全文获取类型
收费全文 | 2572篇 |
免费 | 101篇 |
国内免费 | 6篇 |
专业分类
化学 | 1594篇 |
晶体学 | 123篇 |
力学 | 86篇 |
数学 | 253篇 |
物理学 | 623篇 |
出版年
2023年 | 17篇 |
2022年 | 41篇 |
2021年 | 46篇 |
2020年 | 41篇 |
2019年 | 49篇 |
2018年 | 54篇 |
2017年 | 43篇 |
2016年 | 77篇 |
2015年 | 75篇 |
2014年 | 80篇 |
2013年 | 229篇 |
2012年 | 131篇 |
2011年 | 131篇 |
2010年 | 104篇 |
2009年 | 98篇 |
2008年 | 118篇 |
2007年 | 108篇 |
2006年 | 84篇 |
2005年 | 87篇 |
2004年 | 76篇 |
2003年 | 62篇 |
2002年 | 72篇 |
2001年 | 33篇 |
2000年 | 24篇 |
1999年 | 27篇 |
1998年 | 16篇 |
1997年 | 21篇 |
1996年 | 31篇 |
1995年 | 27篇 |
1994年 | 18篇 |
1993年 | 30篇 |
1992年 | 30篇 |
1991年 | 32篇 |
1990年 | 28篇 |
1989年 | 37篇 |
1988年 | 26篇 |
1987年 | 24篇 |
1986年 | 30篇 |
1985年 | 28篇 |
1984年 | 33篇 |
1983年 | 25篇 |
1982年 | 25篇 |
1981年 | 30篇 |
1980年 | 26篇 |
1979年 | 25篇 |
1978年 | 15篇 |
1977年 | 32篇 |
1976年 | 16篇 |
1975年 | 21篇 |
1974年 | 20篇 |
排序方式: 共有2679条查询结果,搜索用时 15 毫秒
991.
Ezhumalai Yamuna Matthias Zeller Karnam Jayarampillai Rajendra Prasad 《Tetrahedron letters》2012,53(12):1514-1517
We have developed an effective microwave assisted p-TsOH catalyzed synthesis of indolo[2,3-b]dibenzo[b,g][1,8]naphthyridines via a one pot reaction of 3-amino-9-ethylcarbazole and 2-chloro-3-formylquinolines. 相似文献
992.
A series of supramolecular architectures of magnesium tetranitrooctaethylporphyrins mediated by several bidentate axial ligands have been synthesized in excellent yields and structurally characterized. Six conjugated axial ligand with increasing chain lengths have been utilized in the present investigations in which the Mg···Mg nonbonding distance between successive ions also increases from 0.73 to 2.70 nm in the series. To the best of our knowledge, this is the first report where stable metallo-porphyrin polymers with such long spacers have been synthesized in one pot so easily. Linear one-dimensional (1D) polymeric chains were observed in the X-ray structure of the six-coordinated complexes in which porphyrin units are aligned parallel to each other to have so-called "shish kebab" like architectures to maintain offset-stacked overlap. However, after an optimum Mg···Mg nonbonding distance, these 1D chain do not continue, rather they form five-coordinated porphyrin dimers with "wheel-and-axle" like architectures which are then self-aggregated by π-π interactions in a perpendicular manner to fill space created by large bridging ligands more effectively which consequently results in spherical structures. The structures of the molecules in solution and their surface patterns on highly ordered pyrolytic graphite (HOPG) have also been investigated. 相似文献
993.
Satyendra N. Shukla Pratiksha Gaur Mamta Prasad Mahender Prasad Ripul Mehrotra Sheela Mathews 《中国化学会会志》2012,59(4):485-493
The reaction of a new heterocyclic bidentate N containing spacer, (ligand) 5,5′‐methylenebis(pyridine) with ruthenium sulphoxide precursors resulted, dinuclear complexes. We herein report three formulations; [{cis,fac‐RuCl2(so)3}2(μ‐mbp)].3so; [{trans,mer‐RuCl2(so)32}2(μ‐mbp)].3so and [{trans‐RuCl4(so)}2(μ‐mbp)]2?[X]2+; where so = dimethyl‐sulfoxide/tetramethylenesulfoxide; mbp = 5,5′‐methylenebis(pyridine) and [X]+ = [(dmso)2H]+, Na+ or [(tmso)H]+. These complexes were characterized on the basis of elemental analyses, molar conductance measurement, magnetic susceptibility, FT‐IR, 1H‐NMR, 13C{1H}‐NMR, electronic spectroscopy and FAB‐Mass spectrometry. Catalytic activity of these complexes has been investigated in hydrolysis of benzonitrile. All the complexes exhibit good antibacterial activity against gram‐negative bacteria Escherichia coli in comparison to Chloramphenicol. 相似文献
994.
Jamadar A Duhme-Klair AK Vemuri K Sritharan M Dandawate P Padhye S 《Dalton transactions (Cambridge, England : 2003)》2012,41(30):9192-9201
A series of eight pyruvate-based aroylhydrazones was synthesised and characterised. The reaction of the sodium salts of the aroylhydrazones with one equivalent of copper(II) chloride allowed the isolation of neutral 1:1 complexes in which the hydrazones occupy three basal coordination sites of a square pyramidal Cu(II)-centre, with two solvent molecules completing the coordination sphere. Structural details were obtained through the determination of the crystal structures of two representative pyruvate-based aroylhydrazones and three Cu(II) complexes. The evaluation of the antimycobacterial activity of the sodium salts of the eight pryruvate hydrazones showed that the compounds are essentially inactive in their anionic form. The corresponding neutral Cu(II) complexes, however, exhibit promising antimycobacterial activities if tested under high iron (8 μg Fe per mL) conditions. As observed for the related antimycobacterial agent isoniazid, the activity of the complexes decreases if the M. tuberculosis cells are grown under low iron (0.02 μg Fe per mL) conditions. The Cu(II) complexes may thus have a similar mode of action and may require an iron-containing heme-dependent peroxidase for activation. 相似文献
995.
Refractive indices as a function of temperature are measured in a number of liquid crystals which belong to the N-(p-n-butoxy benzylidene)-p-n alkyl anilines, 4O.m compounds of the famous Schiff’s base nO.m liquid crystal compounds with m = 4 to 10 and 12. The temperature gradient of refractive indices, dne/dT and dno/dT, of these compounds are estimated. It is well known that in the case of dno/dT, there exists a crossover temperature, TCO, which exhibits an odd–even effect with the alkyl chain length as expected. Further, using the birefringence data, the length to breadth ratio, k, for these compounds is estimated, which shows not exactly even–odd effect but irregular change with the chain number. The results are discussed with the body of the data. 相似文献
996.
S.N. Behera B.K. Patel G.V.R. Prasad S. Chakravarty R. Mythili M. Vijaylakshmi 《Phase Transitions》2013,86(1-2):133-142
A low energy cluster beam deposition (LECBD) technique has been used to prepare the Sb cluster films on different substrates and are characterized using a variety of probes. Proton induced X-ray emission (PIXE) analysis shows the absence of any foreign trace elemental impurity even at ppm level. Glancing angle X-ray diffraction (GXRD) and transmission electron diffraction (TED) studies reveal the presence of single crystalline feature of Sb with hexagonal symmetry along with Sb-oxides. The transmission electron micrograph (TEM) of the cluster films of thickness 20 Å and 100 Å show size distribution which is more for the 100 Å film compared to that of the 20 Å one. The photoluminescence (PL) studies at 300 K show red shifted peaks along with the one possibly due to HOMO-LUMO transition around 2.4 eV. X-ray photoelectron spectroscopy (XPS) of Sb cluster films show shoulders to the core level peaks of the Sb corresponding to 3d 3/2 and 3d 5/2 which are shifted by 1.0 to 2.0 eV suggesting the formation of Sb-oxide. Raman scattering studies show the shift of A 1g and E g vibrational modes from their bulk value. 相似文献
997.
N. V. S. Rao V. G. K. M. Pisipati P. V. Datta Prasad P. R. Alapati 《Phase Transitions》2013,86(3):187-195
The temperature dependences of the molar volume, the ultrasonic velocity and the refractive index in N(p-n-pentyloxy benzylidene) p-n-decyl aniline, which exhibits nematic, smectic-A and smectic-B, phases, are presented. The isotropic-nematic and smectic-A-smectic-B phase transitions are found to be of first order while the nematic-smectic-A transition is of second order. The thermal expansion coefficient (a) computed from the molar volume data agrees with a second-order N-SA transition. The computed adiabatic compressibility βad, molar sound velocity (or Rao number Rn ) and molar compressibility (or Wada constant A) are presented. 相似文献
998.
Venkata Reddy Mamidi Srimannarayana Kamineni L. N. Sai Prasad Ravinuthala Venkatapparao Thumu Vengal Rao Pachava 《Fiber and Integrated Optics》2013,32(4):325-335
AbstractAn experimental study has been carried out for the characterization of encapsulating materials for fiber Bragg grating-based temperature sensors to prevent the formation of micro-cracks and devitrification on the fiber surface at elevated temperatures by making use of a rigid probe. The developed sensor probe was configured by encapsulating a type 1 fiber Bragg grating with an aluminum nitride tube and is used to measure temperatures from 20°C to 500°C. The adapted encapsulation technique validated the sensor to achieve linearity of 99.979%, sensitivity of 14.03 ± 0.02 pm/°C, and good repeatability; its practical use in harsh environments is predicted. 相似文献
999.
The underlying atomistic mechanisms that govern vacancy aggregation in crystalline silicon are probed using a parametrically consistent, two-scale approach. The essential ingredient in this framework is a direct, quantitative comparison between the predictions of atomistic and continuum simulations for the transient size distribution of vacancy clusters. The former is carried out with parallel molecular dynamics simulation of a silicon system containing 215?000 atoms and 1000 vacancies. The continuum model is based on a sequence of coupled Master equations and is parametrized based on the same empirical potential used to perform the atomistic aggregation simulation. An excellent representation of the cluster size distribution can be obtained with consistent parameters only if the relevant physical mechanisms are captured correctly. The inclusion of vacancy cluster diffusion and a model to capture the dynamic nature of cluster morphology at high temperature are necessary to reproduce the results of the large-scale atomistic simulation. Finally, the continuum model is used to investigate cluster evolution for longer times, which are relevant for process simulation of defect-optimized silicon substrates for microelectronic device fabrication. 相似文献
1000.
S. Krishna Prasad R. Shashidhar K. A. Suresh A. N. Kalkura G. Heppke R. Hopf 《Molecular Crystals and Liquid Crystals》2013,570(1):185-191
We have carried out pressure studies on the eighth and ninth members of the homologous series 4-alkoxybenzoyloxy-4′-cyanoazobenzene (nOBCAB). It is found that the pressure behavior of the monolayer smectic A (A1) phase is entirely different from that of the partially bilayer smectic A (A1) phase. The A1 -nematic phase boundary is a straight line at all pressures and the range of the Ad phase increases with increasing pressure. On the other hand, the Ad -nematic phase boundary is elliptic in shape and the Ad phase gets bounded. These results emphasize the fact that the pressure behavior of the A phase is strongly related to the extent of overlap of the molecules in the layer. 相似文献