Title: Electrical Resistivity and Seebeck Coefficient of YbAl3 Compound
I prepared YbAl3 specimens using a hot-pressing technique and then I measured the Seebeck coefficient and electrical resistivity over the temperature range 150-700K in an attempt to evaluate their potential as thermoelectric materials. My results show that YbAl3 possesses an electrical power factor double those of the state-of-the-art Bi2Te3 thermoelectric materials. So I can conclude that YbAl3 is a promising candidate material for thermoelectric generation using "low temperature" waste heat.
A thermoelectric energy converter is very unique and reliable heat engine in which the electron gas serves as the working fluid, BUT, its more wide-scale application has been limited by its relatively low energy conversion efficiency, so that research effort has concentrated on improving its performance by increasing the thermoelectric -of-merit, Z=a2/rl, of thermocouple materials, where a is the Seebeck coefficient, r the electrical resistivity, l the thermal conductivity; a2/r is referred to as the electrical power factor.
To date, all the established thermoelectric materials are semiconductors in which the thermal conductivity consists mainly of two contributions, a lattice and an electronic component with the former being significantly the larger of the two. Solid state theory has provided theoretical models of the lattice thermal conductivity and over the past four decades research efforts have focused on its reduction. Unfortunately, these efforts have met with limited success due to an accompanying degradation in electrical properties (Rowe and Bhandari 1983). Recently, attention has focused on increasing the electrical power factor and new materials, some with novel structures such as quantum wells (Mensah and Kanyah 1992, Hicks and Dresselhaus 1993) and multiple potential barriers (Moyzhes and Nemchinsky 1992, Rowe and Min 1994) are being proposed in an attempt to achieve this objective.
Traditionally, intermetallic compounds are considered inferior thermoelectric materials to semiconductors because they possess low s-of-merit. However, a member of this family of materials is the rare-earth intermetallic compound YbAl3, which exhibits unusual transport properties and may offer potential for development as a high performance thermoelectric material (Van Deel et al. 1974, Mahan and Sofo 1996). The objective of this work is to investigate the thermoelectric properties of this promising compound.
Specific geometries are usually necessary in the assessment of thermoelectric properties and materials with a relatively large dimension (over 1 x 1 x 1 mm3) are usually required in the construction of thermoelectric elements. Preparation of such “large” specimens of pure YbAl3 proved difficult due to the peritectic nature of its phase diagram. Although large dimension specimens can be prepared using arc-melting technique, materials prepared by this method are invariably a mixture of YbAl2 and YbAl3 with different proportion depending on the starting composition and preparation conditions. However, pure YbAl3 powder can be obtained by crystal growth or solid diffusion (Rowe et al.1997) in an alumina crucible with excess aluminium using a so-called “flux technique” (Canfield and Fisk 1992). A hot-pressing technique was employed to prepare the compact specimens about 6 mm in diameter and 1.5 mm thick. Hot-pressing at 200 MPa was carried out in vacuum (about 10-2 torr) and at a temperature of 700 K for about 4 hours, followed by a heat treatment at approximately 900 K in an aluminium excess environment for 15 hours. The density of the specimen is estimated to be about 88% of its theoretical value. The Seebeck coefficient and electrical resistivity as a function of temperature over the range 150-700 K were measured “simultaneously” using an apparatus described in ref. (Rowe et al. 1997).
The results of the measured Seebeck coefficient and electrical resistivity are shown in 1 and 2, respectively. The corresponding values for the established thermoelectric materials Bi2Te3 based alloys (Yim and Rosi 1972) are also shown in the s for comparison (broken lines). As expected, the electrical resistivity is much lower than that of Bi2Te3 alloys. However, although the Seebeck coefficient of YbAl3 is lower than that of Bi2Te3 based alloys, it is substantially larger than that of the other intermetallic compounds or metal alloys. Consequently, a large electrical power factor is obtained over the whole temperature range investigated as shown in 3. A maximum a2/r value of about 90 x 10-4 W/mK2 is obtained at a temperature of around 250 K, which is almost twice as large as that of the best thermoelectric materials previously reported, and over the temperature range 300-700 K, it exceeds that of Bi2Te3 based alloys by at least 50 %.
The temperature dependence of the Seebeck coefficient for both YbAl3 and Bi2Te3 based alloys appears similar: the Seebeck coefficient initially increases with an increase in temperature until it reaches a maximum around room temperature and then decreases with a further increase in temperature. However, the electrical resistivity of YbAl3 increases with an increase in temperature over the whole temperature range, while that of Bi2Te3 exhibits a similar trend as that of the Seebeck coefficient. Bi2Te3 based alloys are narrow band-gap semiconductors and the temperature dependence of the Seebeck coefficient and electrical resistivity can be explained within the framework of semiconductor transport theory. YbAl3 is a rare-earth intermetallic compound which exhibits unusual electrical properties due to the “Kondo effect”: a minimum is observed in the temperature dependence of the electrical resistivity accompanied by a substantial value of the Seebeck coefficient. As a result, materials which exhibit the “Kondo effect” may possess a significantly large power factor. Furthermore, deviations from the Weidemann-Franz-Lorenz law have also been observed in “Kondo materials” (Bauer 1991), which may facilitate manipulation of the thermal conductivity in a similar way to that in semiconductors (White and Klemens 1992).
In summary, YbAl3 exhibits a substantially larger electrical power factor than any other currently available thermoelectric materials over the temperature range 300-500 K. It offers a distinctive advantage for electrical power generation using waste hot water (< 425 K), where the electrical power density rather than the conversion efficiency is a major consideration (Rowe and Min, 1996). Furthermore, an improved understanding of its substantially large electrical power factor and possible deviation from the Weidemann-Franz-Lorenz law may provide an insight into increasing the thermoelectric -of-merit in other materials.
This work is supported by the New Energy and Industrial Technology Development Organisation (NEDO), the Energy Conversion Centre, Japan. Prof. R.J.D.Tilley, Mr. R. Jones and Dr. D. Pasero are acknowledged for X-ray analysis of specimens.
Bauer, E., Adv. Phys., 40, (1991), p417
Canfield, P.C. and Fisk, Z., Philosophical Magazine, 65, 6, (1992), p1117
Hicks, L.D. and Dresselhaus, M.O., Phys. Rev. B47, (1993), 12 p727
Mensah, S.Y. and Kanyah, G.K., J. Phys: Condens. Mater. 4, (1992), p919
Mahan, G.C. and Sofo, J. O., Proc. Natl. Acad. Sci. USA, Vol 93, July, (1996), p7436
Moyzhes, B.Y. and Nemchinsky, V., Proceedings of 11th International Conference on Thermoelectrics, (1992), Arlington, Tx., USA, p232
Rowe, D.M. and Bhardari, C.M., Modern Thermoelectrics (Holt, Rinehart and Winston, 1983)
Rowe, D.M. and Min, G., Proceedings of 13th International Conference on Thermoelectric, (1994), Kansas City, USA, p339
Rowe, D.M. and Min, G., IEE Pro.-Sci. Meas. Technol. Vol. 143. No. 6., (1996), 351
Rowe, D.M., Min, G., Williams, S.G.K., Kuznestsov, V. and Aourn, A., NEDO Technical Results Report: TR3 (1996-1997), University of Wales, Cardiff, (1997)
Van Daal, H.J., Van Aken, P.B. and Buschow, K.H.J., Phys. Lett., 49A, 3, (1974), p246
White, D.P. and Klemens, P.G., J. Appl. Phys., 71 (9), (1992), p4258
Yim, W.M. and Rosi, F.D., Solid-State Electronics, 15, (1972), p1121
