Mastering chaos to design high-entropy ceramics

Assembly line: a different chemical mixture is created in each of the “tube flow reactor” droplets – under the same boundary conditions. Credit: Empa

Nature yearns for chaos. It’s a nice, comforting phrase when another cup of coffee has spilled onto the computer keyboard and you imagine you might wish the sweet, milky brew back into the cup of coffee, where she had been seconds before. But wishing won’t work. Because, as mentioned, nature yearns for chaos.

Scientists coined the term entropy for this effect, a measure of disorder. In most cases, if the disorder worsens, the processes take place spontaneously and the return to order that previously prevailed is blocked. See the overturned cup of coffee. Even thermal power plants, which generate a huge cloud of steam above their cooling tower from a pile of wood or a pile of coal, work thanks to entropy. The mess increases dramatically in many combustion processes – and humans take advantage of this, drawing some energy in the form of electricity from the ongoing process for their own purposes.

Can entropy stabilize anything?

Crystals are considered the opposite of disorder. In a crystal structure, all of the lattice elements are neatly sorted next to each other in the smallest possible volume. This makes the idea that crystals can be stabilized by the force of entropy and thus create a new class of materials all the more bizarre.

Entropy-stabilized materials are still a young field of research. It started in 2004 with so-called high-entropy alloys, mixtures of five or more elements that can be mixed together. If the mixture is successful and all the elements are evenly distributed, special properties sometimes emerge which do not come from the individual ingredients but from their mixture. Scientists call this “cocktail effects”.

Even in the heat, chaos reigns

Since 2015, it has been known that even ceramic crystals can be stabilized by the “power of disorder”. In this way, even oversized and tiny elements fit into the crystal, which would normally destroy it. The Empa research team has already succeeded in inserting nine different atoms into a crystal. The advantage is that they remain stable even at high temperatures, since “rearranging” them would lead to greater order. The natural search for maximum disorder thus stabilizes the unusual crystal structure – and thus the entire material.

“With up to four components in the crystal, everything is still normal; with five or more components, the world changes,” says Michael Stuer, researcher at Empa’s High-Performance Ceramics Department. Since the Luxembourg-born researcher joined Empa in 2019, he has been working in the field of high-entropy crystal research. “This class of materials opens up a wide range of new opportunities for us,” says Stuer. “We can stabilize crystals that would otherwise disintegrate due to internal stresses. And we can create highly active crystal surfaces that have never existed before and look for interesting cocktail effects.”

Together with his colleague Amy Knorpp, Stuer is now heading into the unknown. Both are specialists in the production of fine crystalline powder, and they have colleagues at Empa for X-ray and surface analysis to precisely characterize the samples they produce. With their help, Michael Stuer now wants to be at the forefront of the international scene. “The number of publications on the subject of high-entropy crystals is increasing very sharply at the moment. And we want to be there from the start,” says the researcher.

islands of knowledge

What is needed now is a systematic approach, expertise and a healthy dose of perseverance. Where to start ? What direction are we taking? “At the moment, there is no consistent expertise, no comprehensive overview of this new area of ​​research,” Stuer says. “Different research groups around the world are working on limited projects. Thus, individual islands of knowledge are emerging that will need to grow together over the next few years.”

Michel Stuer and Amy Knorpp focus on catalytically active materials. The chemical reaction they are interested in consists of combining CO2 and hydrogen to form methane. The objective is to transform a greenhouse gas into a sustainable and storable fuel. “We know that CO2 the molecules adsorb particularly well on certain surfaces and the desired reaction then occurs more easily and quickly,” explains Amy Knorpp. “We are now trying to produce entropic crystals on the surfaces of which such highly active regions exist.

chemical assembly line

To make progress faster, the researchers built a special synthesis apparatus with the help of Empa’s workshop, in which many different chemical mixtures can be tested one after another, like on an assembly line. In the “Segmented Flow Tubular Reactor”, small bubbles pass through a tube in which the respective reaction takes place. At the end, the bubbles are emptied and the powder they contain can be further processed.

“The ‘Tubular Flow Reactor’ has a huge advantage for us: all the bubbles are the same size, which is why we always have ideal and consistent boundary conditions for our syntheses,” explains Stuer. “If we need larger quantities of a particularly promising mix, we simply produce several bubbles with the same mix one after the other.”

Windows on the right side

The precursor powder is then transformed into fine crystals of the desired size and shape by various drying processes. “Crystals are like houses, they have closed exterior walls and some with windows,” explains Michael Stuer. Sometimes the shape of the crystal already indicates the side of the window. For example, when a mixture forms needle-shaped crystals. “The long sides of the needle are the low energy ones. Not much going on there. The crystal edges at the ends of the needles, on the other hand, are the high energy ones. That’s where it gets interesting,” Stuer said.

For their first major project, the Empa researchers teamed up with colleagues from the Paul Scherrer Institute (PSI). They are studying the possible methanization of CO2 from biogas plants and sewage treatment plants in an experimental reactor. The PSI researchers have already gained experience with various catalysts and repeatedly come across a problem: the catalyst, on the surface of which the chemical reaction takes place, weakens over time. This is because the sulfur components of the biogas contaminate the surface or the catalyst surfaces undergo chemical transformation at high temperatures.

This is where researchers are looking for a breakthrough using entropic crystals; after all, these do not decompose even at high temperatures – they are stabilized by chaos. “We remain hopeful that our crystals will last longer in the process and perhaps be more impervious to sulfur pollution,” Stuer said.

draw a map

After that, Empa’s crystal specialists are ready for further challenges, such as high-performance batteries, superconducting ceramics or catalysts for car exhaust and other chemical production processes. “It’s a dark forest that we walk through,” says Amy Knorpp. “But we have a guess in which direction something might be found. Now we’re drawing a map of those systems. Somewhere out there, we think, is a treasure chest of hidden ideas.”

Their recent research is published in CHEMISTRY.


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More information:
From synthesis to microstructure: Engineering of high-entropy ceramic materials of the future, CHEMISTRY (2022). DOI: 10.2533/chimia.2022.212

Provided by the Swiss Federal Laboratories for Materials Science and Technology

Quote: In control of chaos to engineering high-entropy ceramics (2022, August 9) retrieved August 9, 2022 from https://phys.org/news/2022-08-chaos-high-entropy-ceramics.html

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