Difference between revisions of "Surface passivation"
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Revision as of 09:33, 3 July 2017
Just like the flour on the dough hydrogen can keep parts from sticking together (crude analogy)
In advanced gem-gum nanosystems (the far term goal of atomically precise manufacturing) there are many nanoscale machine parts (crystolecules) embedded in the products.
In most cases these crystolecules need to have all the bonds on their surfaces sealed off. The technical term for a non-sealed open covalent bond is "dangling bond".
If the dangling bonds would not be sealed off the crystolecules would:
A) likely seamlessly and irreversibly weld together once they touch for a short period of time.
And would
B) react with any possibly present fluid or gaseous surrounding medium if it is not extremely nonreactive (like noble gasses).
Contents
Methods for passivation
Atoms in general do not behave like a construction toy with pieces that have a defined number of pegs and holes. Metallic bonds and ionic bonds (and electron deficiency bonds in some sense) violate that picture. Depending on the combination of electronegativities some unusual oxidation numbers may occur. (Interesting example Stishovite)
If one restricts oneself to a certain range of elements (or more broadly to certain combinations of elements next to each other) one can make sure to get to very good approximation the construction toy behavior.
Single bond passivation
Hydrogen has one shell electron (and one missing for a full outer shell) and behaves (in the targeted contexts) like a building block with one connection point. One can look at it as a plug that can be used to close up dangling bonds.
The same holds for fluorine (which is located below hydrogen in the periodic table if hydrogen is interpreted as member of the halide group) and all the other halides like chlorine bromine and iodine. One might want to stay away from fluorine since it's compounds are often toxic and its not too abundant. The same goes for bromine and iodine (weak reactive bonds). Chlorine while often toxic is at least highly abundant and accessible.
Multi bond passivation
Going to the left from fluorine and chlorine one finds elements with increasing numbers of shell electrons which increases the number of "connection points". Elements with up to three connection points are suitable for plugging up surfaces (suitable for passivation).
Note that due to the different bond lengths of the passivation elements and the base material stresses are induced that when left uncompensated lead to strains / deformations of the passivated crystolecules.
The next group to the left of the halides are the chalcogens with two missing shell electrons (two holes). They contain oxygen and sulfur. Both very suitable for passivation. The next group to the left of the chalcogens are the the pnictogens with three missing shell electrons (three holes). They contain nitrogen and phosporus.
Snapback stiffness threashold and friction
The more bonds the passivation atoms have to the substrate elements the stiffer are they against sideward force. There seems to be a critical point at which "snapback" starts to occur when two surfaces are simultaneously pressed and slid against each other. This should massively increase energy dissipation (more friction). This might be very useful for dissipation elements (mechanical resistors).
Passivation difficulty
Some materials are very easy to passivate. This includes diamondoid materials in the more narrow sense. Diamond, lonsdaleite (= hexagonal diamond), moissanite (= gem guality silicon carbide), silicon (cubic and hexagonal), boron-nitride and similar compounds.
Others may be more difficult. Polymorphs of silicon dioxide (e.g. quartz) has -OH groups sticking out when passivated. Not entirely unreactive to Water. (Big meshes of oxides may make it harder to create sliding interfaces)
Salts (like e.g. preiclase MgO) may not be passivatable at all and thus have only use as structural framework material.
Where leaving things unpassivated makes sense
Advanced gem-gum nanosystems will have their internal volumes for all practical purpouses perfectly sealed off by walls so dangling bonds not exposed to the outside (usually earths atmosphere or more or less salty water) are totally ok.
A lot of dangling bonds can even help. In the unlikely event that a stray molecule found its way in it will not get very far to critical points like e.g. mechanosynthetic cores or to points where it could act like a wrench in the gears. (Note that not every collision of a molecule with a radical automatically leads to a bonding event - misaligned spins can lead to repulsion force due to Pauli exclusion principle.)
By leaving internal structural elements unpassivated as much as possible one can save some hydrogen which may be important on in places where hydrogen is very scarce like e.g. on Venus and Mercury. Note that hydrogen usage in advanced gem-gum-systems is already low due to the fact that the used crystolecules are 3D objects compared to hydrocarbons which are 1D which makes for much less surface passivation (hydrogen) per internal volume (carbon).
Unpassivated bonds could maybe also used for energy storage and conversion. See: Chemomechanical energy conversion.
When kept at a distance unpassivated surfaces facing each other may have some interesting bearing characteristics.
Related
External links
- Wikipedia: Dangling bond
