Vitreous Carbon

Vitreous Carbon is a ceramic carbon based material. It has the appearance of black glass and it is fragile. It has a closed micro-porosity and is impervious to gas.  Vitreous Carbon has good resistance to thermal shock because of a relatively good thermal conductivity. Vitreous Carbon is sometimes referred to as Glassy Carbon.

Vitreous Carbon is related to graphite crystal but with a very large disorder and submicron voids. It has a low density of 1.51 gm/cc as compared with 2.25 gm/cc for theoretical density of graphite crystal. Vitreous Carbon is unusual in that when it is treated at 2500°C and over it very slightly modifies its crystallographic structure.

Almath Crucibles offers Vitreous Carbon in a Grade A quality only. The operating temperature, when heat treated, of Grade A Vitreous carbon is 2500°C. The normal operating temperature for it is 2000°C and it may be used up to 2800°C in a non-oxygen atmosphere but at this maximum temperature some of the properties of Vitreous Carbon may change.

Grade A is the most useful and the most common grade.

The Grade A of Vitreous Carbon is more stable in dimension during thermal cycles and is preferred when sharp variations of temperature can occur or when maximum operating temperature is above 1000°C.

Grade A Properties
Value
Maximum Service Temperature 2000 oC
Electrical Resistivity 42 x 10-4 ohm cm
Apparent Porosity 1.0 – 3.0%
Coefficient of Thermal Expansion (20 - 200oC) 2.0 - 2.2 x 10-6/oC
Thermal Conductivity (30o) 5.8 W/mK
Apparent Density 1.51 gm/cc
Ash Content < 5 ppm
Permeability Coefficient 10-10 ~ 10-12 cm2/sec
Shore Hardness 100 - 1101
Young's Modulus1 29,500 – 32,400 MPa
Flexural Strength1 147 MPa
Charpy Impact Value 2.1 – 3.6 kgf.cm/cm2

1 4-point bending test; geometry of specimen: circular rod, diameter 3 mm, length 60 mm

Chemical Corrosion Resistance of Vitreous Carbon

  • Hydrofluoric Acid at 40%, 25oC, weight loss < 1% in 1 month
  • Sulphuric Acid at 95%, 150oC, weight loss 0.2% in 6 months
  • Nitric Acid at 52%, 75oC, weight loss 2.5% in 6 months
  • Nitric Acid at 93%, 25oC, weight loss < 1% in 1 month
  • Crucible used for melting at 800oC directly on a Meker torch
  • With soda carbonate mixing (Na + K), pyrosulphate - no erosion within the crucible, the weight loss by outside oxidation is about 50 mg in 30 minutes for a crucible 35 mm diameter x 35 mm (weight 10 grams)

Vitreous Carbon is very resistant to attack by the following reagents: bromine, hydrochloric, hydrofluoric, nitric, sulphuric and chromic acids, fused caustic alkalis, fused potassium bisulphate, fused calcium fluoride and nitric acid and hydrogen peroxide, nitric acid and potassium chlorate.

Some of the applications of Vitreous (glassy) Carbon are:

  • Preparation and crystal growth of Group III-V semiconductors.
  • Production of oxygen free single crystal silicon.
  • Spectrographic and polarographic electrodes.
  • Laboratory beakers, basins, boats and containers for handling acids and caustic alkalis.
  • Glass and carbon joints and pieces, mandrels and other equipment for glass work.
  • Steam fuel and rocket nozzles.
  • Surgical and Medical applications.
  • Microphone electrodes and relay contacts.
    and many others...