全文获取类型
收费全文 | 4111篇 |
免费 | 178篇 |
国内免费 | 33篇 |
专业分类
化学 | 2739篇 |
晶体学 | 29篇 |
力学 | 160篇 |
数学 | 409篇 |
物理学 | 985篇 |
出版年
2023年 | 31篇 |
2022年 | 84篇 |
2021年 | 198篇 |
2020年 | 163篇 |
2019年 | 157篇 |
2018年 | 175篇 |
2017年 | 123篇 |
2016年 | 173篇 |
2015年 | 161篇 |
2014年 | 173篇 |
2013年 | 327篇 |
2012年 | 264篇 |
2011年 | 291篇 |
2010年 | 170篇 |
2009年 | 147篇 |
2008年 | 151篇 |
2007年 | 175篇 |
2006年 | 129篇 |
2005年 | 135篇 |
2004年 | 90篇 |
2003年 | 79篇 |
2002年 | 62篇 |
2001年 | 68篇 |
2000年 | 51篇 |
1999年 | 47篇 |
1998年 | 18篇 |
1997年 | 21篇 |
1996年 | 32篇 |
1995年 | 37篇 |
1994年 | 30篇 |
1993年 | 34篇 |
1992年 | 33篇 |
1991年 | 42篇 |
1990年 | 20篇 |
1989年 | 22篇 |
1988年 | 30篇 |
1987年 | 27篇 |
1986年 | 26篇 |
1985年 | 39篇 |
1984年 | 23篇 |
1983年 | 24篇 |
1982年 | 23篇 |
1981年 | 12篇 |
1980年 | 30篇 |
1979年 | 21篇 |
1978年 | 27篇 |
1977年 | 27篇 |
1976年 | 19篇 |
1973年 | 11篇 |
1968年 | 10篇 |
排序方式: 共有4322条查询结果,搜索用时 31 毫秒
91.
James P. Johnston Bruce P. Mosier Zia U. Khan 《International Journal of Heat and Fluid Flow》2002,23(6):744-749
An experimental study of flow downstream of round, pitched and skewed wall-jets (vortex generating jets) is presented to illustrate the effects of changing the geometric inlet conditions of the jet-holes. In one case the jet-hole has a smoothly contoured inlet, and in the other the inlet was a sharp-edged, sudden contraction. The test region geometry, mean jet flow and cross-flow conditions were otherwise identical. In both cases, dominant streamwise vortex structures are seen in the boundary layer downstream; the flow and turbulence is nearly the same in the far-field starting downstream of x/D=5. In the near-field, for x/D<5, there are significant differences; turbulence levels are higher, and the start of the dominant vortex shape is less clear for the sharp-edged case. This is believed to be the result of flow separation and free shear layer instability inside the jet-hole which are not present for the smoothly contoured case. 相似文献
92.
On the use of electrical-resistance metallic foil strain gages for measuring large dynamic plastic deformation 总被引:2,自引:0,他引:2
The use of electrical-resistance metallic foil strain gages for measuring large plastic strain in dynamic experiments in studied. The maximum nominal strains obtained in this investigation are 35 percent in compression, 25 percent in tension. A linear variation of gage factor with strain is found in this range. The corrected maximum strains are in excellent agreement with permanent strains measured after the tests. Thus foil strain gages can be effectively used to measure the large dynamic plastic strains. 相似文献
93.
The electrochemistry of some quinones has been focused to determine the mode of interaction in presence of polyalcohols. Three compounds of each family (benzoquinones, naphthoquinones and anthraquinones) were investigated through cyclic voltammetry in the presence of ethylene glycol (a diol)) and glycerol (a triol) in dichloromethane and acetonitrile at 25°C. The observed positive shift in both the waves of the quinone with successive addition of alcohol was attributed to hydrogen bonding in the quinone-alcohol couple. “Two electron one step” electron transfer mechanism was proposed for the increase in the first wave height at the expense of second. The depletion of the first anodic wave at higher concentration of polyalcohol was rationalized in terms of protonation-deprotonation mechanism. A prior peak observed in the presence of glycerol was ascribed to the hydrogen bonding of the alcohol with neutral quinone. The difference in basicity strength within a family as well as among the three quinone families was also addressed in view of the interaction effectiveness. 相似文献
94.
Constantin Mateescu Ecaterina Princz Gilles Bouet Mustayeen A. Khan 《Phosphorus, sulfur, and silicon and the related elements》2013,188(4):947-958
The complex formation equilibrium of ethylenediaminetetramethylenephosphonic acid (EDTMP, H8L) with iron (III) has been studied potentiometrically at 25°C and an ionic strength of 0.2 M (NaCl). The successive protonation constants of ligand EDTMP and the complex formation constants were determined with the PSEQUAD program. Keeping in view the biological studies, the speciation in the system Fe (III)—EDTMP was calculated and drawn with the HySS computer program, and pFe values are compared. 相似文献
95.
96.
Kabir-ud-Din P. Ajmal Koya Ziya Ahmad Khan 《Journal of Dispersion Science and Technology》2013,34(4):558-567
Effects of three organic solvents, viz. methyl cellosolve, acetonitrile, and formamide, on the micellization process of Gemini surfactant pentamethylene-1,5-bis(tetradecyldimethylammonium bromide) aqueous solutions, with the volume percentages of the organic solvents up to 50%, have been investigated conductometrically. The studies were made at different temperatures and the data were used to find out different micellization parameters. From the study, it was observed that, although an increment in the amount of the organic solvents delays the micellization, the increase in the critical micelle concentration (cmc) is comparatively less below 20%(v/v) showing the predominance of water character in the bulk phase at lower compositions of the organic solvents. Applying equilibrium model for micelle formation, various thermodynamic parameters were also calculated from the temperature dependence of the cmc values and the results show that the micellization process becomes less spontaneous as the volume % of the organic solvent increases in the system due to the action of water-organic solvent mixed media as better solvent than pure water (solvophobic effect) for the studied Gemini molecules. 相似文献
97.
Adnan J. Al-Rehaily Mohammad S. Ahmad Jamal Mustafa Mai M. Al-Oqail Wafaa H. Hassan Shabana I. Khan Ikhlas A. Khan 《Journal of Saudi Chemical Society》2013,17(1):67-76
Solanopubamine (3β-amino-5α, 22αH, 25βH-solanidan-23β-ol), a steroidal alkaloid was isolated from the alkaloidal fraction of Solanum schimperianum in significant yield. Its structure was established by IR, positive ESI-MS, 1D and 2D NMR. The presence of -3β-NH2 and -23β-OH groups was achieved through methylation, acetylation or coupling with octadecanoic and undec-11-enoic acids to produce six derivatives (2–7). Their structures were confirmed by spectroscopic analyses. Solanopubamine and semi-synthetic analogs are investigated for their in vitro cytotoxicity against a panel of human cancer cell lines and anti-microbial activity. Solanopubamine showed good antifungal activity only against Candida albicans and C. tenuis with MIC of 12.5 μg/mL. Semi-synthesized compounds (2–7) have failed to show anti-tumor and anti-microbial activities. 相似文献
98.
99.
The kinetic theory of plasma has been employed to compute the test-charge potential distributions accounting for quantization effects in magnetized electron-positron-ion (EPI) plasmas. In this regard, the degenerate positrons and electrons are assumed to follow the Fermi-Dirac distribution, while inertial ions are modelled by Maxwellian velocity distribution. By solving the Fourier-transformed Vlasov–Poisson equations, a modified dielectric function and electrostatic potential is obtained. By imposing various constraints on the test-charge speed, the potential profile has been analysed in terms of Debye–Hückel (DH), far-field (FF), and wake-field (WF) potentials. It has been found that the amplitude of DH and FF potentials increases by the inclusion of quantization effects, and it becomes the opposite for the WF potential profile. Furthermore, the variation of positron concentration significantly affects the DH, FF, and WF potentials. The present findings are important to understand the shielding phenomenon in degenerate multi-species plasmas. 相似文献
100.
Ismail A. Haj Waseem Hafiz Muhammad Ishtiaq Muhammad Jamal Sajjad Shaukat Khan Majid 《International Journal of Theoretical Physics》2021,60(5):1720-1741
International Journal of Theoretical Physics - Most of the enciphering structures are based on the transformation of mediums by describing the strict criteria. The presented article is based on... 相似文献