These days, a cellphone is no longer just a phone. Our smartphones become, among other things, GPS systems, calendars, cameras, mini-arcades, instant messengers, and Internet browsers, all with the swipe of a finger. Though it’s easy to take these capabilities for granted, the technology that goes into creating just the shell of such a versatile tool is astounding. Ken Kelton, the Arthur Holly Compton Professor of Physics and director of the new Institute of Materials Science and Engineering, explains how feats of materials science improve even the screens and cases of modern smartphones.
When talking about materials science, where do we start?
That’s a big question with a big answer. Our entire history as a civilization has been built on materials. We map the ages
and our advancement as a society through materials: the Stone Age, the Bronze Age, the Iron Age. Advancements in materials lead to an advancement in technologies. Materials science is in our clothing, in our electronics, in the structures that hold buildings together. It’s everywhere.
So if we take something like a cellphone, can you tell us how materials science has helped shape it?
Sure. I’m glad you asked about that one because there are a lot of materials there. So, the first thing you see is the screen. This is a type of glass called Gorilla Glass, which is made by Corning, the same people who invented Pyrex glassware and CorningWare. Gorilla Glass is designed to be very tough. There are videos on YouTube showing a car running over a cellphone with Gorilla Glass, and the glass doesn’t break. It’s also really thin – about the thickness of a piece of construction paper. An enormous amount of research has gone into just this piece of glass for the front of the cellphone.
Now, if we turn my iPhone 5 over, the back is made of metal. The cases of most cellphones have been made from molded plastic, but metals like aluminum are more resistant to cracking. However, the process of molding the metal into the right form can cause it to weaken and lose some of these mechanical advantages. But in 2010, Apple bought the rights to a new kind of metal. This new metal is unusual because it’s a glass. A metallic glass. Metallic glasses are extremely strong and hard, making them break- and scratch-resistant. My current research focuses on these glasses and the liquids from which they form.
What is metallic glass?
Metallic glass was discovered in 1960 at the California Institute of Technology (Caltech) by Professor Pol Duwez, who was studying liquid metals. Normally metals are crystalline in their structure, but when Duwez cooled these liquid metals at a very high rate – around a million degrees per second – he got something that looked like a regular metal, but when it was studied closer was found to have the structure of a glass. This had never been observed. Ever. There are a few naturally occurring glasses, like obsidian, but nowhere in nature do you find metallic glass. This was something totally new.
So how did this lead to the metallic glass in our smartphones?
In the 1990s, again at Caltech, Professor William Johnson and his research group discovered new kinds of glasses that could be formed at much slower cooling rates. If you ever watch glass blowers cool a piece of glass, they just take the glass out of the furnace, and it cools slowly. If you do that with a typical metal, it crystalizes. But in this case, the researchers used complicated alloys with five or six components, and they found that those metals could cool slowly and form a glass.
This means you can actually blow these liquid metals, like glass, or inject them into a mold, like plastic. Johnson founded a company called Liquidmetal Technologies to market these new glasses, and Apple signed an exclusive agreement with Liquidmetal in 2010. Apple hasn’t stated that they use Liquidmetal yet, but large companies often don’t share information about their technology. For example, Apple only recently admitted to using Gorilla Glass in their phones.
Besides cellphones, how are metallic glasses used - and what's in the works?
The first application of metallic glass was a golf club. You can find a demonstration online in which people drop three balls – one on a piece of stainless steel, one on a piece of titanium, and one on a piece of metallic glass. The ball on both the titanium and stainless steel bounces for maybe 10 seconds. Meanwhile, the ball on the metallic glass continues to bounce for three or four times as long. The reason for that is something called the coefficient of restitution, which is: if I impact a body and deform it, how much does it spring back? For metallic glasses, it’s a huge amount. They’re very elastic materials. When you use one of these clubs, a large amount of energy is imparted to the golf ball. My joke is, “Well, that’s great. I can just hit it farther out of the fairway,” but Tiger Woods could do really well with one.
Metallic glasses are also being studied for bio-implants and stents. Because the glasses can be molded like a plastic, you can get very intricate and tiny shapes. It’s a niche market for now, but I think it’s an expanding one, and obviously if these metallic glasses are moving into something like cellphone or computer cases, they’re going to become a major part of the objects we use every day.