Thermoelectric properties of UB4 from 300 to 850 K

Article abstract of DOI:10.1016/j.jpcs.2004.06.074

Seebeck coefficient and the electrical conductivity for polycrystalline UⁿB (ⁿB₄: natural boron) were measured in the temperature range from 300 to 850 K. Seebeck coefficient increased with temperature from 8 μVK^-1

Full text of DOI:10.1016/j.jpcs.2004.06.074

Journal of Physics and Chemistry of Solids 66 (2005) 652–654
www.elsevier.com/locate/jpcs
Thermoelectric properties of UB4 from 300 to 850 K
a
Yoshimasa Nishi , Yuji Arita , Tsuneo Matsui , Kouta Iwasaki , Takanori Nagasaki
b
a,
a
a
*
aDepartment of Quantum Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
bResearch Center for Nuclear Materials Recycle, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Accepted 14 June 2004
Abstract
Seebeck coefficient and the electrical conductivity for polycrystalline UnB (nB4: natural boron) were measured in the temperature range
from 300 to 850 K. Seebeck coefficient increased with temperature from 8 mVKK1 at 300 K to 40 mVKK1 at 850 K, and the electrical
conductivity decreased with increasing temperature from 2.7!103 ScmK1 at 300 K to 2.3!103 ScmK1 at 850 K. The temperature
dependence of the electrical conductivity suggested that UB4 had semimetallic character. The power factor for UnB4 showed the value of
3.4!10K4 WmK1 KK2 at 850 K. The power factor for isotopically enriched U11B4 showed the value of 7.2!10K4 WmK1 KK2 at 850 K. It
was suggested that the power factor for UB4 was enhanced by the increase in carrier concentration with impurities.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: D. Electrical conductivity
1. Introduction
Furthermore, kph is able to controlled by isotopic
composition.
We have investigated thermoelectric properties of
URu2Si2, U2Ru3Si5 [2] and UB4 [3]. The power factor
for URu2Si2 and U2Ru3Si5 showed the maximum value of
1.2!10K4, 5.4!10K5 WmK1 KK2 at 1100 K, respect-
ively. The thermoelectric properties for UB4 were only
measured in the temperature range from 80 to 300 K.
Power factor and Z increased with temperature and showed
the maximum value of 1.3!10K5 WmK1 KK2 and 4.2!
10K3 KK1 at 300 K, respectively. Many compounds
including boron such as b-boron [4], boron carbide [5–7]
and a-AlB12 [4] indicate high thermoelectric properties at
high temperature. Therefore, UB4 is expected to exhibit
high thermoelectric properties at high temperature. The
thermoelectric properties for UB4, however, were not
reported except our previous study at low temperatures [3].
In this study, Seebeck coefficient and the electrical
conductivity for polycrystalline UnB4 (nB: natural boron)
were measured in the temperature range from 300 to 850 K
to investigate the thermoelectric properties for UB4. The
thermoelectric properties for U10B4 and U11B4, which were
prepared from 10B or 11B enriched boron powder and
depleted uranium were also reported to confirm the isotope
effect for electrical properties.
In the nuclear fuel cycle, a large amount of depleted
uranium (99.8% 238U) is discharged. The depleted uranium
is not adequate for the re-use as nuclear fuel except the
blanket in fast-breeder reactor. Therefore, the utilization of
the depleted uranium is important to complete the nuclear
fuel cycle.
The thermoelectric material directly converts heat into
electricity, and vice versa. The thermoelectric figure of
merit (Z), ZZ(a2$s/k) where a is Seebeck coefficient, s is
electrical conductivity and k is thermal conductivity, is
proportional to m2/3(mc/kph) [1], where m is the effective
mass of the charge carrier, mc is the charge carrier mobility
and kph is the thermal conductivity by phonons. In general,
materials having large average atomic number tend to
exhibit large mc/kph, and f-electrons lead to large effective
mass of the charge carrier. Since uranium has the large
atomic number (92) and 5f-electrons, the thermoelectric
properties of materials including U are of interest.
* Corresponding author. Tel: C81 52 789 4682; fax: C81 52 789 4691.
E-mail address: t-matsui@nucl.nagoya-u.ac.jp (T. Matsui).
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.06.074

Y. Nishi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 652–654
653
2. Experimental
Each specimen was prepared by the arc-melt method in
an argon atmosphere using nB powder (natural boron, Koch
Chemicals Ltd), isotopically enriched 10B and 11B powders
(TRACE Co., Ltd) and depleted-U metal (Power Reactor
and Nuclear Fuel Development Corp.). The isotopic
compositions for boron powders and U metal are shown in
Table 1. A presence of a single phase for each sample was
confirmed by X-ray diffraction analysis.
Seebeck coefficient and the electrical conductivity were
measured in vacuum in the temperature range from 300 to
850 K. Seebeck coefficient was calculated within G10%
errors from the difference of temperature and electromotive
force between the both ends of sample. The electrical
conductivity was measured by the direct current four-probe
method.
Fig. 1. Temperature dependence of the electrical conductivity for UB4. (%)
UnB4, (C) U10B4, (B) U11B4, ($) UnB4 [3].
3. Results and discussion
10B143C and 11B142C) was smaller than that for natural
boron carbide nB4nC. In general, the thermal conductivity of
isotopically enriched materials are enhanced by the
reduction of the probability of the phonon-isotope scatter-
ing. The reduction of the thermal conductivity for
isotopically enriched boron carbide indicated that the
content of impurities in isotopically enriched powders was
larger than that in natural powder. Therefore, it was thought
that the electrical conductivity for U10B4 and U11B4, which
were made from the same isotopically enriched boron
powder as in the previous study [13], were larger than that
for UnB4 due to the impurities.
UB4 crystallizes in the ThB4-type structure (space group
P4/mbm) [8–10]. The lattice parameters of UnB4, U10B4 and
U11B4 are shown in Table 1 together with a literature value
of UnB4 [10]. From the isotopic composition dependence of
the lattice parameters for Ge [11] and C [12], the lattice
parameters are considered to depend on the isotopic
composition as a consequence of the zero-point motion.
The differences in the lattice parameters among three
samples, however, could not be detected.
The measured electrical conductivity (s) is plotted as a
function of temperature in Fig. 1 together with the electrical
conductivity for UnB4 below 300 K. The electrical conduc-
tivity for UnB4 decreased with increasing temperature from
2.7!103 ScmK1 at 300 K to 2.3!103 ScmK1 at 850 K. The
electrical conductivity of UnB4 at 300 K obtained in this
study is in good agreement with that obtained in our
previous study [3]. Although, the electrical conductivity of
metals decreases inversely proportional to the temperature,
the decreasing rate of the electrical conductivity for UB4
was 40% of that for metals. Moreover, the magnitude of the
electrical conductivity is smaller than that for metals. These
behaviors suggest that UB4 is semimetal.
Seebeck coefficient (a) is plotted as a function of
temperature in Fig. 2 together with that for UnB4 below
300 K [3]. Seebeck coefficient for UnB4 was positive
and increased with temperature from 8 mVKK1 at 300 K to
As is seen in Fig. 1, the electrical conductivities for
U10B4 and U11B4 were larger than that for UnB4. According
to our previous study [13], the thermal conductivity for
isotopically enriched boron carbide (10B412C, 11B412C,
Table 1
The lattice parameters and isotope abundances
Sample
Lattice parameter
Isotopic composition
a (nm)
c (nm)
10B (at.%)
11B (at.%)
80.2
0.25
UnB4
U10B4
0.70771(8)
0.70769(8)
0.39813(6)
0.39810(6)
19.8
99.75
0.48
19.8
U11B4
UnB4 [10]
0.70778(10) 0.39819(9)
0.70773(2) 0.39816(2)
99.52
80.2
Fig. 2. Temperature dependence of Seebeck coefficient for UB4. (%) UnB4,
(C) U10B4, (B) U11B4, ($) UnB4 [3].

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Y. Nishi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 652–654
4. Conclusion
The electrical conductivity decreased with increasing
temperature and Seebeck coefficient was larger than that for
metals, which suggested that UB4 indicated semimetallic
behavior.
The power factor for UnB4 showed the maximum value of
3.4!10K4 WmK1 KK2 at 850 K. The power factor of U10B4
and U11B4 were larger than that for UnB4, indicating that the
power factor was enhanced by the impurities. The power
factor of U11B4 showed the value of 7.2!10K4 WmK1 KK2
at 850 K which was close to a standardvalue for practical use,
a2$sZ1!10K3 WmK1 KK2. Therefore, UB4 is a promising
thermoelectric material. Moreover, improvement of the
power factor is expected by the control of the charge carrier
density.
Fig. 3. Temperature dependence of the power factor for UB4. (%) UnB4,
(C) U10B4, (B) U11B4, ($) UnB4 [3]. (—) U [14,15], (– – –) B4C [5],
(-$-$-) B4C [6]. (-$$-) B4C [7], (- - - -) b-boron [4], (//) a-AlB12 [4].
40 mVKK1 at 850 K. The Seebeck coefficient of UnB4 at
300 K obtained in this study is also in good agreement with
that obtained in our previous study [3]. The magnitude of
Seebeck coefficient was larger than that for metals, which
corresponded to semimetallic behavior of the electrical
conductivity. Seebeck coefficients for U10B4 and U11B4 were
larger than that for UnB4 below 600 K, and the deference
tended to be small with increasing temperature above 600 K.
The power factor (a2$s), which is the electrical output of
the thermoelectric devices, is shown in Fig. 3. The power
factor for UB4 increased monotonically with temperature
and showed the value of 3.4!10K4 WmK1 KK2 for UnB4,
6.6!10K4 WmK1 KK2 for U10B4 and 7.2!10K4 WmK1
KK2 for U11B4 at 850 K. Although the power factor for UnB4
was smaller than that of b-boron [4] and U metal [14,15], the
power factor for U10B4 and U11B4 up to 850 K was
larger than that for U metal [14,15], b-boron [4], boron
carbide [5–7] and a-AlB12 [4]. These relatively high
power factors for U10B4 and U11B4 are ascribed to
large carrier concentrations introduced by non-isotopic
impurities.
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