Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 13
TEM
A schematic diagram of a TEM instrument is given in Fig. 1.8. Again, the
system is enclosed in a very high vacuum and the image is viewed by
focusing electrons upon a fluorescent screen after their transmission through
the sample. A resolving power down to 180 pm is obtainable in modern
instruments. Two types of sample may be studied: replicas and foils.
Replicas. After polishing a sample as for optical microscopy, the surface
is etched to reveal the required metallographic detail and produce surface
relief. The surface is then overlaid with a cellulose acetate or similar film
which, when stripped, replicates the surface relief, Fig. 1.9(c). The stripped
replica can be coated by evaporation with carbon and ‘shadowed’ with a
heavy metal such as gold or platinum, which gives enhanced image contrast
after the acetate is removed with a solvating reagent.
For samples containing small particles of a different phase, such as
Gun
C 1
C 2
Specimen
O
I
P
Screen
Camera
1.8 The layout of a TEM, showing condenser (C), objective (O), and
intermediate (I) and projector (P) lenses.