The larger the total amount of impurity, the lower the Rspez. The impurity Na2O can form with alumina a phase known as ß-Al2O3, which has the molecular formula Na2O-11Al2O3 and is known to be an ionic conductor. Based on the molecular formula of ß-Al2O3, it can be seen that even small amounts of Na2O are sufficient to form significant amounts of highly conductive ß-Al2O3. Fe2O3 also has a lower resistivity than pure alumina. Measurements in the field have demonstrated considerable influence of contamination on the resistivity RSpez of Al2O3 and spinel layers in the sealed state. However, it was not possible within the scope of this work to find out which impurity is most responsible for the reduction of Rspez, since the amount of the respective impurities could not be varied independently. However, due to the strong effect of sodium on the formation of ß-Al2O3, it can be assumed that Na2O should most likely be avoided when a high Rspez is required.
In a comparative study, the electrical insulation properties of 20 different alumina ceramics containing admixtures of chromium, titanium and magnesium oxide were investigated. With final sealing, the values for the specific resistance Rspez at 75 percent relative humidity were increased from 5*106 Ohm*m to as high as 5*109 Ohm*m, which makes it possible to use pure aluminum oxide or spinel in power electronics. The dielectric strength behaves largely independent of Rspez and is high enough for the materials mentioned (Al2O3 and spinel) at > 20 kV / mm. To meet the minimum requirements of Rspez > 108 Ohm*m, the total amount of impurities must not exceed 0.4 percent (w/w) for the materials Al2O3 and spinel suitable for power electronics. The amount of Na2O should not exceed 0.1 percent (w/w). Halving the total impurities to 0.2 percent results in a tenfold increase of Rspez to 109 Ohm*m.
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