APPLICATION NOTE
Recording Dynamic Experiments in the SEM.

For many materials, the dynamic response to changes in variables such as temperature, hydration or partial pressure of reactive gases are important to understand on a microscopic scale. In the case of high temperatures, micro-scale changes in phase or chemical state can be key to understanding macroscale properties.

For hydrated cryogenic samples, for example prepared using the ALTO Cryo-SEM
product range, the specimen temperature in a vacuum controls the sublimation rate. This provides a very controlled, non contact, and surface sensitive etching technique. Closer to ambient temperatures, the partial pressure in the SEM chamber can provide information on processes such as deliquescence of salts or watching paint dry!

For a growing variety of specimen types, in-situ tensile, bending or compression testing experiments are also of interest. An in-situ tensile test, where the deforming region can be indentified and the evolution of the microstructure studied and correlated with quantitative stress strain data, can provide substantially more information than a conventional ex-situ deformation experiment. Experiments using Gatan’s Microtest* product range can range from very large stresses traditional to metallurgy applications, to very small stresses for much smaller, fragile specimens. For some systems, special heated or cooled grips and / or platforms can also allow dynamic tensile testing at a range of specimen temperatures.

The SEM can therefore become not just an examination chamber for static specimens, but an experimental chamber where precise control of experimental variables complements the key strengths of high depth of field, flexible field of view, spatial resolution and range of characterisation techniques.

SEM images of powder steel specimen as a function of time and temperature using Gatan’s H1004 hot stage and DigiScan II.

Snap Shots versus Movies?
With dynamic experiments, there is a further challenge of recording the experiment as a function of time. If single “snap-shots” are relied upon to record the evolution of an experiment, the difficult step is often freezing the conditions for that snap-shot, without disturbing the dynamics of the experiment. Furthermore in the case of single snap-shots, the important experimental variables should ideally be recorded at each stage.

Digital streaming of images in the form of movies at “web-cam” type resolutions are popular and often provide suitable image quality with a fast refresh rate. In the case of Gatan’s Microtest product range for in-situ tensile testing, the MTVideo product option can provide a digital movie from a “TV output” of the image from the electron microscope, which is synchronized with the quantitative stress, stress and time data points from the experiment. Each frame of the movie is embedded with these data points, and the relevant frame of the movie can be chosen by moving the cross-hairs on a stress strain graph.

In many applications mentioned above, a higher pixel density in an SEM image is desirable as this best utilizes the spatial resolution within the required field of view. However, there is an obvious “trade-off” for a given image signal to noise ratio between the refresh rate and the pixel density. Faster scanning at high pixel densities will lead to noisy pixels which provide little benefit to the image. In many applications an image refresh rate closer to 1Hz or less is more appropriate than TV frequency sampling. This facilitates both higher signal to noise, and higher pixel density images to be recorded.

Gatan’s powerful solution is to employ DigiScan II™ for digital beam control and image capture. Within DigitalMicrograph™, this provides very flexible pixel density and dwell times, image aspect ratio, and “auto-surveying” contrast and brightness. Furthermore both screen capture software and if appropriate, experiment control software can be run on the same computer and captured together to provide a time lapse movie of the progress of all aspects of the experiment. For dynamic experiments this route avoids interfering with the core functioning of the SEM PC to control the column or stage. Furthermore DigiScan can provide live images of multiple signals simultaneously which can therefore be recorded simultaneously if they are present on the screen.

Gatan’s DigiScan II™ is a 2nd generation digital beam control system powered using DigitalMicrograph™ software, and suitable for most SEMs.

Screen capture software provides a maximum pixel resolution to that of the computer screen, and is flexible in terms of capturing only a selected window, selected areas, or the whole screen.

Two selected frames from “screen capture” movie showing final necking of a copper alloy specimen at room temperature. The load drops as the necking proceeds and the experiment can be paused with a constant load feature to study relaxation taking place. DigiScan II provides high pixel resolution SEM images in tandem with live recording of the load and extension status of the specimen.

* The Microtest product range is developed in collaboration with Deben UK Ltd.