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Journal of the American Ceramic Society—Guo et al.
Vol. 95, No. 2
Fig. 9. The oxide layer structure of MgB2.
the reactants can be very fast.18 The succession of linear por-
tions and sudden increase in the oxidation rate observed in
our experiments indicate that the oxide layer which is formed
on the surface is never protective in our oxidation conditions.
It is inferred that the polycrystalline MgO and t-Mg2B2O5
layer provide preferential diffusion paths along the oxides
grain boundaries. It can be deduced from the TG and XRD
experiments that Stage B is associated with the formation of
a non protective t-Mg2B2O5 oxide layer.
The weight gain rate increases dramatically at first and
then decreases gradually in Stage C (1343–1498 K). Simi-
larly, the acceleration of oxidation is due to the rapid oxida-
tion of Mg vapor released from reaction (2). The XRD
pattern [Fig. 5(c)] shows that t-Mg2B2O5 formed in Stage B
is transformed into monoclinic Mg2B2O5 (m-Mg2B2O5) as
temperature grows up. Figure 6(d) shows that the fine crys-
talline morphology formed in Stage B have been changed
and the samples agglomerated after heated up to 1423 K in
the air flow at a heating rate of 5 K/min. It can be deduced
that the phase transition destroys the diffusion paths in the
oxide layer, leading to the decrease of weight gain rate
observed in Stage C.
The third step of the decomposition of MgB2 occurred in
Stage D, leading to the increase of weight gain rate observed
in Stage D (1498–1553 K). The MgB2 powder samples
melted after being heated up to 1533 K in the air flow at a
heating rate of 5 K/min. We press fresh MgB2 powder into a
pellet for further investigation of MgB2 oxidation. The pellet
was also heated up to 1533 K in the air flow at a heating
rate of 5 K/min. The microstructure and composition of the
oxides layer grown on the cylindrical sample surface were
analyzed using SEM-EDS and XRD. The grown oxide layer
can consist of the stacking of two contributions: an external
oxide layer showing good crystallinity and an internal porous
oxide layer shown in Fig. 6. The internal porous oxide layer
is mainly composed of Mg3B2O6 and Mg2B2O5 identified by
XRD shown in Fig. 7. The external oxide layer is mainly
composed of MgO derived from the EDS measurement.
Moreover, there was a transparent film deposited on the cru-
cible surface. The transparent film is mainly composed of
B2O3. It can be inferred that m-Mg2B2O5 formed in Stage C
melted with the increase of temperature and then gradually
decomposed into MgO, B2O3, and Mg3B2O6 in Stage D. The
liquid B2O3 vaporized at high temperatures and then depos-
ited on the crucible surface when the samples were cooling
down. The condensed MgO layer on the sample surface can
act as a diffusion barrier, hindering the further oxidation.
Therefore, the decrease of weight gain rate is observed at the
end of Stage D. The fourth step of the decomposition of
MgB2 occurred in Stage E, leading to the small increase of
weight gain rate. However, the oxide layer structure of Stage
E is similar to that of Stage D.
this study and the applied research approach. Indeed, a
detailed description of the information of the bulk should be
carried out in future work.
IV. Conclusions
In summary, we have investigated the decomposition and
oxidation behavior of MgB2 using TG, XRD and SEM-
EDS. The experimental results indicate that the decomposi-
tion process of MgB2 includes four successive stages at
atmospheric pressure in the temperature range explored (298
–1673 K). There is only Mg in the vapor phase in the decom-
position process. Moreover, the weight loss decreases rapidly
stage-by-stage with increasing temperature. The mechanism
of MgB2 decomposition was discussed based on the experi-
mental results and reported reactions. The nucleation or
formation of boron-rich phases are the rate-limiting steps in
the decomposition process.
The TG curve shows that the oxidation process of MgB2
comprises five successive stages in the temperature range
explored (298–1673 K). Rapid oxidation of MgB2 starts after
873 K and hence, MgB2 can be handled safely without the
risk of oxidation below this temperature in air. There are
close relationship between the oxidation and decomposition
process of MgB2. Except for the first oxidation stage, the last
four oxidation stages correspond to the four decomposition
stages, respectively. The decomposition reactions occur dur-
ing the oxidation process. The increase in weight gain rate
and the exothermic peak observed in each oxidation stage are
ascribed to the rapid oxidation of the Mg vapor released from
the decomposition of MgB2. The microstructure and compo-
sition of the oxides formed in the oxidation process were
investigated using XRD and SEM-EDS. The oxide layer
structure was presented based on the experimental results.
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Above all, the oxide layer structure is presented in Fig. 9.
As mentioned above, the composition of the bulk at high
temperatures is complicated which is beyond the scope of