全文获取类型
收费全文 | 928篇 |
免费 | 58篇 |
国内免费 | 7篇 |
专业分类
化学 | 635篇 |
晶体学 | 8篇 |
力学 | 45篇 |
综合类 | 3篇 |
数学 | 144篇 |
物理学 | 158篇 |
出版年
2023年 | 8篇 |
2022年 | 11篇 |
2021年 | 43篇 |
2020年 | 54篇 |
2019年 | 36篇 |
2018年 | 39篇 |
2017年 | 34篇 |
2016年 | 49篇 |
2015年 | 42篇 |
2014年 | 55篇 |
2013年 | 85篇 |
2012年 | 60篇 |
2011年 | 79篇 |
2010年 | 51篇 |
2009年 | 40篇 |
2008年 | 38篇 |
2007年 | 38篇 |
2006年 | 17篇 |
2005年 | 20篇 |
2004年 | 19篇 |
2003年 | 15篇 |
2002年 | 14篇 |
2001年 | 12篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1998年 | 8篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 11篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1981年 | 7篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1972年 | 1篇 |
1971年 | 4篇 |
1970年 | 2篇 |
1969年 | 3篇 |
1968年 | 1篇 |
排序方式: 共有993条查询结果,搜索用时 156 毫秒
81.
82.
The effects of Mg, La and Ca promoters on primary and secondary CO_2 and H_2O formation pathways during Fischer-Tropsch synthesis on precipitated Fe/Cu/SiO_2 catalysts are investigated. The chemisorbed oxygen atoms in the primary pathway formed in the CO dissociation steps reacted with co-adsorbed hydrogen or carbon monoxide to produce H_2O and CO_2 , respectively. The secondary pathway was the water-gas shift reaction. The results indicated that the CO_2 production led to an increase in both primary and secondary pathways, and H_2 O production
decreased when surface basicity of the catalyst increased in the order Ca >Mg >La. 相似文献
83.
The synthesis and structural characterization of dendritic macromolecules based on 3,5-dihydroxybenzoic acid are described. The molecular structures and purity of all new compounds were confirmed by 1H NMR, 13C NMR spectroscopy and elemental analysis. The dendritic non-covalent liquid crystalline complexes were prepared through the formation of hydrogen bonds between different generation of dendritic acids (proton donor) and stilbazole derivative containing bipyridyl units (proton acceptor). We found that, the hydrogen-bonded dendritic liquid crystals supramolecules (G1-2py, G2-2py and G3-2py) exhibits nematic and semectic phase. The polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) were used for investigation of the liquid crystalline properties of the hydrogen-bonded dendritic supramolecular complexes. 相似文献
84.
Naser Seyed-Matin Amir Hossein Jalili Masih Hosseini Jenab Seyed Majid Zekordi Ali Afzali Chapar Rasouli Akbar Zamaniyan 《Plasma Chemistry and Plasma Processing》2010,30(3):333-347
The carbon dioxide reforming of methane to synthesis gas under DC-pulsed plasma was investigated. The effects of specific
input energy and feed ratio on the product distribution and also feed conversion was studied. At the input energy of about
11 eV/molecule per methane and/or carbon dioxide the feed conversion of 38% for CH4 and 28% for CO2 and product selectivity of 74% has been attained for H2 and CO at feed flow rate of 90 ml/min. The energy consumption in this work displays potential to further study and optimization
of the process. The importance of the electron impact reactions in the process was discussed. The results show that by prudent
tuning of system variables, the process be able to run in the way of synthesis gas, instead of hydrocarbon production. 相似文献
85.
Two nickel(II) complexes of [1 + 1] macrocyclic Schiff base ligand (L) have been prepared by cyclocondensation reactions between 1,3-diamino-2-propanol and 2-[3-(2-formylphenoxy)-2-hydroxypropoxy] benzaldehyde, using NiX2 (X = Br, and I) salts as template agents, and characterized by elemental analyses, IR, molar conductivity and electronic spectra in both solid and solution states. The single-crystal X-ray diffractions of the complexes are also reported that contain nickel(II) ion in a distorted octahedral geometry coordination of N2O3X (X = Br, I and NO3). In all complexes the ligand behaves as a pentadentate ligand. Cyclic voltammetric studies of nickel(II) complexes indicate a quasi-reversible redox wave in the negative potential range. 相似文献
86.
The syntheses, structures and spectroscopic properties of tricarbonylrhenium(I) complexes with N,N′-bis(2-bromo, 4-bromo, 4-chloro and 3-methoxybenzaldehyde)-1,2-diiminoethane Schiff base ligands have been investigated in this paper. Characterization of these complexes was carried out with FTIR, NMR, UV–Vis spectroscopy, elemental analysis and X-ray crystallography. The electrochemical behavior of the investigated complexes has been studied by cyclic voltammetry. The crystal structures of the 4-chloro, 4-bromo and 4-methoxy substituted complexes are stabilized by intermolecular C–H?Cl and C–H?O hydrogen bonds. The remarkable features of the 2-bromo, 4-bromo and 4-chloro substituted complexes are short intermolecular halogen–oxygen contacts. In the 4-bromo complex, short intermolecular Br?O and O?O contacts link neighboring molecules along the [1 0 0] direction, which are further stabilized by short intermolecular π?π interactions. In 2-bromo complex, intermolecular Br?O interactions link neighboring molecules into 1D extended chains along the [0 1 0] and [0 0 1] directions, forming a 2D network which is parallel to the bc-plane. 相似文献
87.
Jafar Akbari Akbar Heydari Leila Ma’mani Seyed Hassan Hosseini 《Comptes Rendus Chimie》2010,13(5):544-547
An efficient and practical protocol for the chemoselective N-Boc protection of various structurally different aryl, aliphatic and heterocyclic amines was carried out with (Boc)2O using protic 1, 1, 3, 3-tetra-methylguanidinium acetate (10 mol%) as recyclable catalyst under solvent free condition at ambient temperature. No competitive side reactions (isocyanate, urea and N, N-di-Boc) were observed. α-Amino alcohols afforded the N-Boc-derivative without oxazolidinone formation. 相似文献
88.
Fariborz Momenbeik Mostafa Roosta Ali Akbar Nikoukar 《Journal of chromatography. A》2010,1217(24):3770-3773
An environmentally benign and simple method has been proposed for separation and determination of fat-soluble vitamins using isocratic microemulsion liquid chromatography. Optimization of parameters affecting the separation selectivity and efficiency including surfactant concentration, percent of cosurfactant (1-butanol), and percent of organic oily solvent (diethyl ether), temperature and pH were performed simultaneously using genetic algorithm method. A new software package, MLR-GA, was developed for this purpose. The results indicated that 73.6 mM sodium dodecyl sulfate, 13.64% (v/v) 1-butanol, 0.48% (v/v) diethyl ether, column temperature of 32.5 °C and 0.02 M phosphate buffer of pH 6.99 are the best conditions for separation of fat-soluble vitamins. At the optimized conditions, the calibration plots for the vitamins were obtained and detection limits (1.06–3.69 μg mL−1), accuracy (recoveries > 94.3), precision (RSD < 3.96) and linearity (0.01–10 mg mL−1) were estimated. Finally, the amount of vitamins in multivitamin syrup and a sample of fish oil capsule were determined. The results showed a good agreement with those reported by manufactures. 相似文献
89.
We study corrections to the entropy of Einstein–Maxwell dilaton–axion black holes beyond semiclassical approximations. We consider the entropy of the black hole as a state variable and derive these corrections using the exactness criteria of the first law of thermodynamics. We note that from this general frame-work the entropy corrections for “simpler” black holes like Schwarzschild, Reissner–Nordström and anti-de Sitter–Schwarzschild black holes follow easily. This procedure gives us the modified area law as well. 相似文献
90.
We consider several subtle aspects of the theory of neutrino oscillations which have been under discussion recently. We show
that the S-matrix formalism of quantum field theory can adequately describe neutrino oscillations if correct physics conditions are
imposed. This includes space-time localization of the neutrino production and detection processes. Space-time diagrams are
introduced, which characterize this localization and illustrate the coherence issues of neutrino oscillations. We discuss
two approaches to calculations of the transition amplitudes, which allow different physics interpretations: (i) using configuration-space
wave packets for the involved particles, which leads to approximate conservation laws for their mean energies and momenta;
(ii) calculating first a plane-wave amplitude of the process, which exhibits exact energy-momentum conservation, and then
convoluting it with the momentum-space wave packets of the involved particles. We show that these two approaches are equivalent.
Kinematic entanglement (which is invoked to ensure exact energy-momentum conservation in neutrino oscillations) and subsequent
disentanglement of the neutrinos and recoiling states are in fact irrelevant when the wave packets are considered. We demonstrate
that the contribution of the recoil particle to the oscillation phase is negligible provided that the coherence conditions
for neutrino production and detection are satisfied. Unlike in the previous situation, the phases of both neutrinos from Z
0 decay are important, leading to a realization of the Einstein-Podolsky-Rosen paradox. 相似文献