Threaded Rings that Swim in Excitable Media

Cardiac tissue and the Belousov-Zhabotinsky reaction provide two notable examples of excitable media that support scroll waves, in which a filament core is the source of spiral waves of excitation. Here we consider a novel topological configuration in which a closed filament loop, known as a scroll ring, is threaded by a pair of counterrotating filaments that are perpendicular to the plane of the ring and end on the boundary of a thin medium. We simulate the dynamics of this threaded ring (thring) in the photosensitive Belousov-Zhabotinsky excitable medium, using the modified Oregonator reaction-diffusion equations. These computations reveal that the threading topology induces an exotic motion in which the thring swims in the plane of the ring. We propose a light templating protocol to create a thring in the photosensitive Belousov-Zhabotinsky medium and provide experimental confirmation that this protocol indeed yields a thrin

SINTESI E SIMULTANEA DENSIFICAZIONE DI INTERMETALLICI MEDIANTE SINTERIZZAZIONE IN CORRENTE PULSATA

Sono state evidenziate alcune delle più recenti applicazioni in campo metallurgico della sinterizzazione attivata mediante corrente elettrica pulsata, SCP, meglio nota con l’acronimo inglese SPS (“Spark Plasma Sintering”). In particolare l’attenzione è stata focalizzata sull’impiego di tale tecnica per la sintesi e simultanea densificazione dei sistemi intermetallici NiTi e NbAl3 a partire da polveri dei metalli elementari. Nel caso della lega equiatomica NiTi, è stato dimostrato che, attraverso un processo ad un solo stadio, la tecnica SCP consente l’ottenimento di un prodotto finale già caratterizzato da un elevato grado di purezza (5.8-7.2 mol% di NiTi2) ed una elevata densità relativa (99%), più rapidamente (20 min) rispetto ai tempi richiesti dai processi convenzionali della metallurgia delle polveri. Sono state inoltre analizzate le potenzialità della combinazione di tecniche di attivazione meccanica con il processo SCP. Attraverso una opportuna analisi dell’effetto dei principali parametri operativi di entrambe le tecniche è stato possibile l’ottenimento di un prodotto NbAl3 puro e caratterizzato da densità molto elevate (97.5 ± 2.5% rispetto a quella teorica)

A methodology to investigate the intrinsic effect of the pulsed electric current during the spark plasma sintering of electrically conductive powders

A novel methodology is proposed for investigating the effect of the pulsed electric current during the spark plasma sintering (SPS) of electrically conductive powders without potential misinterpretation of experimental results. First, ensemble configurations (geometry, size and material of the powder sample, die, plunger and spacers) are identified where the electric current is forced to flow only through either the sample or the die, so that the sample is heated either through the Joule effect or by thermal conduction, respectively. These ensemble configurations are selected using a recently proposed mathematical model of an SPS apparatus, which, once suitably modified, makes it possible to carry out detailed electrical and thermal analysis. Next, SPS experiments are conducted using the ensemble configurations theoretically identified. Using aluminum powders as a case study, we find that the temporal profiles of sample shrinkage, which indicate densification behavior, as well as the final density of the sample are clearly different when the electric current flows only through the sample or through the die containing it, whereas the temperature cycle and mechanical load are the same in both cases