全文获取类型
收费全文 | 7858篇 |
免费 | 961篇 |
国内免费 | 746篇 |
专业分类
化学 | 1976篇 |
晶体学 | 18篇 |
力学 | 627篇 |
综合类 | 208篇 |
数学 | 2954篇 |
物理学 | 3782篇 |
出版年
2024年 | 13篇 |
2023年 | 65篇 |
2022年 | 142篇 |
2021年 | 193篇 |
2020年 | 161篇 |
2019年 | 200篇 |
2018年 | 184篇 |
2017年 | 280篇 |
2016年 | 339篇 |
2015年 | 289篇 |
2014年 | 447篇 |
2013年 | 549篇 |
2012年 | 452篇 |
2011年 | 474篇 |
2010年 | 337篇 |
2009年 | 480篇 |
2008年 | 514篇 |
2007年 | 510篇 |
2006年 | 453篇 |
2005年 | 397篇 |
2004年 | 374篇 |
2003年 | 330篇 |
2002年 | 316篇 |
2001年 | 256篇 |
2000年 | 261篇 |
1999年 | 196篇 |
1998年 | 204篇 |
1997年 | 171篇 |
1996年 | 132篇 |
1995年 | 86篇 |
1994年 | 98篇 |
1993年 | 63篇 |
1992年 | 59篇 |
1991年 | 60篇 |
1990年 | 43篇 |
1989年 | 53篇 |
1988年 | 38篇 |
1987年 | 37篇 |
1986年 | 33篇 |
1985年 | 52篇 |
1984年 | 30篇 |
1983年 | 10篇 |
1982年 | 32篇 |
1981年 | 34篇 |
1980年 | 21篇 |
1979年 | 14篇 |
1978年 | 14篇 |
1977年 | 17篇 |
1976年 | 21篇 |
1973年 | 9篇 |
排序方式: 共有9565条查询结果,搜索用时 15 毫秒
11.
根据生产任务选择加工设备进行制造资源重组是实现可重构制造系统的关键问题之一,由于设备的选择涉及到多种因素,既有定量指标,又有定性指标,传统的依靠人工经验的方法显得力不从心。本文首先结合实际情况,提出了一套设备选择评价体系,通过对模糊判断矩阵采用最小对数二乘法确定各评价因素的权重系数,针对定性指标和定量指标采用不同的方法确定其性能指标值,通过模糊积分对评判指标进行综合评判,最后进行了实例研究。所提出的方法有效地简化了决策过程,为可重构制造系统设备选择提供了一套行之有效的方法。 相似文献
12.
第五讲新型光纤水听器和矢量水听器 总被引:2,自引:0,他引:2
光纤水听器和矢量水听器作为当前水声研究领域最具有代表性的两大技术倍受业界关注。光纤水听器的重要贡献在于,从一个全新的角度出发,试图解决传统的水声传感和声纳数据传输一体化设计和实现的一系列问题,这有助于改善声纳系统的可靠性,并且有可能降低其制造、使用和维护的总成本。矢量水听器则由于其特有的指向性和矢量一相位处理方法,在低频和甚低频水声微弱目标探测方面具有潜在的优势.经过不懈的努力,光纤水听器和矢量水听器系统已经从实验室逐渐进入到工程应用阶段.这些对未来声纳系统的发展会产生相当重要的影响.文章尝试从声纳设计的角度对这两者的技术现状进行简要综述,包括它们各自的物理基础、工作原理、关键技术和应用领域. 相似文献
13.
We show that the difference between the Schrödinger uncertainty relations (UR) and the Heisenberg UR is fundamental. We propose a modified version of stochastic mechanics that allows clearly demonstrating that the contributions from the anticommutator and the commutator to the Schrödinger UR are equally important. A classification of quantum states minimizing the Schrödinger UR at an arbitrary instant is proposed. We show that the correlation of the coordinate and momentum fluctuations in such correlated-coherent states (CCS) is largely determined by the contributions from not only the commutator but also the anticommutator of the corresponding operators. We demonstrate that the character of this correlation changes qualitatively in time from the antiphase correlation typical for the Heisenberg UR to the inphase correlation for which the contribution from the anticommutator is decisive. We comparatively analyze properties of a free microparticle and a quantum oscillator in CCS and show that the CCS correspond to traveling-standing de Broglie waves in both models. 相似文献
14.
Mathematical economics has a long history and covers many interdisciplinary areas between mathematics and economics. At its center lies the theory of market equilibrium. The purpose of this expository article is to introduce mathematicians to price decentralization in general equilibrium theory. In particular, it concentrates on the role of positivity in the theory of convex economic analysis and the role of normal cones in the theory of non-convex economies. 相似文献
15.
Matroid bundles, introduced by MacPherson, are combinatorial analogues of real vector bundles. This paper sets up the foundations
of matroid bundles. It defines a natural transformation from isomorphism classes of real vector bundles to isomorphism classes
of matroid bundles. It then gives a transformation from matroid bundles to spherical quasifibrations, by showing that the
geometric realization of a matroid bundle is a spherical quasifibration. The poset of oriented matroids of a fixed rank classifies
matroid bundles, and the above transformations give a splitting from topology to combinatorics back to topology. A consequence
is that the mod 2 cohomology of the poset of rank k oriented matroids (this poset classifies matroid bundles) contains the free polynomial ring on the first k Stiefel-Whitney classes. 相似文献
16.
用能量法求多自由度振动系统的角频率 总被引:2,自引:1,他引:1
利用简谐振动能量方程,通过分析振幅矢量的关系,用能量法求多自由度振动系统的角频率或简正振动频率。 相似文献
17.
The angular variations of the five-fold differential cross section obtained by using different wave functions of helium are
compared with experimental data. It is found that in the coplanar geometry two kinematical arrangements, (i) equal energy
sharing between the two ejected electrons with one of them ejected along the momentum transfer direction and the other along
varying direction and (ii) the Bethe ridge condition with fixed sum of ejected electron energies and varying angle between
them, are very sensitive to e-e correlations contained in the target wave function. This comparison has been used to show
that open-shell class of wave functions better incorporate e-e correlations than the closed-shell class. 相似文献
18.
19.
《Magnetic resonance in chemistry : MRC》2003,41(6):417-430
A systematic study of the one‐bond and long‐range J(C,C), J(C,H) and J(H,H) in the series of nine bicycloalkanes was performed at the SOPPA level with special emphasis on the coupling transmission mechanisms at bridgeheads. Many unknown couplings were predicted with high reliability. Further refinement of SOPPA computational scheme adjusted for better performance was carried out using bicyclo[1.1.1]pentane as a benchmark to investigate the influence of geometry, basis set and electronic correlation. The calculations performed demonstrated that classical ab initio SOPPA applied with the locally dense Dunning's sets augmented with inner core s‐functions used for coupled carbons and Sauer's sets augmented with tight s‐functions used for coupled hydrogens performs perfectly well in reproducing experimental values of different types of coupling constants (the estimated reliability is ca 1–2 Hz) in relatively large organic molecules of up to 11 carbon atoms. Additive coupling increments were derived for J(C,C), J(C,H) and J(H,H) based on the calculated values of coupling constants within SOPPA in the model bicycloalkanes, in reasonably good agreement with the known values obtained earlier on pure empirical grounds. Most of the bridgehead couplings in all but one bicycloalkane appeared to be essentially additive within ca 2–3 Hz while bicyclo[1.1.1]pentane demonstrated dramatic non‐additivity of ?14.5 Hz for J(C,C), +16.6 Hz for J(H,H) and ?5.5 Hz for J(C,H), in line with previous findings. Non‐additivity effects in the latter compound established at the SOPPA level should be attributed to the through‐space non‐bonded interactions at bridgeheads due to the essential overlapping of the bridgehead rear lobes which provides an additional and effective non‐bonding coupling path for the bridgehead carbons and their protons in the bicyclopentane framework. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
20.
A sample of 1233 events is used to study the general characteristics of {}^{16}O-Em interactions at 4.5 A GeV/c. Multiplicity and angular distributions of slow particles and correlations among them are discussed. The present data are compared with the corresponding results from the interactions of other projectiles at the same energy and also the same projectile at different energies. The results indicate that black particle production is independent of the energy and mass of the projectile, but for grey particles it is dependent on the mass of projectile. 相似文献