全文获取类型
收费全文 | 2191篇 |
免费 | 71篇 |
国内免费 | 12篇 |
专业分类
化学 | 1466篇 |
晶体学 | 16篇 |
力学 | 85篇 |
数学 | 388篇 |
物理学 | 319篇 |
出版年
2022年 | 19篇 |
2021年 | 39篇 |
2020年 | 32篇 |
2019年 | 50篇 |
2018年 | 32篇 |
2017年 | 22篇 |
2016年 | 71篇 |
2015年 | 45篇 |
2014年 | 53篇 |
2013年 | 95篇 |
2012年 | 97篇 |
2011年 | 119篇 |
2010年 | 75篇 |
2009年 | 86篇 |
2008年 | 119篇 |
2007年 | 111篇 |
2006年 | 109篇 |
2005年 | 112篇 |
2004年 | 94篇 |
2003年 | 81篇 |
2002年 | 60篇 |
2001年 | 29篇 |
2000年 | 35篇 |
1999年 | 39篇 |
1998年 | 21篇 |
1997年 | 32篇 |
1996年 | 29篇 |
1995年 | 26篇 |
1994年 | 25篇 |
1993年 | 18篇 |
1992年 | 27篇 |
1991年 | 16篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 18篇 |
1987年 | 15篇 |
1986年 | 22篇 |
1985年 | 29篇 |
1984年 | 25篇 |
1983年 | 17篇 |
1982年 | 15篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1888年 | 9篇 |
排序方式: 共有2274条查询结果,搜索用时 15 毫秒
81.
The combination of a high‐order unstructured spectral difference (SD) spatial discretization scheme with sub‐grid scale (SGS) modeling for large‐eddy simulation is investigated with particular focus on the consistent implementation of a structural mixed model based on the scale similarity hypothesis. The difficult task of deriving a consistent formulation for the discrete filter within the SD element of arbitrary order led to the development of a new class of three‐dimensional constrained discrete filters. The discrete filters satisfy a set of selected criteria and are completely local within the SD element. Their weights can be automatically computed at run time from the number of solution points within each element and the expected filter cutoff length scale. The novel discrete filters can be applied to any SGS model involving explicit filtering and to a broad class of high‐order discontinuous finite element numerical schemes. The code is applied to the computation of turbulent channel flows at three Reynolds numbers, namely Reτ = 180, 395, and 590 (based on the friction velocity uτ and channel half‐width δ). Results from computations with and without the SGS model are compared against results from direct numerical simulation. The numerical experiments suggest that the results are sensitive to the use of the SGS model, even when a high‐order numerical scheme is used, especially when the grid resolution is kept relatively low and mostly in terms of resolved Reynolds stresses. Results obtained using existing filters based on the projection of the solution over lower‐order polynomial bases are also shown and demonstrate that these filters are inadequate for SGS modeling purposes, mostly because of their inability to enforce the selected cutoff length scale with sufficient accuracy. The use of the similarity mixed formulation proved to be particularly accurate in reproducing SGS interactions, confirming that its well‐known potential can be realized in conjunction with state‐of‐the‐art high‐order numerical schemes.Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
82.
83.
84.
Birgit U. Jaki Anton Bzhelyansky Guido F. Pauli 《Magnetic resonance in chemistry : MRC》2021,59(1):7-15
In the last two decades, quantitative NMR (qNMR) has become increasingly important for the analysis of pharmaceuticals, chemicals, and natural products including dietary supplements. For the purpose of quality control and chemical standardization of a large variety of pharmaceutical, chemical, and medicinal products, qNMR has proven to be a valuable orthogonal quantification method and a compelling alternative to chromatographic techniques. This work reviews a fundamental component of the early development of qNMR, reflected in the pioneering work of the late George M. Hanna during the years between 1984 and 2006 at the US Food and Drug Administration (FDA). Because Hanna performed the majority of his groundbreaking work on a 90‐MHz instrument, his legacy output connects with recent progress in low‐field benchtop NMR instrumentation. Hanna gradually established the utility of qNMR for the routine quality control analyses practiced in pharmaceutical and related operations well ahead of his peers. His work has the potential to inspire new developments in qNMR applied to small molecules of biomedical importance. 相似文献
85.
Single‐Walled Carbon Nanotubes in Highly Viscous Media: A Comparison between the Dispersive Agents [BMIM][BF4], L121, and Triton X‐100 下载免费PDF全文
Dr. Carla Gasbarri Prof. Fausto Croce Dr. Ida Meschini Christopher H. Bowen Dr. Lisa Marinelli Prof. Antonio Di Stefano Dr. Guido Angelini 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(2):546-549
Dispersions of single‐walled carbon nanotubes (SWNTs) have been prepared by using the room‐temperature ionic liquid [BMIM][BF4] (1‐butyl‐3‐methylimidazolium tetrafluoroborate), the triblock copolymer Pluronic L121 [poly(ethylene oxide)5‐poly(propylene oxide)68‐poly(ethylene oxide)5] and the non‐ionic surfactant Triton X‐100 (TX100) in the pure state. The size of the SWNTs aggregates and the dispersion degree in the three viscous systems depend on the sonication time, as highlighted by UV/Vis/NIR spectroscopy and optical microscopy analysis. A nonlinear increase in conductivity can be observed as a function of the SWNTs loading, as suggested by electrochemical impedance spectroscopy. The generation of a three‐dimensional network of SWNTs showing a viscoelastic gel‐like behavior above a critical percolation concentration has been found at 25 °C in all the investigated systems by oscillatory rheology measurements. 相似文献
86.
87.
We study the problem of existence of response solutions for a real-analytic one-dimensional system, consisting of a rotator subject to a small quasi-periodic forcing with Bryuno frequency vector. We prove that at least one response solution always exists, without any assumption on the forcing besides smallness and analyticity. This strengthens the results available in the literature, where generic non-degeneracy conditions are assumed. The proof is based on a diagrammatic formalism and relies on renormalisation group techniques, which exploit the formal analogy with problems of quantum field theory; a crucial role is played by remarkable identities between classes of diagrams. 相似文献
88.
Guido Dell’Acqua 《Journal of mathematical chemistry》2013,51(5):1300-1309
An asymptotic analysis at leading order of the Goldbeter–Koshland switch, the simplest futile cycle, is carried out in detail. After a nondimensionalization of the problem, we find the leading order uniform expansions of the reactants, identifying the proper time scales in a total setting. Comparison with numerical integration confirms the goodness of our analysis. 相似文献
89.
We formally derive and rigorously justify the modulation equations of lowest order for the interaction of two modulated pulses on a one-dimensional nonlinear oscillator chain. We show that solutions with the initial form of the assumed ansatz preserve this form over time intervals with positive macroscopic length, and we show a bound on the possible shift of the envelope caused by the interaction. Thus, we rigorously justify and quantify the statement that under the given conditions there is almost no interaction of the modulated pulses. 相似文献
90.