Charles E. Lyman

Tutorial Review: X-ray Mapping in Electron-Beam Instruments

This review traces the development of X-ray mapping from its beginning 50 years ago through current analysis procedures that can reveal otherwise obscure elemental distributions and associations. X-ray mapping or compositional imaging of elemental distributions is one of the major capabilities of electron beam microanalysis because it frees the operator from the necessity of making decisions about which image features contain elements of interest. Elements in unexpected locations, or in unexpected association with other elements, may be found easily without operator bias as to where to locate the electron probe for data collection. X-ray mapping in the SEM or EPMA may be applied to bulk specimens at a spatial resolution of about 1 μm. X-ray mapping of thin specimens in the TEM or STEM may be accomplished at a spatial resolution ranging from 2 to 100 nm, depending on specimen thickness and the microscope. Although mapping has traditionally been considered a qualitative technique, recent developments demonstrate the quantitative capabilities of X-ray mapping techniques. Moreover, the long-desired ability to collect and store an entire spectrum at every pixel is now a reality, and methods for mining these data are rapidly being developed.

High-performance X-ray detection in a new analytical electron microscope

X‐ray detection by energy‐dispersive spectrometry in the analytical electron microscope (AEM) is often limited by low collected X‐ray intensity (P), modest peak‐to‐background (P/B) ratios, and limitations on total counting time (τ) due to specimen drift and contamination. A new AEM has been designed with maximization of P, P/B, and τ as the primary considerations. Maximization of P has been accomplished by employing a field‐emission electron gun, X‐ray detectors with high collection angles, high‐speed beam blanking to allow only one photon into the detector at a time, and simultaneous collection from two detectors. P/B has been maximized by reducing extraneous background signals generated at the specimen holder, the polepieces and the detector collimator. The maximum practical τ has been increased by reducing specimen contamination and employing electronic drift correction. Performance improvements have been measured using the NIST standard Cr thin film. The 0·3 steradian solid angle of X‐ray collection is the highest value available. The beam blanking scheme for X‐ray detection provides 3–4 times greater throughput of X‐rays at high count rates into a recorded spectrum than normal systems employing pulsepileup rejection circuits. Simultaneous X‐ray collection from two detectors allows the highest X‐ray intensity yet recorded to be collected from the NIST Cr thin film. The measured P/B of 6300 is the highest level recorded for an AEM. In addition to collected X‐ray intensity (cps/nA) and P/B measured on the standard Cr film, the product of these can be used as a figure‐of‐merit to evaluate instruments. Estimated minimum mass fraction (MMF) for Cr measured on the standard NIST Cr thin film is also proposed as a figure‐of‐merit for comparing X‐ray detection in AEMs. Determinations here of the MMF of Cr detectable show at least a threefold improvement over previous instruments.

High resolution transmission electron microscopy of interphase interfaces in NiO-ZrO2(CaO)

Abstract High resolution transmission electron microscopy (HRTEM) observations of the interphase interfaces in the directionally solidified eutectic (DSE) NiO - ZrO 2 ( CaO ) are presented. The planar lamellar interfaces were observed to be extremely clean without any evidence of an interface phase. Characteristic line and/or step defects at the interface have been identified and analyzed. Chemical microanalysis using X-ray emission spectroscopy (XES) indicated clear partitioning of CaO to ZrO 2 . The observations are discussed in view of the limited understanding of the structure and chemistry of interphase interfaces in oxide systems.

X-ray emission spectrometry of phase separation in PtRh nanoparticles for nitric oxide reduction

Abstract Composition determinations of individual sub-10 nm metal particles in Pt Rh/alumina by analytical electron microscopy are consistent with a low-temperature miscibility gap below about 750°C in this system. Pt-rich alloy particles are more active than pure Pt for the reduction of nitric oxide with hydrogen at temperatures ≤ 300°C. Rh-rich particles appear to have low activity for this reaction. Catalysts which contain a bimodal distribution of Pt-rich and Rh-rich particles are not as active as pure Pt. Composition versus size plots of small metal particles appear to comprise a new characterization method for supported metal catalysts.

Analysis of Alloy Nanoparticles

Abstract. Quantitative analysis of particles less than 10 nm in diameter requires a focused electron beam to isolate individual particles for X-ray emission spectrometry. Effects such as phase separation among particles and surface segregation within particles can only be determined by this technique. This analysis can be made quantitative with minimal use of the usual correction factors provided the small particles are supported on ceramic materials about the same thickness as the particles themselves. Two alloy nanoparticle systems are examined here: Pt-Rh and Pt-Re. In each case the catalytic properties resulting from various processing procedures have been correlated with the microstructure within and among individual particles.

The influence of the chlorine content on the bimetallic particle formation in Pt–Re/Al2O3 studied by STEM/EDX, TPR, H2 chemisorption and model reaction studies

The influence of the chlorine content on the bimetallic particle formation in Pt–Re/Al2O3 studied by STEM/EDX, TPR, H2 chemisorption and model reaction studies

Crystal symmetry and coherent twin structure of calcium zirconate

CaZrO 3 has been investigated at room temperature using a variety of electron microscopy techniques. Conventional transmission electron microscopy and high-resolution transmission electron miscroscopy revealed a characteristic coherent twin structure in CaZrO 3. In order to interpret the occurrence of the twin structure and the associated crystallography, the cell parameters and space-group symmetry of CaZrO 3 have been determined independently using convergent-beam electron diffraction. The results are consistent with those previously obtained by neutron diffraction. Both the crystal symmetry and lattice-parameter data have been used to explain the details of the twin structure in CaZrO 3.

Quantitative X-Ray Analysis: The Basics

As discussed in Chapters 5 and 6, the x rays emitted from a specimen bombarded with the finely focused electron beam of the scanning electron microscope (SEM) or electron-probe microanalyzer (EPMA) can be used to identify which elements are present (qualitative analysis). With the proper experimental setup and data-reduction procedures, the measured x rays can also be used to quantitatively analyze chemical composition with an accuracy and precision approaching 1%. This chapter provides an overview of the basic principles and techniques used for determining chemical composition, on the micrometer scale, with the SEM and EPMA. Our intention is to provide the conceptual basis for an understanding of the x-ray microanalytical data-reduction procedures that today are almost always incorporated into black-box computer-based, integrated analysis systems with which the analyst interacts as a user. As a user, the analyst depends on the knowledge and skill of programmers to have devised an accurate, robust analytical procedure from the diverse approaches available in the literature. Despite the apparent disconnection which has arisen between the analyst as user and the underlying physics as incorporated into the algorithms of the software, it is nevertheless extremely important to grasp the underlying physical principles to become a sophisticated analyst rather than a mere user.

On the space group of aluminium oxynitride spinel

Abstract In view of the controversy surrounding the crystal symmetry of spinels, an aluminium oxynitride phase reported to be based on the spinel structure has been investigated using convergent beam electron diffraction (CBED) and computer simulation. A variety of CBED point and space group determination methods have been utilized in search of possible non-centrosymmetry which reduces the space group symmetry from Fd3m to F43m. The occurrence of Gj⊘nnes-Moodie (GM) lines in the debated {2 0 0} reflections is also explored through careful voltage variation experiments. The detailed analyses clearly show that the point group of this spinel is the centrosymmetric m3m and that the space group is Fd3m.

Low-Energy Interfaces in NiO-ZrO2(CaO) Eutectic

Interphase interfaces in the directionally solidified eutectic (DSE) NiO-ZrO2(CaO) have been investigated using transmission electron microscopy (TEM) techniques. Arguments are presented, based on extensive experimental results, to show that the observed interface plane, (111) NiO//(100) ZrO2, corresponds to a minimum in interface energy. The possible relaxation events associated with this interface are identified with the aid of imaging and diffraction analyses. A recently introduced technique of convergent beam electron diffraction for a plan-view bicrystal is attempted in order to identify rigid body translation associated with this interface. Some of the difficulties associated with this technique are discussed.