Effective thermal diffusivity of layered materials measured by modulated photothermal techniques

Abstract

Modulated photothermal techniques provide useful methods based on linear relations to measure the thermal diffusivity of homogeneous materials. We have analyzed theoretically the applicability of such linear relations to two particular cases of layered composites, i.e., two-layer materials and superlattices. In order to measure the through-thickness and in-plane thermal diffusivities of these anisotropic materials, planar and pointlike excitations have been studied. The main result of this article is that the linear relations encountered for homogeneous materials still hold for layered composites, although their slopes do not always give the effective thermal diffusivities of the sample parallel and perpendicular to the layers, as derived from the in-parallel and in-series thermal resistors models, respectively. However, an “apparent” thermal diffusivity is obtained from which the thermal parameters of each layer can be retrieved.

Photothermal Radiometry and Diffuse Reflectance Analysis of Thermally Treated Bones

“…1) is only adequate for homogeneous materials [25][26][27]. This is an important restriction in the thermophysical analysis of materials, especially those with important heterogeneities.…”

“…e 1 (α 1 ) and e 2 (α 2 ) are the thermal effusivities (thermal diffusivity) of the aluminum foil and sample, respectively. When the thickness of the aluminum is small, in such a way that its thermal diffusion length μ 1 , is at least two times greater than its thickness μ 1 > 2L 1 , it is said that it is thermally thin [27]. In this case in Eq.…”

“…On the other hand, if the thermal diffusion length μ 2 of the sample is smaller than half its thickness (μ 2 < L 2 /2), it is said that the sample is thermally thick, and the approximation e −2σ 2 L 2 << 1 can be used. It is important to mention that the thermally thin and thermally thick definitions used here are more strict that those currently used and discussed by Salazar et al [27]. This more restrictive option was imposed to make sure that the specific thermal regime is adequate to analyze the experimental data, with the intention of obtaining a reliable characterization of a non-homogeneous material such as bone.…”

“…In this case in Eq. 1, the approximation e −2σ 1 L 1 ≈ 1 can be performed [27]. On the other hand, if the thermal diffusion length μ 2 of the sample is smaller than half its thickness (μ 2 < L 2 /2), it is said that the sample is thermally thick, and the approximation e −2σ 2 L 2 << 1 can be used.…”

Photoacoustic studies on thermal parameters of liquid crystal mixtures

“…All the photothermal methods depend upon the detection of thermal waves generated in the sample after illumination with pulsed or chopped optical radiation [4][5][6]. Amongst the variety of techniques available [7][8][9][10], the laser-induced photoacoustic (PA) method has gained more popularity due to its simple, elegant experimental technique as well as to its versatility in employing different configurations to measure the required thermophysical parameter with great accuracy [11,12].…”