Title: Electrical Resistivity and Seebeck Coefficient of YbAl3 Compound
I prepared YbAl3 specimens utilizing a hot-pressing technique and so I measured the Seebeck coefficient and electrical electric resistance over the temperature scope 150-700K in an effort to measure their possible as thermoelectric stuffs. My consequences show that YbAl3 possesses an electrical power factor dual those of the state-of-the-art Bi2Te3 thermoelectric stuffs. So I can reason that YbAl3 is a promising campaigner stuff for thermoelectric coevals utilizing “ low temperature ” waste heat.
A thermoelectric energy convertor is really alone and dependable heat engine in which the negatron gas serves as the working fluid, BUT, its more wide-scale application has been limited by its comparatively low energy transition efficiency, so that research attempt has concentrated on bettering its public presentation by increasing the thermoelectric -of-merit, Z=a2/rl, of thermocouple stuffs, where a is the Seebeck coefficient, r the electrical electric resistance, l the thermic conduction ; a2/r is referred to as the electrical power factor.
To day of the month, all the established thermoelectric stuffs are semiconducting materials in which the thermal conduction consists chiefly of two parts, a lattice and an electronic constituent with the former being significantly the larger of the two. Solid province theory has provided theoretical theoretical accounts of the lattice thermic conduction and over the past four decennaries research attempts have focused on its decrease. Unfortunately, these attempts have met with limited success due to an attach toing debasement in electrical belongingss ( Rowe and Bhandari 1983 ) . Recently, attending has focused on increasing the electrical power factor and new stuffs, some with fresh constructions such as quantum Wellss ( Mensah and Kanyah 1992, Hicks and Dresselhaus 1993 ) and multiple possible barriers ( Moyzhes and Nemchinsky 1992, Rowe and Min 1994 ) are being proposed in an effort to accomplish this aim.
Traditionally, intermetallic compounds are considered inferior thermoelectric stuffs to semiconducting materials because they possess low s-of-merit. However, a member of this household of stuffs is the rare-earth intermetallic compound YbAl3, which exhibits unusual conveyance belongingss and may offer possible for development as a high public presentation thermoelectric stuff ( Van Deel et Al. 1974, Mahan and Sofo 1996 ) . The aim of this work is to look into the thermoelectric belongingss of this promising compound.
Specific geometries are normally necessary in the appraisal of thermoelectric belongingss and stuffs with a comparatively big dimension ( over 1 ten 1 ten 1 mm3 ) are normally required in the building of thermoelectric elements. Preparation of such “large” specimens of pure YbAl3 proved hard due to the peritectic nature of its stage diagram. Although big dimension specimens can be prepared utilizing arc-melting technique, stuffs prepared by this method are constantly a mixture of YbAl2 and YbAl3 with different proportion depending on the get downing composing and readying conditions. However, pure YbAl3 pulverization can be obtained by crystal growing or solid diffusion ( Rowe et al.1997 ) in an aluminum oxide crucible with extra aluminum utilizing a alleged “flux technique” ( Canfield and Fisk 1992 ) . A hot-pressing technique was employed to fix the compact specimens about 6 millimeters in diameter and 1.5 millimeter midst. Hot-pressing at 200 MPa was carried out in vacuity ( about 10-2 millimeter of mercury ) and at a temperature of 700 K for approximately 4 hours, followed by a heat intervention at about 900 K in an aluminum surplus environment for 15 hours. The denseness of the specimen is estimated to be approximately 88 % of its theoretical value. The Seebeck coefficient and electrical electric resistance as a map of temperature over the scope 150-700 K were measured “simultaneously” utilizing an setup described in ref. ( Rowe et al. 1997 ) .
The consequences of the measured Seebeck coefficient and electrical electric resistance are shown in 1 and 2, severally. The corresponding values for the established thermoelectric stuffs Bi2Te3 based metals ( Yim and Rosi 1972 ) are besides shown in the s for comparing ( broken lines ) . As expected, the electrical electric resistance is much lower than that of Bi2Te3 metal. However, although the Seebeck coefficient of YbAl3 is lower than that of Bi2Te3 based alloys, it is well larger than that of the other intermetallic compounds or metal metals. Consequently, a big electrical power factor is obtained over the whole temperature scope investigated as shown in 3. A maximal a2/r value of about 90 ten 10-4 W/mK2 is obtained at a temperature of around 250 K, which is about twice every bit big as that of the best thermoelectric stuffs antecedently reported, and over the temperature scope 300-700 K, it exceeds that of Bi2Te3 based metals by at least 50 % .
The temperature dependance of the Seebeck coefficient for both YbAl3 and Bi2Te3 based alloys appears similar: the Seebeck coefficient ab initio increases with an addition in temperature until it reaches a maximal around room temperature and so decreases with a farther addition in temperature. However, the electrical electric resistance of YbAl3 additions with an addition in temperature over the whole temperature scope, while that of Bi2Te3 exhibits a similar tendency as that of the Seebeck coefficient. Bi2Te3 based metals are narrow band-gap semiconducting materials and the temperature dependance of the Seebeck coefficient and electrical electric resistance can be explained within the model of semiconducting material conveyance theory. YbAl3 is a rare-earth intermetallic compound which exhibits unusual electrical belongingss due to the “Kondo effect” : a lower limit is observed in the temperature dependance of the electrical electric resistance accompanied by a significant value of the Seebeck coefficient. As a consequence, stuffs which exhibit the “Kondo effect” may possess a significantly big power factor. Furthermore, divergences from the Weidemann-Franz-Lorenz jurisprudence have besides been observed in “Kondo materials” ( Bauer 1991 ) , which may ease use of the thermic conduction in a similar manner to that in semiconducting materials ( White and Klemens 1992 ) .
In drumhead, YbAl3 exhibits a well larger electrical power factor than any other presently available thermoelectric stuffs over the temperature scope 300-500 K. It offers a typical advantage for electrical power coevals utilizing waste hot H2O ( & lt ; 425 K ) , where the electrical power denseness instead than the transition efficiency is a major consideration ( Rowe and Min, 1996 ) . Furthermore, an improved apprehension of its well big electrical power factor and possible divergence from the Weidemann-Franz-Lorenz jurisprudence may supply an penetration into increasing the thermoelectric -of-merit in other stuffs.
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.
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