Sintered Regolith

Sintered regolith falls into the category of ceramic materials as sintering is the process most common to ceramics. When bricks are made from clay on Earth, first the bricks are heated long enough and hot enough to drive out the water. Then the heating is increased to cause partial melting or vitrification which results in the edges of adjacent grains being bonded together once they have cooled. The unmelted particles provide a stable shape and size during the process which involves some shrinkage and a decrease in porosity.

Lunar Considerations

On Luna water for mixing the various particles would be very expensive and recycling the water would never be 100% effective. It has been demonstrated that the sintering of dry regolith simulant will provide satisfactory bricks, and there is the example of mixing dry powders for powder metallurgy, which is a sintering process performed on Earth and involves putting the material under pressure. Compaction of lunar simulant using vibration has been demonstrated by NASA, but this method may not be as effective or may require more time in the reduced gravity of the moon. More will be known about the quality of bricks possible from lunar materials when the research can be done on Luna. A possibility to consider is the sintering of bricks in an oxygen atmosphere as opposed to in a vacuum.

Radiant Heating

Experiments in radiant heating of regolith simulant have been carried out by NASA Johnson Space Center and Lockheed Engineering & Sciences Co. Results show that radiant heating can reproducibly sinter large, strong bricks in a fused silica mold. Fused silica was chosen for its combination of extremely low thermal conductivity and low density.

Uniform bricks measuring 7.9 x 5.5 x 3.6 cm were produced by heating MLS-1 for two hours at 1,100 degrees Celsius.

Hybrid Microwave Sintering

Research conducted by NASA Johnson Space Center and Lockheed Engineering & Sciences Co. has indicated that the lunar simulant MLS-1 can be successfully sintered using a combination of both microwave and radiant heating.

The crucible was surrounded by silicon carbide bricks, which converted part of the 2.45GHz microwave energy to heat. This radiant heating kept the outside of the regolith hot while microwaves heated the inside at the same temperature of 980 degrees Celsius for 35 minutes. Microwave power was then slowly reduced over several hours. The result was an evenly heated and cooled sintered block with no fractures capable of withstanding up to approximately 1,100 psi.

Oxygen Retrieval and Magnetic Handling

Hybrid microwave sintering in a flowing hydrogen atmosphere resulted in the reduction of iron oxide to produce iron in the sample and water which escaped as vapor. This additional process provides a means of oxygen retrieval as well as increasing the iron content so that bricks can be lifted using a magnet.