The Wondrous World of Perovskites - with Mike Glazer

I was Bragg who just over 100 years ago worked with his father to develop a completely new scientific discipline which we call x-ray crystallography and this is the subject which enables us to study the atomic arrangements in crystalline materials which is the bulk of solid materials this is the reason that we know about the structures of metals and alloys pharmaceuticals drugs of various kinds inorganic organic compounds proteins viruses and of course DNA their work back in 1912 1913 started a complete revolution in science and they obtained together the Nobel Prize in 1915 which is William Lawrence Bragg Lawrence Bragg being the youngest scientific Nobel Prize winner ever at the age of 25 Lawrence Bragg made his discovery when he was only 22 years old in 1912 to working on an experienced experimental results than in Germany but they failed in Germany to solve what they were seeing properly in him Lawrence Bragg who had the vision to realize that there was a simple way of explaining the effects the Germans had and behind me there's two paintings here both father and son work with quite competent artists and I think that it's that combination of being a scientist and being an artist that enable them to work through the formalism the German scientists had and come up with a very simple solution to what they were doing and that's what kicked off x-ray crystallography so it's very important advance the first crystal structure done in 1913 is illustrated here this is Bragg's original model of sodium chloride common salt with an alternating arrangements of sodium and chlorine atoms goes on and on instantly if you'd make it be big enough you get a steelsuit he was highly controversial it's time Lawrence Bragg published that in 1913 and he and his father published the structure over there diamond which is the same structure of silicon one of the most important solids of the 20th century so the two of them really established something very important they obtained nobel prize in 1915 we have one of the Nobel Prize certificates here as you can see well the beautiful lettering the song and now one of the things that oh by the way I'd given you most of you at any rate a pamphlet I haven't got enough for everybody but this is a to do with an exhibition that we mounted a few years ago Warwick University entitled the two bags we had a lot of their equipment and if you look on no pants that you'll see there's a kind of potted history of the the brakes William Bragg a father was here until 1942 he died while in office and his son Lawrence Bragg came in 1954 was here into the 1960s and you could say really in the 20th century the Bragg's were the Royal Institution at Templeton they were ok so one of the things that the lawrenceburg did when he came here was he instituted a series of lectures for schoolchildren something like a hundred to two hundred thousand schoolchildren went through this lecture theatre watching Lawrence Bragg demonstrate science to them and you can see Emily on the right here there's a picture of about 60 years ago of Lawrence Bragg really enjoying himself talking to the young people doing experiment and I put this one up here because you can also see on the left-hand side you can see a shot of the audience there's a school kids in their uniform 60 years ago and take a look at the guy top left there he's having a whale of a time I think I was sitting about there so you can imagine the influence Laurens bag had on me and the reason one of the reasons why I eventually became a crystallographer so now what's this stuff for us guys thoughts at all about what I'm going to try and do is tell you a little bit about the structures these materials and then we're going to look at why they're important and they're going to look at a couple of the physical properties that they have to show you just how versatile this particular material is the Groff Skype it was discovered in Russia the earl's by Gustav Rose and he named it after the the Russian mineralogist live alexeyevitch galovski and that's where it's that these are the typical pictures of course their minerals and chemically calcium titanate CA TI o 3 and that's really sad for most of time just curious minerals but when we get into the 1940s it began to be realized that these materials based on this formula have are very very important and very significant chemically they start from this idea a B X 3 now a and B are positively charged atoms or ions like barium and titanium and layers as a colione and so on and they're positively positively charged X is an anion negatively negatively charged our iron usually oxygen sometimes chlorine fluorine iodine and so on main most products we know of our oxides and so here are a few examples so we have here calcium titanate barium tighten this is a very important industrial material used in capacitors electrolytic capacitors for example many other applications lead zirconate this is potassium copper triploid and then we have sodium business titanate which is some where the a cation is replaced by a mixture of sodium and business 5050 of that and this is a key material today because of its use in us in a field called K's electricity which I will come to I've worked on for many years on a particular series called pzt and I hope to come to that later on this one is a new material which is possible competitor for that my wife Pamela Thomas professor Thomas from work over there is the world's expert on on this particular material okay moving on so what I've done here is I've drawn out the publications which contain the work for us guiding them over a period of time when I started which was 1969 here they were around 50 to 60 publications a year that meant I could read pretty well every paper easy but look what happened it's enormous change this jump here 1987 was the discovery of the high temperature superconductors Nobel Prize to bed laws and Miller everybody wanted to have publications on high temperature superconductors and they are based on basic arrangement crystallographic arrangement we have for our skies and it carried on on the lawn look what happened last year twenty two thousand four hundred publications last year that's more than 60 a day in the course of this lecture there'll be two or three publications coming out how on earth is a scientist today working in Perot skies going to be able to keep up with the literature how are they going to know whether somebody else is working on the same thing with them this is a really serious problem what happens last year six and ourselves next two publications it was a discovery of a new effect in certain classes of proscope it's discovered that some of them you can use as solar cells and now everybody around the world every scientist of us wants to get a paper published on solar cells if the world has gone crazy and there's a subject which is in its infancy it can be very interesting to see where it develops this was discovered by a colleague of mine for a sanely Smith in our department and it's shaken the world really so these probst kites whatever they are which I'm going to show you the more about in a minute I've really you know big stuff this is okay where do we find through Skype what you're standing on it you see from this little slide here it's a mineral called Bridgman it-- postie britain got a Nobel Prize for this and you see that 38% of the earth is prostatitis a type of rhaskos magnesium silicate same form in MGS io 3 a B X C and 93% of the lower mantle is some is prostate so this it's everywhere it's a very common substance and yet a lot of people don't know anything about it now I need to this point talk a little bit about something that's important in crystallography so that we we can understand what we're looking at and I illustrate it with a brick wall you see a lot of bricks and what you'll notice is that bricks repeat in a pattern so they're stuck together this is a type of symmetry that we call translational symmetry and the same thing happens in crystals we have atoms and molecules which repeats on and on and on in three dimensions rather like three dimensional wallpaper and that's symmetries translational symmetry translational symmetry lies at the heart of our understanding of the solid world is the most essential symmetry point in in solace now if I wanted to describe this wall there's assume all the bricks are identical make life easier how could I do it well I could write down the positions of all the bricks and of course in a crystal of you're doing this the bricks are tiny with zillions of them in all directions we would be writing down a lot of paper would be covering sheets and sheets of paper with numbers what we can do instead this is what crystallographers do they say forget that let's just look at one brick describe that and then say let's apply translational symmetry and we generate the rest of the wall and we've saved ourselves an awful lot of work and that's these meat of the concept and what we call the unit cell the unit cell is a part of the crystal contains atoms and molecules and if we stack those unit cells together in all directions we build up a physical crystal so if I take a crystal of quartz for example this is quart everybody knows quartz we have little silicon dioxide units and we repeat them in a particular pattern and eventually they do it enough times you build up an actual crystal like that and we know this because of the work of the bag because developing this x-ray crystallography allows us to determine the positions of the atoms in these crystals sodium chloride over here this is a unit cell of common salt this repeats over and over and over again in all directions to build up your your salt crystal so that is this is the only real sort of thing I need to teach you about in crystallography of the purposes of this talk but we will look at some structures as well so we're going to build up the structure of the basic Prostate for you and there's some diagram the right-hand diagram is a stereo diagram and those of you have got the stereo specs so what I've done is I've drawn a unit cell in the shape of the cube and I put the a cation in the center of the cube that's easy everybody can do that that's most no problem now I'm going to add the be cations and I'm going to put those on the corners of the cube the blue things so we're getting there now we've got to lots of positively charged ions we need some negatively charged ions to balance the whole thing out so we're going to put some negatively charged anions and oxygens perhaps and add those and I put those along the sides halfway along the sides and there is my unit cell of the puros light structure very easy I hope everybody can conditional eyes that very simple let me um let me extend this a little bit so let's just put bonds between the blue and the red between the B cation and the anion and I've drawn those slightly larger and then I'm taking to add in the oxygen the anions in neighboring unit cells so for example in this works here this oh and I'm here belongs in the unit cell not shown to the left this one behind this one in front and so on and what you can see I hope is that if we look at this B cation there are six anions around it to know what we can do is we can join up the dots we can join up the the the red atoms and we form these octahedra so we have these solid shapes with eight faces are called an octahedron what you see is now to describe the structure one of the ways you can do this is say well we've got an octahedron here another one here but they join together by that common atom in fact usually when I draw these things I leave out the anion and we draw it just like that so there is your basic structure of the proscribe we have is an infinite array of octahedron in three dimensions joined by their corners on and on and on very simple highly symmetric structure so why is this important well it's important because we can do things in the structure we can make very very small changes - we can move the atoms around little bit tiny amounts change the basic symmetry change the structure and when we do that we can generate very often remarkable properties which we can use in an industry for example and we're going to see some examples of that so here you are all these octahedra joined up to make an infinite framework like that now these are the sort the things we can do a top diagram we could take one of these cations here and we can move it off-center it's exaggerated but get the idea we put it in one direction and you do that you put more positive charge in the certain direction and the crystal has become electrically polar that means we can do electrical experiments with it and that's what happens in materials like barium titanate structure was solved in 1945 by Helen McGann I'll say a few words about Helen in the moment even before then narrow javo in in Hungary during the war this is a remarkable piece of work he was able to produce this complicated structure you see that octahedra had been tilted form a rather complicated pattern that's calcium titanate that's the original mineral peroxide that's its structure the other thing we can do is we can take an octahedron with a bottom line there and we can stretch it or squash it we can distort the opportunity so with these three things we can create a lot of different structures and then what we can do we can change the chemistry of this we can substitute some of these cations or anions with our app and make mixtures of them so we have a very very rich field that we can explore and see whether they can make materials which are useful to us unit cell distortions and the unit cell itself gets distorted in the courses by a very small amount and we often refer to these as pseudo cubic unit cells so the cube is slightly distorted and there are many versions of this materials thousands of other materials which are based on this idea here just a few Regency so-called Theon Jacobson revels can pop or ravillious and the common thing is they have this corner linked octahedra that sometimes it gaps in between to make more complicated structures this is a very complicated area to study and here the high temperature superconductors on the Left lanceton copper oxide the one that got the Nobel Prize again it's not exactly a Prosecco but it has these layers of octahedra which are corner joins and so it's very closely related and then the famous in coach in bangkok rocks i discovered later it has octahedra they're those a bluish ones and a very dark brewer's half octahedra with a copper atom inside that's the high-temperature superconductors we still don't understand how they work and here's a list of the sort of properties that proselytes show I won't go through all of them die electricity piezo electric fuse I'll talk about pyroelectricity semi conductivity super conductivity magnetism high thermal power catalytic conversion and photovoltaic that's his latest discovery as of last year I may go on if there's loads and loads of properties all because we can move those atoms around in tiny announced remarkable now I owe a debt to two very special women the first one on the Left Kathleen Lonsdale who was my PhD supervisor she was student of wh prank so I've always considered WH Bragg to be my scientific grandfather on the right is Helen McGaw how I went to work with in Cambridge see how the mineral named after her profs I called calcium Senate CA CA SI no physical nagoya and she was a student of the genius John Desmond Bunnell who in turn was a student of wh Bragg and one of the constant jokes between us her and I despite the huge age difference we shared the same grandfather it was Kathleen Lonsdale they recommended me to go and work with Helena gone and it's she introduced me to the Perot skypes which I spent the last 145 years working with so I owe a great debt to these two great scientists now as a scientist when you develop something you're pretending for publication and the what happens then is you expose yourself to the world you've got something wrong it's your neck to get chopped so you always this good bit of some nervous tension with a new piece of work and I think this is what to do with the subject that talent ligaw asked me to deal with and this is something that she was working on when I came now I have a couple of models here she was looking at these octahedra and asking the question I take an octahedral it's in that basic structure and rotate it tilt the octahedron what happens to all the others and she was playing well this is one of her models that she was playing with this is another one that she was working with she never used computers she used models and she used her brain she was the only person I know of who could visualize a crystal structure like this for example in her mind and if you say to her what does it look like down a certain direction she'd take out a piece of paper and draw it for you today we have to use computers to do that but she could do that in my hand a remarkable woman this by the way is models what we call a ballroom spoke model of a proselyte structure more complicated than the basic type it's exactly gadda lynnium affair 8 but it's the same structure see calcium titanate the original kirovsk ide and it has tilted octahedra and displaced cations all mixed together in a little complicated so it was Helen that term posed for me this problem how do you explain that what can happen if I take an octahedron and I tilt it I'm going to try to show you a little bit about that so this is a layer of cross lights a octahedra so this one layer is one axis coming out towards you one out one to the right one for the bottom and you can see they're all lined up nicely so what happens supposing I take the this octahedron here and rotated about the axis coming towards you like this so I've rotated that that octahedron clockwise but it's joined to all these other ones and so the other ones must go the other way what you see first thing you see is that the repeat distance of the unit cell it's not this distance as it was before it's twice as fine it's doubled to a and also the same in that direction so that layer we immediately can see what happens if I just rotate one octahedron about an axis what about the layer of octahedra above this which you can't see well there's two things we can do we take that octahedron there take a think of the octaves above that supposing i rotate it the same way then to be completely hidden and then i call plus c plus look at the sign plus the two the layers are in phase and that is one new structural arrangement and one of these models which one is it you see that one is the model that shows that you can have a look at these later when we at the end of the lecture now there is the prospect with no tilts and then with a single filter that one axis the other thing we can do is tilt the stock even anti-phase as you go along the direction towards you I like a minus sign and that's the other one so just putting one tilt into the structure we generate two structures so here we are three structures no tilts one tilt one till and we can carry on we can say what happens if I now tilt the octahedra in addition about the other two axes so I can put in two tilts that's what these white things are these are two tilts the frilled three possibilities and then what happens if I put the tilt about all three axes here they are the red ones there four of those there are ten possible arrangements and if you now add into this the possibility the angles of tilts are different I worked out that there were 23 possibilities later it was rittle down to 15 by symmetry arguments so they're actually 15 distinct structures of tilty structures without even displacing the cations you now you can put you can now displace the cations make new structures that way change the chemistry of it and you have an enormous variety film but the essence of this the tilting ring in its heart is very simple let me tell you the story I wrote a paper on this in 1972 I sent it for publication that came referees report six pages closely typed criticism comments I was furious I was only young impetuous in that day not like now so I went to hell furious temper so look what this damn referee has done how dare referee criticize my work like this she eventually calmed me down set me down and she read through the referee support and so she said I'll help you with this let's see whether they can satisfy the referee so she helped me on that I realized the manuscript send it back in it was a much better paper that came two pages of closely tied communist again I lost my temper went to heaven and again she sent me down calm me down work through it and paper got even better and that's the paper that was published and it's the piece of work I am probably most famous for because it established a notation we can use to describe these tilty structures and today that's used internationally it was when Helen retired a few years later and she confided with me that she had been the referee [Laughter] the editor by mistake has sent it to her and she wrote back to the editor and saying well Mike Glaser that works for me but if you trust me I can be absolutely objective that tells you a lot about Helen now this thing for many years it sort of sat there in the literature as a curiosity a bit of knife academic research but in the last few years understanding this has become very important because now you can use these different tilt system you can do things to prostate create these tilt systems and has effects on the cations and by doing that you change the physical properties so today when we look at some modern materials they very often have these different tool system plus chemical substitution plus displace cations try to understand the properties so it's become very very important I never imagined that now this diagram are plotted the the radius the ionic radius of the a cation against the B cation and what you can see it kind of breaks into two fields up here the a cation is largest there are no tilts in the structure it has that structure it may have some displaced cation but there are no tools and down here we have tilts the structures down here to business farad for example here as tilts and strontium titanate this one here has no tilt at room temperature when you call it it produces that structure there tools so you can think of temperature really is kind of going down here so this is near the border and you call it it tilts calcium titanate here lie just below the border and it has this Kielty structure here and so on this is lead zirconate you can mix it with titanium along here laser knie is tilted and as you had titanium to it it becomes unfiltered so you'll begin to see patterns here there's another thing that Perot Skype people are interested in it was the tolerance factor is it do the gold spit in 1925 versus of formula and all it is it's a stability factor I've drawn a line which is the government line here and is it more or less fits that they tilt so down here and the note tilts above there it's not perfect but it gives an indication and what that says to you forget about them quantum mechanics forget about electronics and so on size matters geometry rules in this in this field and so you look at this cartoon what you see is that as we get tilting the shape in between the octahedra changes and that allows the cation to move and it's diagrams to left and right so you can force the cation to form polar structures or Antipodes fronts as you want there's a strong link between the octahedra the tilting of the octahedron and the movements of the cations into different structures clearly if the cation is very small and the reason for sky to have no a cation in it if the a count is very small then you can tilt very easily but if the a cation is very large it pushes against the octahedra and stops the tilting so you see how the geometry of this is important perhaps kites are used in industry in a variety of different formats crystal steel or crystals this is lithium niobate here if I can have the overhead a second let me put that on on the lamp on this is a little crystal here small and again this is lithium niobate Li NB o 3 it's a periscope it's use an industry in the optical communications industries very important material you can grow it in much bigger crystals and this is just a small one and I'm grateful to my colleague dr. Prabhakar owners here for giving me this sample to show you so this is a man-made crystals grown from the melt by a special technique let's go back to the yeah the chalk a ski pulling method where you have them you melt the material and then you have a fiber in here with a little seed on it and you the fiber and you pull it out gradually you get a crystal hair silicon's grown silicon crystal the bigger this can be realm of this technique the chalk our ski technique very commonly is so that's single crystals and this is a newer crystal that's grown in this kind of way p.m. NPT is called it's led magnesium niobate mixed with lead titanate and this is a important discovery made about fifteen years ago you can grow big crystals of this and this is used in a number of important industrial applications so they can grow large crystals around usually it's very difficult to grow crystals that you can use and so a very common use of profs kite is in the form of ceramics so this is a micrograph of a typical ceramic it consists of a random collection of very very small crystals this it's a powder it's a compressed powder it's like a pharmaceutical tablet and that's a very common use of brass kai-ting industrial uses and then more recently other works done on thin films on a substrate so you can deposit a very thin film of the Prosecco maybe just one or two you in itself thick and with this you can start to make an interesting models because the interaction between the film and the substrate becomes important and that interaction between them can cause changes of the structure in the film and hence the properties and it's a lot of research over the last twenty thirty years in working on that okay so this is the first of one of the properties I want to talk about is called the pyroelectric effect the formulas given up there on the top what it means this says a change in temperature gives me a change in electrical polarization in other words a charge it senses a change in temperature peyote material doesn't sense temperature it senses changes in temperature you've seen it many times when you walk past the house and the lights go on that's using a pyroelectric material it senses your body heat as you move if you stand still the lights go out soon as you move the lights go on this is the pyroelectric effect and it was first seen a long time ago I've got here 400 BC by Theophrastus with tourmaline crystals and they found when you heat tourmaline crystals it attracts bits of fluff and and dust you get charged one thing about this is that in order for this effect to occur we must have a polar structure in the province that means cations must be displaced all together in a certain direction so one end of the crystal is positively charged in one end of the crystal negatively charged and the color the polar crystal and we need that if they're going to get the pyroelectric effect and this will examples lithium tantalate so I'll light my lithium niobate as you showed you for this dim tantalate is commonly used pyroelectric material and you can grow huge crystals of that and then we have this p.m. NPT that I showed you as a large crystal of there is this formula here it's complicated beasts lead is the a cation the B cation is magnesium two-thirds of it niobium two so magnesium one-third knives and two-thirds with oxygen and then it's mixed with lead titanate form what we call a soil solution it's a complicated piece and yeah you can grow really nice customs of that and use these as pyroelectric detectors so that's an important material these are used as for in intruder alarms motion sensitive red sensing pollution and so on throughout the world this is a very important effect he isn't seeing more on this and the advantages of the system room temperature books at room temperature so you don't need to have to cool these materials as you often do with thermal sensors it's very so very sensitive you can make little devices as in the photograph on the bottom left and with this you can sense people's movements using spectrometry flame and fire sensing environmental monitoring energy management and so on to a lot of wide range of applications and power electrics here's some examples again this on the right there is a crystal lithium tantalate very very good pyroelectric material in the top it's not a pro skite not even the crystal it's a piece of plastic PVDF which is also pyroelectric so you can get plastic and then these two on the bottom left is a slice of ceramic with this rather complicated formula so here we have lead of the a cation and the B flat I'd is 0.58 is a code Ian point to iron point to niobium and point O to titanium and then oxygen to real materials as used in industrial applications and really rather complicated compared with now starting point here so trying to understand these structures and how they work in these environment is really quite tricky I owe this slide to my colleague professor module what was somewhere in the audience there's Roger he's a waving Roger easel probably the world's expert in pile electrics he was my first research terms in Cambridge now progressive Imperial coming and he lent me this slide so this is to show you how one of the ways which you can use ceramic p-8 of Perot skypes in listen and application so what we have here on the Left we have an array of little tiny elements of Paris kite the proscriptive using as this fellow here why is already mentioned to this rather complicated thing here it's got the Konya minun iron knives and titanium he also has a mystery increasing which i won't tell you about so we have some objects here which is emitting radiate infrared radiation a person something warm and we can focus the thermal signal onto this array and on the back of the array we can make electrical connections and attach that to a bit of electronics and plot a picture on the screen and each of these elements responds to the signal there however the problem is we won't see anything because this is a static image so one of the ways in which you can get this to work if notice nobody is moving here this is a static person and actually stationary you can put a chopper in here chop it around and that changes the thermal signal and creates the image on the array so it's simple idea originally these things would develop from the military no it had huge arrays of these things and it was very very expensive but so now they're made very cheaply and I'm going to show you bit more about that in a moment now this is some something Roger and his colleagues did some years back it won on the ward and there's a fire in a room you see and the room filling with smoke and somebody is in there now we switch on the device and we can begin to see what's going on in this room and I here come the fireman into the room because you can make out that they're there and they're now going to find the people that have trapped in this room and you go around there's somebody there you can see some some person sitting in a chair and this work a bit more something is that a baby they're doing something here yes it looks like if they saw the baby in there so this shows you how you can use this type of application in light when lives are threatened in a fire for example so it's a very very useful way of using particular fire pro scouts now there's a company up in Northampton called irises who manufacture a device which you can see on the top left here looks brother like a smoke detector what it does it senses people's motion using an array what they use is a fairly crude array here's the array and when people walk underneath it say let's create a signal as they move very poor resolution but they don't need high resolution for this because you can computer processes and decide whether these are people sometimes you can decide whether it's a cat or a dog or something all those fires are starting somewhere and so you can sense people and the great thing about this is unlike CCD cameras which take an image of you this preserves your personal privacy so you can use this without offending anybody and Iris's manufactures this system is relatively cheap and it's used all over the world in different applications for example sensing in in supermarkets essentially people coming into the the checkouts in order to cue manage manage the queues is used in security environments at airports and so people counseling people going through doorways and coming out through the always and we have a couple of these in store here just to pick one there and one there just above the door you've all been counted in and that's useful because when you go we're going to count you out make sure you'll leave I'd like to deduce actually at this point dr. Tony Holden from Irish company where is he I so there and Tony's going to show you what we found when you came into the lecture room hopefully we've recorded image so if you'd like to do that Tony so you press the appropriate button hopefully we'll see it somewhere that's that okay so the two doors and you can see on the left of each one is the CCD image and on the right hand side you can see the people coming in and L individuals as they come in and zoom wandering around saying am I going to come in am I going to come into this lecture room or not that's I should escape and you can see the numbers at the top there it's counting people coming through these these two doors and this is a very useful thing because at the end of the lecture we don't anybody trapped inside here you've all gone and it's very nice because you see can't tell on those pyrolytic images who those people are we preserved your personal security simple very simple idea brilliant though in its application and this company and our assistants only company in the world that produces this kind of technology and it's a British technology very important so I think around of applause Tony please here's a Houston station scissor counting people are going through a doorway your sensation you can see on the left the normal image on the right the pyroelectric image you see crowds of people go in one way out people going the other way and it's counting them through the entrance of you so you didn't know did you every time you go for use in you're being observed like this but you are okay so that's one of the effects that profs kites are particularly good at the next one is what we call the piezoelectric effect now what that is is a phenomenon found in 1880 by two French businesses Jacques and Pierre Curie Pierre Curie was the husband and Marie Curie and it was a very bright guy and he discovered that in certain crystals like quartz here is quartz an old friend quartz to cut slices of it it has an effect which was termed clean electricity and what it is it converts mechanical to electrical energy you've all seen fire lighters like this to get a spark when you press this up hope mr. Messina if you'd like to guess fires in a circle no batteries in there inside there just couple of pellets of a proselyte ceramic core pzt and when you do that little hammer hits the PSAT and the mechanic lenses due to my movement and my thumb is converted into electrical energy you get a spark and that lasts essentially forever that's keys electricity for the formula Sigma right is a stress we apply to the material and on the left again get a polarization electric charge so it's like the pyroelectric effect except the pilot effect of temperature change this is if you squeeze the material you put pressure on it you generate a charge so that's the plate the piezoelectric effect as you can see here whoops back you can see here because that I stress to it and we can read on a meter the change in the charge and use it electrically now Gabriel Lippmann showed actually you can do this in Reverse you can apply an electric field to the material and you can generate a strain which in other words change the shape of the material it's called the converse effect you can run piezo lectures in these two directions either way as you wish either convert electrical energy into mechanical energy or mechanical energy into electrical energy so this is beginning to look interesting now so where do we use it here's a few of the applications of piays electrics a client in the automotive industry airbag sensors airflow sensor order alarms fuel atomizers buzzers and so on buzzers where your client oscillating electric field material changes shape because beginning process just simple buzzers computers disk drives inkjet printers in consumer markets cigarette lighters nobody uses those today depth finders fishfinders humidifier jewelry cleaners musical instrument speakers telephones what else we got here in medical disposable patient monitors fetal heart monitors ultrasonic imaging in the military that Sounders sonar hydrophones all those sorts of systems uses this piezo electric effect I've even seen this one this is handle of a tennis racket as inside it pz t fibers proselyte fibers in here and the idea of this is when the ball hits the string the string is going to start to oscillate well that mechanical signal goes through these fibers creates an electrical signal that gets fed back in opposite and the fibers change shape and it dance out the oscillation of the strings and other clever applications the market for ceramic plays electrics look at it it's billions predicted for this year in different applications much as fourteen billion dollars this is very big business worldwide business basel appears electrics particularly ceramic peers electrics most of them are this material pzt as i mentioned you'd letter connect tightening that's the material I've spent the last 45 years of my working on I think as of the last couple of weeks I finally understand how it works it's very very complex I will not bore you with all the details this little thing you can do you can make motors so what's happening this is um done by a nice company in Denmark PCB motors and what they do is have a a reading of kegs electric element piezo elements or to go in fact I can show you that if we put this here that we can have this cell visualizer on a second so this is it here it's a tiny little thing and all around here are little piezoelectric elements and you send an electrical signal goes up and down all the way around and little piezoelectric elements change shape and so if I go back to the PowerPoint so it's rather like a Mexican wave you see it goes up and down like that and you put a disc on there it starts to rotate so here's an example with a coffee cup on it and then there it rotates this is my own one here this is one of my models of profs kites which I've put on legal sea rotating and this is a high speed one which professor Roger what Wall has done so you can make interesting motors no magnets not like the original Motors years ago I wanted to get a motor without a hole in the center for doing optical work I couldn't find one except under 10,000 pounds they were very expensive soon this for 3 or 400 euros does the trick very very precisely you control it just on the computer it's a brilliant idea that's using pzt Ruslan who knows what this is so now he tumbles over ok that's very interesting history here originally it was called as Vic IL ic' nobody knows what a stick stands for I do a couple of ideas anti-submarine detects investigation committee mr. Poe nobody's ever found mention of that committee in government documents somebody else said anti-submarine division or anti-submarine division X who knew nobody nobody knows so the idea is ship has a piezoelectric crystals create an ultrasonic wave by putting oscillating electric field on the material vibrates of the slight wave goes out bounces off your enemies submarine comes back and that pressure wave now interacts with your original caves electric using the direct piezoelectric effect creates an electrical signal so now you can time the distance between the wave going out know you're coming back and if you work at high frequency you can focus very narrowly in directions and determine where that submarine is and how far it is this is called a stick it's an interesting story here back in the First World War WH Bragg was employed by the Navy to work on anti submarine detection and set up a unit there but they wrens a trouble with the Navy officials who didn't trust the civilians and so contact between the two was rather fraught and wh Bragg worked the hydrophones in this kind of mixed success during this time the French physicist paul langevin suggested using Kies electrics for this kind of purpose and so work began highly secret work at the end of the first world war into the interwar years into the first second world you more on what was called a stick it was so secret people are not allowed to mention the material they were using the material they were using with cause this is an artificial quartz crystal this is a natural quartz crystal this was a strategic material not not a broad sky this was the material that were using all the time it was so secret they were not allowed to mention quartz they had to call it as device in the Second World War more research was done on new materials for this application and WL Bragg was a consultant in fairly in Scotland with a navy to find new materials which you could use to replace course which is not the world's best piezoelectric and they were playing with different materials one of them was ammonium dihydrogen phosphate not a peroxide ADP but you can grow big crystals of it and here is a crystal it it being this I believe is actually lawrenceburg's crystal from 1944 then the Americans are working on another type of material commercial sort shortly after that in the around 1954 there that barium tightly be a TI o 3 the first Prosecco was used as a transducer used in ultrasonics for detecting the submarines and the application appears electrics is really what sent from n to the u-boat fleet in the Second World War so it's a very important development I'm tightening was the structure that terms Ellenberger solved in 1945 and then we have PM NPT and more recent crystal and a couple of examples of it using medical imaging so we have the famous dancing baby in the top and heart valve in the bottom high-resolution images where you do exactly that it's a so nice exact see the same thing as a stick and sonar done for medical purposes and those are going to finish up just by showing a little bit about this famous PCT this is what we call a phase diagram vertical axis of temperature and on the bottom we have composition so on the left is lead zirconate PB 0 3 on the rightest lead titanate BBJ or 3 and then we mix we change the zirconium for titanium along the horizontal line and you'll notice different fields in high temperature because up there no tilts no displacements is this this structure here when we call it at this region we get the cations lead atoms displace in the vertical direction making a polar structure then on the Left we have again two polar structure - the cations are displaced but along a different direction along the diagonal of the cube this one here in addition has tilted octahedra it's one of these red ones so really quite complex but the vertical nearly vertical line is called the morphotropic phase boundary or MPB and this is where you make your materials with the highest piezo electric coefficient it rises to a maximum at this point and that's the thing that I've studied most of my last 4 years so trying to understand how that works why does itah Maxima because if we can understand it we can start to think about making new materials and in a few years ago a colleague of mine bitten our heater discovered actually in here there's an additional phase called a monoclinic phases a low symmetry phase and this caught a lot of excitement in the scientific world it became the hot topic of its time hundreds of papers suddenly started appearing investigating this monotonic phase who tried to show how that caused this rise of piezoelectricity one of the problems was well as a boundary here you could never find a boundary on that line there and that was one of the mysteries that's one of the things that we worked on to try to on stamp that P said T one of the most important piezoelectric ceramics in the world and there are two things that contribute the PA is electricity one is called the intrinsic effect so this is where you apply a stress or electric field and you move the atoms in the unit cell you see that on the go up and down and by doing that you change the polarization so this is an atomic effect this happens at the atomic level it's called the intrinsic effects and that's one of the ways you can generate piays electricity and in pzt what happens is on the left hand side of that phase diagram in particularly a LED atom is displaced in a certain direction within a plane and it can rotate in that plane as you apply the stress so your PI's red vector changes changes the polarization creates a plays electric effect and there's been a lot of argument so whether this so called polarization rotation effect really happens or not we believe it does happen we have evidence for that and let's part of some papers we've been publishing recently the other way you can get to appears electricity is known as the extrinsic effect this is very complicated not properly understood but one of the models are doing this is where you have a crystal which has two regions in it so this is not unit cells each of these colored regions here contain zillions of unit cells this is on a large scale and you see that this region the crystal is different in that region the crystal the vector polarization points in a different direction if you can make that domain wall move by applying a stress you change the relative balance of the polarization you induce a polarization and there's your piezo electric effect now that's not valid argument there are other people who have other arguments we say well you don't need to move the domain walls just have a lot of domain walls will do this and piece a T in the center of that phase diagram when you look at your duns electron microscope has lots and lots lots of tiny domains in it so that's obviously tied up with it and barium titanate there you can see different domains in barium titanate we can see those in the optical microscope so there's the the MPB in PT and here this is the measurement of the piezoelectric effect you can see that it rises to a max and run center of the phase diagram falls away very rapidly on this side but falls away slowly on that and that's something we've been working on we now know as of this week actually why that happens why that's a long tail there and that's folder and what it is is this is a mixture of intrinsic and extrinsic effects whereas this part is almost purely extrinsic these two different mechanisms give rise to that asymmetry so now we're beginning to really understand the origins of this and can just show you very quickly now when we finish a cartoon this is how we envisage what you might see if you could see this at the atomic level in a crystal grain of pzt so we have unit cells these little hexagons in this case hexagons because if you take a cube look down the body dime it looks hexagonal and what we what we find is that in this crystal is that disordered crystal we have regions Reds these who have colored them green blue red where all the polarization vectors lie in the same direction so those regions are ordered their translational symmetry that we started with but in C in addition we have regions where it's all randomized so this is a horrible mixture P said T is an utter dogs biscuit it really is a complete mess so when I started crystals like this where P really they continues in all directions now when I look at crystals they are all full of mess and it's this mess that's important in understanding these properties this lies at the heart explaining that effect that piezo electric effect we see in peas empty I believe now if we fully understand pretty well for us on how that peter tea works and this is the latest these what we call hybrid pro skites so the a cat I'm here is an organic group I think they've got the formula not quite right their lead is now the B cation sits inside the octahedra the octahedral iodine said of oxygen and at high temperature we have the octahedral all unfiltered the organic thing inside it exists in different orientations so when you average it out you get this sort of mess of a lot of organic fragments we don't know for sure if these are rotating dynamically or whether it's just as you go from one unit cell to the next they're just in different orientations you call it the evidence is you start to get tilting this is a one tilt so it's like that green one then in addition you've seen it has an effect on the organic fragment inside it's less disordered it is there and we lower the temperature so further and we have three tools one of these red ones here and again we simplify the organic fragment that's a structure determination there are other people that have been doing structure determination with slightly different results it's still a matter of controversy we don't understand the structure we don't understand why it gives rise to this photovoltaic effect everybody is excited that you can see at the top we haven't got a conference coming up in Oxford in September third one now to try to discuss this topic people will come from all over the world to that so this is a very exciting area nobody knows really where this is going to end but the number of publications is daunting and then this is the way you make it you make a layer pro Skype you put it on some sort of substrate tin oxide silicon so when you make these sandwiches we start to make devices that you can then get this photo of all take this solar cell effect and you can make this very very cheaply the problem is how to keep it stable and my last slide thank you very much and predict computationally what a given mixture of atoms in a perovskite what properties with it would have

Channel: The Royal Institution

Views: 89,358

Keywords: Ri, Royal Institution, perovskites, geology, crystals, crystallography, lecture, science, bragg, rocks, Mike Glazer

Id: v9bMEUr2II4

Length: 60min 57sec (3657 seconds)

Published: Wed May 10 2017