Thermal diffusivity maps: Case studies in ceramics
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Thermal diffusivity maps: Case studies in ceramics Lanhua Weia) and Grady S. White Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 7 March 1996; accepted 3 April 1997)
A new methodology for mapping thermal diffusivity using a photothermal deflection method is introduced. Two case studies are made: fiber-reinforced composite structures and contact damage zones in alumina. In the former, characterization of thermal microstructural features is demonstrated; in the latter, microcrack density is quantified. Experimental data are analyzed and compared with literature results. Advantages and limitations of the technique are discussed.
I. INTRODUCTION
Thermal diffusivity is an important physical property of materials. Scientifically, it is a fundamental quantity directly relatable to the electron and/or phonon mean free path in a solid.1 Practically, it determines the reliability and performance of devices in many industrial applications, e.g., high-power laser optical components, high-speed electronic circuits, and hightemperature engine component coatings, where the rate of heat dissipation and thermal shock resistance are major considerations.2–5 As a primary physical property, thermal diffusivity correlates closely with other material properties. Such correlations have been widely used in materials characterization, e.g., in using thermal probes to characterize doping content in semiconductors,6 to quantify microcrack density in damage accumulation,7 and to detect delamination in coating systems.8 With the advent of complex materials such as composites, multilayer materials, and functionally graded materials, heterogeneous microstructures have been designed for improved toughness, flaw tolerance, thermal shock resistance, and other advanced application requirements.9–11 Thermal diffusivity is not uniform in these heterogeneous structures. In many cases, it is the local thermal diffusivity (defined as a position-dependent thermal diffusion coefficient)12 that contains valuable information on materials design and performance. Furthermore, when thermal diffusion is used to detect or to characterize variation of other properties of a material system, it is again the local thermal diffusivity that reflects those changes. In such cases, measurements of an averaged thermal diffusivity through the material is inadequate. Some effort has been made to develop techniques for evaluating local thermal properties using imaging techniques.13,14 Two issues are important in any such technique —spatial resolution and imaging quantifia)
Guest scientist, on leave from Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201.
cation. Whereas high spatial resolution (down to micrometers) has been achieved in existing thermal microscopes,13,14 relatively little progress has been made in quantitative interpretations of thermal image contrast. Complications involved in imaging calibration have made such quanti
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