstructured meshes are often adopted because of their geometric flexibility and superiority in gradient calculation. An analytical interface reconstruction algorithm in the PLIC‐VOF method for arbitrary convex polygonal cells has been proposed in this study. The line interface at a given orientation within a polygonal cell is located by an analytical technique. It has been tested successfully for four different geometric shapes that are common in polygonal meshes. The computational efficiency of the present algorithm has been compared with several published schemes in the literature. The proposed algorithm has been shown to yield higher accuracy with reduction in computational complexity. A numerical simulation of a dam‐breaking problem has been performed using the proposed analytical PLIC technique on polygonal meshes. The results are in good agreement with experimental data available in the literature, which serves as a demonstration of its performance in a real multiphase flow. 相似文献
972.
973.
Albert B. Ku 《International Journal of Solids and Structures》1977,13(8):709-715
Upper and lower bounds of buckling load for a nonuniform elastic column under conservative loading are considered. Compatible admissible moment and displacement functions are expressed in terms of a compatible coordinate system. The generalized Timoshenko Quotient and the modified Schreyer and Shih formula are the proposed upper and lower bounds. Both bounds when iterated converge to the exact buckling load. The method described here is simple and convenient and applies to all self-adjoint problems without exception. 相似文献
974.
Poater A Ragone F Mariz R Dorta R Cavallo L 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(48):14348-14353
The current approach to improve and tune the enantioselective performances of transition-metal catalysts for asymmetric synthesis is mostly focused to modifications of the steric properties of the ancillary ligands of the active metal. Nevertheless, it is also known that electrostatic effects can have a remarkable role to promote selectivity in asymmetric synthesis. Using the Rh-catalyzed asymmetric 1,4-addition of phenylboronic acid to 2-cyclohexenone leading to chiral 3-phenylcyclohexanone as an example, we could show that high enantioselectivity can be indeed achieved using catalysts essentially based either on steric or electrostatic effects as the main source of enantioselective induction. In this contribution we suggest that the analysis of the surface of interaction between the catalyst and the substrate could be a useful tool to quantify the power of steric and electrostatic effects of catalysts. 相似文献
975.
Mariz R Poater A Gatti M Drinkel E Bürgi JJ Luan X Blumentritt S Linden A Cavallo L Dorta R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(48):14335-14347
A family of chiral C(2)-symmetric disulfoxide ligands possessing biaryl atropisomeric backbones has been synthesized by using the Andersen methodology. Complete characterization includes X-ray crystallographic studies of all ligands and some of their rhodium complexes. Their synthesis, optical purity, electronic properties, and catalytic behavior in the prototypical rhodium-catalyzed 1,4-addition of phenylboronic acid to 2-cyclohexen-1-one are presented through an in depth study of this ligand class. Density functional theory calculations on the step of the catalytic cycle that determines the enantioselectivity are presented and reinforce the first hypothetical explanations for the high levels of asymmetric induction observed. 相似文献
976.
Roland J. W. Meesters Jeroen J. A. van Kampen Mariska L. Reedijk Rachel D. Scheuer Lennard J. M. Dekker David M. Burger Nico G. Hartwig Albert D. M. E. Osterhaus Theo M. Luider Rob A. Gruters 《Analytical and bioanalytical chemistry》2010,398(1):319-328
Kaletra® (Abott Laboratories) is a co-formulated medication used in the treatment of HIV-1-infected children, and it contains the two antiretroviral protease inhibitor drugs lopinavir and ritonavir. We validated two new ultrafast and high-throughput mass spectrometric assays to be used for therapeutic drug monitoring of lopinavir and ritonavir concentrations in whole blood and in plasma from HIV-1-infected children. Whole blood was blotted onto dried blood spot (DBS) collecting cards, and plasma was collected simultaneously. DBS collecting cards were extracted by an acetonitrile/water mixture while plasma samples were deproteinized with acetone. Drug concentrations were determined by matrix-assisted laser desorption/ionization-triple quadrupole tandem mass spectrometry (MALDI-QqQ-MS/MS). The application of DBS made it possible to measure lopinavir and ritonavir in whole blood in therapeutically relevant concentrations. The MALDI-QqQ-MS/MS plasma assay was successfully cross-validated with a commonly used high-performance liquid chromatography (HPLC)–ultraviolet (UV) assay for the therapeutic drug monitoring (TDM) of HIV-1-infected patients, and it showed comparable performance characteristics. Observed DBS concentrations showed as well, a good correlation between plasma concentrations obtained by MALDI-QqQ-MS/MS and those obtained by the HPLC-UV assay. Application of DBS for TDM proved to be a good alternative to the normally used plasma screening. Moreover, collection of DBS requires small amounts of whole blood which can be easily performed especially in (very) young children where collection of large whole blood amounts is often not possible. DBS is perfectly suited for TDM of HIV-1-infected children; but nevertheless, DBS can also easily be applied for TDM of patients in areas with limited or no laboratory facilities. 相似文献
977.
C. R. Das H. C. Hsu S. Dhara A. K. Bhaduri B. Raj L. C. Chen K. H. Chen S. K. Albert A. Ray Y. Tzeng 《Journal of Raman spectroscopy : JRS》2010,41(3):334-339
Crystalline Si substrates are studied for pressure‐induced phase transformation under indentation at room temperature (RT) using a Berkovich tip. Raman spectroscopy, as a nondestructive tool, is used for the identification of the transformed phases. Raman lines as well as area mapping are used for locating the phases in the indented region. Calculation of pressure contours in the indented region is used for understanding the phase distribution. We report here a comprehensive study of all the phases of Si, reported so far, leading to possible understanding of material properties useful for possible electromechanical applications. As a major finding, distribution of the amorphous phase in the indented region deviates from the conventional wisdom of being in the central region alone. We present phase mapping results for both Si(100) and Si(111) substrates. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
978.
Joaquim Crusats Dr. David Hochberg Dr. Albert Moyano Prof. Dr. Josep M. Ribó Prof. Dr. 《Chemphyschem》2009,10(12):2123-2131
In a closed system an irreversible enantioselective autocatalysis coupled to a mutual inhibition reaction, corresponding to a fast and low exergonic formation of the heterochiral dimer which reverts to the monomers in the final reaction work‐up, yields absolute asymmetric synthesis even in the absence of chiral polarizations. This is due to the very high chiral amplifications of the initial small statistical deviations from the ideal racemic composition. Moreover, this system is sensitive to very small chiral polarizations (energy differences between transition states below the mJ mol?1 range). This behaviour can also be observed in reversible exergonic reactions, because the racemization time scale is substantially longer than that of the transformation of the initial reagents. The effect of the presence of other reactions likely to occur (i.e. non‐catalytic transformations, non‐enantioselective catalysis and homodimer formation) is discussed. Even if these decrease the sensitivity of the network in several chemical scenarios, the emergence of kinetically controlled spontaneous symmetry breaking is not hindered. These features, together with the response of the system to a sequential reaction procedure, suggest that a similar type of network is at the heart of the Soai reaction. 相似文献
979.
Albert J Cadena JM González A Granell J Solans X Font-Bardia M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(3):887-894
We report that the action of Lewis bases, such as triphenylphosphine, pyridine, or trimethylamine, on imine metallacycles derived from amino acids leads to the formation of the first organometallic compound of an NH aldimine, a highly reactive organic species, and the corresponding alpha-ketoester, in a deamination reaction that mimics the metabolism of alpha-amino acids. The synthesis of different cyclopalladated compounds by a reaction between palladium acetate and the Schiff bases 2,4,6-Me(3)C(6)H(2)CH=NCH(R(1))COOR(2) (R(1) = CH(2)Ph, R(2) = Et and R(1) = Ph, R(2) = Me) is also reported. 相似文献
980.
Padwa A Crawford KR Straub CS Pieniazek SN Houk KN 《The Journal of organic chemistry》2006,71(15):5432-5439
Intramolecular Diels-Alder reactions involving a series of N-alkenyl-substituted furanyl amides were investigated. Stable functionalized oxanorbornenes were formed in high yield upon heating at 80-110 degrees C. The cycloaddition reactions include several bromo-substituted furanyl amides, and these systems were found to proceed at a much faster rate and in higher yield than without substitution. This effect was observed by incorporating a halogen in the 3- or 5-position of the furan ring and appears to be general. The origin of increased cycloaddition rates for halo-substituted furans has been investigated with quantum mechanical calculations. The success of these reactions is attributed to increases in reaction exothermicities; this both decreases activation enthalpies and increases barriers to retrocycloadditions. Halogen substitution on furan increases reactant energy and stabilizes the product, which is attributed to the preference of electronegative halogens to be attached to a more highly alkylated and therefore more electropositive framework. 相似文献