全文获取类型
收费全文 | 969篇 |
免费 | 17篇 |
国内免费 | 6篇 |
专业分类
化学 | 438篇 |
晶体学 | 8篇 |
力学 | 15篇 |
数学 | 111篇 |
物理学 | 265篇 |
无线电 | 155篇 |
出版年
2019年 | 9篇 |
2018年 | 5篇 |
2017年 | 8篇 |
2016年 | 12篇 |
2015年 | 6篇 |
2014年 | 9篇 |
2013年 | 29篇 |
2012年 | 33篇 |
2011年 | 39篇 |
2010年 | 21篇 |
2009年 | 12篇 |
2008年 | 46篇 |
2007年 | 62篇 |
2006年 | 63篇 |
2005年 | 45篇 |
2004年 | 38篇 |
2003年 | 22篇 |
2002年 | 52篇 |
2001年 | 25篇 |
2000年 | 30篇 |
1999年 | 11篇 |
1998年 | 17篇 |
1997年 | 23篇 |
1996年 | 13篇 |
1995年 | 11篇 |
1994年 | 14篇 |
1993年 | 18篇 |
1992年 | 17篇 |
1991年 | 18篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 18篇 |
1987年 | 22篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1977年 | 16篇 |
1976年 | 9篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1972年 | 12篇 |
1971年 | 9篇 |
1970年 | 7篇 |
排序方式: 共有992条查询结果,搜索用时 15 毫秒
1.
Fabry-Perot resonators have long been advocated to improve the limited contrast ratio of multiple quantum well optical modulators used in photonic switches based on self electrooptic effect devices (SEEDs) and in other array based optical interconnection schemes. Using data on field dependent GaAs/AlGaAs quantum well absorption and refraction, we have modelled the reflectivity, modulation depth and contrast ratio of resonant modulators. Our results are generally valid for any quantum well modulator and demonstrate 23the important role played by electro-refraction even in regions of strong absorption. Resonators give large contrast ratios but there are trade-offs in the maximum reflectivity change achievable with Fabry-Perot resonators compared to simple modulators. The model gives the optimum number of quantum wells and reflectivity values required to make a resonator at any wavelength for a given quantum well structure. Understanding the limits of Fabry-Perot quantum well modulator performance is important for their application in symmetric self electrooptic effectiveness for photonic switching where modulation and detection properties are both used and for optical interconnection systems. 相似文献
2.
The analysis in this paper looks at two important elements in modelling the market for timber in the United States. First, the issue of directional causality between price and quantity and its implications in a modelling effort is investigated. Second, the extent of the geographic market for timber is discussed and a method of detecting it is suggested. The method for detecting the extent of the geographical market is tractable and can be applied in a straightforward way. Both considerations are applied to the softwood lumber market in the United States. 相似文献
3.
4.
Raman spectra of molten NaClO3 and KClO3 and of aqueous solutions of these salts were measured over the frequency interval from 50 to 1200 cm−1|. Infrared emission spectra of the molten chlorates and of chlorate-nitrate mixtures were recorded, and absorption spectra of aqueous sodium and potassium chlorate also were determined. The ν3(e) and ν4(e) modes of ClO−3 were split in the molten salt and aqueous solution spectra, and a single, weak band was observed between ca. 80 and 200 cm− in the Raman spectra of molten NaClO3 and KClO3. 相似文献
5.
The Fermi and Coulomb holes of the 21
S state of the helium isoelectronic sequence are investigated. Several interesting differences between the results obtained and those which might be expected on the basis of the corresponding 23
S state are pointed out and discussed. 相似文献
6.
Protein tyrosine phosphatase 1B (PTP1B) functions by removing the phosphoryl group from tyrosinephosphorylated proteins in
insulin signaling and metabolism. The regeneration of the active site involves a sulphenylamide intermediate derived from
the intrastrand cross-linking between the catalytic serine and the neighboring backbone nitrogen. Two mechanisms have been
proposed for the formation of the sulphenylamide intermediate and the subsequent reactivation of the catalytic site. In the
current work, the proposed mechanisms have been investigated by the use of density functional theory calculations. Our results
suggest that these two mechanisms have similar overall energy barriers and that the preferred route will be determined by
the availability of hydrogen peroxide or other oxidizing reagents. 相似文献
7.
8.
9.
Ho-Shen Lin Ashraff A. Rampersaud Karen Zimmerman Mitchell I. Steinberg Donald B. Boyd 《中国化学会会志》1993,40(3):273-282
A series of nonpeptide angiotensin II receptor antagonists was synthesized via palladium-assisted cross coupling of aryl stannane and cycloalkenyl triflates and subsequent alkylation of silyl-protected imidazole. Our compounds, which have a terminal five- to seven-membered cycloalkenyl ring, are compared to DuPont EXP7711, an N-[(2′-carboxybiphenylyl)methyl]imidazole, which has a terminal phenyl moiety. Physicochemical properties of the compounds, such as lipophilicity, steric bulk, conformation, and the relative spatial proximity of the 2-carboxyl and the middle phenyl, are quantitated by computational chemistry. Potency in terms of binding affinity to AT1 receptors in rat adrenal glomerulosa and rabbit aorta is maximized when the terminal ring is aromatic. 相似文献
10.
N-Unsubstituted diaryl, alkyl aryl and dialkyl ketimines are prepared in good yield at ambient temperature by the reaction of KOBut on oxaziridines having an α-hydrogen atom on the N-alkyl substituent; the mechanism and stereochemistry of the reaction are discussed. 相似文献