全文获取类型
收费全文 | 267774篇 |
免费 | 3192篇 |
国内免费 | 1567篇 |
专业分类
化学 | 150713篇 |
晶体学 | 3395篇 |
力学 | 11521篇 |
综合类 | 65篇 |
数学 | 30184篇 |
物理学 | 76655篇 |
出版年
2022年 | 3298篇 |
2021年 | 5261篇 |
2020年 | 5362篇 |
2019年 | 6372篇 |
2018年 | 3760篇 |
2017年 | 3531篇 |
2016年 | 4097篇 |
2015年 | 2895篇 |
2014年 | 3699篇 |
2013年 | 9224篇 |
2012年 | 8070篇 |
2011年 | 10157篇 |
2010年 | 9456篇 |
2009年 | 9936篇 |
2008年 | 9750篇 |
2007年 | 9642篇 |
2006年 | 8593篇 |
2005年 | 7883篇 |
2004年 | 8889篇 |
2003年 | 8792篇 |
2002年 | 8616篇 |
2001年 | 9452篇 |
2000年 | 5991篇 |
1999年 | 4677篇 |
1998年 | 3739篇 |
1997年 | 3560篇 |
1996年 | 3886篇 |
1995年 | 2945篇 |
1994年 | 3161篇 |
1993年 | 2874篇 |
1992年 | 3031篇 |
1991年 | 3006篇 |
1990年 | 2904篇 |
1989年 | 2864篇 |
1988年 | 2672篇 |
1987年 | 2682篇 |
1986年 | 2717篇 |
1985年 | 3522篇 |
1984年 | 3513篇 |
1983年 | 3027篇 |
1982年 | 3160篇 |
1981年 | 2938篇 |
1980年 | 2689篇 |
1979年 | 2883篇 |
1978年 | 3137篇 |
1977年 | 3330篇 |
1976年 | 3251篇 |
1975年 | 2964篇 |
1974年 | 3013篇 |
1973年 | 3015篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
981.
C V Fabritius N L Madsen J Clausen J Larsen 《The Journal of the Operational Research Society》2006,57(12):1482-1490
An ontology is a classification model for a given domain. In information retrieval, ontologies are used to perform broad searches. An ontology can be visualized as nodes and edges. Each node represents an element and each edge a relation between a parent and a child element. Working with an ontology becomes easier with a visual representation. An idea is to use the expressive power of 3D representation to provide visualization for the user. In this paper, we propose a new method for positioning the elements of the visualized concept lattice in the 3D world based on operations research (OR) methods. One method uses a discrete location model to create an initial solution and we propose heuristic methods to further improve the visual result. We evaluate the visual results according to our success criteria and the feedback from users. Running times of the heuristic indicate that an improved version should be feasible for on-line processing and what-if analysis of ontologies. 相似文献
982.
In this work, the optical and structural properties of high k materials such as tantalum oxide and titanium oxide were studied by spectroscopic ellipsometry, where a Tauc-Lorentz dispersion model based in one (amorphous films) or two oscillators (microcrystalline films) was used. The samples were deposited at room temperature by radio frequency magnetron sputtering and then annealed at temperatures from 100 to 500 °C. Concerning the tantalum oxide films, the increase of the annealing temperature, up to 500 °C does not change the amorphous nature of the films, increasing, however, their density. The same does not happen with the titanium oxide films that are microcrystalline, even when deposited at room temperature. Data concerning the use of a four-layer model based on one and two Tauc-Lorentz dispersions is also discussed, emphasizing its use for the detection of an amorphous incubation layer, normally present on microcrystalline films grown by sputtering. 相似文献
983.
O.E. Petrova N.B. Tarasova A.Ju. Alyabyev M.N. Davydova N.L. Loseva 《Thermochimica Acta》2006,445(1):67-69
Calorimetric results indicate that nitrocellulose (NC)-induced changes in the metabolism of Desulfovibrio desulfuricans 1388 are caused by both chemical (nitrate) and physical (biofilm formation) factors. Nitrate added to lactate-based culture medium with nitrocellulose competed for the electron flux from lactate and suppressed the bacterial sulfidogenesis and growth. The presence of an insoluble compound (carbon backbone of the polymer) induced the creation of a biofilm-like structure with its own metabolism. 相似文献
984.
The oxide films formed on AISI 304L stainless steel at 300 °C in the oxidation time range between 2 and 4 h have been studied by photoelectrochemistry. Photocurrents were investigated as a function of the wavelength of the incident light and the electrode potential. The investigation allowed the determination of the semiconductive properties of the oxides. The oxide films showed n-type behaviour. A duplex structure of the oxide films has been suggested on the basis of the photocurrent spectra, with an internal oxide layer having an optical gap (Eg2 = 2.16-2.3 eV) depending on the applied potential and oxidation time, higher to that of the external oxide layer (Eg1 ≈ 1.9 eV). Significant variations in the amplitude of the photocurrent were detected as a function of the applied potential and the oxidation time. 相似文献
985.
Yu. D. Perfiliev V. S. Rusakov L. A. Kulikov A. A. Kamnev K. Alkhatib 《Hyperfine Interactions》2006,167(1-3):881-885
To explain line broadening in emission Mössbauer spectra as compared to the corresponding absorber measurements, the model of trapped electrons has been proposed. Auger electrons (emitted, e.g. after electron capture by 57Co or after the converted isomeric transition of 119mSn), as well as secondary electrons, may be trapped in the proximity to the nucleogenic ion. Electrons captured by lattice traps at different distances from the daughter ion induce an asymmetric distribution of quadrupole splitting in the resulting emission spectra, as shown in a few examples. This model is supported by estimates of quadrupole splitting values which may be caused by such trapped electrons located at specified distances from the nucleogenic atom. 相似文献
986.
987.
988.
989.
J. J. EGOZCUE J. L. DIAZ-BARRERO V. PAWLOWSKY-GLAHN 《数学学报(英文版)》2006,22(4):1175-1182
The set of probability functions is a convex subset of L1 and it does not have a linear space structure when using ordinary sum and multiplication by real constants. Moreover, difficulties arise when dealing with distances between densities. The crucial point is that usual distances are not invariant under relevant transformations of densities. To overcome these limitations, Aitchison's ideas on compositional data analysis are used, generalizing perturbation and power transformation, as well as the Aitchison inner product, to operations on probability density functions with support on a finite interval. With these operations at hand, it is shown that the set of bounded probability density functions on finite intervals is a pre-Hilbert space. A Hilbert space of densities, whose logarithm is square-integrable, is obtained as the natural completion of the pre-Hilbert space. 相似文献
990.