全文获取类型
收费全文 | 387篇 |
免费 | 7篇 |
专业分类
化学 | 251篇 |
晶体学 | 1篇 |
力学 | 6篇 |
数学 | 60篇 |
物理学 | 76篇 |
出版年
2021年 | 11篇 |
2019年 | 4篇 |
2017年 | 3篇 |
2016年 | 11篇 |
2015年 | 8篇 |
2014年 | 4篇 |
2013年 | 24篇 |
2012年 | 19篇 |
2011年 | 27篇 |
2010年 | 22篇 |
2009年 | 19篇 |
2008年 | 9篇 |
2007年 | 17篇 |
2006年 | 11篇 |
2005年 | 15篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 16篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1995年 | 12篇 |
1993年 | 4篇 |
1989年 | 3篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1977年 | 4篇 |
1975年 | 6篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1967年 | 5篇 |
1965年 | 4篇 |
1963年 | 5篇 |
1962年 | 3篇 |
1961年 | 4篇 |
1959年 | 3篇 |
1958年 | 4篇 |
1955年 | 4篇 |
1954年 | 2篇 |
排序方式: 共有394条查询结果,搜索用时 15 毫秒
21.
22.
23.
Kochowska E Mazur J Kozak K Janik M 《Isotopes in environmental and health studies》2004,40(3):207-212
The method and the results of radon concentration measurements in water samples are presented. Since May 2000, measurements of radon concentration in well waters in the Kraków area have been carried out--both in urban wells (depth above 50 meters) and in other private wells (depth of several meters). The ionisation chamber AlphaGUARD PQ 2000PRO along with the additional special equipment AquaKIT were used for determination of radon concentration in water samples. A total of 45 wells were examined for radon concentration in water--19 urban wells, 21 private ones (from Nowa Huta, Ojcowska, Wola Justowska area--parts of Kraków) and 5 mineral water sources. Wola Justowska is a region where tectonics faults occur and radon can easily migrate from deep basement structure. All the obtained values of radon concentration are below 12 Bq/l. These preliminary results do not show a direct correlation between geological structure and radon concentration in water samples. However, further investigation is needed and is planned to be undertaken. 相似文献
24.
A new method for on-line batter monitoring using ultrasound techniques is presented. Air or gas incorporation is done during the beating process which produces bubbles in the mixture. The density and the compressibility of the batter vary as a function of mixing time and are quality index of batter. Traditionally, a batter sample of a fixed volume is removed and weighted in order to determine its density. This is a time consuming process. Batters are air filled mixtures of high viscosity which do not support significant transmission of ultrasound. For this reason conventional ultrasonic density sensors for liquids are not suitable for this application. Therefore, a special transducer has been developed. The sensor was constructed using a piezoelectric ceramic at the fundamental frequency of 1 MHz. Instead of measuring density, in this work, changes in compressibility in batters are monitored by measuring the acoustic impedance of the batter. Main advantage of this novel approach is that changes in acoustic impedance are easier to detect than changes in density especially when air incorporation is in small quantities. Experimental results on different liquids and batters with different gas contents are presented and discussed. 相似文献
25.
26.
27.
Tomasz Goslinski Zbigniew Dutkiewicz Michal Kryjewski Ewa Tykarska Lukasz Sobotta Wojciech Szczolko Maria Gdaniec Jadwiga Mielcarek 《Monatshefte für Chemie / Chemical Monthly》2011,13(1):599-608
Abstract
An earlier developed alkylating path leading to tetraalkylated diaminomaleonitrile derivatives was explored. Attempts to explain the reactivity of the representative dialkylated diaminomaleonitrile 2,3-bis[(3-pyridylmethyl)amino]-2(Z)-butene-1,4-dinitrile during the alkylation reaction were performed using X-ray and density functional theory (DFT) studies. The condensed Fukui functions accompanied by softness indices were found to be useful in explaining its reactivity observed during the reaction. The values of the Fukui functions and condensed softness for electrophilic attack calculated from Mulliken, L?wdin, and natural population analyses closely corresponded to the experimental observations. When 2,3-bis[(3-pyridylmethyl)amino]-2(Z)-butene-1,4-dinitrile disodium salt was treated with dimethyl sulfate at lower temperatures the alkylation reaction prevailed, whereas at higher temperatures the alkylating agent acted as a hydride anion acceptor, which favored the elimination reaction. The tetraalkylated dinitrile 2,3-bis[methyl(3-pyridylmethyl)amino]-2(Z)-butene-1,4-dinitrile was used in the synthesis of tribenzoporphyrazine bearing methyl(3-pyridylmethyl)amino groups, which was subsequently subjected to solvatochromic and metallation studies. The changes observed during metallation seem to result from the coordination of the 3-pyridyl group by a palladium ion. This could influence the configuration of the methyl(3-pyridylmethyl)amino moiety, causing more effective donation of a lone pair of electrons from peripheral nitrogen to the macrocyclic ring. 相似文献28.
Benjamin Waldher Jadwiga Kuta Samuel Chen Neil Henson Aurora E. Clark 《Journal of computational chemistry》2010,31(12):2307-2316
The ForceFit program package has been developed for fitting classical force field parameters based upon a force matching algorithm to quantum mechanical gradients of configurations that span the potential energy surface of the system. The program, which runs under UNIX and is written in C++, is an easy‐to‐use, nonproprietary platform that enables gradient fitting of a wide variety of functional force field forms to quantum mechanical information obtained from an array of common electronic structure codes. All aspects of the fitting process are run from a graphical user interface, from the parsing of quantum mechanical data, assembling of a potential energy surface database, setting the force field, and variables to be optimized, choosing a molecular mechanics code for comparison to the reference data, and finally, the initiation of a least squares minimization algorithm. Furthermore, the code is based on a modular templated code design that enables the facile addition of new functionality to the program. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 相似文献
29.
30.