J.A. Small (CSTL, Gaithersburg) and J.R. Michael (Sandia National Laboratory)

Objective: To develop a novel approach to the identification of the crystalline phase of individual micrometer and submicrometer particles in the electron microscope based on the analysis of electron backscatter diffraction patterns.

Problem: The effects of particle geometry and size often limit the accuracy and precision of the quantitative electron probe analysis of individual particles by x-ray emission. Typical uncertainties for particle elemental analyses are on the order of ± 10% relative at best which often makes the identification of the chemical phase of the particles difficult if not impossible.

Recently an EBSD Phase ID system has been commercially developed that enables the rapid identification of the crystallographic phase of unknowns in the scanning electron microscope. In this investigation, we looked at the application of the EBSD Phase ID system to the phase identification of individual micrometer and submicrometer particles rather than the conventional EBSD samples that consist of bulk samples with flat polished surfaces. Unlike conventional samples, the phase identification of individual particles may be complicated by several factors related to particle geometry and size. These factors include: difficulty in obtaining flat-field reference images for individual particles, effects of particle mass/size on EBSD image quality, pattern interference from nearby particles or substrates, and the effects of particle composition on EBSD image quality.

Approach: For this study we analyzed a series of particles with known elemental compositions. These particles included U3O8 (NBS SRM #950b), PbO2, SiC, and PbMoO4 wulfenite. Particles from each of the materials were dispersed onto pyrolitic carbon substrates or carbon tape and analyzed at Sandia National Laboratories in their JEOL 6400 scanning electron microscope at 20 kV acceleration potential. The particles were uncoated and the mounting substrate was at a tilt angle of 70 degrees.

Figure 1. Electron backscattered diffraction pattern taken from a 250 nanometer uranium oxide particle. The particle was identified as orthorhombic U3O8 JCPDS card # 24-1172.

Results and future plans: The results from the EBSD analysis of individual particles indicate that the EBSD system was very successful in identifying the phases of individual submicrometer particles with relatively high average Z as shown in Figure 1. The backscattered pattern in Figure 1 was taken from a U3O8 particle 250 nanometers in diameter that was successfully identified by the Phase ID system as orthorhombic U3O8. In addition the results indicate that the analysis of low Z, submicrometer particles will be challenging since the number of backscttered electrons decreases with decreasing Z. Future efforts in EBSD of individual particles will include studies of particle size and shape effects as well as development of methods to obtain reference images for flat-field processing.

The Phase ID system used in conjunction with an analytical SEM or EPMA provides the analyst with a very powerful and straightforward method to obtain an absolute identification of submicrometer and larger crystalline particles. (see also effects of substrates)

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