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The thermal characterization of composites made up by magnetically aligned carbonyl iron micro-sized particles embedded in a polyester resin matrix is performed using photothermal radiometry technique. The measured experimental data show that the thermal conductivity and thermal diffusivity of the composite, in the direction of the applied magnetic field, increase with the concentration of the particles and are enhanced with respect to their corresponding values for a random distribution of the particles. This thermal enhancement has a maximum at a concentration of particles of 10% and is very low at small and high iron volume fractions, such that for particles concentrations of about 40%, the composite thermal conductivity reduces to its values for random particles. This behavior indicates that for high volume fractions, the effect of the microparticles concentration plays a dominant role over the effect of their alignment. It is shown that the thermal conductivity of the composite can be modeled in terms of the Nielsen model, under an appropriate parameterization of the form factor of the particles. The results of this work could be highly useful for improving the thermal performance of mechanical and electronic devices involving composite materials.

Real time measurement of thermal diffusivity during the evolution of the light curing process in dental resins is reported using photothermal radiometry. The curing is induced by a non-modulated blue light beam, and at the same time, a modulated red laser beam is sent onto the sample, generating a train of thermal waves that produce modulated infrared radiation. The monitoring of this radiation permits to follow the time evolution of the process. The methodology is applied to two different commercially available light curing resin-based composites. In all cases thermal diffusivity follows a first order kinetics with similar stabilization characteristic times. Analysis of this kinetics permits to exhibit the close relationship of increase in thermal diffusivity with the decrease in monomer concentration and extension of the polymerization in the resin, induced by the curing light. It is also shown that the configuration in which the resin is illuminated by the modulated laser can be the basis for the development of an in situ technique for the determination of the degree of curing.

The thermal wave field in a sample of thickness L and optical absorption coefficient β is derived and analyzed, by considering the effects of the multiple reflections of a modulated light beam within the sample. By comparing the corresponding amplitude and phase spectra, in presence and absence of those reflections, it is shown that their effects are strongly determined by the product βL. When the thermal wave field is measured at the illuminated surface, those effects appear remarkably on the amplitude and phase in the thermally thin and thermally thick regimes, respectively, and for βL≲2. The deviation of the amplitude spectrum due to the multiple reflections of incident light beam can be as large as 25%. In contrast, when the thermal wave field is measured at the non-illuminated surface, the contribution of the multiple reflections shows up on both the amplitude and phase in the thermally thick regimes, and for βL≲5. These spectra exhibit deviations of about 20% from their corresponding values without the effects of the light reflections. The obtained results show that the accurate measurement of the thermal and/or optical properties of the sample material, based on the amplitude and phase spectra, requires the consideration of those multiple reflections, especially for the cases where βL≈1. The predictions of the proposed approach for the optical absorption coefficient are in reasonable agreement with the results obtained through an independent technique based on a spectrometer.

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