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Back Scattered Electron Imaging
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The main motive of  this experiment is to demonstrate the influence of the electron detector on the final appearance of the SEM image


Equipment  

 Field emission gun environmental scanning electron microscopy equipped with ET and BSE detectors, to acquire secondary electrons (SE) and back scattered electrons (BSE), respectively.

 

 Specimen

      W–Cr alloy oxidized at 1200°C

 Data presentation and analysis 

SE and BSE images (at  the same field of view) depicting top surfaces of the oxide scales  formed on the W–Cr alloy subjected to exposure at 1200 °C are shown in Fig. 1 and 2, respectively. From SE image (Fig. 1), it appears that the oxide scales is composed of more or less uniform composition. Whereas, from BSE image it is well–revealed that the oxide scales have two different types of oxide grains, which appear either bright gray or dark gray. Such variation in contrast within the BSE image is suggestive of differences in composition or inhomogeneous character of the oxide scales. It is well known fact that theelements that have a high atomic number produce more back scattered electrons than the ones that have a low atomic number. Therefore, the areas of the sample with a high atomic number will look brighter than the ones with a low atomic number. Based on this, it is appropriate to infer that the bright gray grains in the Fig.2 are enriched in W–oxides, whereas the dark gray grains are enriched in Cr–oxides.

 

        

Fig. 1: SEM (SE) image depicting the oxidized surfaces of W-Cr alloy

 

Fig. 2: SEM (BSE) image depicting the oxidized surfaces of W-Cr alloy 

 

 

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