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71.
Summary By combining GPC fractionation with IR spectroscopy of ethylene-propylene copolymer (EP) samples, it has been shown that copolymer composition of EP's used in this study does not dep-end on molecular weight. Thus, contrary to earlier investigations by others, molecular weight distributions (MWD) and averages of EP's have been determined by using only experimental data such as GPC traces, universal calibration curve of GPC, and intrinsic viscosities. A comparative study has been carried out by using the MV method (called median value method) and the method of Ogawa and Inaba (OI method) for determination of MWD's and molecular weight averages of EP's. Application of the MV method results in lower molecular weight averages than the use of the equations proposed by Ogawa and Inaba for calculation of Mark-Houwing constants of EP's. However, polydispersities were found to be the same in both methods. The MV method also yields composition depending reliable pairs of Mark-Houwink constants, a and K, for EP's in 1,2,4-trichloro-benzene at 135 °C. 相似文献
72.
Summary Quantitative aspects of chain extension of telechelics have been studied. Molecular weight independent characteristic terms were defined and simple expressions were derived for number average extension,
, and number average degree of extension,
, as a function of number average functionality (¯Fn) of telechelic prepolymers. Analysis of these expressions indicate that n and ¯DEn are enormously sensitive to insignificant changes in ¯Fn close to ideal ¯Fn = 2.0. Molecular weight of the extended polymer (¯Mr) has also been calculated considering ¯Fn, number average molecular weight of the prepolymer (¯Mn), and molecular weight of the extender (Me). 相似文献
73.
B. I. Bogdanov Iv. Chomakov Y. Iliev P. Tanev 《Journal of the American Ceramic Society》1986,69(8):179-C
The effect of Al(NO3 )3 ·9H2 O, AlCl3 ·6H2 O, Al(CH3 COO)3 , and NH4 F on the specific surface of Al2 O3 obtained from aluminum-ammonium alum by calcining was studied. It was found that the use of these additives makes it possible to obtain Al2 O3 with specific surface varying from 1 to 135 m2 /g after thermal treatment in the interval from 1273 to 1423 K. The changes in the morphology and structure of powederd Al2 O3 obtained from alum containing these additives were studied by electron microscope observations. 相似文献
74.
Iván González Sergio Salazar Jorge Torres Rogelio Lozano Hugo Romero 《Journal of Intelligent and Robotic Systems》2013,70(1-4):93-106
A real-time attitude stabilization control scheme is proposed for the efficient performance of a mini-UAV Quad-Rotor. Brushless DC (BLDC) motor speed sensing is performed by reflective sensors in order to obtain a robust stabilization of the vehicle in hovering mode both indoor and outdoor. The speed measurement has the advantage of introducing this state information directly in the closed loop control which should be very useful for achieving robust stabilization of the mini-UAV. Furthermore a stabilizing control strategy based on Control Lyapunov Function (CLF) is proposed. The control scheme contains two control loops. The inner loop is devoted to control the motors speed while the outer loop is devoted to control the attitude stabilization of a mini-UAV. Assuming that the motors can be considered as a disturbance of the system, then by the standard singular perturbation theory, we may conclude that the system is asymptotically stable. Finally, to verify the satisfactory performance of proposed embedded controller, simulations and experimental results of speed sensing feedback in BLDC motors of the Quad-rotor aircraft in the presence of disturbances are presented. 相似文献
75.
Carol Martínez Iván F. Mondragón Pascual Campoy José Luis Sánchez-López Miguel A. Olivares-Méndez 《Journal of Intelligent and Robotic Systems》2013,72(3-4):517-539
In this paper, we apply a hierarchical tracking strategy of planar objects (or that can be assumed to be planar) that is based on direct methods for vision-based applications on-board UAVs. The use of this tracking strategy allows to achieve the tasks at real-time frame rates and to overcome problems posed by the challenging conditions of the tasks: e.g. constant vibrations, fast 3D changes, or limited capacity on-board. The vast majority of approaches make use of feature-based methods to track objects. Nonetheless, in this paper we show that although some of these feature-based solutions are faster, direct methods can be more robust under fast 3D motions (fast changes in position), some changes in appearance, constant vibrations (without requiring any specific hardware or software for video stabilization), and situations in which part of the object to track is outside of the field of view of the camera. The performance of the proposed tracking strategy on-board UAVs is evaluated with images from real-flight tests using manually-generated ground truth information, accurate position estimation using a Vicon system, and also with simulated data from a simulation environment. Results show that the hierarchical tracking strategy performs better than well-known feature-based algorithms and well-known configurations of direct methods, and that its performance is robust enough for vision-in-the-loop tasks, e.g. for vision-based landing tasks. 相似文献
76.
Iván Cáceres Le Hai Trung Hendrik Dirk van Ettinger Ad Reniers Wim Uijttewaal 《Canadian Metallurgical Quarterly》2010,136(5):299-310
Waves and currents are essential elements in the design of an artificial surfing reef (ASR). ASRs are primarily designed to optimize the surfing conditions (i.e., increase the surfability of the incoming waves) possibly in combination with the shoreline protection from erosion. The currents generated by waves breaking on the ASR play an important role in the surfability through the wave-current interaction (WCI). Depending on the design, the WCI may negatively affect the surfability by causing the waves to break prematurely due to the current-induced wave steepening. In addition, wave breaking tends to become more irregular due to the temporal variability of the underlying currents. To mitigate the negative effects of wave breaking induced currents on the surfability, three ASR layouts are examined through detailed laboratory experiments. The layouts differ in the alongshore separation distance between two symmetrical reef sides. The ensuing flow circulations are examined in detail with both in situ current meters and video observations of surface drifters. This is done for regular incident waves, bichromatic incident waves, and irregular incident waves, all with equal energy. A data analysis shows that for a given layout the mean flow patterns for regular, bichromatic, and irregular waves are qualitatively similar, with oblique rip currents exiting at either side of the reef and strong flow circulations onshore of the gap in between the two reef sides. Increasing the separation distance leads to a significant reduction of the obliquely exiting rip currents at the outer sides of the reef, but an increase in the flow circulation onshore of the gap. This has a positive effect on the surfability by reducing the negative effects associated with the WCI on the wave breaking, thus, providing longer rides. 相似文献
77.
Rice production in coastal wetlands provides critical ecosystem services that range from flood control to wildlife habitat. In the Iberian Peninsula rice was introduced in the 10th Century. Today Iberian rice accounts for about one quarter of the total rice production of the European Union, almost exclusively cultivated in the coastal wetlands of Spain, with permanent flooding. The intensive water management required to produce rice stands at a crucial point since freshwater supply is deteriorating at an unprecedented rate. Here we explore flexible adaptation options to climate change in the Doñana wetlands - a world heritage and biodiversity site - from two points of view: What are the policy options for agricultural water management in view of climate change? How can informed stakeholders contribute to better adaptation? The first question is addressed by simulating water availability to farmers with the WAAPA model under a range of adaptation policy options derived from the view of the local communities. The second question was addressed by means of participatory research. Adaptation options are framed according to the local environmental, social and policy context. Results suggest that perception on the potential role of new water infrastructure and farming subsidies dominates the view of local communities. The choices of the stakeholders that could be simulated with the hydrological model, were quantified in terms of additional water availability for the rice farming, therefore providing a quantitative measure to the qualitative solutions. Information provided during the study shaped the final adaptation options developed. Our research contributes to the definition of sustainable rice production in Europe. 相似文献
78.
Iván Uribe Pérez Tito Luiz da Silveira Tito Fernando da Silveira Heloisa Cunha Furtado 《Journal of Failure Analysis and Prevention》2011,11(1):3-9
This article discusses the problem of graphitization that can occur in carbon steels exposed to high temperature for extended
time when these are present in pressure vessels and piping. Several case studies are described dealing with actual failures
that have been investigated by the authors. 相似文献
79.
The pressure-volume analogy between compressible fluids and macroscopic sand bodies (Ivsic et al. in Phys A, 277:47–61, 2000) is further extended using quantitative determination of corresponding empirical constants based on adapted van der Waals
state equation. The isothermal constants obtained by interpretation of triaxial sand tests at so called “critical state of
sand” are clearly related to the universal ideal gas properties and molar properties of mineral sands. The corresponding constants
for sand and gases or other volatile liquids have the same order of magnitude. The apparent bulk repulsion/attraction effects
in sand bodies are also discussed. 相似文献
80.
Ilaria Armentano Marco Antonio Álvarez-Pérez Bruno Carmona-Rodríguez Iván Gutiérrez-Ospina José María Kenny Higinio Arzate 《Materials science & engineering. C, Materials for biological applications》2008,28(8):1522-1529
In vitro biomineralization process was investigated on functionalized single wall nanotube (SWNT) films. The films were prepared by solvent casting method by using carboxylated and fluorinated nanotubes. SWNT films were characterized by means of electron microscopy, contact angle measurements and optical absorption. The in vitro assays were performed on cultured human alveolar bone-derived cells (HABDC) to determine the capabilities of carboxylated single-walled nanotubes (SWNTs-COOH) and fluorinated single-walled nanotubes (F-SWNTs) to promote the deposit of mineral-like tissue. The results showed that the cellular response of HABDC in secreting a mineralized extracellular matrix and their consequent mineralization is dependent on the degree of functionalization of the SWNTs. Differences were found related to the kind of sidewall functionalization. Both structures promoted hydroxyapatite formation, however, calcium uptake on SWNTs-COOH increased and it was related to crystal density. From our results, it is possible to infer that CNT functionalization opens a path to future developments in new bone graft materials and techniques. 相似文献