全文获取类型
收费全文 | 56652篇 |
免费 | 4984篇 |
国内免费 | 4200篇 |
专业分类
化学 | 24839篇 |
晶体学 | 913篇 |
力学 | 4328篇 |
综合类 | 558篇 |
数学 | 10641篇 |
物理学 | 24557篇 |
出版年
2024年 | 62篇 |
2023年 | 439篇 |
2022年 | 1010篇 |
2021年 | 1119篇 |
2020年 | 1236篇 |
2019年 | 1247篇 |
2018年 | 1147篇 |
2017年 | 1360篇 |
2016年 | 1626篇 |
2015年 | 1321篇 |
2014年 | 1948篇 |
2013年 | 3684篇 |
2012年 | 2447篇 |
2011年 | 2835篇 |
2010年 | 2348篇 |
2009年 | 3435篇 |
2008年 | 3641篇 |
2007年 | 4078篇 |
2006年 | 3530篇 |
2005年 | 2798篇 |
2004年 | 2345篇 |
2003年 | 2572篇 |
2002年 | 2922篇 |
2001年 | 2197篇 |
2000年 | 2088篇 |
1999年 | 1748篇 |
1998年 | 1664篇 |
1997年 | 1050篇 |
1996年 | 946篇 |
1995年 | 839篇 |
1994年 | 900篇 |
1993年 | 629篇 |
1992年 | 727篇 |
1991年 | 461篇 |
1990年 | 444篇 |
1989年 | 335篇 |
1988年 | 314篇 |
1987年 | 300篇 |
1986年 | 251篇 |
1985年 | 236篇 |
1984年 | 237篇 |
1983年 | 146篇 |
1982年 | 195篇 |
1981年 | 190篇 |
1980年 | 124篇 |
1979年 | 150篇 |
1978年 | 127篇 |
1977年 | 120篇 |
1976年 | 76篇 |
1973年 | 60篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Maria Kordaki Athanasia Balomenou 《International Journal of Computers for Mathematical Learning》2006,11(1):99-135
This study focuses on the constructions in terms of area and perimeter in equivalent triangles developed by students aged 12–15 years-old, using the tools provided by Cabri-Geometry II [Labore (1990). Cabri-Geometry (software), Université de Grenoble]. Twenty-five students participated in a learning experiment where they were asked to construct: (a) pairs of equivalent triangles “in as many ways as possible” and to study their area and their perimeter using any of the tools provided and (b) “any possible sequence of modifications of an original triangle into other equivalent ones”. As regards the concept of area and in contrast to a paper and pencil environment, Cabri provided students with different and potential opportunities in terms of: (a) means of construction, (b) control, (c) variety of representations and (d) linking representations, by exploiting its capability for continuous modifications. By exploiting these opportunities in the context of the given open tasks, students were helped by the tools provided to develop a broader view of the concept of area than the typical view they would construct in a typical paper and pencil environment. 相似文献
52.
The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH4/SiF4 mixtures at 300 °C were investigated. This study indicates that the low hydrogen coverage on the growing surface, under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage surface. Moreover, the studies of the role of SiH4 and SiF4 radicals show that the SiH4 radicals are important in the nucleation and growth of grains. However, SiF4 radicals are effective in the structural change of grain boundaries regions and by this way, in the present system, establish the growth of grains under the dominant 〈1 1 0〉 direction. The stress investigation indicates that addition of high flow rate of SiF4 in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic compressive and tensile stress in the both polycrystalline and amorphous silicon films. 相似文献
53.
Nuclear Magnetic Resonance (NMR) was used to investigate the extrusion behaviour of PTFE pastes in a ram extruder. By means
of 1H-NMR imaging (MRI) it is possible to determine the local proton density and therefore, the local fluid concentration. The
19F-MRI provides the local solid concentration. Thus the local saturation and the local porosity can be calculated with the
information of the local fluid and solid concentration. Furthermore displacement profiles can be derived from NMR images by
means of correlation techniques without any preparation or marking of the pastes.
Received: 8 May 2000 Accepted: 1 May 2001 相似文献
54.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
55.
Non-stoichiometric ternary chalcogenides (Zn,Fe)S were prepared in the film form by pyrolytic spray deposition technique, using air/nitrogen as the carrier gas. The precursor solution comprised of ZnCl2, FeCl2 and thiourea. The depositions were carried out under optimum conditions of experimental parameters viz. carrier gas (air/nitrogen) flow rate, concentration of precursor constituents, nozzle substrate distance and temperature of quartz substrate. The deposited thin films were later sintered in argon at 1073 K for 120 min.The structural, compositional and optical properties of the sintered thin films were studied. X-ray diffraction studies of the thin films indicated the presence of (Zn,Fe)S solid solution with prominent cubic sphalerite phase while surface morphology as determined by scanning electron microscopy (SEM) revealed a granular structure.The chemical composition of the resulting thin films as analyzed by energy dispersive X-ray analysis (EDAX) reflected the composition of the precursor solutions from which the depositions were carried out with Fe at% values ranging from 0.4 up to 33.SEM micrographs of thin films reveal that the grain sizes of the thin films prepared using air as carrier gas and N2 as carrier gas are in the vicinity of 300 and 150 nm, respectively.The diffuse transmittance measurements for thin films, as a function of wavelength reveal the dependence of direct optical band gap on Fe content and type of phase. 相似文献
56.
Thin films of, N-N′diphenyl 1-4phenylene-diamineane are prepared using vacuum sublimation technique. The electrical conductivity from room temperature down to 127 K is studied. It is found that the conduction of charge carriers obeys T−1/2 dependence on temperature. The average hopping distance, hopping energy, density of states and their variation due to post-deposition heat treatment are studied. Schottky diodes are fabricated with gold as ohmic contact and aluminium as Schottky contact. From the observed current voltage characteristics the saturation current density, diode ideality factor and the barrier height are determined. Their variation with air annealing is also investigated. 相似文献
57.
In this paper we study the Hankel convolution operators on the space of even and entire functions and on Schwartz distribution spaces. We characterize the Hankel convolution operators as those ones that commute with Hankel translations and with a Bessel operator. Also we prove that the Hankel convolution operators are hypercyclic and chaotic on the spaces under consideration. 相似文献
58.
59.
60.