Instituto Superior Técnico

About: Instituto Superior Técnico is a based out in . It is known for research contribution in the topics: Catalysis & Finite element method. The organization has 10085 authors who have published 30226 publications receiving 667524 citations. The organization is also known as: IST & Instituto Superior Tecnico.

Inhibitor-doped sol–gel coatings for corrosion protection of magnesium alloy AZ31

Abstract: This work presents new anticorrosive coatings for the AZ31 magnesium alloy, based on hybrid sol–gel films doped with a corrosion inhibitor. The sol–gel coatings were prepared by copolymerization of 3-glycidoxypropyltrimethoxysilane and zirconium (IV) tetrapropoxide. 8-Hydroxyquinoline (8-HQ) was chosen as a corrosion inhibitor to be incorporated into the sol–gel films at two different stages of synthesis, either before or after hydrolysis of the sol–gel precursors. The effectiveness of 8-HQ for corrosion suppression on AZ31 was verified by Scanning Vibrating Electrode Technique. Electrochemical Impedance Spectroscopy was used to monitor the evolution of the substrate/film systems in the course of immersion in 0.005 M NaCl. The morphology and the structure of the sol–gel films were characterized with SEM/EDS and TEM techniques. The sol–gel films exhibit good adhesion to the metal substrate and prevent the corrosive attack during 2 weeks under immersion test. Results showed that addition of inhibitor into the sol–gel films enhances the corrosion protection of the magnesium alloy and does not lead to deterioration of the barrier properties of the sol–gel matrix.

Constraining the mass of dark photons and axion-like particles through black-hole superradiance

Abstract: Ultralight bosons and axion-like particles appear naturally in different scenarios and could solve some long-standing puzzles. Their detection is challenging, and all direct methods hinge on unknown couplings to the Standard Model of particle physics. However, the universal coupling to gravity provides model-independent signatures for these fields. We explore here the superradiant instability of spinning black holes triggered in the presence of such fields. The instability taps angular momentum from and limits the maximum spin of astrophysical black holes. We compute, for the first time, the spectrum of the most unstable modes of a massive vector (Proca) field for generic black-hole spin and Proca mass. The observed stability of the inner disk of stellar-mass black holes can be used to derive direct constraints on the mass of dark photons in the mass range 10−13 eV mV 3 × 10−12 eV. By including also higher azimuthal modes, similar constraints apply to axion-like particles in the mass range 6 × 10−13 eV mALP 10−11 eV. Likewise, mass and spin distributions of supermassive BHs – as measured through continuum fitting, Kα iron line, or with the future space-based gravitational-wave detector LISA – imply indirect bounds in the mass range approximately 10−19 eV mV , mALP 10−13 eV, for both axion-like particles and dark photons. Overall, superradiance allows to explore a region of approximately 8 orders of magnitude in the mass of ultralight bosons.

Thermodynamics of Reissner-Nordström-anti-de Sitter black holes in the grand canonical ensemble

Abstract: The thermodynamical properties of the Reissner--Nordstr\"om--anti-de Sitter black hole in the grand canonical ensemble are investigated using York's formalism. The black hole is enclosed in a cavity with a finite radius where the temperature and electrostatic potential are fixed. The boundary conditions allow one to compute the relevant thermodynamical quantities, e.g., thermal energy, entropy, and charge. The stability conditions imply that there are thermodynamically stable black hole solutions, under certain conditions. By taking the boundary to infinity, and leaving the event horizon and charge of the black hole fixed, one rederives the Hawking-Page action and Hawking-Page specific heat. Instantons with negative heat capacity are also found.