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California team tailors ultrafast laser pulse train to weld ceramics

28 Aug 2019

San Diego researchers devise an approach that could solve a key problem in materials processing.

Engineers at the University of California, San Diego (UCSD) believe they have made an important breakthrough for manufacturing ceramic materials, showing for the first time that certain types can be welded using pulses from an ultrafast fiber laser.

While ceramics can be melted in high-temperature furnaces or using high-power continuous-wave lasers, conventional joining methods create so much heat that they cannot be used in close proximity to temperature-sensitive polymers or electronic components.

In addition, the intense heating needed to melt ceramics tends to cause cracking, meaning it is almost impossible to produce ceramic components with complex device geometries.

That could now change. Although ultrafast lasers do not typically transfer much heat to a target material, localized melting is possible by stimulating nonlinear absorption in ceramic materials that are usually opaque or translucent at laser wavelengths - if precisely the right laser pulse parameters are used.

Burnt electronics
Javier Garay, a UCSD professor and lead author on a newly published Science paper detailing the DARPA-funded work, said: “Right now there is no way to encase or seal electronic components inside ceramics, because you would have to put the entire assembly in a furnace, which would end up burning the electronics.”

With collaborators at UCSD and the University of California, Riverside, Garay tailored pulses from a “Satsuma” fiber laser supplied by France-based Amplitude Technologies to show that laser welding of ceramics is in fact possible.

They aimed a series of picosecond pulses along the interface between two ceramic parts, building up enough heat to cause localized melting. According to the team, the key was to optimize the laser pulse characteristics alongside the transparency of the ceramic target.

Co-author Guillermo Aguilar reported: “The sweet spot of ultrafast pulses was two picoseconds at the high repetition rate of one megahertz, along with a moderate total number of pulses. This maximized the melt diameter, minimized material ablation, and timed cooling just right for the best weld possible.”

If the process can be replicated in an industrial setting, it could represent a major development for ceramic materials processing. The extremely hard and shatter-proof materials are also highly biocompatible, making them ideal for medical implants and protective casings for electronic components.

“By focusing the energy right where we want it, we avoid setting up temperature gradients throughout the ceramic, so we can encase temperature-sensitive materials without damaging them,” Garay said.

Nonlinear interplay
So far, tests by the team have confirmed that a join between two small welded ceramic parts was able to hold a vacuum. Garay and colleagues will now look to optimize the approach for larger components, and adapt the method for different ceramic materials and geometries.

“Ultrafast laser welding is more versatile on transparent ceramics because one can focus through the material, allowing the joining of more complex geometries and over multiple interaction zones, [and] increasing the ultimate weld volumes,” they explain in their paper.

The general method should be applicable to a wide range of other oxides, nitrides, and carbide materials, they add. This includes the array of available transparent ceramics with similar optical bandgaps, for example alumina, spinel, YAG, and many others under development.

“The key is the interplay between linear and nonlinear optical properties and laser energy - material coupling,” wrote the team. “The welded ceramic assemblies hold high vacuum and have shear strengths comparable to metal-to-ceramic diffusion bonds.

“Ultrafast lasers with different wavelengths and suitable laser powers likely will become more widely available, expanding the potential usefulness of our technique.”

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