Hui Gu

COMPOSITION AND CHEMICAL WIDTH OF ULTRATHIN AMORPHOUS FILMS AT GRAIN BOUNDARIES IN SILICON NITRIDE

Two different electron energy loss spectroscopy (EELS) quantitative analytical methods for obtaining complete compositions from interface regions are applied to ultrathin oxide-based amorphous grain boundary (GB) films of ∼ 1 nm thickness in high-purity HIPed Si 3 N 4 ceramics. The first method, 1, is a quantification of the segregation excess at interfaces for all the elements, including the bulk constituents such as silicon and nitrogen; this yields a GB film composition of SiN 0.49±1.4 O 1.02±0.42 when combined with the average film thickness from high resolution electron microscopy (HREM). The second method, II, is based on an EELS near-edge structure (ELNES) analysis of the Si– L 2,3 edge of thin GB films which permits a subtraction procedure that yields a completeEELS spectrum, e.g., that also includes the O– K and N– K edges, explicitly for the GB film. From analysis of these spectra, the film composition is directly obtained as SiN 0.63±0.19 O 1.44±0.33 , close to the one obtained by the first method but with much better statistical quality. The improved quality results from the fewer assumptions made in method II; while in method I uniform thickness and illumination condition have to beassumed, and correction of such effects yields an extra systematic error. Method II is convenient as it does not depend on the film thickness detected by HREM, nor suffer from material lost by preferential thinning at the GB. In addition, a chemical width for these films can be deduced as 1.33 ± 0.25 nm, that depends on an estimation of film density based on its composition. Such a chemical width is in good agreement with the structural thickness determined by HREM, with a small difference that is probably due to the different way in which these techniques probe the GB film. The GB film compositions are both nonstoichiometric, but in an opposite sense, this discrepancy is probably due to different ways of treating the surface oxidation layers in both methods.

A quantitative approach for spatially-resolved electron energy-loss spectroscopy of grain boundaries and planar defects on a subnanometer scale

Abstract A quantitative approach for spatially-resolved electron energy-loss spectroscopy (SREELS) is demonstrated by investigating grain boundaries and planar faults in ceramics. This approach combines spatially-resolved energy-loss near-edge structure (ELNES), EELS quantification and associated spatial information on a subnanometer scale, and is based on an improved “spatial difference” method. This is a quantitative “spatial difference” which analyses elements present at defects as well as in the bulk, and which is performed with a systematic procedure to subtract completely the signal of the bulk based on the knowledge of ELNES for reference systems. Criteria to prevent artefacts are highlighted. The processed spectrum is dedicated to a defect, and may include signals from more than one element. Spatial information associated to the defect, such as the chemical width of a grain boundary, is obtained from quantification of the spectrum. Applying this approach to linescans (“Spectrum-Line”) not only achieves very high spatial resolution, but also provides an effective probe size. A spectrum for a planar fault of 0.22 nm width was obtained.

Tensile Ductility of Liquid-Phase Sintered β-Silicon Carbide at Elevated Temperature

Nano-grain sized β-SiC was prepared with various additives by hot-pressing in Ar. The relative bulk densities were more than 95 %. As-sintered bodies were composed of equiaxed grains. Amorphous phase existed at grain boundaries in each as-sintered material. Tension tests were performed at the initial strain rates from 3 X 10 -4 s -1 to 3 X 10 -5 S * at 1973 - 2048 K. Tensile elongation more than 60 % was obtained. In deformed specimens, grain growth was observed. However, anisotropy of grain growth was not observed. The grain-boundary phase was vaporized during tension tests. The β phase of SiC was stable even after tension test. Therefore, critical deformation mechanism was thought to be grain-boundary sliding. Deformation behavior was influenced by the grain-boundary phase.

Indirect EELS imaging reaching atomic scale – CaO planar faults in CaTiO3

Abstract Indirect EELS analysis is based on an enhanced “spatial difference” method which separates ELNES of a defect from ELNES of the matrix [H. Gu, Ultramicroscopy 76 (1999) 159, 173.]. Combined with the “spectrum imaging” technique [C. Colliex et al., Microchem. Acta 114/115 (1993) 71], the spatial difference method is employed in the STEM to investigate CaO planar faults in nonstoichiometric CaTiO3 which can locally form Ca4Ti3O10. Although inelastic imaging used as 1D profiling resolves individual faults separated by 1.36xa0nm from each other, the effective EELS probe size is worse than the elastic HADF imaging by a factor of 3 for the same microscope conditions. However, quantitative analysis of the EELS profiling yields additional information about equivalent probe size, and more importantly, provides an alternative way to separate the EELS spectra. O-Kxa0ELNES in the separated spectra reveal convincingly the differences between the perovskite and the rock-salt structures, and their corresponding ELNES profile maps distinguish between these two structures. In this indirect way, EELS analysis reaches effectively the state-of-art by obtaining a high quality spectrum reflecting the electronic structures in a 0.22xa0nm layer of defect structure.

Structural and Chemical Widths of General Grain Boundaries: Modification of Local Structure and Bonding by Boron-Doping in β-Silicon Carbide

Using a hot-isostatic-pressed boron-doped silicon carbide (SiC) material as an example, we demonstrate that the structural width and the chemical width of general boundaries may be quite different. The high-resolution electron microscopy (HREM) observation did not detect the existence of ∼1 nm thick amorphous film at such grain boundaries (GB). There is only a core structure of 1–2 atomic planes at GB. The chemical width of GB, obtained by the spatially-resolved electron energy-loss spectroscopy (EELS) analysis, is visibly wider than the core region. Furthermore, the spatially-resolved energy-loss near-edge structures (ELNES) analysis not only revealed the chemical bonding between boron and carbon, oxygen and silicon, but also distinguished an extended GB region with chemical bonding modified from that of the grain interiors. Such ELNES analysis defines an ELNES width that is even wider than the chemical width. The three GB widths of different scale construct a comprehensive picture of general boundaries that is remarkably different from general boundaries with amorphous film. Instead of a film confined by the two atomically sharp grain surfaces, there is only one interface, the rough GB core having most of the B–C and Si–O bonds, and the extended grain surface layers, to form such general GB in B-doped SiC.

Compositions and Thicknesses of Grain Boundary Films in Ca-Doped Silicon Nitride Ceramics

Nanobeam analytical electron microscopy and high-resolution electron microscopy have been used to characterize both the local composition as well as the thickness of the amorphous intergranular silicate-rich film in high-purity Si3N4 ceramics doped with different levels of Ca impurities. Grain boundary films could be detected in all ceramics. The films always contained Si (cation) and O and N (anions). Ca was detected at both grain boundary junctions and triple junctions for all materials doped with different levels of Ca. The thickness h of the intergranular films depends sensitively on the Ca content. In undoped materials h is 1.0±0.1 nm. With increasing Ca content the thickness h decreased for a dopant of 80 ppm Ca but increased with further additions of Ca. Variations in film thickness and composition can be semi-quantitatively understood in terms of different long-range interatomic forces acting normal to the film.

Analytical electron microscopy of planar faults in SrO-doped CaTiO3

Oxide-rich planar faults within a perovskite matrix are the prevailing type of extended defects in polycrystalline SrO-doped CaTiO 3 . These defects form, depending on the temperature of sintering, random networks, or ordered structures. The chemistry of the polytypoid, the isolated planar faults, and the perovskite phase have been studied by spatially resolved electron energy-loss and energy-dispersive x-ray spectroscopies using a dedicated scanning transmission electron microscope. We have found that Sr ions from SrO additions preferably substitute Ca in the CaTiO 3 lattice, thus forming a solid solution (Ca 1– x Sr x )TiO 3 . The surplus of Ca ions forms single and ordered CaO-rich planar faults in the host (Ca 1– x Sr x )TiO 3 phase. Whereas the excess Ca segregates in a form of single planar faults at lower temperatures, it forms a stable polytypoidic phase at higher temperatures. For materials having up to 25 mol% of SrO additions, this phase has (Ca 1– x Sr x ) 4 Ti 3 O 10 composition, comprising a sequence of CaO faults followed by three (Ca 1– x Sr x )TiO 3 perovskite layers. Analytical electron microscopy revealed that the composition of the single planar faults, formed at lower temperatures, is identical to that of polytypoids, which are stable at higher sintering temperatures.

Segregation and Local Structure at Grain Boundaries in SiO2-Doped Tetragonal ZrO2 Polycrystalline Materials

SiO2 doping in Y2O3 stabilized tetragonal ZrO2 (TZP) materials introduced significant change in mechanical properties around 0.3 wt.% doping level [1]. In order to understand the influence of grain boundary structure and chemistry by doping, high purity undoped and SiO2-doped 3Y-TZP samples were studied using high-resolution and analytical TEM. Typical grain boundary structures are different for the two types of samples, while amorphous film was not observed at most grain boundaries. A new EDS analysis method was introduced to detect the weak Si and Y signals which overlap with the predominent Zr peaks. It revealed that Si segregation to the grain boundary saturates at 12 at./nm2 (or 1.5 monolayer of SiO2) when the SiO2 doping level reached and surpassed 0.3 wt.%. It is the segregated atoms which enhanced the grain boundary diffusivity and therefore altered the deformation mechanism.

A novel interfacial microstructure in SrTiO3 ceramics with Bi2O3-doping

Abstract A novel interfacial microstructure was found in Bi 2 O 3 -doped SrTiO 3 ceramics. Bi-rich precipitates of sizes between 10 and 100 nm were observed at most grain boundaries. Using spatially-resolved EDS analysis, it was shown that these nano-precipitates had similar composition to Sr 2 Bi 4 Ti 5 O 18 secondary phase formed at triple grain pockets. This was revealed by the same trends in both Bi/Ti and Sr/Ti ratios as a function of particle size. In addition to forming precipitates, Bi segregation was also detected at these boundaries. However, the segregation level was substantially lower compared with that at boundaries without precipitates. The latter type of boundaries was always covered with thin amorphous films. Such novel interfacial structure containing nano-particles at grain boundaries does not hinder the dielectric properties of the material and it cannot be simply incorporated into the grain–boundary–barrier–layer–capacitor (GBBLC) model, which requires continuous insulating layers.

Influence of Ca-Doping on Chemistry and Force Balance at Grain Boundary in Si 3 N 4

High-purity HIPed Si 3 N 4 ceramics doped systematically with Ca additives are analyzed quantitatively by EELS. Using an ELNES related method to obtain film composition, we found that the composition difference from film to film is less significant than segregation. This is interpreted as a change of film width related to grain surface faceting. The hexagonal structure associated with anisotropie dielectric function can alter van der Waals attraction for different surface structures to produce variations in film thickness with a uniform film composition. These grain boundary films are better described as a high pressure amorphous oxynitride phase that allows higher but still limited solubility of calcium nitrogen.