全文获取类型
收费全文 | 114652篇 |
免费 | 5020篇 |
国内免费 | 3236篇 |
专业分类
化学 | 41213篇 |
晶体学 | 1073篇 |
力学 | 8128篇 |
综合类 | 109篇 |
数学 | 34556篇 |
物理学 | 24083篇 |
综合类 | 13746篇 |
出版年
2024年 | 154篇 |
2023年 | 642篇 |
2022年 | 1120篇 |
2021年 | 1184篇 |
2020年 | 1088篇 |
2019年 | 1027篇 |
2018年 | 11320篇 |
2017年 | 11121篇 |
2016年 | 7423篇 |
2015年 | 2152篇 |
2014年 | 1953篇 |
2013年 | 2345篇 |
2012年 | 6572篇 |
2011年 | 13604篇 |
2010年 | 8000篇 |
2009年 | 8100篇 |
2008年 | 8851篇 |
2007年 | 11030篇 |
2006年 | 1878篇 |
2005年 | 2757篇 |
2004年 | 2696篇 |
2003年 | 2886篇 |
2002年 | 1890篇 |
2001年 | 943篇 |
2000年 | 995篇 |
1999年 | 1190篇 |
1998年 | 1149篇 |
1997年 | 1105篇 |
1996年 | 1125篇 |
1995年 | 893篇 |
1994年 | 756篇 |
1993年 | 622篇 |
1992年 | 581篇 |
1991年 | 541篇 |
1990年 | 424篇 |
1989年 | 430篇 |
1988年 | 334篇 |
1987年 | 288篇 |
1986年 | 209篇 |
1985年 | 145篇 |
1984年 | 95篇 |
1983年 | 79篇 |
1982年 | 76篇 |
1981年 | 56篇 |
1980年 | 57篇 |
1979年 | 54篇 |
1914年 | 45篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
科技中介机构在科技产业化的进程中起着重要的桥梁和纽带作用,然而发展中出现的一些问题也亟待解决。文章就如何发挥政府在科技中介发展中的政策环境建设能力,加快科技中介产业化发展作了探讨。 相似文献
52.
C. Sun M.D. Thouless A.M. Waas J.A. Schroeder P.D. Zavattieri 《International Journal of Solids and Structures》2008,45(17):4725-4738
To enable the effective and reliable use of structural adhesive bonding in automotive applications, the cohesive properties of a joint need to be determined over a wide range of loading rates. In this paper, a strategy for determining these properties has been described and used to analyze a set of experimental results presented in a companion paper. In the particular system studied, a crack growing in a toughened quasi-static mode could make a catastrophic transition to a brittle mode of fracture. The cohesive parameters for both the toughened and brittle modes of crack growth were determined by comparing numerical predictions from cohesive-zone simulations to the results of experimental tests performed using double-cantilever beam specimens and tensile tests. The cohesive parameters were found to be essentially rate-independent for the toughened mode, but the toughness dropped by a factor of four upon a transition to the brittle mode. The results of wedge tests were used as an independent verification of the cohesive parameters, and to verify that the quasi-static properties remained rate-independent to very high crack velocities corresponding to conditions of low-velocity impact. The effects of friction, and the use of the wedge test to determine cohesive parameters, were also explored. 相似文献
53.
The shock structure problem is one of the classical problems of fluid mechanics and at least for non-reacting dilute gases it has been considered essentially solved. Here we present a few recent findings, to show that this is not the case. There are still new physical effects to be discovered provided that the numerical technique is general enough to not rule them out a priori. While the results have been obtained for dense fluids, some of the effects might also be observable for shocks in dilute gases. 相似文献
54.
It is shown that time-dependent temperatures in a transient, conductive system can be approximately modeled by a fractional-order
differential equation, the order of which depends on the Biot number. This approximation is particularly suitable for complex
shapes for which a first-principles approach is too difficult or computationally time-consuming. Analytical solutions of these
equations can be written in terms of the Mittag-Leffler function. The approximation is especially useful if a suitable fractional-order
controller is to be designed for the system. 相似文献
55.
This paper presents the applications of digital image correlation technique to the mesoscopic damage and fracture study of
some granular based composite materials including steelfiber reinforced concrete, sandstone and crystal-polymer composite.
The deformation fields of the composite materials resulted from stress localization were obtained by the correlation computation
of the surface images with loading steps and thus the related damage prediction and fracture parameters were evaluated. The
correlation searching could be performed either directly based on the gray levels of the digital images or from the wavelet
transform (WT) coefficients of the transform spectrum. The latter was developed by the authors and showed higher resolution
and sensitivity to the singularity detection. Because the displacement components came from the rough surfaces of the composite
materials without any coats of gratings or fringes of optical interferometry, both surface profiles and the deformation fields
of the composites were visualized which was helpful to compare each other to analyze the damage of those heterogeneous materials.
The project supported by the National Natural Science Foundation of China (10125211 and 10072002), the Scientific Committee
of Yunnan Province for the Program of Steel Fiber Reinforced Concrete, and the Institute of Chemical Materials, CAEP at Mianyang 相似文献
56.
The central zinc(II) atom in the title complex is tetrahedrally coordinated by four nitrogen atoms derived from 4‐methyl‐5‐imidazolecarboxyaldehyde ligands with Zn? N in the range 2.007(3) to 2.026(4) Å. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
57.
A method was proposed for the simultaneous determination of trace cadmium and mercury by vapor generation non-dispersive atomic fluorescence spectrometry using an intermittent flow system. The effects of the parameters on the performance were studied systematically. The parameters such as acid concentration of the reaction medium, flow rate of the carrier gas and shield gas, the observation height and the atomizer temperature, etc. which affected the sensitivity, were optimized. Ascorbic acid, cobalt ion and thiourea were used as enhancement reagents or masking agents to enhance the generation efficiency of the volatile species of Cd and Hg. The mechanisms of their effects on vapor generation were investigated. In the presence of thiourea and ascorbic acid, the influences of some coexisting elements on the determination of cadmium and mercury were investigated. The detection limits (3sigma) were 0.010 microg l(-1) for Cd and 0.019 microg l(-1) for Hg, respectively. The relative standard deviations for Cd and Hg at 1.00 microg l(-1) were 2.6% and 0.97% (n = 11), respectively. The proposed method has been satisfactorily applied to the determination of trace cadmium and mercury in Chinese herbal medicine. 相似文献
58.
59.
60.
文章分析了材料力学实验的现状和目前存在的问题,针对性地进行了实验教学改革的探索,并应用于实验教学当中,取得了良好的教学效果。 相似文献