Seebeck Coefficient and Electrical Resistivity of Single Crystal B12As2at High Temperatures

Abstract

There is increasing interest in thermoelectric materials because of their potential to recover usable energy from waste heat. There is a strong incentive to develop thermoelectric materials that can function at high temperatures. Compounds such as boron carbide, SiGe alloys, and oxides have been extensively studied. Recently, there have been further exciting developments in some novel borides. REB44Si2 exhibits Seebeck coefficients, , greater than 200 V/K at high T , and unlike most compounds, the figure of merit, ZT , shows a steep increase at T > 1000K. A series of homologous RE–B–C(N) compounds; REB17CN, REB22C2N, and REB28:5C4, were discovered to be the long awaited n-type counterpart to p-type boron carbide, one of the few TE materials with a history of commercialization. Very recently, excellent control of the charge carrier type, por n-type (j j > 200 V/K) was demonstrated in YxAlyB14 which is composed of relatively abundant elements. 8) Another boride, B12As2, has seen limited but continued interest spurring recent advances in the production of large single crystals, thin films, and nanowires. A wide band gap of 3.47 eV for single crystal B12As2 was determined from optical measurements. However, its thermoelectric properties are largely unexplored except for a single report with two room temperature (RT) measurements of Seebeck coefficients of 107 and 136 V/K, taken on B12As2 epitaxial films on silicon carbide. Investigations into the electrical conductivity of B12As2 have been more actively explored, but only at RT. For example, the RT resistivity of B12As2 was reported by Xu et al. to be controllable over several orders of magnitude from 10 to 10 cm through silicon doping, but high temperature measurements were not included. Single crystals of B12As2 were obtained via solution growth from a nickel solvent, following the procedure reported by Whiteley et al. First, pure As was placed at the end of a quartz ampoule, and a eutectic mixture of B and Ni powders (45 at. % B, 55 at.% Ni) were combined in a BN boat placed 15 cm from the As powder. The ampoule was then evacuated, sealed and placed in a four-zone horizontal furnace. After melting the Ni–B mixture, As was sublimed so that it would dissolve in the solvent and react with B. As the solution was cooled, B12As2 precipitated out of the Ni solvent in large single crystals, typically 2 to 3mm in length, but occasionally larger. The largest B12As2 crystal was selected, cut and filed into a 6mm by 3.9mm by 1.1mm geometry. The resistivity and thermoelectric power were measured with an ULVAC ZEM-2 using the four probe method and differential method, respectively, at 100, 200, 300, and 400 C. Then the sample was cooled to RT and remeasured at 100, 200, 250, 300, 350, and 400 C. The measuring procedure was repeated, creating a total of four runs. The resistivity of the B12As2 crystal had a nominal value of 15–18 cm at 400K and decreased to just over 4 cm at 700K. The electrical conductivity, , of a single crystal B12As2 is shown in Fig. 1 and plotted as log versus reciprocal temperature. The plot reveals a linear relationship (R 1⁄4 0:966): log 1⁄4 490:87 1 T þ 0:0941: ð1Þ From the slope of the line (slope 1⁄4 Ea=kb), the activation energy in the conductivity was calculated as 42meV, which is two orders of magnitude smaller than the band gap of 3.47 eV, and indicative of extrinsic behavior. High impurity concentrations (>10 cm ) are typical of solution-grown bulk B12As2 crystals 12) and are responsible for the sample’s low resistivity. Hall Effect measurements on B12As2 crystals grown by Whiteley et al. from the same method have revealed carrier concentrations on the order of 10 cm 3 and hole mobilities around 20 cm V s 1 at RT. Gong et al. measured the resistivity of epitaxial B12As2 films with carrier concentrations of the order of 10 and reported a range of resistivity values similar to solution grown bulk crystals (0.74–42.8 cm). Furthermore, this single crystal B12As2 was dark and opaque, despite the wide band gap of 3.47 eV, reinforcing the suggestion of high impurity concentrations. Employing optical techniques, i.e., photoluminescence, Klein et al. previously measured the binding energies of acceptors as 175, 255, and 291meV on higher purity B12As2 crystals. The lower activation energy reported here could possibly be explained by new acceptor sites introduced by additional impurities. -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4