全文获取类型
收费全文 | 22157篇 |
免费 | 3710篇 |
国内免费 | 3582篇 |
专业分类
化学 | 10596篇 |
晶体学 | 406篇 |
力学 | 843篇 |
综合类 | 374篇 |
数学 | 1680篇 |
物理学 | 5944篇 |
无线电 | 9606篇 |
出版年
2024年 | 139篇 |
2023年 | 489篇 |
2022年 | 797篇 |
2021年 | 920篇 |
2020年 | 912篇 |
2019年 | 844篇 |
2018年 | 709篇 |
2017年 | 883篇 |
2016年 | 818篇 |
2015年 | 1098篇 |
2014年 | 1304篇 |
2013年 | 1706篇 |
2012年 | 1827篇 |
2011年 | 1867篇 |
2010年 | 1613篇 |
2009年 | 1586篇 |
2008年 | 1694篇 |
2007年 | 1585篇 |
2006年 | 1461篇 |
2005年 | 1210篇 |
2004年 | 986篇 |
2003年 | 701篇 |
2002年 | 737篇 |
2001年 | 615篇 |
2000年 | 672篇 |
1999年 | 380篇 |
1998年 | 231篇 |
1997年 | 195篇 |
1996年 | 188篇 |
1995年 | 190篇 |
1994年 | 180篇 |
1993年 | 161篇 |
1992年 | 130篇 |
1991年 | 96篇 |
1990年 | 104篇 |
1989年 | 79篇 |
1988年 | 62篇 |
1987年 | 41篇 |
1986年 | 54篇 |
1985年 | 44篇 |
1984年 | 27篇 |
1983年 | 29篇 |
1982年 | 20篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1971年 | 2篇 |
1959年 | 4篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
301.
Jiye Liang Feng Wang Chuangyin Dang Yuhua Qian 《International Journal of Approximate Reasoning》2012,53(6):912-926
Feature selection is a challenging problem in many areas such as pattern recognition, machine learning and data mining. Rough set theory, as a valid soft computing tool to analyze various types of data, has been widely applied to select helpful features (also called attribute reduction). In rough set theory, many feature selection algorithms have been developed in the literatures, however, they are very time-consuming when data sets are in a large scale. To overcome this limitation, we propose in this paper an efficient rough feature selection algorithm for large-scale data sets, which is stimulated from multi-granulation. A sub-table of a data set can be considered as a small granularity. Given a large-scale data set, the algorithm first selects different small granularities and then estimate on each small granularity the reduct of the original data set. Fusing all of the estimates on small granularities together, the algorithm can get an approximate reduct. Because of that the total time spent on computing reducts for sub-tables is much less than that for the original large-scale one, the algorithm yields in a much less amount of time a feature subset (the approximate reduct). According to several decision performance measures, experimental results show that the proposed algorithm is feasible and efficient for large-scale data sets. 相似文献
302.
Zhijuan Cao Fengxian Qiu Qing Wang Guorong Cao Yijun Guan Lin Zhuang Xiaolong Xu Jie Wang Qian Chen Dongya Yang 《Applied physics. B, Lasers and optics》2013,111(1):93-102
An azo chromophore molecule 4-[(benzothiazole-2-yl)diazenyl]phenyl-1,3-diamine (BTPD) was prepared with 2-amino benzothiazole and m-phenylenediamine by diazo-coupling reaction. Then, the chromophore molecule BTPD was polymerized with NJ-210 and isophorone diisocyanate (IPDI) to obtain novel azo benzothiazole polymer (BTPU). The structures of BTPD and BTPU were characterized using the Fourier transform infrared, UV–visible spectroscopy, DSC and TGA. The physical properties of the obtained BTPU were investigated. The refractive index (n) of BTPU was demonstrated at different temperature and wavelength (532, 650 and 850 nm) using attenuated total reflection technique. The transmission loss and dispersion characteristic of BTPU film were investigated using the CCD digital imaging devices and Sellmeyer equation. A Y-branch and 2 × 2 Mach–Zehnder interferometer (MZI) polymeric thermo-optic switches based on the thermo-optic effect of prepared BTPU were proposed and the performance of switches was simulated. The results indicated that the power consumption of the Y-branch thermo-optic switch could be only 0.6 mW. The Y-branch and MZI switching rising and falling times obtained were 8.0 and 1.8 ms. 相似文献
303.
On the Upper Bound of Second Eigenvalues for Uniformly Elliptic Operators of any Orders 总被引:1,自引:0,他引:1
Gao Jia Xiao-ping Yang Chun-lin QianBengbu Tank Institute Bengbu China & School of Science Nanjing University of Science Technology Nanjing ChinaSchool of Science Nanjing University of Science Technology Nanjing China Suzhou Television & Radio Broadcasting University Suzhou China 《应用数学学报(英文版)》2003,19(1):107-116
Abstract Let Ω R~m(m≥1)be a bounded domain with piecewise smooth boundary Ω.Let t and r bepositive integers with t>r+1. We consider the eigenvalue problems(1.1)and(12),and obtain Theorem 1and Theorem 2, which generalize the results in[1,2.5]. 相似文献
304.
为了研究索尼LCX029CRT液晶显示器纯位相调制特性,设计了索尼LCX029CRT液晶显示器的驱动电路,并搭建了测试光路对其纯位相调制特性进行了研究,同时对光路进行了理论分析。实验获得一组移动的干涉条纹,实验测试研究结果表明索尼LCX029CRT液晶显示器具有π/2的相位调制特性。 相似文献
305.
Effect of conversion process ([I]-P→ΛΛ) on[I]--hypernucleus is studied for [I]12-Be. It is found that theconversion process has a certain extent effect on properties of low-lying states of the[I]--hypernuclei. 相似文献
306.
It has been shown that optical activity can occur in microemulsion under external electric field and rotation angle can also be tuned by the electric field. A set of microemulsions (water/Span80/transformer oil) with different water concentration were prepared and their optical activity was measured with the changes of applied electric field and θ, the angle between the electric vector of the incident linearly polarized light and the external electric field, using an automatic polarimeter. The experiments indicate that when none of the external electric field, water concentration and θ are zero, there is optical activity in microemulsions. For a given concentration, rotation angle ψ increases with electric field, and it firstly increases, passes through a maximum at C = C0,then monotonically decreases as C increases when electric field keeps constant. The relationship between the rotation angle and θ is also obtained. It is thought that the electric field-induced destroy of spatial symmetry of microemulsion is responsible for the optical activity of microemulsion. 相似文献
307.
Lei Qian ZhiChen Pan Di Li George Hobbs WeiWei Zhu Pei Wang ZhiJie Liu YouLing Yue Yan Zhu HongFei Liu DongJun Yu JingHai Sun Peng Jiang GaoFeng Pan Hui Li HengQian Gan Rui Yao XiaoYao Xie FernANDo Camilo ANDrew Cameron Lei Zhang Shen Wang 《中国科学:物理学 力学 天文学(英文版)》2019,(5)
<正>To assist with the commissioning [1] of the Five-hundredmeter Aperture Spherical radio Telescope (FAST), we performed a pulsar search, with the primary goal of developing and testing the pulsar data acquisition and processing pipelines. We tested and used three pipelines, two (P1 and P2 hereafter) searched for the periodic signature of pulsars 相似文献
308.
D. Qian Yinwan Li D.M. Casa Y.-D. Chuang T. Tohyama H. Eisaki 《Journal of Physics and Chemistry of Solids》2005,66(12):2212-2215
Momentum dependence of charge excitations across the effective Mott gap in several quasi-low-dimensional model cuprates with different effective dimensionalities is studied using high resolution inelastic X-ray scattering by working near Copper k-edge resonance which allows us to extract the dispersion relations of the particle-hole pair excitations at the gap edge. Besides electron-electron correlation, momentum dependence of the gap-excitations is found to be strongly dependent on the effective dimensionality (or topology) of the 3dx2−y2 network. 相似文献
309.
We analyze the propagation of thickness-twist waves in an unbounded piezoelectric ceramic plate with air gaps between the plate surfaces and two electrodes. These waves are also called anti-plane or shear-horizontal waves with one displacement component only. An exact solution is obtained from the equations of the linear theory of piezoelectricity. Dispersion relations of the waves are obtained and plotted. Results show that the wave frequency or speed is sensitive to the air gap thickness. This effect can be used to manipulate the behavior of the waves and has implications in acoustic wave devices. 相似文献
310.
This paper is concerned with the following rational difference equation xn+1=axn-bxn-1+ex2n/(cXn+dxn-1), with the initial conditions x-1 , x0 ∈ (0, +∞) and a, b, c, d, e ∈ R+ . Locally asymptotically stability, global attractively and boundedness character of the equilibrium point of the equation are investigated. Moreover, simulation is shown to support the results. 相似文献