• PDF / 752,664 Bytes
• 12 Pages / 420.48 x 639 pts Page_size
• 37 Downloads / 289 Views

DOWNLOAD

REPORT

---

HIGH TEMPERATURE THERMOELECTRIC PROPERTIES OF BORON CARBIDE T. L. Aselage, 87185-5800

Division 1842,

Sandia National Laboratories,

Albuquerque, NM

ABSTRACT Boron carbides are refractory solids with potential for application as very high temperature p-type thermoelectrics in power conversion applications. The thermoelectric properties of boron carbides are unconventional. In particular, the electrical conductivity is consistent with the thermally 21 3 activated hopping of a high density (= 10 /cm ) of bipolarons; the Seebeck coefficient is anomalously large and increases with increasing temperature; and the thermal conductivity is surprisingly low. In this paper, these unusual properties and their relationship to the unusual structure and bonding present in boron carbides are reviewed. Finally, the potential for utilization of boron carbides at very high temperatures (up to 2200°C) and for preparing n-type materials is discussed. INTRODUCTION Boron and carbon combine at high temperatures to form a boron carbide phase that extends over a wide range of compositions: from -9 to -19 atomic 1 2 percent (a/o) carbon. . These boron carbides are refractory, with melting temperatures exceeding 2400°C. They are known for their extreme hardness 3 and wear resistance. Boron carbides enriched in the neutron-absorbing 10 isotope B (19% natural abundance) are also utilized as neutron flux moderators. The boron carbide structure tolerates large amounts of 4 5 radiation damage in these reactor applications. ' The hardness, extreme melting temperature, and radiation tolerance arise from the novel structure and bonding of boron carbides. The unusual bonding in the solids also leads to distinctive electronic and thermal transport properties. Here, a combination of "anomalous" properties leads to an unexpectedly large thermoelectric

figure-of-merit,

Z,

at

high temperatures.

Indeed,

values of

Z as large as 0.5 x 10-3 at IO00°C have been reported for a p-type sample 6 of B9C doped with 0.5% Mg. The possibility of utilizing boron carbides in high temperature thermoelectric power conversion devices has been appreciated since the 7 earliest days of thermoelectric materials development. Their refractory nature and tolerance of large amounts of radiation naturally suggest their utilization in space power generation. It is only recently, however, that some understanding of the distinctive properties of boron carbides has been 8 developed. In this paper, the structure, bonding, and properties of boron carbides will be reviewed, and areas for further work described. STRUCTURE AND BONDING

The crystal structure of boron carbides is depicted in Figures la-c. The rhombohedral unit cell is composed of two distinct units: twelve-atom, boron-rich icosahedra that reside at the vertices of the rhombohedron, and 91 0 three-atom chains that lie along the three-fold axis. , Carbon atoms, depicted by black balls in Figure 1, are found at the ends of the chain and within the icosahedra. In the idealized B4 C structure, Figure la, each of the chains