Ceramics-Silikáty 56, (3) 222 - 228 (2012)

Mullite composites have been synthesized at 400^oC, 800^oC, 1000^oC and 1300^oC via the sol-gel technique in the presence of cobalt. The electrical resistivity and activation energy of the composites have been measured and their variation with concentration of the metal ion doping has been investigated. The resistivity of doped mullite decreases rapidly from 400^oC-800^oC more gently from 1000^oC-1300^oC. The lowering of resistivity is due to the 3d orbital electrons and the concentration of cobalt ions. X-ray analysis confirms the presence of Co²⁺ ions in mullite, which entered the octahedral site. The Co²⁺ ion which substituted Al³⁺ ion in the octahedral site of mullite structure appeared to be efficient in reducing the resistivity. This has been confirmed due to the results of activation energy of resistivity/band gap energy, the E_g which was lowest for concentration 0.15 M. As the concentration increases, these ions lower the resistivity of mullite to a minimum.