全文获取类型
收费全文 | 14478篇 |
免费 | 2294篇 |
国内免费 | 2393篇 |
专业分类
化学 | 7933篇 |
晶体学 | 123篇 |
力学 | 632篇 |
综合类 | 152篇 |
数学 | 1328篇 |
物理学 | 3813篇 |
无线电 | 5184篇 |
出版年
2024年 | 28篇 |
2023年 | 362篇 |
2022年 | 488篇 |
2021年 | 621篇 |
2020年 | 642篇 |
2019年 | 621篇 |
2018年 | 477篇 |
2017年 | 555篇 |
2016年 | 717篇 |
2015年 | 775篇 |
2014年 | 845篇 |
2013年 | 1084篇 |
2012年 | 1079篇 |
2011年 | 1178篇 |
2010年 | 906篇 |
2009年 | 918篇 |
2008年 | 998篇 |
2007年 | 834篇 |
2006年 | 809篇 |
2005年 | 770篇 |
2004年 | 565篇 |
2003年 | 513篇 |
2002年 | 547篇 |
2001年 | 476篇 |
2000年 | 416篇 |
1999年 | 312篇 |
1998年 | 233篇 |
1997年 | 168篇 |
1996年 | 203篇 |
1995年 | 144篇 |
1994年 | 156篇 |
1993年 | 124篇 |
1992年 | 97篇 |
1991年 | 109篇 |
1990年 | 96篇 |
1989年 | 55篇 |
1988年 | 53篇 |
1987年 | 33篇 |
1986年 | 37篇 |
1985年 | 41篇 |
1984年 | 20篇 |
1983年 | 22篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1971年 | 1篇 |
1957年 | 3篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 531 毫秒
131.
The Uniform Geometrical Theory of Diffraction (UTD) has been successfully used in pattern analysis of airborne dipole antenna on the A-5 fighter in high-frequency region. In the analysis, several canonical problems are investigated first, then the complicated problem is calculated with compiling these canonical problems. The numerical results are compared with the experimental data, and good agreement has been seen. The effectiveness and practicality of the present analysis are thus verified. 相似文献
132.
The mode coupling in a helical fibre with a circular cross-section undergoing perturbations is investigated in the Serret-Frenet frame. The coupled-mode equations have been derived and are compared with the conventional ones. Applying the coupled-mode equations to a helical fibre with an elliptical core deformation, we obtained, for the first time, a rigorous analysis of the transmission characteristics of the fibre. The numerical examples show that the mode ellipticity and elliptical birefringence increase as the core ellipticity increases and also vary as the pitch or the core offset change. 相似文献
133.
对一类遗传程序设计问题,提出了简单等长的分布式染色体,取代复杂变长的树结构染色体,使演化算子操作更方便,个体复杂度得到有效控制.用该染色体形式解决割草问题,分析了分布式染色体带来的一些性质,并给出了很好的计算结果. 相似文献
134.
It is demonstrated that many novel vacuum effects will be caused if an anisotropic electromagnetic environment, which can break the universal symmetry of vacuum, is achieved. It is thus possible for the momentum to be transferred from the vacuum zero-point field to the anisotropic electromagnetic media. In addition to the effect considered by Feigel more recently [A. Feigel, Phys. Rev. Lett. 92 (2004) 020404], there may exist another vacuum-fluctuation contribution to the momentum of a medium. Such an effect has a relativistic origin (resulting from the relativistic transformation of the optical constants), which, however, was not taken into account by Feigel. 相似文献
135.
136.
A new variational framework of geometric active contours to track multiple non-rigid moving objects in the clutter background is presented. Incorporating motion edge information, it consists of motion detection and tracking stages. The results of experiments are promising and demonstrate the validity of the proposed framework 相似文献
137.
A double hydrophilic block copolymer composed of poly(acrylic acid) (PAA) and poly(4‐vinyl pyridine) (P4VP) was obtained through hydrolysis of diblock copolymer of poly(tert‐butyl acrylate) (PtBA) and P4VP synthesized using atom transfer radical polymerization. Water‐soluble micelles with PAA core and P4VP corona were observed at low (acidic) pH, while micelles with P4VP core and PAA corona were formed at high (basic) pH. Two metalloporphyrins, zinc tetraphenylporphyrin (ZnTPP) and cobalt tetraphenylporphyrin (CoTPP), were used as model compounds to investigate the encapsulation of hydrophobic molecules by both types of micelles. UV–vis spectroscopic measurements indicate that micelles with P4VP core are able to entrap more ZnTPP and CoTPP as a result of the axial coordination between the transition metals and the pyridine groups. The study found that metalloporphyrins encapsulated by the micelles with PAA core could be released on pH increase, while those entrapped by the micelles with P4VP core could be released on pH decrease. This behavior originates from the two‐way pH change‐induced disruption of PAA‐b‐P4VP micelles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1734–1744, 2006 相似文献
138.
The difficulty suffered in optimization-based algorithms for the solution of nonlinear equations lies in that the traditional
methods for solving the optimization problem have been mainly concerned with finding a stationary point or a local minimizer
of the underlying optimization problem, which is not necessarily a solution of the equations. One method to overcome this
difficulty is the Lagrangian globalization (LG for simplicity) method. This paper extends the LG method to nonsmooth equations
with bound constraints. The absolute system of equations is introduced. A so-called Projected Generalized-Gradient Direction (PGGD) is constructed and proved to be a descent direction of the reformulated nonsmooth optimization problem. This projected
approach keeps the feasibility of the iterates. The convergence of the new algorithm is established by specializing the PGGD.
Numerical tests are given.
This author's work was done when she was visiting The Hong Kong Polytechnic University.
His work is also supported by the Research Grant Council of Hong Kong. 相似文献
139.
航天产品可靠性信息的来源多样,可靠性信息的时间性、随机性、有价性、时效性和可追溯性等特征,决定了可靠性数据收集应满足真实性、连续性、完整性等基本要求。同时,对多源信息的收集应遵循先进行需求分析,再确定数据收集点并制定数据收集表格的程度进行。给出了信息的适用性准则和信息分类的三维模型。最后结合具体型号航天产品的可靠性工作的要求,给出了可靠性信息数据库系统的整体方案设计。 相似文献
140.
In the discrete-time Sparre Andersen risk model with investments and dividend payments, the company controls the dividend payments and the proportions of venture investments in order to maximize the cumulative expected discounted dividends prior to ruin. The paper gets the algorithm of the optimal dividend strategy by analyzing a Hamilton-Jacobi-Bellman equation and transforming the value function. Furthermore, the existence of the optimal solution of the transformation function is proved by using compression mapping and fixed point principle. In order to make the calculation easier, this paper also proposes an
innovative random simulation method for the optimal strategy, and proves that the simulation result is the consistent estimate of the real value. Finally, the random simulation method in the Matlab is used for numerical analysis in an example, which shows the innovative simulation method is a very good and helpful method for making dividend payment and investment decisions. 相似文献