Toughening of epoxy using core–shell particles

An epoxy resin, cured using an anhydride hardener, has been modified by the addition of preformed core–shell rubber (CSR) particles which were approximately 100 or 300 nm in diameter. The glass transition temperature, T _g, of the cured epoxy polymer was 145 °C. Microscopy showed that the CSR particles were well dispersed through the epoxy matrix. The Young’s modulus and tensile strength were reduced, and the glass transition temperature of the epoxy was unchanged by the addition of the CSR particles. The fracture energy increased from 77 J/m² for the unmodified epoxy to 840 J/m² for the epoxy with 15 wt% of 100-nm diameter CSR particles. The measured fracture energies were compared to those using a similar amount of carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber. The CTBN particles provided a larger toughening effect when compared to CSR particles, but reduced the glass transition temperature of the epoxy. For the CSR-modified epoxies, the toughening mechanisms were identified using scanning electron microscopy of the fracture surfaces. Debonding of the cores of the CSR particles from the shells was observed, accompanied by plastic void growth of the epoxy and shell. The observed mechanisms of shear band yielding and plastic void growth were modelled using the Hsieh et al. approach (J Mater Sci 45:1193–1210). Excellent agreement between the experimental and the predicted fracture energies was found. This analysis showed that the major toughening mechanism, responsible for 80–90% of the increase in fracture energy, was the plastic void growth.     

Influence of the Nonsingular Stress Terms on Small-Scale Supercritical Transformation Toughness

In the present paper we examine the influence of the secondorder term of the asympotic stress solution for mode-I, steady-state, plane-strain, quasi-static crack growth in a supercritically trasforming material. It was found that the second-order term can affect the toughness, not always in a positive way. The second-order term includes the effects of the loading, of the overall geometry, and of the crack length. The findings can be used for the evaluation of the purely material resistance curves of small-scale transforming ceramics, as well as for the toughness predictions of these materials. The results may be useful for unifying different test results, and for a better understanding of the strength-toughness relation.     

A suboptimal static controller for HVDC links supplying isolated AC networks

Contents The design of a centralized suboptimal static controller for HVDC links connected to load systems without AC generators, using the projective control technique, is presented. Frequency and voltage variations at the inverter side are the inputs of the static controller, while the modulation of the DC current settings and the constant extinction angle are the outputs. The design of this controller used to modulate the active power of the HVDC link to meet the various large load disturbances in the isolated AC network is realized, in a near optimal way, by taking into account the strength of the inverter AC voltage. Using a fundamental frequency digital simulation program, the proposed control system has been applied and tested in a realistic electrical AC/DC power system.

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Ein teiloptimaler statischer Regler für HGÜ-Systeme ohne Wechselstromquellen

Übersicht Der Entwurf eines zentralen teiloptimalen Reglers für HGÜ bei Systemen ohne Wechselstromgeneratoren wird vorgestellt. Als Eingangsgrößen des statischen Reglers werden die Frequenzund die Spannungsänderungen auf der Wechselstromseite des Wechselrichters gewählt, während die Modulation des Gleichstromsollwertes und der konstante Löschwinkel als seine Ausgangsgrößen betrachtet werden. Der Entwurf dieses Reglers, der die Wirkleistung der HGÜ-Anlage moduliert, um die unterschiedlich großen Laststörungen zu beseitigen, ist nahezu optimal realisiert worden unter Beachtung des Spannungswertes auf der Wechselstromseite des Wechselrichters. Durch Anwendung eines Programms für digitale Simulation wurde das vorgeschlagene Regelsystem für ein realisierbares WS/GS-Netz angewandt und geprüft.

     

A new converter equivalent circuit for fast digital simulation of multiterminal HVDC systems

Contents A new converter equivalent circuit suitable for dynamic simulation of multiterminal HVDC systems with multibridge converters is presented. Using this circuit all possible operating modes of the converter can be analyzed by means of one topological matrix only. It is shown that the proposed converter equivalent circuit increases the flexibility of the simulation and considerably reduces the computations required to obtain the state equations of the converter as a function of the conduction state of the valves. Computer results of selected abnormal operating conditions are also presented to demonstrate the inherent accuracy and the capability of the proposed converter equivalent circuit.

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Ein neues Stromrichterersatzschaltbild für schnelle digitale Simulation eines HGÜ Systems mit mehreren Unterwerken

Übersicht Ein neues Stromrichterersatzschaltbild geeignet für die Simulation dynamischer Vorgänge von HGÜ Systemen mit mehreren Unterwerken und vielen Drehstrombrückenschaltungen wird beschrieben. Die Anwendung dieses Ersatzschaltbildes ermöglicht die Analyse aller möglichen Funktionsarten des Stromrichters mit Hilfe nur einer topologischen Matrix. Es beweist sich noch, daß durch das vorgeschlagene Ersatzschaltbild eine höhere Simulationsflexibilität erreicht werden kann. Der Rechenaufwand zur Bildung der Zustandsgleichungen des Stromrichters abhängig vom momentanen Leitungszustand der Ventile wird wesentlich reduziert. Rechenergebnisse für speziell gewählte nicht normale Betriebszustände werden dargestellt, um die existierende genauigkeit und die Fähigkeiten des vorgeschlagenen Ersatzschaltbildes zu demonstrieren.

     

Effects of mesulergine treatment on diet selection, brain serotonin (5-HT) and dopamine (DA) turnover in free feeding rats

1. The effects of mesulergine, a 5-hydroxytryptamine (5-HT) receptor antagonist with dopamine (DA) agonistic properties, on rats diet selection over a seven day period and on 5-HT and DA turnover was studied. 2. Three groups of male Wistar rats were individually caged and ad libitum fed with a standard (SD) and 50% sweet carbohydrate enriched diet (CED). Food intake was measured daily 4 hrs and 24 hrs after i.p. injections of mesulergine (1 and 3 mg/kg) or vehicle. 5-HT and 5-HIAA in hypothalamus (Hy), Striatum (St) and hippocampus (Hi) as well as DA and DOPAC in (Hy) and (St) were assayed at the 8th day of the experiment. 3. There was a dose dependent increase of SD consumption 4 hrs after mesulergine treatment while the CED remained unchanged with total food intake dose dependently increased as a consequence. At 24 hrs measurements SD consumption was increased only for the dose of 1 mg/kg of mesulergine, while a dose dependent decrease of CED intake was observed. Total food intake was unchanged for the dose of 1 mg/kg and decreased with the dose of 3 mg/kg consequently. A dose dependent decrease of rats body weight was observed too. 4. A significant increase of 5-HIAA/5-HT ratio in (Hy) and (St) for the dose of 1 mg/kg and in (Hi) for the dose of 3 mg/kg with no changes of DA turnover were found. 5. The above data suggest a dual mode of action of mesulergine presented as a short term hyperphagia due to simultaneous antiserotonergic and dopaminergic activity and long-term hypophagia due to long-term agonistic effects of dopaminergic neurons.     

Structural study of very thin anodic alumina films on silicon by anodization in citric acid aqueous solution

The formation of thin alumina films on a silicon substrate by anodization in a mild acid, specifically in 1% wt citric acid aqueous solution, is investigated by transmission electron microscopy (TEM). We present a comparative study between two cases of starting material: pure aluminum and an alloy of aluminum with 1% silicon. In both cases the thickness of the Al layer was less than 50 nm. It was observed that under exactly the same conditions, in the first case the anodization was stopping before anodizing the whole film and a remaining non-anodized Al layer was always present, while in the second case, the Al layer was fully anodized, resulting in an alumina matrix with a very high density of silicon nanocrystals of uniform sizes embedded in it. In both cases the alumina film was compact and amorphous.     

FePt and CoPt nanoparticles co-deposited on silicon dioxide-a comparative study

We compare CoPt and FePt nanoparticles grown under identical conditions on oxidized Si?substrates by electron beam co-evaporation. Growth was performed under high vacuum conditions at substrate temperatures of 1023?K and was immediately followed by an annealing step. This process forms CoPt and FePt nanoparticles with mean diameters between ～17 and ～22?nm. In particular, the annealing step results in grain size enlargement for all samples and in a progressive magnetic hardening of the nanoparticles which reach maximum perpendicular coercivities of ～6.6?kOe (for the CoPt) and ～10.2?kOe (for the FePt nanoparticles). We show that, during this annealing step, a progressive transition towards the hard magnetic L1(0) ordered phase takes place in both materials. In contrast to FePt, CoPt nanoparticles must be annealed in order to crystallize in this phase.     

State-variable approach to analysis of power systems including HVDC convertors

A new method is presented for the analysis of power systems including static convertors. The basic feature of the proposed method is that the state equations of each component of a power system can be written separately by considering fictitious sources at their terminals. The values of these sources are calculated from a general algebraic connection equation, applied to any practical system bus and derived by using Kirchhoff's laws. Any model of the system components is easily incorporated using this technique and the response of the whole system is recorded at the expense of less computer time and memory.     

Adhesion of electrodeposited nanocrystalline Ni–W coatings on Cu

A study was performed on the adhesion of nanocrystalline Ni–W alloy coatings, on copper, with the help of a scratch testing apparatus. The nanocrystalline Ni–W coatings were electrodeposited on copper substrates. The tungsten content in the Ni–W alloy coating was found to be 42?wt-% (19?at.-%). The results obtained from the scratch experiments showed that the nanocrystalline Ni–W coatings on copper suffer tensile failure and minor delamination. The mean cohesive and adhesive critical loads were found to be in the range of 19–85?N and 34–102?N, respectively and to increase with increasing thickness of the coatings.