toms@fcs260c2.ncifcrf.gov (Tom Schneider) (04/04/91)
I was reading one of Drexler's papers in the Nanocon 1989 proceedings, where he
notes the possibility of creating diamond for structural components. So I
began to think of a shark with diamond tipped teeth. Wouldn't that have an
advantage over calcium? Wouldn't a diatom or shrimp encased in diamond be able
to survive better? So why don't living things use diamond? Or do they? If
they don't, is this telling us something about the limits of nanotechnology?
That is, maybe diamond can ONLY be made at high pressures and/or temperatures,
so a molecular machine couldn't do it, unless the machine operated under those
conditions.
It's clearly possible for cells to deal with crystals. We know of bacteria
that make magnets, that trigger ice formation (!) and lots of other cases of
mineral deposition, such as bone formation:
@article{Mann1988,
author = "S. Mann",
title = "Molecular recognition in biomineralization",
journal = "Nature",
volume = "332",
pages = "119-124",
year = "1988"}
Cells handle carbon structures all the time, so why are shells and structural
components mostly silicon and calcuim? The person who figures out how to get
a cell to grow a diamond may become rich!
Tom Schneider
National Cancer Institute
Laboratory of Mathematical Biology
Frederick, Maryland 21702-1201
toms@ncifcrf.gov
[I don't know about sharks' teeth, but human teeth are continually
redeposited from minerals in the saliva, in a process involving no
molecular machinery at all. Maybe this puts limits on the materials
that can work.
More fundamentally, there are some limits to the capabilities of
biomechanisms as they work inside cells. In particular, a design
that calls for a specific reaction to happen at one place and not
another is much more complicated than with a nanomechanism that
works like a "pick and place" robot arm. The mechanisms that
the cell has to use look a lot more like those of synthetic
chemistry, and the assembly of a complex design can become
exponential in the worst case. (Although I don't think this
applies to diamond per se).
--JoSH]john@granada.mit.edu (John Olson) (04/05/91)
Diamonds have a big disadvantage over bone in that diamonds are thermodynamically unstable. At all temperatures, graphite is the more stable state for carbon. If you heat up a diamond--even under pressure-- it will mostly revert to graphite. the only reason they last is that the diamond --> graphite reaction is very slow at normal temperatures. And the only reason we see diamond at all is that there is an equilibrium between graphite and diamond. So when you do the Superman synthesis (coal --> diamonds under high pressure and temperature), most of the graphite just stays graphite. Now, that's not to say that a nanomachine (or a cell) couldn't make diamond, but it will be more difficult to synthesize than materials which can be thermodynamically stable. When a diatom makes its shell out of silica, well, silica will precipitate into a solid without the diatom's help. The diatom just has to make a frame for the silica to sit on. Similarly (as JoSH pointed out) for teeth and bones, the minerals will precipitate out on their own.
erich@eecs.cs.pdx.edu (Erich Stefan Boleyn) (04/05/91)
toms@fcs260c2.ncifcrf.gov (Tom Schneider) writes: > ...So why don't living things use diamond? Or do they? If >they don't, is this telling us something about the limits of nanotechnology? >That is, maybe diamond can ONLY be made at high pressures and/or temperatures, >so a molecular machine couldn't do it, unless the machine operated under those >conditions. Not necessarily. Consider how diamond is produced... the intense temperatures and pressures are necessary because there is inherently a large energy input required to produce diamond. If you look at the chemical reaction, the activation energy is large, but there is no product energy, and since the energy required is so large, it is very slow. >It's clearly possible for cells to deal with crystals. We know of bacteria >that make magnets, that trigger ice formation (!) and lots of other cases of >mineral deposition, such as bone formation: Crystals are somewhat easy to produce... so biologically, most of what is done is making the production easier... usually by making the materials easily accessible, maybe even catalyzing the reaction. >Cells handle carbon structures all the time, so why are shells and structural >components mostly silicon and calcuim? The person who figures out how to get >a cell to grow a diamond may become rich! The energy required to do this as opposed to making crytals that have a small associated energy would be enormous. Imagine all of the food required to power those cells! Normally, organisms evolve into routes where the energy expenditure to try out a new idea is sufficiently small so that it is "worth" attempting. The bio-engineering necessary to make this thing work would be quite an achievement... worthy of some of the better science-fiction written. Erich "I haven't lost my mind; I know exactly where it is." / -- Erich Stefan Boleyn -- \ --=> *Mad Genius wanna-be* <=-- { Honorary Grad. Student (Math) }--> Internet E-mail: <erich@cs.pdx.edu> \ Portland State University / Phone #: (503) 289-4635
minsky@media-lab.media.mit.edu (Marvin Minsky) (04/09/91)
In article <Apr.4.22.02.48.1991.29630@athos.rutgers.edu> john@granada.mit.edu (John Olson) writes: > >Now, that's not to say that a nanomachine (or a cell) couldn't make diamond, >but it will be more difficult to synthesize than materials which can be >thermodynamically stable. There have been a number of such remarks. But it is not obvious that diamonds need by significantly harder to Nanofacture (is that a new word) than more stable substances. Because once you have positioned the carbon in the right place, etc., you might be able to zap it wigh an electric pulse much larger than is ever possible chemically -- while perhaps holding it under unusual mechanical pressure. Perhaps one point of nanotechnology has been overlooked: that when you do things one atom at a time, you're not at the mercy of the standard chemical constraints that come from thermodynamic statistics. Maybe there are even substances stronger than diamond that cannot occur in nature because the formation statistics are too unfavorable -- and yet are metastable enough, like diamond, to last a few billion years once formed.
dietz@cs.rochester.edu (Paul Dietz) (04/09/91)
erich@eecs.cs.pdx.edu (Erich Stefan Boleyn) writes: >> ...So why don't living things use diamond? Or do they? If >>they don't, is this telling us something about the limits of nanotechnology? >>That is, maybe diamond can ONLY be made at high pressures and/or temperatures, >>so a molecular machine couldn't do it, unless the machine operated under those >>conditions. > > Not necessarily. Consider how diamond is produced... the intense >temperatures and pressures are necessary because there is inherently a large >energy input required to produce diamond. If you look at the chemical >reaction, the activation energy is large, but there is no product energy, >and since the energy required is so large, it is very slow. Uh, fellows... you guys *are* aware of the work over the last few years on chemical vapor deposition of diamonds, aren't you? Carbon deposited from a near vacuum in the presence of large amounts of atomic hydrogen preferentially forms diamond. The first work (in Russia and Japan) used things like microwave discharges, but they can now make it work using just an acetylene torch flame as the source. This technology is already making its way into commercial products. High pressure is *not* required to make diamond. A source of chemical energy (in this case, atomic carbon and hydrogen) is needed, but that's also true in biological systems, where energy-rich compounds are consumed so that ordered structures (proteins, DNA) can be assembled with low error. Anyway, saying that diamond does not occur in biology because graphite is the lower energy state begs the question: why doesn't *graphite* occur? Or Kevlar, or nylon, or ultrahigh molecular weight polyethylene, or some other high strength materials? Most likely because evolution is constrained to do local search, and has looked at only a very tiny region of the search space. Preexisting solutions get adapted: proteins used to make spider webs, for example, even though other materials would be stronger. Paul F. Dietz dietz@cs.rochester.edu
merkle@parc.xerox.com (Ralph Merkle) (04/09/91)
Diamond was recently named "molecule of the year" by Science (volume 250, December 21 1990). On page 1641 it says: "Vapor deposition methodology now appears to be in an exponential phase of growth. Diamond films can be grown at pressures ranging from tens of torrs to 1 atmosphere. Film growth rates of 1 millimeter per hour are possible. Diverse volatilization methods have become available, including microwave discharges, hot filaments, plasma torches, and ion beams. The deposition of films at lower-than-normal temperatures (around 300 degrees C instead of the standard 700 to 1100 degrees C) has been accomplished through the addition of halogens to reaction mixtures; this is an important step if diamond is to be deposited on temperature- sensitive substrates. All of these variations on the basic CVD [chemical vapor deposition] theme are making possible faster production of better materials with diverse morphologies." In another section, the article states that "...one estimate is that diamond chips might be able to withstand temperatures as high as 5000 degrees C." Clearly, diamond can be synthesized at room temperature and at low pressure. The argument that diamond is "thermodynamically unstable" is irrelevant if the time required for it to adopt a "stable" form exceeds the life of the universe. Many things are "thermodynamically unstable" but last quite a long time and are quite useful. "Diamonds are forever" is perhaps an exaggeration, but does indicate that the useful lifetime of diamond structures, provided that you refrain from putting them in a furnace, should be quite long.
cphoenix@elaine25.stanford.edu (Chris Phoenix) (04/09/91)
In article <Apr.4.22.04.48.1991.29658@athos.rutgers.edu> erich@eecs.cs.pdx.edu (Erich Stefan Boleyn) writes: > Not necessarily. Consider how diamond is produced... the intense >temperatures and pressures are necessary because there is inherently a large >energy input required to produce diamond. If you look at the chemical >reaction, the activation energy is large, but there is no product energy, >and since the energy required is so large, it is very slow. Diamond does not require large temperatures or pressures. See Discover, March 91 (V. 12 #3) p. 66, "Diamonds in the Rough" by Ed Regis. John Angus has caused seed crystals to grow larger and incorporate boron by passing methane gas heated to 1800 degrees over them. With boron in the gas, the diamonds turned blue. He has also replicated Yoichi Hirosi's work, making diamonds by using an oxyacetylene welder's torch, aimed at a molybdenum disc on a water-cooled copper block. The diamonds form without seed crystals.
neufeld@aurora.physics.utoronto.ca (Christopher Neufeld) (04/09/91)
In article <Apr.4.22.02.48.1991.29630@athos.rutgers.edu> john@granada.mit.edu (John Olson) writes: >Diamonds have a big disadvantage over bone in that diamonds are >thermodynamically unstable. At all temperatures, graphite is the more >stable state for carbon. If you heat up a diamond--even under pressure-- >it will mostly revert to graphite. the only reason they last is that the >diamond --> graphite reaction is very slow at normal temperatures. >And the only reason we see diamond at all is that there is an equilibrium >between graphite and diamond. So when you do the Superman synthesis >(coal --> diamonds under high pressure and temperature), most of >the graphite just stays graphite. > Huh? The energy required to make diamond from graphite is really quite modest. To provide a scale, when a gram of graphite burns it releases 7796.6 calories of heat. To change a gram of graphite into diamonds requires all of 27.4 calories of heat. There's little difference in energy between the two phases. The fact that diamond is a higher energy phase, though, means that it is not thermodynamically stable unless you can get some additional energy advantage from the phase change. This happens if the P*dV contribution arising from the lower volume of the diamond phase equals or exceeds the energy difference in the crystal. The reason you have to go to such extreme lengths to make diamond is that it doesn't matter how much energy you inject into graphite, it's not going to change to diamond until diamond is the stable allotrope. Looking at a phase diagram of carbon, one can see that around 13500 atmospheres diamond is the preferred phase at absolute zero. At increasing temperatures the diamond/graphite transition region rises in pressure, reaching 70000 atmospheres around 1400 Celcius, and at about 215000 atmospheres and 3900 Celcius the graphite/diamond transition curve hits the solid/liquid transition line, which is pretty close to vertical (constant temperature) all the way down to the triple point at about 130 atmospheres. Note that this means that the ideal conditions for forming diamond are high pressure and LOW temperature. The hotter you make it, the harder you have to squeeze it to make diamond the stable allotrope. To quote from _Carbon and Graphite Handbook_, and converting from a rather quaint set of units of dubious historical interest: "Conversely, diamond exists quite well in the graphite-stable region at temperatures below [1230-1730 Celcius]. However, when diamond is heated to temperatures above about [1730 Celcius] at low pressure, it rapidly changes to graphite, because the thermal agitation of the atoms becomes energetic enough to swing them loose from the diamond lattice. They regroup in the more stable graphite lattice form." p.54 Reference: _Carbon and Graphite Handbook_ by C.L. Mantell. 1968 SBN 470 567791 Note that if you make any diamond at all, it is because diamond was the stable allotrope in the conditions under which you made it. If you have an efficient system which exposes all the graphite to the same conditions for a reasonable period of time, you should have essentially complete conversion to diamond. The point of all this is that diamond would make a pretty good bone material. It isn't going to transform spontaneously into graphite inside a person's legs. I don't think there is much in the way of biological impediments to the evolution of diamond bones. When you ask why creatures haven't evolved diamond bones you might as well ask why they haven't evolved bulletproof skins, asbestos fur, or for that matter, warp drive. It just wasn't in the evolutionary dice. >Now, that's not to say that a nanomachine (or a cell) couldn't make diamond, >but it will be more difficult to synthesize than materials which can be >thermodynamically stable. > Well, wood is thermodynamically unstable. The low energy configuration even of wet wood is ash, water vapour, and carbon dioxide. -- Christopher Neufeld....Just a graduate student | Flash: morning star seen neufeld@aurora.physics.utoronto.ca Ad astra! | in evening! Baffled cneufeld@{pnet91,pro-cco}.cts.com | astronomers: "could mean "Don't edit reality for the sake of simplicity" | second coming of Elvis!"
erich@eecs.cs.pdx.edu (Erich Stefan Boleyn) (04/10/91)
neufeld@aurora.physics.utoronto.ca (Christopher Neufeld) writes: ...[deleted]... >lengths to make diamond is that it doesn't matter how much energy you >inject into graphite, it's not going to change to diamond until diamond >is the stable allotrope. > Looking at a phase diagram of carbon, one can see that around 13500 >atmospheres diamond is the preferred phase at absolute zero. At >increasing temperatures the diamond/graphite transition region rises in >pressure, reaching 70000 atmospheres around 1400 Celcius, and at about >215000 atmospheres and 3900 Celcius the graphite/diamond transition >curve hits the solid/liquid transition line, which is pretty close to >vertical (constant temperature) all the way down to the triple point at >about 130 atmospheres. Note that this means that the ideal conditions >for forming diamond are high pressure and LOW temperature. The hotter >you make it, the harder you have to squeeze it to make diamond the >stable allotrope. ...[references deleted]... >the stable allotrope in the conditions under which you made it. If you >have an efficient system which exposes all the graphite to the same >conditions for a reasonable period of time, you should have essentially >complete conversion to diamond. Hmmm... I am definitely out of my knowledge depth here... (nice references though ;-) I know about the vapor deposition methods to a point, but the conditions always have had to be very controlled. My comments in an earlier posting were partially referring to possible requirements in a non-pristine environment, but I got sloppy. The idea is to make these work in a chemically rich (or at least non-void) environment, where energy could be sapped off by nearby molecules, charges altered in a somewhat unpredictable pattern, etc. Any operation that takes a decent amount of energy, and/or needs a very specific event to take place, and where thermodynamic or chemical noise is a problem will of course cause a lot of interference, or at least a much higher probability of failed reactions. Anyway, These deposition methods are still energy expensive. Even supposing that you have high amounts of energy available that would not damage your nanotech machinery (energy high enough to establish the carbon- carbon pi sigma bonds are also high enough to pose possible danger to the strutural integrity of your device ?), that is still a lot of energy to direct locally. Specific atom deposition has been done with slightly modified scanning tunneling microscopes, but you have a large structure to feed the energy in, if you need it (I am not too familiar with what has actually been done, but I think they only placed the atoms onto the surface bonded by bulk bonds (Van der Waals, polar, etc.), not molecular). How would the energy feed be done for a nanomachine? Chemically? It would have to provide a good punch, so to speak... current biological-chemical reactions don't tend to be too high-energy... (this is where I don't know what the comparative energy levels would be, I'll check up on it). > The point of all this is that diamond would make a pretty good bone >material. It isn't going to transform spontaneously into graphite inside >a person's legs. I don't think there is much in the way of biological >impediments to the evolution of diamond bones. When you ask why >creatures haven't evolved diamond bones you might as well ask why they >haven't evolved bulletproof skins, asbestos fur, or for that matter, >warp drive. It just wasn't in the evolutionary dice. Moving around molecule subunits is easier than moving atoms, and calcium has always been abundant as a free ion in cells, not to mention being used by many other things. One of the main problems would be to work with raw carbon, and how to isolate that from the rest of the body. Making bones is a sloppy operation. I agree that it could be done, but think that it would involve some more radical changes than are hinted at, no system in the body exists even in semi-isolation. Erich "I haven't lost my mind; I know exactly where it is." / -- Erich Stefan Boleyn -- \ --=> *Mad Genius wanna-be* <=-- { Honorary Grad. Student (Math) }--> Internet E-mail: <erich@cs.pdx.edu> \ Portland State University / Phone #: (503) 289-4635 [I think it's a bit premature to speculate about diamond fabrication inside a body just now! It does seem that diamond fabrication is a reasonable thing to expect a molecular assembler to do, but the most I think we can deduce from this is that it is not UNreasonable to design an assembler with diamond parts (the "matrix" or frame of a mechanical nanocomputer, for example...). BTW, I've personally seen a 6-inch synthetic diamond wafer made for use as a substrate in advanced electronics. By the time diamond is made by nanotechnology, it won't be that uncommon as an engineering material. --JoSH]
landman@eng.sun.com (Howard A. Landman) (04/10/91)
In article <Apr.3.14.33.24.1991.24769@athos.rutgers.edu> toms@fcs260c2.ncifcrf.gov (Tom Schneider) writes: >So why don't living things use diamond? > >Cells handle carbon structures all the time, so why are shells and structural >components mostly silicon and calcuim? There are lots of possible answers. I think the most likely are: 1. It is difficult to build diamond at room temperature and pressure, and/or in aqueous solution. Life just never happened to randomly develop a mutation capable of doing that in a way that had survival value. 2. Carbon is already very valuable to living organisms, but silicon (and maybe even calcium) are not so valuable, nor so flexible in their uses. It makes sense to save the carbon for nanotech purposes (protein, DNA, energy storage) and use the simpler materials for bulk structural purposes. 3. Calcium comes in single-atom units that are easy to deposit. We don't really know how to grow a diamond one atom at a time, nor is there a plentiful source of single carbon atoms in most biological systems. (Energy metabolism is based on pairs of carbon atoms - crudely, burning acetate units.) Perhaps the best source is methionine, but such "active methyl" groups tend to be used for very specialized purposes. (There was a good overview article on folate/B12/methionine metabolism in one volume of Advances in Nutritional Research a while back.) Even then, you have a CH3- unit, so you have to figure out where to dispose of the hydrogens. In summary, there isn't a convenient source of "pure carbon" unbonded to other elements. Remember that "best" in a biological context is a very complicated notion. It involves questions of availability, feasibility, cost, simplicity and reusability (why do you have fingers on your feet?), as well as engineering considerations like strength. Further, you usually have to consider not just the organism but its competitors, food sources, predators and environment. -- Howard A. Landman landman@eng.sun.com -or- sun!landman
neufeld@aurora.physics.utoronto.ca (Christopher Neufeld) (04/13/91)
In article <Apr.9.21.53.27.1991.26805@athos.rutgers.edu> erich@eecs.cs.pdx.edu (Erich Stefan Boleyn) writes: > > Anyway, These deposition methods are still energy expensive. >Even supposing that you have high amounts of energy available that would not >damage your nanotech machinery (energy high enough to establish the carbon- >carbon pi sigma bonds are also high enough to pose possible danger to the >strutural integrity of your device ?), that is still a lot of energy to >direct locally. > > [stuff deleted] > >How would >the energy feed be done for a nanomachine? Chemically? It would have to >provide a good punch, so to speak... current biological-chemical reactions >don't tend to be too high-energy... (this is where I don't know what the >comparative energy levels would be, I'll check up on it). > Recall that the carbon atoms -> diamond synthesis is exothermal, so the nanomachine doesn't have to channel the energy to form the bond, it just has to stand out of the way. "Bond energies" refer to the binding energy of the bond, or the energy which is released when the bond is formed, not the energy required to make the bond. If that were the case, every compound you see would realize that it could get to a lower energy state by atomizing and the world would dissolve to gas. The nanomachine would just have to be designed not to absorb the energy, which will be primarily thermal energy released when the lattice rebounds from the new carbon bond. Very little of that energy is likely to find its way back to the nanomachine, if just because of acoustic impedance mismatches between the crystal and the surrounding medium. >> The point of all this is that diamond would make a pretty good bone >>material. It isn't going to transform spontaneously into graphite inside >>a person's legs. I don't think there is much in the way of biological >>impediments to the evolution of diamond bones. > > Moving around molecule subunits is easier than moving atoms, and calcium >has always been abundant as a free ion in cells, not to mention being used >by many other things. One of the main problems would be to work with raw >carbon, and how to isolate that from the rest of the body. Making bones is >a sloppy operation. > Now I'll stray far from my field. The diamond atoms in the crystal form bonds with their four nearest neighbours in a tetragonal geometry, akin to sp3 hybridization of atomic orbitals. This is also the geometry of a methane molecule, and will be very close to the geometry of a methyl group CH3. Not ever having gone above high school chemistry, I'd design my diamond-building machines to be dominantly atomic step extenders. They would come in with a methyl group on a long stick, and would insert it at the edge of a step from one crystal plane to the other. In doing so they would trade the methyl for the hydrogen on the surface of the crystal, ie. the surface of the crystal would always be fully hydrogenated, with the crystal being built by substituting a methyl group for one or more hydrogens on the surface. Perhaps one in every million nanomachines would be atomic step creators. They would have the capacity to form a bond with the surface of a crystal plane, instead of just the edge of one. This scheme would allow each crystal plane to be manufactured to completion with fast growth along the face of the crystal, and slow growth outward, to limit the formation of voids and nanomachine inclusions. This scheme doesn't involve handling individual unbonded carbon atoms, which are notoriously reactive, just the handling of methyl groups which biological systems do all the time. -- Christopher Neufeld....Just a graduate student | Flash: morning star seen neufeld@aurora.physics.utoronto.ca Ad astra! | in evening! Baffled cneufeld@{pnet91,pro-cco}.cts.com | astronomers: "could mean "Don't edit reality for the sake of simplicity" | second coming of Elvis!"
cloister@milton.u.washington.edu (cloister bell) (04/13/91)
diamond might make for nifty skeletons, but i wouldn't want to have it as bone
material. i mention this because it seems likely that some will construe the
preceeding discussion into the area of artificial prostheses for
reconstructive surgery. diamond has a very definite lattice structure, and
shatters under the right kind of blow. this is why lapidaries can do rough
working of diamond with steel tools, which are softer than the diamonds.
while i have never personally seen a shattered diamond, i can only imagine
that the fragments are rather sharp, and would cause a *lot* of damage to the
fleshy bits before the doctors could get them out.
--
+-------------------------------------------------+---------------------------+
|i thought of a good sig, but it was a sight gag. | cloister@u.washington.edu |
+-------------------------------------------------+---------------------------+
[No one, I trust, would try to make a bone or any other part with a similar
function out of a single crystal of diamond ... imagine making a bone of
glass. Yet you can make tough, non-shatterable objects of fiberglass,
and you could do the same thing with diamond if you wanted.
--JoSH]dietz@cs.rochester.edu (Paul Dietz) (04/13/91)
In article <Apr.9.22.03.31.1991.26994@athos.rutgers.edu> landman@eng.sun.com (Howard A. Landman) writes: >In article <Apr.3.14.33.24.1991.24769@athos.rutgers.edu> toms@fcs260c2.ncifcrf.gov (Tom Schneider) writes: >>So why don't living things use diamond? >There are lots of possible answers. I think the most likely are: ... >2. Carbon is already very valuable to living organisms, but silicon > (and maybe even calcium) are not so valuable, nor so flexible > in their uses. It makes sense to save the carbon for nanotech > purposes (protein, DNA, energy storage) and use the simpler > materials for bulk structural purposes. There are plenty of forms of life that use carbon-containing compounds in structural elements. Plants (cellulose + lignin), arthropods (chitin), mollusks (shells) and even vertebrates (cartilage, horn, egg shells). So this explanation seems dubious. Paul F. Dietz dietz@cs.rochester.edu
burns@latcs1.lat.oz.au (Jonathan Burns) (04/13/91)
I have wondered if a Graphite Assembler might not be an early nanotoy. I imagine something with an adhesive tongue, which wanders over the surface of an unstructured mass of graphite (e.g. soot), peels off a flake of the substance a few layers of atoms thick and a hundred wide, then lays it down flat on an existing surface, as if doing a jigsaw. Considering the tensile strength of pyrolytically-produced graphite fibres, this kind of thing might be surprisingly useful, especially if doped for electronic or optical activity. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Jonathan Burns | It's a bonanza when Veronica plays piannica burns@latcs1.oz | On my granda-momma's oldio piazzica Computer Science Dept | With the whistle of the B and O La Trobe University | Booting out a obligatti-gattigo! - LaFemme et Owl, '51 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
osan@cbnewsb.cb.att.com (andrew.vida-szucs) (04/13/91)
In article <Apr.3.14.33.24.1991.24769@athos.rutgers.edu> toms@fcs260c2.ncifcrf.gov (Tom Schneider) writes: > >I was reading one of Drexler's papers in the Nanocon 1989 proceedings, where he >notes the possibility of creating diamond for structural components. So I >began to think of a shark with diamond tipped teeth. Wouldn't that have an >advantage over calcium? Wouldn't a diatom or shrimp encased in diamond be able >to survive better? So why don't living things use diamond? Or do they? If >they don't, is this telling us something about the limits of nanotechnology? >That is, maybe diamond can ONLY be made at high pressures and/or temperatures, >so a molecular machine couldn't do it, unless the machine operated under those >conditions. > There are chemical deposition methods for creating artificial diamonds that require *no* high pressure conditions whatsoever. In fact they require partial vacuum conditions which may explain why it does not occur in living organisms. These techniques use (i believe) methane gas as the carbon source. They were initially developed by Exxon oh, maybe 20 years ago but only in the past year or so have they been refined to the point where they are commercially viable. There was an article in the Wall Street Journal several months back. It seems that just about *any* shape can be produced using this method. I don't (though am not sure) think that this method will produce gem quality stones, so don't rush out to get your equipment. There is one creature I know of that has *carbide* teeth. It is some sort of animal in a shell whose name I forget at the moment and in its mouth there is a circle of small rectangular (Tungsten??) carbide teeth that it uses to grind away at the reef on which it lives. They exist somewhere in the South Pacific maybe Polynesia or where have you. Any how, that's about it regarding my knowledge of synthetic diamond.
Howard.Landman@eng.sun.com (Howard A. Landman) (04/19/91)
In article <Apr.12.16.34.48.1991.7800@athos.rutgers.edu> neufeld@aurora.physics.utoronto.ca (Christopher Neufeld) writes: >I'd design my diamond-building machines to be dominantly atomic step >extenders. They would come in with a methyl group on a long stick, and >would insert it at the edge of a step from one crystal plane to the >other. In doing so they would trade the methyl for the hydrogen on the >surface of the crystal, ie. the surface of the crystal would always be >fully hydrogenated, with the crystal being built by substituting a >methyl group for one or more hydrogens on the surface. ^^ ^^^^ The two words "or more" are critical here. Without their presence, I can prove mathematically that the above process doesn't work, i.e. is incapable of producing a diamond lattice. There are at least two approaches, one graph-theoretical, one combinatorial. Before you look at my proofs, you should pause to try to think this through. It's actually pretty obvious. THE GRAPH-THEORETICAL PROOF A diamond lattice, considered as a graph, contains many cycles. Substituting a methyl radical for a hydrogen does not create a cycle. Therefore, if we define a "diamond" carbon atom to be one that is contained in at least one cycle, such a step is incapable of creating a new "diamond" atom. In fact, if we start with a tree (connected acyclic graph), each step leaves us with a tree. Starting from methane, all you can build are saturated hydrocarbons. THE COMBINATORIAL PROOF Diamond is pure carbon. Therefore, any process to produce diamond must end up with mostly pure carbon. When you replace a hydrogen with a methyl radical, you add one carbon atom but also add (net) two hydrogen atoms. Thus the asymptotic composition of the product must be two hydrogens for every carbon, which is not diamond, but something more like polyethylene. The first proof implies that we need to have at least one step in any diamond building procedure which closes loops. The second proof implies that we need to have at least one step in any such procedure which removes (net) hydrogen without removing carbon (this could of course be the same step that closes loops), OR that the step which adds a methyl carbon removes ON AVERAGE about three hydrogen atoms. Not so simple sounding anymore, is it? -- Howard A. Landman landman@eng.sun.com -or- sun!landman
neufeld@aurora.physics.utoronto.ca (Christopher Neufeld) (04/20/91)
In article <Apr.19.00.30.48.1991.841@athos.rutgers.edu> Howard.Landman@eng.sun.com (Howard A. Landman) writes: > >In article <Apr.12.16.34.48.1991.7800@athos.rutgers.edu> neufeld@aurora.physics.utoronto.ca (Christopher Neufeld) writes: >>I'd design my diamond-building machines to be dominantly atomic step >>extenders. They would come in with a methyl group on a long stick, and >>would insert it at the edge of a step from one crystal plane to the >>other. In doing so they would trade the methyl for the hydrogen on the >>surface of the crystal, ie. the surface of the crystal would always be >>fully hydrogenated, with the crystal being built by substituting a >>methyl group for one or more hydrogens on the surface. > ^^ ^^^^ >The two words "or more" are critical here. Without their presence, >I can prove mathematically that the above process doesn't work, i.e. >is incapable of producing a diamond lattice. There are at least two >approaches, one graph-theoretical, one combinatorial. Before you >look at my proofs, you should pause to try to think this through. >It's actually pretty obvious. > Of course it is. A methyl is carbon and three hydrogens, so on average you have to discard three hydrogens for every carbon laid down. I'm surprised that you critisize me on the basis of something which I might have left out to make my statement incorrect, but didn't. I said what I did because I thought that it might make more sense for the device to be designed to grow planes normal to the (-1,1,1) direction than the (0,0,1) direction. In that geometry half the methyl additions yield one hydrogen, and the other half yield five hydrogens. This is not likely to be something which a single machine can do, so the principle axis growth is more likely, I just thought I'd cover all possibilities. >OR that >the step which adds a methyl carbon removes ON AVERAGE about three >hydrogen atoms. Not so simple sounding anymore, is it? > It's not simple in the first place. I'm just mentioning a way which might, I think, make it easier. You don't want your nanomachines to crawl over the surface of a diamond crystal with hanging carbon bonds or you'll stick tight. Hydrogenating the surface bonds serves two functions, one to make the crystal less sticky and the add-on units more manageable, and two to set it up with the correct electron orbital geometry. There's still the problem of how to fling those hydrogens out of there in the first place, perhaps with a highly electronegative bonding site which could be tucked in between the carbons to grab out the hydrogen. Candidates are nitrogen and oxygen. Flourine won't work unless you can make it polyvalent. You then pull out the arm with the hydrogens attached, leaving the carbon bonds essentially unchanged but unbonded. "Essentially unchanged" could mean a deflection of a few degrees, with the bond angle of water being a good example. I'm not a chemist, so this might all be nonsense. > Howard A. Landman > landman@eng.sun.com -or- sun!landman -- Christopher Neufeld....Just a graduate student | Flash: morning star seen neufeld@aurora.physics.utoronto.ca Ad astra! | in evening! Baffled cneufeld@{pnet91,pro-cco}.cts.com | astronomers: "could mean "Don't edit reality for the sake of simplicity" | second coming of Elvis!"