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81.
The n.m.r. spectra of triterpenoids related to 12-ursene have been examined using the shift reagent Eu(fod)3. The assignment of all the methyl resonances has been achieved for ursolic acid derivatives and the corresponding compounds of the 3-epi series, which have not been investigated previously. The additive shielding effects of various substituents on the methyl resonance frequencies have been deduced from the results. 相似文献
82.
Electroluminescence has been observed in p-type CuGaSe2 single crystals. At room temperature these crystals exhibit a resistivity of about 10?2 Ω cm. The electrominescence has been obtained by minority carrier injection from an indium electrode. Two peaks have been identified in the emission spectrum whose energies are 1.53 and 1.59 eV, respectively. These peaks could be caused by the recombination of injected minority carriers in acceptor centers which are localized in the forbidden gap near the valence band. 相似文献
83.
La Rivista del Nuovo Cimento (1971-1977) - 相似文献
84.
85.
Solid-phase microextraction and gas chromatography–mass spectrometry for rapid characterisation of semi-hard cheeses 总被引:1,自引:0,他引:1
Verzera A Ziino M Condurso C Romeo V Zappalà M 《Analytical and bioanalytical chemistry》2004,380(7-8):930-936
A headspace solid-phase microextraction (HS-SPME) method in combination with gas chromatography–mass spectrometry (GC–MS) has been used for extraction and identification of components of the volatile fraction of Provola dei Nebrodi, a typical semi-hard Sicilian cheese. Cheese samples from different producers and at different ripening stages have been examined. The effects of various conditions (e.g. sample volume, sample headspace volume, sample heating temperature, extraction time, etc.) on extraction efficiency were studied in order to optimise the technique. The technique used made it possible to identify 61 components: fatty acids from C2 to C14 and their esters, aldehydes, alcohols, methyl ketones, -lactones, aromatic compounds, hydrocarbons and terpenes. The main components were butanoic, hexanoic and octanoic acids. The linear free fatty acids (FFA) from C2 to C10 were quantified by using the standard addition method. Calibration curves constructed for the FFA spiked into cheese were highly linear with a correlation coefficient R2>0.998. Significant statistical differences (P0.05) were evident for the even-carbon-number fatty acids during ripening. 相似文献
86.
Michelangelo?ConfortiEmail author Romeo?Rizzi 《4OR: A Quarterly Journal of Operations Research》2004,2(2):153-159
We review the recent three-volume monograph authored by Alexander Schrijver, Combinatorial Optimization - Polyhedra and Efficiency, Springer-Verlag, 2003, ISBN 3-540-44389-4, 1881 pages (in a slip-case), price: € 89,95 .Received: November 2003, Revised: January 2004, AMS classification:
90C57, 68R10, 05C99 相似文献
87.
M. Romeo 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》2001,77(3):730-748
We derive a solution to the problem of shear horizontal electroacoustic surface waves in a piezoelectric half-space. We formulate a time-domain dynamic problem accounting for time dispersion for both electric and elastic fields and use a separation of variables to express the solution in terms of a wave propagator. Transient surface waves of the B-G type are found to propagate with a constant speed and exponentially decay in space. Their amplitude vanishes at large distances from the boundary as the reciprocal of the depth. Dispersive and non dispersive solutions are compared. 相似文献
88.
The linear rheology of PP-TiO2 composites was investigated as a function of the filler particle size. Small amounts (ϕ = 4.5%) of micron and nano-sized TiO2 spherical particles were added to the polymer by melt compounding. In spite of the low particle volume fractions and the weak polymer-filler interactions, relevant changes have been observed in the linear viscoelastic behaviour of the nanostructured formulations. Microstructural analyses performed through electron microscopy have shown a good degree of dispersion of micron-sized particles. Conversely, the presence of a relevant number of TiO2 nanoparticle clusters has been detected for the nanocomposite. The structure of these aggregates seems to be strongly responsible for the rheological behaviour of these last materials in comparison with microstructured ones. Relevant similarities between the viscoelastic properties of nanocomposite and the flow feature of many soft glassy materials have been noticed, and a simple physical picture has been proposed in order to interpret the rheological response of studied nanocomposites. 相似文献
89.
90.
Maurizio Romeo 《Meccanica》1996,31(6):673-686
The scattering of harmonic transverse waves is studied in a layered solid half-space which is prestressed and electrically biased. Electromechanic modes propagating normally to the stratification planes are analyzed using a wave-splitting method and a sufficient condition is derived for the existence of the solution. The invariant imbedding technique is employed to obtain a differential equation for the reflectivity matrix. This equation is exploited to formulate the inverse problem whose solution is presented in an iterative form. A useful result is then established about the convergence of the series so determined.
Sommario Si studia la propagazione di onde armoniche trasversali in un semispazio occupato da un dielettrico elastico stratificato, soggetto a prestress ed elettricamente polarizzato. Si analizzano in particolare i modi elettromeccanici che si propagano normalmente ai piani di stratificazione facendo uso del metodo di wave-splitting e ottenendo una condizione sufficiente per l'esistenza della soluzione. Si riformula poi il problema mediante una equazione di Riccati per la matrice dei coefficienti di riflessione al fine di presentare una soluzione iterativa per il problema inverso. Si dimostra un utile risultato sulla convergenza di questa soluzione.相似文献