全文获取类型
收费全文 | 1174篇 |
免费 | 28篇 |
国内免费 | 3篇 |
专业分类
化学 | 759篇 |
晶体学 | 21篇 |
力学 | 56篇 |
数学 | 83篇 |
物理学 | 286篇 |
出版年
2024年 | 7篇 |
2023年 | 15篇 |
2022年 | 40篇 |
2021年 | 31篇 |
2020年 | 61篇 |
2019年 | 48篇 |
2018年 | 47篇 |
2017年 | 52篇 |
2016年 | 41篇 |
2015年 | 34篇 |
2014年 | 43篇 |
2013年 | 113篇 |
2012年 | 75篇 |
2011年 | 93篇 |
2010年 | 36篇 |
2009年 | 58篇 |
2008年 | 46篇 |
2007年 | 38篇 |
2006年 | 28篇 |
2005年 | 24篇 |
2004年 | 23篇 |
2003年 | 11篇 |
2002年 | 19篇 |
2001年 | 20篇 |
2000年 | 10篇 |
1999年 | 9篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 6篇 |
1990年 | 12篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1974年 | 5篇 |
1973年 | 6篇 |
1971年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有1205条查询结果,搜索用时 15 毫秒
991.
992.
Arunabha Thakur Amarnath Chakraborty Manjula Devi 《Journal of organometallic chemistry》2010,695(7):1059-13900
Mono and doubly alkynyl substituted ferrocene complexes, [Fc(CH2OCH2CCH)n], 2-3 (2: n = 1; 3: n = 2; Fc = ferrocene) have been synthesized from the room temperature reaction of mono and 1,1′-dihydroxymethyl ferrocene, Fc(CH2OH)n , 1a-b (1a: n = 1; 1b: n = 2) and propargyl bromide, in modest to good yields. These new ferrocene derivatives have been characterized by mass, IR, 1H, 13C NMR spectroscopy, and molecular structures of compound 2 and 3 were unequivocally established by single crystal X-ray diffraction study. The crystal structure analysis revealed that 2 and 3 consist of infinite 1D zig-zag hydrogen bonded chains and 2D microporous hydrogen bonded network of molecules, linked by intermolecular C-H···O hydrogen bonding. The molecular structures of both 2 and 3 are further stabilized by C-H···π interactions. 相似文献
993.
A novel stability indicating LC method was developed and validated for the quantitative determination of riluzole in bulk drugs and in pharmaceutical dosage forms in the presence of its isomers, related substances and degradation products. The drug was subjected to stress conditions of hydrolysis (acid, base and neutral), oxidation, photolysis and thermal degradation. Considerable degradation was observed under base hydrolysis and oxidation. The stress samples were assayed against a qualified reference standard and the mass balance was found close to 99.5%. The developed method was validated with respect to linearity, accuracy, precision, specificity, ruggedness and robustness. 相似文献
994.
The influence of poly(ethylene glycol) (PEG) plasticiser content and molecular weight on the physicochemical properties of films cast from aqueous blends of poly(methyl vinyl ether-co-maleic acid) was investigated using thermal analysis, swelling studies, scanning electron microscopy (SEM) and attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy. FTIR spectroscopy revealed a shift of the CO peak from 1708 to 1731 cm−1, indicating that an esterification reaction had occurred upon heating, thus producing crosslinked films. Higher molecular weight PEGs (10,000 and 1000 Da, respectively), having greater chain length, producing hydrogel networks with lower crosslink densities and higher average molecular weight between two consecutive crosslinks. Accordingly, such materials exhibited higher swelling rates. Hydrogels crosslinked with a low molecular weight PEG (PEG 200) showed rigid networks with high crosslink densities and, therefore, lower swelling rates. Polymer:plasticizer ratio alteration did not yield any discernable patterns, regardless of the method of analysis. The polymer-water interaction parameter (χ) increased with increases in the crosslink density. SEM studies showed that porosity of the crosslinked films increased with increasing PEG MW, confirming what had been observed with swelling studies and thermal analysis, that the crosslink density must be decreased as the Mw of the crosslinker is increased. Hydrogels containing PMVE/MA/PEG 10,000 could be used for rapid delivery of drug, due to their low crosslink density. Moderately crosslinked PMVE/MA/PEG 1000 hydrogels or highly crosslinked PMVE/MA/PEG 200 systems could then be used in controlling the drug delivery rates. We are currently evaluating these systems, both alone and in combination, for use in sustained release drug delivery devices. 相似文献
995.
Unsymmetrically substituted metal-free and metallated benzoporphyrazines including pyridino[3,4]tribenzoporphyrazinic macrocycles have been synthesized in good yields by ring expansion reactions of boron(III) subphthalocyanines with phthalonitriles in functional ammonium ionic liquids in the presence of 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) under different reaction conditions. The concentration dependent UV/vis, fluorescence, NMR and ESI-MS spectroscopic data indicate that zinc(II) pyridino[3,4]tribenzoporphyrazines exist as head-to-tail self-aggregates in the solution phase. 相似文献
996.
L. Kumari W. Z. Li C. H. Vannoy R. M. Leblanc D. Z. Wang 《Crystal Research and Technology》2009,44(5):495-499
Vertically aligned and well interconnected NiO nanowalls with thin nanodiscs of cubic NiO as the in situ formed building blocks were fabricated via a template‐free approach. The alkaline solution of ammonium hydroxide was used as a reductant and Ni foil as both source material and substrate for the nanowall synthesis. Surface morphology analysis confirmed the formation of 15 nm thick and 0.2–1.5 μm wide nanowalls. The nanowalls had the crystal structure of cubic NiO with their growth plane along the [111] direction. An optical band gap of about 3.8 eV for the NiO nanowalls was obtained from the optical absorption measurement. NiO nanowalls exhibited a broad UV emission band centered at around 390 nm. This simple, but efficient synthesis technique can facilitate the growth of well aligned 2D nanostructures with large surface area for possible applications in nanoscale devices. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
997.
S. Pandhija N. K. Rai A. K. Rai S. N. Thakur 《Applied physics. B, Lasers and optics》2010,98(1):231-241
The present paper deals with the detection and quantification of toxic heavy metals like Cd, Co, Pb, Zn, Cr, etc. in environmental
samples by using the technique of laser-induced breakdown spectroscopy (LIBS) and calibration-free LIBS (CF-LIBS). A MATLABTM program has been developed based on the CF-LIBS algorithm given by earlier workers and concentrations of pollutants present
in industrial area soil have been determined. LIBS spectra of a number of certified reference soil samples with varying concentrations
of toxic elements (Cd, Zn) have been recorded to obtain calibration curves. The concentrations of Cd and Zn in soil samples
from the Jajmau area, Kanpur (India) have been determined by using these calibration curves and also by the CF-LIBS approach.
Our results clearly demonstrate that the combination of LIBS and CF-LIBS is very useful for the study of pollutants in the
environment. Some of the results have also been found to be in good agreement with those of ICP-OES. 相似文献
998.
Frequency and temperature dependence of dielectric parameters of a liquid crystalline compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate under planar orientation of the molecules have been investigated in the frequency range 1 Hz-10 MHz. This compound possesses smectic paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmCA*) phases. Dielectric spectroscopy suggests the existence of a relaxation mechanism in the SmA* phase, which behaves as a soft mode. In the SmC* phase two relaxation modes are observed. One mode continues from the SmA* phase with decreasing dielectric strength and the other has characteristics of the Goldstone mode. Two dielectric relaxation modes have been observed for the SmCA* phase. These two modes are related to the antiferroelectric ordering and the helical structure of the SmCA* phase. 相似文献
999.
Archana Shukla R.N.P. Choudhary A.K. Thakur 《Journal of Physics and Chemistry of Solids》2009,70(11):1401-139
The polycrystalline samples of BaTi1−xMnxO3 (BMT) ferroelectric ceramics with x=0, 0.04, 0.07, 0.10 have been prepared using a solid-state reaction technique. The calcination temperature of the samples was optimized by thermal gravimetric analysis and repeated firing. Preliminary structural study using X-ray diffraction technique at room temperature suggests that structure of Mn modified compounds (BMT) change into orthorhombic crystal system from tetragonal crystal system. The field emission scanning electron micrographs (FE-SEM) show uniform grain distribution throughout the surface of the samples. Complex impedance analysis (CIS) has been carried out to investigate the electrical properties of BMT. The real and imaginary part of complex impedance plots exhibit semicircle(s) in the complex plane. The bulk resistance of the material decreases with rise in temperatures similar to a semiconductor. The variation of bulk ac conductivity with frequency shows that the compounds exhibit dispersive type electrical conductivity. 相似文献
1000.
Mechanics of Composite Materials - The properties of jute- and chicken-feather-fiber (CFF)-reinforced polymer hybrid composites have been investigated experimentally. The scanning electron... 相似文献