全文获取类型
收费全文 | 1849篇 |
免费 | 118篇 |
国内免费 | 32篇 |
专业分类
化学 | 1282篇 |
晶体学 | 21篇 |
力学 | 56篇 |
综合类 | 3篇 |
数学 | 185篇 |
物理学 | 452篇 |
出版年
2023年 | 12篇 |
2022年 | 38篇 |
2021年 | 36篇 |
2020年 | 29篇 |
2019年 | 40篇 |
2018年 | 23篇 |
2017年 | 22篇 |
2016年 | 54篇 |
2015年 | 55篇 |
2014年 | 85篇 |
2013年 | 85篇 |
2012年 | 145篇 |
2011年 | 156篇 |
2010年 | 86篇 |
2009年 | 104篇 |
2008年 | 138篇 |
2007年 | 125篇 |
2006年 | 79篇 |
2005年 | 92篇 |
2004年 | 58篇 |
2003年 | 46篇 |
2002年 | 51篇 |
2001年 | 46篇 |
2000年 | 42篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 13篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 28篇 |
1992年 | 13篇 |
1991年 | 17篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 22篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 9篇 |
1973年 | 9篇 |
1971年 | 5篇 |
排序方式: 共有1999条查询结果,搜索用时 15 毫秒
31.
32.
Biflavonoids, widely distributed in natural plants, had strong biological activities including spasmolysis, peripheral vasodilatation, antibradykinin activity and antispasmogenic action against prostaglandin PGE1, inhibition of cyclic GMP and cyclic AMP phosphodiesterase and inhibition of hepatoma cells. Recently, some biflavonoids were demonstrated to enhance suppersion of lymphocyte proliferation, inhibition of phospholipaseCr1, anti-inflammatory activity, anti-HIV activity, anticompleme… 相似文献
33.
We have previously demonstrated that H-bond arrangement has a significant influence on the energetics, structure and chemistry of water clusters. In this work, the effect of H-bond orientation on the dissociation of hydrogen fluoride with seven water molecules is studied by means of graph theory and high level ab initio methods. It is found that cubic structures of HF(H(2)O)(7) are more stable than structures of other topologies reported in the literature. Electronic calculations on all possible H-bond orientations of cubie-HF(H(2)O)(7) show that ionized structures are energetically more favorable than nonionized ones. This is an indication that seven water molecules might be capable of ionizing hydrogen fluoride. 相似文献
34.
Three novel supramolecular arrays of zigzag polyaromatic salts are reported. Both the conformation and disposition of the dications are subjected to various noncovalent interactions. Thus, the presence or absence of the π‐π interacting enclathrated molecules, the efficient packing and the involved hydrogen bonding interactions of anions, as well as the increased hydrophobic property of the dications themselves exert influence. 相似文献
35.
Further studies on the bark of Taiwania cryptomerioides found four new podocarpane derivatives, 1beta,13-dihydroxy-8,11,13-podocarpatriene (1), 14,18-dihydroxy-13-methoxy-8,11,13-podocarpatriene (2), 1beta,14-dihydroxy-13-methoxy-8,11,13-podocarpatriene-2,7-dione (3), and 3beta,14-dihydroxy-13-methoxy-8,11,13-podocarpatrien-7-one (4), together with a known 1beta,13,14-trihydroxy-8,11,13-podocarpatrien-7-one (5). Those structures were elucidated principally from spectral evidence. 相似文献
36.
Four new lignans, 3',4'-O,O-demethylenehinokinin (1), chamalignolide (2), 8'beta-hydroxyhinokinin (3) and 7beta,8beta-epoxyzuonin A (4), as well as (-)-hinokinin (5), and (-)-zuonin A (6), were isolated from the heartwood of Chamaecyparis obtusa var. formosana. The structures of these lignans were unambiguously determined by spectroscopic methods. And the absolute configuration of 1 was elucidated with a circular dichroism (CD) spectrum. 相似文献
37.
Modified nucleosides in human serum. 总被引:1,自引:0,他引:1
E P Mitchell L Evans P Schultz R Madsen J W Yarbro C W Gehrke K Kuo 《Journal of chromatography. A》1992,581(1):31-40
Methylated purines and pyrimidines derived from the degradation of transfer ribonucleic acid have been shown to be excreted in abnormal amounts in the urine of patients with cancer. Recent technology developed by Gehrke and Kuo has allowed the separation and quantification of modified nucleosides in serum using reversed-phase high-performance liquid chromatography with diode-array measurement. Serum levels of ten modified nucleosides were measured in 37 normal healthy adults to establish normal values and to correlate activity with age and sex. In addition, serum levels of patients with several malignancies were measured to determine activity in these diseases. Levels of modified nucleosides in normal individuals were consistently reproducible and showed no significant variation among males versus females or with age. Patients with malignant diseases showed consistent elevations and these were highest in patients with more advanced disease. The evidence of no significant differences in the mean levels of modified nucleosides in serum with age or sex in normal adults and elevations in patients with malignancies demonstrate the potential value of modified nucleosides as cancer biomarkers. 相似文献
38.
T. Itagaki E. Rokuta H.‐S. Kuo K. Nomura T. Ishikawa B.‐L. Cho I.‐S. Hwang T. T. Tsong C. Oshima 《Surface and interface analysis : SIA》2007,39(4):299-303
We observed field emission microscopy (FEM) patterns of noble‐metal (NM) covered W nano‐tips with three different apex structures fabricated by field evaporation. Each of the three tips was terminated with a single atom, three atoms or ten atoms. We investigated the temporal changes in the FEM of these tips to discuss the stabilities in the spatial distributions of the field emission (FE) beams. The single‐atom tip showed two characteristics that were superior to the others. First, the beams emitted from the single‐atom tip were the most collimated among the three tips (the semi‐cone angle of 1.0° , FWHM). Second, adsorption of residual gas had little influence on FE from the single‐atom tip, while the other tips were easily contaminated even at ultra high vacuum, resulting in the emission fluctuation. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
39.
Two crystalline compounds from the bark of Taiwania cryptomerioides Hayata were isolated. One of them was identified as 6,7-dehydroroyleanone and the other was shown to be a new derivative of royleanone having a structure of 6β-acetoxy-7α-hydroxyroyleanone. 相似文献
40.
The intrinsic characteristics of radical pairs produced in squalane and in cetane receiving high gamma-dose are extensively studied with the EPR technique at temperatures from 77°K up to 150°K. The spectra of the paired radicals occur at g=4 with a very low transition probability in contrast to that of isolated radicals which appear at g=2 A well-resolved hyperfine spectrum corresponding to the species (CH3CH2.CH2CH3) is observed in cetane. The isothermal decay rates of radical pairs in cetane below 100°K are significantly slow; however, the decay kinetics at 150°K is first order with rate constant=1.86 min?1. A relatively slower decay rate is obtained for isolated radicals suggesting that the decay mechanism of paired radicals is through geminate recombination. The relative inter-radical distance in radical pairs is known from a decay curve as a function of temperature. The yields of radical pairs are low in both matrices, only few percents of those of isolated radicals. The formation mechanisms of paired radicals with direct radiolytic bond scission process are discussed in connection with the experimental observations. 相似文献