§ 4.5: Producing High Temperature Refractories
Refractories are materials needed for handling high temperature liquids, gases and solids, e.g., for industrial processing. Applications include solar furnaces, casting molds for molten materials, heat exchangers, and aerobraking heat shields.
Industrial refractory needs can be satisfied by sintered calcia (CaO), silica (SiO2), magnesia (MgO), alumina (Al2O3) and titania (TiO2), with the desired porosity. Of course, these stable materials are commonly used on Earth for the same purposes, due to their great resistance to heat, oxidation (they are already fully oxidized), corrosion and abrasion. Minerals such as olivine [(MgFe)2SiO4] and anorthite (CaAl2Si2O8) are also useful for making refractory bricks and ceramics. Some refractories and their ceramics have low expansion due to heat and are attractive for space environments where a wide range of temperatures are experienced.
Production of ceramics and refractories in space from lunar materials has been discussed in a number of papers, including Poisl and Fabes, as well as Shirley et al., and Mackenzie and Claridge, among others.
One particular application of refractories is in transportation for returning cargoes to low Earth orbit by aerobraking with the upper atmosphere, and perhaps slowing down some incoming asteroid payloads by aerobraking. Of course, this is the method used by spacecraft to return to Earth, including the reusable Space Shuttle. The Space Shuttle's tiles are made from silica (SiO2) (with a thin borosilicate coating to provide a smooth, aerodynamic surface for a smooth landing).
Aerobraking tiles are produced from amorphous silica fibers which are pressed and sintered, with the resulting tile having as much as 93% porosity (i.e., very lightweight) and low thermal expansion, low thermal conductivity, and good thermal shock properties. This process can be readily performed in space when we can produce silica of the required purity.
Cheaper materials besides silica fibers can be used. Silica fibers are used on the Space Shuttle in order to keep its weight down, thereby increasing cargo weight capacity. For resources already in space, we don't have this economic need. A number of other materials can be used for heat shields, e.g., alumina (Al2O3) or anorthite (CaAl2Si2O8).
The lack of hydrogen and water in lunar material could reduce the difficulty in making these materials, and improve the quality, as compared to making them on Earth.
A technical analysis of making aerobrake shields from lunar materials is given by Poisl and Fabes.