Nanophase iron - Lunar Homestead

Nanophase iron – Photo – en.wikipedia.org/wiki/Space_weathering – Author – IntrplnetSarah

Nanophase is a term for materials with grain sizes between 1 and 100 nanometers. Nanophase iron (np-Fe⁰) is typically created by impact heating but can also be the result of geological activity and solar wind sputtering. Either way, it’s a source of iron that might be relatively easy to extract.

Meteoroid (and micro-meteoroid) impacts generate heat. This heat releases the hydrogen trapped in the regolith and creates a reducing environment that strips oxygen from the iron-rich materials involved, leaving nanophase-iron behind. The Lunar free iron post has a more thorough description of the process.

Sputtering is when solar wind ions remove individual atoms from regolith grains. These removed atoms are either lost to space or re-deposited on nearby grains.

The geological processes that create np-Fe⁰ are simply iron reduction of iron-bearing minerals in very hot or molten rocks.

My thoughts

Not sure how we can use the nanophase iron directly.
- Can’t mechanically separate it out. Way too small.
- Melting it out may be an option.
  - Melt the dust then use powerful magnets to pull the iron particles out of the molten glass?
    - Would also get mineral particles with nanophase iron rims.
  - Then use the melted (iron-free) glass to make things?
We can definitely use nanophase iron indirectly though.
- We can use microwaves to melt/sinter very fine Lunar dust.
  - Create bricks
  - Create small structural components
  - Create small parts
  - Hermetically seal structures (melt a layer of dust)
    - Sounds like something to research in addition to the iron.
    - We can create roads/landing pads
- Microwaves might be more efficient but are hardly low tech. We might be better off using simple focused solar thermal heating. Easy to maintain and to build using Lunar resources (iron, aluminum, glass).

Abundance

Native Fe in Lunar soils is 10X greater than in the rocks the soils were derived from. Meteoroid impacts only contribute 1% to Lunar material, with meteoroid iron contributing a fraction of that 1%. The majority of this iron is from auto-reduction of FeO in minerals during impact strikes (8).
Nanophase iron can be found in two places, agglutinate glass and vapor/sputter deposited rims (patina) on individual soil grains (7).
Lunar soil grading (8)
- 95% is finer than 1 millimeter
- 50% is finer than 60 micrometers (the thickness of a human hair)
- 10% – 20% is finer than 20 micrometers.
Primarily created by impact strikes and found in agglutinates.
- Nanophase-iron is most likely to show up in the smallest sized fraction of the regolith (2, pg 434).
  - Particles 4 – 33 nanometers had concentrations of 0.20% Fe⁰ by weight (1, pg 320).
  - Particles 4 – >33 nanometers had concentrations of 0.33% Fe⁰ by weight (1, pg 320).
  - Particles 2 – >100 nanometers had concentrations of 0.33% Fe⁰ by weight (1, pg 320).
- These small fines (which include the np-Fe⁰) are then held together in the form of agglutinates (2, pg 434) or coating the surface of mineral and rock particles (4).
  - Agglutinates make up 5% – 65% of Lunar soil, with an average of 25% – 30% (1, pg 298). Lunar Bases and Space Activities of the 21st Century states that the average is over 50% (3, pg 500).
  - One study put agglutinate abundance at 45% -47% for fine (<10 to 45 micrometer) soil (unknown if by weight or volume)(2, pg 432)
  - Agglutinate particles are the primary carriers of nanophase iron and are usually smaller than 1mm (2, pg 439).
  - Fe in agglutinates = few nanometers to several hundred nanometers (7).
Up to 90% of grains have rims (7).
- The npFe0 particles (blebs) on rims average 3nm in diameter (1-15nm in range) (7).
- Iron deposited by sputter vaporization on rims does not need hydrogen to produce a reducing environment. The temperatures reached in vaporization are sufficient to separate out the FeO (7).
- By volume, the majority of metallic iron in lunar soils can be found in agglutinates. However, the npFe0 on the rims can have a larger impact on the physical and optical properties than those found in the agglutinate (7).
  - Finer grains have higher surface to volume (or rim to grain) ratio. Since iron concentrates in the rims, the concentration of nanophase iron increases the smaller the particles get (7).
  - Agglutinitic glass is fragile and will quickly break down into smaller grain sizes, resulting in the iron enrichment of the smallest particle sizes (7).
- For grain sizes <50 micrometers, the amount of rim iron can equal the amount of iron found in the agglutinate glass (8).
Nanophase metallic iron has several effects on the optical properties of Lunar regolith, More nanophase metallic iron makes regolith darker, reduces the intensity of silicate mineral absorption bands, and shifts the spectral slope towards the red (longer) wavelengths (6). We can use this to locate areas rich in np-Fe⁰.
- Nanophase iron can potentially cause misinterpretation of remotely obtained spectral data (2, pg 414). Although not defined, I would guess that problems arise because the nanophase-iron makes the top layer of regolith (0.5mm ?) look more iron rich than the regolith beneath it actually is.
- Larger np-Fe⁰ particles (40-50 nanometers) contribute less to spectral change than smaller particles (6).
- Nanophase iron vapor deposited on the rims of small particles are thought to be the primary cause of optical and spectral changes (6).
Some studies have suggested that magnetic fields at the Lunar surface may deflect the solar wind so that regolith at the center has less weathering and the edges of the field has more (2, pg 440). We may find more and/or larger nanophase iron particles at the edges of localized magnetic fields (2, pg 440). These could be locations for serious iron extraction.
Lunar regolith is composed of 40%- 50% dust-sized particles (<50 micrometers) (8).

Total (Fe⁰) in combined Lunar samples (highland and mare) is 0.54±0.18% (equivalent weight) (10).
- 0.20±0.10% (equivalent weight) of combined Lunar samples was nanophase iron (40-330A), mostly in agglutinate glass

- 0.17±0.08% (equivalent weight) was meteoroid particles (predominately >330A)
- 0.17±0.08% (equivalent weight) was from source material (predominately >330A)
Mare maximum concentration of metallic iron in regolith (FeO 15% by weight) (10)
- About 1.0% (equivalent weight) of total regolith
- For soils with FeO >10% (by weight) then Fe⁰_RM> Fe⁰_MM
  - This means that iron rich soil (like mare) will contain more metallic iron created by reduction caused by impact events than metal from the impactor itself.
Highlands maximum concentration of metallic iron in regolith (FeO 5% by weight)
- About 0.7% (equivalent weight) of total regolith (10)
- For soils with FeO <10% (by weight) then Fe⁰_RM< Fe⁰_MM
  - This means that iron poor soils will have more metallic iron from meteoroids and less from native iron.

Chemical composition

Nanophase iron is pure iron (Fe⁰).
Majority of agglutinitic glass Fe grains had not been completely reduced to iron metal (7).
The iron on the surface of small (<0.4 micrometers) Apollo 16 regolith plagioclase particles has been shown to be nanophase iron (Fe⁰) instead of meteoroid components or iron implanted by the solar wind (5).
Highly reactive
- If inhaled can react with water to form HROS (Highly Reactive Oxygen Species) that attack lung cells.

Physical composition

Typical size range of nanophase iron particles (6).

- In agglutinates = 10-100 nanometers
- In vapor deposition rims on particles = 1-10 nanometers
40% of total observed metallic iron is 40-330A (10).

Physical properties

Magnetism
- Super-paramagnetic (9).
- The presence of nanophase iron makes fine particles (<50 micrometers) magnetic (8).
- Studies have successfully magnetically beneficiated soil particles in the 90-150 micrometer range (8).
- Efficiency of magnetic separation decreased as grain size decreased, down to 45 micrometers (8). Smaller sized particles would clump together because all the particles have a surface covering of ferromagnetic nanophase iron (8).
- A hand magnet will easily attract grains in the 10-20 micrometer range, even plagioclase (because of the rim deposits) (8).
Thermal
- Melting point Fe⁰= 1538°C, 2800°F, 1811 K (11, pg 4-66)
- Boiling point Fe⁰ = 2861°C, 5182°F, 3134 K (11, pg 4-66)
- Specific heat capacity (the amount of heat energy required to raise the temperature of a substance per unit of mass)
  - 449 J/(kg K) (11, pg 4-66)
  - 0.450 J/(g °C)
  - This means it takes 449 joules to increase the temperature of 1 kilogram of iron one degree of Kelvin.
Density
- 7.87 g/cm³(11, pg 4-66)
Hardness
- Nanophase metals are harder (up to 5 times) and more brittle than larger grain metals with the same composition (pages.mtu.edu/~suits/nanophase.html).
Compression/Tension
- Young’s modulus (measures how a material withstands changes in length when under lengthwise compression or tension) = 211.4 GPa (soft) / 152.3 GPa (cast)
- Bulk modulus (measures how much a material will compress under a given amount of external pressure. Expressed as the ratio of the change in pressure to the fractional volume compression) = 169.8 GPa

Notes

Nanophase sells Fe₂O₃ in nanophase form. (nanophase.com/products/iron-oxide/)

This dust (glass and fine grain soil particles containing nanophase iron) can be microwave melted or sintered (8). The presence of nanophase iron on fine soil (<50 micrometers) allows the soil to melt when exposed to low level microwave radiation (2.45 MHz) (8).
- Microwave energy can couple with the nanophase iron in Lunar dust to create temperatures >1000 deg C in a matter of minutes (8). (1000 dec C/min)(2000 deg C) (8).
- Can be used to hermetically seal underground structures (8). This might be an alternate research path if iron doesn’t pan out. Or maybe a parallel track so Homesteaders have several options.
- Microwave melting or sintering might have significant energy savings over conventional heating (8).

The magnetic properties of fine Lunar dust could also lead to solution in dealing with the dust, both inside and outside the habitat.
Fine regolith particles contain the majority (>80%) of solar wind implanted elements (due to higher surface areas) (8). The dust is where we should go for hydrogen, helium, nitrogen, carbon, and the rest of the rare elements we’re going to need.
Regolith simulants can’t really simulate the presence of very small (3-10 nanometer) nanophase iron in the agglutinate particles and on the surface of the grains.
Nanophase iron oxide is being researched as a way to deliver drugs (such as chemotherapy) directly to an area of the body using external magnets. Neat!

Data

Concentrations of metallic iron (Fe⁰) (percent equivalent weight) by particle diameter range (79 samples)(1, pg 320).

Particle diameter 40 – 330 angstroms (4 – 33 nanometers)
- Average = 0.20%
- Standard deviation = 0.10
Particle diameter 40 – >330 angstroms (>33 nanometers)
- Average = 0.33%
- Standard deviation = 0.15
Particle diameter 20 – >1000 angstroms (>100 nanometers)
- Average = 0.54%
- Standard deviation = 0.18

Dry magnetic separation of 1g samples (9)

Two highland soils
- Pure anorthite (magnetic susceptibility of χ= -0.39 µcc/gm) was separated from the diamagnetic fraction of immature highland regolith (<1mm).
  - Small amounts of iron-rich glassy components were separated out.
- Increasing the amount of anorthite (percent by weight) in a sample decreases the magnetic susceptibility and the recovery of anorthite of the sample.
  - Immature soil (8.8 I_s/FeO)
    - About 500 µcc/gm at 70% to 2 µcc/gm at 90% anorthite
    - 0.8 lbs anorthite/pound of soil at 80% anorthite vs <0.2 lbs anorthite/pound of soil at 90% anorthite.
  - Mature soil (106 I_s/FeO)
    - >1,000 µcc/gm at 70% to about 3 µcc/gm at 92% anorthite
    - 0.7 lbs anorthite/pound of soil at 80% anorthite vs almost 0 lbs anorthite/pound of soil at 90% anorthite.
- Alumno-silicate materials (such as anorthite) are diamagnetic.
Three high-Ti mare soils
- Ilmenite (magnetic susceptibility of χ approx. = 60 µcc/gm) and pyroxene were separated from the paramagnetic fractions of mare basalts (<1mm)
- Increasing the amount of ilmenite and pyroxene in a sample increases the magnetic susceptibility and decreased the recovery of these minerals of the sample.
  - Immature soil (14 I_s/FeO)
    - Ilmenite
      - Spike around 400 µcc/gm at 30% ilmenite
      - 0.12 lbs ilmenite/pound of soil at 24% ilmenite vs 0.06 lbs ilmenite/pound of soil at 28% ilmenite.
    - Pyroxene
      - Spike around 800 µcc/gm at 26% ilmenite
      - The graph is hard to read but basically a slight decrease in the concentration by percent weight corresponds with an increase in recovery.
  - Submature soil (35 I_s/FeO)
    - Ilmenite
      - Spike around 200 µcc/gm at 30% ilmenite
      - 0.13 lbs ilmenite/pound of soil at 24% ilmenite vs 0.05 lbs ilmenite/pound of soil at 30% ilmenite.
    - Pyroxene
      - Spike around 800 µcc/gm at 26% ilmenite
      - The graph is hard to read but basically a slight decrease in the concentration by percent weight corresponds with an increase in recovery.
  - Mature soil (78 I_s/FeO)
    - Ilmenite
      - Spike around 50 µcc/gm at 20% ilmenite
      - The graph is hard to read but basically a slight decrease in the concentration by percent weight corresponds with an increase in recovery.
    - Pyroxene
      - Spike around 800 µcc/gm at 26% ilmenite
      - The graph is hard to read but basically a slight decrease in the concentration by percent weight corresponds with an increase in recovery.
- Ilmenite and pyroxene are paramagnetic. This is because of the Fe²⁺.
Agglutinates (magnetic susceptibility of χ> 130 µcc/gm) and other fused soil components containing metallic iron were separated from the strongly magnetic fractions of all soils (<1mm).
- Apparent magnetism can range from diamagnetic to paramagnetic.
- Small amounts of metallic iron in agglutinates can overwhelm the magnetism of most Lunar materials.
Soil maturity affects the magnetic susceptibility of a sample. This makes sense as the amount of nanophase iron in a sample will increase with maturity.
- Soils of the lowest maturity are the best candidates for all materials, from alumno-silicates to ilmenite and pyroxene.
- As iron content increases in maturing soils, the soil becomes more magnetic. This soil magnetism overwhelms the magnetism of the diamagnetic and paramagnetic components and reduces their recovery.
Rocks are generally less magnetic that soil because of the higher nanophase iron in the soil.
Samples
- 67511 (immature highland) (8.8 I_s/FeO)
  - magnetic susceptibility of χ= -147.5 µcc/gm
  - Iron oxide (% weight) = 14.16
  - Fe⁰ (% weight) = 0.08
  - Soil maturity = 8.8 I_s/FeO
- 65701 (mature highland) (106 I_s/FeO)
  - magnetic susceptibility of χ= 433 µcc/gm
  - Iron oxide (% weight) = 4.9
  - Fe⁰ (% weight) = 1.55
  - Soil maturity = 106 I_s/FeO
- 71061 (immature mare hi-Ti) (14 I_s/FeO)
  - magnetic susceptibility of χ= 196 µcc/gm
  - Iron oxide (% weight) = 13.84
  - Fe⁰ (% weight) = 0.24
  - Soil maturity = 14 I_s/FeO
- 71501 (submature mare hi-Ti) (35 I_s/FeO)
  - magnetic susceptibility of χ= 434 µcc/gm
  - Iron oxide (% weight) = 16.54
  - Fe⁰ (% weight) = 0.49
  - Soil maturity = 35 I_s/FeO
- 10084 (mature mare hi-Ti) (78 I_s/FeO)
  - magnetic susceptibility of χ= 786 µcc/gm
  - Iron oxide (% weight) = 14.15
  - Fe⁰ (% weight) = 0.87
  - Soil maturity = 78 I_s/FeO

Concentrations, sizes, and origins of metallic iron (Fe⁰) in 79 Lunar Soil Samples (10).

Total (Fe⁰)
- 0.54±0.18% (equivalent weight) of Lunar soil is Fe⁰.
- Fe⁰_TOT = Fe⁰_RM + Fe⁰_MM + Fe⁰_SM
Fe⁰_RM
- Concentration of metallic iron produced by exposure-induced reduction of ferrous iron
- 0.20±0.10% (equivalent weight) of samples
- 40-330A in diameter
- Predominately associated with agglutinate glass
Fe⁰_MM
- Concentration of metallic iron produced by metallic phases of micrometeorites involved in forming agglutinates
- 0.17±0.08% (equivalent weight) of samples
- Predominately >330A
- 70% (equivalent weight) of total Fe⁰ in samples was Fe⁰_MM
Fe⁰_SM
- Concentration of metallic iron in source material (bedrock and breccias) for soil
- 0.17±0.08% (equivalent weight) of samples
- Predominately >330A
For soils with FeO >10% (by weight) then Fe⁰_RM> Fe⁰_MM
For soils with FeO <10% (by weight) then Fe⁰_RM< Fe⁰_MM
Highlands maximum concentration of metallic iron in regolith (FeO 5% by weight)
- About 0.7% (equivalent weight) of total regolith
Mare maximum concentration of metallic iron in regolith (FeO 15% by weight)
- About 1.0% (equivalent weight) of total regolith
40% of total observed metallic iron is 40-330A

Primary literature

Lunar Sourcebook
A Global Lunar Landing Site Study to Provide the Scientific Context for Exploration of the Moon
Lunar Bases and Space Activities of the 21st Century
Space Resources: Vol 3 Materials
Iron Isotope and the Origin of Nanophase Iron in Lunar Regolith
Space weathering simulations through controlled growth of iron nanoparticles on olivine
The Lunar Regolith
The Lunar Dust Problem: From Liability to Asset
Magnetic Beneficiation of Lunar Soils
Origins and size distribution of metallic iron particles in the lunar regolith
CRC Handbook of Chemistry and Physics 97^th Edition

Not used but relevant

Iron Isotope and the Origin of Nanophase Iron in Lunar Regolith (www.lpi.usra.edu/meetings/lpsc2012/pdf/1148.pdf)