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PROFESSOR CIMA: All right. So what are they like on the microscopic,
the chemical, scale? And as you know, they look kind of like this.
So here on the left is sort of our 2D glass, this with a glass former that's
got a coordination number of 3 there. I built a little silica one, SiO2, here.
Let me get this up, and I got to zoom out. OK.
And here, I don't know if you can tell-- oh, the colors come out
terrible there-- but obviously the tetrahedral ones are silicon.
And the ones that just have two bonds here are oxygen.
And I think you can see-- here's sort of a number of tetrahedra.
There's 1, 2, 3 there, 4, 5, 6, 7, 8, 8 tetrahedra of quartz, SiO2.
Often people think of a silicate-base glass as the stacking of these
tetrahedra, each of which shares a corner. See, so like this tetrahedra shares a corner
with this tetrahedra. And, of course, if this is a regular array,
you get quartz, crystalline quartz, not quartz crystal, just to confuse
you. What's quartz crystal?
The stuff your mom has, right, or my dad had. Yeah, it's amorphous quartz.
It's just some weird-- it's actually different.
It's actually got lead in it. We'll talk about why this has lead in it.
So this is crystalline quartz, perfect array. So, glasses are made from network formers
like SiO2. They form compounds with coordination number of 3 or 4.
So SiO2, just like I showed you there, silicon wants to be in a tetrahedral
environment, so it's got four atoms surrounding it at
that tetrahedra there. And so that's the coordination number for
silicon is 4. For boron, [INAUDIBLE]
3, it needs 3 bonds, so it's coordination number is 3.
And, lo and behold, both of these are what we call network formers because
they can form these networks. Go back here.
Yes? STUDENT: Is the transparency because of smaller
order? Or what comes--
PROFESSOR CIMA: So what determines the transparency in a glass?
Why is SiO2 transparent? Well, does it have a band?
Do the molecular orbitals span the entire crystal?
Well, we don't have a crystal. So, the reason why it's transparent is that
the-- if you look at the silicon oxygen bond, the
energy to the anti-bond, the empty anti-bond, is larger than the wavelength
of light. So I can violate this rule of transparency
if I can find a glass that that's not true, that the bond energy
is weak enough that the anti-bonding level is within an energy associated
at the wavelength of light. Yes, sir?
STUDENT: Well, I read somewhere that aerogel is made by picking the oxygen
out of the molecule of silicon. But it's still not--
PROFESSOR CIMA: No. No, that's not true.
Aerogel is not made that way. Yeah.
You learned that on the internet. Don't believe everything you learn on the
internet. But, what aerogel is full of is pores, and
the pores are extremely small. And in fact, they're so small that they're
smaller than the wavelength of light.
Now if you take a look at aerogel and you look carefully at it, it's
transparent, but it looks kind of blue. What's that happening?
What's happening there? Why's the sky blue?
Scattering, right, light scattering. So what happens with an aerogel is light scattering
off those little bubbles.