# A metal that heats ''quickly'' and maintains temperature

1. Jul 22, 2011

### jc.int

hello,
I've got an idea for a project but I'd need to know if there's some kind of metal (it would be great if it was a common/affordable one, not very rare/expensive) that heats rather quickly with sunlighth (like iron) but when it gets at a temperature (say, 100ºC) doesn't continue heating (or heats slowlier).

Thank you,
Jaime

2. Jul 22, 2011

### QuantumPion

What you are looking for is a metal with a high emissivity so that it absorbs sunlight well and, if I understand you correctly, high thermal conductivity so that it reaches equilibrium temperature quickly. I think lead would meet your needs, although dull iron or steel would also work.

3. Jul 23, 2011

### meplumber

Traditional cast iron is what you are looking for in my humble opinion.

Think about the cast iron radiators in a hydronic or steam heating system. The radiators heated relatively quickly and then held their heat and emitted it over time due to the large thermal mass of the radiator. The rate of transfer of the heat (cooling) is directly related to the surrounding air temperature. The rate of heat absorbtion of the metal decreases as the metal temp approaches the temp of the heat source.

Disclaimer: I am not an engineer. Just a plumbing, heating, and cooling contractor.

4. Jul 27, 2011

### SecretTheme

Hello! I am not a scientist but maybe I have a direction for you to go. There is a new product coming out (no, I am not selling it) called Coffee Joulies. They are stainless steel "beans" with a proprietary substance inside that absorbs heat quickly but releases it slowly. The info says "Joulies are filled with a proprietary substance called a “Phase Change Material” (PCM) that melts at 140°F...)

So perhaps you need to find a metal that has a certain, low, melting point like gallium and then encase it. Good luck!

5. Jul 27, 2011

### Winzer

Someone correct me if I am wrong but you may be looking for a material with a
relatively low diffusivity.

Diffusivity is a material property that is the ratio of THERMAL CONDUCTANCE to VOLUMETRIC HEAT CAPACTIANCE. That is:

$$\alpha = \frac{k}{\rho c_p}$$

So a higher diff allows for a metal to be heated up quicker, but not hold the heat as quickly. A lower metal diff will take a longer time to heat up, but will hold the heat better. You should be able to find a list of these online, or you may have to calculate them yourself.

6. Aug 10, 2011

### caedar

Winzer, do you mean high diffusivity? Heat capacitance is the amount of energy required to heat up the material by one degree, so we want that to be very low for it to heat quickly. We also want high thermal conductance so that the material heats quickly throughout the bulk.

To summarize everything that I think is correct that has been said so far, you are looking for a material with high thermal conductivity, low heat capacity, and high emissivity. However, the OP also wants something that heats very slowly after 100 degrees C. For this we need something that drastically changes in either emissivity or heat capacity near 100 degrees C. This generally means that a phase change is required. However, I'm not aware of many materials that have serious changes in emissivity on phase change, however heat capacity is known to change drastically upon phase change. SecretTheme had a good point when referring to coffee joulies, I think you may need to encase a 2nd material that undergoes a phase change in a shell for this to work.