MTU RESEARCHERS STUDY SETTLING
HOUGHTON, MI--Two researchers in Michigan Techs Department of Metallurgical and Materials Engineering are using a $320,000 four-year grant from the NASA Microgravity Materials Science program to try to solve a problem with materials settling during various manufacturing processes. Dr. Thomas Courtney and Dr. Shu-Zu Lu are spearheading the effort. A simple example of settling can be found after a large snowfall. Ten inches of snow will soon be much less, because the snow settles. These results are predictable because the pores within the snow depth are compressible. Surprisingly, the same results can be found in solid-liquid materials even though liquids are uncompressible. Courtney and Lu are most concerned with interconnected structures where the settling rates are much slower. Interconnected structures are those in which both the solid and the liquid phases are connected. For example, one could reach any liquid point by traveling only through the liquid phase, or one could travel only through the solid phase to reach any other solid point.
One explanation Courtney is investigating is that self-weight causes material to shrink. Courtney believes that this explanation is incorrect because "great depth and time are needed for material to deform under its own weight."
Another explanation being investigated is a connection between coarsening and settling. As a material coarsens, the particles grow, resulting in fewer, but larger particles. Coarsening can be observed in ice cream. When ice cream gets older, the ice particles coarsen, taking on a more grainy texture. Courtney believes that during sedimentation, some particles break free and drop or rise, depending on their density, resulting in the material settling.
The researchers are using 3 systems to test and model: tungsten and nickel, lead and tin, and a water-salt system. They know that the ultimate cause of settling is gravity, but hope to find the mechanism by which it happens. "Since we know settling is caused by gravity," Courtney explains, "a microgravity environment should minimize or eliminate the problem, but while gravity is the driving force for settling, it does not control the rate of settling."
The work with lead and tin is specifically relevant to the electronics industry in the use of solder. Solder is a solid alloy, often made of lead and tin, which is melted to bond metal surfaces, such as an electronic chip to a circuit board. As it melts, it becomes a solid-liquid material. Settling causes the solid and liquid to separate, potentially weakening the solder joint.
The problem of settling is not exclusive to the electronic industry; it is also observed in polymer synthesis and materials manufacture. During heat treating of solid-liquid mixtures, settling causes segregation of the solid and liquid phases. This segregation is a concern in production operations, such as liquid phase sintering, that utilize solid-liquid two-phase mixtures. Courtney hopes that by discovering the mechanism of settling, some problems common to the manufacture of solid-liquid phase sintered products will be alleviated.
For more information contact Dr. Courtney at 906/487-2036 or by email at thc@mtu.edu.
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07/09/99MTN131