Infrared spectroscopy studies molecular structure by detecting resonances of atoms or atomic groups that swing back and forth, or twist, or go in and out, along their atomic bond to other atoms or atomic groups. In the case of polymers, groups do this with respect to the backbone, and this often defines where their infrared absorption peaks are. In practice, infrared absorption and emissivity are the same thing.
I do a lot with radiative transfer of thermal energy around polymers, and am used to expecting higher emissivities for polymers that have more complicated structures. There are more kinds of bonds and more kinds of groups hanging off of one another in a more complicated polymer. I just told a co-worker that the more ink it takes to draw the structure of a polymer, the higher its emissivity tends to be.
But then I thought of graphite. It’s hard to imagine a less complicated structure. Yet it has a high emissivity. Why? What’s wrong with my simple rule of thumb in this case?
Perhaps the semiconductive, semi-insulative nature of graphite means it tends to absorb electromagnetic waves like a lossy resistor network absorbs radio waves? Eddy current losses, I mean?
Actually, graphite doesn’t have a high emissivity. Its emissivity is 0.7-0.8, according to the table I link to.
You might be thinking of carbon black, which has an emissivity of 0.95 to 1.0. It’s the classic case of a real-life black body.
But, I hasten to point out, only in bulk form. microscopic soot particles— like the ones in a candle flame, the ones responsible for giving the flame its yellow glow – actually have a complex, wavelength-dependent emissivity, because their size is comparable to the wavelength of light.
as for why they have these emissivities, I really don’t know. But I think that looking at it in terms of atomic transitions probably isn’t the best way to understand them.
Presumably, it’s due to their physical microstructure. Carbon black has innumerable crevices which light can get trapped in instead of being reflected away. A single soot particle is a bit like a smooth marble; a whole bunch of them packed together is like a container of marbles, with lots of gaps between them.
Arrays of carbon nanotubes can do even better. Light that enters the forest has many chances to bounce around and be absorbed, and thus a low probability of leaving. Vantablack is named after “vertically aligned nanotube arrays” and is among the blackest coatings known, with correspondingly high emissivity (>0.9995).
No, I mean, it has a simpler chemical structure than zinc selenide or barium fluoride or similar ionic compounds, and yet it is mostly absorbing, whereas those are very transmissive indeed. Why isn’t graphite transparent in the thermal infrared?
Its structures are small relative to 2 or 20 um wavelengths.
I understand absorption in terms of resonances, but that’s only part of what’s going on. How do I think of the rest?
This table has it as 0.97 for powder, 0.98 for a filed surface.
But in any case, I suspect the reason is related to the crystal structure - graphite is quite “bouncy” in a lattice sense - it has a high phonon movement. AT least, that’s the little I remember from crystallography class, 30 years ago.
That’s for carbon. If you look up graphite on various sites you find it between 0.7 and 0.8. Carbon black – which doesn’t have graphite’s lamellar (platelike planar) structure – the factor responsible for its slipperiness and utility as a dry lubricator. It’s amorphous carbon black that has the extremely high emissivity.
It’s specifically for “carbon:graphite, filed surface” and “graphite : powder”. They have other values for other carbon forms.
The 0.98 value for a filed graphite surface is not unique to that cite. Others (pdf) do too. They also give the general emissivity of graphite as 0.7-0.8.
Also, no need to explain the meaning of lamellar to me or, I suspect, most anyone on this board.
The spectral emissivities of graphite and carbon have been determined at a wavelength of 0.653μ in the temperature range of 1285 to 2035°K and have been found to be temperature dependent in accordance with a prediction from a relation between reflectivity and resistivity. For polished graphite of high purity having a resistivity of about 1120 microhm‐cm the value obtained is 0.78; the value obtained for a polished high density graphite having a resistivity of 1740 microhm‐cm is 0.78; and that for polished spectroscopic carbon having a resistivity of 5080 microhm‐cm is 0.79. If graphite is sublimed from a polished surface, the latter becomes roughened, so that the emissivity increases. In one particular experiment it attained a value of 0.90.
So, polished is 0.78; in line with the other cited figures. When the same surface undergoes a roughening process, it increases to 0.90.
Apparently thin, smooth single sheets of graphite are reasonably transparent:
So, there is some kind of interesting electronic effect (it’s not completely transparent)
Anywhere from a geology class, a medieval history class, reading about a weird skin disease, to hanging out with SCA people or playing D’nD. Those are all contexts I’ve seen or used the word.
Not surprising. The roughened surface would have lots of powdered material, which wouldn’t have the same structure as the rest of the graphite. In other words, it’s like covering the graphite with the carbon black.
Powdered graphite still has the graphite crystalline structure, it’s not the same as amorphous carbon. It’s just in every orientation relative to the incident radiation.
