Study of annealing effects on structural and sorption properties of low energy mechanically alloyed AB5's

Abstract

In this work, various AB5's were mechanically alloyed using a low energy mill until final or completion milling stages were reached. This process leaves micro- and nano-structured materials with highly distorted microstructures. Then, further annealing is needed to obtain an intermetallic suitable for hydrogen thermal compression process. After milling, the samples were annealed and analyzed by X-ray diffraction and Differential Scanning Calorimetry. Hydriding properties were studied using volumetric methods. After annealing at 200 °C for 24 h no changes occur in neither structural nor hydriding properties. For samples annealed at 400 °C, relaxation effects of the structure were observed. It occurs due to the release of strain produced by annealing on the microstructure. It leads to the improvements in both structural and hydriding properties. Strong recrystallization effect was present between 400 and 600 °C. At 600 °C, the main improvements were larger crystallite size, lower strain values and pressure-composition isotherms with well-defined plateaus. Despite this behavior, no evidence of crystallization was observed by Differential Scanning Calorimetry measurements from room temperature up to 500 °C. These topics, the amount of energy supplied during mechanical alloying and the correlation between the structural and sorption properties are discussed according to the governing mechanisms of recrystallization observed in each temperature range. From these results, an improved synthesis-thermal treatment method is outlined.