A. Ocariz

Thermal diffusivity of anisotropic materials by photothermal methods

In this paper we analyze the possibility of extending the photothermal methods developed for thermal diffusivity measurements of isotropic materials [A. Salazar and A. Sanchez‐Lavega, Rev. Sci. Instrum. 65, 2896 (1994)] to the case of anisotropic specimens. A full theoretical treatment of the photothermal signal generation for the case of an anisotropic sample with its principal axes aligned with the sample surface is presented. Three photothermal detection schemes have been studied: infrared radiometry, photothermal reflectance, and optical beam deflection (mirage effect). The fundamental result we have obtained is that when using infrared radiometry, photothermal reflectance and collinear mirage setups, the thermal magnitude retrieved is the resistivity to heat diffusion (a tensor defined as the inverse of the thermal diffusivity tensor). Only the perpendicular mirage experiment allows one to directly retrieve the thermal diffusivity along any direction of the material. Experimental measurements perform...

PHOTOTHERMAL STUDY OF SUBSURFACE CYLINDRICAL STRUCTURES. II. EXPERIMENTAL RESULTS

We present an extensive experimental study of the photothermal signal produced by subsurface cylinders immersed in opaque solids. The results provided by three different modulated photothermal techniques (thermoreflectance, infrared radiometry, and mirage) for a variety of sample and cylinder thermal properties show very good agreement with the theoretical models presented in Part I of this work. A single subsurface cylinder can be quantitatively characterized by means of photothermal measurements (i.e., the determination of its size, depth, orientation, and thermal properties). We also present experimental data on a series of buried cylinders parallel to the surface that show multiple-scattering effects on the incident thermal wave.

Photothermal study of subsurface cylindrical structures. I. Theory

A study of the photothermal signal generated by subsurface cylindrical structures is presented in two parts: theoretical modelization (Part I), and experimental measurements on calibrated samples (Part II). In this first part we develop a theoretical study of the scattering of planar and spherical thermal waves by a buried single infinite cylinder, parallel to the sample surface, as a function of its size and depth, and the thermal characteristics (thermal conductivity and diffusivity) of both the sample and cylinder. Temperature field solutions to the heat conduction equation are explored in two ways: by means of the Green’s function method and by a series expansion. We discuss the limits and accuracy provided by both types of calculations, and we compare the resulting temperature field for extended and pointlike illumination. Our study shows that photothermal methods allow one to locate and characterize the geometrical and thermal properties of the buried cylinder.

Novel results on thermal diffusivity measurements on anisotropic materials using photothermal methods

We have applied two photothermal techniques (infrared radiometry and mirage) and conventional procedures to measure the thermal diffusivity of a highly anisotropic sample of pyrolytic graphite. Unexpected and strongly differing results are obtained with each technique. A theoretical analysis indicates that whereas the mirage technique gives a measure of the elements of the thermal diffusivity tensor, infrared radiometry yields the elements of another thermal magnitude represented by the inverse of this tensor.We have applied two photothermal techniques (infrared radiometry and mirage) and conventional procedures to measure the thermal diffusivity of a highly anisotropic sample of pyrolytic graphite. Unexpected and strongly differing results are obtained with each technique. A theoretical analysis indicates that whereas the mirage technique gives a measure of the elements of the thermal diffusivity tensor, infrared radiometry yields the elements of another thermal magnitude represented by the inverse of this tensor.

Photothermal characterization of vertical and slanted thermal barriers: A quantitative comparison of mirage, thermoreflectance, and infrared radiometry

We present a quantitative and comparative study of three different photothermal techniques based on the signal produced on vertical/slanted thermal barriers under a variety of barrier/sample conditions. Models based on integral methods are developed to calculate the amplitude and phase of the sample surface temperature and mirage deflection in surrounding air. The geometries studied include: vertical, tilted, buried, and finite size barriers separating identical or different media. The models incorporate the probe and pump beam sizes, the thermal resistance of the barrier and the optothermal characteristics of the sample. Experimental measurements are performed on a variety of fabricated barriers with three modulated photothermal techniques: the thermoreflectance, infrared radiometry, and mirage detection. We discuss in a comparative way the limits, drawbacks and the applicability of each technique. Model fits to the experimental results allow characterization of the thermal barrier (spatial localization,...

A STUDY OF THE PHOTOTHERMAL SIGNAL PRODUCED BY A SERIES OF SUBSURFACE CYLINDERS IN OPAQUE MATERIALS

We present a theoretical and experimental study of the photothermal signal produced by a series of buried cylinders in an optically opaque sample. The models are valid for any value of the thermal properties of the cylinder and sample (the extreme cases treated are isolating and conducting cylinders), and the modulated excitation can be extended (plane wave generation) or punctual (spherical wave generation). The model results are compared with measurements performed on calibrated samples using two photothermal techniques: “mirage” deflection and infrared radiometry. Semiquantitative agreement between theory and experiment is found showing that this methodology can be used as a tool for the nondestructive evaluation of close subsurface cylinders, which represents a further step to the photothermal study of some materials of interest as the fiber-reinforced composites.

PHOTOTHERMAL CHARACTERIZATION OF ANISOTROPIC MATERIALS WITH BURIED PRINCIPAL AXES

We study theoretically and experimentally the ability of photothermal techniques to thermally characterize the anisotropic materials whose principal axes are not contained at the sample surfaces. We first determine the thermal diffusivity tensor when the directions of the principal axes are known. Inversely, we calculate the directions of the principal axes in a sample whose thermal diffusivities are given. Measurements validating our theory are presented for a sample of pyrolytic graphite (PG), which has a high anisotropy, by means of two photothermal techniques: IR radiometry and the mirage effect.

Photothermal mirage characterization of vertical interfaces separating two different media

A theoretical and experimental study about the ability of the photothermal modulated mirage technique to characterize vertical interfaces (thermal resistances) separating two different opaque and thermally thick media with thermal conductivities larger than the medium surrounding the sample and with any value in their thermal diffusivities has been carried out. For the calculation of the mirage deflection, a theoretical model that incorporates the mirage parameters (sizes of probe and pump beams, probe beam height) and which is valid for any value of the thermal resistance characterizing the interface is presented. The model is validated by experimental measurements on a fabricated sample having a controlled interface width that separates two pieces of copper and steel. Fitting the model calculations and the experimental data allows one to retrieve the value of the thermal resistance.