全文获取类型
收费全文 | 719篇 |
免费 | 33篇 |
国内免费 | 3篇 |
专业分类
化学 | 485篇 |
晶体学 | 6篇 |
力学 | 17篇 |
数学 | 75篇 |
物理学 | 172篇 |
出版年
2023年 | 3篇 |
2022年 | 9篇 |
2021年 | 13篇 |
2020年 | 9篇 |
2019年 | 12篇 |
2018年 | 8篇 |
2016年 | 26篇 |
2015年 | 26篇 |
2014年 | 21篇 |
2013年 | 48篇 |
2012年 | 43篇 |
2011年 | 51篇 |
2010年 | 32篇 |
2009年 | 27篇 |
2008年 | 35篇 |
2007年 | 32篇 |
2006年 | 37篇 |
2005年 | 29篇 |
2004年 | 30篇 |
2003年 | 17篇 |
2002年 | 23篇 |
2001年 | 8篇 |
2000年 | 14篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 3篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1972年 | 9篇 |
1971年 | 4篇 |
1961年 | 3篇 |
1940年 | 5篇 |
排序方式: 共有755条查询结果,搜索用时 15 毫秒
91.
Zinc-phosphite and -phosphate based microporous materials are crystalline open framework materials with potential industrial applications. Although (31)P MAS NMR has been used for characterization of these materials, the local environments around zinc centres have never been directly probed by solid-state NMR due to the many unfavourable NMR characteristics of (67)Zn. In this work, we have characterized the local structure around the Zn centres in several representative microporous zinc phosphites and zinc phosphates by acquiring natural abundance (67)Zn solid-state NMR spectra at ultrahigh magnetic field of 21.1 T. The observed line-shapes are mainly determined by the second order quadrupolar interaction. The NMR tensor parameters were extracted from the spectra and are related to the local geometry around the Zn centre. Computational study of the electric field gradient (EFG) tensor at Zn was performed using hybrid density functional theory (DFT) calculations at B3LYP level of theory on model clusters. The calculations using Projector Augmented-Wave (PAW) method were also carried out with the CASTEP code wherever it was possible. The work has shown that it is possible to study Zn environments in porous materials which often have very low Zn concentration by natural abundance (67)Zn SSNMR at very high magnetic fields. 相似文献
92.
Consider a compact Riemannian manifold M of dimension n whose boundary ?M is totally geodesic and is isometric to the standard sphere S n?1. A natural conjecture of Min-Oo asserts that if the scalar curvature of M is at least n(n?1), then M is isometric to the hemisphere $S_{+}^{n}$ equipped with its standard metric. This conjecture is inspired by the positive mass theorem in general relativity, and has been verified in many special cases. In this paper, we construct counterexamples to Min-Oo??s Conjecture in dimension n??3. 相似文献
93.
Miroslav Kuchta Kent‐Andre Mardal Mikael Mortensen 《Numerical Methods for Partial Differential Equations》2019,35(1):375-393
Multiscale or multiphysics problems often involve coupling of partial differential equations posed on domains of different dimensionality. In this work, we consider a simplified model problem of a 3d‐1d coupling and the main objective is to construct algorithms that may utilize standard multilevel algorithms for the 3d domain, which has the dominating computational complexity. Preconditioning for a system of two elliptic problems posed, respectively, in a three‐dimensional domain and an embedded one dimensional curve and coupled by the trace constraint is discussed. Investigating numerically the properties of the well‐defined discrete trace operator, it is found that negative fractional Sobolev norms are suitable preconditioners for the Schur complement of the system. The norms are employed to construct a robust block diagonal preconditioner for the coupled problem. 相似文献
94.
Hssaine Amamou Andre BoisBelkacem Ferhat Roland RedonBruno Rossetto Pascal Matheron 《Journal of Quantitative Spectroscopy & Radiative Transfer》2002,75(6):747-763
Expressions for fundamental spectral line parameters are reported for Gaussian and Lorentzian shape profiles for homogeneous plasma at equilibrium in presence of the self-absorption. These expressions for Lorentzian profiles are applied to the determination of the ratios of transition probabilities and the ratios of optical thicknesses by a new method that we propose in this article. The self-absorption is computed by spectral line profiles with a Simplex algorithm program fitting. Applications to some experimental lines illustrate the appropriate corrections. 相似文献
95.
96.
97.
98.
Andre S. Barci V. Barneoud D. Beraud R. Charvet A. Samman H. El Genevey J. Gizon A. Gizon J. Kossakowski R. Tréherne J. Weiss B. 《Zeitschrift für Physik A Hadrons and Nuclei》1982,308(2):181-182
Preliminary results on residual nuclei production in 30 MeV/A12C interaction with medium heavy targets are presented. Gamma-ray spectroscopic techniques were used. Evidence for charge and angular momentum transfer is found.
相似文献99.
100.
S A Wickline E D Verdonk B E Sobel J G Miller 《The Journal of the Acoustical Society of America》1992,91(5):3018-3025
It has been reported previously that acute and mature myocardial infarction in dogs can be differentiated in vitro and in vivo by ultrasonic tissue characterization based on measurement of the frequency dependence of ultrasonic backscatter. To characterize human infarction with an index of the frequency dependence of backscatter that could be obtained in patients, cylindrical biopsy specimens from 7 normal regions and 12 regions of infarction of 6 fixed, explanted human hearts in 2-deg steps around their entire circumference with a 5-MHz broadband transducer were insonified. One to six consecutive transmural levels were studied for each specimen. The dependence of apparent (uncompensated for attenuation or beam width) backscatter, /B(f)/2, on frequency (f) was computed from spectral analyses of radio-frequency data as /B(f)/2 = afn, where from theoretical considerations the magnitude of n decreases as scatterer size increases. Apparent integrated backscatter was computed as the average of /B(f)/2 from 3 to 7 MHz. The average value for n for normal tissue (0.9 +/- 0.1) exceeded that for tissue from regions of infarction (0.6 +/- 0.1; p less than 0.05). Infarct manifested a significant decrease of n from epicardial to endocardial levels (epi----mid----endo: 0.9----0.7----0.2; p less than 0.05) whereas normal tissue manifested similar values for n at each transmural level (0.8----1.1----0.9; p = NS). Average integrated backscatter across all transmural levels for infarct was significantly greater than for normal tissue (-48.3 +/- 0.5 vs -53.4 +/- 0.4 dB, infarct versus normal; p less than 0.05). The presence of fibrosis was associated with smaller values of n and greater integrated backscatter.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献