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 above 2000 K during pulsed-current heating in vacuum. Niobium coupon specimens (3 mm × 0.5 mm × 100 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 linear thermal expansion and average coefficients of thermal expansion (CTE) were evaluated under three temperature conditions, yielding a maximum linear expansion of 1.88 × 10 ^− 2 between 340 K and 2355 K and an average CTE of 9.30 × 10 ^− 6 K ^− 1 at a mean temperature of 1344 K. The relative deviation from literature values remained below 1.1 × 10 ^− 7 K ^− 1 , and the combined standard uncertainty of the CTE measurement was 1.73 × 10 ^− 7 K⁻¹ (1.85%), including a 0.02% contribution from temperature non-uniformity across the specimen surface. These results confirm the method’s validity for rapid, precise, non-contact thermal expansion measurement above 2000 K.