Silica glass is defined as a type of glass with a simple structure composed of slightly distorted SiO4 tetrahedra linked together at corners, with each oxygen serving as a bridging oxygen between neighboring tetrahedra. Historically the most common and familiar form of glass is clear (optically transparent) silica glass which is composed largely of silicon dioxide (SiO2). The definition of glass does not restrict either the composition or the optical properties of the material, implying a wide variety of different materials that are considered glass. In fact, theoretically, any crystalline solid that can be brought to a liquid state, can be forced into an amorphous solid state through rapid solidification via extraordinary cooling rates. Non-silica glasses, in particular metallic glasses, can obtain unique electric, optical, or thermal properties from their crystalline counterparts through glassification. Non-metallic glasses can obtain similarly unique properties by adjusting elemental compositions and introducing dopants.

Structure and Properties of Dense Silica Glass

In the amorphous form, silica glass has become a prototype system for understanding the disordered state. The study of silica glass under high pressure is particularly important and challenging as it has been used as a zeroth order model of silicate magma in the earth's interior. Numerous theoretical and a variety of experimental techniques have been employed to characterize the complex and sometimes anomalous behaviour of silica glass. In the amorphous form, silica glass has become a prototype system for understanding the disordered state. The study of silica glass under high pressure is particularly important and challenging as it has been used as a zeroth order model of silicate magma in the earth's interior. Numerous theoretical and a variety of experimental techniques have been employed to characterize the complex and sometimes anomalous behaviour of silica glass. The transformation from 4- to 6-fold coordination is not direct. The change in the Si environment commences at 12 GPa via the formation of substantial 5-fold coordinated local structures and largely completed at 60 GPa. The abrupt increase of acoustic velocity around 120 GPa was reproduced and related to a slight increase in coordination number higher than 6.[1]

Is the second peak indicative of 6-fold coordination? The O-K absorption spectrum is a probe of the O 2p projected density of states (PDOS) in the conduction band. In quartz, the Si is 4-fold coordinated. The average (longitudinal and transverse) sound velocity of silica glass at high pressure can be estimated from the theoretical bulk modulus and density determined from the calculated equation of states. Clear changes in the slope in the plot of acoustic velocity against pressure are observed at ca. 30–40 and at 130–150 GPa. These changes have been attributed to the change in CN from 4→6 and 6→6+, respectively. The theoretical results support this interpretation. A unified explanation of the electronic, acoustic and structure of silica glass based on the structure of silica glass obtained from First-Principles molecular dynamics calculations was presented. Apart from reproducing the experimental results connections between the electronic and acoustic properties and the structure have been established. The theoretical results provide a clear picture on the compression mechanism of silica glass and emphasize on the existence of 5- fold coordination of Si in it. The appearance of a second band in the XRS may not be used as the indication of the occurrence of 6-fold coordination. A dense polymorph with higher than 6-fold coordination may exist above 140 GPa in silica glass.

Three-dimensional printing of silica glass with sub-micrometer resolution

Modern life is unthinkable without glass, with applications ranging from glassware and windows to optical lenses and fibers. Silica glass has excellent material properties such as thermal and chemical stability, hardness, and optical transparency in a wide wavelength range. Yet, as a result of the stability and the brittleness of it, fabricating three-dimensional (3D) silica glass objects with features at the micrometer scale remains challenging, although structures at this scale are critical for many exciting applications, for example, in nanophotonics, nanoelectromechanical systems, and nanofluidics. The mandatory sintering process at such elevated temperatures severely limits the application space and integration compatibility of these methods. This is because any substrate materials or pre-manufactured structures onto which the 3D-printed silica-glass structures are to be directly integrated must withstand the thermal treatment, which essentially eliminates most materials of interest. In this work, we report a process for 3D printing of silica glass that is solid and optically transparent as-printed and features sub-micrometer resolution. In this process, we take advantage of our finding that hydrogen silsesquioxane without any additives can be selectively crosslinked into silica glass in 3D by exposure to sub-picosecond laser pulses with a wavelength of 1040?nm, which is a nonlinear absorption process as HSQ has no linear sensitivity to light with wavelengths above 248?nm.[2]

Taken together, the results in the present work show that our 3D printing technology makes it possible to additively manufacture transparent and solid 3D silica glass structures with sub-micrometer features on a substrate surface without the need of thermal post-processing. These capabilities are going well beyond the capabilities of existing surface micromachining techniques, including those that utilize growth, deposition, lithography, etching, and liftoff of silica glass layers and those that use direct cross-linking of HSQ via linear-absorption of electrons or deep UV light. While beyond the scope of this work, further investigation of the mechanism behind multiphoton cross-linking of HSQ to silica glass using sub-picosecond laser pulses will be of interest for both research and applications. Such investigations could provide insights that may help to reduce the high ratio of 4-membered silicon-oxygen rings and avoid photoluminescence in the as-printed glass, and that could contribute to a deeper understanding of the light-matter interaction in transparent materials.

Elastic Moduli of Permanently Densified Silica Glasses

An extensive Brillouin Light Scattering study on permanently densified silica glasses after cold compression in diamond anvil cell has been carried out, in order to deduce the elastic properties of such glasses and to provide new insights concerning the densification process. In order to discuss the permanent structural changes responsible for such evolution of the elastic moduli, first let us recall some general aspects concerning the densification process. At ambient pressure, silica glass has an open three-dimensional network structure, consisting of corner-shared SiO4 tetrahedra which forms wide Si–O–Si bond-angles and ring size distributions. [3]

In order to compare the structure of silica glass at similar densities during in situ experiments and after permanent densification, data displays simultaneously in situ and ex situ Raman spectra at ρ = 2400 Kg.m?3 and ρ = 2660 Kg.m?3. Using Brillouin spectroscopy, we have shown herein that after cold compression the longitudinal wave velocity decreases with the densification ratio at the beginning of the permanent densification. Such behavior was also reported recently on SiO2 samples heated and quenched at high pressure in a multi-anvil apparatus. In the recent article of Keryvin et al. which proposes a mechanical model of silica glass under hydrostatic pressure, the authors use a linear function to describe the evolution of the elastic moduli versus densification ratio. It is also specified by the authors that the lack of information between 4% and 20% prevents a more accurate law. Our results clearly reveal that the evolution of elastic moduli is far from linear. Taking into account the polynomial laws proposed previously would considerably improve such modelling, whether for hydrostatic compression or indentation.

References

[1]Wu M, Liang Y, Jiang JZ, Tse JS. Structure and properties of dense silica glass. Sci Rep. 2012;2:398. doi: 10.1038/srep00398. Epub 2012 May 8. PMID: 22570763; PMCID: PMC3347315.

[2]Huang PH, Laakso M, Edinger P, Hartwig O, Duesberg GS, Lai LL, Mayer J, Nyman J, Errando-Herranz C, Stemme G, Gylfason KB, Niklaus F. Three-dimensional printing of silica glass with sub-micrometer resolution. Nat Commun. 2023 Jun 7;14(1):3305. doi: 10.1038/s41467-023-38996-3. PMID: 37280208; PMCID: PMC10244462.

[3]Deschamps T, Margueritat J, Martinet C, Mermet A, Champagnon B. Elastic moduli of permanently densified silica glasses. Sci Rep. 2014 Nov 28;4:7193. doi: 10.1038/srep07193. PMID: 25431218; PMCID: PMC4246209.