By guest aurthor Bridget O’Neal from 3D Printing
In the recently published ‘Sol-Gel Based Nanoparticles for 3D Printing of Optical Glass,’ Peter Palencia and Koroush Sasan of Lawrence Livermore National Laboratory are innovating further in the realm of high-performance optics, using 3D printing and printable titanium-germanium-silica inks made from colloidal sol-gel feed stocks.
Although glass is such a valuable material to scientists attempting to transmit, refract, and reflect light, Palencia and Sasan point out the many challenges in using it for work like theirs. For this study, they focused on a ternary formulation of core-shell SiO2 nanoparticles coated with a TiO2 shell and GeO2 nanoparticles that could be used as a feedstock for direct ink writing (DIW) lenses.
“At the atomic level, the refractive index of glass can be adjusted to a certain value based on the composition of oxides within the glass. For example, sodium oxide and titanium oxide can be added to the glass to increase the refractive index,” explain the authors. “However, the introduction of the different dopants into the glass create potential problems as they not only drastically change the optical properties of the glass, but also its thermodynamic stability and overall viability to form a transparent glass.”
With additive manufacturing processes, a new and ‘revolutionary approach’ allows for the creation of a wide range of materials, whether researchers are using cells in tissue engineering or printing with resin.
“Although these methods have seen success in the production of other materials, a robust method to 3D print an optical lens has not been developed,” stated the authors.
Bubbles are a typical challenge—resulting in stress, which then causes cracking. Other methods have not been fully researched—such as glass created via UV curable ink. In previous research by the authors, however, using fumed silica, they had some success but still found it to be ‘an inconsistent mixture that affects all aspects of the glass.’ It tends to cluster together too, creating pieces too large for extrusion. Here, the researchers found a solution in eschewing fumed silica and replacing it with sol-gel based silica nanoparticles. It can be used for direct ink writing:
“Its transparency, refractive index, and coefficient of thermal expansion is comparable to a commercial lens,” stated the authors.
The authors found that to create functional optical glass, they had to rely on a combination of germanium, silica, and titanium. The dopants (germanium oxide and titanium oxide) caused an increase in the refractive index, while still upholding functionality and performance.
In contrast to the previous use of silica inks, the new mixture promoted stability, while great care still had to be taken during heating to avoid stress cracks.
“By varying the concentration of GeO2 and TiO2, we tested the viability of glass in terms of transparency and printability,” concluded the researchers. “The glass was transparent up to 7 mol% TiO2 and 14 mol% GeO2. Also, a higher colloidal stability of the glass could be seen through the zeta potential of the nanoparticles.
“While these glasses are not completely crack-free, a range of temperatures were identified as cause of cracking. Experiments to optimize ternary glass through the addition of surfactants and to reduce cracking during heat treatment are ongoing.”