Backscattered Electron Kikuchi Patterns Research Articles

Microdiffraction phase identification in the scanning electron microscope (SEM)

The identification of crystallographic phases in the scanning electron microscope (SEM) has been limited by the lack of a simple way to obtain electron diffraction data of an unknown while observing the microstructure of the specimen. With the development of charge coupled device (CCD)-based detectors, backscattered electron Kikuchi patterns, alternately referred to as electron backscattered diffraction (EBSD) patterns, can be easily collected. Previously, EBSD has been limited to crystallographic orientation studies due to the poor pattern quality collected with video rate detector systems. With CCD detectors, a typical EBSD can now be acquired from a micron or submicron sized crystal using an exposure time of 1–10 s with an accelerating voltage of 10–40 kV and a beam current as low as 0.1 nA. Crystallographic phase analysis using EBSD is unique in that the properly equipped SEM permits high magnification images, EBSDs, and elemental information to be collected from bulk specimens. EBSD in the SEM has numerous advantages over other electron beam-based crystallographic techniques. The large angular view (∼70°) provided by EBSD and the ease of specimen preparation are distinct advantages of the technique. No sample preparation beyond what is commonly used for SEM specimens is required for EBSD.

Phase Identification of Individual Particles by Electron Backscatter Diffraction (EBSD)

Backscattered electron Kikuchi patterns (BEKP) were first observed by Alam et al. in 1954. J.A. Venables and C.J. Harland made the initial observation of BEKP and in the scanning electron microscope in 1973. In 1996, Goehner and Michael developed an electron backscatter diffraction (EBSD) system that uses a 1024 × 1024 pixel CCD camera coupled to a thin scintillator rather than photographic film. In their system, the quality of the raw patterns is improved by the use of “flat fielding” which normalizes the raw image to a “flat field” reference image that contains the image artifacts, including background, but not the crystallographic information. Automated pattern analysis is carried out using a Hough transform to locate bands and band edges in the pattern. The resulting crystallographic information is coupled with the elemental information from energy or wavelength dispersive x-ray spectrometry and the phase is identified is made through a link to a database such as the Powder Diffraction Files published by ICDD. An indexed pattern of the suspected phase is then synthesized for comparison to the unknown. This system is marketed commercially by Noran Instruments and offers the first practical method for rapid identification of unknown crystallographic phases in the SEM.

Chemical Characterization of Magnetic Materials at High Spatial Resolution

ABSTRACTThe fabrication of nanoscale magnetic materials and novel heterostructures has placed great demands on the analytical tools used to characterize the chemical and structural variations in these samples. Attempts to determine the composition, phase, crystallinity, and interface structure of nanoscale materials often involve tradeoffs between accuracy, sensitivity, and spatial resolution. Traditional techniques such as electron energy-loss spectroscopy (EELS), energy dispersive x-ray spectrometry (EDS), secondary ion mass spectrometry (SIMS), and scanning Auger microscopy (SAM) will be surveyed in this context. Powerful new tools for probing the chemical heterogeneity of materials at ultrafine length scales will also be discussed, including energy-filtered transmission electron microscopy (EFTEM) and spectrum imaging, as well as recent advances in new methods such as backscattered electron Kikuchi patterns (BEKP) and microcalorimetry.

Comparison of ferroelectric domain assemblages in Pb(Zr,Ti)O3 thin films and bulk ceramics

Ferroelectric domain assemblages of bulk and thin film Pb(Zr,Ti)O3 (PZT) ceramics are characterized as a function of grain size. Specifically, domain morphologies of chemically prepared, bulk PZT 95/5 (rhombohedral symmetry) ceramics are compared to tetragonally distorted perovskite PZT thin films of composition ranging from PZT 20/80 to PZT 53/47. Transmission electron microscopy (TEM), electron channel contrast imaging (ECCI) and backscattered electron Kikuchi patterns (BEKP) are the primary means of structural characterization. Domain morphologies in TEM foils, which have minimal mechanical constraints, are to first order very similar to domain morphologies determined by ECCI of PZT thin films mechanically constrained to substrates and to those of bulk ceramics exhibiting internal stress. While quasistatic 90° type domain patterns as a function of grain size are similar for both bulk and thin film ferroelectrics, 90° domain dynamics are substantially different for PZT thin film and bulk materials. 90° type domains are readily mobile under an applied electric field in large grain, bulk ceramics, but movement of these 90° type domains is shown to be severely limited in PZT thin films.

Solidification and welding metallurgy of Thermo-Span alloy

The welding metallurgy of Thermo-Span alloy has been evaluated and compared with that of two common superalloys, alloy 909 and alloy 718. The solidification behaviour and fusion zone hot cracking tendency were evaluated using differential thermal analysis and varestraint testing. Gleeble thermal cycle simulations were used to assess the hot ductility of the alloy during weld thermal cycles. Solidification microstructures were characterised by optical and electron microscopy. Differential thermal analysis indicated that melting of wrought Thermo-Span initiates at ternperatures near 1265°C and continues to ∼1413°C. In a manner similar to that of alloy 909, Thermo-Span solidifies with the formation of primary austenite followed by a terminal eutecticlike constituent near 1225°C. Varestraint testing indicates that the hot cracking tendency of Thermo-Span is similar to that of alloys 718 and 909. Hot ductility testing revealed that Thermo-Span displays ductilities on heating and cooling similar to those of alloy 909 and other nickel based superalloys. Solidification microsegregation patterns were determined by electron probe microanalysis, and the terminal solidification phases were identified by analytical electron microscopy and backscattered electron Kikuchi patterns to be an austenite–Laves eutecticlike constituent.

Phase Identification in a Scanning Electron Microscope Using Backscattered Electron Kikuchi Patterns.

Backscattered electron Kikuchi patterns (BEKP) suitable for crystallographic phase analysis can be collected in the scanning electron microscope (SEM) with a newly developed charge coupled device (CCD) based detector. Crystallographic phase identification using BEKP in the SEM is unique in that it permits high magnification images and BEKPs to be collected from a bulk specimen. The combination of scanning electron microscope (SEM) imaging, BEKP, and energy dispersive x-ray spectrometry holds the promise of a powerful new tool for materials science.

YBa/sub 2/Cu/sub 3/O/sub 7-x/ 45° [001] tilt grain boundaries induced by controlled low-energy sputtering of MgO substrates: transport properties and atomic-scale structure

Grain boundaries can act as weak links in the high T/sub c/ materials. If properly controlled, these grain boundaries can be used in various device applications. We have been able to reproducibly form 45/spl deg/ [001] tilt grain boundary junctions in YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films. The films were grown-on MgO substrates using a pre-growth substrate treatment. A low energy broad beam argon ion source was used to irradiate a select region of (100) MgO substrates. The film on the milled portion of the substrate grows predominantly with a grain orientation rotated 45 degrees about the c-axis with respect to the grain on the unmilled portion. Backscattered electron Kikuchi patterns have been used to confirm that the rotation occurs across the entire milled portion of the substrate. Transport properties of these films are discussed and related to high resolution electron microstructural and microchemical analyses of the grain boundaries. This technique has potential use in device applications as a method for controlled grain boundary engineering.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Advances in backscattered-electron Kikuchi patterns for crystallographic phase identification

The development of a charged coupled device (CCD)-based detector for the acquisition of high quality backscattered electron Kikuchi patterns (BEKP) has allowed crystallographic phase identification studies to be conducted in the scanning electron microscope (SEM). High quality BEKPs suitable for analysis have been obtained from a wide variety of materials. Phase identification through a combination of BEKP and energy dispersive x-ray spectrometry (EDS) has been performed with relatively little specimen preparation compared to electron diffraction in the transmission electron microscope. This paper will discuss the measurement of the depth resolution of the BEKPs. Crystallographic phase identification through a combination of BEKP and energy dispersive x-ray spectrometry is demonstrated by the identification of a Laves phase in an Fe-Ni-Cr-Nb alloy. The CCD-based detector has been described in detail previously and will not be discussed here. The thickness of the surface region that gives rise to the BEKP has been reported to be quite thin, thus requiring care in surface preparation.

Texture measurements in Al2O3 using BEKP

The study of preferred crystallographic orientation (texture) in ceramics is assuming greater importance as their anisotropic crystal properties are being used to advantage in an increasing number of applications. The quantification of texture by a reliable and rapid method is required. Analysis of backscattered electron Kikuchi patterns (BEKPs) can be used to provide the crystallographic orientation of as many grains as time and resources allow. The technique is relatively slow, particularly for noncubic materials, but the data are more accurate than any comparable technique when a sufficient number of grains are analyzed. Thus, BEKP is well-suited as a verification method for data obtained in faster ways, such as x-ray or neutron diffraction. We have compared texture data obtained using BEKP, x-ray diffraction and neutron diffraction. Alumina specimens displaying differing levels of axisymmetric (0001) texture normal to the specimen surface were investigated.BEKP patterns were obtained from about a hundred grains selected at random in each specimen.

Appllied Crystallography in the Scanning Electron Microscope Using a CCD Detector*

Abstract The development of a new charge coupled device (CCD)-based detector for the scanning electron microscope (SEM) has allowed high quality backscattered electron Kikuchi patterns (BEKP) suitable for crystallographic analysis to be collected. These BEKPs can be used for crystallographic texture, phase and microstress analysis. This CCD detector system, can also be equipped with a special filter for removing backscattered electrons, which allows us to image low intensity, highly divergent x-ray diffraction (Kossel) patterns. The Kossel patterns are utilized for the accurate measurement of d-spacings suitable foe residual stress and lattice parameter measurements.

Crystallographic phase identification in the SEM: Backscattered electron kikuchi patterns

The use of the recently developed charge coupled device (CCD)-based detector for the acquisition of high quality backscattered electron Kikuchi patterns (BEKP) has allowed on-line crystallographic phase identification studies to be conducted in the scanning electron microscope. High quality patterns suitable for analysis have been obtained in a wide variety of materials with little special specimen preparation. Phase identification through a combination of BEKP and energy dispersive x-ray spectrometry (EDS) is demonstrated by the identification of crystals present on ruthenium oxide thin films on Si. The crystals were identified as RuO2, a tetragonal phase.The CCD-based detector has been described previously and will only be briefly described here. The detector consists of a scientific grade slow scan CCD coupled to a YAG scintillator by a fiber optic reducer. During operation the CCD is cooled to -40°C by a thermoelectric cooler. This detector allows high quality patterns to be recorded with beam currents as low as 10-10A and exposure times of 1 to 10 seconds.