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q ABSTRACTThe derivative discharge curves for ~-and 7-MnO2 down to ,~0.7V open cell voltage were resolved into one characteristic set of four normal distribution functions (peaks). Only peak positions shift to lower voltages for ~-compared to 7-MnO2. When the ratio of electrolyte to MnO2 is increased, the first peak increases while the other peaks decrease (the second passes through a maximum) and at a mass ratio above 14 only the first peak remains. The first peak represents direct reduction of Mn 4+ to soluble Mn e+ which ends at pH 7.5, the second is attributed to the formation of Mn(OH)~, and the third to the formation of MnOOH. Each reaction takes place throughout the depolarizer pellet before the next becomes significant. Electrochemical tests for the evaluation of the dry cell activity of MnO2 samples should be performed at the same mass ratio of electrolyte to MnO2 as used in commercial dry cells. Higher ratios lead to an overestimation of samples with Iow activity.

Derivative discharge curves of ~-, 8-, or ~-MnO2 can be resolved into a set of six normal distribution functions, designated as peaks O, A, B, C, D, and E, which can each be attributed to a specific reaction. Peak O situated at the highest cell voltages is attributed to the reduction of adsorbed oxygen. The following peak A which is linked to the formation of dissolved Mn + +-ions ends when the solubility product of Mn(OH)2 is attained in the electrolyte. For peak B the over-all reaction MnO2 % Mn + + + 2H20 + 2e--> 2Mn(OH)2 is proposed. In 9M KOH the electrochemical reduction of ~-MnO2 proceeds in one step to Mn(OH)2, ~-MnO2 is at first (peak C) reduced to Mn304 and then (peak D) to Mn(OH)2, and 7-MNO2 reduces in three successive steps to an amorphous substance, Mn.~O4 and Mn(OH)2. As in practically all cases the presence of Mn(OH)2 has been observed, it is proposed that bivalent manganese is the primary reduction product which reacts in alkaline electrolyte with unreduced MnO2 to give secondary reduction products the kind of which depends on the type of MnO2 involved, the electrolyte, and time of contact. Mixing MnO2 with appropriate amounts of Mn(OH)2 gives, under comparable conditions, the same products as obtained in electrochemical reduction.Following the proposition of Balewski and Brenet (1) the first derivatives of discharge curves obtained during cathodic reduction of manganese dioxides under Leclanch6-type dry cell conditions have been examined with a pulse galvanostatic analyzer (2, 3). The derivative discharge curves obtained can be resolved into a set of up to six normal distribution functions: peaks O, A, B, C, D, and E. Their relative importance is mainly governed by the mass ratios of electrolyte to manganese dioxide in the depolarizer mix pills (3). This observation and especially the changes of the peak areas with increasing mass raUos suggest that each peak may be attributable to a specific chemical reaction taking place in the depolarizer mixes during the cathodic reduction.It is the purpose of the present communication to make proposals for this attribution. In order to obtain further evidence, discharge reactions of ~-, ~-, and ~-MnO2 in 9M KOH and in modified Leclanch6-type dry cell electrolytes have been investigated and the various reduction products formed in alkaline electrolyte have been examined by x-ray diffraction. ExperimentalSamples and discharge procedure.--The properties of the manganese dioxides used in the experiments are compiled in Table I and the compositions of the depolarizer mixes are presented in Table II. The pulse galvanostatic analyzer, preparation of the depolarizer mixes, and discharge tests together with their evaluation are described in foregoing publications (2, 3).

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The steady-state creep rate of mixed-conducting oxides, Lal_,SrxMn03, for x = 0.1,O. 15, and 0.25, and Lao.7Ca0.3Mn03 has been investigated at temperatures between 1150 and 13OOOC. Creep parameters and transmission electron microscopy observations indicate that deformation is controlled by lattice diffusion of one of the cations. The dependence of the creep rate on Sr concentration, combined with a point-defect model, confirms this hypothesis; however, the oxygen partial pressure dependence of creep (from 10-1 to 2 x 104 Pa) cannot be accounted for within the framework of a simple poin t-de fec t model.