Effect of the Particle Size of the Components of CaWO4–Al2O3 Composites on Their Transport Properties

We have studied the effect of the grain size of aluminum oxide and calcium tungstate on the transport properties of (1 – x)СaWO₄–xAl₂O₃ composites with a mole fraction of aluminum oxide x ≤ 0.35. The phase composition and thermodynamic stability of the composites were ascertained by X-ray diffraction, thermogravimetry, and differential scanning calorimetry. Their surface morphology was examined by electron microscopy and their elemental composition was determined by X-ray microanalysis. The electrical conductivity of the composites was determined by electrochemical impedance measurements and analyzed as a function of temperature, oxygen pressure in the gas phase, dispersoid (aluminum oxide) concentration, and the grain size of their components. The conductivity of the (1 – x)СaWO₄–xAl₂O₃ composites containing 5–10 mol % aluminum oxide was found to exceed that of CaWO₄ by more than one order of magnitude. Varying the average grain size of the Al₂O₃ nanopowder from 21 to 82 nm caused no appreciable changes in the conductivity of the composites, which was due to the polydispersity of the aluminum oxide, whereas reducing the average grain size of CaWO₄ from 6.4 to 1.6 μm led to a twofold increase in the conductivity of the composites.