Microscopy Conference 2023

Prof. Dr.

Session chairs

Prof.

Correlative light and electron microscopy of poly(ʟ-lactic acid) spherulites for automatic detection

MS 7: Ceramics and composites

Abstract text (incl. figure legends and references) Crystalline polymers, such as polyethylene, are used in various applications because of their lightweight, low cost and excellent processability. The physical properties of crystalline polymers largely depend on their hierarchical solid-state structures with sizes spanning the nanometer to micrometer range. Understanding the hierarchical structure is essential for the structural design of crystalline polymeric materials. For this reason, it is desirable to have a method to analyze each level of the hierarchical structure at various scales. Polarized optical microscopy (POM) has often been used to observe spherulites because the molecular orientation in the spherulite can be observed from the change in polarization state when linearly polarized light is incident on the sample. However, the resolution of POM is limited to a feature size of ∼250nm, making it difficult to detect small spherulites on a submicron scale. The limitation of POM can be complemented using electron microscopy, such as transmission electron microscopy (TEM). One of the technical problems associated with TEM observations is the ultra-thin sectioned samples, which are often deformed during microtoming. We observed poly(ʟ-lactic acid) spherulite using POM and TEM in the same field of view. We attempted to combine the advantages of each technique - namely, molecular orientation information by POM and detailed scale morphology information by TEM [1]. To match the field of view of the TEM and POM images, a large number of TEM images were stitched together to produce a high-resolution large-area TEM image. After that, the deformations of the TEM ultrathin sections were corrected by reference to the POM images at the exact position of the sample, and large-area TEM images without deformations were successfully produced (Fig. 1). We demonstrated a method to detect spherulites from the acquired large-area TEM image using YOLO, a well-known object detection system using convolutional neural networks. We obtained spherulites' number density and space-filling factor (relative crystallinity) within the desired region (Fig. 2). Such local structural information is key to understanding the nucleation and growth mechanisms of polymer spherulites. This detection process is speedy and much more powerful than manual processing and therefore is expected to analyze large amounts of image data. Furthermore, the present method would be more effective than the conventional methods in studying the heterogeneously distributed spherulites in materials (e.g., preferential nucleation at the filler surface in polymer composites). References: [1] Konyuba, Yuji, et al. "Correlative light and electron microscopy of poly (ʟ-lactic acid) spherulites for fast morphological measurements using a convolutional neural network." Microscopy 71(2) (2022): 104-110. Fig. 1. Same field of view of PLLA by polarized microscope (POM) and transmission electron microscope (TEM). (a) POM image. (b) Large-area TEM image of the same field of view as the POM image (deformations caused during ultrathin section preparation have been corrected). (c) Merged image of the POM and TEM images. Fig. 2. Results of spherulite detection by the convolutional neural network. (a) Image of a 250 µm × 250 µm area from the large-area TEM image. (b) TEM image after spherulite detection. Each red frame shows a detected spherulite and its width and height.

Poster

Cryo-STEM analysis of electron beam sensitive anodic aluminum oxides

MS 4: Functional thin films

Abstract text (incl. figure legends and references) Aluminum alloys with predefined alloying elements are chosen regarding their performance in certain areas of industrial applications. Unfortunately, the natural forming protective alumina layer is suppressed in most alloys. Thus, the anodization of aluminum alloys is mandatory to achieve chemical resistance. The existence of segregations and stress fields inside the Al matrix hinders the formation of highly ordered anodic aluminum oxides (AAOs) as achievable using high-purity aluminum samples. Depending on the thickness of the sample and anodization process, the electron beam damage of AAOs during transmission electron microscopy (TEM) investigations varies at a high level. This leads to severe artefacts and finally to the evaporation of the sample when performing experiments at room temperature. Previous scanning transmission electron microscopy (STEM) studies have shown that cryogenic temperatures can partially reduce the beam damage in electron beam sensitive materials as organics or AAOs [1,2]. In this study, the sample is characterized by nanobeam diffraction pattern (NBDP) analysis and electron dispersive x-ray spectroscopy (EDS) at cryogenic temperatures. The focus is set on the correlation of structural and chemical characterization of the participating segregations inside the AAOs and at the interface. For comparison the same measurements are done inside the Al 6xxx alloy close to the interface to the AAO. Polished Al 6xxx samples are anodized with various anodization processes. An enhancement of the imaging quality of the high angle annular dark field (HAADF) detector is achieved by coating the pores with ZnO using atomic layer deposition (ALD). The sample preparation and milling are conducted with a focused ion beam using either Ga-ions or a mixture of Si/Au-ions. The samples were analyzed with a Gatan Double Tilt Liquid Nitrogen Cooling Holder 636 using a Titan Themis 60 – 300 at a temperature of ~96K. EDX spectra and energy-filtered NBDP are recorded with a quadrupole SuperXG2 detector and a CCD-GIF camera, respectively. An exemplified measurement of the correlation between chemical and structural information is given in figure 1. In total, a dataset of 10.000 NBDP is acquired on a part of the subsequently conducted EDX map. The beam damage during acquisition was reduced in comparison to measurements at room temperature. The combination of both methods shows the amorphous AAO tubes which are coated with polycrystalline ZnO. However, the segregations inside the AAOs are remaining crystalline and are only partially anodized. Based on the results, the segregations remain crystalline inside the amorphous AAOs. The decrease of chemical concentration assumes that one has to distinguish between segregation consisting of valve metals (e.g. Si, Al, Mg, etc.) and other elements (e.g. Mn). [1] H. Ayoola, C.-H. Li, S. D. House, C. S. Bonifacio, K. Kisslinger, J. Jinschek, W. A. Saidi, and J. C. Yang.Ultramicroscopy 219 (2020), 113127 [2] A. M. Jasim, X. He, Y. Xing, T. A. White, and M. J. Young. ACS Omega 6.13 (2021), 8986–9000. Fig. 1: STEM – EDX maps, HAADF and NBDP to analyze segregations inside the formed AAOs. As indicated in the HAADF signal, a smaller map for NBDP is recorded along the segregation. Areas which are enriched with Al are amorphous, whereas Zn enriched regions show polycrystalline NBDP. The diffraction pattern of the segregations indicate that the corresponding structure remains crystalline.

Electro-optical properties for BCZT and vanadium-doped BCZT nanoparticles using valence electron energy loss spectroscopy

MS 1: Energy-related materials and catalysts

Abstract text (incl. figure legends and references) Valence electron energy loss spectroscopy, in transmission electron microscopy mode (VEELS-TEM) is an important tool to elucidate the complex dielectric function (CDF) and opto-electronic properties (O-EP) of insulators. The motivation of this work is to present a comparative study of CDF and the O-EP of lead-free Ba0.8Ca0.2Ti0.9Zr0.1O3 (BCZT) and Ba0.8Ca0.2Ti0.85Zr0.1V0.05 (BCZTV) electro-ceramics. The samples were prepared by the modified Pechini method to obtain nanoparticles. A single tetragonal perovskite structure, considering the P4mm space group was stabilized at 700 oC for 1 h (1). This structure was confirmed by Rietveld refinement of the x-ray diffraction pattern for both compositions using Fullprof suite software. The microstructural parameters such as average crystallite size and shape in the nanometer range were simulated using the spherical harmonics method implemented in the GFourier program. The VEELS–TEM experiment scanned the energy interval from 0 to 50 eV was obtained with an electron energy loss spectrometer (EELS GAT–777 STEMPack) attached to a JEM2200FS (200 kV). All measurements were performed at room temperature. The system has an energy resolution of 1.00 eV, determined by the full-width-half-maximum of the zero-loss peak. The electron probe size was below 1 nm. The energy dispersion was set to 0.05 eV/channel, to maintain the energy resolution. The spectrum was collected with a parallel beam (convergence semi-angle α = 0 mrad). The corresponding collection semi-angle was β = 20.3 mrad. For the EELS acquisition, 10 consecutive exposures were accumulated. The deconvolution and subtraction of the zero-loss peak (ZLP) from the VEELS region, the Fourier log method to remove plural scattering and Kramers-Kronig analysis performed in the single scattering distribution were carried out using the routines available in the Digital Micrograph™ Gatan Microscopy Suite (GMS 3) software. Absolute thickness of samples was determined via log ratio method with a value of 51.1 nm. The spectroscopic analysis started with the chemical identification of the atoms that conforms the BCZT solid-solution. Bulk and surface plasmon were located at 27.2 eV and 12.9 eV for BCZT and 26.9 eV and 12.3 eV for BCZTV, respectively, in the normalized energy loss function. Dielectric constant was calculated from the real part of the complex dielectric function, while the inter-band transitions were identified in the joint density of states function. The refraction index n and the extinction coefficient k, as a function of energy, were obtained from the complex dielectric function. The band gap energy was determined using a polynomial fit in the optical absorption coefficient plot with an Eg = 3.2 eV (BCZT) and 3.18 eV (BCZTV) considering the relativistic correction using the difference method. The Cole-Cole plot corroborates the electronic contributions by the presence of two semicircles due to the inter-band transitions and plasmon region. As an important remark is that Vanadium plays an important role in the microstructural effects, in the covalent character and also in the opto-electronic properties of BCZT compounds. G. Herrera-Perez would like to thank Mexico Catedra-CONACYT Grant No. 2563, SNI 1-CONACYT for complementary support and CONACyT–SEP Basic Research Project No. 253605. References: (1) G. Herrera-Pérez, et al. Micron 149 (2021) 103124.

The nanoscale structure of relaxor ferroelectric Na1/2Bi1/2TiO3-BaTiO3 ceramics: A TEM investigation

Abstract text (incl. figure legends and references) The EU directive for the Restriction of Hazardous Substances (RoHS) has driven researchers to look for alternatives in order to replace lead-containing components in electronic devices, due to growing concerns over health and environmental risks. The lead-free (1-x)Na1/2Bi1/2TiO3-xBaTiO3 (NBT-BT) system has gained considerable attention, especially for high-power and high-frequency applications, such as ultrasonic transducers. The relaxor ferroelectric solid solution exhibits enhanced piezoelectric properties at its morphotropic phase boundary (MPB) near x = 0.06. Relaxors are characterized by the presence of nanometer-sized polar regions (PNRs/nanodomains), which allow for a high electromechanical response and high permittivity values. In the relaxor-ferroelectric transition of NBT-BT ceramics, the electromechanical properties are highly linked to a complex domain structure, which strongly varies with composition. The local structure and phase assemblage of NBT-BT ceramics with 6, 9 and 12 mol % BaTiO3 (BT) was studied by using different imaging techniques and electron diffraction in transmission electron microscopy (TEM). Deploying dark-field (DF) imaging of superlattice reflections, the relaxor nanostructure is visualized, consisting of densely packed tetragonal P4bm nanodomains and a minor fraction of rhombohedral R3c nanodomains. The 5-30 nm sized P4bm regions display an elongated shape and are oriented along the {001}pc planes. Mapping the polar displacement in high-resolution high-angle annular dark field (HAADF) images reveals a heterogeneous local structure with short-range fluctuations of a few nanometres in size (PNRs). The nanoscale structure correlates to the frequency dispersion of the temperature-dependent permittivity, typical for relaxors. With increasing BT fraction, long-range ferroelectric lamellar domains of tetragonal P4mm symmetry emerge. Within these ferroelectric domains, the nanodomain contrast considerably declines, which is associated with the disappearance of octahedral tilting and a transition to the non-tilted P4mm structure, reflected in a more ferroelectric nature of BT-rich compositions. Nevertheless, a reduced population of both P4bm and R3c nanodomains remains embedded within the lamellar ferroelectric domains, indicating a complex hierarchical domain arrangement. The findings emphasize the important role of a direct microscopic analysis in order to elucidate on the nanostructure-property correlation in relaxor ferroelectric solid solutions.

Probing the 3-dimensional structure of lead zirconate-based antiferroelectrics with a combined approach of TEM and NMR spectroscopy

Abstract text (incl. figure legends and references) The classical model of the prototype antiferroelectric material PbZrO3 with antiparallel pairs of Pb displacements of equal magnitude has long been considered a principal guide for the understanding of antiferroelectricity. However, this simplistic picture has raised great concern as it is yet to be found in a real material, and a detailed examination of the 3-dimensional Pb positions is still lacking. Here, not only the in-plane displacement but also the out-of-plane environment of Pb in PbZrO3 and (Pb,La)(Zr,Sn,Ti)O3 (PLZST) perovskite oxides is considered by jointly employing high-resolution scanning transmission electron microscopy (HRSTEM) and solid-state nuclear magnetic resonance (NMR) spectroscopy. Displacement studies of PbZrO3 using the high-angle annular dark-field (HAADF) technique revealed an ordered four-fold modulation, which consists of two distinct segments that are nearly perpendicular to each other, contrasting the Kittel model. This ordered in-plane configuration is complemented by two discrete 207Pb NMR signals reflecting alternating displacement magnitudes out-of-plane. On the other hand, PLZST is characterized by an inhomogeneous distribution of the Pb displacement modulation layers and Pb-O bond width calculated from annular bright-field (ABF) image, further manifested by a distribution of 207Pb resonance frequencies revealing a significant increase in the displacement distribution width. The combination of TEM and NMR techniques allows for a complementary exploration of the 3-dimensional crystal structure, correlating the microstructural displacements with the bulk displacement distribution both qualitatively and quantitatively. The findings rationalize our understanding of the complex polarization configuration in PbZrO3-based antiferroelectrics that go beyond the Kittel model and highlight their highly disordered nature instead.

TEM characterization of pseudomorphic polymer-derived ceramic papers: sample preparation and microstructure analysis

LS 7: Advances in sample preparation

Abstract text (incl. figure legends and references) Through the pyrolysis of polymeric precursor infiltrated cellulose-based paper templates, ceramic composites with paper-like morphology can be successfully manufactured. Such polymer-derived ceramic papers (PDCPs) are pseudomorphic after their template, closely resembling the original paper structure, with randomly interwoven carbonized fibers encased by a polymer-derived ceramic layer. For this study, several PDCPs were prepared with varying synthesis parameters by pyrolysis of cellulose paper templates dip-coated with two different polysilazane-based single-source precursors, each modified with one of three different transition metals (Fe, Ni, Pd). The resulting ceramic papers were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) analyses in combination with X-ray diffraction (XRD) as well as Raman and Fourier-transform infrared (FTIR) spectroscopy. In order to reveal the in-situ generated microstructure, electron transparent thin-foils of the filigree and inherently brittle ceramic composites were prepared for TEM investigations. For this purpose, conventional polishing in combination with ion thinning as well as ultramicrotomy slice-cutting was comparatively employed to assess their suitability for these materials and to evaluate preparation artifacts introduced. Overall, ultramicrotomy preparation led to a much better sample quality, allowing the core-shell structure of the composite ceramic fibers to be resolved in great detail (Figure 1). This points out that ultramicrotomy has high potential for TEM sample preparation of ceramic material systems, although it is seldomly used. Based on the results, the phase evolution and the role of the cellulose template during pyrolysis is discussed. Figure 1: SEM BSE image (left) of a SiFeO(C,N)-based ceramic paper pyrolyzed in ammonia at 1000 °C and TEM BF image (middle) of an ultra-thin fiber cross-section prepared by ultramicrotomy preparation revealing the core-shell structure of the ceramic composite fibers. The electron diffraction patters (right) show that the ceramic layer is amorphous and contains numerous nanosized α-Fe particles, while the cellulose-derived fiber consists of semicrystalline turbostratic carbon.

Investigations on the amorphization of lithium-containing glass-ceramics during mechanochemical recycling

MS 6: Geoscience and construction materials, cultural heritage

Abstract text (incl. figure legends and references) Due to its critical role in batteries, lithium has recently become a rather important element. Moreover, many efforts are being made to mine primary lithium sources, while lithium recycling and recovery are almost lacking. In addition to batteries, glasses and glass ceramics are an important and as yet unexploited source of lithium. Recently, we published an article on a novel mechanochemical recycling approach using end-of-life glass-ceramics from spend cooktops as feedstock for lithium extraction and simultaneous zeolite synthesis [1]. The experimental results indicate an effective particle size reduction and amorphization of the crystal structure of the high quartz lattice by intensive milling during the alkaline leaching process as the main driving forces for the reaction, significantly accelerating lithium extraction in parallel to zeolite synthesis. The aim of this study is to investigate the physiochemical changes and amorphization of glass-ceramic material during ball milling without any chemical side reaction (mechanical activation). In order to study the amorphization process, the glass-ceramic powder was mechanically activated in a planetary ball mill at 600 rpm in deionized water. After different times of 30, 60 and 120 min of ball milling, samples were taken and characterized by powder X-ray diffraction (PXRD), N2 adsorption/desorption analysis and transmission microscopy (TEM) including selected area electron diffraction (SAED). Analysis of N2 adsorption/desorption by the Brunauer-Emmet-Teller (BET) method showed an increase in surface area with time, which can be attributed to a significant reduction in particle size due to intensive ball milling. With increasing time of mechanical activation, the PXRD measurements showed a decrease and broadening of the diffraction peaks. Moreover, the calculation on the crystallite size using Scherrer&quot;s equation resulted in a clear trend to smaller crystallites with an increase of mechanical activation. Finally, TEM studies reveal the microstructural features of the samples, while the SAED patterns confirm the PXRD measurements and show the transformation of a well-ordered material into a (nano)crystalline or partially amorphous material. The analytical results of BET, XRD and TEM/SAED measurements confirm a stepwise amorphization of the high-quartz structure and the transformation of a well-ordered source material into a (nano)polcrystalline material. Therefore, amorphization can be rationalized as the driving force for the mechanochemical reaction during lithium extraction and zeolite synthesis. [1] Necke, T.; Wolf, D.M.; Bachmann, A.L.; Berberich, K.; Kleebe, H.-J.; Weidenkaff, A. Mechano&shy;chemical Lithium Extraction and Zeolite Synthesis from End-of-Life Glass−Ceramics, ACS Sustainable Chemistry &amp; Engineering (advance online publication) 2022. https://pubs.acs.org/doi/10.1021/acssuschemeng.2c02342

Prof. Dr. rer. nat.

Microstructural evolution of novel Si(M)(BC)N polymer-derived ceramics upon different heat treatments

Abstract text (incl. figure legends and references) Polymer-derived ceramics (PDCs) are promising candidates used as protective coatings on hard metal substrates for applications, requiring operating temperatures &lt;1300°C. The polymer-to-ceramic synthesis route gives access to prepare ceramic coatings, such as environmental (EBC) or thermal barrier coatings (TBC) via several inexpensive liquid-phase deposition methods. Typically, PDCs consist of a highly refractory phase, such as early transition metal nitrides, carbides or borides, which are finely dispersed in a silica-former matrix phase (e.g., silicon carbide, silicon nitride, silicon(boro)carbonitride). As the microstructure and phase composition of a material strongly influence its high-temperature properties, a profound understanding of the microstructural development during its different processing steps is indispensable. During the synthesis of a PDC the starting preceramic polymers are firstly crosslinked at temperatures ranging from 200-1000°C and subsequently annealed at temperatures &lt;1500°C, which induces phase separation and crystallization processes. To obtain dense monolithic materials, as-pyrolyzed powder samples are consolidated via spark plasma sintering. Dense monolithic ceramics are highly needed to gain information on mechanical properties and oxidation behaviour. In this context a series of novel PDCs were synthesized according to the polymer-to-ceramic route. As-pyrolyzed and subsequently annealed powder samples as well as spark plasma sintered (SPS) monolithic bulk materials in the Si(M)(BC)N system (M=Hf,Ta) were compared regarding their microstructural development upon different heat-treatments. Pyrolysis of Si(M)N powder samples was carried out at 1000°C under NH3 gas followed by an annealing at 1600°C in N2 atmosphere. As-pyrolyzed Si(M)(BC)N powders were sintered via SPS at 1950°C in vacuum employing different sintering parameters (different pressure &amp; dwell times). The synthesized materials have been investigated via electron microscopy concerning their temperature-dependent microstructural evolution upon pyrolysis and subsequent heat treatments. Si(M)N powder samples as well as dense Si(M)(BC)N bulk materials were analysed via scanning (SEM) and transmission electron microscopy (TEM) with respect to their chemical composition, crystallization behaviour as well as their phase composition and distribution. The annealed Si(M)N powder samples revealed a microstructure consisting of two regions named bulk and surface regions, which differentiate in their intrinsic microstructure and phase composition. Bulk regions contain a silicon nitride-based matrix phase with finely dispersed nano-sized transition metal carbides (TMCs). In comparison, surface regions underwent phase separation, decomposition reactions and diffusion-controlled coarsening phenomena, showing a completely different microstructure, consisting of silicon carbide and grain coarsened TMCs. Similar microstructures were also found in the studied SPS dense monolithic materials. Former powder particles are consolidated via sintering, showing exactly the same above-mentioned two regions within the internal microstructure: surface and bulk. Furthermore, it is discussed, whether the microstructural evolution in powder samples compared to dense monolithic materials consolidated via SPS, show an almost identical microstructural development. A detailed microstructural evolution model for both, powder samples and dense monolithic materials is introduced.

Piezotronic ZnO bicrystals: A TEM study on interfacial segregation and properties of (0001) inversion boundaries

Abstract text (incl. figure legends and references) Zinc oxide is well-known for its non-linear current-voltage (I‑V) characteristics and its respective application as varistor (variable resistor) ceramic.1 The varistor effect itself is attributed to dopant-induced electrostatic potential barriers at ZnO grain boundaries.1,2 In case of so‑called piezotronic materials, semiconducting and piezoelectric properties are combined in order to tune such potential barriers. Therefore, ZnO – featuring piezoelectricity as well as intrinsic semiconduction – has received renewed scientific interest as piezotronic material and is considered to hold a large potential for the development of novel devices, such as strain-triggered transistors, diodes or sensors.3,4 However, the dopant-related formation of varistor-type potential barriers in ZnO as well as the related semiconducting properties are still not completely elucidated, in particular, against the background of novel piezotronic applications, which imply conditions different from the established polycrystalline ZnO varistor ceramics. In the presented study, ZnO bicrystals with (0001) inversion boundaries were analyzed using atomic resolution (S)TEM methods, with a special focus on orientation and dopant segregation.5,6 In doing so, different synthesis methods were applied and their effect on the interface properties was investigated. Based on the TEM findings and corresponding I-V measurements, a distinct correlation between interface coherency, degree of segregation and degree of non-linearity was observed. Since this correlation is attributed to basic principles of segregation and interfacial properties, the presented results are not only seen as a valuable contribution to the field of piezotronics and varistor ceramics, but also as adaptable for other material systems. D. R. Clarke, "Varistor Ceramics," J Am Ceram Soc, 82[3] 485-502 (1999). Y. Sato, T. Yamamoto, and Y. Ikuhara, "Atomic Structures and Electrical Properties of ZnO Grain Boundaries," J Am Ceram Soc, 90[2] 337-57 (2007). X. D. Wang, J. Zhou, J. H. Song, J. Liu, N. S. Xu, and Z. L. Wang, "Piezoelectric Field Effect Transistor and Nanoforce Sensor Based on a Single ZnO Nanowire," Nano Lett, 6[12] 2768-72 (2006). Z. L. Wang and W. Z. Wu, "Piezotronics and Piezo-Phototronics: Fundamentals and Applications," Natl Sci Rev, 1[1] 62-90 (2014). M. Trapp, P. Keil, T. Frömling, J. Rödel, and H.-J. Kleebe, "Segregation and Properties at Curved vs Straight (0001 over bar ) Inversion Boundaries in Piezotronic ZnO Bicrystals," J Am Ceram Soc (2019). P. Keil, M. Trapp, N. Novak, T. Frömling, H.-J. Kleebe, and J. Rödel, "Piezotronic Tuning of Potential Barriers in ZnO Bicrystals," Adv Mater, 30[10] (2018).

The prevention of SiC-fiber decomposition via the incorporation of a thin buffer layer in ZrB2-based ultra-high temperature ceramics

Abstract text (incl. figure legends and references) A ZrB2–based ultra-high temperature ceramic (UHTC), containing short Hi-Nicalon SiC fibers, was separated from the outermost oxidation resistant ZrB2-MoSi2 layer by sandwiching it with a Mo-impermeable buffer layer, in order to prevent fiber decomposition. This additional layer consisted of ZrB2 doped with either Si3N4 or with the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. Scanning in addition to transmission electron microscopy imaging (SEM/TEM) and elemental mapping via energy-dispersive X-ray spectroscopy (EDS) showed that this tailored sample geometry provides an effective Mo-diffusion barrier, preventing the SiC fibers from deterioration due to their reaction with Mo or Mo-compounds. In contrast, the structure of the SiC fibers in a reference sample without such a buffer layer is strongly degraded by MoSi2 diffusion into the fiber core. The comparison of the three buffer-layer systems showed a moderate alteration of the fiber structure in the case of Si3N4 addition, whereas in the PDC-doped samples hardly any structural change within the SiC fibers was observed. A stepwise reaction mechanism is proposed, based on the continuous progression of a reaction zone that propagates towards the ZrB2-MoSi2 top layer. The progression of such a reaction zone, which is seen as a consequence of the different eutectic melts forming in the different layers, i.e., first in the SiC-fiber containing bulk, followed by the buffer layer itself, and finally in the top ZrB2-MoSi2 layer at high temperature, allows for an effective separation of the oxidation resistant top layer from the SiC fibers that are embedded in the bulk. Subsequent oxidation of the UHTCs at 1500°C and 1650°C for 15 min did not affect the efficacy of all three buffer layers containing Si3N4, SiCN or SiHfBCN, since no structural changes within the corresponding buffer layer as well as of the fibers were observed, as compared to the non-oxidized samples.

PS 23
Poster session MS 7: Ceramics and composites

Appointment

Session chairs

Programme

PS 23Poster session MS 7: Ceramics and composites

Appointment

Session chairs

Programme

PS 23
Poster session MS 7: Ceramics and composites