全文获取类型
收费全文 | 38380篇 |
免费 | 2139篇 |
国内免费 | 1611篇 |
专业分类
化学 | 17338篇 |
晶体学 | 413篇 |
力学 | 1919篇 |
综合类 | 147篇 |
数学 | 9793篇 |
物理学 | 12520篇 |
出版年
2023年 | 196篇 |
2022年 | 285篇 |
2021年 | 363篇 |
2020年 | 422篇 |
2019年 | 425篇 |
2018年 | 1524篇 |
2017年 | 1762篇 |
2016年 | 1262篇 |
2015年 | 1078篇 |
2014年 | 1031篇 |
2013年 | 1359篇 |
2012年 | 4048篇 |
2011年 | 3271篇 |
2010年 | 2458篇 |
2009年 | 2226篇 |
2008年 | 1248篇 |
2007年 | 1227篇 |
2006年 | 1162篇 |
2005年 | 4866篇 |
2004年 | 4269篇 |
2003年 | 2544篇 |
2002年 | 784篇 |
2001年 | 536篇 |
2000年 | 324篇 |
1999年 | 367篇 |
1998年 | 261篇 |
1997年 | 221篇 |
1996年 | 191篇 |
1995年 | 153篇 |
1994年 | 137篇 |
1993年 | 118篇 |
1992年 | 246篇 |
1991年 | 236篇 |
1990年 | 204篇 |
1989年 | 166篇 |
1988年 | 126篇 |
1987年 | 78篇 |
1986年 | 73篇 |
1985年 | 55篇 |
1984年 | 44篇 |
1983年 | 38篇 |
1976年 | 81篇 |
1975年 | 37篇 |
1974年 | 42篇 |
1973年 | 48篇 |
1972年 | 39篇 |
1970年 | 35篇 |
1969年 | 37篇 |
1968年 | 35篇 |
1966年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
41.
42.
The compression behaviour of Ni77P23 amorphous alloy is investigated at room temperature in a diamond-anvil cell instrument using in-situ high pressure energy dispersive x-ray diffraction with a synchrotron radiation source. The equation of state is determined by fitting the experimental data according to the Birch-Murnaghan equation. It is found that the structure of Ni77P23 amorphous alloy is stable under pressures up to 30.5 GPa. Within the pressure range from zero to the experimental one, the pressure-induced structural relaxation is reversible. 相似文献
43.
We develop a production policy that controls work-in-process (WIP) levels and satisfies demand in a multistage manufacturing system with significant uncertainty in yield, rework, and demand. The problem addressed in this paper is more general than those in the literature in three aspects: (i) multiple products are processed at multiple workstations, and the capacity of each workstation is limited and shared by multiple operations; (ii) the behavior of a production policy is investigated over an infinite-time horizon, and thus the system stability can be evaluated; (iii) the representation of yield and rework uncertainty is generalized. Generalizing both the system structure and the nature of uncertainty requires a new mathematical development in the theory of infinite-horizon stochastic dynamic programming. The theoretical contributions of this paper are the existence proofs of the optimal stationary control for a stochastic dynamic programming problem and the finite covariances of WIP and production levels under the general expression of uncertainty. We develop a simple and explicit sufficient condition that guarantees the existence of both the optimal stationary control and the system stability. We describe how a production policy can be constructed for the manufacturing system based on the propositions derived. 相似文献
44.
The distortion theorem for biholomorphic convex mappings in bounded symmetric domains are considered. Especially the distortion theorem for biholomorphic convex mappings in classical domain of type IV and two exceptional domains are given. 相似文献
45.
采用固源Si分子束外延,在较高的生长温度于Si(100)衬底上制备出Si1-xGex/Si量子阱发光材料。发光样品的质量和特性通过卢瑟福背散射、X射线双晶衍射及光致发光评估。背散射实验中观察到应变超晶格的反常沟道效应;X射线分析表明材料的生长是共度的、无应力释放的,结晶完整性好。低温光致发光主要是外延合金量子阱中带边激子的无声发射和横光学声子参与的激子复合。并讨论了生长温度对量于阱发光的影响。 相似文献
46.
LI Zi-jian LIU Ning LIU Zhi-qiang LIU Shu-ying HAN Qi-de ZHANG You-yi . Institute of Vascular Medicine Peking University Third Hospital Key Laboratory of Molecular Cardiovascular Sciences Ministry of Education Beijing P. R. China . Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun P. R. China 《高等学校化学研究》2006,22(3):319-323
Introduction Adrenoceptors(ARs),membersofthesuperfami lyofG proteincoupledreceptors,arethemostimpor tantreceptorsinvolvedincardiacregulation[1].ARsin cardiacmuscleincludebothαARsandβARssub types,whileinmostmammalianheart,βARsarethe predominantsubtypes.… 相似文献
47.
We prove that RANDOM EDGE, the simplex algorithm that always chooses a random improving edge to proceed on, can take a mildly exponential number of steps in the model of abstract objective functions (introduced by Williamson Hoke [Completely unimodal numberings of a simple polytope, Discrete Appl. Math. 20 (1988) 69-81.] and by Kalai [A simple way to tell a simple polytope from its graph, J. Combin. Theory Ser. A 49(2) (1988) 381-383.] under different names). We define an abstract objective function on the n-dimensional cube for which the algorithm, started at a random vertex, needs at least exp(const·n1/3) steps with high probability. The best previous lower bound was quadratic. So in order for RANDOM EDGE to succeed in polynomial time, geometry must help. 相似文献
48.
The impact of fabrication errors on a planar waveguide demultiplexer is analyzed based on an analytical method. The explicit
expression of the transfer function taking into account phase and amplitude errors is presented in order to analyze the loss
and crosstalk of the demultiplexer caused by fabrication errors. A basic requirement for the demultiplexer with a certain
crosstalk criterion can be easily obtained. Using an etched diffraction grating demultiplexer as an example, it is shown that
the analytical results have a good agreement with results from a numerical method. 相似文献
49.
Q. Gong R. N tzel P.J. van Veldhoven T.J. Eijkemans J.H. Wolter 《Journal of Crystal Growth》2005,280(3-4):413-418
We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux. 相似文献
50.
Yafang Gong 《分析论及其应用》2006,22(4):329-338
Based on Bernstein's Theorem, Kalandia's Lemma describes the error estimate and the smoothness of the remainder under the second part of Hoelder norm when a HSlder function is approximated by its best polynomial approximation. In this paper, Kalandia's Lemma is generalized to the cases that the best polynomial is replaced by one of its four kinds of Chebyshev polynomial expansions, the error estimates of the remainder are given out under Hoeder norm or the weighted HSlder norms. 相似文献