Dietmar Koch

Evolution of Porosity by Freeze Casting and Sintering of Sol-Gel Derived Ceramics

Freeze cast of aqueous ceramic powder slurries is described as a versatile process to fabricate complex-shaped ceramic parts. Since freezing of aqueous sols or powder suspensions include the nucleation and growth of ice crystals the evolving microstructure in particular the pore characteristics which are left behind after elimination of the solvent can be controlled by the freezing process. The freezing kinetics have then to be used to manifest the conditions for the formation of the intended porosity. The temperature profile in the freezing slurry was measured and calculated, in particular the movement of the freezing front through the slurry was determined. The results show that a homogeneous microstructure is reached in the surface region of the consolidated part. Individual ice crystals are detected within a distance of some hundred micrometers from the surface. The final pores are dendritic in shape with an elliptical cross section. The pores can grow up to several millimeters in length under the process conditions used in this study. The limits of freeze-sensitive slurry compositions should be investigated in further studies and the approach should be followed to increase the porosity by additional foaming steps.

Freeze gelation: a new option for the production of biological ceramic composites (biocers)

Abstract Freeze gelation was used for immobilization and conservation of living microorganisms in inorganic solids. The irreversible sol–gel transition of a mixture of colloidal silica, ceramic powder and biocomponent by freezing (freeze gelation or freeze casting) enables the production of low-cost, porous, crack-free green bodies with nearly zero shrinkage in which microorganisms are immobilized safely. First investigations to the survival rate, activity and storage ability will be presented for biocers with Bacillus sphaericus and Saccharomyces cerevisiae under the selected freeze and drying conditions. The structure of these new biocers were characterized by SEM micrographs, physical adsorption and mercury porosimetry.

Development of a Novel Zinc/Air Fuel Cell with a Zn Foam Anode, a PVA/KOH Membrane and a MnO

A new type of zinc/air fuel cell comprising a Hg/Pb free Zn foam anode, a PVA/KOH electrolyte membrane and a MnO2/SiOC-based cathode was developed in this work. The electrochemical activity of the zinc foam and air electrode was investigated in 6 M KOH under half-cell conditions. The pristine ZnO layer of the foam matrix favoured direct oxidation of the zinc particles to zinc oxide in 6 M KOH. In the laboratory cell, a specific energy of about 500 mWh g(-1) zinc was measured at 5 mA discharging current with a zinc foam, a PVA/KOH/H2O membrane and a MnO2+ Vulcan/carbon paper cathode. A correlation between cell performances and porosity of the zinc foam was found. However, stability of the Zn foam and SiOC GDL materials towards KOH should be improved.

A novel one-pot process for near-net-shape fabrication of open-porous resorbable hydroxyapatite/protein composites and in vivo assessment

We present a mild one-pot freeze gelation process for fabricating near-net, complex-shaped hydroxyapatite scaffolds and to directly incorporate active proteins during scaffold processing. In particular, the direct protein incorporation enables a simultaneous adjustment and control of scaffold microstructure, porosity, resorbability and enhancement of initial mechanical and handling stability. Two proteins, serum albumin and lysozyme, are selected and their effect on scaffold stability and microstructure investigated by biaxial strength tests, electron microscopy, and mercury intrusion porosimetry. The resulting hydroxyapatite/protein composites feature adjustable porosities from 50% to 70% and a mechanical strength ranging from 2 to 6 MPa comparable to that of human spongiosa without any sintering step. Scaffold degradation behaviour and protein release are assessed by in vitro studies. A preliminary in vivo assessment of scaffold biocompatibility and resorption behaviour in adult domestic pigs is discussed. After implantation, composites were resorbed up to 50% after only 4 weeks and up to 65% after 8 weeks. In addition, 14% new bone formation after 4 weeks and 37% after 8 weeks were detected. All these investigations demonstrate the outstanding suitability of the one-pot-process to create, in a customisable and reliable way, biocompatible scaffolds with sufficient mechanical strength for handling and surgical insertion, and for potential use as biodegradable bone substitutes and versatile platform for local drug delivery.

Combination of biological mechanisms for a concept study of a fracture-tolerant bio-inspired ceramic composite material

The biological materials nacre and wood are renowned for their impressive combination of toughness and strength. The key mechanisms of these highly complex structures are crack deflection at weak interfaces, crack bridging, functional gradients and reinforcing elements. These principles were applied to a more fracture-tolerant model material which combined porous stiff ceramic layers, manufactured by freeze casting, infiltrated and bonded by a polymer phase reinforced with fabric layers. In the hybrid composites, crack deflection occurred at the ceramic–fabric interface and the intact fabric layers served as crack-bridging elements. Fabric-reinforced epoxy layers stabilized the fracture behaviour and delayed catastrophic failure of the material. The influence of the different components was analysed by varying the ceramic, fabric and interface properties. More ductile fabrics lead to larger strain to failure and more crack bridging but reduced the composite strength and stiffness after initial cracking. Higher elastic mismatch between the components improved crack deflection and bridging but resulted in deterred load transfer and a lower strength. The stiffness and strength of the ceramic layers influenced the elastic properties of the laminar composite and the initial crack resistance. Flaw tolerance was increased with polymer infiltration. We show with our hybrid ceramic–fabric composite as a bio-inspired concept study how fracture toughness, work of fracture and tolerance for cracking can be tailored when the contributing factors, i.e. the ceramic, the fabric and their interface, are modified.

Evaluation of Mechanical Properties and Comprehensive Modeling of CMC with Stiff and Weak Matrices

The mechanical properties of ceramic matrix composites (CMC) depend on the individual properties of fibers and matrix, the fiber-matrix interface, the microstructure and the orientation of the fibers. The fiber-matrix interface of ceramics with stiff matrices (e.g. CVI-derived SiC/SiC) must be weak enough to allow crack deflection and debonding in order to achieve excellent strength and strain to failure (weak interface composites WIC). This micromechanical behavior has been intensively investigated during the last 20 years. With the development of CMC with weak matrices (weak matrix composites WMC) as e.g. oxide/oxide composites or polymer derived CMC the mechanical response can not be explained anymore by these models as other microstructural mechanisms occur. If the fibers are oriented in loading direction in a tensile test the WMC behave almost linear elastic up to failure and show a high strength. Under shear mode or if the fibers are oriented off axis a significant quasiplastic stress-strain behavior occurs with high strain to failure and low strength. This complex mechanical behavior of WMC will be explained using a finite element (FE) approach. The micromechanical as well as the FE models will be validated and attributed to the different manufacturing routes.

Development of Ceramic Membranes and Adsorbents from Silicon‐Organic Precursors

Manufacturing ceramic powders and compacts by pyrolizing organosilicon polymers results in an amorphous microporous Si-O-C matrix with favorable high temperature strength and chemical stability compared to conventional porous organic matrices. Using this method the authors present the production of defect-free ceramics with ideally oriented channels.

Elastic properties of braided ceramic matrix composites

Abstract Fibre reinforced ceramic components are most efficiently produced by the braiding technique. Since the braiding angle can be adjusted during the preform fabrication process in a wide range as required by the loading situation of the composite, the evaluation and prediction of the elastic properties as a function of variable braiding angles is of high importance. Based on the classical laminate theory an inverse method for the determination of the elastic properties of an equivalent unidirectional ply is elaborated and applied to predict the effect of variable braiding angles. Orthogonal and non-orthogonal braided carbon/carbon composites were tested under in-plane tensile and shear loading in order to validate the analytical concept. A very good correlation between theoretical and experimental results was obtained.

Determination of elastic properties for a wound oxide ceramic composite

Abstract Thanks to its low cost and high flexibility, in the last few years the winding technique has been successfully adapted for the production of complex Ceramic Matrix Composite (CMC) components with load-oriented fibre alignment. Since the winding angle can be adjusted in any direction (from 0° to 90°) during the fabrication process, it is important for the design of components to evaluate the elastic properties of CMCs as a function of the winding angle. In this study, an inverse method based on the Classic Laminate Theory (CLT) has been used for the prediction of the elastic properties, i.e. Young’s modulus, shear modulus and Poisson’s ratio, for a wound oxide CMC material, called WHIPOX® (Wound HIghly Porous OXide ceramic matrix composite). For this purpose the characteristics of an equivalent unidirectional layer (UD-layer) with consideration of fibre volume content (FVC) and porosity were calculated. On the basis of microstructural analysis the computed WHIPOX®UDproperties have been divided into two sets of elastic properties for small (below 30°) and large winding angles (30° and above). Full coverage of the mechanical properties in different wound orientations, non-orthogonal with ±3°/±87°, ±15°/±75°, ±30°/±60° and orthogonal with ±45° and 0°/90°, were evaluated with in-plane tension, and Iosipescu-shear tests. A good correlation between experimental and analytically calculated results is shown in this paper.

Mechanismen und Modellierung der Verformung und Schädigung keramischer Faserverbundwerkstoffe

Auf der Basis der Kontinuumschädigungsmechanik wird das mechanische Verhalten eines bidirektional verstärkten Carbon/Carbon-Verbundwerkstoffs in einem experimentell gestützten Modell beschrieben. Der betrachtete Werkstoff repräsentiert eine Gruppe keramischer Faserverbundwerkstoffe, deren mechanische Eigenschaften von einer porösen und schwachen Matrix stark beeinflusst sind. Mit dem Modell, das in das kommerziell verfügbare Finite-ElementeProgramm MARC implementiert wurde, ist es möglich, das Werkstoffverhalten unter quasistatischer Zug-, Scherund Druckbelastung für beliebige Belastungsrichtungen relativ zur Faserorientierung in sehr guter Übereinstimmung mit den experimentellen Ergebnissen vorherzusagen.