Thermal diffusivities measured by photodisplacement detection of transient thermal gratings

Abstract

Abstract. Laser-induced transient thermal gratings generated on polycrystalline copper and nickel surfaces were measured by photothermal displacement techniques. The goal was to test the potential of this method for determining thermal diffusivities. It is shown that the literature bulk values of the thermal diffusivities are reproduced by a simplified description of the time decay of the grating in terms of a superposition of decoupled lateral and vertical decay rates. 44.50.+t, 65.70.+y Transient thermal gratings (TTGs) have been used for more than two decades to investigate thermal properties of absorbing liquids and solids [1][2][3]. Gratings on surfaces were first reported by Cachier [4], and later by Hoffman et al. [5], but received only recently revived attention [6,7]. The great advantage of TTGs is that they introduce a periodicity in one direction, suggesting the use of the one-dimensional heat conduction equation for evaluating the data. There were systematic deviations, however, between experimental results obtained by the grating technique and commonly accepted literature values for the thermal diffusivity, for example for ruby [3], colored glass and silicon [7]. We suspect that these deviations are caused by the use of the one-dimensional solution of the heat conduction equation for data analysis, which leads to the simple relation PACS:between the decay time ~y of the grating and its periodicity A, x being the thermal diffusivity. The simplicity of the one-dimensional solution originates from the periodic source term. However, it is a poor approximation since it only takes into account the heat diffusion across the grating and ignores the heat flow into the other two directions, in particular the vertical one. It is the purpose of this paper to point out that by adding the diffusion rates for the other two directions a much better approximation can be achieved, which describes the experimental data very well and leads to perfect agreement between the measured diffusivity and the bulk value quoted in the literature. This will be demonstrated for polished polycrystalline copper and nickel surfaces as test cases.TTGs have hitherto been identified by the diffraction of either the generating light itself [2,4] or of a probe beam [5][6][7][8]. In the experiments reported here we utilized the photodisplacement technique [9][10][11][12] for physically probing the periodic thermal deformation of the grating with a HeNe laser beam of sufficiently narrow beam waist. This technique combines a good signal-to-noise ratio with high accuracy. It measures the grating period directly, and the decay time can be recorded at each point of the grating. The thermal diffusivity can then be extracted from these two quantities without the need of knowing the absolute amplitude of the thermoelastic deformation.

Transient surface grating technique for thermal diffusivity measurement

“…Its gradient, evaluated according to Eq. (l) and [4], implies an effective thermal diffusivity of 9 . 0~1 0~~m~s~' , which is entirely plausible, though somewhat higher than that of common glasses (eg D~.…”

Non-equilibrium transient thermal grating relaxation in metal

“…In the course of the thermal grating decay, heat is transferred over a distance between the peaks and nulls of the grating that can be varied typically from a fraction of a micron to tens of microns. 5 Hitherto transient gratings have been successfully used to measure thermal diffusivity of bulk materials [6][7][8] and thin films. [9][10][11] In metals with high electron mobility such as gold and copper, thermal conductivity is proportional to electrical conductivity 12 and thus, the transient grating technique can be used for noncontact monitoring of the electrical resistivity of thin metal films, an application that has shown promise for process control in microelectronics.…”

Desenvolvimento de transdutor em fibra óptica com estrutura hí­brida LPG-FBG para medição de propriedades térmicas de materiais.

“…: (PADDOCK;EESLEY, 1986a;CAHILL, 2004;WILSON et al, 2012;HUANG et al, 2011;TAM, 1995) Deformação de superfície (Surface deflection, Photodisplacement) REF. : (AMERI et al, 1981;MIRANDA, 1983;JÁUREGUI;MATTHIAS, 1992 Haupin, no mesmo ano, propôs um instrumento baseado no HWM para medição rápida de materiais sólidos isolantes. Naquele aparato uma junção de termopar foi colocada no centro da amostra e possuía duas funções, a de fonte de calor, pois era aquecida por meio da aplicação de uma corrente alternada, e a de termômetro, na medição da temperatura.…”