Thermal diffusivity measurements of thin plates and filaments using lock-in thermography

Abstract

Photothermal radiometry has been widely used to measure the thermal diffusivity of bulk materials. In the case of thin plates and filaments, a one-dimensional heat propagation model including heat losses has been developed, predicting that the thermal diffusivity can be obtained by recording both the surface temperature amplitude and phase profile slopes ("slope method"). However, this method has given highly overestimated values of the thermal diffusivity of poor-conducting films and filaments. In this paper we analyze the effect of the experimental factors affecting the thermal diffusivity measurements of thin plates and filaments using infrared thermography, in order to establish the experimental conditions needed to obtain accurate and reliable values of the diffusivity of any kind of material using the slope method. We present the calculations of the surface temperature of thin isotropic and anisotropic plates heated by a modulated and tightly focused laser beam, showing that the slope method is also valid for this kind of pointlike heating. Special attention is paid to the effect of surface heat losses (convective and radiative) on the diffusivity measurements of small-dimension and poor-conducting materials. Lock-in thermography measurements performed in the best experimental conditions on a wide set of samples of different thermal properties (thin isotropic and anisotropic plates and filaments) confirm the validity of the slope method to measure accurately the thermal diffusivity of samples of these shapes.

Thermal diffusivity of TBC: Results of a small round robin test and considerations about the effect of the surface preparation and the measuring approach

“…In all the simulations the difference between the 'true' and the estimated thermal diffusivity resulted smaller than 1%, always smaller than (or comparable to) the intra-lab and interlaboratory experimental uncertainty. These results are in good agreement with those reported by A. Mendioroz and co-authors [32].…”

“…One aspect only partially studied in the literature [31,32] common to most of the photothermal and thermographic techniques concerns with the effect of the blackening coating used for guaranteeing the absorption of the heating radiation just within a very shallow outer layer and to make opaque the TBC in the sensitivity range of the IR detector/camera.…”

Can TBC porosity be estimated by non-destructive infrared techniques? A theoretical and experimental analysis

“…For experiments lasting within very few seconds or less, convective heat losses do not take place; moreover, for small temperature increases caused by the heating pulse, radiative heating losses resulted sufficiently small to adopt the adiabatic model. A thin graphite layer (b 4 μm) on the TBC surface can be painted to prevent the laser radiation to penetrate within the TBC without altering significantly the final result [31][32][33]. In the specific case when an IR camera is used to detect the surface temperature decay after the pulse, the uncertainty is related to the frame rate and the integration time (the time period where the detectors are active in receiving the signal) of the camera.…”

Phase lock-in thermography for thermal diffusivity measurement and layer surface characterization

“…The phase shift has several advantages over the temperature amplitude such as it is independent of the sample's reflectivity and the light power 2,3 . Nowadays, there are several methods to measure these parameters but each one has its disadvantages: can only be applied to bulk materials 4,5,6 , requires use of a small focused beam 7 and requires the variation of the sample surface temperature which can be influenced by the heat loss effect 8 .…”