全文获取类型
收费全文 | 15072篇 |
免费 | 2317篇 |
国内免费 | 1353篇 |
专业分类
化学 | 8230篇 |
晶体学 | 89篇 |
力学 | 664篇 |
综合类 | 67篇 |
数学 | 1440篇 |
物理学 | 4233篇 |
无线电 | 4019篇 |
出版年
2024年 | 54篇 |
2023年 | 403篇 |
2022年 | 463篇 |
2021年 | 606篇 |
2020年 | 616篇 |
2019年 | 518篇 |
2018年 | 459篇 |
2017年 | 434篇 |
2016年 | 630篇 |
2015年 | 716篇 |
2014年 | 817篇 |
2013年 | 999篇 |
2012年 | 1198篇 |
2011年 | 1287篇 |
2010年 | 851篇 |
2009年 | 861篇 |
2008年 | 932篇 |
2007年 | 877篇 |
2006年 | 819篇 |
2005年 | 686篇 |
2004年 | 526篇 |
2003年 | 413篇 |
2002年 | 390篇 |
2001年 | 305篇 |
2000年 | 291篇 |
1999年 | 352篇 |
1998年 | 297篇 |
1997年 | 295篇 |
1996年 | 288篇 |
1995年 | 247篇 |
1994年 | 199篇 |
1993年 | 185篇 |
1992年 | 149篇 |
1991年 | 125篇 |
1990年 | 101篇 |
1989年 | 62篇 |
1988年 | 58篇 |
1987年 | 56篇 |
1986年 | 30篇 |
1985年 | 35篇 |
1984年 | 30篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1970年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Double Michael additions of lithium enolaie of 1,4-dioxaspiro[4.5]dec-6-en-8-one to four acrylates afforded bicyclo[2.2.2]octan-2-ones with high regio- and stereoselectivities in moderate yields. 相似文献
12.
We have obtained expressions of the accelerating effect in Kerr-Newman Kasuya field. These expressionsinclude four parameters: mass m, angular momentum a, electric charge q, and magnetic charge φ. Furthermore we studyits special case (vi = 0). We get the following conclusion. In the gravitation field of souse mass with electric charge qand magnetic charge b, the acceleration of test particle has not only radial component but also transverse component.When θ = 0, the acceleration is minimum, and when θ = π/2, the acceleration is maximum. Furthermore, we discussthe effects of electric charge q and magnetic charge φ respectively. 相似文献
13.
Fiber-optic reflex sensor for in-line production measurement 总被引:1,自引:0,他引:1
The paper describes experiments concerning distance measurement with fiber-optic sensor and improvement of the measurement principle. The sensor probe was well-designed with a small structure and multi-function. The novel optical fiber probe arrangement as well as the possibilities of use for complex measurement problems are explained using the measurement of an internal screw thread as an example. The experimental results show that the measurement uncertainty of the thread minor diameter can reach ±10 μm, and the stability of the measurement system is better than 0.07%. 相似文献
14.
A Modified Quasi-Newton Method for Structured Optimization with Partial Information on the Hessian 总被引:2,自引:0,他引:2
This paper develops a modified quasi-Newton method for structured unconstrained optimization with partial information on the
Hessian, based on a better approximation to the Hessian in current search direction. The new approximation is decided by both
function values and gradients at the last two iterations unlike the original one which only uses the gradients at the last
two iterations. The modified method owns local and superlinear convergence. Numerical experiments show that the proposed method
is encouraging comparing with the methods proposed in [4] for structured unconstrained optimization
Presented at the 6th International Conference on Optimization: Techniques and Applications, Ballarat, Australia, December
9–11, 2004 相似文献
15.
Xiangfei Chen Jianping Yao Fei Zeng Zhichao Deng 《Photonics Technology Letters, IEEE》2005,17(7):1390-1392
A novel single-longitudinal-mode (SLM) fiber ring laser that incorporates an equivalent phase-shifted fiber Bragg grating acting as an ultra-narrow bandpass filter in the laser cavity is proposed. The equivalent phase-shifted fiber Bragg grating has an ultra-narrow transmission bandwidth which ensures an SLM lasing. Stable SLM operation without mode hopping is demonstrated. 相似文献
16.
该文讨论一类带有奇异系数的双重调和方程{△^2u-μu/|x|^s=f(x,u),x∈Ω,u=δu/δv=0,x∈δΩ,这里Ω包含R^N是包含0的有界光滑区域,u∈H0^2(Ω),μ∈R是参数,0≤s≤2,△^2=△△表示双重拉普拉斯算子,当f(x,u)=u^p,p=2N/N-4时,上述问题就是一个临界双重调和问题,该文运用Sobolev-Hardy不等式和变分方法,得到它的解的存在性的一些结果。 相似文献
17.
Wang Qing-Lun Zhao Bin Liao Dai-Zheng Yan Shi-Ping Cheng Peng Jiang Zong-Hui 《Transition Metal Chemistry》2003,28(3):326-330
The copper(II) complex Na2[Cu(pba)] · 6H2O has been synthesized and its structure determined. It consists of a novel two-dimensional network of [Cu(pba)]2– anions connected to each other through the sodium ion as a bridge. Spectroscopic and magnetic properties have been measured and a spin distribution calculation has been carried out with the GAUSSIAN-94. Theoretical calculations indicate the rather larger spin density on nitrogen atoms should be responsible for the satellite lines appearing beside the main hyperfine coupling signs of copper(II). The antiferromagnetic interaction may be due to the intermolecular interaction and/or different copper(II) ions through the Na+ bridge in the same layer. 相似文献
18.
Nickel complexes 1–4 ligated with 2,9-disubstituted-1,10-phenanthroline were synthesized and characterized by FT-IR spectra and elemental analysis.
The molecular structure of complex 2 was confirmed by X-ray crystal diffraction analysis. Activated with methylaluminoxane (MAO), those complexes showed moderate
activities for ethylene oligomerization.
Published in Kinetika i Kataliz, 2007, Vol. 48, No. 5, pp. 710–714.
This article was submitted by the authors in English. 相似文献
19.
20.
基于DSP+CPLD的高精度信号发生器 总被引:2,自引:0,他引:2
介绍了基于直接数字式频率合成(DDS)原理的全数字信号发生器(DSP),利用DSP芯片快速、高精度的运算优势以及CPLD芯片灵活的编程逻辑、大容量存储功能的特点,采用通用可编程芯片以及数字波形合成技术,形成高稳定、高精度、高动态的数字合成信号.该信号发生器可产生0~25 kHz的正弦波、三角波和方波,输出电压峰峰值为0~5 V,频率步进1 Hz,幅度步进0.001 V. 相似文献