Additive-Manufacturing of 3D Glass-Ceramics down to Nanoscale Resolution

1Laser Research Center at Vilnius University, Sauletekio Ave. 10, Vilnius, LT-10223, Lithuania 2Femtika Ltd., Saulėtekio Ave. 15, Vilnius, LT-10224, Lithuania 3Faculty of Chemistry and Geosciences, Vilnius University, Naugarduko Str. 24, Vilnius, LT-03225, Lithuania 4Swinburne University of Technology, John St., Hawthorn 3122 Vic, Australia 5Melbourne Centre for Nanofabrication, the Victorian Node of the Australian National Fabrication Facility, 151 Wellington Rd., Clayton 3168 Vic, Australia *Correspondence: darius◦gaileviciusΘff◦stud◦vu◦lt *Correspondence: sjuodkazisΘff◦swin◦edu◦au *Correspondence: mangirdas◦malinauskasΘff◦vu◦lt


Introduction
We show that a popular hybrid organic-inorganic sol-gel resist SZ2080 can be converted into a material with entirely different properties obtained via polymer-to-ceramic transition guided by a high temperature sintering and oxidation.The silica and zirconia precursors present in the resist in the ∼20% mass of inorganic component will lead to emergence of silica and zirconia crystalline phases in the final sintered ceramic material.Importantly, a proportional downscaling of the 3D polymerized object takes place with significant volume change of 40-50% dependent on annealing protocol without distortion of the proportions of the initial 3D design.The temperature-guided resizing and the composition change can be potentially tailored to form 3D free-form patterns of complexity which is not amenable by other micro-/nano-fabrication methods.
In this study, we explore a possibility to apply ultrafast 3D laser nanolithography 1 in conjunction with heat-treatment 2 to acquire ceramic 3D structures in micro-and nano-scale.Laser fabrication (Fig. 1) allows production of initial 3D structures with relatively small (hundreds-of-nm) feature sizes out of hybrid organic-inorganic material SZ2080 3 .Then, a post-fabrication sintering at different temperatures up to 1500 • C in the air atmosphere facilitates decomposition of organic part (80%) and results in the glass-ceramic hybrid material.Resolution in the final 3D structure is superior to that of the as-fabricated structures.Both, filled volume as well as complex free-form 3D objects can be resized and their structural composition changed.
Ultrafast lasers are extending their applications towards advanced nanoscale processing of materials 4,5 .Recently, additive manufacturing of 3D micro-/nano-structures followed by heat-treatment protocols for downscaling their dimensions while keeping their initial geometry was reported 6 , yet the attention was not paid in experimentally validated explanation of material conversion process 7 , thus the potential advantages besides resizing were not investigated 8,9 .Alternatively, the physical addition of metal nanoparticles resulted in roughening of the structures after turning them fully into inorganic coatings 10 .This limits their applications for functional 3D nanostructures where pure inorganic materials and/or optical quality and structural uniformity of the patterns and workpieces are required 11 .Lastly, up to know there is no report on true-3D ceramic 12 or glass 13,14 structures with higher than tens-of-micrometers resolution despite specially designed pre-ceramic resins 15 .
Here, we show a straight-forward method to make 3D glass and ceramic structures with resolution down to the nanoscale with final composition tailored by the initial materials.

Experimental
To prove this we used the prepolymer synthesized as described in 3 , yet we do not use any photonitiator.We made 3D laser direct writen structures from this material and performed heat-treatment in temperature range of 1000 − 1500 • C and analyzed the changes in Raman spectra and also performed X-ray diffraction measurements (XRD).
More specifically, for our experiment, we fabricated 3D structures with different geometries.These included: bulk -cubes, periodic -3D woodpile micro lattices, free form structures -micro-sculptures, combining bulk and nanometer feature elements with complex bends, and macroscopic hexagonal 3D lattices which are usually used as artificial cell scaffolds.For laser structuring, we used a 300 fs pulse duration 515 nm wavelength 200 kHz pulsed laser beam focused through different numerical value objectives lenses, which are in the range of 0.8 NA and 1.4 NA.Other relevant fabrication parameters were tuned to be approximately near the middle of the fabrication window 16 .We developed the resulting structures in methyl-isobutyl-ketone for one hour and treated them with a Piranha solution to remove organic contaminants from their surface.The geometrical changes and stability were tracked by making optical images and scanning electron microscope (SEM) micrographs.For the (micro-)Raman spectra analysis we choose our hexagonal scaffolds 17 that due their millimeter scale are easy to handle.We measured micro-Raman spectra on the beams and intersection of hexagonal scaffolds in each relevant step: before heat treatment, after heat treatment at different temperatures T ≤ 1500 • C. To corroborate and interpret the results we also performed an XRD analysis on calcinated and then powdered SZ2080.

Results
The micro-Raman measurements are summarized in Fig. 2. As the temperature increases the spectral shape changes and evolves via qualitatively two distinct form-factors (Fig. 2).Close examination of the initial spectrum and comparison to that for T = 1000 • C reveals that they differ by the molecular vibrations which can be associated with the carbon-carbon, carbon-oxygen, carbon-hydrogen bonds.After heat-treatment those spectral lines vanishes (Fig. 2b).The new spectral form coincides with that typical for silica glass; we measured a control sample of fused silica.
Increasing the temperature further in steps of ∆T = 100 • C results in a new qualitative change of spectrum at T = 1200 • C.More pronounced peaks emerge with increased temperature.At the highest temperature (Fig. 2c), a few pronounced peaks become apparent.They coincide with vibrational signatures of the cristobalite 18 and tetragonal zirconia 19 phases, which are formed at high temperatures 20,21 .
Figure 3 shows X-ray diffraction (XRD) data of annealed powder samples.At the lowest treatment temperature T = 1000 • C, broad peaks indicate formation of a glassy amorphous phase and only initial seeds of a crystalline phase.Also, the high background of XRD signal confirms a substantial amount of amorphous material.As the temperature is increased, the peaks become exceedingly pronounced distinguishably showing an increasing dominance of the crystalline phase.The XRD peaks match well with reference data for cristobalite 22 and tetragonal zirconia 23 , proving that annealed material is a mix of inorganic crystalline phases.From the XRD pattern, the period d which corresponds to the most pronounced peaks at the diffraction angle 2θ , given by the Bragg's condition d = λ /(2 sin θ ) can be estimated.The size L of the nano-crystalline phase follows from the Scherrer's equation L = Kλ /(B(2θ ) cos θ ; where K = 0.89 for spherical crystals and B(2θ ) [rad] is the peak's angular bandwidth at full width half maximum and the Cu K α emission at λ = 1.5406Å was used.The most pronounced peaks are observed after treatment at T = 1500 • C at Bragg diffraction angle 2Θ ≈ 30.14 • corresponds to the crystallite size of L 117.5 nm for the t-ZrO 2 and similarly at the angle 2Θ ≈ 21.82 • the size of cristobalite nano-crystallites was L 30.8 nm.A size evolution of the t-ZrO 2 crystallites traced by the most prominent 111 XRD peak was L ≈ 1, 14, 118 nm as temperature was increasing trough 1000, 1200, 1500 • C, respectively.As for the geometrical stability of the structures, they remain stable at least up to T ≤ 1200 • C.After which, the structures appear to show signs of melting and deformation.Sharp features become noticeably rounded by surface tension of the molten phase.We show that for the primary "glassy" phase the structures retain their shape without shape distortions (Fig. 4).The effect of much higher temperatures is shown in the optical images in Fig. 2a.

Conclusions and Outlook
It is shown that by a high-temperature calcination of 3D polymerized structures, initially made by 3D laser writing in the organic-inorganic SZ2080 polymer resist produces either silica-based glass or a polycrystalline ceramic pure inorganic material.Glass phase dominate in the samples annealed at moderate temperatures up to ∼ 1200 • C. When samples were annealed above ∼ 1200 • C, formation of the polycrystalline silica and zirconia was observed, in particular, the cristobalite and t-zirconia phases.
The presented modifications of silica-zirconia-rich resist SZ2080 from glass to polycrystalline ceramic by annealing shows a principle of the thermally guided 3D material printing which has nanoscale resolution.Isotropic down-sizing of the initial 3D polymerized objects with a volume fraction of 0.5-to-1 simplifies fabrication since there is no need to alter proportions of the initial material as it is widely used in DLW 3D nanolithography of photonic crystals, micro-optics and biomedical scaffolds in order to eliminate the effect of anisotropic shrinkage.
One can foresee a possibility to create required polymerizable mixtures which will lead to the final compounds of tailored composition -stoichiometry and size of polycrystalline phases according to the phase diagram 24,25 .Mechanical and chemical properties of the final structures and objects will acquire new features, especially resilience at harsh physical and chemical environments.Since nanoscale materials can initiate precipitation and guide growth of nano-crystallites, a wide field for experimentation horizons are widened by the presented modality of additive manufacturing.
after heat treatment as shown in the SEM micro-graphs S.Fig.3. Some random cracks can be observed in the annealed structures, however, since the structures were not attached to the substrate and had to be handled multiple times when transferring them mechanically from substrate to substrate (glass substrate -fabrication, corundum substrate -furnace, adhesive carbon tape -SEM imaging) we cannot exclude the cause of cracks being due to a mechanical strain from handling.
We summarize that for the particular case of the DLW structures having feature sizes smaller than 3 µm, they are strongly affected by the thermal re-flow process, but the structures with features larger than approximately 10 µm rescaled by maintaining the original outline shape.

Figure 1 .
Figure1.Illustration of the main steps in synthesis of ceramics out of hybrid SZ2080 followed from laser induced polymerization that occurs during direct laser writing.During first stage of calcination, organic part is removed from the matrix and an inorganic glass matrix forms.As temperature is increased further, crystallization occurs and polycrystalline ceramic phase forms.Crystal structure of cristobalite and t-ZrO 2 are shown in bottom row.

2 / 9 PreprintsFigure 2 .
Figure 2. Raman spectrum of laser structured SZ2080 before and after heat-treatment (for 1 h in air).Disappearance of sharp peaks represents the changed make-up of the material after heat-treatment.(a) Change of Raman spectrum at several annealing temperatures; optical images of 3D-micro scaffolds (T = 1000, 1400 • C) from the sites where Raman spectra were measured.(b) Detailed spectrum of the initial structure and heat treated structure at T = 1000 • C in comparison to the fused silica.(c) Raman spectrum after the highest temperature T = 1400 • C annealing with peak matching to the cristobalite and t-ZrO 2 .

3 / 9 PreprintsFigure 3 .
Figure 3. X-ray diffraction (XRD) analysis of SZ2080 resist heat treated at different temperatures for 1 hours in air in ambient pressure.Broad peaks become more pronounced and evolve with temperature into sharp signature peaks of the two known crystalline phases of silica and zirconia.

Figure 4 .
Figure 4. Micro-graphs of different initial and treated structures (1000 • C for 2h).Down-sizing of solid volumetric and free-form structures (with correspondingly high and low initial volume fractions of polymer).From top to bottom: (a) a free-form sculpture Vytis (Coat of arms of Lithuania), (b) homogeneous cube structure, (c) photonic crystal (periodic) structure with cage and (d) hexagonal scaffold.

Figure 2 .
Figure 2. SEM micrographs of a fine-feature ballerina dancer (free-form) sculpture * : untreated (a) and thermally annealed ceramic structure (b).Also, a periodic 3D grating with a cage is shown in a glassy phase (c) and in a ceramic phase (d).

8 / 9 PreprintsFigure 3 . 9 / 9 Preprints
Figure 3. SEM micrographs of large areas scaffold after heat treatment at different magnifications; temperature and duration of annealing are indicated on the SEM images.