全文获取类型
收费全文 | 78768篇 |
免费 | 4337篇 |
国内免费 | 2684篇 |
专业分类
化学 | 41498篇 |
晶体学 | 1216篇 |
力学 | 4671篇 |
综合类 | 204篇 |
数学 | 6929篇 |
物理学 | 31271篇 |
出版年
2023年 | 518篇 |
2022年 | 1232篇 |
2021年 | 1237篇 |
2020年 | 1269篇 |
2019年 | 1209篇 |
2018年 | 1240篇 |
2017年 | 1073篇 |
2016年 | 1842篇 |
2015年 | 1640篇 |
2014年 | 2122篇 |
2013年 | 3986篇 |
2012年 | 4063篇 |
2011年 | 4889篇 |
2010年 | 3325篇 |
2009年 | 3316篇 |
2008年 | 3919篇 |
2007年 | 3680篇 |
2006年 | 3509篇 |
2005年 | 3061篇 |
2004年 | 2781篇 |
2003年 | 2340篇 |
2002年 | 2194篇 |
2001年 | 3459篇 |
2000年 | 2565篇 |
1999年 | 1955篇 |
1998年 | 1441篇 |
1997年 | 1354篇 |
1996年 | 1208篇 |
1995年 | 1097篇 |
1994年 | 997篇 |
1993年 | 850篇 |
1992年 | 1188篇 |
1991年 | 1144篇 |
1990年 | 1023篇 |
1989年 | 895篇 |
1988年 | 821篇 |
1987年 | 882篇 |
1986年 | 729篇 |
1985年 | 956篇 |
1984年 | 870篇 |
1983年 | 611篇 |
1982年 | 597篇 |
1981年 | 555篇 |
1980年 | 511篇 |
1979年 | 628篇 |
1978年 | 652篇 |
1977年 | 665篇 |
1976年 | 576篇 |
1975年 | 483篇 |
1974年 | 522篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
研究了Yb3 掺杂铝氟磷酸盐 (AFP)玻璃的吸收光谱、荧光光谱 ,测量了Yb3 离子的荧光有效线宽 (Δλeff>5 5nm)以及2 F5 2 能级的荧光寿命 (τmax=2ms)及随掺杂浓度的变化 .应用倒易法计算了Yb3 的发射截面 ,其发射截面可达 0 6 6 82 3pm2 ,且激光增益系数τfσemi达 1 2 89ms.pm2 .评估了Yb3 在AFP玻璃中的激光性能 ,发现其具有较理想的激发态最小粒子数 (0 15 )、饱和抽运强度 (8 3kW cm2 )和最小抽运强度 (1 2 4 5kW cm2 )值及良好的热稳定性 .研究结果表明掺Yb3 氟磷酸盐玻璃是实现高功率超短可调谐激光器的理想增益介质 . 相似文献
82.
83.
G. Z. Ruan S. Y. Wang Y. Yamamoto S. S. Zhu 《Journal of Optimization Theory and Applications》2004,123(2):409-429
In this paper, a model of a linear multilevel programming problem with dominated objective functions (LMPPD(l)) is proposed, where multiple reactions of the lower levels do not lead to any uncertainty in the upper-level decision making. Under the assumption that the constrained set is nonempty and bounded, a necessary optimality condition is obtained. Two types of geometric properties of the solution sets are studied. It is demonstrated that the feasible set of LMPPD(l) is neither necessarily composed of faces of the constrained set nor necessarily connected. These properties are different from the existing theoretical results for linear multilevel programming problems. 相似文献
84.
85.
New heuristics for over-constrained flight to gate assignments 总被引:1,自引:0,他引:1
We consider the over-constrained Airport Gate Assignment Problem where the number of flights exceed the number of gates available, and where the objectives are to minimize the number of ungated flights and the total walking distances. The problem is formulated as a binary quadratic programming problem. We design a greedy algorithm and use a Tabu Search meta-heuristic to solve the problem. The greedy algorithm minimizes ungated flights while we devise a new neighbourhood search technique, the Interval Exchange Move, which allows us flexibility in seeking good solutions, especially when flight schedules are dense in time. Experiments conducted give good results. 相似文献
86.
W Y Hung S Kucherenko N J Samsatli N Shah 《The Journal of the Operational Research Society》2004,55(8):801-813
In this paper, we present a new modelling approach for realistic supply chain simulation. The model provides an experimental environment for informed comparison between different supply chain policies. A basic simulation model for a generic node, from which a supply chain network can be built, has been developed using an object-oriented approach. This generic model allows the incorporation of the information and physical systems and decision-making policies used by each node. The object-oriented approach gives the flexibility in specifying the supply chain configuration and operation decisions, and policies. Stochastic simulations are achieved by applying Latin Supercube Sampling to the uncertain variables in descending order of importance, which reduces the number of simulations required. We also present a case study to show that the model is applicable to a real-life situation for dynamic stochastic studies. 相似文献
87.
In 1983, a preconditioner was proposed [J. Comput. Phys. 49 (1983) 443] based on the Laplace operator for solving the discrete Helmholtz equation efficiently with CGNR. The preconditioner is especially effective for low wavenumber cases where the linear system is slightly indefinite. Laird [Preconditioned iterative solution of the 2D Helmholtz equation, First Year's Report, St. Hugh's College, Oxford, 2001] proposed a preconditioner where an extra term is added to the Laplace operator. This term is similar to the zeroth order term in the Helmholtz equation but with reversed sign. In this paper, both approaches are further generalized to a new class of preconditioners, the so-called “shifted Laplace” preconditioners of the form Δφ−k2φ with
. Numerical experiments for various wavenumbers indicate the effectiveness of the preconditioner. The preconditioner is evaluated in combination with GMRES, Bi-CGSTAB, and CGNR. 相似文献
88.
89.
90.
Y. A. Coutinho J. A. Martins Simões C. M. Porto 《Zeitschrift fur Physik C Particles and Fields》1997,73(4):705-709
In this paper we investigate the effects of heavy Majorana neutrinos in the reaction e + e ? → W + W ?. We consider neutrino masses in the 1–10 TeV region. We show that at LEP II and NLC energies it is possible to use this processes to verify indirect evidence of heavy neutral particles with mixing angles of the order sin2 α = 0.01. We discuss the unitarity restrictions that can be obtained for vector singlet and fermion-mirror-fermion models. 相似文献