全文获取类型
收费全文 | 24712篇 |
免费 | 4140篇 |
国内免费 | 3662篇 |
专业分类
化学 | 18253篇 |
晶体学 | 504篇 |
力学 | 1299篇 |
综合类 | 314篇 |
数学 | 2668篇 |
物理学 | 9476篇 |
出版年
2024年 | 55篇 |
2023年 | 365篇 |
2022年 | 657篇 |
2021年 | 763篇 |
2020年 | 864篇 |
2019年 | 957篇 |
2018年 | 774篇 |
2017年 | 778篇 |
2016年 | 1062篇 |
2015年 | 1154篇 |
2014年 | 1319篇 |
2013年 | 1771篇 |
2012年 | 2282篇 |
2011年 | 2191篇 |
2010年 | 1745篇 |
2009年 | 1694篇 |
2008年 | 1925篇 |
2007年 | 1623篇 |
2006年 | 1614篇 |
2005年 | 1447篇 |
2004年 | 1165篇 |
2003年 | 921篇 |
2002年 | 959篇 |
2001年 | 758篇 |
2000年 | 643篇 |
1999年 | 498篇 |
1998年 | 331篇 |
1997年 | 253篇 |
1996年 | 258篇 |
1995年 | 217篇 |
1994年 | 227篇 |
1993年 | 183篇 |
1992年 | 131篇 |
1991年 | 134篇 |
1990年 | 87篇 |
1989年 | 86篇 |
1988年 | 83篇 |
1987年 | 52篇 |
1986年 | 71篇 |
1985年 | 65篇 |
1984年 | 54篇 |
1983年 | 42篇 |
1982年 | 33篇 |
1981年 | 30篇 |
1980年 | 32篇 |
1979年 | 24篇 |
1978年 | 18篇 |
1977年 | 22篇 |
1976年 | 19篇 |
1975年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
枸橼酸莫沙必利的合成 总被引:1,自引:0,他引:1
分别以邻羟基对氨基苯甲醇钠,对氟苯甲醛,邻苯二甲酰亚胺为起始原料,制得3个中间体,用其中2个中间体经偶合,环化后与第3个中间体反应,再经成盐制得产品,其摩尔总收率为31.5%。 相似文献
32.
33.
Jun Xiang Ge 《International Journal of Infrared and Millimeter Waves》1994,15(10):1681-1688
In 1991, we developed a new type of quasi-optical power combiner, called a compound quasi-optical power combiner, at Ka-band. In this paper, the circuit of such a compound quasi-optical power combiner is analysed. Its equivalent circuit model is proposed. The circuit equations, the balance condition, the injection locking and the stabilized condition of the compound quasi-optical power combiner are studed by the equivalent circuit model. As an example, a compound quasi-optical power combiner which consists of two single—cavity, two—device power combiners is analysed 相似文献
34.
Xia Yin Weiguang Zhang Qijiao Zhang Jun Fan Chian Sing Lai Edward R. T. Tiekink 《应用有机金属化学》2004,18(3):139-140
The centrosymmetric structure of {Cd[S2CN(CH2Ph)2]2}2 features both bridging and chelating dithiocarbamate ligands so that a square pyramidal S5 coordination geometry results. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
35.
In the crystal of the title compound, the quinone fragment of the host (1) participates in weak D-A interactions with a neighboring naphthalene moiety of 1 and with the guest molecule via fairly short intermolecular face-to-face contacts. An intermolecular electrostatic interaction between a quinone-oxygen atom and a neighboring quinone ring is suggested by short O...C contacts. There exist several C-H...O hydrogen bonds, one of which seems to contribute to the formation of a centrosymmetric dimer of 1. 相似文献
36.
37.
38.
年龄相关的种群扩散系统的最优分布控制 总被引:5,自引:1,他引:4
本文讨论年龄相关的种群扩散系统的最优分布控制 ,证明了最优分布控制的存在性 ,得到了分布控制为最优的充分必要条件及确定最优控制的最优性组 . 相似文献
39.
Applying the approximate Fokker-Planck equation we derived, we obtain the analytic expression of thestationary laser intensity distribution Pst(Ⅰ) by studying the single-mode laser cubic model subject to colored cross-correlation additive and multiplicative noise, each of which is colored. Based on it, we discuss the effects on the stationarylaser intensity distribution Pst(Ⅰ) by cross-correlation between noises and “color“ of noises (non-Markovian effect) whenthe laser system is above the threshold. In detail, we analyze two cases: One is that the three correlation-times (i.e.the self-correlation and cross-correlation times of the additive and multiplicative noise) are chosen to be the same value(τ1 = τ2 = τ3 = τ). For this case, the effect of noise cross-correlation is investigated emphatically, and we detect thatonly when λ≠ 0 can the noise-induced transition occur in the Pst(Ⅰ) curve, and only when τ≠ 0 and λ≠ 0, can the“reentrant noise-induced transition“ occur. The other case is that the three correlation times are not the same value,τ1 ≠τ2 ≠τ3. For this case, we find that the noise-induced transition occurring in the Pst (Ⅰ) curve is entirely differentwhen the values of τ1, τ2, and τ3 are changed respectively. In particular, when τ2 (self-correlation time of additivenoise) is changing, the ratio of the two maximums of the Pst(Ⅰ) curve R exhibits an interesting phenomenon, “reentrantnoise-induced transition“, which demonstrates the effect of noise “color“ (non-Markovian effect). 相似文献
40.
This paper is concerned with a class of nonlinear delay partial difference equations with variable coefficients, which may
change sign. By making use of frequency measures, some new oscillatory criteria are established. This is the first time oscillation
of these partial difference equations is discussed by employing frequency measures.
相似文献