Pulsed Heat Injection Applications

With few exceptions, these new approaches involve the time-gating of thermal image sequences following the application of heat, usually a pulse, in order to characterize defects at various depths below the target surface. The term time-resolved infrared radiometry (TRIR) is generally used to describe the technique, as illustrated graphically in Fig. 9.3. 

In general, a typical pulsed thermography system consists of the following elements  

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Cryogenic traps are convenient accumulation and injection devices for fast gas chromatography and interfaces for coupled-column gas chromatography, where a heartcut sample is collected and focused from the first column, and reinjected into the second column. The main requirement for a cryogenic trap used in these applications is efficient accumulation over time with rapid injection of the collected analytes as a narrow pulse in both time and space. Commercially available systems using a capacitance discharge for heating provide injection bandwidths of 5-20 ms. 

One published application of time-resolved asynchronous FT-IR spectrometry involves the reorientation dynamics of ferroelectric liquid crystals induced by the reversal of the external electric field [26,27]. For the study to be described here, the liquid crystals were held in a 2-pm CaFa cell whose windows had been coated with indium tin oxide, which served as the electrode, and with a polyimide film that had been oriented by stroking it in one direction. After the sample had been heated to 90° C to ensure that it was all in the isotropic liquid phase, it was injected into the cell, which had been heated to the same temperature. The cell was then allowed to cool to 40°C at a rate of 1°C min to obtain a homogeneously oriented liquid crystal. An IR polarizer was set at an angle of 45° to the orientation direction of the liquid crystal to obtain the maximum change in the absorption spectrum. Rectangular electric pulses with 20-V peak voltages and a 20-ps period were applied to the cell. 

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