Abstracts Book - IMRC 2018

• SD4-P005
SPINODAL TRANSFORMATION IN NANOSTRUCTURED CERAMIC
OXIDES PRODUCED BY MECHANICAL MILLING
Araceli Huerta 1
1 Escuela Superior de Física y Matemáticas IPN, Departamento de Física, Mexico.
Two ceramic systems are synthesized by means of mechanical milling and spark
plasma sintering (SPS). They consist of magnetite particles (Fe3O4, magnetic
phase) in a wüstite matrix (FexO, insulator phase) and magnesioferrite particles
(MgFe2O4, magnetic phase) in a magnesia and/or magnesiowüstite matrix (MgO
and/or Mg1-xFexO, insulator phase). The Fe base system is synthesized by
milling pure powders of C, Fe, Fe3O4 and Fe2O3 in different amounts in a lowenergy
ball mill. As for the Magnesia-magnesioferrite system, the starting
powders are MgO and Fe2O3 in different proportions by using high and ultrahigh
energy mills. As-milled powders have a nanocrystalline structure in both
systems. Low energy milling gives rise to an increasingly higher volume fraction
of wüstite as a function of milling time in the Fe1-xO-Fe3O4 system. Increasing
amounts of magnesiowüstite (MgxFe1-xO) form as a function of milling time.
Sintering is carried out at different temperatures varying from 673 to 1273 K in
vacuum by SPS. Sintering at low temperatures allows retention of nanosized
grains containing a fine dispersion of magnetic particles in a wüstite and
magnesiowüstite matrix. There is a clear indication that the new phase
nucleates via spinodal decomposition. Specific observations in transmission
electron microscopy show composition fluctuations in samples sintered at
relatively low temperature. Satellites in electron diffraction patterns and high
resolution electron microscopy are used to characterize the process.
Additionally, measurement of magnetic properties is used to determine both
the nature of the developed phases and the grain sizes. Mechanical milling
develops a nanocrystalline grain structure consisting mostly of a solid solution
of O and Fe and/or Mg and minor amounts of other phases. Spark plasma
sintering gives rise to a phase decomposition process producing a dispersion of
magnetic spinel particles in a nonconductive matrix (FexO or Mg1-xFexO).
Sintering at lower temperatures produces nanosized grain structures.
Measurement of magnetic properties reflects the constitution of the sintered
samples and the effect of grain size. It also allows determination of the
transformation sequence both during mechanical milling and sintering.