Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!elroy.jpl.nasa.gov!decwrl!stanford.edu!rutgers!aramis.rutgers.edu!athos.rutgers.edu!nanotech From: neufeld@aurora.physics.utoronto.ca (Christopher Neufeld) Newsgroups: sci.nanotech Subject: Re: Diamonds? Keywords: diamonds, bones Message-ID: Date: 9 Apr 91 02:40:43 GMT Sender: nanotech@athos.rutgers.edu Organization: University of Toronto Physics/Astronomy/CITA Lines: 75 Approved: nanotech@aramis.rutgers.edu In article 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!"