全文获取类型
收费全文 | 5025篇 |
免费 | 229篇 |
国内免费 | 391篇 |
专业分类
化学 | 4005篇 |
晶体学 | 103篇 |
力学 | 303篇 |
综合类 | 5篇 |
数学 | 65篇 |
物理学 | 1164篇 |
出版年
2024年 | 6篇 |
2023年 | 60篇 |
2022年 | 65篇 |
2021年 | 53篇 |
2020年 | 81篇 |
2019年 | 91篇 |
2018年 | 87篇 |
2017年 | 122篇 |
2016年 | 227篇 |
2015年 | 176篇 |
2014年 | 175篇 |
2013年 | 275篇 |
2012年 | 334篇 |
2011年 | 346篇 |
2010年 | 258篇 |
2009年 | 379篇 |
2008年 | 264篇 |
2007年 | 314篇 |
2006年 | 269篇 |
2005年 | 218篇 |
2004年 | 187篇 |
2003年 | 127篇 |
2002年 | 99篇 |
2001年 | 111篇 |
2000年 | 90篇 |
1999年 | 96篇 |
1998年 | 93篇 |
1997年 | 74篇 |
1996年 | 62篇 |
1995年 | 80篇 |
1994年 | 75篇 |
1993年 | 61篇 |
1992年 | 48篇 |
1991年 | 54篇 |
1990年 | 45篇 |
1989年 | 29篇 |
1988年 | 57篇 |
1987年 | 44篇 |
1986年 | 49篇 |
1985年 | 49篇 |
1984年 | 64篇 |
1983年 | 34篇 |
1982年 | 66篇 |
1981年 | 40篇 |
1980年 | 30篇 |
1979年 | 42篇 |
1978年 | 16篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1973年 | 5篇 |
排序方式: 共有5645条查询结果,搜索用时 15 毫秒
1.
A preconditioning approach based on the artificial compressibility formulation is extended to solve the governing equations for unsteady turbulent reactive flows with heat release, at low Mach numbers, on an unstructured hybrid grid context. Premixed reactants are considered and a flamelet approach for combustion modelling is adopted using a continuous quenched mean reaction rate. An overlapped cell‐vertex finite volume method is adopted as a discretisation scheme. Artificial dissipation terms for hybrid grids are explicitly added to ensure a stable, discretised set of equations. A second‐order, explicit, hybrid Runge–Kutta scheme is applied for the time marching in pseudo‐time. A time derivative of the dependent variable is added to recover the time accuracy of the preconditioned set of equations. This derivative is discretised by an implicit, second‐order scheme. The resulting scheme is applied to the calculation of an infinite planar (one‐dimensional) turbulent premixed flame propagating freely in reactants whose turbulence is supposed to be frozen, homogeneous and isotropic. The accuracy of the results obtained with the proposed method proves to be excellent when compared to the data available in the literature. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
J. I. Ramos 《国际流体数值方法杂志》1991,12(9):881-894
Two domain-adaptive finite difference methods are presented and applied to study the dynamic response of incompressible, inviscid, axisymmetric liquid membranes subject to imposed sinusoidal pressure oscillations. Both finite difference methods map the time-dependent physical domain whose downstream boundary is unknown onto a fixed computational domain. The location of the unknown time-dependent downstream boundary of the physical domain is determined from the continuity equation and results in an integrodifferential equation which is non-linearly coupled with the partial differential equations which govern the conservation of mass and linear momentum and the radius of the liquid membrane. One of the finite difference methods solves the non-conservative form of the governing equations by means of a block implicit iterative method. This method possesses the property that the Jacobian matrix of the convection fluxes has an eigenvalue of algebraic multiplicity equal to four and of geometric multiplicity equal to one. The second finite difference procedure also uses a block implicit iterative method, but the governing equations are written in conservation law form and contain an axial velocity which is the difference between the physical axial velocity and the grid speed. It is shown that these methods yield almost identical results and are more accurate than the non-adaptive techniques presented in Part I. It is also shown that the actual value of the pressure coefficient determined from linear analyses can be exceeded without affecting the stability and convergence of liquid membranes if the liquid membranes are subjected to sinusoidal pressure variations of sufficiently high frequencies. 相似文献
3.
An algorithm for a time accurate incompressible Navier–Stokes solver on an unstructured grid is presented. The algorithm uses a second order, three‐point, backward difference formula for the physical time marching. For each time step, a divergence free flow field is obtained based on an artificial compressibility method. An implicit method with a local time step is used to accelerate the convergence for the pseudotime iteration. To validate the code, an unsteady laminar flow over a circular cylinder at a Reynolds number of 200 is calculated. The results are compared with available experimental and numerical data and good agreements are achieved. Using the developed unsteady code, an interaction of a Karman vortex street with an elliptical leading edge is simulated. The incident Karman vortex street is generated by a circular cylinder located upstream. A clustering to the path of the vortices is achieved easily due to flexibility of an unstructured grid. Details of the interaction mechanism are analysed by investigating evolutions of vortices. Characteristics of the interactions are compared for large‐ and small‐scale vortex streets. Different patterns of the interaction are observed for those two vortex streets and the observation is in agreement with experiment. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
4.
The apparent molal volumes and compressibilities of NaCl, KCl, and CsCl in mixtures of 18-Crown-6 and water have been calculated from density and speed-of-sound measurements at 25°C. The partial molal volumes and compressibilities of the salts when all cations have formed complexes with 18-Crown-6 molecules have been evaluated. The sign and magnitude of the volume and compressibility changes of complex formation strongly suggest that the charge of the cation becomes very effectively screened by the crown ether. 相似文献
5.
P. Terzieff 《Monatshefte für Chemie / Chemical Monthly》1996,127(8-9):841-844
Summary Alloys of the ternary system gold-indium-zinc with a variable content of goldx
Au at a constant ratio ofx
In:x
Zn=1:1 have been investigated by susceptibility measurements from 300 K up to 1500 K with emphasis on the liquid state. The magnetic susceptibilities of the homogeneous liquids were found to vary in a non-linear manner with temperature and composition. At temperatures close to the liquid-solid transition, the susceptibilities pass through a characteristic minimum which tends to disappear with increasing temperature. The anomaly located atx
Au 0.6 seems to be related to a particular valence electron concentration ofe/a 1.6.
Magnetische Suszeptibilität von flüssigen Gold-Indium-Zink-Legierungen
Zusammenfassung Legierungen des ternären Systems Gold-Indium-Zink mit einem variablen Goldgehaltx Au bei konstantem Indium-Zink-Verhältnis (x In:x Zn=1:1) wurden zwischen 300 und 1500 K auf ihre magnetische Suszeptibilität untersucht. Das Hauptaugenmerk wurde dabei auf den flüssigen Zustand gelegt. Die magnetischen Suszeptibilitäten der homogenen Schmelzen variieren nichtlinear mit Temperatur und Zusammensetzung. Bei Temperaturen nahe dem übergang flüssig-fest tritt ein charakteristisches Minimum der Suszeptibilität auf, das mit steigender Temperatur verschwindet. Die Anomalie bei einemx Au vonca. 0.6 scheint mit einer speziellen Valenzelektronenkonzentration (e/a) von etwa 1.6 verknüpft zu sein.相似文献
6.
Combinatorial library of isoxazoles was prepared by 1,3-dipolar cycloaddition on solid support. p-Cyano derivatives showed nematic and/or smectic A phases. A bilayer smectic phase for 2h is proposed by the combination of molecular mechanics calculation and X-ray diffraction experiment. 相似文献
7.
M. R. Gregg 《Chromatographia》1985,20(2):129-133
Summary Electrode surface modification by electrochemical pretreatment of glassy carbon electrodes was shown to enhance significantly the sensitivity of the electrodes for the detection of timolol and oxprenolol, but reduce slightly the sensitivity to prenalterol. This method may permit the detection of exprenolol and timolol with increased sensitivity, or may allow their detection at lower applied potentials than is presently possible. Electrode surface modification may prove to be a valuable aid to the detection of compounds that are considered to be outside the practical limits of electrochemical detection. 相似文献
8.
Thomas F. Brodasky 《Journal of separation science》1978,1(5):239-244
Two new acyltetramic acids related to streptolydigin have been isolated from fermentations of Streptomyces lydicus. The principal members of this complex were resolved by TLC on silica gel. However, the methods of detection, permanganate spray or bioautography, were not suitable for both crude fermentation broths and purified extracts. Gas chromatography is unsuitable for the detection of either underivatized or silylated streptolydigins. High performance liquid chromatography (HPLC) particularly on triethylaminoethyl cellulose is rapid and sensitive and is the method of choice for the analysis of both crude and purified samples. Using high performance liquid chromatography, two components were detected in the complex, which are not observed using any of the other chromatographic procedures. 相似文献
9.
Kenichiro Todoroki 《Analytica chimica acta》2006,555(1):14-19
A fluorimetric liquid chromatographic method for the determination of 5-hydroxyindoles based on the benzylamine derivatization process mediated through an online photocatalytic oxidation has been developed. In this study, we used a photocatalytic column comprising tefzel tubing packed with TiO2-coated glass beads, as a pre-column derivatization reactor. The fluorescence derivatization of 5-hydroxyindoles using benzylamine proceeded during their passage through the reaction column under near-UV irradiation. The 5-hydroxyindole derivatives were separated continuously on a reversed-phase liquid chromatography within 50 min, using 100 mM acetate buffer (pH 4.6)-acetonitrile (72:28, v/v; isocratic elution) containing 3 mM sodium octanesulfonate; the samples were detected fluorimetrically at 465 nm upon excitation at 350 nm. The detection limits (signal-to-noise ratio = 3) of the 5-hydroxyindoles were in the range from 160 to 360 fmol per 5 μL injection. We have applied this method, which requires minimal sample pre-treatment, to the determination of 5-hydroxyindole-3-acetic acid in human urine. 相似文献
10.
An accurate estimation of pKa values in methanol-water binary mixtures is very important for several separation techniques such as liquid chromatography and capillary electrophoresis that use these solvent mixtures. In this study, the pKa values of 11 polyphenolic acids have been determined in methanol-water binary mixtures (10%, 20% and 30% (v/v)) by potentiometry, liquid chromatography (LC) and LC-DAD methodology.The results show a similar trend for the pKa values of all the studied compounds, as they increase with increasing concentration of organic modifier, which allows a linear relationship between pKa values and mole fraction of methanol to be obtained. The pKa values obtained in aqueous medium have been compared with those given in the literature, and also with the values predicted by the SPARC on-line pKa calculator. The data obtained have been used to test the feasibility of an estimation of dissociation constants in a methanol-water medium from the relationship between pKa values and the organic cosolvent fraction in the mixtures. 相似文献