Ultrasound-triggered supercooling mitigation and thermal regeneration of a miniature gallium fixed-point cell for blackbody integration

In-orbit infrared calibration requires spaceborne blackbodies with traceable on-board temperature references. A miniature gallium fixed-point cell can provide such a reference through a melting plateau linked to the ITS-90 gallium point, but deep and variable supercooling increases the required cooling depth. This places an additional burden on payload thermal design. A Ni particulate addition was first examined as an internal nucleation strategy. It showed a lower mean supercooling than the separately acquired natural-cooling reference, but the effect weakened over repeated cycles and was accompanied by a distinct melting-plateau response. This study evaluated external through-wall ultrasonic triggering as a non-invasive regeneration strategy for a sealed miniature high-purity gallium fixed-point cell, while also examining whether the triggered solidification affected the subsequent melting plateau used as the thermometric reference. The effects of different ultrasonic frequencies and generator input levels on gallium supercooling were experimentally evaluated. Meanwhile, acoustic–structure simulations and conditional Rayleigh–Plesset and Keller–Miksis calculations were used to examine possible physical mechanisms underlying ultrasound-triggered solidification. Measurements obtained from the same cell after 28 kHz ultrasonic regeneration showed no statistically detectable change in the mean characteristic melting-plateau temperature relative to natural regeneration, with a difference of 0.2 mK. Ultrasonic triggering reduced mean supercooling from 9.044 K under natural cooling to 1.902–2.573 K across the tested frequencies, with a maximum reduction in cooling depth of 79.0%. The reduced cooling requirement indicates a lower thermal-control demand for blackbody regeneration.