全文获取类型
收费全文 | 54531篇 |
免费 | 9830篇 |
国内免费 | 7937篇 |
专业分类
化学 | 39359篇 |
晶体学 | 784篇 |
力学 | 3632篇 |
综合类 | 481篇 |
数学 | 6147篇 |
物理学 | 21895篇 |
出版年
2024年 | 164篇 |
2023年 | 1029篇 |
2022年 | 1943篇 |
2021年 | 2091篇 |
2020年 | 2184篇 |
2019年 | 2166篇 |
2018年 | 1959篇 |
2017年 | 1958篇 |
2016年 | 2717篇 |
2015年 | 2806篇 |
2014年 | 3467篇 |
2013年 | 4305篇 |
2012年 | 5101篇 |
2011年 | 5155篇 |
2010年 | 3699篇 |
2009年 | 3750篇 |
2008年 | 4122篇 |
2007年 | 3483篇 |
2006年 | 3241篇 |
2005年 | 2660篇 |
2004年 | 1939篇 |
2003年 | 1639篇 |
2002年 | 1631篇 |
2001年 | 1342篇 |
2000年 | 1097篇 |
1999年 | 996篇 |
1998年 | 762篇 |
1997年 | 680篇 |
1996年 | 669篇 |
1995年 | 577篇 |
1994年 | 505篇 |
1993年 | 398篇 |
1992年 | 352篇 |
1991年 | 315篇 |
1990年 | 283篇 |
1989年 | 199篇 |
1988年 | 174篇 |
1987年 | 140篇 |
1986年 | 125篇 |
1985年 | 91篇 |
1984年 | 56篇 |
1983年 | 59篇 |
1982年 | 30篇 |
1981年 | 35篇 |
1980年 | 21篇 |
1979年 | 19篇 |
1978年 | 9篇 |
1977年 | 14篇 |
1957年 | 16篇 |
1925年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
On free entropy dimension of finite von Neumann algebras 总被引:3,自引:0,他引:3
92.
Summary This paper uses Hamiltonian structures to study the problem of the limit of three-dimensional (3D) elastic models to shell
and rod models. In the case of shells, we show that the Hamiltonian structure for a three-dimensional elastic body converges,
in a sense made precise, to that for a shell model described by a one-director Cosserat surface as the thickness goes to zero.
We study limiting procedures that give rise to unconstrained as well as constrained Cosserat director models. The case of
a rod is also considered and similar convergence results are established, with the limiting model being a geometrically exact
director rod model (in the framework developed by Antman, Simo, and coworkers). The resulting model may or may not have constraints,
depending on the nature of the constitutive relations and their behavior under the limiting procedure.
The closeness of Hamiltonian structures is measured by the closeness of Poisson brackets on certain classes of functions,
as well as the Hamiltonians. This provides one way of justifying the dynamic one-director model for shells. Another way of
stating the convergence result is that there is an almost-Poisson embedding from the phase space of the shell to the phase
space of the 3D elastic body, which implies that, in the sense of Hamiltonian structures, the dynamics of the elastic body
is close to that of the shell. The constitutive equations of the 3D model and their behavior as the thickness tends to zero
dictates whether the limiting 2D model is a constrained or an unconstrained director model.
We apply our theory in the specific case of a 3D Saint Venant-Kirchhoff material andderive the corresponding limiting shell and rod theories. The limiting shell model is an interesting Kirchhoff-like shell model
in which the stored energy function is explicitly derived in terms of the shell curvature. For rods, one gets (with an additional
inextensibility constraint) a one-director Kirchhoff elastic rod model, which reduces to the well-known Euler elastica if
one adds an additional single constraint that the director lines up with the Frenet frame.
This paper is dedicated to the memory of Juan C. Simo
This paper was solicited by the editors to be part of a volume dedicated to the memory of Juan Simo. 相似文献
93.
Wei Zhou Ruonong Fu Rongji Dai Zaifu Huang Yanfei Chen 《Journal of separation science》1994,17(10):719-722
A new kind of side chain liquid crystalline polysiloxane containing a crown ether with a longer spacer (PSC-11) has been prepared and coated on a fused silica capillary column. The main chroma-tographic characteristics including efficiency, polarity, and selectivity have been examined. The phase exhibits the retention properties of both liquid crystal and crown ether stationary phases and possesses higher efficiency and better selectivity than PSC-3, which has a shorter spacer between the main polysiloxane chain and liquid crystalline side chain. 相似文献
94.
Patterned uniformly (100)-orientated silicon nanocrystallite (SiNC) films were fabricated based on hydrogen ion implantation technique and typical electrochemical anodic etching method. The surface morphology and microstructure characteristics of the films were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and atomic force microscopy. The efficient field emission with low turn-on field of about 3.2 V/μm at current density of 0.1 μA/cm2 was obtained. The emission current density from the SiNC films reached 1 mA/cm2 under a bias field of about 11 V/μm. The experimental results demonstrate that the SiNC films have great potential applications for flat panel displays. 相似文献
95.
Urn模型的离散概率分布被用于构造逼近格式,具有Bernstein多项式的许多特性。本文用B网技术对Urn模型概率分布给出新的解释,指出了Urn模型B网的诸多性质,导出分布函数曲线的导数公式,构造了Urn分布的对偶基及与此相关的拟插值算子。 相似文献
96.
Jun Xiang Ge 《International Journal of Infrared and Millimeter Waves》1994,15(10):1681-1688
In 1991, we developed a new type of quasi-optical power combiner, called a compound quasi-optical power combiner, at Ka-band. In this paper, the circuit of such a compound quasi-optical power combiner is analysed. Its equivalent circuit model is proposed. The circuit equations, the balance condition, the injection locking and the stabilized condition of the compound quasi-optical power combiner are studed by the equivalent circuit model. As an example, a compound quasi-optical power combiner which consists of two single—cavity, two—device power combiners is analysed 相似文献
97.
Palladium-catalyzed cross-coupling reactions of aryl perfluorooctanesulfonates with amines are introduced. Application of the fluorous tag in multistep synthesis of triaryl-substituted pyrimidine is also described. 相似文献
98.
X. Y. Yang H. C. Gao X. L. Tan H. Z. Yuan G. Z. Cheng S. Z. Mao S. Zhao L. Zhang J. Y. An J. Y. Yu Y. R. Du 《Colloid and polymer science》2004,282(3):280-286
1H chemical shift changes of sodium 4-decyl naphthalene sulfonate (SDNS) at 313 K show that its critical micellar concentration lies between 0.82 and 0.92 mmol/dm3, which is in the same range as that of the previous study at 298 K. The spin–lattice relaxation time, spin–spin relaxation time and two-dimensional nuclear Overhauser enhancement spectroscopy experiments give information about the structure of the SDNS micelle and the dynamics of the molecules in the micelle. The size of the SDNS micelle remains almost unchanged in the temperature range from 298 to 313 K as deduced by analyzing the self-diffusion coefficient. Special arrangement of the naphthyl rings of SDNS in the micelles affects the packing of these hydrophobic chains. The methylene groups of the alkyl chain nearest the naphthalene groups penetrate into the aromatic region, which results in a more tightly packed hydrophobic micellar core than that of sodium dodecyl sulfonate. 相似文献
99.
A method has been described for the determination of arsenic species (arsenite and arsenate) by hydride generation-atomic fluorescence spectrometry (HG-AFS). The experimental conditions that influence the fluorescence intensity and the reduction of arsenic were investigated and optimized, and the influences from foreign ions and their elimination were studied. The detection limit was found to be 79.7 ng L(-1). The proposed method was applied to the determination of arsenic species in water leachate of traditional Chinese medicines with a recovery range of 91.1-109.5%. 相似文献
100.