Direct Measurement of the In-Plane Thermal Diffusivity of Semitransparent Thin Films by Lock-In Thermography: An Extension of the Slopes Method

Abstract

We present an extension of the well-known slopes method for characterization of the in-plane thermal diffusivity of semitransparent polymer films. We introduce a theoretical model which considers heat losses due to convection and radiation mechanisms, as well as semitransparency of the material to the exciting laser heat source (visible range) and multiple reflections at the film surfaces. Most importantly, a potential semitransparency of the material in the IR detection range is also considered. We prove by numerical simulations and by an asymptotic expansion of the surface temperature that the slopes method is also valid for any semitransparent film in the thermally thin regime. Measurements of the in-plane thermal diffusivity performed on semitransparent polymer films covering a wide range of absorption coefficients (to the exciting wavelength and in the IR detection range of our IR camera) validate our theoretical findings.

Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement

“…In‐plane and cross‐plane thermal conductivities . The in‐plane and cross‐plane thermal conductivities of the Hec/PVP hybrid Bragg stacks were characterized by lock‐in thermography and photoacoustic measurements, respectively. Since the density, ρ , and specific heat, C p , are prerequisites for the thermal conductivity analysis, they were also determined experimentally by using helium pycnometry and differential scanning calorimetry (DSC) (Section S2), respectively.…”

Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement

“…In-plane and cross-plane thermal conductivities. The inplane and cross-plane thermal conductivities of the Hec/PVP hybrid Bragg stacks were characterized by lock-in thermography and photoacoustic measurements, [11,28] respectively. Since the density, 1, and specific heat, C p , are prerequisites for the thermal conductivity analysis, they were also determined experimentally by using helium pycnometry and differential scanning calorimetry (DSC) (Section S2), respectively.…”

Characterizing the Thermal Diffusivity of Single, Micrometer-Sized Fibers via High-Resolution Lock-In Thermography

“…Since only the slopes of ln T̃ 0 and Ψ are needed, we can use the excitation point of the laser as the coordinate origin. This method is known in the literature as the slope method and has been confirmed for various samples. ,,− We refer to ln T̃ 0 as the linearized amplitude.…”

“…In combination with the lock-in frequency, the respective slopes lead to the thermal diffusivity. This evaluation method has been proven for films as well as fibers. ,− By changing the excitation frequency, materials with diffusivities spanning several orders of magnitude can be measured …”