Measurement of thermal diffusivity and thermal effusivity of thick solids by photoacoustic technique

A. Yoshida1, A. Imuta1 and T. Yamada1

1Osaka Prefecture University, Japan

Keywords: thick solid, human skin, photoacoustic measurement, thermal diffusivity, thermal effusivity
property: thermal diffusivity
material: solid

In recent years, methods that use heat-transfer phenomenon have been increasingly applied in the field of medical technology. And accurate data of thermophysical properties of human skin is required. However, traditional measurement methods of thermophysical properties for industrial materials are unavailable to human body, so simple, quick and noninvasive measurement technique which puts less strain on human body is required. This study focuses on photoacoustic technique as a technique which meets above requirement and aims at in vivo measurement of thermal effusivity and thermal diffusivity of human skin. As the fundamental research, thick solids are treated and the accuracy is investigated.

We assume the three layer model that is constructed of sample, plate and gas in the theory. And equation of photoacoustic signal is based on thermal diffusion equation. In the case that the beam is absorbed in plate, equation of phase lag between incident light and signal (f) is composed of plate thickness (lp), absorption coefficient (Kp), thermal diffusivity (ap) and thermal effusivity ratio between plate and sample (s=es/ep)

f = F( f : Kp,lp,ap,s ) (1)

where plate properties are given so f shifts only with change in s. Therefore, es is determined by comparing observed data and theoretical curve. In the case that beam is absorbed in plate and sample, f' is composed of 6 parameters containing sample absorption coefficient (Ks) and thermal diffusivity (as)

f' = F( f : Kp, Ks,lp,ap,as,s ) (2)

where plate properties and Ks are given and we use above measurement result es, so as is determined in a similar way.

The outline of actual measurement is as follows. We used a laser diode (1850nm) to minimize the effects of the optical properties of human skin. Laser beam whose intensity is modulated periodically hits the sample's surface. Then, the periodical heat in the sample that is produced by absorption of beam reaches the gas near its surface. The volume of gas change and acoustic wave is detected by microphone.

We measured thermophysical properties of a sample that properties are given for validation of the method. We used a rigid polyvinyl chloride (PVC) sheet as thin plate and a epoxy resin as sample. The result shows the present technique is sufficiently accurate.

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