Symbols

a Crystallographic a-direction (within the basal plane)

b Empirical constant c Crystallographic c-direction C Elastic moduli

C Specific heat

Cp Specific heat at constant pressure E Young’s modulus G Shear modulus h Plank’s constant k Boltzmann’s constant KIc Critical stress-intensity factor KT Thermal conductivity at temperature T la Mean graphite crystal dimensions in the

a-direction

lc Mean graphite crystal dimensions in the

c-direction

m Charge carrier effective mass N Charge carrier density

P Fractional porosity q Electric charge R Gas constant S Elastic compliance (1/C)

T Stress T Temperature

a Coefficient of thermal expansion a Thermal diffusivity

aa Crystal coefficient of thermal expansion in the a-direction

ac Crystal coefficient of thermal expansion in the c-direction

ay Synthetic graphite coefficient of thermal

expansion parallel to the molding or extrusion direction

a? Synthetic graphite coefficient of thermal expansion perpendicular to the molding or extrusion direction Dth Thermal shock figure of merit g Cosine of the angle of orientation with respect to the c-axis of the crystal во Debye temperature A Charge carrier mean-free path

m Charge carrier mobility

n Poisson’s ratio

nf Charge carrier velocity at the Fermi surface r Bulk density

s Electrical conductivity s Strength

sy Yield strength

t Relaxation time

v Frequency of vibrational oscillations

2.10.1 Introduction

Graphite occurs naturally as a black lustrous mineral and is mined in many places worldwide. This natural form is most commonly found as natural flake graphite and significant deposits have been found and mined in Sri Lanka, Germany, Ukraine, Russia, China, Africa, the United States of America, Central America, South America, and Canada. However, artificial or synthetic graphite is the subject of this chapter.

The electronic hybridization of carbon atoms (1s2, 2s2, 2p2) allows several types of covalent-bonded structures. In graphite, we observe sp2 hybridization in a planar network in which the carbon atom is bound to three equidistant nearest neighbors 120° apart in a given plane to form the hexagonal graphene structure. Covalent double bonds of both а-type and p-type are present, causing a shorter bond length than that in the case of the tetrahedral bonding (а-type sp3 orbital hybridization only) observed in diamond. Thus, in its perfect form, the crystal struc­ture of graphite (Figure 1) consists of tightly bonded (covalent) sheets of carbon atoms in a hexagonal lattice network.1 The sheets are weakly bound with van der Waals type bonds in an ABAB stacking sequence with a separation of 0.335 nm.

The invention of an electric furnace2,3, capable of reaching temperatures approaching 3000 °C, by Acheson in 1895 facilitated the development of the process for the manufacture of artificial (synthetic) polygranular graphite. Excellent accounts of the properties and application of graphite may be found elsewhere.4-6