全文获取类型
收费全文 | 711篇 |
免费 | 14篇 |
国内免费 | 1篇 |
专业分类
化学 | 451篇 |
晶体学 | 7篇 |
力学 | 45篇 |
数学 | 61篇 |
物理学 | 162篇 |
出版年
2022年 | 6篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2017年 | 8篇 |
2016年 | 7篇 |
2015年 | 9篇 |
2014年 | 18篇 |
2013年 | 38篇 |
2012年 | 21篇 |
2011年 | 32篇 |
2010年 | 12篇 |
2009年 | 21篇 |
2008年 | 24篇 |
2007年 | 27篇 |
2006年 | 29篇 |
2005年 | 39篇 |
2004年 | 36篇 |
2003年 | 25篇 |
2002年 | 22篇 |
2001年 | 4篇 |
2000年 | 12篇 |
1999年 | 21篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 6篇 |
1992年 | 15篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 7篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 19篇 |
1984年 | 11篇 |
1983年 | 18篇 |
1982年 | 17篇 |
1981年 | 17篇 |
1980年 | 20篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1972年 | 8篇 |
1969年 | 8篇 |
1964年 | 4篇 |
排序方式: 共有726条查询结果,搜索用时 343 毫秒
51.
52.
A series of novel 4-amino-1,4-benzodiazepine-2,5-diones was synthesized via two pathways. The first method involved reductive alkylation of unsymmetrical hydrazines with glyoxylic acid, followed by Fisher esterification. The resulting N-aminoglycinate ethyl ester was subsequently o-nitrobenzoylated, reduced, and thermally cyclized to obtain 4-dialkylaminobenzodiazepinones. In the second method methylhydrazine was acetylated at Nα then benzoylated at Nβ to give 1,2-diacylhydrazines. Alkylation with ethyl bromoacetate and reduction of the nitro group, followed by thermal cyclization yielded 4-acetamidobenzodiazepinones. All title compounds were evaluated in mice in MES seizure and sc Met seizure threshold tests for anticonvulsant activity, and in the rotorod test for neurotoxicity. Activity and toxicity were both minimal. 相似文献
53.
Rangel Elidiane C. Cruz Nilson C. da Kayama Milton E. Rangel Rita C. C. Marins Nazir Durrant Steven F. 《Plasmas and Polymers》2004,9(1):1-22
Diamond-like carbon (DLC) films were grown from radiofrequency plasmas of acetylene-argon mixtures, at different excitation powers, P. The effects of this parameter on the plasma potential, electron density, electron temperature, and plasma activity were investigated using a Langmuir probe. The mean electron temperature increased from about 0.5 to about 7.0 eV while the mean electron density decreased from about 1.2 × 109 to about 0.2 × 109 cm–3 as P was increased from 25 to 150 W. Both the plasma potential and the plasma activity were found to increase with increasing P. Through actinometric optical emission spectrometry, the relative concentrations of CH, [CH], and H, [H], in the discharge were mapped as a function of the applied power. A rise in [H] and a fall in [CH] with increasing P were observed and are discussed in relation to the plasma characteristics and the subimplantation model. The optical properties of the films were calculated from ultraviolet-visible spectroscopic data; the surface resistivity was measured by the two-point probe method. The optical gap, E
G, and the surface resistivity, s, fall with increasing P. E
G and s are in the ranges of about 2.0–1.3 eV and 1014–1016 /, respectively. The plasma power also influences the film self-bias, V
b, via a linear dependence, and the effect of V
b on ion bombardment during growth is addressed together with variation in the relative densities of sp2 and sp3 bonds in the films as determined by Raman spectroscopy. 相似文献
54.
55.
56.
57.
The extent to which the occurrence of falls, the dominant feature of human attempts to balance a stick at their fingertip,
can be predicted is examined in the context of the “Dragon-King” hypothesis. For skilled stick balancers, fluctuations in
the controlled variable, namely the vertical displacement angle θ, exhibit power law behaviors. When stick balancing is made
less stable by either decreasing the length of the stick or by requiring the subject to balance the stick on the surface of
a table tennis racket, systematic departures from the power law behaviors are observed in the range of large θ. This observation
raises the possibility that the presence of departures from the power law in the large length scale region, possibly Dragon-Kings,
may identify situations in which the occurrence of a fall is more imminent. However, whether or not Dragon-Kings are observed,
there is a Weibull-type survival function for stick falling. The possibility that increased risk of falling can, at least
to some extent, be predicted from fluctuations in the controlled variable before the event occurs has important implications
for the development of preventative strategies for the management of phenomena ranging from earthquakes to epileptic seizures
to falls in the elderly. 相似文献
58.
The synthesis, characterization and in situ catalytic performance of new unsymmetric N,N′‐disubstituted imidazolium‐based dicationic salts in Mizoroki–Heck coupling of acrylates with aryl bromides under aerobic conditions are described. A series of flexible dicationic salts with varying steric and electronic properties were synthesized in good to excellent yields. All the salts were well characterized using spectroscopic techniques. X‐ray diffraction analysis of two salts with the same dicationic backbone and different counter anions shows that the ligand adopts two different conformations which are influenced by the nature of the anion. Thus, the ligand is capable of changing its conformation according to the change in environment due to its flexible nature. All the synthesized imidazolium salts were found to be active in in situ palladium‐catalysed Mizoroki–Heck coupling under aerobic conditions. Amongst the salts, the hydroxyl‐functionalized imidazolium salt, incorporating the features of both bidentate chelating O,O ligand and carbene, shows the maximum catalytic activity. A variety of aryl and heteroaryl methyl and ethyl cinnamates were synthesized using these imidazolium salts as preligands. In addition, NMR studies confirm in situ generation of normal N‐heterocyclic carbenes from the C‐2 position of imidazol‐2‐ylidene ring. The mercury poisoning test was also performed to ascertain the nature of catalytically active palladium species. Aerobic conditions, low catalytic loading (0.5 mol%), shorter reaction times, broad functional group tolerance and good to excellent isolated yields are some of the significant features of the novel catalytic systems described here. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
59.
Stephen Brand John Milton Martin F. Jones Christopher M. Rayner 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):367-368
A new method for the synthesis of cyclic and acyclic optically active S,O-acetals is reported, along with subsequent stereocontrolled transformations, culminating in a synthesis of the important antiviral nucleoside Lamivudine. 相似文献
60.
Yuanzhong Yang Reinhard I. Boysen Chadin Kulsing Maria T. Matyska Joseph J. Pesek Milton T. W. Hearn 《Journal of separation science》2013,36(18):3019-3025
The retention behavior of a set of polar peptides separated on a silica hydride stationary phase was examined with a capillary HPLC system coupled to ESI‐MS detection. The mobile phases consisted of formic acid or acetic acid/acetonitrile/water mixtures with the acetonitrile content ranging from 5 to 80% v/v. The effects on peptide retention of these two acidic buffer additives and their concentrations in the mobile phase were systematically investigated. Strong retention of the peptides on the silica hydride phase was observed with relatively high‐organic low‐aqueous mobile phases (i.e. under aqueous normal‐phase conditions). However, when low concentrations of acetic acid were employed as the buffer additive, strong retention of the peptides was also observed even when high aqueous content mobile phases were employed. This unique feature of the stationary phase therefore provides an opportunity for chromatographic analysis of polar peptides with water‐rich eluents, a feature usually not feasible with traditional RP sorbents, and thus under conditions more compatible with analytical green chemistry criteria. In addition, both isocratic and gradient elution procedures can be employed to optimize peptide separations with excellent reproducibility and resolution under these high aqueous mobile phase conditions with this silica hydride stationary phase. 相似文献