Image: Zak Zak, flickr

Under pressure

By exploiting some of the most fundamental laws of nature, scientists are making new materials, paving the way for better technologies.

Experimental physics is all about understanding the laws of nature.

That way, you can exploit them for your own gain. Or, perhaps more accurately, for the benefit of society.

How else could you reach pressures equivalent to the lower mantle of the Earth without drilling hundreds of kilometers underground?

Pressure high enough to turn a crystal into a metal without the aid of magic tricks?

“Pressure equals force per unit area,” explains Associate Professor Jodie Bradby from the ANU Research School of Physics and Engineering.

“So if you make the point you are using to apply force really small, you can get pressures that are quite extreme.”

Just how extreme is mind boggling.

Imagine the pressure that builds up in a popcorn kernel before it pops. This is the same pressure required to brew a shot of espresso coffee. 

Popcorn kernel popping

Now multiply that around 1.5 million times, and you get the type of pressure that Associate Professor Bradby and her team uses to make new materials. 

“We apply these extreme pressures to materials such as silicon, germanium and more recently carbon.

“And what happens when you apply that amount of pressure, is that the material wants to deform in some way.”

To reach these extreme pressures, scientists use a diamond point that is 1,000 times smaller than the head of a pin.

“With silicon, it does something really interesting, it changes from being a semiconductor, which is what we rely on for our computers and our phones, into being a metallic form of silicon, which blows people’s minds.”

But scientists aren’t just squishing and poking silicon with diamonds to change it from a crystal to a metal—even if it is really cool.

“We are trying to exploit these changes to produce more efficient solar cells,” says Associate Professor Bradby.

“Most solar cells are made of silicon. But they are made of the common garden variety silicon which has a cubic crystal structure.

“If you were designing a material from scratch to use for a solar cell, you wouldn’t use silicon.

“This is because the spectrum of light absorbed by silicon is not perfectly matched to the light coming down from the sun.”

Whilst some of the light energy from the sun can be absorbed by a solar cell, much of it travels through the solar cell without being absorbed and turned into electricity.

The materials being produced at The Australian National University, may provide a solution to this problem.

“With silicon, we can take the normal cubic crystal structure and put it under pressure to create a metal,” says Associate Professor Bradby.

“But if we release that pressure, it transitions to a new crystal structure, which has different light absorbing properties to regular silicon. Think about how diamond and graphite have such dramatically different properties even though they are both made of pure Carbon – this is because of their different crystal structures.

And this different form of Silicon does not just have a different crystal structure, it is also a nanomaterial.

But what is a nanomaterial exactly, and how small are we talking about?

“The standard is that anything 100 nanometers and below is a nanomaterial.”

To put that size into perspective, “Imagine taking the thickness of a sheet of paper and dividing that a thousand times”.

That is just how small we are talking about when we talk about nanomaterials.

“The really important thing with a nanomaterial is that its properties change because of its size.”

Nanomaterials can be used in anything from sporting goods, to healthcare and electronics.

And whilst it may take some time for advances like this in silicon to make their way into solar cell manufacturing, one thing is for sure.

If you learn the laws of physics, exploiting them to your advantage will open up a world of possibilities, including driving the advancement of technology.