全文获取类型
收费全文 | 653篇 |
免费 | 35篇 |
国内免费 | 1篇 |
学科分类
数理化 | 689篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 43篇 |
2020年 | 27篇 |
2019年 | 31篇 |
2018年 | 33篇 |
2017年 | 19篇 |
2016年 | 41篇 |
2015年 | 30篇 |
2014年 | 29篇 |
2013年 | 56篇 |
2012年 | 51篇 |
2011年 | 52篇 |
2010年 | 35篇 |
2009年 | 28篇 |
2008年 | 46篇 |
2007年 | 27篇 |
2006年 | 21篇 |
2005年 | 17篇 |
2004年 | 15篇 |
2003年 | 16篇 |
2002年 | 15篇 |
2001年 | 2篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 4篇 |
1995年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1985年 | 2篇 |
1975年 | 2篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1965年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有689条查询结果,搜索用时 218 毫秒
91.
Lucie Bielská Jakub Hofman 《International journal of environmental analytical chemistry》2015,95(7):635-649
The proper choice of exposure times is critical if the freely dissolved concentration of chemicals in soil porewater is to be measured via the equilibrium solid-phase microextraction (SPME) as the times to equilibrium may vary depending on compound and soil properties. To reveal the effects of compound hydrophobicity, ageing and soil organic matter content on times to equilibrium, the SPME uptake was measured for five freshly added and aged hydrophobic organic compounds (phenanthrene, pyrene, lindane, p,p′-DDT and polychlorinated biphenyl (PCB) 153) in two contrasted soils (arable and forest soil). The tested compound-soil systems behaved kinetically different. Longer equilibrium times were observed with increasing hydrophobicity of compounds for aged compared to freshly added chemicals and for the forest soil in comparison to the arable soil. The calculated soil–porewater partition coefficients (i.e. sorption coefficients, Kd) of chemicals differed between soil types mainly due to various organic carbon (OC) contents as evidenced by the comparable Koc values (i.e. Kd values normalised to soil OC content). Similar Koc values were also found with the various extent of ageing, indicating that both the freshly added and aged compounds linearly partitioned between the soil organic matter and porewater. Our results suggest that, for a respective compound, variations in equilibrium times may be expected depending upon the residence time and the organic matter content in soil where the longest equilibrium times seems to appear for a combination of aged compounds and high organic soils. With regard to this outcome, the effect of the level of sample depletion due to the SPME extraction (LDSPME) on equilibrium times was assessed. At LDsSPME of up to 10%, equilibrium times increases linearly with LDsSPME for p,p′-DDT and PCB 153. For phenanthrene (LDSPME<10%), and for lindane and pyrene (1.2% < LDSPME > 40%), no clear relationships were observed. 相似文献
92.
Dariusz Marek Adam Domaski Joanna Domaska Jakub Szygua Tadeusz Czachrski Jerzy Klamka 《Entropy (Basel, Switzerland)》2021,23(5)
In this article, a way to employ the diffusion approximation to model interplay between TCP and UDP flows is presented. In order to control traffic congestion, an environment of IP routers applying AQM (Active Queue Management) algorithms has been introduced. Furthermore, the impact of the fractional controller and its parameters on the transport protocols is investigated. The controller has been elaborated in accordance with the control theory. The TCP and UDP flows are transmitted simultaneously and are mutually independent. Only the TCP is controlled by the AQM algorithm. Our diffusion model allows a single TCP or UDP flow to start or end at any time, which distinguishes it from those previously described in the literature. 相似文献
93.
Jakub Trawiński 《液相色谱法及相关技术杂志》2019,42(3-4):63-73
Photostability of an antidepressant agomelatine under the simulated solar radiation was studied. Quantitative and qualitative analysis of this process was performed with the use of UHPLC-DAD system coupled with a high resolution hybrid ESI-Q-TOF mass spectrometer. In contrast to the foregoing studies agomelatine turned out to be relatively photolabile compound. During the experiment six transformation products were formed, and their structures were elucidated on the basis of MS/MS fragmentation spectra. Four of these photoproducts were found to be a result of aromatic and aliphatic hydroxylation. Additionally, identified products were submitted to the in silico toxicity evaluation, which showed that some of them could be more mutagenic or toxic to rodents than the parent compound. 相似文献
94.
95.
Marlena Pięta Jacek Kędzia Dorota Kowalczyk Jakub Wojciechowski Wojciech M. Wolf Tomasz Janecki 《Tetrahedron》2019,75(17):2495-2505
An efficient, versatile, enantioselective synthesis of 1,3-disubstituted and 1,3,6-trisubstituted 5-methylidenedihydrouracils applying Horner-Wadsworth Emmons methodology was developed. Starting 1,3-disubstituted 5-diethoxyphosphoryluracils were subjected to reduction of the double bond or addition of various Grignard reagents and obtained Horner-Wadsworth Emmons reagents were used for the olefination of formaldehyde. Enantioselective synthesis of 1,3,6-trisubstituted 5-methylidenedihydrouracils was accomplished by introducing (R,R)- or (S,S)-di(1-phenylethylamino)phosphoryl groups as chiral auxiliary. Additions of Grignard reagents in the presence of these groups were highly and complimentary diastereoselective (de?~?80%). Further separation of the diastereomeric mixtures by column chromatography enabled synthesis of (R)- and (S)-1,3,6-trisubstituted-5-methylidenedihydrouracils with ee?≥?98%. Furthermore, absolute configuration of the adducts and final products was established using single crystal X-ray analysis. Stereochemical course of the addition reactions is also discussed. 相似文献
96.
Agata Kozioł Jakub Frątczak Ewa Grela Maryla Szczepanik Beata Gabryś Katarzyna Dancewicz 《Natural product research》2020,34(10):1399-1403
AbstractTerpenoid derivatives, which contain a preserved carane system in their structure, exhibit a broad spectrum of biological activities. Among them, we can distinguish insecticides, structures with pharmacological application etc. In the presented paper, the substrate - (–)-cis-caran-trans-4-ol was transformed using the reactions of typical organic synthesis to obtain novel derivatives. Most importantly, bromolactone ((–)-(1R,4R,6S)-2'-(bromomethyl)-4,7,7-trimethylspiro[bicyclo[4.1.0]heptan-3,3'-furan]-5'(4'H)-one) with the preserved carane system was synthesized. This bromolactone was tested for antifeedant activity against the lesser mealworm, Alphitobius diaperinus Panzer, and peach potato aphid (Myzus persicae). In addition, its moderate antibacterial activity was observed against the Bacillus subtilis strain (with Minimal Inhibitory Concentration of 200 µg/mL). 相似文献
97.
Martowicz Adam Bryła Jakub Staszewski Wieslaw J. Ruzzene Massimo Uhl Tadeusz 《Nonlinear dynamics》2019,97(3):1911-1935
Nonlinear Dynamics - The work is primarily devoted to the peridynamic model elaborated for a solid body made of shape memory alloys (SMAs). The superelasticity effect is taken into consideration as... 相似文献
98.
Mária Bubeníková Petra Ecorchard Lórant Szatmáry Ondřej Mrózek Petra Salačová Jakub Tolasz 《Journal of Radioanalytical and Nuclear Chemistry》2018,315(2):263-272
The simple method for preparation of nanocomposites of graphene oxide or carboxylated graphene oxide on polystyrene matrix and graphene oxide on PA66 matrix were developed and structures of nanocomposites were characterized by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, high-resolution scanning electron microscopy, transmission electron microscopy and scanning transmission electron microscopy. The nanocomposites were investigated for radiostrontium removal from aqueous solutions. The Sr(II) sorption was dependent on pH and ionic strength at pH < 7. The Sr(II) equilibrium data were fitted by Freundlich and Langmuir models. The nanocomposites prepared were evaluated as suitable for the radiostrontium removal from contaminated wastewater. 相似文献
99.
Helicenophyrins: Expanded Carbaporphyrins Incorporating Aza[5]helicene and Heptacyclic S‐Shaped Aza[5]helicene Motifs 下载免费PDF全文
Dr. Bartosz Szyszko Monika Przewoźnik Dr. Michał J. Białek Dr. Agata Białońska Prof. Dr. Piotr J. Chmielewski Dr. Jakub Cichos Prof. Dr. Lechosław Latos‐Grażyński 《Angewandte Chemie (International ed. in English)》2018,57(15):4030-4034
Incorporation of phenanthrene into a hexaphyrin(1.1.1.1.1.0) frame resulted in intramolecular ring fusion, thus giving rise to chiral helicenophyrins. These molecules contain helicene and porphyrin features by incorporating either an aza[5]helicene or heptacyclic S‐shaped aza[5]helicene. 相似文献
100.
Ján Jakubík 《Czechoslovak Mathematical Journal》2008,58(1):1-14
Freytes proved a theorem of Cantor-Bernstein type for algbras; he applied certain sequences of central elements of bounded
lattices. The aim of the present paper is to extend the mentioned result to the case when the lattices under consideration
need not be bounded; instead of sequences of central elements we deal with sequences of internal direct factors of lattices.
This work was supported by Science and Technology Assistance Agency under the contract No APVT-51-032002. 相似文献