Solidification Using a Baffle in Sealed Ampoules

The Solidification Using a Baffle in Sealed Ampoules (SUBSA) facility objective is to advance understanding of the processes involved in semiconductor crystal growth. SUBSA offers a gradient freeze furnace for materials science investigations that can reach 850 °C. Samples are contained in transparent quartz or ceramic ampoules with high-definition video imaging available in real time, along with remote commanding of thermal control parameters.

SUBSA operates within the MicrogravityThe condition of perceived weightlessness created when an object is in free fall, for example when an object is in orbital motion. Microgravity alters many observable phenomena within the physical and life sciences, allowing scientists to study things in ways not possible on Earth. The International Space Station provides access to a persistent microgravity environment. Sciences Glovebox (MSG) facility for containment, utilities provision, remote commanding, and video imaging capabilities. It was originally operated successfully onboard ISSInternational Space Station Flight UF2 in 2002 during the MSG rack’s first mission increment. SUBSA was refurbished and updated for reflight with modernized software, data acquisition, high-definition video, and communication interfaces to the ISS and MSG facility. In 2020, SUBSA was refurbished again and updated for reflight.

The SUBSA furnace operates in an ambient air environment and can process 16-mm diameter cylindrical ampoules up to 290 mm in length. Samples are typically melted at a controlled rate, held at maximum temperature to stabilize, and then solidified at a precise cooling rate in gradient freeze mode. The furnace hot zone is a 20-cm length stainless steel tube having an actively heated portion of 10 cm. There is an adjacent transparent zone of 9.5 cm length in which the solid-liquid interface is formed and observed. The sample cartridge head acts as a cold side thermal sink to the MSG facility’s cold plate, which is held at a nominal temperature of 22 °C.

Thermal parameters can be adjusted based on real-time video imaging for maximum benefit to the science team. The behavior of the solid-liquid interface is observed and recorded in real time using a video system that offers remotely controlled zoom, fine focus, image setting adjustments, and selectable frame rate. Four thermocouple sensor channels are available for measuring sample thermal profiles at a user-defined frequency rate, along with optional overlay of science data onto the video feed. Precision heating and cooling control supports up to eight process segments, including various combinations of dwell, heat-up, or cool-down periods with resolution and stability of 0.1 °C on the setpoints and 0.1 °C/hr on the ramp rates. Individual process segments can also be adjusted while within that segment.