Sintering of calcium fluoride

The Role of Plastic Flow by Dislocation Motion in the Sintering of Calcium Fluoride ... 

C. S. Yust and C. S. Morgan, Dislocation Movement in Sintering of Calcium Fluoride, paper presented at 65th Annual American Ceramic Society Symposium, May 1963. 

It is difficult to achieve a gas-tight seal to the anode current connections, which must also be electrically insulated against the cover of the electrolyzer, to which the cathodes are attached. Formerly, fused calcium fluoride was used as sealing material. This is, however, brittle and sometimes even porous recently sintered corundum has been used and for the external packing teflone (polymerized tetrafluoro-ethylene), buna, and polyvinylchloride. 

Mineralizers are added to the raw feed to aeeelerate the kinetics of reactions by modifying the solid and liquid state sintering. The temperature of decomposition of calcium carbonate is lowered in the presence of mineralizers. In the synthesis of C3S and C2S, DTA has shown that some phosphates, earbonates, sulfates, and ehlorides deerease the deearbonation temperature and that of the formation of the silieates.[ °l Caleium fluoride acts both as a flux and a mineralizer in promoting the formation the tricalcium silicate phase. DTA thermograms have been applied to study the effect of mineralizers on the melting behaviors and crystallization temperatures of clinker. Early formation of liquid can be established by thermal techniques. 

While the studies discussed so far were concerned with the sintering of metals, the question of material transport in sintering is as pertinent for ionic solids as it is for metals. At the temperatures where sintering is important extensive plastic deformation by dislocation motion is possible in both single crystals and polycrystalline aggregates of ionic solids. It was therefore decided to adapt the Brett and Seigle technique of sintering with fiducial markers to an ionic crystalline solid, calcium fluoride. 

In the assemblies sintered with an external load the necks formed between the spheres and the top plate, both of which contained markers, were used for observation and measurement. The only purpose of the marker-free lower plates in the assembly was as a control. In the final polishing of the samples for microscopic examination diamond powder was used, which could possibly become embedded in the calcium fluoride and give the impression that markers were present. No such effect was observed. 

The results of this investigation indicate that material transport during sintering of polycrystalline calcium fluoride takes place by slip due to dislocation motion when the stress exerted by surface tension forces or by a combination of surface tension forces and an external force is high. When the stress becomes smaller than a critical stress, which is estimated to be in the neighborhood of 10 dyn/cm (1.4 psi) for calcium fluoride with an average grain size of 40 //, material transport takes place by diffusional flow. 

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