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1.
M. Georgelin A. Pocheau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):169-174
The polycrystalline perovskitelike manganese oxides La1-xAxMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation
state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates
for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie
temperature.
Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998 相似文献
2.
Chemical techniques were employed to synthesize CdS nanoparticles embedded in polymer (PEG 300) and sol-gel silica matrices. Systematic growth of particles (radius 3–9 nm) was obtained by adjusting post-deposition annealing temperature and time to examine the dependence of surface-state–related luminescence on particle size. Photoluminescence (PL) peak energy showed a linear dependence with a gentle slope in the weak confinement region and a steep slope in the strong confinement region, the divergence being observed near the excitonic Bohr radius for CdS. The empirical relation proposed for the weak confinement region could be used for estimating chemically prepared CdS nanoparticle size with a high degree of reliability from PL peak energy. 相似文献
3.
Summary The normal incidence reflectivity spectrum of excitons in GaAs/Ga1−x
Al
x
As multiple quantum wells is calculated within the local response approximation. It is shown that the reflectivity lineshape
strongly depends on the sample geometry. Using realistic parameters of a multiple quantum well structure, we obtain an excellent
fit of the experimental reflectivity curve, thus giving exciton energies, oscillator strengths and exciton broadening parameters.
Riassunto Lo spettro di riflettività ad incidenza normale degli eccitoni nei pozzi quantistici multipli di GaAs/Ga1−x Al x As è calcolato nell’ambito dell’approssimazione locale. Si mostra con la forma della linea di riflettività dipende in gran misura dalla forma geometrica del campione. Usando parametri realistici di una struttura a pozzi quantistici multipli, si ottiene un’ottima approssimazione della curva di riflettività sperimentale, che fornisce così energie eccitoniche, forze dell’oscillatore e parameri di ampliamento eccitonico.
Резюме В рамках приближения линейного отклика вычисляется спекрт отражательной способности экситонов при нормальном падении для множественных квантовых ям в GaAs/Ga1−x Al x As. Показывается, что форма линии отражательной способности сильно зависит от геометрии образца. Используя реалистические параметры структуры множественных квантовых ям, мы получаем хорошее соответствие с экспериментальной кривой для отражательной способности. Получаются энергии экситонов, силы осцилляторов и парам⪟тры экситонного уширения.相似文献
4.
Jeffrey N Brown 《Analytica chimica acta》2003,486(2):159-169
A new application of reversed-phase octadecyl (C18) solid phase extraction disks has been developed to separate the colloidally-associated polycyclic aromatic hydrocarbons (PAHs) from those that were truly dissolved in the samples of fresh water. A correction for the retention of small amounts of colloidal material on the C18 disks was required, which would have otherwise lead to minor underestimates in the degree of partitioning between the two phases. Using the humic substance Aldrich Humic Acid (AHA) as a model colloid and the 16 PAHs on the US Enrivonmental Protection Agency priority pollutant list, the partitioning coefficients of the PAHs between the colloidal and truly dissolved phases were shown to be proportional to the hydrophobicity of the PAHs, as measured by their octanol water partition coefficients (Kow). The values for the partition coefficients obtained (cKdoc′) were similar to those previously reported in the literature using alternative methods, confirming that the technique was producing acceptable results. The technique allows the in situ partitioning of PAHs between the truly dissolved and colloidal phases in fresh water bodies to be determined. It will provide an invaluable cross-check of the laboratory-based methods which often require substantial manipulation of the sample and potentially alter the partitioning between the phases. 相似文献
5.
The enthalpies and free energies of proton affinity (PA) were calculated by the PM3 method for 27 thiophenoxide anions. For thiophenols, linear correlation has been found between the PAs and the values of pK
a
measured by different authors in aqueous acetone, aqueous ethanol, and methanol media. The dependences found permit one to predict a priori the protolytic properties of thiophenols. The differential effects of aqueous ethanol media with different ethanol contents on thiophenol acidity were evaluated based on quantum-chemical data.Original Russian Text Copyright © 2004 by A. N. Pankratov and A. V. Shalabai__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 800–806, September–October, 2004. 相似文献
6.
本文报道综合利用长叶烯(1)催化异构化为异长叶烯(2)及合成系列香料的方法;通过氧化反应合成不同香型烯醇酯类等;Prins反应合成系列化合物。鉴定了10种香料化合物的香型。 相似文献
7.
Anna Rizzolo Patrizia Lombardi Maristella Vanoli Stefano Polesello 《Journal of separation science》1995,18(5):309-314
Capillary GC/sensory analysis was used to judge if dynamic headspace on sliced pulp and on intact fruit, and solvent extraction could collect the “character impact” and the “contributory” aroma compounds in peaches. Capillary GC/sensory data showed that the headspace techniques selectively recovered the “contributory” volatile compounds, which are strictly related to the characteristic odor of the various peach cultivars, whereas solvent extraction better quantified the “character impact” compounds (lactones). 相似文献
8.
L. N. Vakhitova A. V. Skrypka K. V. Bogutskaya N. A. Taran A. F. Popov 《Theoretical and Experimental Chemistry》2007,43(6):389-395
The reactivity of hydroxide and peroxide anions with 4-nitrophenyl N,N-dimethylcarbamate in aqueous-alcoholic detergent media
and in “oil in water” microemulsions has been studied. Aqueous-alcoholic solutions of cationic (CTAB) and neutral (Triton
X-100) surface active substances have promise for decomposition of substrates by a nucleophilic mechanism.
__________
Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 6, pp. 358–363, November–December, 2007. 相似文献
9.
Studies on the complexation of copper(II) by phenolic acids, as ligand models of humic substances were done by potentiometry. The acids under study were: 3,4-dihydroxyhydrocinnamic acid or hydrocaffeic acid (1), 3,4-dihydroxyphenylacetic acid (2) and 3,4-dihydroxybenzoic acid or protocatechuic acid (3). Acidity constants of the ligands and the formation constants of metal-ligand complexes were evaluated by computer programs. The carboxylic group of the phenolic acids has different pKa1 values, being the dissociation constants intrinsically related with the distance between the function and the aromatic nucleus. The results obtained allow concluding that acidity constants of the catechol moiety of the compounds are similar with pKa2 and pKa3 values between 9.47-9.41 and 11.55-11.70. The complexation properties of the three ligands towards copper(II) ion are quite similar, being the species found not different either in nature or stability. Although the model ligands have some structural differences no significant differences were found in their complexation properties towards copper(II). So, it can be postulated that complexation process is intrinsically related with the presence of a catechol group. 相似文献
10.
Pectinic substances from the aerial part of Mentha x piperita were isolated and characterized and found to be a mixture of β-(1→4)-glucogalactan (MPG) and two α-(1→4)-rhamnopolygalacturonans (MPP′-1 and MPP′-2). It was shown that the pectin and its components exhibited membrane-stabilizing,
antiatherogenic, and antioxidant activity. Hemicelluloses from M. piperita were a lignocarbohydrate complex.
__________
Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 411–416, September–October, 2007. 相似文献