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A unique method has been developed for rapidly determining, nondestructively, the localized thermal conductivity, and thermal diffusivity of graphites near room temperature. The method employs transient heating and requires access to only one surface of a solid. It involves heating a small area on the surface of an apparent “semi-infinite” solid with radiant energy and simultaneously observing the parabolic-shaped temperature history of this area's center using an infrared radiometer as the sensor. This history is related to the thermal inertia of the solid, so that a comparison of it with a history for a “standard” material will yield the conductivity and diffusivity if the density and specific heat of the solid-of-interest are known. Typically, the period of time to generate a measurement is 10 s, and the temperature rise is less than 10 C. Application of this technique to coated ATJ graphite indicates that it is capable of discerning inertia differences equal to 15 per cent, conductivity differences of 15 per cent, and diffusivity differences of 10 per cent. Approximately 70 per cent of the error contributing to these precision values is caused by variability in the applied coating thickness. Precision could ultimately be expected to increase to about 3 per cent. The probable accuracy of the technique is limited to that of the standard and is about 9 per cent for an inertia measurement when calibrated against Armco iron. Three noteworthy features of this method include: (1) point-to-point conductivity/diffusivity/inertia determinations, which afford an additional tool for nondestructively evaluating material variability; (2) extension of its application to poor thermal conductors, which will increase the method's precision; and (3) probable application to materials in elevated temperature environments.
graphite, infrared testing, nondestructive testing, transient measurements, thermal conductivity, thermal diffusivity, thermal inertia, thermal measurements, thermal properties
Schultz, A. W.
Senior staff scientistPersonal member ASTM, Avco Corp., Lowell, Mass.