Nanocomposite makes printing glass a synch

Glass, while it possesses an unmatched combination of transparency, mechanical, thermal, and chemical resistance, along with thermal and electrical insulating properties, is notoriously difficult to shape into complex structures. Now, however, researchers Bastian E. Rapp and colleagues from Karlsruhe Institute of Technology have created a composite comprising silica nanopowder in a polymeric matrix that promises easy printing of a wide variety of complex, freestanding glass structures [Kotz et al., Nature (2017), doi: 10.1038/nature22061].

The crucial starting material is the nanocomposite — a liquid prepolymer in which silica glass nanoparticles 40 nm in diameter are suspended. The prepolymer can be formed into any structure using 3D printing and cured to fix its shape. The mixture is then heated to remove the polymeric binder before finally converting the silica nanoparticles into glass through a high-temperature treatment known as sintering.

“We have made high-quality fused silica glass, one of the oldest materials used by the human race, accessible to modern 3D-printing methods,” says Rapp. “Our approach is the very first method that allows structuring of fused silica glass at resolutions sufficient for optical applications.”

The silica glass is nonporous, as optically transparent as commercial glass made by conventional methods, and smooth. In fact, with surface roughness of only a few nanometers, the fused silica glass structures have the clarity and reflectivity necessary for optical devices like lenses and filters. Moreover, colored glasses can be easily created by adding metal salts to the initial mixture: chromium nitrate salts (Cr(NO₃)₃) for green, vanadium chloride (VCl₃) for blue, or gold chloride (AuCl₃) for red.

The new process gets around the previous size and resolution limits on the formation of glass structures, producing complex architectures such as honeycombs, pretzels, and even a microscale model of castle gate, without any need of harsh chemicals (Fig. 1).

“3D printing is currently restricted mostly to polymers,” points out Rapp. “So, the novelty in our approach is in the design of the nanocomposite, which is processable using standard desktop 3D printers.”

The nanocomposite precursor mixture is highly stable and can be stored for weeks in a refrigerator before being used in a regular, bench-top 3D printer. The glass structures produced by the team are also, as would be expected of any fused silica glass, resistant to swelling, defects or changes in optical properties when exposed to hazardous chemicals like acids, alkalis, or alcohols.

“[Our approach] opens up applications [of fused silica glass] ranging from high-performance optics to chemistry-on-achip applications, from making decorative glass objects to potentially whole facade elements,” says Rapp.

Lithography-based additive manufacturing is well known for its outstanding capabilities in terms of feature resolution and surface quality of printed parts but there has been a lack of available materials for demanding academic and industrial applications, points out Jürgen Stampfl of the Institute of Materials Science and Technology at TU Wien.

“Now Kotz et al. have added quartz glass to the spectrum of 3D-printable photopolymerizable materials,” he comments. “Of high importance is the excellent transparency of the material, which is crucial for targeted use in microfluidics or chemical process engineering.”

The researchers are now looking at the scalability of their approach — how well the process could work for manufacturing larger meter-scale objects. The team is spinning out a company to commercialize the technology and tackle the manufacturing challenges, says Rapp.

This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.06.003