A modification of Ångström's method that employs photothermal radiometry to measure thermal diffusivity : application to chemical deposited diamond

Abstract

A modification of the one dimensional A ° ngstro¨m’s method that employs photothermal radiometry has been used to determine the longitudinal thermal diffusivity of three thin long bars of chemical vapor deposited diamond. Long bar specimens permit us to use a simple one-dimensional treatment that employs a linear least squares fitting procedure on both magnitude and phase data as a function of position, provided that the condition for ignoring end effects is fulfilled. Any differences in diffusivities obtained from magnitude data and from phase data can be attributed to surface heat losses; the values of diffusivity obtained with the two types of data showed no significant difference. The diffusivities obtained agree reasonably well with the mean values calculated from measurements made by several other laboratories on the same specimens. The heat source was the beam of an argon-ion laser focused onto the specimen surface either with a cylindrical lens to form a line focus or with a spherical lens to form a point focus. The differences in diffusivities obtained when a line source was used and when a point source was used were not statistically significant. A theoretical calculation indicates that the measurements on the specimen were made sufficiently far from the heat source for the one-dimensional treatment to be valid whether the line source or the point source were used: either source is expected to give the same result as was observed experimentally. A point source is preferable because the optical configuration of the experiment is simpler and larger signals are obtainable.