The properties of the ideal material, that is a material that most closely corresponds to an infinitely large graphite crystal, are reviewed in this section. Such a material does not exist in the real world and the properties given below are either calculated or based on the actual properties of graphite crystals closely approaching this ideal structure.
As already mentioned and will be seen in later chapters, a wide range of materials comes under the heading of carbon or graphite and these materials often have properties that are much different from those of the ideal graphite crystal. Obviously it is necessary to define the material accurately when speaking of the properties of “carbon” or “graphite”.
Anisotropy of the graphite crystal: The peculiar crystal structure of graphite results in a considerable anisotropy, that is the properties of the material may vary considerably when measured along the ab directions (within the plane) or the c direction (perpendicular to the planes). Such anisotropy, especially in electrical and thermal properties, can often be put to good use as will be seen in later chapters.
The physical properties of graphite are summarized in Table 3.1. It should be stressed that to obtain accurate measurements of the properties of materials much above 3000K is a trying task. In the case of graphite, many of these measurements are based on carbon-arc experiments which are difficult to perform and interpret. The results must be viewed accordingly and some of these results and conclusions are still controversial.
Table 3.1. Physical properties of graphite
Crystalline form: hexagonal
Lattice parameters: ao=0.245nm
co=0.671nm
Color: Black
Density at 300K, 1 atm: 2.26g/cm3
Atomic volume: 5.315 cm3/mol
Sublimation point at 1 atm: 4000K
Triple point: 4200K
Boling point: 4560K
Heat of fusion: 46.84 kJ/mol
Heat of vaporization to monoatomic gas: 716.9 kJ/mol
Pauling electronegativity: 2.5
Density: the density of the perfect crystal listed in Table 3.1 is the theoretical density. Most graphite materials will have lower densities due to the presence of structural imperfections such as porosity, lattice vacancies and dislocations.
With the exception of boron nitride, graphite materials have a lower density than all other refractory materials as shown in Table 3.2. This is a advantageous characteristic especially in aerospace applications.
Melting, sublimation, and triple point: The melting point of a crystalline material such as graphite is the temperature at which the solid state is in equilibrium with the liquid at a given pressure. “Normal” melting point occurs at a pressure of one atmosphere. Graphite does not have a normal melting point since, at one atmosphere, it does not melt but sublimes when the temperature reaches approximately 4000K. To observe melting, a pressure of 100 atm and a temperature of 4200K are necessary.
Table 3.2. Density of some refractory materials
g/cm3
Graphite 2.26
Molybdenum 10.22
Rhenium 21.04
Tantalum 16.6
Tungsten 19.3
Titanium diboride 4.50
Hafnium carbide 12.20
Tantalum carbide 13.9
Boron nitride 2.25
Aluminum oxide 3.97
Zirconium oxide 5.89