全文获取类型
收费全文 | 1929篇 |
免费 | 55篇 |
国内免费 | 9篇 |
专业分类
化学 | 1204篇 |
晶体学 | 21篇 |
力学 | 38篇 |
数学 | 147篇 |
物理学 | 367篇 |
无线电 | 216篇 |
出版年
2023年 | 12篇 |
2022年 | 14篇 |
2021年 | 24篇 |
2020年 | 23篇 |
2019年 | 42篇 |
2018年 | 22篇 |
2017年 | 28篇 |
2016年 | 45篇 |
2015年 | 29篇 |
2014年 | 36篇 |
2013年 | 93篇 |
2012年 | 108篇 |
2011年 | 125篇 |
2010年 | 61篇 |
2009年 | 51篇 |
2008年 | 116篇 |
2007年 | 108篇 |
2006年 | 137篇 |
2005年 | 99篇 |
2004年 | 68篇 |
2003年 | 50篇 |
2002年 | 60篇 |
2001年 | 45篇 |
2000年 | 35篇 |
1999年 | 23篇 |
1998年 | 21篇 |
1997年 | 18篇 |
1996年 | 25篇 |
1995年 | 25篇 |
1994年 | 20篇 |
1993年 | 26篇 |
1992年 | 31篇 |
1991年 | 24篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 20篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 24篇 |
1984年 | 20篇 |
1983年 | 14篇 |
1982年 | 16篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 19篇 |
1976年 | 15篇 |
1975年 | 16篇 |
1974年 | 20篇 |
1973年 | 21篇 |
排序方式: 共有1993条查询结果,搜索用时 531 毫秒
161.
Merritt Smith Kaitlyn Stambaugh Lauren Smith Hye-Jin Son Amanda Gardner Scott Cordova Krystal Posey Donald Perry Alexandru S. Biris 《Vibrational Spectroscopy》2009,49(2):288-297
Adsorption properties of acetylsalicylic acid (AA), ibuprofen and acetaminophen deposited from volatile solvents with varying protic/aprotic properties on vacuum-evaporated silver films were characterized using surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman spectroscopy (SERS). SERS preferentially enhances monolayer Raman shifts, while SEIRA can enhance the infrared absorbance of the monolayer and multilayers. To our best knowledge, this is the first reported study of these molecules using a combination of SERS/SEIRA. SERS revealed that AA and ibuprofen adsorbed ionically in monolayers, independent of the deposition solvents used in the process. SEIRA experiments showed that AA multilayers condensed molecularly using a deposition solvent with polar bonds. However, when an alkane deposition solvent with non-polar bonds such as n-heptane was used, AA adsorbed as acetylsalicylate ions in the first few multilayers, while ibuprofen always adsorbed as the free acid in the multilayer. These ionization trends depend upon the affinity of AA and ibuprofen for the underlying silver film. TPD experiments on silver powders further demonstrated that ibuprofen affinity for silver was less than AA. Furthermore, SEIRA indicated that acetaminophen adsorbed as multilayers of metastable polymorphs using protic or polar aprotic deposition solvents. Protic deposition solvents gave higher quality SERS spectra of an acetaminophen monolayer in comparison to polar aprotic deposition solvents. Such studies could find significant applications in biochemical and nanotechnology processes such as drug delivery, catalysis, and tissue engineering and will contribute to the understanding of the impact and fate of analgesics released into the environment. 相似文献
162.
Grant Martin Steve Leibson 《电子产品世界》2008,(10)
本系列深度分析文章将调查13种不成功的处理器种群,探索了造成每一个处理器种群死亡的主要设计错误。每一个主要设计错误还用一个或几个例子来阐述。 相似文献
163.
Schoolyard Shade and Sun Exposure: Assessment of Personal Monitoring During Children's Physical Activity
下载免费PDF全文
![点击此处可从《Photochemistry and photobiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jennifer K. Vanos Grant R. McKercher Kylie Naughton Marc Lochbaum 《Photochemistry and photobiology》2017,93(4):1123-1132
Childhood exposure to ultraviolet radiation (UVR) is a major risk factor for the development of melanoma later in life. However, it is challenging to accurately determine personal outdoor exposure to UVR, specifically erythemally weighted UVR (UVEry), due to technological constraints, variable time–activity patterns, and the influence of outdoor environmental design. To address this challenge, this study utilized mobile and stationary techniques to examine the UVEry exposures of 14 children in a schoolyard in Lubbock, TX, in spring 2016. The aims of the study were to examine the influence of artificial shade on personal UVEry exposures and to assess full sun exposure ratios (ERs) within the same playground microenvironment. On average, personal wrist dosimeters worn during play in the sun measured 18% of the total onsite UVEry measured by a stationary UV pyranometer. Shade was found to significantly reduce the personal UVEry exposures by 55%, UVB280–315 nm exposures by 91%, and the overall solar radiation by 84%. Substantial benefits can be garnered through focused design of children's recreational space to utilize shade—both natural and artificial—to reduce UVR exposures during play, and to extend safe outdoor stays. Finally, although the wrist is a practical location for a dosimeter, it often underestimates full exposures, particularly during physical activity. 相似文献
164.
This report describes the use of Pd(II)/bis-sulfoxide 1 catalyzed intra- and intermolecular allylic C-H amination reactions to rapidly diversify structures containing a sensitive β-lactam core similar to that found in the monobactam antibiotic Aztreonam. Pharmacologically interesting oxazolidinone, oxazinanone, and linear amine motifs are rapidly installed with predictable and high selectivities under conditions that use limiting amounts of substrate. Additionally, we demonstrate for the first time that intramolecular C-H amination processes may be accelerated using catalytic amounts of a Lewis acid co-catalyst [Cr(III)(salen)Cl 2]. 相似文献
165.
Lauren M. Chapman Laura T. Kliman Christopher L. Hamblett 《Tetrahedron letters》2010,51(11):1517-5418
We have developed a reaction that affords the selective preparation of hexahydro-2H-pyrido[2,1-a]isoquinoline dienes, allenes, or alkenes via an intramolecular Heck cyclization of an aryl chloride with an alkyne. Tricyclic isoquinoline core structures of this nature are difficult to access by alternative methods. The unsaturated product formed can be partially controlled by choice of ligand and reaction solvent. 相似文献
166.
Lauren M. Fleury 《Tetrahedron letters》2010,51(18):2427-5683
A protocol for the generation of allyl Grignard reagents via the catalytic activation of allyl halides is described herein. Subsequent nucleophilic addition to carbonyl derivatives provided the desired homo allylic alcohols in excellent yields (84-99%). Evidence suggests that titanocene dichloride catalyzes the formation of an allyl Grignard species which reacts solely with the carbonyl electrophile as evidenced by the complete absence of Würtz coupling. This methodology will have wide-ranging applicability in the generation of highly reactive organometallic reagents. 相似文献
167.
The complex [Pt(9S3)(SbPh3)(Ph)](PF6) forms directly from [Pt(9S3)(SbPh3)2](PF6)2 during the room temperature crystallization of the latter in nitromethane. The crystal structure shows a five-coordinate Pt(II) center containing the tridentate thiacrown ligand, a Sb donor from the triphenylstibine ligand, and a σ-coordinating phenyl group. The phenyl group forms via Sb-C bond cleavage from one of the SbPh3 ligands in the bis complex. 相似文献
168.
Alexander Zawaira Marco Gallotta Natasha Beeton-Kempen Lauren Coulson Patrick Marais Michelle Kuttel Jonathan Blackburn 《Computational Biology and Chemistry》2010,34(1):42-52
In this work, a model for the interaction between CYP2B4 and the FMN domain of rat P450-oxidoreductase is built using as template the structure of a bacterial redox complex. Amino acid residues identified in the literature as cytochrome P450 (CYP)–redox partner interfacial residues map to the interface in our model. Our model supports the view that the bacterial template represents a specific electron transfer complex and moreover provides a structural framework for explaining previous experimental data.We have used our model in an exhaustive search for complementary pairs of mammalian CYP and P450-oxidoreductase (POR) charge clusters. We quantitatively show that among the previously defined basic clusters, the 433K–434R cluster is the most dominant (32.3% of interactions) and among the acidic clusters, the 207D–208D–209D cluster is the most dominant (29%). Our analysis also reveals the previously not described basic cluster 343R–345K (16.1% of interactions) and 373K (3.2%) and the acidic clusters 113D–115E–116E (25.8%), 92E–93E (12.9%), 101D (3.2%) and 179E (3.2%).Cluster pairings among the previously defined charge clusters include the pairing of cluster 421K–422R to cluster 207D–208D–209D. Moreover, 433K–434R and 207D–208D–209D, respectively the dominant positively and negatively charged clusters, are uncorrelated. Instead our analysis suggests that the newly identified cluster 113D–115E–116E is the main partner of the 433K–434R cluster while the newly described cluster 343R–345K is correlated to the cluster 207D–208D–209D. 相似文献
169.
Krow GR Liu N Sender M Lin G Centafont R Sonnet PE DeBrosse C Ross CW Carroll PJ Shoulders MD Raines RT 《Organic letters》2010,12(23):5438-5441
CD spectra for homooligomers (n = 4, 6, 8) of (1S,4R,5R)-5-syn-carboxy-2-azabicyclo[2.1.1]hexane (MPCA), a methano-bridged pyrrolidine β-carboxylic acid, suggest an ordered secondary structure. Even in the absence of internal hydrogen bonding, solution NMR, X-ray, and in silico analyses of the tetramer are indicative of conformations with trans-amides and C(5)-amide-carbonyls oriented toward the C(4) bridgehead. This highly constrained β-amino acid could prove useful in the ongoing development of well-defined foldamers. 相似文献
170.
Oluwatosin O. Dada Lauren M. Ramsay Jane A. Dickerson Nathan Cermak Rong Jiang Cuiru Zhu Norman J. Dovichi 《Analytical and bioanalytical chemistry》2010,397(8):3305-3310
We report a multiplexed capillary electrophoresis system employing an array of 32 capillaries with a micromachined sheath-flow
cuvette as the detection chamber. The sample streams were simultaneously excited with a 473-nm laser beam, and the fluorescence
emission was imaged on a CCD camera with a pair of doublet achromat lens. The instrument produced mass detection limits of
380 ± 120 yoctomoles for fluorescein in zone electrophoresis. Capillary isoelectric focusing of fluorescent standards produced
peaks with an average width of 0.0029 ± 0.0008 pH. Capillary coating stability limits the reproducibility of the analysis. 相似文献