Thermoelectric properties of some metal borides

Article abstract of DOI:10.1016/j.jssc.2003.02.005

Polycrystalline AlMgB₁₄ and some hexaborides (CaB₆, SrB₆, YbB₆, SmB₆, and CeB₆) were synthesized to examine their thermoelectric properties. Single phase of orthorhombic AlMgB₁₄, which contains B₁₂ icosahedral clusters as building blocks, was obtained at sintering temperatures between 1573 and 1823K. Seebeck coefficient (α) and electrical conductivity (σ) of the phase were about 500μV/K and 10^-11/Ωm at room temperature, respectively. These values are comparable to those of metal-doped β-rhombohedral boron. On the other hand, metal hexaborides with divalent cation possessed large negative α ranging from -100 to -270μV/K at 1073K. Calculated power factors of CaB₆ and SrB ₆ exceeded 10^-3W/K²m within the entire range of temperature measured. As a result, they can be thought as promising candidates for n-type thermoelectric material.

Full text of DOI:10.1016/j.jssc.2003.02.005

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 471–475
Thermoelectric properties of some metal borides
Masatoshi Takeda,ꢀ Tadahiro Fukuda, Ferrer Domingo, and Takahiro Miura
Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka 940-2188, Japan
Received 1 October 2002; accepted 27 February 2003
Abstract
Polycrystalline AlMgB14 and some hexaborides (CaB6, SrB6, YbB6, SmB6, and CeB6) were synthesized to examine their
thermoelectric properties. Single phase of orthorhombic AlMgB14, which contains B12 icosahedral clusters as building blocks, was
obtained at sintering temperatures between 1573 and 1823 K. Seebeck coefficient (a) and electrical conductivity (s) of the phase were
about 500 mV/K and 10ꢁ1 1/O m at room temperature, respectively. These values are comparable to those of metal-doped b-
rhombohedral boron. On the other hand, metal hexaborides with divalent cation possessed large negative a ranging from ꢁ100 to
ꢁ270 mV/K at 1073 K. Calculated power factors of CaB6 and SrB6 exceeded 10ꢁ3 W/K2 m within the entire range of temperature
measured. As a result, they can be thought as promising candidates for n-type thermoelectric material.
r 2003 Elsevier Inc. All rights reserved.
Keywords: AlMgB14; Hexaboride; Seebeck coefficient; Electrical conductivity; Power factor
1. Introduction
absolute value of the a decreases. The doping of certain
metals, such as V, Cr and Fe, changes a from positive to
negative [5,6,14], but the TE properties of the n-type
boron-rich semiconductors were insufficient compared
to p-type ones. Whereas further doping will be effective
for improvement, it is difficult to increase doping level,
because maximum amount of dopant is limited to a few
percent due to the restriction of the doping site in the
crystal structure. It is, therefore, of practical and
scientific interest to investigate TE properties of borides
other than boron carbide or b-boron type structure,
which have higher metal-to-boron ratio.
In the present study, we synthesized and examined
some metal borides, more specifically, AlMgB14 and
metal hexaborides (CaB6, SrB6, YbB6, SmB6, and
CeB6). The AlMgB14 is an orthorhombic phase contain-
ing B12 icosahedral clusters identical with those in b-
boron, but arrangement of the cluster is different. In the
AlMgB14 structure, there are large openings that can
accommodate boron, aluminum, or magnesium atoms.
The hexaborides have almost the same metal-to-boron
ratio as AlMgB14 phase, but the clusters constituting the
hexaboride are octahedrons. They possess semiconduct-
ing or metallic character in accordance with the
constituent metal [15]. We examined TE properties of
these metal borides, and discussed their potential as TE
materials.
One of the promising application fields for boron-rich
solids is high-temperature thermoelectric (TE) conver-
sion [1]. Intensive studies on TE properties have been
carried out on boron carbides [2] including single
crystals [3] and thin films [4], and metal-doped b-
rhombohedral boron (b-boron) [5,6] because of their
large Seebeck coefficient (a) and low thermal conduc-
tivity. They contain 12-atom icosahedral clusters (B12)
as building blocks, and their large a and hopping type
conduction are attributed to the existence of the clusters
[7]. At high temperatures, the boron carbides’ a is
temperature independent or slightly increases with
temperature, while the electrical conductivity (s) in-
creases [3]. As a consequence, relatively large TE figure-
of-merit, Z; can be obtained at high temperatures.
However, all boron carbides and doped ones [8–11]
studied are p-type semiconductor except for n-type
behavior reported in Ni-doped ones [12,13]. As for the
b-boron, metal-doped ones have been studied to
improve TE properties and to search for n-type
materials. Significant increase in s by the doping results
in the improvement of total performance, although the
ꢀCorresponding author. Fax: +81-258-47-9770.
E-mail address: takeda@mech.nagaokaut.ac.jp (M. Takeda).
0022-4596/$ - see front matter r 2003 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2003.02.005

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472
2. Experimental
Polycrystalline bulk of AlMgB14 was prepared by
pulsed electric current sintering (PECS) method. Prior
to the sintering, precursor powders of Al–B and Mg–B
with a metal/boron ratio of 1/7 were prepared because
melting temperatures of Al and Mg are extremely low
compared to that of boron. For the preparation of the
precursors, Al (99.99% purity), Mg (99.9% purity), and
B (99% purity) powders were used. Mixture of the
precursors was then sintered at 50 MPa for 15 min under
Ar (99.99% purity) atmosphere.
We prepared powders of hexaborides according to the
borothermal reaction,
MOx þ ð6 þ xÞB-MB6 þ xBOm;
ð1Þ
where M is Ca, Sr, Yb, Sm, or Ce. A mixture of metal-
oxide and B powders was pressed into a pellet, and
heated at 1473 K for 24 h in vacuum. The metal-oxides
used as starting materials were CaO (99.9% purity), SrO
(99% purity), Yb2O3 (99.99% purity), Sm2O3 (99.9%
purity), and CeO2 (99.99% purity). The resultant MB6
powders were then sintered by the PECS at 2073 K and
50 MPa under Ar atmosphere.
Fig. 1. Temperature dependence of electrical conductivity of sintered
AlMgB14. For comparison, those of boron carbide, b-boron, and
Mg2B105 are also shown. The arrows indicate the small kink (see text).
The phases of the powders and the sintered bodies
were determined by XRD, and microstructure of the
bulks was observed by SEM. Electron probe micro
analysis (EPMA) was used to examine contamination
and segregation in the sintered specimens. Only the
constituent elements were detected by the EPMA, but it
was difficult to determine precisely the composition of
the specimens, because of the low sensitivity of the
EPMA to light elements such as B. In the present paper,
we use the nominal compositions of the specimens.
The s of the specimens cut into rectangular plates was
measured by standard four-probe technique. The a of
them was calculated from the slope of several thermo-
electric voltage vs. temperature difference measurements
and corrected for the probe leads. These thermoelectric
properties were measured from room temperature up to
1073 K.
Fig. 2. Temperature dependence of Seebeck coefficient of sintered
AlMgB14. For comparison, those of boron carbide, b-boron, and
Mg2B105 are also shown.
boron (B105Mg2), which were also synthesized by the
same sintering method as that used for the AlMgB14 and
the metal hexaborides. The AlMgB14’s s is comparable
to that of Mg2B105 and exhibits similar temperature
dependence, in which there is a small kink. Temperature
dependence of the s for AlMgB14 and Mg2B105 is likely
to be explained by hopping-type conduction rather than
Arrhenius-type conduction below the temperature at
which the small kink appeared. Hopping-type conduc-
tion mechanisms are usually applied to explain transport
property of boron-rich semiconductors [16,17]. Tem-
perature dependence of boron carbides’ s can be
explained by bipolaron hopping expressed as
3. Results and discussion
3.1. AlMgB14
XRD measurements revealed that single phase of
orthorhombic AlMgB14 phase was obtained at sintering
temperatures between 1573 and 1823 K, while higher
sintering temperatures led to the appearance of g-AlB12-
type phase. Temperature dependence of the TE proper-
ties was measured for the AlMgB14 sintered at 1773 K.
Temperature dependence of the s and the a are shown in
Figs. 1 and 2, respectively, together with corresponding
data for a boron carbide, b-boron, and Mg-doped b-
sT ¼ A expðꢁE=kTÞ;
ð2Þ

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while variable-range-hopping expressed as
1=4
s ¼ s0 expfꢁðT0=TÞ
g
ð3Þ
is applied to explain those of metal-doped b-borons’ s in
many cases. However, difference in temperature depen-
dence among these two hopping conductions and
Arrhenius-type conduction is small when temperature
range is not wide enough. It is therefore difficult to
determine conduction-type of AlMgB14 from the s:
As shown in Fig. 2, the a of the AlMgB14 is positive
and large, which is similar to those of other boron-rich
semiconductors containing icosahedral clusters. Almost
temperature independent or slightly increasing tempera-
ture dependence of a at high temperatures is predicted
by theories on hopping conduction [16,18–20]; actually,
boron carbide exhibits such temperature dependence. As
for the AlMgB14, a exhibits weak temperature depen-
dence at lower temperature measured, while it decreases
more rapidly at higher temperature. Such a situation is
also observed in Mg2B105. It seems that the weak
temperature dependence in a is consistent with hopping-
type behavior in s: And the decrease in a at high
temperatures may be attributed to the increase of
thermally activated carriers with temperature.
Fig. 3. Temperature dependence of electrical conductivities of
hexaborides.
3.2. Hexaborides
According to EPMA analysis, there were B and CaB6
phases in the sintered CaB6 specimen, while no B peak
could be observed by XRD. Since the amount of B
phase is small, the measured thermoelectric properties
are considered to represent those of CaB6 phase. As for
the other hexaborides prepared, no secondary phase was
detected by XRD, SEM, and EPMA. Relative density of
the specimens was more than 90%, and there was no
significant difference among the specimens studied.
Figs. 3 and 4 show temperature dependence of s and
a; respectively. As shown in the figures, the divalent
hexaborides (CaB6, SrB6, and YbB6) possess large
negative a with relatively large s; while the a for
trivalent hexaboride (CeB6) is small. The trivalent
hexaborides are understood as metallic systems, and
divalent ones are considered as semiconductors or
semimetals [15,21,22]. The small a and large s of CeB6
are typical character of metals. The intermediate-valent
hexaboride (SmB6) possess small a above room tem-
perature, although large negative a was reported at low
temperatures [23]. These results suggest that the trivalent
and intermediate-valent hexaborides are not suitable for
high-temperature TE material.
Fig. 4. Temperature dependence of Seebeck coefficients of hexabor-
ides.
change the electronic property of the hexaborides from
small gap semiconductor to semimetal, resulting from
slight band overlapping [22,25]. The metallic tempera-
ture dependence of the s observed in the divalent
hexaborides except for YbB6 appears to be attributed to
the band overlapping. And the large negative Seebeck
coefficients indicate a low concentration of n-type
carriers at the Fermi level. The different behavior
between YbB6 and other divalent hexaborides may be
caused by the compositional and structural difference,
namely non-stoichiometry and structural defect, which
affect the band overlapping of the divalent hexaborides.
Further investigation is needed to elucidate the origin of
the difference. Nevertheless, present results and pre-
viously reported theoretical and experimental results
In the case of divalent hexaborides, early theoretical
study predicted the divalent hexaborides to be polar
semiconductors, with a complete charge transfer from
metal atom to the B6 octahedron [24]. Recent band
calculations, however, have shown that the changes in
inter- and intra-octahedron B–B distances can easily

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474
suggest that the negative Seebeck coefficient is a
common feature of the divalent hexaborides. Imai et al.
predicted a negative Seebeck coefficient in divalent
hexaborides on the basis of first principle calculations
and Mott’s theory [26], and negative a was reported in
CaB6, YbB6, and EuB6 [27–30]. To the best of our
knowledge, TE properties of SrB6 have never been
reported. As shown in Fig. 4, SrB6 also possesses large
negative a:
trivalent and intermediate-valent ones possess as as
small as those of typical metals. In particular, CaB6 and
SrB6 were found to be promising candidates for n-type
TE materials in boron-rich solids, because their PFs
were larger than boron carbide and comparable to those
of TE materials currently used. Measurements on
thermal conductivity and further improvements are
necessary for the TE application. The substitution of
metal atoms in the hexaborides with other atoms might
be effective to reduce the thermal conductivity and
improve the electrical properties. Such attempts are now
in progress.
3.3. Power factor
Fig. 5 shows the power factors (PF), expressed as a2s;
for AlMgB14 and divalent hexaborides together with
those of the boron carbide and the Mg2B105 for
comparison. The AlMgB14’s PF is comparable to that
of the Mg2B105 and increases with temperature. How-
ever, their low ss lead to their PF insufficient for the TE
application.
In contrast, the divalent hexaborides possess large PF
values. Particularly, the PFs of CaB6 and SrB6 are larger
than that of boron carbide in the entire range of
temperature we measured, and their PF are almost
temperature independent, which suggests the possibility
to be used within a wide temperature range.
Acknowledgments
This work was supported in part by Yazaki Memorial
Foundation for Science & Technology and by New
Energy and Industrial Technology Development Orga-
nization (NEDO).
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