The Phase Equilibria and Seebeck Coefficient of (Co,M)3AlC (M=Fe or Ni)

The phase equilibria and the Seebeck coefficient were measured at the vicinity of the carbide (Co,M)3AlC (M=Fe or Ni) in the Co-Al-C, Co-Fe-Al-C and Co-NiAl-C systems. The samples were well annealed at 1473 K and examined through X-ray diffraction measurements, energy dispersive X-ray analysis and carbon analysis. Co3AlC with a perovskite structure did not exist at its stoichiometric composition, but was located within a narrow compositional range near Co3AlC0.67. The maximum Seebeck coefficient was 23.3 (mu)V/K at 873 K for Co3AlC0.63. The replacement of a small amount of Co by Fe resulted in the expansion of the single-phase area of (Co,Fe)3AlCx, and the Seebeck coefficient of the single phase reached its maximum, 32.6 (mu)V/K, at 873 K and Co3.506Fe0.025AlC0.77 in the quaternary Co-Fe-Al-C system. The addition of Ni did not stabilize this carbide, and the maximum Seebeck coefficient in the quaternary Co-Ni-Al-C system was 26.9 (mu)V/K at 873 K.