Oxide and proton conductivity in aluminum-doped tricalcium oxy-silicate

Aluminum doping in tricalcium silicate, Ca3(SiO4)O, has been studied by high-resolution laboratory X-ray powder diffraction and the Rietveld method. Two nominal series have been designed and studied. Oxygen-fixed Ca3−x/2Alx/2(Si1−x/2Alx/2O4)O series has been prepared as single-phase up to x = 0.03. However, oxygen-variable Ca3(Si1−xAlxO4)O1−x/2□x/2 series has not been stabilized for any composition. The samples show oxide anion conductivity with a small p-type electronic contribution under oxidizing conditions. Typical total conductivities for these solids are 10− 5–10− 4 S cm− 1 at 1100 K. The oxide ion transference numbers are very high, ∼ 0.98, under reducing conditions, i.e. dry 5%H2–N2/air gradient. The oxide ion transference numbers are slightly lower, ∼ 0.91 under oxidizing conditions, i.e. dry O2/air gradient. These compounds display a very important proton contribution to the overall conductivities under humidified atmospheres. The proton transference number ranges between 0.72 and 0.55 at 873 and 1023 K, respectively.