Measurement of thermal diffusivities through processing of infrared images

Abstract

The measurement of the thermal diffusivity of a thin layer in the direction of its plane is usually a difficult operation. The standard ‘‘flash technique’’ is very appropriate for diffusivity measurement in the direction of the thickness of the sample but adaptations of this method to in-plane measurements remain very sensitive to the position and form of heat excitation and temperature sensors. The new procedure proposed here consists of applying any geometrically nonuniform heat impulse on the front face of the sample and recording the entire transient temperature image on the rear face thanks to an infrared camera. The influence of axial diffusion can be avoided for periods much longer than the axial diffusion characteristic time. Integral transforms on the radial space variables (Fourier transform) are very suitable for treating the temperature field and to estimate radial diffusivity. The main advantage of this method is to avoid any experimental precaution (no knowledge of the geometrical form of the excitation - replacement of the sensor positioning by an image calibration). Furthermore, the considerable number of data produced by the camera is processed using a statistical approach. The validation of the method is made on a homogeneous sample by comparison between the in-plane direction measurements (obtained with the present procedure) and the thickness direction measurements (obtained by the classical flash technique).

Simple possibilities of thermal diffusivity estimation for small-sized samples, with a laser pulse heating and infrared cameras

“…One way to process the front surface temperature field (T(x,y,z=0,t)) is to consider the Fourier cosine transform on the space variables (x,y) (see [3] , [4]), such as: The Fourier transform method can be applied if the heating is a pulse but not necessary a point source. In the case of a point source, the short time approximation (when l and L are assumed to be large or infinite) of the Fourier transform of expression (1) is: …”

Comparison of some thermographic techniques applied to thermal properties characterization of porous materials

“…Another dynamic approach to measure diffusivity in a one-side (reflection) scheme is to shoot a laser pulse (1 ms duration) on the surface of the specimen with a spatial top-hat shape (1 cm diameter) and measure heat diffusion on the surface [2,3]. Discrete Cosine Transform is used to analyze the spatial shape variation of surface temperature in time.…”

Comparison of thermal diffusivity measurement techniques

“…The methods for the thermal diffusivity measurement proposed over the last four decades, belong mainly to photothermal and photoacoustic [6,7] techniques, generally confined to the laboratories. In the last years, due to the significant progress in infrared technology as well as in electronics, some thermographic techniques for the in-situ thermal diffusivity evaluation have been developed [8][9][10][11][12][13][14][15][16].…”