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00:00OK, now we turn to silica ceramics.
00:08These are materials made mostly from silicon and oxygen.
00:14I mentioned that most likely the positive carions are very small and anions are much bigger.
00:25And the two most abundant elements in the earth.
00:32The key building block is the silicon oxygen tetrahedron written as silicon oxide.
00:43One silicon and four, one, two, three, four, this building unit.
00:50In this tetrahedron, a silicon atom sits in the center, right?
00:55And then four atoms make some tetrahedron shape.
01:01Can you see that one in this zinc blended structure?
01:07Do you remember this structure here?
01:09And the silicon 4 plus is sitting inside.
01:16And then tetrahedron, see, they are like the oppositely direct.
01:28And this polymorphic foams are quartz.
01:32When they make a high-odered one.
01:35That's the one.
01:38It makes several crystalline foams.
01:43And the first one is quartz and then crystallite.
01:48And what are they?
01:51Like I've never heard about those forms.
01:54Like tri-d-mite or something.
01:58Which means they are not gonna be on your mid-term exam.
02:02Because I don't even know what they are.
02:05But textbook says that way.
02:08Okay.
02:09The common crystalline foams are obviously quartz.
02:14And uh, crystallite.
02:16And um, some tree-d-mite, whatever.
02:25These structures are relatively open.
02:28And which means to the atoms are not packed very tightly.
02:33So, uh, there are many defects.
02:37We can somehow crack them through very small crack.
02:42So the density is relatively also low.
02:46However, the silicon oxygen bonds is very strong.
02:50Right?
02:51Remember that?
02:52Like a glass.
02:54When you think about glass or quartz.
02:57They don't melt out by simply like heating it up.
03:03With normal temperature we can go up to.
03:06Right?
03:07It says like, uh, the silica has a high melting point.
03:12Temperature around like 1710.
03:161710 degrees Celsius.
03:20That's quite high temperature.
03:24Right?
03:26And um, in many silicate minerals.
03:31This is silicate form.
03:34That tetrahedron shape.
03:37Connected by sharing their corners oxygen atoms.
03:41Sharing these atoms.
03:43They can also share edges or faces in some cases.
03:47Uh, so this could be like some face or some edges.
03:54To keep the whole structure electrically neutral.
03:58Positively charged ions such as calcium.
04:01That, that's gonna be shown in the next, some next slides.
04:06Magnesium and aluminum and others are added.
04:10These are called like cation.
04:12Right?
04:13The, the, some high number cation.
04:17Could be sodium or something like that.
04:19Ionically bond silicate to one another.
04:24Right?
04:25So to balance them.
04:27Here, that's four negative.
04:29So we need like a two of calcium ion.
04:32Or two of magnesium.
04:34And, um, so they need to make some stoichiometry.
04:39These, this cation impurity or adding, they do very important jobs.
04:52First, they balance one.
04:54That's the main job here.
04:55Right?
04:56And also, they help link the tetrahedron together to form larger, stronger structures.
05:03See?
05:04Um, let's see.
05:06See?
05:07Like some, here they showed only this one.
05:11But, uh, I can, I think they have something like this.
05:16Like a layer of structure.
05:18And they connect to each other.
05:20Let's go back here.
05:23Okay.
05:24And, um, the, and, in some mineral, the forsterite, the formula is magnesium silicate.
05:36Magnesium silicate.
05:38The magnesium ions both balance charge and connect the tetrahedron units.
05:44So that's kind of like a connection unit.
05:48Um, silica can also be non-crystalline.
05:55Here we see some crystalline one and glass is non-crystalline.
06:01Um, is it like a liquid or solid?
06:06Based on this definition, non-crystalline, it's kind of liquid.
06:10But nobody believes that glass is kind of liquid.
06:14In science, in science, uh, term, it is liquid.
06:18But, I don't know, that's gonna be some, um, topic to discuss over.
06:24But here, so, um, it has this, like a pure amorphous silica is, is called like a fused silica.
06:35Or vitreous silica.
06:38And, uh, some, some other common glasses contain like a impurity such as a sodium or calcium, cation, aluminum, and boron, too.
06:51You know, in solar cell research, people use the glasses for the, for substrates, for, uh, the loading all the, the, the thin film solar cell materials.
07:05Sometimes we have to heat up the glass to do some annealing process or mixing elements in to form the, the right material for the solar cell absorber layer.
07:22Then, this cation moves to that, the, the, the main solar cell materials that may act as a kind of the, what is that, impurity or some kind of a doping material.
07:41Some materials are, like, affecting in good way or some, some, some, some elements are, some cations are not good material for the, for that kind of purpose.
07:56So we have to get rid of them.
07:58And then also the, the, they add intentionally some boron material to make it stronger and, uh, less fragile.
08:09And, uh, those, uh, glass materials are very, very expensive.
08:15And, uh, also layered silicate exists than, uh, two dimensional sheets, right?
08:25We call this layered silicates and, uh, they include clays, mica, and, uh, some other things.
08:34Mica is very important thing.
08:36You know, that, that guy who got a Nobel prize for the, the graphite material, he used that Mica for very flat material.
08:47They used only, like, uh, what is that? Scotty tape to make a very thin layer of metal oxide, which has a lower surface energy.
08:58So, uh, that graphite material can be easily, uh, taken off from the surface by dissolving that Mica material.
09:10So, uh, if you know, like, uh, the properties of this layered silica, that might be a good way to work for, like, a 2D material.
09:23And then, see, as I mentioned before, like, some, some, uh, cation, smaller cation is holding that anion, the face one or some edge one.
09:38And then they, uh, they are connected through this cation part.
09:43That is called, like, a kaolinate something.
09:48Uh, I don't think this is important, but I want to know, like, some 2D materials can be, um, the, what is that?
10:01Like, generalized by, uh, considering this silicate, the material.
10:14Um, and then that, like, some polymorphic forms of carbon.
10:23So, the carbon has several solid forms, which is, which is called, like, a polymorphs.
10:32You know, carbon, if we say, like, carbon, carbon is a kind of regular carbon material.
10:39The very black, carbon black.
10:43And also, diamond.
10:45And what else?
10:46Like, graphene, graphite, right?
10:49They are all, like, a polymorph.
10:53So, the first one is diamond.
10:56Tetrahedral bonding of carbon.
10:59Uh, do you remember this structure?
11:01Like, they are tetrahedron structure.
11:05And then, the, they all look like a zinc blendy, right?
11:09In, in case of zinc blendy, this is different from this one.
11:14But, in this case, they are all identical.
11:18So, um, it's not called, like, a zinc blendy.
11:23It's called, like, a diamond structure.
11:25But, silicon is also, uh, uses the same structure.
11:29So, when we add some sort of, um, doping material, sometimes, uh, they sit on this side.
11:37Then, it makes, like, some zinc blendy structure.
11:42And, uh, like, uh, the diamond is one of the hardest material we know, right?
11:50So, we use this to, to cut something.
11:53Cutter.
11:54Or, the, we use it for very, what is that, like, um, very expensive gemstone.
12:03Right?
12:04So, it's, it's, it's very shiny.
12:08And, uh, also very small crystals.
12:11Used to grind, cut other, something material, diamond, thin film.
12:17Um, the, um, a few years ago.
12:23And, uh, I know many scientists are, like, making very thin diamond layer,
12:30uh, crystallized one.
12:32And, also, some of them are, the, partially conductive.
12:36And, then, they use that, that layer as a, uh, as a substrate for growing some other material.
12:44Or, sensing something.
12:45And, also, the second form of the, uh, carbon polymorph is, uh, graphite.
12:51Graphite.
12:52Graphite is, also, very, like, a light material.
12:56And, then, highly conductive.
12:58And, uh, through this, 2D, uh, direction.
13:02And, um, but, this, this, interaction is, like, a van der Waals force.
13:08So, they do not have, like, a strong attraction to each other.
13:12And, then, also, the, it has highly conductive through this, this way.
13:17But, it's not that much, like, conductive through this way.
13:21But, still conductive.
13:26Uh, like, planes slide easily over one another.
13:30Like, a good lubricant.
13:32Right?
13:33So, people use, put this one into, uh, engine oil or something.
13:39To make it smoother.
13:49Okay, and, then, like, a crystallinity in polymers.
13:53We saw that some crystallinity in metal or some ceramic material.
13:58But, can we guess?
14:00We can actually get some crystallinity in polymers.
14:04Or, the atomic arrangement involving molecular scale, molecular chain.
14:09Is it possible?
14:11Yes, it is.
14:12So, people, the, figure out, some, the, crystals can be found in polymer structure, too.
14:20Here, like, this is the example polyethylene, uh, unicell.
14:25When we have solid, uh, the ethylene material.
14:28Then, they can be ordered like this.
14:31And, they can be packed through this way.
14:34And, uh, they make some kind of a crystal structure.
14:38Like this.
14:39And, um, the, when we do this, like, polyethylene chains adopt all trends.
14:46Like, a zigzag conformation.
14:48And, pack into an orthorhombic unicell.
14:52That's quite interesting, right?
14:54Even though individual polymer chains are, like, enormous compared to atoms.
15:00The idea of repeating building blocks still applies.
15:05The unicell captures how chains are, like, arranged relative to one another in three dimensions.
15:14Then, why do you care for this?
15:17Crystallinity strongly affects properties.
15:21Higher crystallinity in a polymer generally leads to higher density.
15:27That, that might be possible, right?
15:30So, like, a higher density and stiffness and tensile strength.
15:35We will learn about this tensile something in the later chapter.
15:39And then, improved chemical resistance.
15:42When they have a highly packed, then they might have less void or cracks.
15:48So, the chemical can penetrate, uh, when, uh, the, the, the, the corrosion, corrosion chemicals cannot penetrate into the crack in a proper position.
16:03Because they are highly packed, right?
16:06So, highly, uh, highly crystalline polymers can be, uh, opaque.
16:12Because regular periodic structures scatter light, uh, efficiently.
16:18Whereas, amorphous polymers can be, optically, might be transparent.
16:23So, um, uh, but I haven't seen that kind of example so far.
16:33So, I, uh, actually, I'm not interested in that point either.
16:39But, if we can control this kind of density by applying some voltage or by changing the temperature.
16:50Then, that polyethylene can be highly packed or less packed depending on the, the temperature or some applied voltage.
16:59Then, it can be changing from the opaque to trans, uh, the translucent and also transparent.
17:08So, we can change the, uh, the, the smart, uh, we can use it for a smart window.
17:16Uh, note that, like, a polymer crystallization is never 100%.
17:23We talk about, like, degrees of, like, crystallinity ranging from 0 to, like, perhaps 90% or less than 90.
17:33Textbook says that way.
17:35Textbook says that way.
17:36And processing condition, like cooling rate, molecular weight, and then also some, some sort of treatment.
17:46Like, um, keeping the temperature at, at, at, at certain temperature for a long time may, um, help crystallizing their structures.
17:57Now, let's connect, like, crystal structure to engineering application.
18:05The properties of crystalline materials are often tightly linked to crystal structure, right?
18:12But that's quite, obviously, I guess, obvious, I guess.
18:17And some applications require single crystals to, uh, leverage an isotropic behavior.
18:23Let's take a look at the quartz first.
18:26And, um, the fractures more easily along some crystal plane than others.
18:33So, that means, uh, like, if we choose a certain direction that can be highly resistive to the, uh, the, the, the fracture.
18:44So, uh, uh, we can, like, um, some silicate tetrahedral sharing corners to form all those three dimensional networks.
18:55The fracture behavior is, uh, orientation dependent.
18:59So, quartz tend to crack more readily along specific crystallographic planes.
19:06Another one is a diamond one.
19:11Hmm, single crystal uses, used for abrasives.
19:17Diamond adopts the diamond cubic structure and essentially a zinc blending.
19:23That's what I mentioned in the previous, I guess, uh, framework structure.
19:27And, uh, that's very strong sp3 covalent bonds.
19:32It's extraordinary hardness depends on the, uh, the integrity of its single crystal structure
19:40and the orientation of the, uh, the cutting facets relative to the crystal lattices.
19:47And, um, so the, in high temperature aerospace application or some, um, not, not just simply aerospace one
20:00and in nuclear power material too.
20:03So, I remember when I visited Dusan long time, Dusan long time ago, they were, like, making some, this, the, uh, turbine blades for the nuclear power system.
20:17And then this one blade, the price of this one blade is almost the same as a small car.
20:27So, you know, which one is more complicated, like this one or a car.
20:33So, people are expanding the whole line to make just a car.
20:40And then they have one, uh, like building block for a blade to make the same price.
20:50Which one would you like to work for?
20:53So, uh, but anyway, I'm, uh, I'm sorry for that kind of, uh, I may mis- uh, misconduct some information about those too.
21:06So, in high temperature and also that the nuclear power, this turbine blades can be fabricated as a single crystals of, uh, nickel-based super alloys
21:18or some other kind of, uh, material elements.
21:23And then polycrystals, uh, most engineering materials, however, are polycrystalline.
21:33Not just a single crystalline here, polycrystalline.
21:37So, here you see a lot of crystallines are, uh, connecting to each other, right?
21:43The, um, it means like many small single crystals or grains that vary in orientation.
21:53On a micrograph here, you know, on a micrograph, each grain is a single crystalline, very small one.
22:00And, uh, the borders between them are grain boundaries.
22:04If the grains are randomly oriented, the macroscopic properties are isotropic.
22:11Because, uh, they have, uh, they randomly oriented all the direction to all direction.
22:18They do not depend on directions.
22:21Because the anisotropy of individual grains averaged out.
22:26But, um, grain size matters.
22:30Grains can range from nanometers to centimeter here.
22:35Nanometer to centimeter.
22:38Up to the scale, I, I don't know exactly.
22:43According to the, uh, whole patch relationship.
22:49That reducing grain size generally increase yield strength.
22:53Because grain boundaries act as a various to, uh, dislocation motion.
23:01There are some trade-offs.
23:04Very small grains can impact toughness and, uh, high temperature stability.
23:11While a very large grain can lead to anisotropy and a lower strength based on the textbook.
23:20In a welded plate, for example, you might see large columnar grains in the fusion zone.
23:32And smaller, uh, equiaxed grains in the heat affected zone.
23:39Reflecting thermal, uh, gradients and, uh, solidification dynamics.
23:45Those microstructural differences, uh, like translated directly into variations in local mechanical properties.
23:55Let's, um, contrast the single crystals and polycrystals more explicitly.
24:09In a single crystal, properties vary with directions.
24:13Right?
24:14So, single crystal varies with, uh, the anisotropic.
24:19That's more common for the single one.
24:23You know, like when we observe this kind of, like structure, we see four atoms on this side.
24:30And then, uh, four atoms on this side.
24:34But somehow here, just one atom.
24:36But if we do the, uh, the repeating unit, they may have a different, uh, number.
24:42of things.
24:44Let's take a look at this, this plane.
24:49Then five, right?
24:51So, uh, depending on what plane I'm taking, that, that atom, the number of atoms are quite changing.
25:03So, we also have to consider the area.
25:08So, what kind of a density.
25:11So, we don't know, like, uh, which direction has a higher density and which direction has a lower density.
25:18That lower density means they do not have a longer, the stronger bonding.
25:24So, they might have a lower, uh, interaction, attraction.
25:29So, if we have a single crystal, it, it cannot be anisotropic.
25:35It cannot be isotropic.
25:37So, it is anisotropic.
25:39So, it says like a modulus of elasticity in BCC ion.
25:44Uh, the copper is also the same way.
25:48So, they have like a different strength depending on the crystal direction.
25:55And in the case of polycrystals, it's isotropic.
26:00Because, uh, that crystals are randomly oriented.
26:07So, their unisotropic directions are all scattered to, to all the direction.
26:14And then they, that makes, say, like an isotropic.
26:19Canceled out.
26:21Is it correct?
26:24Like, um, here like, by, polycrystals with a random grain orientations are approximately isotropic.
26:35On the macroscopic scale.
26:37Each grain may be like anisotropic, right?
26:40Each grain is mainly like anisotropic.
26:43But, collectively, the directional dependencies cancel.
26:50That said, if a polycrystal has a strong texture.
26:54Meaning, many grains share similar, uh, the orientations.
26:59Then anisotropy reappears at the, uh, component level.
27:05That's quite obvious.
27:08It's not that difficult to understand, is it?
27:12And, uh, polymorphism.
27:15Many materials can adopt more than one crystal structure, depending on the temperature as shown here.
27:21It's iron.
27:23That's quite interesting material.
27:25Um, it's changing their structure.
27:28Here, see, at, uh, the above 912 degree Celsius.
27:33It changes, like, um, alpha to gamma ion.
27:38And then, uh, it should go here.
27:41See?
27:42And it's changing the crystal structure.
27:47Body-centered cubic to face-centered cubic.
27:49And then, body-centered cubic again.
27:52And, uh, that, that can be changed depending on some pressure too.
28:00And, uh, and so also, like, uh, titanium, it's changing their structure is a tool, like a classic example.
28:10At room temperature, alpha-titanium has a hexagonal close-packed structure.
28:16And then, when we increase the temperature over like 882, and then it becomes like a beta-titanium, uh, which is like a body-centered cubic.
28:28And this transformation changes sleep system.
28:32We will learn about that sleep system in the later chapter.
28:35Affecting that ductility, and the strength, and, uh, the formability.
28:47So, um, all those, like, crystal structures can be checked by x-ray diffraction.
28:55And then, uh, I think you guys all know about this x-ray diffraction.
29:06So, I don't need to describe it detailed on this x-ray one.
29:10Let me just go through this black one.
29:13And, uh, and, uh, x-ray is going through this.
29:17And then, when we have, like, different, the planes of Adam's location, and then arrangement.
29:26And then they have, uh, for example, this green line, and then this red line.
29:32The same pathway up to this guy, and up to this part.
29:37And then reflecting, and reflecting.
29:40This, this distance are the same.
29:42But it reflected more traveling distance over, over here and here.
29:48And that's related to this distance too.
29:52So, depending on that, uh, this gap.
29:56We will see certain, uh, depending on the, the, this theta.
30:02We will see a sharp peak.
30:04That's a, like, constructive wave.
30:07And then that's, like, a Bragg equation.
30:11We already know about it, right?
30:13So, uh, if we do some alpha ion body centered cubic.
30:18We see their plane.
30:21That will be discussed in, uh, chapter three extended part.
30:25Like a one one zero plane, and two zero zero plane, and two one one plane.
30:30So, you also need to, uh, take the class over, like, the three extended part.
30:37And, uh, that's gonna be introduced in that, uh, the, the class recorded file too.
30:47So, one zero zero, one one zero.
30:50And then this is, like, two zero zero.
30:54And, uh, two one one.
30:57After learning that one, you may, uh, be able to see what this, like, a plane is meaning.
31:04So, uh, yeah, we learned a lot of things.
31:09Like, face centered cubic, and body centered cubic, and hexagonal class spec.
31:14And then, the, uh, AFP, uh, atom, the, I'm sorry, APF.
31:23Uh, what is that?
31:26Like a vector, right?
31:28So, um, um, um, also, we can, uh, predict the density of material.
31:35So, up to zero point five, four, two, like, uh, zero point seven, what, two?
31:43Or something?
31:45And, uh, interatomic bonding in ceram, ceram, ceramics.
31:50Uh, the crystals, and some materials can have more than one crystal structure.
32:01Polymorphism and allotropy.
32:04And, uh, the, if it's, uh, element scale, then that is called, like, allotropy.
32:11Otherwise, uh, polymorphism.
32:13You know what element is?
32:15So, um, um, if you can see someone, something in the periodic table, that's element.
32:24Like carbon, oxygen, um, they are elements.
32:28But ethylene is not a, not a, uh, not an element.
32:35Because ethylene is not on the, the periodic table.
32:40Okay, uh, please, uh, take a look at this, uh, textbook too.
32:49And I also, I will let you know which, like, the question at the end of the chapter should
32:56be solved for the midterm exam.
32:59Okay, thank you.
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