Simultaneous noncontact precision imaging of microstructural and thickness variation in dielectric materials using terahertz energy

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Simultaneous noncontact precision imaging of microstructural and thickness variation in dielectric materials using terahertz energy

Roth, Don J. / Seebo, Jeffrey P. / Winfree, William P.

This paper describes a noncontact, single-sided, terahertz electromagnetic measurement and imaging technique that simultaneously characterizes microstructural (for example, spatially lateral density) and thickness variation in dielectric (insulating) materials. The technique was demonstrated for two materials - space shuttle external tank spray-on foam insulation and a silicon nitride ceramic. With high signal-to-noise ratio echoes, the technique is very precise, as was shown experimentally and through error analysis. Specialized signal processing techniques are required to improve the signal-to-noise ratio of front surface echoes off foam. In this investigation, a wavelet denoising process followed by software amplification was used to make front surface reflections off the foam usable for this technique. A thin layer of plastic can be placed on the foam surface to increase dielectric mismatch and initially locate the front surface echo (to know the time range to gate). The technique can have significant applicability to characterization of density variations and crushed foam detection for the spray-on foam insulation of the thermal protection system of the space shuttle external tank As such it supports the Columbia Accident Investigation Board recommendation to initiate an aggressive program to eliminate foam shedding. Additional application would be for dielectric materials needing precision density and thickness characterization in a nondestructive fashion. Scale-up to curved/cylindrical shapes and determination of practical limit of thickness variation are left for further investigation.