Graphite can survive sudden thermally induced loads (thermal shock), such as those experienced when an arc is struck between the charge and the tip of a graphite electrode in an electric arc melting furnace, or on the first wall of a fusion reactor. To provide a quantitative comparison of a material’s resistance to thermal shock loading, several thermal shock figures of merit (D) have been derived. In its simplest form, the Figure of merit (FoM) may be expressed as

K Sy

a E where K is the thermal conductivity, sy the yield strength, a the thermal expansion coefficient, and E is the Young’s modulus. Clearly, graphite with its unique combination of properties, that is, low ther­mal expansion coefficient, high thermal conductivity, and relatively high strain to failure (s/E), is well suited to applications involving high thermal shock loadings. Taking property values from Table 1 for Toyo Tanso IG-43 and for POCO AXF-5Q gives FoM values of D = 99923 and D = 67 875, respec­tively (from eqn [13]). Another FoM takes account of the potential form of failure from thermally induced biaxial strains, Dth, and may be written as

K Sy

aE(1 — n) where K is the thermal conductivity, sy the yield strength, a the thermal expansion coefficient, E the Young’s modulus, and n is Poisson’s ratio. Larger values of Dth indicate improved resistance to thermal shock. Using the values above and dividing by (1 — n) from eqn [14] gives FoM values of Dth = 124 904 and 84 844 for IG-11 and AXF-5Q, respectively. The thermal shock FoM, Dth, has been reported48 for several candidate materials for fusion reactor first wall materials (see Chapter 4.18, Carbon as a

Fusion Plasma-Facing Material). Wrought beryl­lium has a value of ~1 x 104, pure tungsten a value of ^0.5 x 105, and carbon-carbon composite material ~1 x 106. If the thermal shock is at very high tem­perature, the material’s melting temperature is a key factor. Again, graphite materials do well as they do not exhibit a melting temperature; rather they pro­gressively sublimate at a temperature higher than the sublimation point (3764 K).