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3(a). Specimen Stub and Freezing Chamber

Table of Contents:

1. Introduction
2. Physical Consideration
(a) Cryogenic
(b) Environmental
3. Cryo SEM Preparation System
(a) Specimen Stub & Freezing Chamber
(b) Transfer Device & Preparation Chamber
(c) Microscope Stage (SEM) & Applications

SPECIMEN STUB

The specimen stub has a large usable area (8mm x 12mm) upon which a wide variety of specimens may be mounted. The provision of a movable shroud enables the specimen to be protected from condensing water and hydrocarbon molecules. The shroud is opened or closed by rotation of the transfer rod.

FREEZING CHAMBER

Facilities for freezing specimens, mounted on the stub, are built into the preparation unit. The freezing chamber is supplied fitted with a polystyrene Dewar. This is used to contain the cryogen, e.g. liquid nitrogen (LN2). In order to obtain improved cooling rates with LN2 it is desirable to freeze in sub-cooled liquid nitrogen. This is obtained by plain boiling nitrogen at -196oC in an insulated container and subjecting to vacuum, that of a rotary pump.

At the triple point pressure the nitrogen will change to be solid phase, this takes place after approximately sixty seconds under vacuum. Rapid back leaking preferably of an inert gas (the system is fitted with facilities to utilise argon gas at all points) will return the system to ambient pressure with the liquid nitrogen in a sub-cooled state at -210oC.

The transfer device is fitted to freezing chamber port, and, using the transfer rod, the specimen stub (bearing the sample) is plunged into the cryogenic liquid. When the sample is frozen, the stub shroud is closed and the stub is withdrawn into the transfer device chamber, which is immediately evacuated.

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