--- Log opened Thu Dec 23 00:00:09 2021
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10:25 < L29Ah> .t https://pubmed.ncbi.nlm.nih.gov/30606221/
10:25 < saxo> PEDF-derived peptide promotes tendon regeneration through its mitogenic effect on tendon stem/progenitor cells - PubMed
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15:51 < docl> .t https://www.newsweek.com/spacecraft-foam-interstellar-proxima-centauri-1521614
15:51 < saxo> Spacecraft Made from Ultra Thin Foam Could Reach Proxima Centauri in Just 185 Years
15:51 < docl> "The spacecraft would be made from aerographite. This is a carbon-based foam that is around 15,000 times more lightweight than aluminium. It is versatile and light enough that it could be used to create solar sails—which harness energy from the sun for propulsion, a process called solar photon pressure."
15:54 < docl> why not just use it as a solar collector here in earth orbit
15:57 < muurkha> graphite aerogel?
15:57 < superkuh> Because it's not a photovoltaic or the like. It doesn't does anything but provide mechanical support at low density.
15:58 < muurkha> https://en.wikipedia.org/wiki/Aerographite makes it sound a little different from carbon aerogel
15:59 < docl> I wonder how it differs from the stuff in this article https://www.newscientist.com/article/mg22630235-400-spacecraft-built-from-graphene-could-run-on-nothing-but-sunlight/
15:59 < docl> .t
15:59 < saxo> Spacecraft built from graphene could run on nothing but sunlight | New Scientist
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16:00 < docl> maybe it's that the latter is an oxide?
16:00 < muurkha> graphene isn't an oxide, though graphene oxide also exists and is a promising material for some such things
16:00 < docl> graphene can be an oxide? without dissolving into CO2?
16:00 < muurkha> yeah
16:00 < muurkha> well, it can be oxidized, that is
16:01 < muurkha> if you oxidize it fully you get just CO₂, yes
16:01 < muurkha> this idea of etching away ZnO or SnO templates with H₂ is interesting
16:01 < muurkha> ZnO and SnO are semiconducting ceramics with much higher boiling points than their respective metals
16:02 < muurkha> so when you reduce them with hot H₂, the resulting metal is in the gas phase and wafts away with the steam
16:03 < muurkha> anyway apparently aerographite is actual graphite, with tube walls on the order of 15 nm thick
16:03 < muurkha> while graphene and carbon nanotubes are more like 0.15 nm thick
16:04 < muurkha> (or, arguably, aerographite is actual glassy carbon, but at these scales the approximation of treating atomic structures as continuous materials starts to break down; 15 nm is well under the critical dimension for flaw-tolerance, for example)
16:06 < docl> hmm. at 15nm the stuff still manages to be 15000x lighter than aluminum
16:06 < muurkha> well, that's a function of the large spaces in between the 15-nm-walled tubes
16:07 < muurkha> (as well)
16:08 < muurkha> you could presumably make it arbitrarily sparse and thus light in zero G, but even on Earth you ought to be able to make it lighter by another order of magnitude or two
16:08 < muurkha> at least in smallish samples
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16:14 < docl> I wonder if can be made PV enough with some doping? 1% efficient would be pretty good in this context, 5000 m^2/kg
16:16 < muurkha> dunno, it's probably too conductive
16:16 < muurkha> I mean the intrinsic conductivity of glassy carbon is pretty high
16:25 < docl> hmm. resistance is inversely proportional to the cross section of the material, and were talking 15nm
16:26 < muurkha> that's right
16:27 < docl> I wonder if there's a way to make it act with high impedence? big spiral shaped collectors with a magnetic field?
16:27 < muurkha> for useful PV you need two conductive layers separated by a thin layer that is much less conductive
16:28 < muurkha> across which light drives some sort of charge carrier
16:28 < docl> could the thin layer be vacuum?
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16:29 < muurkha> maybe?  then it wouldn't matter how thick it was
16:29 < superkuh> In that case the charge carrier would have to be carbon ions from electrical field breakdown of imperfection on the surface vaporizing it.
16:29 < superkuh> This is ... not what this material is for.
16:29 < muurkha> no, the work function of carbon is not infinite, superkuh
16:29 < docl> couldn't just be electrons?
16:29 < muurkha> but something like BaO would be a much better choice
16:29 < superkuh> muurkha, with a vacuum gap between.
16:29 < muurkha> right
16:30 < muurkha> you know about the photoelectric effect?
16:30 < superkuh> I know about how high voltage and surfaces work under vacuum more than I know about the photoelectric effect. But sure.
16:31 < superkuh> I'm just saying there'd be no charge carrying ions available until the electrical field was high enough to cause surface irregularities to break down, arc, and vaporize some material.
16:31 < muurkha> that isn't true though
16:31 < muurkha> http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/photoelec.html
16:32 < docl> I know the kindergarten version... light striking something ejects electrons when the photon energy exceeds a certain threshold, with excess energy going to the electron velocity
16:32 < superkuh> I guess maybe the break in communication is me using a strict interpretation of the word ion? I'm not talking about electron field emission.
16:33 < muurkha> if a cesium surface (w.f. 2.1 V) absorbs a 4 eV photon, the photoelectron can force its way across a gap of up to 1.9 volts
16:33 < muurkha> if you put a honeycomb in front of your cesium-plated surface, separated from it by vacuum, it will let most of the light through (you point it at the sun) but capture most of the photoelectrons
16:34 < muurkha> a metal honeycomb, I mean, maintained at some MPPT-determined voltage like 1.5 V
16:34 < muurkha> electrons make perfectly good charge carriers; charge carriers don't need to be ions
16:35 < muurkha> at high voltages it wouldn't work at all except when the occasional X-ray strikes
16:36 < muurkha> I'm not sure what the quantum efficiency of photoelectron emission is
16:36 < docl> so what I'm hearing is, put two 1mm plates of the stuff close enough together and you get a big PV element (that's also a big capacitor)
16:36 < superkuh> No.
16:37 < muurkha> well, if you put two *plates* close together, one of them will block the light from getting to the other
16:39 < muurkha> you want one of the "plates" to let light through to hit the other, while still catching most of its photoelectrons
16:39 < muurkha> you could surely arrange to transfer heat between the two plates without transferring electrons to keep them at the same temperature
16:42 < muurkha> in vacuum it doesn't matter how far apart they are, as long as they aren't close enough to cause field emission; what matters is the uphill voltage the electrons have to climb before being collected
16:44 < muurkha> (and the relative temperatures also matter, because if the collector is hot enough, it'll lose whatever electrons it collects)
16:45 < muurkha> (maybe the absolute temperatures matter too?)
16:52 < muurkha> apparently cesium on silver oxide has a lower work function reaching down into the infrared: https://en.wikipedia.org/wiki/Phototube
16:59 < muurkha> guess I'll write this up for Dernocua.  how should I credit you, docl?
16:59 < docl> Luke Parrish
17:00 < docl> :)
17:00 < muurkha> thanks!
17:02 < docl> so phototubes are "photoemissive" but it's really the same thing as photovoltaic, just with vacuum instead of an insulator?
17:04 < muurkha> with vacuum instead of a depletion region around a pn semicondcutor junction, yes, as I understand it
17:08 < docl> maybe a vacuum tube : transistor analogy applies here :)
17:10 < muurkha> yeah, transistors are kind of like vacuum tubes; that's why two of the BJT electrodes are called "base" and "collector"
17:10 < muurkha> uh, "emitter" and "collector"
17:10 < muurkha> "base" is the... chunk of germanium your point contacts are resting on
17:11 < muurkha> FETs are more similar to triodes than BJTs are tho
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17:38 < muurkha> superkuh: explain?
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19:18 < muurkha> so, after doing the calculations, probably you can get more watts per gram out of this gadget than out of silicon, but less watts per square meter, and also less watts per gram than from thin-film solar cells
19:19 < muurkha> full workup is in text/photoemissive-power.md in http://canonical.org/~kragen/dernocua.git
19:19 < muurkha> if I'm overlooking something, superkuh, I'd be delighted to hear about it
20:27 < muurkha> to be more precise, it seems like solar conversion efficiency could reach 6%
20:28 < muurkha> (I finally found a good table of photoemissive cathode material quantum efficiencies!)
20:29 < muurkha> should be about a 5 minute read
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--- Log closed Fri Dec 24 00:00:10 2021