全文获取类型
收费全文 | 622篇 |
免费 | 5篇 |
国内免费 | 4篇 |
专业分类
化学 | 312篇 |
力学 | 115篇 |
数学 | 120篇 |
物理学 | 84篇 |
出版年
2018年 | 6篇 |
2015年 | 7篇 |
2014年 | 8篇 |
2013年 | 28篇 |
2012年 | 11篇 |
2011年 | 30篇 |
2010年 | 11篇 |
2009年 | 19篇 |
2008年 | 22篇 |
2007年 | 15篇 |
2006年 | 16篇 |
2005年 | 20篇 |
2004年 | 13篇 |
2003年 | 12篇 |
2002年 | 22篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 15篇 |
1992年 | 10篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 7篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 9篇 |
1984年 | 19篇 |
1983年 | 9篇 |
1982年 | 14篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 15篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 12篇 |
1975年 | 15篇 |
1974年 | 15篇 |
1973年 | 22篇 |
1972年 | 7篇 |
1971年 | 4篇 |
1970年 | 9篇 |
1969年 | 8篇 |
1968年 | 8篇 |
1967年 | 9篇 |
1965年 | 6篇 |
排序方式: 共有631条查询结果,搜索用时 15 毫秒
571.
Activation barriers (DeltaHMe(double dagger)) for adding methyl radicals to ions of the general formula CH3CR=OCH3+ have been measured by looking at the threshold energies for the reverse reaction, dissociative photoionization of ethers of the general formula RC(CH3)2OCH3. Dissociation by loss of a methyl radical has more favorable thermochemistry than loss of R*, yet the onset of R* loss occurs at lower energies than loss of CH3*. In other words, the more endothermic dissociation exhibits a lower appearance energy. Contrathermodynamic ordering of appearance energies is observed for R = Et, nPr, iPr, tBu, and neopentyl. The sum of the appearance energy difference, DeltaAE, and the thermochemical difference (DeltaDeltaH, calculated using G3 theory) gives a lower bound for the barrier for adding methyl radical to CH3CR=OCH3+. More specifically, the difference between that activation barrier and the one for adding R* to (CH3)2C=OCH3+, DeltaHMe(double dagger)-DeltaHR(double dagger), equals DeltaAE + DeltaDeltaH and has values in the range 20-24 kJ mol(-1) for the homologous series investigated. There is no systematic trend with the steric bulk of R, and available evidence suggests that DeltaHR(double dagger) does not have a value >5 kJ mol(-1). The difference in barrier heights, DeltaHMe(double dagger)-DeltaHiPr(double dagger) for CH3* plus iPrC(CH3)=OX+ vs iPr* + (CH3)2C=OX+, has the same value, regardless of whether X = H or CH3. Mixing of higher energy electronic configurations provides a qualitative theoretical explanation for some (but not all) observed trends in barrier heights. 相似文献
572.
573.
574.
The need for a new avionic interconnection scheme that can provide superior performance, installability and maintainability as compared with copper has become evident. Fiber optics, which has been proposed for this role, is expected to improve environmental immunity, decrease system weight, and increase bandwidth. It has also been suggested that sensors based on fiber optics can offer significant advantages in aerospace applications. Realizing these advantages is a difficult challenge because mature sensing technologies have done a good job at reasonable cost. Furthermore, it has been demonstrated that a wide variety of aircraft electrical sensors can be addressed by a few standard interfaces. Therefore, future fiber optic sensor suites will also need to be interrogated through only a few standard interfaces, and standardization of the fiber optic sensor interface will be a primary issue in furthering fiber optic sensing technology for aerospace applications. This paper discusses the fiber optic sensor standardization process underway in the Avionic Systems Division of the Society of Automotive Engineers. 相似文献
575.
Structure and absolute stereochemistry of everheptose 总被引:1,自引:0,他引:1
A K Ganguly O Z Sarre D Greeves J Morton 《Journal of the American Chemical Society》1973,95(3):942-945
576.
Morton Litt 《Journal of Polymer Science.Polymer Physics》1970,8(12):2105-2113
A relatively novel method of calculating molecular weight changes and broadening during a polymerization is proposed. It differs from others in that the distribution is considered implicitly rather than explicitly. This allows direct use of kinetic rate equations in the derivation of weight-average and number-average degree of polymerization etc.; the mathematics simplifies to simple integration rather than sums of complicated series. The method is illustrated by applying it to several cases. These range from various models where only monomer concentration changes, to where both monomer and initiator or monomer and chain transfer agent are changing. Cases previously unsolvable can now be handled. 相似文献
577.
578.
579.
580.