全文获取类型
收费全文 | 3486篇 |
免费 | 241篇 |
国内免费 | 423篇 |
专业分类
化学 | 3308篇 |
晶体学 | 27篇 |
力学 | 111篇 |
综合类 | 3篇 |
数学 | 29篇 |
物理学 | 672篇 |
出版年
2024年 | 13篇 |
2023年 | 81篇 |
2022年 | 43篇 |
2021年 | 42篇 |
2020年 | 56篇 |
2019年 | 48篇 |
2018年 | 44篇 |
2017年 | 70篇 |
2016年 | 152篇 |
2015年 | 155篇 |
2014年 | 162篇 |
2013年 | 200篇 |
2012年 | 240篇 |
2011年 | 298篇 |
2010年 | 225篇 |
2009年 | 238篇 |
2008年 | 193篇 |
2007年 | 231篇 |
2006年 | 190篇 |
2005年 | 202篇 |
2004年 | 209篇 |
2003年 | 147篇 |
2002年 | 125篇 |
2001年 | 94篇 |
2000年 | 86篇 |
1999年 | 91篇 |
1998年 | 67篇 |
1997年 | 85篇 |
1996年 | 73篇 |
1995年 | 69篇 |
1994年 | 35篇 |
1993年 | 34篇 |
1992年 | 29篇 |
1991年 | 26篇 |
1990年 | 23篇 |
1989年 | 17篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有4150条查询结果,搜索用时 15 毫秒
101.
Nonisothermal kinetics of the solid‐solid phase transition in (n‐C10H21NH3)2ZnCl4(C10Zn), (n‐C16H33NH3)2ZnCl4(C16Zn) and their binary system were determined by Kissinger and Ozawa methods from DSC measurements. The activation energy Ea of the binary system shows a waving dependence on WC10Zn%, which is caused by not only an intermediate (C10H21NH3)(n‐C16H33NH3)ZnCl4 but also three solid solution ranges (α, β, γ) in the phase diagram of C10Zn‐C16Zn. The variations of the layer d‐spacing are also convenient for the above result. 相似文献
102.
Hideko Koshima Teruo Matsuura 《Journal of photochemistry and photobiology. A, Chemistry》1996,100(1-3):85-91
This paper deals with the solid state bimolecular photoreactions occurring at the interface between crystallites of two different organic compounds prepared as a polycrystalline mixture by melting-resolidification. The photoreactions include the photoaddition of NH compounds to aromatic and olefinic double bonds, hydrogen abstraction, condensation reaction via photoelectron transfer and [2+2] photoaddition. Particular attention is focused on the characterization of the polycrystalline mixture, the selectivity of the photoreactions in the solid state compared with those in the solution phase and the factors controlling the photoreactions. 相似文献
103.
104.
Jon Volden Yngvar Thomassen Tyge Greibrokk Syvert Thorud Paal Molander 《Analytica chimica acta》2005,530(2):263-271
The storage stability of the occupationally frequently occurring compounds, methylethylketone, methylisobutylketone, benzene, toluene, tetrachloroethylene, n-butylacetate, -pinene, β-pinene, limonene and n-decane, has been investigated on the adsorbents Tenax TA, Chromosorb 106 and Carbotrap using thermally desorbable tube type samplers, commonly utilized in ambient and workroom atmospheric measurements. Fifty and 500 ng of each compound were loaded on the various adsorbents tubes, stored at both ambient (20 °C) and refrigerated (4 °C) temperatures and analysed by means of thermal gas chromatography with mass spectrometric detection on days 0, 7, 14 and 28 after exposure. A 90% storage recovery was chosen as acceptance criteria for storage stability, and statistical testing by Student's t-test, analysis of variance and Bonferroni post hoc tests were employed to investigate the effect of the categorical variables storage time, storage temperature and analyte loading on the different adsorbents. Chromosorb 106 showed the overall best behaviour with recoveries of 90% or better for all analytes during the 28-day test period. Tenax TA and Carbotrap yielded lower recoveries and were more influenced by variations in storage time, storage temperature and analyte loading. Refrigerated temperatures were best avoided for storage on Tenax TA, but may increase the recovery of some compounds on Carbotrap (e.g. n-butylacetate). The blank build-up on the adsorbents was also investigated, and Carbotrap and Tenax TA showed no signs of artefact development over time. Chromosorb 106, however, contained inherently more artefacts that build up over time, which in spite of the excellent storage capability, may limit its use in field studies where long storage times are normal. 相似文献
105.
A solid-phase synthetic route to quinoxaline, thiazine, and oxazine analogs is described. N-Alloc-3-amino-3-(2,4-difluoro-5-nitrophenyl)propanoic acid was tethered to Rink resin via its carboxylic acid group. The 4-arylfluorine was displaced with a primary amine, alcohol, or thiol to create, respectively, a resin bound aniline, phenol, or thiophenol derivative with one diversity element and one single atom (e.g., N, S, or O) diversity point. A fused heterocyclic system was subsequently created via a benzyne heterocyclization initiated by dehydrofluorination with strong base. Acid treatment released the desired products in high yield and moderate purity. 相似文献
106.
Sébastien L.X. Martina 《Tetrahedron letters》2004,45(16):3279-3282
Novel solid-supported phosphonate reagents have been prepared and evaluated for the synthesis of α,β-unsaturated esters with a preference for the Z-alkene. The optimal reagent was a hybrid of both Still-Gennari and Ando reagents, and showed good to high yields and fair to good Z-selectivity for the conversion of both aliphatic and aromatic aldehydes. 相似文献
107.
Low concentrations of Pt and Pd in industrial (µgg–1 level) and environmental samples (ngg–1 level) together with the complexity of the matrix causing many interferences during the determination of noble metals often require elaboration and application of pre-concentration/matrix separation procedures before detection of the analyte. Different pre-concentration/matrix separation procedures applied prior to the determination of Pt and Pd by atomic spectrometric techniques are reviewed and critically compared taking into account potential interferences. The methods studied are divided into 5 groups including precipitation and coprecipitation, liquid–liquid extraction, solid phase extraction, electrochemical pre-concentration and biosorption. The main analytical problems occuring during sample preparation and storage are discussed. 相似文献
108.
Kwang-il ChungJung-Do Lee Eui-Jung KimWoo-Seong Kim Jung-Hwan ChoYong-Kook Choi 《Microchemical Journal》2003,75(2):71-77
Although a lithium metal anode has a high energy density compared with a carbon insertion anode, the poor rechargeability prevents the practical use of anode materials. A lithium electrode coated with Li2CO3 was prepared as a negative electrode to enhance cycleability through the control of the solid electrolyte interface (SEI) layer formation in Li secondary batteries. The electrochemical characteristics of the SEI layer were examined using chronopotentiometry (CP) and impedance spectroscopy. The Li2CO3-SEI layer prevents electrolyte decomposition reaction and has low interface resistance. In addition, the lithium ion diffusion in the SEI layer of the uncoated and the Li2CO3-coated electrode was evaluated using chronoamperometry (CA). 相似文献
109.
110.
C. C. Camarasu 《Chromatographia》2002,56(1):S131-S135
Summary A sensitive headspace SPME method for the extraction of residual solvents from pharmaceutical products has been developed
and optimized. It was found that minimizing sample and headspace volume has a beneficial effect on extraction efficiency.
At the same time the method reproducibility was seriously affected by reducing sample and headspace volume. The added air
volume was not found to have any significant influence on method sensitivity. The method showed reproducibilities of less
than 10% and detection limits as low as 1 ppb for benzene and dichloromethane. The headspace SPME method is around 1000 times
more sensitive than static headspace. The optimized parameters were headspace volume 1.5 mL, sample volume 10 μL, and extraction
time 30 min. The method was successfully applied to the identification of unknown residual solvents in three different proprietary
active drug substances and was successfully applied to the confirmation of the presence of benzene in a proprietary drug substance.
Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001 相似文献