Science Articles

Living in a material world

Mon, 26th Sep 2016

When you create something new, how do you find out its properties?

Rosalind Davies

Scientist in lab

Living in a material world

Imagine the situation: youíre a chemist, trying hard every day to make a new material. Maybe you add something different to your reaction ingredients, or maybe you just heat them for a bit longer, or to a higher temperature than normal. How do you know whether youíve succeeded, and made something never seen before? Your reaction product might end up a different colour, or be a bit smellier, but it also might not. People are known for getting salt and sugar confused in their own kitchens, so you canít rely on your eyes alone. There are many different methods that chemists use to characterise a material, finding out whether it could be the next big thing.

Taking a closer look

First you might start off by using a more powerful set of eyes: the field of microscopy extends far beyond the optical microscopes you could use to look at leaves or bugs on a glass slide. Using an optical microscope, you are limited by the wavelength of the photon of light that you are using to see your material. An electron has a wavelength of up to 100,000 shorter than that of a visible light photon, and so if you use electrons you can see even smaller, revealing changes in material you hadnít been able to see before. High resolution images produced in this way have been used in graphene research to check to see if there are defects in the graphene layer.

graphene

Whatís it made of?

If just looking at the outside of your material doesnít hold any clues, then you could try and have a peek at whatís inside. You might think that because you know what ingredients you put into your reaction mixture, that you know what has come out at the end, but thatís not always the case. Gases can easily boil off, and so itís a good idea to check what elements are present in your product. There are lots of techniques that will help you find the chemical composition of your product.

Mass spectrometry involves firing electrons at your material in order to break it up into its constituent parts that are positively charged. These fragments are then ordered by weight, usually using a magnetic field. They are then detected and the results youíll see will be all the elements that make up your material .These act as a fingerprint for your substance, and this technique is used in airport security locations to test for the fingerprints of drugs or explosives.

Infrared radiation can also be used to investigate how the atoms in your material are bonded together. Some chemical bonds will absorb infrared radiation, causing them to vibrate. It is this absorption that causes the greenhouse effect, as the bond between carbon and oxygen in carbon dioxide has this property. Using this technique will be able to give you information about, for example, whether the nitrogen atoms in your material are bonded to oxygen or hydrogen.

Same thing, different structure?

Sometimes you might have two materials that have the same chemical composition, but where the atoms inside them have arranged themselves differently. In this case, you need to investigate the crystal structure to be able to tell the difference between them. In the same way that you can look at your own internal bone structure by using X-rays, materials chemists use X-ray diffraction to look at the structure of crystals. A material is crystalline if the atoms that make it up are arranged in a regular, repeating pattern. The whole material can then be described by repeating one small section of it over and over again, in all directions.

When X-ray radiation is shone on a sample, the waves that make up the radiation are scattered by the atoms in the crystal. The waves scattered by different layers of the crystal interact, and produce a pattern on the detector. X-rays are used because their wavelength is a similar size to the distance between crystal layers. This technique is particularly useful when investigating the arrangement of atoms in minerals and metals. It was used by the Mars rover Curiosity to find minerals in the Martian soil that are also present near volcanoes in Hawaii.

The Curiosity rover

What does it do?

OK, so youíve made your material, and it looks, feels, and smells different to anything youíve seen before. But how does it behave? The discovery is only the first step Ė then you have to find out what it does.

Lots of mechanical testing is how you would imagine it: you find out how hard something is by hitting it; how soft it is by squeezing it; and how bendy it is by trying to bend it! Chemically, the techniques are similar too. You can monitor the weight of your material whilst you heat it up, measuring what gases are given off and when. This will give you an indication as to how stable it is at high temperature. You could also try putting the gas back in, and see if the material regains the weight it lost. This could be useful for storing carbon dioxide, or hydrogen. You might try finding out if the material can conduct electricity, or ions. Maybe your new material will be inside every smartphone battery within a few years.

Many of these tests take a long time. You will need to find out how your material behaves over multiple uses, whether it starts to break, or decompose.

What next?

Development of these different characterisation techniques has revolutionised the world of materials chemistry. Being able to gain an insight into the chemical make-up and structure gives you a clue as to what it is about a material that makes it behave how it does. This focuses your research on improving those properties, and lets you know when youíre on track for something promising. So with these techniques becoming more widespread and giving even more detailed insights Ė surely itís only a matter of time before the next big breakthrough?

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