“Heat capacity (usually denoted by a capital C, often with subscripts), or thermal capacity, is the measurable physical quantity that shows the amount of heat required to change a substance’s temperature by a given amount. In the International System of Units (SI), heat capacity is expressed in units of joule(s) (J) per kelvin (K).”
All right, to the point now. I’ve always had the impression that a brass heatsink was better. ’ It’s heavier so it will absorb more heat’. Specific heat capacity of brass is 0.38 kJ/Kg/Kelvin (0.38J/g/K) and that of aluminium is 0.87 kJ/kg/Kelvin (0.87 J/g/K). Given the thermal conductivity difference between brass (109-125) vs aluminium (204-250) I have begun questioning this concept. I’m currently believing that the better the themal conductivity of the metal, the faster the heat will be drawn away from the LED.
Brass is approximately 8.5/2.7=3.15 times the mass of aluminum for a given volume. With aluminium having a specific heat capacity of approximately (0.87/.038=2.29) 2.29 times that of brass, the mass of brass over tha of aluminium has little impact on the absorbtion of heat.
Is the above concept true then would you agree that brass has no place being the pill of a flashlight?
1) Heat does not penetrate all materials at the same rate.
2) Because heat does not travel through all materials at the same rate, heat does not dissipate (convection, conduction and radiation) from all materials at the same rate.
3) Heat does dissipate from materials, so the change in temperature will be less (much) than expected.
Given the thermal properties of the two materials, and assuming reasonable conduction from the pill to the host, I would actually assume Aluminum to be the superior material.
I remember acceleration equations in physics being easy back when we were allowed to ignore air resistance…
Initially the most important quality of a heat sink is to conduct as much heat away from the source as possible. Any resistance to this flow of heat will create a thermal differential, just as electrical resistance will create a voltage differential. Once the heat is quickly spread out, away from the source it may be wise to use another, cheaper metal that has a higher heat capacity.
BTW, metals that are good electrical conductors are also good thermal conductors. Brass and copper are very good conductors of heat and are a good choice for first contact, in our case, with the led.
Thermal paste is important for good physical contact, but also good thermal contact. That is why I think it is a good idea to use artic SILVER between an led and a copper sink rather than artic ALUMINUM.
Oops, I meant to edit my post and instead deleted it entirely. Glad you responded before I did that.
You make some good points. The rate at which heat flows (heat current) for a given temperature gradient is a function of the thermal conductivity of the material. I was just talking about the ability of a given material to absorb heat and the resultant change in temperature for that material.
Heat does dissipate (it’s not a closed system so it’s not a classic calorimetry problem in which we can ignore the surrounding environment and find the equilibrium temperature) and how quickly that happens depends not only on the temperature gradient but the properties of the dissipating medium and the modes of transfer (convection, conduction, radiation). In the case of heat loss by radiation, by the Stefan-Boltzman law, the heat current is a function of the surface area of the radiating body, the fourth power of the absolute temperature of the body, and the body’s emissivity. I just looked at a table of emissivities and hadn’t realized it can vary so much for a single material (e.g., aluminium).
We do always make simplifying assumptions in physics. Sometimes that annoys students because they want to know why we’re not talking about air resistance and such but I always say we need to learn the basic principles first. Newton’s laws apply whether there’s air resistance or not, we just need to consider that force when we remove that simplifying assumption from our model.
I was of the understanding that brass was used for flashlights (like P60 pills), because of the availability, the cost and the ease to machine thin wall parts with it. I don't think it has a blasted thing to to with heat transfer. Silver would be the best for a heat sink, copper second, aluminum third and all the rest are b*st**d step children. At least that's what I've been told over the years by many people more knowledgeable than I.
The problem I have with something like copper is that it is used as a heat sink and then put into an aluminum body light. That's like being on a four lane highway and then entering a construction zone when you hit the aluminum. The lanes go down to two lanes and there's a big traffic (heat) backup. The copper heat sink will now retain more heat, as it can't get rid of it fast enough through the aluminum and if it's left that way, it actually does more damage than if it was all aluminum. Of course, a light that put out that much heat would either be turned off by the user or the batteries would drain, before the heat became excessive, but the fact still remains that there is no real solution other than active cooling, if we desired high powered flashlights to stay on for long periods of time.