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학습트랜스크립트
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00:24I don't think we categorize this one as like the ceramic if we say something as
00:35a source ceramic then it's what is that it's like a highly resistive to
00:42corrosion and also it contains all kinds of the oxide the anion somewhere but we
00:54will see how how it goes so the slides want us to move to move from the metals to
01:05ceramics ceramics are open ionic and mixed ionic and the covalent in that term
01:18that so this like ion compound could be considered as a ceramic but we will see
01:25where whereas metal atoms can slide into dense arrangements without regard for
01:34any charge right so we didn't consider that's negative metal or positive metal
01:41they are all just metal they are mixing together or something and the electrons
01:48are like a flowing over the all over the sea so that's what we we considered in the
01:56metal like ceramic cations and anions must arrange it to maximize the opposite
02:05charge attraction right so we have positive charge and negative charge then
02:11they are attracted to each other right so if we take a look at the pictures below
02:17then the bigger one normally becomes like a negative one negative charged and the
02:23center this what is that like blue one should be like a positive so the positive
02:32and negative they are making some although it it looks like a day it's it's making
02:39some contacts yeah they they make contacts right we this is anion there so they might
02:51make a contact or a certain distance between them so maximize maximizing right that's the most
03:02important thing attraction while minimizing like the charge repulsion so in this case we
03:12need to consider this maximizing interaction and minimizing repulsion between them
03:18this introduces like a constraints the relative sizes of ions matter right so the sometimes the
03:31cations are a little bit bigger for example lithium is very small but the sodium is a little bit
03:39bigger than that so if we consider this as a the proton H plus good it it's not like H plus
03:47but if we consider that as a proton then this could be lithium plus and this
03:55could be sodium plus or like this could be lithium plus and then the next one sodium and potassium
04:03something like that so the relative sizes of ions that quite matter right and the charge balance must be satisfied charge balance but that's also considered in general chemistry class right so you you may know about like a charge balance so let's say if it's closed one that's the that's the
04:10that's the thing we can see every thing we can see everything here if it should be negative and nar
04:40If it's negative one, negative one, negative one, the centered one should be some sort of metal and positive four, right?
04:50That's what it says.
04:52So charge balance should be satisfied.
04:56And bonding directionality may appear where covalency is significant, right?
05:03So it says like ionic and covalent is some sort of mixture.
05:08So ionic compound doesn't have that much bonding direction.
05:16So if we have positive and negative, this is the bonding direction.
05:20But they can be rotated through all the geometry.
05:27But in the case of covalent bond, for example, H, O, H.
05:32So this direction cannot be rotated because it has a certain rule between this electron and electron repulsion, right?
05:44So that's around 104.5 degree.
05:48So it's fixed.
05:52But in this case, this can rotate.
05:55And negative could be rotated anywhere around here.
06:03So the consequence is that ceramic crystal structures can be more complex.
06:11Not like the simple cubic or body-centered cubic and face-centered cubic.
06:20It's more complicated.
06:22How?
06:23Like we will see.
06:24They often feature anions forming a close-packed framework.
06:30Framework?
06:31It looks like a framework.
06:32Frequently, FCC, right?
06:37With cations occupying interstitial sites, we will see that in the later real structure.
06:46After looking at the real structure, the choice of which interstitial sites to occupy, tetrahedral, octahedral, or cubic,
06:56depends primarily on cation-to-anion radius.
07:01That's quite an important thing.
07:03So we will discuss over this for like three or four slides after this.
07:09And the charge consideration.
07:12We will see.
07:13So this is what it says.
07:15Like at the previous slide at the end, I mentioned that.
07:18So here, the radius of cation, radius of cation, divided by radius of anion.
07:27This number is very important to determine the structure of the ceramic.
07:35So if the value of RC over RA is close to these numbers, their coordination number will be either 2, 3, 4, 6, 8.
07:54So the cation and anion, the radius ratio is smaller than 0.155, this one, greater than 0.155, and then smaller than 0.225, this one.
08:15And 225, some other value, other value, other value.
08:19So the value is categorized in certain area.
08:26So 2, 3, 4, 6, 8.
08:32Like this.
08:35Isn't it like easy to understand this?
08:40Do you see any complexity over here to understand that?
08:45It's just, we don't know much about the reason at this moment.
08:49But we can simply guess if it's too small and they cannot, they don't have to make a geometry change it because it's close enough to make the bonds.
09:03So each cation is coordinated by enough anions to neutralize its charge locally.
09:16This is electrostatic stabilization, right?
09:19So, and also, and also, the anion's anion distances remain large enough to avoid strong repulsion, right?
09:28So, have you heard about, like, VSEPR, valence-shell-electron pair repulsion, so the electrons are showing repulsion because they are negatively charged.
09:44So, they want to make the longest distance to minimize their repulsion, so this picture leads to a geometric predictor, easy one, right?
10:04So, if I ask you to describe what's the radius ratio rule, then you want to describe this kind of thing.
10:20You don't have to go detail over this number, because I don't want you to memorize any number or some sort of stupid things.
10:30So, if needed, I will give this table, so you don't have to memorize things.
10:36So, it estimates which coordination environment for 6-8 neighbors can accommodate a certain number of the anion surrounding,
10:56a cation of a given size relative to the anion.
11:00So, I think you know about this, like, anions are, in most cases, greater, the insides are greater than the cation, normally, right?
11:20If you don't know about it, then you have to go through the general chemistry first, again.
11:26So, here's the table, the aluminum, and some sort of this positive cation, and here are, like, a negative anion, simply ionic bonds, right?
11:46But this book mentions this kind of bonding material as ceramics.
11:52So, we will see how it goes, then, let's go for the next slide, to see.
12:09Not all ceramics are purely ionic.
12:13Many have significant covalent character, too.
12:15So, the degree of covalency correlates with small electronegativity differences between the constituent elements.
12:27So, this figure shows, like, this picture shows the electronegativity.
12:33Do you memorize a certain number for each element?
12:38In general chemistry, I was told, like, fluorine is kind of the standard material to determine the electronegativity.
12:49They fixed the number for, like, 4.0 at the very beginning.
12:56But, in new technology, they figured out it's a little bit more than the actual value we fixed at that old moment.
13:07So, that might be why they put, like, 4.1 rather than 4.0 in the previous, like, the scale.
13:17So, partially, it could be, like, covalent, but the degree of covalency correlates, right?
13:26So, with the smaller electronegativity differences between the constituent elements, that's what a textbook says.
13:34So, for highly ionic systems like alkali halide or electrostatic dominate and the radius ratio rule works reasonably quite well.
13:53For materials like silicon carbide or many 3 to 5 semiconductors, gallium arsenide or indium phosphide, covalency is quite strong.
14:06And tetrahedral coordination aligned with sp3 hybridization is favored.
14:15So, here, like, then how do we, like, determine the percent of ionic character and how, what percentage is the covalency, something, right?
14:39So, in the next, here, some sort of, some example, and the ceramic crystal structures are categorized, like, oxide structure.
15:04So, the helpful, the mental model is to start with anions open, O2 negative, so, oxide structure, forming a closed-packed structure, typically, like, FCC, yeah.
15:22Then, where do the carions go? Can you guess?
15:28The closed-packed anions lattice has, like, interstitial sites, tetrahedral sites, coordination by 4 anions, and octahedral sites, coordinated by 6 anions, right?
15:41Octahedral, you remember that, octahedral? This is octahedral, right?
15:47So, the coordination, like, negative one, then center should be here, that cation should go.
15:59So, the cations choose sites that fit their size and also charge constraints, right?
16:07To balance the charged things.
16:10So, let's tie this to some real structures in the rock salt structure, like sodium chloride or magnesium oxide, something like that.
16:30Then, in the case of sodium chloride or the structure, the coordination is 8, more like the simple cubic arrangement, where each ion sits at the center of a cube, right?
16:53So, for example, then chloride are located at the corner, and cation is sitting in the middle of these two planes in a three-dimensional way, right?
17:14So, we will see this kind of, like, each structure a little bit later.
17:20So, we start from oxide structures, but we also can consider, like, chloride or some other kinds of anions at the end.
17:33So, factors that determine crystal structure.
17:36So, we just saw, like, a coordination number, right?
17:41Then, that coordination number is very important to determine the structure of crystal, right?
17:48So, for example, if that, here, it's obvious to say this.
17:55So, if we have very small cation that can be movable from inside this space everywhere,
18:05and even negative should touch each other to make the contact of this positive one.
18:14In that case, it should be unstable.
18:18But if it's, like, chargely balanced in all directions, that could be stable.
18:28And also, if they can have some sort of repulsion distance, then that's also stable.
18:35If it's too large, then the bonds are quite weak.
18:39So, in that case, it's not stable.
18:43It's not stable either.
18:45And here, maintenance of charge neutrality.
18:52So, it's easy to understand.
18:54Like, calcium is 2+, and fluoride is 1-.
18:58So, we have 2 fluoride.
19:00Isn't it obvious to say this, right?
19:04So, this is the table to summarize the previous slides.
19:12So, I hope you understand this coordination and structure selection.
19:23So, this is the kind of summary for the description I emphasized for the last slides.
19:36So, here, this is the thing we have to take a look.
19:42So, the coordination number is increasing depending on R-canion divided by R-anion.
19:49Value is falling into some value interval.
19:57So, the value is around, if the value is 1.2, then triangular form will be favorable, right?
20:07So, here, the main focus is on like 4, 6, and 8 because they are like most cases in nature.
20:19So, if we take a look at some ceramic material, most of them are falling into this 4, 6, 8 categories.
20:32Here, it says like it looks like a zinc blendy tetrahedral.
20:41So, I mentioned that like it looks like silicon or carbon bonds like a diamond.
20:47And also, the tetrahedral, see?
20:52Do you see like this tetrahedral is in the center?
20:56In the center, we have a certain the cation one, and then this 4, and the up and down, total 6.
21:07That's octahedral, right?
21:09You can draw like something like this.
21:14That's octahedral.
21:16And then that is considered as sodium chloride.
21:19Zinc blend is zinc sulfide and sodium chloride.
21:23And this one, the cesium chloride.
21:26So, when you read the textbook, they categorize this 4, 6, 8 coordination as zinc blendy and sodium chloride and cesium chloride structure.
21:46So, not just simply the coordination number 4, but also it describes as zinc blending structure.
21:59Isn't it like some weird, like that's representing material like zinc blending, zinc sulfide and sodium chloride structure.
22:09So, if somebody says like sodium chloride structure, that means it's octahedral structure for ceramics.
22:18And cesium chloride, I drew this before in the previous slides.
22:23Do you remember that?
22:25So, that's a cesium chloride.
22:28So, although textbook describes that way, I actually do not know that's like a common for materials engineers.
22:40And, you know, these days they are all mixed up.
22:45So, every scientist or engineer uses most like common words because of that scientific papers we share.
23:01But, I don't, I don't do much work on this area.
23:09So, I don't know much about like a sodium chloride structure or something.
23:14But, I prefer, I prefer octahedral structure rather than sodium chloride structure.
23:25But, that's what happens here.
23:30If you solve some problem in the chapter question, they also mention some sodium chloride structure or what's the structure for this material.
23:44Then, you have to answer like sodium chloride.
23:47Although, that's not sodium chloride.
23:49So, here's an example.
23:58To appreciate the number, coordination number here.
24:04Let's calculate this one.
24:07So, the coordination number is like six here.
24:15Then, it's going up and down.
24:18So, they make some contacts.
24:21How do we know they are making contacts or not?
24:24So, in this case, they made contacts.
24:27That's how they drew, right?
24:30So, this distance, and that's anion radius, anion radius, and cation radius, cation radius.
24:42Right?
24:45And, this is considered as A.
24:48And, that's anion, two of the anions.
24:53Then, this one.
24:59Right?
25:00So, that's what we have here.
25:03And, then, also, that another equation comes right away.
25:13And, then, we do some calculation.
25:18See, the ratio becomes 0.414.
25:24So, 0.414.
25:25So, 0.414.
25:27So, let's go back to the...
25:29Sorry.
25:300.414.
25:31In that case, it could be tetrahedral or octahedral.
25:41But, it says like octahedral here, coordination number.
25:45That's what it says.
25:55So, bond hybridization, that's what we discussed before.
26:01But, actually, I don't care much about this bond hybridization.
26:06So, I know like there might be some ionic characters or, and also like covalent characters.
26:15But, we can simply guess which one is overwhelming the other.
26:25So, whenever we see like some sort of positive and anion, caryon-anion interaction, that's most likely the ionic bonds.
26:36And, some part, it shows like the covalent bonds too.
26:42It's a calculation way.
26:44So, they have like electronegativity values.
26:48And, then, put just into the equation.
26:52So, if needed, if this equation is needed, then I will give that equation in the midterm exam.
27:01So, you don't have to memorize any equations for the test.
27:07So, 11.5% is ionic character.
27:16That means 89% is covalent bonding, silicon carbide.
27:22But, I don't know this has like the ionic characters.
27:28Most people mention that this one has like the covalent bonding.
27:34So, here example problem.
27:41On the base is ionic radii.
27:43What crystal structure would you predict for ion oxide?
27:48So, this is the question I just mentioned.
27:51Like, what crystal structure?
27:53Then, what should we answer?
27:55Like octahedral or tetrahedral?
27:57That also describes the structure shape, right?
28:02But, in this case, in the textbook, they answer as like sodium chloride or zinc blendy, zinc sulfide structure.
28:10Or, a cesium chloride structure.
28:14So, here, ion oxide, ion and oxide.
28:21R cat divided by R and I.
28:25Right?
28:26So, that's gonna be 0.550.
28:32Oh, that means it should have like a coordination number of 6.
28:39Because, we just saw like a 0.414 is a starting point for the coordination number 6.
28:47Right?
28:48The crystal structure is sodium chloride.
28:51What are you, what is this?
28:53Right?
28:54So, octahedral could be an answer too.
28:58So, either one is fine.
29:00That's the sodium chloride structure or the octahedral structure.
29:07Whatever you prefer.
29:12Rock salt structure.
29:14Sodium chloride.
29:15So, rock salt structure could be the answer for the previous problem.
29:20Right?
29:21Right?
29:22Like, in the rock salt structure.
29:25Imagine the anions chloride in sodium chloride or the oxide in magnesium oxide.
29:39The cations occupy all the, here, cations, octahedral sites in that lattice.
29:50So, this results like a 6 coordination.
29:536 to 6 coordination.
29:55Right?
29:56Each cation is surrounded by 6 anions at the corners of an octahedron.
30:02And each anion is surrounded by 6 cations.
30:07That's easy to see.
30:11I think you can visualize this structure by just looking at it.
30:23It's not that difficult to understand, I guess.
30:26We saw, like, this kind of structure many, many times from, like, undergraduate, graduate courses.
30:34And every, like, material, science books, and the solid books, they show this kind of structure.
30:42And, uh, it's, it's, it's, it's enough to see that three-dimensional way.
30:47Right?
30:48Octahedral way.
30:49Here, here, here.
30:51Magnesium oxide is the same as sodium chloride structure, somehow.
31:00So, rock salt.
31:10Uh, I'm not familiar with these words, like, rock salt or sodium chloride structure.
31:20Uh, but textbook is very, uh, fluent, fluently mentioned that kind of words in describing
31:29this kind of structures.
31:31Which means, like, that might be very common for material science and, uh, or engineers.
31:37But this book has been written for a long time ago.
31:40So, I don't know, people are still working this way.
31:45Because the, for chemistry, I, I learned some specific design or, uh, the expression way.
31:55But I don't see that one in, in, in the recent articles.
32:04They are all mixed up.
32:05Because, you know, chemistry is not the only subject for chemists only.
32:11So, a lot of material science will do the chemists.
32:15And also, the other way, the same way.
32:18Like, a material science that chemists are working on that way, too.
32:22So, they are all mixed up.
32:25Right?
32:26So, for example, for, like, um, electrochemistry, we have notation way.
32:33Electron flowing this way.
32:35Right?
32:36So, the calcium and calcium 2 plus ion.
32:42But I don't see this kind of notation in the article any longer.
32:47They don't have to describe this one.
32:50So, they just put, like, anode reaction, cathode reaction, something like that.
32:57Okay.
32:58Let's move on to the, the many different structures.
33:04Here, they are, like, describing many different structures.
33:08And that's what we saw, like, a cesium chloride.
33:11Cesium is located in the center of, like, this, the cubic, body-centered cubic-like structure.
33:18But, this is not, like, the, ax, I'm sorry.
33:28So, um, the ratio is up quite high.
33:32And then, that's the way we describe this one.
33:39And, ax2, fluorite structure.
33:46So, uh, we saw this, the, what is that, like, charge balance, so on.
33:52Then also, um, in the case of calcium 2 plus and F negative.
33:58So, uh, each cation has to have two times, uh, many, uh, many, uh, fluoride, right?
34:13So, the fluoride should be, uh, doubled.
34:20That's why we have many fluoride, but in the center, we only have, like, a small amount of calcium.
34:29Calciums are, like, centered in the, the volume.
34:34So, it's fully considered as one.
34:38But, at the corner, they are considered as only, like, a quarter or half, something like that, right?
34:45So, that's why we saw more fluoride.
34:48It should be, like, two times greater.
34:56And then, also, they, they are cut somewhere.
34:59So, uh, we need more fluoride to satisfy the actual number for each car, each ion.
35:11And this is perovskite structure, the same way.
35:16And, uh, the complex oxide.
35:20See, like, uh, this one is also considered as, uh, what? Ceramic.
35:25But, you know, like, ceramics are the, um, not conductive material.
35:34So, it could be semiconductor.
35:36So, perovskite one could be semiconductor under the light or certain condition.
35:44But, it's not, uh, conductor.
35:49For, uh, the whole, uh, condition.
35:55Thinkable and Unicell, I mentioned that, um, you can just visualize this using the software, somehow.
36:04And density computation is the same way, as we mentioned in the previous metal cases.
36:12So, uh, in this case, cation and anion, they have different weight, right?
36:19And, uh, the volumes are the same thing, the same way.
36:23So, as long as you understood the previous density calculation way, it should be the same way.
36:32Okay, uh, the densities are, like, um, categorized here.
36:40So, we learned about the density of metals and ceramics and polymers.
36:46That's quite obvious to understand this one, right?
36:49So, metal, they have quite the high, uh, density.
36:55It's heavy.
36:57And ceramics is a little bit lower than metal.
37:01Due to, like, um, the oxides has, like, higher volume, right?
37:06And then, polymer, they have, uh, a lot of void volume.
37:11They are not, like, crystallized structure.
37:14So, it has a lower density.
37:17And composites are locating somewhere in between them.
37:21So, I hope you understand this one very clearly.
37:27Uh, I don't think that they, uh, they have any difficult part for understanding, right?
37:35So, um, let me wrap up here.
37:45Let's see what's in the next part.
37:49Silicate ceramic.
37:51This is a more complicated one.
37:53I will describe the rest of the, uh, the slides on the third, the recording.
38:03Thank you.
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