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00:00OK, let me continue dislocation and crystal structure.
00:07This is quite important.
00:09We are still working on this dislocation, like imperfection,
00:14how the materials can be changing when we have like a force onto that material.
00:27Here, that structure is very sensitive to like close packed planes.
00:35You know what that close packed plane?
00:38So we learned about like the simple cubic and body centered one.
00:46I'm sorry.
00:50And also face centered one.
00:52And the face centered the cubic and hexagonal close packed.
01:02Here, we also check the like high density for the atomic density on the plane.
01:12For example, let's say we have some sort of the body centered one.
01:22Then the highest atomic density could be either this plane or some of this plane.
01:36I didn't calculate the area for each, so I don't know exactly.
01:40But normally, if we cut this way, there are like five atoms and then this area.
01:49And here, this one has like four atoms and this area.
01:54So depending on the size of the area, then four divided by some area and five divided by area.
02:13So which one has a higher atomic density on a certain area.
02:21That close packed one is the highest the number of atomic, the number on the unit area.
02:32That's the one.
02:33So in this case, we can see here, like in the case of body centered one, that close packed one, the planes are not existing.
02:45It says it may be saying like most directions are almost the same.
02:52But in the case of face centered cubic, you remember that ABC, ABC content stacking, right? FCC.
03:02And many close packed planes, the highest, it's contacting each other.
03:08Maybe some of them did not make a direct contact on this one.
03:13So if we compare that the atomic contact density that way, that plane, see this like the AB AB staking, you know, in that case, all the atoms are like directly contacted to each other.
03:38So that's close packed plane.
03:45In the case of FCC, that's ABC, ABC.
03:49In that case, many close packed planes in three dimensionally, that's what we saw in the unit set structure.
03:59But in the case of hexagonal close pack, only one plane, the baseline and then the second one.
04:07So we don't have to look at some other directions.
04:11So all the stacking, they are like planes, like close packed planes.
04:20So in the case of the hexagonal close packed one, this basal plane, that's the hexagonal close pack.
04:32So and in that, through that direction, it can make some slip.
04:41Remember that when we have like a tensile strength, tensile force onto the material, then they can make some slip.
04:54And the direction is this way, right?
05:01So here, let's take a look at the magnesium and aluminum.
05:05When we have like a tensile direction to the outside, then hexagonal close packed has only one plane.
05:13So simply they just cut into that plane.
05:18But in the case of face centered cubic, it has many, many different, many close packed planes through all the direction.
05:30So here, so they make all the changes through the material.
05:38Because it has many, they slip, slip, the side.
05:44So it's not like cutting, it's releasing or getting soft, it looks like a soft material onto this, right?
05:57That's, that's the main difference between this hexagonal close packed on the face centered cubic material.
06:05So, um, if that element prefers a hexagonal close pack, then when we have a force onto that material, it just like, it shows, what is that?
06:16Like a breathless.
06:18It's, it's cutting suddenly.
06:20But in the case of face centered cubic, it's like a polymer.
06:25It's, it's, it's more ductile and soft.
06:28So it shows some, we can see when it, it, it, it will be broken.
06:35So, if we want to use this material for, uh, our uses for mechanical purpose, which one would you like to use?
06:46Like a sudden breaking or, uh, you can guess where it can, um, where.
06:54And then we can, where we, we need to change the material.
06:59So, uh, in most cases, I, I prefer to use this aluminum rather than this magnesium.
07:06But, you know, until the certain force, they, they might, the, they might resist.
07:15And then it doesn't show this kind of release.
07:19So sometimes the magnesium is better because it doesn't, uh, it doesn't expand.
07:28But in the case, the aluminum, it's expanding itself.
07:33So sometimes the length will be changing.
07:36And, uh, that's not the way we would like to have for a certain purpose, right?
07:42So depending on where we would like to use, we can choose magnesium or aluminum.
07:49That's quite important.
07:52You may, you may see some sort of the questions in your midterm exam, uh, over this topic.
08:00Uh, not this topic, the previous slide, the catalyst and the surface defects.
08:06So, uh, we, we just observed that like, uh, some vacancy materials is going to the top or some other area.
08:17And then, um, that's just atom, right?
08:21Our groups of atom, they make some, many different types of the islands.
08:28You're like a vacancy empty, empty, uh, empty spot.
08:33And then they will grow onto the, like a terrace, terrace shade.
08:38And, uh, this is the edge on the latch.
08:43And then it's a step, like 계단.
08:47And add a term.
08:50It's add is like adding.
08:52Adding, atom.
08:54Add a term.
08:56And, uh, kink on this side.
09:00And crevice.
09:02Tum, right?
09:04So, um, depending on how it looks like, we have different names for that.
09:10This is the, uh, the, like a same image.
09:15To, uh, this is like, could be like TEM image.
09:20Because, uh, it's a very, very high resolution, right?
09:24So, uh, the active sites on catalysts are normally surface defects.
09:29So, this is quite important if you are working on catalysts.
09:33So, um, a lot of people are working on the high crystal material.
09:39The high crystallinity for their material.
09:43But if we make that way, that's not good for catalysts purpose.
09:48In the case of catalysts, we should have, like, higher surface area.
09:53And also the specific atom and direction to react with the product.
10:01I'm sorry.
10:02Reactant to produce, like, products.
10:05Okay, uh, then how do we watch all this, like, um, the atom or some defects on the surface?
10:21Normally, that's located on the surface or very shallow surface.
10:26So, crystallites, grains, and grain boundaries.
10:30We, uh, we already observed them very considerably in size.
10:35Can be, uh, quite large.
10:37And large single crystal or quartz or diamond or silicon.
10:42They are all, uh, they can be seen through, like, a SAM image or TEM image.
10:49And also we have some diffraction pattern to see how the crystallinity could be.
11:01And, um, the aluminum light posts and garbage can see the individual grains.
11:07And crystallite grains can be quite small, necessary to observe with a microscope.
11:15You know, like, uh, there are many different kinds of microscope, like, electron microscope.
11:20And also the, like, uh, just the regular optical microscope.
11:29You know, optical microscope has, like, lower resolution compared to the electron microscope.
11:35So, depending on the grain size or the size of the material itself,
11:42you can apply that the optical microscope or the electron microscope or the higher resolution microscope.
11:52Here, like, uh, optical microscopy.
11:57Useful up to 2,000 times magnification.
12:012,000 times.
12:03How large is that?
12:04Yeah, that's quite large.
12:06But, uh, it's not that large to observe, like, uh, atomic scale or, uh, groups of atom, atoms scale.
12:16That's not good for with this optical microscopy.
12:20Um, if you took another classes from me, that surface chemistry, surface, uh, like,
12:26science, and, uh, I already explained that, like, um, the, if we use the 300, 300 nanometer light,
12:39most likely, like, the resolution could be around, like, 150 nanometer.
12:45But this is not enough to see, uh, the grain size.
12:50It could be smaller than 150 nanometer.
12:55And polishing removes the surface feature.
12:58And, uh, the, you know, polishing.
13:02I, I also mentioned, uh, in that class.
13:05Cleaning.
13:06You know, what's the best cleaning method?
13:09Like, polishing.
13:11Or grinding, touching.
13:13So, mechanical cleaning is the best way to clean the surface.
13:17They may change some surface morphology, because we are giving some force on top, onto the surface.
13:26But that's the best way to clean it up.
13:29So, not, like, a chemical etching is not the best way.
13:34Sometimes, when we have, like, a pollutant on the surface, that chemical etching may not, uh, go through or cannot touch the surface between that pollutant and the substrate.
13:50So, the mechanical way is the best way.
13:53If we combine them, that might be the best way, too.
13:56Like, etching changes reflectance depending on the crystal orientation.
14:01Um, so, here, if we have, like, uh, you see, the top shape is, uh, like, uh, this way.
14:15And then, the light is reflecting.
14:18So, uh, that might be dark place.
14:25And, uh, when we have this one, that maybe brighter one.
14:31And then, it's scattering all the other directions.
14:37That might be the darkest, I think.
14:39This could be this.
14:41And some of them could be this.
14:43And this.
14:48They say, like, uh, copper and zinc.
14:51Zinc, I love it.
14:52That's 29.
14:53And, uh, zinc is 30, I guess.
14:56I don't know exactly.
14:57You can check.
14:58And, uh, so, when we use optical microscopy, we should get some kind of information through this.
15:06So, grain boundaries can be seen through this optical microscopy.
15:11Our imperfections and are more susceptible to etching may be revealed as the dark lines.
15:21Dark lines.
15:22Because I see why it's shown as a dark line through this microscope.
15:28Change in crystal orientation across the boundaries.
15:33So, ASTM grain, that's, that's quite important.
15:37So, uh, we just check some one inch, one inch, the area.
15:45And then, uh, count how many grains, the, how many grains are located in this, like a certain area.
15:56One, two, three, four, five, six, seven, eight.
15:59Right?
16:00So, eight equal two N minus one.
16:04Then it goes to the other way.
16:07Nine equal two N.
16:09Uh, what is that?
16:11Like around three, right?
16:14Then this number goes to this table.
16:20So around three, then the number of grains are, in our case, that was around the, uh, per square inch at a hundred times.
16:32So, but in that case, that's not, was like a hundred times.
16:37What was it?
16:38Uh, let me check.
16:40The, the, the magnification scale might be different.
16:47So here, depending on the, the, the magnification, um, times, then we can check, like a, how many grains can be existing in that, uh, area.
17:03See, like a, this textbook, uh, this, um, book was written in 1948, but they are still, uh, useful, I guess.
17:18But we don't use it that way.
17:19But we don't use it that way, right?
17:20Right?
17:21When we put into the, the software, they count it.
17:25And also we don't even count how many, the, the, the, the crystallographic grains are, uh, existing in certain area.
17:35For the optical microscope, we use, like a, we can use like polarized light.
17:44So, to see the light scattering direction.
17:51And metallurgical scopes often use polarized light to increase the contrast, right?
17:56So, uh, polarized light has a, just a one direction light.
18:00Some, um, otherwise it has like a many different kind of scattering light, light.
18:07So, it's very, um, um, difficult to figure out the, the contrast.
18:16Also used for transparent samples, such as polymers.
18:21That's quite interesting.
18:23Then, uh, optical micro, the resolution is around, like, um, 100 nanometer.
18:29For high resolution, then we can use the x-ray, but that's not easy to focus.
18:36So, we normally use electron one.
18:39That's SAM, right?
18:40Scattering electron microscopy.
18:43So, um, the resolution is, uh, this good.
18:48And atomic resolution is possible, and electron beam can be easily focused by the magnetic lenses.
18:57We don't know the, the detailed information about the, the system itself.
19:02But we still, we can get some advantages from the electron microscopy.
19:09They are available everywhere this day, right?
19:12And, uh, scanning tunneling microscope, like a TAM.
19:17Atoms can be arranged and images through this.
19:21I remember this was on news, like a, uh, nine o'clock news.
19:26The main news, when it was discovered.
19:30So, people can see the atom.
19:33I don't know, it's really, uh, atom now.
19:38You know, like, it was like 1995.
19:41But that was like, uh, the written in Chinese character.
19:45That's quite interesting.
19:47It's not like atom, A-T-O-M.
19:49What does that mean?
19:50Um, I don't know.
19:51So, uh, we are, we are like summarizing what we have learned in this class.
19:56Uh, point, line, area defects exist in solids, and the number of type of defects can be varied through this, uh, temperature and everything.
20:11Defects affect material properties.
20:12The grain boundaries control crystal slip.
20:13So, mechanical properties and chemical properties, too.
20:14Defects may be desirable or undesirable.
20:16For the catalyst purpose, we need them, right?
20:33But for mechanical purpose, we don't need them.
20:37Right?
20:38So, depending on the application way, we can choose either high-crystallinity or low-crystallinity
20:57to maximize the materials, the role.
21:06Okay, uh, that's the end of chapter four.
21:10And then I will prepare the chapter five soon.
21:14Thank you for your, uh, watching.
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