全文获取类型
收费全文 | 13354篇 |
免费 | 928篇 |
国内免费 | 248篇 |
专业分类
化学 | 9116篇 |
晶体学 | 107篇 |
力学 | 288篇 |
综合类 | 8篇 |
数学 | 1592篇 |
物理学 | 2411篇 |
无线电 | 1008篇 |
出版年
2024年 | 24篇 |
2023年 | 159篇 |
2022年 | 201篇 |
2021年 | 311篇 |
2020年 | 333篇 |
2019年 | 315篇 |
2018年 | 300篇 |
2017年 | 285篇 |
2016年 | 546篇 |
2015年 | 502篇 |
2014年 | 513篇 |
2013年 | 992篇 |
2012年 | 958篇 |
2011年 | 1036篇 |
2010年 | 595篇 |
2009年 | 519篇 |
2008年 | 875篇 |
2007年 | 834篇 |
2006年 | 694篇 |
2005年 | 642篇 |
2004年 | 506篇 |
2003年 | 490篇 |
2002年 | 446篇 |
2001年 | 288篇 |
2000年 | 273篇 |
1999年 | 204篇 |
1998年 | 163篇 |
1997年 | 138篇 |
1996年 | 165篇 |
1995年 | 114篇 |
1994年 | 131篇 |
1993年 | 129篇 |
1992年 | 106篇 |
1991年 | 67篇 |
1990年 | 46篇 |
1989年 | 58篇 |
1988年 | 45篇 |
1987年 | 42篇 |
1986年 | 50篇 |
1985年 | 73篇 |
1984年 | 64篇 |
1983年 | 44篇 |
1982年 | 39篇 |
1981年 | 30篇 |
1980年 | 26篇 |
1979年 | 15篇 |
1978年 | 23篇 |
1977年 | 21篇 |
1975年 | 21篇 |
1974年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
本文介绍了一种鱼类耳石样品的扫描电镜制样方法,通过对草鱼耳石进行包埋,抛磨,并用合适的腐蚀剂处理,用SEM成功地观察了耳石的日轮及超微结构,该方法简便易行,保证所观察耳石样品的微观结构完整与清晰。 相似文献
43.
44.
Preparation and microwave absorption properties of electroless Co-Ni-P coated strontium ferrite powder 总被引:1,自引:0,他引:1
A new type of Co-Ni-P coated strontium ferrite nanocomposite was prepared with electroless plating enhanced by ultrasonic wave at room temperature. The plating process was studied carefully. The morphology, crystal structure and microwave absorption properties of the Co-Ni-P coated powder were studied with field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX) and vector network analyzer. The results show that the strontium ferrite powder was successfully coated with Co-Ni-P alloy and possesses excellent microwave absorption properties. The maximum microwave loss of the composite powder reaches −44.12 dB. The bandwidth with the loss above −10 dB exceeds 13.8 GHz. 相似文献
45.
We analyze the critical length for design purposes of six-dimensional spaces invariant under translations and reflections
containing the functions 1, cos t and sin t. These spaces also contain the first degree polynomials as well as trigonometric and/or hyperbolic functions. We identify
the spaces whose critical length for design purposes is greater than 2π and find its maximum 4π. By a change of variables,
two biparametric families of spaces arise. We call shape preservation region to the set of admissible parameters in order
that the space has shape preserving representations for curves. We describe the shape preserving regions for both families.
To our friend Mariano Gasca on occasion of his 60th birthday
Research partially supported by the Spanish Research Grant MTM2006-03388, by Gobierno de Aragón and Fondo Social Europeo. 相似文献
46.
47.
Gianluigi Luppi Rodrigo J. Corrêa Angelo C. Pinto Quirinus B. Broxterman Claudia Tomasini 《Tetrahedron》2006,62(51):12017-12024
The first total synthesis of (R)-convolutamydine A has been achieved by the organocatalytic addition of acetone to 4,6-dibromoisatin. The absolute configuration was determined by single crystal X-ray diffraction. DFT studies were used to model the transition states for the aldol reaction and equilibrium geometries of the post-aldol reaction intermediates. The DFT study revealed that the aldol bond forming reaction was considerably endothermic. 相似文献
48.
M. T. Crespo J. M. Los Arcos C. E. Granados M. L. Aceña 《Journal of Radioanalytical and Nuclear Chemistry》1989,130(1):99-110
A new method of analysis of the efficiency of four filters of MnO2 in series which takes into account the reactions induced in plutonium by the sorbent is described. The system of differential equations describing the behaviour of plutonium in water as it passes through the filters is solved and the parameters of the filters are determined for a best fitting of the solutions corresponding to the experimental uncertainties. The method is checked by numerical simulation and then applied to water solutions with a known plutonium concentration and to some experimental results given by other authors. In all occassions the results are consistent with the possibility of determining the contents of plutonium in sea and continental waters with uncertainties of the order of 10%. 相似文献
49.
提出了一种相似度的新定义,使双极WAT模型的实现更加简洁明了。增加一个阈值使系统能判别非存储模式或因其信息量过低而无法鉴定的输入模式,阈值的大小与要求的容错能力有关。双极相似度及阈值通过光学方法实现。阈值的光学实现使系统仍具有照明不变性。通过电路实现取及WTA网络的迭代运算。 相似文献
50.
测量了GaAs/AlGaAs多量子阱红外探测器在零偏压、小偏流和大偏压等条件下的光电流谱,并结合理论计算的光跃迁能量讨论了光电流谱的多峰结构和异常增强特性。 相似文献