全文获取类型
收费全文 | 442851篇 |
免费 | 28283篇 |
国内免费 | 18255篇 |
专业分类
化学 | 248083篇 |
晶体学 | 6526篇 |
力学 | 19580篇 |
综合类 | 32篇 |
数学 | 53379篇 |
物理学 | 161789篇 |
出版年
2020年 | 3566篇 |
2019年 | 4197篇 |
2018年 | 4736篇 |
2017年 | 5270篇 |
2016年 | 7883篇 |
2015年 | 6469篇 |
2014年 | 8009篇 |
2013年 | 21014篇 |
2012年 | 16305篇 |
2011年 | 20644篇 |
2010年 | 17983篇 |
2009年 | 17770篇 |
2008年 | 16881篇 |
2007年 | 16503篇 |
2006年 | 15385篇 |
2005年 | 14907篇 |
2004年 | 14315篇 |
2003年 | 12472篇 |
2002年 | 12077篇 |
2001年 | 13470篇 |
2000年 | 10189篇 |
1999年 | 8679篇 |
1998年 | 7331篇 |
1997年 | 7147篇 |
1996年 | 7225篇 |
1995年 | 7321篇 |
1994年 | 7151篇 |
1993年 | 7276篇 |
1992年 | 7261篇 |
1991年 | 6857篇 |
1990年 | 6278篇 |
1989年 | 6331篇 |
1988年 | 6331篇 |
1987年 | 5691篇 |
1986年 | 5465篇 |
1985年 | 6943篇 |
1984年 | 7202篇 |
1983年 | 5674篇 |
1982年 | 6614篇 |
1981年 | 6250篇 |
1980年 | 6231篇 |
1979年 | 6027篇 |
1978年 | 6394篇 |
1977年 | 6194篇 |
1976年 | 6657篇 |
1975年 | 5521篇 |
1974年 | 5508篇 |
1973年 | 6214篇 |
1972年 | 3951篇 |
1971年 | 3110篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
双掺(Tm3+,Tb3+)LiYF4激光器1.5 μm波长激光阈值分析 总被引:1,自引:0,他引:1
由速率方程推出了双掺(Tm^3 ,Tb^3 )离子准四能级系统的激光阈值解析式,讨论了Tm^3 和Tb^3 离子之间的相互作用。分析了1.5μm波长附近的激光阈值和Tm^3 、Tb^3 离子的掺杂原子数分数及晶体长度的关系。结果表明,对于对应Tm^3 离子^3H4→^3F4跃迁的约1.5μm波长的激光,激活离子Tm^3 的掺杂原子数分数过大时,交叉弛豫作用将使系统阈值迅速增加。Tb^3 离子的加入,一方面能抽空激光下能级,起到降低阈值的作用;另一方面亦减少了激光上能级的寿命,使阈值升高。故Tb^3 离子有最佳掺杂原子数分数。对于Tm原子数分数为y=0.01的Tm:LiYF4晶体,Tb^3 离子的最佳掺杂原子数分数为0.002左右,同时表明,激光阈值与晶体长度有关。最佳晶体长度与Tm^3 、Tb^3 离子的掺杂原子数分数以及晶体的衍射损耗和吸收损耗有关。 相似文献
142.
R. Notoya 《Russian Journal of Electrochemistry》2002,38(3):255-260
It is found that the density (number of the component ion or molecule in 1 cm3) of a molten salt could be expressed as a function of the product of the electron donating and accepting properties of the component anion and cation. This equation is very useful for estimating the density of any molten salt at high temperature with the standard deviation accurately. 相似文献
143.
144.
Demixing during film casting of blends of polystyrene, polymethylmethacrylate, and a symmetric diblock copolymer of styrene and methylmethacrylate is discussed. The concentration fluctuations in the homogeneous solutions were calculated in mean field approximation. The structures in the homogeneous and demixed solutions and in the dry films were measured by small-angle x-ray scattering, and the morphologies of the dry films were characterized by transmission electron microscopy. The structure of the dry blends is evidently already pre-formed in solution. 相似文献
145.
Investigated is the dependency of the crack tip opening displacement (CTOD) on the local microstructure of the heat affected zone in high-strength low-alloy (HSLA) microalloyed steel. Since the initiation of the crack tip location could not be controlled in fatigue, any possible correlation between heat iput in welding and fracture toughness could be smeared. Modified CTOD data are defined; they show that the fracture resistance of the weld joint decreased as the heat input increased. 相似文献
146.
147.
An analysis is made of the conditions of formation and the stability region of static-soliton-type magnetic nonuniformities
in a (001)-crystal plate with combined anisotropy. It is shown that if demagnetizing fields in the plate are taken into account,
static solitons can localize at certain defects for appropriate parameters of the material. The soliton stability region was
found to be bounded by two extreme values of the material parameters, namely, those at which the soliton is unstable against
collapse and at which it expands. It was also found that the soliton stability region differs considerably from that predicted
theoretically in the model disregarding the finite size of the sample and the presence of defects.
Fiz. Tverd. Tela (St. Petersburg) 40, 1498–1502 (August 1998) 相似文献
148.
ON A PAIR OF NON-ISOMETRIC ISOSPECTRAL DOMAINS WITH FRACTAL BOUNDARIES AND THE WEYL-BERRY CONJECTURE
ONAPAIROFNONISOMETRICISOSPECTRALDOMAINSWITHFRACTALBOUNDARIESANDTHEWEYLBERRYCONJECTURESLEEMAN,B.D.CHENHUAManuscriptrec... 相似文献
149.
The barotropic compressible Navier–Stokes equations in an unbounded domain are studied. We prove the unique existence of the solution (u, p) of the system (1.1) in the Sobolev spaceHk + 3 × Hk + 2provided that the derivatives of the data of the problem are sufficiently small, wherek ≥ 0 is any integer. The proof follows from an analysis of the linearized problem, the solvability of the continuity equation, and the Schauder fixed point theory. Similar smoothness results are obtained for a linearized form of (1.1). 相似文献
150.
P. Seelig A. Dax S. Faber M. Gerlach G. Huber T. Kühl D. Marx P. Merz W. Quint F. Schmitt H. Winter M. Würtz 《Hyperfine Interactions》1998,114(1-4):135-139
The investigation of the 1s HFS provides a good possibility for testing QED effects in a combination of a strong electric
and magnetic field. Here, we report about the laserspectroscopic measurements of the ground state hyperfine splitting in 207Pb81+. To handle this M1-transition in the infrared optical regime with its long lifetime, we developed a new detection technique
using a bunched ion beam. For the observation of fluorescence light, a new mirror system is adapted to the emission characteristics
from an ion beam at relativistic velocities.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献