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Haiyan Wang, center, the Basil S. Turner Professor of Engineering in Purdue University’s School of Materials Engineering, reviews a transmission electron microscope image with doctoral student Xuejing Wang, left, and postdoctoral research associate Jie Jian. (Purdue University/Charles Jischke)

Probing FAST processing for high-tech ceramic components

Researchers from Purdue University are studying the fundamental mechanisms behind a technique known as field-assisted sintering technology (FAST), where applying an electric field enhances ceramic sintering and thus potentially reduces costs while dramatically speeding the manufacturing process.

In particular, the team is looking to enhance the manufacture of high-tech ceramic components for a range of military and commercial applications. Such components tend to be expensive to manufacture because they are traditionally created from a powder that is sintered–or fused together–at around 1500°C for several hours. A form of FAST, called flash sintering, takes only seconds and reduces the processing temperature by 50% or less, to roughly 800°C.

“Our scope is to understand why within a matter of seconds and at such a low temperature you can facilitate sintering, which conventionally needs a much longer time at higher temperature,” said Haiyan Wang, the Basil S. Turner Professor of Engineering in Purdue University’s School of Materials Engineering. “Despite successful demonstrations of flash sintering, it remains poorly understood at the atomic scale.”

Understanding the mechanisms behind the FAST process could speed commercial development of bulk ceramic-sintering processing for a range of applications. The same mechanisms also apply to other applications beyond flash sintering, such as research into rechargeable lithium-ion batteries and fuel cells, where charged ions play a critical role in the overall functionalities.

The project, which officially began in January, is being funded with a $3 million four-year grant from the U.S. Office of Naval Research and is led by Wang. (See her describe the research here.)

“We need to develop a comprehensive scientific understanding along with predictive modeling tools and a set of rules and guidelines related to electrically assisted materials manufacturing of ceramics,” Wang said.

Critical to the research at Purdue are laboratory tools called in-situ transmission electron microscopy and in-situ scanning electron microscopy. The techniques allow researchers to see what’s going on in the crystal structure of the ceramic material at the atomic scale while it is being sintered. The crystal structure is made up of faceted grains 100 nm in diameter, which is too small to be seen with conventional light microscopes. Researchers will take real-time video of the process.

“Macroscopic descriptions of flash-sintering-related phenomena exist, but micro- or nano- scale understanding is still lacking,” said Wang.

Although ceramics are electrically insulating, they can conduct ions, or charged atoms. This flow of ions is sufficient to sinter the material at far lower temperatures than ordinarily required. The ions hop from sites on a ceramic’s crystal lattice structure. If oxygen atoms are missing from some sites in the crystal lattice, a positive charge is created at these sites, attracting the negatively charged ions.

The researchers are studying the phenomenon in a ceramic called yttria-stabilized zirconia and two other ceramics that have a large number of “oxygen vacancies” in their crystal structure.

“The energy required for this hopping is quite high, but an interesting twist is that this type of ceramic has unique oxygen hopping channels,” said Wang said.

Three objectives have been identified for the research team: understanding the interactions between the electric field and matter under processing conditions; providing theories and software for science and engineering; and developing rules and guidelines for materials engineers to use in manufacturing settings.

In addition, findings could be mined and used to generate predictive models with machine-learning methods, which could help engineers and technologists choose the correct materials to use for a given task. The research also could shed light on a phenomenon called electromigration, which can affect the performance of electronic devices.

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