A team of scientists from DTU Energy Conversion at the Technical University of Denmark has recently made a scientific breakthrough which will add a whole new dimension to the properties of cerium(IV) oxide, also known as ceria with the chemical formula CeO2. They have made the very well-known, very stable and very reliable ceria become vastly more reactive with a factor ten to the six.
“We talk about making ceria mass diffusion one million times faster. It is pretty amazing, especially because ceria is well known for its self-constraint. Ceria doesn’t grow, it doesn’t react and it doesn’t interdiffuse, especially when it is highly doped. But under this new condition it is completely different. It becomes Mr. Hyde from Dr. Jekyll and Mr. Hyde,” explains senior researcher Vincenzo Esposito and researcher De Wei Ni, both part of the team at DTU Energy Conversion.
The potential of the discovery is huge.
Ceria and its cousin cerium(III) oxide, Ce2O3, which is more stable at standard temperatures and pressures than CeO2, has been analyzed in depth and thoroughly described in thousands of scientific articles worldwide, as the CeO2 -> Ce2O3 process has a very wide range of industrial uses.
In its pure solid form ceria is used in self-cleaning oven walls as a hydrocarbon catalyst during the high-temperature cleaning process, as part of catalytic converters in automotive applications and for polishing glass and stone etc., while ceria doped with Gd2O3, also known as gadolinia, is mainly used in applications like fuel cells and conversion sensors. But with the new discovery and the new properties the usefulness of ceria has the potential to grow manifold.
If the new reactive condition can be controlled, that is. Because ceria has a major flaw, which is to expand and contract dramatically and a tendency to break or even disintegrate totally under catastrophic stress conditions. A tendency which has been known and avoided for years, but no one has considered using it for practical purposes. Until now.
The taste of a breakthrough
“We have discovered this new phenomenon of fast mass diffusion in ceria and it has this smell, this taste, this flavor of a major scientific discovery, which is so satisfying for a scientist. We have not only seen the phenomenon, we have described it and what is more important: We can also control it!” says Vincenzo Esposito and De Wei Ni. They made the final test in the first days of September and proved that ceria could be both changed and tamed.
De Wei Ni and Vincenzo Esposito are currently working on a scientific article on the process and would like to wait until its publication before describing the details on how they did it. But they are willing to tell how they got there, as the discovery has had many fathers.
Many articles have over the years mentioned the tendency of ceria to expand and contract in reduction/oxidation cycles, but none delved into which other phenomena occur when the ceria reacts that way. Only that it should be avoided because it causes severe stress and cracks.
Five years ago two scientists at DTU Energy Conversion, Andreas Kaiser and Zeming He, began pushing the limits of the materials in some sintering experiments
“Andreas and Zeming observed the phenomenon, like hearing thunder where there is no lightning. But they did not understand why there was a thunder. This is my personal contribution to this work. I have looked at literature for two years, and finally I found some work on different fluorites, including uranium oxide, zirconia etc., which are very similar to ceria in terms of structure”, says Vincenzo Esposito.
Different materials, same process
The articles by Cheng, a very famous scientist on the field of sintering, described in 1992 and 1996 how and why there was very little mass diffusion in ceria, and why the material is so stable, that it doesn’t react in conventional conditions. Vincenzo Esposito took Cheng’s theory and translated it into the new condition of highly doped ceria, which is highly defective under different chemical conditions.
“This is very interesting, because according to Chen’s theory, something should happen when we introduce the defects and this is partially happening. Also in our condition, but there was something more…”
With inspiration from Cheng the team of DTU scientists, including De Wei Ni and Vincenzo Esposito, used a few equations to predict the grain mobility, which gave a kind of ideal rate of diffusion of the material.
In ceramic form, doped ceria consists of a multitude of very tiny grains. These grains (little crystalline particles) are in the micro-range, one millionth of a meter, and maybe even smaller, in ceria under normal conditions. And the piece of ceramic stays together because all these grains are attached to each other.
“The glue keeping the grains together is due to mass diffusion between all those boundaries, but whenever you work in reducing conditions, the material expands a lot, and then when we cool it down in oxygen, there is a very dramatic contraction, which leads to fractures between the grains.”
The fatal disintegration
It’s like heating a glass rod until it is very very hot, and then suddenly immerse it in very cold water.
“Only the cooling of the rod is less severe. If you have a really hot rod and you put into water, it breaks preferentially at few points. Because you have cracks and the major flaws propagates into the material and goes through the veins of the glass. That is a catastrophic event. But oxidizing reduced ceria is more catastrophic, because the stress has an effect on every single interface of your grains, meaning that all grains keeping your piece of ceramics together are disconnected simultaneously. It basically disintegrates into dust”, says Vincenzo Esposito
“As I said, this has the flavor of discovery. Not because we have observed this phenomenon, but because we can explain it. This disintegration has been probably observed by many other groups, but nobody really took note of it. We have. We have a very deep knowledge about this material and based on our understanding of the process, we can go to different materials, different conditions and start again. We know what to look for in other chemical compositions, and when knowing what is going on, we can actually control it.”
What De Wei Ni and Vincenzo Esposito have done is to change the concentration of defects in ceria during the heating at high temperatures. They put the material in the condition at these high temperatures, where ceria have a lot of diffusion, and then they slowly, very slowly change the thermodynamic conditions of the doped ceria to avoid the disintegration. And they can only do it because they know what is going on.
“Andreas and Zeming opened the door, when they heard thunder. We have seen the lightning, and when you understand something about lightning, then you can say that okay, to avoid getting burned, you have to place something that react with the energy, a sort of lightning rod. We have made that rod!”
The implications were highlighted by a very recent experiment, where De Wei Ni and Vincenzo Esposito constrained ceria as thin film on another really stable material, a sapphire. A sapphire is a single crystal of aluminum oxide which is extremely stable. And they made the film unstable and reactive.
“Usually there is no way to change the microstructure of ceria thin films when it is deposited on such materials. But last week we prepared these two very very stable doped ceria thin film on a sapphire. And then we put the film in this new condition and gave the materials the possibility to change, to get into the Mr. Hyde shape. And we were very surprised because after a little time it reacted and formed a completely new microstructure! And we hadn’t even put the materials under severe conditions, only a bit over 1000 degree celsius. But we saw degenerated porosity and other interesting phenomena.”
Top five per cent in the world
De Wei Ni and Vincenzo Esposito and their coworkers are not sure what implications the phenomenon in the thin films has. But when the team described the phenomenon with reactive doped ceria in the article “Enhanced mass diffusion phenomena in highly defective doped ceria” in the journal Acta Materialia, issue 61 of 2013, they received special mentions by the Acta Materialia editorial board, which believed the new discovery to be in the top 5% of on-going research in this field.
The anonymous peer reviewer was also impressed:
“Moreover, the investigation of sintering doped CeO2 under low pO2 is of great interest for the energy community, and may have important implications in the preparation of large-scale materials for applications such as solid oxide fuel cell anodes or in solar-to-fuel conversion under rapid redox cycling.”
Such high praise is not given lightly.
“This is very good, because there has been a lot of scientific production on this material and this kind of case studies since the 1960-1970s. And the reviewer has some really good points, as our discovery can explain some of the microstructural changes which happen when using solar concentrators”, says De Wei Ni and Vincenzo Esposito.
De Wei Ni and Vincenzo Esposito are not sure how their discovery and taming of reactive doped ceria can be used in other technologies, but they know that the Dr. Jekyll turned Mr. Hyde has a great potential.
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From the abstract in journal Acta Materialia, issue 61 of 2013
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The densification and grain growth of the solid state ionic conductor material Ce0.9Gd0.1O1.95-δ (i.e. GDC10, gadolinium-doped ceria, with Gd 10 mol.%) are analysed for nanometric and fine powders of various particle sizes, both in air and in a 9 vol.% H2–N2 mixture. Due to a dominant solute drag effect in aliovalent highly doped ceria, the starting morphology of the powders controls the diffusion mechanisms of the material in air. Conversely, highly enhanced densification and grain growth are achieved by firing the materials at reduced temperatures (800 < T < 1200° C) in low oxygen activity atmospheres (pO2 < 10-12 atm). Solute drag is not the rate-limiting step in highly defective GDC and the densification mechanisms are nearly independent of the starting powder properties. Fast diffusion is activated under low oxygen activity with high grain boundary mobility (e.g. Mgb ~ 10-10 m3 N-1 s-1 at 1100° C). The change of the dominant sintering mechanisms under low oxygen activity is attributed to the formation of a large concentration of oxygen vacancies (V €O), electronic defects (Ce’Ce, i.e. Ce3+) and reduced Gd/Ce cation mismatch. High densification and electric conductivity are achieved in Ce0.9Gd0.1O1.95-δ at low temperatures (~ 1000° C) and low oxygen activity, preserving the mechanical integrity of the material.”
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