High-Speed Shadowgraph Imaging for Measuring the Thermal Expansion of Niobium Under Pulsed-Current Heating Beyond 2000 K

A high-speed shadowgraph technique was developed to measure the linear thermal expansion of metallic solids up to approximately 2400 K during pulsed-current heating in vacuum. Niobium coupon specimens (3 mm × 100 mm × 0.5 mm) were resistively heated with a direct current of over 100 A for up to 2.3 s. The system incorporates a 405 nm bandpass filter, a Type-C thermocouple welded to the specimen surface, and a high-speed CMOS camera to enable high-contrast silhouette imaging under intense thermal radiation emitted by the specimen. Specimen elongation was determined by subpixel contour extraction of silhouette images, and the specimen temperature was recorded via the welded thermocouple. The relative linear thermal expansion (ε) and average coefficients of thermal expansion (α) were determined at three temperatures, yielding a maximum ε of 1.87 × 10^–2 between 334 K and 2352 K and a corresponding α of 9.28 × 10^–6 K⁻¹ at a mean temperature of 1343 K. In all three cases, the relative deviations from literature values were less than 1.2 × 10^–7 K⁻¹, which fall within the combined standard uncertainty of up to 1.83 × 10^–7 K⁻¹ (1.97%).