Electronic Material Properties - und Geowissenschaften ...

In case of the undoped sintered B6O sample, the individual B6O grains are typically well
faceted with sharp edges. The grain-size distribution in this sample is bimodal with
larger grains (1-2 µm) embedded in a fine-grain B6O matrix. All B6O grains typically
reveal numerous stacking faults (similar to B4C particles). In the case of the doped,
sintered B6O sample, the grain size was greatly increased (by a factor of about 5x).
Stacking faults are a common feature of this B6O material even upon long-term anneal.
A secondary phase formation was observed in the Al-containing sample. With respect to
the phase diagram, the two aluminium borate compounds that are most likely to form
upon sintering are Al4B2O9 and/or Al18B4O33.
Conventional TEM (bright field; Figure 3) and HRTEM (Figure 4) were performed on the
crystalline secondary phase in addition to selected area electron diffraction (SAD). The
diffraction pattern given in Figure 4 can be indexed corresponding to a primitive
orthorhombic unit cell (a,b = 0.7617 and c = 0.2827 nm; Pbma) as well as with respect
to an A-centered orthorhombic symmetry (Amam). EDS analysis does not allow the
distinction between both phases, because this technique is rather insensitive to boron
(no quantitative analysis possible). The A-centered phase; however, should follow the
extinction rules (k+l=2n). Under the assumption that the rather weak {010}-type
reflections visible in the SAD-pattern
(Figure 4) are a result from 2 nd -order Laue
reflections, it is concluded that extinction
occurs. Therefore, the secondary crystalline
Al-compound observed in this material is
the Al18B4O33 phase. It should be noted that
the secondary phase shows an identical
orientation within an area of 20-50 microns
within the sample. This observation
indicates that this Al-phase is characterized
by a rather low nucleation rate, which is
uncommon for most secondary phases.
Fig. 3: TEM image of the secondary Al-phase,
Al18B4O33, observed in the doped, sintered B6O
sample.
Fig. 4: HRTEM image of the secondary Alphase
present in B6O. Note the presence of
stacking faults as well the weak reflections in
the SAD pattern (extinction).
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