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PROFESSOR: So these are the
scores from test number two.

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00:00:24,880 --> 00:00:26,560
The celebration.

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00:00:26,560 --> 00:00:29,290
And as you can see, the
average has gone down.

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00:00:33,480 --> 00:00:37,800
So as I've said in the past, I
think everybody in this room

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00:00:37,800 --> 00:00:41,400
has the intellectual capacity
to be here.

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00:00:41,400 --> 00:00:43,415
Some people choose
to be there.

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I don't know why they choose to
be there, why they choose

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00:00:49,710 --> 00:00:53,140
not to go to recitation, why
they choose not to try the

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00:00:53,140 --> 00:00:56,840
homework, why they choose not
to go to office hours, why

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00:00:56,840 --> 00:00:58,250
they choose not to read.

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00:00:58,250 --> 00:01:00,500
But I guarantee you,
when you make that

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choice you will be here.

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00:01:03,980 --> 00:01:09,940
And I have no requirements
whatsoever to pass a certain

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number of people.

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00:01:11,870 --> 00:01:14,790
I can give everybody
A's if I want.

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00:01:14,790 --> 00:01:17,890
And I can take a large number
of people-- you

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00:01:17,890 --> 00:01:19,030
know, look at this.

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00:01:19,030 --> 00:01:21,870
The way this is right now,
the failure rate will be

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00:01:21,870 --> 00:01:23,120
abnormally high.

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00:01:28,710 --> 00:01:32,900
So I would warmly recommend that
if you're down here, that

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00:01:32,900 --> 00:01:35,235
you look in the mirror and ask
yourself a few questions.

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00:01:42,250 --> 00:01:42,620
OK.

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00:01:42,620 --> 00:01:45,160
Let's get to the lesson.

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00:01:45,160 --> 00:01:46,980
We're going to start
a new unit today.

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00:01:46,980 --> 00:01:50,210
The new unit is going to be
the second half of the

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00:01:50,210 --> 00:01:52,100
classification of solids.

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00:01:52,100 --> 00:01:54,670
We looked at solids and we
reasoned that there were

35
00:01:54,670 --> 00:01:57,890
ordered solids and we've looked
at crystals and their

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00:01:57,890 --> 00:01:59,530
arrangements and so on.

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00:01:59,530 --> 00:02:01,180
And today, we're going
to start talking

38
00:02:01,180 --> 00:02:02,630
about disordered solids.

39
00:02:02,630 --> 00:02:06,635
And they're characterized
by no long-range order.

40
00:02:09,640 --> 00:02:11,140
They have short-range order.

41
00:02:11,140 --> 00:02:15,680
You might know who your next
nearest neighbor is, or your

42
00:02:15,680 --> 00:02:18,070
second next nearest neighbor,
but you certainly don't know

43
00:02:18,070 --> 00:02:22,300
who your 10th nearest neighbor
is or who your 100th nearest

44
00:02:22,300 --> 00:02:23,490
neighbor is.

45
00:02:23,490 --> 00:02:28,060
And these solids are called
amorphous solids.

46
00:02:28,060 --> 00:02:29,630
And there's a simple
one salable

47
00:02:29,630 --> 00:02:31,660
Angle-Saxon word for this.

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00:02:31,660 --> 00:02:33,520
It's called glass.

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00:02:33,520 --> 00:02:34,770
So we're going to talk
about glasses.

50
00:02:37,190 --> 00:02:40,090
What kind of materials
can form glasses?

51
00:02:40,090 --> 00:02:42,710
Well, obviously, materials
that have trouble

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00:02:42,710 --> 00:02:43,940
crystallizing.

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00:02:43,940 --> 00:02:47,450
And they come from a variety
of walks of life.

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00:02:47,450 --> 00:02:53,430
So we have in organic
compounds.

55
00:02:53,430 --> 00:02:56,710
Some inorganic compounds can
form disordered solids.

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00:02:56,710 --> 00:02:58,710
And a good example
is silicates.

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00:02:58,710 --> 00:03:02,190
And this is what you know as
window glass, glass that's

58
00:03:02,190 --> 00:03:05,420
used in bottles, cookware,
and so on.

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00:03:05,420 --> 00:03:11,280
There are organic compounds
that can form disordered

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00:03:11,280 --> 00:03:13,590
solids and a variety
of polymers.

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00:03:13,590 --> 00:03:20,860
Things like food wrap and so on,
are also prone to forming

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00:03:20,860 --> 00:03:22,370
disordered solids.

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00:03:22,370 --> 00:03:24,030
Some elements.

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00:03:24,030 --> 00:03:27,190
Some simple elements can
form disordered solids.

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00:03:27,190 --> 00:03:28,990
A good example is sulfur.

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00:03:28,990 --> 00:03:33,400
Sulfur can form solid in
crystalline form, but more

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00:03:33,400 --> 00:03:38,260
often than not, it can form
disordered solids.

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00:03:38,260 --> 00:03:41,970
And what I'm going to show you
towards the end of the lecture

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is metal alloys.

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00:03:43,770 --> 00:03:48,150
Metal alloys can form disordered
solids, and you're

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00:03:48,150 --> 00:03:54,540
going to see metallic glasses.

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00:03:54,540 --> 00:04:05,450
And they're typically 80%
metal and 20% metalloid.

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00:04:05,450 --> 00:04:09,470
That is somewhere along that red
staircase on your Periodic

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00:04:09,470 --> 00:04:12,850
Table that divides the metals
from the nonmetals.

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00:04:12,850 --> 00:04:16,580
So a good example here
is iron 80--

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00:04:16,580 --> 00:04:18,270
this is on mole basis--

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00:04:18,270 --> 00:04:22,540
and boron 20, boron being
the metalloid.

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00:04:22,540 --> 00:04:24,640
And some, some do both.

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00:04:24,640 --> 00:04:29,570
Some compounds can form both
crystal and amorphous.

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00:04:29,570 --> 00:04:34,600
So one example of that
is SiO2, silica.

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00:04:34,600 --> 00:04:41,420
When it forms crystalline solid,
we call it quartz.

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00:04:41,420 --> 00:04:52,800
Crystalline SiO2 is quartz,
and amorphous SiO2 is the

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00:04:52,800 --> 00:04:57,290
silicate glass that we know
for windows and bottles.

84
00:04:57,290 --> 00:05:03,110
And so as I've said many times,
the term "glass" is

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00:05:03,110 --> 00:05:05,625
related to atomic arrangement.

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00:05:10,660 --> 00:05:12,660
It has nothing to do
with the ability

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00:05:12,660 --> 00:05:13,840
to see through something.

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00:05:13,840 --> 00:05:18,160
Transparency has
no place here.

89
00:05:18,160 --> 00:05:19,700
So what are the conditions?

90
00:05:19,700 --> 00:05:24,130
I mean, if you have silica and
it can form crystalline or

91
00:05:24,130 --> 00:05:29,970
amorphous forms, how does
it decide which to do?

92
00:05:29,970 --> 00:05:34,130
And so I'm going to
look at conditions

93
00:05:34,130 --> 00:05:36,540
promoting glass formation.

94
00:05:44,600 --> 00:05:46,950
And I'm going to form glasses
on the basis of

95
00:05:46,950 --> 00:05:47,622
solidification.

96
00:05:47,622 --> 00:05:51,160
So I'm going to start from the
liquid and go to the solid.

97
00:05:51,160 --> 00:05:54,380
So I'm asking the question, why,
on some occasions, does

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00:05:54,380 --> 00:06:00,770
the liquid go to a solid that's
amorphous and in other

99
00:06:00,770 --> 00:06:06,450
occasions it will go to a solid
that is crystalline?

100
00:06:06,450 --> 00:06:08,310
As in the case of quartz.

101
00:06:08,310 --> 00:06:12,700
So there are primarily three
factors that it boils down to.

102
00:06:12,700 --> 00:06:16,380
And I like to make the analogy
to the game of musical chairs.

103
00:06:16,380 --> 00:06:18,850
So imagine I've got chairs
placed around

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00:06:18,850 --> 00:06:20,460
this central table.

105
00:06:20,460 --> 00:06:21,500
You know how the game goes.

106
00:06:21,500 --> 00:06:23,250
There are more people
than chairs.

107
00:06:23,250 --> 00:06:26,980
And the music starts and you
walk around, and then at some

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00:06:26,980 --> 00:06:30,570
point the music stops and people
race for the chairs.

109
00:06:30,570 --> 00:06:34,610
And some people are going to get
a seat and they get booted

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00:06:34,610 --> 00:06:35,970
out of the game.

111
00:06:35,970 --> 00:06:40,040
And then one of the chairs is
removed and then so it goes.

112
00:06:40,040 --> 00:06:41,150
You know this game.

113
00:06:41,150 --> 00:06:43,880
I know you don't want to admit
to playing it, but your little

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00:06:43,880 --> 00:06:44,680
siblings play it.

115
00:06:44,680 --> 00:06:46,140
But of course, you
don't play it.

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00:06:46,140 --> 00:06:48,730
But imagine if you
were to play it.

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00:06:48,730 --> 00:06:50,470
The same situation here.

118
00:06:50,470 --> 00:06:55,080
I've got silicate floating
around in the liquid state,

119
00:06:55,080 --> 00:06:59,190
and the chairs are the crystal
lattice sites.

120
00:06:59,190 --> 00:07:04,000
So the question is, what
promotes the ability to get to

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00:07:04,000 --> 00:07:05,640
the lattice sites or not?

122
00:07:05,640 --> 00:07:07,810
So obviously, one of the
first things we think

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00:07:07,810 --> 00:07:10,770
about is atom mobility.

124
00:07:10,770 --> 00:07:14,280
Atom or compound mobility.

125
00:07:14,280 --> 00:07:16,870
This is in the liquid phase.

126
00:07:16,870 --> 00:07:19,160
And the way I want to write
this, I want to talk about

127
00:07:19,160 --> 00:07:21,110
promoting glass formation.

128
00:07:21,110 --> 00:07:24,550
So obviously, high mobility is
going to enhanced crystal

129
00:07:24,550 --> 00:07:25,760
formation, isn't it?

130
00:07:25,760 --> 00:07:27,220
If you've got high mobility,
you're going to be able to

131
00:07:27,220 --> 00:07:29,080
find the right lattice site.

132
00:07:29,080 --> 00:07:32,070
So I'm going to talk about
if atom mobility promotes

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00:07:32,070 --> 00:07:35,760
crystallization, the reciprocal
of atom mobility

134
00:07:35,760 --> 00:07:38,250
will promote glass formation.

135
00:07:38,250 --> 00:07:41,410
The second thing is the
arrangement of the chairs.

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00:07:41,410 --> 00:07:43,920
If the chairs are in a simple
line, it's easy

137
00:07:43,920 --> 00:07:45,570
to get to the chairs.

138
00:07:45,570 --> 00:07:47,080
All other things being equal.

139
00:07:47,080 --> 00:07:50,210
But if the chairs are arranged
in some complex formation,

140
00:07:50,210 --> 00:07:52,410
it's harder to get
to the chairs.

141
00:07:52,410 --> 00:07:55,720
So that has an analogy and
that's the complexity of the

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00:07:55,720 --> 00:07:56,970
crystal structure.

143
00:08:09,396 --> 00:08:11,650
The more complex,
the more likely

144
00:08:11,650 --> 00:08:14,120
you are to form glasses.

145
00:08:14,120 --> 00:08:20,460
And then the last thing, an
analogy to the musical chairs,

146
00:08:20,460 --> 00:08:24,710
is the rate at which
the music stops.

147
00:08:24,710 --> 00:08:27,630
If the music stops suddenly you
could get trapped in the

148
00:08:27,630 --> 00:08:28,950
liquid state.

149
00:08:28,950 --> 00:08:31,760
If I gradually turn down
the volume, you

150
00:08:31,760 --> 00:08:32,910
know, OK, wait a minute.

151
00:08:32,910 --> 00:08:33,850
The music's going to stop.

152
00:08:33,850 --> 00:08:37,100
You start moving towards
the seat.

153
00:08:37,100 --> 00:08:38,360
So that's the analogy.

154
00:08:38,360 --> 00:08:39,610
Here is the cooling rate.

155
00:08:43,230 --> 00:08:46,020
So a high cooling rate is
going to make it more

156
00:08:46,020 --> 00:08:49,480
difficult for the system to
find the crystal structure

157
00:08:49,480 --> 00:08:53,830
because as the cooling starts,
the atoms start thinking, gee,

158
00:08:53,830 --> 00:08:55,100
it's time to form the solid.

159
00:08:55,100 --> 00:08:58,490
But the thermal energy is
removed from the system before

160
00:08:58,490 --> 00:09:01,400
the atoms have had a chance to
find a crystal structure.

161
00:09:01,400 --> 00:09:06,070
So the reciprocal of atom
mobility, we write in a

162
00:09:06,070 --> 00:09:10,080
positive term and call
that the viscosity.

163
00:09:10,080 --> 00:09:13,920
Something, a fluid, a liquid,
that has low atom mobility has

164
00:09:13,920 --> 00:09:14,810
high viscosity.

165
00:09:14,810 --> 00:09:19,830
So I'll take this out and
instead represent it this way.

166
00:09:19,830 --> 00:09:25,350
This is the way to think
about the formation of

167
00:09:25,350 --> 00:09:27,270
the amorphous state.

168
00:09:27,270 --> 00:09:28,610
And why are we studying
glasses?

169
00:09:28,610 --> 00:09:33,150
Well, in the old days it was
bottles and food and

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00:09:33,150 --> 00:09:34,120
cook ware and so on.

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00:09:34,120 --> 00:09:35,870
Today, fiber optics.

172
00:09:35,870 --> 00:09:39,390
Fiber optics is based on
silicate chemistry.

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00:09:39,390 --> 00:09:40,370
Very important.

174
00:09:40,370 --> 00:09:43,310
So understanding silicate
chemistry has high-tech

175
00:09:43,310 --> 00:09:43,980
implications.

176
00:09:43,980 --> 00:09:49,260
So let's look at, for the first
study, the silicates.

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00:09:49,260 --> 00:09:56,070
The silicates for their value
in fiberoptic technology.

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00:09:56,070 --> 00:10:03,120
So these are based on SiO2.

179
00:10:03,120 --> 00:10:04,520
And some nomenclature.

180
00:10:04,520 --> 00:10:07,670
This is called silica.

181
00:10:07,670 --> 00:10:14,880
So the oxide of an atom is named
by adding the term "a."

182
00:10:14,880 --> 00:10:17,520
So silicon gives us silica
as the oxide.

183
00:10:17,520 --> 00:10:24,010
So here's the atom silicon,
the basic element.

184
00:10:24,010 --> 00:10:25,870
And if we fully oxidize--

185
00:10:25,870 --> 00:10:28,700
see, here I've taken silicon, I
put two oxygens, so I made a

186
00:10:28,700 --> 00:10:29,990
neutral compound--

187
00:10:29,990 --> 00:10:31,000
that's silica.

188
00:10:31,000 --> 00:10:32,540
And then if I fully oxidize--

189
00:10:32,540 --> 00:10:36,380
the maximum number of oxygens
I can put around is four--

190
00:10:36,380 --> 00:10:40,050
and this is going to have
a net charge of 4 minus.

191
00:10:40,050 --> 00:10:43,800
And this fully oxidized anion
is called the silicate.

192
00:10:43,800 --> 00:10:46,690
And that's where we get the name
of the family of glasses.

193
00:10:46,690 --> 00:10:49,100
So this is fully oxidized.

194
00:10:57,440 --> 00:10:58,830
So now let's look at
the structure.

195
00:11:01,750 --> 00:11:03,420
Silicate glasses.

196
00:11:03,420 --> 00:11:04,680
sp3 hybridized.

197
00:11:07,770 --> 00:11:10,760
Just like carbon above it.

198
00:11:10,760 --> 00:11:12,450
sp3 hybridized.

199
00:11:12,450 --> 00:11:17,050
So that gives us the four struts
off of the central

200
00:11:17,050 --> 00:11:22,240
silicon at 109 degree angles
to one another.

201
00:11:22,240 --> 00:11:26,650
And let's put some flesh
on the bones here.

202
00:11:26,650 --> 00:11:29,130
So I'll put a silicon
here in the center.

203
00:11:29,130 --> 00:11:30,460
One, two, three, four.

204
00:11:30,460 --> 00:11:33,130
But instead of putting silicons
everywhere, I'll put

205
00:11:33,130 --> 00:11:36,430
oxygens at the end
of the struts.

206
00:11:36,430 --> 00:11:39,800
And then I'll put the second
silicon, and you can see the

207
00:11:39,800 --> 00:11:44,120
oxygen is acting as a bridge
between the two silicons.

208
00:11:44,120 --> 00:11:46,190
One, two, three, four.

209
00:11:46,190 --> 00:11:48,230
Let's do one more.

210
00:11:48,230 --> 00:11:49,020
Another silicon.

211
00:11:49,020 --> 00:11:52,000
One, two, three, four.

212
00:11:52,000 --> 00:11:55,350
So again, you can see the
oxygen acts as a bridge

213
00:11:55,350 --> 00:11:57,270
between the two silicons.

214
00:11:57,270 --> 00:12:01,690
So you see four oxygens per
silicon, but I've got two

215
00:12:01,690 --> 00:12:03,090
silicons per oxygen.

216
00:12:03,090 --> 00:12:08,820
Hence, the structure looks
like SiO4, but the

217
00:12:08,820 --> 00:12:11,540
stoichiometry of the
compound is SiO2

218
00:12:11,540 --> 00:12:13,210
because the oxygens bridge.

219
00:12:13,210 --> 00:12:16,600
So this doesn't give you any
clue as to what the structure

220
00:12:16,600 --> 00:12:17,450
should be, does it?

221
00:12:17,450 --> 00:12:19,970
You really have to write
this thing out.

222
00:12:19,970 --> 00:12:24,000
Now, here's where the thing
gets interesting.

223
00:12:24,000 --> 00:12:25,900
This is a three-dimensional
network.

224
00:12:25,900 --> 00:12:28,900
All of these oxygens
bridge to silicons.

225
00:12:28,900 --> 00:12:33,030
And I was talking here
about viscosity.

226
00:12:33,030 --> 00:12:36,060
You can see that this thing in
the liquid state is huge.

227
00:12:36,060 --> 00:12:38,950
It's like a giant battleship,
and this is just one.

228
00:12:38,950 --> 00:12:40,640
So now we can have
another silicate.

229
00:12:40,640 --> 00:12:41,670
It can form chains.

230
00:12:41,670 --> 00:12:43,910
It can form meshes.

231
00:12:43,910 --> 00:12:46,300
And these meshes entangle.

232
00:12:46,300 --> 00:12:50,280
So viscosity very high.

233
00:12:50,280 --> 00:12:52,770
So when it goes to solidify--

234
00:12:52,770 --> 00:12:56,380
David, if we can go to the
document camera, please--

235
00:12:56,380 --> 00:12:58,280
so here you see the silicate.

236
00:13:01,520 --> 00:13:04,760
So the yellows represent
silicon.

237
00:13:04,760 --> 00:13:08,160
And you can see the
oxygens, here, are

238
00:13:08,160 --> 00:13:10,690
at 109 degree angles.

239
00:13:10,690 --> 00:13:12,650
So this is the SiO4.

240
00:13:12,650 --> 00:13:15,890
We'll do some nanotechnology
here.

241
00:13:15,890 --> 00:13:17,900
So here's the SiO4.

242
00:13:17,900 --> 00:13:19,160
One, two, three, four.

243
00:13:19,160 --> 00:13:20,590
And now I'm going to
make the bridge.

244
00:13:20,590 --> 00:13:22,370
But look.

245
00:13:22,370 --> 00:13:27,500
The bond between the oxygens and
the silicon is defined in

246
00:13:27,500 --> 00:13:28,700
three dimensions.

247
00:13:28,700 --> 00:13:31,250
It's a 109 degree angle.

248
00:13:31,250 --> 00:13:35,160
But the bond between the two
oxygens is not defined.

249
00:13:35,160 --> 00:13:37,350
It's only defined in
two dimensions.

250
00:13:37,350 --> 00:13:40,440
So you can see that if I put
all of these oxygens on the

251
00:13:40,440 --> 00:13:41,970
same plane--

252
00:13:41,970 --> 00:13:43,960
so somebody borrowed this
and look, they lost

253
00:13:43,960 --> 00:13:45,310
an oxygen for me.

254
00:13:45,310 --> 00:13:46,890
Got a missing oxygen, here.

255
00:13:46,890 --> 00:13:49,640
If you find this thing, please
bring it to my office.

256
00:13:49,640 --> 00:13:52,910
So now all five oxygens
are on the same plane.

257
00:13:52,910 --> 00:13:56,040
One, two, three,
four, and five.

258
00:13:56,040 --> 00:13:59,100
And then up here are
the two silicons.

259
00:13:59,100 --> 00:14:00,620
They're in the same plane.

260
00:14:00,620 --> 00:14:02,370
The oxygens are in
the same plane.

261
00:14:02,370 --> 00:14:04,350
What I'm showing you here
is the beginning

262
00:14:04,350 --> 00:14:06,780
of crystalline quartz.

263
00:14:06,780 --> 00:14:10,000
Crystalline quartz,
this stuff.

264
00:14:10,000 --> 00:14:12,690
But now what happens
if we cool quickly?

265
00:14:12,690 --> 00:14:17,630
This bond is only specified in
two dimensions, which means I

266
00:14:17,630 --> 00:14:20,290
can hold this bond at
the proper value and

267
00:14:20,290 --> 00:14:22,310
this is free to rotate.

268
00:14:22,310 --> 00:14:23,460
This is free to rotate.

269
00:14:23,460 --> 00:14:25,520
Look at the autofocus
on this thing.

270
00:14:25,520 --> 00:14:26,770
Who designed this?

271
00:14:30,720 --> 00:14:32,980
Boy, what a stupid machine.

272
00:14:32,980 --> 00:14:35,720
So anyway, so here we are.

273
00:14:35,720 --> 00:14:36,480
Here is the point.

274
00:14:36,480 --> 00:14:39,810
This bond is free to rotate,
and when it rotates, look--

275
00:14:39,810 --> 00:14:43,420
now this oxygen is no longer in
the plane with this oxygen.

276
00:14:43,420 --> 00:14:47,110
And as a result of that freedom
to rotate, we can end

277
00:14:47,110 --> 00:14:49,050
up running out of
thermal energy.

278
00:14:49,050 --> 00:14:51,840
And this is nowhere near its
regular crystalline array.

279
00:14:51,840 --> 00:14:54,250
So this gives you the
indications of the high

280
00:14:54,250 --> 00:14:57,920
viscosity, moderate cooling rate
will end up giving you

281
00:14:57,920 --> 00:14:59,410
the amorphous silica.

282
00:14:59,410 --> 00:15:04,140
But in all cases, this bond
is the linkage through.

283
00:15:04,140 --> 00:15:07,550
So we end up with this
three-dimensional network.

284
00:15:07,550 --> 00:15:11,820
So what do we do with to
show that we have some

285
00:15:11,820 --> 00:15:13,010
evidence for this?

286
00:15:13,010 --> 00:15:16,050
David, may we cut back to
the slides, please?

287
00:15:16,050 --> 00:15:18,050
So how do you characterize?

288
00:15:18,050 --> 00:15:19,660
I'm going to use x-rays.

289
00:15:19,660 --> 00:15:25,430
So here's an x-ray diffraction
pattern of the-- the upper one

290
00:15:25,430 --> 00:15:28,050
is cristobalite, which is
one of the polymorphs of

291
00:15:28,050 --> 00:15:30,360
crystalline SiO2.

292
00:15:30,360 --> 00:15:34,050
And you can see you have
distinct peaks indicative of

293
00:15:34,050 --> 00:15:35,940
satisfying Bragg's Law.

294
00:15:35,940 --> 00:15:38,050
There's some width to
them, but that's

295
00:15:38,050 --> 00:15:40,080
because it's a real crystal.

296
00:15:40,080 --> 00:15:42,740
Down here, this is the
amorphous silica.

297
00:15:42,740 --> 00:15:44,280
You don't see all of
these features.

298
00:15:44,280 --> 00:15:46,930
You see only one very
broad peak.

299
00:15:46,930 --> 00:15:48,840
Now if I said it's crystal--

300
00:15:48,840 --> 00:15:50,710
pardon me-- if it's glass, you'd
say, well, it has no

301
00:15:50,710 --> 00:15:51,470
long-range order.

302
00:15:51,470 --> 00:15:54,040
So why do we have even
this one feature?

303
00:15:54,040 --> 00:15:57,310
What's this one feature
indicative of?

304
00:15:57,310 --> 00:16:00,050
Doesn't matter how much disorder
there is, I still

305
00:16:00,050 --> 00:16:03,600
know that no matter what, I'm
always going to have these

306
00:16:03,600 --> 00:16:05,960
four oxygens as my nearest
neighbors.

307
00:16:05,960 --> 00:16:10,220
So these four oxygens minus any
long-range order gives us

308
00:16:10,220 --> 00:16:11,610
this one line here.

309
00:16:11,610 --> 00:16:13,440
So we have evidence for it.

310
00:16:13,440 --> 00:16:16,660
Now let's look at the
energetics, because clearly,

311
00:16:16,660 --> 00:16:19,480
these are two different
states.

312
00:16:19,480 --> 00:16:22,350
One of these is lower energy
than the other.

313
00:16:22,350 --> 00:16:24,220
Which one is it?

314
00:16:24,220 --> 00:16:26,360
How to think about
the problem?

315
00:16:26,360 --> 00:16:28,260
There's a simple way
to do it with the

316
00:16:28,260 --> 00:16:30,550
tools we have in 3.091.

317
00:16:30,550 --> 00:16:35,530
Energetics can be given
by bond formation.

318
00:16:35,530 --> 00:16:40,000
Energetics via bond density.

319
00:16:44,070 --> 00:16:46,640
When things form bonds
the energy

320
00:16:46,640 --> 00:16:47,940
of the system decreases.

321
00:16:47,940 --> 00:16:50,680
So which one is going to have
higher bond density?

322
00:16:50,680 --> 00:16:53,370
Well, the higher bond density
clearly is going to be

323
00:16:53,370 --> 00:16:56,790
exhibited by the one that
has the tighter packing.

324
00:16:56,790 --> 00:16:58,730
And which one has
tighter packing?

325
00:16:58,730 --> 00:17:01,375
The disordered solid or
the ordered solid?

326
00:17:01,375 --> 00:17:06,180
The ordered solid has much
more dense packing.

327
00:17:06,180 --> 00:17:23,110
So the ordered crystal exhibits
tighter packing,

328
00:17:23,110 --> 00:17:26,130
therefore, this means more
bonds per unit volume.

329
00:17:33,860 --> 00:17:38,790
So that means, to me, that the
energy of the crystalline

330
00:17:38,790 --> 00:17:42,650
state must be more negative
than the energy

331
00:17:42,650 --> 00:17:43,910
of the glassy state.

332
00:17:47,950 --> 00:17:49,750
That makes sense so far.

333
00:17:49,750 --> 00:17:52,710
But now I want to show you one
other thing that we've just

334
00:17:52,710 --> 00:17:55,740
inferred from this
little exercise.

335
00:17:55,740 --> 00:18:00,200
If we get more bonds per unit
volume, then can you see that

336
00:18:00,200 --> 00:18:04,540
we've made an association
between the binding energy of

337
00:18:04,540 --> 00:18:11,210
any arbitrary ensemble and
it's molar volume?

338
00:18:11,210 --> 00:18:14,240
So volume now, is a very easy
thing to measure, isn't it?

339
00:18:14,240 --> 00:18:16,350
You can see it with
the naked eye.

340
00:18:16,350 --> 00:18:21,210
So the volume of something with
a given composition--

341
00:18:21,210 --> 00:18:22,760
and we're talking about a mole,
we're talking about

342
00:18:22,760 --> 00:18:25,270
equal numbers of atoms--

343
00:18:25,270 --> 00:18:29,470
so the molar volume is
indicative of binding energy.

344
00:18:29,470 --> 00:18:32,220
It's a one-to-one
correspondence.

345
00:18:32,220 --> 00:18:40,200
If you like, the Vmolar is a
measure of disorder, meaning

346
00:18:40,200 --> 00:18:43,440
the higher the molar volume,
the greater the disorder.

347
00:18:43,440 --> 00:18:47,020
Or put the other way, the
smallest molar volume is that

348
00:18:47,020 --> 00:18:49,440
exhibited by the crystal.

349
00:18:49,440 --> 00:18:52,980
So we have some traces here that
come from the reading.

350
00:18:52,980 --> 00:18:55,370
So this comes from the archival
lecture notes that

351
00:18:55,370 --> 00:18:58,940
were written by my predecessor,
Professor Witt.

352
00:18:58,940 --> 00:19:01,990
There's a little typo here that
obviously heating means

353
00:19:01,990 --> 00:19:05,050
increasing temperature, not
decreasing temperature.

354
00:19:05,050 --> 00:19:06,740
Make a little correction here.

355
00:19:06,740 --> 00:19:10,760
So what we're plotting is the
volume, the molar volume of

356
00:19:10,760 --> 00:19:14,610
silicate glass as a function
of temperature.

357
00:19:14,610 --> 00:19:17,710
And imagine we start with
some blob of glass--

358
00:19:17,710 --> 00:19:19,230
of some known mass--

359
00:19:19,230 --> 00:19:22,580
so we can divide and figure out
what the molar volume is,

360
00:19:22,580 --> 00:19:24,160
and we started cooling it.

361
00:19:24,160 --> 00:19:26,680
We cool down until we get
to the crystallization

362
00:19:26,680 --> 00:19:28,520
temperature of quartz.

363
00:19:28,520 --> 00:19:31,460
And when we get to that
temperature crystalline quartz

364
00:19:31,460 --> 00:19:35,940
forms and along with
it, a tremendous

365
00:19:35,940 --> 00:19:37,570
decrease in the volume.

366
00:19:37,570 --> 00:19:41,320
Unlike water ice, which is a
rare exception, where the ice

367
00:19:41,320 --> 00:19:45,890
occupies a larger volume than
the liquid, for most systems,

368
00:19:45,890 --> 00:19:48,510
the solid is more compact
than the liquid.

369
00:19:48,510 --> 00:19:49,850
And that's what you see here.

370
00:19:49,850 --> 00:19:52,810
There's an abrupt drop here
at the melting point.

371
00:19:52,810 --> 00:19:56,550
And then when we continue to
cool, there's some thermal

372
00:19:56,550 --> 00:19:57,230
contraction.

373
00:19:57,230 --> 00:20:01,380
And as you can imagine a hot
solid occupies a greater

374
00:20:01,380 --> 00:20:03,540
volume than a cold solid.

375
00:20:03,540 --> 00:20:05,960
And so this is the cooling curve
and you can retrace it,

376
00:20:05,960 --> 00:20:08,600
heat up, and when we get to the
melting point, there's a

377
00:20:08,600 --> 00:20:12,290
tremendous expansion
and then off we go.

378
00:20:12,290 --> 00:20:19,310
So that's the classical form
of the crystallization of a

379
00:20:19,310 --> 00:20:24,530
material that is undergoing the
normal process of liquid

380
00:20:24,530 --> 00:20:26,490
to solid transformation.

381
00:20:26,490 --> 00:20:28,090
I'm going to use some
tea colors today.

382
00:20:28,090 --> 00:20:29,240
So we went down the red line.

383
00:20:29,240 --> 00:20:31,230
Now let's go down
the green line.

384
00:20:31,230 --> 00:20:32,800
So we're going to go down
the green line.

385
00:20:32,800 --> 00:20:35,360
We go down the green line, we're
going to use the same

386
00:20:35,360 --> 00:20:38,890
stuff, this silicate network,
only we're going to cool a

387
00:20:38,890 --> 00:20:40,490
little bit faster.

388
00:20:40,490 --> 00:20:43,080
And we cool a little bit faster,
we can zoom right on

389
00:20:43,080 --> 00:20:45,490
past the normal melting
point and create

390
00:20:45,490 --> 00:20:47,500
a supercooled liquid.

391
00:20:47,500 --> 00:20:51,350
And that supercooled liquid
gets lower and lower in

392
00:20:51,350 --> 00:20:55,130
temperature and all the while
the volume is shrinking.

393
00:20:55,130 --> 00:20:58,570
Again, a hot liquid what
occupies a smaller volume than

394
00:20:58,570 --> 00:21:00,120
cold liquid.

395
00:21:00,120 --> 00:21:02,510
You know this from a mercury--

396
00:21:02,510 --> 00:21:04,920
the old days, you remember
these old thermometers?

397
00:21:04,920 --> 00:21:06,590
You probably have never seen
one of these things.

398
00:21:06,590 --> 00:21:07,540
It's all done digitally.

399
00:21:07,540 --> 00:21:10,260
But we used to have these liquid
and bulb thermometers.

400
00:21:10,260 --> 00:21:13,500
You'd have numbers on here,
and what happens is the

401
00:21:13,500 --> 00:21:16,240
temperature goes up, the liquid
in here rises to a

402
00:21:16,240 --> 00:21:17,860
higher temperature.

403
00:21:17,860 --> 00:21:21,200
And at a lower temperature
this contracts.

404
00:21:21,200 --> 00:21:24,650
Isn't the solid expanding,
too?

405
00:21:24,650 --> 00:21:25,760
Uh-huh.

406
00:21:25,760 --> 00:21:27,720
So what's the other thing
you learned from this?

407
00:21:27,720 --> 00:21:40,330
It must mean that coefficient of
thermal expansion, which we

408
00:21:40,330 --> 00:21:43,650
affectionately will call CTE,
the coefficient of thermal

409
00:21:43,650 --> 00:21:48,800
expansion of the liquid, must
be much greater than the

410
00:21:48,800 --> 00:21:51,310
coefficient of the thermal
expansion of a solid.

411
00:21:51,310 --> 00:21:53,700
Otherwise, the two would expand
and you wouldn't get

412
00:21:53,700 --> 00:21:56,640
any sensible measurement
out of this, right?

413
00:21:56,640 --> 00:21:58,670
Well, we see that on a curve.

414
00:21:58,670 --> 00:22:01,450
We see that on a curve, because
when we plot volume

415
00:22:01,450 --> 00:22:05,500
versus temperature, when we're
up here in the liquid regime

416
00:22:05,500 --> 00:22:06,930
we have a steep slope.

417
00:22:06,930 --> 00:22:10,880
This slope here is dv by dt.

418
00:22:10,880 --> 00:22:13,110
And when we get down on
the solid regime,

419
00:22:13,110 --> 00:22:14,300
it's a gradual slope.

420
00:22:14,300 --> 00:22:17,120
It's still a slope, but it's
a gradual because the

421
00:22:17,120 --> 00:22:19,510
coefficient of thermal
expansion down

422
00:22:19,510 --> 00:22:24,010
here is very low.

423
00:22:24,010 --> 00:22:27,080
And that's what you're
seeing here.

424
00:22:27,080 --> 00:22:31,580
So at some temperature the
system changes from a

425
00:22:31,580 --> 00:22:35,290
supercooled liquid to a solid.

426
00:22:35,290 --> 00:22:37,840
But it's a disordered solid
because we've quenched in all

427
00:22:37,840 --> 00:22:41,330
of that remaining
liquid disorder.

428
00:22:41,330 --> 00:22:43,360
And take a look at this--

429
00:22:43,360 --> 00:22:48,830
you've changed from the slope
that's characteristic of the

430
00:22:48,830 --> 00:22:51,950
coefficient of thermal expansion
of a liquid down

431
00:22:51,950 --> 00:22:54,830
here to the gentle slope
coefficient of thermal

432
00:22:54,830 --> 00:22:56,340
expansion of solid.

433
00:22:56,340 --> 00:22:57,730
You know what this proves?

434
00:22:57,730 --> 00:23:00,980
This proves that glass
is a solid.

435
00:23:00,980 --> 00:23:03,490
There are many people out there,
even in the popular

436
00:23:03,490 --> 00:23:06,500
press, who will say, glass
is just a very,

437
00:23:06,500 --> 00:23:08,030
very viscous liquid.

438
00:23:08,030 --> 00:23:08,720
Nonsense.

439
00:23:08,720 --> 00:23:09,850
Look at this.

440
00:23:09,850 --> 00:23:12,140
It has this coefficient
of thermal expansion.

441
00:23:12,140 --> 00:23:14,600
And don't fall for any of that
nonsense they tell you when

442
00:23:14,600 --> 00:23:16,900
you go to the cathedrals in
Europe and the glass is

443
00:23:16,900 --> 00:23:19,220
thicker at the bottom because
it's been dripping for 400

444
00:23:19,220 --> 00:23:21,030
years, and that proves--

445
00:23:21,030 --> 00:23:23,020
you know why the glass is
thicker at the bottom?

446
00:23:23,020 --> 00:23:25,030
Because they made
it by spinning.

447
00:23:25,030 --> 00:23:28,320
And when they spun it, it was
graded in thickness from the

448
00:23:28,320 --> 00:23:29,310
center out.

449
00:23:29,310 --> 00:23:31,550
And now, if you're the glazier
and you're putting the glass

450
00:23:31,550 --> 00:23:34,590
in a window, which way would you
put a pane of glass that

451
00:23:34,590 --> 00:23:35,710
had variable thickness?

452
00:23:35,710 --> 00:23:37,430
Would you put the thickest
part up or the

453
00:23:37,430 --> 00:23:39,470
thickest part down?

454
00:23:39,470 --> 00:23:40,940
It's thicker on the bottom
because that's

455
00:23:40,940 --> 00:23:42,190
the way it was made.

456
00:23:44,720 --> 00:23:47,610
Lord help the tour guide when
there's an MIT student taking

457
00:23:47,610 --> 00:23:50,830
3.091 on that tour.

458
00:23:50,830 --> 00:23:54,890
All right, so here we are.

459
00:23:54,890 --> 00:23:57,530
Look at what you have here?

460
00:23:57,530 --> 00:23:59,330
You see this?

461
00:23:59,330 --> 00:24:01,590
At this temperature--

462
00:24:01,590 --> 00:24:03,540
let's say down here where
the lines end, it's room

463
00:24:03,540 --> 00:24:04,340
temperature.

464
00:24:04,340 --> 00:24:06,070
At room temperature
the volume of the

465
00:24:06,070 --> 00:24:08,080
crystalline solid is low.

466
00:24:08,080 --> 00:24:10,740
The volume of the amorphous
solid is higher.

467
00:24:10,740 --> 00:24:12,470
That's proving this.

468
00:24:12,470 --> 00:24:15,560
Vmolar is a measure
of the disorder.

469
00:24:15,560 --> 00:24:19,730
And sure enough, the glass that
was cooled quickly ends

470
00:24:19,730 --> 00:24:23,360
up quenching in more of the
liquid state disorder, and you

471
00:24:23,360 --> 00:24:27,320
see that in terms of what I
call the excess volume.

472
00:24:27,320 --> 00:24:30,560
The excess volume is a measure
disorder because the

473
00:24:30,560 --> 00:24:33,370
crystalline solid
non-zero volume.

474
00:24:33,370 --> 00:24:38,390
So we can define, we can go from
this directly over and

475
00:24:38,390 --> 00:24:41,140
say that v, that's
the excess--

476
00:24:41,140 --> 00:24:41,710
that's a pun.

477
00:24:41,710 --> 00:24:43,910
You know, instead
of writing this?

478
00:24:43,910 --> 00:24:45,190
This is the way they
do in high school.

479
00:24:45,190 --> 00:24:46,850
This is the excess volume.

480
00:24:46,850 --> 00:24:47,180
Bologna.

481
00:24:47,180 --> 00:24:48,810
We don't write like that.

482
00:24:48,810 --> 00:24:53,560
Excess volume is equal
to v of the glass

483
00:24:53,560 --> 00:24:56,880
minus v of the crystal.

484
00:24:56,880 --> 00:25:00,320
And the greater the excess
volume, the greater the degree

485
00:25:00,320 --> 00:25:02,360
of disorder.

486
00:25:02,360 --> 00:25:03,430
So let's do one more.

487
00:25:03,430 --> 00:25:05,590
Let's go down the orange line.

488
00:25:05,590 --> 00:25:09,130
The projector isn't giving us a
good yellow component, here.

489
00:25:09,130 --> 00:25:10,540
OK, so this is the
orange line.

490
00:25:10,540 --> 00:25:12,060
And what's the difference
between the orange line and

491
00:25:12,060 --> 00:25:12,740
the green line?

492
00:25:12,740 --> 00:25:14,760
The difference between the
orange line and the green line

493
00:25:14,760 --> 00:25:18,030
is that in the orange line
we're going to cool more

494
00:25:18,030 --> 00:25:20,400
slowly than we did on
the green line.

495
00:25:20,400 --> 00:25:21,270
Because green means go.

496
00:25:21,270 --> 00:25:23,060
So that's the fast one, right?

497
00:25:23,060 --> 00:25:23,890
So here we are.

498
00:25:23,890 --> 00:25:27,060
We still get supercooled liquid,
but we get down to a

499
00:25:27,060 --> 00:25:31,610
lower temperature before we have
ceased to have higher and

500
00:25:31,610 --> 00:25:34,990
higher viscous liquid and
have formed the solid.

501
00:25:34,990 --> 00:25:38,320
The knee in that curve occurs
at a lower temperature.

502
00:25:38,320 --> 00:25:42,810
And that value is called the
glass transition temperature.

503
00:25:42,810 --> 00:25:45,130
Why do we call it the glass
transition temperature instead

504
00:25:45,130 --> 00:25:47,720
of just saying it's a
solidification temperature?

505
00:25:47,720 --> 00:25:50,740
Because when I say
solidification, before today

506
00:25:50,740 --> 00:25:52,670
you would say, OK, liquid
became a solid.

507
00:25:52,670 --> 00:25:55,910
But after today, if someone says
to you, solidification,

508
00:25:55,910 --> 00:25:59,730
you say, do mean ordered solid
or disordered solid?

509
00:25:59,730 --> 00:26:01,160
So we distinguish.

510
00:26:01,160 --> 00:26:05,040
So every crystallization is a
solidification, but every

511
00:26:05,040 --> 00:26:07,990
solidification is not
a crystallization.

512
00:26:07,990 --> 00:26:09,550
So here we are.

513
00:26:09,550 --> 00:26:13,120
Look at this-- this has a
smaller excess volume.

514
00:26:13,120 --> 00:26:16,010
Because if it was a slower
cooling, that meant that's the

515
00:26:16,010 --> 00:26:21,220
equivalent of giving more time,
more thermal energy, for

516
00:26:21,220 --> 00:26:23,400
things to find their crystalline
position, which

517
00:26:23,400 --> 00:26:26,500
means there's less excess
volume quenched in.

518
00:26:30,450 --> 00:26:31,060
OK.

519
00:26:31,060 --> 00:26:34,380
So let's just get that down,
define these things.

520
00:26:34,380 --> 00:26:37,505
So this is a measure
of disorder.

521
00:26:41,250 --> 00:26:43,310
Or glassiness, if you like.

522
00:26:43,310 --> 00:26:44,040
OK.

523
00:26:44,040 --> 00:26:48,430
So now let's define these two
different temperatures so that

524
00:26:48,430 --> 00:26:50,760
we have the distinction.

525
00:26:50,760 --> 00:26:54,460
So we have, first of all, the
classical one, which is

526
00:26:54,460 --> 00:26:55,710
crystallization.

527
00:26:59,680 --> 00:27:04,636
And crystallization represents
the reactions of liquid.

528
00:27:04,636 --> 00:27:07,760
In both cases, we're going to
convert a liquid to a solid,

529
00:27:07,760 --> 00:27:10,210
but a liquid goes to a
crystalline solid.

530
00:27:14,020 --> 00:27:16,820
And that occurs at a
unique temperature

531
00:27:16,820 --> 00:27:19,530
called the melting point.

532
00:27:19,530 --> 00:27:21,120
Imagine if I talked
to you about the

533
00:27:21,120 --> 00:27:23,150
freezing point of water.

534
00:27:23,150 --> 00:27:28,220
Freezing point of water at
atmospheric pressure is 0

535
00:27:28,220 --> 00:27:29,450
degrees centigrade.

536
00:27:29,450 --> 00:27:32,780
If I asked you, well, if I cool
the water really quickly

537
00:27:32,780 --> 00:27:35,980
or if I cool it slowly, does it
make any difference to the

538
00:27:35,980 --> 00:27:37,330
freezing point of water?

539
00:27:37,330 --> 00:27:38,210
No.

540
00:27:38,210 --> 00:27:39,710
Why not?

541
00:27:39,710 --> 00:27:41,770
Why isn't all this happening?

542
00:27:41,770 --> 00:27:45,210
Because water is a tiny molecule
and so cooling rate

543
00:27:45,210 --> 00:27:48,730
has no impact on the temperature
of conversion.

544
00:27:48,730 --> 00:27:51,050
Those water molecules
will always find

545
00:27:51,050 --> 00:27:52,430
their lattice sites.

546
00:27:52,430 --> 00:27:56,270
So this is a function
only of composition.

547
00:27:56,270 --> 00:27:59,600
If I put something in the water
and I make it impure, I

548
00:27:59,600 --> 00:28:01,250
know I can change its
freezing point.

549
00:28:01,250 --> 00:28:04,410
If I put salt in water, I will
depress its freezing point.

550
00:28:04,410 --> 00:28:08,190
But if I put pure water, it
will free at 0 degrees

551
00:28:08,190 --> 00:28:09,720
centigrade.

552
00:28:09,720 --> 00:28:11,340
Later on, we'll learn that
there are some pressure

553
00:28:11,340 --> 00:28:14,860
effects, but today that's not
going to elucidate anything.

554
00:28:14,860 --> 00:28:15,870
It'll just be a distraction.

555
00:28:15,870 --> 00:28:18,380
So pure water, always
the same thing.

556
00:28:18,380 --> 00:28:21,350
But now if you go to something
like silica, which has the

557
00:28:21,350 --> 00:28:26,840
capability of forming a glass,
the glass formation is a

558
00:28:26,840 --> 00:28:28,670
different reaction.

559
00:28:28,670 --> 00:28:31,120
Glass formation is written
in this manner.

560
00:28:31,120 --> 00:28:33,960
We will start with supercooled
liquid.

561
00:28:33,960 --> 00:28:36,850
It's liquid, but I'm already
going to stipulate that it's

562
00:28:36,850 --> 00:28:40,090
liquid that's been cooled
below the melting point.

563
00:28:40,090 --> 00:28:42,710
Supercooled means it's
cooled below the

564
00:28:42,710 --> 00:28:44,430
normal melting point.

565
00:28:44,430 --> 00:28:52,800
So a supercooled liquid is going
to form a glassy solid,

566
00:28:52,800 --> 00:28:55,840
and this occurs at tg.

567
00:28:55,840 --> 00:29:00,500
tg, which is the glass
transition temperature.

568
00:29:11,380 --> 00:29:15,205
And that is very much a function
of cooling rate.

569
00:29:18,050 --> 00:29:20,010
And, of course, the function
of composition.

570
00:29:20,010 --> 00:29:24,280
Obviously, if we change the
composition from SiO2, we're

571
00:29:24,280 --> 00:29:26,240
no longer comparing
apples to apples.

572
00:29:26,240 --> 00:29:30,410
So composition is important in
both instances, but only in

573
00:29:30,410 --> 00:29:34,730
the case of supercooled liquid
do we form the glassy solid.

574
00:29:34,730 --> 00:29:37,700
And the degree of disorder
is a function

575
00:29:37,700 --> 00:29:38,710
of the cooling rate.

576
00:29:38,710 --> 00:29:41,720
And how is it a function
of the cooling rate?

577
00:29:41,720 --> 00:29:45,420
As the dt by dt, right?

578
00:29:45,420 --> 00:29:51,220
Change in temperature with time
as dt by dt goes up, the

579
00:29:51,220 --> 00:29:54,910
degree of disorder goes up.

580
00:29:54,910 --> 00:29:58,750
So if I want to quench in the
liquid state, can you imagine

581
00:29:58,750 --> 00:30:02,200
if I had liquid and I could
instantaneously cool it?

582
00:30:02,200 --> 00:30:05,640
I would quench in all of
the liquid disorder.

583
00:30:05,640 --> 00:30:11,120
If I quench it less rapidly,
there will be some time for

584
00:30:11,120 --> 00:30:16,970
the atoms to strive for a
degree of crystallinity.

585
00:30:16,970 --> 00:30:22,210
So all of this we've said with
reference to silicate glasses.

586
00:30:22,210 --> 00:30:23,690
Now, there are other glasses.

587
00:30:23,690 --> 00:30:27,190
What are other glass
forming oxides?

588
00:30:27,190 --> 00:30:28,870
Well, what do I have
to look for?

589
00:30:28,870 --> 00:30:32,690
I should look for other
compounds that form bridging

590
00:30:32,690 --> 00:30:34,070
oxygens, right?

591
00:30:34,070 --> 00:30:34,970
That's the key here.

592
00:30:34,970 --> 00:30:36,750
What's the unifying feature?

593
00:30:36,750 --> 00:30:40,440
It's bridging oxygens.

594
00:30:40,440 --> 00:30:47,390
Bridging oxygen leads
to glass formation.

595
00:30:47,390 --> 00:30:50,760
So what are other compounds
that could give us such

596
00:30:50,760 --> 00:30:51,790
bridging oxygens?

597
00:30:51,790 --> 00:30:54,670
Well, you could be lazy and say,
well, if silica does it,

598
00:30:54,670 --> 00:30:56,870
then why don't I look up
and down the column

599
00:30:56,870 --> 00:30:57,850
on a Periodic Table.

600
00:30:57,850 --> 00:31:00,270
If you go up to carbon that's no
good because it forms gas,

601
00:31:00,270 --> 00:31:06,190
but if you go underneath you
will form germania glasses.

602
00:31:06,190 --> 00:31:10,670
So this is germania, and the
glasses are germinate glasses.

603
00:31:10,670 --> 00:31:13,270
You can also go to
group three.

604
00:31:13,270 --> 00:31:14,030
B2O3.

605
00:31:14,030 --> 00:31:18,330
B2O3 forms sp2 hybrids.

606
00:31:18,330 --> 00:31:21,210
And the sp2 hybrids
off of each boron,

607
00:31:21,210 --> 00:31:23,670
we have three oxygens.

608
00:31:23,670 --> 00:31:26,560
And that oxygen can bond
to another boron.

609
00:31:26,560 --> 00:31:29,420
One, two, three.

610
00:31:29,420 --> 00:31:32,730
And this is all going to lie
flat in a plane, isn't it?

611
00:31:32,730 --> 00:31:37,780
But this oxygen bridge is only
specified in two dimensions,

612
00:31:37,780 --> 00:31:41,970
whereas the boron struts are
specified in three dimensions,

613
00:31:41,970 --> 00:31:46,340
which means this oxygen bond
between the two borons doesn't

614
00:31:46,340 --> 00:31:49,000
have to lie in the plane.

615
00:31:49,000 --> 00:31:53,090
It could tilt this up and if it
does, this is going to have

616
00:31:53,090 --> 00:31:55,020
a greater volume.

617
00:31:55,020 --> 00:31:56,410
You can see this with
the naked eye.

618
00:31:56,410 --> 00:31:58,230
I mean, I could have just taught
the lessen by putting

619
00:31:58,230 --> 00:31:59,460
this slide up.

620
00:31:59,460 --> 00:32:03,190
If those are equivalent numbers
of borons and oxygens,

621
00:32:03,190 --> 00:32:06,910
it's plainly obvious that on
the right side you've got

622
00:32:06,910 --> 00:32:08,290
excess volume.

623
00:32:08,290 --> 00:32:10,830
This occupies a much larger
volume than the

624
00:32:10,830 --> 00:32:11,780
image to the left.

625
00:32:11,780 --> 00:32:15,420
The image to the left
is crystalline B2O3.

626
00:32:15,420 --> 00:32:18,440
The image to the right is the
same stuff, except in many

627
00:32:18,440 --> 00:32:22,250
instances this
boron-oxygen-boron bond

628
00:32:22,250 --> 00:32:23,700
doesn't lie in the plane.

629
00:32:23,700 --> 00:32:26,810
It tilts out of the plane and
it causes all sorts of

630
00:32:26,810 --> 00:32:30,400
distortions and leads
to excess volume.

631
00:32:30,400 --> 00:32:32,670
So this is called
a borate glass.

632
00:32:32,670 --> 00:32:36,140
All right, so if we can do it
with the borates, we can do it

633
00:32:36,140 --> 00:32:39,360
with anything that will
form covalent bonds.

634
00:32:39,360 --> 00:32:44,190
So we can do it with P2O5,
phosphate glasses.

635
00:32:44,190 --> 00:32:47,930
V2O5, vanadate glasses.

636
00:32:47,930 --> 00:32:50,740
As2O5, arsenate glasses.

637
00:32:50,740 --> 00:32:54,390
And Sb2O5, stibnite glasses.

638
00:32:54,390 --> 00:32:58,320
And these are used, actually,
as additives to some of the

639
00:32:58,320 --> 00:33:01,400
glass that's put in computer
screens so that they will

640
00:33:01,400 --> 00:33:05,840
gobble up excess oxygen on
cooling and avoid bubble

641
00:33:05,840 --> 00:33:09,900
formation, which then makes
the glass foggy.

642
00:33:09,900 --> 00:33:14,880
And a foggy computer screen is
no fun to try to look through.

643
00:33:14,880 --> 00:33:17,370
So what are the properties
of these oxide glasses?

644
00:33:17,370 --> 00:33:20,070
Well, first of all, they're
chemically inert.

645
00:33:20,070 --> 00:33:21,780
Why are they chemically inert?

646
00:33:21,780 --> 00:33:24,350
Because they've got strong
covalent bonds.

647
00:33:24,350 --> 00:33:26,730
So if you try to react something
with them, it's

648
00:33:26,730 --> 00:33:28,710
going to take a very
special compound

649
00:33:28,710 --> 00:33:30,100
that can trigger reactions.

650
00:33:30,100 --> 00:33:34,750
Which is why they're used for
bottling beverages and

651
00:33:34,750 --> 00:33:36,590
packaging foods.

652
00:33:36,590 --> 00:33:42,100
You put vegetables and fruits
in glass jars going back to

653
00:33:42,100 --> 00:33:43,490
ancient times.

654
00:33:43,490 --> 00:33:46,090
Until recent times, with the
advent of the soda can, there

655
00:33:46,090 --> 00:33:47,860
were glass bottles and so.

656
00:33:47,860 --> 00:33:49,320
They're electrically
insulating.

657
00:33:49,320 --> 00:33:51,970
Why are they electrically
insulating?

658
00:33:51,970 --> 00:33:53,300
Electronic structure.

659
00:33:53,300 --> 00:33:57,020
These are all covalent bonds,
high band gap, which is why

660
00:33:57,020 --> 00:34:00,040
the amorphous version is
used in window glass.

661
00:34:00,040 --> 00:34:02,760
High band gap, which means
light goes through.

662
00:34:02,760 --> 00:34:05,260
How do I think about whether
something is transparent?

663
00:34:05,260 --> 00:34:08,400
Do a little finger
demonstration.

664
00:34:08,400 --> 00:34:12,300
This is the band gap of the
glass, and this is the band

665
00:34:12,300 --> 00:34:14,740
energy, the photon energy.

666
00:34:14,740 --> 00:34:17,850
If the photon energy is small
it goes right through.

667
00:34:17,850 --> 00:34:20,060
That's called transparency,
see.

668
00:34:20,060 --> 00:34:22,080
You can study quantum mechanics,
but you know what?

669
00:34:22,080 --> 00:34:22,620
This is it.

670
00:34:22,620 --> 00:34:24,670
That's all you have to know.

671
00:34:24,670 --> 00:34:27,370
Now what happens if the
band gap is small--

672
00:34:27,370 --> 00:34:29,430
like around 2eV--

673
00:34:29,430 --> 00:34:32,050
and here's the photon energy?

674
00:34:32,050 --> 00:34:36,140
That's called absorption
and re-emission.

675
00:34:36,140 --> 00:34:38,800
Transparency, absorption.

676
00:34:38,800 --> 00:34:40,050
That's all it takes.

677
00:34:42,890 --> 00:34:44,420
You think I'm kidding.

678
00:34:44,420 --> 00:34:45,970
That's all you need to know.

679
00:34:45,970 --> 00:34:47,460
So now, the next thing.

680
00:34:47,460 --> 00:34:48,740
Mechanically brittle.

681
00:34:48,740 --> 00:34:50,990
Why are they mechanically
brittle?

682
00:34:50,990 --> 00:34:52,150
Strong bonds.

683
00:34:52,150 --> 00:34:55,580
No opportunity for slip.

684
00:34:55,580 --> 00:34:58,640
Please don't tell me-- this is
what students tell me every

685
00:34:58,640 --> 00:35:01,870
year and they get zero for the
stupid answer-- that the

686
00:35:01,870 --> 00:35:05,580
reason glass is brittle is it's
a distorted solid and

687
00:35:05,580 --> 00:35:07,570
therefore has no dislocations.

688
00:35:07,570 --> 00:35:08,740
Uh-uh.

689
00:35:08,740 --> 00:35:13,720
Dislocations take the stress
required to cause slip and

690
00:35:13,720 --> 00:35:16,120
reduce it to a lower value.

691
00:35:16,120 --> 00:35:18,660
But you cannot cause this
thing to slip once it is

692
00:35:18,660 --> 00:35:19,680
solidified.

693
00:35:19,680 --> 00:35:22,830
The only way to get this silicon
to move relative to

694
00:35:22,830 --> 00:35:26,350
that silicon in a
vertical shear--

695
00:35:26,350 --> 00:35:28,750
let's say I want to make this
silicon move up and this

696
00:35:28,750 --> 00:35:30,890
silicon move down-- there's
only one way.

697
00:35:30,890 --> 00:35:32,160
It's called break that bond.

698
00:35:32,160 --> 00:35:35,710
When you break that covalent
bond, that's called fracture.

699
00:35:35,710 --> 00:35:38,820
So there is no slip allowed
because we have strong

700
00:35:38,820 --> 00:35:39,660
covalent bonds.

701
00:35:39,660 --> 00:35:42,420
If you want to reshape glass,
what do you have to do?

702
00:35:42,420 --> 00:35:45,940
You have to heat it back up
above its glass transition

703
00:35:45,940 --> 00:35:47,150
temperature.

704
00:35:47,150 --> 00:35:50,330
But you do not shape glass
once it is solidified.

705
00:35:50,330 --> 00:35:52,910
You may have tried in the home,
and then you end up with

706
00:35:52,910 --> 00:35:55,520
something called shards.

707
00:35:55,520 --> 00:35:56,920
And that's the reason.

708
00:35:56,920 --> 00:36:00,010
Optically transparent, we
know that one already.

709
00:36:00,010 --> 00:36:01,600
OK.

710
00:36:01,600 --> 00:36:04,090
Last thing is the very
high melting.

711
00:36:04,090 --> 00:36:09,190
Melting point of silica is over
1,500 degrees Celsius.

712
00:36:09,190 --> 00:36:10,240
You're saying, well,
wait a minute.

713
00:36:10,240 --> 00:36:13,290
He's telling me we're going to
make beverage containers,

714
00:36:13,290 --> 00:36:15,640
we're going to make food
containers, we're going to

715
00:36:15,640 --> 00:36:19,090
make all sorts of useful
objects, but that's going to

716
00:36:19,090 --> 00:36:21,520
cost a lot of energy
to go away up there

717
00:36:21,520 --> 00:36:23,610
to melt this glass.

718
00:36:23,610 --> 00:36:25,350
But it does have desirable
properties--

719
00:36:25,350 --> 00:36:26,950
chemically inert, and so on.

720
00:36:26,950 --> 00:36:29,060
So what can we do?

721
00:36:29,060 --> 00:36:30,670
Back here.

722
00:36:30,670 --> 00:36:34,040
Glass transition temperate is a
function of cooling rate and

723
00:36:34,040 --> 00:36:35,300
a function of composition.

724
00:36:35,300 --> 00:36:39,680
So next step is let's modify
the composition of the

725
00:36:39,680 --> 00:36:44,290
silicate network in order
to drop it's processing

726
00:36:44,290 --> 00:36:47,930
temperature so that we can make
beverage containers at

727
00:36:47,930 --> 00:36:48,990
acceptable energy.

728
00:36:48,990 --> 00:36:50,930
So what's it going to take?

729
00:36:50,930 --> 00:36:54,410
I'm going to have to do
something about those bonds.

730
00:36:54,410 --> 00:36:57,020
Because those bonds are what
caused me to have to go to

731
00:36:57,020 --> 00:36:58,210
such high temperatures.

732
00:36:58,210 --> 00:37:09,750
So in order to reduce the
processing temperature of

733
00:37:09,750 --> 00:37:18,110
silicate glasses via change
in composition--

734
00:37:18,110 --> 00:37:19,470
so this is material science--

735
00:37:26,240 --> 00:37:29,040
change the composition in
order to get desirable

736
00:37:29,040 --> 00:37:29,650
properties.

737
00:37:29,650 --> 00:37:33,370
And specifically, I want
to lower the processing

738
00:37:33,370 --> 00:37:33,860
temperature.

739
00:37:33,860 --> 00:37:37,730
Even when I get these things
liquid, they don't deform very

740
00:37:37,730 --> 00:37:42,500
well, because all these networks
are entangled.

741
00:37:42,500 --> 00:37:44,140
So I'll give you another
analogy.

742
00:37:44,140 --> 00:37:47,020
Suppose you're in the kitchen
and you're going

743
00:37:47,020 --> 00:37:48,920
to cook some pasta.

744
00:37:48,920 --> 00:37:52,840
So you take some linguine and
it's about a foot long.

745
00:37:52,840 --> 00:37:56,520
So you got this pound of
linguine and you cook it in

746
00:37:56,520 --> 00:37:59,630
the boiling water and you
pour in into a colander.

747
00:37:59,630 --> 00:38:02,550
And you may or may not rinse
it with cold water.

748
00:38:02,550 --> 00:38:04,010
Whatever.

749
00:38:04,010 --> 00:38:06,660
Just leave it in the colander
for a little while and what

750
00:38:06,660 --> 00:38:11,630
you'll find is the whole thing
turns into one big mass.

751
00:38:11,630 --> 00:38:13,660
I'm not talking about the
stuff got all gooey.

752
00:38:13,660 --> 00:38:16,500
I'm just saying it just
hangs together.

753
00:38:16,500 --> 00:38:19,400
And if you're clever you can
just gently slide it out.

754
00:38:19,400 --> 00:38:22,080
And it'll slide out of
this one big mass.

755
00:38:22,080 --> 00:38:26,930
What's holding those strands of
pasta together, by the way?

756
00:38:26,930 --> 00:38:27,730
Well, are they covalent bonds?

757
00:38:27,730 --> 00:38:28,690
Are they ionic bonds?

758
00:38:28,690 --> 00:38:30,100
Are they metallic bonds?

759
00:38:30,100 --> 00:38:33,230
I don't know, if you've got
metallic pasta, you've got

760
00:38:33,230 --> 00:38:35,580
digestive problems. So
what's the bonds?

761
00:38:35,580 --> 00:38:38,220
It's van der Waals
bonds, right?

762
00:38:38,220 --> 00:38:41,840
Now, I can cause them to
slip again, can't I?

763
00:38:41,840 --> 00:38:43,325
I can put a little
water in there.

764
00:38:43,325 --> 00:38:44,585
And what does the water do?

765
00:38:44,585 --> 00:38:47,310
It goes in between and then it's
got hydrogen bonds and

766
00:38:47,310 --> 00:38:48,350
they slide.

767
00:38:48,350 --> 00:38:52,010
I can put a little oil in there
and then that slides.

768
00:38:52,010 --> 00:38:52,480
or.

769
00:38:52,480 --> 00:38:55,220
The other thing I
could do is--

770
00:38:55,220 --> 00:38:56,850
you know, have you ever seen
some people, they break the

771
00:38:56,850 --> 00:38:58,910
pasta before they boil it?

772
00:38:58,910 --> 00:38:59,730
I did this experiment.

773
00:38:59,730 --> 00:39:03,050
So you take the pound,
divide it two halves.

774
00:39:03,050 --> 00:39:05,510
By my math, that's
two half pounds.

775
00:39:05,510 --> 00:39:08,650
So you cook the one half pound
one foot in length and you

776
00:39:08,650 --> 00:39:10,770
cook the other half pound--

777
00:39:10,770 --> 00:39:11,680
break them in three.

778
00:39:11,680 --> 00:39:13,850
So they're about four-inchers.

779
00:39:13,850 --> 00:39:15,920
And then you put them each
in a separate colander.

780
00:39:15,920 --> 00:39:17,750
Which one do you think is
going to be much more

781
00:39:17,750 --> 00:39:19,880
difficult to move around?

782
00:39:19,880 --> 00:39:21,080
It's the long stuff, right?

783
00:39:21,080 --> 00:39:22,690
The long stuff entangles.

784
00:39:22,690 --> 00:39:24,280
So we're going to do the same
thing here with the

785
00:39:24,280 --> 00:39:24,640
processing.

786
00:39:24,640 --> 00:39:28,180
And basically, you can study
and learn pretty much

787
00:39:28,180 --> 00:39:30,960
everything you need to know
about polymeric networks in

788
00:39:30,960 --> 00:39:33,390
the kitchen with a
pound of pasta.

789
00:39:33,390 --> 00:39:34,780
All you need to know.

790
00:39:34,780 --> 00:39:36,235
All right, so let's
go and look at it.

791
00:39:36,235 --> 00:39:40,580
I want to reduce the amount, the
length, of those chains.

792
00:39:40,580 --> 00:39:43,720
If I reduce the length of those
chains, I can process at

793
00:39:43,720 --> 00:39:45,450
much, much lower temperatures.

794
00:39:45,450 --> 00:39:48,640
So what's my weapon, here?

795
00:39:48,640 --> 00:39:51,090
My weapon here is oxygen.

796
00:39:51,090 --> 00:39:55,400
Oxygen is going to go in there
in the form of the oxide ion.

797
00:39:55,400 --> 00:39:59,770
And the oxide ion, O double
minus, has a very, very high

798
00:39:59,770 --> 00:40:02,600
affinity for the bonding.

799
00:40:02,600 --> 00:40:09,840
And so what'll happen is
oxide ion attacks the

800
00:40:09,840 --> 00:40:11,900
oxygen-silicon bond.

801
00:40:11,900 --> 00:40:15,900
It attacks the oxygen-silicon
bond and breaks it.

802
00:40:15,900 --> 00:40:20,360
It breaks it in two and then
incorporates itself into the

803
00:40:20,360 --> 00:40:23,360
network in the following way--
you see I have to silicons

804
00:40:23,360 --> 00:40:25,060
joined across an oxygen?

805
00:40:25,060 --> 00:40:28,760
This free oxide ion will come
in here, interrupt that

806
00:40:28,760 --> 00:40:32,060
network in the following
manner.

807
00:40:32,060 --> 00:40:35,700
So it's now broken the
silicon chain.

808
00:40:35,700 --> 00:40:37,660
Now, I've got conservation
of charge.

809
00:40:37,660 --> 00:40:39,080
This was 2 minus.

810
00:40:39,080 --> 00:40:41,410
I don't see any exposed
charge here.

811
00:40:41,410 --> 00:40:42,800
I need 2 minus.

812
00:40:42,800 --> 00:40:46,276
This'll be minus 1, this'll
be minus 1.

813
00:40:46,276 --> 00:40:49,610
I have conservation of charge,
I have conservation of mass.

814
00:40:49,610 --> 00:40:52,290
And what I've done is I've
broken the chain.

815
00:40:52,290 --> 00:40:53,540
This is called chain scission.

816
00:40:57,150 --> 00:41:00,770
Shorter chains, higher
fluidity.

817
00:41:00,770 --> 00:41:04,780
Higher fluidity, which means
lower processing temperature.

818
00:41:04,780 --> 00:41:05,980
Now, where am I going
to get my--

819
00:41:05,980 --> 00:41:09,600
I can't go to the lab and get
a bottle of oxide anions.

820
00:41:09,600 --> 00:41:12,090
I have to have charged
neutral species.

821
00:41:12,090 --> 00:41:17,060
And so what I'm going to look
for is an oxide ion donor.

822
00:41:17,060 --> 00:41:18,630
I need an oxide ion donor.

823
00:41:18,630 --> 00:41:21,130
And where do I find an
oxide ion donor?

824
00:41:21,130 --> 00:41:23,110
Well, better be something
that's going to be a

825
00:41:23,110 --> 00:41:24,810
cation, isn't it?

826
00:41:24,810 --> 00:41:27,642
Because if I've got an oxide
anion, I need a cation.

827
00:41:27,642 --> 00:41:28,892
And what kind of a cation?

828
00:41:31,440 --> 00:41:34,270
How do I make an oxide anion?

829
00:41:34,270 --> 00:41:38,130
I have to have electrons
to give to the oxygen.

830
00:41:38,130 --> 00:41:40,480
So I need a good
electron donor.

831
00:41:40,480 --> 00:41:43,890
Or to use a simple Anglo-Saxon
word, a good metal.

832
00:41:43,890 --> 00:41:47,470
So a good metal oxide like,
say, calcium oxide.

833
00:41:47,470 --> 00:41:51,320
So if I take calcium oxide and
I dissolve calcium oxide in

834
00:41:51,320 --> 00:41:54,220
silica, it dissociates
to give calcium

835
00:41:54,220 --> 00:41:56,920
cation plus oxide anion.

836
00:41:56,920 --> 00:42:01,450
And then the oxide anion
migrates over here to our pal

837
00:42:01,450 --> 00:42:09,400
the oxygen bridge and results in
two of these broken pieces.

838
00:42:09,400 --> 00:42:13,750
So we call this one a bridging
oxygen, as I've been saying up

839
00:42:13,750 --> 00:42:15,440
until now, BO.

840
00:42:15,440 --> 00:42:20,500
And this one is called
a terminal oxygen.

841
00:42:20,500 --> 00:42:23,330
This is bridging oxygen, this
is terminal oxygen.

842
00:42:23,330 --> 00:42:25,010
Or some people, I
don't know why--

843
00:42:25,010 --> 00:42:27,525
they have no literary skills--
they call it

844
00:42:27,525 --> 00:42:29,240
non-bridging oxygen.

845
00:42:29,240 --> 00:42:31,950
I hate that because it tells
you what it's not.

846
00:42:31,950 --> 00:42:35,890
So you might see non-bridging
oxygen.

847
00:42:35,890 --> 00:42:40,990
So all of these up here, the
silicates and so on, these

848
00:42:40,990 --> 00:42:45,150
compounds that have the
ability to form oxygen

849
00:42:45,150 --> 00:42:49,470
bridges, these are called
network formers.

850
00:42:49,470 --> 00:42:52,190
These are all network formers.

851
00:42:54,860 --> 00:42:58,220
Because they have the capacity
to make oxygen bridges.

852
00:42:58,220 --> 00:43:02,140
And then compounds like calcium
oxide that have the

853
00:43:02,140 --> 00:43:06,670
ability to donate oxide ions
that break bridges, these are

854
00:43:06,670 --> 00:43:07,985
called network modifiers.

855
00:43:14,110 --> 00:43:15,530
So good examples--

856
00:43:15,530 --> 00:43:16,450
any good metal.

857
00:43:16,450 --> 00:43:20,540
So I can look at lithium oxide,
sodium oxide, these

858
00:43:20,540 --> 00:43:23,490
will all disassociate
to give oxide anion.

859
00:43:23,490 --> 00:43:26,180
If calcium will work and you
believe Mendeleyev, then you

860
00:43:26,180 --> 00:43:29,490
should probably think about
magnesium oxide, calcium

861
00:43:29,490 --> 00:43:32,590
oxide, and anything
in that series.

862
00:43:32,590 --> 00:43:37,770
A good ionic oxide like
lanthanum oxide, yttrium

863
00:43:37,770 --> 00:43:39,715
oxide, these will be
oxide ion donors.

864
00:43:39,715 --> 00:43:42,620
If you want to get yourself
fired, use scandium oxide

865
00:43:42,620 --> 00:43:46,150
because it's frightfully
expensive.

866
00:43:46,150 --> 00:43:47,640
And you can go to Group four.

867
00:43:47,640 --> 00:43:51,030
You can even use something like
lead oxide or tin oxide.

868
00:43:51,030 --> 00:43:53,405
Even they will donate
oxide ions.

869
00:43:53,405 --> 00:43:56,670
In fact, you can put a lot of
lead oxide in, you'll modify

870
00:43:56,670 --> 00:44:00,450
this thing so much that it'll
start to allow you to cut

871
00:44:00,450 --> 00:44:05,800
crystalline facets and this'll
be called lead crystal.

872
00:44:05,800 --> 00:44:06,940
Why do we use lead?

873
00:44:06,940 --> 00:44:09,640
Well, number one, it modifies
so I can cut it and I get a

874
00:44:09,640 --> 00:44:13,740
straight line instead of that
conchoidal glass edge that if

875
00:44:13,740 --> 00:44:16,950
you've ever cut yourself you'll
never do it again.

876
00:44:16,950 --> 00:44:19,610
The other thing is lead, because
it's got so many

877
00:44:19,610 --> 00:44:24,250
electrons, has a very, very
high index of refraction.

878
00:44:24,250 --> 00:44:27,640
So when you make your billion
and you've got your crystal

879
00:44:27,640 --> 00:44:30,400
chandelier hanging, of course
you want the cut crystal with

880
00:44:30,400 --> 00:44:32,200
a high index of refraction
because it's going to make

881
00:44:32,200 --> 00:44:35,160
that candlelight look
really elegant and

882
00:44:35,160 --> 00:44:36,280
romantic and so on.

883
00:44:36,280 --> 00:44:39,410
And if you make a super boatload
of money, then you're

884
00:44:39,410 --> 00:44:40,490
not going to go with
lead crystal.

885
00:44:40,490 --> 00:44:42,270
You're going to make diamond
pendants, right?

886
00:44:42,270 --> 00:44:44,240
Because they've got
a higher index.

887
00:44:44,240 --> 00:44:46,730
And we all want the best
index, don't we?

888
00:44:46,730 --> 00:44:50,260
OK, so I think this is probably
a good place to stay.

889
00:44:50,260 --> 00:44:54,230
So let's jump to the end,
because we've got

890
00:44:54,230 --> 00:44:55,460
a few things here.

891
00:44:55,460 --> 00:44:58,000
So I said I was going to show
you about metallic glass.

892
00:44:58,000 --> 00:44:59,520
Here's metallic glass.

893
00:44:59,520 --> 00:45:03,830
1959, Pol Duwez, who was a
professor at Caltech, reasoned

894
00:45:03,830 --> 00:45:09,060
that if he were able to cool
liquid metal very quickly, at,

895
00:45:09,060 --> 00:45:12,600
say, a million degrees per
second, he could freeze the

896
00:45:12,600 --> 00:45:17,910
random orientations of atoms
in the liquid state and

897
00:45:17,910 --> 00:45:20,350
prevent them from finding even
something, either simple

898
00:45:20,350 --> 00:45:23,540
cubic, body-centered cubic,
face-centered cubic lattice.

899
00:45:23,540 --> 00:45:25,930
So he worked with
gold silicon.

900
00:45:25,930 --> 00:45:28,220
Gold silicon has a very,
very deep eutectic.

901
00:45:28,220 --> 00:45:32,170
Even though gold melts to over
1,000, mixed with silicon, it

902
00:45:32,170 --> 00:45:35,210
gets down to about 400
degrees Celsius.

903
00:45:35,210 --> 00:45:38,110
So he dropped this through
a little orifice here.

904
00:45:38,110 --> 00:45:41,510
This is molten and it drops
onto a water-cooled copper

905
00:45:41,510 --> 00:45:43,490
wheel spinning at a
very high speed.

906
00:45:43,490 --> 00:45:46,850
And it makes a ribbon some
tens of microns wide.

907
00:45:46,850 --> 00:45:50,400
And what came out here
was disordered solid.

908
00:45:50,400 --> 00:45:51,520
It was metallic glass.

909
00:45:51,520 --> 00:45:56,250
This was the birth of rapid
solidification, and this is

910
00:45:56,250 --> 00:45:58,390
metallic glass.

911
00:45:58,390 --> 00:46:00,250
This is metallic glass.

912
00:46:00,250 --> 00:46:03,730
This has no long-range order,
no grain boundaries, no

913
00:46:03,730 --> 00:46:06,650
dislocations, but it's
not transparent.

914
00:46:06,650 --> 00:46:06,980
Why?

915
00:46:06,980 --> 00:46:09,020
Because it's a metal.

916
00:46:09,020 --> 00:46:11,750
And what do you know about
band gaps and metals?

917
00:46:11,750 --> 00:46:12,390
And it's different.

918
00:46:12,390 --> 00:46:13,130
It was a different Q-factor.

919
00:46:13,130 --> 00:46:15,190
It has different mechanical
properties.

920
00:46:15,190 --> 00:46:17,100
It has no ductility in
a classical sense.

921
00:46:17,100 --> 00:46:18,090
This is just for
your reference.

922
00:46:18,090 --> 00:46:19,650
This is aluminum foil.

923
00:46:19,650 --> 00:46:21,440
And you know what aluminum is.

924
00:46:21,440 --> 00:46:22,690
It's got ductility.

925
00:46:28,480 --> 00:46:31,740
It's got no grain boundaries
so the corrosion properties

926
00:46:31,740 --> 00:46:32,400
are different.

927
00:46:32,400 --> 00:46:36,320
It's got no magnetic
wall boundaries.

928
00:46:36,320 --> 00:46:39,510
So that a transformer made of
this stuff is half the mass of

929
00:46:39,510 --> 00:46:43,900
a transformer made of
crystalline glass.

930
00:46:43,900 --> 00:46:45,150
It has some other uses.

931
00:46:47,560 --> 00:46:51,300
So it's used in magnetoelastic
resonators for theft

932
00:46:51,300 --> 00:46:52,300
prevention-- oh, I'm sorry.

933
00:46:52,300 --> 00:46:52,970
I'm in Cambridge.

934
00:46:52,970 --> 00:46:54,380
I can't say theft prevention--

935
00:46:54,380 --> 00:46:56,350
for inventory control.

936
00:46:56,350 --> 00:47:01,560
And so you see that it's 20%
boron and all of this metal--

937
00:47:01,560 --> 00:47:03,040
iron, chrome, and moly--

938
00:47:03,040 --> 00:47:06,200
and they put this inside the
object in the store.

939
00:47:06,200 --> 00:47:07,980
And then there's a permanent
magnet there,

940
00:47:07,980 --> 00:47:12,480
iron-cobalt-chromium, that sets
the metglass to a field.

941
00:47:12,480 --> 00:47:15,480
And then when you walk through
the uprights, there's a

942
00:47:15,480 --> 00:47:21,140
58-kilohertz signal that causes
this thing to ring.

943
00:47:21,140 --> 00:47:25,040
So it excites and listens
for the ring.

944
00:47:25,040 --> 00:47:27,100
And if you still have that
little piece of metglass in

945
00:47:27,100 --> 00:47:30,610
there that hasn't been
demagnetized--

946
00:47:30,610 --> 00:47:31,880
Bing!

947
00:47:31,880 --> 00:47:33,120
And then, oh, jeez, I'm sorry.

948
00:47:33,120 --> 00:47:33,780
I really am.

949
00:47:33,780 --> 00:47:35,770
I meant to pay for this.

950
00:47:35,770 --> 00:47:38,150
Explain it to the
police officer.

951
00:47:38,150 --> 00:47:38,560
All right.

952
00:47:38,560 --> 00:47:41,930
So that's inventory control.