S. Majumdar et al. / Thermochimica Acta 473 (2008) 45–49
49
varied starting from 400 ◦C. The optimum condition producing
non-pyrophoric, spherical powder of minimum agglomeration was
found to be as temperature 550 ◦C, time 30 min under reducing
atmosphere. Fig. 10 shows the formation of 1–1.5 m spherical
powder at the optimized conditions. The powder was easily flow-
able and thus suitable for preparation of cobalt slugs and pellets
by powder metallurgical processes. Fig. 11 is the XRD plot of the
cobalt powder produced at 550 ◦C showing the presence of both
hcp and fcc phases in the powder. Strong (1 1 1) reflections from fcc
cobalt was detected along with the (1 0 0), (1 0 1), etc., hcp reflec-
tions. Though the allotropic transition from hcp to fcc phase is at
417 ◦C, significant amount of high temperature fcc phase is retained
in the cobalt crystals. The peak broadening at each reflection is also
the indicative of formation of fine size cobalt powder.
4. Conclusions
Isothermal kinetics of thermal decomposition of cobalt oxalate
to cobalt was found to obey Avrami-Erofe’ev equations. The
increase in activation energy with increasing temperature revealed
that the decomposition involved multi-step reaction mechanisms.
The powder size was less than 100 nm at the decomposition
temperatures below 370 ◦C. Decomposition at 550 ◦C for 30 min
produced the cobalt powder suitable for making slugs and pellets
by powder metallurgical processing techniques. Existence of both
hcp and fcc phases was observed in the cobalt powder at room
temperature.
Fig. 10. Micron size cobalt powder produced after decomposition at 550 ◦C for
30 min.
Acknowledgements
The authors wish to thank Prof. I. Samajdar and Dr. S. L. Kamat
of Indian Institute of Technology Bombay, Powai, Mumbai, India
for extending his help in carrying out XRD and SEM experiments,
respectively.
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