| Solid ionic electrolytes are a major concern in fuel cell development, but only a few compounds are known to be superionic. The fluorite structure, in particular, has been the basis for several superionic conductors of F −, I−, and O2− ions.; Rare earth carbides of the form LnC2 (where Ln refers to any element of the lanthanide series) have the fluorite structure when they are above their transition temperatures, which vary from 350°C (EuC2 ) to 1450°C (LuC2). The carbon atoms in these compounds reside as anions in tetragonal positions equivalent to the positions of the mobile ions, F− and O2− in the known superionic conductors CaF2 and Zr0.8Y0.2O 2. These cubic lanthanide carbide compounds could potentially be good ionic conductors for carbon. The discovery of a material with a high carbon ion conductivity would be a major scientific advance, opening the possibility of an entirely new class of fuel cells that could convert carbon directly to CO/CO2 and produce electric power without combustion.; In order to stabilize the cubic fluorite structure to low temperatures, a mixture of two different lanthanide dicarbides must be formed. The lanthanide carbides having a stabilized fluorite structure that have been produced in this research are mixtures of La0.5Er0.5C2, Ce0.5Er0.5C2, and La0.5Y 0.5C2.; Aluminum carbide, Al4C3, has also been investigated as a potential carbon ionic conductor. Although Al4C3 does not possess the fluorite crystal structure, it is of interest because the carbon atoms reside as single 4− ions rather than C22− ion pairs found in most carbides.; The lanthanide dicarbides were synthesized by reacting mixtures of Ln 2O3 and amorphous 13C under vacuum at high temperatures (>1600°C), using a number of newly developed synthesis techniques.; The diffusion coefficients for La0.5−Er0.5C 2 have been found to be approximately 2.0 • 10−13 cm2/sec at 850°C increasing to 1.7 • 10 −12 cm2/sec at 1150°C, which values are not in the range of superionic conductors (10−4–10 −7 cm2/sec). The diffusion coefficient for carbon in aluminum carbide was measured to be 3.2 • 10−13 cm2/sec at 850°C increasing to 4.9 • 10−12 cm2/sec at 1150°C. From these measurements the activation energy for carbon diffusion was calculated to be 95 kJ/mol in lanthanum erbium carbide and 135 kJ/mol in aluminum carbide. These are the first carbon diffusion results ever reported for any member of these two classes of materials. (Abstract shortened by UMI.)... |
