We replaced standard liquid nitrogen (LN) cooled cold-finger with a peltier cold-finger, designed and built in the University of Bristol Earth Science Workshop. The original JEOL cold finger assembly was retained but the liquid nitrogen flask and vacuum seal are replaced.

The latest incarnation comprises two high power peltier modules and the minimum temperature is limited by the ability to cool the hot side of the modules. More information about Peltier modules can be found here. The peltier modules are water cooled using the lab chiller circuit and temperatues of -33C are achieved at the peltier modules and -27C at the cold finger.

Previous work by Hirsch et al. (1994) and others suggested liquid nitrogen temperatures were not required, with tempertures on the order of -15°C to -70°C effective depending on the configuration. We compared the peltier cooled cold finger to the liquid nitrogen cooled cold finger and found the reduction in carbon buildup to be comparable once the cold finger was cooled below -25°C.

The peltier cooled cold finger is able to work continuously for many days. At -25C water remains a vapor phase in the vacuum and therefore the severe degradation observed when warming up a liquid-nitrogen cold finger is not observed.

For further information see Published article 

References

Hirsch et al. (1994). Contamination in a Scanning Electron Microscope and the Influence of Specimen Cooling. SCANNING Vol. 16, 101-110