Dl11 Lunar ilmenite for solar panels

As discussed above, most photovoltaic cells are made with silicon or gallium arsenide. However, ilmenite - particularly ilmenite which is formed in a reducing atmosphere such as the Moon's - has excellent potential as a material for photovoltaic cells. Ilmenite comprises up to 20 percent of the lunar regolith in its richest locations. It has been studied for use as a feedstock for the hydrogen reduction process to manufacture oxygen. However, its use as a semiconductor and for solar cell manufacture is a relatively new idea.

There is an important difference between lunar ilmenite and terrestrial ilmenite. Lunar ilmenite is formed under reducing conditions, and consequently its iron is in the 2+ valence state.7 On Earth, the iron in ilmenite is only partially in the 2+ state, because there was more oxygen available during its formation. The iron in the 2+

7 In the 2+ valence state, the atom has lost two of its (negatively charged) valence electrons, and thus has a net positive charge of 2.

state is responsible for the good semiconducting qualities of ilmenite, thus lunar ilmenite would be preferred for semiconductors.

Engineers at Texas A&M University have grown monocrystalline ilmenite in the lab under reducing conditions to learn more about the benefits of lunar ilmenite as a semiconductor (Sankara, 1995). They use the Czochralski technique, which is commonly used in the semiconductor industry to grow bulk silicon monocrystals. It utilizes the chemical reaction

The procedure involves first grinding the material to mix it, then pre-heating it to 1,000°C in a reducing atmosphere, with about 258 torr (34,474 Pa) of nitrogen. This is not a perfect duplication of the lunar environment, but is one that can be maintained for long periods with the equipment that is available. X-ray diffraction plots are made of the sintered material. If it has not reacted into ilmenite, then the pre-heating cycle is repeated until there are good ilmenite lines in the X-ray diffraction plot.

Inductive8 heating is used because of the very high temperatures (1,405-1,410°C) required. In inductive heating, radio frequency is fed into a coil. The radio frequency couples with the crucible and causes eddy currents that heat it to very high temperatures. The temperature can be controlled by changing the power fed into the coils.

For this process to work, the right material must be chosen for the crucible. Platinum does not work well in a reducing atmosphere, so iridium is used instead. A graphite crucible, which is resistant to the high temperature and reducing atmosphere, surrounds the iridium crucible. The graphite is heated, and it heats the iridium which in turn heats the ilmenite inside.

In order to start crystal growth, a crystal "seed" must be immersed in the melt. They use a material that has a close lattice match to ilmenite to dip into the liquid, in order to grow a boule (a cylinder) with several single-crystal sections, one of which can be isolated and used as a seed for the next phase. The seed is dipped into the liquid at the correct temperature using sophisticated control systems. The temperature and the pull rate (how fast the seed is pulled out of the liquid) have to be monitored and continuously adjusted to achieve good crystal growth.

By controlling the operating parameters, the crystal can be grown to any diameter needed. A photograph of a completed boule is shown in Figure D.3. Wafers are cut from the boule and polished, and these become the substrate material for semiconductor devices. Very accurate control is required to avoid defects in the crystal structure. The crystal grows at a rate of about 2 mm per hour, so it takes about 20 hours to grow a few centimeters of material.

The researchers have also experimented with neural networks for control systems so that the process of creating ilmenite semiconductors can be automated. (This would be a good candidate for automation, as explained in Appendix A.) The goal is to eliminate human involvement in the process and make it fully automatic. The

8 Induction is the process by which an object having electrical or magnetic properties produces similar properties in a nearby object, without direct contact.

Figure D.3. A completed boule of synthetic ilmenite.

team has modeled zero-G growth on the computer, in preparation for future zero-G experiments.

Because large-band-gap semiconductors such as ilmenite may display electrical resistivities that are two to three orders of magnitude greater than the resistivity of silicon, they can reduce leakage currents and hence reduce the power that is lost to heat dissipation. Extensive cooling equipment is required for silicon-based electronics on spacecraft. Much of the cooling equipment could be eliminated by using a lunar ilmenite semiconductor.

Another problem with silicon and gallium arsenide is that their conversion efficiency is limited at the shorter light wavelengths of the space environment. Ilmenite, with its higher band gap, will have a higher response to this full spectral range and thus may be more suitable for space-based solar cells.

Radiation damage is the major life-limiting factor for a photovoltaic array, especially in space. A typical silicon solar array may be oversized by as much as 40 percent to assure sufficient power over the life of the mission, whereas the large band gap of ilmenite makes it naturally radiation-resistant.

Lunar ilmenite would also be an excellent material for solid-state lasers for optical data storage systems. Optical data systems are limited by the wavelength of the laser light used in read and write operations. Because of its higher band gap, ilmenite can be used for shorter-wavelength lasers, which will enable higher data densities.

The initial work on synthesizing and characterizing lunar ilmenite for its semiconducting properties has been completed, but much work remains. For example, we need to know more about its structure and the nature of the oxidation states of the iron. Then modifications can be made in the system to suit the needs of technology. Another area of research is the annealing of the material to stabilize it and enhance atomic ordering.

D.l.1.2 Thin films

Thin-film technologies are being developed to reduce the mass of light-absorbing material required in a solar cell. This can lead to reduced processing costs compared with bulk materials, but also tends to reduce energy conversion efficiency. To address this challenge, multi-layer thin films have been used, which demonstrate efficiencies above those of bulk silicon wafers.

Thin-film solar cells developed directly on the lunar surface, using ilmenite-based semiconductors and robotic manufacturing equipment, are one of the enabling technologies for a lunar electric grid (Chapter 7) to be constructed.

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