Jingcheng Ma, along with a team of researchers at the University of Illinois, Urbana-Champaign, found a way to make ultra-thin water-resistant surface coatings robust enough to survive scratches and dings.

Tech Briefs: What got you started on this project?

Jingcheng Ma

Jingcheng Ma: This is something that many people have tried before — to develop a thin hydrophobic (water-resistant) coating for the condenser of a steam power plant to enhance its energy efficiency. It was first proposed more than 100 years ago. This type of coating, when applied to the surface of the condenser, makes it more water-resistant, which optimizes heat transfer. We knew that this was a materials problem because these coatings are usually not durable. This has been an unsolved problem for a very long time.

We wanted to make a thin coating; however most previous ultrathin coatings developed defects once they cured onto a surface. Steam penetrates through these defects, leading to the gradual delamination of the coating.

We started our research project two years ago to try to understand why thin coatings degrade so easily. That was when we realized that the problem comes from tiny pinhole defects. After that, it was clear that our goal would be to find a pinhole-free coating.

This project started with a conversation between my advisor, Professor Nenad Miljkovic and Professor Christopher Evans, a polymer specialist who has explored self-healing in bulk polymers. The two of them decided to try applying this self-healing polymer chemistry to the pinhole problem.

Tech Briefs: Why do hydrophobic coatings help heat transfer?

Ma: In every steam power plant, liquid water is heated and becomes steam to drive a turbine. Then it cools down in the condenser and returns to liquid water — that's the whole steam cycle. This condenser is very important to the overall efficiency of the cycle. James Watt in his innovating work on the steam engine back about 250 years ago, realized that if the efficiency of the condenser is low, it results in a low energy efficiency of the engine.

What we're trying to do here is the essentially the same thing he was working on — to enhance the heat transfer efficiency in the condenser. We want to make the vapors condense more quickly and leave more quickly so that the heat can transfer from the steam to the condenser at a faster rate, which translates to higher energy efficiency.

Tech Briefs: How does a coating achieve that?

Ma: In every power plant, the materials used in these condensers are metals like copper or aluminum because they're robust, they conduct heat very quickly, and they're relatively cheap. But since these surfaces attract water, when you condense the steam, liquid films form on the condenser. The liquid film is a thermal barrier, which impedes the heat transfer from the vapor to the metal.

So, the problem is to remove this layer of water, which can’t be done if the film is hydrophilic — the water will stay. So, we want to have a water-repellent surface. If the surface is hydrophobic, the steam will condense into discrete water droplets that can quickly roll down from the surface of the heat exchanger, which exposes the surface and enables a higher heat transfer rate.

So, the key idea behind our coating is that it enables faster condensation from steam to liquid water, which results in an improved heat transfer performance of the condenser. And that further enhances the overall heat transfer coefficient — energy efficiency — of the steam power plant.

Tech Briefs: What about this coating makes it hydrophobic?

Ma: Making a hydrophobic surface is not a very hard thing to do for polymer people. Probably the most well-known hydrophobic coating is Teflon™. It lowers the surface energy, which makes it water-repellent. We use a hydrophobic silicone chemistry to achieve the characteristic we need.

Tech Briefs: What about the high temperatures in the power plant steam generators?

Ma: The temperature of the steam power plant condenser is usually about 100 °C — the temperature of the steam. When we fabricate this material, it is annealed at a temperature of 100 ° C, so it should be able to withstand the temperature of the condenser wall.

Tech Briefs: Do you see other uses for this coating?

Ma: Anti-icing, anti-fogging, anti-bacterial, anti-fouling — all these applications rely on the nature of the hydrophobic surface. For example, anti-icing basically means if you have ice on a surface, say on an airplane wing plane, it can be removed more easily because the hydrophobic surface doesn’t like water, so it doesn’t like ice as well — they’re the same chemistry.

Anti-fouling doesn't rely on the surface being hydrophobic, but if you have a pinhole-free surface, you don't have corrosion, because you don’t have charge transfer between the corrosive environment and the substrate. This creates a perfect barrier for protecting the materials beneath the coating.

Tech Briefs: What about the antibacterial applications?

Ma: Those are actually the same. Water doesn’t like hydrophobic coatings because the surface is very non-active, it has low surface-energy. So, it's not only water that doesn't like it, but also almost anything. For example, tape will not stick to it, and significantly, bacteria will not adhere as well, bacteria need an active surface to stick to.

Tech Briefs: Can you make it transparent so I could use it on my car's windshield?

Ma: Yes, we actually demonstrated this. We tried deposition on glass, and found that the coating, which is only about 10 nanometers thick, does not block the transparency of the window — compare that to the diameter of a hair, which is about 10,000 nanometers thick — the coating is essentially invisible.

Tech Briefs: How is it self-healing?

Ma: It depends on the network of the polymer. Polymers have lots of molecular chains packed together. Usually, these form a polymer network that is kind of permanent — the network structure stays unchanged after they are fabricated. So, if you scratch the surface, you make a pinhole in it, you break the network, and the network will not heal. What's new about this material is that we add a relatively active component: boric acid, to these small molecules. This enables the chains to bond dynamically — a bond is formed at one time, then the bond switches to another molecular chain continuously — it's almost like the material is flowing. So, when you break this network, the boric acid will restart a connection between different molecular chains so that it just heals, and you have another network.

Tech Briefs: Does this self-healing occur in a bulk film as well as this thin film?

Ma: Yes, we can demonstrate that. Just like with Silly Putty™, you can cut it into four pieces, press them together and you've got a new piece. So, the question is how you can make such a self-healing polymer in an ultra-thin film. I don’t think many people are trying to do that — not everyone is working on heat transfer, which requires an ultra-thin film. The other co-lead author, Ellie Porath, takes most of the credit for making the film.

Tech Briefs: Do you have any idea about when this might be implemented commercially?

Ma: That's a that's a very good question, I'm glad you asked. Right now, we're only demonstrating on a small wafer, which is hugely different than a large condenser at high temperature. We are trying to test this coating with a power plant that supplies about 70% of the electricity used on our campus. They are open to testing new technologies related to the power industry. So, we will try to start from there. We are going to make a new condenser and test the coating to see how long it will last and how much of an economic gain it will provide — that's the first step. I’m glad they’re offering us this opportunity so that we can learn more about the realistic problems of using this coating system. In general, it is not easy to collaborate with the power plant industry because they tend to be relatively conservative — stable electricity output is the most important thing so there can be a tendency for them to keep the system unchanged.

Tech Briefs: Have you thought about how you're going to coat a large condenser as opposed to a small wafer?

Ma: That's a very good question, because for our project we used spin coating. We deposit liquids on the plane surface and spin it very quickly so that the coating gets thinner. But it is impossible to use that same technique for complicated geometries. So, we're trying to develop other scalable deposition techniques like dip coating: You can make a huge batch of solution, dip the surface in it and the coating reaction begins in the liquid phase, then you will have a conformal coating on the surface.

Tech Briefs: What future questions do you intend to explore?

Ma: Our film, commonly called a vitrimer, is thinner than 10 nanometers, and is completely different from previous research, which was done on bulk materials, say one centimeter by several millimeters high, which you can see using your bare eyes. You can apply our material not only to large plain surfaces, but also to nanomaterials — you can modify the surface properties of these smaller objects.

Another area we need to work on, is that we are not sure if the structure of this material is exactly the same as the bulk material. A common question for most of the polymer materials is whether the structure of a film will be different when it's thin compared to the bulk.

So, we’re trying to do two things: better understand the structure and expand the applications for these kinds of materials.

An edited version of this interview appeared in the November 2021 issue of Tech Briefs.