Transparent ceramics made with aluminum | Hacker News Reader

Transparent ceramics made with aluminum

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Metal will never be transparent. Metal means metal bonds which means free electrons. Free electrons are opaque to visible light.

Just to add on for others reading your comment: Although metals are opaque, conductors can still be transparent. A few techniques:

- Very thin layers (like graphene) such that absorption is on the order of a percent

- Narrow bandwidth materials that are opaque to infrared but transparent to visible

- Bad metals and non-band conductors that have correlated electron hopping from site to site

Transparent conductors like ITO are critical materials for displays and solar cells, since light needs to enter/exit one side of the device.

See this experiment for a real transparent metal. Admitedly, the conditions are kinda extreme :)

https://www.youtube.com/watch?v=BIGMfai_ICg

Don't mind the intro, the interesting part starts at around 2:00

I'd never realised the two were connected. What's a good learning reference for bandgap physics? It's something I've never quite understood properly.

You don't need bandgaps to understand why conductors (other than thin films of ITO) don't transmit light. You derive it from Maxwell's equations and the fact that in a conductor, current density = conductivity * electric field. Griffiths' Introduction to Electrodynamics is the standard undergraduate textbook on electromagnetism, and explains this reasonably well. For bandgaps, Ashcroft and Mermin's Introduction to Solid State Physics is what I'm reading from right now, but you don't need it to understand why metals (conductors) don't transmit light.

Ashcroft and Mermin is only an 'introduction' in the graduate student sense of 'introduction'. I don't think you can make sense of it without previous grounding in both quantum mechanics and solid state physics.

Well, he did ask about band gaps. Is there a way to understand band gaps without quantum mechanics? I think you only need a little Fourier analysis to get the basics of band gaps.

Anyway, that text and along with Kittel's are the references for an undergraduate solid state course that I'm taking. No prior exposure to solid state physics for me and only introductory quantum mechanics (first half of Griffiths' QM); I find the text totally approachable.

Semiconductor Device Fundamentals by Robert F. Pierret was my go-to for undergraduate text. The diagrams speak a thousand words and Pierret is a funny dude. Builds everything up from basic quantum.

Don't mistake it with his graduate text, Advanced Semiconductor Fundamentals, though. That's also a great text, but very short and focuses almost exclusively on the quantum aspect without getting too much into the higher level meat of putting it together to form devices.

For a comprehensive guide, though, Physics of Semiconductor Devices by Simon M. Sze was my reference bible. It's big and bulky, very heavy on the first principles math and physics, and has everything from quantum to devices and variants on devices.

I don't think in quite understand. You mean they don't let photons to pass through? In which case do electrons in orbit? Or nuclei for example? Or does this need advanced physics knowledge?

Roughly light is electromagnetic radiation and puts force on electrons it comes in contact with. If the electrons are fixed in a non conductor they don't move much and so don't absorb the energy. If they can move as in most metals the force accelerates them and they absorb energy from the radiation, stoping of reducing it.

My physics is only high school level so as I see, in the case of fixed - does the photon not get scattered/reflected or does it pass through the electron? And why no orbital transition?

Reflection on the surface of polished metals occurs because of the collective movement of electrons at the surface, caused by the incoming photons. Light of most wavelengths, especially low-energetic infrared doesn't have enough energy to cause orbital transitions in most elements.

What about partial ? or only to some wavelengths ?

X-rays penetrate metal to some extent, though they also scatter off the atoms. Radiography of high value/risk metal components (such as the stressed parts of gas turbine engines) is a mainstream non-destructive testing technique.

huh, is this why sometimes metal gets unbelievably hot if left in the sun? but that should be true of any opaque material

For a given temperature rise, metals feel hotter than many other materials because they have a high thermal conductivity and enough heat capacity to deliver a lot of heat quickly.

As another though experiment, consider that you dread walking barefoot across cold tile floors but can bear to walk across carpeted floors in the same house. These two materials are at the same temperature.

Also consider aluminum foil you just pulled out of the oven. It's thinness runs contrary to the large thermal capacity of a solid chunk of metal- you can touch it immediately because you are such a large heat sink compared to it that it can't burn you even while it has only just started (rapidly) cooling from 350°F.

A conductive surface will reflect an EM wave because a lot of math but it can be proven

On a quantum level, when a photon encounters a material you have to ask whether the material is able to absorb a photon with that wavelength. When electrons are bound to a specific atom (or are in specific molecular bonds) then there are only certain energy levels possible: that's one of the core features of quantum systems, and it's the reason that each material has its own characteristic "absorption spectrum" (or emission spectrum: same idea). Photons whose wavelength corresponds to an energy that doesn't very closely match what's necessary to raise an electron from one specific level to another will just pass on through. (Nuclei are in bound states with discrete energy levels, too, so they work the same way.)

But one of the essential features of a metal is that the atoms all share a bunch of electrons that are free to move around more or less any way they'd like throughout the material. Because the electrons aren't trapped in one specific bound state, they have an essentially continuous range of energies available to them (just speed up or slow down a little to change your energy), so they are able to absorb photons of any wavelength at all.

[Now, to actually understand why you get reflection rather than stopping with absorption would take me a little more work to figure out how to explain. My instinct keeps being to go back to the classical explanations at that point, but I wanted to focus on quantum here to address your question about electrons in orbit.]

Thank you. That's very helpful. I studied the quantized transfer but never bothered to ask what happens if electron is hit by the energy isn't exactly what's required for an orbital transition.

If the film is thin enough metal is transparent. See for instance 50/50 mirrors and gold plated windows.

Of course you can now argue that if the photons hit the metal they will not pass through, but that's not how it works: the photons will excite an electron to a higher orbit and it may drop back to a lower orbit on the other side of the film making the metal appear transparent or it may reflect.

edit: saiya-jin I can't reply to your comment but yes, the direction is preserved. The same happens with a mirror, the photons ejected will be ejected at the correct angle even though the photoelectric effect has absorbed the photons. That's why metals reflect the way they do!

https://www.scientificamerican.com/article/how-do-mirrors-re...

The small fraction of photons that is absorbed will heat up the mirror.

that doesn't make sense - if that would be the case, the back-emitted photon would have a random vector, not pertaining the same one as original one (thus preserving the picture beyond the sheet of metal).

you are stating somehow the direction of photon is preserved when absorbed by electron - absurd idea even for layman physics (not claiming I know how this works, but this can't be the way)

> you are stating somehow the direction of photon is preserved when absorbed by electron

Well, that is how lasers work :)

https://en.wikipedia.org/wiki/Stimulated_emission

I barely understand even layman-level physics, but it would seem to me that this “absurd” idea is just a natural implication of the conservation of momentum?

edit: to all the downvoters of saiya-jin — let the one who has never defended their incorrect intuitions in physics cast the first downvote!

> let the one who has never defended their incorrect intuitions in physics cast the first downvote!

Amen. This stuff is wildly counter-intuitive, we only properly know how mirrors work since we understand the photo electric effect, and even with that understanding it is still quite tricky because it requires insight into how stuff works at a level where direct observation is no longer possible without access to enormous resources.

I've had that same question before.

This is in fact how it works - otherwise mirrors wouldn't work!

This is the only relevant part:

>"In a quantum-mechanical picture, light consists of photons, or packages of optical energy. The photons of the light reflected from a metal (or a dielectric mirror) are identical to the incident ones, apart from the changed propagation direction."

This doesn't explain anything about how it works quantum mechanically.

A minute current is induced which then results in the creation of a new photon identical to the original.

They are identical in terms of their quantum mechanical properties.

I'm not so sure about that, ever hard of plasmonics?

This definitely looks like something that has the potential to be really useful, and gives the example of replacing gorilla glass. While I get that this is a very tough glass, I did not see anything about it's hardness in relation to gorilla glass, as hardness is the property that provides the scratch resistance that is so highly sought after in smartphone screens.

Toughness is how much energy a material can absorb, whereas hardness is the resistance to deformation. Think a rubber band vs. glass.

According to wikipedia's AlON page it has a knoop hardness of ~1800 which from my understanding of things (I may have misread sources or be mistaken, I'm not a material scientist or even amateur) looks to be about equivalent to sapphire glass and much higher than gorilla glass (~600).

edit: in fact while I skipped the intro it states specifically that AlON has ~85% the hardness of sapphire, which more or less checks out. Suffice to say it has excellent hardness, way beyond gorilla glass.

I could find no data on relative permittivity though, and I assume that would be a factor for touchscreens.

I believe hardness is actually a bad thing in terms of replacing gorilla glass in phones. It increases the likelihood of shattering when dropped, and is the main reason for sapphire not being adopted.

> It increases the likelihood of shattering when dropped

AlON seems significantly tougher than glass as well as being way harder. So it looks to be both more scratch-resistant and more shatter-resistant.

> is the main reason for sapphire not being adopted.

Do you have sources (actual sources, not Corning fearing for their business) for that?

I'm no materials scientist. I just recall some articles I read when I was curious why I hadn't seen sapphire showing up in phone screens. Here is the first article Google pulls up.

http://time.com/3377972/why-apple-didnt-use-sapphire-iphone-...

I agree with you that AlON appears to be harder than glass. I just question if that is necessarily a good thing for consumer electronics. I expect we want something that is very tough but relatively flexible/soft.

> So it looks to be both more scratch-resistant and more shatter-resistant.

Refractive index 1.79, Abbe number 58… Why are my eyeglasses not made of this?

"main reason for sapphire not being adopted"

Edmund Optics sells a sapphire window thats unfortunately round, too thin (2mm) and too small (75 mm about 3 inches) to replace the glass in an iphone; the primary problem is that far too small optical window costs $650. On one hand a sheet large enough for a phone would cost more, on the other hand industrial production would be cheaper, much handwaving and it could be done but it'll cost $1K per phone, perhaps.

There have been phone screens made of sapphire, e.g. there was a special edition sapphire version of the HTC U Ultra (https://www.theverge.com/circuitbreaker/2017/5/4/15544426/ht...) which cost 150 Euro more than the glass version. I guess for just screen protection you don't need as high quality as for optical applications.

The Corning product is an extremely interesting material, the hardness varies with the depth leading to scratch resistance and a reasonably high level of resistance against shattering.

Is there are reason why artificial diamond coating hasn't taken off? Cost?

I wish it was possible to buy a phone with the option of a screen who’s glass was optimised for toughness rather than hardness, or at least an aftermarket replacement.

I can live with a screen protector, but broken glass is the bane!

I would assume "rugged(ized)" phone series have this sort of tradeoffs

I have a ruggedised case on my phone. Trade-offs hey.

Apple don’t make a ruggedised phone, if I wanted that I’d have to by an Android.

I would buy an Android, but I don’t want to.

Something like this https://www.catphones.com/ ?

The deal is that it isn’t aluminium. Seriusly, what’s with the clickbate headline? If you consider everything that contains aluminium atoms in the structure to be actual aluminium, then Saffire is also transparent aluminium.

The authors are diliberatly misleading their readers in order to cincrease interest. That’s a shitty thing to do in science even if do have a cool materiale on It’s own merits.

The headline relates to a line in an old movie (which is mentioned in TFA At the bottom of the page).

We've updated the title above.

To add to this, the "transparent aluminum" is really aluminum oxynitride crystals.

The mental imagery of denting, bending and crushing a transparent glass-like material and having it react the same way aluminium would is immensely satisfying for some reason

Yup, but it won't happen with "transparent aluminum", it behaves more like a ceramic does, so don't expect to indulge your bending and denting urges on it.

Am I the only one to recoil at the thought of someone chewing transparent aluminum foil?

The potential is tremendous in aviation. Imagine transparent aircraft skins -- the superstructure and internals (fuel tanks, hydraulics, etc.) could be inspected without disassembly (which itself adds stress to the structure). Though I'm not sure passengers would take kindly to aircraft with transparent skins. Sometimes it's best that things are hidden under a bonnet.

You could wrap the cabin interior with OLED panels that are normally active and opaque during flight.

Then, when you're on a longhaul and it's time to wake the passengers up - turn them off in midair. Good morning!

Brilliant. If the sudden increase in luminosity didn't wake you, the screams of fellow passengers looking through the invisible floor at the ground/Ocean 30,000 feet below would surely do the trick!

Other examples of "transparent aluminum": ruby and sapphire, both of them varieties of the mineral corundum, or Al2O3.

Slashdot, 2004:

https://slashdot.org/story/04/08/23/1141217/transparent-alum...

tl;dr: Transparent alumin_a_, not transparent alumin_um_.

On a completely separate note, what was the deal with the need for transparent aluminum in Star Trek IV?

Looking at the footage, they easily could have made the tank bigger if they just used all of the space available and didn't need to be able to "see" them from outside their tank. All they would have needed the aluminum for is to keep water out of where water shouldn't go and regular aluminum (or other material) would have worked just as well.

Maybe its stronger than regular

Aluminum corrodes a bit in sea water although it didn't need to last that long.

I don't think they used transparent aluminum in Star Trek IV.

I thought they paid for whatever they used (plexiglass?) with the recipe for transparent aluminum.

Although it could just be that I'm forgiving the poor writing.

This. They didn't have money, so they bartered with information.

Wouldn't that mean that nobody ever invented it?

No humans maybe.

And if humans were in fact the source of Starfleet's knowledge of transparent aluminum then it'd be an instance of a causal loop, of which there are many examples in fiction.

> of which there are many examples in fiction.

Hell, there's several examples just in Star Trek.

I believe the story was the person who invented transparent aluminum was the man they approached and just gave him the idea sooner (or maybe this is how he always came up with it, timelines can be funny)

Yes, it seems that way from the script. But then, why a transparent material?

(The answer of course is they're building an aquarium tank, and aquarium tanks are see through or have windows into it so humans can spectate. If they wanted to just bring whales back, they probably would have just flooded some crew room.)

Because a company that sells transparent materials like plexiglass can see the use of an innovative formula for a new transparent material.

It seems logical to me - it just had nothing directly to do with their whale-transporting plans.

It's been a while since I've watched the movie, but I don't think they specifically needed transparent aluminum, but it's how Scotty paid for them getting what they needed. They didn't have money to pay for it, so this was a barter, the knowledge of how to make it for making them some.

https://www.reddit.com/r/startrek/comments/3luo7c/in_star_tr...

Saying this is transparent Aluminum is the same as saying Glass is transparent Silicon. It's not, it's just a ceramic.

The title is a reference to a line in the movie "Star Trek IV: The Voyage Home" where Scotty trades the formula for "transparent aluminum" (from the future) in a barter transaction for plexiglass sheets (in the present) that the cast need to achieve their goal in the movie.

https://en.wikipedia.org/wiki/Star_Trek_IV:_The_Voyage_Home

https://en.wikipedia.org/wiki/Transparent_aluminum#Transpare...

This isn't new, I've read articles on AlON before. Synthetic Sapphire is more interesting in that its use is growing quickly due to demand for LED and laser substrates. Large single crystals are grown and sliced into wafers: https://en.wikipedia.org/wiki/Kyropoulos_process

New producers of the High purity Alumina (HPA) needed to produce sapphire are coming online, perhaps prices will come down enough that phone screens are an application, but I suspect that this is marketing speculation from the HPA makers to attract investors. For example this presentation from a HPA company speculated in 2015 that the iphone 7 would use a sapphire screen, which turned out to be wrong: https://www.altechchemicals.com/sites/altechchemicals.com/fi...

Does the fact that there is aluminum inside of the ceramic mean that it has a higher conductance? I couldn't find a reference online (there seems to be a few papers over the conductance of two very thin plates of AL203, but no characterization)

While it isn't a metal its still amazing stuff. Sometimes I wish I had stuck with my PhD in Chemistry which would have been working for someone who made equally wild materials.

At last, the whales can be saved!

Synthetic sapphire is a trivial example of a transparent ceramic made with aluminium. Its formula is something very close to Al_2 O_3.

I believe the first Apple Newton had transparent aluminium for the screen. I can't find confirmation of that but I remember going to a sales training session for it (long story) and they told people to say the screen was transparent aluminium, like they talked about in one of Star Trek movies.

This is about the same material which transparent circuit boards are fabricated with: http://www.dk-ceramics.com/transparent-pcb/

Jerry Seinfeld's new material has become super arcane.

I think transparent aluminum foil would be awesome - I could tell if my potatoes are cooked on the BBQ without opening the packet.

tl;dr the substance is probably a ceramic, aluminum oxynitride. Which seems a bit of a cop out like calling regular glass transparent sodium because it contains some. It seems quite cool stuff though.

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