Video Transcript – Research in the Functional Materials Lab at Boise State
Hi, everyone. I am Professor Rick Ubic and I’m going to introduce you to research in our functional materials lab at Boise State.
This is some of the group. Clearly, this was taken before COVID so it’s an old photograph. I haven’t been able to get the whole group together during COVID to take a picture like this, so it’s not been updated, but still.
This is me. This is Steve Johns, who is now a postdoc in the lab. He was a student at the time. This is Evan Smith, also was a student, is now a postdoc in the lab.
Both graduated last year. This is one of our undergraduate students who has now graduated, and this is a local teacher who was in our group as part of the Research Experience for Teachers, which we run.
So one of the longest running projects we’ve had in the lab is correlative models (that’s empirical models) of various structural aspects of perovskites.
Perovskites are a class of ceramic material. Their stoichiometry is ABX3 generically. A and B are cations, X is an anion. It can be a lot more complicated than this. This is the general archetypal stoichiomtry. This is a perovskite.
What we’ve done is we’ve looked at many aspects of structure and structural distortions in perovskites, always using real experimental data – either data that we generated in the lab by actually making things and testing them, or we cull data from literature. There’s a lot of perovskites out there in the literature, so we can use those data as well.
And what we’ve done, we’ve looked at things like lattice constant. So this shows predicted lattice constant versus experimental lattice constant. There are 132 data points on this curve, all ABX3 type perovskites, and the fit it is really beautiful.
We’ve also looked at structural distortions. So this is a second order Jahn Teller distortion, sometimes called a lone-pair distortion, which is kind of what this is showing. We’ve looked at things on the A site, this site here.
A long-range and short-range ordering of cations on the A site, so you have more than one cation species residing on that site, for example, and if they order, it involves a volume expansion. We were very surprised by that when we saw our model predicting that because when things order generally they pack more efficiently and volume goes down, so the expansion was very surprising to us when our model first showed us that.
We can also find the effective vacancy size of A-site vacancies. We do similar things on the B site. We’ve looked at long-range ordering on the B site. That involves the volume contraction, quite sensibly what you’d expect. We’ve looked at the effective anion vacancy size (oxygen vacancy size). We’ve seen tetragonal and trigonal distortions. This is a tetragonal one. And then we’ve also modeled the intrinsic polarization that would arise from these distortions.
This graph over here, showing us the vacancy size, the A-site vacancy size, as a function of this geometrical term called tolerance factor, which is just a bunch of values you could look up from a single table so it’s easy to get these data.
And what our model shows is that you have a beautiful curve that describes everything from calcium titanate up to barium titanate and a lot of things in between, anything that fits this general sort of stoichiometry will work with our model. Very simple model also, they’re not high-powered, computational intensive models. You can run them on Excel, probably on a hand calculator if you wanted to.
We’ve done a lot of modeling but we’re not done. There’s still modeling to do that we haven’t done. We’ve looked at ordering, I said, on the A and B sites for only one-to-one ordering so far, so that’s two species that order one-to-one.
We haven’t done one-to-two ordering. So we’ve done things like lanthanum zinc titanate, so zinc-titanium-zinc-titanium (1:1), but we haven’t done things like barium magnesium tantalate, where it’s magnesium-tantalum-tantalum, magnesium-tantalum-tantalum, or the one-to-two ordering.
So we haven’t tackled that yet. We haven’t been able to tackle a predictive model for Jahn Teller distortions. We can see that it’s there because our model shows that it’s there, but only after we have experimental data to compare the model to.
So we’d like to be able to predict, obviously, these things, like we can with other aspects of the structural distortions. We’d also like to extend the vacancy model beyond this sort of compositional range.
And then, combining oxygen vacancies and A-site vacancies together. That’s not really been done.
Okay, one of the other projects we have, still with perovskites is perovskite solar cells. There is a student starting this year making hybrid organic-inorganic perovskite solar cells, and these are usually based on this compound, this methylammonium lead iodide compound. That’s the optically active layer.
There are lots of variations of this and his project is basically finding variations of this because essentially it’s a nice material, it works really well, except it has a big stability problem.
You can get efficiencies up to 20% with materials like this, they played chemical games to get there, but this is the real issue – the stability. Methylammonium lead iodide, in the presence of water (that’s humidity in the air) or ultraviolet light (neither of which is a good Achilles heel for something that’s meant to be a photovoltaic device), obviously. This will degrade into lead iodide and methylammonium iodide.
So the device will degrade and stop working. So there are chemical ways of trying to prevent that, replacing some of the iodide with other anions like bromide or chloride, replacing some of the methylammonium with other organics or even with inorganics.
We can substitute some of the lead with tin because lead is a big environmental no no these days, right, so tin is a bit more friendly to work with. It has its own set of properties, but it’s a go-to material to replace lead.
And we can also play with the electrode materials, the electron acceptor and
other electrodes. We can change zinc and nickel (using metal electrodes) into organic electrodes PCBM and PEDOT:PSS. These are organic materials that are a bit easier to work with.
So we can do that as well. So lots of chemical games to play to increase the efficiency and the longevity of these kinds of solar cells.
Another project that’s been started this year is the room-temperature fabrication of electroceramics. Most ceramics, not just electroceramics, require very high temperatures to to process, eventually to to sinter or densify, and if you need to do that, that limits how you can use them. If you want to put electroceramics on a silicon wafer and synthesize everything all at once, then it has to have a very limited temperature window, and if you can do this at room-temperature, it would save a lot of energy and a lot of money and time.
So basically what we do is, we take a composite material. We have an electroceramic material that’s the main phase and we wet it with a water-soluble ceramic. There are water-soluble ceramics out there. And so we wet this, the active phase, and we densify it in the normal press and out pops a dense ceramic. We’ve made it about 90% dense so far. I think we can do better if we keep working at the process – a lot of synthesis parameters and variables to optimize. But anyway, this shows great promise for tackling room-temperature synthesis of ceramics.
Okay, the last project I’ll talk about kind of doesn’t fit under the category of functional ceramics, but never mind that, it’s in our lab and we’ve done it for a long, long time. It’s nuclear graphite.
Nuclear graphite is, it seems like it should be very simple. Graphite, how hard, you know, how difficult is graphite? It’s carbon, right, how complicated could the structure be?
Well, nuclear graphite is made in a very complicated microstructure in order to make it isotropic because graphite normally is very anisotropic.
And that gives it a very complicated microstructure and then you irradiate it with neutrons and that just makes everything go crazy.
We’re still trying to work out what goes on structurally inside the material on an atomic scale when it’s irradiated at different temperatures, and one of the things we found is that we get dislocations forming, this is a kind of crystalline defect, under irradiation. This is under electron irradiation, but it’s a substitute for neutron irradiation, and you can see these guys here forming. These are dislocations forming and moving.
But even so, we find that we need a mechanism to explain large volume changes that you can’t explain with traditional models in graphite. And what we found is this “ruck-and-tuck” defect, which we published last year, which you can see. It’s almost like a rug would deform, right? Graphite is big sheets of rugs basically.
And this clearly involves a very large displacement in this direction, which is what we see. So we’re happy we found finally experimental evidence of that mechanism that was proposed several years ago.
Okay, so that, basically, is the same thing in a nutshell, so I will stop there.
I hope you all learn something in your visit to Boise state – virtual visit – and if any questions I’d be happy to to hear from any of you to take questions or discuss potential projects.