White Graphene: It is white but not graphene…
White graphene is the trivial name given to a material called hexagonal boron nitride (hBN). It occurs in several forms, the hexagonal form is the one of primary interest to us. Think of the basic unit of a hexagon that repeats creating a flat sheet, just like graphene, it is another two-dimensional (2D) material. There are some differences though, read on to find out more…
In graphene there is a carbon atom at each of the six corners, in hBN there are three boron atoms and three nitrogen atoms alternating at each of the six corners.
This hexagonal structure is made possible by a special type of atomic bond called sp2 hybridisation, you can find out more here. This sp2 bonding is very stable and gives the flat sheet its strength and also flexibility. hBN is nearly as strong as graphene. These bonds also allow the transmission of heat through the material and hBN is a very good conductor of heat as a result.
As well as conducting heat, graphene and hBN also have very high melting points. Graphene has the highest melting point at over 4000 degrees centigrade and hBN is not far behind at 2800 degrees centigrade. In fact neither material actually melts, they both decompose rather than go through the liquid phase as water would do.
Like its sister material, graphene, hBN is a flat sheet of tightly bound atoms that are connected together so closely that they form an impenetrable barrier to gases and liquids. This means that hBN is resistant to chemical attack making it a good candidate material for corrosion resistant coatings.
Graphene is permeable to hydrogen ions and it turns out that hBN is too as this research shows. Being permeable to ions is a rather useful property for the electric battery business so it should come as no surprise to find that researchers have found that white graphene electrolytes have found a niche application in batteries where high temperatures would decompose lesser alternatives.
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Just like graphene forms layers that stack and make graphite, hexagonal Boron Nitride also forms layers that stack to make an analogous material called white graphite. These layers are held in place by the weak Van Der Waals force and a consequence of this is that the layers can slide over one another. This means white graphite can be used as a lubricant where machined parts need the friction of contact reduced. White graphite comes into its own where a high temperature, corrosive environment would destroy other lubricants.
hBN is not a substitute for graphene though. One of the main differences is that while graphene is one of the most electrically conductive materials known, hBN is such a poor conductor of electrons that it can be considered as an insulator.
The fact that graphene and hBN are similar and different is exciting researchers around the world. They are starting to ask the question, ‘what would happen if we can combine these materials in layers?’
The discovery of graphene opened the door to a world of two-dimensional materials and now researchers are exploring the possibility of creating entirely new materials by layering different 2D sheets in new combinations.
This field is called hetero-materials and offers the promise of completely novel materials that have never existed in nature before. Watch this column, I’ll be writing about these in future.
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>