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排序方式: 共有360条查询结果,搜索用时 31 毫秒
71.
Amauri J. Paula Rodrigo Parra Maria A. Zaghete Jos A. Varela 《Solid State Communications》2009,149(39-40):1587-1590
Because of the environmental concerns, the manufacture of ceramics based on lead titanate zirconate [Pb(Zr1−xTix)O3−PZT] has been condemned because of the lead toxicity. In this context, the electromechanical properties of sodium, potassium and lithium niobate [(Na0.5−x/2K0.5−x/2Lix)NbO3−NKLN] at the morphotropic phase boundary granted these materials the position of most suitable candidate to replace PZT. However, the production of these ceramics is rather critical mainly because of a natural tendency of forming secondary phases. To help with the studies of the synthesis of this lead-free piezoceramic, this work presents an evaluation of the crystallization of the (Na0.47K0.47Li0.06)NbO3 phase by solid-state reactions. TG-DTA, XRD, dilatometric and ferroelectric hysteresis analyses indicated that a secondary phase (K3Li2Nb5O15) crystallizes at temperatures above 850 C and also during the sintering of the powders compacts at 1080 C. To prevent the formation of this phase, the addition of Na2Nb2O6.nH2O microfibers obtained through a microwave hydrothermal synthesis was performed in the sintering process. After to this addition, the suppression of the K3Li2Nb5O15 phase occurred and an increase of the NKLN electrical properties was then obtained. 相似文献
72.
C.S. Riccardi R.C. Lima M.L. dos Santos P.R. Bueno J.A. Varela E. Longo 《Solid State Ionics》2009,180(2-3):288-291
Cerium carbonate hydroxide (orthorhombic Ce(OH)CO3) hexagonal-shaped microplates were synthesized by a simple and fast microwave–hydrothermal method at 150 °C for 30 min. Cerium nitrate, urea and cetyltrimethylammonium bromide were used as precursors. Ceria (cubic CeO2) rhombus-shape was obtained by a thermal decomposition oxidation process at 500 °C for 1 h using as- synthesized Ce(OH)CO3. The products were characterized by X-ray powder diffraction, field-emission scanning electron microscopy, thermogravimetric analysis and Fourier transformed infrared spectroscopy. The use of microwave–hydrothermal method allowed to obtain cerium compounds at low temperature and shorter time compared to other synthesis methods. 相似文献
73.
Dicyclohexylcarbodiimide-promoted self-condensation of 2,3:4,5-di-O-isopropylidene-d-galactonic acid (3) led to the macrocyclic oligomeric cyclo[(2,3:4,5-di-O-isopropylidene-(1→6)-d-galactonate)2] (4) and cyclo[(2,3:4,5-di-O-isopropylidene-(1→6)-d-galactonate)3] (5), having, respectively, 14- and 21-membered rings. The macrocycles 4 and 5 were also synthesized by cyclization of the respective linear dimer 11 and trimer 14 ω-hydroxy acids precursors prepared by stepwise additions of 3. Compounds 4 and 5 are biomaterials that may be described as macrolactone-cyclodextrins. 相似文献
74.
D. Morales-Acosta L.G. Arriaga L. Alvarez-Contreras S. Fraire Luna F.J. Rodríguez Varela 《Electrochemistry communications》2009,11(7):1414-1417
Pt–Pd/MWCNT with Pt:Pd atomic ratio 40:60 and Pt/MWCNT electrocatalyst were synthesized and evaluated as oxygen reduction reaction (ORR) cathodes for Direct Ethylene Glycol Fuel Cells (DEGFC) applications. As reference, a commercial Pt/C material was also tested. We found that Pt–Pd/MWCNT has high tolerance capability to EG and higher selectivity for the ORR compared to the Pt-alone materials. As a result, the shift in onset potential for the ORR, Eonset, at Pt–Pd/MWCNT was considerably smaller than the shift at Pt/MWCNT or Pt/C. The average particle size (from XRD) was 3.5 and 4 nm for Pt/MWCNT and Pt–Pd/MWCNT, respectively. A moderate degree of alloying was determined for the Pt–Pd material. An advantageous application of Pt–Pd electrocatalysts should be in DEGFCs. 相似文献
75.
V. Vijayasundaram V. Ramasamy PL. RM. Palaniappan 《Journal of Thermal Analysis and Calorimetry》2009,98(1):183-188
The paper presents the changes in the thermal properties of control, arsenic exposed and DMSA treated Labeo rohita bones by using thermo analytical techniques. The result shows that the mass loss due to the thermal decomposition occurs
in three distinct steps due to loss of water, organic and inorganic materials. The arsenic exposed bones present a different
thermal behaviour compared to the control bones. The residue masses are increased due to arsenic exposure, while the DMSA
treatment reduces the residue mass level. These thermal characteristics can be used as a qualitative method to check the metal
accumulation in samples. 相似文献
76.
Ramiro Varela Camino R. Vela Jorge Puente María Sierra Inés González-Rodríguez 《Annals of Operations Research》2009,166(1):125-146
We confront a practical cutting stock problem from a production plant of plastic rolls. The problem is a variant of the well-known
one dimensional cutting stock, with particular constraints and optimization criteria defined by the experts of the company.
We start by giving a problem formulation in which optimization criteria have been considered in linear hierarchy according
to expert preferences, and then propose a heuristic solution based on a GRASP algorithm. The generation phase of this algorithm
solves a simplified version which is rather similar to the conventional one dimensional cutting stock. To do that, we propose
a Sequential Heuristic Randomized Procedure (SHRP). Then in the repairing phase, the solution of the simplified problem is
transformed into a solution to the real problem. For experimental study we have chosen a set of problem instances of com-mon
use to compare SHRP with another recent approach. Also, we show by means of examples, how our approach works over instances
taken from the real production process.
All authors are supported by MEC-FEDER Grant TIN2007-67466-C02-01 and by contract CN-05-127 of the University of Oviedo and
the company ERVISA, and by FICYT under grant BP04-021. 相似文献
77.
Manuel Abad Cecilia Rossana Cimadamore José Patricio Díaz Varela 《Central European Journal of Mathematics》2009,7(2):299-309
In this paper, every monadic implication algebra is represented as a union of a unique family of monadic filters of a suitable
monadic Boolean algebra. Inspired by this representation, we introduce the notion of a monadic implication space, we give
a topological representation for monadic implication algebras and we prove a dual equivalence between the category of monadic
implication algebras and the category of monadic implication spaces.
相似文献
78.
González-Pérez A Varela LM García M Rodríguez JR 《Journal of colloid and interface science》2006,293(1):213-221
In the present paper, we analyze the dependence of the second critical micelle concentration (second cmc) of ionic amphiphiles on the number of atoms in the hydrocarbon molecular chain, n(c). A molecular thermodynamic model for the interaction energy between the end caps of rodlike micelles, g(N), is introduced and the linear dependence of this object on n(c) analyzed, thus leading to a Stauff-Klevens-like behavior of the second cmc. The predictions agree with previously reported data for n-alkyldimethylbenzylammonium chloride (C(n)BACl) for n-alkylpyridinium chloride (C(n)PyCl) and n-alkyltrymethylammonium bromide (C(n)TABr) at 35 degrees C. These conclusions are reinforced by the conductivity, density, and ultrasound velocity measurements of the second cmc of several C(n)PyCl (n=12, 14, 16) and n-alkylpyridinium bromides (C(n)PyBr, n=12, 14, 15, 16) presented in this paper. 相似文献
79.
L.S. Cavalcante A.Z. Simões M.R.M.C. Santos J.A. Varela 《Journal of solid state chemistry》2006,179(12):3739-3743
Ca(Zr0.05Ti0.95)O3 (CZT) thin films were grown on Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates by the soft chemical method. The films were deposited from spin-coating technique and annealed at 928 K for 4 h under oxygen atmosphere. CZT films present orthorhombic structure with a crack free and granular microstructure. Atomic force microscopy and field-emission scanning electron microscopy showed that CZT present grains with about 47 nm and thickness about 450 nm. Dielectric constant and dielectric loss of the films was approximately 210 at 100 kHz and 0.032 at 1 MHz. The Au/CZT/Pt capacitor shows a hysteresis loop with remnant polarization of , and coercive field of 18 kV/cm, at an applied voltage of 6 V. The leakage current density was about at 3 V. Dielectric constant-voltage curve is located at zero bias field suggesting the absence of internal electric fields. 相似文献
80.
Varela JA Rubín SG Gonzalez-Rodríguez C Castedo L Saa C 《Journal of the American Chemical Society》2006,128(29):9262-9263
Functionalized bicyclic 1,3-cyclohexadienes can be easily prepared by a new cascade reaction which involves the Ru-catalyzed addition of acyclic alkenes to 1,6-diynes to give (Z)-hexatrienes, followed by a pure thermal 6e-pi electrocyclization. 相似文献