Silicon

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1:14Both. Earn up to$120 a year in statement credits of participating partners. Up to$10 each month when you pay with the Amex Gold card. Learn more at americanexpress.com slash explore-gold. Enrollment required. Terms apply. When Kohler, global design leader in luxurious kitchen and bath products, asked me to be their ambassador for timeless, elegant, durable cast iron, I said, I'm in. Soon after, I was in their Kohler, Wisconsin foundry, watching molten iron poured, enamel applied by hand, and the beautiful finished pieces ready to ship. Since 1883, Kohler cast iron has been crafted by incredible artisans, and seeing it firsthand gave me a whole new appreciation for their craftsmanship.

1:57Now, I'm proud to lend my stamp of approval to my favorite Kohler cast iron products for their durability, beauty, and enduring style. Shop my curated pics at Kohler.com. As the Kohler cast iron ambassador, I say, long live cast iron. This is In Our Time from BBC Radio 4 and this is one of more than a thousand episodes you can find in the In Our Time archive. A reading list for this edition can be found in the episode description wherever you're listening. I hope you enjoy the programme. Hello. The chemical element silicon is at the heart of some of the most useful and beautiful objects on the planet.

2:44Created billions of years ago in dying stars, it's one of the most abundant elements in the Earth's crust and a building block of the universe. More recently, silicon has become the foundation of modern electronics and we live now in the silicon age. Less appreciated is the role it plays in the biology of life on Earth and the cycling of elements between land, oceans and atmosphere that sustains us. With me to discuss silicon are Kate Hendry, Oceanographer at the British Antarctic Survey and Bi Fellow of Queen's College, University of Cambridge, Andrea Seller, Professor of Chemistry at University College London, and Monica Grady, Professor Emerita in Planetary and Space Sciences at the Open University.

3:34Welcome. Monica, let's start at the very beginning. Can you take us to where it started silicon, when it started and how it was created? What is this stuff? Well, silicon was created in one of the later generation of stars. If we go back 13.2 something billion years to the Big Bang, which is when hydrogen and helium and a little bit of lithium were created, And then the stars came and the original stars burnt hydrogen to helium. And then the next generation of stars were burning helium to carbon. And then carbon, nitrogen and oxygen were burning. And when I say burning, I'm not talking a temperature of an ordinary fire.

4:20I'm talking billions, millions of kelvins, very, very hot. And you get to a stage when oxygen has been created and that burns by adding a helium nucleus to it to produce silicon. And this is the penultimate stage before a star will collapse. And what part did silicon then play in the formation of planets, particularly in our solar system? Because we've been going for only 4.5 billion years, our solar system, we're very much sort of later generation from the Big Bang. And many, many stars have contributed to the interstellar medium. So this is the space between the stars. It's not space. It's full of stuff.

5:12It's full of dust, which is silicate, dust, silicon and oxygen grains together. Just explain to me, silicon, I understand, is an element. Silicate is what? Well, as you say, silicon is an element, and that's just silicon atoms all bonded together. A silicate is a compound where you have silicon bonded to oxygen, making long lattices and sheets. And then you've got other bits and pieces in there as well, magnesium, iron, calcium, and so on. And these are minerals. And this stuff is floating around in the intercellar space, as you say. It is. There's tiny, tiny little grains mixed with ice, water ice, and also still lots of hydrogen.

5:59And you have these big clouds, molecular clouds, with all this stuff in. And they might start to collapse. And when they collapse, they fall in on themselves and they'll make a disk. and there are some wonderful pictures that you can see of this actually in operation that have been taken by some of our huge telescopes. And you can see these disks, you look at them front on and you can see gaps in the disk where planets are forming, where the dust is clumping together. And as it clumps, it gets hotter and it starts to melt and we've got these planet-forming processes and like the earth it was originally made of these silicates they've started to melt iron goes into the metal because there's iron in the silicates goes to the center to form a core and we've got the crust which is it's mainly material which is left over from this melting process and it goes on in asteroids as well so that means without silicon or without silicates the earth wouldn't exist.

7:07Absolutely. Right. Okay. Andrea Seller, can you take us now down to silicon at the atomic level? What does it comprise and why does that matter? So silicon is element number 14, which means that it has 14 positively charged protons in its core And then different numbers of neutrons. But the key thing is that the organizing principle for chemistry, in a way, is the periodic table. And silicon sits directly below carbon. And so one of the key things when we talk, for example, when we hear about silicates, is the fact that silicon is almost invariably connected to four things. So if you have the element itself, and the element is sort of a strange, it's a metalloid.

8:01It sits somewhere between a nice electrically conductive metal and on the other hand, a rather non-conductive, let's say, non-metal. It sort of sits in a sort of sweet spot in the middle. in all of its structures, whether it be the element or in its compounds, you've always got silicon attached to four things. And that's the motif which really underpins an awful lot of the chemistry that it undergoes. What does that four things imply when you say it underpins the chemistry? Why is that number special? So the number is special essentially because you have four electrons in the sort of outermost part of the silicon, which are available for bonding.

8:46And so what that means is that you can easily form four links. It's possible to make more, but certainly silicates, you know, other compounds, and we will perhaps come to silicones and things like that. But you've always got this number four, which recurs. Now, one of the beauties of silicon is that on the one hand, you've got this metalloid type behavior, but you've also got loads and loads of chemistry. And one of the things that that allows you to do is to obtain silicon in extraordinary levels of purity. And that is a whole area of technology which is underpinned on the one hand, as you said before, the electronics industry, but also to help us define how we measure stuff.

9:37And the element silicon, because of this sort of strange combination of forming a very, very nice crystalline structure and at the same time having lovely chemistry that allows you to purify things, it has allowed us to define one of the key seven units that are used to measure everything in our world. And that's the mole, the unit of chemistry. The MOL mole. The M-O-L-E, as in the Italian word mole, as meaning the quantity, from which molecule, as in a little quantity, comes. That's very useful, thanks. And just so we clarify, silicon, the element, silicate, it's silicon mixed with other things.

10:26What about silica? So silica is sometimes referred to as quartz. It means silicon with just oxygen. And so I've got a beautiful sort of little clump of crystals of silica or quartz. And you can see that they're very clear. They look rather glassy, although they've got the odd crack. But crucially, what they've got, they've got points. and you can see that they've got very clear facets. And the interesting thing about quartz is that you have left-handed crystals and you have right-handed crystals. And that's because of the structure of these silicon oxygen aggregates is that the silicon is surrounded by four oxygens and these form spirals which run through the structure.

11:20And what it means is that you've got left-handed crystals and right-handed crystals. They're quite odd. Kate Hendry, Andrea was talking then about purity in silicon, but we don't find pure silicon very often. What forms occur naturally on the Earth? So we've heard a little bit about silicates already. So silicon likes to bond with atoms of other elements, normally oxygen. So when there are four oxygens around silicon, that's a sort of basic silicate structure. What can happen then is that the oxygen can bond with other metals, magnesium and iron. And in fact, in some cases, when these build these big framework structures that we've heard about, aluminium can sort of swap in for the silicon, if you like.

12:08And those are the sort of silicate minerals. There's lots of different ways those silicate shapes can come together to form these crystals. And there's a whole array of different minerals. But that's probably one of the most common ways that we find silicon on the Earth. And we've heard about silica as well. We've heard a bit about how sometimes what happens when silica forms a structure is that rather than one silicon surrounded by four oxygens, the silicons sort of start to share the oxygens. So the ratio of silicon to oxygen changes and you get just usually one silicon to two oxygens. So that's what happens in quartz, as you've heard about already.

12:43but sometimes those silicon and oxygen units can come together in a more higgledy-piggledy way and that's what we call an amorphous silica and that's basically glass so that can happen in nature so if you call a lava a molten rock very quickly it doesn't form a crystal it will form this glass so it's volcanic glass but also something that i am really interested in is that biology can do this too so biological processes can actually make this amorphous silica which we call biogenic silica. Well, I was going to ask you, what's the link between silicon and life on Earth? Well, every living organism needs at least a little bit of silicon, including us.

13:23So we need a trace amount of silicon in our diet for our bones to be healthy. We don't normally have a problem getting silicon from our diet. There's plenty in drinking water, for example. There's quite a lot in beer as it happens. So it's actually quite a good source of silicon. But in the plants that we eat as well. So we're generally fine with getting enough silicon. But some organisms actually need a lot more silicon in their diets, if you like. And those are the organisms that make this biogenic silica. It's an absolute requirement for them to have it. So plants produce silica in their leaves.

13:59Sometimes if you rub a leaf and you can feel it being quite rough, that's the little blobs of silica that they produce in their leaves. Thought to be a sort of defence mechanism against herbivores. Animals can produce silica too. very basic animals. So it's the sponges. So these are the sort of really ancestral animals that live on the seafloor and in freshwater environments. They make their skeletons out of little needle-like elements called spicules, which in many groups are made out of silica. Are they actually, are they sort of gathering the silica up or are they producing silica itself? So they take in silicon from their environment and this is the silicon that's dissolved in the water that they're living in.

14:35So dissolved silicon in the earth's oceans and in fresh waters is in the form of silicic acid you can just think of that as dissolved silicon it's a sort of dissolved silicate in its own right really it's got the same sort of structure but they sponges and other organisms can take that dissolved silicon in through their cells in a sort of active process right okay so we've had silicon we've had silica we've had silicates we'll come on to silicones later but monica before we do let's just stick with silicates and how they develop on Earth. And in particular, I'm interested in what they produce as minerals.

15:12Well, this is these compounds where you've got silicon bonded to oxygen, bonded to another silicon, bonded to another oxygen, and you get these enormous, great big... You can make sheets, you can make rings, you can make cages of silicon and oxygen. And occasionally you add a magnesium, you add an iron, a calcium, an aluminium, and you make lots and lots of different minerals. And you also make very beautiful minerals. Andre had his colourless quartz. But if you put a bit of, you know, a pinch of something else in there, you get amethyst, the beautiful purple semi-precious gemstone. And you can get citrine and tiger quartz, yellow and brown and all these different colours.

15:57And it's just what you've added in to the silicon and the oxygen. So jewellery basically depends upon silicon? Well, jewellery depends upon elements from the period number four in the table. No, it's not. It's the group four. Because a lot of our jewellery, of course, is diamonds, which is carbon. And we know that silicon is just below. And where carbon bonds to carbon bonds to carbon bonds to carbon to make the diamond structure, here we have silicon bonded to oxygen bonded to oxygen bonded to silicon to make the silicate structure to get those beautiful crystals as well. Now, let's get back to the purification of silicon.

16:43And there's a reason for this, Andrea, because for most ordinary people, let's face it, silicon is associated with three things. With sealants that you put around your bathtub. Silicones. Silicones. Then there is the gel that some people use to enhance various body parts. Again, silicones. Good. So we're really hitting the silicones here. And then finally, there are semiconductors and silicon valley. Now, first of all, can you explain what a silicone is? And secondly, how do we purify silicon so that it can be used in microchips, in semiconductors, essentially? So at the heart of all of this is the chemistry of the element silicon.

17:34And in the case of silicones, what you're doing is you're exploiting the fact that, first of all, silicon likes to form long chains or even rings, alternating silicon and oxygen. Now, of course, that means that if you have a silicon, you are attached to two oxygens. That means you've got room for two more things. And those two more things will typically be carbon. And so now you can start to make long chains, i.e. plastics. And those plastics are quite interesting because they can be made extremely elastic, deformable, rubber-like. And so the silicones are, you can make kind of gelatinous ones.

18:22You can make quite stiff ones. You can make doingy ones, to use the technical term, right? The kind of thing that you'll have in your kitchen, right? The sort of kitchen spatulas, that sort of thing. And that's really because of the availability of the chemistry and the fact that we've refined that so much. But to actually get to the element is quite interesting. And so, like so many metals, you've got to smelt it. And the way in which that smelting is done turns out to be using carbon. So you start by mixing essentially coke or coal and you take quartz, right, the very nice, fairly pure silicon oxide.

19:10You grind them together and then you heat them very hot. The result is you get carbon dioxide coming off and you are left with pretty good pure silicon at the end of it. That silicon is about, you can get it 98%, 99 % pure, but that's nowhere near good enough for what you really want if you want to do that semiconductor industry, that microprocessor, all that kind of technology. For the extreme purity you need, you go one step further. And the one step further is an extraordinary process that was invented by a Polish metallurgist called Czokralski. And he was one day idly working at his desk and he had his inkwell on the desk and also a crucible containing some molten metal.

20:12And without realizing it, he dipped his pen in molten metal, pulled it out, and out came an extraordinary thread of beautiful crystalline metal. And this is the inspiration for how silicon is purified. What you do is you take a crystal of ultra-pure silicon and you dip it into a bath which contains molten silicon. Now, silicon melts at 1 ,400 degrees. So, you know, this is not a process for the faint-hearted. You've got to keep oxygen away because it'll instantly turn to the oxide. And now what you do is you rotate both the crystal and the pot and you pull the crystal upwards. And as you do so, the silicon atoms, as they solidify, as they crystallize onto the surface, they crystallize in a perfect crystal.

21:13They form a single, enormous crystal, several, I mean, tens of centimeters across. and above all because it's quite slow if there are any impurities that stick to the crystal they have time to come off and be replaced by the silicon itself and so now you get to levels of purity which are one in a billion it's extraordinary we're going to we're going to come back a little bit later and work out what the implications of that were for electronics in in particular But, Kate, I want to move on to something else here, back to life on Earth and silicon, diatoms. Tell us what diatoms are in the oceans and how they regulate our atmosphere and what's silicon got to do with it?

22:06So diatoms are a group of organisms I've not yet mentioned. They're really important in terms of the silicon cycle. So they're one of these organisms that make their shells out of silica. What diatoms are is that they're algae, so they're photosynthetic. They live in the surface of the oceans and are fresh waters. And they use the energy from the sun to make organic matter out of CO2 from the atmosphere, essentially. So they essentially, in the oceans, they perform the same role as plants. They are the primary producers of the ecosystems. and they're responsible for, we estimate up to about 40 % of the formation of organic matter in the oceans are these diatoms.

22:49So they're incredibly important for locking up carbon. So not only do they fuel ecosystems, but they are really important for carbon cycling and that's that link with climate because they draw that CO2 down from the atmosphere. When diatoms die, they sink. They're actually fairly efficient at sinking out of the surface ocean where it's light, where they grow, down into the depths. So they take that organic matter with them and that can be buried away from the atmosphere for thousands of years. So they're really critical for carbon. When Kohler, the global design leader in luxurious kitchen and bath products, came to me and said, Martha, we need an ambassador for our timeless, elegant, durable cast iron products.

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26:15There's a lot we still need to understand about how diatoms make silica, to be honest. Scientists are working very hard on trying to understand those pathways. What we do know is that they take and dissolve silicon from their environment. They transport the silicon across their cell membranes. If the concentration of silicon in the water is pretty high, it can diffuse. But normally it's this active uptake that they use. From there, they use a special organelle, special device within their cells called a silica deposition vesicle, SDV. and what they do is they concentrate the dissolved silicon so high that it starts to essentially spontaneously precipitate out of solution to make this silica.

26:59And like I say, they make their shells out of silica and they are absolutely beautiful. They have ornate structures. They have holes and spikes in all sorts of beautiful patterns and all sorts of shapes and sizes. That's all very much genetically controlled. So every species is identifiable by its shape and by its ornate structures. I mean, this point about beauty is really important. I mean, in the 1860s, a man called Adolf Schmidt produced an extraordinary atlas of the structures of these things. And they're just amazing because they're so geometric. They're just wild. But may I just add something to what Kate just said?

27:37Because on the one hand, you've got the problem of kind of concentrating the silica, having a mechanism to be able to pull it out of the water, shove it into the cell. Then you've got to get it concentrated enough that you can precipitate it. But then the other thing you've got to do is to put it into the right shapes. And one of the things which is quite mysterious and a huge area of active research is how does biology control crystallization? And this is done with these essentially with soft biological structures, self-organizing and providing templates. And how that works in detail is something which people are working on.

28:21But it's amazing. And Kate, you've got one which looks like a pair of trousers that you're fond of. What's that do? So, well, it's a species called Eucampia Antarctica. It lives in the waters around Antarctica, as its name might suggest. And it's a type of diatom called eccentric diatom. So these are sort of flat discs, if you like, in their very basic form. But they can have all sorts of ornamentation to them. And these ones in particular grow these sort of almost horn-like structures. So if you sort of orientate it in the right way, it does look a bit like a pair of trousers. And sometimes they're a little bit shorter, so they look a little bit more like a pair of shorts instead.

28:54but it's just a child in me. I just quite like that. I think they're quite fun. And they produce a lot of oxygen. Is that right? So the algae around the oceans will also produce oxygen. So that's another link with life, absolutely. As I say, it's also this really strong link with carbon cycling in particular that we're interested in at the moment. And I said right at the beginning that diatoms control the silicon cycle. They're really important for the silicon cycle. They've essentially revolutionised how silicon is cycled on the planet. They evolved, well, they appeared in the fossil record very patchily in the Mesozoic.

29:32So this is a time of the dinosaurs. But then after that, after the dinosaurs died out 65 million years ago, the diatoms really took off then. So, you know, you might think we're living in the age of mammals, but in my view, we're living in the age of the diatoms. And since then, because they're so good at taking up that dissolved silicon from the ocean, They've essentially stripped silicon out of the surface ocean entirely. So if you go in most parts of the ocean today, you'll find pretty low levels of silicon in the surface ocean. And that means that the availability of that dissolved silicon can actually limit how much they grow.