Dr. Flint answers the question: What is Graphite?
Important work is currently being done at the National University of Singapore (NUS) to standardize graphite characterization. The reason is that graphite consumers have difficulty getting reproducible results from different graphite sources. The result is that they must formulate their products using the product from a specific supplier. The reason for the lack of reproducibility is that graphite with the same grade and particle size often behaves very differently because many other characteristics have not been described. This short description tries to explain why. Currently graphite is categorized by grade and a particle size; stated differently, by the mineralogical composition and morphology of the crystal.
Thickness: Graphene is a single layer of honeycomb carbon one atom thick; essentially a plane of infinite linked poly aromatic hydrocarbons. A graphite crystal is a book of graphene; a crystal that may be composed of hundreds to millions of layers of graphene.
Aggregates: Graphite as it is produced can be that book or an entire library as graphite crystals often grow intimately together and are not processed into individual flakes. Thus, an individual particle of graphite may be one crystal or an entire population of crystals.
Stacking: How the graphene is layered, also, makes a difference in the characteristics of the graphite. The layers maybe closer or further apart, broken within the crystal, or be displaced.
Bending: a crystal of graphite bends very easily and processing often bends, folds and balls the graphite.
Size: A size statement only gives information on the particle size and nothing about individual crystals that may aggregates or bent. In addition, the size is often quoted as being a certain percentage smaller than a particular size. This, again, says very little about the size distribution. As an example, a graphite product assured to be 100% greater than 200 micrometers could be composed of individual flakes or aggregates or any combination. A product of 100% less than 200 micrometers with an average size of 150 micrometers may be made up only of 180 micrometers and have 20 micrometers particles, or all 150 micrometers. Or, the particle may be made of up 300 micrometer particles that have been bent and folded until they fit through the 200 micrometer screen.
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Intergrowth: Often, graphite from the higher metamorphosed rocks will contain contaminates that have grown in and around or through a graphite crystal resulting in processed graphite that is not a continuous but with holes, gaps or shapes other than the theoretical hexagonal crystal.
Encapsulation: when graphite is not processed correctly the graphite can bend and fold around contaminate particles affectively preventing removal of this contamination by physical or chemical means.
The quality of the graphite depends on the grade of the graphite but also on the amounts of other materials present. Some contaminates are important for some applications and not others. In addition, how the contamination is present can be important. Typical contaminates are silica, pyrite or other sulfides, host rock, sulfur, mica, kaolin or other clays, carbonates: malachite, calcite, dolomite and others, garnets, tourmaline, topaz, corundum, and many other minerals. These may occur as individual particles contained separated within the population of graphite particles, locked to the graphite, or contained between the graphene layers. Thus, the graphite grade often tells only part of the story.
Assay reports often come with a statement of organic and graphitic carbon. These numbers can be confusing because it is actually a statement of how much carbon burns at a low temperature under the assumption that it is organic carbon. Disordered, functionalized and graphite edges can also burn at these low temperatures albeit at a slower rate. A graphite from a geothermal vein or metamorphosed rocks are highly unlikely to contain organic carbon. Such a classification should be reconsidered and labelled differently than graphite sourced from high ranking coals or artificial graphite.
Dr. Flint has been active in the graphite/graphene industry for over 25 years with experience ranging from engineering review, test work, pilot plants, process design, ... <Read more about Dr. Ian Flint>