Heat treatment – graphitising
The final step in graphite manufacture is a conversion of baked carbon to graphite called graphitising, i.e. heat-treating the material at temperatures in the region of 2600 C – 3300 C. During the graphitising process, the more or less pre-ordered carbon (turbostatic carbon) is converted into a three-dimensionally ordered graphite structure. Depending on the raw materials and the processing parameters, various degrees of convergence to the ideal structure of a graphite single crystal are achieved. Since graphitisation increases the lattice order and produces smaller layer distances, it simultaneously leads to a considerable growth of ordered domains. However, the degree of order that can be reached depends largely on the crystalline pre-order of the solid used. These reduced lattice layer distances are macroscopically noted as a contraction in volume. This graphitisation-shrinkage is approximately 3 to 5%. Due to this shrinkage, density of the graphite increases.
General: The Graphitisation Process
• The final step in the production of graphite is the graphitisation process
• Amorphous or baked carbon is converted to electrographite by a thermal treatment at approximately 3000C
• Essentially any amorphous carbon material can be graphitised. The potential crystallite growth and ordering are latent within the baked carbon structure.
• Under the influence of temperature the crystallites grow and rearrange in an ordered pattern of stacked parallel planes. This transformation is accompanied by a change in the physical properties of the material.
• The greated the degree of crystallite growth during heating up, the better the graphitability(graphitization degree), which effects the final resistivity achieved • There is a variation among different needle cokes concerning the graphitability
• The graphitisation degree depends on the structure of the basic material (graphitability) and the applied graphitisation temperature. It is determined by x-ray measurements
CHART Change in properties during graphitization
Typical expansion during graphitisation
GRAPH Development of crystallite alignment during graphitisation change by picture Harry Marsch
Heath treatment Graphitisation – Acheson
Acheson furnace
The stock is arranged in blocks within a horizontal bed, usually perpendicular to the axis of the furnace. The space between these segments is filled with a resistor material consisting of a coke/graphite granular mixture. The current is supplied to the load by two water-cooled head electrodes at the narrow sides of the furnace, which is thermally insulated by a mixture of coke, sand, carbon black and/or sawdust, thus protecting the material against oxidation. As the electrical resistance of the furnace decreases with an increasing degree of graphitisation, the power to the head electrodes is controlled and adjusted by transformers. Niowadays, nearly all graphitising furnaces are DC-operated since powerful rectifiers are available. For fine-grained material, an uncontrolled release of decomposition products from the charge does not cause problems, so that the normal maximum temperature of 2800C can be reached within a few days. Depending on the size of the furnace and its operation mode, one working cycle including cooling lasts two to three weeks.
GRAPH Acheson furnace
