Depending of the annealing conditions, various
forms of well-aligned
PG are available. At present, HOPG, HOPG-flex and HAPG , which differ by
their defect- and grain- structure, as well as mechanical properties, are
commercialized.
Highly Oriented Pyrolitic Graphite
as the first commercially available form of well-aligned PG was developed in the middle of the last century. The goal was to reproduce a single crystal of graphite, which occurs very rarely in nature and only in small chips less than 1 mm in size. The result of this very extensive scientific work [Moore A.W., 1973] was a mosaic crystal with an almost perfect structure at the crystallographic level.
This ideal graphite structure consists of graphene-like carbon planes in which the carbon atoms are covalently bonded in a honeycomb structure.
The carbon planes stack together by van-der-Waals forces into a three-dimensional structure with a hexagonal unit cell
Although HOPG has an almost ideal structure at the atomic level, the real material has a rather complex two-level grain structure. Low-angle boundaries subdivide the material into regions with a perfect structure (crystallites).
These crystallites, in turn, are combined into large blocks (grains) separated by large-angle boundaries [Ohler, 2000]. Crystallites in classical HOPG are of a size from a few to a few tens of microns, while grains are from a few tens to a hundred of microns. Acoustical microscopy reveals enhanced segregation of defects predominantly along the grain boundaries.
Structure of HOPG in a Scanning Electron Microscope (provided by our dealer www.nanoscopy.ru)
Structure of HOPG in Acoustical Microscope Scale 1000 x 100µm 125 µm under the surface
The manufacturing process includes annealing of Pyrolytic Carbon (PC) - a laminar material, produced by decomposing a carbon-containing gas, usually methane, and depositing carbon on a hot surface. After deposition, the material has only two-dimensional ordering, where the basal carbon planes are randomly arranged relative to each other in the perpendicular direction. This leads to a larger interlayer distance in PC compared to PG (about 3,4Å instead of 3.354-3.358 Å). PC has mosaic spread of about 30 degree and clearly visible growth cones. High temperature annealing at temperatures of about 3000℃ under stressed conditions provides the structure with 3-dimentional ordering and transforms PC into PG.
HOPG crystals are produced by annealing under compressive deformation in c-axis direction. The crystals are limited in size due to the limited annealing volume. The mosaic spread from 0,4 degree to 3,5 degree are commercially available. (See more in the page “HOPG crystals”). Crystals made by traditional technology are often referred to as bulk-HOPG.
A plate of Pyrolytic Carbon before annealing at 3000ºC
A plate of Highly Oriented Pyrolytic Graphite after annealing at 3000ºC under uniaxial pressure
classical Technology&Neu Forms
The minimum mosaic spread of bulk HOPG is 0,3 degree. There are significant difficulties in the obtaining of bulk HOPG in focusing geometry. In the traditional technology, bent HOPG crystals are manufactured by a high temperature annealing on concave/convex dies. Anisotropy of thermal expansion coefficient of HOPG causes problems in manufacturing of the required shapes with a good accuracy. As the result, only a very limited set of cylindrical shapes with relatively big radii were commercialized
Trying to decrease mosaicity of graphite crystals and to widen the range of available geometries, Optigraph purposely varied the annealing process. The research resulted in two more forms of well-aligned PG named HOPG-flex (Flexible Highly Oriented Pyrolytic Graphite) and HAPG (Highly Annealed Pyrolytic Graphite). HOPG-flex and HAPG became basis materials for Graphite Optics.
Experimental confirmation of the structural differences of the PG forms was given by Atomic Force Microscopy
The number of defects in the visual field of 10x10 μm² of Atomic-force microscope (averaged by 30 photos for HAPG and by 10 photos for other materials). (the results are kindly provided by our dealer www.nanoscopy.ru )
Defects in the honeycomb graphite structure corresponds usually to carbon atoms in sp³ hybridization. Such defects, as well as grain boundaries, anchor carbon planes. It is clearly seen from the table, where the number of interlayer defects and grain boundaries correlates with the number of broken carbon planes on the basal surface of a sample. The defects and grain boundaries supply the crystal with the rigidity . As the result, forms with fewer defects and larger grains (HAPG and HOPG-flex) have flexible films used for manufacturing of Graphite Optics.
Direct methods also confirm that the size of grains and crystallite in HOPG-flex and HAPG is several times larger that for HOPG.
Electron Microscopy (SEM) fresh HAPG surface
Back Scattering Diffraction (EBSD) ofs fresh HAPG surface
The physical properties of all well-aligned PG forms are quite similar. Due to the layered structure, they all are very anisotropic. They have excellent thermal conductivity in the basal plane, and rather moderate value in the perpendicular direction. Pyrolytic graphite is electrically conductive in the basal plane and insulating in the perpendicular direction.
The most investigated form is bulk HOPG, which has been on the market for over 50 year. The main physical characteristics (at 20ºC) of bulk HOPG are summarized in the Table
Table physical characteristics (at 20ºC) of bulk HOPG
Decreased defect content shifts the properties of flexible forms of well-aligned PG toward ideal graphite: thermal conductivity of HAPG is larger than that of bulk HOPG (2000±100 W/mK compared to 1700±100 W/mK) and the interlayer distance is smaller (3.354 Å instead of 3.357 Å).
The manufacturing process itself purifies the material, and all forms of well-aligned PG contain extremely low total impurities, not exceeding a few ppm. No additional cleaning procedures are required.