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32iSHS 2019
Moscow, Russia
SELF-PROPAGATING HIGH-TEMPERATURE SYNTHESIS OF Fe2VAl-BASED THERMOELECTRIC MATERIALS
K. Abe*', N. Okinaka', and T. Akiyama'
a
aHokkaido University, Sapporo, Hokkaido, 060-8628, Japan
*e-mail: k_abe@eng.hokudai.ac.jp
DOI: 10.24411/9999-0014A-2019-10003
INTRODUCTION
Thermoelectric conversion is a promising candidate among various heat recovery methods for the next generation. Thermoelectric materials have been recently focused for direct conversion from thermal energy to electric one by Seebeck effect. Thermoelectric properties are commonly evaluated using the dimensionless figure of merit ZT:
where Z is the figure of merit (K-1), S is the Seebeck coefficient (VK-1), T is the absolute temperature (K), p is the electric resistivity (fim), and k is the thermal conductivity (Wm-1K-1). To improve the thermoelectric properties, higher Seebeck coefficient (absolute value), and lower electrical resistivity and thermal conductivity are needed.
Fe2VAl has been focused as environment-friendly-material because it contains relatively plentiful and non-toxic elements. Theoretically, SHS method to produce Fe2VAl is much better than a conventional one because of lower energy consumption [1]. We already reported the possibility of producing Fe2VAl using self-propagating high-temperature synthesis (SHS) [2]. During SHS, the products melted and macroscopically separated into two parts (Fe2VAl and AhO3) due to gravity [2]. To improve its thermoelectric properties of the SHSed Fe2VAl, ball-milling treatment and substitution of fourth element can be the effective ways. This is because after ball-milling and sintering, the grain size decreases and the thermal conductivity decreases by the effect of phonon scattering. Substitution of a fourth element such as Si significantly increases its Seebeck coefficient.
The purpose of this study is to improve the thermoelectric properties of SHSed Fe2VAl by substitution of Si to Al-site and post-treatment of ball-milling and SPS.
EXPERIMENTAL
The following thermite reaction was used for SHS in this study:
Here, x is molar substitution ratio of Si to Al site (x = 0-0.10).
Commercially available powders of Fe (3-5 |im), V2O5 (75 |im), Al (3 ^m), and Si (45 p,m) were weighed and then they were well mixed for 1 h before the SHS experiment. Next, the resulting mixture was placed in a vertically-placed graphite crucible and a carbon foil igniter was put on the powders. The powders were electrically flashed at a room temperature in an argon atmosphere, as shown in Fig. 1.
The products were fragmented to particles below 300 p,m and then the particles were milled with a planetary ball-milling system using stainless balls in a stainless pot at a ball-to-sample mass ratio of 10:1. The ball-milling was conducted for 12 h at 350 rpm. The milled samples
ZT = S2T/px
(1)
2Fe + (1/2)V2O5 + ((8/3)-x)Al + xSi ^ Fe2VAli-xSix + (5/6)AhO3
(2)
XV International Symposium on Self-Propagating High-Temperature Synthesis
were finally SPSed at 1273 K for 1 min in vacuum at a uniaxial pressure of 25 MPa. Thermoelectric properties were evaluated using a Seebeck coefficient/electrical resistance measurement system (ZEM-3, ULVAC-RIKO, Yokohama, Japan) and a laser flash thermal constant analyzer (TC-7000, ULVAC-RIKO, Yokohama, Japan).
Graphite crucible Filter
► R. P.
Graphite crucible
Control box
Fig. 1 Schematic image of the SHS apparatus.
RESULTS AND DISCUSSIONS
The calculated adiabatic flame temperatures for Eq. (2) were 2327 K at every x value. This temperature corresponds to the melting point of Al2O3, meaning that Al2O3 was partially molten during SHS. The melting point of Fe2VAl is around 1800 K, implying all Fe2VAh-xSix products were molten during SHS.
300 400 500 600 Temperatiire. T K
Fig. 2. Electrical resistivity (p) of the SHSed Fe2VAh-xSix.
Fig. 3 Seebeck coefficient (S) of the SHSed Fe2VAh-xSix.
Figure 2 shows the temperature dependence of the electrical resistivity of SHSed Fe2VAh-xSix. The electrical resistivity decreased with temperature increasing because of its semiconductor-like tendency. When the fraction of Si increased, the electrical resistivity decreased. In Fe2VAh-xSix, the dominant carrier is electron and substitution of Si to the Al position increased the number of carrier because Si has four and Al has three valence electrons. The electrical resistivity is inversely proportional to the carrier concentration, so the electrical resistivity decreased as x value increased.
Figure 3 shows the temperature dependence of the Seebeck coefficient of SHSed Fe2VAh-xSix. Generally, Si-substituted Fe2VAh-xSix is n-type (negative Seebeck coefficient)
ÏSHS2019
Moscow, Russia
thermoelectric material [3]. Every Fe2VAli-xSix sample successfully showed a negative Seebeck coefficient. In addition, the absolute value of the Seebeck coefficient was much higher in Si-substituted Fe2VAli-xSix compared to Fe2VAl (x = 0). It is said that the Fermi level in Fe2VAl is located at the valley of pseudogap [4, 5] and the Seebeck coefficient is easily increased by substitution of small amount of a fourth element such as Si. Figure 4 shows the
temperature dependence of the thermal conductivity of SHSed Fe2VAli-xSix. The thermal conductivity was reduced as the Si amounts in Fe2VAli-xSix was increased. When Si is substituted in Fe2VAl, lattice strain resulting from the difference in crystal radius between Al (0.53 A) and Si (0.40 A) is introduced. Phonon scattering at the crystal lattice was more encouraged than that of stoichiometric Fe2VAl, resulting in the lower thermal conductivity. In this study, we succeeded to produce Si-substituted Fe2VAli-xSix and the thermoelectric properties were improved by the Si substitution. However, the thermoelectric properties of Fe2VAli-xSix, which was
30
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20
15
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1 1 1 1
200 300 400 500 600 Temperature, T/K
Fig. 4. Thermal conductivity (k) of the SHSed Fe2VAli-xSix.
700
produced by conventional arc-melting method or mechanical alloying method, is much better than those of SHSed Fe2VAli-xSix. We already found AhO3 was still left in the alloy part [2] and it might be the cause of the different thermoelectric properties.
CONCLUSIONS
Si-substituted Fe2VAl were synthesized via SHS, planetary ball milling, and SPS. The electrical resistivity was decreased as the amount of fourth-element substitution due to the increase in the number of dominant carrier. Fe2VAli-xSix showed higher negative Seebeck coefficient values than stoichiometric Fe2VAl. The thermal conductivity of Fe2VAli-xSix was decreased as the amount of fourth-element substitution because the phonon scattering was increased by the introduction of lattice strain which was caused by the difference in atomic radius between Al and Si.
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