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81.
Kenji Fujito Shuichi Kubo Hirobumi Nagaoka Tae Mochizuki Hideo Namita Satoru Nagao 《Journal of Crystal Growth》2009,311(10):3011-3014
We succeeded in preparing very thick c-plane bulk gallium nitride (GaN) crystals grown by hydride vapor phase epitaxy. Growth of the bulk GaN crystals was performed on templates with 3 μm GaN layer grown by metal organic chemical vapor deposition on (0 0 0 1) sapphire substrates. Colorless freestanding bulk GaN crystals were obtained through self-separation processes. The crystal's diameter and thickness were about 52 and 5.8 mm, respectively. No surface pits were observed within an area of 46 mm diameter of the bulk GaN crystal. The dislocation density decreased with growth direction (from N-face side to Ga-face side) and ranged from 5.1×106 cm−2 near the N-face surface to 1.2×106 cm−2 near the Ga-face. A major impurity was Si, and other impurities (O, C, Cl, H, Fe, Ni and Cr) were near or below the detection limits by SIMS measurements. 相似文献
82.
Abstract— Results concerning absorption-emission spectra and fluorescence quantum yields at 77 and 298K. triplet absorption spectra, and quantum yields of intersystem crossing and photoisomerization at 298 K, are presented for 11- cis β-apo-14'-carotenal (C22 -Ald), the immediate higher homologue of 11- cis retinal. The absorption spectra are characterized by two band-systems with maxima at 390–400 and 270–280 nm, respectively. Upon cooling from 298 to 77 K, the intensities of these two band-systems undergo changes in opposite directions indicating 12-s- cis: 12- s-trans conformational changes. No intermediate band-system analogous to the one located at 270–310 nm in cis retinals is observed for 11- cis C22 -Ald. In nonpolar hydrocarbon solvents (e. g. cyclohexane) at room temperature, quantum yields of fluorescence (0.01), intersystem crossing (0.6) and photoisomerization (0.4) are all quite pronounced. The photophysical and photochemical properties of 11- cis C22 -Ald are discussed in the light of similarities and dissimilarities with those of all- trans C22 -Ald and 11- cis retinal under comparable conditions. 相似文献
83.
Deo P Deo N Somasundaran P 《Langmuir : the ACS journal of surfaces and colloids》2005,21(22):9998-10003
Interactions of surfactants with hydrophobically modified polyelectrolytes in aqueous solutions are important in several applications such as detergents, cosmetics, foods, and paints. Fundamental questions arise on the mechanisms of complexation of the polyelectrolyte and surfactant that control their behavior. In this work, the complexation was studied by examining interactions in aqueous solutions of a hydrophobically modified polymer, poly(maleic acid/octyl vinyl ether) (PMAOVE), with sodium dodecyl sulfate (SDS) by monitoring viscosity, pyrene solubility, light scattering, and analytical ultracentrifugation. When the anionic surfactant SDS was added to aqueous solutions of the similarly charged polymer PMAOVE, the surfactant was incorporated into the hydrophobic nanodomains of PMAOVE even far below the cmc of the surfactant. On the basis of viscosity, pyrene solubility, and analytical ultracentrifugation data, it is proposed that PMAOVE undergoes structural unfolding and at higher SDS concentrations mixed micelles are formed. 相似文献
84.
Abstract— The spectra of absorption, fluorescence and excitation of monolayers and thin films containing chlorophyll a together with a carotenoid (cis-β-carotene, trans-β-carotene, fucoxanthin, or zeaxanthin), were measured at — 196°C. The concentration ratios used, (Chl)/(Car), were 6:1, 4:1, 3:1, 2:1, 1:1 and 1:3, and the area densities, 3·70, 2·55, 1·76, 0·71, 0·37 and 0·17 nm2/pigment molecule. In dilute monolayers, (3·70 nm2/molecule), with a constant concentration ratio (Chl)/(Car) = 3:1, evidence of three β-carotene forms, with absorption bands at 460, 500 and 520 nm (C460, C500 and C520), and of a chlorophyll a form with an absorption band at 669–672 (Chl669–672) was found. On increasing the density to 0·2–0·3 nm2/molecule, a conversion of C460 and C520 into C500, was observed, and several more additional (probably more strongly aggregated) chlorophyll a forms appeared, with absorption bands at 672–733 nm. With excess carotene [(Chi)/(Car) = 1:3] the forms C460, C500, C520 and Chl669–672 were present even in the most dense films (0·2–0·3 nm2/molecule). The same was found with other carotenoids: if one of the pigments was in excess, aggregated forms of the other tended to disappear. In the transfer of energy from carotenoids to chlorophyll a, C500 was found to be the main donor. In layers with a concentration ratio (Chl)/(Car) = 3:1, the efficiency of transfer was less than 10 per cent at the lowest density used (3·70 nm2/molecule); it increased to 50 per cent, as the density was increased to 0·20 nm2/molecule. When the relative concentration of the carotenoid was increased to (Chl)/(Car) = 1:1, the efficiency of energy transfer dropped to 25 per cent even at 0·20 nm2/molecule. It seems that the efficiency of energy transfer between carotene molecules (prior to its transfer to chlorophyll a) is low, and effective transfer occurs only between β-carotene and immediately adjacent chlorophyll a molecules. 相似文献
85.
Some bis(cyclopentadienyl) complexes of Ti(IV)and Zr(IV) have been synthesized by the metathetical reactions of Cp2MCl2 with the sodium salt of monofunctional bidentate Schiff's base ligands in 1:1 and 1:2 stoichiometry in anhydrous THF. All the derivatives of the types Cp2MCl2?n{O(C6H4)CH6H5}n and Cp2MCl2?n{OC(CH3)(R)6H5}n (where R = CH3 or C6H5 and n = 1 or 2) have been characterized by elemental analyses, molecular weight measurements, FT‐IR and ‐NMR [1H and 13C{1H}] studies. The FAB mass spectra of a few representative complexes and the molecular weight measurements in freezing benzene indicate their monomeric nature in solution. Spectral studies appear to indicate mono‐ and bi‐dentate mode of attachment of the ligand in solution and in the solid state, respectively. Thermogravimetric analyses (TGA) of Cp2TiCl{O(C6H4)CH} ( 1 ) and Cp2ZrCl{O(C6H4)CH} ( 7 ) indicate their low thermal stability. Transformations of some of these complexes to pure titania (TiO2)/zirconia (ZrO2) have been observed at relatively low sintering temperatures. Powder X‐ray diffraction (XRD) and scanning electron micrograph (SEM) studies suggest the nanocrystalline nature of these materials. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
86.
Surya Subianto Namita Roy Choudhury Naba K. Dutta 《Journal of polymer science. Part A, Polymer chemistry》2008,46(16):5431-5441
Phosphonic acid‐bearing styrene–ethylene/butylene–styrene (SEBS) block copolymer was synthesized by bromination and subsequent palladium‐catalyzed phosphonation of SEBS. The phosphonated block copolymer was characterized by spectroscopic, thermal, and conductivity measurements. The new polymer shows good ion‐exchange capacity of ~0.7 meq/g and proton conductivity of around 2–4 mS/cm (at room temperature and 100% relative humidity) which is in good agreement with literature value of other phosphonated materials. This value was obtained despite a relatively low degree of phosphonation, demonstrating the ability of the phase separated nature of block copolymers to promote proton conductivity. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5431–5441, 2008 相似文献
87.
The Kernel of a Hankel Operator on the Bergman Space 总被引:1,自引:0,他引:1
In this paper we characterise the kernel of a little Hankeloperator on the Bergman space in terms of the inner divisors, and obtain a characterisationfor finite rank little Hankel operators using the invariantsubspace theory technique. 1991 Mathematics Subject Classification47B35. 相似文献
88.
Uranium is reported to be present in phosphate fertilizers. The recovery of uranium from the fertilizers is important because
it can be used as fuel in nuclear reactors and also because of environmental concerns. For both these activities suitable
method of uranium determinations at trace levels in these fertilizers are required. Studies have been initiated for such TXRF
determination of uranium and the results are reported in the present paper. For TXRF determinations the fertilizer samples
were processed with nitric acid and the uranium present in it was removed by solvent extraction using tri-n-butyl phosphate
as the extractant. The organic phase containing uranium was equilibrated with 1.5% suprapure nitric acid to bring out uranium
in aqueous phase. This aqueous phase was mixed with internal standard Y and the TXRF spectra were measured by depositing samples
on float glass supports. The amounts of uranium in four fertilizer samples of Hungarian origin were determined by processing
these TXRF spectra. Uranium concentrations in two fertilizer samples were found to be in the range of 4–6 μg/g, whereas two fertilizer samples did not show the presence of uranium. The precision of the TXRF determination of uranium
was found to be better than 8% (1σ). 相似文献
89.