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Electron Backscattered Kikuchi Patterns of a Plutonium Alloy Captured for the First Time

New Technique Minimizes Surface Oxidation, Allows for Better Electron Penetration to Underlying Metal

Plutonium is one of the most complex elements known to science. It may experience more than six atomic structure changes, transforming itself from one crystallographic phase to another, and each phase has different properties. Understanding the phase transformations, and in particular the microstructural evolution and aging behavior of plutonium and its alloys, is important to maintaining the safety and reliability of the weapons in the nuclear stockpile. The relative crystallographic orientation of separate grains within a material, also called crystallographic texture, has long been known to strongly influence material properties. Researchers have typically obtained bulk texture data through x-ray or neutron diffraction, while transmission electron microscopy has provided local texture information. Recent technological advances in microscopy techniques have provided a means to evaluate crystallographic texture using commonmetallurgical preparation techniques. This development has been in the field of electron backscattered diffraction through the automated collection and indexing of electron backscattered Kikuchi patterns.

Los Alamos researchers have come up with a way to minimize surface oxidation on plutonium samples during electron backscattered diffraction analysis. Oxidation results in a buildup on the surface of the sample that obstructs electron penetration to the underlying metal. The researchers have designed a device to transfer the sample to the scanning electron microscope chamber under vacuum. The device includes an entry-port adapter (the adapter's polycarbonate block has been illuminated with pink light in this photo; it doesn't really glow) and the vacuum suitcase (the black cylinder sitting atop the entry-port adapter). The scanning electron microscope chamber is the large cylinder located behind the adapter.

Electron backscattered diffraction analysis is ideally suited for plutonium investigations because of the complex phases and phase transformations possible. However, using electron backscattered diffraction to analyze plutonium has proved elusive for several reasons, one of which is rapid surface oxidation. This results in the buildup of an amorphous surface layer that acts as an obstacle for electron penetration to the underlying crystalline metal. A team of Los Alamos researchers has come up with a way to minimize surface oxidation when using electron backscattered diffraction to analyze plutonium. They have designed a device to transfer the sample to the scanning electron microscope chamber under vacuum. The device includes a vacuum suitcase and scanning electron microscope entry-port adapter.

The team's research shows that maintaining a low-oxygen environment after cleaning a sample's surface is important to successfully observing electron backscattered diffraction patterns. The new technique has allowed researchers for the first time to evaluate crystallographic texture through the automated collection and indexing of electron backscattered Kikuchi patterns.

When incident electrons penetrate a sample in electron backscattered diffraction, they may experience a variety of elastic scattering events until the sample absorbs their energy, or they may scatter elastically and escape from the specimen surface as backscattered electrons. The diffracted backscattered electrons are emitted in a series of bands, known as Kikuchi bands, which relate to the crystal planes and their orientation within the sample.

To capture Kikuchi patterns, a phosphor screen is placed in a scanning electron microscope vacuum chamber close to a steeply inclined sample. When the backscattered electrons bombard the phosphor screen, a network of Kikuchi bands is imaged by a camera. The positions of the bands, which contain information relating to the symmetry and orientation of the crystal, are identified.

It is thought that sharp Kikuchi patterns are formed from backscattered electrons that have experienced only elastic interaction (low-loss electrons). This indicates that electron backscattered diffraction is surface-sensitive and the technique and quality of the patterns is strongly dependent on sample preparation. This is especially true for high-density materials such as plutonium

This view inside the scanning electron microscope chamber shows a sample (shown magnified in the inset) on a puck assembly. A phosphor screen (the disk-shaped object) and a charge-coupled device camera are shown to the right of the sample. When backscattered electrons bombard the phosphor screen, a network of Kikuchi bands is imaged by the camera.

Researchers estimate that within a distance of nine nanometers below the surface of a plutonium sample, 95 percent of the incident electrons would experience a collision resulting in a permanent energy loss for the electron. This energy loss would result in lost structural information provided by the electron. Therefore, researchers expect that the bulk of the electron backscattered diffraction information for plutonium comes from only the top several nanometers of the surface.

The material used by the Los Alamos team in its initial evaluation was a plutonium-gallium alloy containing the face-centered-cubic atomic structure (the same structure that is predominately used in nuclear weapons). For most nonreactive metals, simple metallographic polishing followed by transfer through the air to the scanning electron microscope is sufficient for successful electron backscattered diffraction capture.

However, this isn't the case for highly reactive metals such as plutonium, and additional surface cleaning, such as ion etching- a method to remove the surface of a material by bombarding it with accelerated ions in a vacuum-is necessary.


These images show the electron backscattered Kikuchi patterns captured from two grains. The positions of the bands contain information relating to the symmetry and orientation of the crystals.


For this reason, the research team used a multifunctional ultrahigh-vacuum Auger/energy loss spectroscopy (EELS) instrument to characterize and remove surface layers contaminated with chemical impurities, in particular carbon and oxygen. (For more information on the Auger/EELS instrument, see Actinide Research Quarterly, 4th Quarter, 2000.)

In addition to small plutonium peaks, researchers observed large carbon and oxygen peaks and a small fluorine peak. To remove the surface layers concentrated with contaminants, the sample surface was bombarded repeatedly by argon ions used in the ion-etching process. Along with removing the surface layers concentrated with oxygen and carbon, ion etching enhanced the grain boundary contrast (i.e., the grains were topographically distinct).

One of the most interesting aspects of the ion-etched microstructure was that the bulk impurities tended to etch at a different rate than the plutonium-gallium metal. This resulted in toothlike features that protruded up to two micrometers (two-millionths of a meter) above the surface. By minimizing ion etching to remove only those surface layers concentrated with carbon and oxygen, the surface protrusions, which interfere with the backscattered electrons used for diffraction, were likewise minimized. Most environmentally sensitive materials that require surface cleaning can be transferred from the ion etcher through air to the scanning electron microscope vacuum chamber. For materials that oxidize rapidly, this jeopardizes surface integrity. For the plutonium-gallium sample, atmospheric transfer did not result in successful Kikuchi pattern acquisition, and this was attributed to the formation of a thick, amorphous surface oxide layer as a result of air exposure. Even an atmospheric exposure of only one second after ion etching led to a surface that did not exhibit Kikuchi patterns from the underlying plutonium-gallium metal.


These images obtained through scanning electron microscopy show low- and high-magnifications of an ion-etched microstructure. Bulk impurities on the surface of a sample result in toothlike protrusions (seen in the photo at right) that interfere with the backscattered electrons used for diffraction. By minimizing the ion etching to remove only those layers concentrated with carbon and oxygen, the surface protrusions were likewise minimized.

After the initial patterns were captured, researchers left the sample in the scanning electron microscope vacuum chamber for 68 hours and then successfully reanalyzed the sample for Kikuchi patterns. The surface oxide on the plutonium-gallium metal did not thicken enough within the vacuum environment to prevent researchers from observing the diffraction patterns, illustrating the minimal effect the vacuum environment has on surface oxidation.

Researchers believe this new sample preparation and characterization technique will provide a powerful means to further understand phase-transformation behavior, aging behavior, and texture in the complicated plutonium and plutonium-alloy systems.

Contributors to this article are: Carl Boehlert and Jeremy Mitchell (NMT-16), and Roland Schulze (MST-6). Other researchers on the project are Thomas Zocco (NMT-5); and Jeffrey Archuleta, Ramiro Pereyra, and Doug Farr (NMT-16).


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