A MODIFICATION OF ANGSTROMS METHOD THAT EMPLOYS PHOTOTHERMAL RADIOMETRY TO MEASURE THERMAL-DIFFUSIVITY - APPLICATION TO CHEMICAL-VAPOR-DEPOSITED DIAMOND

A modification of the one dimensional Angstrom's method that employs p hotothermal radiometry has been used to determine the longitudinal the rmal diffusivity of three thin long bars of chemical vapor deposited d iamond. Long bar specimens permit us to use a simple one-dimensional t reatment 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 d iffusivities obtained from magnitude data and from phase data can be a ttributed to surface heat losses; the values of diffusivity obtained w ith the two types of data showed no significant difference. The diffus ivities obtained agree reasonably well with the mean values calculated from measurements made by several other laboratories on the same spec imens. 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 fo cus 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 s ource was used were not statistically significant. A theoretical calcu lation indicates that the measurements on the specimen were made suffi ciently 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 experiment ally. A point source is preferable because the optical configuration o f the experiment is simpler and larger signals are obtainable. (C) 199 8 American Institute of Physics.