全文获取类型
收费全文 | 1107篇 |
免费 | 44篇 |
国内免费 | 4篇 |
专业分类
化学 | 668篇 |
晶体学 | 26篇 |
力学 | 33篇 |
数学 | 85篇 |
物理学 | 343篇 |
出版年
2023年 | 13篇 |
2022年 | 19篇 |
2021年 | 26篇 |
2020年 | 16篇 |
2019年 | 27篇 |
2018年 | 30篇 |
2017年 | 18篇 |
2016年 | 38篇 |
2015年 | 35篇 |
2014年 | 38篇 |
2013年 | 100篇 |
2012年 | 77篇 |
2011年 | 80篇 |
2010年 | 58篇 |
2009年 | 47篇 |
2008年 | 65篇 |
2007年 | 57篇 |
2006年 | 56篇 |
2005年 | 50篇 |
2004年 | 39篇 |
2003年 | 38篇 |
2002年 | 28篇 |
2001年 | 8篇 |
2000年 | 11篇 |
1999年 | 10篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 13篇 |
1995年 | 17篇 |
1994年 | 13篇 |
1993年 | 7篇 |
1992年 | 6篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1985年 | 3篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1978年 | 4篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1970年 | 3篇 |
1968年 | 3篇 |
1967年 | 2篇 |
1937年 | 2篇 |
1930年 | 10篇 |
1929年 | 4篇 |
1906年 | 2篇 |
排序方式: 共有1155条查询结果,搜索用时 15 毫秒
131.
Ward DE Gillis HM Akinnusi OT Rasheed MA Saravanan K Sasmal PK 《Organic letters》2006,8(12):2631-2634
Meso 1,9-diketones (six to seven stereocenters) are readily obtained by stepwise or simultaneous two-directional aldol reactions of tetrahydro-4H-thiopyran-4-one with a thiopyran-derived aldehyde or dialdehyde. Enantioselective enolizations of these diketones with the lithium amide from (R,R)-bis(1-phenylethyl)amine occur with simultaneous kinetic resolution to give the mono-TMS enol ethers in >90% yields (BORSM) and >95% ee. The products are applicable to polypropionate synthesis. [reaction: see text] 相似文献
132.
Tapan Kuman Kar K. S. Chaudhuri 《International Journal of Mathematical Education in Science & Technology》2013,44(4):543-556
The present paper deals with the problem of non-selective harvesting of a prey-predator system in which both the prey and the predator species obey the law of logistic growth and each predators functional response to the prey approaches a constant as the prey population increases. Boundedness of the exploited system is examined. The existence of its steady states and their stability are studied using eigenvalue analysis. The existence of bionomic equilibria has been considered. The problem of determining the optimal harvest policy is then solved by using Pontryagin's maximal principle. Finally, some numerical examples are given to illustrate the results. 相似文献
133.
A brief survey is given to show that harmonic averages enter in a natural way in the numerical solution of various variable coefficient problems, such as in elliptic and transport equations, also of singular perturbation types. Local Green’s functions used as test functions in the Petrov-Galerkin finite element method combined with harmonic averages can be very efficient and are related to exact difference schemes. 相似文献
134.
Debasish Ghosh Pradip Ghosh Golap Kalita Takuto Noda Chisato Takahashi Masaki Tanemura 《固体物理学:研究快报》2013,7(7):489-492
An efficient cathode material with high transparency (93%) based on conducting polymer poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and single wall carbon nanotubes (SWCNTs) has been developed for the fabrication of highly transparent and flexible field electron emitters (FEE). This kind of material showed superior field emission (FE) performance with very high current density (10–3A/cm2) at very low electric field. The FE performance of the hybrid materials was dramatically improved compared to either SWCNTs and PEDOT:PSS. Thus the hybrid structures of conducting polymer and SWCNTs might be a good choice for use as a cathode material to enhance the FE performance and for potential application in future portable displays. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
135.
Abstract In this article, various issues involved in a ground-to-satellite optical communication link (i.e., acquisition time, uncertainty area, and channel noise) are discussed. Acquisition time of a free-space optical link is evaluated for coherent (sub-carrier BPSK and QPSK) and non-coherent (OOK and -PPM) modulation schemes over weak turbulent channel with transmit diversity. In the analysis, both uncorrelated and correlated beams are considered. It is seen that an increase in transmit diversity order helps to improve the acquisition time, irrespective of turbulence strength in the atmosphere. At high correlation, a marginal change in acquisition time is observed with the increase in diversity order. 相似文献
136.
J. Karátson 《Numerical Functional Analysis & Optimization》2013,34(5-6):590-611
The superlinear convergence of the preconditioned CGM is studied for nonsymmetric elliptic problems (convection-diffusion equations) with mixed boundary conditions. A mesh independent rate of superlinear convergence is given when symmetric part preconditioning is applied to the FEM discretizations of the BVP. This is the extension of a similar result of the author for Dirichlet problems. The discussion relies on suitably developed Hilbert space theory for linear operators. 相似文献
137.
Asymmetric transfer hydrogenation of ketones using Ru(II)arene/amino alcohol catalyst system proceeds with significant rate enhancement by ultrasound promotion. Comparison of the silent reactions carried out at 25 degrees C with reactions under sonochemical activation at 25 degrees C clearly showed enhancement in catalytic activity by 5-10 times without significantly affecting the enantioselectivity. 相似文献
138.
S. Ravi Manoranjan Kar S. M. Borah P. S. R. Krishna 《Crystal Research and Technology》2008,43(12):1318-1322
Neutron powder diffraction patterns were recorded on CaMn1‐xCuxO3 (x = 0 and 0.20) compounds at different temperatures down to 11K. All the patterns were analyzed by employing Rietveld refinement technique and using the Fullprof program. The observed crystallographic peaks could be refined by using Pbnm space group and no structural transition has been observed down to 11K. An additional peak at 2θ = 16.7° has been observed with decrease in temperature below TN and its intensity was found to increase with decrease in temperature. It could be indexed to magnetic (101) plane. The magnetic ordering is found to be G‐type antiferromagnetic behaviour. The magnetic moment at 11K for the samples x = 0.0 and 0.20 are found to be 2.69 and 2.42μB. The doped Cu ions are found to be in Cu2+ state and take part antiferromagnetic interactions with Mn ions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
139.
T. Ghosh S. Bandyopadhyay K. K. Roy S. Kar A. K. Lahiri A. K. Maiti K. Goswami 《Crystal Research and Technology》2008,43(9):959-963
Thermally processed lead iodide (PbI2) thin films were prepared by the vacuum evaporation method in a constant ambient. Measured thickness of the film was verified analytically from the optical transmittance data in a wavelength range between 300 and 1600 nm. From the Tauc relation for the non‐direct inter band transition, the optical band gap of the film was found to be 2.58 eV for film thickness 300 nm. X‐ray diffraction analysis confirmed that PbI2 films are polycrystalline, having hexagonal structure. The low fluctuation in Urbach energy indicates that the grain size is quite small. The present findings are in agreement with the other results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
140.
Reactions between superdislocations involved in deformation in the basal, prismatic, and type-I and II pyramidal planes in single-crystal Ti3Al are considered. The types of dislocation interactions are established that result in the formation dislocation barriers (microcrack nuclei). The force and energy conditions for microcracks to arise are found. The interaction between a and 2c + a superdislocations results in microcracks with the plane of opening lying in basal and pyramidal planes; the interaction of 2c + a superdislocations in different pyramidal planes results in the formation of microcracks in prismatic and pyramidal planes; and the interaction of a superdislocations in basal and/or pyramidal planes does not cause the formation of dislocation barriers. The types of microcracks are classified in terms of the orientation of deformation axes of single crystals, and the regions of the stereographic triangle are determined characterized by a preferential type of crack opening. 相似文献