全文获取类型
收费全文 | 362篇 |
免费 | 7篇 |
专业分类
化学 | 121篇 |
晶体学 | 1篇 |
力学 | 17篇 |
数学 | 56篇 |
物理学 | 78篇 |
无线电 | 96篇 |
出版年
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 2篇 |
2017年 | 5篇 |
2016年 | 6篇 |
2015年 | 5篇 |
2014年 | 3篇 |
2013年 | 17篇 |
2012年 | 9篇 |
2011年 | 10篇 |
2010年 | 5篇 |
2009年 | 11篇 |
2008年 | 18篇 |
2007年 | 14篇 |
2006年 | 8篇 |
2005年 | 20篇 |
2004年 | 14篇 |
2003年 | 30篇 |
2002年 | 9篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 11篇 |
1998年 | 7篇 |
1997年 | 14篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1934年 | 2篇 |
1886年 | 1篇 |
排序方式: 共有369条查询结果,搜索用时 15 毫秒
91.
Hyperpolarized (129)Xe (xenon) gas dissolved in a perfluorooctyl bromide (PFOB) emulsion stabilized with egg yolk phospholipid (EYP) is a possible contrast agent for quantitative blood flow measurements using magnetic resonance imaging. The NMR line shape of xenon dissolved in PFOB emulsion depends strongly on the exchange of spins between PFOB and water. The exchange in this system depends on three factors: the geometrical factors (i.e., droplet size and surrounding water volume), the permeability of the EYP monolayer surrounding the droplet, and the diffusion coefficients of xenon in the two media. A theoretical model which predicts the line shape of xenon in the emulsion based on the Bloch-Torrey equations is presented. Fitting the full width at half maximum (FWHM) of the theoretical line shapes with the FWHM of the experimental spectra obtained from emulsions with different water dilutions allows estimation of the volume-weighted average diameter of the PFOB droplets (3.5+/-0.8) microm and the permeability of the EYP membrane surrounding the droplet (58+/-14) microm / s. 相似文献
92.
B. C. Karrer F. C. Peiris Brenda Vanmil Ming Luo N. C. Giles Thomas H. Myers 《Journal of Electronic Materials》2005,34(6):944-948
We have determined the optical properties of a series of Cl-doped ZnSe epilayers grown on GaAs substrates using ellipsometry
and prism coupling. Initially, the carrier concentrations were determined using Hall measurements for samples between 6.30×1016 cm−3 and 9.50×1018 cm−3. Using a variable angle spectroscopic ellipsometer in the energy range between 0.7 eV and 6.5 eV, we then obtained experimental
spectra for each of the samples. By incorporating a three-layer model to simulate the experimental data, we determined the
complex dielectric functions for these Cl-doped ZnSe epilayers. In order to facilitate this modeling procedure, we have complemented
the ellipsometric results with prism coupling experiments that measured the film thickness and the index of refraction (at
discrete wavelengths) very precisely. For the fundamental band gap, we observe a blue shift with respect to the doping concentration,
which can be explained by the Burstein-Moss effect. In addition, we have determined the critical point parameters related
to these specimens by fitting the second derivatives of both the real and the imaginary parts of the dielectric functions.
Similar to several doped III-V semiconductors reported thus far, we find that in Cl-doped ZnSe epilayers, both E1 and E1 + Δ
1 red shift, as well as a broadening with respect to the doping concentration. 相似文献
93.
A.J. Giles J.D. Caldwell R.E. Stahlbush B.A. Hull N.A. Mahadik O.J. Glembocki K.D. Hobart K.X. Liu 《Journal of Electronic Materials》2010,39(6):777-780
To understand the nature of various extended defects and their impact on the electronic or optoelectronic characteristics
of semiconductor devices, the investigation of spectral properties is required. However, electroluminescence spectroscopy
does not provide spatial or structural information. The lack of such information can lead to incorrect assignment of a luminescence
band and therefore misinterpretation of the nature of the emitting defect. Here we report on the collection and analysis of
real-color and spectrally selective monochromatic electroluminescence (EL) images from 4H-SiC PiN diodes. The former provides
the approximate spectral properties from the color of the various defects with high spatial resolution, while the latter enables
simultaneous collection of both structural and spectral properties from extended defects, thereby assisting in providing the
correct assignment of the various spectral features observed. This effort enabled the observation of the formation of a green
EL emission located at C-core partial dislocations (PDs) that occurred during the stacking fault (SF) expansion process. 相似文献
94.
中面对海外上市和迟早要经历改革的中国股市,中国的经理人现在就应该着手,自上而下实施价值管理,让企业走上价值创造之路国国内的资本市场已接近8年来的低点,这与中国经济的持续强劲增长形成了鲜明对比。在股价可能决定一家企业是通过并购取得成长还是被竞争者吞吃的市场环境中,价值管理能力的重要性将日益显现。 相似文献
95.
Kendall E. Giles Michael W. Trosset David J. Marchette Carey E. Priebe 《Computational Statistics》2008,23(4):497-517
One problem in many fields is knowledge discovery in heterogeneous, high-dimensional data. As an example, in text mining an
analyst often wishes to identify meaningful, implicit, and previously unknown information in an unstructured corpus. Lack
of metadata and the complexities of document space make this task difficult. We describe Iterative Denoising, a methodology
for knowledge discovery in large heterogeneous datasets that allows a user to visualize and to discover potentially meaningful
relationships and structures. In addition, we demonstrate the features of this methodology in the analysis of a heterogeneous
Science News corpus. 相似文献
96.
K-M Osei-Bryson K Giles B Kositanurit 《The Journal of the Operational Research Society》2003,54(7):790-797
In the Knowledge Discovery Process, classification algorithms are often used to help create models with training data that can be used to predict the classes of untested data instances. While there are several factors involved with classification algorithms that can influence classification results, such as the node splitting measures used in making decision trees, feature selection is often used as a pre-classification step when using large data sets to help eliminate irrelevant or redundant attributes in order to increase computational efficiency and possibly to increase classification accuracy. One important factor common to both feature selection as well as to classification using decision trees is attribute discretization, which is the process of dividing attribute values into a smaller number of discrete values. In this paper, we will present and explore a new hybrid approach, ChiBlur, which involves the use of concepts from both the blurring and χ2-based approaches to feature selection, as well as concepts from multi-objective optimization. We will compare this new algorithm with algorithms based on the blurring and χ2-based approaches. 相似文献
97.
A. Lloyd S. J. Parry N. M. Lynn I. S. Giles 《Journal of Radioanalytical and Nuclear Chemistry》2005,264(1):83-90
Summary The work describes the selection of an activable tracer that could be used to study dispersion patterns in an English estuary and was carried out in connection with the development and testing of a particle-tracking computer model. Understanding estuarine dynamics will assist in the decision-making process for estuarine management and in contingency planning. Neutron activation analysis (NAA) has been used to characterize the concentration and natural variation of 40 trace elements in suspended particulate matter and inter-tidal bed sediment in order to identify a suitable tracer. Results have shown that europium, terbium and ytterbium would be suitable to use in a tracer study as they have low background concentrations in sediments and little variability, (Eu: 1.31 mg . kg-1±13%, Tb: 0.90 mg . kg-1±16% and Yb: 2.91 mg . kg-1±18%). The ratios of these elements are constant throughout the estuary and experiments have shown that increases in concentration of any of these elements due to the addition of artificial elemental tracer could be quantitatively determined by changes in the ratio constant. 相似文献
98.
B M Gordon M S Uhrig M F Borgerding H L Chung W M Coleman J F Elder J A Giles D S Moore C E Rix E L White 《Journal of chromatographic science》1988,26(4):174-180
The major components of an alkaloid-free, flue-cured, tobacco essential oil sample are isolated and identified. This is accomplished by utilizing modern hyphenated analytical methods. The instrumentation developed to accomplish this are an automated multidimensional gas chromatograph/mass spectrometer/flame ionization detector (MDGC/MS/FID) and a multidimensional gas chromatograph/matrix isolation/Fourier transform infrared spectrometer (MDGC/MI/FTIR). A total of 306 compounds is identified in the essential oil, of which 80 are found as tobacco constituents for the first time. 相似文献
99.
M. B. Giles 《国际流体数值方法杂志》1997,24(8):739-757
This paper analyses the accuracy and numerical stability of coupling procedures in aeroelastic modelling. A two-dimensional model problem assuming unsteady inviscid flow past an oscillating wall leads to an even simpler one-dimensional model problem. Analysis of different numerical algorithms shows that in general the coupling procedures are numerically stable, but care is required to achieve accuracy when using very few time steps per period of natural oscillation of the structure. The relevance of the analysis to fully three-dimensional applications is discussed. © 1997 by John Wiley & Sons, Ltd. 相似文献
100.
M. Giles A. Cua N. Sasen M. Llorente L. Fornaro 《Crystal Research and Technology》2004,39(10):906-911
Lead bromide polycrystalline films were grown by the physical vapor deposition method (PVD). Glass 1″x1″ in size, uncoated, and coated with Indium Tin Oxide (ITO), was used as substrate and rear contact. The starting material was evaporated at temperatures from 395°C to 530°C under high vacuum atmosphere (6 x 10‐3 Pa) and during 8 days. The substrate temperature was prefixed from 190°C to 220°C. Film thickness yielded values from 40 to 90 μm. Optical microscopy and scanning electron microscopy (SEM) were performed on the films. Grain size resulted to be from 1.0 to 3.5 μm. SEM and X‐ray diffraction indicate that films grow with a preferred orientation with the (0 0 l) planes parallel to the substrate. The Texture Coefficient (TC) related to the plane (0 0 6) was 7.3. Resistivity values in the order of 1012 Ωcm were obtained for the oriented samples, but a strong polarization indicates severe charge transport problems in the films. Film properties were correlated with the growth temperature and with previous results for films of other halides. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献