Erbium boride composites with high ZT values at 800 K
- PDF / 180,054 Bytes
- 4 Pages / 432 x 648 pts Page_size
- 31 Downloads / 213 Views
Erbium boride composites with high ZT values at 800 K Frederick C. Stober1,2 and Barbara R. Albert1 1 Eduard-Zintl-Institute of Inorganic and Physical Chemistry, Technische Universität Darmstadt, Petersenstr. 18, 64287 Darmstadt, Germany 2 Bosch Solar CISTech GmbH, Münstersche Str. 24, 14772 Brandenburg an der Havel, Germany
ABSTRACT Single phase erbium borides ErB2, ErB4, and ErB12 show Seebeck coefficients and power factors with absolute values that are significantly lower than those of a stable Er-B multi phase composite obtained through high temperature solid-solid reaction from the elements (molar ratio Er:B = 1:6). According to quantitative Rietveld analysis the composite consists of erbium diboride (1 %), tetraboride (83 %), and dodecaboride (16 %), and the measurement of the electrical conductivities, Seebeck coefficients, and thermal conductivities leads to ZT values as high as 0.53 at 830 K. Such refractory materials can be used for energy conversion in a range of high temperatures that are otherwise difficult to address. INTRODUCTION Boron, boron carbide, and many metal borides are semi-conducting or metallic materials that are thermally very stable and that show interesting properties for thermoelectric applications. Elemental boron was reported to show Seebeck coefficients as high as 900 μV/K at room temperature [1], but due to its low electrical conductivity the ZT values are very small. As reported so far, the class of borides and boride silicides has no representative with ZT values higher than 0.25 [2]. Dresselhaus et al. have explained that significant increases in ZT can be expected when nano-structured materials are prepared [3]. But, for high-temperature applications it is often difficult to ensure the stability of nanoscale structures due to the high mobility of atoms at temperatures close to the melting or decomposition temperature. Agglomeration and phase separation can be expected to occur for non-refractory materials in thermoelectric generators. Now, we synthesized several composites of borides that are crystalline and micro- or nano-structured, and tested them for their thermoelectric properties. In addition, the corresponding binary borides were prepared as bulk materials [4]. Since such composites form at temperatures above 2000 K from the elements they can be expected to remain very stable under possible application conditions between 600 and 900 K. Here, we will report on the hightemperature thermoelectric properties of an Er-B composite that consists of three phases, and we will compare it to the single phase binary borides, ErB2, ErB4, and ErB12. These are compounds well-known from the Er-B phase diagram, and they are stable up to temperatures as 1912 K (ErB2), 2227 K (ErB4), and 1742 K (ErB12) [5]. The only other binary known compound in the Er-B system, ErB66, was not observed to form under synthesis conditions described in this work. The existence of erbium hexaboride is controversial, according to literature. Here, ErB6 was not obtained.
41
EXPERIMENT Samples of the composit
Data Loading...
