全文获取类型
收费全文 | 2000篇 |
免费 | 112篇 |
国内免费 | 57篇 |
专业分类
化学 | 632篇 |
晶体学 | 43篇 |
力学 | 28篇 |
综合类 | 16篇 |
数学 | 409篇 |
物理学 | 1041篇 |
出版年
2023年 | 9篇 |
2022年 | 14篇 |
2021年 | 16篇 |
2020年 | 14篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 19篇 |
2016年 | 19篇 |
2015年 | 25篇 |
2014年 | 32篇 |
2013年 | 76篇 |
2012年 | 39篇 |
2011年 | 50篇 |
2010年 | 53篇 |
2009年 | 178篇 |
2008年 | 178篇 |
2007年 | 146篇 |
2006年 | 135篇 |
2005年 | 103篇 |
2004年 | 136篇 |
2003年 | 104篇 |
2002年 | 80篇 |
2001年 | 68篇 |
2000年 | 81篇 |
1999年 | 95篇 |
1998年 | 45篇 |
1997年 | 29篇 |
1996年 | 40篇 |
1995年 | 28篇 |
1994年 | 43篇 |
1993年 | 35篇 |
1992年 | 43篇 |
1991年 | 31篇 |
1990年 | 20篇 |
1989年 | 16篇 |
1988年 | 17篇 |
1987年 | 15篇 |
1986年 | 14篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有2169条查询结果,搜索用时 15 毫秒
81.
We have investigated the photoluminescence (PL) properties of trace amounts of Pr and Tb in single-crystal samples of yttria-stabilized zironia (YSZ), and found that Pr of the order of 10−6 mass% and Tb of the order of 10−5 mass% in YSZ can be detected by the PL spectroscopy. The PL spectra of the YSZ samples for the 280 nm excitation were comprised of several peaks and a broad emission. The peaks were attributed to transitions of Pr3+ and Tb3+ in the YSZ samples, whereas the broad emission seemed to be attributed to the yttria-associated oxygen vacancies. The peak intensities corresponded to the amounts of Pr and Tb in the YSZ samples, the amounts of which were analyzed by glow discharge mass spectrometry. In the PL excitation spectra, but not in the photoabsorption spectra, small peaks at 376 and 381 nm were observed, and were attributed to the transitions of Tb3+ in the YSZ samples. The results of the PL excitation spectra corresponding to the Pr3+ line emissions suggest that the charge transfer occurs between the YSZ and Pr ion in it. The trace amounts of these lanthanoids in YSZ would disturb the decay process of the photoinduced electrons to the yttria-associated oxygen vacancies. 相似文献
82.
Nanosized ZnGa2O4:Cr3+ powder is synthesized through hydrothermal method. The average particle size is 20 nm and they are spherical in shape. The excitation band from the charge transfer between Cr3+-O2− shows a blueshift behavior due to quantum confinement effect. X-ray diffraction pattern, Fourier transform-infrared spectrum, and electron paramagnetic resonance signal indicate that nanosized ZnGa2O4:Cr3+ phosphor shows many defect-related energy states and heavy lattice distortion in comparison with bulk ZnGa2O4:Cr3+ phosphor. Many defect states result in more nonradiative loss and shorter decay time. 相似文献
83.
84.
Polycrystalline Sn1−m>x m>Mnm>x m>O2 (0≤m>x m>≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (m>x m>=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases. 相似文献
85.
Physical aspects of an operation of the GaAs-based InGaAs/GaAs quantum-well (QW) VCSELs with the intentionally detuned optical
cavities have been considered in the present paper using the comprehensive three-dimensional self-consistent optical–electrical–thermal-gain
simulation. In GaAs-based structures, very good DBR resonator mirrors and a very efficient methods to confine radially both
the current spreading and the electromagnetic field with the aid of oxide apertures may be applied. It has been found using
the above simulation that even currently available immature technology enables manufacturing the above devices emitting radiation
of wavelengths over 1.20 μm. In particular, while the room-temperature 1.30-μm lasing emission is still beyond possibilities
of the InGaAs/GaAs QW VCSELs, these structures may offer analogous 1.25-μm emission, especially for the high-power and/or
high-temperature operation. 相似文献
86.
S. M.D. Queirós L. G. Moyano J. de Souza C. Tsallis 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(2):161-167
We present results about financial market observables, specifically
returns and traded volumes. They are obtained within the current nonextensive statistical mechanical framework based on the
entropy
mg src="/content/3337q1h181mrk7pt/10051_2006_Article_205_TeX2GIFEqu1.gif" alt="$S_{q}=k\frac{1-\sum_{i=1}^{W} p_{i} ^{q}}{1-q}\, (q\in \Re) \left(S_{1} \equiv S_{BG}=-k\sum_{i=1}^{W}p_{i}\,\ln p_{i}\right)$" align="middle" border="0">
. More precisely, we present stochastic dynamical mechanisms which mimic probability density functions empirically observed.
These mechanisms provide possible interpretations for the emergence of the entropic
indices q in the time evolution of the corresponding observables. In addition to this, through multi-fractal analysis of return
time series, we verify that the dual relation qstat+qsens=2 is numerically satisfied, qstat and qsens being associated to the probability density function and to the sensitivity to initial conditions respectively. This type
of simple relation, whose understanding remains ellusive, has been empirically verified in various other systems. 相似文献
87.
88.
采用密度泛函理论(DFT)中的B3LYP方法得到了(AlB2)m团簇的平衡几何结构. 计算并分析了基态掺杂团簇的平均结合能、电离势、能隙和前线分子轨道. 结果表明:掺杂团簇(AlB2)m (m=1~6)整体上具有较高化学活性,(AlB2)5团簇具有金属特征. Al原子总是向团簇外围扩散并且以配位数较少的方式与主团簇结合,团簇表现出以AlB2分子为基元生长的迹象. B-Al键长大于B-B键长. 电荷总是从Al原子转移到B原子. (AlB2)m团簇中B原子的2p轨道在成键中起主要作用,并使(AlB2)m团簇趋于形成离域π键. 相似文献
89.
The coexistence of TE-TM surface waves in uniaxially anisotropic left-handed materials 总被引:1,自引:0,他引:1
We investigate theoretically the characteristics of surface waves in uniaxially anisotropic left-handed materials. The constraints for the existence of TE and TM polarized surface waves in uniaxially anisotropic left-handed materials are identified. We discuss mainly whether TE and TM polarized surface modes may coexist in the same frequency domain at the interface between one isotropic regular medium and another uniaxially anisotropic left-handed medium. It is shown that the answer to the coexistence of TE and TM surface modes is really positive. The Poynting vector and the density of energy associated with surface modes are calculated. Depending on the system parameters either TE or TM surface modes has the time averaged Poynting vector directed opposite or same to the mode phase velocity. In the presence of anisotropy, negative refraction does not need to be left-handed. We show that the characteristics of surface waves in uniaxially anisotropic left-handed media are significantly different from that in isotropic left-handed media. 相似文献
90.
The effect of K+ ions on GdTaO4:Eu3+ thin-film phosphors was investigated in order to improve their luminescent properties. The GdTaO4:Eu0.1, Km>x m> thin films were synthesized by sol-gel process, and characterized through measuring their microstructure and luminescence. The results indicated that photoluminescence (PL) intensity of GdTaO4:Eu3+ film was improved remarkably by K doping. There were two maxima in the curve of PL intensity against K+ dopant concentration, where one was improved up to 2.1 times at m>x m> = 0.001 and the other was enhanced up to 2.7 times at m>x m> = 0.05. The first maximum was regarded as the alteration of the local environment surrounding the Eu3+ activator by incorporation of K+ ions, and the second maximum was due to the flux effect. Additionally, the luminescence increased with the increase of firing temperature from 800 °C to 1200 °C. 相似文献