Manufacturing in Space

Space-based manufacturing of materials poses numerous advantages for U.S. space exploration and utilization. Launch costs may be significantly reduced by launching raw materials for space-based fabrication and structures that are too large for transport may be fabricated in situ. The freeze-casting technique is simple, scalable, cost-effective, and employs easily obtained materials. Moreover, complex manufacturing equipment is unnecessary and the range of producible, near net-shaped porous materials is nearly limitless. To date, the freeze-casting technique has been utilized to fabricate porous polymer, ceramic, metal, and composite materials. The large number of materials that have already been fabricated using this technique is illustrative of the fact that the principles governing freeze-casting are not strongly dependent on the composition of the materials (or chemical interactions), but rather on the fundamental, physical nature of the solidification process. The freeze-casting technique has the potential to extend and sustain human activities beyond low-Earth orbit by providing a method to manufacture structural and functional materials in space, thus increasing the reliability and safety of access to space while also decreasing overall costs.

Improving Terrestrial Fabrication

The solidification of metals and alloys in microgravity has been studied for over thirty years. The research offers significant insight into the physical properties governing the freeze-casting process. However, solidification of slurries is a bit more complex. Indeed, despite major advancements in the freeze-casting technique, the underlying principles that govern solidification behavior are not well understood. Current research approaches strive to improve the mechanical and functional properties of resulting porous materials, but these attempts are limited by our current knowledge base.

A major advantage of the freeze-casting technique is that it inherently allows for the adjustment of microstructural properties through the manipulation of solidification conditions. This is significant because it means that mechanical and physical properties of final products may be tailored to suit desired applications simply by modifying the conditions of solidification, e.g. solidification temperature, solid content of the suspension, particle size, just to name a few.

Of course, there are some limitations, but in principle, since the ice dendrites serve as templates for pores in the final product, learning how to precisely control their growth is paramount to achieving a desired final structure. Microgravity research helps us to better understand the fundamental processes at work, which can lead us to the development of improved terrestrial fabrication techniques.

Planetary Science

In freeze-cast materials, ice lenses are defects that form during solidification and compromise a resulting material’s compressive response. In geology, ice lensing is studied as the mechanism responsible for frost heaves. Similarly, for planetary scientists, ice lenses initiate geomorphic events that hold insights into climate history. Our parabolic flightwork provided evidence that gravity may contribute to ice lens development in freeze-cast materials (Fig. 1).

Fig. 1. Longitudinal cross-sections of a sintered TiO2 sample sintered from a 20 wt% TiO2 aqueous suspension directionally solidified under terrestrial conditions, showing periodic ice lenses perpendicular to the freezing direction. White areas represent sintered TiO2 particle walls and dark areas represent pores, templated by individual ice lenses (red arrows). The ~70 mm thick white band at the bottom of the image is the bottom of the sample, consisting of densely sintered TiO2, which was in contact with the freezing substrate.

For the CubeSat mission, we will freeze salt water solutions and aqueous particle suspensions. We anticipate that the collection of temperature and image data during solidification will offer important insight into ice segregation patterns observed in oceans, soil, and on planetary surfaces, while also helping us to better understand the formation mechanisms of ice lens defects in freeze-cast materials.