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Optical properties of metals at high temperatures
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DumHed
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Optical properties of metals at high temperatures

Does anyone know anything much about this?

I've seen and heard of heaps of instances of metal becoming translucent at high temperatures.
The most common story is being able to see through turbocharger housings or exhaust manifolds on cars when they've been run flat out for a while.
I've heard in the past that the optical properties of metal change enough at high temperatures that light can pass through, but I'm not sure about it.

Another possibility is that Infra Red radiation from inside the turbo, etc can cause enough of a change in colour temperature of the external metal that it looks like an image is visible through it.

Strangely, this is one thing that doesn't seem to have any solid info on the net, from either perspective.

It's well known that a thin enough piece of metal is translucent (the aluminium layer in a CD, or very thin gold leaf), but I haven't seen anything that fits this into the general optical characteristics of metals.

Any ideas?

There's a big discussion going on a car forum after someone said they could see the turbine in the turbo after a dyno run.

I also posted this on the Mythbusters forums: http://community.discovery.com/groupee/forums?a=tpc&s=6941912904&f=9701967776&m=2441941218&r=2441941218#2441941218
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Post Tue Nov 08, 2005 10:36 am 
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Knightrous
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Just regurgitating somethings I learnt in Chemistry classes at school, I think the only way you could see through metal is if the atoms where to viabrate enough from the excessive energy (Heat) trying to break the bonds that it might make it appear translucent. Sorta like when you wave you hand in front of your eyes fast enough that you get that ghosting effect.

A real interesting topic you've got here Andrew.


EDIT: By the hand ghosting example, I mean that rapid moving objects can appear transparent, not that your breaking the bonds of the atomic structures in your hand Razz
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Post Tue Nov 08, 2005 10:50 am 
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Spockie-Tech
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Re: Optical properties of metals at high temperatures

quote:
Originally posted by DumHed:
story is being able to see through turbocharger housings when they've been run flat out for a while.
I've heard in the past that the optical properties of metal change enough at high temperatures that light can pass through,

Another possibility is that Infra Red radiation from inside the turbo, etc can cause enough of a change in colour temperature of the external metal that it looks like an image is visible through it


An interesting perspective..

If a photon from the original item gets absorbed, bounced around, and re-emitted on the other side of the opaque material, does that count as "seeing" the item that emitted the original Photon ? Does that mean the intermediate material is "transparent" ?

Semantics I suspect. If you look at something through a piece of paper and just see the shadow on the paper, are you seeing the original item ?

If by "see" you mean direct path of photons from emitter to receptor (eyes) with no absorbtion and remission along the way, then I doubt that many metals would allow that. unless they formed a very pure crystal structure (like diamond), although I suspect the atomic spacing in a metal crsytal structure would be sufficiently varied to scramble any light waves on the way through.;

That however, is just whistling in the dark, perhaps one the Kero's with their knowledge on the various phases of Metallurgy might be able to assist further ?
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Post Wed Nov 09, 2005 1:33 pm 
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DumHed
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yeah it's a real grey area for information, which seems strange.

I'd expect there to be some hard info saying yes or no, but there's not really anything.


quote:
If a photon from the original item gets absorbed, bounced around, and re-emitted on the other side of the opaque material, does that count as "seeing" the item that emitted the original Photon ? Does that mean the intermediate material is "transparent" ?


I don't think that counts as transparent, but in the end the effect would be that you can see objects through hot metal, which is what I'm trying to prove / disprove.
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Post Wed Nov 09, 2005 1:58 pm 
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Nick
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I'm no metalurgist but I rekon all metals are opaque at visible wavelengths and the only way you would "see" thru metal is after you had turned it into a cloud of plasma. Exceptions would be when the metal is only a few wavelengths thick.

Its possible to make some metals take the structural form of a glass by supper rapid cooling from the molten state, but even then, I don't recall any special optical properties.

In the case of the turbo, I agree with andrew; it could have been a thermal "shadow", caused by the turbine blades (presumably stationary?) either heating or cooling the housing, thereby altering the colour of the metal.


quote:
If a photon from the original item gets absorbed, bounced around, and re-emitted on the other side of the opaque material, does that count as "seeing" the item that emitted the original Photon ? Does that mean the intermediate material is "transparent" ?


An example of seeing through an opaque object would be an image intensifier tube. the receptor is opaque but the photons are encouraged to tunnel thru
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Post Wed Nov 09, 2005 2:17 pm 
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Daniel
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If the metal becomes translucent at high temperture and still remains a solid, when it gets to liquid form how would the foundarymen see where the metal is when they are pouring a casting?

As much as car enthusiest like to gloat about their parts, turbo housings and exhuast tubes arn't made out of super rare alloys that process magical properties. Are turbo housings cast steel or aliminium? They seem to me to be a gravity die casting as you won't get that sort of tolerance with lost wax or investment, but I have never actually seen one in person.

Lets just say you are refering to steel parts. Most steels undergo a cysticallagraphic change at about 785 degress C. If the exhaust gasses are able to heat the the pipes and turbo up that far what excatly is happening to your pistons and conrods? Surely that will be a lot warmer and well into the glass transition zone (read as becoming really floppy. Not good for engine parts).

As for Nicks thermal shodow, that sound like a good idea until you think about the turbos impeller may be spinning a several thousand rpm. It would be like watching jolts blade. Thermal conductivity isn't that fast.

And I severlly doubt anybody is building anything out of metal glass. Being able to create anything out of metal glass is extremly difficult, plus it is extremly brittle, it tends to shatter when it is being cast, and is as transparent as tar.

So the main reason why I think you can't find any information about it on the net is because it is a myth. Just like my school mate telling me he was going 250km/h down the sunshine motorway in a car with no speedo. I'll ask the metalurgist here if he knows any more.

Post Wed Nov 09, 2005 2:56 pm 
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Nick
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I qualified the thermal shadow idea with "presumably stationary" Smile. Turbos spin at insane speeds and even with a window in the housing, you would only see a blur when its running. If the guy on Andrew's car forum is claiming to see a running turbo through the housing, that's WAY into the realm of B.S.!

Perhaps the person in question sniffed a bit of NO from a leaky injection system? Laughing
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Post Wed Nov 09, 2005 3:21 pm 
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prong
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what is interesting about this is the amount of people who have claimed to have seen it, not just car enthusiasts, but all sorts of people, reputable people even!

as I keep telling Andrew, we need to heat up a sheet of steel and put a bright light behind it and see what happens!

Post Wed Nov 09, 2005 3:29 pm 
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kkeerroo
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Here is a story from my Army Reserve days. It was told to me by my section commander during my induction training at Sunny Camp Kapooka.
During a leason on the F89 Light Support Weapon (or more commonly called a Minimi) we where instructed that the barrels must be changed to a spare barrel after 200 or so rounds of continous firing. This is done because the barrel can get so hot that it will deform and destroy the barrel. My section commander, who was giving the lesson, then went on to tell of how, when his combat engineer squadron first got issued the weapons and was told this they thought they would see if it was true.
So they took one of these brand new light machine guns down to the range, made up a belt of 2000 rounds and went for it. After 600 rounds the barrel started to glow red and the rounds began to fall bellow the target. After 800 the barrel was white hot and the range of the weapon had decreased to less than 100m. At close to 1000 rounds the barrel was noticably sagging and, I quote, "you could see the rounds passing through the barrel like glowing lights". Shortly after this the weapon jammed.
I cannot confirm this as the most rounds I ever fired at once was just over 200 blank rounds during a yippie shoot (a shoot at the end of an exercise toget rid of all your left over ammo unless you want to carry it home). Apart from a lot of white oxidation on the barrel which took ages to clean and a temporary loss of hearing nothing much happened. But oh what fun!
It seems to me that what you can see is just localised heating of the metal.
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Post Wed Nov 09, 2005 4:03 pm 
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prong
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I would think the barrell would have to much thermal mass for you to see localised heating from bullets... hmmm

Post Wed Nov 09, 2005 4:34 pm 
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DumHed
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could the localised heating happen fast enough to be visually discernible though?
I guess in that sort of situation maybe the projectile would actually be in contact with the barrel and cause rapid frictional heating.

quote:
Originally posted by Daniel:
If the metal becomes translucent at high temperture and still remains a solid, when it gets to liquid form how would the foundarymen see where the metal is when they are pouring a casting?


Translucent materials are still visible, especially when they're glowing!


quote:
As much as car enthusiest like to gloat about their parts, turbo housings and exhuast tubes arn't made out of super rare alloys that process magical properties. Are turbo housings cast steel or aliminium?

Due to the fact that turbos glow anything up to white hot in operation I think it's fairly obvious that they're not aluminium.
They tend to be either high silicon or high nickel content steel alloys, and can run at extremely high temperatures for long periods of time.
The most expensive turbo housings are inconel, as are most modern turbocharger turbine wheels.
There are even some in various forms of stainless steel, for thinner walls and lighter weight (from the turbo Formula 1 days)


quote:

Lets just say you are refering to steel parts. Most steels undergo a cysticallagraphic change at about 785 degress C. If the exhaust gasses are able to heat the the pipes and turbo up that far what excatly is happening to your pistons and conrods? Surely that will be a lot warmer and well into the glass transition zone (read as becoming really floppy. Not good for engine parts).


Exhaust gases can be up around those temperartures, but the internal engine parts are well cooled, and designed to transfer heat away from themselves.
Pistons are aluminium, and in most turbo engines are cooled by jets of oil from underneath.
The part that runs the hottest is the exhaust valve, which must dissipate its heat into the water cooled cylinder head via the valve guide (tube its stem runs though) and the actual valve seat when it's closed.
There are upgraded valve seats made from beryllium alloys to improve this heat transfer, and some engines (including the Nissan SR20DET) have sodium filled exhaust valves, which use the phase change of the sodium as a heat pipe through the valve stem to transfer heat from the tip of the valve to the guide, and cylinder head.


quote:

As for Nicks thermal shodow, that sound like a good idea until you think about the turbos impeller may be spinning a several thousand rpm. It would be like watching jolts blade. Thermal conductivity isn't that fast.

I don't think it'd be possible to see the turbine spinning, because it can be running in excess of 100,000rpm under boost, and will be doing 10,000+ at idle unless it's a very large turbo (which can actually stop at idle)


quote:

So the main reason why I think you can't find any information about it on the net is because it is a myth.


Myths are the kind of thing that's all over the net though, because people always like to prove or disprove something. That's what makes it strange to not be able to find anything.
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Post Wed Nov 09, 2005 4:36 pm 
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Daniel
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quote:
It seems to me that what you can see is just localised heating of the metal.


That would be friction of the bullet rubbing on the soggy barrel. If the barrel was sagging then there would be a bugger load of friction which would get it from red hot to yellow hot and then the rest of the coller barrel will draw the heat away.

I don't think they will be having friction between the impeller and the housing inside a turbo unit. But that would be interesting to watch.
I also don't think they are firing bullets out the exhaust. Maybe the exhaust system is badly set up and the exhaust is balling down the piping. If you get large clumps of hot exhaust it might cause the surface temperture to change back and forth which might make the exhaust pipe appear to change color. Anyone who has been playing with NOX might think they are seeing though it. The same might happen with a turbo, but it wouldn't sound good.

Post Wed Nov 09, 2005 4:41 pm 
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Daniel
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quote:
Originally posted by DumHed:
Translucent materials are still visible, especially when they're glowing!


Yes, I have seen a cast steel pour and I didn't see though the metal.


quote:
Due to the fact that turbos glow anything up to white hot in operation I think it's fairly obvious that they're not aluminium.
They tend to be either high silicon or high nickel content steel alloys, and can run at extremely high temperatures for long periods of time.
The most expensive turbo housings are inconel, as are most modern turbocharger turbine wheels.
There are even some in various forms of stainless steel, for thinner walls and lighter weight (from the turbo Formula 1 days)


The nickle steel spec sheet I just looked says that nickel steel is rated to a max 870 degree C. I'd like to compare that to a color chart.
I guess the Nickel increases the phase change temperture. Taking it about that tempreture would be a bad idea. unless you know it would cool down correctly.


quote:
Exhaust gases can be up around those temperartures, but the internal engine parts are well cooled, and designed to transfer heat away from themselves.
Pistons are aluminium, and in most turbo engines are cooled by jets of oil from underneath.
The part that runs the hottest is the exhaust valve, which must dissipate its heat into the water cooled cylinder head via the valve guide (tube its stem runs though) and the actual valve seat when it's closed.
There are upgraded valve seats made from beryllium alloys to improve this heat transfer, and some engines (including the Nissan SR20DET) have sodium filled exhaust valves, which use the phase change of the sodium as a heat pipe through the valve stem to transfer heat from the tip of the valve to the guide, and cylinder head.


Ok, I know nothing about car engines. Very Happy




quote:
Myths are the kind of thing that's all over the net though, because people always like to prove or disprove something. That's what makes it strange to not be able to find anything.


I thought we would have heard about engineers trying to use this sort of thing to make steath fighter planes.

Post Wed Nov 09, 2005 4:57 pm 
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DumHed
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I think the effect (if it's there) is only small, and will only occur in certain conditions.

I don't think a very slightly translucent aircraft that's completely glowing red hot classes as stealth.


quote:
Yes, I have seen a cast steel pour and I didn't see though the metal.


What was behind it though?
I'm thinking that if something can be seen through the metal it could only be hotter surfaces forming internal details in the turbo / manifold.
In a stream of molten metal, even if it is translucent or even transparent all you will see is more of the same glowing metal inside it.
There's no detail to view, so it will look the same as if it was solid.

If you shine bright light on a glowing piece of metal it looks normal, because of the large amount of light being reflected compared to what is being radiated.

The dim lit situations where glowing metal is most visible would allow much easier viewing of any slight image showing through the metal.

Compare it to a dark room, viewed from outside a window where there is bright ambient light. In that case a window is effectively opaque due to swamping by reflection, but if there's more light in the room than there is outside the window is suddenly very transparent.

I think that if the properties of metal can change enough to allow some light through, then an object like a turbo, with a hollow area inside containing shapes that run hotter than the external casing would provide the perfect scenario for viewing its translucency.

I can't think of any other situations where the same conditions exist.

Another interesting thing is that one of the major reasons why metal is opaque is the sea of "free" electrons that exist in it - which is also what gives its conductivity.
As temperature increases, so does electrical resistance, so I would guess that opacity will reduce as well (maybe in too small an amount to notice though).
It could also be worth noting that nickel iron alloy is ~14 times less conductive than pure iron.
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Post Wed Nov 09, 2005 5:29 pm 
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kkeerroo
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I thought I would do a bit of research on why metals glow when hot. Here's a website: http://www.squ1.com/concepts/color-temperature.html
Basicaly, any material that is above 0 degrees Kelvin emits electro-magnetic radiation (light, radio waves etc). When metal is heated the atoms vibrate and bang into each other. The higher the temp, the more vibrating and the harder they bump into each other. If they hit hard enough the atoms can "lose" an electron which flys off to hit another atom. When it hits it gives some of its energy to the atom which causes one of its electrons to jump to the next valence level. This electron then jumps back to its natural state and in the process gives off a photon of light. Simple!
The hotter the metal, the more electrons flying around, the more photons given off per second meaning a higher frequency of light. So a metal starts off red and moves up the frequencies to violet. But the metal does not give off just one colour off light but also the colours from the lower frequencies as well. Which is why the metal goes from red to yellow to white.
But just because the metal is giving off light does not mean that it is allowing light to pass through it. As long as the crystal structure of the metal remains the same then its optical properties are also unchanged. If the crystal structure is changed then your turbo is stuffed.
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Post Wed Nov 09, 2005 6:14 pm 
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