Note on Diffusion of Cobalt Oxide and Ferrous Oxide in Polycrystalline Magnesium Oxide

Abstract

N CONNECTION with studies on the diffusion of FeO into singlecrystal MgO, a preliniinary investigation was made into the diffusiou mechanism of FeO and COO into polycrystalline MgO. In particular it was desirous to determine if there was cvideucc of grain-boundary diffusion. The possibility of grainboundary diffusion was suggested by Zaplatynsky.Polycrystallinc MgO of 98'% of theorctical density, 50p in grain size, and 99.99% pure was cut into bars 0.3 by 0.3 by 2.0 cm. I'cO (wiistitc) previously prepared and in fine-powder form was tightly packed around each MgO bar. The specimens were placed in an clectrical resistance furnace a t 1100°C for 25 hours in a controllcd atmosphere of hydrogen and water vapor, ensuring the stability of FeO. After the specimens were removed from the furnace, thcy wcre sectioned, polished, and observed with a reflected-light microscope. A sharp, straight boundary was observed between the diffused and nondiffused part of the MgO bar. There was no evidence of grain-boundary diffusion but only of bulk diffusion similar to the diffusion observed with single I crystals2 and a t a rate comparable with single crystals (see Fig. 1). In both the single and polycrystalline MgO the diffusion process appeared to take place by a counterdiffusion of the cations Fig. 2. Diffusion boundary of COO into "pure" polycrystalline MgO. MgO.Above boundary, solution of COO and MgO; below boundary, "pure" through a relatively rigid oxygen lattice. Because of the nonstoichiometric property of FeO, cation vacancies were present. The diffusiou process appeared to prefer bulk movement by means of vacancies over grain-boundary diffusion.Cobalt oxide was also packed around the polycrystalline MgO in the same manner as,with FeO. The specimens were placed in a gas furnace open to the atmosphere for 25 hours at 1600OC. Results showed a straight boundary indicating only bulk diffusion and no grain-boundary diffusion (Fig. 2). Bars of sintered Fig. 1. Diffusion boundary of FeO into "pure" polycrystalline MgO. Above boundary, solid solution of FeO and MgO; below boundary, "pure" MgO.

Diffusion and Reaction in Solids

“…To use Equation 7to predict the concentration gradient, constant K must first be evaluated. The procedure developed by Danckwerts (6) and Crank (4) for moving boundary diffusion problems in gas-liquid and gas--= 0.812 erfc (v) (17) a0 where rj = -j--.…”

“…Discussion. The question of the relative importance of reaction and diffusion resistances in solid systems has interested many investigators (3,17) but quantitative results apparently have not been obtained for any specific systems. The results given in Figure 12 could be used with experimental data to evaluate k and D and thus establish the significance of diffusion and reaction.…”