全文获取类型
收费全文 | 175篇 |
免费 | 11篇 |
专业分类
化学 | 107篇 |
晶体学 | 14篇 |
力学 | 3篇 |
数学 | 24篇 |
物理学 | 38篇 |
出版年
2024年 | 4篇 |
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 4篇 |
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 7篇 |
2017年 | 3篇 |
2016年 | 10篇 |
2015年 | 9篇 |
2014年 | 8篇 |
2013年 | 17篇 |
2012年 | 19篇 |
2011年 | 15篇 |
2010年 | 16篇 |
2009年 | 7篇 |
2008年 | 17篇 |
2007年 | 3篇 |
2006年 | 8篇 |
2005年 | 10篇 |
2004年 | 6篇 |
2003年 | 1篇 |
2002年 | 1篇 |
2001年 | 1篇 |
1998年 | 3篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1989年 | 1篇 |
1986年 | 1篇 |
排序方式: 共有186条查询结果,搜索用时 0 毫秒
61.
Radulović N Blagojević PD Rabbitt K Menezes Fde S 《Natural product communications》2011,6(7):1015-1022
Analysis (GC and GC/MS) of an essential oil sample obtained from dry leaves of Nepeta x faassenii Bergmans ex Steam, a hybrid species produced by crossbreeding N. mussinii Spreng. with N. nepetella L., led to the identification of 109 constituents that represented 95.9% of the oil. The major constituents were 4aalpha,7alpha,7aalpha-nepetalactone (67.8%), 1,8-cineole (6.6%), germacrene D (4.8%), beta-pinene (2.7%), (E)-beta-ocimene (2.6%), 4aalpha,7beta,7aalpha-nepetalactone (2.3%) and (E)-beta-farnesene (1.0%). Chemical composition of the oil was compared, using multivariate statistical analyses (MVA) with those of the oils of other Nepeta taxa, in particular N. mussinii and N. nepetella. This was done in order to explore the mode of inheritance of the monoterpene biosynthetic apparatus of N. faassenii. Chemical composition of the volatiles of a Nepeta taxon (different populations) can be subject to variation due to environmental and geographical factors. To accommodate this fact in the MVAs, along side with N. faassenii essential oil, additional 6 oils (3 different populations of N. nuda L. and N. cataria L. from Serbia) were included in this study (isolated and analyzed (chemically and statistically)). The MVA analyses recognized N. faassenii as being closely related to both N. mussinii and N. nepetella. If the relative content of oil constituents per plant and not per chromatogram were used as variables in the MVA (this was done by simple multiplication of the yields and relative percentages of components) a higher degree of mutual similarity (in respect to the monoterpene biosynthesis) of N. faassenii to N. mussinii, than to the other parent species, was observed. 相似文献
62.
N Fischer K Hüll TM Klapötke J Stierstorfer G Laus M Hummel C Froschauer K Wurst H Schottenberger 《Dalton transactions (Cambridge, England : 2003)》2012,41(36):11201-11211
A modification of the synthesis of sodium 5,5'-azotetrazolate pentahydrate, described by Thiele in 1898, yields the unknown and unexpected corresponding 5N-oxido derivative sodium 5,5'-azoxybistetrazolate pentahydrate (Na(2)zTO·5H(2)O, ). Purification was achieved by recrystallization based on the better solubility of Na(2)zTO·5H(2)O in water. Different nitrogen-rich salts, such as the diammonium (), the dihydroxylammonium (), the bis-diaminoguanidinium (), the bis-triaminoguanidinium () and the diaminouronium salt (), have been prepared using metathesis reactions starting from barium 5,5'-azoxybistetrazolate pentahydrate () and ammonium, hydroxylammonium, diaminoguanidinium or diaminouronium sulfate and triaminoguanidinium chloride, respectively. The nitrogen rich azoxy-derivatives were characterized using NMR, IR and Raman spectroscopy, mass spectrometry and elemental analysis. Additionally the solid state structures of , , and were determined by single crystal X-ray diffraction. The heats of formation of and and their corresponding azo-tetrazolate derivatives were calculated by the atomization method based on CBS-4M enthalpies. With these values and the crystal densities, several detonation parameters such as the detonation velocity, detonation pressure and specific impulse were calculated (EXPLO5) and compared. The sensitivities towards shock (BAM drophammer), friction (BAM friction tester) and electrostatic discharge of the described compounds were determined. 相似文献
63.
Molecular assays such as immunoassays are often performed using solid carriers and fluorescent labels. In such an assay format
a question can be raised on how much the fluorescence of the label is influenced by the bio-affinity binding events and the
solid carrier surface. Since changes in fluorescence intensity as labels bind to surfaces are notoriously difficult to quantify
other approaches are preferred. A good indicator, independent of the fluorescence intensity of the label, is the fluorescence
lifetime of the marker fluorophore. Changes in fluorescence lifetime reliably indicate the presence of dynamic quenching,
energy transfer or other de-excitation processes. A microsphere based assay system is studied under two-photon excitation.
Changes in fluorescence lifetime are studied as labeled protein conjugates bind on microsphere surfaces – both direct on the
surface and with a few nanometer distance from the surface. Fluorescence signal is measured from individual polystyrene microspheres
and the fluorescence lifetime histogram is simultaneously recorded. The results indicate that self-quenching and quenching
by the polystyrene surface are both present in such a system. However, the effect of the surface can be avoided by increasing
the distance between the surface and the label. Typical distances achieved by a standard sandwich type of assay, are already
sufficient to overcome the surface induced quenching in fluorescence detection. 相似文献
64.
Nanocomposite samples of poly(methyl methacrylate) (PMMA) polymer with a γ-Fe2O3 (maghemite) filler have been synthesised and studied by ferromagnetic resonance (FMR) and differential scanning calorimetry (DSC) methods. Two types of samples have been investigated: containing 5 wt%, 10 wt% of a maghemite filler. DSC measurements have revealed an increase in the glass transition temperature Tg and a decrease in the heat capacity cp with an increase in the concentration of nanoparticles. A FMR study in the 4–300 K temperature range has shown the presence of an asymmetric spectrum that has been analyzed in terms of two Gaussian-shaped components arising from the assumed magnetic anisotropy of the nanoparticles. The FMR investigation has exposed the temperature range of a superparamagnetic regime (60–290 K) and the blocking temperature of TB ~ 60 K. In that range a shift in the resonance line δHr and the linewidth ΔH is related by δHr ~ (ΔH)n, where n = 2.79 indicates a fair amount of disorder in the maghemite system. An analysis of the FMR spectra in terms of two component lines has shown the importance of the dipole–dipole interaction for higher concentrations of maghemite and for T > 220 K. 相似文献
65.
A multicomponent vanadate compound Zn2InV3O11 has been synthesized and investigated using the electron paramagnetic resonance (EPR) technique. The Zn2InV3O11 compound is isostructural with a previously studied magnetic system M2FeV3O11 (M = Mg, Zn, Co, Ni). According to the nominal stoichiometry of the Zn2InV3O11 compound all ions are non-magnetic, but the registered EPR spectra in the 4–300 K temperature range have revealed the presence of a rich variety of paramagnetic centers involving predominantly monomeric and dimeric units, as well as clusters of V4+ ions. The most intense EPR component, attributed to the VO2+ vanadyl ion in an axial symmetry, has displayed a well resolved hyperfine structure. The presence of a complicated spectrum at low magnetic field (g 4.4) and a broad line at g 2 has confirmed the existence of vanadium dimers. A spectral feature attributed to vanadyl spin clusters has been observed at temperatures above 80 K in the 270–310 mT magnetic field range. The role of oxygen deficiency in the appearance of the observed magnetic centers has been discussed. Comparison with the previously studied Mg2InV3O11?δ compound has been made and the influence of specific cations on magnetic defect centers has been considered. 相似文献
66.
Niko Naumann 《Advances in Mathematics》2007,215(2):569-600
We construct the algebraic stack of formal groups and use it to provide a new perspective onto a recent result of M. Hovey and N. Strickland on comodule categories for Landweber exact algebras. This leads to a geometric understanding of their results as well as to a generalisation. 相似文献
67.
68.
N. Guskos G. Zolnierkiewicz A. Guskos J. Typek J. Blyszko W. Kiernozycki U. Narkiewicz M. Podsiadly 《Journal of Non》2008,354(35-39):4510-4514
Samples of micro-silica/cement containing iron oxide, Fe2O3, and doped with carbon-coated cobalt nanoparticles and free radical DPPH were prepared and studied by the magnetic resonance method. The concrete’s main components (silica and cement) produced very complicated FMR/EPR (ferromagnetic and electron paramagnetic resonance) spectra. The temperature dependence of the FMR/EPR spectra was recorded in the 90–300 K temperature range. The cement/micro-silica matrix produced a very broad FMR line originating from iron oxide particles and two EPR lines originating from iron(III) ions in the crystal field of low-symmetry (centered at geff ~ 4.3) and from manganese(II) ions (geff ~ 2) of hyperfine structure. Additionally, a very narrow line and a very broad EPR/FMR line were registered and, respectively, attributed to DPPH and cobalt nanoparticles. The isolated paramagnetic iron(III) and manganese(II) centers displayed increasing intensity of the EPR spectra with decreasing temperature, while no influence of the magnetic nanoparticles was observed. The intensity of the FMR spectrum of iron oxide decreased strongly and the resonance field was effectively shifted toward low magnetic fields with decreasing temperature. The observed FMR behavior is similar to what was registered for iron oxide magnetic nanoparticles. The introduction of magnetic nanoparticles influenced the EPR spectrum of the free radical DPPH significantly: its intensity decreased above 260 K and increased slightly below this temperature, while the resonance field changed with decreasing temperature. This behavior may be associated with the porous state of cement and/or the reaction of the multi-component magnetic system. The FMR/EPR method could be very useful for the characterization of matrices containing small amounts of magnetic nanoparticles. 相似文献
69.
Niko Tratnik 《International journal of quantum chemistry》2019,119(21):e26006
The weighted (edge-)Szeged index and the weighted (vertex-)PI index are modifications of the (edge-)Szeged index and the (vertex-)PI index, respectively, because they take into account also the vertex degrees. As the main result of this article, we prove that if G is a connected graph, then all these indices can be computed in terms of the corresponding indices of weighted quotient graphs with respect to a partition of the edge set that is coarser than the Θ*-partition. If G is a benzenoid system or a phenylene, then it is possible to choose a partition of the edge set in such a way that the quotient graphs are trees. As a consequence, it is shown that for a benzenoid system, the mentioned indices can be computed in sublinear time with respect to the number of vertices. Moreover, closed formulas for linear phenylenes are also deduced. 相似文献
70.