Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 88

directions and into a very large number of fragments. In the case of crushing to
obtain a thin slice for the TEM, the residual grains must be small enough to present
transparent fragments to the electrons, either over all of the grains or on their
extremities. Depending on the material crushed, the fracture can be transgranular,
intergranular, or interatomic. Liquid nitrogen cryogenics is used to harden a ductile
or soft material to make it brittle and capable of being crushed.
Wedge cleavage is the result of an interatomic separation along crystallographic
planes with low Miller indices. A single crystal can cleave according to different
crystal planes of high atomic density. The start of the fracture generates a high dis-
location rate, then a fracture occurs and propagates throughout the crystal along an
atomic plane. Figure 5.3 shows the cleavage planes of gallium arsenide (GaAs).

[001]

[100]

[010]

Cleavage plane

- -

[110]

(110)

[110]

Cleavage plane

(110)

Fig. 5.3 Cleavage planes (110) and (1-10) of a GaAs single crystal relative to the crystallographic
directions

Ultramicrotomy and cryo-ultramicrotomy use a knife as a cutting tool. The crack
is started by the cutting edge of the knife, which is in the shape of a prism. The
increasing thickness of the knife opens the crack and acts as a stress on this crack.
The crack propagates within the sample because of areas of low cohesion and mate-
rial heterogeneities. The most fragile bond in the material breaks first. The fracture
is directional, corresponding to the introduction of the knife (Fig. 5.4) .
Distortion forces are involved during cutting. In Fig. 5.5 we see that cutting force
R under knife angle

has divided into two components, force F c , which causes the
fracture, and force F t , which causes compression of the material. This compression
is greater if the knife angle is large. This angle can vary between 35 ◦ and 55 ◦ .The
section fractures into small fragments if the material is brittle and is compressed if
the material is soft.
For cryo-ultramicrotomy, the mechanical action is the same type as that described
for ultramicrotomy. Since the material is frozen, this is an ice-based heterogeneous
compound, which behaves like a brittle material.
The freeze-fracture technique is applicable to multiphase samples that have an
aqueous phase. Water is transformed by freezing into the solid phase. Frozen water

θ