全文获取类型
收费全文 | 20321篇 |
免费 | 2625篇 |
国内免费 | 2348篇 |
专业分类
化学 | 15529篇 |
晶体学 | 248篇 |
力学 | 921篇 |
综合类 | 159篇 |
数学 | 2998篇 |
物理学 | 5439篇 |
出版年
2024年 | 37篇 |
2023年 | 275篇 |
2022年 | 557篇 |
2021年 | 664篇 |
2020年 | 702篇 |
2019年 | 736篇 |
2018年 | 612篇 |
2017年 | 561篇 |
2016年 | 990篇 |
2015年 | 1032篇 |
2014年 | 1113篇 |
2013年 | 1470篇 |
2012年 | 1789篇 |
2011年 | 1848篇 |
2010年 | 1396篇 |
2009年 | 1255篇 |
2008年 | 1454篇 |
2007年 | 1306篇 |
2006年 | 1162篇 |
2005年 | 1023篇 |
2004年 | 913篇 |
2003年 | 698篇 |
2002年 | 760篇 |
2001年 | 547篇 |
2000年 | 373篇 |
1999年 | 334篇 |
1998年 | 252篇 |
1997年 | 199篇 |
1996年 | 184篇 |
1995年 | 157篇 |
1994年 | 125篇 |
1993年 | 108篇 |
1992年 | 92篇 |
1991年 | 87篇 |
1990年 | 70篇 |
1989年 | 49篇 |
1988年 | 39篇 |
1987年 | 40篇 |
1986年 | 25篇 |
1985年 | 35篇 |
1984年 | 34篇 |
1983年 | 24篇 |
1982年 | 12篇 |
1981年 | 20篇 |
1980年 | 18篇 |
1979年 | 17篇 |
1978年 | 12篇 |
1976年 | 7篇 |
1975年 | 8篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
271.
We report the achievement of the first images to our knowledge obtained with a fractal zone plates (FraZPs). FraZPs are diffractive lenses characterized by the fractal structure of their foci. This property predicts an improved performance of FraZPs as image forming devices with an extended depth of field and predicts a reduced chromatic aberration under white-light illumination. These theoretical predictions are confirmed experimentally in this work. We show that the polychromatic modulation transfer function of a FraZP affected by defocus is about two times better than one corresponding to a Fresnel zone plate. 相似文献
272.
Pattern时延差编码水声通信体制利用信息码元的时延值调制信息,他的抗多途扩展干扰能力与码型种类及码型脉宽有关.扩频通信可获得扩频增益,可胜任远程水声通信,但其通信速率低是使用受限的重要原因.提出将扩频通信与Pattern时延差编码水声通信体制相结合,并采用M元扩频、多通道工作方式,构成一种新的适用于水声环境的通信方案,既可获得扩频通信的优良性能,又可提高通信速率.湖试结果验证了本文提出的M元混沌扩频多通道Pattern时延差编码水声通信方案的鲁棒性及可行性. 相似文献
273.
Solvent and electric field dependence of the photocurrent generation in donor:acceptor blend system 下载免费PDF全文
This paper reports that the blend films of poly
(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV)
and N,N'-bis(1-ethylpropyl)-3,4: 9,10-perylene bis
(tetracarboxyl diimide) (EP-PDI) with the weight ratio of 1:2.5
have been prepared by spin-coating from chloroform (CF) and
chlorobenzene (CB) solutions respectively. The absorption
spectra and the morphology of the blend films show that large
crystal-like EP-PDI aggregates are formed in film prepared from
CB solution, which corresponds to a new absorption shoulder near
590nm, while there is no shoulder around 590\,nm in the
UV--Vis absorption spectra of the blend film from CF solution.
The electric-field dependence spectra of the photocurrent
generation quantum yield of the film from CB solution shows that
at weak electric field the EP-PDI aggregates act as more
efficient sensitizers, but at strong electric field the quantum
yields become almost invariable over the entire spectral range
no matter what the state of EP-PDI, monomer or aggregate. At
strong electric field, the photocurrent generation yields of
both films from CF and CB solution saturate and their yield
spectra become spectrally similar, mentioning that at strong
electric field the photoexcitons dissociate efficiently and the
free charges are collected by the electrodes almost completely. 相似文献
274.
张文 李宁蒋玲 缪巍林镇辉 姚骑均史生才陈健 吴培亨S.I.Svechnikov Yu. B. Vachtomin S. V. Antipov B. M. voronov G. N. Gol'tsman 《中国物理快报》2007,24(6):1778-1781
A quasi-optical superconducting NbN hot-electron bolometer (HEB) mixer is measured in the frequency range of 0.5-2.5 THz for understanding of the frequency dependence of noise temperature of THz coherent detectors. It has been found that noise temperature increasing with frequency is mainly due to the coupling loss between the quasioptical planar antenna and the superconducting HEB bridge when taking account of non-uniform distribution of high-frequency current. With the coupling loss corrected, the superconducting HEB mixer demonstrates a noise temperature nearly independent of frequency. 相似文献
275.
Wen J Zhao H Lv L Yuan B Wang G Wen X 《The Journal of the Acoustical Society of America》2011,130(3):1201-1208
Recently, by introducing locally resonant scatterers with spherical shape proposed in phononic crystals into design of underwater sound absorption materials, the low-frequency underwater sound absorption phenomenon induced by the localized resonances is observed. To reveal this absorption mechanism, the effect of the locally resonant mode on underwater sound absorption should be studied. In this paper, the finite element method, which is testified efficiently by comparing the calculation results with those of the layer multiple scattering method, is introduced to investigate the dynamic modes and the corresponding sound absorption of localized resonance. The relationship between the resonance modes described with the displacement contours of one unit cell and the corresponding absorption spectra is discussed in detail, which shows that the localized resonance leads to the absorption peak, and the mode conversion from longitudinal to transverse waves at the second absorption peak is more efficient than that at the first one. Finally, to show the modeling capability of FEM and investigate shape effects of locally resonant scatterers on underwater sound absorption, the absorption properties of viscoelastic materials containing locally resonant scatterers with ellipsoidal shape are discussed. 相似文献
276.
The nature of the magnetic ground state near the insulator-metal transition (IMT) in La(1-x)Ba(x)CoO3 was investigated via neutron scattering. Below the critical concentration, x(c)~0.22, a commensurate antiferromagnetic (AFM) phase appears initially. Upon approaching x(c), the AFM component weakens and a ferromagnetic (FM) ordered phase sets in while in the rhombohedral lattice. At x(c), a spin flip to a new FM structure occurs at the same time as the crystal symmetry transforms to orthorhombic (Pnma). The Pnma phase may be the driving force for the IMT. 相似文献
277.
Rand DA Shaw SE Ochoa JR Ripin DJ Taylor A Fan TY Martin H Hawes S Zhang J Sarkisyan S Wilson E Lundquist P 《Optics letters》2011,36(3):340-342
A cryogenic Yb amplifier using two laser materials, Gd3Sc2Al3O12 and Y3Al5O12 (YAG), has been used to obtain 70 W average power at 5 kHz pulse repetition frequency; the output was compressed to 1.6 ps, compared with an input compressible to 1.4 ps. The gain broadening obtained by combining two media enables shorter pulses than using Yb:YAG alone but retains the power-scaling advantages of cryogenic Yb:YAG. 相似文献
278.
The resolution of electron energy loss spectroscopy (EELS) is limited by delocalization of inelastic electron scattering rather than probe size in an aberration corrected scanning transmission electron microscope (STEM). In this study, we present an experimental quantification of EELS spatial resolution using chemically modulated 2×(LaMnO(3))/2×(SrTiO(3)) and 2×(SrVO(3))/2×(SrTiO(3)) superlattices by measuring the full width at half maxima (FWHM) of integrated Ti M(2,3), Ti L(2,3), V L(2,3), Mn L(2,3), La N(4,5), La N(2,3) La M(4,5) and Sr L(3) edges over the superlattices. The EELS signals recorded using large collection angles are peaked at atomic columns. The FWHM of the EELS profile, obtained by curve-fitting, reveals a systematic trend with the energy loss for the Ti, V, and Mn edges. However, the experimental FWHM of the Sr and La edges deviates significantly from the observed experimental tendency. 相似文献
279.
Qinqin Liu Juan YangXiaonong Cheng Xiujuan SunChuanliang Zang 《Physica B: Condensed Matter》2011,406(18):3458-3464
ZrW2O8 displays the unusual property of an isotropic bulk contraction in volume as a function of temperature. We report here on the positive thermal expansion (PTE) property caused by substituting Mo for W sites in ZrW2O8 at room temperature. The room temperature crystal structure of ZrW2−xMoxO8 compounds, which were synthesized using a low temperature route, could be divided into ordered phase with α-ZrW2O8 structure (0≤x≤0.5) and disordered phase with β-ZrW2O8 (0.5<x≤1.5) and c-ZrMo2O8 structure (1.5<x≤2). ZrW2−xMoxO8 adopting β-ZrW2O8 structure shows abnormal PTE property at room temperature due to the existence of water molecules, while ZrW2−xMoxO8 adopting the other two structures (α-ZrW2O8 and c-ZrMo2O8) shows negative thermal expansion (NTE) property from room temperature until decomposition for α-ZrW2O8 structure or trigonal phase transition for c-ZrMo2O8 structure. The reason lies in the fact that the oxygen migration caused by the Mo substitution leads to the re-arrangement of W(Mo)O4 tetrahedra lying along the 3-fold axis, only particular W/Mo ratio (0.5<x≤1.5) produces a special crystal structure, which allows the entrance of water molecules. 相似文献
280.
This paper investigates the synchronization of complex systems with delay that are impulsively coupled at discrete instants only. Based on the comparison theorem of impulsive differential system, a distributed impulsive control scheme is proposed to achieve the synchronization for systems with delay. In the control strategy, the influence of all nodes to network synchronization relies on its weight. The proposed control scheme is applied to the chaotic delayed Hopfield neural networks and numerical simulations are presented to demonstrate the effectiveness of the proposed scheme. 相似文献