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PROFESSOR CIMA: We were talking about the fact to that when we add network
modifiers the glass, we do a couple of things. But the most important thing here, for this
graph, is the fact that when we add the network modifier, that excess volume
gets smaller because it's lower viscosity.
So as I cool this thing, it has a chance to pack better.
Well, you see that here. So here's fused silica.
And what I'm plotting here is the permeability-- this is essentially permeability--
versus 1 over T. And what I see is that, interestingly enough, fused
silica, for helium gas, has the highest permeability compared with
soda lime glass. Here's my favorite one, here.
Soda lime-- sodium oxide and calcium oxide added to the
glass. It's orders of magnitude.
So here's the stuff that melts at a higher temperature, and helium can go
right through. Not right through it, but it goes much easier
than through soda lime glass. And the reason is because, the excess volume,
when I and network modifier, decreases.
Now why do people worry about this stuff? This is still small, here.
Well, it turns out that if you're in the business of making packaging for
electronics and you want to go into the business of selling iPads or iPods
or whatever-- people do stupid stuff with them.
They put them in water and they spill their Coke on them or spill their
coffee on them. You don't want anything permeating that package.
So even small amounts, even trace amounts, can mess up the electronics
inside of them. And I'm not talking about the outside cover.
I'm talking about the chip that's in there that has a package around it.
And you worry about the permeability, at these scales, for anything getting
through to them. You want to make a pacemaker, for example.
Can you imagine that? You can put your iPod inside of somebody.
That has got to be hermetically sealed. And they test it by helium leak checks.
All right. So we're going to get into more of this next
time, but we can already apply some of our knowledge about this.
So up here, I'm plotting diffusivity as a function of 1 over T. You can see
you get these slopes like this. They are because it's an activated process.
So when I go to higher temperatures, I get a hired diffusivity.
Now, here's something interesting. When I look at this plot, here's iron.
And this is hydrogen and iron. And not surprisingly, at room temperature,
it's BCC iron. And as I raise the temperature, diffusivity
goes up. But boom.
It turns out, iron, at high temperature, it's stable phases, the
stable crystalline structure is FCC. And so, maybe not too surprisingly, you get
a change in the diffusivity. Because why?
Because if you look at the diffusivity, it's got things
like this in there. It depends on the crystal structure.
Now, why does it go down? Can anybody tell me?
Why should hydrogen in FCC diffuse less rapidly than in BCC?
PROFESSOR CIMA: Iron, in both cases, just to change the crystal structure.
Why should it go down? Take a guess.
Yeah. STUDENT: The iron packs more densely in FCC.
PROFESSOR CIMA: That's exactly right. There's less room for the hydrogen to get
around. So what do we say?
74% packing density in FCC, and in BCC it's 68% of the
theoretical packing density. The same thing happens here for carbon.
Carbon is interstitial in iron. We talked about that before when we were talking
about point defects, and you can see the same thing happens-- it drops
when you go from BCC to FCC, it drops.