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
|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
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.