全文获取类型
收费全文 | 3678篇 |
免费 | 293篇 |
国内免费 | 217篇 |
专业分类
化学 | 2722篇 |
晶体学 | 39篇 |
力学 | 115篇 |
综合类 | 28篇 |
数学 | 770篇 |
物理学 | 514篇 |
出版年
2023年 | 35篇 |
2022年 | 44篇 |
2021年 | 47篇 |
2020年 | 116篇 |
2019年 | 91篇 |
2018年 | 96篇 |
2017年 | 80篇 |
2016年 | 134篇 |
2015年 | 109篇 |
2014年 | 120篇 |
2013年 | 211篇 |
2012年 | 128篇 |
2011年 | 167篇 |
2010年 | 169篇 |
2009年 | 165篇 |
2008年 | 148篇 |
2007年 | 177篇 |
2006年 | 152篇 |
2005年 | 196篇 |
2004年 | 181篇 |
2003年 | 140篇 |
2002年 | 133篇 |
2001年 | 99篇 |
2000年 | 136篇 |
1999年 | 101篇 |
1998年 | 91篇 |
1997年 | 88篇 |
1996年 | 75篇 |
1995年 | 86篇 |
1994年 | 65篇 |
1993年 | 47篇 |
1992年 | 50篇 |
1991年 | 48篇 |
1990年 | 46篇 |
1989年 | 36篇 |
1988年 | 42篇 |
1987年 | 40篇 |
1986年 | 36篇 |
1985年 | 48篇 |
1984年 | 37篇 |
1983年 | 29篇 |
1982年 | 33篇 |
1981年 | 32篇 |
1980年 | 24篇 |
1979年 | 29篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有4188条查询结果,搜索用时 31 毫秒
11.
12.
Pulsed laser deposition with a Nd:YAG laser was used to grow thin films from a pre-synthesized Ti3SiC2 MAX-phase formulated ablation target on oxidized Si(1 0 0) and MgO(1 0 0) substrates. The depositions were carried out in a substrate temperature range from 300 to 900 K, and the pressure in the deposition chamber ranged from vacuum (10−5 Pa) to 0.05 Pa Argon background pressure. The properties of the films have been investigated by Rutherford backscattering spectrometry for film thickness and stoichiometric composition and X-ray diffraction for the crystallinity of the films. The silicon content of the films varied with the energy density of the laser beam. To suppress especially the silicon re-sputtering from the substrate, the energy of the incoming particles must be below a threshold of 20 eV. Therefore, the energy density of the laser beam must not be too high. At constant deposition energy density the film thickness depends strongly on the background pressure. The X-ray diffraction measurements show patterns that are typical of amorphous films, i.e. no Ti3SiC2 related reflections were found. Only a very weak TiC(2 0 0) reflection was seen, indicating the presence of a small amount of crystalline TiC. 相似文献
13.
We isolate several classes of stationary sets of [k]ωand investigate implications among them. Under a large cardinal assumption, we prove a structure theorem for stationary sets. 相似文献
14.
张国胜 《数学的实践与认识》2006,36(7):356-367
设W(1n)W2(n)…W(nn)是严平稳随即序列{Xn,n 1}的前几个变量的顺序统计量,Nn Mn是两正整值r.v,n 1.在较弱一些的混合条件下,本文给出了W(NMn)n标准化后的极限分布,从而使一些提法更趋一般化,主要结果推进了Rao(1984)和张国胜(1993)的工作.由于混合条件的减弱,使得对m-相关和正态平稳过程的具体应用讨论更加充分. 相似文献
15.
B. V. Pal’tsev I. I. Chechel’ 《Computational Mathematics and Mathematical Physics》2006,46(5):820-847
The convergence rate of a fast-converging second-order accurate iterative method with splitting of boundary conditions constructed by the authors for solving an axisymmetric Dirichlet boundary value problem for the Stokes system in a spherical gap is studied numerically. For R/r exceeding about 30, where r and R are the radii of the inner and outer boundary spheres, it is established that the convergence rate of the method is lower (and considerably lower for large R/r) than the convergence rate of its differential version. For this reason, a really simpler, more slowly converging modification of the original method is constructed on the differential level and a finite-element implementation of this modification is built. Numerical experiments have revealed that this modification has the same convergence rate as its differential counterpart for R/r of up to 5 × 103. When the multigrid method is used to solve the split and auxiliary boundary value problems arising at iterations, the modification is more efficient than the original method starting from R/r ~ 30 and is considerably more efficient for large values of R/r. It is also established that the convergence rates of both methods depend little on the stretching coefficient η of circularly rectangular mesh cells in a range of η that is well sufficient for effective use of the multigrid method for arbitrary values of R/r smaller than ~ 5 × 103. 相似文献
16.
Materials from the Mn(0.5−x)CaxTi2(PO4)3 (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, 31P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R
space group. Mn2+/Ca2+ ions occupy only the M1 sites in the Ti2(PO4)3 framework. The divalent cations are ordered in one of two M1 sites, except for the Mn0.50Ti2(PO4)3 phase, where a small departure from the ideal order is observed by XRD and 31P MAS NMR. The electrochemical behaviour of Mn0.50Ti2(PO4)3 and Mn(0.5−x)CaxTi2(PO4)3 phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti2(PO4)3 framework. In the 0≤y≤2 range, the OCV curves of Li//LiyMn0.50Ti2(PO4)3 cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li(1+y)Ti2(PO4)3 and Li//LiyMn0.50Ti2(PO4)3 shows that the intercalation occurs first in the unoccupied M1 site of Mn0.50Ti2(PO4)3 at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn0.50Ti2(PO4)3 on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn0.50Ti2(PO4)3, the stability of Mn2+ either in an octahedral or tetrahedral environment facilitates cationic migration. 相似文献
17.
Yan Shun MA Qing Hua MENG Hong Yu SHI Yuan De LONG Tian Bao HUANG 《中国化学快报》2006,17(6):807-808
Dencichine (β-N-oxalyl-L-α, β-diaminopropionic acid, L-ODAP), a nonprotein amino acid, is a haemostatic agent present in important Chinese medicinal herbs such as Panax notoginseng, as well as other Panax species1. It was first isolated from the seeds … 相似文献
18.
19.
We establish solvability of the boundary-value problems describing stationary and periodic flows of viscoplastic media. In the case of stationary flows we study the question of convergence of the Galerkin method. For the problem of periodic flows we prove a version of the second Bogolyubov theorem. 相似文献
20.
Kenji Handa 《Journal of statistical physics》1996,83(3-4):555-571
Entropy production per site in a (nonreversible) spin-flip process is studied. We give it a useful expression, from which a property stronger than affinity of the entropy production per site follows. Furthermore, quasi-invariance of nonequilibrium measures in the spin-flip processes is discussed via entropy production. 相似文献