全文获取类型
收费全文 | 3020篇 |
免费 | 392篇 |
国内免费 | 223篇 |
专业分类
化学 | 2066篇 |
晶体学 | 44篇 |
力学 | 110篇 |
综合类 | 8篇 |
数学 | 434篇 |
物理学 | 973篇 |
出版年
2023年 | 39篇 |
2022年 | 48篇 |
2021年 | 81篇 |
2020年 | 113篇 |
2019年 | 104篇 |
2018年 | 93篇 |
2017年 | 84篇 |
2016年 | 125篇 |
2015年 | 156篇 |
2014年 | 143篇 |
2013年 | 261篇 |
2012年 | 270篇 |
2011年 | 288篇 |
2010年 | 167篇 |
2009年 | 152篇 |
2008年 | 197篇 |
2007年 | 187篇 |
2006年 | 140篇 |
2005年 | 137篇 |
2004年 | 92篇 |
2003年 | 110篇 |
2002年 | 67篇 |
2001年 | 76篇 |
2000年 | 58篇 |
1999年 | 48篇 |
1998年 | 44篇 |
1997年 | 35篇 |
1996年 | 45篇 |
1995年 | 30篇 |
1994年 | 36篇 |
1993年 | 36篇 |
1992年 | 24篇 |
1991年 | 24篇 |
1990年 | 10篇 |
1989年 | 28篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 20篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1979年 | 2篇 |
1977年 | 3篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1940年 | 1篇 |
1938年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有3635条查询结果,搜索用时 234 毫秒
991.
Density functional theory calculations were performed on C82 hydroxylated fullerene. B3LYP and PBE0 functionals with 6-31G** basis set were utilised to get chemical shieldings, chemical shifts and the isotropic Fermi contact coupling on each atomic site. A relation between nuclear magnetic resonance (NMR) properties and reactivity of the molecule, obtained through the electronic Fukui function, was observed. Interestingly, the most stable configurations of OH groups adsorbed on C82 surface were obtained when the hydroxyl groups are adsorbed on deshielded (isotropically and anisotropically) sites. For open-shell systems, a relation between isotropic Fermi contact, spin density and average Fukui function was found, that is, sites with a great amount of Fukui function (analytical and the one obtained through finite difference) and spin density have the largest isotropic Fermi contact coupling data. With the adsorption of the first hydroxyl molecules, spin densities and Fukui functions show preferential sites to adsorb the following OH groups close to previously adsorbed. Additionally, theoretical spectra of chemical shifts of C82(OH)n (n = 1, 2, 3 and 4) were obtained and they were compared with experimental reports, getting a reasonable comparison. For example, regarding 13C NMR chemical shifts obtained in C82OH molecule, 80 ppm (B3LYP) and 79 ppm (PBE0) were calculated on hydroxylated carbon, which is in good agreement with experimental results in C60 fullerols. 相似文献
992.
大气压介质阻挡放电(DBD)可以在常压下产生非平衡等离子体,已经成为热点研究领域.通过脉冲或交变电源激发放电,研究电源输出特性、电源与放电发生器负载间的匹配和外界条件对放电的影响对于理解放电现象和提高放电效率具有重要意义.本文采用Lissajous图形法,分别研究了驱动电压、气流速率等因素影响同轴DBD发生器介质层等效电容及负载幅频特性的规律.结果表明,气流速率和驱动电压等外界条件影响DBD发生器的负载特性:介质层等效电容随气流速率增大而减小,随驱动电压增大而增大;幅频特性曲线均表现出RLC回路谐振现象,谐振频率随气流速率增大而增大,随驱动电压增大而减小.通过对比发现,介质层等效电容随频率的变化曲线与幅频特性曲线具有一致的特征,介质层等效电容是影响电路谐振频率动态变化的主要因素.提出了一种有关介质层等效电容的形成机制. 相似文献
993.
Yi‐Huei Kao Mu‐Huan Chi Chia‐Chan Tsai Jiun‐Tai Chen 《Macromolecular rapid communications》2014,35(1):84-90
A simple and versatile method is developed for preparing anisotropic polymer particles by pressing polymer microspheres at elevated temperatures. Polystyrene (PS) microspheres are used to demonstrate this approach. Depending on the mechanical deformation and wetting of the polymers on the substrates, polymer structures with special shapes such as barrel‐like or dumbbell‐like shapes can be prepared. The morphology of polymer structures can be controlled by the experimental parameters such as the pressing pressure, the pressing temperature, and the pressing time. The wetting of the polymers on the substrates dominates when the samples are annealing at higher temperatures for longer times.
994.
The coefficients of a linear system, even if it is a part of a block-oriented nonlinear system, normally satisfy some linear algebraic equations via Hankel matrices composed of impulse responses or correlation functions. In order to determine or to estimate the coefficients of a linear system it is important to require the associated Hankel matrix be of row-full-rank. The paper first discusses the equivalent conditions for identifiability of the system. Then, it is shown that the row-full-rank of the Hankel matrix composed of impulse responses is equivalent to identifiability of the system. Finally, for the row-full-rank of the Hankel matrix composed of correlation functions, the necessary and sufficient conditions are presented, which appear slightly stronger than the identifiability condition. In comparison with existing results, here the minimum phase condition is no longer required for the case where the dimension of the system input and output is the same, though the paper does not make such a dimensional restriction. 相似文献
995.
996.
José L. Gámez-Merino Gustavo A. Muñoz-Fernández Daniel Pellegrino Juan B. Seoane-Sepúlveda 《Linear algebra and its applications》2012,436(1):237-242
In this paper we prove a characterization of continuity for polynomials on a normed space. Namely, we prove that a polynomial is continuous if and only if it maps compact sets into compact sets. We also provide a partial answer to the question as to whether a polynomial is continuous if and only if it transforms connected sets into connected sets. These results motivate the natural question as to how many non-continuous polynomials there are on an infinite dimensional normed space. A problem on the lineability of the sets of non-continuous polynomials and multilinear mappings on infinite dimensional normed spaces is answered. 相似文献
997.
998.
Leonid A. Kurdachenko José M. Mu?oz-Escolano Javier Otal 《Milan Journal of Mathematics》2012,80(1):227-241
The source of this paper is a classical theorem by B. H. Neumann on groups whose conjugacy classes are boundedly finite. In a natural way this leads to the study of groups with restrictions on the normal closures of their cyclic subgroups. More concretely, in this paper we study groups G such that the normal closure of every cyclic subgroup ${{\langle{g}\rangle}}$ has a divisible Chernikov G-invariant subgroup D of minimax rank r such that gD has at most b conjugates in the factorgroup G/D. We prove that such groups are Chernikov-by-abelian and bound their invariants in terms of r and b only. 相似文献
999.
1000.
María del Mar Cordero-Rando Manuel Barea-Zamora Juan M. Barberá-Salvador Ignacio Naranjo-Rodríguez Juan A. Muñoz-Leyva José L. Hidalgo-Hidalgo de Cisneros 《Mikrochimica acta》1999,132(1):7-11
A zeolite-modified carbon paste electrode (CPE) has been used for the determination of 4-nitrophenol by differential pulse
voltammetry (DPV). The electrochemical reduction of 4-nitrophenol at −1.0 V is carried out in a Britton-Robinson medium at
pH 3.5. The cyclic voltammetric (CV) behaviour has been investigated to study the nature of the process. Studies on the effect
of pH were carried out over the pH range 2–9 with the Britton-Robinson buffer solution, and the influence of pH on peak height
and peak potential was analyzed. A linear relationship between peak intensity and concentration is obtained in the range 0.2–10 mg L−1, with a detection limit of 0.04 mg L−1; a relative standard deviation of 1.5% for a 5 mg L−1 4-nitrophenol concentration and a relative error of 2.6% were also obtained (n=11).
Received March 3, 1998. Revision December 10, 1998. 相似文献