Lab profile: Ricardo H. R. Castro, University of California-Davis

The surface of a material plays a role at all scales – but is all-important at the nanoscale. Here the interfaces between solids in a crystalline material or between solid and liquid – if particles are in solution – or between solid and gas determine the physical properties and stability of a nanomaterial. Understanding the interdependence between interfacial energies and these factors could lead to much needed control of nanomaterial properties, believes Ricardo Castro, who leads the Nanoceramics Thermochemistry Laboratory (NTL) based in the Department of Materials Science & Engineering at University of California-Davis.

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But as well as attempting to thermodynamically control the properties and processing of nanoceramic materials, Ricardo Castro is applying the same approach to nanomaterials in extreme environments such as nuclear reactors.

In recent years, Ricardo Castro has received the Robert L. Coble Award from the American Ceramic Society and the American Ceramic Society’s Engineering Division Global Young Investigator Award. Among his other commitments, he is also editor of the Elsevier journal Materials Letters.

Ricardo Castro talked to Materials Today about his research and future plans…

How long has your team been running?

The group was founded in 2009, when I joined UC Davis as a faculty member.

How many staff makes up your team?

The number of staff, including students, postdocs, and visiting scholars, has fluctuated over the years but is usually about 10-15 scientists in total.

What are the major themes of research in your lab?

Our group aims to develop fundamental understanding of how to control nanomaterials, in particular ceramics, by tuning the thermodynamics of their interfaces. As this may sound a bit esoteric, let me elaborate.

One of the characteristics of nanomaterials is the relatively high proportion of interfacial area. In a crystalline material, these could be a solid-vapor, a solid-solid, or even a solid-liquid interface. While at the microscale the thermodynamic effect of these interfaces is unimportant, for nanomaterials it has a preponderant effect on their properties and processing behavior.

Let me give you an example: in heterogeneous catalysis, available surface area (solid-vapor interface) on a catalytic particle is directly related to its performance; hence, nanoparticles are of key interest. However, the same surface area brings excess energy to the nanoparticle because chemical bonds and coordination are not fully satisfied, so dangling bonds are highly reactive. This excess energy brings so much instability that at high operating temperatures nanoparticles tend to collapse and grow into microsized counterparts, ultimately losing their nanoscale benefits. To solve this problem, we are focusing on lowering the surface energy, eliminating (or at least minimizing) the excess energy. We tackle this by using ionic dopants prone segregate on the surface of particles. The mechanism is similar to a detergent in water – where detergent molecules lower the surface tension – but here it is an all-solid system.

When we first proposed this, there was resistance from the community because ionic dopants were known to help inhibit nanoparticle coarsening, but were thought to work in a kinetic manner. To prove our hypothesis, we developed sophisticated microcalorimetric techniques that measure the surface energy of nanoparticles directly. You would imagine that methods to measure surface energies of crystalline solids would be readily available, but this was not true before our work on the topic. Calorimetry is the ‘art’ of measuring heat (or quantifying thermodynamics) involved in systems or reactions. We used a combination of highly sensitive calorimetry and gas-sorption analysis, along with a complex mathematical framework describing the thermodynamics of the system, to determine the surface energy of nanoparticles accurately. With this advance, we were able to demonstrate that, for instance, rare-earth elements can work as ‘detergents’ for spinel nanoparticles, decreasing their surface energies and inhibiting resistance to coarsening – enabling application as catalysts even at high temperatures.

But we don’t stop with nanoparticles. Nanocrystalline dense oxides (with only solid-solid interfaces or grain boundaries) are equally important for technological applications and face similar limitations in terms of coarsening at high temperatures. Recently, we have shown that nanocrystalline ceramics can show unprecedented mechanical properties. The hardness of a spinel ceramic can double as the grain size is refined down to the nanoscale. The demonstration of this nano-effect relied on refined processing of nanoparticles into bulk nanoceramics by using a high pressure densification strategy designed in our lab, known as ‘deformable punch spark plasma sintering’ (DPSPS). Even though we can make truly dense nanoceramics with DPSPS, their stability at high temperatures is questionable for the same reasons that nanoparticles are unstable. Our lab also targets the control of solid-solid interface energies by using dopants prone to segregation. This method is so efficient that we have created nanoceramics with virtually no excess energy, i.e. zero grain boundary energy.

There are other fronts in our lab exploring thermodynamic matters of nanomaterials and how it affects their properties. The interface energetic perspective has allowed us to understand mechanical properties of ceramics to the point we can increase toughness and impact resistance of these materials that were previously considered ‘brittle’. Moreover, we have been studying the energetic evolution during gas adsorption on the surface of oxides, helping to explain sensorial and catalytic mechanisms. In the past years, we have also linked this thermodynamic perception of nanomaterials to better understand radiation damage. We have shown that interfacial thermodynamics is a determinant factor in defining whether a nanomaterial is more stable or less stable when compared to its coarse-grained counterpart.

How and why did you come to work in these areas?

I started working on nanomaterial thermodynamics simply because there was no other theory that could satisfactorily explain the nano-behaviors I was observing experimentally during my PhD. My advisor was not a thermodynamics expert, so I had to dig around and find my own way into this world. In particular, I noticed that was very little actual thermodynamic experimental data on nanomaterials. There was one professor (Alexandra Navrotsky), who was leading the effort, but there were still major challenges to be faced. To solve them, we had to develop new calorimetric techniques and methods to measure interface energies and fully characterize nanostructures.

What has been your highest impact/most influential work to date?

Some would say that our most influential work is that we’ve enabled the manufacturing of nanocrystalline dense samples with unprecedented grain sizes. This has allowed us, for instance, to prove that the hardness of ceramics increases linearly with the square root of the grain size, and this effect has no breakdown, as previously thought. The smaller the grain the harder the stuff, always! Others would say it is our groundbreaking work on the radiation tolerance of nanocrystalline zirconia that showed interface thermodynamics play a major role.

However, I believe our influence goes beyond that, towards the fundamental understanding of the stability of nanomaterials. For instance, we have proven that the stabilization of a nanoparticle against coarsening can be immensely improved by using dopants prone to segregate on the surface. This way, a catalyst support, such as MgAl₂O₄, with high surface areas can be stabilized even when exposed to 1000 °C. We demonstrated this by showing that the surface energy in this system can indeed be decreased to minimize driving force for growth, tricking Mother Nature.

The excess energy from interfaces is the major problem for nanomaterials. But by focusing on strategies to lower these energies, we have shown that grain boundary energies in ceramics (solid-solid interfaces) can be practically zeroed with the right dopant. This means virtually  ‘thermodynamically stable’ nanomaterials. We have just published this result, and we are waiting to see what the community thinks of this ‘heresy’.

What facilities and equipment does your lab have?

Our lab is fully equipped with wet-chemistry facilities and characterization techniques, including calorimeters, mechanical testers, surface area analyzers, compositional analyzer (XRF), more calorimeters, x-ray diffraction (XRD) for phase identification, thermomechanical analyzer, and so on. The NTL is also a partner of the Peter A. Rock Thermochemistry Laboratory, which further amplifies our facilities.

The goal of having all of this is to be sure that what we measure in terms of calorimetry (thermodynamics) is actually meaningful. Calorimeters are like computer software in one sense: if you put garbage data in, it will give you garbage data out. Calorimeters are not smart, they will give you heat signals from whatever you put in. It is up to us to design the experiment to ensure that the heat signal can be interpreted to understand nanomaterials. Hence, having phase control, composition control, quantification of interface areas, etc. is mandatory to actually learn something from the experiments. And I have to say that, in particular for nanomaterials, having what we call a ‘clean’ sample is a major challenge. We often spend more time on that than actually doing thermodynamic analysis!

Do you have a favorite piece of kit or equipment?

Our calorimeters, of course! I know these sound like 1900s technology, but even with recent advances in the accuracy and sensitivity of these instruments we are still only scratching the surface of what calorimetry can do for us in terms of understanding nanomaterials’ behavior. Calorimeters can quantify the energetics of nanomaterials, allowing us to understand it and develop strategies to manipulate the energy to induce specific properties. In the end, everything is linked to the thermodynamics of the system. Mother Nature wants to minimize energy, which makes nanomaterials highly unstable and difficult to deal with… unless we learn Nature’s own tricks and exploit them.

What is the key to running a successful lab?

I don’t run the lab, my students do! During my PhD, my advisor gave me all the freedom I needed to develop something new and extraordinary. If I had been micromanaged, I would not have found the things I did. In my lab, I do the same and I tell my students “I’m giving you freedom, but I expect responsibility!” Sometimes it backfires, with some students taking that freedom and just enjoying it, but the risk pays off when students thrive.

How do you plan to develop your lab in the future?

Our line of work has many branches, as it touches many different fields – all related to thermodynamic quantities somehow. Right now, we are starting to focus on how to improve mechanical properties by targeting thermodynamics. These are not necessarily good buddies, as the theories of mechanics have all been developed based on defect dynamics. But we trust this new perspective can bring unprecedented results.

But we can’t ignore our global energy crisis, so we are also targeting the thermodynamics of nuclear materials and those of surface reactions for catalytic conversion.  

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Key publications

1. ??R.H.R. Castro, D. Gouvea. Sintering and nanostability: The thermodynamic perspective. Journal of the American Ceramic Society 99 (2016) 1105-1121.
2. D.N.F. Muche, J. W. Drazin, J. Mardinly, S. Dey, R.H.R. Castro. Colossal grain boundary strengthening in ultrafine nanocrystalline oxides. Materials Letters 186 (2017) 298-300.
3. R.H.R. Castro. On the Thermodynamic Stability of Nanocrystalline Ceramics. Materials Letters 96 (2013) 45-56.
4. Md. M. Hasan, S. Dey, N. Nafsin, J. Mardinly, P. P. Dholabhai, B. P. Uberuaga, R.H.R. Castro. Improving the Thermodynamic Stability of Aluminate Spinel Nanoparticles with Rare Earths. Chemistry of Materials, 28 (2016) 5163-5171.
5. S. Dey, J. Drazin, B. Uberuaga, R.H.R. Castro. Radiation Tolerance of Nanocrystalline Ceramics: Insights from Yttria Stabilized Zirconia. Scientific Reports 5 (2015) 7746.
6. N. Nafsin, R.H.R. Castro. Direct measurements of quasi-zero grain boundary energies in ceramics. Journal of Materials Research 32 (2017) 166–173.
7. L.J. Wu, S. Dey, J. Mardinly, Md. Hasan, R.H.R Castro. Thermodynamic Strengthening of Heterointerfaces in Nanoceramics. Chemistry of Materials 28 (2016) 2897-2901.