全文获取类型
收费全文 | 13672篇 |
免费 | 1730篇 |
国内免费 | 738篇 |
专业分类
化学 | 1488篇 |
晶体学 | 18篇 |
力学 | 825篇 |
综合类 | 54篇 |
数学 | 1651篇 |
物理学 | 5741篇 |
无线电 | 6363篇 |
出版年
2024年 | 20篇 |
2023年 | 135篇 |
2022年 | 220篇 |
2021年 | 241篇 |
2020年 | 277篇 |
2019年 | 214篇 |
2018年 | 232篇 |
2017年 | 447篇 |
2016年 | 527篇 |
2015年 | 550篇 |
2014年 | 955篇 |
2013年 | 874篇 |
2012年 | 975篇 |
2011年 | 1051篇 |
2010年 | 775篇 |
2009年 | 816篇 |
2008年 | 954篇 |
2007年 | 998篇 |
2006年 | 852篇 |
2005年 | 672篇 |
2004年 | 611篇 |
2003年 | 546篇 |
2002年 | 421篇 |
2001年 | 447篇 |
2000年 | 356篇 |
1999年 | 304篇 |
1998年 | 263篇 |
1997年 | 250篇 |
1996年 | 209篇 |
1995年 | 152篇 |
1994年 | 127篇 |
1993年 | 120篇 |
1992年 | 97篇 |
1991年 | 83篇 |
1990年 | 58篇 |
1989年 | 47篇 |
1988年 | 51篇 |
1987年 | 27篇 |
1986年 | 20篇 |
1985年 | 34篇 |
1984年 | 30篇 |
1983年 | 17篇 |
1982年 | 18篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 843 毫秒
51.
Asymptotic chi-squared test statistics for testing the equality of moment vectors are developed. The test statistics proposed are generalized Wald test statistics that specialize for different settings by inserting an appropriate asymptotic variance matrix of sample moments. Scaled test statistics are also considered for dealing with nonstandard conditions. The specialization will be carried out for testing the equality of multinomial populations, and the equality of variance and correlation matrices for both normal and nonnormal data. When testing the equality of correlation matrices, a scaled version of the normal theory chi-squared statistic is proven to be an asymptotically exact chi-squared statistic in the case of elliptical data. 相似文献
52.
An analytic and numerical study of the behavior of the linear nonhomogeneous wave equation of the form ε2utt = Δu + tf with high wave speed (ε 1) is carried out. This study was initially motivated by meteorological observations which have indicated the presence of large spatial scale gravity waves in the neighborhood of a number of summer and winter storms, mainly from visible images of ripples in clouds in satellite photos. There is a question as to whether the presence of these waves is caused by the nearby storms. Since the linear wave equation is an approximation to the full system describing pressure waves in the atmosphere, yet is considerably more tractable, we have chosen to analyze the behavior of the linear nonhomogeneous wave equation with high wave speed. The analysis is shown to be valid in one, two, and three space dimensions. Partly because of the high wave speed, the solution is known to consist of behavior which changes on two different time scales, one rapid and one slow. Additionally, because of the presence of the nonhomogeneous forcing term tf, we show that there is a component of the solution which will vary only on a very large spatial scale. Since even the linearized wave equation can give rise to persistent large spatial scale waves under the right conditions, the implication is that certain storms could be responsible for the generation of large-scale waves. Numerical simulations in one and two dimensions confirm analytic results. 相似文献
53.
54.
描述了简并V型三能级原子与单模相干态光场的Raman相互作用,获得了处于激发态单态的原子与相干态光场相互作用的结果.利用探测原子与光场的相互作用将原子和光场制备成最大缠结态,并注入待测原子,通过原子与腔模构成的Bell基矢演化,对腔场进行选择性探测,获得探测原子相互作用后可能的量子状态,然后对待测原子与腔场进行联合探测,接着对探测原子的量子状态实施幺正变换,就将探测原子制备到待测原子的初始量子态上,从而实现未知原子态的隐形传送. 相似文献
55.
56.
用实验的方法,研究电流并联负反馈对放大器性能的改善,提出电流型、并联型负反馈放大器性能参数的测量方法。 相似文献
57.
本文给出了一个判定最大无关组的充要性定理及其证明.同时对用矩阵的行变换求最大无关组这一问题进行了点滴分析并介绍了一个解齐次线性方程组的简便方法。 相似文献
58.
本文以fp 同伦方法为工具,借助于一些适当的变换,研究有序的(B)空间中的集值映象方程的多正解问题;在文中的有关工作中,还使用了集值映象的拟导数的某些性质. 相似文献
59.
本文介绍了多路逆磁测量方法及利用这一方法在HER装置上测量热电子环沿轴向的长度与晃荡电子密度的轴向分布的实验结果。 相似文献
60.
The GaSb layers investigated were grown directly on GaAs substrates by molecular beam epitaxy (MBE) using SnTe source as the
n-type dopant. By using admittance spectroscopy, a dominant deep level with the activation energy of 0.23-0.26 eV was observed
and its concentration was affected by the Sb4/Ga flux ratio in the MBE growth. A lowest deep-level concentration together with a highest mobility was obtained for GaSb
grown at 550°C under a Sb4/Ga beam equivalent pressure (BEP) ratio around 7, which should correspond to the lowest ratio to maintain a Sb-stabilized
surface reconstruction. In the Hall measurement, an analysis of the temperature-dependent mobility shows that the ionized
impurity concentration increases proportionally with the sample’s donor concentration, suggesting that the ionized impurity
was introduced by an SnTe source. In addition, optical properties of an undoped p-, a lightly and heavily SnTe-doped GaSb
layers were studied by comparing their photoluminescence spectra at 4.5K. 相似文献