全文获取类型
收费全文 | 242643篇 |
免费 | 2409篇 |
国内免费 | 869篇 |
专业分类
化学 | 130161篇 |
晶体学 | 3688篇 |
力学 | 9889篇 |
综合类 | 2篇 |
数学 | 27044篇 |
物理学 | 75137篇 |
出版年
2020年 | 1764篇 |
2019年 | 2035篇 |
2018年 | 2469篇 |
2017年 | 2400篇 |
2016年 | 3814篇 |
2015年 | 2588篇 |
2014年 | 3891篇 |
2013年 | 11145篇 |
2012年 | 8064篇 |
2011年 | 9987篇 |
2010年 | 6562篇 |
2009年 | 6459篇 |
2008年 | 8970篇 |
2007年 | 9120篇 |
2006年 | 8363篇 |
2005年 | 7822篇 |
2004年 | 6978篇 |
2003年 | 6198篇 |
2002年 | 6131篇 |
2001年 | 7196篇 |
2000年 | 5358篇 |
1999年 | 4215篇 |
1998年 | 3547篇 |
1997年 | 3575篇 |
1996年 | 3287篇 |
1995年 | 3128篇 |
1994年 | 2944篇 |
1993年 | 2999篇 |
1992年 | 3292篇 |
1991年 | 3327篇 |
1990年 | 3144篇 |
1989年 | 3099篇 |
1988年 | 3159篇 |
1987年 | 3029篇 |
1986年 | 2923篇 |
1985年 | 3985篇 |
1984年 | 4145篇 |
1983年 | 3399篇 |
1982年 | 3749篇 |
1981年 | 3645篇 |
1980年 | 3517篇 |
1979年 | 3551篇 |
1978年 | 3777篇 |
1977年 | 3606篇 |
1976年 | 3814篇 |
1975年 | 3388篇 |
1974年 | 3517篇 |
1973年 | 3819篇 |
1972年 | 2327篇 |
1971年 | 1786篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
The stationary and the time-dependent homogeneous ordered states in convection may both become unstable against localized perturbations. Defects are then created and they may contribute to the disorganization of the homogeneous state. We present an experimental study of defects in some homogeneous stationary structures as well as in the traveling-wave states of convection of a nematic liquid crystal. We show that the core of the defects is a germ of the unstable state and it can become unstable under the external stress. Then, either fully homogeneous states with the symmetry of the core, or complex disordered states can develop from the local instability of defects in processes quite similar to displacive transitions in solids. Some of the main features are qualitatively similar to numerical simulations of an appropriate Landau-Ginzburg equation. 相似文献
192.
Summary We study the integration theory for general integral metrics when restricted to upper integrals q, finding improvements in the relation between the classes of the q-integrable and the ql-integrable functions. We give new results and notions which lead to the desirable characterizations of q-integrable functions as ql-integrable f with q(|f|) < ∞, and of ql-integrable functions via the integrability of their upper truncations, under natural conditions which are fulfilled in most
finitely additive integration theories. 相似文献
193.
194.
195.
F.A. Fernández-Lima C.R. Ponciano E. Pedrero E.F. da Silveira 《Applied Surface Science》2006,252(23):8171-8177
Experimental results of laser sputtering of cesium and rubidium iodide secondary ions are presented. A TOF mass spectrometer, operating in linear mode, continuous extraction for positive or negative ions, was used for the analysis of (CsI)nCs+, (CsI)nI−, (RbI)nRb+ and (RbI)nI− ion emission as a function of the laser irradiance. Experimental data show that the cluster ion emission yields decrease exponentially with n, for all the laser irradiances applied. Theoretical analysis of the clusters structure was performed using density functional theory at the B3LYP/LACV3P level, for the positive and negative cluster series. A quasi-equilibrium evolution of the clusters is proposed to extract a parameter characteristic of the cluster recombination process: the effective temperature. The hypothesis of the atomic species’ recombination (during the expansion of a high density highly ionized cloud) leading to cluster formation is confirmed to some extent in a second set of experiments: the UV laser ablation of a mixed and non-mixed cesium iodide and potassium bromide targets. These experiments show that the emission yields contain contributions from both the recombination process and from the sample stoichiometry, even for high laser irradiances. 相似文献
196.
A planar map is a 2-cell embedding of a connected planar graph, loops and parallel edges allowed, on the sphere. A plane map is a planar map with a distinguished outside (“infinite”) face. An unrooted map is an equivalence class of maps under orientation-preserving homeomorphism, and a rooted map is a map with a distinguished oriented edge. Previously we obtained formulae for the number of unrooted planar n-edge maps of various classes, including all maps, non-separable maps, eulerian maps and loopless maps. In this article, using the same technique we obtain closed formulae for counting unrooted plane maps of all these classes and their duals. The corresponding formulae for rooted maps are known to be all sum-free; the formulae that we obtain for unrooted maps contain only a sum over the divisors of n. We count also unrooted two-vertex plane maps. 相似文献
197.
A. Ferreira da Silva I. Pepe S.A. Tomás W.M. de Azevedo R. Ahuja 《Applied Surface Science》2006,252(15):5365-5367
In this paper we present spectroscopic properties of doped and undoped titanium dioxide (TiO2) as nanofilms prepared by the sol-gel process with rhodamine 6G doping and studied by photoacoustic absorption, excitation and emission spectroscopy. The absorption spectra of TiO2 thin films doped with rhodamine 6G at very low concentration during their preparation show two absorption bands, one at 2.3 eV attributed to molecular dimmer formation, which is responsible for the fluorescence quenching of the sample and the other at 3.0 eV attributed to TiO2 absorption, which subsequently yields a strong emission band at 600 nm. The electronic band structure and optical properties of the rutile phase of TiO2 are calculated employing a fully relativistic, full-potential, linearized, augmented plane-wave (FPLAPW) method within the local density approximation (LDA). Comparison of this calculation with experimental data for TiO2 films prepared for undoped sol-gels and by sputtering is performed. 相似文献
198.
199.
Tunable properties of light propagation in photonic crystal fibers filled with liquid crystals, called photonic liquid crystal
fibers (PLCFs) are presented. The propagation properties of PLCFs strongly depend on contrast between refractive indices of
the solid core (pure silica glass) and liquid crystals (LCs) filing the holes of the fiber. Due to relatively strong thermo-optical
effect, we can change the refractive index of the LC by changing its temperature. Numerical analysis of light propagation
in PLCF, based on two simulation methods, such as finite difference (FD) and multipole method (MM) is presented. The numerical
results obtained are in good agreement with our earlier experimental results presented elsewhere [1]. 相似文献
200.
E. Baggio Saitovitch F. J. Litterst I. Souza Azevedo R. B. Scorzelli 《Hyperfine Interactions》1989,50(1-4):529-535
The Mössbauer spectra of YBa2(Cu1?x Fe x )3O7 at room temperature show several doublets attributed to Fe in Cu(1) sites with different oxygen configurations. Here we present a systematic study performed at 4.2 K forx=0.005, 0.01, 0.03, 0.05, 0.10, 0.15. To obtain information about the magnetic ordered state two samples, withx=0.005 andx=0.15, have been studied at 4.2 K underB ext=5 T. The Mössbauer spectra indicate that the iron moments are polarized forx=0.005, while in the ordered state (x=0.15) they have an antiferromagnetic or spin-glass-like arrangement with high anisotropy. 相似文献