selected for 5 min holding time for the first set of experiments.
The sample compacted at 650 C has achieved a
densification of only 85 %. The samples compacted at 750
and 850 C were further compacted for 2 and 0 min
holding time at respective temperatures.
Powder XRD analysis was performed to determine the
crystal phases: Fig. 1a–i presents several XRD patterns
from the as prepared and compacted Mg2Si nanopowder
and compacts obtained under different SPS parameters (as
detailed in Table 1). XRD pattern from pristine bulk Mg2Si
is shown in Fig. 1a and it confirms some contamination of
oxides and Si impurities. Figure 1b clearly shows after ball
milling Mg2Si has major crystalline phase, and it also
revealed that traces of MgO and Si phases are still present
in the ball-milled Mg2Si powder. The presence of MgO and
Si originate from raw starting materials and partly due to
the oxidation of Mg2Si during the BM process [24]. XRD
patterns from Mg2Si powder and compacted samples were
indexed using reference patterns for Mg2Si (JCPDS#
00-035-0773), MgO (JCPDS# 00-045-0946), and Si
(JCPDS# 04-001-7247). Figure 1c–i clearly show that the
peak intensity of free Si is reducing during the SPS processing.
It is also important to note that the XRD peaks of
BM Mg2Si powder are significantly broader as compared to
the sintered samples revealing the increase in crystallite
size of the compacted samples under sintering conditions.
The presence of MgO phase is detrimental for TE performance
of silicide, as reported by Fiameni et al. [25]. The
pristine materials contain about 6 % of the MgO phase as
reported in earlier works [25]. All the samples sintered at
750 C present lower MgO amounts (10 %) with respect to
the samples sintered at 850 C (in as much as approximately
19 %). Rietveld refinement was employed to estimate
the crystallite size and the average crystallite size is
reported in the range from 120 to 240 nm (See Table 1).
The geometrical (d = m/V) and theoretical density values
(Rietveld refinement) are reported in Table 1 for samples