Oct., 1963 c.s.r. measurernetits on ZnO given by Ilokes in another
I t seems likc>lythat the d.c. coiiductivities measured by Glemza :tiid liokes' !\ere doinhated by surface effects (especially i n the presence of adsorbed oxygen, wheii bloc.king layers tend to form), since high-freqtieticy iiiethous usually are necessary in order to estimate the biilk c.oriductivity of powder coinpacts.6 Under such conditions, tlie recovery of conductivity nould tic a coiiscquc'iice of the diffusion of excess ziiic from the bulk to the surface of the graiiis until the initial distributioti was restored. There is little doiibt that the diffusioii of interstitial zinc at 350' is sufficiently rapid t o account for the observed effect,2 cspecially 1%lieti the influence of the field of the adsorbed oxygen ioiis is taken into account.' 1 kperimeiits carried out in this Laboratory also support the above interpr(1tatioli. I t was found that a traction of tht. oxygcii adsorbed 011 a zinc-rich powder a i 2.5" coiild be dcsorbcd a t temperaturcs below %00", IirrtJas the rcniaindcr \\as irreversibly hc)ld, e v m at 400". So further irreversible adsorption could thcn t w ot)srrvcd at 25" utiless the sample \\as preheated in t'ucuo. 111 srlvh a case, the anioutit irreversibly ads o r h d (riot rcniovcd a t 200-400') was proportioiial to the square root, of the time of prehcatitig (the tenipcraturc employed was 400"). Tlicsca results suggcst that thc irrevcrsihlc adsorption represents oxidsi ion of superficial ewess ziti?, which later may be replenished by diffusion from the bulk a t higher temperatures, allon irig more osygoii to be irreversibly adsorbed at 2 j o . l
