The rapid progress in space exploration and hypersonic aviation has intensified the demand for materials capable of withstanding extreme temperatures (>2000 degrees C) to protect underlying metallic structures. Ceramic matrix composites, especially carbon-carbon (C/C) composites, with protective SiC coatings, possess good oxidation resistance, but their effectiveness is limited to similar to 1600 degrees C due to the active oxidation of SiC above 1500 degrees C. This limitation drives the need for new materials for next-generation hypersonic thermal protection systems. ZrB2-B4C-SiC-LaB6 composites are attractive for ultra-high-temperature applications, and they are known to provide oxidation resistance at > 2000 degrees C. In this study, ZrB2-B4C-SiC-LaB6 composites of two different configurations, 3-layered functionally graded material (FGM) and equivalent nongraded composite (NGC), were fabricated using hot pressing. A detailed comparative analysis of the structural, mechanical, and thermal shock behaviour of the FGM and NGC was performed. The interlayer regions of the FGM remained intact even after the 3-point bending test and thermal shock from Delta T = 800 degrees C. The highest average bending strength (similar to 376 MPa) was achieved in the FGM when the ZrB2-rich layer experienced tensile stress, which was similar to 20 % higher than the NGC samples. The thermal shock behavior of the composites was studied using a combined 3-point bending test and non-destructive ultrasonic phase spectroscopy. While the critical thermal shock temperature was similar for both material types (similar to 350 degrees C), the overall thermal shock resistance of the NGC material was marginally better owing to the favorable thermal residual stress distribution in the as-fabricated materials.

Transparent polycrystalline magnesium aluminate (MAS) spinel ceramics are of great interest for industry and academia due to their excellent optical and mechanical properties. However, shaping of MAS is notoriously challenging especially on the microscale requiring hazardous etching methods. Therefore, a photochemically curable nanocomposite is demonstrated that can be structured using high-resolution two-photon lithography. The printed nanocomposites are converted intro transparent MAS by subsequent debinding, sintering, and hot isostatic pressing. The resulting transparent spinel ceramics exhibit a surface roughness Sq of only 10 nm and can be shaped with minimum feature sizes of down to 13 µm. This technology will be important for the production of microstructured ceramics used for optics, photonics, or photocatalysis.

Transparent ceramics such as magnesium aluminate spinel (MAS) are an outstanding class of materials that combine high optical transparency with remarkable mechanical, chemical and thermal strength. They are particularly interesting for micro-optical applications, since MAS offers a high refractive index in combination with a low optical dispersion, which is inaccessible for other materials including glasses and polymers. However transparent polycrystalline MAS is notoriously difficult to microstructure. Methods such as hot pressing or slip casting only allow simple geometries like plates or domes to be manufactured. More complex geometries require time-consuming and cost-intensive postprocessing. We have therefore developed a thermoplastic nanocomposite that can be structured with high accuracy by injection molding. The nanocomposite can subsequently be transformed into a transparent polycrystalline MAS ceramic with a transparency close to the theoretical maximum by thermal debinding, sintering and hot isostatic pressing (HIP). This innovative process makes transparent ceramics for optics and photonics available at low cost and with high production rates.

Glass devices of pure and clean silica have excellent properties such as a wide range of high transparency in the visible and IR range and a good resistance against a large number of chemicals. Conventional glass processing is mostly based on melting processes, and that is the reason for the use of high temperatures for processing SiO2. In order to lower these temperatures, on one hand, network formers can be used, whereby the material composition is influenced or sintering techniques based on particulate systems can be applied. By the use of silica nanopowders, the process temperature can be reduced and the forming process is no longer limited to the melting process.

Ta–Al2O3 composite samples with different compositions are prepared using Field Assisted Sintering Technique (FAST). Two different alumina powders are used to investigate the influence of the starting powders particle size on the microstructural features and the resulting electrical conductivity of the prepared composite materials. Percolation threshold of the two material systems is influenced by the metal fraction, as well as the alumina particle size of the starting powder. The percolation threshold for the fine- and the coarse-grained alumina is found to be at 15 vol.-% Ta and 7.5 vol.-% Ta, respectively. Microstructural investigations show significant differences in terms of particle shape of both, Ta and Al2O3 after sintering, most likely being the reason for the different percolation thresholds of the investigated materials. Anisotropy effects resulting from the processing using FAST and the influence on electrical properties are also shown.

Field‐assisted sintering technique (FAST) is used for the preparation of Nb–Al2O3 composite materials. The electrical conductivity is investigated depending on the particle size of the used starting powders and under varying volume contents of the refractory metal in the starting powder mixture. The percolation threshold is investigated and found to be influenced not only by the metal fraction but also by the particle size of the alumina used for sample preparation. For the fine‐ and coarse‐grained alumina, a percolation threshold of 17.5 and 10 vol% Nb is estimated, respectively. Furthermore, the microstructure is investigated to gain a basic understanding of the dependency between microstructural features and the resulting material properties on the macroscopic scale. Also, the influence of the sintering process and the resulting microstructure–properties relationship is considered. It could be shown that the electrical properties are anisotropic because of anisotropy effects caused by the FAST process.

Lithium-ion batteries with solid electrolytes offer safety, higher energy density and higher long-term performance, which are promising alternatives to conventional liquid electrolyte batteries. Lithium aluminum titanium phosphate (LATP) is one potential solid electrolyte candidate due to its high Li-ion conductivity. To evaluate its performance, influences of the experimental factors on the materials design need to be investigated systematically. In this work, a materials design strategy based on machine learning (ML) is employed to design experimental conditions for the synthesis of LATP. In the variation of parameters, we focus on the tolerance against the possible deviations in the concentration of the precursors, as well as the influence of sintering temperature and holding time. Specifically, models built with different design selection strategies are compared based on the training data assembled from previous laboratory experiments. The best one is then chosen to design new experiment parameters, followed by measuring the corresponding properties of the newly synthesized samples. A previously unknown sample with ionic conductivity of 1.09 × 10−3 S cm−1 is discovered within several iterations. In order to further understand the mechanisms governing the high ionic conductivity of these samples, the resulting phase compositions and crystal structures are studied with X-ray diffraction, while the microstructures of sintered pellets are investigated by scanning electron microscopy. Our studies demonstrate the advantages of applying machine learning in designing experimental conditions by the synthesis of desired materials, which can effectively help researchers to reduce the number of required experiments.

Al-SiC and hybrid Al-SiC-graphite MMCs with up to 70 vol% SiC and 10 vol% graphite were fabricated using spark plasma sintering. Detailed studies of the microstructure, Vickers hardness, longitudinal elastic constant C11, as well as thermal shock resistance of the MMC samples were carried out. Non-destructive ultrasound phase spectroscopy was used to measure the C11 elastic constant of the MMCs, and the measured values were compared with different micromechanical models for particle reinforced composites. For both two- and three-component MMCs, the Hashin Shtrikman lower bound fitted best to the experimental results in the absence of any porosity. Thermal shock resistance of the MMCs was carried out by heating in air to 500 degrees C and subsequently quenching in water for ten times. The extent of thermal shock-induced structural alteration was determined via a systematic study of the change in density, hardness, and C11 of the samples. It is observed that the thermal shock resistance of the MMCs depends upon multiple factors such as SiC content, graphite content, and the amount of residual porosity. While the reduction in mechanical properties of the Al-SiC MMCs due to graphite addition is according to the expected line, the present study shows that graphite addition also reduces the thermal shock resistance of the MMC.

The aim of this work is to compare the stiffness of a graded composite and equivalent monolithic particulate reinforced composite experimentally. Hybrid Al-SiC-graphite composites were fabricated using spark plasma sintering. Longitudinal elastic constants were measured using non-destructive ultrasound phase spectroscopy. It is seen that for the graded composites, the stiffness depends solely on the reinforcement content, while the gradient structure has no influence. Comparison with analytical micromechanical models show that the Hashin Shtrikman lower bound predicts the elastic constants most accurately.

Cracks terminating at free surfaces are affected by local stresses in the surface region. Under residual compression, the crack front must retard, whereas residual tensile stresses lead to an advance, both compared with the crack contour in the absence of stresses. This effect can be used for an estimation of residual surface stresses in silica generated during the silica/water reaction and caused by volume swelling. A strong shielding stress intensity factor of about –2.5 MPa√m was found for Double Cleavage Drilled Compression specimens heat-treated for 192 h at 250°C in water vapour under saturation pressure and in liquid water. This result is a clear indication for compressive stresses developing in the water diffusion zone at the surface.

Transparent ceramics like magnesium aluminate spinel (MAS) are considered the next step in material evolution showing unmatched mechanical, chemical and physical resistance combined with high optical transparency. Unfortunately, transparent ceramics are notoriously difficult to shape, especially on the microscale. Therefore, a thermoplastic MAS nanocomposite is developed that can be shaped by polymer injection molding at high speed and precision. The nanocomposite is converted to dense MAS by debinding, pre-sintering, and hot isostatic pressing yielding transparent ceramics with high optical transmission up to 84 % and high mechanical strength. A transparent macroscopic MAS components with wall thicknesses up to 4 mm as well as microstructured components with single micrometer resolution are shown. This work makes transparent MAS ceramics accessible to modern high-throughput polymer processing techniques for fast and cost-efficient manufacturing of macroscopic and microstructured components enabling a plethora of potential applications from optics and photonics, medicine to scratch and break-resistant transparent windows for consumer electronics.

In previous reports, we dealt with the SIF computations of semielliptical surface cracks introduced in glass specimens by grinding. It can simply be shown that subcritical extension of such cracks must result in a reduction of stresses applied to the specimen. We apply this effect on tensile tests and on torsion tests by Aaldenberg et al. carried out on cylindrical specimens at temperatures between 550°C and 700°C in normal lab air environment. The torsion moments Mt measured by Aaldenberg et al. decreased with time t for a constant drill angle. This effect was interpreted by Aaldenberg et al. as a proof for a relaxation effect by the presence of molecular water. In the present report, we discuss the possibility that the reduction of the torsion moment is caused by subcritical crack growth that results in comparable effects.

Solid oxide fuel cells (SOFC) convert chemical energy from hydrogen, methane, or other hydrocarbons directly into electrical energy and heat. Advantages are low noise during operation as well as relatively low pollutant emissions. This makes them interesting for stationary applications, eg combined heat and power plants for domestic use and for mobile applications, when there is a demand for integrating auxiliary power units. The high operating temperatures of about 850 degrees C and the simultaneous presence by both, reducing and oxidizing atmospheres place high demands on the components of a SOFC. Due to these requirements, glass-ceramics are proposed as sealants between interconnector and electrolyte. They provide lower costs and lower weight than commercially used silver solders. Furthermore, they have the following impressive benefits: The sealants are electrical insulating, chemical stable and by careful materials selection and adapted manufacturing processes, they adhere well on steel and on ceramic substrates. In order to characterize the adhesion of glass-ceramic sealants on steel and on zirconia substrates, layer-like composites are fabricated by screen-printing and subsequent sintering in air. It turns out that the formation of crystalline phases at the interface is crucial for the adhesion behavior.

The measurement of material resistance against crack propagation is mostly performed with (in principle) one-dimensional cracks as usual for standard test specimens as for instance DCB (Double Cantilever Beam) specimens. Such a crack exhibits a constant stress intensity factor along the crack front that allows simple computations of crack extension. In the case of 2-dimensional cracks, e.g. semi-elliptical surface cracks, the calculation of strength and service life under subcritical crack growth is considerably more complicated. The main reason for this is the fact that the stress intensity factor changes along the crack front. In this report, averaged K-factors for virtual crack propagation in both axial directions of the semi-ellipse are determined. For this purpose, virtual crack area increments according to Cruse and Besuner are used. The stress intensity factors for the deepest point of the crack and the surface terminating points are given by polynomials obtained via curve fitting of the numerical results. In addition, the averaged K-factors under concentrated forces were estimated.

Silica shows the effect of subcritical crack growth in humid environments. Measurements in liquid water show increasing subcritical crack growth velocities when the temperature is increased as was shown by Wiederhorn and Bolz. Since this has been generally found for glasses, this effect is called normal subcritical crack growth. For measurements on silica in water vapour environment the astonishing effect of decreasing crack-growth rate ν at an increased temperature was observed for constant partial water pressure in the humid environment. This surprising result observed in v–K experiments by Suratwala and Steele is called anomalous subcritical crack growth behavior. In the present report we consider the effects of reduced water solubility at silica surfaces and crack-tip shielding as the reasons for anomalous subcritical crack growth. Three influences were found, namely the water vapour pressure, the temperature-dependent surface solubility of water, and crack-tip shielding. From our computation, we can conclude that silica shows normal subcritical crack growth, when it is taken into account that the real physical stress intensity factor KI is used that describes the stresses in the singular crack-tip field, i.e. when crack growth velocity v is plotted vs. Ktip.

A systematic methodology for developing hybrid Al–SiC-graphite functionally graded composite materials with the highest feasible SiC-content in the top-most layer, without resulting in residual porosities, has been established. The distribution of ceramic reinforcements in individual layers is very homogeneous, and the interlayer regions are free from any defect. Monolithic composites, having composition identical to the overall average compositions of the FGMs, were also fabricated, and their mechanical properties were compared with the corresponding FGM properties. Due to the locally high SiC-content, the top-layer hardness of the FGM is significantly higher than the corresponding monolithic composite. A systematic study of the flexural stress-strain behavior of individual mono-layers and the overall FGM was carried out at different orientations. For identical mono-layer compositions, the orientation of the FGM has a strong influence on its flexural stress-strain behavior. The failure stress is significantly higher when the layer containing high SiC-content is at the compressively loaded side during the 4-point bend test.

Lithium aluminum titanium phosphate (LATP) is known to have a high Li-ion conductivity and is therefore a potential candidate as a solid electrolyte. Via sol-gel route, it is already possible to prepare the material at laboratory scale in high purity and with a maximum Li-ion conductivity in the order of 1·10 −3 s/cm at room temperature. However, for potential use in a commercial, battery-cell upscaling of the synthesis is required. As a first step towards this goal, we investigated whether the sol-gel route is tolerant against possible deviations in the concentration of the precursors. In order to establish a possible process window for sintering, the temperature interval from 800 °C to 1100 °C and holding times of 10 to 480 min were evaluated. The resulting phase compositions and crystal structures were examined by X-ray diffraction. Impedance spectroscopy was performed to determine the electrical properties. The microstructure of sintered pellets was analyzed by scanning electron microscopy and correlated to both density and ionic conductivity. It is shown that the initial concentration of the precursors strongly influences the formation of secondary phases like AlPO 4 and LiTiOPO 4 , which in turn have an influence on ionic conductivity, densification behavior, and microstructure evolution.

When water diffuses into silica glass it reacts chemically forming nanometre sized pores that change the physical properties of the glass, for example, affect its strength. Here we discuss the effect of water on Young's modulus, and show how it is reduced by the water reaction, whereby a proportional behaviour applies to small amounts of water involved in the reaction. The value of the elastic modulus will be not linear with the hydroxyl-quantity in the glass for very high concentrations. The relationship between hydroxyl concentration and Young's modulus can be determined from measurements of sound wave velocity and will be represented by damage and pore models from literature.

In this study, silica based slurries for stereolithographic printing of glass structures are developed and characterized. Stereolithography has the potential to print complex structures with high resolution. Therefore, acrylate based photocurable slurries have been developed and their viscosities are examined as a function of the solid loading. A critical shear rate can be derived, which must not be exceeded during the printing process. Therefore, rheological characterizations provide important insights into the printing process and the ability to produce samples with precise structures. Other properties such as polymerization time and curability kinetic were investigated with time dependent attenuated total reflection infrared spectroscopy (ATR-IR). Afterwards, the slurries were printed on a commercial printer operating with visible light. For debinding the printed green bodies, the decomposition temperatures were derived from thermogravimetric analysis in order to obtain stable and transparent samples.

In this report the general influence of different elastic modules ahead of a crack tip and in the bulk on the stress intensity factors will be discussed again. From the FE results obtained in Report [1], and the theoretical solution by Merkle [3] K = K$_{appl}$$\sqrt{\frac{E}{E_{0}}}$ it becomes clear that the FE-results for relative zone sizes $\omega$/a >0 must deviate from the exact solution but are also in rather good agreement with the theoretic results for finite $\omega$/a. In this report also results are compiled for cracks fully embedded in zones with reduced module. Such cracks show stress intensity factors completely different from a $\sqrt{ E/E_{0}}$ -dependency. Nevertheless, it can be concluded that the stress intensity factors on the basis of the J-integral agree with those obtained by COD evaluation. As an approximate description of the observed E/E$_{0}$-dependency we suggest $\frac{K}{K_{appl}}$ $\cong$$\frac{2E}{E+E_{0}}$