# Glassy carbon

A large sample of glassy carbon, with 1 cm3 graphite cube for comparison
A small rod of glassy carbon
Vitreous-glassy carbon crucibles

Glass-like carbon, often called glassy carbon or vitreous carbon, is a non-graphitizing, or nongraphitizable, carbon which combines glassy and ceramic properties with those of graphite. The most important properties are high temperature resistance, hardness (7 Mohs), low density, low electrical resistance, low friction, low thermal resistance, extreme resistance to chemical attack and impermeability to gases and liquids. Glassy carbon is widely used as an electrode material in electrochemistry, as well as for high temperature crucibles and as a component of some prosthetic devices, and can be fabricated as different shapes, sizes and sections.

The names glassy carbon and vitreous carbon have been introduced as trademarks; therefore, IUPAC does not recommend their use as technical terms.[1]

Vitreous carbon can also be produced as a foam. It is then called reticulated vitreous carbon (RVC). This foam was first developed in the mid to late 1960s as a thermally insulating, microporous glassy carbon electrode material. RVC foam is a strong, inert, electrically and thermally conductive, and corrosion resistant porous form of carbon with a low resistance to gas and fluid flow. Due to these characteristics, the most widespread scientific use of RVC is in electrochemistry (Sumita, 1985; Friedrich et al., 2004; Rogulski et al., 2004). Additionally, RVC foams are characterized by an exceptionally high void volume, high surface area, and very high thermal resistance in non-oxidising environments, which allows for heat sterilization and facilitates manipulation in biological applications.

## History

Glassy carbon was first observed in the laboratories of The Carborundum Company, Manchester, UK, in the mid-1950s by Bernard Redfern, a materials scientist and diamond technologist. He noticed that Sellotape he used to hold ceramic (rocket nozzle) samples in a furnace maintained a sort of structural identity after firing in an inert atmosphere. He searched for a polymer matrix to mirror a diamond structure and discovered a resole resin that would, with special preparation, set without a catalyst. Using this phenolic resin, crucibles were produced. Crucibles were distributed to organisations such as UKAEA Harwell.

Bernard Redfern left The Carborundum Co., which officially wrote off all interests in the glassy carbon invention. While working at the Plessey Company laboratory (in a disused church) in Towcester, UK, Redfern received a glassy carbon crucible for duplication from UKAEA. He identified it as one he had made from markings he had engraved into the uncured precursor prior to carbonisation. (It is almost impossible to engrave the finished product.) The Plessey Company set up a laboratory first in a factory previously used to make briar pipes, in Litchborough, UK, and then a permanent facility at Caswell, near Blakesly, UK. Caswell became the Plessey Research Centre and then the Allen Clark Research Centre. Glassy carbon arrived at the Plessey Company Limited as a fait accompli. Redfern was assigned J.C. Lewis, as a laboratory assistant, for the production of glassy carbon. F.C. Cowlard was assigned to Redfern's department later, as a laboratory administrator. Cowlard was an administrator who previously had some association with Silane (Silane US Patent assignee 3,155,621 3 Nov 1964). Neither he nor Lewis had any previous connection with glassy carbon. The contribution of Bernard Redfern to the invention and production of glassy / Vitreous carbon is acknowledged by his co-authorship of early articles.[2] But references to Redfern were not obvious in subsequent publications by Cowlard and Lewis.[3] Original boat crucibles, thick section rods and precursor samples exist.

Redfern's UK patent application were filed on 11 January 1960 and Bernard Redfern was the author of US patent US3109712A, granted 5 November 1963, priority date 11 January 1960, filing date 9 January 1961.[4] This came after the rescinded British patent. This prior art is not referenced in US patent 4,668,496, 26 May 1987 for Vitreous Carbon. Patents were filed "Bodies and shapes of carbonaceous materials and processes for their production" and the name "Vitreous Carbon" presented to the product by the son of Redfern.

Glassy/Vitreous Carbon was under investigation used for components for thermonuclear detonation systems and at least some of the patents surrounding the material were rescinded (in the interests of national security) in the 1960s.

Large sections of the precursor material were produced as castings, moldings or machined into a predetermined shape. Large crucibles and other forms were manufactured. Carbonisation took place in two stages. Shrinkage during this process is considerable (48.8%) but is absolutely uniform and predictable. A nut and bolt can be made to fit as the polymer, processed separately and subsequently give a perfect fit.

Some of the first ultrapure samples of Gallium Arsenide were zone refined in these crucibles. (Glassy carbon is extremely pure and unreactive to GaAs).

Doped/impure glassy carbon exhibited semiconductor phenomena.

Uranium carbide inclusions were fabricated (using U238 carbide at experimental scale).

On October 11, 2011, research conducted at the Carnegie Geophysical Laboratory led by Stanford’s Wendy L. Mao and her graduate student Yu Lin described a new form of glassy carbon formed under high pressure with hardness equal to diamond, a kind of diamond-like carbon. Unlike diamond, however its structure is that of amorphous carbon so its hardness may be isotropic. Research is ongoing.[5]

## Structure

The structure of glassy carbon has long been a subject of debate. Early structural models assumed that both sp2- and sp3-bonded atoms were present, but it is now known that glassy carbon is 100% sp2. However, more recent research has suggested that glassy carbon has a fullerene-related structure.[6]

Note that glassy carbon should not be confused with amorphous carbon. This from IUPAC: "Glass-like carbon cannot be described as amorphous carbon because it consists of two-dimensional structural elements and does not exhibit ‘dangling’ bonds."[1]

It exhibits a conchoidal fracture.

## Electrochemical properties

Glassy carbon electrode (GCE) in aqueous solutions is considered to be an inert electrode for hydronium ion reduction:[7]

${\displaystyle {\ce {\overbrace {H3O+_{(aq)}} ^{hydronium}+e^{-}<=>[{\ce {GCE}}]H._{(aq)}}}}$       ${\displaystyle E^{\ominus }=-2.10\ \mathrm {V} }$ versus NHE at 25 °C

Comparable reaction on platinum:

${\displaystyle {\ce {{H3O+_{(aq)}}+Pt_{(s)}+e^{-}<=>Pt:H_{(s)}}}}$       ${\displaystyle E^{\ominus }=\ 0.000\mathrm {V} }$ versus NHE at 25 °C

The difference of 2.1 V is attributed to the properties of platinum which stabilizes a covalent Pt-H bond.[7]

## Physical properties

Properties include 'high temperature resistance', hardness (7 Mohs), low density, low electrical resistance, low friction, and low thermal resistance.[citation needed]

Due to their specific surface orientation, glassy carbon is employed as an electrode material for the fabrication of sensors. Glassy carbon paste, glassy carbon, carbon paste etc. electrodes when modified are termed as chemically modified electrodes.