全文获取类型
收费全文 | 2792篇 |
免费 | 66篇 |
国内免费 | 7篇 |
专业分类
化学 | 2048篇 |
晶体学 | 35篇 |
力学 | 47篇 |
数学 | 336篇 |
物理学 | 399篇 |
出版年
2022年 | 35篇 |
2021年 | 26篇 |
2020年 | 46篇 |
2019年 | 31篇 |
2018年 | 21篇 |
2017年 | 28篇 |
2016年 | 41篇 |
2015年 | 45篇 |
2014年 | 62篇 |
2013年 | 141篇 |
2012年 | 129篇 |
2011年 | 172篇 |
2010年 | 96篇 |
2009年 | 67篇 |
2008年 | 150篇 |
2007年 | 165篇 |
2006年 | 172篇 |
2005年 | 166篇 |
2004年 | 152篇 |
2003年 | 117篇 |
2002年 | 119篇 |
2001年 | 48篇 |
2000年 | 29篇 |
1999年 | 28篇 |
1998年 | 30篇 |
1997年 | 27篇 |
1996年 | 34篇 |
1995年 | 24篇 |
1994年 | 34篇 |
1993年 | 29篇 |
1992年 | 25篇 |
1991年 | 37篇 |
1990年 | 25篇 |
1989年 | 18篇 |
1988年 | 19篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 31篇 |
1984年 | 35篇 |
1983年 | 22篇 |
1982年 | 33篇 |
1981年 | 24篇 |
1980年 | 23篇 |
1979年 | 27篇 |
1978年 | 21篇 |
1977年 | 26篇 |
1976年 | 19篇 |
1975年 | 18篇 |
1974年 | 20篇 |
1973年 | 18篇 |
排序方式: 共有2865条查询结果,搜索用时 15 毫秒
181.
Fletcher NH McGee WT Tarnopolsky AZ 《The Journal of the Acoustical Society of America》2002,111(3):1437-1444
The periodic re-voicing of the bell clappers of the Australian National Carillon in Canberra provided an opportunity for the study of the acoustic effects of this operation. After prolonged playing, the impact of the pear-shaped clapper on a bell produces a significant flat area on both the clapper and the inside surface of the bell. This deformation significantly decreases the duration of the impact event and has the effect of increasing the relative amplitude of higher modes in the bell sound, making it "brighter" or even "clangy." This effect is studied by comparing the spectral envelope of the sounds of several bells before and after voicing. Theoretical analysis shows that the clapper actually strikes the bell and remains in contact with the bell surface until it is ejected by a displacement pulse that has traveled around the complete circumference of the bell. The contact time, typically about 1 ms, is therefore much longer than the effective impact time, which is only a few tenths of a millisecond. Both the impact time and the contact time are reduced by the presence of a flat on the clapper. 相似文献
182.
Fletcher NH Tarnopolsky AZ Lai JC 《The Journal of the Acoustical Society of America》2002,111(3):1189-1196
Free rotational aerophones such as the bullroarer, which consists of a wooden slat whirled around on the end of a string, and which emits a loud pulsating roar, have been used in many ancient and traditional societies for ceremonial purposes. This article presents an experimental and theoretical investigation of this instrument. The aerodynamics of rotational behavior is elucidated, and relates slat rotation frequency to slat width and velocity through the air. Analysis shows that sound production is due to generation of an oscillating-rotating dipole across the slat, the role of the vortices shed by the slat being relatively minor. Apparent discrepancies between the behavior of a bullroarer slat and a slat mounted on an axle in a wind tunnel are shown to be due to viscous friction in the bearings of the wind-tunnel experiment. 相似文献
183.
The Tamao oxidation of alkoxysilanes was investigated computationally to determine the role of fluoride, a key additive, in this reaction. A sequence of fluoride equilibria as well as possible transition states, mediated by basic and neutral peroxide, respectively, were examined, and a potential energy surface was calculated which was consistent with the typical synthetic methods required for the conversion of alkoxysilanes to alcohols. 相似文献
184.
Several analogues of the 9-phenylthioxanthyl (S-pixyl) photocleavable protecting group have been synthesized, containing substituents on the 9-aryl ring and on the thioxanthyl backbone. Each analogue protected the 5'-hydroxy moiety of thymidine in good to excellent yield. The protected substrates were deprotected in 1:1 water:acetonitrile with irradiation at 300 nm, resulting in recovered thymidine in excellent yield, except for the nitro-substituted analogues which gave substantially lower yields. Substrates with 2,7-dibromo or 3-methoxy substitution on the thioxanthyl backbone were also deprotected efficiently with irradiation at 350 nm. Shorter irradiation times were observed in the less nucleophilic solvent mixture of 1:9 trifluoroethanol:acetonitrile, with no formation of secondary photooxidation products. Photodeprotection with high yields was also achieved in the absence of solvent, with no secondary photoproducts. 相似文献
185.
Pasa-Tolić L Lipton MS Masselon CD Anderson GA Shen Y Tolić N Smith RD 《Journal of mass spectrometry : JMS》2002,37(12):1185-1198
In the era of systems biology, computational and high-throughput experimental biological approaches are increasingly being combined to provide global snapshots of entire genomes and proteomes under tissue- and disease-specific conditions. The aim is to identify proteins changing in concentration and/or post-translational state and/or location, and develop a better molecular level understanding of the operation of biological systems. Here we describe an approach for comparative proteomics that builds upon the combination of high-performance nano-scale separations with the high-mass measurement accuracy, mass-resolving power and sensitivity of Fourier transform ion cyclotron resonance mass spectrometry to provide broad dynamic range, comprehensive and quantitative proteome measurements. 相似文献
186.
We exploit the approximate (broken) symmetries of time translation, time reversal, and Hamiltonian structure to obtain general scaling laws governing the process of pattern formation in weakly damped Faraday waves. Using explicit parameter symmetries we determine, for the case of two-frequency forcing, how the strength of observed three-wave interactions depends on the frequency ratio and on the relative phase of the two driving terms. These symmetry-based predictions are verified for numerically calculated coefficients, and help explain the results of recent experiments. 相似文献
187.
Iodomethyl-, chloromethyl-, and fluoromethyldimethylsulfonium salts, 4b-d, have been synthesized and are observed to be highly reactive molecules that exhibit extraordinary diversity with respect to the nature of their reactivity, undergoing facile direct substitution (S(N)2) reactions, but also being highly susceptible to electron-transfer reactions. Cyclic voltametry experiments indicated that the iodomethyldimethylsulfonium compound, 4b, is a potent electron acceptor, even surpassing the reactivity of perfluoro-n-alkyl iodides in that capacity. The iodo- and chloromethyldimethylsulfonium salts, 4b,c, as well as the analogous iodomethyltrimethylammonium salt, 3a, are shown to be reactive SET acceptors. 相似文献
188.
Background
It is well established that nucleus basalis magnocellularis (NbM) lesions impair performance on tests of sustained attention. Previous work from this laboratory has also demonstrated that pedunculopontine tegmental nucleus (PPTg) lesioned rats make more omissions on a test of sustained attention, suggesting that it might also play a role in mediating this function. However, the results of the PPTg study were open to alternative interpretation. We aimed to resolve this by conducting a detailed analysis of the effects of damage to each brain region in the same sustained attention task used in our previous work. Rats were trained in the task before surgery and post-surgical testing examined performance in response to unpredictable light signals of 1500 ms and 4000 ms duration. Data for PPTg lesioned rats were compared to control rats, and rats with 192 IgG saporin infusions centred on the NbM. In addition to operant data, video data of rats' performance during the task were also analysed. 相似文献189.
190.