Efeito do processamento por fricção e mistura mecânica (FSP) na cinética de armazenagem de hidrogênio da liga ZK60 contendo 1,5% mischmetal

Abstract

The use of hydrogen as a fuel stored in a solid-state in metallic materials has been widely studied to produce clean, safe, and renewable energy. It is a safer storage method, and the ease of transport is superior compared to those in liquid or gas forms. In this case, the magnesium alloy has the highest theoretical gravimetric capacity among the alloys used to store hydrogen (7.6% by mass). However, the actual amount of hydrogen absorbed and desorbed and the kinetics in which these solid-state reactions occur depend strongly on how the alloy is thermomechanically or chemically modified. In this work, the ZK60 alloy with an addition of 1.5% by mass of mischmetal (ZK60-1.5% Mm), which has a 6.45% gravimetric capacity, was processed by friction stir processing (FSP) in an attempt to accelerate the kinetics of hydrogen entering and leaving this alloy. The manual filing process was used in the alloy cast and processed by FSP to decrease the path taken by hydrogen in the alloy, thus accelerating the kinetics of hydrogen entry and exit. A PCT-type system was used using the Sieverts method for hydrogen absorption and desorption at 350 ºC. Samples of the alloy cast and processed by FSP were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Samples processed by FSP and files have storage capacity and hydrogenation kinetics slightly higher than those of the only fused alloy filings. The experimental results of hydrogen entry and exit were applied in a mathematical model that used Arrhenius' law. Using the Luus-Jaakola stochastic method, it was possible to obtain the alloy's activation energy (Ea) values and kinetic law coefficient (k0). After all the constants present in the kinetic model were known, the alloy's kinetic behavior at different absorption temperatures (300 ºC and 325 ºC) and desorption (250 ºC and 300 ºC) was simulated. It was possible to show that the temperature used for the experimental tests (350 ºC) is the one that most accelerates the kinetic behavior of the alloy.