Modifying the structure and properties of polyethylene foams using thermal treatments

BACKGROUND: The physical properties of polymer foams depend on many factors: density, cellular structure, matrix polymer morphology, etc. Therefore, these properties can be adapted by appropriate control of the structure. However, this simple and attractive concept has some limitations because the cellular structure of foams cannot be fully controlled during manufacturing. Therefore, in order to make possible the control of properties, it is highly desirable to develop simple procedures, such as thermal treatments, to modify the cellular structure. In the work reported, low‐density polyethylene foams were thermally treated at temperatures below the melting temperature of the base polymer. The cellular structure, polymer base morphology and several thermal and mechanical properties were studied before and after the thermal treatments. RESULTS: It is shown that the anisotropy of the cellular structure is reduced by using adequate treatments. This modification of the structure influences physical properties that are sensitive to the cell shape, such as thermal expansion, elastic modulus and collapse stress. CONCLUSION: A simple procedure to allow further control of the structure and properties of polyethylene‐based foams has been presented. The use of adequate thermal treatments is able to modify the cellular structure and hence the physical properties of these materials. Copyright © 2009 Society of Chemical Industry     

Preparation of Al3Ti and L12 Al3Ti-Base Alloys Microalloyed with Fe by a Melting/Casting Rapid-Solidification Technique

Intermetallic compounds of Al₃Ti and L1₂ (Al₃Ti base) microalloyed with Fe have been obtained using a melting/casting rapid solidification technique. Microstructural characterization of these alloys has been carried out through analytical SEM observations and also x-ray diffraction studies. Contaminants in these alloys have also been studied and their nature explored. Other morphology structural features such as grain size have been studied.     

Calculation of optimum directional overcurrent relays settings: theparallel lines case

A simple and practical criterion is proposed to consider the transient changes due to nonsimultaneous operation of directional overcurrent relays in parallel lines. The criterion consists of using two different time coordination intervals: one for relay coordination pairs that do not belong to parallel lines and another for pairs belonging to parallel lines. The method can be implemented in programs that use the optimization technique for the calculation of relay settings. Numerical results showing the performance of the method are also presented     

Mesostructured mixed Mo–V–Nb oxides for propane ammoxidation

Mesostructured Mo–V–Nb mixed oxide phases were prepared by reacting inorganic precursors in the presence of cationic, anionic and alkylamine surfactants. The occurrence of these mesostructured phases was explained in part by charge-matching considerations at the inorganic–organic interface. Other interactions, such as covalent bonding between the surfactant headgroup and metal atoms, could direct the self-assembly process. Despite of the limited thermal stability of the mesophases, the air-calcined phases were catalytically active and selective for propane ammoxidation to acrylonitrile and acetonitrile. The selectivities to acrylonitrile and acetonitrile were as high as 26 and 49 mol%, respectively, at 66% propane conversion.     

Sensorless control of induction motors by artificial neuralnetworks

In this letter, we propose a voltage-source inverter control working in the open loop of an induction motor measuring the stator current and using an artificial neural network. This technique has the mission to estimate the speed and torque of the rotor without using sensors. With this, a simple and cheap method of control is obtained, with as much precision and robustness as other more complex ones     

Photovoltaic potential induced by 10 μm laser radiation in Siphotodiodes

Photon drag and photovoltaic response observed in p⁺/n and n⁺/p silicon junctions excited by moderated-power-density 10-μm radiation are discussed. A multistep multiphotonic process through intermediate states located within the energy gap is proposed as the mechanism responsible for the photovoltaic potential     

FATIGUE CRACK CLOSURE OF CORNER CRACKS LOCATED AT HOLES LOADED IN TENSION OR BENDING

Abstract— The objective of this paper is to report fatigue crack growth and closure behavior for corner cracked holes. An optical interference technique is employed with transparent polymer specimens to determine the three-dimensional crack surface displacement field. The local crack closure behavior is examined along the crack front under remote tension and pure bending conditions. It was observed that the crack opening loads along the hole and the free surface were significantly higher than the opening values at interior crack locations.     

Fougere method applied to antenna data

In the letter the performance of the Fougere method for autoregressive (AR) spectral analysis is studied. This method is compared with the Burg method and the least-squared (LS) method (Marple algorithm). It is shown that when the signal consists of two equal-level sinusoids in white noise and the data are complex-valued, as they will be in antenna applications, the Fougere method improves the results obtained by the Burg method in all cases. Also, for low signal/noise ratio (SNR), the Fougere method gives better results than LSM.