全文获取类型
收费全文 | 1255篇 |
免费 | 46篇 |
国内免费 | 8篇 |
专业分类
化学 | 909篇 |
晶体学 | 33篇 |
力学 | 28篇 |
数学 | 69篇 |
物理学 | 270篇 |
出版年
2024年 | 2篇 |
2023年 | 14篇 |
2022年 | 17篇 |
2021年 | 18篇 |
2020年 | 39篇 |
2019年 | 43篇 |
2018年 | 31篇 |
2017年 | 39篇 |
2016年 | 56篇 |
2015年 | 38篇 |
2014年 | 91篇 |
2013年 | 99篇 |
2012年 | 92篇 |
2011年 | 125篇 |
2010年 | 60篇 |
2009年 | 72篇 |
2008年 | 88篇 |
2007年 | 85篇 |
2006年 | 60篇 |
2005年 | 43篇 |
2004年 | 47篇 |
2003年 | 45篇 |
2002年 | 36篇 |
2001年 | 12篇 |
2000年 | 9篇 |
1999年 | 4篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1981年 | 1篇 |
排序方式: 共有1309条查询结果,搜索用时 31 毫秒
191.
MCM-41-anchored sulfonic acid (MCM-41-SO3H) used as a solid acid catalyst has been reported in recent years for various synthetic protocols. The superior advantage of MCM-41-SO3H is that it can be recovered and reused several times without loss of its efficiency. In this tutorial review, we attempt to give an overview of the use of MCM-41-SO3H as a solid and heterogeneous catalyst in the synthesis of various organic compounds that have industrial and pharmaceutical applications. 相似文献
192.
This review discusses the advances in the use of silica-supported polyphosphoric acid (PPA-SiO2) as a green and reusable heterogeneous catalyst in the preparation of various organic compounds and pharmaceutical intermediates. PPA-SiO2 could be recovered and reused several times without significant loss of their efficiency. In this review, we attempt to give an overview of the applications of PPA-SiO2 as a catalyst in condensation, cyclization, and addition reactions for the preparation of various organic compounds. 相似文献
193.
Xiao D Rajian JR Cady A Li S Bartsch RA Quitevis EL 《The journal of physical chemistry. B》2007,111(18):4669-4677
The intermolecular spectra of three imidazolium ionic liquids were studied as a function of temperature by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids comprise the 1,3-pentylmethylimidazolium cation ([C(5)mim]+), and the anions, bromide (Br-), hexafluorophosphate (PF(6)-), and bis(trifluoromethanesulfonyl)imide (NTf(2)-). Whereas the optical Kerr effect (OKE) spectrum of [C(5)mim][NTf(2)] is temperature-dependent, the OKE spectra of [C(5)mim]Br and [C(5)mim][PF6] are temperature-independent. These results are surprising in light of the fact that the bulk densities of these room temperature ionic liquids (RTILs) are temperature-dependent. The temperature independence of the OKE spectra and the temperature dependence of the bulk density in [C(5)mim]Br and [C(5)mim][PF(6)] suggest that there are inhomogeneities in the densities of these liquids. The existence of density inhomogeneities is consistent with recent molecular dynamics simulations that show RTILs to be nanostructurally organized with nonpolar regions arising from clustering of the alkyl chains and ionic networks arising from charge ordering of the anions and imidazolium rings of the cations. Differences in the temperature dependences of the OKE spectra are rationalized on the basis of the degree of charge ordering in the polar regions of the RTILs. 相似文献
194.
A gigawatt laser-induced Coulomb explosion has been observed in carbon disulphide (CS(2)) clusters generating energetic, multiply charged [C](m+) (m = 1-4) and [S](n+) (n = 1-6) atomic ions of carbon and sulphur. The Coulomb explosion shows wavelength dependence. Comparison of these results with our earlier work shows that the polarizability and dipole moment might help in energy absorption from the laser field but they are not mandatory conditions for this low-intensity Coulomb explosion. The results show that in a field of 10(9) W/cm(2), absorption of 266 and 355 nm laser radiation by CS(2) clusters leads to multiphoton dissociation/ionization whereas at 532 nm the whole cluster explodes generating multiply charged atomic ions. 相似文献
195.
Tak V Kanaujia PK Pardasani D Kumar R Srivastava RK Gupta AK Dubey DK 《Journal of chromatography. A》2007,1161(1-2):198-206
A qualitative method was developed for the determination of degraded products of nerve agents by using ion-pair liquid chromatography electrospray ionization tandem mass spectrometry (IP-LC-ESI-MS(n)). Generally, alkylphosphonic acids (APAs) and O-alkyl alkylphosphonic acids (AAPAs) give deprotonated molecular ion [M-H](-) in negative mode. Interestingly, first time we obtained the molecular radical anion [M](.-) of phosphonic acids in negative mode by using tri-n-butyl amine as an ion-pairing agent. We interpreted this observation as an indication of electrochemical reduction of phosphonic acids in electrospray needle. Three variables such as sheath gas flow, electrospray needle voltage and pH of the mobile phase were investigated to enhance the molecular radical anion [M](.-) signal of each analyte. The Doehlert design was used to obtain the region in which the optimum value of such variables is simultaneously achieved. Limit of detection achieved was 0.5 microg mL(-1) for AAPAs and 10 microg mL(-1) for APAs. Excellent precision was observed with less than 8.61% RSD. Finally, the method was applied for the detection of ethyl methylphosphonic in aqueous extract of soil sample. 相似文献
196.
Jana S Dutta B Bera R Koner S 《Langmuir : the ACS journal of surfaces and colloids》2007,23(5):2492-2496
A complex moiety containing copper (II) has been anchored covalently into the organic-modified Si-MCM-41 to prepare a new catalyst. The amine group containing organic moiety 3-aminopropyl-triethoxysilane has been first anchored on the surface of Si-MCM-41 via silicon alkoxide route. The amine group upon condensation with salicyldehyde affords a bidentate ligand in the mesoporous matrix for anchoring copper(II) ions. The prepared catalyst has been characterized by UV-vis, electron paramagnetic resonance (EPR), and infrared (IR) spectroscopic analysis, small-angle X-ray diffraction, and N2 sorption study. A remarkable difference in the pore structure has been observed after the immobilization of copper(II) complex in Si-MCM-41. The catalyst showed excellent catalytic efficiency in epoxidation reactions with various olefinic compounds including styrene and allyl alcohol, using tert-BuOOH as oxidant. Notably, styrene shows unprecedented high conversion (97%) as well as epoxide selectivity (89%) with tert-BuOOH over the Cu-MCM-41 catalyst. 相似文献
197.
Infiltration of Liquid Metals in Porous Compacts: Modeling of Permeabilities During Reactive Melt Infiltration 总被引:1,自引:0,他引:1
Reactive infiltration is a fast and cost-effective technique for manufacturing ceramic-matrix composites (CMCs). CMCs are used in elevated temperature applications like rocket engine casings, jet nozzles, gas turbine blades and nuclear cladding. There is an urgent need for minimizing experimental costs as well as optimizing process parameters during manufacture, so that we have minimized manufacturing costs and reduced infiltration times. Towards this end, the objective of this research was to develop an integrated micro-macro model of reactive flow of molten silicon in a porous preform consisting of carbon-coated silicon carbide fibers and then optimize process parameters computationally. The overall objective of the research was to arrive at a modified equation of Darcy's law for flow through a porous medium with the help of numerical/computational modeling. This paper deals with the flow of silicon through porous carbon at the macro level. The macro flow of silicon was integrated with an available micro model by determining the transient porosity from the micro model and using it in Darcy's law written for the macro flow of silicon. From the results of this study, we recommend suitable process parameters such as initial temperature of the solid reactant and the specific kind of reactants to be used for achieving complete infiltration. These conclusions are drawn after observation of the rate of decrease of permeability with more reaction. 相似文献
198.
Steven M. Richter Rajesh R. Shinde Ganesh V. Balgi Eva M. Sevick-Muraca 《Particle & Particle Systems Characterization》1998,15(1):9-15
The transport properties of particulate process streams and their final product quality, are directly affected by critical parameters of particle size distribution, f(x), and volume, mass, or number density of particles or dispersed phase droplets. A method is proposed for the potential on-line monitoring of particle size distribution and volume fraction in real time, using frequency-domain photon migration measurements (FDPM). Theory, experimental measurements, and results for the determination of particle size distributions for both a polystyrene latex and a titanium dioxide suspension determined using the photon migration technique are presented. The critical issues associated with the application of photon migration to particulate and dispersed phase processes are discussed, including the effects of interparticle interactions on the transport of light. 相似文献
199.
N-Phenylbenzamide-2,2′-dicarboxylic acid (PBDA) and its copper(II), nickel(II), cobalt (II), zinc(II) and manganese(II) chelates have been synthesized and characterized by their physical measurements, infrared and electronic spectra and magnetic moment data. In an acute anti-inflammatory test, the cobalt chelate was most active (31.1% inhibition) followed by the zinc and copper chelates, whereas the copper chelate (22.3% inhibition) was most active in an adjuvant arthritis test. Again the cobalt chelate was most active in the cotton-wad granuloma test. Gastric irritancy was markedly reduced after chelation by copper, followed in order by zinc, cobalt, manganese and nickel chelates. © 1997 John Wiley & Sons, Ltd. 相似文献
200.
Tarun Patodia Rajesh Sahu Balram Tripathi Takayuki Ichikawa Sushil Kumar Jain Ankur Jain 《Macromolecular Symposia》2024,413(1):2300089
Commercialization of Li–S in present scenario is obstructed by poor performance of cathode and its compatibility with electrolyte used. Here in this work, in order to improve the electrochemical performance all solid state Li–S battery, solid electrolyte (SE) formed by composition of lithium sulfide (Li2S) and phosphorus pentasulfide (P2S5) combinedly called LPS is used. The modified carbon in the form of graphene oxide (GO) and reduced graphene oxide (rGO) as additive is used to provide better electron conduction pathway. High conductivity of the order 10−4 S cm−1 of prepared LPS overcomes the major drawback of insulating nature of sulfur. The coin cells are fabricated by using above mentioned material as a cathode material, LPS as SE, and lithium foil as anode. The prepared nanocomposites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) to study structural and morphological properties. Energy dispersive X-ray spectroscopy (EDS) images of the cathode surface confirms the uniform spreading of material. The electrochemical performance of coin cell is studied by Galvanostatic charge-discharge plot at 0.1 C to check the compatibility of composite and electrolyte prepared. The cells having additive material GO and rGO with host sulfur show better results as compared to the cell having pristine sulfur. 相似文献