Effect of particle size on hydrogenation properties of a gas-atomized AB5-type alloy

The effect of particle size on the hydrogenation properties of gas-atomized MmN(3.6)Co(0.56)Mn(0.56)Al(0.28) was investigated. Its gas-phase and electrochemical hydrogenation properties are different from those of specimens prepared by conventional methods. The initial shape of the alloy particles is spherical. After hydrogenation, significant changes take place in the larger particles. They become irregular, and a fresh surface is created. For particles smaller than 10 mum, no cracks are observed and they can hardly react with hydrogen gas. For particles larger than 10 mum, the gas-phase hydrogenation rate is faster than for the smaller particles, but the P-C-T curves are about the same. As regards electrochemical behavior, 75-38-mum particles have the hi hest discharge capacity (282 mAh/g). For the particles 9 smaller than 38 mum, the maximum capacity decreases to 238 mAh/g. The maximum capacity of the particles smaller than 10 mum is only similar to8 mAh/g. If the particles smaller than 10 mum are excluded, i.e. for particles in the range 38-10 mum, the discharge capacity increases to 268 mAh/g. It is concluded that 10 mum is a critical size for practical application of gas-atomized particles. Surface characterization of the particles of different sizes explains the inferior performance of the smaller particles. (C) 2002 Elsevier Science B.V. All rights reserved.