全文获取类型
收费全文 | 233篇 |
免费 | 3篇 |
专业分类
化学 | 116篇 |
晶体学 | 23篇 |
力学 | 22篇 |
数学 | 29篇 |
物理学 | 46篇 |
出版年
2023年 | 1篇 |
2022年 | 5篇 |
2021年 | 2篇 |
2020年 | 5篇 |
2019年 | 3篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 10篇 |
2015年 | 9篇 |
2014年 | 10篇 |
2013年 | 35篇 |
2012年 | 20篇 |
2011年 | 13篇 |
2010年 | 10篇 |
2009年 | 11篇 |
2008年 | 4篇 |
2007年 | 7篇 |
2006年 | 5篇 |
2005年 | 14篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1991年 | 1篇 |
1989年 | 1篇 |
排序方式: 共有236条查询结果,搜索用时 9 毫秒
81.
Moez Guettari Amal Belaidi Stéphane Abel Tahar Tajouri 《Journal of solution chemistry》2017,46(7):1404-1417
The objective of this work is to study the behavior of a neutral polymer, polyvinylpyrrolidone (PVP), in a mixture of water and ethanol. A comparison of the experimental results with a theoretical model of effective solvent interaction with polymer (ESIP) was made. To do so, dynamic light scattering experiments were used to measure the hydrodynamic radius of PVP (M w = 3.6 × 105 g·mol?1) as a function of the ethanol fraction, x A, in the medium at 25 °C. We show that the polymer adopts an ideal chain–globule–coil conformation transition as the ethanol molar fraction varies. This transition is attributed to the change of the solvent quality which results from water and ethanol complex formation. On the other hand, the ternary PVP/water/ethanol system was described by the ESIP model. From the polymer–effective solvent interaction, the second virial coefficient of the polymer/effective solvent and the preferential adsorption parameter were calculated. The obtained results are in agreement with the reported experiments. 相似文献
82.
Chiheb Mhiri Lamia Boubakri Riadh Ternane Lamjed Mansour Abdel Halim Harrath J. Al-Tamimi Lassaad Baklouti Naceur Hamdi 《Journal of heterocyclic chemistry》2020,57(1):291-298
Three-component reaction of arylaldehydes with malononitrile and 4-hydroxycoumarine using CH3COONH4 as a catalyst at reflux was used for the synthesis of novel substituted pyrano[3,2-c]chromene derivatives. The structure of these compounds was assigned by spectroscopic data such as (IR, 1HNMR, 13CNMR, and mass spectral data). The cation binding properties of chromene derivatives 4a-c towards Cu2+, Ni2+, and Zn2+ were studied in methanol. The results showed that Zn2+ is the most complexed in this series of cations, and 4c is best complexed with either Ni2+ and Zn2+. Antimicrobial properties of new pyrano[3,2-c]chromene derivatives are investigated, the compound 4c presents against Micrococcus luteus LB 14110 an MIC value of 0.0185 mg/mL quite better to that of ampicillin (0.0195 mg/mL) used as standard. Concerning acetylcholinesterase inhibition activity (AChEI), compound 4c presents an interesting AChEI activity with an inhibition of 52%. 相似文献
83.
I. Bhouri Baouabe H. Bournot N. Mahjoub Saïd H. Mhiri G. Le Palec 《Heat and Mass Transfer》2011,47(3):323-342
An experimental study and a numerical modelling analysis were carried out simultaneously to study the flow field structure
issuing from a chimney around an obstacle. The main purpose of this study is to evaluate the impact of the jet emitted from
a chimney (bent or straight) on the dynamics and the turbulent features of the surrounding flow. The consideration of these
features is particularly pertinent to the understanding of mixing between the interacting flows which may be very important
in controlling pollutant dispersion in the atmosphere. The experimental data are depicted by means of a PIV technique; whereas
the numerical three-dimensional model is simulated through the resolution of the different governing Navier–Stokes equations.
The volume finite method, together with the second order turbulent closure model (RSM), was adopted. Variations in obstacle
form (cylindrical or parallelepiped) and chimney configuration (bent or straight) were tested and features studied were: the
global jet plume, the windward and leeward jet spread; the size, location and magnitude of the reverse flow region; the penetration
and the deflection of the jet trajectory around the obstacle. All these considerations allowed us to characterize well the
impact of the injection of the jet emitted from the chimney within the crossflow, and its spreading around the obstacle and
within the whole domain. Such characterization is very important with regard to pollutant dispersion and consequently to the
environmental impact. Indeed, the different species contained within the emitted fumes are mainly directed by the velocity
components and their mixing and progression within the domain and around the obstacle are closely related. 相似文献
84.
A highly enantioselective asymmetric hydrogenation of various trisubstituted enamides derived from chroman-3-ones promoted by cationic Ru-Synphos catalysts is reported. This atom-economical and clean method provides an efficient route to optically active 3-aminochroman derivatives, which are important pharmacophores found in numerous drug candidates, in high chemical yields and enantiomeric excesses up to 96%. 相似文献
85.
The essential oils obtained by hydrodistillation from leaves and fruits of Daucus sahariensis Murb. were analyzed by GC/MS. The main constituents of the essential oil from the leaves were myristicin (34.3%), alpha-pinene (5.4%), cis-chrysanthenyl acetate (5.3%) and epi-alpha-bisabolol (4.8%), and those from the fruits myristicin (43.9%), alpha-pinene (13.1%), limonene (9.4%), and cis-chrysanthenyl acetate (7.4%). Myristicin, the main constituent of both essential oils, is generally absent in the oils from other Daucus species, permitting the hypothesis that this compound is a chemical marker of this Saharan species. 相似文献
86.
Artus M Tahar LB Herbst F Smiri L Villain F Yaacoub N Grenèche JM Ammar S Fiévet F 《J Phys Condens Matter》2011,23(50):506001
Highly crystalline CoFe(2)O(4) nanoparticles with different diameters ranging from 2.4 to 6.1 nm have been synthesized by forced hydrolysis in polyol. The size can be controlled through adjusting the nominal water/metal molar ratio. X-ray diffraction, transmission electron microscopy, x-ray absorption spectroscopy and (57)Fe M?ssbauer spectrometry were employed to investigate the structure and the microstructure of the particles produced. Magnetic measurements performed on these particles show that they are superparamagnetic with a size-dependent blocking temperature. At 5 K, high saturation magnetization (~85 emu g(-1)) approaching that of the bulk was found for the larger particles, whereas a very large coercivity (14.5 kOe) is observed for the 3.5 nm sized particles. 相似文献
87.
A comparative study of the magnetic properties and magnetic structures of the natrochalcite, NaFe(2)(D(3)O(2))(MoO(4))(2) (FeD) to those of the isostructural NaCo(2)(D(3)O(2))(MoO(4))(2) (CoD) and NaNi(2)(D(3)O(2))(MoO(4))(2) (NiD) is presented. The structural change is a shrinking of the unit cell in the order of the ionic radii of the transition metal, FeD > CoD > NiD. While NiD and CoD are canted-antiferromagnets with T(N) = 28 and 21 K, respectively, FeD is an anisotropic 2D-Ising antiferromagnet (T(N) = 17 K) with a spin-flop field of 14 kOe at 2 K and the presence of a hysteresis loop reaching only (1)/(4) of the saturation magnetization in 70 kOe. The critical field decreases almost linearly on warming to T(N). The neutron diffraction patterns of FeD below T(N) display numerous magnetic Bragg peaks which cannot be assigned to any one magnetic structure but fits well to two superposed sets, one with a temperature independent line width and has a propagation vector k(1) = (0, 0, 0) while for the other there is a clear dependence and k(2) = (0, 0, ?). In the k(1) = (0, 0, 0) magnetic structure the moments are parallel to each other within one chain and lie along the a-axis but are antiparallel to those in neighboring chains. In contrast CoD and NiD, for which k = (0, 0, 0), have their moments aligned along the b-axis and ac-plane, respectively. The second magnetic structure, k(2) = (0, 0, ?), is characterized by four sublattices, two per layer, where the moments are in the ab-plane and canted with a resultant along the a-axis which is compensated by those of the adjacent layers. For the k(2) = (0, 0, ?) structure, the scattering coherent length decreases, and the moments tend progressively toward the a-axis upon increasing temperature. The coexistence of two concomitant magnetic structures is unprecedented for compounds containing transition metal moment carriers. 相似文献
88.
Cartigny D Berhal F Nagano T Phansavath P Ayad T Genêt JP Ohshima T Mashima K Ratovelomanana-Vidal V 《The Journal of organic chemistry》2012,77(10):4544-4556
A general asymmetric hydrogenation of a wide range of 2-alkyl- and 2-aryl-substituted quinoxaline derivatives catalyzed by an iridium-difluorphos complex has been developed. Under mild reaction conditions, the corresponding biologically relevant 2-substituted-1,2,3,4-tetrahydroquinoxaline units were obtained in high yields and good to excellent enantioselectivities up to 95%. With a catalyst ratio of S/C = 1000 and on a gram scale, the catalytic activity of the Ir-difluorphos complex was maintained showing its potential value. Finally, we demonstrated the application of our process in the synthesis of compound (S)-9, which is an inhibitor of cholesteryl ester transfer protein (CETP). 相似文献
89.
A new organic compound with the formula C39H46Cl2N6O was synthesized from aqueous solution of 4,4′-diaminodiphenylmethane and hydrochloric acid at room temperature. This compound crystallizes in the monoclinic space group C2: a = 25.6525(16), b = 5.7107(3), c = 13.7777(8) Å, β = 118.862(2)°, V = 1767.64(22) Å3 and Z = 2, giving D x = 1.288 g cm?3. The refinement converged to R 1 = 0.027 and wR 2 = 0.0649. The structural arrangement can be described as infinite layers parallel to bc plane. In these layers cations and non protonated molecules are linked with N-H...N hydrogen bonds building [C39H44N6]2+ organic clusters. These clusters, in turn, are linked by N-H...Cl and O-H...Cl hydrogen bonds. 相似文献
90.
Force constants and dipole moment derivatives have been computed for the molecules CH3 X(X=F, Cl, Br, I) using MO wavefunctions with pseudo-potentials for the interaction between the inner shell and valence electrons. The values obtained at the SCF approximation level from a set of gaussian valence orbitals contracted in double-zeta form and enriched with polarization functions compare well with experimental assignments of force and interaction constants, as well as integrated intensities from infrared data. The transferability of atomic force fields and polar tensors (second derivatives of total energies and first derivatives of dipole moments with respect to atomic displacements in cartesian coordinates) is discussed. 相似文献