全文获取类型
收费全文 | 3189篇 |
免费 | 380篇 |
国内免费 | 262篇 |
专业分类
化学 | 1853篇 |
晶体学 | 58篇 |
力学 | 708篇 |
综合类 | 44篇 |
数学 | 350篇 |
物理学 | 818篇 |
出版年
2024年 | 15篇 |
2023年 | 45篇 |
2022年 | 142篇 |
2021年 | 149篇 |
2020年 | 148篇 |
2019年 | 110篇 |
2018年 | 88篇 |
2017年 | 99篇 |
2016年 | 156篇 |
2015年 | 97篇 |
2014年 | 137篇 |
2013年 | 331篇 |
2012年 | 138篇 |
2011年 | 125篇 |
2010年 | 132篇 |
2009年 | 138篇 |
2008年 | 131篇 |
2007年 | 167篇 |
2006年 | 152篇 |
2005年 | 139篇 |
2004年 | 118篇 |
2003年 | 136篇 |
2002年 | 100篇 |
2001年 | 85篇 |
2000年 | 89篇 |
1999年 | 61篇 |
1998年 | 64篇 |
1997年 | 84篇 |
1996年 | 74篇 |
1995年 | 79篇 |
1994年 | 64篇 |
1993年 | 52篇 |
1992年 | 37篇 |
1991年 | 27篇 |
1990年 | 20篇 |
1989年 | 17篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1982年 | 9篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 3篇 |
1977年 | 9篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有3831条查询结果,搜索用时 15 毫秒
991.
992.
工程结构优化的神经网络方法 总被引:14,自引:1,他引:14
本文阐述了神经网络优化计算的基本原理,构造了工程结构优化的神经网络模型。采用模拟退火技术进行模型求解,且巧妙地将退火温度T的倒数作为Lagrange乘子,以改善增广目标函数的收敛性。实例计算表明,由非线性模拟神经元组成的大规模并行、互连的网络在工程结构的优化设计中是可行且有效的。 相似文献
993.
Jian‐Li Guo Chong Liu Bin‐Bin Xie Yan‐Ying Zhao Ke‐Mei Pei Hui‐Gang Wang Xuming Zheng Yue‐Jie Ai Xue‐Bo Chen Wei‐Hai Fang Chi Shun Yeung 《Journal of Raman spectroscopy : JRS》2012,43(10):1477-1486
The photophysics and photochemistry of pyrazine (C4H4N2, D2h) after excitation to the S2 (1 1B2u, 1ππ*) electronic state were studied by using the resonance Raman spectroscopy and complete active space self‐consistent field method calculations. The B‐band resonance Raman spectra in cyclohexane solvent were obtained at 266.0, 252.7, and 245.9 nm excitation wavelengths to probe the structural dynamics of pyrazine in the S2 (1 1B2u, 1ππ*) state. Three electronic states 1 1B3u, 1 1B1g, and 1 1B2g were found to couple with the S2 (1 1B2u, 1ππ*) state. Two conical intersection (CI) points CI[S2(B2u)/S1(B3u)] and CI[S1/S0] and one transition state of the isomerization between pyrazine and pyrimidine were predicted to play important roles in the photochemistry of pyrazine. On the basis of the calculations, the mechanism of the photoisomerization reaction between pyrazine and pyrimidine has been proposed. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
994.
The x-ray anomalous dispersion and optical rotation in the crystalline solid solution NaClO3: NaBrO3
Sodium chlorate and sodium bromate crystals of thes ame structural configuration have opposite senses of optical rotation.
It is also known that these halates exhibit continuous miscibility in solid solution. In the present work, the conjecture
of Bijvoet that in the solid solution of these halates, the crystalline configuration would continue itself, has been directly
verified by two approaches: (i) by optical rotation measurements on single crystals which show an annulling trend in proportion
with composition and (ii) by measuremerts of Bijvoet inequality for several reflections from a single crystal (rich in chlorate)
which shows an agreeable trend with the calculated values for chlorate.
This paper forms a part of Mohanlal’s thesis accepted by the Madurai University for the Ph.D. degree.
We emphasize optical rotation is dextro, if the plane of polarisation is rotated clockwise, for an observer looking towards
the source of light. 相似文献
995.
G. Wisz P. Sawicka-Chudy P. Potera M. Sibiński Ł. Głowa 《Molecular Crystals and Liquid Crystals》2018,672(1):81-91
AbstractThin films copper oxides are perspective materials for many optoelectronic applications, including photovoltaics. The samples were deposited on glass and silicon substrates by magnetron sputtering method using Modular Platform PREVAC. After deposition the samples were thermally treated by annealing in oxygen atmosphere for 60?min at 450?°C. Morphology confirms that all the films have crystalline structure. Optical measurements show that the films have wide band gap within the range 2.20÷2.48?eV before and 2.03÷2.40?eV after annealing. The article presents the discussion about the influence of annealing on Cu2O thin film parameters. 相似文献
996.
H. Karaaa M. Parlak O. Karabulut U. Serincan R. Turan B. G. Aknolu 《Crystal Research and Technology》2006,41(12):1159-1166
Structural, optical and electrical properties of Ge implanted GaSe single crystal have been studied by means of X‐Ray Diffraction (XRD), temperature dependent conductivity and photoconductivity (PC) measurements for different annealing temperatures. It was observed that upon implanting GaSe with Ge and applying annealing process, the resistivity is reduced from 2.1 × 109 to 6.5 × 105 Ω‐cm. From the temperature dependent conductivities, the activation energies have been found to be 4, 34, and 314 meV for as‐grown, 36 and 472 meV for as‐implanted and 39 and 647 meV for implanted and annealed GaSe single crystals at 500°C. Calculated activation energies from the conductivity measurements indicated that the transport mechanisms are dominated by thermal excitation at different temperature intervals in the implanted and unimplanted samples. By measuring photoconductivity (PC) measurement as a function of temperature and illumination intensity, the relation between photocurrent (IPC) and illumination intensity (Φ) was studied and it was observed that the relation obeys the power law, IPC αΦn with n between 1 and 2, which is indication of behaving as a supralinear character and existing continuous distribution of localized states in the band gap. As a result of transmission measurements, it was observed that there is almost no considerable change in optical band gap of samples with increasing annealing temperatures for as‐grown GaSe; however, a slight shift of optical band gap toward higher energies for Ge‐implanted sample was observed with increasing annealing temperatures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
997.
A. S. Batsanov Yu. T. Struchkov A. A. Gakh A. A. Fainzil'berg 《Russian Chemical Bulletin》1994,43(4):588-590
An X-ray structural study of 2,5-bis(fluorodinitromethyl)-1,3,4-oxadiazole (1), one of the representatives of azoles with strong electron-withdrawing substituents, has been carried out. It has been found that in the orthorhombic modification of1 the conformations of the fluorodinitromethyl groups are different. Intermolecular contacts in the crystal are realized through the heterocycles arranged at large angles to each other. The presence of strong electron-withdrawing substituents results in shortening of the C-O bonds and in a certain increase in the OCN angle in the 1,3,4-oxadiazole ring.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 639–641, April, 1994. 相似文献
998.
本文根据分子拓扑学原理,用色图方法探讨了卤代烷烃结构与折光指数的关系,提出一个普遍适用于卤代烷烃折光指数的定量关系式。应用这一定量关系,不仅能够描述卤代烷烃折光指数的变化规律、预测卤代烷烃的折光指数,而且能够合理表征物质结构与性能之间的关系。 相似文献
999.
1000.
基于代数特征值逆问题理论,提出了一种利用静力试验数据修正有限元模型方法。由于修正后的模型为满阵,根据有限元模型的力学特征对修正后的模型加以约束,使其不仅是对称的,而且还保持稀疏、带状特性,其计算过程简洁,计算结果表明该方法可靠,效果令人满意。 相似文献