MEASUREMENTS OF THE THERMAL-DIFFUSIVITY OF ALUMINUM USING FREQUENCY-SCANNED, TRANSIENT, AND RATE WINDOW PHOTOTHERMAL RADIOMETRY, THEORY ANDEXPERIMENT

The thermal diffusivity of various types of aluminum has been measured , using a completely noncontact experimental configuration based on in frared photothermal radiometry. Photothermal response transients, conv entional frequency scans, and pulse duration- or repetition rate-scann ed rate windows have been investigated. It has been shown that the con ventional frequency scan is not suitable for measurements of aluminum with a short thermal transport time such as foils, due to an extremely degraded signal-to-noise ratio (SNR). Also, it has been found that th e conventional frequency scan method is less sensitive to the actual v alue of thermal diffusivity than the rate-window scan. The rate-window method. furthermore, gives superior SNR especially for thin metals an d yields excellent agreement between the theory and the data. An advan tage of the pulse duration-scanned rate window mode is that it does no t require knowledge of the instrumental transfer function as an input. The transient response gives the worst SNR but is best for the physic al interpretation of the photothermal signals. In addition, it has bee n shown that the infrared photothermal radiometric transmission mode i s less sensitive to surface roughness than the reflection mode and, th erefore, is preferable for thermal diffusivity measurements of aluminu m and of good thermal conductors, in general.