Mixed ionic electronic conductivity and flash sintering

In this contribution, we present and discuss similarities and differences between two phenomena connected to creation and transport of point defects. One is flash sintering (FS) on which this conference is focused on and the other is switching in some non-volatile memory devices. The explanation of flash sintering (FS) is based on the understanding that the solids being sintered contain point defects, both mobile ions and quasi free conducting electrons i.e. that they are mixed-ionic-electronic-conductors (MIECs). We shall refer specifically to oxide MIECs. The explanation also requires understanding how these MIECs respond to temperature changes, to changes in the oxygen chemical potential (or oxygen partial pressure) and how chemical diffusion can take place. The oxygen chemical potential can be fixed by the ambient or by polarization under an electric current applied on polarizing electrode. To present those properties we address a specific memory system which has much in common with FS, show the similarity where it exists and point at the differences. The memory system is constructed of one single nano grain (crystallite), which is an insulator at room temperature, placed between two metal electrodes, subjected to a high electrical field. The temperature exhibits runaway, very similar to FS, the current increasing rapidly after an incubation period. The increase in the current of a few orders of magnitude is facilitated by both the increase in temperature and the reduction of the oxide. In this single grain system, the I-V relations are not linear (Ohmic) but rather super linear, while in FS they may be ohmic changing only due to the temperature increase. The key difference between this system and FS is by the absence of grain boundaries within the bulk and no need for densification and grain growth. Yet not only electrical current but also material transport take place in the single grain system under an applied electric field. Heat losses in the single grain system are due to conduction of electrode and in FS mainly by radiation. Due to the small size of the system the incubation starts at room temperature despite the insulating nature of the grains, however the electrical field is of the order of 106 V/cm. The response time is very short and can be as low as 1 ns. The region of high vacancy concentration in the single grain system and in FS during the transient step (II) can be either near the anode or the cathode. It is only under conditions close to steady state of step III that the vacancies concentrate near the cathode. When the single crystal is replaced by a polycrystalline solid, mobile ions that are injected into it under an applied voltage form a percolation pattern with one branch reaching the opposite electrode, accompanied by a few shorter branches. This is reminiscent of possible percolation path during FS. We shall discuss also the interaction of water vapor with oxides and the possible impact on surface as well as bulk conductivity

Strong Ultraviolet Pulse From a Newborn Type Ia Supernova

Type Ia supernovae are destructive explosions of carbon oxygen white dwarfs. Although they are used empirically to measure cosmological distances, the nature of their progenitors remains mysterious, One of the leading progenitor models, called the single degenerate channel, hypothesizes that a white dwarf accretes matter from a companion star and the resulting increase in its central pressure and temperature ignites thermonuclear explosion. Here we report observations of strong but declining ultraviolet emission from a Type Ia supernova within four days of its explosion. This emission is consistent with theoretical expectations of collision between material ejected by the supernova and a companion star, and therefore provides evidence that some Type Ia supernovae arise from the single degenerate channel.Comment: Accepted for publication on the 21 May 2015 issue of Natur

Activated Met Signalling in the Developing Mouse Heart Leads to Cardiac Disease

BACKGROUND: The Hepatocyte Growth Factor (HGF) is a pleiotropic cytokine involved in many physiological processes, including skeletal muscle, placenta and liver development. Little is known about its role and that of Met tyrosine kinase receptor in cardiac development. METHODOLOGY/PRINCIPAL FINDINGS: In this study, we generated two transgenic mice with cardiac-specific, tetracycline-suppressible expression of either Hepatocyte Growth Factor (HGF) or the constitutively activated Tpr-Met kinase to explore: i) the effect of stimulation of the endogenous Met receptor by autocrine production of HGF and ii) the consequence of sustained activation of Met signalling in the heart. We first showed that Met is present in the neonatal cardiomyocytes and is responsive to exogenous HGF. Exogenous HGF starting from prenatal stage enhanced cardiac proliferation and reduced sarcomeric proteins and Connexin43 (Cx43) in newborn mice. As adults, these transgenics developed systolic contractile dysfunction. Conversely, prenatal Tpr-Met expression was lethal after birth. Inducing Tpr-Met expression during postnatal life caused early-onset heart failure, characterized by decreased Cx43, upregulation of fetal genes and hypertrophy. CONCLUSIONS/SIGNIFICANCE: Taken together, our data show that excessive activation of the HGF/Met system in development may result in cardiac damage and suggest that Met signalling may be implicated in the pathogenesis of cardiac disease