Influence of Al2O3 on the electrical conductivity and structure of calcium-silicate-based melts

Electrical conductivity and structure of the CaO-SiO2-based mold flux melts with various Al2O3 contents were investigated. The results show that the electrical conductivity increases with the addition of Al2O3 from 2 wt% to 4 wt%, but decreases with the further increase of Al2O3 from 4 wt% to 8 wt%. Correspondingly, the apparent activation energy reduces firstly from 55.12 +/- 1.20 kJ mol to 41.09 +/- 0.38 kJ mol, and then increases from 41.09 +/- 0.38 kJ mol to 98.99 +/- 1.42 kJ mol. The structure analyses suggest that complex structural units, such as Si-OAl, Al-O0, Si-O-Si and Q3(Si), reduce first, but increase with the further addition of Al2O3. Conversely, these simple structural units, such as Al-O-, Q0(Si), Q1(Si) and Q2(Si) vary in the opposite way with the change of Al2O3 content. From the variations of electrical conductivity, activation energy and structural units, it can be found that when Al2O3 works as network breaker to simplify the melt structure, the energy barrier for transportation of conducting ions/ionic reduce, which results in the increase of electrical conductivity; while when Al2O3 becomes into network former, the conductivity increases, correspondingly.