全文获取类型
收费全文 | 3333篇 |
免费 | 187篇 |
国内免费 | 83篇 |
专业分类
化学 | 2497篇 |
晶体学 | 16篇 |
力学 | 180篇 |
数学 | 399篇 |
物理学 | 511篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 53篇 |
2021年 | 88篇 |
2020年 | 118篇 |
2019年 | 148篇 |
2018年 | 203篇 |
2017年 | 150篇 |
2016年 | 275篇 |
2015年 | 167篇 |
2014年 | 239篇 |
2013年 | 453篇 |
2012年 | 289篇 |
2011年 | 257篇 |
2010年 | 205篇 |
2009年 | 172篇 |
2008年 | 184篇 |
2007年 | 139篇 |
2006年 | 74篇 |
2005年 | 70篇 |
2004年 | 74篇 |
2003年 | 52篇 |
2002年 | 41篇 |
2001年 | 18篇 |
2000年 | 20篇 |
1999年 | 17篇 |
1998年 | 10篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 1篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1963年 | 1篇 |
排序方式: 共有3603条查询结果,搜索用时 15 毫秒
971.
First-order tests of special relativity are based on a comparison of clocks synchronized with the help of slow clock transport with those synchronized by the Einstein procedure. This comparison enables the measurement of the one-way velocity of light and is equivalent to a measurement of the time dilatation factor. The accuracy of present measurements is of the order 10–7, yielding an upper limit of 3 cm/sec for the ether drift. 相似文献
972.
Analysis of labile Cu2+ in fresh waters using the Cu2+-catalysed oxidation of 1,10-phenanthroline by superoxide anion radical has been investigated. It was found that certain metal ions, notably Fe3+, Fe2+ and Pb2+ enhance the chemiluminescence (CL) intensity of this reaction by up to an order of magnitude when present in the μM concentration range. This enhancement is considered to arise through coordination of the metal ion to 1,10-phenanthroline, which prevents free rotation of the benzene rings in the excited state intermediate thought to be responsible for light emission. This introduces a potential interference when analyzing fresh waters, which usually contain Fe3+ concentrations of this magnitude. However, the enhancement effect saturates at about 4 μM Fe3+, so that reliable results can be obtained if the water sample is supplemented with Fe3+ to reach this level. Application of the enhanced technique to a river water, and a reservoir to which CuSO4 had been added to control algal growth, are described. In both cases, only a small fraction of total dissolved Cu2+ is labile with respect to the chemiluminesence method. 相似文献
973.
Reza Akhtar 《K-Theory》2004,32(3):269-291
Let k be a field and X a smooth projective variety of dimension d over k. Generalizing a construction of Kato and Somekawa, we define a Milnor-type group
which is isomorphic to the ordinary Milnor
We prove that
is isomorphic to both the higher Chow group CHd+s (X,s) and the Zariski cohomology group
相似文献
974.
We report here a general synthetic methodology for palladium-catalyzed carbonylative annulation of o-alkynylphenol to construct 2-substituted-3-aroyl-benzo[b]furan. On the basis of the results, this methodology could be applied to a wider selection of iodide substrates to generate desired products. In accordance with mechanistic studies, this process involves coordination of cationic and less hindered acyl palladium complexes with o-alkynylphenols to create a desired cascade triad (coordination, nucleophilic addition, and reductive elimination). Consistent with this mechanism, addition of 1 equiv of AgBF(4) to palladium catalyst Pd(Ph(3)P)(4) generates an ideal candidate for this unique transformation. 相似文献
975.
[structure: see text] N-Fmoc-L-p-azidotetrafluorophenylalanine was prepared from achiral starting materials using an acetamidomalonate synthesis and enzymatic resolution. A photoactive peptide containing this fluorinated residue could be assembled using solid-phase Fmoc chemistry. 相似文献
976.
[reaction: see text] A palladium-catalyzed coupling reaction of phenyliodonium zwitterions with aryl boronic acids has been developed. The unique characteristics of the mild reaction conditions and convenient synthetic accessibility of phenyliodonium zwitterions make this method a valuable tool for generating diversified 3-aryl-4-hydroxycoumarins. 相似文献
977.
We have developed an accurate and highly efficient method for upscaling and simulation of immiscible displacements in three-dimensional
(3D) heterogeneous reservoirs, which is an extension of the technique that we developed previously for 2D systems. The method
utilizes wavelet transformations (WTs) to upscale the geological model of a reservoir, based on the spatial distribution of
the single-phase permeabilities and the locations of the wells in the reservoir. It generates a non-uniform grid in which
the resolved structure of the fine grid around the wells, as well as in the high-permeability sectors, are preserved, but
the rest of the grid is upscaled. A robust uplayering procedure is used to reduce the number of the layers, and the WTs are
used to upscale each layer areally. To demonstrate the method’s accuracy and efficiency, we have applied it to the geological
model of a highly heterogeneous reservoir put forward in the tenth Society of Petroleum Engineers comparative solution project
(the SPE-10 model), and carried out simulation of waterflooding in the upscaled model. Various upscaling scenarios were examined,
and although some of them resulted in efficient simulations and accurate predictions, the results when non-uniform upscaling
is used based on the WT technique are in excellent agreement with the solution of the same problem in the fine grid of the
SPE-10 model. Most importantly, the speed-up factors that we obtain are several orders of magnitude. Hence, the method renders
it unnecessary to use massively parallel computations for such problems. 相似文献
978.
In this research the fluid dynamics characteristics of a stellar turbulent jet flow is studied numerically and the results
of three dimensional jet issued from a stellar nozzle are presented. A numerical method based on control volume approach with
collocated grid arrangement is employed. The turbulent stresses are approximated using k–ε and k–ω models with four different inlet conditions. The velocity field is presented and the rate of decay at jet centerline is noted.
Special attention is drawn on the influence of corner angle and number of wings on mixing in stellar cross section jets. Stellar
jets with three; four and five wings and 15–65° corner angles are studied. Also the effect of Reynolds number (based on hydraulic
diameter) as well as the inflow conditions on the evolution of the stellar jet is studied. The Numerical results show that
the jet entrains more with corner angle 65° and five wings number. The jet is close to a converged state for high Reynolds
numbers. Also the influence of the inflow conditions on the jet characteristics is so strong. 相似文献
979.
G. Reza Bebahani David Dunnion Patrick Falvey Kenneth Hickey Mary Meade Yvonne McCarthy Martyn C. R. Symons W. Earle Waghorne 《Journal of solution chemistry》2000,29(6):521-539
We report enthalpies of solution of formamide, N,N-dimethylformamide,N,N-dimethylacetamide, acetic acid, methyl acetate, and acetonitrile in water +dimethylsulfoxide mixed solvents. These, along with literature data for additional solutes,are analyzed in terms of the extended coordination model of solvation. We alsoanalyze infrared data for several of these solutes. These analyses show thatN,N-dimethylformamide and N,N-dimethylacetamide are preferentially hydrated,while the other solutes appear to be preferentially solvated by dimethylsulfoxide.In all cases, the extent of preferential solvation is relatively small. It is also foundthat the degrees of preferential solvation recovered from analyses of the enthalpydata correspond closely to those recovered from the infrared data, although thelatter refer only to the polar chromophores on the solute molecules. It is foundthat the extent to which the solutes disrupt the solvent-solvent interactions variessystematically with the area of the nonpolar surfaces of the solute molecules. 相似文献
980.
Ali Reza Ashrafi 《Journal of Applied Mathematics and Computing》2000,7(1):115-124
For a finite groupG, #Cent(G) denotes the number of centralizers of its elements. A groupG is called n-centralizer if #Cent(G) =n, and primitiven-centralizer if # Cent(G)\text = # Cent\text(\fracGZ(G))\text = n\# Cent(G){\text{ = \# }}Cent{\text{(}}\frac{G}{{Z(G)}}){\text{ = }}n. In this paper we compute the number of distinct centralizers of some finite groups and investigate the structure of finite groups with exactly six distinct centralizers. We prove that ifG is a 6-centralizer group then % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaSaaaeaaca% WGhbaabaGaamOwaiaacIcacaWGhbGaaiykaaaacaqGGaGaeyyrIaKa% aeiiaGqaciaa-readaWgaaWcbaGaa8hoaaqabaGccaGGSaGaaeiiai% aa-feadaWgaaWcbaGaa8hnaaqabaGccaGGSaGaaeiiaiaabQfadaWg% aaWcbaGaaeOmaaqabaGccaqGGaGaey41aqRaaeiiaiaabQfadaWgaa% WcbaGaaeOmaaqabaGccaqGGaGaey41aqRaaeiiaiaabQfadaWgaaWc% baGaaeOmaaqabaGccaqGGaGaae4BaiaabkhacaqGGaGaaeOwamaaBa% aaleaacaqGYaaabeaakiaabccacqGHxdaTcaqGGaGaaeOwamaaBaaa% leaacaqGYaaabeaakiaabccacqGHxdaTcaqGGaGaaeOwamaaBaaale% aacaqGYaaabeaakiaabccacqGHxdaTcaqGGaGaaeOwamaaBaaaleaa% caqGYaaabeaaaaa!62C4!\[\frac{G}{{Z(G)}}{\text{ }} \cong {\text{ }}D_8 ,{\text{ }}A_4 ,{\text{ Z}}_{\text{2}} {\text{ }} \times {\text{ Z}}_{\text{2}} {\text{ }} \times {\text{ Z}}_{\text{2}} {\text{ or Z}}_{\text{2}} {\text{ }} \times {\text{ Z}}_{\text{2}} {\text{ }} \times {\text{ Z}}_{\text{2}} {\text{ }} \times {\text{ Z}}_{\text{2}} \] . 相似文献