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Author: Subject: Optical furnace for growing crystals
Admagistr
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[*] posted on 27-1-2022 at 16:07
Optical furnace for growing crystals


I am looking into the idea of creating a smaller and simplified optical furnace for some crystals and gemstone production. The basis of this furnace is a quartz tube, halogen bulbs, ellipsoidal mirrors. The amount of energy required to prepare ruby is about 4000 W, depending on the technical arrangement. The starting powder material is pressed and sintered and then gradually remelted in this furnace, resulting in an optically flawless crystal if everything goes right. Would anyone tech-savvy know how to make ellipsiod mirrors so that they concentrate all the light energy received from halogen lamps into one spot, and what particular IR halogen lamps to choose? Of course it depends on the diameter of the quartz tube, I have several of even large diameters, the largest one I have is about 13.5 cm diameter from my memory. I also have smaller quartz tubes, perfect optical quality, I guess 5 cm diameter. I'd have to check... I'm thinking of using four halogen IR bulbs, each with a power of 1000 W, and concentrating their energy in one focal point, where a ruby crystal would be formed.
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[*] posted on 29-1-2022 at 12:26


You have some interesting thoughts & projects! I'm by no means an expert, but just out of curiosity, why do you abandon the Nernst lamp for the halogen bulbs? It's all about the getting the IR focused on the sample?

...I'm sourcing material for a small mini-furnace along the lines in the previous post " A high-temperature mini-furnace for temperatures above 2000 C." and thought the Nernst lamp should fit the bill, can even work without protective atmosphere...
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[*] posted on 29-1-2022 at 15:09


I floated this idea a while ago. ( can't remember when and can't find it on here anymore) But the idea is sound. I spoke to a furnace designer refractory specialist since my first post and they confirmed the idea would work.

Ultra Refractory vacuum kiln.png - 25kB
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[*] posted on 29-1-2022 at 20:02


Quote: Originally posted by Chemetix  
I spoke to a furnace designer refractory specialist since my first post and they confirmed the idea would work.


Which suggests they don't much about optics. You've drawn parallel rays coming off your element, and while a few will be parallel, by far most will not be and will not be focused.

You might look to Nd:YAG laser cavity reflectors for inspiration - the sort used to transfer light from a linear flashlamp to the laser rod.

Bear in mind that the focii of an ellipse are points, so with any source you put there the radiating surface cannot be at the focus, only close to it.




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[*] posted on 29-1-2022 at 21:38


"You've drawn parallel rays coming off your element, and while a few will be parallel, by far most will not be and will not be focused. "

It's not about optical purity, this isn't a telescope. The rays are there to represent the majority of the EM flux, which emanating from the floor of the kiln will be more or less a constant temperature. Yes it's focused too close to the parabola for the rays to be close to parallel but that doesn't matter. The focal point only has to be a concentration of energy not a perfect intersection of rays for image resolution. What we worked out was that there will be a spot that will concentrate most of the energy from the floor heating on a small area. Ending up with a large magnitude of energy concentration. Taking 1000W.m^-2 and concentrating this to 10x makes a small area with an energy density far higher than a conventional kiln.

Fixating on the optical purity of the system misses the point. It's still powerful enough to make a small area very very very hot.
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[*] posted on 30-1-2022 at 00:21


Although you will be focussing a lot of heat onto the target,
I believe that you can not heat an area o a temperature higher than the heater element temperature.




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[*] posted on 30-1-2022 at 01:06


Quote: Originally posted by Sulaiman  
Although you will be focussing a lot of heat onto the target,
I believe that you can not heat an area o a temperature higher than the heater element temperature.


Not general true, look at solar power towers, can even melt steel, Melting steel with solar power...

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[*] posted on 30-1-2022 at 02:58


And what's the temperature of that heater element?



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[*] posted on 30-1-2022 at 04:05


Quote: Originally posted by Fulmen  
And what's the temperature of that heater element?


Well that's a gotcha question ;)

.. but seriously, are there any thermodynamics here, or any other physics, that impose such a a limit?

[Edited on 2022-1-30 by JohnnyBuckminster]
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[*] posted on 30-1-2022 at 06:43


Quote: Originally posted by JohnnyBuckminster  
You have some interesting thoughts & projects! I'm by no means an expert, but just out of curiosity, why do you abandon the Nernst lamp for the halogen bulbs? It's all about the getting the IR focused on the sample?

...I'm sourcing material for a small mini-furnace along the lines in the previous post " A high-temperature mini-furnace for temperatures above 2000 C." and thought the Nernst lamp should fit the bill, can even work without protective atmosphere...


Thanks! The Nernst rod is not out of the picture, I'm still looking for it, I listed the halogen IR lamp because it is available, but I only found it in tube form, even at power 2000 watts! The idea is to concentrate the heat into one spot-focus and place the emerging crystal as close to the focal point as possible. I'd coat the elliptical mirror with thin gold leaf, the kind sold to artists who want to gold leaf something. Gold reflects IR radiation best of all metals, and even firefighters use gold-plated shields as heat protection when they put out fires. By being very thin, the gold leaf is light in weight and therefore affordable....
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[*] posted on 30-1-2022 at 08:37


Quote: Originally posted by JohnnyBuckminster  
.. but seriously, are there any thermodynamics here, or any other physics, that impose such a a limit?

Yes, black body radiation, Stefan–Boltzmann law, solid angles and other stuff.




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[*] posted on 30-1-2022 at 12:44


Quote: Originally posted by Chemetix  
The rays are there to represent the majority of the EM flux,


Thats exactly my point - those rays do not represent the majority of the EM flux, but only a small fraction. Every point on that floor is radiating in all directions, distributing energy in all directions. Which means the energy following your ray diagram is only a small fraction of the total energy.

But you are more than welcome to build it and test it - thats what experimenting is all about.




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[*] posted on 31-1-2022 at 01:13


Quote: Originally posted by Sulaiman  
Quote: Originally posted by JohnnyBuckminster  
.. but seriously, are there any thermodynamics here, or any other physics, that impose such a a limit?

Yes, black body radiation, Stefan–Boltzmann law, solid angles and other stuff.


Postulate:

It is not possible to heat an object to a temperature higher than the heater element temperature.

I hear this argument every now and then, and I always get bogged down, so let’s investigate.

Assume the element radiates according to Stefan-Boltzmann law, and by applying Wien's displacement law we can find the temperature of the element.

Now, to make things a bit cleaner, replace the heater elements by six identical pulsed CO2 lasers arranged like in the figure below. Each laser is directed at a 1 g target consisting of Al2O3 powder. A CO2 laser will emit at around 10 um, and we assume that the pulse energy is 1 J.



The heat capacity for solid Al2O3 at room temperature is 90 J/(mol K), if all losses are ignored, the temperature of the target will rise about 1 K for each pulse.

If all six lasers are fired simultaneously, the temperature of the target will rise 6 K. Repeat this 100 times and the target will be at around 600 K. This is nothing spectacular, CO2 lasers are commonly used in industry for welding a range of different metals.

But, and here it comes, if our heater elements, the lasers, emit at a peak wavelength of 10 um, according to Wien's displacement law that corresponds to a temperature of 290 K. How can an emitter at 290 K melt metals at around 1 500 K?

What is the difference if I replace the lasers with heated elements, like Nernst lamps, and mirrors to direct the energy at a focal spot? Obviously, for all practical reasons, the energy required to increase the temperature 1 K of a sample at 1 000 K will be far greater compared to increasing the temperature 1 K for a sample at room temperature. This is more of a fundamental question. Can you generally claim the stipulated postulate?




[Edited on 2022-1-31 by JohnnyBuckminster]
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[*] posted on 31-1-2022 at 05:29


JohnnyBuckminster:

Its the second law of thermodynamics that forbids heat flowing from a lower temperature to a high temperature under equilibrium conditions.

Yes you can melt steel with sunlight, because the sun's surface is at about 6,000 C , while steel melts at about 1500C

Wien's displacement law is applicable to blackbody radiation. The radiation from a laser does not have the spectrum of a black body so Wien's law can not be used to convert the laser frequency to a meaningful blackbody temperature. For example blue LEDs are not at thousands of degrees.

I should add that it is possible to assign a temperature to electrons (and other particles) based on their velocity or energy. There are even materials which are described as having a negative temperature based unusual definitions of temperature.

Perhaps an odd property of diffuse (Lambertian) matt and extended surfaces is they appear to have the same brightness when viewed through an optical system near or far away. This effect prevents the intensity of heating effect form a heating element being increased by focusing the radiation from the heating element using mirrors or lens.

Because lasers are not Lambertion (they can have diffraction limited parallel beams) they can be focused to increase their intensity. Heating elements are Lambertion and can not be focused to increase their intensity.

I will try to find a better description of this effect. I will post a link to it if I do find a better description.




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[*] posted on 31-1-2022 at 08:58



Thanks for your comments wg48temp9, I enjoy these kind of discussions, so I dive straight in

1. Its the second law of thermodynamics that forbids heat flowing from a lower temperature to a high temperature under equilibrium conditions.

The 2nd law of Thermodynamics does not prevent heat to flow from a colder to a hotter end as long as the net entropy generated in the Universe increases. A laser has a typical efficiency of 50 %, so in the case of the furnace with six lasers focusing on a target, a lot of entropy will be created in the surroundings. With heating elements it will get even worse, say only 5 % reach the target and 95 % of the energy is wasted to the surroundings.

2. Yes you can melt steel with sunlight, because the sun's surface is at about 6,000 C , while steel melts at about 1500C

Let’s build a Dyson Sphere and collect every single photon emitted from the sun and direct the radiation at a block of Kryptonite 1 metre in diameter placed in a thermos, i.e. a closed system. This will be an engineering challenge! If the temperature can’t exceed 6,000 C, and we are bound by the 1st law of Thermodynamics, what happens with the “excess” energy hitting the Kryptonite block? We have the same energy, but it is localised to a smaller space.

2. Wien's displacement law is applicable to blackbody radiation. The radiation from a laser does not have the spectrum of a black body so Wien's law can not be used to convert the laser frequency to a meaningful blackbody temperature. For example blue LEDs are not at thousands of degrees.

Imaging an observer located inside the furnace. All he ever will know is what he can see, and thus, he is clueless about what's behind the furnace brick wall. One day he notices that the target material in the centre of his world gets warmer, and he traces this effect to an inflow of radiation through “ports” in the furnace wall. Using a spectrograph he concludes that there is an inflow of radiation of wavelength 10 um that causes the heating effect.

Let’s throw in a few more lasers at wavelengths to mimic blackbody radiation and direct them at the target, the observer inside the furnace will now record an energy distribution similar to a black body radiation peaking at 10 um. He will probably use Wien’s displacement law and speculate about the nature of this 290 K background radiation.

For the observer it doesn’t matter if it is a laser or a heating element that injects energy into his world, the effect is the same, it gets warmer.

3. Perhaps an odd property of diffuse (Lambertian) matt and extended surfaces is they appear to have the same brightness when viewed through an optical system near or far away. This effect prevents the intensity of heating effect form a heating element being increased by focusing the radiation from the heating element using mirrors or lens.

This might be the actual reason, it is not forbidden because it would violate any of the laws of thermodynamics, it is just not possible to collect the energy and localise it to a smaller volume, in some analogy with a diffraction limited spot…. need to think a bit more about this...

Thanks for your your comments, food for thoughts...
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[*] posted on 31-1-2022 at 09:30


I remember a similar discussion years ago on an astronomy newgroup: Could you build a lens or mirror large enough to set something on fire, using the light of the moon? If I remember correctly, the answer was no and the reason was the angular size of the moon.
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