Since graphite is a metallic type conductor of electricity, it might be anticipated that the electrons and holes that are responsible for the conduction of electricity would also transport heat. However, all evidence indicates that heat is transferred by lattice vibrations (phonons) rather than by electrons or holes.
Thermal Properties
Thermal conductivities of graphite
In common with other properties, the thermal conductivity of single crystals of graphite is highly anisotropic. At room temperature the ratio of k measured transverse to the [0 0 l] direction to that measured parallel to the [0 0 l] direction is about 5. This anisotropy in the crystallites is reflected in the anisotropy of polycrystalline graphites. For most nuclear graphites the thermal conductivity measured parallel to the grain [k(||)] ranges from 0.3 to 0.6 cal/(sec) (cm) (°C). The thermal conductivity measured perpendicular to the grain [k(^) ] is somewhat lower. A value for the ratio k(||) / k(^) =2 is common, but the exact value depends upon the degree of orientation.
Measurement of thermal conductivity
The determination of k below about 500°C can be accomplished most conveniently by measuring the heat transmitted by a sample in which a temperature gradient is maintained. This method is satisfactory up to temperatures where radiative heat losses become appreciable. Above 500°C radiative losses make accurate measurements difficult. The few measurements that have been reported suggest that k decreases approximately as 1/T.
Thermal expansion
The coefficient of thermal expansion (CTE) of different polycrystalline graphites near room temperature ranges from 2*10-6 to 6*10-6 per °C transverse to the extrusion direction and 0.5*10-6 to 5*10-6 per °C parallel to the extrusion direction. Thus the volume coefficient is much smaller than that of single crystals. The expansion of graphite is not a linear function of temperature. It is customary, however, to speak of an average coefficient of thermal expansion that is just the thermal dilatation divided by the temperature change. It has been found [19] empirically that the average CTE(||) or CTE(^) measured over one range of temperature can be extrapolated to a higher temperature by the addition of a term that depends upon the higher temperature. The table 6.4 below gives the term to be added to the CTE measured over the range 20 to 100°C. The values apply to a wide range of polycrystalline graphites with an accuracy of ± 0.2*10-6 per °C.
Table 6.4 Factors for calculation of mean coefficient of thermal expansion
Thermal shock
During rapid cooling of a body, the outer layers, which are at a lower temperature than the interior, contract more rapidly than the interior. As a result, tensile stresses develop at the surface, and compressive stresses, in the interior. A similar stress distribution can be generated when the body is heated rapidly from within. In the case of sudden heating of the body from the exterior, the exterior experiences a compressive stress, and the interior, a tensile stress. Generally, it is experimentally more convenient to quench heated samples than to heat samples rapidly. Hence rapid cooling is commonly used as a test of thermal-shock resistance.
When a heated body is suddenly quenched, the magnitude of the thermal gradient at the surface depends on the thermal conductivity of the body. For a given thermal gradient, the thermal strains depend on the magnitude of the coefficient of thermal expansion. A low value for the elastic modulus yields a lower thermal stress for a given thermal strain. Finally, a high tensile strength enables the material to withstand the thermal stress. Thus, the thermal-shock resistance for sudden cooling is high in a material that has a low coefficient of thermal expansion (a), a high thermal conductivity (k), a high tensile strength (S), and a low modulus of elasticity (E). Values of the figure of merit KS/aE for graphite and a number of other materials are given in Table 6.5. Graphite clearly outranks other high-temperature refractory materials in thermal-shock resistance on the basis of this figure of merit. As a dramatic example of shock resistance, graphite specimens have been heated to 1650°C and quenched in water without damage.
Table 6.5 Relative thermal-shock resistance
