全文获取类型
收费全文 | 30057篇 |
免费 | 2531篇 |
国内免费 | 1941篇 |
专业分类
化学 | 16347篇 |
晶体学 | 521篇 |
力学 | 1778篇 |
综合类 | 141篇 |
数学 | 3249篇 |
物理学 | 12493篇 |
出版年
2024年 | 51篇 |
2023年 | 373篇 |
2022年 | 690篇 |
2021年 | 652篇 |
2020年 | 675篇 |
2019年 | 689篇 |
2018年 | 661篇 |
2017年 | 539篇 |
2016年 | 950篇 |
2015年 | 853篇 |
2014年 | 1173篇 |
2013年 | 1751篇 |
2012年 | 2124篇 |
2011年 | 2368篇 |
2010年 | 1716篇 |
2009年 | 1567篇 |
2008年 | 1786篇 |
2007年 | 1755篇 |
2006年 | 1700篇 |
2005年 | 1435篇 |
2004年 | 1167篇 |
2003年 | 970篇 |
2002年 | 975篇 |
2001年 | 1322篇 |
2000年 | 901篇 |
1999年 | 791篇 |
1998年 | 596篇 |
1997年 | 576篇 |
1996年 | 509篇 |
1995年 | 422篇 |
1994年 | 405篇 |
1993年 | 328篇 |
1992年 | 313篇 |
1991年 | 280篇 |
1990年 | 232篇 |
1989年 | 211篇 |
1988年 | 159篇 |
1987年 | 128篇 |
1986年 | 102篇 |
1985年 | 121篇 |
1984年 | 101篇 |
1983年 | 59篇 |
1982年 | 48篇 |
1981年 | 41篇 |
1980年 | 30篇 |
1977年 | 21篇 |
1976年 | 29篇 |
1975年 | 27篇 |
1974年 | 21篇 |
1973年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
PERSISTENCE AND GLOBAL STABILITY FOR TWO-SPECIES NONAUTONOMOUS PREDATOR-PREY SYSTEM WITH DIFFUSION AND TIME DELAY 总被引:3,自引:0,他引:3
In this paper, a non-autonomous predator-prey model with diffusion andcontinuous time delay is studied, where the prey can diffuse between two pat-ches of a heterogeneous environment with barriers between patches, but for thepredator, the diffusion does not involve a barrier between patches, further itis assumed that all the parameters are time-dependent. It is shown that thesystem can be made persistent under some appropriate conditions. Moreover,sufficient conditions that guarantee the existence of a unique periodic solutionwhich is globally asymptotic stable are derived. 相似文献
102.
We describe one convenient synthesis route to boron nitride (BN) nanotube by the reaction of boron powder, iron oxide, and ammonium chloride at 600 °C for 12 h. Characterized by XRD, FTIR, XPS, TEM and SAED, the composition and morphology of the products are confirmed. The possible reaction mechanism is also discussed. 相似文献
103.
A new synthetic method for the manufacture of glutaric dialdehyde is investigated. Glutaric dialdehyde was prepared by the
addition-hydrolysis reaction of benzimidazolium salt with saturated dihalide as the di-Grignard reagent. The yield of glutaric
dialdehyde by this method can reach 73%. Both infrared spectra and melting point of the compound were consistent with those
reported earlier.
Translated from Journal of Northwest University (Natural Science Edition), 2005, 35(2) (in Chinese) 相似文献
104.
X. Y. Yang H. C. Gao X. L. Tan H. Z. Yuan G. Z. Cheng S. Z. Mao S. Zhao L. Zhang J. Y. An J. Y. Yu Y. R. Du 《Colloid and polymer science》2004,282(3):280-286
1H chemical shift changes of sodium 4-decyl naphthalene sulfonate (SDNS) at 313 K show that its critical micellar concentration lies between 0.82 and 0.92 mmol/dm3, which is in the same range as that of the previous study at 298 K. The spin–lattice relaxation time, spin–spin relaxation time and two-dimensional nuclear Overhauser enhancement spectroscopy experiments give information about the structure of the SDNS micelle and the dynamics of the molecules in the micelle. The size of the SDNS micelle remains almost unchanged in the temperature range from 298 to 313 K as deduced by analyzing the self-diffusion coefficient. Special arrangement of the naphthyl rings of SDNS in the micelles affects the packing of these hydrophobic chains. The methylene groups of the alkyl chain nearest the naphthalene groups penetrate into the aromatic region, which results in a more tightly packed hydrophobic micellar core than that of sodium dodecyl sulfonate. 相似文献
105.
Qi Sheng Liu Lian Dong Liu Jing Min Shi 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(1):m58-m60
In the title centrosymmetric binuclear complex, [Cd2(C17H11N3O)4(H2O)2](ClO4)4, the CdII ion assumes a distorted octahedral geometry. There are π–π stacking interactions between the pyridine and 1,10‐phenanthroline ring systems of adjacent ligands at the same CdII centre. Intermolecular hydrogen bonds between the coordinated aqua ligand and the O atom of a keto group connect adjacent complex cations into extended chains. Hydrogen bonds also exist between the complex cations and the perchlorate anions. Compared with the fluorescence spectrum of the organic ligand, the complex displays strong fluorescent emission and an ipsochromic shift of the emission peaks, which may be attributed to the structural character. 相似文献
106.
O. Maksimov P. Fisher M. Skowronski P.A. Salvador M. Snyder J. Xu X. Weng 《Journal of Crystal Growth》2008,310(11):2760-2766
MgO films were grown on (0 0 1) yttria-stabilized zirconia (YSZ) substrates by molecular beam epitaxy (MBE). The crystalline structures of these films were investigated using X-ray diffraction and transmission electron microscopy. Growth temperature was varied from 350 to 550 °C, with crystalline quality being improved at higher temperatures. The MgO films had a domain structure: (1 1 1)[1 1 2¯]MgO(0 0 1)[1 0 0]YSZ with four twin variants related by a 90° in-plane rotation about the [1 1 1]MgO axis. The observed epitaxial orientation was compared to previous reports of films grown by pulsed laser deposition and sputtering and explained as resulting in the lowest interface energy. 相似文献
107.
The CRONE suspension, French acronym of “suspension à Comportement Robuste d’Ordre Non Entier”, results from a traditional suspension system whose spring and damper are replaced by a mechanical and hydropneumatic system defined by a fractional (so-called non-integer) order force–displacement transfer function. ‘ling, frequency-domain robust control design methodology, optimal approach, stability analysis and performance are presented in this paper. 相似文献
108.
109.
By means of a new force sensor based on optical beam deflection (OBD), the mechanical effects of laser-matter interaction underwater at different incident laser energy are investigated in detail. The experimental results show that a target underwater is impacted in turn by laser-plasma ablation force and high-speed liquid-jet impulse induced by bubbles collapse in the vicinity of a solid boundary. Furthermore, the amplitudes of the two forces increase monotonously with laser energy. According to the ablation force detected by the experiment and the theoretical relationship between laser intensity and ablation pressure, the value of liquid-jet impact against a solid boundary can be easily obtained. In addition, based on the model of a collapsing bubble, some characteristic parameters, such as the liquid-jet impact velocity, the maximum bubble radius, the bubble energy can also be obtained at different laser energy, which are valuable in the corresponding research fields. 相似文献
110.