全文获取类型
收费全文 | 2689篇 |
免费 | 805篇 |
国内免费 | 305篇 |
专业分类
化学 | 1093篇 |
晶体学 | 22篇 |
力学 | 120篇 |
综合类 | 45篇 |
数学 | 574篇 |
物理学 | 1945篇 |
出版年
2024年 | 12篇 |
2023年 | 40篇 |
2022年 | 106篇 |
2021年 | 98篇 |
2020年 | 81篇 |
2019年 | 79篇 |
2018年 | 114篇 |
2017年 | 143篇 |
2016年 | 132篇 |
2015年 | 134篇 |
2014年 | 237篇 |
2013年 | 198篇 |
2012年 | 210篇 |
2011年 | 249篇 |
2010年 | 202篇 |
2009年 | 183篇 |
2008年 | 188篇 |
2007年 | 198篇 |
2006年 | 166篇 |
2005年 | 167篇 |
2004年 | 122篇 |
2003年 | 120篇 |
2002年 | 76篇 |
2001年 | 76篇 |
2000年 | 66篇 |
1999年 | 47篇 |
1998年 | 52篇 |
1997年 | 53篇 |
1996年 | 46篇 |
1995年 | 42篇 |
1994年 | 28篇 |
1993年 | 15篇 |
1992年 | 29篇 |
1991年 | 16篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1979年 | 1篇 |
排序方式: 共有3799条查询结果,搜索用时 15 毫秒
991.
In this paper an analysis of the far-field pattern of a focal-plane array in the case of two different lenses, a coma-free lens and a hyperbolic lens, and their comparison, are presented. Ray tracing and aperture integration are used. The calculated field pattern at W band is presented. The focal-plane elements are assumed to be corrugated horn. The f / D of the lens is 1.3. The coma-free lens is designed by step integration. The calculation results show that the coma-free lens is superior to the hyperbolic lens in the imaging performance. The coma-free lens can contain more pixels under the condition that the beam size and beam quality are almost constant over the range of angles scanned and the differences among beams gain less 1.8 dB. The aperture efficiency and directivity are also presented. 相似文献
992.
993.
The study of millimeter-wave low sidelobe linear arrays with time modulation technique is presented in this paper. Special
emphasis is on the realization of low sidelobes with uniform amplitude excitations, which are easier for the practical implementation.
Two types of time modulated linear arrays with uniform amplitude excitations, namely, the variable aperture size (VAS) linear
arrays with optimized time sequences and the linear arrays with optimized random time sequences were studied. The optimized
time sequences are obtained via the differential evolution (DE) algorithm and the genetic algorithm (GA). Many factors that
affect the array performance are discussed.
Supported by the National Natural Science Foundation of China (No. 60571023), the SRF for ROCS, SEM, and the innovative Research
Team Program of UESTC, China. 相似文献
994.
应用波的相干原理分析了相控阵雷达的工作原理.以波的形成、波的发射和回波的接收等三个环节为主线,简要地介绍了由固态技术、光电子技术、计算机技术和信号处理技术所形成的相控阵雷达关键技术的背景、现状,并展望了相控阵雷达关键技术的未来. 相似文献
995.
In the current work, we succeeded in incorporation of Pt ions into sisal fiber (SF) – a biological matrix with a characteristic
morphology of microtube array, and in subsequent in situ synthesis of Pt nanoparticles of ca. 3.6 nm. Carbonization of the SF with Pt nanopaticles at 400°C produced Pt-nanoparticle/carbon-fiber
composite, preserving the initial microtube-array morphology of SF. It is interesting that the walls of neighboring microtubes
and the middle lamella between these microtubes were fused by carbonization, and a homogeneous wall was formed. Although the
size of Pt nanoparticles was enhanced to ca. 5.3 nm after carbonization, the solid matrices (from cellulose to carbon) acted
as effective barriers against the growth of Pt nanoparticles. The Pt-nanoparticle/carbon-fiber composite combines several
important aspects, including the morphologies of fiber and microtube-array, carbon matrix, and Pt nanoparticles. Thus it might
be a novel type of catalyst and have potential applications in many fields. 相似文献
996.
采用一种设备简单、原料低廉的新型方法,在镀有ZnO先驱薄膜的(0001)蓝宝石上利用水热 法制备出了柱状ZnO阵列薄膜.用扫描电镜(SEM),X射线衍射(XRD)对样品的形貌和结构进行 了表征,结果显示ZnO薄膜为柱状阵列,基于蓝宝石衬底沿c轴择优生长,且(0004)摇摆曲线 半高宽度(FWHM)约为1.8°.此ZnO阵列薄膜具有很强的紫外发射光谱(PL).
关键词:
柱状ZnO阵列薄膜
水热法
(0001)蓝宝石
PL谱 相似文献
997.
A new type of laser array named four-beamlets laser array is proposed. Based on the diffraction integrals, analytical propagation formulae for the four-beamlets laser array passing through paraxial aligned and misaligned optical systems are derived, respectively. The four-beamlets laser array can also be expressed as a superposition of a series of Hermite Gaussian modes by using polynomial expansion. As a numerical example, the propagation properties of a four-beamlets laser array in free space are illustrated graphically. The fractional Fourier transform of the four-beamlets laser array is also studied. Our model provides a convenient way to describe laser array with four-beamlets. 相似文献
998.
We present the theoretical investigation of spontaneous magnetization of a coupled 2×2 πring array. It is indicated by free energy calculation that the system has the lowest energy when the four π rings have the full antiparallel configuration. Furthermore, the numerical evaluation results show that the system which favours full antiparallel spontaneous magnetization is a quantum effect deriving from the phase cohering of the superconducting quantum wavefunctions in the four superconducting rings through the shared Josephson junctions. 相似文献
999.
A decentered flattened Gaussian beam (DEFGB), is defined by a tensor method. The propagation formula for a DEFGB passing through an axially nonsymmetrical paraxial optical system is derived through vector integration. The derived formula can be reduced to the formula for a generalized decentered elliptical flattened Gaussian beam under certain condition. As an example application of the derived formula, the propagation characteristics of a DEFGB in free space are calculated and discussed. As another example we have studied the properties of superposition with radial array consisted by DEFGB. 相似文献
1000.
The radiation properties of 2 X 2 element planar array of equilateral triangular patch microstrip antenna in plasma medium
are studied. The array factor and far-zone EM-mode and P-mode radiation fields of the array geometry are derived using vector
wave function techniques and pattern multiplication approaches. The total field patterns and various characteristics of pattern
such as half power beam width (HPBW), first null beam width (FNBW) and direction of maximum radiation are computed for two
different values of progressive phase excitation difference between the elements. The results of this array geometry are obtained
both in plasma medium and in free space and compared with those of single element equilateral triangular patch microstrip
antenna. 相似文献