A new method for measuring activities in slags containing a volatile component
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the expectation when gravity acts on the microstructure and when grain shape accommodation gives more settling and a higher packing density (near 90 vol pct solid) at the energy minimum, tiT]
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Interest in this topic was initiated by Peter Voorhees. Research funding on the gravitation role in liquid-phase sintering was provided by NASA through the Lewis Research Center, with Richard DeWitt, Project Manager, and Thomas Glasgow, Project Scientist.
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0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.~8 o.b9 1.00 separation distance
Fig. 2 - - T h e normalizedenergy per unit volume as a function of the fractional grain separation distance for two agglomerating solid spheres in a liquid matrix. The calculations for a 7ss/TsL ratio of 1.93 show a subtle energy minimum for a relatively small level of bonding.
microgravity for samples with 4 vol pct solid. Some of this agglomeration may be due to initial specimen preparation techniques. However, the 180-minute results from Koharat~~ also show that agglomeration is favored in the absence of a gravitational settling force. For the case where the grains are different in size, the bond can be planar but the equilibrium shape would be curved with the radius of curvature dependent on the grain size ratio and surface energies, t~4] For small differences in grain size (up to approximately 40 pct size difference), the results are largely unchanged. For large grain size differences, there is a larger energy reduction that further favors agglomeration. In a few cases of liquidphase sintering, there are reports of zero dihedral angle, such as in ZnO-BaO, t~5] suggesting that no agglomeration should occur since 7ss/TsL >- 2.00. However, experiments showed that coalescence occurs even in this system, probably due to grain rotation into a low crystallographic misorientation, t16]Thus, even in zero dihedral angle systems, the suggested agglomeration effect may occur. Finally, as a point of comparison, consider the relative energy involved in these agglomeration calculations. For tin grains in a lead-tin eutectic liquid, the density difference is approximately 1.2 g/cm 3 (tin is less dense). For a 20-p.m-diameter grain size, the energy difference associated with two-sphere agglomeration at a 7ss/TsL = 1.93 is estimated at 5.10 -14 J (surface energy of 0.13 j/m2). Although this is a small energy, it is comparable to the energy for a 1-mm gravitational displacement of this same tin grain in the eutectic liquid. However, in microgravity, a displacement near 100 m is necessary to equal this same agglomeration energy. Accordingly, on a relative basis, the agglomeration energy becomes large under proposed microgravity conditions and may lead to extensive agglomeration, even in dilute solid-liquid mixtures during coarsening. Based on Eq. [8], at long times, the clusters will pack to 60 to 70 vol pct solid, similar to what is observed at short times in the settled region. [4] This prediction differs from
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