Scanning Electron Microscopy and Transmitted Electron Backscatter Diffraction Examination of Asbestos Standard Reference Materials, Amphibole Particles of Differing Morphology, and Particle Phase Discrimination from Talc Ores

Abstract

Since 1972, when the US Occupational Health and Safety Administration established the first limits on occupational exposure to asbestos fibers, numerous analytical methods employing several microscopy techniques have been developed to identify a group of minerals defined by legislation as asbestos. While transmission electron microscopy (TEM) is implemented in standardized analytical methods, these methods specify the use of selected area electron diffraction. Because of this constraint, the diffraction data a TEM can provide are often underutilized due to challenges associated with collecting and interpreting individual diffraction patterns. It has been shown that transmission electron backscatter diffraction (tEBSD) produces diffraction patterns nearly identical to electron backscatter diffraction, but from smaller crystal domains. This paper explores the utility of tEBSD for characterization of asbestiform particles from reference asbestos materials, a suite of amphibole minerals of varying morphologies to determine if there is a correlation between mineral habit (i.e., crystal form), microscopic particle shape preferred orientation, and mineral specimens from an industrial talc deposit to provide a case study of the utility and limitations of the technique.

Application of Transmitted Kikuchi Diffraction in Studying Nano-oxide and Ultrafine Metallic Grains

“…TKD is gaining a growing attention since its introduction in 2010. − It offers higher spatial resolution and lower interaction volume than conventional EBSD to study nanoscale crystalline materials. These are due to electron beam vertical penetration, in contrast with horizontal spreading of the beam in EBSD as a result of high tilt angles.…”

“…Automated data acquisition over large areas could be achieved by TKD, utilizing already available EBSD hardware and software. Additionally, TKD could be combined with other techniques such as energy-dispersive spectroscopy (EDS) in SEM for phase characterization, as performed by Brodusch et al Overall, it is a promising technique to study nano- and ultrafine grains considering the availability of localized sample preparation methods, such as focused ion beam (FIB), to prepare electron-transparent samples already used in transmission electron microscopy (TEM) and atom probe microscopy (APM). − …”

“…No standard tilt angle has been defined in TKD, unlike conventional EBSD, where a standard −70° tilt angle is applied (between sample surface normal and electron beam). Tilt angles from 10 to 60° have been reported. − However, 10–20° angles have been mostly used to prevent shadowing on the detector while patterns with sufficient intensity were produced. Higher tilt angles result in a beam broadening that increases the interaction volume and reduces the spatial resolution.…”

“…In most TKD cases, electron-transparent slices prepared via methods such as FIB with a typical thickness of 100 nm provided satisfactory patterns. − It should be noted that chemical sample preparation methods such as electropolishing may result in improved diffraction signals since ion-milling techniques produce an amorphous layer at the sample surface.…”

Transmission electron imaging and diffraction of asbestos fibers in a scanning electron microscope

“…20 The data collection procedure is practically identical to that used in electron backscatter diffraction (EBSD) except that 4D STEM applications are not limited to those that utilize only Kikuchi scattering. Although conventional and transmission EBSD studies have been done on asbestos, 21,22 4D STEM methods have not to our knowledge. 4D STEM is well suited to examine asbestos for several reasons.…”