Comparisons of N 2 and Ar based spray forming showed that with all other factors remaining constant, the billet top temperature was lower with N 2 than with Ar, because of the differences in thermal diffusivity of the two atomising gases : Ar has a thermal conductivity of 0.0179 W / mK which is approximately a third less than N 2 with a thermal conductivity of 0.026 W / mK.
32.
Where " ? " is the thermal diffusivity of the mantle lithosphere ( ), " a " eff is the effective thermal expansion coefficient for rock ( ), " T " 1 is the temperature of ascending magma compared to the temperature at the upper boundary ( 1220 �C for the Atlantic and Indian Oceans, 1120 �C for the eastern Pacific ) and " d " r is the depth of the ridge below the ocean surface.
