A travelling photothermal technique employing pyroelectric detection to measure thermal diffusivity of films and coatings

Abstract

A travelling thermal wave technique employing optical excitation and pyroelectric detection of thermal waves propagating along a material film/coating on a substrate is described. The method enables direct measurement of thermal diffusivity. The technique involves measurement of the phase lag undergone by an optically excited thermal wave as it propagates along the coating. The set up has been automated for convenient and fast data acquisition and analysis. The technique has been adapted to measurement of thermal diffusivity of a commercial paint sample coated on glass and copper substrates. It is found that thermal diffusivity of the coating is independent of the thermal conductivity of the substrate. Dependence of thermal diffusivity on coating thickness shows exponential increase, with value reaching a constant at a characteristic high thickness. Measurements have been carried out on a few other samples with wide variations in thermal diffusivity, and the results compared with available reports or results obtained following other techniques. Analyses of the results show that the technique allows measurement of thermal diffusivity of coatings and films with uncertainties better than ±2.5%.

Thermal diffusion in thin plates and coatings: influence of thickness and substrate material

“…The method involved measurement of the phase lag undergone by a thermal wave while propagating over a finite distance through the coating. In the present experiment we used laser-based photothermal excitation to generate thermal waves in the film and a pyroelectric detector to detect them after traveling over a defined distance [17]. The phase difference of the pyroelectric output signal was continuously measured as a function of the separation between the points of excitation and detection.…”

“…The thermal diffusivities of the coatings or plates were measured employing a traveling thermal wave technique described elsewhere [15,17]. The method involved measurement of the phase lag undergone by a thermal wave while propagating over a finite distance through the coating.…”

“…Among these authors, Bhusari et al [15] reported an exponential dependence for thermal conductivity on thickness for Al films, while Chattopadhyay et al [16] reported similar dependence for aluminum films and a-SiC x N y films. Philip et al [17] have reported exponential variation for thermal diffusivity with thickness for paint coatings in the low thickness regime (<0.2 mm).…”

“…Later, by measuring the phase lag of a thermal wave propagating along a free-standing diamond sheet, Kosky [14] reported its thermal diffusivity. Necessary theory of the technique was developed by this author, which was extended later to other materials and films by other researchers [15][16][17]. The latter authors used an optical beam deflection technique or pyroelectric detection to monitor traveling thermal waves for these measurements.…”

“…A traveling photothermal technique, described earlier [17], was employed to measure thermal diffusivities of thin coatings, films, or plates with thickness in the micrometer to millimeter regime. Our results show that the thickness dependence of thermal diffusivity of thin material coatings, plates, or films can be described by a general expression irrespective of the thermal conductivity of the material in the bulk form.…”