全文获取类型
收费全文 | 573篇 |
免费 | 30篇 |
专业分类
化学 | 472篇 |
晶体学 | 7篇 |
力学 | 7篇 |
数学 | 77篇 |
物理学 | 40篇 |
出版年
2023年 | 2篇 |
2022年 | 13篇 |
2021年 | 25篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 6篇 |
2017年 | 3篇 |
2016年 | 17篇 |
2015年 | 20篇 |
2014年 | 21篇 |
2013年 | 24篇 |
2012年 | 22篇 |
2011年 | 49篇 |
2010年 | 27篇 |
2009年 | 32篇 |
2008年 | 37篇 |
2007年 | 43篇 |
2006年 | 30篇 |
2005年 | 29篇 |
2004年 | 24篇 |
2003年 | 16篇 |
2002年 | 27篇 |
2001年 | 5篇 |
2000年 | 6篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1991年 | 9篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1959年 | 1篇 |
1958年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有603条查询结果,搜索用时 328 毫秒
41.
Gutierrez-Lugo MT Woldemichael GM Singh MP Suarez PA Maiese WM Montenegro G Timmermann BN 《Natural product research》2005,19(7):645-652
Bioassay guided isolation of an antibacterial extract prepared from the fermentation broth of a Micromonospora sp. P1068 led to the isolation of eight compounds identified as (3R) 3,4',7-trihydroxy-isoflavanone (1), 3-hydroxydehydrodaidzein, daidzein (2), 3-methyl-1H-indole-2-carboxylic acid (3), 1H-indole-3-carboxaldehyde (4), 3-(p-hydroxyphenyl)-N-methylpropionamide, N-methylphloretamide (5), phenyl acetic acid (6), 2-hydroxy phenyl acetic acid (7) and 4-hydroxy-5-methoxy-benzoic acid (8). Compounds 1 and 5 were found to be novel chemical entities while 3 was isolated from a natural source for the first time. All compounds were evaluated for their antimicrobial activities against a panel of clinically significant microorganisms. Compound 4 was active against Staphylococcus aureus (MIC, 32 microg/ml), Enterococcus faecium (MIC, 32 microg/ml) and Escherichia coli (MIC, 64 microg/ml). 相似文献
42.
We propose and analyze a new scheme of realizing both spin filtering and spin pumping by using ac-driven double quantum dots in the Coulomb blockade regime. By calculating the current through the system in the sequential tunneling regime, we demonstrate that the spin polarization of the current can be controlled by tuning the parameters (amplitude and frequency) of the ac field. We also discuss spin relaxation and decoherence effects in the pumped current. 相似文献
43.
Subías G García J Blasco J Grazia Proietti M Renevier H Concepción Sánchez M 《Physical review letters》2004,93(15):156408
We have investigated the charge ordering (CO) in magnetite below the Verwey transition. A new set of half-integer and mixed-integer superlattice reflections of the low-temperature phase have been studied by x-ray resonant scattering. None of these reflections show features characteristic of CO. We demonstrate the absence of CO along the c axis with the periodicity of either the cubic lattice q=(001) or the doubled cubic lattice q=(001/2). This result suggests that the Verwey transition is caused by strong electron-phonon interaction instead of an electronic ordering on the octahedral Fe atoms. 相似文献
44.
Fan Y Mandelis A Spirou G Vitkin IA 《The Journal of the Acoustical Society of America》2004,116(6):3523-3533
In conventional biomedical photoacoustic imaging systems, a pulsed laser is used to generate time-of-flight acoustic information of the subsurface features. This paper reports the theoretical and experimental development of a new frequency-domain (FD) photo-thermo-acoustic (PTA) principle featuring frequency sweep (chirp) and heterodyne modulation and lock-in detection of a continuous-wave laser source at 1064 nm wavelength. PTA imaging is a promising new technique which is being developed to detect tumor masses in turbid biological tissue. Owing to the linear relationship between the depth of acoustic signal generation and the delay time of signal arrival to the transducer, information specific to a particular depth can be associated with a particular frequency in the chirp signal. Scanning laser modulation with a linear frequency sweep method preserves the depth-to-delay time linearity and recovers FD-PTA signals from a range of depths. Preliminary results performed on rubber samples and solid tissue phantoms indicate that the FD-PTA technique has the potential to be a reliable tool for biomedical depth-profilometric imaging. 相似文献
45.
The development of new strategies for solid-phase synthesis of 3-aza-6,8-dioxabicyclo[3.2.1]octane scaffolds, named BTKa, is described. The preparation was made possible by the combination of three components: amines, alpha-halo-acetophenones, and sugar or tartaric acid derivatives. By anchoring each of the three components it was possible to synthesize BTKa compounds either as amino alcohols or amido esters. The compatibility of the protocols with different classes of amines and substituted alpha-halo-acetophenones was demonstrated. 相似文献
46.
The Ugi four-component condensation between anilines, aromatic aldehydes, isocyanides, and α-oxoacids afforded the expected adducts which were cleaved in mild conditions to give 2,N-diarylglycines in high yields. 相似文献
47.
Riccardo Narizzano Fulvia Risso Gloria VenturelliCarla Devia Eugenio CarliniStefano Maggiolo 《Journal of chromatography. A》2009,1216(39):6762-6766
Two methods for the analysis of dimethyl fumarate (DMF) on silica gel, leather and other similar products are presented. The methods are based on solvent extraction or head space solid phase microextraction coupled with gas-chromatography–mass spectrometry. The methods are developed in order to satisfy the mandatory requirements of the European Community and with the aim to have easy and reliable methods to provide rapid responses. 相似文献
48.
Gloria Dion Anne Harvey Carol Jackson Patricia Klag Jinghua Liu Craig Wright 《School science and mathematics》2001,101(8):427-438
This survey investigated the current status of calculator use in classrooms and schools. The results indicated that the prevailing policy in the sample of high schools is to allow the use of calculators during classroom learning activities and tests. Scientific calculators are more frequently used than graphing calculators in algebra I and geometry; whereas, graphing calculators are more frequently used in algebra II and precalculus/trigonometry. At the time of this survey, school policies regarding the use of graphing calculators with symbolic algebra capabilities were still not determined. 相似文献
49.
Björn Schwarz Issic K. C. Leung Nils Buchholtz Gabriele Kaiser Gloria Stillman Jill Brown Colleen Vale 《ZDM》2008,40(5):791-811
In this paper, qualitative results of a case study about the professional knowledge in the area of argumentation and proof of future teachers from universities in three countries are described. Based on results of open questionnaires, data about the competencies these future teachers have in the areas of mathematical knowledge and knowledge of mathematics pedagogy are presented. The study shows that the majority of the future teachers at the participating universities situated in Germany, Hong Kong and Australia, were not able to execute formal proofs, requiring only lower secondary mathematical content, in an adequate and mathematically correct way. In contrast, in all samples there was evidence of at least average competencies of pedagogical content reflection about formal and pre-formal proving in mathematics teaching. However, it appears that possessing a mathematical background as mandated for teaching and having a high affinity with proving in mathematics teaching at the lower secondary level are not a sufficient preparation for teaching proof. 相似文献
50.
A series of 2-aryl-2H-pyrazolo[4,3-c]quinolin-3-ones derivatives 6 and 7 was conveniently prepared. A multistep synthesis was carried out starting from dichloro- and bromoanilines (1a-b) and diethyl 2-(ethoxymethylene)malonate using a slightly modified Gould-Jacobs reaction. In this work we present a novel chlorination strategy to prepare quinoline derivatives 4 in excellent yields as key intermediates in the synthesis of the target compounds. Several reaction conditions were evaluated to optimize the formation of pyrazoloquinolinone nucleus. Differences in chemical behavior of both chloroquinolinones 4a-b with aryl and benzyl-hydrazines are also discussed. 相似文献