heating copper with laser diode

Hello all,
I was wondering if you could help me with a laser question, I was looking at using a laser diode as a heating element.
I was wondering how much power it would take to heat a thin piece of copper about the size of a 5 or 10c coin and as thin as say a couple of pieces of paper, to 200 degrees Celsius approx. (the laser can be near or better still touching the copper, creating an enclosed heating element)

I was interested to know if this would be achievable within perhaps a 5 or 6sec time frame, how much power would be needed (could it be done with mains AC 240v, 12v DC or perhaps even just a couple of aa battery) And also what would be the cheapest laser and lens assembly that would be able to achieve this without self-destructing?

If it is cost effective to do so i would like to do some experiments with such a laser to see if it would suit my purpose, how much would i be looking at for such a laser and what would a typical life span be if producing heat at those temps?

If anyone has or can make a laser and driver assembly that could be suitable id love to buy one ready made, im a computer tech and have a good understanding of electronics in general but lasers i only have a basic understanding of, optic's and radiation are not really my thing.
I have seen demo's on the net of blu-ray laser diodes being used to heat objects but we cant believe everything we see on youtube :can: The ultimate source for Lasers and Accessories have a kit for $100 that they show mounted in a bic lighter but they have no specs on the heat that could be expected to be produced so i was a bit skeptical.

Im betting that most of you would have successfully produced the kind of temps i am wanting to achieve, any help would be much appreciated.
Regards,
Jon.

Some quick math.

200deg C is ~175K increase over RT. Specific heat of copper is .385 J/(g*K), so for even just a 1g piece of copper (which is tiny), you need about 67J of energy to reach that temperature. To get there in 6.7 seconds, you therefore need 10W of power absorbed into the copper, which means you need more than 10W of laser power, since the copper won't absorb all the laser light. 2g doubles the energy, and therefore the power, required. So clearly, the power required gets ugly fast.

In the visible regime, it's going to reflect a lot of light, so you're going to need a lot of power coming out of your laser. For 10W (minimum power needed even with 100% absorption, which isn't possible), you've already gone well past the handheld lasers most common around here. As a starting place, I would look for the absorption curves of copper, especially at IR wavelengths (higher powers are more easily available) and see about what powers you're looking at, assuming you need at least 10W absorbed into the copper.

I believe there are likely much easier and cheaper ways to heat a piece of metal than shining a laser at it. It seems pretty likely to me that if the laser is placed close, the wasted power of the laser, coming out of the diode as heat, will do more to heat the metal than the actual laser light.

First off, is there any problem with a normal heating element? Converting electrical energy directly into thermal energy.

But I'll assume you want to use the laser because you can heat without contact (even though you said it wouldn't matter if it was in contact)

Some out loud calculations:

I'll go with the bigger coin of the two, a nickel, and take it as about 22mm diameter. And taking paper to be about .1mm thick, and 5 sheets, we've got a disk .5mm thick and 22mm diameter. Which is 190mm^3, 0.19cm^3.

Copper is 8.94g/cm^3, so .19cm^3 of it is 1.7g.

Copper's atomic weight is 65.6g/mol, so 1.7g of copper accounts for 0.026mol.

Copper's specific heat capacity is 24.4J/mol/°K. So to raise 0.026mol of copper 1°K (1°C) would require .63J. Since as you want to raise the temperature to 200°C (assuming room temp is 25°C) you need to raise the temperature 175°C, requiring 111J.

Now, to get 111J into something in 5 seconds, would require 22W (since as a watt is a joule per second). And that's with the major assumption that you'll be able to get 100% absorption.

So at this stage, I would really just be going for a purpose built heating element (glorified resistor).

Copycat. :na:

Thanks for your reply. I kind of expected lasers to be a rather inefficient way of producing heat but i didnt expect that the resistance in the diode would produce more heat then the laser radiation itself.

I was just using Copper as an example because it conducts heat well but for antibacterial applications silver would be better as it is an even better conductor.

The advantages over say using something like nichrome wire as a heating element would be that the heat would be completely free of impurities apart from the metal thats being heated. So for uses in areas such as biology or other science it could be an alternative heating method.
But like you say, there are cheaper ways to heat metal.

So im guessing that the small burning blu-ray laser kits that i have been seeing must not get very hot, and it would be faster to light a match with the sun and a standard magnification glass then one of these laser kits?

Thanks very much for your info.

pullbangdead said:

Copycat. :na:

Click to expand...

I was typing intermittently, I didn't see yours until I posted. And anyway, my calculations were better.

aXit said:

I was typing intermittently, I didn't see yours until I posted. And anyway, my calculations were better.

Click to expand...

Better? Pfft.

They were both equally accurate in their own rights, but mine was much more succinct and faster, and only required looking up one number. :fightin:

Both really good answers as far as laser power required is concerned. Thanks allot for the help. Saved me hours of research and gave me something to think about :thinking:

Cheers.

Your best bet, if you had a fair amount of money, time and a pretty big working space would be to get hold of one of the sleds that hold 24 of the 445nm 1W diodes, get two of those which gives you 48 diodes and have them all aiming at the copper sheet.

Should chuck out about 48W's of power, and you could carbon black (I think it's called that) the back surface of the copper to absorb heat better. While this sounds fairly unrealistic it could work with some time and a lot of effort.

Other route is just look into a C02 gas laser which puts out 100 or more W's, that would probably go through thin copper though, have to include some some sort of beam expander or something, I'm not sure of the divergence of a C02 laser so you could just set the plate at the focusing distance where the dot is the same size as the surface

you cant just put a current through the copper?
you could try a 50W co2 laser with a beam diameter the size of the coins face. I dont imagine using light to heat a reflective surface is efficient, but it could be completely different in terms of absorption in IR range.

Why would you even bother using a laser for such a purpose? If you want to generate heat, even with just light, you don't need a laser unless you need to be cutting things. Even then, you'll need a far more powerful CO2-type laser to do any kind of real heating; these laser diodes are really pretty weak.

Want to heat things up? Get an incandescent light bulb and a magnifying glass. Get a heat lamp, or a heat gun. Don't bother with a laser unless you specifically need one, because they're expensive, dangerous, and quite frankly a waste to spend on heating up stuff.

I'd recommend a light bulb as well. Something that peaks in the IR might be best. It seems that copper likes IR...


How about that... a peak right around 808nm....