# Transparent Metal

1. Nov 13, 2005

### [initial steve]

There is a HUGE debate going on over at another forum i frequent, several people are claiming that their turbos become translucent at extreme temperatures. They claim they are able to see the turbine wheel spinning through the housing.

Another group is claiming that what you are seeing is not a result of the metal becoming transparent, but a shaddow caused by the black body radiation theory.

One of the sites senior tech guru's has stated that it is well known that metal becomes transparent over 1000 degrees.

im not sure if i can link to the other forum on here, if i can, let me know so you can all see the debate going on.

ANY help would be much apriciated.

Cheers
Steve

2. Nov 13, 2005

### Integral

Staff Emeritus
Metal at 1000 (C or F) is not transparent. Ask anyone who has ever worked in a foundry. It does however glow with a characteristic color. For 1000 (F or C) the color is between a a dull orange and red. The temperature of the radiating surface will be VERY sensitive to variability of the internal heat source. So if the 'turbo' is hot enough to glow with blackbody radiation, you may well be able to "see" shadows of the spinning turbine blades. They would cause the outer surface to darken as the cooler blade passes the inside surface. Now it is very likely that the shadow of the blade is actually trailing the actual blade by a measurable distance. This distance would depend on the type and thickness of the housing material.

So are you seeing the tribune? No, you are seeing a "shadow" caused by the lower temperature of the turbine blade. I would expect the blades to be at a much lower temperature then the housing because they are in continual contact with the lower temperature air they are compressing.

Make sense?

I don't think a link to your discussion would be inappropriate. ...This isn't the transparent metal thread at Anantech is it?

3. Nov 13, 2005

### pallidin

The turbine wheel is not a solid body.
A "dominant" illumination influence from "behind" the wheel and the human eye "persistance of vision" will create the effect.
This is identical in concept to viewing a scene behind a spinning fan.

4. Nov 13, 2005

### [initial steve]

Hi Integral, thanks for the quick response.

The link to the topic is:
http://www.nissansilvia.com/forums/index.php?showtopic=120630

its 9pages long now, only read it if your very bored hehe

A few questions about your post. What the some people are arguing about black body radiation is that the side directly opposite the turbine wheel is emitting IR radiation, which is being absorbed by the turbine wheel, and this is creating a shaddow on the other side of the housing.

One point that has been raised is that nobody can actually say whether or not the turbine wheel itself is cooler or hotter than the housing.

The effect is acutally on the exhaust side, so we have the extremely hot exhaust gasses being pushed over the turbine wheel. If it is the case that the blades are actually hotter, could the cooler outside air be bringing the temperature of the housing down, while the pulses of exhaust gasses coming form the engine be heating the housing up?

People who claim to have seen the effect, also claim that it is impossible to photograph (how convenient)

5. Nov 13, 2005

### [initial steve]

can anyone tell me why light cant pass through 1000 degree metal.

does temperature have any effect at all on metals refractive index?

6. Nov 14, 2005

### Integral

Staff Emeritus
Light is not transmitted though hot metal for the same reason it is not transmitted through a cold metal, the photons are adsorbed by the atomic structure. At high temperatures the metal becomes a source of photons but it does not become transparent. I have worked with Stainless at 1000C, it was not transparent. Though variations in temperature were very evident.

I tried reading the linked forum, these guys may know something about cars, but they sure could stand a grammar lesson or two!

I will stay close to my original post, though it may not be cooler turbine blades but just hot swirling gasses passing though that cause a variation in heat transfer to the outer casing. If, as is stated in one post that the turbine is spinning at 70k then I do not think that it would be any more then a blur under the best of conditions. What you are seeing is the result of the variations in temperature inside the housing,

7. Nov 14, 2005

### [initial steve]

excellent, thanks for your reply :) Makes perfect sense to me :)

8. Nov 14, 2005

### [initial steve]

this is the response i got from posting what you said.

"yeah, they're doing the same thing as we are.
How do they know that the atomic structure of the metal behaves in the exact same way when it's hot?
What about different metals?
Does it change things if the internal objects are at a much higher temperature?

It's obvious from that guy's first post that he doesn't know how a turbo works, or how it's constructed, so what he's saying may or may not be relevant to the scenario in question."

and this:

"so in other words they themselves havn't seen it so they aren't sure as to what is happening ? and there not likely to have the same conditions as the person that saw the turbine wheel in the first place so it's there ASSUMPTION as to what is happening ?
at the end of the day I really couldn't care
other people have told me they have seen this same effect on there turbo though there turbo died shortly after ! bent shaft "

These guys are absolutely convinced metal takes on glass like properties when its hot 0_0

9. Nov 14, 2005

### Staff: Mentor

Perhaps you could as them to explain the science of it...

Or even post a picture.

10. Nov 14, 2005

### [initial steve]

they have come up with the claims, they are asking me to prove it cant happen 0_o crazy i know.

They also claim that the effect cant be captured on video or still camera.

there is nothing i can do to convince these guys.. :(

11. Nov 14, 2005

### Integral

Staff Emeritus
He's right I know nothing about turbo's but that is still a lot more then these guys know about physics.

While I have never seen a red hot turbine, I have seen red hot metal, do these guys think that the metal in a turbo is some how special? As long as the metal retains its basic shape and strength it has not changed physical properties significantly. Due to thermal expansion the density may have dropped a bit, but not so much as to render the metal porous. Having worked with hot Stainless Steel I can testify that, at those temperatures, it is soft and easily deformed. I can also testify that its radiant color is very sensitive to small variations in temperature. That is what causes the illusion that you can see the turbine though the metal.

Here is a link you should post for your buddies on the auto fourm:http://www.iespell.com/" [Broken]

Last edited by a moderator: May 2, 2017
12. Nov 14, 2005

### Integral

Staff Emeritus
This is a fact that we have experianced here. There are some things people believe like a religion, there is nothing you can say or do that will make them change. No matter how wrong they are. It is not worth the fight. Good luck.

13. Nov 14, 2005

### brewnog

Ooh, interesting discussion!

I see it most days at work, when we're running the engines at overload. I was astonished the first time I saw it, I did ask myself whether the volutes were actually becoming translucent! Can't say I've ever tried to photograph the effect, I might give it a go next time I've got the camera out.

14. Nov 14, 2005

### ahrkron

Staff Emeritus
They have a couple of nice pictures in the forum quoted by [initial steve].

15. Nov 14, 2005

### [initial steve]

I can forgive people for thinking that metal can become translucent, looking into a glowing stainless steel manifold you can see some very intresting effects :)

May i ask what you do?

Steve

16. Nov 14, 2005

### Staff: Mentor

Oh, I think I see what the problem is (a few people mentioned it) - they are interpreting the dark areas as being thicker so you can't see through them as well. But the dark areas are thicker, which could make them cooler, making them darker. I had no idea they got so hot, though...

edit: I've read a little of the site - your instincts are right about the refractive index: if something is opaque, it doesn't have an index of refraction.

Also, refraction and reflection are not the same thing.

Also, glass isn't a liquid, it is a....well....glass. That's a classification of amorphous solids.
I found an error you made: "Your theory ONLY works if the material emitting light is opaque to begin with.. steel is not." - you mean translucent, not opaque.

One more thing:
Um, we know because we've studied materials science and engineering in school. Some of us even (not me, personally) make a living dealing with such aspects of science. :uhh:

Knowing what happens to metal when you heat it - on the atomic level - is pretty important for engineers.

Last edited: Nov 14, 2005
17. Nov 15, 2005

### DumHed

Do you have any information on what actually happens to the molecular bonds at different temperatures, especially to do with the relationship between the metal's optical properties and its electrical conductivity?

I've been looking around and can't find any decent info, or for that matter any metalurgists or engineers who can explain this in any detail.

The turbo translucency thing is interesting because there are a lot of reputable people who claim that it happens, as well as a lot of people who claim that it can't happen.
As a kid I learnt that it does happen, and I have seen effects that look a lot like translucency (ie, seeing internal details of glowing metal objects)

From what I've read on the technical side I'm 50:50 as to whether it's a direct optical effect or an illusion.
I'm quite happy to believe either given some solid info explaining peoples' ability to perceive internal details of turbos and manifolds.

I don't think it's possible to see through a sheet of heated metal in any major detail, or see more than a very small portion of the visible spectrum, but I think that if metal can take on any translucent properties a glowing turbo is exactly the scenario that would make it most visible.

A turbo's turbine housing has a scroll shaped volute with a double skin over most of it, forming an annular nozzle around the turbine.
Different areas of the housing run at quite different temperatures due to gas velocity and density, and from my testing it appears that the turbine runs quite a bit hotter than the housing.

In a normal scenario, with a solid metal object (or contained molten liquid) there will be no well defined temperature differentials to form a visible "edge" so even if the metal is translucent it won't be visible over the emitted glow.

It's also feasable that an illusion of translucency is formed by surface temperature variations due to the varying levels of IR radiation internally, but that couldn't generate any sort of perspective or parralax effects when viewed from different angles.

I'm building a higher powered gas combustor, which will hopefully be able to heat a housing up at least as hot as they run on cars under full power, which will certainly come close to answering the question for me.

18. Nov 15, 2005

### Staff: Mentor

Welcome to PF!
Probably the best place to start is by looking through some materials science course notes. http://mpdc.mae.cornell.edu/Courses/MAE212/mae212.html" [Broken] is a site I found via Google. Of interest are sections 5 (atomic structure) and 14 (temperature effect on crystal structure).

I'm afraid there isn't anything to find on the optical properties of metal with regard to transparency/translucency because, quite simply, there aren't any. The closest you may get are with the http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Rutherford-Alpha&Beta.html" [Broken]. In both cases, however, metal foils/coatings are made so thin that there really are gaps that light can pass through. And recognize that such filters reduce the amount of light transmitted by several thousand times. They look opaque unless you are actually looking at the sun or the filament of a light bulb (you cannot see a frosted bulb through a solar filter). (I'll post some pics if I can...)

That illustrates a two pretty major flaws in the reasoning of the guys on that site, btw: If heating metal made it translucent, it would still require an intense source of light behind it to see through it - you wouldn't even see it through a micrometer solar filter, so how could you see it through a centimeter of steel? And, any metal parts inside the turbo wouldn't be putting out much more light than the casing, so they wouldn't be visible even if it was completely transparent but emitting light itself (which, as the pics of hot glass showed, isn't really possible anyway).

Last edited by a moderator: May 2, 2017
19. Nov 15, 2005

### brewnog

The other thing which confused me was just how easy it would be to see something spinning at 90,000rpm, let alone anything enshrouded inside a steel casing!

I'm currently a development engineer, working with medium-large diesel and gas engines.

20. Nov 15, 2005

### Astronuc

Staff Emeritus
In a metal, it is still atomic bonds. The metal lattice expands as temperature increases (thermal expansion). As electronic properties, the electrical conductivity will increase up to some temperature and then it will decrease. Metals are opaque after 10's of nm, or less. (Need ZapperZ to confirm exactly). Heated metals radiate in infrared and visible (optical) range, but they never become transparent, nor even translucent.

Thermal expansion - http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thexp.html#c1
Thermal conductivity - http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thercond.html#c2
Electrical conductivity as a function of temperature is described by the Wiedemann-Franz Law in one of the sections.

Electrical conductivity - http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html#c1

What appears to be translucency is due to heat transfer by conduction and radiation, and perhaps the radiant heat adds to the image of the outer casing, but that certainly does not mean transparency, nor translucency, either of which infer the initial photon passes through an material without absorption.

As for it not being videoed, well if one can see it, it can be video-taped. There might be an issue with sampling rate, so one might need a special CCD.

21. Nov 15, 2005

### DumHed

The problem with the cameras is not the sampling rate, but the fact that they are somewhat sensitive to IR radiation, whereas eyes are not.
We can see subtle changes in the visible spectrum radiating from glowing objectes, but most cameras are swamped by the IR and produce quite washed out pictures.

I think the illusion of translucency being caused by internal radiation is very plausible, but it must be a very convincing effect to have so many people fooled.
It seems that the majority of people who work with turbos are of the opinion that it does happen, and many other people also think it does.

There are plenty of people, like me, who as kids learned that metal becomes translucent at high temperatures. I've never really questioned it because I've seen and heard of many instances of it happening. So basically it either can happen, or certain situations cause a very convincing illusion.

Hopefully I'll have some decent testing done over the weekend :)

Last edited: Nov 15, 2005
22. Nov 15, 2005

### Gokul43201

Staff Emeritus
I can't attest to what is being seen, but I can say this : metals do not become transparent at high temperatures. The low transmissibility and high reflectivity of a metal comes from its very large dielectric constant.

In the visible range, the dielectric constant of a metal goes roughly like $$\frac {\epsilon(\omega)}{\epsilon_0} \approx 1 - \frac{\omega_p^2}{\omega^2}$$,
where $\omega_p$ is the plasma frequency, given by $e\sqrt{n/\epsilon_0m^*}$. Here, n is the free electron density and $m^*$ is the electron effective mass. Plugging in numbers for iron (primary constituent of steel), gives you a plasma frequency that's over an order of magnitude above the frequency range of visible light. This makes the extinction coefficient in iron be $\alpha \approx c/3\omega \approx 0.5 ~ microns$. So, the fraction of light transmitted through even a 1mm thick sheet is roughly one part in $10^{1000}$.

The only effect that temperature has on the dielectric constant is a very weak influence on the effective mass. Even assuming this influence to be as strong as the change in lattice parameter, that only implies a 1% change in $m^*$ over a 1000K temperature change. This changes the plasma frequency and hence, the extinction coefficient by about 0.5%.

Moral : Raising the temperature of iron/steel raises the value of the extinction coefficient by at most a few nm. That's essentially a negligible effect. Doesn't look to me like anything's turned transparent here !

Last edited: Nov 15, 2005
23. Nov 15, 2005

### DumHed

thanks for that info!
Do you think there could be enough transmission of far IR radiation to cause surface temperature changes with enough resolution to be viewed as a rough image of the internal structure?

Also, is it possible to work out the dielectric constant for alloys given rough information on their composition?
Is electrical conductivity closely related to the dielectric constant and therefore the metal's transmissibility?

24. Nov 16, 2005

### inha

Given a composition and lattice structure it is possible to work out the dielectric function but it's a complex computational process because you need to calculate the band structure to get the dielectric function. IIRC marlon has done these calculations or atleast knows more about the details.

25. Nov 16, 2005

### DumHed

hmm it sounds like the way to go is to replicate the effect and then decide what must be causing it :)