Physical Principles of Electron Microscopy: An Introduction to TEM ...
Physical Principles of Electron Microscopy: An Introduction to TEM ...
Physical Principles of Electron Microscopy: An Introduction to TEM ...
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The Transmission <strong>Electron</strong> Microscope 89<br />
Figure 3-16. Schematic diagram <strong>of</strong> a rotary vacuum pump, illustrating one complete cycle <strong>of</strong><br />
the pumping sequence. Spring-loaded vanes A and B rotate clockwise, each drawing in gas<br />
molecules and then compressing them <strong>to</strong>ward the outlet.<br />
During the remainder <strong>of</strong> the rotation cycle, the air is compressed (Fig. 3-16c)<br />
and driven out <strong>of</strong> the outlet tube (Fig. 3-16d). Meanwhile, air in the opposite<br />
half <strong>of</strong> the cylinder has already expanded and is about <strong>to</strong> be compressed <strong>to</strong><br />
the outlet, giving a continuous pumping action. The rotary pump generates a<br />
“rough” vacuum with a pressure down <strong>to</strong> 1 Pa, which is a fac<strong>to</strong>r � 10 5 less<br />
than atmospheric pressure but still <strong>to</strong>o high for a tungsten-filament source<br />
and <strong>to</strong>tally inadequate for other types <strong>of</strong> electron source.<br />
To produce a “high” vacuum (� 10 -3 Pa), a diffusion pump (DP) can be<br />
added. It consists <strong>of</strong> a vertical metal cylinder containing a small amount <strong>of</strong> a<br />
special fluid, a carbon- or silicon-based oil having very low vapor pressure<br />
and a boiling point <strong>of</strong> several hundred degrees Celsius. At the base <strong>of</strong> the<br />
pump (Fig. 3-17), an electrical heater causes the fluid <strong>to</strong> boil and turn in<strong>to</strong> a<br />
vapor that rises and is then deflected downwards through jets by an internal<br />
baffle assembly. During their descent, the oil molecules collide with air