Glassy carbon is a special form of carbon that is non-graphitisable, meaning that even at temperature extremes it cannot be converted into crystalline graphite. As its name suggests, glassy carbon carries a combination of ceramic and glassy characteristics. Its chemical structure is made up completely of sp2-bonded carbon atoms, with some experts suggesting that its structure is related to that of fullerene.
Properties of glassy carbon
High resistance to heat
Considering that this material is produced from high-temperature input, it follows that glassy carbon is highly heat-resistant. Its strength increases further when subjected to heat of up to 2400°C, and it doesn’t become brittle after this kind of heat exposure.
With zero open porosity and a permeability coefficient of 1E-9 cm²/s, it is as impermeable as silica glass material and has low permeability to gases.
Owing to its tightly knit atomic structure, this material has a uniquely glossy surface that works well in permeation-prohibited systems and sensory electrodes. Because of this glassy exterior, the material is easy to clean and polish, thereby maintaining its lifespan longer.
Exceptional strength and material hardness
At 480 MPa compressive strength and 230 Vickers hardness, this material is likened to high-performance ceramics.
High resistance to corrosion
The tightness of the atomic bonds in glassy carbon prevents entry or insertion of external compounds (even fluids). This gives the material a natural, self-protecting property against corrosion, whether alkali or acid.
This material also has the highest oxidation resistance of all carbon forms, even when exposed to concentrated nitric acid or pure oxygen.
High degree of purity
Unlike alloy metals, this material is made of pure carbon.
Its raw density is relatively low at around 1.5 g/cm3. This may be attributed to the interstitial voids present within its chemical structure.
- Electrical resistivity: 4.5E-5 Ω·m at 30°C
- Thermal conductivity: 6.3 W/(m·K) at 30°C
- Elastic modulus: 35 GPa at 20°C
- Coefficient of thermal expansion: 2.6E-6 1/K at 20°C
Production and processing
Glassy or vitreous carbon may be produced from pyrolysis of furfuryl alcohol, phenolic resins, and similar polymers under temperatures that can reach up to 3000°C.
At the lower heat register of 300°C, these precursor materials start to lose hydrogen and other non-carbon components to leave a purer form of carbon. By 1000°C, virtually all the non-carbon components have become eradicated. This leaves pure carbon polymers which form the building blocks for glassy carbon.
The material may be processed further into its activated form. The latter has an atomic structure containing voids that are more intersected than the natural form of glassy carbon.
More recent developments in processing have led to the discovery of glassy carbon with boosted strength but a higher degree of elasticity. This was achieved by adding extreme pressure – roughly 250,000 times the standard pressure – and slightly elevated temperatures of close to 1000°C.
Applications of glassy carbon
The unique properties of glassy carbon make it a suitable material for a wide variety of uses. Its most popular application is in electrochemistry in the form of glassy carbon electrodes (GCEs), which may be used for sensors. GCEs are used in potentiometric analysis such as acid-base titrations, as well as in the biomedical field.
Compared to gold, glassy carbon has a wider window of electrochemical activity when immersed in water. Because of this, an electrode made of glassy carbon has less sensitivity to interference than an Au electrode.
Due to its zero open porosity, high degree of impermeability, and mirror-like surface, glassy carbon is perfect for vacuum systems and environments where fluid permeability is prohibited.
Meanwhile, its extreme heat resistance lends itself useful in temperature management, such as in crucibles and furnaces.
Other ideal areas of application include metallurgy, semiconductor and electronics, microanalysis, aerospace, nuclear sciences, laboratory-based research, and ultratrace analysis.