What is an explosive?

07 July 2015

Interview with

Professor Chris Bishop, Microsoft Research

Explosives are capable of causing catastrophic damage. But how do they actually work, Gasoline explosions, simulating bomb drops at an airshow.and what's the difference between the gunpowder that the Chinese invented and more modern "high explosives"? Ginny Smith went to meet Chris Bishop, from Microsoft Research, to explode some explosive myths and find out how these chemicals do what they do...

Chris - It's a substance that's either a solid or sometimes a liquid which can undergo a chemical reaction and can turn into a gas extremely quickly. The gas occupies a volume about a thousand times greater than the solid or liquid and so it wants to expand very rapidly.

Ginny - So, all the different kinds of explosive work on that same kind of basic principle?

Chris - That's right. But the precise way in which that works can be very different for different kinds of explosives.

Ginny - Can you give me some examples?

Chris - Well, the place to start I think is with the very first explosive which is gunpowder - a mixture of charcoal and sulphur, and saltpetre.

Ginny - And what do those three things react together to do?

Chris - Well, the charcoal is a fuel just like the charcoal on your barbecue. It can react with oxygen to produce carbon dioxide and that releases a lot of energy. The oxygen though doesn't come from the air. In the case of gunpowder, it comes from the potassium nitrate. The potassium nitrate, we call it an oxidiser. When it undergoes this chemical reaction, the oxygen is released and combines with the carbon, the charcoal, and that produces energy.

Ginny - So effectively, because the oxidiser is in the mixture, your fuel doesn't have to mix with air in order to burn like a candle say, does?

Chris - That's exactly right. In fact, the burning process is much more rapid. We give it a special name. We call it deflagration but it just means burning essentially.

Ginny - Do you have any examples of this that we could have a look at?

Chris - Well, we've got some modern gunpowder here.

Ginny - It looks a bit like you've taken a pencil lead and kind of smashed it up a bit so you've got little pieces of that pencil lead. You've got quite an impressive looking blowtorch there. Are you going to use that to set fire to the gunpowder?

Chris - I am, but I'll just use a very, very small flame.

Ginny - Well, that was quite pretty. We got a sort of oomph and a bit of flame going up into the air.

Chris - So, that's what we call a low explosive. So, it burned quite quickly, just in a fraction of a second because that's very good quality gunpowder. There was certainly no bang. To get a low explosive to produce an explosion, it needs to be confined so it needs to be trapped inside some sort of container. So, if you think about a firework, it will be in something like a cardboard tube. You have a few grams of gunpowder inside the cardboard tube. When the gunpowder is ignited, it undergoes this chemical reaction that produces lots of gas, but the gas is now trapped inside that tube. And so, the pressure builds up until the point where the tube bursts and that's when we get the bang.

Ginny - So, that's a low explosive. High explosives sound a bit more exciting.

Chris - So, high explosives are very different in terms of their physics and often in terms of the chemistry. High explosives really were discovered in the middle of the 19th century. An Italian Chemist, Ascanio Sobrero was experimenting with taking various organic compounds and treating them with nitric acid. One day, he tried glycerine. When he treated this with nitric acid, he obtained nitro-glycerine. So, it was a different chemical compound. It has very different properties.

Ginny - How does nitro-glycerine explode?

Chris - In nitro-glycerine, instead of having this fine powdered mixture of a fuel and oxidiser, the oxidiser effectively is combined into the same molecule. So the molecule has a carbon backbone, three carbon atoms in a row and then attached to those are nitro groups. That's nitrogen and oxygen groups. And so, the oxygen combines with the carbon within the same molecule. So, it's a much more intimate mixture than even the world's best gunpowder because these are mixed at the molecular level. The nitrogen-nitrogen bond is one of the strongest bonds in chemistry. If you take one of those nitrogen molecules and you try to pull the two atoms apart, you need an enormous amount of energy. So, it follows that when two atoms of nitrogen come together to make a nitrogen molecule, they release that energy. And so, one of the big sources of energy in high explosives is the formation of nitrogen gas.

Ginny - Have you got an example of a high explosive we could have a look at?

Chris - We could start by having a little look at some nitro-glycerine. So, I've got a few millilitres of it to show you.

Ginny - It looks like it's underwater and it's just a blob. It's colourless. It looks oily, I guess. Like if you put oil in water.

Chris - So this time, instead of initiating it using heat and producing deflagration, I'm going to hit this with a hammer. So, we're going to provide a very sharp, hard shock and we'll see that nitro-glycerine can behave in a very different way.

Ginny - That was quite an impressive bang and I saw when I was looking at it, there was a little bit of a flash. What was happening? What was different?

Chris - What happened then was a process called detonation. It proceeds through a shockwave. In fact, it's a supersonic shockwave that travels through the nitro-glycerine and causes the chemical reaction to happen. The chemical reaction releases energy and that reinforces that shockwave. We now have a detonation proceeding at many thousands of meters a second perhaps even up to 25 times the speed of sound.

Ginny - So, what does the shockwave actually do as it's propagating? How does it do its damage?

Chris - Well, if you were standing still and a shockwave went past you, what you would notice is a sudden rise in the pressure. So if you imagine, you have something like a wall. At the moment, it arrives at the wall, on one side, you have high pressure, and on the other side, you have ordinary atmospheric pressure. Now, it doesn't take much of a pressure difference across a large area to produce a very large force. And it's that force which can push down a wall and do all sorts of damage.

So here's another way of illustrating what we mean by a high explosive or a detonation. I have something here called shock tubing. This is a commercial product that's used very extensively by the mining and quarrying industries. And it's plastic tubing, it's 3 millimeters in diameter, it's a bright yellow colour. It's got a 1 millimetre diameter hole down the middle and the inside of that hole is coated in a very light dusting of high explosive, it's called RDX. It's actually the most powerful commonly used military explosive, but the quantities here are tiny. It's a few grams for every kilometre of shock tubing. So, in fact I'm going to set this off and actually hold it in my hand while I set it off. And the way I'm going to set it off is not by setting fire to it, if I set fire to this it would just burn like any piece of plastic tubing. Instead we need a shock to initiate detonation.

Ginny - So the equivalent of hitting it with a hammer?

Chris - That's right, but I've got a slightly more sophisticated way of doing it here. This is a so-called blasting machine. It's effectively a little electronic handheld device, and it's going to charge up a capacitor to two and a half thousand volts and then discharge the capacitor to produce effectively a little lightning strike. And that lightning strike is a very sharp, short release of energy that will initiate the shock wave that will then travel down this shock tubing. And the speed at which the wave travels down the tubing is around just over 2000 metres a second.

Ginny - So I'm not going to be able to see it propagating along this bit of tube?

Chris - No what you'll see when we fire this is a bright flash. The whole tube will light up, but you won't be able to see the shock propagating from one end to the other because it will just be much too fast for the human senses to detect.

Ginny - Okay, well let's give it a go. So you're sticking the bit of - it looks like a kind of plastic cable effectively - into the end of your blasting device. And you've got a couple of buttons you're going to press there, so shall we do a countdown?

Chris - Alright.

Ginny - 3, 2, 1... Wow! That was quite a nice noise. I like that one. It's a bit of a higher pitch and I saw the whole of the length of the plastic tube sort of flashing for a second and then there was a little bit of smoke coming out of the end. What was going on inside there, was it similar to the nitro-glycerine?

Chris - It's very similar. It's a different molecule but again it contains carbon and nitrogen and oxygen and again the shockwave propagates through the explosive and as it does so it causes this chemical reaction to happen, whereby the molecule breaks apart and then the atoms recombine to make new molecules. Nitrogen gas, carbon dioxide, and that recombination, that formation of new chemical bonds, releases energy. And that energy release then sustains that shockwave which then continues to propagate at this very high speed.

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