Tikalon Blog by Dev Gualtieri

Transparent Amorphous Oxide

January 29, 2018

Fake news is not a recent phenomenon. From childhood, I've seen many impossible stories in the tabloids arrayed at supermarket checkouts. Also, several alternative facts were a part of my childhood science education. One of these was the idea that glass was a liquid that continued to flow long after it cooled to room temperature.

Glass is not a crystalline solid as are most of the inorganic materials that we encounter daily. Examples of these crystalline solids are metals and table salt. The atoms in crystals are regularly arrayed in a lattice structure, while the atoms in glass are randomly located, in analogy to the atoms in a liquid. So, a glass must be a liquid, right? The cited "proof" of glass being a liquid is that the stained glass panels in centuries-old cathedrals are thicker at the bottom, so the glass flowed in response to gravity just as a liquid would.

Glassmaking started before 3000 BC. As noted in Pliny's Natural History, both furnace technology and glassmaking were extensively developed in the 1st century AD

An image of Hasdrubal Aufdinger, the glaser, from the house book of Landauer Zwölfbrüderstiftung, vol. 1 (Nuremberg 1511-1706).

(Collection of the City Library Nuremberg, via Wikimedia Commons)

As it turns out, the varied thickness of the glass is just an artifact of how the glass was manufactured. The artisans just arranged the thicker part at the bottom by artistic convention. As explained on the website of the Corning Glass Museum, the calculated flow of glass is infinitesimally small.[1] I visited the Corning Glass Museum, which is a three hour's drive from my home town, when I was in high school. This was perhaps one catalyst for my career in materials science.

So, where do we place the neither fish nor fowl, neither crystalline solid nor liquid, material called glass? Glass is an amorphous solid, a solid that's a frozen image of a liquid. Metals can be produced as glasses, aptly named metallic glass. One commercially important metallic glass was developed by AlliedSignal (now, Honeywell) and is trademarked as Metglas®. Metallic glasses have magnetic properties that make them suitable for transformer cores and magnetic shielding (see photo).

Metglas® transformer cores.

These cores are the SA1 composition with boron (1-5%), iron (85-95%), and silicon (5-10%).[2]

(photo by the author.)

Metallic glasses closely resemble metals, so they don't possess the transparency that makes most glass valuable. There's a wide range of transparent glass compositions, each developed to serve a particular function. These functions can be as simple as window glazing, or as technically advanced as conductive glass for computer and cellphone displays. The following table gives a few examples of transparent glass compositions.

Name	Major Components	Uses
Soda-lime glass	SiO₂/Na₂O/CaO/Al₂O₃	Window glass, bottles
Flint glass	SiO₂/Na₂O/CaO/PbO or BaO	Lenses
Crown glass	SiO₂/Na₂O/CaO	Lenses
Borosilicate glass	SiO₂/B₂O₃/Na₂O/Al₂O₃	labware
Aluminosilicate glass	Al₂O₃/SiO₂	labware
Fused quartz	SiO₂	labware
ITO	In₂O₃/SnO₂	Displays

While an amorphous, glassy, structure can be locked-in at room temperature, heating of amorphous materials allows the movement of atoms and attendant crystallization. In the case of metallic glasses, heating to high temperatures allows the formation of nanoscale crystallites in the amorphous matrix that affect material properties such as heat capacity, electrical conductivity and magnetism. Magnetic metallic glasses have typically low coercivity, but creation of nanocrystallites can increase coercivity substantially. Crystallization will convert an originally transparent amorphous polymer to a cloudy, translucent material.

Integrated optical devices need to be made from transparent materials that resist changes in their properties during high temperature processing. Such thermal treatment can be required to deposit or anneal a material in an integrated optical device, causing glassy transparent oxides to crystallize and ruin their transparency. To overcome this problem, a team of engineers and materials scientists from the Toyohashi University of Technology (Toyohashi, Aichi, Japan), the Japan Science and Technology Agency, Precursory Research for Embryonic Science and Technology (Saitama, Japan), and the Massachusetts Institute of Technology (Cambridge, Massachusetts) has investigated a high refractive index thermally-stable amorphous tantalum-yttrium oxide with low infrared absorption for use in magneto-optical devices.[3-4]

The amorphous tantalum yttrium oxide is formed by the addition of a small amount of yttrium oxide (Y₂O₃) to tantalum oxide (Ta₂O₅). Tantalum oxide is widely used as a high refractive index material since it's a thermally stable oxide with high optical transmissivity.[3] While tantalum oxide has a 300°C higher crystallization and glass-transition temperature than other high refractive index oxides such as titanium oxide (TiO₂) and hafnium oxide (HfO₂), it's still not stable at the 800 °C oxygen anneal required for magneto-optical garnet crystallization. It's only after such crystallization that these garnets exhibit high Faraday rotation.[3]

Crystallization is sometimes suppressed by adding elements of a smaller ionic radius to a host material. For example, the crystallization temperature of HfO₂ is increased when La₂O₃ is added, lanthanum being larger than hafnium.[3] To suppress the crystallization of Ta₂O₅, the larger yttrium ion in the form of Y₂O₃ can be added.[3] This was the approach taken in the current study with the addition of about 14% Y₂O₃, which was enough to increase the crystallization temperature to about 850°C.[4]

Magnetron sputtering was used to fabricate layers of the high refractive index amorphous yttrium-tantalum oxide as part of a dielectric mirror used in an iron garnet magneto-optical device operating at near-infrared wavelengths.[3] The magnetic garnet required thermal treatment at about 750°C.[4] Aside from magneto-optical devices, the tantalum-yttrium oxide material would be useful for spintronic, magnonic, and multiferroic devices as well.[3] Says Taichi Goto, an assistant professor at the Toyohashi University of Technology,

"We used amorphous tantalum yttrium oxide to form a dielectric mirror and combined it with magnetic garnet. Actually, other than magnetic garnet, there are more materials that have not been combined with dielectric mirrors because of the high-temperature thermal treatment that is required. I hope that our findings will help make such materials usable too."[4]

Element maps of Ta, Y, and O in an amorphous Ta-Y-oxide.

Element maps of tantalum, yttrium, and oxygen in an amorphous Ta-Y-oxide. These maps, created using the EDX technique, demonstrate the amorphous nature of the oxide. (Portion of fig. 5 of Ref. 3, licensed under the Creative Commons Attribution 4.0 International License.)

Transparent Amorphous Oxide

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