Scientists Create Material More Insulating than the Vacuum

Dec 10, 2009 by Lisa Zyga weblog
In a typical thermos, a vacuum is used to reduce heat transfer. Scientists have found that layers of photonic crystals in a vacuum can reduce the thermal conductance to about half that of a pure vacuum. Credit: Wikimedia.

(PhysOrg.com) -- With its complete lack of atoms, a vacuum is often considered to be the best known insulator. For this reason, vacuums are regularly used to reduce heat transfer, such as in the lining of a thermos to keep beverages hot or cold. However, in a recent study scientists have found a material even less able to conduct heat: a stack of photonic crystals layered within a vacuum can create a material with a thermal conductance just half that of empty space alone.

Basically, heat can be transferred from one material to another in three main ways: , conduction, and radiation. Conduction and convection both require some kind of material medium for heat to pass through; therefore, the lack of material in a pure vacuum greatly minimizes the effectiveness of these two processes. However, heat can also be transferred through , a form of light that is invisible but can be felt as heat. In the example of the thermos, infrared radiation can travel through the vacuum to the thermos' outer wall; when absorbed by the outer wall, the radiation causes the molecules in the outer wall to vibrate and release heat.

Shanhui Fan of Stanford University and his colleagues wondered if any material could block infrared radiation better than a vacuum can. Last year, the scientists theoretically calculated that photonic crystals might be the answer. Photonic crystals, which can be found in nature as well as be created in the lab, consist of periodic bands of that affect how light travels through them. Significantly, photonic crystals can have band gaps that forbid propagation of certain frequency ranges of light. In this case, they could be used to block infrared radiation.

The scientists found that a 100-micron-thick structure made of a stack of 10 photonic crystal layers, each 1 μm thick and separated by 90-μm gaps of vacuum, could reduce the thermal conductance to about half that of a pure . In a more recent study, Fan and his colleagues calculated the fraction of all frequencies that the photonic crystal allows through. They were somewhat surprised to find that the thermal conductance doesn't depend on the thickness of the layers but only on how fast light travels through the material, or its index of refraction.

Photonic crystals have previously been shown to have promising applications in communications and computing, and the new research suggests their thermal properties could make them useful for a variety of other applications. For example, solar-thermal applications that capture the sun's heat for use as an energy source could likely benefit from a material that can allow visible light to pass through, while keeping the inside.

More information: W T Lau et al. "Universal features of coherent photonic thermal conductance in multilayer photonic band gap structures." 2009 Phys. Rev. B 80 155135.


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User comments : 27

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Smellyhat
4.8 / 5 (11) Dec 10, 2009
Shanhui Fan of Stanford University and his colleagues wondered if any material could block infrared radiation better than a vacuum can.


That was a particularly unfortunate line, as the vacuum is notoriously bad at absorbing photons. It was possible to figure out from the article what was actually being done, scientifically, but the headline and the latter part of the text seem to lose track of the difference between the conduction and the radiation of heat. Implicitly, this bandgap metamaterial, which blocked the radiation of heat, must have been placed within a vaccum, which would continue to prevent the mechanical conduction of heat energy as in a regular thermos.
ThomasS
4.3 / 5 (3) Dec 10, 2009
wow great article
codesuidae
5 / 5 (3) Dec 10, 2009
I am curious as to how effective a couple layers of polished aluminum foil would be at blocking radiation. It seems to me that these photonic crystal structures should be compared not to an empty vacuum, but to a vacuum with a basic, non-exotic IR reflective barrier.

Anyone have any data on that? I suppose glass vacuum bottles are shiny for a reason.
StarDust21
5 / 5 (1) Dec 10, 2009
I am curious as to how effective a couple layers of polished aluminum foil would be at blocking radiation. It seems to me that these photonic crystal structures should be compared not to an empty vacuum, but to a vacuum with a basic, non-exotic IR reflective barrier.

Anyone have any data on that? I suppose glass vacuum bottles are shiny for a reason.


yea idk, wouldn't a simple mirror do the job to reflect radiation?
Osmosium
Dec 10, 2009
This comment has been removed by a moderator.
agafford
5 / 5 (3) Dec 10, 2009
yes, the comparison should be to simple radiation shields. multilayer quilted aluminum foil is quite effective in atmosphere and would be more so in vacuum. the low emissivity of the typical materials used in vac flasks buys quite a lot in terms of insulation.
sender
1 / 5 (1) Dec 10, 2009
laser-fields soufnd well suited to insulation for inertial coninement theromonuclear fusion, surprised it hasn't been used so far
weirmeir
3.6 / 5 (5) Dec 10, 2009
eventually we knew you earth dwellers would combine the photon torpedo and the dilithium crystal to form a super efficient thermos for coffee.
Feldagast
5 / 5 (1) Dec 10, 2009
how much more would a thermos cost with this technology?
thermodynamics
4 / 5 (4) Dec 10, 2009
codesuidae, StarDust21, Osmosium, and agafford all have it right. This has been done for decades. It is called multi-layered insulation and is described at the following web site:

http://www.techni...dex.html

Nothing new here other than the use of nano-materials. However, since they don't compare it with MLI, we have no idea if it is even as good.
antialias
3.5 / 5 (2) Dec 11, 2009
Could be a different principle involved. Whereas the silver layer in thermos bottles absorb the IR and then throw the photons back some remain (some vibrations/heat transfer is caused)

Nanomaterials (especially metamaterials) could work in a different way by not absorbing the IR but making it head back towards its source. since this does not require an absorption/eission process it would mean that no phononic energy is absorbed and hence result in less heat transfer.

This way I could see the stated reduction of heat transfer working. (Putting materials in vacuum does not increase isolation properties by istelf as those materials will quickly reach a thermic equilibrium between absorption and emission)

But that's all speculation since I'm not sure what types of phononic crystals they are using.
RayCherry
not rated yet Dec 11, 2009
I don't know the obsorption ratio of the MLI materials, but I have read plenty on PhysOrg about the alteration of light paths by use of meta/nano materials. This article seems to indicate that the heat/IR is redirected back to the source with less loss than reflective materials that 'bounce' the energy along the originating path.

Not to get started with the AWT crowd on here, but does not a vacuum lose intergrity once it is filled with high frequency photons on collision courses? Wouldn't the amount of interference produce some standing waves, and those provide conduction (or less resistance) as a consequence?

If the meta materials reduce the amount of IR than makes it to the vacuum, then it may also reduce any conductivity that could build up there.

Obviously, this not not for normal domestic or industrial applications. The inhibitive cost of meta-materials would probably make it unviable where the vessel being insulated has a entry/exit closed by a less insulating material.
RayCherry
not rated yet Dec 11, 2009
Also ... do these meta-materials scale up to handle radiation of larger particles? Could they produce nuclear radiation management? Would it be possible to handle nuclear waste in such a way as the irradiation of the waste was controlled, concentrated and produced a useful energy source?

Recycling rather than dumping ... its en vogue.
Lennart
not rated yet Dec 11, 2009
Can a coat of photonic crystals on a missile/rocket prevent it from being blown up by a laser defence system then?
antialias
3.5 / 5 (2) Dec 11, 2009
Nuclear radiation comes in different flavors
1) Alphas (ionized helium cores): Since these are particles you can't use metamaterials to redirect them (though a sheet of paper usually is enough to block them)

2) Betas (electrons): Also not redirectable by meta materils (you need waves for these materials to be effective). Betas are harder to block. A bit of concrete/lead does the trick...mostly.

3) Gammas (high energy photons): these you could, conceivable, feed back with metamaterials. However you do not want to do that since these gammas could impact other atoms and cause them to split, too - rteleasing more gammas (you'd create an avalanche effect - effectively leading to a meltdown/nuclear explosion)

Can a coat of photonic crystals on a missile/rocket prevent it from being blown up by a laser defence system then?

No, unless you know EXACTLY which frequency the laser will be using. And probably not even then.
joekid
not rated yet Dec 11, 2009
It would be nice if these photonic cystals could be made into fibers or flexible sheets; perhaps carbon nano-tube technology could used. It would be nice if a astronaut could wear a suit that had less bulk and mass.
GBogumil
not rated yet Dec 11, 2009
Isn't this the same technology used by sunglasses to block UV?
codesuidae
5 / 5 (1) Dec 11, 2009
@RayCherry, re "high frequency photons on collision courses"; photons are bosons, so regardless of their frequency they won't collide. IIRC, a sufficient energy density will have an effect on space, but that would be some very hot coffee indeed.

@antialias, re Nanomaterials (especially metamaterials) could work in a different way by not absorbing the IR but making it head back towards its source. since this does not require an absorption/eission process it would mean that no phononic energy is absorbed and hence result in less heat transfer.

I don't think photons work like that. Short of bending space I think the only way to make a photon change direction is to absorb and re-emit it. It may still be that an exotic material could reflect a greater proportion of the photons than a typical metal sheet.
antialias_physorg
not rated yet Dec 11, 2009
codesuida: Check out metamaterials on wikipedia. especially look for the chapter "negative refraction index" or google the 'cloaking device'-type of applications using metamaterial arrays.
hix1050
not rated yet Dec 12, 2009
It looks like no one is aware of vacuum superinsulation, multiple layers of reflective material, invented in 1951, and with a conductance of less than 40 uW/m-K, and which is in just about every dewar you've ever handled.
codesuidae
not rated yet Dec 12, 2009
@antialias, as I understand it, in order for a material to reflect or refract a photon, it must interact with the photon by absorbing it. If the photon isn't absorbed, then it simply follows a straight path through space.

I did run into a reference about photons not being completely absorbed by an electron, but I'm not sure exactly what that means, and I couldn't easily find more information about it.

I'm just a physics spectator though, photons do a lot of stuff I don't know about; if you know how they are reflected or refracted without first being absorbed by an electron/positron I'd be curious to know how it happens.
antialias_physorg
not rated yet Dec 13, 2009
When a photon moves from one material to another it is not necessarily absorbed and reemitted (e.g. a photon can move through a pane of glass). But when you move between materials with different refraction indices the path of the light gets bent (air/glass (lens/prism), water/air, air/meta-material, vacuum/meat-material, ... ).

This change in direction is not due to absorption/emission but due to the different speeds at which the EM components spread in the 2 materials (Note: photons are not particles - and no: they are not waves either. Particle-waves is a crass misonomer which confuses people a lot. Particle-waves are a state that is neither particle nor wave - it just exhibits properties LIKE both under certain circumstances)

With a negative refraction index (like in meta materials) you can construct materials that lead photons in any desired path...around objects or, through a layered meta material, right back where it came from without - any absorption taking place.
flaredone
not rated yet Dec 13, 2009
.. well suited to insulation for inertial confinement thermonuclear fusion, surprised it hasn't been used so far..
In thermonuclear weapons a kind of "photonic crystal" mirror for neutrons is used. These neutrons are reflected back into exploding core by thin layers of uranium sheets, which are encasing the weapon. But these layers doesn't reflect EM waves, but de-Broglie waves of fast moving neutrons. Until recent years it was pretty secret technology.
NeptuneAD
not rated yet Dec 13, 2009
I am getting sick of this spam, nice article though.
Also is the vacuum in a thermos an absolute vacuum?
KBK
not rated yet Dec 14, 2009
"They were somewhat surprised to find that the thermal conductance doesn't depend on the thickness of the layers but only on how fast light travels through the material, or its index of refraction."

~~~~~~~~~~~

And that, by far, is the most interesting, revealing, and important part of the article -to pay attention to.
antialias
not rated yet Dec 14, 2009
Yes - that is why I speculated in my first comment that there is something else going on besides an absorption/emission mechanism (which would show up as a property which is proportional to the thickness/volume of the material used)
KBK
not rated yet Dec 15, 2009
One thing that is well recognized in the coatings industry is that, when painting:

If you put a DARK color over LIGHT...and view from that upper dark coat:---- The view point of the dark coating will REDSHIFT.

If you PUT a LIGHT color over DARK...and view from that upper light coating, the view will BLUESHIFT.

Only when the pigment reaches dye status, ie transparent pigment (one where the pigment/dye structure is below the dye's primary color light frequency --in size) that is sensitive to degradation from light, to some extent, does that observational equation change at all.

Opacity of pigment does not play into this as much as you would think. It has never failed to produce that aspect in the history of art painting.

Yet, science fails to recognize this point that is at the heart of the art world-every day.

Now, what is the (+) or (-) point, with regard to energetic spin in these particular structures? What is it's pH?

All these things are telling. It's quite alchemical.
Javinator
not rated yet Dec 23, 2009
@antialias

Wrt radiation.

Betas would usually not be blocked with lead in practice due to Bremsstahlung (ie. X-rays produced by the betas when you try to shield them with heavier elements). Betas are actually completely shielded with about a centimeter of plastic or plywood.

Gammas are bad news, but they don't cause nuclear fission. Gammas generally cause ionization. The reflection of neutrons inwards could lead to this avalanche effect (as someone else mentioned), but not gammas.

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