In the previous explanation, we delved into the mechanism and characteristics of metallic bonding (extremely important).
I explained that metals are formed by multiple atoms sharing electrons ($e^-$) and being held together by Coulomb forces.
So, are the bonded atoms "bonded in a random shape"? Not exactly, to a certain extent.They are joined together in a regular, consistent shape.
This time, we will consider the shape of these bonds that exhibit a regularity.
The content is basic high school chemistry, but it's extremely important, and I think even mechanical engineers might have forgotten it, so I hope you understand it.
Crystal, crystal lattice (lattice)
To recap from last time, before considering crystal lattices, let's recall metallic bonding (I'll explain why I'm using metallic bonding as an example later).
In metallic bonds, free electrons are shared throughout the entire material, resulting in a bond structure like the one shown in the following diagram (for example, the metallic bond of iron atoms).
Each metal atom contributes electrons to the other, and these electrons are shared (the electron shell is shared throughout the entire material) to form a bond.
If we connect the electron shells in the upper diagram with lines instead of circles, metallic bonding can be represented as shown in the lower diagram.
As you can see from the diagram, you can see that they are arranged in a regular pattern.
Although the diagram shows them neatly arranged in a 3x4 grid, in reality they are arranged in a slightly more complex way.
However, for the sake of easier understanding, let's assume for now that things are simply and neatly aligned.
Up until now, we've only considered things in terms of two dimensions, but actual matter is not composed solely of surfaces; it fundamentally exists in three dimensions.
Now let's consider how the metals are bonded together in three dimensions.
First, let's imagine a metallic bond surface arranged in a 2x4 grid as an example.
If we consider that there are three identical bonding surfaces, and we superimpose these three surfaces, we get the following diagram.
This creates a three-dimensional solid.
When you look at a diagram that has been created by overlapping surfaces to form a three-dimensional object, you will notice that it is a collection of regular three-dimensional shapes.
To make it easier to understand, remove the circles representing atoms from the diagram, leaving only the lines.
As you can see from the diagram, it looks like eight identical boxes (cubes) stacked in a regular pattern.
like thisA crystal is a mass of the same shape arranged in a regular pattern.They call it,The box-like structure that forms the basis of a crystal is called a crystal lattice or space lattice.It is called.
In the diagrams so far, for the sake of ease of understanding, we have used simple shapes like building blocks based on a simple box (cube), but actual materials often have much more complex shapes (with a greater variety and number of constituent crystal lattices).
For example, ice crystals, which many people are familiar with, have a base shape that is a complex stacking of polygons (there is a clear regularity to it, not randomness).
The same applies to other things like quartz crystals.
However,No matter how complex the shape, if it is a crystalline structure, the underlying shape (crystal lattice) is arranged in a regular pattern.
Therefore, this concept is very important, so I want you to understand it well.
It's important to note here that not all atoms and bonding methods result in a crystalline structure. Please keep in mind that crystallization may occur depending on the atoms and bonding method.
Incidentally, the reason why metallic bonding is used as an example when explaining crystals and crystal lattices isThis is because many metal atoms and metallic bonds often form crystals.
MoreoverMore than 70% of all metal atoms and metallic bonds are made up of three simple crystal lattices.You have it.
Iron, aluminum, titanium, and magnesium, in particular, are prime examples of metal materials commonly used in human civilization, possessing simple crystal structures and lattices.
So next, let's introduce three types of crystal lattices that are commonly found in metallic bonding.
Typical crystal lattices of metals
Before explaining specific metal crystal lattices, let's briefly explain what changes when the crystal lattice and crystal structure differ.
This might not be a clear explanation, but different crystal lattices and crystals result in completely different properties of a material.
For example, Even with the same substance, iron, differences in crystal lattice and crystal structure can alter properties such as tensile strength, hardness, wear resistance, impact resistance, fatigue resistance, and thermal conductivity.
I will now explain in detail why the properties of metals, including iron, change depending on the differences in their crystal lattices and crystal structures.
Furthermore, not only the mechanical properties but also the conductivity and electromagnetic properties change (I'm not an expert on electrical properties, sorry).
Therefore, in order to know the properties of a material with a crystalline structureIdentifying atoms, bonding methods, and molecular structures is insufficient; a thorough understanding of the crystal lattice and crystal structure is also necessary.That becomes extremely important.
Let's start with the simplest and easiest crystal lattice.
Body Centered Cubic (BCC)
The term "body-centered cubic lattice" suddenly came up, but let's first take a look at what kind of crystal lattice shape it actually is.
In the description of crystal lattices and crystals, identical, aligned bonding planes were superimposed.
In a body-centered cubic lattice, the arrangement of atoms in the second layer differs from that shown in the diagram.
I'll explain using BB pellets I had at home as an analogy for atoms (sorry the pellets are dirty).
First, let's visualize the metallic bonding in the first row, 2 rows and 2 columns, using a model.
Next, let's consider the second row.
Now, to consider the second stage, we must consider an important law of nature."Nature tends to act in the direction of what is easiest," and in the case of atomic bonding, what is easiest is for atoms to stay as close together as possible and become more compact.
Therefore, the second row of four atoms arranged in a 2x2 grid is positioned in the central depression of the four atoms, which is close to the first row of atoms.
The third layer is arranged in the same order as the first layer (imagine it as putting a lid on it).
I colored the second row red to make it easier to understand.
This is a body-centered cubic lattice.
The diagram illustrates this as follows.
As I've shown in the diagram, my personal impression isSince there is one atom at the center of the box (cube), it is called a body-centered cubic lattice.I think so.
English is also important.BCC is an abbreviation of Body, Centered, and Cubic.It is called.
A crystal like the one shown in the following diagram is formed when many of these body-centered cubic (FCC) lattices come together.
First, the image of the shape is important, so I will explain the characteristics of the fine grid later.
Examples of materials that form this crystal lattice and crystal structure include iron (Fe), chromium (Cr), molybdenum (Mo), tungsten (W), and vanadium (V) at room temperature (all extremely important materials).
However, since the shape of the crystal lattice of each substance changes depending on the temperature and other conditions and treatments, I don't think it's necessary to memorize them.
Faced Centered Cubic (FCC)
Just like with the body-centered cubic lattice, let's start by visualizing the face-centered cubic lattice.
First, as preliminary preparation for the image, I will include the diagram that was introduced in the explanation of the crystal lattice.
In the face-centered cubic lattice, the arrangement of atoms differs only in the second row compared to the diagram.
First, let's visualize the atomic arrangement of the first layer using BB pellets (arranged in a 2x2 grid, similar to a body-centered cubic lattice).
The arrangement on the second tier is a bit tricky, so I'll explain it in detail.
Atoms tend to have shorter bond distances between them, so in a body-centered cubic lattice, only one atom is placed in the center of the first layer.
In a face-centered cubic lattice, a different approach is taken to reduce the distance between atoms.
This different approach involves placing atoms on top of two connected atoms. Since the first layer has four atoms, there are also four indentations, so four atoms are placed in the second layer.
Let's actually try placing the second row.
Another way to think about it is that the atoms are arranged in the same order as the first layer, but shifted by a 45-degree angle.
The third layer is arranged in exactly the same order and direction as the first layer (like a lid).
Some atoms are invisible from only one direction, so try changing their orientation.
This is a face-centered cubic lattice.
The diagram illustrates this as follows.
As I've shown in the diagram, my personal impression isSince there is one atom at the center of each face of the box (cube), it is a face-centered cubic lattice.I think so.
English is also important.FCC is an abbreviation for Face, Centered, and Cubic.It is called.
When many face-centered cubic lattices (FCCs) come together, they form a crystal like the one shown in the following diagram.
First, the image of the shape is important, so I will explain the characteristics of the fine grid later.
Materials that form this crystal lattice and crystal structure include iron (Fe), gold (Au), silver (Ag), copper (Cu), platinum (Pt), aluminum (Al), nickel (Ni), and manganese (Mn) at high temperatures (all extremely important materials).
However, since the shape of the crystal lattice of each substance changes depending on the temperature and other conditions and treatments, I don't think it's necessary to memorize them.
We've now explained two of the three types of crystal lattices, leaving one left. However, the shape of the last one is slightly more complex, and this page is getting long, so I'll explain it next time.
Summary
Let's summarize it as usual.
First, let's talk about crystals and crystal lattices.
Depending on the atoms and bonding methods, multiple atoms may be arranged in a regular, identical shape.
A unit with a regular shape is called a crystal lattice, and a cluster of identical crystal lattices is called a crystal.
Not all atoms and bonding methods possess a crystalline structure or crystalline lattice.
Most (over 70%) of the metal atoms and metallic bonds have a crystalline structure or crystalline lattice.
Metal crystal lattices can be broadly divided into three types.
Next is a summary of metal crystal lattices.
Even with the same substance (metal), the properties can change significantly depending on the crystal structure and the shape of the crystal lattice.
One of the most representative types of metallic crystals is the body-centered cubic lattice (BCC).There is.
One of the most representative types of metallic crystals is the face-centered cubic lattice (FCC).There is.
I apologize for always repeating the same thing, but understanding is more important than memorization (though I apologize to students and exam takers, as memorization is also necessary).
Even if you don't remember it, if you understand the mechanism, you can figure it out yourself, and if you forget, you can just look it up.
This topic is something many people learn early on in high school chemistry, but I think many people don't know how it's actually used.
In my mechanical design work, this is an extremely important and familiar issue: even with the same material, changes in crystal structure and crystal lattice significantly alter the material's mechanical properties, such as its strength. Therefore, I generally have a good understanding of the crystal structure and crystal lattice of the materials I use.
What is particularly important isThe hardness of a material varies greatly depending on its crystal structure and lattice (which I will explain in detail later). I think it's safe to say that there isn't a single mechanical drawing that doesn't include hardness information.
Furthermore, even with the same material, the heat treatments and surface treatments (such as carburizing, carbonitriding, sulfurization, and plating) that can be applied vary depending on the crystal structure and crystal lattice shape, so it can be quite difficult if you don't understand this.
This is extremely important, so I really want you to understand it.
Next time, we will explain the last of the three representative types of metallic bonding: the hexagonal close-packed lattice.
Finally, I'd like to introduce something different and unusual.
I've explained some typical crystal lattices, but for those who want to understand them more deeply, I recommend trying to build a model like I did (it's easy). After all, I think that working with your hands while thinking is one of the best ways for humans to acquire knowledge.
This particular lattice isn't very complex, so it's easy to understand without making a model, but having a model is useful when considering hexagram close-packed lattices or interconnected lattices (crystals).
Anything will do as long as you have a ball and some adhesive, but I'll introduce what I used.
Tokyo Marui BBs 0.2g 1600 rounds
The manufacturer, weight, and precision don't really matter, but I recommend white.
Loctite instant adhesive
Any adhesive will do, but it's best to use a branded one as it will be difficult to work with if it doesn't have a certain level of performance.
To those who found this article helpful in understanding design:
While there is basically no textbook covering this content, and it is my own original work, I will introduce the textbook that I have been using since I was a student.
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