Electrical properties and thermal expansion of strontium aluminates

• Sr3Al2O6, SrAl2O4 and Sr4Al14O25 ceramics are semiconductors.
• Electrical conductivity is as low as 10−6−4×10−5 S/cm at 1273 K in dry air.
• SrAl2O4 is a mixed conductor with predominant ionic conductivity.
• Sr3Al2O6 shows significant contribution of protonic transport in wet atmospheres.
• Average TECs vary in the range (8.5–11.1)×10−6 K−1 and are p(O2)-independent.

Strontium aluminate ceramics, including Sr3Al2O6, SrAl2O4 and Sr4Al14O25, synthesized by glycine–nitrate combustion and sintered at 1773 K in air, were characterized by thermal analysis, dilatometry and electrical measurements in controlled atmospheres. All studied strontium aluminates are semiconductors with electrical conductivities as low as 10−6−4×10−5 S/cm at 1273 K in dry air. Electrical measurements in controlled atmospheres in combination with ion transference number determination demonstrated that SrAl2O4 is a mixed conductor with predominant ionic conductivity and increasing n-type and p-type electronic contributions under highly reducing and oxidizing conditions, respectively. While the behavior of electrical conductivity of Sr3Al2O6 in dry atmospheres was qualitatively similar to that of SrAl2O4, a significant increase of conductivity in wet atmospheres was attributed to a protonic contribution to electrical conduction, in correlation with thermogravimetric data and the tendency of this material to form a hydrogarnet at low temperatures. The average thermal expansion coefficients of strontium aluminates, (8.5–11.1)×10−6 K−1 at 333–1373 K, increase with increasing strontium content in the sequence Sr4Al14O25 < SrAl2O4 < Sr3Al2O6 and are essentially independent of oxygen partial pressure.