Up to recently, we analyzed materials before and after certain testing. However, there is a need to obtain detailed information on how material undergoes structural and chemical changes under aggressive environments at elevated temperatures. in situ closed-cell gas reaction, CCGR, scanning transmission electromicroscopy was developed specifically for this.
To study the dynamic changes in real time at elevated temperatures in a gas environment, up to full atmospheric pressure in a wide range of material system, such as catalysts, structural materials, carbon nanotubes, and so on. Furthermore, the reactions can be studied at different length scales, anywhere from microns down to atomic scale level. This is very important, because at lower magnifications we can learn a lot about overall system behavior and extract kinetic information.
While at the atomic scale level, we can learn about reaction mechanisms and kinetics that occur at the surface as well as the interfaces that are site-specific. This is truly remarkable, to obtain atomic resolution images at atmospheric pressure, and it cannot be really done by any experimental techniques. This protocol highlights how to perform in situ closed-cell gas reaction using in situ electromicroscopy.
It also highlights sample preparation and its challenges for different materials. Direct powder deposition by drop casting from a colloidal solution. Crush the powder if powder particles are too large.
Mix a small amount of powder and two milliliters solvent. Amount is determined by experiments. Sonicate solution for five minutes to create a colloidal suspension.
Place the E-chip on the E-chip retaining fixture. Drop cast the suspension using a micro-pipette directly onto the E-chip. Clean the gold contact with an absorbent paper point while viewing throw a stereo microscope.
Direct powder deposition through a mask. Place a new clean E-chip on the E-chip retaining fixture. Use and mask and place it directly onto E-chip within the fixture.
Use the top plate to clamp a new clean E-chip and a mask together within the fixture. Deposit a small amount of the powder using a spatula directly on the silicon nitride membrane of the new clean E-chip through the mask. Gently vibrate the fixture to shake the particles down to the E-chip.
This could be done by sonication unit and placing the fixture in the dry beaker. Shake off the excess powder, dissemble the system and inspect the placement of powder on the E-chip. Deposition methods by either electron beam evaporation, ion, or magnetron sputtering.
Create a pattern mask using a spacer chip with silicon nitride membrane and silicon nitride micro-porous TEM window. Use cyanoacryolate glue to attach to the silicon nitride micro-porous TEM window face-down over the 50 by 250 micron opening, following the manufacturer's recommendation. Then repeat the procedure to prepare as many pattern masks as needed for the E-chips.
Place a new clean E-chip on the E-chip fixture. Place the pattern mask on the E-chip. Cover the top plate and clamp it.
Use either electron beam evaporation, ion sputtering or magnetron sputtering deposition technique. Dissemble the system and inspect the E-chip with deposited material. Focused ion beam or FIB milling.
Prepare a TEM lamella using the FIB. Place the TEM lamella on the E-chip. Make sure that by attaching the sample to the E-chip, you do not damage the silicon nitride membrane.
Preparation of the CCGR-TEM holder. Download desired calibration file. Check the resistance of the E-chip.
Measure the resistance of the silicon carbide heater to ensure that it is within the resistance range for that particular E-chip calibration as provided by the CCGR manufacturer. Remove clamp from the top of the holder. Clean the tip of the CCGR-TEM holder using absorbent paper points or compressed air, making sure no debris remains on the O ring groves.
Place the spacer chip into the CCGR-TEM holder. Place the E-chip, with the heater contacts making proper connections to the electrical contacts with the flex cable on the holder. Then torque the set screws.
Measure again the resistance of silicon carbide heater after assembling the CCGR-TEM holder to ensure that it is within the resistance range for that particular E-chip calibration as provided by CCGR manufacturer. Preparation of experimental setup. Bake and pump down the system over the night, either with or without the holder connected.
Load the holder and connect the tubing. For the experiment, pump and purge the system with the inert gas. For example, twice from 100 Torr to 0.5 Torr.
Preform a final pump and purge from 100 Torr to 0.001 Torr. Residual gas analyzer. During pump and purge procedure, turn on RGA system to warm up the filament.
Integrating an RGA with an in situ closed cell gas reactor system on a stem, provides the critical measurements to correlate gas composition with dynamic surface evolution of materials during reactions. Continuous gas monitoring is essential for performing reactions on catalysts and structural materials with mixed gases or while alternating gases and pressures, and especially when water vapor is incorporated into the gas mixture. Attach the purge gas to the VDS and turn the lever knob to the exhaust, then turn to the park position.
Pressure the VDS by flowing inert gas three times, or until no more liquid is present. Turn the knob to park position and attach VDS to the manifold. Turn the knob to fill position and then remove purge gas line.
Set the vapor pressure to 18.7 Torr in the gas control software. Pump the VDS to vacuum, which is 0.1 Torr. Fill the VDS with water, which is 2 millimeters via syringe and tubing.
Note that if higher purity vapor is needed, additional purging steps are required. Running the reaction. Make sure all of the gases are connected to the manifold.
Using the atmosphere software under naming, set the correct gases for the reaction and save the row file. Under E-chip set up, select the correct calibration file for the E-chip and run calibration. Under pump and purge, see preparation of experimental setup.
Under Gas control, select the gas composition. Under temperature, select the heating rate and target temperature. Start the experiment.
Start flowing gas. Start imaging. Record gas composition.
Representative results. Example of the in situ CCGR-STEM results. Bright field STEM images show an example of this surface evolution of a platinum nano-particle on a titanium support when exposed to water vapor.
Structural changes in the platinum particle show rearrangement of structure associated with minor shape changes while monitoring gas composition. End the experiment. Turn off the temperature.
Stop flowing gas. End session.Summary. CCGR-STEM, for example, is beneficial for accelerated development of next generation catalysts with high durability, that are important for a number of catalytic conversion processes, such as a catalytic fast pyrolysis single step conversion of ethanol to N-butane and on to jet fuel or CO2 hydrogeneration and so on.
Moreover CCGR-STEM can be used to investigate high temperature oxidation behavior of structural materials in aggressive environments, to mimic materials'behavior similar, for instance, to gas turbine engine environments or to the next generation fission or fusion reactors.
