Thermoelectric power factors of nanocarbon ensembles as a function of temperature

Abstract

Thermoelectric power factors of nanocarbon ensembles have been determined as a function of temperature from 400 to 1200 K. The ensembles, composed of mixtures of nanographite or disperse ultrananocrystalline diamond with B4C, are formed into mechanically rigid compacts by reaction at 1200 K with methane gas and subsequently annealed in an argon atmosphere at temperatures up to 2500 K. The ensembles were characterized using scanning electron microscopy, Raman, x-ray diffraction, and high resolution transmission electron microscopy techniques and found to undergo profound nanostructural changes as a function of temperature while largely preserving their nanometer sizes. The power factors increase strongly both as a function of annealing temperature and of the temperature at which the measurements are carried out reaching 1 μW/K2 cm at 1200 K without showing evidence of a plateau. Density functional “molecular analog” calculations on systems based on stacked graphene sheets show that boron substitutional doping results in a lowering of the Fermi level and the creation of a large number of hole states within thermal energies of the Fermi level [P. C. Redfern, D. M. Greun, and L. A. Curtiss, Chem. Phys. Lett. 471, 264 (2009)]. We propose that enhancement of electronic configurational entropy due to the large number of boron configurations in the graphite lattice contributes to the observed thermoelectric properties of the ensembles.

Reaction Joining of Aluminum‐Doped Lanthanum Strontium Manganese Oxide to Yttria‐Stabilized Tetragonal Zirconia for Gas Sensor Applications

“…The DC electrical conductivity was performed using a computer controlled four‐point probe test setup. Details of the test setup are described elsewhere . Samples were heated to the test temperature and held for 2 h. At each test temperature, at 60 s intervals, a 10 mA current was applied through the sample using two outer probes.…”

Induction annealing and subsequent quenching: Effect on the thermoelectric properties of boron-doped nanographite ensembles

“…4 These samples were then placed into the graphite susceptor for annealing and allowed to reach the target temperature in 1 h. The sample was then annealed for 4 h at the target temperature followed by helium gas quenching. The samples, are composed of mixtures of nanographite ͑30 nm, from MTI Corporation͒ with or without the addition of B 4 C ͑50 nm, from American Elements͒ powders.…”

“…4 These NGEs consist of randomly oriented nanographite without anisotropic properties. In earlier work carried out at Argonne National Laboratory, it was recently discovered that the thermoelectric power factors of boron-doped nanographite ensembles ͑NGEs͒ increased strongly both as a function of annealing temperatures and measuring temperatures.…”