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511.
Combined dynamic and static light scattering (DLS, SLS) and cryogenic transmission electron microscopy (cryo-TEM) were used to investigate extruded cationic vesicles of dioctadecyldimethylammonium chloride and bromide (DODAX, X being Cl(-) or Br(-)). In salt-free dispersions the mean hydrodynamic diameter, D(h), and the weight average molecular weight, M(w), are larger for DODAB than for DODAC vesicles, and both D(h) and M(w) increase with the diameter (varphi) of the extrusion filter. NaCl (NaBr) decreases (increases) the DODAB (DODAC) vesicle size, reflecting the general trend of DODAB to assemble as larger vesicles than DODAC. The polydispersity index is lower than 0.25, indicating the dispersions are rather polydisperse. Cryo-TEM micrographs show that the smaller vesicles are spherical while the larger ones are oblong or faceted, and the vesicle samples are fairly polydisperse in size and morphology. They also indicate that the vesicle size increases with phi and DODAB assembles as larger vesicles than DODAC. Lens-shaped vesicles were observed in the extruded preparations. Both light scattering and cryo-TEM indicate that the vesicle size is larger or smaller than phi when phi is smaller or larger than the optimal phi approximately 200 nm. 相似文献
512.
Flávio José L. dos Santos Antônio Flávio de C. Alcântara Dalton L. Ferreira-Alves Dorila Piló-Veloso 《Structural chemistry》2008,19(4):625-631
This work describes the synthesis of new derivatives of 6α,7β-dihydroxyvouacapan-17β-oic acid (1) employing the Swern method for the oxidation of C-6 and C-7 of methyl 6α,7β-di-hydroxyvouacapan-17β-oate (2) and the formation of methyl 6,7-dioxovouacapan-17β-oate (3). NMR structural studies associated with theoretical calculations of reaction intermediates and products are also reported.
The mixture of methyl 7β-hydroxy-6-oxovouacapan-17β-oate (4; 21%) and methyl 6α-hydroxy-7-oxovouacapan-17β-oate (5; 79%) was the product from the first step of the oxidation of 2. The lower energy of 5, calculated by HF/6-31G* and DFT/BLYP/6-31G* methods, reinforces the thermodynamic control proposed for the reaction. After
further Swern oxidation of the mixture of 4 and 5, the isomeric form methyl 6-hydroxy-7-oxovouacap-5-en-17β-oate (6) was obtained. Theoretical calculations indicate a lower energy for the mono-enol 6 than the 1,2-diketo 3 and the other mono-enol methyl 7-hydroxy-6-oxovouacap-7-en-17β-oate (7). 相似文献
513.
Teresa Antón-Cánovas Prof. Dr. Francisco Alonso 《Angewandte Chemie (International ed. in English)》2023,62(4):e202215916
C−H bond formylation is the most immediate way to incorporate the versatile formyl group into (hetero)aromatics. However, the type of reagents and severe conditions involved in the classical formylation methods often curtail their application, especially in the presence of other functional groups. Herein, we present the Eschenmoser's salt, a commercially available (dimethylamino)methylating chemical, as a useful reagent for the C−H formylation of indolizines and other compounds. The method is straightforward and mild, furnishing indolizinecarbaldehydes in modest-to-good yields with exclusive and remote regioselectivity. Furthermore, these compounds can be easily transformed into push-pull dyes and are highly selective in the colorimetric detection of nitrite, a substance extensively employed as preservative in the food industry, the concentration of which is crucial to control to prevent harmful effects in living organisms. The assay is simple, allowing the naked-eye detection of nitrite in solution or on a cotton swab for a wide range of concentrations. 相似文献
514.
This paper introduces a method for approximating real world data by means of non-integer harmonics series (NIHS). The NIHS terms are sinusoidal functions of arbitrary amplitude, frequency and phase shift that can be computed by direct numerical algorithms. Applications go from obtaining time derivatives up to forecasting future values. Three illustrative examples with the Dow Jones Industrial stock market index, the Europe Brent Spot Price FOB and the daily temperature records of New York city are studied. The results show the effectiveness of the NIHS when dealing with real world time data series.
相似文献515.
Dennys Reis Ana C. Trindade Maria Helena Godinho Laura C. Silva Maria do Carmo Gonçalves Antônio M. Figueiredo Neto 《Brazilian Journal of Physics》2017,47(1):19-25
The nanostructure of urethane/urea elastomeric membranes was investigated by small-angle X-ray scattering (SAXS) in order to establish relationships between their structure and mechanical properties. The networks were made up of polypropylene oxide (PPO) and polybutadiene (PB) segments. The structural differences were investigated in two types of membranes with the same composition but with different thermal treatment after casting. Type I was cured at 70–80 °C and type II at 20 °C. Both membranes showed similar phase separation by TEM, with nanodomains rich in PB or PPO and 25 nm dimensions. The main difference between type I and type II membranes was found by SAXS. The type I membrane spectra showed, besides a broad band at a 27-nm q value (modulus of the scattering vector), an extra band at 6 nm, which was not observed in the type II membrane. The SAXS spectra were interpreted in terms of PPO, PB soft segments, and urethane/urea links, as well as hard moiety segregation in the reaction medium. This additional segregation (q = 7 nm), although subtle, results in diverse mechanical behavior of in both membranes. 相似文献
516.
Margarida Machado Paulo Flores J. C. Pimenta Claro Jorge Ambrósio Miguel Silva António Completo Hamid M. Lankarani 《Nonlinear dynamics》2010,60(3):459-478
The aim of this work is to develop a dynamic model for the biological human knee joint. The model is formulated in the framework
of multibody systems methodologies, as a system of two bodies, the femur and the tibia. For the purpose of describing the
formulation, the relative motion of the tibia with respect to the femur is considered. Due to their higher stiffness compared
to that of the articular cartilages, the femur and tibia are considered as rigid bodies. The femur and tibia cartilages are
considered to be deformable structures with specific material characteristics. The rotation and gliding motions of the tibia
relative to the femur cannot be modeled with any conventional kinematic joint, but rather in terms of the action of the knee
ligaments and potential contact between the bones. Based on medical imaging techniques, the femur and tibia profiles in the
sagittal plane are extracted and used to define the interface geometric conditions for contact. When a contact is detected,
a continuous nonlinear contact force law is applied which calculates the contact forces developed at the interface as a function
of the relative indentation between the two bodies. The four basic cruciate and collateral ligaments present in the knee are
also taken into account in the proposed knee joint model, which are modeled as nonlinear elastic springs. The forces produced
in the ligaments, together with the contact forces, are introduced into the system’s equations of motion as external forces.
In addition, an external force is applied on the center of mass of the tibia, in order to actuate the system mimicking a normal
gait motion. Finally, numerical results obtained from computational simulations are used to address the assumptions and procedures
adopted in this study. 相似文献
517.
Frederico J. M. F. Custódio Patrick D. Anderson Gerrit W. M. Peters António M. Cunha Han E. H. Meijer 《Rheologica Acta》2010,49(1):23-44
Residual stresses are a major issue in the mechanical and optical behavior of injection-molded parts. In this study, we analyze
their development in the case of gas-assisted injection molding (GAIM) of amorphous polymers. Flow-induced residual stresses
are computed within a decoupled approach, in which elastic effects are neglected in the momentum balance, assuming a generalized
Newtonian material behavior. In a staggered procedure, the computed viscous flow kinematics are used to calculate normal stresses
employing a compressible version of the Rolie-Poly model. For the computation of thermally and pressure-induced residual stresses,
a linear thermo-viscoelastic model is used. A 3-D finite element model for GAIM is employed, which is able to capture the
kinematics of the flow front and whose capabilities to predict the thickness of the residual material layer have been validated
by Haagh and Van de Vosse (Int J Numer Methods Fluids 28:1355–1369, 1998). In order to establish a clear comparison, the development of residual stresses is analyzed using standard injection molding
and GAIM for a test geometry. 相似文献
518.
Teresa Avilés António Dinis Michael G. B. Drew Vitor Félix 《Monatshefte für Chemie / Chemical Monthly》2000,131(12):1305-1310
Summary. The self-assembly of Ag[BF4] with trans-azobenzene in dichloromethane yields a new coordination polymer ([Ag(μ-trans-azobenzene)H2O][BF4])n which was characterized by X-ray single crystal diffraction. The crystal consists of 1-D zigzag cationic chains made up from
[Ag(H2O)]+ units linked by trans-azobenzene bridges and BF−
4 anions. Hydrogen bonding interactions between the chains and BF−
4 anions occur via intermolecular C*H⋯F and O*H⋯F contacts, and the crystal displays a 2-D supramolecular structure.
Received May 31, 2000. Accepted June 30, 2000 相似文献
519.
Miguel R. Oliveira Pan?o António Luis N. Moreira Diamantino G. Dur?o 《Experiments in fluids》2012,53(1):105-119
This paper analyzes the transient characteristics of intermittent sprays produced by the single-point impact of multiple cylindrical jets. The aim is to perform a transient analysis of the intermittent atomization process to study the effect of varying the number of impinging jets in the hydrodynamic mechanisms of droplet formation. The results evidence that hydrodynamic mechanisms underlying the physics of ligament fragmentation in 2-impinging jets sprays also apply to sprays produced with more than 2 jets during the main period of injection. Ligaments detaching from the liquid sheet, as well as from its bounding rim, have been identified and associated with distinct droplet clusters, which become more evident as the number of impinging jets increases. Droplets produced by detached ligaments constitute the main spray, and their axial velocity becomes more uniformly distributed with 4-impinging jets because of a delayed ligament fragmentation. Multijet spray dispersion patterns are geometric depending on the number of impinging jets. Finally, an analysis on the Weber number of droplets suggests that multijet sprays are more likely to deposit on interposed surfaces, thus becoming a promising and competitive atomization solution for improving spray cooling. 相似文献
520.
Ueveton Pimentel da Silva Gabriela Milane Furlani Antônio Jacinto Demuner Otávio Luis Marques da Silva 《Natural product research》2019,33(18):2681-2684
Euphorbia heterophylla L. is regarded as a major weed worldwide. Its high aggressiveness in agricultural environment prompted us to investigate the allelopatic activity and chemical constitution of extracts from roots of this plant. Hexane extract showed low phytotoxic activity. Methanol extract at 2.0 mg mL?1 inhibited 100% of germination, root and shoot growth of the indicator plants Sorghum bicolor and Lactuca sativa. β-sitosterol, stigmasterol, and esters of lupeol, germanicol, taraxasterol, pseudotaraxasterol, α-amyrin and β-amyrin were isolated from the hexane extract and their structures elucidated on the basis of MS and 1H, 13C and DEPT-135 NMR data. GC-MS analysis of the derivatized methanol extract allowed for identifying a series of allelopathic organic acids potentially involved in allelopathic interactions of E. heterophylla. This is the first study on the allelopathic activity of extracts and identification of metabolites from roots of E. heterophylla. 相似文献