Electrical Conductors
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| Element | Electrical resistivity @ 20°C | Density (near r.t.) | Found on Luna? |
| Silver | 15.87 nΩ•m | 10.49 g•cm−3 | Unknown |
| Copper | 16.78 nΩ•m | 8.96 g•cm−3 | Unknown |
| Gold | 22.14 nΩ•m | 19.3 g•cm−3 | Unknown |
| Aluminum | 26.50 nΩ•m | 2.70 g•cm−3 | Yes |
| Magnesium | 43.90 nΩ•m | 1.738 g•cm−3 | Yes |
| Iron | 96.10 nΩ•m | 7.15 g•cm−3 | Yes |
| Chromium | 125.00 nΩ•m | 7.86 g•cm−3 | Yes |
| Titanium | 420.00 nΩ•m | 4.506 g•cm−3 | Yes |
| Manganese | 1440.00 nΩ•m | 7.21 g•cm−3 | Yes |
| Sodium | 209.6 1/mohm-cm | 0.971 g•cm−3 | Abundant |
| Lower number => better conductor | r.t. = room temperature |
Contents
Silver
Silver is the best known conductor, but in an oxygen rich environment it tarnishes. Silver is used in specialized equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high frequencies.
In the Lunar outdoors, (in a vacuum where it can't tarnish), silver would be a marginally better, if heavier, conductor than copper, and a way better, but much heavier, conductor than aluminum.
Silver is not readily available on the Moon.
Copper
As a general conductor copper is the most commonly used on Earth because it's cheap, reasonably flexible, reasonably light and the 2nd best conductor and the best per unit weight. Copper allows for ease of soldered and crimped/clamped connections. It corrodes worse than silver, this is usually seen in older wires that have turned green.
Copper is not readily available on the Moon.
Gold
Gold is not an especially good conductor at all, though it is better than aluminum but not per unit weight due to it's much higher density. It is very expensive, but compared to the cost of transport to the Moon from Earth, the cost is not significant. Gold is usually only used as a conductor in very specialized applications such as very fine wires like those used to wire bond integrated circuits to their lead frames.
A more important everyday use of Gold is in connectors
For connectors gold reigns supreme for several reasons
- 1. It doesn't tarnish (important on Earth, important indoors on Luna)
- 2. It is soft, so you can make the connectors tight and they dig into each other forming a good connection.
Gold is not readily available on the Moon.
Aluminum
Aluminum is commonly used as a conductor here on earth, in fact you use it every day without realizing it. High Tension cables have a steel core and an aluminum outer layer. It's used because losses are fairly low at 110kV and the weight of the cable and cost of the towers is important. Steel cored aluminum cable allows a longer span and is the most common form of aluminum wire used for HT lines on Earth.
One major disadvantage of Aluminum on Earth is that it corrodes rapidly in an oxygen atmosphere. The corrosion does not go right through however, only forming a thin layer on the surfaces exposed to oxygen. On the moon, in a vacuum environment, it would be an excellent material to use.
As a general purpose electrical conductor it's not great, but considering that copper is not readily available on the Moon there appears to be no choice but to use aluminum. This means regular supply voltages higher than 110V would probably be better. In some places on Earth it is illegal to use aluminum for general wiring mostly as a result of fire risk caused by contractors using too light a gauge in the past.
Aluminum is abundant on the Moon.
Magnesium
Magnesium is not an especially good conductor, being less conductive than aluminum but it is lighter. It has some other major drawbacks which make it completely unsuitable for use in electrical installations. This metal burns in oxygen, or nitrogen (forming magnesium nitride) or even carbon-dioxide (forming magnesium oxide and carbon). Once burning, it's very difficult to extinguish. Magnesium also reacts with water.
This is a highly flammable metal, but while it is easy to ignite when powdered or shaved into thin strips, it is difficult to ignite in mass or bulk.
There are many other uses for magnesium but it is completely unsuitable for electrical wiring.
Magnesium is readily available on the Moon.
Iron
Iron is widely used in many applications, not an especially good conductor, very prone to corrosion in a oxygen atmosphere.
Iron is rarely used in its pure form for anything. Even railroads use steel, an iron alloy, for their tracks. Not suitable for flexible wiring, too brittle. The lower gravity on Luna may make it usable for some outdoor electric railroad applications.
Iron is abundant on the Moon.
Chromium
Not a very good electrical conductor by any means, but it does have its uses in this field. Chromium Boride (CrB) is used as a high temperature electrical conductor.
There are many other uses for this metal.
Chromium is readily available on the Moon. (Boron is not known to be available)
Titanium
Titanium's properties as an electrical conductor can best be described as hopeless.
There are many far more suitable uses for this metal.
Titanium is readily available on the Moon.
Manganese
Manganese is an even more hopeless electrical conductor than titanium.
There are many far more suitable uses for this metal.
Manganese is readily available on the Moon.
Sodium
Sodium is about one half as good an electrical conductor as Aluminum and is so abundant on the Moon that it is expected to be a significant by-product of the production of Volatiles. At room temperature it is a soft shiny metal and is easily worked.
It melts at a very low temperature (98 C) which can be both an advantage and a limitation. The low melting point means that very little energy is necessary to purify and work it. In use, care must be taken not to overheat it, but even this can be an advantage. Aluminum, in contrast, requires a very large amount of high quality energy to smelt.
On Earth, Sodium is used as an electrical conductor only in unusual circumstances. The primary problem is that it reacts violently with water, even the water in human skin. This is not a problem on the Moon as there is no water at all.
Sodium Scenario
Many of Sodium's properties that make it hard to use on Earth can be a boon on the Moon. This is easier to understand with a scenario of a possible lunar use. Consider the following case:
In our Timelines we are at the early part of the Miner Period. We have enough volatiles production to generate useful by-products and a prototype shop in which to hand-build the materials and tools we need.
We are just completing a moderate-sized solar power system of 100 kilowatts on high ground. We need to use the power for an industrial operation located one kilometer away and down a slope.
We have manufactured three kilometers of power cable to do the job and now have it on large reels made from discarded decent vehicle struts. The cable is made from three materials, (Sodium, Potassium, and Iron) and is 100% lunar materials.
The cable is multi-layered and coarsely stranded. The center single strand is iron for strength. The next layer is Potassium strands that provide little conduction but they bulk out the cable and help with several thermal problems. Next is a single layer of strands of Sodium that are the main electrical conductors. The final outer layer is of finer Potassium wires, also abundant on the Moon, lying in the troughs formed by the Sodium strands. Most of the voids between strands are filled with fine glass fibers made from industrial slag. These fibers help the cable keep its shape, keep it flexible, and help heat get out. The cable is a two square centimeter in cross-section which is about the diameter of a large finger. It has no outer insulation at all.
To lay the cable, we have modified a rover that was originally driven by people but is now robotic. It has a complex trenching rig at the rear that can bury cable a meter into the regolith in a very controlled manner. On its back is a mechanical a handler for one oversized cable reel. On its front is a small bulldozer blade.
The solar power station has a power conditioner that puts out 1000 volts of three-phase AC power at 2 kilohertz. The there phase configuration is optimal for industrial applications. The low RF frequency greatly reduces the amount of iron needed in transformers and motors.
The rover starts the first of three trenches at the power conditioner. In one slow and complex operation:
- It cuts a trench a meter deep and 100 mm wide
- It sieves the regolith into three sizes, fines, course, and rocks.
- It packs 20 mm of fines into the bottom of the trench
- It lays the cable down in an easy serpentine pattern.
- It packs 100 mm of fines on top of the cable
- It dumps the course material back into the trench
- It tops of the trench with the rocks.
- During these operations it keeps the trench open with two aluminum side plates.
The meter depth provides electrical insulation, radiation shielding, and thermal stability. The packed fines help remove heat from the cable and insure that sharp rocks do not damage it. The serpentine pattern allows the cable to thermally expand and contract without damage. The top rocks give a clear visual indication that something is buried here.
The rover works away from the power station until it reaches the slope. It then lowers the bulldozer blade and shifts the weight of the cable reel back. It then starts working straight down the slope rolling a mound of regolith in front to control its decent.
This process is repeated three times. At the cable ends sensors are buried to measure the current and the temperature of the cable and the surrounding regolith. The exposed ends of the cables are covered with glass or ceramic insulating tubes. The ends of the cables are placed into aluminum tubes and crimped solid. The aluminum then bolts into the power converter connections.
In normal operation the Sodium carries almost all the power. The waste heat generated in it is transferred to the packed regolith fines. Getting rid of waste heat is the most difficult problem in power generation on the Moon. The large cable core insures that the Sodium wires will have good contact over a large area.
In the case of a failure that generates very high currents in the conductors, the Potassium wires will be the first to start to melt as Potassium has a 25 C lower melting point than Sodium. This melting will adsorb an enormous amount of heat, but only for a short time. The cable will be able to withstand a few seconds of a full short and by then the power conditioner should be able to detect the fault and open the connection. The outer layer of Potassium wires should melt on the first overload and from then on it will help improve the transfer of heat to the regolith.
The strengths of this scenario are the very large amount of lunar material used and the very small amount of Earth material needed. Its weakness is simply that we do not know enough about the thermal properties of man-disturbed regolith to know if we can successfully dump the waste heat. Thinking the problem through this way does help us understand what questions we need to ask.
