LLNL researchers have reported the synthesis of 3D printed transparent glass components using a "slurry" of silica particles extruded through a direct-ink writing process. From left: LLNL chemical engineer and project lead Rebecca Dylla-Spears and LLNL materials engineer Du Nguyen.

Lawrence Livermore National Laboratory scientists and academic collaborators from the University of Minnesota and Oklahoma State University have demonstrated the synthesis of transparent glass through 3D printing, a development that could ultimately lead to altering the design and structure of lasers and other devices that incorporate optics.

Other research institutions have shown 3D printing of glass is possible, however, prior demonstrations have involved extruding molten glass filaments through a heated print-head or using lasers to selectively melt and fuse glass powders. With these methods, the powders and filaments don't fully meld together in the short times they are heated during the printing process, which leads to porous or non-uniform structures that would not be suitable for optical applications.

Lawrence Livermore's approach does not rely on printing molten glass; instead the researchers create custom inks that are formed from concentrated suspensions of glass particles with highly controlled flow properties so they can be printed at room temperature. The printed components then undergo a carefully designed thermal treatment to densify the parts and remove evidence of the printing process. Finally, the processed parts are given an optical quality polish. Researchers claim this approach improves the odds of achieving optical uniformity.

"For printing high-quality optics, you shouldn't be able to see any pores and lines, they have to be transparent," said LLNL materials engineer Du Nguyen, who went through numerous mixtures of materials before finding the right combination. "Once we got a general formulation to work, we were able to tweak it so the material could merge during the printing process. Most other groups that have printed glass melt the glass first and cool it down later, which has the potential for residual stress and cracking. Because we print at room temperature, that's less of an issue."

LLNL's method uses a "slurry" of silica particles extruded through a direct-ink writing process. The printed product comes out opaque, but after drying and heat treatment it becomes transparent. In contrast to 3D printing with molten glass, the researchers state, the approach doesn't require high temperatures during printing, thereby allowing for higher resolution features.

The research could allow scientists to print glass that incorporates different refractive indices in a single flat optic, as opposed to the special shapes that are required for constant composition glasses to achieve similar lensing characteristics. Due to the ability to program the composition, Nguyen said, printed components would be easier and cheaper to finish.

"Polishing complex or aspheric lenses is pretty labor-intensive and requires a lot of skill, but polishing a flat surface is much easier," Nguyen said. "By controlling the refractive index in the printed parts, you alter the bending of light, which enables a lens that could be polished flat."

Rather than replace traditional optics, researchers said they want to explore new applications with composition gradients that don't exist on the market today. Designing for novel optical components instead of using off-the-shelf optics could reduce the size, weight or cost of optics systems.

While the research could expand the design space for optical engineers, it also may have applications outside of optics, including glass microfluidic devices that have complex and previously unobtainable layouts, researchers said. Glass is a prized material for microfluidics due to its optical transparency, chemical resistance, mechanical properties, and ability to tailor its surface chemistry and functionality. However, glass is difficult to machine and etch to make complex microfluidic device geometries feasible. The 3D printing of glass could change that, and the team demonstrated 3D printing of a simple microfluidic network.

Now that they've proved printing transparent glass is possible, researchers are turning their attention to making actual high-quality optics and gradient index lenses by varying the composition of the glass. The next hurdle is Gradient Refractive Index (GRIN) optics, which will require more process understanding and control.

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