全文获取类型
收费全文 | 397篇 |
免费 | 26篇 |
国内免费 | 5篇 |
专业分类
化学 | 237篇 |
晶体学 | 2篇 |
力学 | 17篇 |
数学 | 71篇 |
物理学 | 63篇 |
无线电 | 38篇 |
出版年
2023年 | 7篇 |
2022年 | 7篇 |
2021年 | 20篇 |
2020年 | 26篇 |
2019年 | 18篇 |
2018年 | 18篇 |
2017年 | 15篇 |
2016年 | 38篇 |
2015年 | 22篇 |
2014年 | 26篇 |
2013年 | 42篇 |
2012年 | 29篇 |
2011年 | 30篇 |
2010年 | 19篇 |
2009年 | 16篇 |
2008年 | 12篇 |
2007年 | 21篇 |
2006年 | 11篇 |
2005年 | 7篇 |
2004年 | 7篇 |
2003年 | 3篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 4篇 |
1995年 | 1篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1987年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 5篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
排序方式: 共有428条查询结果,搜索用时 15 毫秒
151.
D. Kiani 《manuscripta mathematica》2007,124(2):269-274
Suppose that D is a division ring with center F and N is a non-central normal subgroup of GL
n
(D). In this paper we generalize some known results about maximal subgroups of GL
n
(D) to maximal subgroups of N. More precisely, we prove that if M is a maximal subgroup of N such that F[M] satisfies a polynomial identity and contains an algebraic element over F or and either n ≥ 2 or n = 1 and M is not abelian, then [D : F] < ∞.
This research was partially supported by a grant from IPM (No. 85160047). 相似文献
152.
Various two vertex sharing macropolyhedral boranes were computed at the B3LYP/6-311 + G**//B3LYP/6-31G* level of theory to determine the preferred fragments for the thermodynamically most stable isomers. These are nido-10 and arachno-9 vertex fragments for neutral macropolyhedral boranes. The thermodynamically most stable isomers of the nido:nido-, arachno:nido- and arachno:arachno-macropolyhedral borane classes are structurally related to each other by the successive removal of one open face vertex as in the case of simple polyhedral boranes. For these classes, the stabilities of the thermodynamically most stable macropolyhedra relative to isomeric simple polyhedra follow similar trends with respect to the number of skeletal electrons. 相似文献
153.
154.
Enhanced Efficiency of Dye–sensitized Solar Cells Based on Bulk Synthesized TiO2 Nanorods Annealed at Different Temperatures
下载免费PDF全文
![点击此处可从《中国化学会会志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Hani Sayahi Masood Hamadanian Farshid Mohsenzadeh Kioumars Aghapoor Mohammad Ali Kiani 《中国化学会会志》2015,62(9):811-816
In this study, bulk TiO2 nanorods are synthesized by hydrothermal method in order to be used in the dye‐sensitized solar cells (DSSCs). These nanorods are annealed at different temperatures and deposited electrophoretically. The influence of post heat treatment has been thoroughly investigated on fabricated DSSCs using electrochemical impedance spectroscopy (EIS). The results have revealed that the diameter, size and density of the prepared bulk nanorods are function of annealing temperatures. Optimization of the prepared DSSCs has led to an improved efficiency (ca. 3.82%) under AM 1.5 simulated sunlight. 相似文献
155.
Keivan Kiani 《Current Applied Physics》2013,13(8):1651-1660
Thanks to the brilliant mechanical properties of single-walled carbon nanotubes (SWCNTs), they are suggested as high speed nanoscale vehicles. To date, various aspects of vibrations of SWCNTs have been addressed; however, vibrations and instabilities of moving SWCNTs have not been thoroughly assessed. Herein, vibrational properties of an axially moving SWCNT with simply supported ends are studied using nonlocal Rayleigh beam theory. Employing assumed mode and Galerkin methods, the discrete governing equations pertinent to longitudinal, transverse, and torsional motions of the moving SWCNT are obtained. The resulting eigenvalue equations are then numerically solved. The speeds corresponding to the initiation of the instability within the moving nanostructure are calculated. The roles of the speed of the moving SWCNT, small-scale parameter, and aspect ratio on the characteristics of longitudinal, transverse, and torsional vibrations of axially moving SWCNTs are scrutinized. The obtained results show that the appearance of the small-scale parameter would result in the occurrence of both divergence and flutter instabilities at lower levels of the speed. 相似文献
156.
It has been shown recently that the normal branch of a DGP braneworld scenario self-accelerates if the induced gravity on the brane is modified in the spirit of f(R) modified gravity. Within this viewpoint, we investigate cosmological viability of the Hu-Sawicki type modified induced gravity. Firstly, we present a dynamical system analysis of a general f(R)-DGP model. We show that in the phase space of the model, there exist three standard critical points; one of which is a de Sitter point corresponding to accelerating phase of the universe expansion. The stability of this point depends on the effective equation of state parameter of the curvature fluid. If we consider the curvature fluid to be a canonical scalar field in the equivalent scalar-tensor theory, the mentioned de Sitter phase is unstable, otherwise it is an attractor, stable phase. We show that the effective equation of state parameter of the model realizes an effective phantom-like behavior. A cosmographic analysis shows that this model, which admits a stable de Sitter phase in its expansion history, is a cosmologically viable scenario. 相似文献
157.
Afshin Rajabi Khorrami Parvin Kiani Ayoob Bazgir 《Monatshefte für Chemie / Chemical Monthly》2011,9(6):287-295
Abstract
l-Proline was found to be a versatile organocatalyst for the synthesis of new spirooxindole derivatives in good yields under mild reaction conditions using 1-propanol as a solvent. 相似文献158.
The synthesis of air‐sensitive 2, 6‐diformyl‐4‐tert‐butylthiophenol dioxime H3L3 was achieved by a Pd‐mediated S–C cleavage of the corresponding S‐tert‐butyl protected thioether. The novel ligand forms a dinuclear, neutral PdII2 complex, which is stabilized by two N ··· HO hydrogen bonds to give a pseudo‐macrocyclic structure. The crystal structure of a PdII complex of an oxidized isothiazole derivative of H3L3 is also reported. 相似文献
159.
Zirconium nitride (ZrN) coatings were prepared on Si (100) by single ion beam sputtering in N2 and Ar mixture at different substrate temperatures. Structure and morphology of the ZrN coatings were analyzed using X‐ray diffraction and atomic force microscopy. Rutherford backscattering technique was utilized for the determination of composition and thickness of the coatings. Electrical properties of the ZrN coatings were determined by four point‐ probe and Hall test. The results showed that the growth of ZrN with a preferred (111) orientation was achieved. The coating thickness depended on the substrate temperature and coating surface roughly remained smooth. The resistivity of the coatings varied from 1× 10‐3 to 14× 10‐3 Ω cm depending on the substrate temperature. A correlation between resistivity and charge carrier density was established to explain the electrical behavior of the coatings. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
160.
High-order accurate numerical simulations are performed to investigate the effects of wavy leading edges (WLEs) on aerofoil–gust interaction (AGI) noise. The present study is based on periodic velocity disturbances predominantly in streamwise and vertical directions that are mainly responsible for the surface pressure fluctuation of an aerofoil. In general, the present results show that WLEs lead to reduced AGI noise. It is found that the ratio of the wavy leading-edge peak-to-peak amplitude (LEA) to the longitudinal wavelength of the incident gust (λg) is the most important factor for the reduction of AGI noise. It is observed that there exists a tendency that the reduction of AGI noise increases with LEA/λg and the noise reduction is significant for LEA/λg≥0.3. The present results also suggest that any two different cases with the same LEA/λg lead to a strong similarity in their profiles of noise reduction relative to the straight leading-edge case. The wavelength of wavy leading edges (LEW), however, shows minor influence on the reduction of AGI noise under the present gust profiles used. Nevertheless, the present results show that a meaningful improvement in noise reduction may be achieved when 1.0≤LEW/λg≤1.5. In addition, it is found that the beneficial effects of WLEs are maintained for various flow incidence angles and aerofoil thicknesses. Also, the WLEs remain effective for gust profiles containing multiple frequency components. It is discovered in this paper that WLEs result in incoherent response time to the incident gust across the span, which results in a decreased level of surface pressure fluctuations, hence a reduced level of AGI noise. 相似文献