Thermal History Of Particles Research Articles

The gravitino problem in extended gravity cosmologies

The gravitino problem is investigated in the framework of extended gravity cosmologies. In particular, we consider f(R) gravity, the most natural extension of the Hilbert–Einstein action, and f({mathcal{T}}) gravity, the extension of teleparallel equivalent gravity. Since in these theories, the expansion laws of the Universe are modified, as compared to the standard Lambda CDM cosmology, it follows that also the thermal history of particles gets modified. We show that f(R) models allow to avoid the late abundance of gravitinos. In particular, we found that for an appropriate choice of the parameters characterizing the f(R) model, the gravitino abundance turns out to be independent of the reheating temperature. A similar behavior is achieved also in the context of f({mathcal{T}}) gravity. In this perspective, we can conclude that geometric corrections to standard General Relativity (and to Teleparallel Equivalent of General Relativity) can improve shortcomings both in cosmology and in unified theories beyond the standard model of particles.

Thermal leptogenesis inf(R)cosmology

The cosmological consequences of $f(R)$ gravity are reviewed in the framework of thermal leptogenesis. In $f(R)$ cosmology the expansion laws of the Universe are modified with respect to the standard cosmology, which also changes the thermal history of particles. In these cosmological scenarios it turns out that the condition for the out-of-equilibrium decay of the lightest heavy neutrino gives rise to a modulated upper bound on the lightest light neutrino mass, i.e. ${\stackrel{\texttildelow{}}{m}}_{1}\ensuremath{\lesssim}{10}^{\ensuremath{-}3}A(T={M}_{1})\text{ }\text{ }\mathrm{eV}$, where $A(T)$ is the ratio between the expansion rates of the Universe evaluated in $f(R)$ cosmology and in standard cosmology, and ${M}_{1}$ is the lightest heavy neutrino mass. This factor can be parametrized in terms of the temperature $T$ as $A(T)=(T/{T}_{*}{)}^{p}$, where ${T}_{*}$ is the transition temperature at which the expansion law in $f(R)$ cosmology deviates from the standard one, and $p$ is related to the parameters characterizing the $f(R)$ model under consideration. Results imply that if $A\ensuremath{\gg}1$ then the thermal leptogenesis scenario can accommodate the case of a degenerate mass spectrum of the light neutrinos (for a low enough transition temperature), which is otherwise impossible with standard cosmology. In our analysis we consider $f(R)$ models of the form $f(R)=R+\ensuremath{\alpha}{R}^{n}$.

Nanoscale characterization of FeAl-HVOF coatings

FeAl intermetallic is one of the highly attractive aluminides due to its high specific properties and good oxidation resistance. The present work investigates the production of iron aluminide coatings obtained by means of the High Velocity Oxygen Fuel process. Two well differentiated structures have been achieved by the modification of particle size distribution, reaching lower oxidation at splat boundaries with particle sizes in the range of 40–60 μm. Such features have been studied by using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) as well. TEM has also been employed to illustrate the grain size and to reveal the degree of the compound ordering in the as-sprayed form; despite rapid solidification being a way to induce disordering in intermetallics, since the feedstock is a Fe–40 at.% Al milled powder, it becomes slightly ordered at the high temperatures during in-flight time. Moreover, a further attempt to study how the order affects to the coating structure, the initial powder was annealed in order to induce such ordering before spraying. While the ordering observed in the as-sprayed milled powder came from the thermal history of particles during the spraying process, the ordering of the as-annealed powder is much more evidenced by the visible superlattice spots.

Computational simulation of metal droplet break-up, cooling and solidification during gas atomisation

In an atomisation process for powder production, metal droplets go through undercooling, recalescence, peritectic and segregated solidification before fully solidified. The cooling process is further complicated by droplet break-up during the atomisation. This paper describes a numerical model which combines both cooling and break-up in a single computation. The dynamic history of droplets is solved as discrete phase in an Eulerian gas flow. The coupling between droplet and gas flows are two-way, in which the heat and momentum exchanges affecting the gas flow are treated as source/sink terms in the fluid equations. The droplet models were employed a gas atomisation process for metal powder production and good agreement is achieved with the results in open literature. The model results further confirm that thermal history of particles is strongly dependent on initial droplet size. Large droplets will not go through undercooling while small droplets have identifiable stages of undercooling, unclearation and recalescence. The predictions demonstrate that droplets have very similar profiles during gas atomisation and the major factor influencing the atomisation and solidification process of droplets are in-flight distance.

The thermal history of interplanetary dust particles collected in the Earth's stratosphere

Abstract— Fragments of 24 individual interplanetary dust particles (IDPs) collected in the Earth's stratosphere were obtained from NASA's Johnson Space Center collection and subjected to pulse‐heating sequences to extract He and Ne and to learn about the thermal history of the particles. A motivation for the investigation was to see if the procedure would help distinguish between IDPs of asteroidal and cometary origin. The use of a sequence of short‐duration heat pulses to perform the extractions is an improvement over the employment of a step‐heating sequence, as was used in a previous investigation. The particles studied were fragments of larger parent IDPs, other fragments of which, in coordinated experiments, are undergoing studies of elemental and mineralogical composition in other laboratories. While the present investigation will provide useful temperature history data for the particles, the relatively large size of the parent IDPs (∼40 μm in diameter) resulted in high entry deceleration temperatures. This limited the usefulness of the study for distinguishing between particles of asteroidal and cometary origin.