Published on September 4th, 2018 | By: Faye Oney0
Nanoparticles lower thermal shock stress for ceramics at extreme temperaturesPublished on September 4th, 2018 | By: Faye Oney
[Image above] (first block) on as-received alumina ceramics and (b) on an engineered hydrophobic surface. Credit: Divya J. Prakash and Youho Lee
Remember the controversy nearly a decade ago, when consumers pulled glass baking dishes from their hot ovens and placed them on a wet countertop? They cracked and sometimes shattered, sending glass shards flying across the kitchen. That phenomenon is called thermal shock—and it also happens to ceramic materials in industrial settings where the impact can be a lot more serious—and costly, especially in nuclear power plants.
Now, researchers at the University of New Mexico are using a different approach to reducing heat transfer to ceramic material surfaces—and it doesn’t involve complicated processes or expensive equipment.
Researchers Youho Lee, assistant professor in the nuclear engineering department, and Divya J. Prakash, successfully reduced the heat transfer to alumina coupons using an inexpensive nanoparticle coating that repels water, described in an open-access article on AIP Publishing.
Using high-speed video of water droplets on a hot alumina surface, researchers noticed highly turbulent bubbles appeared on the surface, according to Lee. They also observed that the transfer of heat reduced the strength of the alumina when they conducted subsequent bending tests.
“When heat transfer is fast, the collision moments are characterized by violent bubbles and jets on the surface,” Lee says in the article.
When they increased the temperature to 325oC (617oF), the ceramic material’s strength decreased even further—although, at temperatures beyond 325oC, the water droplets formed a vapor film and had less impact on the strength of the material.
In order to reduce heat transfer (and ultimately decrease thermal shock), Lee coated the surface of the ceramic material with nanoparticles, which created an insulated vapor film to repel water. In repeated experiments, the scientists observed a vapor film forming on the surface, replacing the turbulent bubbles that previously appeared.
But more important, the vapor film-coated ceramic material did not decrease in strength after they applied the nanoparticle coating.
“We use exactly the same material but control the heat transfer, allowing the material to see a more benign temperature gradient, alleviating tensile stresses and so radically improving thermal shock behavior,” Lee explains in the article.
“What we did was very simple, with no expensive, fancy equipment or materials,” he adds. “The innovation of this study was to prevent dramatic heat transfer by promoting the vapor film formation, which insulated the material from thermal shock.”
Scientists have previously studied ways to change outcomes and effects of thermal shock on ceramic materials. But improving thermal shock resistance requires changing properties of the material, Lee says. “If you improve the material in one way, you sacrifice other properties.”
The findings could be useful in nuclear power plants to increase thermal-shock tolerance for higher temperatures, or even in industrial applications that use extreme temperatures to heat ceramic materials.
The open-access paper, published in AIP Advances, is “Heat transfer foot print on ceramics after thermal shock with droplet impingement: Development of thermal shock tolerant material with hydrophobic surface” (DOI: 10.1063/1.5041809).
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