Academic Centre for Materials and Nanotechnology

Advanced microstructure characterization based on the development of new generations of computer-controlled electron microscopes started with the beginning of XXI century. New technologies have improved spatial resolution and increased a rate of collecting and processing of large quantities of data. Orientation microscopy is nowadays a well-established technique for studying and quantifying microstructures and determining crystallographic orientations and topography by use of Electron BackScatter Diffraction (EBSD) in Scanning Electron Microscope (SEM) [e.g. 1]. The technique is based on the automatic acquisition and indexing of a set of diffraction patterns correlated with sample coordinates. EBSD has made an extensive contribution to the crystallographic and structural analyses of ceramics providing information on: absolute or relative grain orientations, misorientation angles, planes of grain boundaries, identification of unknown phases etc. The fully automated EBSD facilities in SEMs are nowadays commercially available and fairly common in materials characterization laboratories. However, EBSD spatial resolution during SEM measurements strongly depends on a type of electron gun as well as material being investigated, permitting only microstructures with features in the range down to ~100 nm to be "mapped". In order to overcome such problems, Orientation Mapping (OM) technique in transmission electron microscope (TEM), based on Convergent Beam Electron Diffraction (CBED) patterns, was developed [2, 3, 4]. Despite some restrictions, OM in TEM offers spatial resolution less than 10 nm and can be used for quantitative analysis of microstructures at nanoscale .

Orientation imaging microscopy (OIM) is a technique which allows crystallographic information to be obtained from samples both in scanning and transmission electron microscopes (SEM/TEM). Microstructure can be quantitatively characterized, different phases identified, and grain boundary character and morphology described. This technique based on electron backscatter diffraction (EBSD) in the SEM has shown in the last decade a spectacular advance in speed and quality of the data as well as post-processing procedures. In most cases the investigated samples are conductive and one encounters no problems connected with specimen charging. Controlling the charge and quality of the surface are the challenges for the operator running experiments on insulators. Specimen charging obscures both the forward scatter electron image and decreases the EBSD pattern quality making its analysis difficult or even impossible. Also final steps of sample preparation largely determine whether or not the pattern quality is sufficient for indexing. The influence of gas pressure and gas path length on the image quality of diffraction pattern is illustrated with the results of experiments carried out on the (PbLa)(ZrTi)O 3 ceramics. Ferroelectric domains in the PLZT grains have been identified both by EBSD in the SEM and BF images in the TEM. Domains in particular grains have the same crystal orientations but different polarisation directions.

Electrodeposition process is a very promising method for producing metal matrix composites reinforced with ceramic particles. In this method insoluble particles suspended in an electrolytic bath are embedded in a growing metal layer. This paper is focused on the investigations of the nickel matrix nanocomposite coatings with hard a-Al 2 O 3 nano-particles, electrochemically deposited from modified Watts-type baths on steel substrates. The influence of various current densities on the microstructure, residual stresses, texture, hardness and corrosion resistance of the deposited nickel/alumina coatings was investigated. The surface morphology, cross sections of the coatings and distribution of the ceramic particles in the metal matrix were examined by scanning electron microscopy. The phase composition, residual stresses and preferred grain orientation of the coatings were characterized using X-ray diffraction techniques. The coating morphology revealed that a-Al 2 O 3 particles show a distinct tendency to form agglomerates, approximately uniformly distributed into the nickel matrix.

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Wetting of Al (99 %) pads by Sn-8.8Zn, Sn-8.7Zn-1.5In, and Sn-8.7Zn-1.5Ag (wt%) alloys was studied by means of the sessile drop method. The tests were carried out at 250°C, using Amasan ALU33Ò flux, for up to 60 min. We used a setup that allows fast transfer of a sample to and from the hot zone of the furnace. Solidified alloy-substrate couples were cross sectioned and examined by scanning electron microscopy and energy dispersive X-ray analysis. The studied alloys wet Al pads as the wetting angles, determined after cleaning the flux residue from solidified alloy-substrate couples, are lower than 35 degrees. Wetting angles of Sn-8.7Zn-1.5Ag (wt%) alloy are higher and wetting areas are smaller than those of the remaining alloys, possibly due to a small gap between the melting and test temperatures. There is no clear dependence of the wetting angle and spreading area on time.

ABSTRACT Modern scanning electron microscopes (SEMs) increase the ability to study a wide range of materials. Especially, an application of low vacuum conditions enables characterization of nonconductive samples without complicated preparation procedure. However, the operator must be aware of several problems he may encounter collecting electron diffractions in the SEM with variable pressure. The charge control and quality of the surface are the challenges when running experiments on insulators. Specimen charging obscures forward scatter electrons images and decreases the EBSD pattern quality making indexing difficult or even impossible. Another crucial question is how to limit the influence of so called "skirt effect" caused by ionization of gas molecules followed by electron beam broading above the sample. The influence of several important parameters (gas pressure, a type of gas, working distance and energy of electron beam) on the EBSD pattern quality must also be considered. When it is properly done, a coupling of crystallographic information with the chemical data obtained from Energy Dispersive Spectroscopy (EDS) in the LV-SEM allows to perform phase identification of insulators. The paper presents some ideas how to deal with the (Pb, La)(Zr, Ti)O3 ceramics in high resolution Quanta 3D SEM (with thermally assisted Schottky type FEG) equipped with EDAX-TSL system in low vacuum environment. The problems occurring during EBSD analysis of the PLZT ceramics are discussed and some solutions are suggested. Paper summarizes the results obtained from PLZT ferroelectric ceramics in the low vacuum FEGSEM and shows how to optimise experimental parameters in order to achieve the best quality of orientation maps acquired from nonconductive samples.

Crystal lattice rotations induced by shear bands developed in an AA1050 aluminium alloy have been examined in order to investigate the influence of the finegrained structure on the slip propagation across the grain boundaries and the resulting texture evolution. Samples of the AA1050 alloy were pre-deformed in ECAP up to 6 passes via route C, then machined and further compressed in a channel-die up to ~25% at room temperature. The microstructure and texture were characterized by SEM equipped with a high resolution EBSD facility.

The effect of calcination temperature on the structural properties and phase formation of synthesized CaO-Al2O3 nanopowder was investigated and discussed. The calcination products were identified by differential thermal analysis (DTA) and the crystalline phase formation was analyzed by X-ray diffraction (XRD). The obtained results showed that the crystallization started at 460 C. Finally, the microstructures of the nanoparticles were observed by scanning (SEM) and transmission electron (TEM) microscopes. The investigation showed that an increase in the calcination temperature led to the appreciable increase in the crystallite size and the crystallinity of the final product. The obtained data confirmed that the prepared materials were mayenite with different surface area in the range of 71.18 m 2 /g to 10.34 m 2 /g after annealing in the temperature range of 470 C to 960 C.

In this paper, a transient microstructure-oriented numerical simulation of a planar Direct Internal Reforming Solid Oxide Fuel Cell (DIR-SOFC) is delivered. The performance criteria in a direct steam reforming for a fuel starvation scenario are analyzed in order to optimize the underlying process. The proposed two-dimensional multiscale model takes into account mass and heat transport, electrochemistry, as well as the intrinsic steam-reforming kinetics. In the paper, the methane/steam reforming process over the Ni/YSZ catalyst is experimentally investigated to verify the used chemical reaction model. A three-dimensional digital microstructure representation of the commercial anode is analyzed using a Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) and the nickel-pore contact surface is calculated to relate the reforming reaction rate to the catalyst's active area. Based on the complete DIR-SOFC model, a parametric study is carried out, to simulate the dynamic response of a fuel cell for different design and operating conditions. The results prove the dominant impact of inlet fluid temperature and methane content on the calculated distribution of hydrogen across the channel, while the collected current density was found to be a less important factor. The simulations indicate, that in the case of the direct reforming, fuel starvation is likely to occur in the upstream of the anode channel, where the reforming reaction does not keep up with producing hydrogen. The obtained results provide a significant insight into safe and efficient control strategies for Solid Oxide Fuel Cells.

High entropy Cr x AlFeCoNi alloys with x = 0, 0.5, 1.0 and 1.5 were synthesized using arc-melting and sintering preparation techniques. Three crystal structures (fcc, bcc and s) were observed using XRD technique, while EDX measurements showed the presence of up to three chemically different phases (FeCr-rich phase with fcc structure, AlNi-rich phase with bcc structure and Cr-rich phase with bcc and/or s structures). The reasons for the observed phase coexistence were addressed to total energy electronic structure calculations using KKR-CPA method accounting for chemical disorder effects. Such theoretical analysis confirmed that the multi-phase system was energetically more favorable than the single-phase one. Furthermore, DSC measurements allowed to identify two phase transitions in melted samples, unlike sintered ones due to high-temperature nitrogen corrosion. This process turned out to be highly selective, resulting in the formation of the scales consisting of AlnNm - phases at the cost of total Al loss in the HEA alloy. Crown

High entropy Cr x AlFeCoNi alloys with x ¼ 0, 0.5, 1.0 and 1.5 were synthesized using arc-melting and sintering preparation techniques. Three crystal structures (fcc, bcc and s) were observed using XRD technique, while EDX measurements showed the presence of up to three chemically different phases (FeCr-rich phase with fcc structure, AlNi-rich phase with bcc structure and Cr-rich phase with bcc and/or s structures). The reasons for the observed phase coexistence were addressed to total energy electronic structure calculations using KKR-CPA method accounting for chemical disorder effects. Such theoretical analysis confirmed that the multi-phase system was energetically more favorable than the single-phase one. Furthermore, DSC measurements allowed to identify two phase transitions in melted samples, unlike sintered ones due to high-temperature nitrogen corrosion. This process turned out to be highly selective, resulting in the formation of the scales consisting of Al n N m ephases at the cost of total Al loss in the HEA alloy.

The models of solid oxide fuel cells (SOFCs), which are available in the open literature,may be categorized into two non-overlapping groups: microscale or macroscale. Recent progressin computational power makes it possible to formulate a model which combines both approaches,the so-called multiscale model. The novelty of this modeling approach lies in the combination ofthe microscale description of the transport phenomena and electrochemical reactions’ with thecomputational fluid dynamics model of the heat and mass transfer in an SOFC. In this work,the mathematical model of a solid oxide fuel cell which takes into account the averaged microstructureparameters of electrodes is developed and tested. To gain experimental data, which are used toconfirm the proposed model, the electrochemical tests and the direct observation of the microstructurewith the use of the focused ion beam combined with the scanning electron microscope technique(FIB-SEM) were conducted. The numerical results are ...

In this research, we investigate the connection between an observed enhancement in solid oxide fuel cell stack performance and the evolution of the microstructure of its electrodes. A three dimensional, numerical model is applied to predict the porous ceramic-metal electrode performance on the basis of microstructure morphology. The model features a non-continuous computational domain based on the digital reconstruction obtained using focused ion beam scanning electron microscopy (FIB-SEM) electron nanotomography. The Butler–Volmer equation is used to compute the charge transfer at reaction sites, which are modeled as distinct locally distributed features of the microstructure. Specific material properties are accounted for using interpolated experimental data from the open literature. Mass transport is modeled using the extended Stefan–Maxwell model, which accounts for both the binary, and the Knudsen diffusion phenomena. The simulations are in good agreement with the experimental ...

In this research, a fully three-dimensional, multiphase, microstructure-scale heterogeneous (non-continuous) electrode, Solid Oxide Fuel Cell (SOFC) stack model is implemented in order to assess the impact of homogeneity disturbance in an SOFC anode. The Butler–Volmer model is combined with recent empirical relations for conductivity and aspects of the Maxwell–Boltzmann kinetic theory describing the transport of mass within the porous medium. Methods for the localized quantification of electrode morphology parameters (such as triple phase boundary length) are implemented. The exchange current distribution in the electrode, the partial pressures and the electric potential fields for each phase are computed numerically. In order to simulate heterogeneity, transfer barriers of varying placement and size are added to an otherwise homogeneous, virtual microstructure based on data from FIB-SEM tomography. The results are compared to a model based on the continuous electrode theory, and th...

The effect of calcination temperature on the structural properties and phase formation of synthesized CaO-Al2O3 nanopowder was investigated and discussed. The calcination products were identified by differential thermal analysis (DTA) and the crystalline phase formation was analyzed by X-ray diffraction (XRD). The obtained results showed that the crystallization started at 460 °C. Finally, the microstructures of the nanoparticles were observed by scanning (SEM) and transmission electron (TEM) microscopes. The investigation showed that an increase in the calcination temperature led to the appreciable increase in the crystallite size and the crystallinity of the final product. The obtained data confirmed that the prepared materials were mayenite with different surface area in the range of 71.18 m2/g to 10.34 m2/g after annealing in the temperature range of 470 °C to 960 °C.

ÐThe microstructure and micro-mechanical properties of superplastically deformed Y-TZP ceramics and Y-TZP/Al 2 O 3 composites were investigated. The grain growth on average of the pure Y-TZP ceramics during deformation at the temperature 1008C below the sintering temperature was very limited, while extensive growth of Y-TZP grains in Y-TZP/alumina composites were found under the same condition. The segregation of Y 3+ in single phase Y-TZP and the glass phase present at triple points in the composites was believed to be responsible for the dierent grain growth behavior. The change of mechanical properties of the materials were found to mainly relate to the microstructural change of the materials during deformation. In the case of single phase Y-TZP ceramics, both Vicker's hardness and indentation toughness do not apparently change by deformation, but the indentation toughness of the Y-TZP/alumina composites decreased noticeably due to the extensive grain growth of Y-TZP and thereafter the occurrence of monoclinic zirconia phase.

The aim of the study was to characterize nanostructured Al2O3-13TiO2 coatings deposited by plasma spraying on a grit blasted steel substrate. The Al2O3-13TiO2 coatings were characterized using scanning and transmission electron microscopy and X-ray diffraction techniques. Obtained coatings possessed a unique microstructure consisting of fully melted regions with the microstructure similar to a typical plasma sprayed lamellar morphology as the conventional coatings and areas comprising unmelted or partially melted nanosized particles. The analysis showed that most of the α-Al2O3 phase from the nanostructured powders transformed into γ-Al2O3 phase after plasma spraying process. Moreover, the presence of amorphous phase was also observed.