Study the mechanics of materials on the smallest scale

May 27, 2022

When looking to figure out how tough a material is or understand why it acts the way it does, Hoover says you might want to start small.

“I got my doctorate at Northwestern University working in a field called cohesive fracture mechanics with a focus on the so-called size effect,” says Hoover. “It’s a way of understanding that small structures made of certain materials have higher strength than large structures made of the same materials, and it’s important to understand this phenomenon when you’re designing large-scale projects from these. materials.”

Christian Hoover, an assistant professor in the School of Sustainable Engineering and the Built Environment at ASU’s Ira A. Fulton Schools of Engineering, will expand his research into vitreous and organometallic frameworks over the next five years through the funding for a National Science Foundation CAREER Award. Photo by Monica Williams/ASU
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His path then took him to the Massachusetts Institute of Technology for his postdoctoral work, where he studied the mechanics of materials at the nano and micro scale. He has pushed to understand the properties of materials and has published work on the mechanics of glass, bitumen, cement, polymers, and clay minerals since arriving at Arizona State University to teach civil engineering in schools in engineer Ira A. Fulton.

Hoover, an assistant professor in the School of Sustainable Engineering and the Built Environment, one of seven Fulton schools, says he will apply the concepts he learned from his doctoral and postdoctoral work as he is moving forward with a five-year research project focused on vitreous, metallo-organic, or gMOF.

The project is supported by Hoover’s 2022 National Science Foundation Early Career Development Program Award, or CAREER Award, which comes with $600,000 in funding.

The CAREER Award program supports academic faculty members who are early in their careers and who demonstrate leadership potential as researchers and educators. They are selected to serve as role models in their academic departments and to advance research in their field.

“Understanding the origins of the behaviors of these materials is the first step to being able to exploit these materials for specific applications,” says Hoover.

MOFs are porous hybrid materials that start out as crystals, or vsMOFs, reinforced with an additional organic structure, are highly sought after for several applications. They can be created with little negative environmental impact, and with small-scale features and structures optimized to achieve a variety of unique optical, electrical, and magnetic behaviors.

Perhaps one of their greatest attractions is their large absorption surfaces, due to their high porosities, which can be used for carbon dioxide capture, separation and storage applications.

However, the mechanical disadvantages of vsMOFs that inhibit their widespread use. For example, for use in such industrial carbon capture and sequestration applications as an adsorbent medium, the vsMOFs must be able to withstand the high pressures that result from the continuous flow of CO2. During the tests, pieces of vsMOFs have been shown to collapse quite easily because the highly porous structure makes the vsVery delicate and fragile MOF. The fragility of vsMOFs also prevent them from being made into larger chunks.

A solution to the mechanical defects of vsMOFs is to create a gMOF system by fusion and densification of the vsMOF without destroying the organic structure. This is how gMOFs are manufactured and why they offer designers the ability to specify the porosity they want in order to achieve the desired mechanical performance. A major advantage of gMOFs is that they can be made into larger parts of any shape, large enough for a variety of applications.

“You can think of them as glass that’s also reinforced with an organic structure,” says Hoover.

The main knowledge gap with gMOFs is that most of their mechanical behaviors are unknown. An Ashby diagram, or behavior diagram, allows materials scientists to track properties such as density or yield strength of materials and classify them with similar elements. The behaviors of gKnown MOFs are mostly unique and overlap new and unexplored areas in these graphs. Through his research, Hoover hopes to find out what makes these materials behave so uniquely.

Hoover says receiving a CAREER Award is an honor because the NSF bases its decisions about who receives the awards on recommendations from prominent reviewers with deep roots in the mechanical research community.

“It means that members of your community have deemed you and your ideas worthy enough to receive this award,” says Hoover. “So it’s a huge honor.”

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