This approach has applications in industry where solid materials need to be melted.
There is a need to develop an efficient method for processing lunar regolith in support of future missions to colonize the Moon. A system for heating lunar regolith (“moon soil”) using microwaves for processing has been developed. It relies on an enhanced heating effect based on a large temperature gradient forming when a sample of lunar regolith under microwave radiation emits heat from its surface rapidly as the core is melting. Once the core melts, the sample absorbs microwave energy more readily. This molten lunar regolith would then exit the sample tube, and the lunar regolith could then be introduced into molds for forming a desired structure or building block.
The approach can be demonstrated by using a waveguide cavity excited in a TE10n mode. A thin-walled cylindrical quartz or sapphire tube goes vertically entirely through the cavity near a location of maximum electric field. The lunar regolith initially fills the tube. When the TE mode is excited, the sample will begin to heat.
It was previously demonstrated that volumetric microwave heating of an alumina sample will cause a large temperature gradient to be formed within the sample. This gradient arises because the sample is uniformly heated and the sample surface radiates heat away while the energy in the interior takes time to propagate to the surface. The time for this heat to propagate depends on the thermal conductivity of the sample. Since the thermal conductivity of lunar regolith is very low, the gradient produced is expected to be rather large. As the sample heats, the interior will reach the melting point of one or more of the material components of the sample. Once melting occurs, the sample will absorb microwave energy more efficiently. Since the electric field strength is uniform along the axis of the tube, the center of the sample along the entire length of the tube should melt first. By heating this molten region above its melting point, it should flow more easily and exit the waveguide at the bottom of the tube.
Once the molten regolith begins to flow out of the cavity, it can be placed in any desired shaped mold to build up a structure, layer by layer. It can also be allowed to drop to the ground to form slowly a solid layer that could be the beginning of a road. As the molten sample gradually moves out of the tube, new sample material will be inserted at the top of the tube. One way to practically deploy this 3D microwave system, nicknamed the “Sinterator,” on the lunar surface is to attach it to a leg of the All-Terrain Hex-Limbed Extra-Terrestrial Explorer (ATHLETE) robotic mobility system. The ATHLETE can be used as a precision positioning tool to 3D-print large-scale, in situ structures on planetary surfaces such as the Moon or Mars.
This work was done by Martin B. Barmatz, David E. Steinfeld, and A. Scott Howe of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49271