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31.
H. L. Luo Viorica Spânu Guan-mian Chen S. Y. Dai X. C. Huang G. Filoti 《Hyperfine Interactions》1990,54(1-4):599-602
The 0.05 (wt%) Co-coated hematite having cigar-shaped particles was investigated by means of Mössbauer effect at temperatures between 77 K and 300 K by ascending and descending temperatures, respectively. The Mössbauer spectra indicated the coexistence of two distinct magnetic phases below 223 K or 232 K, depending on the starting point. The angle between the spin direction and the crystallographic trigonal axis was calculated taking into account the quadrupole splitting values. 相似文献
32.
G. I. Rusu A. Airinei Viorica Hamciuc G. G. Rusu Petronela Râmbu Mihaela Diciu 《Journal of Macromolecular Science: Physics》2013,52(2):238-253
Polysulfone (PSF)-polydimethylsiloxane (PDMS) block copolymers (PSF-b-PDMS) with various compositions were prepared by condensing chloro-terminated polysulfone oligomers with α, ω -dihydrogensilyl-polydimethylsiloxane in refluxing chlorobenzene solution in the presence of urea as a hydrogen chloride acceptor. The temperature dependences of the electrical conductivity and thermoelectric power (Seebeck coefficient) of copolymers were studied using thin films deposited from a dimethylformamide solution (spin coating method) onto glass substrates. It was established that the various copolymers showed typical semiconducting properties. Some correlations between these properties (activation energy, ratio of carrier mobilities, etc) and molecular structure of the copolymers were found. A model based on band gap representation was found to be suitable for explanation of the electronic transport mechanism through the studied copolymers in thin films. The study of the transmission and absorption spectra (in the spectral range from 300 nm to 1400 nm) evidenced that the indirect allowed optical transitions were predominant. The values of the optical energy band gap (ranged between 1.50 eV and 1.70 eV) are in agreement with values of width of the forbidden band calculated from the temperature dependence of the electrical conductivity. 相似文献
33.
Carmen Racles Viorica Gaina Mihai Marcu Maria Cazacu Mihaela Simionescu 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(9):1605-1617
Abstract Organofunctional siloxane oligomers containing maleimide were synthesized by condensation reaction of α. ω-hydroxypropylsiloxane oligomers with n-[4-(chlorocarbonyl)phenyl]maleimide. A model compound [bis(maleimide-ester)disiloxane] was obtained to facilitate the characerization of the oligomers. Then, the heterogeneous catalyzed ring-opening cationic polymerization technique was applied to obtain an oligomer starting from the model compound and octamethylcyclotetrasiloxane. The structures of the resulting oligomers were confirmed by elemental analysis, IR and 1H-NMR spectroscopy. They were characterized by determining softening points, solubility and studying their thermal behavior by thermogravimetric (TGA and differential scanning calorimetry (DSC) measurements. 相似文献
34.
Yan Liu Viorica Lopez-Avila Marcela Alcaraz Werner F. Beckert 《Journal of separation science》1994,17(7):527-536
As part of a continuing evaluation of new analytical and sample preparation techniques conducted by the US Environmental Protection Agency (EPA), the use of capillary gas chromatography with atomic emission detection (GC-AED) for the simultaneous determination of organotin, organolead, and organomercury compounds in environmental samples was investigated. Pentylmagnesium bromide was used to pentylate ionic organotin, organolead, and organomercury compounds; the pentyl derivatives were then separated by GC and determined by AED. Several important GC-AED parameters, including the type of injector inlet, carrier gas flow rate, and helium make-up gas flow rate, were optimized for the simultaneous determination of these organometallic compounds. Their minimum detectable concentrations were approximately 1.0 to 2.5 ng/mL using a 0.5-μL on-column injection. The calibration curves exhibited good linearity between 2.5 and 2500 ng/mL for organotin and organolead compounds, and between 2.5 and 10000 ng/mL for organomercury compounds. 相似文献
35.
Cristofor I. Simionescu Virgil BÎrboiu Bogdan C. Simionescu Viorica Talmaciu Corina Sava 《Journal of polymer science. Part A, Polymer chemistry》1986,24(5):851-860
Bulk radical copolymerization of methyl acrylate (MeA, M1) with styrene (St, M2) in presence and absence of ZnCl2 as complexing agent was studied. 1H-NMR spectra were used to establish copolymer composition and sequence distribution. The methoxy group signal was observed to be split due to pentads, but the analysis of sequence distribution is possible only at triad level. Both composition and sequence distribution data confirmed that bulk radical copolymerization respects quite well the terminal addition model; the values of r1 = 0.14 ± 0.02 (from composition data) and r1 = 0.25 ± 0.03 (from sequence distribution data) and r2 = 0.83 ± 0.10 (from composition data) were found. The presence of ZnCl2 increases the probability of alternating addition, e.g., for [ZnCl2]/[MeA] = 0.2, r1 = 0.03 ± 0.02 and r2 = 0.17 ± 0.03. The radical copolymer obtained in bulk in the absence of ZnCl2 presents a coisotactic configuration with σ = 0.75 ± 0.03, but the presence of the complexing agent reduces the probability of coisotactic addition, e.g., for [ZnCl2]/[MeA] = 0.2, σ = 0.52 ± 0.03. 相似文献
36.
Vasile I Parvulescu Viorica Parvulescu Uwe Endruschat Pascal Granger Ryan Richards 《Chemphyschem》2007,8(5):666-678
Sol‐gel synthesis of silica and silica–tantalum oxide embedded platinum nanoparticles is carried out using Pt colloids as templates. These colloids are prepared by reduction with Na[AlEt3H] and stabilized with different ligands (ammonium halide derivatives, non‐ionic surfactants with polyether chains, and 2‐hydroxy‐propionic acid). The aim of the present study is to prepare mesoporous silica embedded Pt colloids combining the “precursor concept” with the model of catalyst preparation using preformed spheres. Nanoparticles of Pt incorporated in high surface area mesoporous materials are formed after calcination. Further, it is observed that calcination of these catalysts causes partial aggregation and oxidation of the parent colloids, a process that is largely dependent on the nature of the stabilizing ligands. Several methods have been used for characterization of these materials: adsorption‐desorption isotherms at 77 K, H2 chemisorption, X‐ray diffraction(XRD), 29Si and 13C magic angle spinning (MAS) NMR, ammonia diffuse reflectance Fourier transform infrared spectroscopy (NH3‐DRIFT), transmission electron microscopy (TEM), and X‐ray photoelectron spectroscopy (XPS). It is found that both metal oxide systems exhibit Brønsted acidity (weaker for silica and quite strong for silica–tantalum oxide). In addition, NH3‐DRIFT experiments demonstrate the oxidative properties of the surface. Part of the adsorbed NH4+ species is oxidized to N2O. Testing these catalysts in the reduction of NO and NO2 with isopentane under lean conditions indicate that the activity of these catalysts is indeed dependent on the size of the platinum particles, with those of size 8–10 nm demonstrating the best results. The support likely contributes to this effect, particularly after Ta incorporation into silica. 相似文献
37.
Salceanu Daniela-Crina Pincu Elena Bruni Giovanna Marini Amedeo Meltzer Viorica 《Journal of Thermal Analysis and Calorimetry》2018,132(2):1095-1103
Journal of Thermal Analysis and Calorimetry - The physico-chemical characterization of norfloxacin commercial sample (NF), norfloxacin anhydrous form A (NFanhA), norfloxacin pentahydrate... 相似文献
38.
In this study the electron beam (e-beam) radiation synthesis in “paste-like condition” and characterization of the network structure of acrylic acid (AA) sodium salt/xanthan gum (XG)/carboxymethylcellulose (CMC) superabsorbent hydrogels incorporating graphene oxide (GO) was investigated. The effects of the AA concentration on gel fraction, sol-gel analysis, swelling degree and network parameters, as well as the relationship between these parameters and radiation dose was also established.Gel fraction exceeds 90%, and the p0/q0 ratio shows a moderate degradation process. The swelling kinetic data were used to determine, first the swelling degree, second the diffusion characteristics, and third the average molecular weight between crosslinks (Mc), as well as. The diffusion data revealed a Fickian mechanism transport, for each hydrogel compositions. The network parameters (Mc and ξ) increased with absorbed dose, while cross-linking density and the radiation-chemical yields were decreased. 相似文献
39.
As a development of the theory of linear error bounds for lower semicontinuous functions defined on complete metric spaces,
introduced in Azé et al. (Nonlinear Anal 49, 643–670, 2002) and refined in Azé and Corvellec (ESAIM Control Optim Calc Var 10, 409–425, 2004), we propose a similar approach to nonlinear error bounds, based on the notion of strong slope, the variational
principle, and the change-of-metric principle, the latter allowing to obtain sharp estimates for such error bounds through
a reduction to the linear case. 相似文献
40.
In this study was to investigate,by phase-transfer catalysis,the activity of single and mixed ammonium and phosphonium salts grafted on a “gel-type“ stryene-7% divinylbenzene copolymer in the oxidation of benzyl alcohol with hydrogen peroxide.A wide variety of catalysts with different quaternary groups and different quaternary chain length substitutents were examined.The activity of single“onium“salts increases as a consequence of the association of ammonium and phosphonium salts grafted onn the same polymeric support.The activity of polymer-supported ammonium and phosphonium salts increases with the number of carbon atoms contained in the alkyl radicals of the -onium and of the functionalization degree with phosphonium groups. 相似文献