全文获取类型
收费全文 | 238490篇 |
免费 | 2789篇 |
国内免费 | 866篇 |
专业分类
化学 | 127149篇 |
晶体学 | 2867篇 |
力学 | 9835篇 |
综合类 | 2篇 |
数学 | 27039篇 |
物理学 | 75253篇 |
出版年
2020年 | 1770篇 |
2019年 | 2044篇 |
2018年 | 2476篇 |
2017年 | 2400篇 |
2016年 | 3834篇 |
2015年 | 2598篇 |
2014年 | 3903篇 |
2013年 | 11191篇 |
2012年 | 8109篇 |
2011年 | 10031篇 |
2010年 | 6584篇 |
2009年 | 6489篇 |
2008年 | 9005篇 |
2007年 | 9160篇 |
2006年 | 8399篇 |
2005年 | 7855篇 |
2004年 | 6999篇 |
2003年 | 6096篇 |
2002年 | 5627篇 |
2001年 | 6957篇 |
2000年 | 5030篇 |
1999年 | 3892篇 |
1998年 | 3188篇 |
1997年 | 3311篇 |
1996年 | 2995篇 |
1995年 | 3058篇 |
1994年 | 2882篇 |
1993年 | 2924篇 |
1992年 | 3241篇 |
1991年 | 3249篇 |
1990年 | 3060篇 |
1989年 | 3025篇 |
1988年 | 3084篇 |
1987年 | 2954篇 |
1986年 | 2855篇 |
1985年 | 3937篇 |
1984年 | 4100篇 |
1983年 | 3335篇 |
1982年 | 3702篇 |
1981年 | 3590篇 |
1980年 | 3474篇 |
1979年 | 3515篇 |
1978年 | 3752篇 |
1977年 | 3581篇 |
1976年 | 3789篇 |
1975年 | 3369篇 |
1974年 | 3490篇 |
1973年 | 3795篇 |
1972年 | 2304篇 |
1971年 | 1758篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
61.
David E. Delgado Lauren F. Sturdy Craig W. Burkhart Kenneth R. Shull 《Journal of Polymer Science.Polymer Physics》2019,57(18):1246-1254
The utility of the quartz crystal microbalance (QCM) as a high‐frequency rheometer operating at 15 MHz was demonstrated. High‐frequency data obtained from a series of rubbery materials were compared with results obtained from traditional dynamic mechanical analysis (DMA) at much lower frequencies. The high‐frequency data enable meaningful shift factors to be obtained at temperatures much further above glass‐transition temperature (T g) than would otherwise be possible, giving a more complete picture of the temperature dependence of the viscoelastic properties. The QCM can also be used to quantify mass uptake and changes in viscoelastic properties during sample oxidation. The viscoelastic response spanning the full range of behaviors from the rubber to glassy regimes was found to fit well with a six‐element model consisting of three power‐law springpot elements. One of these elements is particularly sensitive to the behavior in the transition regime where the phase angle is maximized. The value of this quantity is obtained from the maximum phase angle, which can be obtained from a temperature sweep at fixed frequency, proving a means for more detailed frequency‐dependent rheometric information to be obtained from a fixed‐frequency measurement at a range of temperatures. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1246–1254 相似文献
62.
We will focus on the existence of nontrivial solutions to the following Hamiltonian elliptic system where are numbers belonging to the interval [0, 2), V is a continuous potential bounded below on by a positive constant and the functions f and g possess exponential growth range established by Trudinger–Moser inequalities in Lorentz–Sobolev spaces. The proof involves linking theorem and a finite‐dimensional approximation. 相似文献
63.
Queueing Systems - In this paper, we use importance sampling simulation to estimate the probability that the number of customers in a d-node GI|GI|1 tandem queue reaches some high level N in a busy... 相似文献
64.
65.
The electrochemical behavior of austenitic stainless steel (Type 304) in 3 M sulfuric acid with 3.5% recrystallized sodium chloride at specific concentrations of butan-1-ol was investigated with the aid of potentiodynamic polarization, open circuit measurement and weight loss technique. Butan-1-ol effectively inhibited the steel corrosion with a maximum inhibition efficiency of 78.7% from weight-loss analysis and 80.9% from potentiodynamic polarization test at highest concentration studied. Adsorption of the compound obeyed the Freundlich isotherm. Thermodynamic calculations reveal physiochemical interactions and spontaneous adsorption mechanism. Surface characterizations showed the absence of corrosion products and topographic modifications of the steel. Statistical analysis depicts the overwhelming influence and statistical significance of inhibitor concentration on the inhibition performance. 相似文献
66.
67.
Denis D. Borisov Grigory R. Chermashentsev Roman A. Novikov Yury V. Tomilov 《Tetrahedron letters》2019,60(10):746-750
New “four-component” self-assembly of polyaromatic thiophene structures based on styrylmalonates and 5-phenylthiophene-2-carbaldehyde has been developed. This process is promoted by GaCl3 and involves two [2?+?3]-annulation steps on the CHO-groups and para-substitution into one Ph-ring. The main feature of discovered process is a high diastereoselectivity with a significant increase in molecular complexity. The resulting polyaromatic structures containing two thiophene moieties in each structure have intense color and strong absorption in a near UV spectral region with absorption maxima in the range of 257–360?nm. 相似文献
68.
Pierre Thureau Simone Sturniolo Miri Zilka Fabio Ziarelli Stéphane Viel Jonathan R. Yates Giulia Mollica 《Magnetic resonance in chemistry : MRC》2019,57(5):256-264
Structure determination of functional organic compounds remains a formidable challenge when the sample exists as a powder. Nuclear magnetic resonance crystallography approaches based on the comparison of experimental and Density Functional Theory (DFT)-computed 1H chemical shifts have already demonstrated great potential for structure determination of organic powders, but limitations still persist. In this study, we discuss the possibility of using 13C-13C dipolar couplings quantified on powdered theophylline at natural isotopic abundance with the help of dynamic nuclear polarization, to realize a DFT-free, rapid screening of a pool of structures predicted by ab initio random structure search. We show that although 13C-13C dipolar couplings can identify structures possessing long range structural motifs and unit cell parameters close to those of the true structure, it must be complemented with other data to recover information about the presence and the chemical nature of the supramolecular interactions. 相似文献
69.
A new Schiff base hydrazone (Z)‐2‐(2‐aminothiazol‐4‐yl)‐N′‐(2‐hydroxy‐3‐methoxybenzylidene) acetohydrazide (H2L) and its chelates [VO (HL)2]·5H2O, [Cu (HL)Cl(H2O)]·2H2O and [Fe(L)Cl(H2O)2]·3H2O have been isolated and characterized using different physico‐chemical methods, for example infrared (IR), electron paramagnetic resonance (EPR), thermogravimetric analysis and DTG in the solid state, and 1H‐NMR, 13C‐NMR and UV in solution. Magnetic and UV–visible measurements proposed that the coordination environments are square pyramidal, tetrahedral and octahedral geometries for oxovanadium (IV), Cu (II) and Fe (III), respectively. The ligand acts as mono‐negative NO towards oxovanadium (IV) and Cu (II) ions, and bi‐negative ONO for Fe (III) ion. The geometries of the ligand and its complexes were performed using Gaussian 9 program with density functional theory. The EPR spectral data of oxovanadium (IV) and Cu (II) chelates confirmed the mentioned geometries. The molecular modeling was done, and illustrated bond lengths, bond angles, molecular electrostatic potential, Mulliken atomic charges and chemical reactivity for the inspected compounds. Theoretical IR and 1H‐NMR of the free ligand were calculated. Furthermore, thermodynamic and kinetic parameters for thermal decomposition steps were studied. Docking study of H2L was applied against the proteins of both bacterial strains Staphylococcus aureus and Escherichia coli, as well as the protein of xanthine oxidase as antioxidant agent by Schrödinger suite program utilizing XP glide protocol. Furthermore, antimicrobial, antioxidant and DNA‐binding activities of the compounds have been carried out. 相似文献
70.
We consider the problem of optimizing heat transport through an incompressible fluid layer. Modeling passive scalar transport by advection-diffusion, we maximize the mean rate of total transport by a divergence-free velocity field. Subject to various boundary conditions and intensity constraints, we prove that the maximal rate of transport scales linearly in the r.m.s. kinetic energy and, up to possible logarithmic corrections, as the one-third power of the mean enstrophy in the advective regime. This makes rigorous a previous prediction on the near optimality of convection rolls for energy-constrained transport. On the other hand, optimal designs for enstrophy-constrained transport are significantly more difficult to describe: we introduce a “branching” flow design with an unbounded number of degrees of freedom and prove it achieves nearly optimal transport. The main technical tool behind these results is a variational principle for evaluating the transport of candidate designs. The principle admits dual formulations for bounding transport from above and below. While the upper bound is closely related to the “background method,” the lower bound reveals a connection between the optimal design problems considered herein and other apparently related model problems from mathematical materials science. These connections serve to motivate designs. © 2019 Wiley Periodicals, Inc. 相似文献