Crystallization kinetics of stable and metastable nitric acid trihydrate (NAT) were investigated by time dependent X-ray powder diffraction (XRD) measurements. Kinetic conversion curves were evaluated adopting the Avrami model. The growth and morphology of the respective crystallites were monitored in situ on the cryo-stage of an environmental scanning electron microscope (ESEM) under a partial pressure of nitrogen gas (0.3 Torr, 40 Pa). The results show a close relationship between the presence of ice in the sample and the crystallization mechanism of NAT, which results in different shapes and sizes of NAT crystal particles.

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Low temperature ESEM has the potential to expand the range of experiments carried out in this versatile instrument. We are exploring methodologies for the control and characterisation of specimens over a broad sub-zero temperature range and under different environmental conditions. The aim is to carry out in situ dynamic experiments as well as studies of temperature dependent changes in systems with sub-zero glass transition temperatures.An important aspect of low temperature ESEM is control of chamber environment, which consequently affects specimen stability. For example we have made a systematic study of ice sublimation as functions of temperature and pressure, using a modified Gatan Alto 2100 cryosystem mounted in an FEI XL30 ESEM FEG. The experimental set up for low temperature ESEM can be seen in Figure 1.First we studied sublimation rate as a function of sample temperature. The sublimation rate was calculated by measuring the change in diameter of small beads embedded in ice as it sublimed. In this experiment a constant pressure of 0.8 torr of nitrogen was used throughout, whilst the sample temperature was varied between -90°C and -55°C. The experimental results are compared to the theoretical sublimation rate calculated from Tabor [1] in Figure 2. Both show an exponential relationship between temperature and sublimation rate, although the significant offset in temperature between experiment and theory is due to a departure from the ideal gas law.For conventional high vacuum cryo SEM, it is generally found that sublimation of ice begins at around -100 ºC [2]. However in low vacuum, using nitrogen, we find this to be nearer -75 ºC.One explanation for this difference is the interaction of sublimated water vapour molecules with each other and with nitrogen gas present inside the chamber. This turns out to be a significant factor, leading to our second investigation: the effect of chamber pressure on sublimation rate. For example recent work has shown that at -55 ºC the sublimation rate decreases from 0.35 µm/s to 0.1 µm/s on increasing the pressure of nitrogen from 0.2 torr to 0.6 torr. Further work is being carried out to characterize this behaviour.Our findings have highlighted some key points about carrying out low temperature work in a low vacuum environment. These are;• The temperature for onset of sublimation is higher than for high vacuum SEM • The rate of any sublimation can be controlled as a function of chamber pressure As a corollary to this last point, we have developed a methodology for introducing water vapour as an additional factor in stabilising frozen-hydrated samples at higher temperatures. This is described elsewhere [3].

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.

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