Comparison of thermal diffusivity measurement
techniques
by P.G. Bison, F. Cernuschi*, E. Grinzato, S. Marinetti
CNR-ITEF c.so Stati Uniti 4, 35127 Padova, Italy - paolo.bison@itef.pd.cnr.it
*CESI Via Reggio Emilia 39, 20090 Segrate (MI), Italy
keywords: thermal diffusivity, thermal waves, flash method,
ABSTRACT
Researchers are more and more involved in the measurement of thermal diffusivity. This
parameter that governs the thermal diffusion process may account for many physical-
mechanical-chemical characteristics of the material under test. Examples are hardened
degree in steel and carriers properties in semiconductors. In Thermal Nondestructive
Testing, the detected thermal diffusivity variation accounts for the presence of defects
buried in the material. In Cultural Heritage studies, dating and classifying structures may
be helped by the knowledge of components diffusivity, that may be used as a proof of the
historical variation in manufacturing, or the supplying of raw materials.
Researchers look for measurement techniques that are fast, reliable and possibly executed
in situ. Basing on these requirements, many experimental lay out and processing
algorithms were proposed in the field of thermal testing. Pulse or modulated heating is
used to stimulate the material under test, in form of extended source or focused laser
beams. IR radiometers (spot or imager) or laser deflection sensing are instruments
generally used to measure the diffusivity of the material through the temperature field
produced by the heating.
In this work, fruit of the collaboration between two laboratories, we present different
techniques to measure thermal diffusivity. One of them is the well established and
standardised Laser Flash technique. Others are not new but, lacking of a standardisation,
they may suffer of the variation in the implementation of the equipment and execution of
the test. This is the case of techniques using thermal waves like the 'mirage effect' as well
as those using thermoelectric devices or lamps or lasers to produce modulated heating.
Finally, there are quite new techniques that utilise pulsed heating, like the Laser Flash,
but try to simplify the very stringent requirements of this technique. The purpose of this
work is to establish the degree of accuracy of the presented techniques.
The work is divided in two main sections. In the first a brief description of any technique
both in the experimental layout and in the processing algorithms is given. In the second,
results of any technique, obtained on the same sample of stainless steel AISI 304, are
reported. Uncertainties evaluation of any measurement is reported together with a
discussion on the pros and cons of the related technique.