Mechanical properties of alkaline earth-doped lanthanum gallate

Lanthanum gallate doped with alkaline earths was prepared from combustion-synthesized powders. Mechanical properties of the doped gallates were evaluated as a function of composition and temperature. The indentation fracture toughness of Sr-substituted gallates was significantly better than the Ca- and Ba-substituted materials, but the toughness of all the doped gallates was significantly lower than yttria-stabilized zirconia, a typical electrolyte material. Small improvements in room temperature toughness and strength were measured in (La0.9Sr0.1)xGa0.8Mg0.2O3−δ, (“LSGM-1020”) samples with significant A-site cation non-stoichiometry (x = 0.9). The flexural strength of stoichiometric LSGM-1020 decreased from ≈150 MPa at room temperature, to ≈100 MPa at higher temperatures (600–1000°C). The notched-beam fracture toughness of LSGM-1020 decreased from ≈2.0–2.2 MPa√m at room temperature, to ≈1.0 MPa√m at 600°C. The decrease in mechanical properties over this temperature range was correlated to changes in crystal structure that have been identified by neutron diffraction. These crystallographic changes were also accompanied by significant changes in the thermal expansion behavior and elastic modulus. For off-stoichiometric LSGM-1020 with A/B cation stoichiometry of 0.90, strength and toughness also decreased with temperature, but the retained toughness (≈1.5 MPa√m) at elevated temperatures was higher than the toughness of the stoichiometric LSGM material.