Investigation of Si-Fe transformer sheets by scanning electron acoustic microscopy (SEAM)

Scanning electron acoustic microscopy (SEAM) is a fairly new technique in the microcharacterization of materials in which an intensity-modulated electron beam is used to generate an acoustic signal within the probed specimen location. These signals can be used for location and identification of many material parameters. Detection of the sound at the modulation frequency,f, results in so-called linear images, whereas the use of signals at even harmonics of this frequency (especially:2f) yields images as a consequence of nonlinear interactions. These nonlinear techniques were first introduced to the microscopy of semiconductors and have proven now to be very advantageous for the investigation of metals, ln this paper both the linear and the nonlinear technique are applied to the characterization of Si-Fe-transformer steel sheets. It can be shown that the linear mode exhibits only the grain structure of the material, whereas the nonlinear mode reveals the magnetic domain structure of the sheets. Thus by one experiment it is possible to immage grains and domains separately. Furthermore, this is possible even without any specimen preparation,especially without removal or polishing of the ceramic coating.