全文获取类型
收费全文 | 26987篇 |
免费 | 6163篇 |
国内免费 | 6938篇 |
专业分类
化学 | 8046篇 |
晶体学 | 728篇 |
力学 | 1953篇 |
综合类 | 375篇 |
数学 | 431篇 |
物理学 | 10980篇 |
无线电 | 17575篇 |
出版年
2024年 | 294篇 |
2023年 | 840篇 |
2022年 | 1024篇 |
2021年 | 1051篇 |
2020年 | 715篇 |
2019年 | 971篇 |
2018年 | 618篇 |
2017年 | 901篇 |
2016年 | 903篇 |
2015年 | 1123篇 |
2014年 | 2144篇 |
2013年 | 1601篇 |
2012年 | 1690篇 |
2011年 | 1864篇 |
2010年 | 1684篇 |
2009年 | 1909篇 |
2008年 | 2334篇 |
2007年 | 1864篇 |
2006年 | 1815篇 |
2005年 | 1756篇 |
2004年 | 1538篇 |
2003年 | 1593篇 |
2002年 | 1348篇 |
2001年 | 1250篇 |
2000年 | 891篇 |
1999年 | 765篇 |
1998年 | 698篇 |
1997年 | 647篇 |
1996年 | 661篇 |
1995年 | 679篇 |
1994年 | 582篇 |
1993年 | 428篇 |
1992年 | 446篇 |
1991年 | 391篇 |
1990年 | 377篇 |
1989年 | 384篇 |
1988年 | 106篇 |
1987年 | 58篇 |
1986年 | 33篇 |
1985年 | 30篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 23篇 |
1981年 | 23篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1975年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
61.
采用化学自燃烧法制备了不同Ag+掺杂浓度的Y2O3:Eu纳米晶体粉末样品([Y3+]∶[Eu3+]∶[Ag+]=99∶1∶X,X=0—3.5×10-2),以及通过退火处理得到了相应的体材料.根据X射线衍射谱确定所得纳米和体材料样品均为纯立方相.实验表明在纳米尺寸样品中随着Ag离子浓度的增加,荧光发射强度随之增加,当X=2×10-2时达到最大值,其发光强度比X=0时提高了近50%.当Ag离子浓度继续增加,样品发光强度保持不变.在相应的体材料样品中则没有观察到此现象.通过对各样品的发射光谱,激发光谱,X射线衍射图谱,透射电镜(TEM)照片和荧光衰减曲线的研究,分析了引起纳米样品荧光强度变化的原因是由于Ag离子与表面悬键氧结合,从而使这一无辐射通道阻断,使发光中心Eu3+的量子效率提高;Ag+的引入所带来的另一个效应是使激发更为有效.这两方面原因使发光效率得到了提高. 相似文献
62.
63.
We employ a first-principles plane wave method with the
relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter
(HGH) scheme in the frame of DFT to calculate
the equilibrium lattice parameters and the thermodynamic properties of
AlB2 compound with hcp structure. The obtained lattice parameters are in
good agreement with the available experimental data and those calculated by
others. Through the quasi-harmonic Debye model, obtained successfully are
the dependences of the
normalized lattice parameters a/a0 and c/c0 on pressure P, the normalized
primitive cell volume V/V0 on pressure P, the variation of the thermal
expansion α with pressure P and temperature T, as well as the Debye
temperature \ThetaD and the heat capacity CV on pressure P and
temperature T. 相似文献
64.
The acoustic and perceptive characteristics of discourse units, including clauses, simple sentences, compound sentences and paragraphs, were studied. The acoustic characteristics were based on the analysis of a speech corpus consisted of ten monologues. The Perceived Boundary Strength (PBS) of the monologues was labeled on a five-point scale by 25 participants. The main results are as follows: (1) There is a significant difference in the PBS of clauses (corresponding to intonational phrase), sentences (including simple sentences and compound sentences) and paragraphs. But the PBS of simple sentences and compound sentences is the same, and the pitch reset and pause are also not significantly different between these two kinds of boundaries. (2) The pitch-based cue used to distinguish clauses, sentences and paragraphs appears to be the difference between pre- and post-boundary syllables, but not simply the pitch value of either syllable alone. (3) Declination mainly occurs within intonational phrases, whereas there is no consistent global declination in sentences or in paragraphs. (4) Pauses are longer and more varied at the boundary of larger discourse units. Moreover, there is a significant positive correlation between pause duration and degree of pitch reset at clause boundaries, but not at sentence or paragraph boundaries. 相似文献
65.
66.
67.
A new type of THz waveguides, which employs a solid polyethylene rod as the core and polyethylene tubes in a periodic array of square lattice as the cladding, is proposed. Optical properties of this new THz waveguide, especially in dispersion, confinement loss and single mode property, are investigated in detail with the plane wave expansion method and the beam propagation method. Numerical results demonstrate that the new THz waveguide can reach not only low dispersion but also low confinement loss at single mode propagation. Therefore, the square lattice structure is a better candidate as THz waveguides than the triangular ones. 相似文献
68.
69.
70.
胡志勇 《电子工业专用设备》2002,31(3):148-150
当今的回转头式贴装设备能够以相对低廉的价格 ,提供高速度和准确的贴装 ,以及先进的识别操作。 相似文献