全文获取类型
收费全文 | 2196篇 |
免费 | 100篇 |
国内免费 | 22篇 |
专业分类
化学 | 1450篇 |
晶体学 | 16篇 |
力学 | 39篇 |
数学 | 226篇 |
物理学 | 587篇 |
出版年
2024年 | 2篇 |
2023年 | 13篇 |
2022年 | 17篇 |
2021年 | 62篇 |
2020年 | 53篇 |
2019年 | 45篇 |
2018年 | 37篇 |
2017年 | 39篇 |
2016年 | 100篇 |
2015年 | 64篇 |
2014年 | 80篇 |
2013年 | 177篇 |
2012年 | 158篇 |
2011年 | 178篇 |
2010年 | 98篇 |
2009年 | 90篇 |
2008年 | 127篇 |
2007年 | 130篇 |
2006年 | 119篇 |
2005年 | 117篇 |
2004年 | 105篇 |
2003年 | 88篇 |
2002年 | 79篇 |
2001年 | 39篇 |
2000年 | 41篇 |
1999年 | 26篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 21篇 |
1995年 | 14篇 |
1994年 | 16篇 |
1993年 | 30篇 |
1992年 | 17篇 |
1991年 | 6篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 2篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1975年 | 4篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有2318条查询结果,搜索用时 31 毫秒
71.
This study was undertaken to establish one of the most reliable sampling methods and to precisely evaluate the bias involved in the collection of airborne pollutant samples. For the purpose of our study, we investigated the performance of three different types of sampling techniques by measuring major aromatic volatile organic compounds (VOC) in outdoor air; the target analytes specifically include benzene, toluene, ethylbenzene, and xylene (commonly called BTEX). As the first step of our approach, we designed and developed a multi-channel sampling system consisting of a six-port mass flow controller (SJU-MFC) system. Because this system allowed the collection of up to six replicate samples, our measurement results were analyzed and screened statistically for the derivation of high-quality BTEX data. The feasibility of this sampling system was further tested through a comparison with concurrent measurement data sets obtained by two additional, but independent, sampling techniques: (1) automatic continuous sampler (ACS) and (2) on-line GC (O-GC) system. Based on the data sets collected concurrently by three different sampling methods, we attempted to evaluate the compatibility of sampling techniques. Although the results obtained by SJU-MFC system were not statistically different from those of the O-GC system, they were moderately distinguishable from those of ACS. Such patterns were seen consistently, when examined by correlation analysis. The overall results of our study thus generally point out that the compatibility of data sets, when the proper caution is taken, improve significantly among different sampling methodologies. 相似文献
72.
Peter Brenneisen Jinsook Oh Meinhard Wlaschek Jutta Wenk Karlis Briviba Christina Hommel Gemot Herrmann Helmut Sies Karin Scharffetter-Kochanek 《Photochemistry and photobiology》1996,64(5):877-885
Abstract— The wavelength dependence for the regulation of two major matrix-metalloproteinases, interstitial collagenase (MMP-1) and stromelysin-1 (MMP-3), and their major inhibitor, tissue inhibitor of metalloproteinases (TIMP-1), was studied in human dermal fibroblasts in vitro. Monochromatic irradiation at 302, 307, 312 and 317 nm with intensities ranging from 20 to 300 J/m2 increased MMP-1 and MMP-3 mRNA steady-state levels and the secretion of the corresponding proteins up to 4.4-fold, whereas almost no increase was observed at wavelengths <290 nm. In contrast, the synthesis of TIMP-1 increased only marginally. This unbalance may contribute to the severe connective tissue damage related to photoaging of the skin. The wavelengths responsible for MMP-1 and MMP-3 induction reported here are distinct from the absorption spectrum of DNA and are different from results previously reported in the literature. Importantly, they overlap with wavelengths whose intensity is predicted to increase on the earth's surface upon ozone depletion. Intensities and particular wavelengths used in our studies in vitro can be absorbed readily by fibroblasts within the skin in vivo and, thus, are relevant for risk assessment and development of protective agents. 相似文献
73.
Biosurface fabrication using the Fab′ fragment of immunoglobulin (IgG) was carried out by self-assembly (SA) technique. The pepsin-digested monoclonal antibody (Mab) against bovine insulin containing the F(ab′)2 fragment and residual proteins was separated using affinity chromatography and dialysis. To prevent the nonspecific binding of F(ab′)2 onto gold (Au) substrate, the native disulfide bridge was reduced using dithiothreitol (DTT) to convert F(ab′)2 into Fab′, which made the immobilization to be carried out via the native thiol (–SH) group. The fabricated biosurface using SA technique showed the formation of stable thin film through AFM topography. Through the concentration change of DTT and Fab′, the absorption characteristics against the Au surface were investigated using surface plasmon resonance (SPR) with the flow cell. The amount of immobilized antibody fragment and the antigen binding capacity were regulated with respect to the reduction state and concentration of F(ab′)2. Based on the biosurface of the fabricated Fab′, the insulin-detection was carried out by the measurement of SPR. The proposed antibody surface could successfully detect the bovine insulin at the concentration from 100 ng/mL to 10 μg/mL. 相似文献
74.
75.
76.
The reaction of methyleneaziridine 1 with carbon pronucleophiles (2, H-CR3) proceeds smoothly in the presence of a palladium catalyst affording the corresponding hydrocarbonation products 5 in good to high yield. 相似文献
77.
Moonhyun Oh 《Journal of organometallic chemistry》2003,687(1):78-84
The cationic manganese tricarbonyl complexes containing η6-2-methylhydroquinone (2a), η6-2,3-dimethylhydroquinone (3a), η6-2-t-butylhydroquinone (4a), η6-tetramethylhydroquinone (5a) and η6-4,4′-biphenol (6a) are readily deprotonated to the corresponding neutral (η5-semiquinone)Mn(CO)3 (2b-6b) and anionic (η4-quinone)Mn(CO)3− (2c-5c) complexes. The X-ray structures of 2b-6b feature strong intermolecular hydrogen bonding interactions that result in the formation of supramolecular organometallic networks. Significantly, the substitution pattern at the semiquinone ring affects the stereochemistry of the hydrogen bonding interactions. NMR spectra of 2b, 3b and 5b reveal dynamic hydrogen bonding in solution. 相似文献
78.
Vinyl sulfones are stereoselectively prepared in high yields in a one-pot by the Wittig-Horner reaction via phosphorylation of methyl phenyl sulfone at low temperature. 相似文献
79.
Tetrachlorosilane was found to be a mild as well as selective catalyst for dithioacetalization of the carbonyl compounds. 相似文献
80.