Results obtained with the iron oxide particles of different sizes (sub-micron, FeP, 40–80 lm and 80–160 lm) in Table 2 and Fig. 4, illustrated that the highest reactivity corresponded to the commercial sub-micron powders during the first redox cycle. This suggests that if these iron oxide powders could be well dispersed into a LSF731 matrix, avoiding agglomeration and sintering, a significant increase in the reactivity of the composite material should be observed. To analyze the effect of the nominal cluster size on the