The electrical conductivity of the MnO-CaO-SiO
2-Al
2O
3 system at the composition range 5∼25%MnO and 0.78∼1.43CaO/SiO
2 was measured, and the following results were obtained. (1) The specific conductivity “κ” of the 6%Al
2O
3 samples is 0.1∼0.36 Ω
−1·cm
−1 at 1,400°C. It increases with a rise of temperature, and at the same basicity, increases with the ratio of the MnO content. When the content of MnO is constant, the conductivity increases with CaO/SiO
2, but when this ratio, i.e. the basicity, is above 1.12, it decreases with the increase of CaO/SiO
2. (2) From the results at 6%Al
2O
3 constant, the isoelectrical conductivity lines were plotted on the diagram of CaO-MnO-SiO
2 system. (3) Within the range of Al
2O
3: 1.5∼15%, “κ” tends to attain the largest value at 6∼10%Al
2O
3. (4) From the relation between logκ and 1⁄
T, it was confirmed that the conduction is ionic, and the activation energy was calculated.
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