Engineering, Construction, and Operations in Space IV
American Society of Civil Engineers, pp. 1220-1229, 1994
Carlton C. Allen, John C. Graf, and David S. McKay
Sintering of full-scale "bricks" from lunar soil simulant materials can be accomplished by radiant heating to 1100íC for approximately 2 hours, followed by slow cooling. Small-scale precompaction and the use of a thermally insulating mold are critical for producing strong, crack-free products. Sintering can also be accomplished using a combination of microwave and radiant heating, though the technique is extremely sensitive to thermal profile and configuration. Sintering in hydrogen is synergistic with oxygen production and yields samples containing enough iron metal to permit handling by a magnet.
A future lunar base will require large amounts of dense, strong construction material for thermal and dust control, as well as for radiation protection. Sintered lunar soil, a fine-grained mixture of crushed rock and glass, has been proposed to meet this need (Shirley et al., 1989). Our continuing research effort, initially reported at Space 92 (Allen et al., 1992), is focused on practical methods of sintering to produce lunar "bricks".
Development of any lunar resource can only be justified in relation to the cost of solving the same functional problem using material transported from Earth. We thus set out to demonstrate how large, strong, crack-free sintered bricks could be produced using a minimum of mass and energy. We conducted our experiments without binders or additives, and with little size fractionation or precompaction.
We report here the results of three investigations on the sintering of simulated lunar soil. Radiant heating under carefully controlled conditions can reproducibly yield large, strong bricks. Hybrid microwave sintering, utilizing a combination of microwave and radiant heating, is shown to give promising results. Finally, sintering in hydrogen produces samples containing enough iron metal to allow magnetic handling.
We conducted sintering experiments on two lunar soil simulants. MLS-1 (Minnesota lunar simulant) is a high-titanium crystalline basalt with a chemical composition which approximates Apollo 11 soil (Goldich, 1970). The rock was ground and sieved to a size distribution close to that of lunar soil sample 10084 over a size range from approximately 1 mm to <10 Ám (Weiblen et al., 1990). The specific gravity of MLS-1 particles is 3.1 g / cm3.
JSC-1 is a glass-rich basaltic ash with a composition similar to lunar mare soil (McKay et al., 1994). JSC-1 was also prepared with a grain size distribution close to that of the lunar regolith. Samples used in the current experiments were passed through a 1.168 mm sieve. The specific gravity of JSC-1 particles is 2.9 g / cm3.
The sintering behavior of most materials is significantly improved by compaction, which increases the grain-to-grain contact. Commercial sintering operations generally use large presses to compact material prior to or during heating. Pressure compaction, however, requires equipment which would be too large and heavy to transport to the Moon. We therefore experimented with alternatives to pressure compaction.
Most of our initial experiments were run on minimally compacted samples. The crushed rock was poured into a mold, and was then lightly tamped and leveled. Uncompressed MLS-1 powder has a porosity of approximately 40%. Powder poured into a mold and tamped by hand will reach a porosity of 30 - 31%. Uniaxial compaction with a force of 45,000 psi yields a porosity of 23 - 24%.
We also investigated the use of vibration as an alternative to tamping or uniaxial compaction. A single axis vibration generator operating at 80 Hz with 3 g peak acceleration compressed MLS-1 powder to 30% porosity in five minutes. We employed vibrational compaction in several of our experiments, as noted below.
All radiant heating experiments were conducted in a Lindbergh Model 51333 laboratory furnace, equipped with a controlled atmosphere retort. The retort was heated from above and below. Experiments were run at temperatures of 1000 - 1125íC, for 0.5 - 3 hours.
The basic experiment consisted of heating lunar soil simulant in a brick-shaped mold. Initial testing utilized a mold constructed of 0.5 cm thick inconel (steel), with internal dimensions of 8.2 cm (L) x 4.5 cm (W) x 4.5 cm (H). The mold included an inconel base and a removable inconel top. Rock powder was poured into the mold, hand tamped, and leveled.
A second type of mold was made of porous fused silica. This material was chosen for its combination of low density and extremely low thermal conductivity. Several molds were constructed, with walls 3 cm thick and internal dimensions as large as 13.5 cm (L) x 6.0 cm (W) x 4.5 cm (H). The molds were placed on either steel or fused silica base plates, and were used without tops. Rock powder was poured into the molds and hand tamped or compressed by vibration for 3 - 5 minutes.
In each experiment the furnace was preheated to its operating temperature with the retort in place. The sample in its mold was placed into the retort, the retort door was sealed and a flow of argon was immediately initiated. Argon flow was maintained throughout the heating phase of the experiment. The sample was heated for a predetermined time, following which the furnace power and argon flow were shut off. The furnace was allowed to cool at its natural rate for approximately 18 hours, at which time the retort was opened and the sample was removed. In a typical experiment the furnace cooled from 1100íC to 400íC in the first five hours, and reached 100íC by the termination of the test.
The sintering of geological samples by microwave heating was initially investigated by Meek et al. (1985). We have run a series of investigations into the sintering of crushed MLS-1 basalt in a laboratory microwave furnace. The CEM MDS-81 furnace operates at a frequency of 2.45 GHz and delivers approximately 600 W of microwave energy to the sample. The furnace utilizes an inner chamber of refractory brick, which protects the stainless steel inner walls from overheating. Sample temperatures were approximately monitored by a thermocouple placed in a grounded steel housing immediately below the sample mold.
Each sample of crushed MLS-1 basalt was placed in a cylindrical graphite mold 3.6 cm in diameter by 3.2 cm high. The powder was hand tamped to achieve a porosity of approximately 30%. The mold was capped with a graphite lid 0.26 cm thick. All heating was done in air. However, the graphite mold served as an oxygen "getter," somewhat reducing the effective oxygen fugacity of the sample.
Controlled, even sintering of rock powder by direct microwave heating proved impossible, due to the combined effects of thermal runaway (Kenkre et al., 1991) and self-insulation. The microwave coupling efficiencies of the minerals in MLS-1 rise dramatically with sample temperature. As a result initial heating is slow, but becomes increasingly rapid at temperatures above approximately 400íC. Microwaves penetrate the sample, and heating occurs throughout its volume. However, the center is well insulated by surrounding material, and heats faster than the outside. Typically, our samples sintered strongly or melted in the centers but remained unsintered on the edges.
To achieve uniform sintering we developed a hybrid heating technique, combining microwave and radiant heating. We surrounded the sample crucible with seven silicon carbide blocks, measuring 7.6 x 1.0 x 1.8 cm, in a "picket fence" arrangement (Figure 1). The silicon carbide converted part of the microwave energy to heat. Our samples were heated at full power for periods of up to two hours, and then allowed to cool slowly in the mold under reduced microwave power.
Figure 1. Hybrid microwave furnace (from Cooper, 1992; used with permission)
Two experiments were run in which MLS-1 was sintered in hydrogen. This was done to demonstrate the synergy between sintering and oxygen production from lunar soil. In addition, we sought to make sintered samples containing enough iron metal to allow lifting with a magnet, a potential material handling technique at a lunar base.
Ground MLS-1 was placed in a cylindrical steel die and compressed under approximately 47,000 psi. Each cylinder's diameter, height, and weight were measured to determine density. The initial porosities of the pressed pellets were approximately 24%. The compressed "green bodies" had sufficient cohesion to permit handling.
Both experiments were run in a Deltech vertical tube muffle furnace with a 3.8 cm inside diameter. Sample weight change was monitored with a Cahn microbalance. The furnace was heated to 1050íC and the cylindrical sample was placed on a perforated platinum support and suspended in the hot zone. The furnace was sealed and evacuated, purged with argon and then evacuated a second time and purged with helium, in order to remove air. Hydrogen was then allowed to flow upward past the sample at a rate of 122 cm3 / min for two hours. At the conclusion of each run the furnace was turned down and the sample was allowed to cool in flowing helium overnight.
Sintered samples were closely examined for evidence of cracking and delamination. All samples were weighed and measured prior to and after sintering to determine changes in density. Selected samples were cut and prepared as polished thick sections for optical and scanning electron microscopy (SEM). The compressive strengths of several samples were determined in accordance with the standard test method used for concrete (ASTM, 1986).
Ten experiments utilizing MLS-1 powder in the inconel mold were only partially successful. Samples were strongly sintered at 1100íC in two hours, in agreement with our previous results on much smaller samples (Allen et al., 1992). However, the bricks consistently contained large horizontal and vertical cracks, indicative of heating and/or cooling stresses. One sample, sintered at 1125íC for two hours, melted and had to be chipped out of the mold. Strong, uniform "bricks" of MLS-1 basalt were produced in three experiments by sintering in a fused silica mold on a steel base plate. The resulting bricks, measuring 7.9 (L) x 5.5 (W) x 3.6 cm (H), were sintered for 2 hours at 1100íC.
The MLS-1 bricks heated in this manner are crack-free, with the exception of minor expansion cracking near the top surface (Figure 2). The dimensions of the bricks did not change during sintering, indicating no significant increases in density.
Figure 2. MLS-1 basalt brick sintered at 1100íC for 2 hours. Length = 7.9 cm
The largest bricks were made from the glass-rich JSC-1 simulant, heated to 1100íC in two experiments. The crushed rock was initially compacted in the mold by vibration for five minutes, to a density of 2.45 g / cm3. The samples were sintered for 2.5 hours in a fused silica mold with a fused silica base plate and open top. A silica fabric liner was inserted to prevent the rock from sintering to the mold.
During sintering the bricks underwent uniform shrinkage, which increased the density to a maximum of 2.68 g / cm3. The bricks, which measure 13.1 (L) x 5.6(W) x 4.6 (H) cm, are uniform and crack-free. Apparently the abundant glass in this simulant promotes bonding among rock and mineral grains.
Electron micrographs of polished sections through one brick (Figure 3) show extensive bonding at grain interfaces, but no large-scale melting.
Figure 3. Cross-section of sintered MLS-1 basalt brick, showing bonding at grain-to-grain contacts. SEM back-scattered electron image. Frame width = 550 Ám
Sixty-three experiments were conducted in an attempt to reproducibly sinter MLS-1. Many samples suffered from over- or under-heating, nonuniform sintering, or stress cracking due to uneven cooling.
We achieved optimum results by heating at full power for 85 minutes, with the sample held at 980íC for 35 minutes. At the end of this time the sample was carefully cooled by ramping down the microwave power over a period of several hours. The cylindrical samples were uniformly sintered and crack-free. Sample density increased by an average of 11%. Compressive strengths near 1100 psi were measured.
MLS-1 samples heated in hydrogen at 1050íC for two hours were strongly sintered. They displayed weight losses of approximately 3.1 wt%, attributable to the reduction of iron oxides and associated oxygen release. This weight loss is equivalent to approximately 80% of the oxygen potentially available from FeO and Fe2O3 in the minerals. Despite this sample's compaction and sintering, hydrogen was able to effectively penetrate grains throughout the sample, and oxygen (as water vapor) was able to diffuse out.
Both of the 8 g reduced samples contained enough metallic iron to allow them to be lifted by a small magnet. The samples contained numerous micrometer-scale iron metal blebs, particularly in grains originally composed of ilmenite and magnetite. If the entire 3.1% weight loss corresponds to reduction of iron oxides, metallic iron comprised approximately 10.8% of the final sample weight.
Sintering of small test samples of lunar simulant basalt has been studied in detail (Allen et al., 1992), but "scaling up" to the size of a brick has proved extremely challenging. Crushed rock is an effective thermal insulator, which often leads to uneven heating and thermal cracking. The wide range of grain sizes typical of lunar soil can produce inefficient sintering and localized stress concentrations. Minimizing precompaction limited the number of grain-to-grain contacts available for sintering.
These drawbacks have been overcome by a combination of strategies. Thermal cracking has been minimized by relatively long heating and cooling periods, coupled with the use of fused silica molds with extremely low thermal conductivity. Temperature control has proven to be critical - a mere 25íC can span the difference between minimal sintering and near-total melting. The JSC-1 lunar soil simulant, with its glassy component, sinters significantly more uniformly than the totally crystalline MLS-1. Finally, vibratory compaction provides a relatively low-energy method of increasing grain-to-grain contact and improving sintering performance.
Crushed rock can be heated to the melting point in a microwave furnace, but sintering requires careful control of a number of factors. Thermal runaway, combined with the low thermal conductivity of crushed basalt, makes uniform sintering just below the sample's melting point extremely difficult. Once sintering has occurred the sample must be carefully cooled in order to minimize thermal stresses which lead to cracking.
A hybrid system utilizing internal microwave heating combined with external radiant heating was effective for sintering 3-cm cylinders of MLS-1. The optimum heating time proved to be 85 minutes, including heatup, followed by a slow cooldown. These factors represent a delicate balance between microwave and radiant heating, in a material prone to thermal runaway. Thus, any microwave sintering method is likely to be very sensitive to changes in sample composition, size and configuration.
One of several promising routes for extracting oxygen from lunar soil involves reduction of iron oxide by hydrogen at a temperature of 1050 - 1100íC (Allen et al., 1994). Our experiments have demonstrated that, under such conditions, compacted lunar soil simulant will not only yield oxygen but can also be effectively sintered. Thus, a lunar sintering operation could yield oxygen as a byproduct, or vice versa.
We also conducted these experiments to demonstrate the possibility of magnetic handling of lunar bricks. Such a system could prove advantageous versus more complex mechanical handling schemes. Bricks sintered in a hydrogen atmosphere do indeed contain enough iron metal to permit lifting by a relatively small magnet, and the lifting requirement would be reduced by a factor of six in lunar gravity.
Our experiments show that construction materials with specific properties can be made from lunar soil by a several sintering processes. Additional research is required to further reduce the energy and mass requirements of these processes, and to investigate the production of still larger bricks in complex shapes. The techniques described here produce samples large enough to permit measurement of thermal and radiation shielding and magnetic properties. The heating and cooling data can support first-level designs for flight experiments, which could demonstrate sintering technology on the lunar surface.
We acknowledge Joy Hines, David Altemir and Peter Nolan for their substantial contributions to the initial phases of our study. Dr. David Clark (University of Florida) provided valuable suggestions for our microwave experiments. Reviews by Bonnie Cooper, Brent Sherwood, and Bradley Smith significantly improved the manuscript. This work was supported by the Johnson Space Center Director's Discretionary Fund.
Allen C.C., Hines J.A., McKay D.S., and Morris R.V. (1992) Sintering of lunar glass and basalt. Engineering, Construction, and Operations in Space III (W.Z. Sadeh, S. Sture and R.J. Miller, Eds.), American Society of Civil Engineers, New York, 1209-1218.
Allen C.C., Bond G.G., and McKay D.S. (1994) Lunar oxygen production - a maturing technology. This volume.
ASTM (1986) Standard test method for compressive strength of cylindrical concrete specimens; Standard C 39-86, American Society for Testing and Materials, Philadelphia, PA, 6 pp.
Cooper B.L. (1992) Microwave sintering of regolith simulant. Space Resource News, 1,#8, pp. 1-3.
Goldich S.S. (1970) Lunar and terrestrial ilmenite basalt. Science, 171, p. 1245.
Kenkre V.M., Skala L., Weiser M.W., and Katz J.D. (1991) Theory of microwave interactions in ceramic materials: the phenomenon of thermal runaway. Journal of Materials Science, 26, pp. 2483-2489.
McKay D.S., Carter J.L., Boles W.W., Allen C.C., and Allton J.H. (1994) JSC-1: a new lunar soil simulant. This volume.
Meek T.T., Vaniman D.T., Cocks F.H., and Wright R.A. (1985) Microwave processing of lunar materials: potential applications. Lunar Bases and Space Activities of the 21st Century, Lunar and Planetary Institute, Houston, TX, pp. 479-486.
Shirley F., Buoni C., Lennon J.W., Mezey E.J., and Weller A.E. (1989) A preliminary design concept for a lunar sintered regolith production facility. Battelle, Columbus, Ohio.
Weiblen P.W., Murawa M.J., and Reid K.J. (1990) Preparation of simulants for lunar surface materials. Engineering, Construction and Operations in Space II, American Society of Civil Engineers, New York, pp. 428-435.
Lockheed Engineering & Sciences Co., Houston, TX
NASA Johnson Space Center, Houston, TX