4.1.2 Metals are Crystals
|What are Crystals?|
|Now we have the first key to some of our "why" questions. Many of the properties of metals come straight from the fact that all metals are crystals.|
|In particular, ductility, i.e. the ability to deform plastically, is tied to metals being crystals. The big question then is:|
|The first thing you must learn is that a real crystal has nothing to do with the fancy glasses, known as
"crystal", that you put on your festive dinner
table. These so-called "crystals" are in fact the exact
opposite of real crystals. They are made from amorphous
Exactly the same thing holds for all those cute "crystal" things at airport shops all over the world.
|What I mean by real crystals are all solid materials with an orderly arrangement of most of their atoms as shown schematically below.|
|On occasion you find obvious crystals in nature. Things like rock crystals (= quartz), diamonds, rubies, and so on. More often, however, you find these kind of crystals in jeweler's shops or in mineral exhibits in museums.|
and minerals are commonly addressed as "crystals" because they have some peculiar geometric shape; they come as cubes, pyramids, octagons,
and so on. |
You might believe that crystals are rare since most of us find crystals rather infrequently outside jewelry stores. When did you find your last diamond or ruby on the beach or in the mountains?
Your believe is wrong. The exact opposite is true.
|Wood, jellyfish, coconuts, fingernails and you are not crystals. Outside of biology, things like obsidian aren't crystals either. But amorphous minerals are the big exception. Crystals are the rule.|
|Most of the rocks around us and every metal is a crystal. So how come nobody noticed this?|
Because the overwhelming majority of natural crystals are not
showy single crystals with an easily recognizable "crystal"
shape, but poly crystals, an agglomeration of small crystals
stuck together haphazardly. On top of that, these crystals are as full of defects as a politician
The figure below illustrates that much better than many words:
|It's hard to draw a three-dimensional version of the crystals above. If I would attempt that, the picture would be utterly confusing.|
|It's easy, however, to imagine
what a three-dimensional perfect single crystal could look like. Just stack many layers of
atoms as seen in the figure above on top of each other and your three-dimensional crystal
looks in this projection exactly like the two-dimensional
The only difference is that instead of just one atom you now have a whole stack. And that's what we are going to see in a suitable microscope. We must take care to orient the crystal in such a way that we look down a row of atoms. What we then will see for iron is shown below.
|Just to demonstrate the point a bit more, I will also show you some pictures of my other favored material: silicon (Si).|
|The figures below shows images taken with a scanning tunneling microscope
or STM. We see only the atoms of the surface here. |
You do not just see the atoms on the surface with an STM, you can even use it to move atoms. In this way you can make the smallest possible drawings, e.g. the Japanese sign for "atom"
|In some of the pictures the scale is given in nanometers (nm). Nano is Greek for midget and means really, really small. We don't need a scale, however, since we know that atoms, roughly, are about 0.3 nm large.|
|As promised I will not insult your intelligence by always going on about a "thousandth part of a millionth
part of a meter", and so on. I presume that you know that "nano" means "billionth
part" of whatever, or "whatever multiplied by 109". |
A nanometer is the billionth part of a meter or the millionth part of a millimeter, and yes, it is really small but not "unimaginably" small. Use the link to refresh your memory if needs be.
Note that right now you do not need to unimagine what a nanometer is. You actually see it. Just keep in mind:
|An atom is roughly 1 nm large (it's more like 1/3 of a nanometer actually), and a nm is therefore also the natural scale for crystal lattice dimensions.|
|Single vs. Poly Crystals|
|Single and rather perfect crystals are indeed quite rare. Dirty poly crystals, full of defects, are all over the place.|
|If we definitely need single crystals, for example in
silicon micro technology or for titanium alloy
turbine blades, we have to make them with great
Usually, however, we are much better off with poly crystals. Not only are they far easier to make and thus far cheaper than single crystals, they are actually better for many products, like car bodies or swords. Perfection is not always good; being disordered and messy like the poly crystal in the drawing above, does have advantages for making swords, as we will see.
|Just to make sure you remember: not everything is a crystal. You, just as about all other inwardly slimy biological objects, are not crystalline, neither is glass (typically silicon dioxide with some "dirt", see the figure above).|
polymers like Teflon, nylon, vinyl, acrylic, or "PET"
(polyethylenterephthalat, the stuff plastic bottles are made from), are not crystalline either
but also amorphous. |
Nevertheless, crystalline materials are far more common than amorphous materials and, remember, all metals are crystals! Well, almost all. But we need not worry about the few and rather weird exceptions to the rule.
|A big "why" question comes up at this point.|
and generalized, the question is:Why do most atoms form
a crystal most of the time whenever they stick together? |
|Going a bit beyond that questions, the next obvious questions is|
|In a first step to getting answers we simply note that most of our 90 or so different atoms like to bond to other atoms. Bonding between atoms is just another word for saying that the atoms like to share some electrons in some way.|
|Bonding with a suitable partner (or with more
than one partner for the more promiscuous atoms) via exchanging or sharing electrons, simply
makes a better state of being. Bonding with a suitable
partner is just more comfy, as most of us know. Atoms, it appears, are human, too. Or was
that the other way around? |
While the majority of atoms likes to bond with other ones, a few atoms prefer to be antisocial and stay in splendid isolation at all circumstances. Those snobs we call "noble" elements. Foremost in this group we have the noble gases, for example helium (He), argon (Ar), or xenon (Xe). Then we have the noble metals, in particular gold (Au) and platinum (Pt). It makes you wonder how the antisocial behavior of "noble" atoms relates to human nobility, the kind of people who typically induced other people to work for them in exchange for letting them live. The common denominator might be found in the word "antisocial".
do we define a "better state of being" or being "comfy" in cold scientific
terms? You know when you feel comfy but how can you put a number on it? |
In the world of atoms or other particles, being comfy simply means having as little (free) energy as possible. If you are a couch potato, why should your atoms be otherwise?
The energy of bonded atoms is usually lower than that of atoms hanging around in isolation, and that's why atoms like to bond. If you know your atom, you can actually predict its bonding preferences and behavior.
|With some quantum theory we can calculate how much energy atoms will gain by pairing up, by forming threesomes, and so on. This means we have chemistry covered (in principle).|
|Chemistry, after all, is about getting atoms together to form something
new. Take one silicon (Si) atom and two oxygen (O) atoms, get them close enough, and they
will combine to form a Silicon dioxide (SiO2) molecule. Get a bunch of those and
they will produce solid quartz, one of our most important materials
One hydrogen (H) atom and two oxygen (O) atoms produce water (H2O), quite important too. 2 atoms of carbon (C) plus 5 atoms of hydrogen (H) and 1 atom of oxygen (O) produce ethanol (C2H4OH). Mix ethanol with water and add a little bit of this and that and you have red wine, extremely important for making me comfy.
© H. Föll (Iron, Steel and Swords script)