全文获取类型
收费全文 | 1339篇 |
免费 | 35篇 |
国内免费 | 24篇 |
专业分类
化学 | 913篇 |
晶体学 | 17篇 |
力学 | 17篇 |
数学 | 289篇 |
物理学 | 162篇 |
出版年
2022年 | 16篇 |
2021年 | 29篇 |
2020年 | 12篇 |
2019年 | 18篇 |
2018年 | 23篇 |
2017年 | 20篇 |
2016年 | 38篇 |
2015年 | 31篇 |
2014年 | 44篇 |
2013年 | 103篇 |
2012年 | 71篇 |
2011年 | 77篇 |
2010年 | 49篇 |
2009年 | 37篇 |
2008年 | 62篇 |
2007年 | 87篇 |
2006年 | 73篇 |
2005年 | 63篇 |
2004年 | 70篇 |
2003年 | 46篇 |
2002年 | 53篇 |
2001年 | 33篇 |
2000年 | 25篇 |
1999年 | 16篇 |
1998年 | 18篇 |
1997年 | 15篇 |
1996年 | 23篇 |
1995年 | 22篇 |
1994年 | 11篇 |
1993年 | 19篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 13篇 |
1986年 | 5篇 |
1985年 | 14篇 |
1984年 | 16篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 18篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1974年 | 3篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1970年 | 2篇 |
排序方式: 共有1398条查询结果,搜索用时 656 毫秒
101.
Extraction of formic and acetic acids from aqueous solution by dynamic headspace-needle trap extraction temperature and pH optimization 总被引:1,自引:0,他引:1
A combined method of dynamic headspace-needle trap sample preparation and gas chromatography for the determination of formic and acetic acids in aqueous solution was developed in this study. A needle extraction device coupled with a gas aspirating pump was intended to perform sampling and preconcentration of target compounds from aqueous sample before gas chromatographic analysis. The needle trap extraction (NTE) technique allows for the successful sampling of short chain fatty acids under dynamic conditions while keeping the headspace (HS) volume constant. Two important parameters, including extraction temperature and effect of acidification, have been optimized and evaluated using the needle trap device. The method detection limits for the compounds estimated were 87.2microg/L for acetic acid and 234.8microg/L for formic acid in spite of the low flame ionization detection response for formic acid and its low Henry's law constant in aqueous solution. Precision was determined based on the two real samples and ranged between 4.7 and 10.7%. The validated headspace-needle trap extraction method was also successfully applied to several environmental samples. 相似文献
102.
Simple-structured, well-functioned disposable poly(dimethylsiloxane) (PDMS) microchips were developed for capillary isoelectric focusing with whole column imaging detection (CIEF-WCID). Side-by-side comparison of the developed microchips with well-established commercial capillary cartridges demonstrated that the disposable microchips have comparable performance as well as advantages such as absence of lens effect and possibility of high-aspect-ratio accompanied with a dramatic reduction in cost. 相似文献
103.
104.
105.
106.
Dawid Lichosyt Dr. Paweł Dydio Prof. Janusz Jurczak 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(49):17673-17680
Herein we report the synthesis and detailed studies of the anion‐binding properties of two 20‐membered macrocyclic tetramide receptors: one symmetrical, containing two identical azulene‐based bisamide units, the other a hybrid, containing a dipicolinic bisamide unit and an azulene‐based bisamide unit. Analysis of the crystal structures of the macrocyclic receptors revealed their preference for adopting similar well‐preorganized bent‐sheet conformations, both as free receptors and in their complexes with anions. Studies of the optical properties of both receptors revealed abilities to selectively sense phosphate anions (H2PO4?, HP2O73?), allowing for naked‐eye detection of the presence of these guests in DMSO. Binding studies in solution confirmed that the receptors bind strongly to a series of anions even in highly demanding media, such as mixtures of DMSO with water or with methanol. Comparison of the anion affinity of linear analogues with that of the macrocyclic receptors evidenced the importance of macrocyclic topology. Quantitative analysis revealed that the macrocyclic receptors are selective for H2PO4? over other anions. The affinity to H2PO4? seen for the symmetrical receptor, containing two azulene‐based subunits, is much higher than for the hybrid macrocycle containing both the azulene‐based and pyridine‐derived subunits. This highlights that the azulene‐based building block serves efficiently as both a binding site and a structure‐preorganizing motif. 相似文献
107.
108.
109.
110.
Janusz Adamus 《Proceedings of the American Mathematical Society》2002,130(11):3165-3170
Kwiecinski has proved a geometric criterion for flatness: A morphism of germs of analytic spaces is not flat if and only if its fibre power has a vertical component, for some . We show how to bound using Hironaka's local flattener: If is not flat, then has a vertical component, where is the minimal number of generators of the ideal in of the flattener of .