Most solids used in everyday life — such as steel — have regular, repeating patterns of atoms. These materials are called crystalline solids.
However, there is a different type of solid — called amorphous solids — which, unlike crystalline solids, have no organized order of atoms.
Amorphous solids have shown unique properties, such as high strength and hardness, that have attracted the attention of the science world. However, the structure of the material is still a mystery.
"We understand crystalline [solids] a lot more than we understand amorphous materials," said Ryan Ott, associate scientist at the U.S. Department of Energy's Ames Laboratory. "Particularly we don't understand how you go from a liquid, something that's molten, to this amorphous solid."
Ott's research is specifically focused on metallic amorphous solids, also known as metallic glass. He studies how the atoms are arranged in metallic glass, what affects their structures, the characterization of these structures and the properties they exhibit.
"A lot of what our research has really found is that ... the amorphous structure is much more complicated and has more complex ordering than a lot of people originally imagined," Ott said.
To obtain information about the structure, Ott uses a technique called X-ray scattering, which is different than X-rays used at the doctor's office. In medical X-rays, parts of the body absorb the X-rays. Bones appear white because X-rays are absorbed by bones more than they are absorbed by fat and other tissues.
The X-ray scattering that Ott uses is different in that it is not based on absorption. Instead, when X-rays hit an amorphous material, the waves bounce off the atoms of the material in different directions, depending on their atomic arrangement.
It would be like a classroom with desks arranged in an organized order and students trying to quickly find a seat at the same time before the bell rings, but the students run out of time and are frozen in whatever position they were in last.
In this example, the desks in the classroom are the normal locations of atoms in a crystalline solid, while the students are the frozen atoms in an amorphous solid. Scientists cannot see the exact positions of the students if they are not at their desks, but they can measure the average locations of the students relative to each other.
Therefore, Ott said, X-ray scattering allows scientists to see a snapshot of the atomic structure.
"From that we can get information about the structure," Ott said. "The way the atoms are stacked, that's what allows us to understand the structure more."
Understanding the structure of amorphous solids is important because the structure controls the properties of the material. Properties can give an indication of its potential uses.
A particularly important property of metallic glass is its high strength. For example, if a bowling ball is thrown at a car — which is made of a crystalline solid metal, like steel — the metal will dent. However, if the same is done with a metallic glass, the material will be more resistant to permanently altering its shape.
Metallic glass also lack ductility.
"Say you take a clothes hanger and you bend it, that's ductility," Ott said. "You can bend that and it doesn't break. Metallic glasses don't like to do that, they will break."
Therefore, while metallic glass is less likely to alter its shape by being hit by a bowling ball, if the applied force exceeds the material's high strength, the material could shatter.
In addition, some metallic amorphous material can be magnetized and demagnetized easily, which is known as soft magnetic properties. Metallic glass has been used in the magnetic strips attached to books in libraries to prevent theft. The soft magnetic properties allow it to be easily deactivated at the check-out counter.
Another unique aspect of metallic glass is that when it is heated to what is called the glass transition temperature, the material will soften and become a substance similar to silly putty.
The metallic glass can then be formed into shapes, and when cooled quickly, it retains that shape and size and also regains its original strength and hardness properties.
Ott said this property gives metallic amorphous solids an advantage over crystalline solids because unlike crystalline materials, the amorphous material does not shrink when cooled.
For this reason, Ott explained that metallic glass could be used to make precision parts with intricate details, such as gears with cut teeth that have to precisely mesh together. The shapes will hold their final form after the cooling process.
There are several methods for cooling amorphous materials.
"We use this process called sputtering, where you actually can put down amorphous materials as thin films," Ott said.
During sputtering, the starting material — usually crystalline — is bombarded with argon particles. The atoms of the material are ejected into the gas phase and deposited onto a piece of silicon. These deposits result in a thin film of metallic glass on the silicon.
The advantage to sputtering is the extremely fast cooling rate of about 1 million degrees per second. However, the drawback is that it can only synthesize smaller pieces of material.
Ott said these films are less than 100 microns thick, which is about the width of an average human hair.
This is a major limitation of amorphous solids because they cannot be made into a shape of any size, like cast iron.
In addition, if the material temperature is heated too high above its glass transition temperature, it will eventually crystallize. So any applied uses of the amorphous material cannot involve exposure to high temperatures.
If these disadvantages could be altered, the material would be even more desirable for commercial use. Ott said knowledge of the structure has expanded, and scientists are getting better characterizations and descriptions of the structure.
"I think our longer-term goal is to be able to actually predict these structures, control these structures and then use them to tailor properties," Ott said.
If the properties can be tailored, people will likely see this mysterious amorphous material used more often in daily applications.