全文获取类型
收费全文 | 5639篇 |
免费 | 150篇 |
国内免费 | 15篇 |
专业分类
化学 | 2931篇 |
晶体学 | 125篇 |
力学 | 167篇 |
数学 | 393篇 |
物理学 | 1516篇 |
无线电 | 672篇 |
出版年
2023年 | 48篇 |
2022年 | 122篇 |
2021年 | 135篇 |
2020年 | 164篇 |
2019年 | 150篇 |
2018年 | 156篇 |
2017年 | 182篇 |
2016年 | 219篇 |
2015年 | 148篇 |
2014年 | 208篇 |
2013年 | 432篇 |
2012年 | 318篇 |
2011年 | 322篇 |
2010年 | 221篇 |
2009年 | 203篇 |
2008年 | 257篇 |
2007年 | 224篇 |
2006年 | 168篇 |
2005年 | 169篇 |
2004年 | 127篇 |
2003年 | 113篇 |
2002年 | 121篇 |
2001年 | 69篇 |
2000年 | 62篇 |
1999年 | 47篇 |
1998年 | 41篇 |
1997年 | 47篇 |
1996年 | 51篇 |
1995年 | 36篇 |
1994年 | 40篇 |
1993年 | 52篇 |
1992年 | 65篇 |
1991年 | 44篇 |
1990年 | 48篇 |
1989年 | 61篇 |
1988年 | 47篇 |
1987年 | 46篇 |
1986年 | 41篇 |
1985年 | 70篇 |
1984年 | 60篇 |
1983年 | 46篇 |
1982年 | 51篇 |
1981年 | 58篇 |
1980年 | 50篇 |
1979年 | 50篇 |
1978年 | 46篇 |
1977年 | 52篇 |
1976年 | 57篇 |
1975年 | 44篇 |
1974年 | 44篇 |
排序方式: 共有5804条查询结果,搜索用时 171 毫秒
991.
The present study is focused on the influence of vacuum thermal treatment on surface/interface electronic properties of Si/Ge multilayer structures (MLS) characterized using X-ray photoelectron spectroscopy (XPS) technique. Desired [Si(5 nm)/Ge(5 nm)]×10 MLS were prepared using electron beam evaporation technique under ultra high vacuum (UHV) conditions. The core-level XPS spectra of as-deposited as well as multilayer samples annealed at different temperatures such as 100 °C, 150 °C and 200 °C for 1 h show substantial reduction in Ge 2p peak integrated intensity, whereas peak intensity of Si 2p remains almost constant. The complete interdiffusion took place after annealing the sample at 200 °C for 5 h as confirmed from depth profiling of annealed MLS. The asymmetric behaviour in intensity patterns of Si and Ge with annealing was attributed to faster interdiffusion of Si into Ge layer. However, another set of experiments on these MLS annealed at 500 °C suggests that interdiffusion can also be studied by annealing the system at higher temperature for relatively shorter time duration. 相似文献
992.
993.
A two-dimensional (2-D) analytical subthreshold model is developed for a graded channel double gate (DG) fully depleted SOI n-MOSFET incorporating a gate misalignment effect. The conformal mapping transformation (CMT) approach has been used to provide an accurate prediction of the surface potential, electric field, threshold voltage and subthreshold behavior of the device, considering the gate misalignment effect to be on both source and drain side. The model is applied to both uniformly doped (UD) and graded channel (GC) DG MOSFETs. The results of an analytical model agree well with 3-D simulated data obtained by ATLAS-3D device simulation software. 相似文献
994.
Prashant K. Sharma Ranu K. Dutta Samar Layek 《Journal of magnetism and magnetic materials》2009,321(17):2587-2591
The ZnO:Fe nanoparticles of mean size 3-10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe-Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe3+ nature of the iron atom in ZnO matrix. 相似文献
995.
Within a relativistic mean-field model with nonlinear isoscalar–isovector coupling, we explore the possibility of constraining the density dependence of nuclear symmetry energy from a systematic study of the neutron skin thickness of finite nuclei and neutron star properties. We find the present skin data supports a rather stiff symmetry energy at subsaturation densities that corresponds to a soft symmetry energy at supranormal densities. Correlation between the skin of 208Pb and the neutron star masses and radii with kaon condensation has been studied. We find that 208Pb skin estimate suggest star radii that reveals considerable model dependence. Thus precise measurements of neutron star radii in conjunction with skin thickness of heavy nuclei could provide significant constraint on the density dependence of symmetry energy. 相似文献
996.
Optical time division multiplexing (OTDM) is emerging and promising alternative for future high-speed photonic networks because of its ability to accommodate higher bit rate and flexible bandwidth. Among other factors the performance of an OTDM system largely depends upon the switching characteristics of a de-multiplexer (DEMUX). Symmetric Mach-Zehnder (SMZ) have been found to be most suitable than all the available de-multiplexing switches because of compact size, thermal stability, and low power operation. In this paper, we simulate four-channel OTDM systems (all channel multiplexer (MUX) and DEMUX) with a Mach-Zehnder modulator and SMZ DEMUX to investigate the impact of signal power, pulse width and control signal power on BER. 相似文献
997.
We report structural as well as optical studies on Si/Ge bilayer structures having different individual layer thicknesses. The Raman spectrum of [Ge (5 nm)/Si (5 nm)] bilayer structure shows amorphous nature, while the [Si (5 nm)/Ge (5 nm)] bilayer structure shows a mixed nanocrystalline/amorphous behaviour of the layers. As the thickness of the individual layers increases to 10 nm, the introduction of large number of Si atoms at the interface results in reduction of Ge crystallization as well as higher intensity of interfacial SiGe alloy formation. This may be regarded as a consequence of the island growth induced surface roughening in the later case (i.e. in [Si (10 nm)/Ge (10 nm)] bilayer) as also revealed by corresponding atomic force microscopy (AFM) images. These results are also supported by Photoluminescence (PL) spectra recorded using two different photon energies of 300 and 488 nm along with the optical absorption measurements giving higher values of band gap as compared to their corresponding bulks, revealing the effect of quantum confinement in the deposited layers. 相似文献
998.
Sharma S Vandana Ghoshal SK Arora P Dilbaghi N Chaudhury A 《Applied biochemistry and biotechnology》2009,159(2):310-316
In the present frame of work, Macrophomina phaseolina is encapsulated in silica matrices at various concentrations by low temperature sol-gel technique using tetraethylorthosilicate
(TEOS) as precursor. The optical and photophysical properties of these samples have been studied by second harmonics of Nd:YAG
laser at 532 nm. UV-visible absorption spectra of samples have been recorded and it is found that the absorption increases
with increase in concentration of fungus. Further, a decrease in output transmission intensity of the laser has been observed
with increase in fungus concentration. The temporal response of these samples has also been examined. The results show that
the fungus concentration can be measured within ∼15–20 min. This method of optical sensing of fungus in test sample is faster
than other techniques, such as the conventional colorimetric method which takes about 1 h. 相似文献
999.
Ajaya Kumar Singh Bhawana Jain Reena Negi Yokraj Katre Surya P. Singh Virender K. Sharma 《Transition Metal Chemistry》2009,34(5):521-528
The kinetics of Ru(III) chloride-catalyzed oxidation of β-Alanine (NH3
+CH2CH2COOH, β-Ala) by N–bromophthalimide (NBP) in aqueous perchloric acid medium was studied at 35 °C. The rate law followed a first-order and zero-order
dependence with respect to [NBP] and [β-Ala], respectively. The reaction followed first-order kinetics with respect to [Ru(III)]
chloride at a range of low concentrations while the order changed from first- to zero-order at high concentration of [Ru(III)]
chloride; demonstrating the catalytic effect for the oxidation of β-Ala by NBP. The rate decreased with increase in acidity.
Chloride ions positively influenced the rate of the reaction. Neither phthalimide (NHP) nor Hg(II) influenced the reaction
rate. Ionic strength (I) and dielectric constant (D) of the medium had no significant effect on the rate. Activation parameters
of the reactions were determined by studying the reaction at different temperatures (30–50 °C). The colorimetric, FTIR, and
GC-MS techniques were used to identify methyl cyanide (CH3CN) and CO2 as products of the reaction. In the reaction, approximately 2.3 moles of NBP oxidized one mole of β-Ala. A reaction scheme
of the oxidation of β-Ala by NBP in the presence of Ru(III) chloride was found to be in consistent with the rate law and the
reaction stoichiometry. 相似文献
1000.
Bharat Baral Pawan Kumar Brooke A. Anderson Michael E.
stergaard Pawan K. Sharma Patrick J. Hrdlicka 《Tetrahedron letters》2009,50(42):5850-5852
A short and high-yielding synthetic route to [3-15N]-labeled uridine phosphoramidite 1 (26% overall yield from uridine) has been developed. This will enable automated synthesis of isotopically labeled RNA strands and facilitate their use in structural studies. 相似文献