全文获取类型
收费全文 | 596篇 |
免费 | 41篇 |
专业分类
化学 | 421篇 |
力学 | 7篇 |
数学 | 73篇 |
物理学 | 136篇 |
出版年
2023年 | 12篇 |
2022年 | 6篇 |
2021年 | 20篇 |
2020年 | 16篇 |
2019年 | 9篇 |
2018年 | 17篇 |
2017年 | 7篇 |
2016年 | 24篇 |
2015年 | 29篇 |
2014年 | 33篇 |
2013年 | 41篇 |
2012年 | 37篇 |
2011年 | 53篇 |
2010年 | 34篇 |
2009年 | 19篇 |
2008年 | 39篇 |
2007年 | 21篇 |
2006年 | 34篇 |
2005年 | 24篇 |
2004年 | 20篇 |
2003年 | 23篇 |
2002年 | 12篇 |
2001年 | 8篇 |
2000年 | 8篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1987年 | 2篇 |
1985年 | 9篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1960年 | 3篇 |
1958年 | 2篇 |
1931年 | 1篇 |
1930年 | 1篇 |
1907年 | 2篇 |
1881年 | 1篇 |
排序方式: 共有637条查询结果,搜索用时 265 毫秒
1.
Jonathan M. Levin Lawrence L. Wald Marc J. Kaufman Marjorie H. Ross Luis C. Maas Perry F. Renshaw 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,130(2):292-295
Residual effects of an initial bolus of gadolinium contrast agent have been previously demonstrated in sequential dynamic susceptibility contrast MR experiments. While these residual effects quickly reach a saturation steady state, their etiology is uncertain, and they can lead to spurious estimates of hemodynamic parameters in activation experiments. The possible influence ofT1effects is now investigated with experiments in whichT1weighting is varied as well as with serial regionalT1measurements. Little evidence for significant residualT1effects is found, suggesting instead that susceptibility effects underlie these observations. An initial saturation dose of contrast agent minimizes this effect. 相似文献
2.
In this article we focus on the multiplicities of resonances, ratios of resonant over non-resonant states and rescattering processes in heavy ion collisions. Therefore we utilize a hadron transport model (UrQMD v1.3). We find that rescattering of decay particles is of great importance when studying resonances in a hadronic medium. 相似文献
3.
Dieter Seebach Laurent Schaeffer Franois Gessier Pascal Bindschdler Corinna Jger Delphine Josien Sascha Kopp Grald Lelais YogeshR. Mahajan Peter Micuch Radovan Sebesta BerndW. Schweizer 《Helvetica chimica acta》2003,86(6):1852-1861
Multigram amounts of suitably protected β2‐amino acids with 17 of the 20 proteinogenic side chains are prepared by diastereoselective reactions of Li, B, or Ti enolates of the corresponding 3‐acyl‐4‐isopropyl‐5,5‐diphenyloxazolidin‐2‐ones (acyl‐DIOZ; 1 ) with appropriate electrophiles (amidomethylation, hydroxyalkylation, (benzyloxycarbonyl)methylation) in yields of 55–90% and with diastereoselectivities of 80 to >97% (Scheme). The primary products 2 – 8 thus obtained are converted to protected β2‐amino acids by standard procedures (Table 1). Many of the DIOZ derivatives are highly crystalline compounds (31 X‐ray crystal structures in Table 2). The chiral auxiliary DIOZ, readily prepared in either enantiomeric form, is recovered with high yield. 相似文献
4.
Zusammenfassung Die Aufnahme von Nitrilotriessigsäure-Komplexen von Strontium, Blei, Yttrium, Wismut und Thorium durch den Anionenaustauscher Dowex-1 wurde untersucht, indem die Elutionsgeschwindigkeiten radioaktiver Isotope dieser Elemente aus Austauschersäulen gemessen wurden. Für die Versuche wurde mit Nitrilotriacetat (NTA) beladener Austauscher verwendet; als Elutionsmittel dienten NTA-Lösungen verschiedener Konzentration. Die ermittelten Verteilungskoeffizienten (Austauscher/Lösung) geben Hinweise auf die Ladungszahlen der für die Aufnahme in den Austauscher maßgeblichen Metall-NTA-Komplexe und damit auf deren Zusammensetzung. Sie liefern auch Aussagen über die Wechselwirkung zwischen den komplexen Anionen und dem Austauscher. Anwendungsmöglichkeiten der Aufnahme von NTA-Komplexen in den Anionenaustauscher für radiochemische Trennungen werden aufgezeigt.Mit 3 Abbildungen 相似文献
5.
Optical sensor systems for bioprocess monitoring 总被引:3,自引:0,他引:3
Bioreactors are closed systems in which microorganisms can be cultivated under defined, controllable conditions that can be optimized with regard to viability, reproducibility, and product-oriented productivity. To drive the biochemical reaction network of the biological system through the desired reaction optimally, the complex interactions of the overall system must be understood and controlled. Optical sensors which encompass all analytical methods based on interactions of light with matter are efficient tools to obtain this information. Optical sensors generally offer the advantages of noninvasive, nondestructive, continuous, and simultaneous multianalyte monitoring. However, at this time, no general optical detection system has been developed. Since modern bioprocesses are extremely complex and differ from process to process (e.g., fungal antibiotic production versus mammalian cell cultivation), appropriate analytical systems must be set up from different basic modules, designed to meet the special demands of each particular process. In this minireview, some new applications in bioprocess monitoring of the following optical sensing principles will be discussed: UV spectroscopy, IR spectroscopy, Raman spectroscopy, fluorescence spectroscopy, pulsed terahertz spectroscopy (PTS), optical biosensors, in situ microscope, surface plasmon resonance (SPR), and reflectometric interference spectroscopy (RIF). 相似文献
6.
Ash WW Band HR Blume HT Camporesi T Chadwick GB Coombes RW Delfino MC Fernandez E Ford WT Gettner MW Goderre GP Groom DE Heltsley BK Hurst RB Johnson JR Lau KH Lavine TL Leedy RE Lippi I Maruyama T Messner RL Moromisato JH Moss LJ Muller F Nelson HN Peruzzi I Piccolo M Prepost R Qi N Read AL Ritson DM Rosenberg LJ Shambroom WD Sleeman JC Smith JG Venuti JP von Goeler E Verdini P Wald HB Weinstein R Wiser DE Zdarko RW 《Physical review letters》1985,55(20):2118-2121
7.
Fernandez E Ford WT Qi N Read AL Smith JG Camporesi T De Sangro R Marini A Peruzzi I Piccolo M Ronga F Blume HT Hurst RB Venuti JP Wald HB Weinstein R Band HR Gettner MW Goderre GP Meyer OA Moromisato JH Shambroom WD Sleeman JC von Goeler E Ash WW Chadwick GB Clearwater SH Coombes RW Kaye HS Lau KH Leedy RE Lynch HL Messner RL Moss LJ Muller F Nelson HN Ritson DM Rosenberg LJ Wiser DE Zdarko RW Groom DE Lee H Delfino MC Heltsley BK Johnson JR Lavine TL Maruyama T Prepost R 《Physical review letters》1985,54(15):1620-1623
8.
When spontaneous particle creation occurs in a strong gravitational field, it seems clear on physical grounds that the particle creation must back-react on the gravitation field. It is generally believed that in the semiclassical approximation this effect can be described by assigning an effective stress energy to the created particles, which acts as a source of the gravitational field via Einstein's equation. In this essay, I discuss an axiomatic approach for determining the renormalized value of this effective stress energy.This essay was awarded the third prize for 1977 by the Gravity Research Foundation. 相似文献
9.
Bauer O Guerasimova A Sauer S Thamm S Steinfath M Herwig R Janitz M Lehrach H Radelof U 《Rapid communications in mass spectrometry : RCM》2004,18(16):1821-1829
Peptide nucleic acid (PNA) is a novel class of DNA analogues in which the entire sugar-phosphate backbone is replaced by a pseudopeptide counterpart. Owing to its neutral character and the consequent lack of electrostatic repulsion, PNA exhibits very stable heteroduplex formation with complementary nucleic acid that is essentially ionic strength independent and enables hybridization under minimum salt conditions. This feature as well as its superior ion stability and easy ionization compared to DNA renders PNA very attractive for hybridization-based matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) applications. We have developed an approach to DNA characterization that takes advantage of multiplexed PNA hybridizations analyzed by MALDI-TOFMS. Our motivation was the further development of oligonucleotide fingerprinting, an efficient technique for cDNA and genomic DNA library characterization. Through positive 'charge-tagging' of PNA the efficiency of detection in MALDI-TOFMS was considerably enhanced permitting an unparalleled degree of multiplexing. Results from the simultaneous hybridization of 21 charge-tagged PNA hexamer oligonucleotides showed that genomic DNA and cDNA clones are successfully characterized on the basis of their hybridization profiles. The degree of multiplexing achieved may render a significant increase in throughput and hence efficiency of oligonucleotide fingerprinting possible. 相似文献
10.
On the plastic deformation of bulk syndiotactic polypropylene 总被引:1,自引:0,他引:1
The plastic deformation of syndiotactic polypropylene (sPP) bulk samples has been investigated. A structural comparison of
the initial states before and after plastic deformation by necking is carried out by X-ray diffraction observations. Independent
of the initial states (amorphous, semi-crystalline with different crystal phases), only the planar all-trans crystal form of sPP is present in the deformed samples after necking. Form these results, molecular mechanisms of the plastic
deformation in the neck zone of semi-crystalline polymers will be discussed.
Received: 11 February 1997 Accepted: 15 August 1997 相似文献