Terahertz Mapping of Microstructure and Thickness Variations
- Wednesday, 23 December 2009
Previously, it was not possible to separate microstructural and thickness effects using electromagnetic methods.
A noncontact method has been devised for mapping or imaging spatial variations in the thickness and microstructure of a layer of a dielectric material. The method involves (1) placement of the dielectric material on a metal substrate, (2) through-the-thickness pulse-echo measurements by use of electromagnetic waves in the terahertz frequency range with a raster scan in a plane parallel to the substrate surface that do not require coupling of any kind, and (3) appropriate processing of the digitized measurement data.
More specifically, the method provides for mapping, in a coordinate system defined by the raster scan, of spatial variations of the thickness normal to the substrate surface and spatial variations of the through-the-thickness velocity of the terahertz mapping signal. (In general, variations in the velocity of this or any signal through a material are associated with variations in density and/or other characteristics associated with local microstructure of the material.) The method has been demonstrated on nominally flat metal-backed specimens of two dielectric materials: a silicon nitride ceramic and a spray-on foam of the type used on the external tanks of a space shuttle. The method should also be applicable to other dielectric materials, and it may be feasible to extend the method to cylindrical, beveled, and other non-planar shapes.
In a prior method of terahertz mapping or imaging of this type as applied to space-shuttle external-tank foam bonded to the metal tank surface, one maps variations in the time of flight of the terahertz signal through the thickness of the foam, the time of flight being typically defined as the time between the echo from the substrate surface and the front foam surface. That approach yields information on the combined effects of thickness and through-the-thickness velocity; it does not enable separate determination of variations in thickness and variations in velocity.
In contrast, the present method provides for generation of velocity-variation images free of thickness-variation effects and thickness-variation images free of velocity-variation effects. In this method, terahertz pulse-echo measurements with a raster scan are made as in the prior method. The raster-scan plane is chosen so that there is a suitable air gap between the terahertz transceiver and the front surface of the dielectric material. One difference between the present and prior methods is that two sets of data are acquired: For one set, the sample is absent and the times of the echoes from the substrate alone are measured. For the other set, the dielectric sample is placed on the metal substrate and echoes from both the substrate and front surface of the dielectric are measured as in the prior method.
Another difference between the present and prior methods lies in processing of the two sets of measurement data. The processing is effected by specialpurpose software that performs signalenhancement and data-fusion functions to obtain enhanced values of the times of front-surface and substrate echoes in the presence of the dielectric sample and of the substrate echo in the absence of the sample. Then by use of equations that are readily derived from the basic signal time-of-flight equations, the thickness of the sample and the through-thethickness velocity in the sample are computed from the various echo times.
This work was done by Donald J. Roth, Jeffrey P. Seebo, and William P. Winfree of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18254-1.
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