The effect of heat treatments on the semiconductor properties of a commercially electrodeposited manganese dioxide

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Abstract
The temperature dependence of d.c. conductivity and thermoelectric power has been studied for a series of thermally treated samples of an electrodeposited, microcrystalline, γ‐phase MnO2. Measurements were made on samples compacted at 3.86 × 105 KN/m2. Measurements of a.c. impedance up to 50 MHz were made on the powder samples to test for particle homogeneity.The results indicated that a hopping electron transport model was probably appropriate for these samples and that the majority carriers were electrons, possibly ionised from donor sites provided by oxygen deficiency. Conductivity, surprisingly, was found to increase in an oxygen environment and it is suggested that oxygen adsorption greatly enhances the electron mobility.The d.c. conductivity and thermoelectric power were found to be strongly dependent on sample outgassing temperature. The former reached a maximum at about 320 °C, the latter became less negative with increasing outgassing temperature. It is tentatively proposed that carrier density and “surface” mobility increased as terminal OH groups forming strong traps on manganese sites were removed by thermal treatment.Measurements of a.c. impedance on samples outgassed at 35 °C and 365 °C displayed dispersion around 20 MHz indicative of a high resistance surface skin possibly due to a hydroxylated and reduced surface respectively.