全文获取类型
收费全文 | 27014篇 |
免费 | 3876篇 |
国内免费 | 2365篇 |
专业分类
化学 | 15743篇 |
晶体学 | 561篇 |
力学 | 2124篇 |
综合类 | 305篇 |
数学 | 3475篇 |
物理学 | 11047篇 |
出版年
2024年 | 34篇 |
2023年 | 171篇 |
2022年 | 413篇 |
2021年 | 566篇 |
2020年 | 715篇 |
2019年 | 728篇 |
2018年 | 712篇 |
2017年 | 881篇 |
2016年 | 1141篇 |
2015年 | 1025篇 |
2014年 | 1151篇 |
2013年 | 2643篇 |
2012年 | 1579篇 |
2011年 | 1588篇 |
2010年 | 1351篇 |
2009年 | 1512篇 |
2008年 | 1580篇 |
2007年 | 1540篇 |
2006年 | 1527篇 |
2005年 | 1310篇 |
2004年 | 1335篇 |
2003年 | 1152篇 |
2002年 | 1265篇 |
2001年 | 903篇 |
2000年 | 890篇 |
1999年 | 775篇 |
1998年 | 655篇 |
1997年 | 539篇 |
1996年 | 480篇 |
1995年 | 497篇 |
1994年 | 381篇 |
1993年 | 332篇 |
1992年 | 308篇 |
1991年 | 232篇 |
1990年 | 216篇 |
1989年 | 164篇 |
1988年 | 145篇 |
1987年 | 138篇 |
1986年 | 137篇 |
1985年 | 99篇 |
1984年 | 102篇 |
1983年 | 48篇 |
1982年 | 64篇 |
1981年 | 53篇 |
1980年 | 37篇 |
1979年 | 51篇 |
1978年 | 23篇 |
1977年 | 13篇 |
1976年 | 9篇 |
1973年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
新疆玛纳斯湖水蒸发过程研究 总被引:3,自引:0,他引:3
本文依据Na^ ,K^ ,Mg^2 //Cl^-,SO4^2--H2O五元水盐体系相图,对新疆玛纳斯盐湖水进行原料分析、理论研究、25℃等温蒸发实验和自然蒸发实验,研究了该湖水蒸发过程中盐类的析出规律及湖水中各组分浓度变化的规律,为进行工艺实验及将来的综合开发利用提供了可靠依据。 相似文献
22.
23.
In this paper, we propose and demonstrate a new and original model for theoretical calculation and experimental measurement of the noise power spectral density (NPSD) in phase-modulated optical links. The phase modulation is operated in the RF frequency range by an external phase modulator. The NPSD is derived for the first time in interferometric systems, by considering all effects such as the 1/f FM noise of the laser and white noise applied to light from a 1550 nm DFB laser, with phase modulation. The results show for the first time the influence of the phase modulation index, modulation frequency and interferometric delay in the phase-to-intensity noise conversion. The experimental and simulation results of conversion of FM-noise to intensity noise in an optical link by considering the external RF phase modulation are shown with good agreement. 相似文献
24.
25.
26.
Claudio Simeone 《General Relativity and Gravitation》2002,34(11):1887-1893
A consistent quantization with a clear notion of time and evolution is given for the anisotropic Kantowski–Sachs cosmological model. It is shown that a suitable coordinate choice allows to obtain a solution of the Wheeler–DeWitt equation in the form of definite energy states, and that the results can be associated to two disjoint equivalent theories, one for each sheet of the constraint surface. 相似文献
27.
In this work we study nonnegativity and positivity of a discrete quadratic functional with separately varying endpoints. We introduce a notion of an interval coupled with 0, and hence, extend the notion of conjugate interval to 0 from the case of fixed to variable endpoint(s). We show that the nonnegativity of the discrete quadratic functional is equivalent to each of the following conditions: The nonexistence of intervals coupled with 0, the existence of a solution to Riccati matrix equation and its boundary conditions. Natural strengthening of each of these conditions yields a characterization of the positivity of the discrete quadratic functional. Since the quadratic functional under consideration could be a second variation of a discrete calculus of variations problem with varying endpoints, we apply our results to obtain necessary and sufficient optimality conditions for such problems. This paper generalizes our recent work in [R. Hilscher, V. Zeidan, Comput. Math. Appl., to appear], where the right endpoint is fixed. 相似文献
28.
A Dirac picture perturbation theory is developed for the time evolution operator in classical dynamics in the spirit of the Schwinger–Feynman–Dyson perturbation expansion and detailed rules are derived for computations. Complexification formalisms are given for the time evolution operator suitable for phase space analyses, and then extended to a two-dimensional setting for a study of the geometrical Berry phase as an example. Finally a direct integration of Hamilton's equations is shown to lead naturally to a path integral expression, as a resolution of the identity, as applied to arbitrary functions of generalized coordinates and momenta. 相似文献
29.
30.
甲基紫掺杂聚乙烯醇薄膜材料在两束相干光照射下生成相位光栅.当改变一束相干光光程,通过监测相位光栅的一级衍射信号强度的变化,可以检测相位光栅的生长和擦除过程.在此实验基础上,讨论了甲基紫掺杂聚乙烯醇薄膜材料多重全息存储的原理与结果. 相似文献