Improvements to a cryosystem to observe ice nucleating in a variable pressure scanning electron microscope

Abstract

The variable pressure scanning electron microscope (VPSEM) has expanded the scope of the SEM to allow the imaging of dynamic, electrically insulating systems. The use of water vapor as the imaging gas present in the chamber allows the successful imaging of hydrated samples. As awareness of the system capabilities becomes more well known, greater pressure has been put onto the microscopist to push the boundaries of both temperature and resolution for the study of diverse hydrated samples whose dynamics may not occur at the usual room temperatures in a VPSEM. In this article we discuss the stages in the development of a cryosystem that has led to the successful observation of the nucleation of ice from a solution in situ. This investigation also leads to further possibilities of imaging hydrated samples in the little explored temperature range of 188-238 K (from -85 to-35 degrees C). This study includes the exploration of how the temperature of various surfaces inside the microscope will change the system's ability to keep a sample hydrated or in its native state.

Observation of a Brine Layer on an Ice Surface with an Environmental Scanning Electron Microscope at Higher Pressures and Temperatures

“…Only a few studies have examined noncontaminated ice at high pressures using the ESEM. 24 Leu and co-workers examined ice particle sizes in thin ice films; 25,26 Zimmermann and co-workers observed ice nucleation on various solid supports, such as silver iodide, kaolinite, or montmorillonite; 27 and Varanasi and co-workers observed frost formation on hydrophobic surfaces coated, for example, with a thin layer of (tridecafluoro-1,1,2,2tetrahydrooctyl)trichlorosilane at 260 K. 28 Neshyba and coworkers showed well-resolved surfaces of hexagonal ice crystals possessing strands which are characteristic for growing and ablating facets. 29 Pedersen and co-workers observed ice crystals growing into contact.…”

Cryogenic EBSD on ice: preserving a stable surface in a low pressure SEM

“…The cold trap effectively functions as a ‘cryo‐pump’ removing water vapour from the chamber, and reducing the water vapour pressure near the sample. Thus the removal of water vapour by the cold trap is expected to lead to some sublimation of the sample (Waller et al ., 2008). The cold trap was therefore maintained at about 20°C less than the sample.…”

“…Sublimation could lead to an increase of chamber pressure, although continuous pumping and the presence of a cold trap will counteract any pressure increase (Cullen & Baker, 2001). The sublimation rate can be reduced by working at high‐pressure conditions in a variable pressure SEM (Waller et al ., 2005, 2008). Waller et al .…”

“…These results show that spatial resolution is better with higher pressures. If high pressures are available, EBSD data can be collected with a focused beam using nitrogen gas to compensate charging (Waller et al ., 2008). This is shown for example in Fig.…”

ESEM Methodology for the Study of Ice Samples at Environmentally Relevant Subzero Temperatures: “Subzero ESEM”

“…The idea of mixing two gases in ESEM, with one being water vapor to prevent ice sublimation and the second being a signal amplifying gas, had been presented earlier (Fletcher et al, 1998). Later work demonstrated an experimental set-up and preliminary results using nitrogen gas and water vapor, aimed at observing frozen-hydrated samples in the −85 to −35°C thermal range (Waller et al, 2008). This was subsequently applied to the study of atmospheric ices (e.g., Grothe et al, 2009).…”