全文获取类型
收费全文 | 2659篇 |
免费 | 222篇 |
国内免费 | 474篇 |
专业分类
化学 | 2359篇 |
晶体学 | 15篇 |
力学 | 145篇 |
综合类 | 61篇 |
数学 | 230篇 |
物理学 | 545篇 |
出版年
2024年 | 7篇 |
2023年 | 36篇 |
2022年 | 169篇 |
2021年 | 131篇 |
2020年 | 158篇 |
2019年 | 95篇 |
2018年 | 96篇 |
2017年 | 132篇 |
2016年 | 146篇 |
2015年 | 103篇 |
2014年 | 139篇 |
2013年 | 256篇 |
2012年 | 168篇 |
2011年 | 142篇 |
2010年 | 119篇 |
2009年 | 149篇 |
2008年 | 134篇 |
2007年 | 167篇 |
2006年 | 145篇 |
2005年 | 114篇 |
2004年 | 103篇 |
2003年 | 92篇 |
2002年 | 65篇 |
2001年 | 54篇 |
2000年 | 59篇 |
1999年 | 49篇 |
1998年 | 46篇 |
1997年 | 35篇 |
1996年 | 32篇 |
1995年 | 33篇 |
1994年 | 38篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 13篇 |
1990年 | 19篇 |
1989年 | 5篇 |
1988年 | 15篇 |
1987年 | 11篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有3355条查询结果,搜索用时 296 毫秒
11.
12.
There is a high correlation between molecular surface area (TSA) of triorganotin and triorganolead compounds and their toxicity towards a bacterium (Escherichia coli) and an alga (Selenastrum capricornutum). Parallel attempts to correlate other Group IVA organometals incorporating silicon or germanium were unsuccessful. It was further demonstrated, however, that a high correlation was obtainable between certain series of compounds with the same organic substituent but different metal centers involving all Group IVA elements. In both instances, the inability to obtain a quantitative structure-activity relationship (QSAR) for all systems studied appears to be a function of the solubility of the compounds. While organotin TSA values have been found to correlate well with their toxicities toward various organisms, this study clearly suggests that this type of QSAR can be readily extended to include other organometal systems, provided that there is no solubility problem and the toxicity is a function of the hydrophobicity of the organometal compounds. 相似文献
13.
14.
15.
This paper examines the capabilities of cryogenic sample handling to examine composition and structure of hydrogel materials where siloxane components are central to the analysis. XPS analysis of multicomponent polymers with cryogenic sample handling following exposure to aqueous environments has revealed the composition and kinetics of near surface reorganization for siloxane and fluorocarbon containing polymers. In this study we report results from a ToF-SIMS protocol for cryogenic sample handling applied to the analysis of surface changes upon hydration/dehydration of hydrogel polymers. Comparison of results from angle dependent XPS and ToF-SIMS are discussed for a range of commercial soft contact lens materials. Both methods detected changes in surface chemistry between the hydrated (frozen) and dehydrated surfaces. Analysis of the hydrated surfaces detected polymer components indicative of the commercial formulation as well as ice clusters. Analysis of the dehydrated materials detected changes in surface chemistry relative to the hydrated surface in addition to loss of water due to sample dehydration. A quantitative standard additions method for ToF-SIMS data was used to determine submonolayer amounts of PDMS impurities at the surface of the hydrogels. ToF-SIMS analysis of a series of seven poly (allyl methacrylate-g-dimethylsiloxane), AMA-g-DMS, graft copolymers in the hydrated state revealed high mass oligomeric ion distributions for systems with bulk PDMS content greater than 25 wt.%. This marks the first time that detection of high mass oligomeric ion distributions from hydrated (frozen) surfaces has been reported. Analysis of the dehydrated surface detected formation of high mass oligomeric ion distributions for systems with PDMS bulk content greater than 15 wt.%, but only detected these ion distributions in wet (frozen) samples when the bulk concentration was greater than 25 wt.%. 相似文献
16.
17.
采用AMI方法研究了10种新型的磺酰脲类除草剂的电子结构,并以原子的Mulliken净电荷和除草剂在不同浓度(100,10mg/L)下对油菜、稗草两种作物的根、茎部位的抑制率为训练样本集。构造并训练得到具有活性预测能力的BP神经网络.结果表明,该BP网络不仅能对训练样本很好拟合。亦能对未知化合物的活性作出很好的预测. 相似文献
18.
19.
The enthalpies and free energies of proton affinity (PA) were calculated by the PM3 method for 27 thiophenoxide anions. For thiophenols, linear correlation has been found between the PAs and the values of pK
a
measured by different authors in aqueous acetone, aqueous ethanol, and methanol media. The dependences found permit one to predict a priori the protolytic properties of thiophenols. The differential effects of aqueous ethanol media with different ethanol contents on thiophenol acidity were evaluated based on quantum-chemical data.Original Russian Text Copyright © 2004 by A. N. Pankratov and A. V. Shalabai__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 800–806, September–October, 2004. 相似文献
20.
Teik-Cheng Lim 《Czechoslovak Journal of Physics》2004,54(5):553-559
A theoretical relation between the Bauer-Maysenholder-Seeger (BMS) and the Biswas-Hamann (BH) potential function has been developed herein for the case of 2-body interaction. By equating derivatives of these two potential functions for the 2-body part, a loose form of BMS is expressed in terms of BH parameters with a scaling factor. The validity of the developed parametric relationships has been graphically demonstrated, and the suitability discussed with reference to the extent and direction of bond distortion. 相似文献