03-09-2023, 05:48 PM
(03-09-2023, 04:31 PM)SkunkedAgain Wrote: Thanks Jim, appreciate the additional insight... very interesting how the impurities aid the melting by gathering the heat... Until I got a camera, I always assumed it was dark under the ice, especially with snow on it.. but my camera has shown me how much light actually gets through the ice, so it's a lot easier to see how the solar heating is melting the ice from within... I'm learning a lot that I hadn't really considered before, but it all makes sense when you stop and think about it... I think I have forgotten or didn't learn the first time in material science what makes up a grain? I know they like to go to the space station to produce really large grain sized materials and like you mentioned the rapid cooling (quench) makes the small grain size... I just don't remember what makes grain boundaries and why can some be large or small with the same material??? I think I'd learn a lot more out of that lecture today than when I was just trying to pass the class... Guess it's time for another google search... Later J
As far as how a grain forms, the easiest way for me to visualize it comes from thin films, again in electronics. You take a Si wafer and put it in a vacuum chamber and then heat an Al source hot enough for it to melt and then evaporate. Then you end up with a flux of evaporated Al atoms moving rapidly out from the source. They are individual atoms, but they are gaseous because of their temperature. Most of that energy is converted into kinetic energy which is why they move so fast.
When they hit the wafer surface, which usually is at room temp, their kinetic energy is absorbed by the large mass at room temp and the "stick". However the very surface of the wafer is getting hotter by by absorbing that kinetic energy. As more and more atoms hit the surface they either stick to another stuck Al atom or a bare spot on the surface. When several Al atoms stick to each other you get what's called a nucleation site or the beginning of a grain. As a grain grows larger it will spread across the surface and encounter stuck atoms which get absorbed into the grain.
As the process continues the very surface continues to heat up which allows the grain to continue to grow and it will grow as a single crystal as long as enough energy (heat) is present. Gradually the whole surface becomes covered with growing grains and each new atom helps a grain to grow as it lands on top of it.
When the surface begins to be covered with grains their edges begin to push against other grains and their size becomes limited. This pushing against each other is at the grain boundaries. If there is enough energy, some grains will be able to consume others and get larger. but most will not.
If however, we start the process by heating the wafer to say 350 degrees C, something different happens. When an atom hits the surface it doesn't instantly stick, but wanders around on the surface slowly losing kinetic energy. Because the average time for sticking is longer, fewer nucleation sites are formed and more material moves into the lattice of the few grains that are forming. This results in fewer but larger grains.
Of course with ice the formation of grains is 3 dimensional, but the basic ideas apply. I don't even try to think in 3 dimensions so I like the surface example.
You don't necessarily add heat to ice to get larger grains, but the temperature remains nearer to the ideal temp longer and the grains get larger.
The one that is hard to predict is the thawing and re-freezing thing. In some conditions it causes grain growth and in others it causes breakdown of the existing structure. The thing that is important for us is that the conditions that lead to weakening are far more common or likely that the ones that lead to strengthening!
BTW, I don't remember all that much from the classes either. There are a few things that I have been re-exposed to over the years that now make them easier to think about. A form a reviewing you might call it.