全文获取类型
收费全文 | 2069篇 |
免费 | 510篇 |
国内免费 | 112篇 |
专业分类
化学 | 512篇 |
晶体学 | 1篇 |
力学 | 152篇 |
综合类 | 12篇 |
数学 | 37篇 |
物理学 | 918篇 |
无线电 | 1059篇 |
出版年
2024年 | 6篇 |
2023年 | 29篇 |
2022年 | 66篇 |
2021年 | 81篇 |
2020年 | 47篇 |
2019年 | 61篇 |
2018年 | 62篇 |
2017年 | 84篇 |
2016年 | 104篇 |
2015年 | 124篇 |
2014年 | 130篇 |
2013年 | 186篇 |
2012年 | 218篇 |
2011年 | 172篇 |
2010年 | 123篇 |
2009年 | 140篇 |
2008年 | 130篇 |
2007年 | 139篇 |
2006年 | 160篇 |
2005年 | 97篇 |
2004年 | 98篇 |
2003年 | 77篇 |
2002年 | 57篇 |
2001年 | 47篇 |
2000年 | 52篇 |
1999年 | 21篇 |
1998年 | 31篇 |
1997年 | 18篇 |
1996年 | 20篇 |
1995年 | 19篇 |
1994年 | 16篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1980年 | 2篇 |
排序方式: 共有2691条查询结果,搜索用时 15 毫秒
91.
湍流大气中准直激光束的光斑特征Ⅰ.特征半径 总被引:2,自引:3,他引:2
利用数值方法研究了不同光传播起伏条件下湍流大气中准直激光束的光斑特征 ,结果表明 ,光斑稳定半径(Rrbt)和等效半径 (Reff)具有很好的相关性 ,虽然Rrbt 可应用于更一般的光斑 ,但它们都不适合衡量光斑质量 ;而锐度半径 (Rshp)可以恰当地反映光斑质量。对于准直激光束 ,在极弱起伏条件下 ,约有一半的光斑其质量变好或变差 ,但变化幅度很小 ;随起伏条件的加强 ,越来越多的光斑质量变差 ,但仍有一部分光斑质量变好 ,且程度越来越高 相似文献
92.
三维大气电波折射修正实验研究 总被引:4,自引:0,他引:4
外测系统电波折射修正常用的球面分层法和水平分层法都没有考虑到大气水平不均匀性的影响因此其修正精度不是太高。而三维大气法不仅考虑了大气在垂直方向上的变化,而且也考虑了在水平方向上的变化,所以其修正精度较高。 相似文献
93.
94.
Li G Artamonov M Rabitz H Wang SW Georgopoulos PG Demiralp M 《Journal of computational chemistry》2003,24(5):647-656
High-dimensional model representation (HDMR) is a general set of quantitative model assessment and analysis tools for improving the efficiency of deducing high dimensional input-output system behavior. RS-HDMR is a particular form of HDMR based on random sampling (RS) of the input variables. The component functions in an HDMR expansion are optimal choices tailored to the n-variate function f(x) being represented over the desired domain of the n-dimensional vector x. The high-order terms (usually larger than second order, or equivalently beyond cooperativity between pairs of variables) in the expansion are often negligible. When it is necessary to go beyond the first and the second order RS-HDMR, this article introduces a modified low-order term product (lp)-RS-HDMR method to approximately represent the high-order RS-HDMR component functions as products of low-order functions. Using this method the high-order truncated RS-HDMR expansions may be constructed without directly computing the original high-order terms. The mathematical foundations of lp-RS-HDMR are presented along with an illustration of its utility in an atmospheric chemical kinetics model. 相似文献
95.
96.
激光在大气中传输时吸收损耗的计算 总被引:5,自引:0,他引:5
大气随机信道对激光传输性能的影响是制约激光通信的重要因素之一,因此开展对大气信道的研究对实现激光通信的实用化具有非常重要的意义。通过研究,给出了激光在大气中传输吸收损耗计算的数学模型,并经过仿真计算,得到了一些有益结论,并以此提出了激光通信设计时的一些相应的解决途径。 相似文献
97.
湍流大气中光波闪烁的圆环孔径平均因子 总被引:2,自引:1,他引:2
利用弱起伏条件下球面波在大气湍流中传输的光强起伏(闪烁)理论和圆环孔径滤波函数,获得了圆环孔径平均因子的精确表达式。将圆环孔径平均因子关于一个无量纲参量(孔径半径与菲涅耳尺度的比值)拟合成2阶多项式,再找出多项式系数与孔径内外径之比的函数关系,获得了圆环孔径平均因子关于该参量和孔径内外径之比的函数关系式。通过误差分析确定了拟合关系式的适用范围,在此范围内,拟合式与精确式的相对误差小于25%。分别用Tatarskii谱和修正Hill谱分析了湍流内尺度对圆环孔径平均因子的影响,结果显示:在其他条件不变的情况下,内尺度越大,孔径平均效应相对越小。 相似文献
98.
99.
Quan Shi Narendiran Vitchuli Joshua Nowak Zhan Lin Bingkun Guo Marian Mccord Mohamed Bourham Xiangwu Zhang 《Journal of Polymer Science.Polymer Physics》2011,49(2):115-122
The electrospinnability of polyethylene oxide (PEO) was manipulated by atmospheric plasma treatment of pre‐electrospinning solutions. Conductivity, viscosity, and surface tension of PEO solutions increased after plasma treatment, and the plasma effect remained longer when the solution concentrate increased. Both untreated and treated solutions were then electrospun, and the morphology of the resultant nanofibers was observed by SEM. Atmospheric plasma treatment improved the electrospinnability of PEO solutions and led to less beads and finer diameter distribution in the resultant nanofibers. Additionally, plasma treatment of the pre‐electrospinning solutions affected the crystal structure of resultant nanofibers. These results suggest that atmospheric plasma treatment is a feasible approach to improve the electrospinnability of polymer solutions and can used to control the structure of electrospun nanofibers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 相似文献
100.
T. Portolés J. V. Sancho F. Hernández A. Newton P. Hancock 《Journal of mass spectrometry : JMS》2010,45(8):926-936
The potential applications of a new atmospheric pressure source for GC‐MS analysis have been investigated in this work. A list of around 100 GC‐amenable pesticides, which includes organochlorine, organophosphorus and organonitrogenated compounds, has been used to evaluate their behavior in the new source. Favoring the major formation of the molecular ion in the source has been the main goal due to the wide‐scope screening possibilities that this fact brings in comparison with the traditional, highly fragmented electron ionization spectra. Thus, the addition of water as modifier has been tested as a way to promote the generation of protonated molecules. Pesticides investigated have been classified into six groups according to their ionization/fragmentation behavior. Four of them are characterized by the abundant formation of the protonated molecule in the atmospheric pressure source, mostly being the base peak of the spectrum. These results show that wide‐scope screening could be easily performed with this source by investigating the presence of the protonated molecule ion, MH+. The developed procedure has been applied to pesticide screening in different food samples (nectarine, orange and spinach) and it has allowed the presence of several pesticides to be confirmed such as chlorpyriphos ethyl, deltamethrin and endosulfan sulfate. The availability of a quadrupole time‐of‐flight instrument made it feasible to perform additional MS/MS experiments for both standards and samples to go further in the confirmation of the identity of the detected compounds. Results shown in this paper have been obtained using a prototype source which exhibits promising features that could be applied to other analytical problems apart from those illustrated in this work. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献