Calcium ferrite, uptake oxides

The rates of uptake of molybdenum, tellurium, and rubidium oxide vapors by substrates of calcium ferrite and a clay loam have been measured in air over a temperature range of 900° to 1500°C. and a partial pressure range of about 10r7 to 10 atm. The measured rates of uptake of molybdenum and tellurium oxide vapors by molten calcium ferrite and of rubidium oxide vapor by both molten clay loam and calcium ferrite were controlled by the rates of diffusion of the oxide vapors through the air. The measured rates of uptake of molybdenum and tellurium oxide vapors by molten clay loam were controlled by a combination of a slow surface reaction and slow diffusion of the condensate into the substrate. 

Figures 2 and 3 show the uptake of molybdenum oxide vapor by various sized particles of clay loam and calcium ferrite at 1400°C. Over the temperature and oxide partial pressure ranges used in these experiments, molybdenum oxide vapor consists of a mixture of M0O3, Mo2Oe, and Mo309 molecules whose proportions are temperature and pressure dependent. Data which can be used to calculate these ratios are available from a report of a mass-spectrometric study of the sublimation of M0O2 by Bums et al. (6). Usually, for convenience, the total vapor concentration (/xgram/liter) of all the polymers of M0O3 vapor were used in this report in place of partial pressures. In some situations it was necessary to compute the partial pressure of each polymeric species.

Figures 4 and 5 show the uptake of tellurium oxide vapor by various sized particles of clay loam and calcium ferrite at 1400°C. Over the temperature and pressure ranges used here, Te02 is the predominant vapor species (4).

Figures 6 and 7 show the uptake of rubidium oxide vapor by various sized particles of clay loam and calcium ferrite at 1400°C. There is uncertainty as to the molecular species found in rubidium oxide vapor under the experimental conditions. Data furnished by Bedford and Jackson indicate that the vapor should consist of a mixture of RbO and Rb molecules with RbO predominating (4). Experimental work by Norman and Staley indicate that the vapor should consist predominately of Rb with minor amounts of Rt O molecules (13). To calculate vapor pressures it was assumed that the vapor consisted only of RbO. Since the molecular weight of Rb is only 16% less than that of RbO, no large error is introduced by this assumption even if the vapor consists of Rb molecules.

For the uptake of molybdenum oxide vapor by the clay loam particles, the uptake of rubidium oxide vapor by the calcium ferrite, and the uptake of the tellurium oxide by both the clay loam and the calcium ferrite, the uptake vs. time plots are curved, and it is not immediately obvious what the rate-determining steps are. Plots of amounts of uptake at constant time and initial rates of uptake vs. particle diameters and diameters squared were therefore made. 

These plots show that for the uptake of both tellurium and rubidium oxide vapors by the calcium ferrite particles the rate-determining step is also the diffusion of the oxide vapor molecules through the air. The curvature of the plots showing amounts of uptake vs. time is caused by... 

Figures 8 and 9 show that the calculated rates of uptake of molybdenum oxide vapor by the calcium ferrite particles are close to the experimentally measured rates over large ranges of pressure and temperature. The correspondence between the observed and calculated rates helps substantiate the conclusion that the rate-determining step is indeed the diffusion of the molybdenum oxide vapor through the air.

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