Detlev Stöver

Nickel coarsening in annealed Ni/8YSZ anode substrates for solid oxide fuel cells

Abstract In order to study the nickel coarsening in porous Ni/8YSZ anode cermets of solid oxide fuel cells (SOFC), a series of exposure tests was carried out with anode substrates used in SOFC development at the Research Centre Julich. The changes in electrical conductivity as well as in the microstructure of the material were investigated. The microstructure of the cermets was characterized by digital on-line image analysis and microstructural parameters were determined for the metallic and the ceramic phase as well as for porosity. A decrease of 33% of the initial electrical conductivity was measured after exposing the cermet in Ar/4% H2/3% H2O at 1000°C for 4000 h, which is linked to the agglomeration of the metallic particles of this material. The durability of anode cermets for operations of up to 40 000 h was estimated on the basis of the measured grain coarsening and the decrease of conductivity. The microstructural parameters — like volume fraction, particle and pore size — were used to model the electrical conductivity by theoretical microstructure–property relationships.

High-porosity titanium, stainless steel and superalloy parts

The production of highly porous parts from titanium, stainless steel, and nickel-based superalloys is of increasing interest in lightweight constructions. A new space-holder method uses carbamide (urea) and ammonium hydrogen carbonate to produce samples with porosities between 60 and 80 %. Depending on the shape and size distribution of the space holder, spherical and angular pores in the range of 0.1-2.5 mm were obtained.

Components manufacturing for solid oxide fuel cells

Abstract A worldwide overview of processing technology of solid oxide fuel cell (SOFC) components is given and the fabrication techniques of ceramic components are summarized for the different types of SOFCs. Generally, a tendency towards up-scalable and automatizable processes is observed. In addition, critical points of interconnect materials and interconnect fabrication are stressed. Especially for planar cell designs, the chromium contamination of the cathode and interfacial corrosion is regarded as the weak points to be solved for demonstration of planar SOFC units.

Development of a micromechanical life prediction model for plasma sprayed thermal barrier coatings

Abstract A widely used method to produce thermal barrier coating (TBC) systems is the vacuum plasma spraying of a highly dense bondcoat layer with a defined surface roughness and the atmospheric plasma spraying (APS) of a porous (10–15%) Y 2 O 3 -stabilized zirconia top coat. In thermal cycling operation these systems often fail by crack initiation and propagation close to the bondcoat–top coat interface. This failure is attributed to stresses arising from the formation of a thermally grown oxide (TGO) layer on the rough bondcoat surface. The actual stress situation is rather complex due to TGO formation, creep effects in both bondcoat and top coat and due to the roughness of the bondcoat. All these factors have been take into account in the present work by using a finite element method (FEM) to calculate stress development during thermal loading. These results can then be introduced into a crack propagation model to estimate crack development during the thermal cycling operation. The predictions of this approach are compared to experimental results on the influence of bondcoat roughness on coating life. In these experiments TBC systems with bondcoat layers having three different levels of roughness were cycled in a gas burner rig until failure.

Properties of Ni/YSZ porous cermets for SOFC anode substrates prepared by tape casting and coat-mix® process

Abstract Two different processing techniques were tested for the development of the anode cermet (ceramic–metal) of the planar high temperature solid oxide fuel cell (SOFC): the coat-mix (CM) process; and tape casting (TC). The specimens produced by both methods were sintered and reduced to cermets. The correlation between microstructure, effective electrical conductivity, porosity and gas permeability were studied in these cermets and compared with each other. The superiority of the CM process is established for the manufacturing of anode cermet substrates for SOFC. A TC cermet substrate with high porosity and adequate gas permeability was prepared, but in this case, the effective electrical conductivity was much lower than that of the corresponding CM cermet substrate.

Effects of deposition temperature and thermal cycling on residual stress state in zirconia-based thermal barrier coatings

Abstract Advanced ceramic multilayered coatings are commonly used as protective coatings for engine metal components to improve performance, e.g. thermal barrier coatings (TBCs). Zirconia-based TBCs were produced by plasma spraying process and characterized in terms of microstructure, porosity, elastic modulus, adherence and residual stresses. In this contribution the residual stresses in multilayered coatings applied on Ni based superalloys for use as thermal barrier coatings were studied both by numerical modelling and experimental stress measurement. The thermal residual stresses generated during the spraying process of duplex TBCs were simulated by using an heat transfer finite element program and an elasto-plastic biaxial stress model. The TBC system was subjected to different thermal cycling conditions (maximum temperature, heating up and cooling down rates, dwell time at maximum temperature, etc.). The stress distribution within the TBC was also modelled after thermal cycling. The stress state in the as-deposited and in thermally cycled coatings was verified using an X-ray diffraction technique. The measurements were in good agreement with the residual stress modelled calculations. It was observed that the residual stresses were dependent on the thermal history of the TBC (as-deposited and thermally cycled). It is proposed that thermal cycling allowed the stresses to relax by microcracking and creep mechanisms at high temperature such that on cooling down to room temperature, an in-plane biaxial compressive stress will arise on the zirconia top coating due to the difference on the coefficients of thermal expansion between the metallic substrate and ceramic coating material.

Powder metallurgical fabrication processes for NiTi shape memory alloy parts

Abstract NiTi components with reproducible and stable shape memory properties are attractive for various technical applications (e.g. couplings). With the aim of producing NiTi components on an industrial scale, near-net-shape fabrication routes are preferred considering the limited machinability of NiTi alloys. Powder metallurgy (PM) is known to provide the possibility of material-saving and automated fabrication of at least semi-finished products as well as net-shape components. As promising PM routes hot isostatic pressing (HIP) and metal injection moulding (MIM) were used for the fabrication of NiTi compacts. Microstructural investigations, chemical analysis, X-ray diffraction (XRD) measurements and also differential scanning calometry (DSC) measurements were performed in order to characterize the produced parts. Additionally, the mechanical properties of HIPed samples were measured by tensile tests at room temperature. The components from both fabrication routes show reversible austenite↔martensite transformations which are a prerequisite for shape memory effects.

Stress distributions in plasma-sprayed thermal barrier coatings as a function of interface roughness and oxide scale thickness

Abstract During thermal cyclic loading, plasma-sprayed thermal barrier coatings (TBCs) often show failure within the top coat close to the interface. In all cases this results from crack propagation of pre-existing cracks near the bond coat (BC)–top coat interface. Stresses developing on a microscopic scale near the BC–TBC interface of plasma-sprayed thermal barrier coatings govern crack growth in an initial phase of the failure process. Using a finite element (FE) method the local dependence of stresses in the vicinity of this rough interface was investigated. Measurements of real roughness profiles provided geometrical parameters needed for the calculations. A significant difference in the stress distributions was found for peak and valley locations of the BC roughness profile. The effect of BC oxidation on stress development was more pronounced in the case of less roughness. Analytical fits of the FE results revealed how the parameters of roughness and the oxide thickness correlate with the stress levels. In the next stage of research these fits will serve as input data for a microstructural based lifetime model.

Total electrical conductivity and defect structure of ZrO2–CeO2–Y2O3–Gd2O3 solid solutions

The use of a bilayer composite YSZ–CGO (yttria-stabilized zirconia–gadolinia-doped ceria) electrolyte for intermediate-temperature solid-oxide fuel cells (SOFCs) is still problematic, due to solid-state reaction and interdiffusion phenomena occurring between YSZ and CGO. To overcome this problem, a gradation in the microstructure between the single materials is proposed. To optimize the composition of the interlayer with respect to conductivity, different compositions of the system CGOxYSZ1−x were prepared and characterized in order to study their transport properties. Conductivity was found to reach a minimum for solid solutions where Zr and Ce are present in equimolar quantities. The latter are, however, the most appropriate candidates to be applied as interlayers between the two single materials. Keeping the Ce/Zr atomic ratio approximately equal to one, different solid solutions were prepared and characterized by varying the concentration of the trivalent cations present, in order to clarify their role in the transport properties of the material. For one of the better conducting systems the cell performance of the composite YSZ-interlayer–CGO film electrolyte is simulated and compared to that of the YSZ–CGO film electrolyte.

Easy assessment of the biocompatibility of Ni-Ti alloys by in vitro cell culture experiments on a functionally graded Ni-NiTi-Ti material.

The biocompatibility of nickel-titanium alloys was investigated by single-culture experiments on functionally graded samples with a stepwise change in composition from pure nickel to pure titanium, including an Ni-Ti shape memory alloy for a 50:50 mixture. This approach permitted a considerable decrease of experimental resources by simultaneously studying a full variation of composition. The results indicate a good biocompatibility for a nickel content up to about 50%. The cells used in the biocompatibility studies comprised osteoblast-like osteosarcoma cells (SAOS-2, MG-63), primary human osteoblasts (HOB), and murine fibroblasts (3T3).