Bonding That Makes Graphite Conductive
Each carbon atom in a graphite layer uses three of its four valence electrons to form sigma bonds with three neighbours in a planar hexagonal arrangement, the sp2 configuration. The fourth electron occupies a delocalised pi orbital that extends above and below the layer, and those electrons are free to move through the sheet. This is the origin of the electrical conductivity of graphite: it behaves as a semimetal rather than an insulator, and it is the reason a graphite electrode can pass an arc current that would destroy a ceramic conductor.
Anisotropy of the Layered Lattice
Because the layers are strongly bonded within the plane and only weakly bonded between planes, nearly every property of graphite is directional. Electrical resistivity is much lower along the layers than across them, thermal conductivity shows the same pattern, and mechanical strength and thermal expansion also differ with direction. In a manufactured electrode the coke particles are aligned to a greater or lesser degree by extrusion, so the electrode is itself anisotropic. This is why specifications state whether a property was measured parallel or perpendicular to the axis, and why needle coke, which aligns strongly, produces a low-expansion electrode.
Thermal Behaviour and Sublimation
Graphite does not melt at ordinary pressure. Under a protective atmosphere it sublimes at roughly 3,650 degrees Celsius, which is far above the 3,000 to 3,500 degrees Celsius measured in a steelmaking arc, so the electrode stays solid while the charge melts. The high thermal conductivity of graphite, in the range of about 100 to 150 watts per metre kelvin at working temperature, spreads the arc heat away from the tip and limits the peak temperature in the body. The combination of a very high sublimation point and high conductivity is the thermal foundation of the whole product.
Resistivity and Joule Heating
Even a good conductor generates heat when it carries current. The electrical resistivity of electrode graphite is typically around 5 to 9 micro-ohm metres, and the ohmic loss in the column is proportional to that resistivity and to the square of the current. Because the current is very large, even a small increase in resistivity raises the temperature of the electrode significantly, and a damaged joint with high contact resistance becomes a hot spot. Chemistry and processing set the resistivity: higher graphitisation temperature and better crystal order lower it.
Chemical Stability and Oxidation Onset
Graphite is chemically inert towards most molten metals and slags, which is why it can sit in a steel bath without reacting. Its weakness is oxygen. In air, measurable oxidation begins at around 450 to 600 degrees Celsius, and above that temperature carbon reacts with oxygen, carbon dioxide and water vapour to form carbon monoxide and carbon dioxide. This is the mechanism of sidewall loss in a furnace. The reaction is slow at low temperature and fast at high temperature, so furnace practice that keeps the electrode cooler and shields it with foam slag directly extends electrode life.
Frequently Asked Questions
Q: Why is graphite electrically conductive?
Because each carbon atom contributes one delocalised electron to a pi orbital that extends across the layer, allowing charge to move freely within the hexagonal sheets.
Q: What is the sublimation temperature of graphite?
Roughly 3,650 degrees Celsius at atmospheric pressure, well above the 3,000 to 3,500 degrees Celsius of a steelmaking arc, so the electrode remains solid in service.
Q: Why is graphite anisotropic?
Strong covalent bonds act within the layers while only weak van der Waals forces act between them, so conductivity, strength and thermal expansion all differ with direction.
Q: What is the electrical resistivity of electrode graphite?
Typically 5 to 9 micro-ohm metres, with ultra-high-power grades at the low end. Lower resistivity means less Joule heating inside the column.
Q: At what temperature does graphite start to oxidise?
In air, measurable oxidation starts at roughly 450 to 600 degrees Celsius. Above that, oxygen and carbon dioxide react with carbon and the sidewall slowly disappears.
Q: Does graphite react with molten steel or slag?
It is chemically inert towards most molten metals and slags, which is why it can carry current directly into the bath without being consumed by chemical attack.