1
0
Q. Zhang et al. / Journal of Alloys and Compounds 464 (2008) 9–12
Table 1
Calculated lattice parameters and relative densities for Mg2−xCaxSi compounds
This is in accordance with the previous reports that the scat-
tering of the lattice vibration is predominant [9]. The decreasing
mobility results in the decreasing electrical conductivity in spite
of the increasing carrier concentration with increasing temper-
ature. Correspondingly, all the measured Seebeck coefficients
increase with temperature, reach peak values at about 700 K
and decrease at higher temperatures because of an increasing
number of thermally excited minority carriers. For x = 0, a max-
x
0
0.01
0.03
0.05
0.07
0.1
a ( A˚ )
6.349 6.354
6.363
6.367
6.375
6.384
Relative
98
95
97
92
91
95
density (%)
−
1
imum value of −271 V K is obtained at about 640 K. All the
compounds are n-type conduction with the negative Seebeck
coefficients.
tance of the sample measured with the forward and reversed currents, in order to
eliminate any induced thermoelectromotive force. For the Seebeck coefficient
measurements, a small heater was powered to produce a temperature differ-
ence ꢀT between two ends of the sample. The thermoelectric voltage ꢀV and
With increasing calcium content, the electrical conductivity
increases up to the maximum value at x = 0.03, then decreases.
Calcium has a lower ionization energy relative to magnesium.
The presence of calcium as the ionized impurities in the samples
enhances the scattering of carriers, results in a slower decrease of
electrical conductivity with increasing temperatures. However,
with higher contents of calcium, the overall electrical conduc-
tivities of the samples decrease due to the existence of MgO as
shown in Fig. 1. The valence electron number of calcium is equal
to that of magnesium, making it neither a donor or an acceptor.
However, the lower electronegativity of Ca makes the outer-shell
electrons of calcium atoms be more easily lost than those of
magnesium. Therefore the alloying of Ca in Mg2Si based com-
pounds increases the electronic carrier concentration and hence
the electrical conductivity, while the Seebeck coefficient is obvi-
ously decreased. The total electrical properties weighted by the
power factors are calculated. The highest value for pure Mg2Si
◦
◦
temperature difference ꢀT were measured from ꢀT ≈ 0 C to about 5 C by an
Agilent 34401A multimeter. The Seebeck coefficient was then calculated by the
slope of the line of ꢀV versus ꢀT fitted with about 100 pairs of measured data.
The thermal diffusivity a and the specific heat capacity Cp of the samples were
measured by a laser flash apparatus (Netzsch LFA 457) and a thermal analyzer
(
Netzsch DSC 404), respectively. The thermal conductivity was calculated from
the relationship κ = ρaCp, where ρ is the density of the material.
3
. Results and discussion
The XRD results of Mg2−xCaxSi (x = 0, 0.01, 0.03, 0.05, 0.07,
0
.1) samples are presented in Fig. 1. The major peaks of the
binary silicide, Mg2Si, can be indexed to the face-centered cubic
structure with the space group of Fm3m according to JCPDS
3
5-0773. With increasing content of calcium, the fluidity during
meltingandthereforethehomogeneityoftheternarycompounds
become poorer. We see the increasing amount of the MgO phase
with increasing x value in Fig. 1. Because the ionic radius of cal-
cium is larger than that of magnesium, a little left shift indicates
the solubility of calcium according to the Bragg equation. The
lattice parameters of Mg2−xCaxSi samples are estimated from
the XRD data in Fig. 1 and summarized in Table 1. The lattice
parameter for x = 0 is consistent with the previous report [12],
and increases with increasing calcium content.
The electrical conductivity σ and Seebeck coefficient α of the
Mg2−xCaxSi samples are plotted in Fig. 2 versus measuring tem-
perature. Theelectricalconductivitiesofallthesamplesdecrease
with increasing temperature, showing metal-like conduction.
−
3
−1 −2
is 2.37 × 10 W m
K , which is higher than the results in
other works [6,13]. The solubility of calcium degraded the power
factors of all the compounds. The negative effect of the decrease
in the Seebeck coefficient exceeds the positive effect of the
increase in the electrical conductivity.
The thermal conductivities of Mg2−xCaxSi samples are pre-
sented in Fig. 3(a) plotted against temperature. The κ values of
all samples decrease with increasing temperature at 300–600 K.
The results in Fig. 3(a) show that the alloying of calcium in
Mg2Si could not reduce the thermal conductivity. There are
many effects on the thermal conductivity when calcium is
alloyed in Mg2Si, including the increase of κ due to the elec-
tronic contribution and the decrease of κ due to the enhancement
of phonon scattering by alloyed calcium atoms and also by the
MgO inclusions. In Fig. 3(b), we see that the ratios of electron-
to phonon-conduction, σ/κ, of all calcium alloyed ternary sili-
cides are remarkably higher than the pure Mg2Si. This means
that the electronic conduction for the samples containing cal-
cium is more dominative in the total thermal conductivity than
that for pure Mg2Si.
Fig. 4 shows the dimensionless figure of merit of the
Mg2−xCaxSi samples. The ZT values for all samples increase
with increasing temperature, mainly because the α increase with
temperature shown in Fig. 2(b) and the σ/κ ratios are nearly
independent of temperature as shown in Fig. 3(b). In Fig. 4
we see that the alloying of calcium in Mg2Si decreases ZT due
to the decrease of the Seebeck coefficient. The highest ZT is
0
.41 for the pure Mg2Si and 0.34 for Mg1.99Ca0.01Si at 660 K,
Fig. 1. XRD patterns for Mg2−xCaxSi compounds.
respectively. Although the ZT values measured in the present
Zhang
