Electrical and mechanical properties of alumina doped with transition metal oxides sintered at 1400°C

Corundum (alpha-alumina) is one of the most exploited materials in ceramic industry due to its good physico-chemical properties, mechanical properties, and, importantly, due to its low cost. Advantageous properties, such as hardness, dielectric and thermal properties, and refractoriness, open the door to a wide range of applications of corundum. Porous alumina ceramics is often used for filters and as a catalytic substrate, whereas dense alumina ceramics is frequently used in automotive and aerospace industry. Hence, there is a great interest in exploring this type of ceramics. The aim of this work is to explain the influence of preparation parameters (mechanical activation, addition of transition metal oxides, and sintering conditions) on the final electrical and mechanical properties of alumina. As a starting powder, a mix of a few alumina modifications (α-, κ-,γ - Al2O3) was used. It was doped with 1 wt % of Mn2O3, Cr2O3, and NiO, respectively. Three of the obtained mixtures were homogenized and three were ball milled for 60 min. Particle size analysis, SEM, and EDS were performed in order to demonstrate changes in the microstructure after milling. DTA/TG analyses were used to determine all characteristic temperatures. After sintering at 1400 oC in an air atmosphere, mechanical, electrical, and physico-chemical properties of the obtained ceramics were determined. Results showed that the mechanical treatment lead to a decrease in temperatures of phase transitions and sintering for approximately 100 oC. Incorporation of cations of transition metals into the crystal lattice of alumina was noticed. Activated and sintered samples showed a higher tensile strength and higher relative dielectric permittivity then the sample doped with Cr2O3. The main conclusion of this investigation is that the mechanical activation has the dominant influence on lowering the characteristic temperatures of all detected phase transitions and sintering processes. Using a mixture of several crystalline phases of alumina as a starting powder alleviates the incorporation of transition-metal cations into the alumina crystal lattice, leading to changes in microstructures of the prepared ceramics. Finally, changes in the microstructure and lattice disordering have the dominant influence on the final mechanical properties.