全文获取类型
收费全文 | 575篇 |
免费 | 5篇 |
国内免费 | 1篇 |
专业分类
化学 | 465篇 |
晶体学 | 7篇 |
力学 | 5篇 |
数学 | 47篇 |
物理学 | 57篇 |
出版年
2020年 | 7篇 |
2019年 | 3篇 |
2018年 | 3篇 |
2015年 | 6篇 |
2014年 | 8篇 |
2013年 | 29篇 |
2012年 | 16篇 |
2011年 | 17篇 |
2010年 | 12篇 |
2009年 | 14篇 |
2008年 | 19篇 |
2007年 | 29篇 |
2006年 | 34篇 |
2005年 | 36篇 |
2004年 | 46篇 |
2003年 | 33篇 |
2002年 | 24篇 |
2001年 | 15篇 |
2000年 | 10篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 7篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 4篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 12篇 |
1975年 | 17篇 |
1974年 | 8篇 |
1973年 | 4篇 |
1972年 | 3篇 |
1971年 | 4篇 |
1969年 | 3篇 |
1968年 | 5篇 |
排序方式: 共有581条查询结果,搜索用时 15 毫秒
531.
532.
Ken-Ichi Kitayama 《Fiber and Integrated Optics》2000,19(2):167-186
The architecture of fiber-radio mm-wave wireless access systems critically depends upon the optical mm-wave generation and transport techniques. Four optical mm-wave generation and transport techniques: 1) optical self-heterodyning, 2) external modulation, 3) up- and downconversion, and 4) optical transceiver, will be assessed. From the technical viewpoints, their advantages and disadvantages are discussed. The economical assessment, focusing on the cost of a base station BS ( ), will suggest that the optical transceiver looks the most promising in the long run, but in the near future, however, the external modulation will be cost-effective. The experimental results of 60 GHz testbeds using the external modulation will support the conclusion. 相似文献
533.
534.
[reaction: see text] Optically active (M)-2,11-dihydroxy-1,12-dimethylbenzo[c]phenanthrene-5,8-dicarbonitrile was synthesized from (M)-1,12-dimethyl-2,11-dinitrobenzo[c]phenanthrene-5,8-dicarbonitrile by the reduction and hydroxylation of nitro groups. The compound was converted to several oxygen-functionalized [3 + 3]cycloalkynes with -OH, -OSiMe2-t-Bu, -OAc, -OTf, or -ONf groups, which are chiral arylene ethynylene macrocycles containing three helicenes. The aggregation behaviors of these [3 + 3]cycloalkynes were examined in CHCl3, THF, and acetone using 1H NMR, CD, and vapor pressure osmometry (VPO) studies and were compared with that of the parent [3 + 3]cycloalkyne. An increasing strength of aggregation in CHCl3 was observed in the following order of the substituted derivatives: -H > -ONf > -OTf > -OAc > -OSiMe2-t-Bu. In THF the following strength of aggregation was observed: -OTf > -ONf > -OAc > -H > -OSiMe(2)-t-Bu > -OH. The aggregation of the functionalized [3 + 3]cycloalkynes is stronger for the compounds with electron-withdrawing substituents than for those with electron-donating substituents. (M)-1,12-dimethylbenzo[c]phenanthrene-2,5,8,11-tetraol was also synthesized from the same intermediate. This electron-rich helicene was readily oxidized to 5,6-quinone in air, and the quinone was suggested to form a self-charge-transfer complex in solid state. 相似文献
535.
alpha-Aminoallylation of aldehydes in aqueous ammonia has been developed; commercial aqueous ammonia was successfully used, and this method does not require anhydrous conditions thus leading to easy and practical operations. 相似文献
536.
Sasaki M Takaishi S Miyasaka H Sugiura K Yamashita M 《Journal of the American Chemical Society》2005,127(43):14958-14959
Scanning tunneling microscopy (STM) has been measured in quasi-one-dimensional (Quasi-ID) bromo-bridged Ni(III) complexes. In the STM image of [Ni(pn)2Br]Br2 (pn = (R)-1,2-diaminopropane), the bright spots are fluctuated zigzags. Such a result indicates that the methyl groups of the pn ligands are not arranged on the right- and left-hand alternatively along the chains, but in the domain structures or fluctuated structures. This is the first direct observation of the disorder of the methyl group of pn ligands in real space. 相似文献
537.
36-W diode-pumped continuous-wave 1319-nm Nd:YAG ceramic laser 总被引:4,自引:0,他引:4
Lu J Lu J Murai T Takaichi K Uematsu T Xu J Ueda K Yagi H Yanagitani T Kaminskii AA 《Optics letters》2002,27(13):1120-1122
Continuous-wave output at 1319 nm was obtained from a laser-diode-pumped 1% ceramic Nd:YAG laser. As much as 36.3-W output was obtained under pump power of 290 W, with an optical-to-optical conversion efficiency of 12.5%. 相似文献
538.
539.
Further asymptotic formulas for the non-null distributions of three statistics for multivariate linear hypothesis 总被引:1,自引:0,他引:1
Nariaki Sugiura 《Annals of the Institute of Statistical Mathematics》1973,25(1):153-163
Summary New asymptotic expansions of the non-null distributions of the likelihood ratio, Hotelling's and Pillai's statistics for multivariate
linear hypothesis are given in terms of normal distribution function and its derivatives, assuming the matrix of noncentrality
parameters is of the same order as the sample size. 相似文献
540.
New polyamides with high molecular weights consisting of flexible rod-like mesogenic units (4,4'-diphenyl and 3,4'-diphenyl moieties having bent linkages in the central parts) and aliphatic spacers were synthesized and the thermotropic liquid-crystalline properties were investigated. Differential scanning calorimetry, texture observations by polarizing microscopy, miscibility tests and X-ray analysis suggested that the polymers showed well-defined thermotropic smectic mesophases. The polyamides containing 4,4'-diphenyl moieties had higher transition temperatures and formed more stable liquid-crystalline phases than the polyamide composed of the 3,4'-diphenyl unit with a kinked bond. 相似文献