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排序方式: 共有339条查询结果,搜索用时 15 毫秒
311.
Raul Quijada Rosângela B. Scipioni Raquel S. Mauler Griselda B. Galland Márcia S. L. Miranda 《Polymer Bulletin》1995,35(3):299-306
Summary This study investigated the copolymerization of ethylene with 1-hexene using the homogeneous Et[Ind]2ZrCl2 and [Ind]2ZrCl2 catalysts. The Et[Ind]2ZrCl2 catalyst gave a higher catalytic activity than the [Ind]2ZrCl2 and also showed a better incorporation of 1-hexene for the same comonomer concentration in the feed. Thermal analysis (DSC) and viscosity measurements showed that an increase of the 1-hexene incorporated in the copolymer results in a decrease of the melting point, crystallinity and molecular weight of the polymer formed. The reactivity ratios for ethylene and 1-hexene confirmed the more successful incorporation of the comonomer for the polymerization catalyzed by Et[Ind]2ZrCl2. 相似文献
312.
de Andrade AK Duarte RM Guedes Lima SJ Passos TA Lima KC Montes MA 《Microscopy research and technique》2011,74(1):23-27
This study evaluates the margin of a nanofill, a nanohybrid, and a conventional microhybrid composite in restorations in occlusal cavities of posterior teeth after 12 months. Forty-one patients, each with three molars affected by primary caries or the need to replace restorations, participated in this research. The teeth were restored with a nanofill (Filtek Z350), a nanohybrid (Esthet-X), and a microhybrid as a control (Filtek Z250). Ten patients were selected randomly, and the three restorations were molded with a low-viscosity polyvinyl siloxane material. The molds were poured with epoxy resin, gold-sputter coated, observed by scanning electron microscopy, and classified as: "perfect margin," "marginal irregularity," "marginal gap," "marginal fracture," or "artifact." For statistical analysis, the Wilcoxon and Friedman nonparametric tests and paired-samples t-test were used (significance level of 5%). The performance of the three materials was compared after 1 week and 12 months. No statistically significant differences were detected for all criteria (P > 0.05). When each composite was compared over time, statistically significant differences were found for the criterion, perfect margins (Esthet-X and Filtek Z350, P < 0.05). The materials performed satisfactorily over the 12-month-observation period, but all composites under investigation showed a certain amount of deterioration relating to marginal quality over time. 相似文献
313.
A key topic in classical control theory is the Internal Model Principle (IMP). A particular case of the IMP for tracking periodic references or attenuating periodic disturbances in closed-loop control systems is a technique called repetitive control. This work proposes and describes an educational laboratory plant to show the students the advantages of repetitive controllers in systems with periodic references or disturbances. The plant has been designed to be low cost, easy to build, and subject to periodic disturbances with a clear physical explanation. More specifically, it consists of a pulsewidth modulation (PWM) electronic amplifier, a small dc motor, and a magnetic setup that generates a periodic load torque under constant mechanical speed operation. The control objective for the closed-loop control system is to regulate the mechanical speed to a constant value in spite of the periodic load torque disturbance. In order to accomplish this performance specification, a detailed design of a digital repetitive controller is presented, and some basic experimental results are provided to prove its good behavior. The paper also includes some repetitive control concepts and facts that teaching experience shows as essential to understand the design process. 相似文献
314.
Ángela Anglada María J. Rivero Inmaculada Ortíz Ane Urtiaga 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2008,83(10):1339-1346
BACKGROUND: The effect of four dye‐auxiliary chemicals, typically employed in acid dyeing, on the performance of UV/H2O2 decolouration of the model non‐biodegradable dye C.I. Acid Orange 7 (AO7) was investigated. The initial concentration of AO7 was 0.150 g L?1, while the concentration of the auxiliary compounds (NaCl, Na2SO4, Na2CO3 and CH3COOH) was varied in the range 1–10 g L?1. RESULTS: The negative influence of the presence of the dye‐auxiliary compounds studied on the decolouration rate of AO7 decreased in the following order: CH3COOH > Na2SO4 > NaCl > Na2CO3. Results were quantified in terms of the observed kinetic rate constant, kobs (s?1), of AO7 decolouration as a function of dye auxiliary chemical concentration. The decolouration rate of AO7 decreased as the concentration of dye‐auxiliary compound increased in the range 1–5 g L?1, while higher concentrations had a minor effect. Upon addition of 5 g L?1 of CH3COOH, NaCl and Na2SO4, the kinetic rate constant decreased by 39%, 30% and 12%, respectively. CONCLUSIONS: It was concluded that the presence of NaCl, Na2SO4 and above all of CH3COOH should be considered in the design of the treatment of real dye‐bath effluents by UV/H2O2. Copyright © 2008 Society of Chemical Industry 相似文献
315.
Ángela Fernández Konstantin M. Likin Pedro Turullols Jorge Teniente Ramón Gonzalo Carlos del Río Javier Martí-Canales Mario Sorolla Romualdo Martín 《Journal of Infrared, Millimeter and Terahertz Waves》2000,21(12):1945-1957
Two mirror lines have been used to transmit the microwave power from the powerful microwave generators to the TJ-II plasmas. Both lines have been tested at nominal power level and they are in operation now. This paper deals with the final design of the transmission lines and their testing. Before starting operation at high power level, measurements of the wave beam parameters at low power level were performed. Two horn antennas were designed to simulate the gyrotron output. The results are presented in this paper. A computer code based on Huygens diffraction theory was developed to simulate the propagation of the beam along the mirror lines. A comparison of the theoretical and the experimental results is also shown here. 相似文献
316.
Ana Paula Spier Caren Serra Bavaresco Ângela T.S. Wyse Denise Carvalho João José Freitas Sarkis 《Food chemistry》2007,103(2):565-571
ATP, ADP and AMP (adenosine tri, bi and mono-phosphate, respectively) hydrolysis in rat serum were assessed in vitro in the presence of certain flavonoids: resveratrol, quercetin and rutin. Also, the effects of purple grape juice (PGJ) and the effect of PGJ before arginine treatment were observed on serum nucleotide hydrolysis. The in vitro nucleotide hydrolysis by rat serum was increased in the presence of resveratrol for ATP and ADP hydrolysis. There was a decrease of nucleotide hydrolysis in the presence of other flavonoids tested in vitro. The effects of PGJ treatment after 15 days showed that nucleotide hydrolysis increased for ATP, ADP and AMP. We also investigated whether animals that had received injections of arginine and presented reduction of nucleotide hydrolysis activities, would present a retrieval of this reduction by receiving PGJ before injections were administered. Evidently resveratrol may increase nucleotide hydrolysis by serum and PGJ may be capable of preventing the decrease in nucleotide hydrolysis caused by arginine treatment. 相似文献
317.
Àgata Lapedriza Santi Seguí David Masip Jordi Vitrià 《Pattern Analysis & Applications》2008,11(3-4):299-308
In this paper we present a new method for Joint Feature Selection and Classifier Learning using a sparse Bayesian approach. These tasks are performed by optimizing a global loss function that includes a term associated with the empirical loss and another one representing a feature selection and regularization constraint on the parameters. To minimize this function we use a recently proposed technique, the Boosted Lasso algorithm, that follows the regularization path of the empirical risk associated with our loss function. We develop the algorithm for a well known non-parametrical classification method, the relevance vector machine, and perform experiments using a synthetic data set and three databases from the UCI Machine Learning Repository. The results show that our method is able to select the relevant features, increasing in some cases the classification accuracy when feature selection is performed. 相似文献
318.
Rosângela B. Garcia Toshiyuki Nagashima Jr Ana K. C. Praxedes Fernanda N. Raffin Túlio F. A. L. Moura E. Sócrates T. do Egito 《Polymer Bulletin》2001,46(5):371-379
Summary
Xylan, a hemicelllulose extracted from corn cobs, was used to prepare micro and nanoparticles. First, a chemical evaluation
of xylan extract was performed. Then, particles were prepared by a coacervation method based on neutralization of an alkaline
solution with an acid solution. The influence of polymer content (2.85 to 100 mg/ml) and surfactant presence (0.6 to 1.8%(v/v)) on the manufacturing process was evaluated. It was demonstrated that neutralization of the xylan solution with HCl or acetic
acid was able to generate micro and nanoparticles and that surfactant concentration influences both the particle size stability
and morphology. Therefore, the optimal concentration of surfactant was 1.5%(v/v).
Received: 16 January 2001/Revised version: 16 May 2001/Accepted: 16 May 2001 相似文献
319.
320.
This paper deals with the basic requirements that should be fulfilled by a technique for segmenting video sequences in coding applications. The specific problems of coding-oriented video segmentation are analyzed. This way, intra-frame and inter-frame segmentation approaches are studied. In the inter-frame mode, the problems of temporal label coherence and connectivity in the coding framework are discussed and solutions are presented. 相似文献