全文获取类型
收费全文 | 1595篇 |
免费 | 40篇 |
国内免费 | 7篇 |
专业分类
化学 | 914篇 |
晶体学 | 8篇 |
力学 | 19篇 |
数学 | 213篇 |
物理学 | 488篇 |
出版年
2021年 | 10篇 |
2020年 | 25篇 |
2019年 | 17篇 |
2018年 | 16篇 |
2016年 | 31篇 |
2015年 | 21篇 |
2014年 | 21篇 |
2013年 | 60篇 |
2012年 | 63篇 |
2011年 | 86篇 |
2010年 | 40篇 |
2009年 | 39篇 |
2008年 | 67篇 |
2007年 | 54篇 |
2006年 | 52篇 |
2005年 | 55篇 |
2004年 | 55篇 |
2003年 | 43篇 |
2002年 | 55篇 |
2001年 | 31篇 |
2000年 | 29篇 |
1999年 | 22篇 |
1998年 | 18篇 |
1997年 | 17篇 |
1996年 | 39篇 |
1995年 | 40篇 |
1994年 | 30篇 |
1993年 | 42篇 |
1992年 | 45篇 |
1991年 | 32篇 |
1990年 | 32篇 |
1989年 | 35篇 |
1988年 | 17篇 |
1987年 | 17篇 |
1986年 | 21篇 |
1985年 | 29篇 |
1984年 | 21篇 |
1983年 | 24篇 |
1982年 | 17篇 |
1981年 | 20篇 |
1980年 | 21篇 |
1979年 | 19篇 |
1978年 | 20篇 |
1977年 | 16篇 |
1976年 | 8篇 |
1975年 | 19篇 |
1974年 | 25篇 |
1973年 | 26篇 |
1972年 | 10篇 |
1970年 | 13篇 |
排序方式: 共有1642条查询结果,搜索用时 0 毫秒
21.
Long-lived metastable negative ions of CO2 have been observed in the gas phase as direct products of electron and cesium collisions with organic molecules which contain “bent” CO2 as a basic unit. The long autodetachment life-times are attributed to two related factors - first, the potential energy curve of CO−2(2A1) lies below that of CO2(1A1) at 134°, and, second, an unfavorable Franck-Condon overlap exists between the bent ion (134°) and the linear neutral parent. 相似文献
22.
23.
24.
25.
26.
27.
28.
N. E. Hussey J. R. Cooper Chen Changkang J. W. Hodby 《Czechoslovak Journal of Physics》1996,46(3):1201-1202
Low-field magnetizationM(H) measurements can be used to probe the nature of the screening currents and the interlayer coupling in high-T c cuprates. Here we compare theM(H) behaviour of single crystals of Bi2Sr2CaCu2O8 and fully oxygenated and oxygen reduced YBa2Cu3O7??. In YBa2Cu3O7, theM(H) behaviour is consistent with anisotropic 3D superconductivity whilst in Bi2Sr2CaCu2O8, the surface screening currents are strongly affected by the presence of vortices, implying that the CuO2 planes are coupled via a weak Josephson interaction. In oxygen-deficient YBa2Cu3O6.7 (T c =63K), theM(H) behaviour at low temperatures is similar to that found for Bi2Sr2CaCu2O8, implying that the removal of oxygen from the chains has resulted in a dimensional crossover of the superconducting state in YBa2Cu3O7??. As the temperature approachesT c , the 3D behaviour is eventually restored as thec-axis coherence length ξ c becomes comparable with the interlayer spacingd. 相似文献
29.
30.
Using statistically designed experiments, 12,500 observations are generated from a 4-pieced Cobb-Douglas function exhibiting increasing and decreasing returns to scale in its different pieces. Performances of DEA and frontier regressions represented by COLS (Corrected Ordinary Least Squares) are compared at sample sizes ofn=50, 100, 150 and 200. Statistical consistency is exhibited, with performances improving as sample sizes increase. Both DEA and COLS generally give good results at all sample sizes. In evaluating efficiency, DEA generally shows superior performance, with BCC models being best (except at corner points), followed by the CCR model and then by COLS, with log-linear regressions performing better than their translog counterparts at almost all sample sizes. Because of the need to consider locally varying behavior, only the CCR and translog models are used for returns to scale, with CCR being the better performer. An additional set of 7,500 observations were generated under conditions that made it possible to compare efficiency evaluations in the presence of collinearity and with model misspecification in the form of added and omitted variables. Results were similar to the larger experiment: the BCC model is the best performer. However, COLS exhibited surprisingly good performances — which suggests that COLS may have previously unidentified robustness properties — while the CCR model is the poorest performer when one of the variables used to generate the observations is omitted. 相似文献