The list of terms used to describe various silica-based materials is confusing, long and often misunderstood; these terms include fused silica, silica, quartz glass, fused quartz and quarts.
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This article will explore the unique properties of fused silica and quartz, as well as a few related materials to clear up the confusion surrounding these terms.
Quartz vs. Silica
One of the most important things to know regarding fused silica and quartz is that they both primarily consist of silica, also known as silicon dioxide. Silica is the primary constituent of most types of glass and has the chemical formula SiO2.
Mineral quartz is the main form in which silica is found in nature; mineral quartz makes up an appreciable fraction of the Earth’s crust and is a hard, transparent crystalline material. Besides silica, quartz also contains naturally occurring impurities in various proportions that are dependent on its geological origin.
‘Silica’ refers to a specific chemical compound, silicon dioxide, with the chemical formula SiO2. Meanwhile, quartz is a naturally occurring crystalline mineral that primarily consists of silica but contains some impurities.
Crystalline and Amorphous Solids
In order to understand the differences between different silica-based materials, there first needs to be an understanding of the fundamental differences between amorphous solids and crystalline solids.
The way that the atoms are arranged inside the solids is how to define the difference. The constituent atoms are arranged in regular, repeating patterns known as crystal lattices in a crystalline solid. One example of a crystalline silica-based material is quartz: oxygen and silicon atoms are arranged in a well-defined ordered structure.
The atoms in an amorphous solid, however, have no long-range order. In an amorphous solid, the seemingly random arrangement of molecules resembles those of a liquid, except that they do not move around and are instead fixed in place.
Amorphous solids make up most materials that we think of as “glass”. Indeed, the term “glassy” can be used to describe any material with an amorphous atomic structure.
A material’s characteristics can be profoundly influenced based on whether its atoms are oriented randomly or are arranged in an orderly manner. The glass transition effect exhibited by amorphous solids is one of the most striking examples of this.
Beyond the realm of silica or other oxide-based materials, disordered “glassy” metals are often chosen for use because of their unusual mechanical characteristics when compared to other conventional metals.
Quartz and other silica-based materials can be characterized both in terms of whether they are amorphous or crystalline, as well as by their chemical composition.
Defining Silica-Based Materials
Now that the fundamentals have been established, this paper will explore the differences between fused silica, quartz and other silica-based materials.
Quartz
Quartz, as mentioned above, is the main form of silica that occurs naturally. Quartz is a crystalline solid, meaning that it has very distinct properties from glass while it still resembles glass both in terms of its chemical makeup and appearance.
There are limited industrial applications of quartz (meaning the crystalline mineral). However, quartz crystal oscillators are found in electronic systems, most familiarly in wristwatches.
Industrial quartz applications sometimes use manufactured “synthetic quartz”. This material perhaps would be more accurately described as crystalline silica but is often simply called “quartz.”
Fused Silica and Fused Quartz
The word “fused” here refers to a processing step: fused silica is nominally pure silica that has been melted and cooled to form a glassy, amorphous solid.
Fused silica does not contain any additives while still resembling other glasses in many ways. Fused silica, as a specialty material, has several high-performance applications.
There is often interchangeable use of the terms “fused quartz” and “fused silica”. However, “fused quartz” more specifically refers to an amorphous solid formed by melting naturally occurring quartz. This means that while fused silica is ostensibly pure SiO2, fused quartz contains impurities dependent on what quartz was used.
Silica Glass and Quartz Glass
Both terms are often used in more generic ways and are usually considered interchangeable. These terms could both refer either to fused quartz or fused silica.
Applications of Fused Silica
The amorphous structure of fused silica gives it several highly desirable and distinct electrical, mechanical and thermal properties while still being chemically similar to quartz.
It is common for glasses to contain additives like alkaline Earth, alkali or other oxides to improve physical and chemical properties and to lower the glass processing (melting) temperature; however, fused silica is very pure. This results in it having higher working temperatures while offering different characteristics from other glasses.
Fused silica does not expand or contract much when heated or cooled because it has a very low coefficient of thermal expansion. This means that fused silica can withstand very rapid heating or cooling without cracking and is highly resistant to thermal shock.
The thermal characteristics of fused silica make it highly valuable for high-temperature industrial components such as glass manufacture, boats for steelmaking, trays and crucibles.
There is a very wide spectrum of light in which fused silica is transparent, extending between far-infrared to deep ultraviolet. This makes fused a key component in a range of lenses, mirrors and other UV- or IR-transmitting optics, as well as in optical fibers.
Fused silica is resistant to most acids (excluding hydrofluoric acid) and is also extremely chemically inert. Being chemically inert lends fused silica to biomedical applications, often in the form of porous silica.
The combination of strength, transparency and thermal stability makes fused silica a strong candidate for developing new applications, including etched microwave circuits, photolithography substrates and as a protective layer in semiconductor devices.
References
- Glassy metal set to rival steel : Nature News. https://www.nature.com/news/2011/110109/full/news.2011.4.html.
- Vert, T. Refractory Material Selection for Steelmaking. (John Wiley & Sons, 2016).
- Khalaf, A. L., Shabaneh, A. A. A. & Yaacob, M. H. Carbon Nanotubes and Graphene Oxide Applications in Optochemical Sensors. in Synthesis, Technology and Applications of Carbon Nanomaterials 223–246 (Elsevier, 2019). doi:10.1016/B978-0-12-815757-2.00010-3.
- Wang, S., Zhou, C., Zhang, Y. & Ru, H. Deep-etched high-density fused-silica transmission gratings with high efficiency at a wavelength of 1550 nm. Appl. Opt. 45, 2567 (2006).
This information has been sourced, reviewed, and adapted from materials provided by Mo-Sci Corp.
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