Tip:
Highlight text to annotate it
X
ROFESSOR CIMA: OK. So now it's back to this curve.
Here's another interesting property of glass is that this curve turns out,
even though this glass transition temperature is a thermodynamic
transition, your ability to measure it depends on kinetics.
And the reason is is as you're dropping the temperature, the
viscosity of the glass is getting higher and higher and higher.
And the viscosity is related to how easy it is for these molecules to flow
over one another. And, at some point, the time to get this sort
of equilibrium curve exceeds the time that you have available, like your
lifetime. So, it turns out that the actual curve you
measure depends on the cooling rate.
So this is faster cooling rate. And what you notice here is, depending on
the cooling rate, you get different molar volumes.
And we say that this difference between the volume of the glass and
the volume of the crystal-- so what is it, what'd I call it, V minus Vxstal
is the excess volume. And so the volume, faster cooling, will be
larger, or the density will be less, 1 over the volume, will be less as you
cool it faster. Question?
STUDENT: Why do you have a V bar on your graph? PROFESSOR CIMA: Oh, V bar.
That's supposed to be molar volume. I got to get a consistent set of notation
here. But when I put a bar there, I normally mean
molar volume. Sometimes you'll see it like that.
It's not a vector. Good point, sorry.
Any other questions? Yes, sir.
STUDENT: On the thermal expansion coefficients, which v is velocity and
which V is the volume, or are they both-- PROFESSOR CIMA: These are both volume.
They're both volume. So what this says is-- so what's the units
of thermal expansion? 1 over temperature, right, because this volume
cancels with that volume. All right.
So properties of glass. No long-range order means no diffraction.
Right? Can't have a diffraction unless you have a
crystal. They are brittle.
Why do you think they're brittle? How many slip systems do you think there are
in glass? Any close packed planes?
No, there's no order. So they're brittle because there's no slip
systems, no slip, no dislocations.
I mean, you can think of it, it's all a dislocation, right?
They're generally electronically insulating. There are some exceptions to that that are
interesting. They tend to be chemically stable.
Not always true. They're often optically transparent.
But that's not always true either. So the only ones that really are true are
these two. But the most common ones fit that down there.