全文获取类型
收费全文 | 24978篇 |
免费 | 4609篇 |
国内免费 | 2754篇 |
专业分类
化学 | 17894篇 |
晶体学 | 348篇 |
力学 | 1432篇 |
综合类 | 134篇 |
数学 | 2733篇 |
物理学 | 9800篇 |
出版年
2024年 | 94篇 |
2023年 | 598篇 |
2022年 | 1063篇 |
2021年 | 1021篇 |
2020年 | 1109篇 |
2019年 | 1109篇 |
2018年 | 1028篇 |
2017年 | 836篇 |
2016年 | 1302篇 |
2015年 | 1246篇 |
2014年 | 1458篇 |
2013年 | 1877篇 |
2012年 | 2282篇 |
2011年 | 2274篇 |
2010年 | 1497篇 |
2009年 | 1433篇 |
2008年 | 1602篇 |
2007年 | 1402篇 |
2006年 | 1294篇 |
2005年 | 1074篇 |
2004年 | 789篇 |
2003年 | 607篇 |
2002年 | 616篇 |
2001年 | 490篇 |
2000年 | 420篇 |
1999年 | 507篇 |
1998年 | 359篇 |
1997年 | 366篇 |
1996年 | 374篇 |
1995年 | 310篇 |
1994年 | 264篇 |
1993年 | 262篇 |
1992年 | 180篇 |
1991年 | 186篇 |
1990年 | 159篇 |
1989年 | 117篇 |
1988年 | 102篇 |
1987年 | 83篇 |
1986年 | 79篇 |
1985年 | 64篇 |
1984年 | 66篇 |
1983年 | 44篇 |
1982年 | 37篇 |
1981年 | 25篇 |
1980年 | 27篇 |
1978年 | 15篇 |
1976年 | 17篇 |
1975年 | 15篇 |
1973年 | 18篇 |
1968年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
51.
A one-pot procedure for the conversion of mono-substituted arenes and heteroarenes into the ortho-cyano derivatives was achieved through directed lithiation followed by electrophilic cyanation with phenyl cyanate. This reaction method proved to be applicable to halogen-lithium exchanged intermediates, so especially useful for the synthesis of benzonitriles. The scope of the reaction sequence was explored using a number of substrates. 相似文献
52.
The properties of pulsed laser vapor doping on p-Si(1 0 0) with a KrF (248 nm) excimer pulsed laser (248 nm) and BCl3 gas are reported in this paper. The doped samples are characterized by the resistance measured using a four-probe method, since the sheet resistance changes with the carrier concentration of the sample. The doping effects with the variation of laser energy density, pulse number, and the pressure of BCl3 were investigated in terms of the sheet resistance. In this way, the optimized parameters were obtained and used for the positive heavy doping on p-Si(1 0 0) and p-Si(1 1 1). Then, using a square mesh under the above conditions, an image doping was completed. Finally, the metal–semiconductor Ohmic contacts were realized by plating Ag and Cu films on the doped surface. 相似文献
53.
Lower-dimensional linear complementarity problem approaches to the solution of a bi-obstacle problem
A globally convergent Broyden-like method for solving a bi-obstacle problem is proposed based on its equivalent lower-dimensional linear complementarity problem. A suitable line search technique is introduced here. The global and superlinear convergence of the method is verified under appropriate assumptions. 相似文献
54.
55.
1689-nm diode lasers used in medical apparatus have been fabricated and characterized. The lasers had pnpn InP current confinement structure, and the active region consisted of 5 pairs of InGaAs quantum wells and InGaAsP barriers. 相似文献
56.
本文探讨了非平稳NA序列部分和的精确渐近性.以前的文献在讨论NA序列此类极限性质时都附加有强平稳条件的限制,这必然会给一些问题的研究带来不便.周知,非平稳NA序列在许多实际问题中是大量存在的,所以解除强平稳条件的束缚具有较大的理论和实际意义,这正是本文的目的之所在,同时本文也将已有的一些结果包含成为特殊情形. 相似文献
57.
For one‐dimensional simple symmetric random walk, the Hausdorff and packing dimensions of sets of sample paths with prescribed rate of returns to the origin are determined. This gives a multifractal decomposition of the underlying sample space. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
58.
59.
To improve aircraft crash safety, conditions critical to occupants survival during a crash must be known. In view of the importance of this problem, studies of post-crash dynamic behavior of victims are necessary in order to reduce severe injuries. In this study, crash dynamics program SOM-LA/TA, incorporating a dynamic model of the human body with a finite element model of the seat structure was used. Modifications were performed in the program for reconstruction of an occupant's head impact with the interior walls or bulkhead. A viscoelastic-type contact force model was used to represent the compliance characteristics of the bulkhead. Correlated studies of analytical simulations with impact sled test results were accomplished. A parametric study of the coefficients in the contact force model was then performed in order to obtain the correlations between the coefficients and the Head Injury Criteria. A measure of optimal values for the bulkhead compliance and displacement requirements was thus achieved in order to keep the possibility of a head injury as little as possible. This information could in turn be used in the selection of suitable materials for the bulkhead, instrument panel, or interior walls of an aircraft. 相似文献
60.
Ji-pu Ma Tseng Yaun Rong Functional Analysis Research Center Harbin Normal University Harbin China 《中国科学A辑(英文版)》2007,50(9):1233-1239
Let E, F be two Banach spaces, and B(E, F), Φ(E, F), SΦ(E, F) and R(E,F) be the bounded linear, Fredholm, semi-Frdholm and finite rank operators from E into F, respectively. In this paper, using the continuity characteristics of generalized inverses of operators under small perturbations, we prove the following result Let ∑ be any one of the following sets {T ∈ Φ(E, F) IndexT =const, and dim N(T) = const.}, {T ∈ SΦ(E, F) either dim N(T) = const. < ∞ or codim R(T) = const.< ∞} and {T ∈ R(E, F) RankT =const.<∞}. Then ∑ is a smooth submanifold of B(E, F) with the tangent space TA∑ = {B ∈ B(E,F) BN(A) (∪) R(A)} for any A ∈ ∑. The result is available for the further application to Thom's famous results on the transversility and the study of the infinite dimensional geometry. 相似文献