Molecular Nanotechnology and Future
June 26, 2010 2 Comments
Molecular nanotechnology (MNT), the design and assembly of macroscopic objects literally atom by atom has become a burgeoning field of study, and may constitute the “last technological revolution”. The sweeping changes anticipated from these developments have been treated elsewhere. A major barrier, however, to nanotechnological development is the vast numbers of atoms in even a minute macroscopic object. Although individual molecules can be manipulated by scanning probe microscopes, building a macroscopic object in this manner would take a significant fraction of geologic time. Hence, practical assembly must involve vast numbers of parallel operations. Note, for example, that ordinary solution chemistry is massively parallel by its very nature, as it relies on the thermal motion of colliding molecules to test vast numbers of possible rearrangements over short time scales.
To solve this problem, Drexler has proposed molecular scale “assemblers”, molecular machines capable of building copies of themselves. In this way the huge number of molecular assemblers necessary for parallel construction can themselves be assembled in a reasonable time. Of course, fabricating assemblers capable not just of atomistic control but of self-replication is far beyond current capabilities, though again it is a routine capability of biological systems.
Pollution Control: A Nanotechnological Driver
Thus, although energetically cheap element separation at the molecular level is conceptually possible, it seems at first sight a long way off because of the enormous R&D effort needed to develop a full MNT capability. Drexlerian molecular assemblers are not imminent, and unless developmental pathway(s) can be identified, the relevance of MNT to space development over the near to mid-term is unclear.
It seems such pathways exist, however. First, it appears an interim middle ground exists between present-day “shake and bake” synthesis approaches and full-scale Drexlerian assemblers. One such pathway is molecular self-assembly, which is receiving much study, and which conceptually provides a way of atomic-scale structuring without assemblers. Another way is to use primitive assemblers, such as gangs of scanning-probe microscopes themselves probably made by conventional microlithography techniques, to “sculpt” molecularly perfect structures on surfaces. Although the number of atoms that can be individually arranged this way remains minuscule, it may be practical for constructing highly selective, tailored catalytic surfaces. In this way the intrinsic parallelism of solution chemistry can be exploited; synthesis yields could be vastly improved through effectively excluding the non-catalyzed reaction pathways. A major problem in conventional chemical synthesis is the number of unwanted byproducts that form because the synthesis reactions are too unselective; this not only decreases yield but leads to separation and disposal problems. Furthermore, the decrease in yield is substantial for syntheses requiring multiple steps. (Again, this approach is anticipated by biosystems, which use highly specific catalysts–enzymes–to direct particular synthesis pathways in living cells.)
In addition, such “nanostructured” materials have the advantage of no moving parts, which eases the engineering difficulties. Nonetheless, the developmental problems remain formidable, and financial incentives must exist if they are to be developed over relevant timescales.
Such an economic driver exists: pollution control: As noted already, it is merely another aspect of separating atoms. Thermal-based approaches, however, as used in traditional pyrometallurgy, are obviously impractical because of the low concentrations involved. Although isothermal phase changes can be used (e.g., precipitation), they still have serious limitations; they require additional reagents (which probably were purified by pyrometallurgy), and there is little control over the precipitates as their nature is set by the laws of chemistry. To wit, there are definite limits (set by the solubility products) to the concentrations that can be treated; the nature of the precipitated phase may be inconvenient (e.g., through being vulnerable to oxidation); and finally, changes in solution composition can cause unwanted phases to form, depending on the species present, their concentrations, and the stability fields of possible solid phases.
Selectivity is also an issue; it is common to have low levels of a toxic ion (e.g., Pb) among a much larger concentration of an innocuous ion (e.g., Ca), and a practical extractive process thus must strongly discriminate in favor of the rare ion.
More promising approaches to separation involve nanostructured materials, such as highly selective semipermeable membranes, which could filter out and concentrate particular solutes (e.g., heavy metals). Already, molecular sieves such as zeolites are used to separate gaseous N2 from O222, but wider use of such separation is hindered by the expense of crystallizing the sieves. Specific adsorbers, with molecular binding sites highly specific for certain ligands, furnish another example. Note also that such devices do not involve nanotechnological machines–i.e., devices with molecular-scale moving parts; they instead operate passively.
Both selectivity and extraction of solutes at low concentration requires precision at atomic scales, indeed, current limitations in the applications of membrane technology largely result from fabrication difficulties. The materials are expensive, rather delicate, and molecularly imprecise. Hence the desirability of atomically precise assembly provides incentives for near-term nanofabrication techniques.
In addition, a vast and growing literature exists on highly specific complexing agents (typically macrocyclic compounds such crown ethers and calixarenes) for various metal ions, both for potential therapeutic uses as well as for environmental and hydrometallurgical applications. However, many such compounds are not currently economic due to their costly syntheses. Hence, directed catalysis, as by nanotailored catalytic surfaces as described above, may make such compounds economic and provide another economic motivation for “interim” nanotechnology.
Initially, pollution control will drive these technologies because it is the high value application; the value of the extracted material itself will be insufficient to pay for the technology. Applications initially will lie in such areas as the clean-up of industrial wastewater streams, which is required before their discharge into surface waters. Environmental remediation, such as the clean-up of dump and mining sites (in particular, the amelioration of acid drainage resulting from the oxidation of reactive tailings), are also obvious near-term applications.
As these technologies mature and their costs fall, however, they will ultimately blur the distinction between a “pollutant” and a “resource”; that is, the value of the extracted material will become important in itself. Indeed, as demand increases many sources containing metals in aqueous solution may become attractive. (Note also that the byproduct of such extraction processes would be pure water, which hardly poses a disposal problem.) Seawater is an obvious possibility, but highly saline natural brines may be more attractive. Indeed, deep, saline groundwaters such as those associated with oilfields currently pose a disposal problem.
Because these technologies involve extraction from solutions, it may also prove economic to leach materials containing useful elements, and recover metals from the leachates. This could be looked on as an extension of present hydrometallurgy, as with the present-day cyanide-based solution extraction of Au, or the leaching of Cu with dilute H2SO4. For a further example, during World War II an experimental process for magnesium production involved the dissolution of olivine ((Mg,Fe)SiO4) by a strong mineral acid, such as HCl. Obviously Fe could be a byproduct (unwanted at the time); these authors also noted that Ni and Co, which commonly substitute for Fe and Mg at concentrations up to ~2000 and ~130 ppm, respectively, could be recovered. Finally, another unwanted byproduct was silica gel formed from the disaggregated mineral, but this may itself prove useful in a silicate-based nanotechnology, as discussed below. Olivine is a ubiquitous mineral; it makes up most of planetary mantles, and is locally abundant at the surface of both the Earth and Moon.
One hindrance to wider application of such solution-based extractive processes has been the necessity for selective extraction of solutes from dilute solutions. This is the very problem of pollution control again, and underscores the fundamental fact that what’s a “resource” and what’s a “pollutant” is merely a matter of perspective. Note also that biosystems have anticipated a solution-based approach to extracting raw materials; consider digestion.
In addition, over the longer term MNT is likely to change substantially what elements are desired. In particular, current technology relies heavily on metals for structural members. It is well-known, however, that ordinary macroscopic materials are a couple of orders of magnitude weaker than the ultimate strength limits set by chemical bonds because of their extremely high densities of defects, such that the strengths are determined instead by such things as grain boundaries and dislocations. Under such circumstances metals are useful because they are highly tolerant of microflaws, even at extreme densities; incipient cracks tend to “heal” via plastic deformation rather than propagate. However, metals are intrinsically weak because of this readiness to deform; brittle materials are potentially far stronger, but liable to catastrophic failure via propagation of Griffith cracks unless they are essentially defect-free at a molecular level. MNT should allow fabricating such defect-free materials, with profound potential consequences.
Most theoretical studies have focused on tetrahedral (sp3) carbon frameworks (“diamondoid”) as the structural basis of MNT. This is partly motivated by the enormous strength/weight ratio theoretically possible with such networks, but the familiarity and vast knowledge base of organic chemistry also provide a motivation. However, silicates, compounds of Si and O, are a potentially valuable alternative31. Silicates are based on an SiO4 tetrahedron that easily enters 3-D coordination; that is, each vertex can be shared with an adjacent tetrahedron such that all oxygens “bridge” between two silicon atoms. Furthermore, the Si-O bond is strong and directional, due to its partial covalent character. Moreover, in contrast to “diamondoid” carbon, silicates can polymerize at STP, even from aqueous solution; hence a silicate-based MNT may well be nearer term.
Finally, the crust of the Earth is largely made of silicates; oxygen and silicon, respectively, make up 60.4 and 20.5 atom percent of the crust, and thus raw materials are literally everywhere. However, conventional ores are seldom silicates, simply because of the difficulty of breaking up the Si-O bonds with current pyrometallurgy. Indeed, the waste from conventional mining largely consists of comminuted silicates; ore minerals are typically sulfides, and must be separated from the silicate “gangue” by grinding and flotation. The left-over silicate debris (“tailings”) currently constitutes an environmental problem; it is unesthetic, commonly constitutes a dust hazard, and the oxidation of residual sulfides commonly leads to acidic drainage. Its very comminution, however, suggests that tailings might be ideal feedstock for a silicate-based MNT, and certainly there would be no environmental objection to its reprocessing.
Ironically, therefore, the silicates that make up the bulk of the Earth, and that have been ignored in traditional resource scenarios, may yet prove to be among the most valuable raw materials for a truly mature technology. Indeed, the metals such as Fe, Al, Mg, and so on that make up a large percentage of common rocks may ultimately become a (largely) unwanted byproduct of a silicate-based nanotechnology.
Implications for Space Development
- Terrestrial Materials
Environmental demands are only going to increase in the coming years, and although this has been viewed as increasing the potential demand for space-based resources, it also increases the demand for technologies to ameliorate environmental problems. As described above, this may have the paradoxical effect of quelling demand for off-earth resources, at least for use on Earth. When (say) Ni and Co can be extracted from a wastewater stream at parts-per-million levels, there seems little incentive to mine them from a sideritic asteroid. Indeed, as argued above metals may become a largely unwanted byproduct of a maturing nanotechnology, as the greater intrinsic strengths of brittle materials can be exploited.
Another growing issue is the pressure for total product life-cycle closure, such that the disposal and byproduct costs of a commodity are initially factored into its price. Even if metal is imported for terrestrial use, the ultimate disposal of that metal, due to wear and replacement, represents a cost that must be dealt with.
The above considerations, however, do not apply to energy derived from space, as with the oft-repeated proposals for solar power satellites. Nonetheless, it should be noted that the vast energy demand of present technology is largely because energy is used as heat, the most disorganized and wasteful form of energy. Not only does the application and extraction of heat pervade contemporary processing, as in the traditional pyrometallurgy sketched above, but mechanical motion is nearly always ultimately fueled by a Carot-limited heat engine. Even immature MNT should yield much more efficient energy usage, because little will be used directly as heat; fuel cells, for example, which are not Carnot-limited, are another obvious application of the cheap fabrication of nanostructured materials. Again, a salutary example is provided by biological systems; consider the capabilities of photosynthesizing plants, which, moreover, use only ~1% of the incident sunlight.
- Effects in space development
Calling MNT “convenient” for space applications is likely to be a major understatement; it may indeed be vital for a viable off-Earth civilization. The value of the extreme strengths of MNT-based materials is only one aspect; as described above for terrestrial uses, MNT also makes practical a wide variety of raw materials. I had previously argued, based on several millennia of terrestrial experience, that anomalous concentrations of elements–“ores”–would be necessary for space-based resource extraction, just as they have been on Earth. With the advent of MNT, this seems merely another naive extrapolation of current technology. In addition, because structural metals are likely to be unimportant even with a relatively immature MNT, the ready availability of even high-quality Ni-alloy steels on sideritic asteroids may prove irrelevant.
Of course, C is also abundant in carbonaceous chondrite-like bodies, and thus asteroidal bodies may still prove to be extremely attractive sources of raw materials, quite apart from their low gravity wells. Conversely, C is nearly absent from many rocky Solar System bodies, the Moon in particular, so a diamondoid-based MNT seems unattractive there. (Parenthetically, however, it might be noted that the largest off-Earth reservoir of C in the inner Solar System is the CO2 atmosphere of Venus, which thus may have unexpected long-term value.)
However, a silicate-based nanotechnology is likely to find many applications in space, as silicates dominate rocky bodies such as the Moon just as they do the Earth. Indeed, the regoliths mantling bodies like the Moon, which consist of silicate debris comminuted by eons of meteoritic impact, may prove to be ideal feedstocks. A silicate MNT devised to handle terrestrial mining debris should be readily adaptable to such regoliths.
The separation of elements at an atomic level is an obvious near-term application of molecular nanotechnology. Viewed in one way, this is the problem of resource extraction; but viewed in another, it is the problem of pollution control. Indeed, pollution control is likely to be an economic driver for molecularly precise fabrication, because of the ongoing financial incentives involved.
This has two major implications for space development. First, materials from lunar or asteroidal mines are unlikely to be significant for terrestrial use; when desired elements can be recovered at ppm levels from aqueous solutions, whether wastewater streams, leachates, or natural brines, bringing them in from space is unlikely to make economic sense. Moreover, under a “total product lifetime closure” approach, even space-derived material will have hidden environmental costs due to its ultimate costs of disposal, and such costs will have to be addressed in any case.
Second, by the same token such technologies vastly broaden the potential sources of raw material in space for development in space. When even low concentrations of a desired element can be exploited, “ores” in the traditional sense become unnecessary. However, MNT is also likely to change considerably the desired elements; in particular, structural metal is likely to become unimportant, whereas carbon will become highly sought after. More unexpectedly, the silicates that make up the bulk of the rocky bodies in the inner Solar System may also prove extremely valuable for MNT applications. Hence, the comminuted, rocky regoliths of bodies such as the Moon may prove to be ideal feedstocks, especially as a silicate nanotechnology is likely to be developed in any case for terrestrial applications.
[Credit: Implications of Molecular Nanotechnology for Space Resources by Stephen L. Gillett]