全文获取类型
收费全文 | 118598篇 |
免费 | 6814篇 |
国内免费 | 5488篇 |
专业分类
化学 | 54168篇 |
晶体学 | 1230篇 |
力学 | 9004篇 |
综合类 | 528篇 |
数学 | 36341篇 |
物理学 | 29629篇 |
出版年
2024年 | 102篇 |
2023年 | 806篇 |
2022年 | 1082篇 |
2021年 | 1295篇 |
2020年 | 1631篇 |
2019年 | 1624篇 |
2018年 | 11435篇 |
2017年 | 11183篇 |
2016年 | 7792篇 |
2015年 | 2719篇 |
2014年 | 2535篇 |
2013年 | 3343篇 |
2012年 | 7392篇 |
2011年 | 14148篇 |
2010年 | 8066篇 |
2009年 | 8462篇 |
2008年 | 9193篇 |
2007年 | 11108篇 |
2006年 | 2577篇 |
2005年 | 3290篇 |
2004年 | 3051篇 |
2003年 | 3268篇 |
2002年 | 2319篇 |
2001年 | 1246篇 |
2000年 | 1241篇 |
1999年 | 1102篇 |
1998年 | 947篇 |
1997年 | 885篇 |
1996年 | 1014篇 |
1995年 | 747篇 |
1994年 | 648篇 |
1993年 | 569篇 |
1992年 | 490篇 |
1991年 | 432篇 |
1990年 | 346篇 |
1989年 | 294篇 |
1988年 | 237篇 |
1987年 | 218篇 |
1986年 | 207篇 |
1985年 | 183篇 |
1984年 | 138篇 |
1983年 | 106篇 |
1982年 | 87篇 |
1981年 | 79篇 |
1980年 | 68篇 |
1979年 | 62篇 |
1978年 | 53篇 |
1977年 | 43篇 |
1975年 | 47篇 |
1914年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
This paper outlines the results of a study undertaken to investigate the nature of Community Operational Research (COR). The objective was to contact the people involved in COR through a series of personal interviews and a questionnaire survey, in order to clarify many of the issues that have emerged within the profession relating to the practice of OR within the community and voluntary sector. A total of 20 individuals were successfully contacted, and they have been responsible for at least two-thirds of the projects in the COR area. 相似文献
962.
Hakim Boumaza 《Mathematical Physics, Analysis and Geometry》2007,10(2):97-122
We study a continuous matrix-valued Anderson-type model. Both leading Lyapunov exponents of this model are proved to be positive
and distinct for all energies in (2, +∞) except those in a discrete set, which leads to absence of absolutely continuous spectrum
in (2, +∞). This result is an improvement of a previous result with Stolz. The methods, based upon a result by Breuillard
and Gelander on dense subgroups in semisimple Lie groups, and a criterion by Goldsheid and Margulis, allow for singular Bernoulli
distributions.
相似文献
963.
本文考察了自体移植静脉的纵向应力应变关系。将狗股静脉移植于股动脉,在术后不同时间切取移植静脉进行纵向单轴拉伸实验。结果表明,移植静脉的应力-应变曲线比正常静脉靠近应力坐标轴并在术后逐渐向左移动。这显示了移植血管管壁组织在术后的逐渐硬化。 相似文献
964.
In 1987 the Canadian Department of National Defence enunciated the Total Force concept. The Total Force is comprised of Regular and Reserve components. The intention is to make maximum use of the Reserve to reduce defence expenditures and at the same time to ensure that military capability remains adequate to support national policy objectives. This paper discusses some of the governing parameters that affect the modelling of the composition of the Total Force and analyses the mix of regular and reserve forces. The interplay between the key factors and their marginal costs will be stressed. The models are employed to study two units in the Canadian Forces, a maintenance support unit and a tactical unit with high operational activity cost. The lessons drawn from these studies are highlighted. 相似文献
965.
Mei‐Jiau Huang 《国际流体数值方法杂志》2005,48(5):521-539
The technique of splitting a fat vortex element (with a core width larger than some threshold) into some thin ones in order to fix the convergence problem of the core‐spreading vortex methods is convenient and efficient. In particular, it keeps the method purely Lagrangian. In the present investigation, the splitting process is further viewed as part of the physical diffusion process. A new splitting method in which several weaker child vortices surround a thinned but still strong parent vortex is proposed. It is found that because of the survival of the parent vortex, the error arising from the splitting events can be largely reduced. The computational amount on the other hand is kept reasonably large by merging similar and close‐by vortices. The merging scheme designed herein not only involves fewer restrictions but also allows merging vortices of opposite rotations through the viewpoint of remeshing. The validity and accuracy of these techniques, proposed particularly for simulations undergoing lots of splitting and merging events, are verified by successfully simulating the interactions between two Burgers vortices under an external straining field. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
966.
967.
Wei‐Lie Xiao Jian‐Xin Pu Rui‐Rui Wang Liu‐Meng Yang Xiao‐Li Li Sheng‐Hong Li Rong‐Tao Li Sheng‐Xiong Huang Yong‐Tang Zheng Han‐Dong Sun 《Helvetica chimica acta》2007,90(8):1505-1513
Seven new highly oxygenated nortriterpenoids, rubriflorins D–J ( 1 – 7 ), were isolated from the leaves and stems of Schisandra rubriflora, and their structures were elucidated on the basis of extensive analysis of spectroscopic data. These new compounds feature the opening of ring A compared with related known nortriterpenoids isolated from the genus Schisandra and showed weak activity against HIV‐1. 相似文献
968.
Hua Ning LIU Wen Peng ZHANG 《数学学报(英文版)》2007,23(5):915-924
The main purpose of this paper is to use the generalized Bernoulli numbers, Gauss sums and the mean value theorems of Dirichlet L-functions between a quadratic residue and its inverse modulo p value formula. to study the asymptotic property of the difference (a prime), and to give an interesting hybrid mean 相似文献
969.
Yi‐Ming Xie Ji‐Huai Wu 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(5):m220-m221
The title compound, [HgCl2(C10H8N2)]n, features two‐dimensional [HgCl2(4,4′‐bipy)]n neutral networks (4,4′‐bipy is 4,4′‐bipyridine), based on an octahedral Hg atom coordinated by four μ2‐Cl atoms and two μ2‐4,4′‐bipy ligands in trans positions, yielding a HgCl4N2 octahedron. The structure has mmm symmetry about the Hg atoms, with most of the atoms on at least one mirror plane, but the unsubstituted C atoms of the 4,4′‐bipy rings are disordered across a mirror plane. Photoluminescent investigations reveal that the title compound displays a strong emission in the green region, which probably originates from a ligand‐to‐ligand charge‐transfer transition. 相似文献
970.
针对90t RH精炼装置,以水力学模拟研究了吹气孔直径对真空循环精炼过程中钢液流动和混合特性的影响.模型和原型的相似比为15.模型吹气孔直径分别为0.8 mm和1.2 mm.结果表明,1.2 mm孔径下环流量和主要工艺参数间的关系为Ql Q80.26Du0.69Dd0.80;在给定的吹气量和插入管内径下,增大吹气孔直径并不显著改变精炼过程中钢液流动特征,环流量随吹气孔直径的增大而略有增大,其关系式为Ql=2.40Q 0.23gD0.72u D0.88d d0.13in;相应地,混合时间稍有缩短;0.8和1.2 mm孔径下,τm与搅拌能密度的关系分别为τm ε5和τm ε0.49. 相似文献