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Visualization of the powder/liquid interface at atomistic scale during the evaporation in Cold Sintering Process. Credit: van Duin Group / Penn State

A better understanding of the mechanisms behind the cold sintering process (CSP) will lead to faster adoption and the cold sintering of many new materials, according to a team of Penn State researchers.

Cold sintering was developed in the laboratory of Penn State professor of materials science and engineering Clive Randall, approximately three years ago. CSP densifies powder ceramics, such as zinc oxide, at temperatures many hundreds of degrees lower than traditional sintering. This not only lowers the carbon footprint of the sintering process but also allows the sintering of materials that would evaporate at traditional temperatures, such as plastics. To date, CSP has successfully been applied to over 70 materials systems.

Ceramics are everywhere in our daily life from cookware in our kitchens to insulators in television. Most of the ceramics require very high temperatures (around 1000 Celsius) during the processing stage; therefore, is very costly. Using CSP, these processing temperatures have been decreased to 300 Celsius for several ceramics types. The mechanism behind CSP had been a mystery for a couple of years, and several research groups around the world have been working on unveiling the mechanism.

To elucidate the mechanisms behind CSP, Mert Sengul, Ph.D. candidate and lead author of a recent report in the journal Angewandte Chemie, used atomistic modeling to simulate the mixing of the zinc oxide powder with a small amount of liquid.  

“Experimentalists take the powder of the material and add water or a water/acid mixture,” Sengul explained. “They would like to dissolve the powder a little bit. They then apply pressure and temperature.”

This process sends the dissolved material into the pores where they fill the pores. In the next step they evaporate the solvent to densify the material. When this happens extraordinary grain growth results. Large grain size is good because it means there are fewer grain boundaries to interrupt the electrical conductivity.

“When the water is in contact with the powder, it usually dissociates into hydrogen and hydroxides that bind to the surface. Normally, that is not a good thing,” said Sengul. “But in our simulations we found that the hydroxide on the surface actually helps the densification by accelerating the diffusion.”

Sengul believes that understanding the mechanisms behind CSP will lead to labs around the world finding new composite materials systems with unforeseen properties.

Additional authors on the paper, titled “Water-Mediated Surface Diffusion Mechanism Enables the Cold Sintering Process: A Combined Computational and Experimental Study,” are Jing Guo, a visiting postdoc now an assistant professor in China, Clive Randall and Adri van Duin, both professors at Penn State.

Funding was provided by the Department of Energy, Air Force Office of Scientific Research, the National Science Foundation and ARPA-E.

Contact corresponding author Adri van Duin at acv13@psu.edu.