Pure metals and metallic alloys
From apm
Why these are to first approximation a bad choice for stiff cog&gear style nanomachinery nanomachinery and gemstone-like compounds are the way to go instead:
- tendency to surface diffusion – some atoms do not stay where put at room temperature
- tendency to oxidation – for all but a few noble metals which are generally too rare for large scale structural purposes
- incompatibility with nanoscale passivation – making seamless covalent welding the default. (Related: Catenation)
- ...
Stoichometric intermetallic compounds sometimes fair somewhat better:
Despite still being electrically conductive and optically reflective in the human visible spectral range,
their bonds have a bore covalent character.
This makes them less ductile and more brittle, which is ...
- an undesirable property form macroscale parts with many high strain internal defects
- a desirable property for nanoscale parts with on average no defects
Some gemstones that contain non-mentals too are also still metallic in optical and conductivity and optical properties (similar to inter-metallic compounds). Examples include:
- pyrite FeS (iron sulfides)
- hematite and magnetide (iron oxides)
- titanium nitride and some other XY-type compounds where X=metal Y=nonmetal
