Peter Warbichler

Imaging of nanometer-sized precipitates in solids by electron spectroscopic imaging

Abstract Electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM) can be efficiently used to detect precipitates in solids. In this work we used a GATAN imaging filter which has been attached to a 200 kV TEM to record elemental maps using inner-shell ionization edges. We have investigated a niobium alloy with nanometer-sized titanium-oxide precipitates and steels with vanadium-carbide and chromium-carbide precipitates. These precipitates could be visualized using inner-shell ionization edges (Ti L 23 , Nb M 45 , Cr L 23 , V L 23 , V M 23 and Fe L 23 ). We have compared different ESI techniques to check their validity for precipitate imaging. First, energy-filtered images can yield an enhanced contrast compared to the conventional TEM bright field, but are very sensitive to diffraction contrast in crystalline specimens and to sample thickness variation. Second, elemental maps have been recorded by using the three-window method (two pre-edge images and one post-edge image). Third, ratio images have been acquired by using the two-window method (one pre-edge window and one post-edge window). These ratio images show elemental contrast with lower noise than the elemental maps and are nearly free of the diffraction artifacts. We have successfully used ratio images to detect very small precipitates of diameters ranging from 2 to 10 nm in the materials mentioned above. However, ratio images have to be used carefully, because they are susceptible to artifacts.

Quantitative analysis of EFTEM elemental distribution images

Energy-filtering TEM (EFTEM) can be used to record elemental distribution images at nanometer resolution and with short acquisition times. In this paper we show how elemental maps can be converted into concentration maps. In order to demonstrate the application of the quantification procedures, we have chosen a sample consisting of CVD grown titanium carbonitride layers on a hard metal. Two approaches have been tested: Absolute quantification which is successfully applied to biological (amorphous) specimens yields a concentration map in terms of atoms per unit area. However, it turned out that this method is not suitable for crystalline materials due to diffraction and/or thickness variation effects. In the second method, atomic ratio maps are calculated from two elemental maps by ratioing the elemental maps and dividing them by the partial ionization cross-sections (or k-factors). This method yields concentration maps in terms of atomic ratios offering the advantage that diffraction and/or thickness variation effects are eliminated. Therefore, this method is well suited for the quantification of crystalline materials science specimens. In the second part of the paper we describe how related sets of elemental maps can be examined and combined in one chemical phase map. This can be provided by scatter diagram analysis (2-dimensional) and automatic classification procedures (n-dimensional) that show how intensities of corresponding pixels are correlated. These techniques have been applied to a typical material science specimen (Si-nitride ceramics with SiC and carbon inclusions) so that the reader may get a feeling for the advantages and limitations of these techniques in EFTEM-investigations. Finally, it is shown that the scatter diagram technique can be also applied to atomic ratio maps thus providing fully quantitative chemical phase maps.

Improved imaging of secondary phases in solids by energy-filtering TEM

Abstract Energy-filtering transmission electron microscopy (EFTEM) has been used for imaging of secondary phases in materials, e.g. precipitates and grain boundary phases. The investigations have been performed with a Philips CM20 equipped with a Gatan imaging filter at 200 kV acceleration voltage. In order to visualize the secondary phases in a steel sample we have recorded jump ratio images using the ionization edge of the matrix element (Fe M 23 -edge, division of post-edge image by pre-edge image). This method reduces unwanted diffraction contrast in crystalline specimens considerably when compared with the elemental maps which have been recorded with the three window method. In this work we show that an essential improvement of the jump ratio method is still possible by recording the jump ratio images under rocking beam illumination, because residual diffraction contrasts are completely eliminated in these images. This new method allows the rapid and unequivocal visualization of all chemical heterogeneities in crystalline solids (and also amorphous materials) at a nanometer scale.

On the application of energy-filtering TEM in materials science: III. Precipitates in steel

Abstract Energy-filtering transmission electron microscopy (EFTEM) has been used for imaging of precipitates and grain boundary phases in advanced ferritic-martensitic 10% Cr steel containing W and Mo (G-X12CrMoWVNbN 10 1 1). The investigations have been performed with a Philips CM20 equipped with a GATAN imaging filter at 200 kV acceleration voltage. The secondary phases have been imaged by recording jump-ratio images at the FeM 2, 3 edge under rocking beam illumination. This method reduces unwanted diffraction contrast in crystalline specimens completely, thus allowing one to visualize all compositional heterogeneities from micrometre to a nanometre scale. Due to a heat treatment of 10000 h at a temperature of 480°C the steel contains many different precipitate and grain boundary phases such as chromium carbides (M 23 C 6 ), vanadium nitrides, niobium carbonitrides (MX) and MoW Laves phase precipitates. The distribution of the secondary phases is shown by recording jump-ratio images and elemental maps of the elements Cr, V, Nb, Mo and W. Since, especially, the heavy elements are difficult to image, the optimum instrumental settings for reliable data acquisition are presented. Possible artifacts of the technique are also discussed. In addition, the quantitative composition of the precipitate phases has been determined using EEL and EDX spectrometry.

Quantitative Energy-Filtering Transmission Electron Microscopy (EFTEM)

Abstract. Energy-filtered transmission electron microscopy (EFTEM) can be used to acquire elemental distribution images at high lateral resolution within short acquisition times. In this paper we discuss the unique advantages of EFTEM in terms of information content, resolution, sensitivity and detection limits. Applications in different fields of research, both in materials science and in life sciences, demonstrate the potential of EFTEM for characterizing nano-sized structures. In the first example, we show how secondary phases in a steel specimen can be easily detected by recording jump ratio images of the matrix element under rocking beam illumination. Secondly, we describe how elemental maps can be converted into concentration maps by calculating atomic ratio maps. Thirdly, we show how the energy-loss near edge structures (ELNES) can be used for mapping chemical bonding states thus differentiating between various modifications of an element. Finally, we present recent results on investigations of deposits in lung tissues and how useful EFTEM can be for defect analysis in semiconductor devices.

On the occurrence of Z-phase in a creep-tested 10 % Cr steel

Abstract Electron microscopy studies on a creep-tested 10% Cr steel have revealed the presence of a complex nitride precipitate which has been previously assigned as modified Z-phase. This paper describes how the Z-phase can be efficiently detected by energy-filtering transmission electron microscopy. The results of electron diffraction, electron energy-loss spectroscopy and energy-dispersive X-ray analysis studies lead to a quantitative description of the modified Z-phase of the type Cr(V, Nb) N.

Imaging of the core-shell structure of doped BaTiO3 ceramics by energy filtering TEM

Energy-filtering transmission electron microscopy (EFTEM) was applied to investigate the core-shell structure of a X7R-dielectrics. The investigations were performed with a Philips CM20 equipped with a GATAN imaging filter at 200 kV acceleration voltage. The BaTiO 3 ceramics, of grain sizes 0.1 to 0.5 μm, was doped with low concentrations of the elements Zr, Yb, Sr and Mg. The cores contained ferroelectric domains whilst the shells were completely free of domains. EDXS and EELS analyses revealed that the cores were pure BaTiO 3 whilst the shells contained the doping elements. EFTEM was used to visualize the small concentration differences between the core and the shell within the BaTiO 3 grains. The core-shell structure can be visualized by recording jump-ratio images at the Ti-L 2,3 ionization edge under rocking beam illumination. The distribution of the doping elements can be shown by recording jump-ratio images with the Zr-M 4,5 and Yb-N 4,5 ionization edges. The jump-ratio images recorded under rocking beam illumination have the advantage, that the elemental distributions are not disturbed by diffraction effects such as in case of EFTEM elemental maps measured with the three-window method. Therefore, the core-shell structure could not be detected in the EFTEM elemental maps. This new method allows the rapid and unequivocal visualization of the core-shell structure of BaTiO 3 grains down to nanometer resolution. The methodology presented in this work can be applied to all X7R-dielectrics and is easily useable for rapid routine characterization of these materials.

Mapping the chemistry in nanostructured materials by energy-filtering transmission electron microscopy (EFTEM)

Energy-filtered transmission electron microscopy (EFTEM) can be used to acquire elemental distribution maps at high lateral resolution within short acquisition times, which makes it quite efficient for a detailed characterization of nanostructures, as illustrated with examples concerning a nanostructured substituted La-based cermet compound and a nanoscale multilayer. In the first example, we show how phases in a rapidly cooled substituted LaNi5 can be visualized by recording jump ratio images. Secondly, EFTEM was capable of imaging individual nanoscale layers in a magnetic multilayer consisting of 2 nm terbium and 3 nm iron.

Investigation of multi-layer coatings on cemented carbide cutting tools by analytical electron microscopy

Multi-layer coatings on cemented carbide substrates have been investigated by analytical electron microscopy (AEM). The samples were taken from cross-sections through the coating thus enabling a study of individual layers and interfaces. Various phenomena are shown: (1) tungsten diffusion and graphite inclusions in TiC layers, (2) oxygen impurities in Ti(C, N) coatings and (3) the occurrence of titanium oxinitrides in the interface between the Al2O3 and TiN-layers.

Application of EELS to the microanalysis of materials

Electron energy loss spectroscopy (EELS) is used in analytical electron microscopy (AEM) because it can provide results on the chemical composition and structure of a small volume of material. The practical application of EELS was demonstrated by the investigation of a refractory hard metal of the type WC-TiC-Co and by the investigation of a BaTiO3 ceramic material. To demonstrate the present status of quantitative analysis by EELS, the spectra of Be2SiO4, TiB2 and BaTiO3 were quantified and the results indicate that quantitative analysis is feasible for major concentrations of light elements and also of heavier elements even in the presence of severe edge overlap.