Non-contact measurements of thermophysical properties of niobium at high temperature

Abstract

Four thermophysical properties of both solid and liquid niobium have been measured using the vacuum version of the electrostatic levitation furnace developed by the National Space Development Agency of Japan. These properties are the density, the thermal expansion coefficient, the constant pressure heat capacity, and the hemispherical total emissivity. For the first time, we report these thermophysical quantities of niobium in its solid as well as in liquid state over a wide temperature range, including the undercooled state. Over the 2340 K to 2900 K temperature span, the density of the liquid can be expressed as ρ_L (T) = 7.95 × 10³ − 0.23 (T − T _m)(kg · m⁻³) with T _m = 2742 K, yielding a volume expansion coefficient α_L(T) = 2.89 × 10⁻⁵ (K⁻¹). Similarly, over the 1500 K to 2740 K temperature range, the density of the solid can be expressed as ρ_s(T) = 8.26 × 10³ − 0.14(T − T _m)(kg · m⁻³), giving a volume expansion coefficient α_s(T) = 1.69 × 10⁻⁵ (K⁻¹). The constant pressure heat capacity of the liquid phase could be estimated as C _PL(T) = 40.6 + 1.45 × 10⁻³ (T − T _m) (J · mol⁻¹ · K⁻¹) if the hemispherical total emissivity of the liquid phase remains constant at 0.25 over the temperature range. Over the 1500 K to 2740 K temperature span, the hemispherical total emissivity of the solid phase could be rendered as ε_TS(T) = 0.23 + 5.81 × 10⁻⁵ (T − T _m). The enthalpy of fusion has also been calculated as 29.1 kJ · mol⁻¹.