FE-CR AL2O3 METAL-CERAMIC COMPOSITES - NATURE AND SIZE OF THE METAL PARTICLES FORMED DURING HYDROGEN REDUCTION/

Fe-Cr/Al2O3 metal-ceramic composites prepared by hydrogen reduction at different temperatures and for different periods have been investigat ed by a combined use of Mossbauer spectroscopy, x-ray diffraction, tra nsmission electron microscopy, and energy-dispersive x-ray spectroscop y in order to obtain information on the nature of the metallic species formed. Total reduction of Fe3+ does not occur by increasing the redu ction time at 1320 K from 1 to 30 h, and the amount of superparamagnet ic metallic species is essentially constant (about 10%). Temperatures higher than 1470 K are needed to achieve nearly total reduction of sub stitutional Fe3+. Interestingly, iron favors the reduction of chromium . The composition of the Fe-Cr particles is strongly dependent on thei r size, the Cr content being higher in particles smaller than 10 nm.