全文获取类型
收费全文 | 388454篇 |
免费 | 3223篇 |
国内免费 | 963篇 |
专业分类
化学 | 207923篇 |
晶体学 | 5762篇 |
力学 | 17559篇 |
综合类 | 15篇 |
数学 | 43176篇 |
物理学 | 118205篇 |
出版年
2019年 | 3253篇 |
2018年 | 4282篇 |
2017年 | 4255篇 |
2016年 | 6495篇 |
2015年 | 3921篇 |
2014年 | 6260篇 |
2013年 | 15993篇 |
2012年 | 12004篇 |
2011年 | 14866篇 |
2010年 | 10499篇 |
2009年 | 10304篇 |
2008年 | 13815篇 |
2007年 | 14065篇 |
2006年 | 13192篇 |
2005年 | 11965篇 |
2004年 | 10801篇 |
2003年 | 9747篇 |
2002年 | 9705篇 |
2001年 | 10634篇 |
2000年 | 8276篇 |
1999年 | 6399篇 |
1998年 | 5581篇 |
1997年 | 5540篇 |
1996年 | 5163篇 |
1995年 | 4804篇 |
1994年 | 4933篇 |
1993年 | 4525篇 |
1992年 | 5208篇 |
1991年 | 5291篇 |
1990年 | 5063篇 |
1989年 | 4960篇 |
1988年 | 4987篇 |
1987年 | 4817篇 |
1986年 | 4579篇 |
1985年 | 6038篇 |
1984年 | 6397篇 |
1983年 | 5336篇 |
1982年 | 5634篇 |
1981年 | 5468篇 |
1980年 | 5384篇 |
1979年 | 5460篇 |
1978年 | 5889篇 |
1977年 | 5708篇 |
1976年 | 5806篇 |
1975年 | 5419篇 |
1974年 | 5520篇 |
1973年 | 5644篇 |
1972年 | 3927篇 |
1971年 | 3371篇 |
1970年 | 3088篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
A finite element solution of the Navier-Stokes equations for steady flow under the magnetic effect through a double-branched two-dimensional section of a three-dimensional model of the canine aorta is discussed. The numerical scheme involves transforming the physical co-ordinates to a curvilinear boundary-fitted co-ordinate system. The shear stress at the wall is calculated for a Reynolds number of 1000 with the branch-to-main aortic flow rate ratio as a parameter. The results are compared with earlier works involving experimental data and found to be in reasonable qualitative agreement. The steady flow, shear stress and branch flow under the effect of a magnetic field have been discussed in detail. 相似文献
93.
Ph. Komninou Th. Kehagias A. Delimitis G.P. Dimitrakopulos J. Kioseoglou E. Dimakis A. Georgakilas Th. Karakostas 《Superlattices and Microstructures》2004,36(4-6):509
The structural properties of InN thin films, grown by rf plasma-assisted molecular beam epitaxy on Ga-face GaN/Al2O3(0001) substrates, were investigated by means of conventional and high resolution electron microscopy. Our observations showed that a uniform InN film of total thickness up to 1 μm could be readily grown on GaN without any indication of columnar growth. A clear epitaxial orientation relationship of , was determined. The quality of the InN film was rather good, having threading dislocations as the dominant structural defect with a density in the range of 109–1010 cm−2. The crystal lattice parameters of wurtzite InN were estimated by electron diffraction analysis to be a=0.354 nm and c=0.569 nm, using Al2O3 as the reference crystal. Heteroepitaxial growth of InN on GaN was accomplished by the introduction of a network of three regularly spaced misfit dislocation arrays at the atomically flat interface plane. The experimentally measured distance of misfit dislocations was 2.72 nm. This is in good agreement with the theoretical value derived from the in-plane lattice mismatch of InN and GaN, which indicated that nearly full relaxation of the interfacial strain between the two crystal lattices was achieved. 相似文献
94.
A. G. Chentsov 《Journal of Mathematical Sciences》2007,140(6):873-904
The paper considers the representations of attraction sets in topological spaces and their relations with the relaxation of
accessibility problems under the conditions of sequentially relaxed constraints. The author studies the structure of approximate
(in essence, asymptotic) solutions and generalized elements and establishes the possibility of their real identification for
a certain version (related to the Stone-Čech compactification and the Wallman relaxation) of relaxation of the initial problem.
__________
Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 26, Nonlinear
Dynamics, 2005. 相似文献
95.
W. G. Habashi G. Baruzzi M. F. Peeters M. M. Hafez 《Numerical Methods for Partial Differential Equations》1991,7(2):193-207
Finite element solutions of the Euler and Navier-Stokes equations are presented, using a simple dissipation model. The discretization is based on the weak-Galerkin weighted residual method and equal interpolation functions for all the unknowns are permitted. The nonlinearity is iterated upon using a Newton method and at each iteration the linear algebraic system is solved by a direct solver with all unknowns fully coupled. Results are presented for two-dimensional transonic inviscid flows and two- and three-dimensional incompressible viscous flows. Convergence of the algorithm is shown to be quadratic, reaching machine accuracy in very few iterations. The inviscid results demonstrate the existence of nonunique numerical solutions to the steady Euler equations. 相似文献
96.
Hydrogels of N‐vinylimidazole (VI) and sodium styrenesulfonate (SSS) were synthesized in aqueous solution by radical crosslinking copolymerization with N,N′‐methylene‐bis(acrylamide) as crosslinker. Swelling in several saline solutions was measured for hydrogel samples synthesized with different comonomer concentrations (CT = 10, 25, or 40%) and with SSS mole fractions covering a broad range (fSSS = 0–0.7), while the crosslinker ratio was 2 wt % in all cases. The degree of swelling in aqueous solution with a specific ionic strength (μ), plotted versus the SSS composition of the feed, shows a minimum for any set of samples synthesized with a fixed CT. The dependence of swelling on μ shows both polyelectrolyte (fSSS beyond the minimum) and antipolyelectrolyte behaviors (in the low fSSS limit). It was found that the nonGaussian factor of the crosslinking density and the polymer‐solvent interaction parameter increase with fSSS for any CT. Moreover, in the low fSSS limit, the osmotic swelling pressure is governed not only by the ionic contribution, but also by the polymer‐solvent mixing and, the concentration of mobile counterions inside the gel is not proportional to the net fixed charge but to the addition of cationic and anionic side groups, what discards the formation of ionic pairs. The antipolyelectrolyte effect is interpreted as due to the increasing protonation of VI as μ goes up. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1683–1693, 2007 相似文献
97.
I. P. Raevskii L. A. Reznichenko M. P. Ivliev V. G. Smotrakov V. V. Eremkin M. A. Malitskaya L. A. Shilkina S. I. Shevtsova A. V. Borodin 《Crystallography Reports》2003,48(3):486-490
(Na, K)NbO3 crystals with a perovskite structure and a KNbO3 content up to 40 mol % were grown from flux with the use of the solvent NaBO2. The dielectric measurements of the crystals grown revealed phase transitions that had never been observed before in ceramic samples. 相似文献
98.
Results are presented of an experimental investigation of vortex ring formation by a fluid drop contacting a free surface
with negligible velocity. The pool fluid is mixed with fluorescein dye, and a laser sheet is used to illuminate a plane of
the flow. A series of representative images is recorded by a CCD camera and speculation is made regarding specific sources
of vorticity flux through the free surface. Two scaling analyses previously presented by other investigators are demonstrated
to be equivalent under the assumptions of this experiment, and they provide the motivation for a series of test runs in which
the duration of the coalescence process, τ*, is related to variations in drop diameter L and fluid surface tension σ. Experimental results are in agreement with the analyses, showing τ*∼σ-1/2 and τ*∼L
3/2.
Received: 22 December 1995 / Accepted: 15 October 1996 相似文献
99.
G. Weber 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):639-642
Summary For accurate speciation analyses it is important to know the stability of the respective species, especially in the case of metal complexes. Factors affecting the chromatographic stability of such metal species are investigated. By using thermodynamic models for complex formation and chromatographic retention equilibria the influence of species concentration, stoichiometry and excess of ligand is calculated and compared with experimental results for iron complexes (lactate, gluconate and citrate species). Iron citrate is the only species, that is chromatographed as 1:2 complex (metal: ligand), while iron lactate and gluconate are transformed to 1:1 species. Problems resulting from the coelution of different species are discussed.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
100.
Experiments for the determination of mono-, di and tri-butyltin (MBT, DBT and TBT) by hydride generation/gas chromatography/atomic absorption spectrometry in various matrices (sediment, suspended matter, mussel, algae and water) have revealed that poor butyltin recoveries are obtained in sediments displaying high sulphur and hydrocarbon contents; very poor recoveries were also observed for TBT in sediments with high chlorophyll pigment contents as well as in algal samples. It was however not clear whether the hydride generatin was inhibited by these infering compounds, as was previously assumed in the case of hydrocarbons, or whether interferences affected the atomization rate. Further studies were performed to solve this problem in order to validate this method in the case of analyses of, for example, oil-contaminated sediment and algae. This paper presents the results obtained. It is concluded here that the poor recoveries were due to an inhibition of hydride generation rather than to interference at the atomization stage. 相似文献