Measurement of thermal diffusivity by optical excitation and infrared detection of a transient thermal grating

Abstract

A contactless technique is described for measuring the thermal diffusivity in a direction parallel to the surface of a thin sample by pulsed-laser excitation and thermal-radiation detection of a thermal grating on the surface. The transient thermal grating (TTG) with periodicity Λ in the range 20–1000 μm is created with a novel interferometer which does not require a long-coherence-length laser. Fast thermometry is achieved with an infrared detector focused on a spot ∼30 μm in diameter to examine in detail the shape and the time dependence of the TTG. The performance of the system is demonstrated on a thin Ti foil with Λ ranging over an order of magnitude. Data are also presented for an 11 μm thick diamond film with a factor-of-20 higher diffusivity than for Ti. The results indicate that the system is well suited to measuring local diffusivities in regions as small as ∼100 μm in thin, free-standing films.