The term "thermal effusivity" may not be familiar, but it is actually one of the most important thermal properties.

 

The thermal property measured by Bethel’s thermal property measurement device, the Thermal Microscope TM3, is the thermal effusivity. Why does the TM3 measure thermal effusivity?

 

When explaining to customers, we say:

“The Thermal Microscope TM3 outputs 'thermal effusivity' as the final result.”

However, most customers respond with:

“What is 'thermal effusivity'?”

This is understandable because, generally, what is covered in high school or university textbooks are thermal conductivity and thermal diffusivity.

 

**◇ What is Thermal Effusivity?** 

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It refers to the “capacity of a material to absorb heat from another material when they are in contact.”

 

For example, when you touch metal on a cold day and it feels cold, it's because the "metal," which has a high thermal effusivity, is absorbing the heat from your hand.

 

Conversely, if you touch something made of wood or plastic, it does not feel as cold as metal because the thermal effusivitys of "wood" and "plastic" are lower than that of metal.

 

In other words, materials with a high thermal effusivity feel colder when touched.

 

Similarly, during a hot summer, touching the metal buckle of a seatbelt feels burning hot for the same reason. (Although the seat itself should be at the same temperature, it does not feel as hot.)

 

**◇ Converting Thermal Effusivity to Other Thermal Properties** 

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Most customers are interested in measuring 'thermal conductivity,' but if the 'thermal effusivity,' 'specific heat,' and 'density' are known, 'thermal conductivity' and 'thermal diffusivity' can be calculated.

 

**◇ Principles of Measuring Thermal Effusivity** 

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Another frequently asked question is the "principle of measuring thermal effusivity."

 

The steps for measuring the thermal effusivity using the Thermal Microscope TM3 are as follows:

 

1. A Mo (molybdenum) film is deposited on the sample surface.

2. The Mo film is periodically heated using a heating laser.

3. A detection laser is irradiated coaxially with the heating laser.

   The reflectivity of Mo varies with temperature, so the intensity changes of the detection laser irradiated coaxially with the heating laser become periodic.

4. The thermal effusivity is calculated from the phase difference between the waveform of the heating laser and the reflected waveform of the detection laser.

 

For a sample with a high thermal effusivity, the Mo film absorbs more heat, resulting in less temperature change in the Mo film. Consequently, the phase difference between the heating laser and the reflected waveform of the detection laser becomes smaller. Conversely, for a sample with a low thermal effusivity, the phase difference becomes larger.

 

**◇ A Minimum Resolution of 3 μm!** 

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The measurement in a tiny area with a resolution of 3 μm, which is performed by the Thermal Microscope TM3, is currently impossible with other devices.

 

For those who want to measure small areas, "resolution" is likely to be the main concern. If you wish to solve this problem, please feel free to consult Bethel.

 

This article is a revised version of one originally published on August 31, 2014, at:

[https://blog.thermal-measurement.info/archives/52061322.html]