# Thermal Diffusivity

1. Mar 31, 2009

### Skyeagle

Hi guys, need your help on the below question:

There is this parameter known as thermal diffusivity.

Thermal Diffusivity = k/ p*C, where k= thermal conductivity, p=density and C= Heat Capacity.

The thermal diffusivity value for Steel is 20.11 um^-2/s (melting point at 1810K) and the value for titanium is 8.85 um^-2/s (melting point at 1933K).

Can I say that in this case, titanium will melt faster than steel because the heat that is mostly absorbed by the titanium will cause the melting, whereas steel melt slower because the heat is conducted away?

Or in another words, any material with a lower thermal diffusivity value will melt faster?

Thanks and Best Rgds

2. Mar 31, 2009

### Mapes

We can say for sure that a temperature difference will propagate faster through steel than titanium.

However, melting involves an externally applied heat source and a phase change, so things get more complicated. Are you assuming constant temperature, constant heat flux, or another boundary condition to enforce melting? This would affect the melting comparison. Also, the heat must propagate through the molten material, so the properties of liquid steel and titanium are also relevant. Finally, the heat of fusion of both metals is also relevant, as this energy must be carried away through the liquid phase.

I don't believe one can reach a conclusion based on information given, unfortunately.

Incidentally, your values are off by six orders of magnitude; shouldn't the units be mm2/s?

3. May 4, 2009

### Edpuente

The heat is not conducted away, it is needed to increase the temperature needed to melt the material. Melting has to do with characteristics of the material, such as how the molecules acomodate and form crystals. The way the crystals are arrenged has to do with the energy requiered for them to move among them (Melting).

Ceramics has lower thermal diffusivity and does not melt faster! Such as the ones used in the space shuttle that are used to protect it from the heat when they reenter the atmosphere.

4. May 5, 2009

### jaap de vries

Thermal diffusivity measures the ability to conduct thermal energy relative to its ability to store thermal energy (Incropera and DeWitt). It basically tels you how quickly a material responds to changes in their thermal enviroment. It does not say anything about the melting point.