全文获取类型
收费全文 | 301篇 |
免费 | 9篇 |
专业分类
化学 | 195篇 |
晶体学 | 1篇 |
力学 | 4篇 |
数学 | 37篇 |
物理学 | 35篇 |
无线电 | 38篇 |
出版年
2021年 | 2篇 |
2019年 | 1篇 |
2018年 | 5篇 |
2017年 | 2篇 |
2016年 | 3篇 |
2015年 | 10篇 |
2014年 | 9篇 |
2013年 | 11篇 |
2012年 | 18篇 |
2011年 | 24篇 |
2010年 | 17篇 |
2009年 | 8篇 |
2008年 | 18篇 |
2007年 | 27篇 |
2006年 | 22篇 |
2005年 | 16篇 |
2004年 | 22篇 |
2003年 | 18篇 |
2002年 | 12篇 |
2001年 | 6篇 |
2000年 | 6篇 |
1999年 | 2篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有310条查询结果,搜索用时 15 毫秒
111.
Chernichenko K Nieger M Leskelä M Repo T 《Dalton transactions (Cambridge, England : 2003)》2012,41(30):9029-9032
Two 2-[bis(pentafluorophenyl)boryl]-N,N-dialkylanilines reported here exemplify a new class of intramolecular frustrated B/N Lewis pairs. A structure closely related to this class structure was synthesized in 2003 by Piers et al. but was unable to activate H(2). The new aminoboranes can activate hydrogen at near ambient conditions; besides, one of them can hydrogenate imines and enamines in a catalytic fashion demonstrating the validity of the original Piers' approach to hydrogen activation with ansa-aminoboranes. 相似文献
112.
Cascade linear phase recursive half-band filters implement the most efficient digital down converter
fred harris Elettra Venosa Xiaofei Chen Markku Renfors 《Analog Integrated Circuits and Signal Processing》2012,73(2):531-543
The digital down converter (DDC) is a fundamental component in modern DSP based receivers. The basic architecture is the DSP implementation of the Edwin Armstrong heterodyne circuit. It is formed by three processes, a quadrature heterodyne, a pair of low-pass filters, and an M-to-1 down sampler. The index M is the ratio of input to output bandwidth of the filtering process. When M is large, the filtering is often performed in two sub-filters: a cascade integrator comb (CIC) filter that performs most of the down sampling followed by a pair of half-band filters that perform spectral correction, some final bandwidth reduction, and the remainder of the down-sampling. The attraction of the CIC filter is that it performs the filtering without multiplies. In this paper we present two alternate filter structures that offer significant computational advantages over the conventional CIC based DDC. These architectures offer the minimum power implementation of a DDC and likely will find great value in battery operated radio receivers. 相似文献
113.
Winkler PM Vrtala A Steiner G Wimmer D Vehkamäki H Lehtinen KE Reischl GP Kulmala M Wagner PE 《Physical review letters》2012,108(8):085701
First order phase transitions involve nucleation, formation of nanoscale regions of a new phase within a metastable parent phase. Using the heterogeneous nucleation theorem we show how clusters formed by nucleation on single molecules evolve from the gas phase and determine the critical size beyond which condensation starts to form aerosol particles. Our experiments reveal the activation of molecules into droplets to happen via formation of critical clusters substantially larger than the seed molecule. The nanosized critical clusters were found to be well predicted by the Kelvin-Thomson relation pointing directly to the key step in the phase transition. 相似文献
114.
We study the quantization of two versions of unimodular gravity, namely fully diffeomorphism-invariant unimodular gravity and unimodular gravity with fixed metric determinant, utilizing standard path integral approach. We derive the BRST symmetry of effective actions corresponding to several relevant gauge conditions. We observe that for some gauge conditions, the restricted gauge structure may complicate the formulation and effective actions, in particular, if the chosen gauge conditions involve the canonical momentum conjugate to the induced metric on the spatial hypersurface. The BRST symmetry is extended further to the finite field-dependent BRST transformation, in order to establish the mapping between different gauge conditions in each of the two versions of unimodular gravity. 相似文献
115.
Purpose
To evaluate the liver-to-muscle signal intensity and R2* methods to gain a transferable, clinical application for liver iron measurement.Materials and Methods
Sixteen liver phantoms and 33 human subjects were examined using three 1.5-T MRI scanners from two different vendors. Phantom-to-muscle and liver-to-muscle signal intensity ratios were analyzed to determine MRI estimated phantom and hepatic iron concentration (M-PIC and M-HIC, respectively). R2* was calculated for the phantoms and the liver of human subjects. Seven patients' biochemical hepatic iron concentration was obtained.Results
M-PIC and R2* results of three scanners correlated linearly to phantom iron concentrations (r=0.984 to 0.989 and r=0.972 to 0.981, respectively), and no significant difference between the scanners was found (P=.482 and P=.846, respectively) in vitro. The patients' R2* correlated linearly to M-HIC of the standard scanner (r=0.981). M-HIC values did not differ from those obtained from the biopsy specimens (P=.230). The difference in M-HIC was significant, but the difference in R2* was not significant between the scanners (P<.0001 and P=.505, respectively) in vivo.Conclusion
Both methods, M-HIC and R2*, are reliable iron concentration indicators with linear dependence on iron concentration in vivo and in vitro. The R2* method was found to be comparable among different scanners. Transferability testing is needed for the use of the methods at various scanners. 相似文献116.
Markku Talja Mika Polamo Markku Leskel 《Journal of molecular catalysis. A, Chemical》2008,280(1-2):102-105
Four titanium complexes derived from 2-(2-ethylanilino)-, 2-(3,5-dimethylanilino)pyridine, 2-(4-n-butylanilino)- and 2-(2-t-butylanilino)pyridine were synthesized and characterized by spectroscopic methods. These complexes were used to catalyze the polymerization of ethylene in the presence of MAO as cocatalyst. The effect of the complex structures on the polymerization behavior was studied. All the alkylphenylaminopyridinato titanium complexes used in this study yielded higher molar masses than the unsubstituted bis(phenylaminopyridinato) titanium dichloride complex. Correspondingly, activities were lower and molar mass distributions were broader than those in the case of the unsubstituted bis(phenylaminopyridinato) titanium catalyst. The fluxional behavior of alkylphenylaminopyridinato titanium catalysts is probably the reason for the broad molar mass distributions. This might be due to the electron-donating effect from the alkyl substituent because the alkyl substituent enhances the active site isomerization rate. 相似文献
117.
Milja S. Hannu-Kuure Raija Oilunkaniemi Markku Ahlgrén 《Journal of organometallic chemistry》2003,687(2):538-544
In addition to well-known dinuclear phenylselenolato palladium complexes, the reaction of [PdCl2(PPh3)2] and NaSePh affords small amounts of novel trinuclear and hexanuclear complexes [Pd3Se(SePh)3(PPh3)3]Cl (1) and [Pd6Cl2Se4(SePh)2(PPh3)6] (2). Complex 1 is triclinic, P1?, a=13.6310(2), b=16.2596(2), c=16.9899(3) Å, α=83.1738(5), β=78.9882(5), γ=78.7635(5)°. Complex 2 is monoclinic, C2/c, a=25.7165(9), b=17.6426(8), c=27.9151(14) Å, β=110.513(2)°. There are no structural forerunners for 1, but the hexanuclear complex 2 is isostructural with [Pd6Cl2Te4(TeR)2(PPh3)6] (R=Ph, C4H3S) that have been observed as one of the products in the oxidative addition of R2Te2 to [Pd(PPh3)4]. Mononuclear palladium complexes may play a significant role as building blocks in the formation of the polynuclear complexes. 相似文献
118.
Enrique Colacio Jose Manuel Dominguez Vera Jose Maria Moreno Jose Ruiz Markku R. Sundberg 《Transition Metal Chemistry》1992,17(5):397-400
Two new imidazolate-bridged dinuclear CuII complexes, Na[Cu2(L1)2(im)] and K[Cu2(L2)2(im)] (where H2L1=6-amino-1,3-dimethyl-5(2-carboxyphenyl)azouracil and H2L2=1,3-dimethyl-5(2-carboxyphenyl)azobarbituric acid) have been prepared and characterized by magnetic susceptibility and spectroscopic
measurements. Both compounds exhibit the expected antiferromagnetic behaviour with 2J=−54.8 and −30.4 cm−1, respectively. Because of the lack of suitable crystals for single crystal X-ray analysis, we have calculated the lowest
energy structures using a program based on the MMX force field. On the basis of the results, the magnitude of the magnetic
interaction is discussed.
TMC 2623 相似文献
119.
Salvador Dueñas Helena Castán Héctor García Alfonso Gómez Luis Bailón Kaupo Kukli Jaakko Niinistö Mikko Ritala Markku Leskelä 《Microelectronic Engineering》2009,86(7-9):1689-1691
Electrical characterization of zirconium oxide (ZrO2) based metal-oxide-semiconductor (MOS) structures has been carried out. ZrO2 films have been atomic layer deposited (ALD) by using novel cyclopentadienyl-based precursors, which have recently revealed themselves as very adequate in terms of thermal stability and high permittivity of the dielectrics deposited. Our results demonstrate good quality of the films, especially when mixed alkylamido-cyclopentadienyl precursors are used on SiO2/Si substrates. Conduction mechanisms in these MIS capacitors were studied, with moderately or highly-doped silicon used as substrate. 相似文献
120.
Stable operation of a cw optical parametric oscillator near the signal-idler degeneracy 总被引:1,自引:0,他引:1
The frequency stability of a cw optical parametric oscillator (cw OPO) near the signal-idler degeneracy has been studied. The strong tendency of a near-degenerate OPO to mode hop has been suppressed by using a bulk Bragg grating as a spectral filter in the OPO cavity. An experimental demonstration of stable parametric oscillation in a single longitudinal mode of the OPO cavity is reported, together with the capability of tuning the signal-idler difference frequency from 1 to 4 THz. The OPO has potential use in cw terahertz generation. 相似文献