Classification of Graphite Powder: Types, Grades and Industrial Uses
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What Graphite Powder Is
Graphite powder is milled natural or synthetic graphite graded to a controlled particle size and carbon content. Its carbon layers are strongly bonded within the plane but stacked by weak forces between planes, which gives the powder electrical and thermal conductivity, low friction, chemical inertness and thermal stability. It is therefore used in lubricants, conductive coatings, battery electrodes, powder metallurgy blends and refractory mixes.
Classification by Origin: Natural and Synthetic
Natural graphite powder comes from mining, beneficiation and milling. Synthetic graphite powder is made by graphitising petroleum coke, needle coke or pitch coke at roughly 2500-3000 °C in an Acheson-type furnace, then crushing and classifying the product.
Natural flake graphite powder - hexagonal platelets with high anisotropy and the best conductivity and lubricity in the plane of the flake.
Natural amorphous graphite powder - microcrystalline, lower crystallinity and lower cost, used in foundry coatings and carbon brushes.
Natural vein graphite powder - compact and very high in purity, supplied in smaller volumes for specialised duties.
Synthetic graphite powder - primary material graphitised from coke, or secondary material reclaimed from electrode scrap; tighter control of purity, shape and ash at a higher price.
Classification by Carbon Content
Fixed carbon on a dry basis is the second classification axis; ash, moisture, volatile matter and sulphur are reported with it, because together they decide the duty a grade can serve.
| Grade | Fixed carbon | Typical use |
|---|---|---|
| Low carbon | below 80% | foundry coatings, friction materials, refractory ramming mixes |
| Medium carbon | 80-94% | brake linings, lubricant fillers, crucible bodies |
| High carbon | 94-99% | carbon brushes, conductive coatings, powder metallurgy |
| High purity | 99.9% and above | battery anodes, nuclear duties, electronic pastes |
Flake grades are commonly specified against GB/T 3518 and microcrystalline grades against GB/T 3519, with fixed carbon, ash, moisture and particle size declared lot by lot.
Classification by Particle Size
Particle size governs dispersion, surface area and reactivity. Coarse grades flow freely; fine and submicron grades disperse more evenly but agglomerate more readily and generate more dust.
| Designation | Approximate size | Behaviour |
|---|---|---|
| Coarse | +100 mesh, above about 150 µm | fast wetting, low surface area |
| Medium | -100 to +325 mesh, about 44-150 µm | general lubrication and coatings |
| Fine | -325 mesh, below about 44 µm | suspensions, pastes, plastics |
| Submicron | below 1 µm | colloidal graphite, electronic and sensor coatings |
Coarse grades are checked by sieve analysis and fine or submicron grades by laser diffraction, and the two methods are not interchangeable.
Choosing a Grade for the Duty
Colloidal graphite powder is a submicron grade dispersed in water, oil or solvent with a small amount of dispersant. It is used for lubricating and conductive coatings, mould release and die wall lubrication. Where a coating is ordered, state the carrier, the solids content and the required dry film thickness rather than the particle size alone.
Conductivity first: a high crystallinity flake or primary synthetic grade with fixed carbon above 99%.
Lubrication under high load: a fine flake or colloidal grade with low ash and low moisture.
Battery anode: a spheroidised or synthetic grade with tight size distribution and low impurity levels.
Foundry and refractory duty: a medium or low carbon grade with controlled sulphur.
Frequently Asked Questions
Q: What is the difference between flake graphite powder and amorphous graphite powder?
Flake graphite has a well ordered hexagonal crystal structure and delivers higher electrical and thermal conductivity, while amorphous graphite is microcrystalline with lower crystallinity, lower conductivity and a lower price.
Q: Which carbon content is needed for conductive coatings?
Most conductive coatings use grades with fixed carbon of 94% or higher. Residual ash and moisture raise resistivity, so both should be specified together with the carbon content.
Q: How is the particle size of graphite powder measured?
Coarse grades are checked by sieve analysis and fine or submicron grades by laser diffraction, so a mesh figure and a micrometre figure can only be compared when the test method is stated.
Q: Can graphite powder be used as a high temperature lubricant?
Yes. In inert gas or vacuum it remains stable up to about 3000 °C and keeps a low friction coefficient, but in air it starts to oxidise well below that temperature.
Q: What makes graphite powder high purity?
High purity usually means fixed carbon of 99.9% or above, obtained by acid or thermal purification. Trace iron, sulphur and ash must also be reported for battery, nuclear and electronic duties.
