H. Müllejans

Strength and fracture toughness of aluminum/alumina composites with interpenetrating networks

The mechanical properties of metal reinforced ceramics, especially Al/Al2O3 composites with interpenetrating networks, are described. Key parameters to tailor the characteristics of these materials are the ligament diameter and volume fraction of ductile reinforcement. Fracture strength and fracture toughness data are given as a function of both variables and are compared with the corresponding values for the porous preforms. A simple model accounts for the influence of metal volume and metal ligament diameter on the plateau toughness of the composites. The increase in fracture strength from the porous preform to the composite is found to be much larger than the gain which can be predicted from the increase in fracture toughness alone. A discussion of fracture strength in these composites therefore must include at least two issues, crack propagation through the matrix as well as crack initiation at metal filled pores.

Investigations of the chemistry and bonding at niobiumsapphire interfaces

Spatially resolved electron energy-loss data have been recorded at the interface between niobium and sapphire (α-Al 2 O 3 ), a model metal/ceramic couple. The spatial-difference technique is used to extract interface specific components of the energy-loss near-edge structure (ELNES), which are dependent on the chemistry and bonding across the interface. Multiple scattering calculations of aluminum, oxygen, and niobium clusters were performed to simulate the measured Al L 2,3 ELNES. Two samples fabricated by different techniques were examined. The first interface was made by diffusion bonding pure crystals. Its interface spectrum is identified with tetrahedral coordination of the Al ions at the interface. The calculations match the experimental edge structures, supporting the notion of aluminum to niobium metal bonding and concurring with a structural model in which the basal plane of sapphire at the interface is terminated by a full monolayer (i.e., 67% excess) of aluminum. The second sample was produced by molecular beam epitaxy. The spectrum of this interface is consistent with an atomistic structure in which the interfacial basal plane of sapphire is terminated by oxygen. An unoccupied band of states within the band gap of Al 2 O 3 is observed, signifying chemical bonding between metal and ceramic.

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.

Improved quantification of grain boundary segregation by EDS in a dedicated STEM

Abstract The segregation of impurities at grain boundaries is a well known phenomenon in materials science. In some cases the grain boundary segregation causes embrittlement of the material. The effect depends on the amount of impurity coverage of the grain boundaries. A suitable technique for the quantification of the grain boundary segregation of impurities is energy dispersive X-ray spectroscopy in a dedicated scanning transmission electron microscope. We have investigated a model system, the segregation of bismuth at grain boundaries in copper, and have found inconsistent quantitative results from the energy dispersive X-ray spectroscopy measurements under different experimental conditions. The inconsistencies were caused by beam broadening in the specimen which depends on the specimen thickness. A new method is proposed to quantify impurity segregation. An effective scanwidth is calculated for the scanning transmission electron microscope, depending on specimen thickness, as determined by electron energy loss spectroscopy. This approach takes beam broadening into account. The application to different grain boundaries in Cu doped with Bi yields quantitative results which are independent of experimental conditions.

Bismuth segregation at copper grain boundaries

Abstract The dependence of Bi segregation on the grain-boundary (GB) geometry for various Bi concentrations in Cu at different temperatures is investigated systematically. The GB segregation of Bi in Cu(Bi) bicrystals was determined quantitatively using energy dispersive X-ray spectroscopy (EDS) in a dedicated scanning transmission electron microscope (STEM). The Miller indices of the corresponding GB planes were determined by high resolution transmission electron microscopy (HRTEM). The energies of all experimentally investigated GBs were calculated by atomistic computer simulations. Gibbs segregation free energies were determined from the experimentally measured amount of Bi segregation using the classical McLean segregation theory. The free energies decrease monotonically with increasing calculated GB energies. The Fowler–Guggenheim segregation theory yields an attractive interaction between the segregated Bi atoms. No Bi-induced changes in the bonding at the GBs could be detected by electron energy loss spectroscopy (EELS).

Improvements in detection sensitivity by spatial difference electron energy-loss spectroscopy at interfaces in ceramics

Abstract Many problems in materials science require that a small amount of an impurity is detected, for example the segregation of such an impurity to an internal interface. For such inhomogeneous specimens two electron energy-loss spectra can be recorded: one from the interface and one from the nearby matrix. The second spectrum serves as a reference and can be subtracted from the first to remove the background and to reveal the excess of an element at the internal boundary. This method is labelled “spatial difference”. In this paper spatial difference is compared to other methods of background removal in electron energy-loss spectra. After a general discussion of the methods their detection sensitivity is compared quantitatively by calculating the signal-to-noise ratio with a simple numerical model. The results are compared to experimental results for the detection of Ca at grain boundaries in alumina and N inclusions in diamond. It is found that spatial difference has the highest signal-to-noise ratio in these model experiments and therefore improves the detection sensitivity as compared to the other methods. Spatial difference of inhomogeneous specimens in a scanning transmission electron microscope is straightforward. A clear advantage of the spatial difference involves the study of near-edge structure of the impurity element, yielding information on the atomistic structure and chemical bonding of internal interfaces.

Optical properties of AlN determined by vacuum ultraviolet spectroscopy and spectroscopic ellipsometry data

Precise and accurate knowledge of the optical properties of aluminum nitride (AlN) in the ultraviolet (UV) and visible (VIS) regions is important because of the increasing application of AlN in optical and electro-optical devices, including compact disks, phase shift lithography masks, and AlN/GaN multilayer devices. The interband optical properties in the vacuum ultraviolet (VUV) region of 6–44 eV have been investigated previously because they convey detailed information on the electronic structure and interatomic bonding of the material. In this work, we have combined spectroscopic ellipsometry with UV/VIS and VUV spectroscopy to directly determine the optical constants of AlN in this range, thereby reducing the uncertainty in the preparation of the low-energy data extrapolation essential for Kramers–Kronig analysis of VUV reflectance. We report the complex optical properties of AlN, over the range of 1.5–42 eV, showing improved agreement with theory when contrasted with earlier results.

Dispersion forces and Hamaker constants for intergranular films in silicon nitride from spatially resolved-valence electron energy loss spectrum imaging

The van der Waals (vdW) dispersion forces represent one of the fundamental long range inter- facial and surface forces in materials. The dispersion forces, for a set of materials in close proximity, arise from the electronic structure of the materials wherein the electrons in interatomic bonds acting as oscillat- ing dipoles exhibit an attractive interaction energy. These vdW dispersion forces, represented by a propor- tionality constant, the full spectral Hamaker constant (A), can be calculated directly from optical property based electronic structure spectra such as the interband transition strength (Jcv) using the Lifshitz theory. Si3N4 exhibits equilibrium intergranular films (IGFs) whose thickness is determined by a force balance where the contribution of the van der Waals dispersion force is dictated by the IGF chemistry. Using spatially resolved-valence electron energy loss (SR-VEEL) spectroscopy in the STEM with a 0.6 nm probe permits the in situ determination of vdW forces on the IGFs in viscous sintered polycrystalline systems. In addition local variations in IGF chemistry and dispersion forces throughout the microstructure of individ- ual silicon nitride samples can be determined using these methods. From multiplexed zero loss/plasmon loss optimized SR-VEEL spectra across IGFs with subsequent single scattering deconvolution, Kramers Kronig analysis and London dispersion analysis, the index of refraction and Hamaker constants can be determined. The method proved to be accurate and reproducible with comparison to VUV measurements for the bulk materials and repeated measurements on numerous individual IGFs. For these optimized Si3N4 materials, the dispersion forces varied over a range from 2 to 12 zJ. These showed standard devi- ations on the order of 1 zJ for systems with IGFs. Additional systematic errors can not be excluded. Local variations in Hamaker constants within the microstructure of a single sample correlate to the distribution of IGF thicknesses observed, i.e. the thickness varies inversely with Hamaker constant. The technique of measuring Hamaker constants in situ represents an important new tool for dispersion force and wetting studies. For the first time it is observed that the thickness of the IGF scales with the local Hamaker con- stant of the investigated grain boundary region. # 1998 Acta Metallurgica Inc.

Spatially resolved electron energy-loss studies of metal–ceramic interfaces in transition metal/alumina cermets

Composites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy‐loss spectroscopy (EELS), and, to a lesser extent, by high‐resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal‐ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near‐edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al‐terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O3M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion‐bonded Nb/Al2O3 interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.

Interband electronic structure of a near- grain boundary in -alumina determined by spatially resolved valence electron energy-loss spectroscopy

Valence electron energy-loss spectroscopy in a dedicated scanning transmission electron microscope has been used to obtain the interband transition strength of bulk -Al2O3. The interband electronic structure was obtained from critical point modelling. Comparison to established results from vacuum ultraviolet spectroscopy was used to improve the analysis of the energy-loss spectra and quantitative agreement between both methods was obtained. Spatially resolved measurements of a near-611 tilt grain boundary in -Al2O3 were analysed with the same procedure. This revealed an increase in the electron occupancy of the O 2p valence band to conduction band transitions which can be associated with an increased ionic character of the bonding at the grain boundary with respect to the bulk material. This is consistent with the results of other studies which determined the atomic structure and then calculated the electronic band structure of the same near-611 tilt grain boundary. Quantitative analysis of valence electron energy-loss spectroscopy can be regarded as a new electronic structure tool for application to localized structures such as internal interfaces in our quest to better understand their micro- and macroscopic properties.