Theoretical studies of molecular
nanotechnology (MNT) have focused on tetrahedral carbon
("diamondoid") frameworks as an ultimate goal. It seems
natural to wonder, though, whether silicon, carbon's second row
homolog in the periodic table, could form a reasonable basis for
MNT, especially given the importance of Si in semiconductor
technology. Although Si itself is unpromising--the Si-Si bond is
not nearly so stable as is C-C, and Si shows minimal tendency to
form the double and delocalized bonds so typical of C--silicates,
compounds of Si and O, indeed show promise for MNT, because of:
The strong and directional character of the Si-O bond,
due to its partly covalent character.
Their ease of polymerizing into 3D structures
("tectosilicates"). Silicates enter tetrahedral
coordination at STP, even out of aqueous solution, in
stark contrast to sp3 carbon.
Their thermal stability: certain tectosilicates, such as
quartz and feldspars, will even crystallize directly from
silicate melts at hundreds of degrees C.
Their stability to oxidizing conditions; whatever its
other virtues, C burns.
Their sheer abundance. O and Si make up, respectively,
60.4 and 20.5 atom percent of the Earth's crust. Nearly
all common minerals are silicates.
Silicates at ordinary pressures are based on an SiO44-
tetrahedron; these tetrahedra can share one through all four
vertices to build up a vast array of structures, including
chains, sheets, and infinite 3D networks. They thus in essence
form giant-molecule anions in which charge balance is preserved
by cations that fit between the anionic structures.
This structural variety only in part accounts for the
diversity of silicates, however, as other atoms can substitute
for Si. In natural systems, Al substitution is ubiquitous; one
class of alumino-tectosilicates, the feldspars,are the most
common compounds in Earth's crust. Other
alumino-tectosilicates,the zeolites, have great technological
importance as "molecular sieves" and catalysts, due to
their molecular-sized internal voids. Finally, other tetrahedral
substitutions (e.g., B, Ga, P, transition metals) vastly increase
the potential structures for MNT.
Siloxanes ("silicones"), "hybrids" of
silicates and organic compounds that consist of siloxy (Si-O-Si)
chains with H or organic side-groups attached to Si, show how
silicate-based structures could be integrated with C-based
structures. Cubosiloxane, H8Si8O12,
the siloxy analog of cubane, C8H8, in which
each C-C bond is replaced by an Si-O-Si bond, shows their
diversity; moreover, cubosiloxane is both considerably easier to
synthesize and more stable than cubane.
Though silicate raw materials are literally everywhere,
conventional mining waste seems especially attractive. Despite
their ubiquity, silicates are usually unattractive as ore
minerals due to the energy required to break the Si-O bond.
However, most ores consist largely of silicate minerals in which
the valued non-silicate ore minerals constitute only a
minor fraction. Such ore rock must be crushed and ground merely
to separate out the ore minerals; hence, not only is the energy
needed to break the rock up mechanically been largely
wasted, but the very finely comminuted waste from such mineral
dressing operations (the "tailings") presents a serious
disposal problem. Such material should furnish an excellent
feedstock for silicate-based MNT, however, especially as it is
relatively reactive due to its comminution.
Last, silicate-based MNT has major potential applications in
space development, as many extraterrestrial bodies (e.g., the
Moon) are dominated by silicates but have scant carbon. Indeed,
the surface material of the Moon, which largely consists of
silicate minerals comminuted by eons of meteorite impact, would
also be an excellent feedstock to a silicate MNT.
Stephen L. Gillett, Dept. Geol. Sciences, Mackay School of Mines,
University of Nevada, Reno, NV 89557, ph:702-784-4760, fax:
702-784-1833, email: email@example.com