Fritz Aldinger

Bismuth based oxide electrolytes— structure and ionic conductivity

Abstract Bismuth oxide systems exhibit high oxide ion conductivity and have been proposed as good electrolyte materials for applications such as solid oxide fuel cells and oxygen sensors. However, due to their instability under conditions of low oxygen partial pressures there has been difficulty in developing these materials as alternative electrolyte materials compared to the state-of-the-art cubic stabilised zirconia electrolyte. Bismuth oxide and doped bismuth oxide systems exhibit a complex array of structures and properties depending upon the dopant concentration, temperature and atmosphere. In this paper we comprehensively review the structures, thermal expansion, phase transitions, electrical conductivity and stability of bismuth oxide and doped bismuth oxide systems. ©

Thermodynamic calculation of the ternary system Al-Si-C

Abstract The phase diagram of the ternary system Al-C-Si was optimised using experimental data from the literature. The binary Al-C and Al-Si systems were taken from the literature. The Si-C system was also optimised using the least square method. The calculated sections agree very well with the experimental data.

Progress in SiAlON ceramics

Abstract In view of the considerable progress that has been made over the last several years on the fundamental understanding of phase relationships, microstructural design, and tailoring of properties for specific applications of rare-earth doped SiAlONs, a clear review of current understanding of the basic regularities lying behind the processes that take place during sintering of SiAlONs is timely. Alternative secondary phase development, mechanism and full reversibility of the α′ to β′ transformation in relation with the phase assemblage evolution are elucidated. Reaction sintering of multicomponent SiAlONs is considered with regard of wetting behavior of silicate liquid phases formed on heating. Regularities of SiAlONs behavior and stability are tentatively explained in terms of RE element ionic radii and acid/base reaction principle.

Experimental investigation and thermodynamic calculation of the Al–Mg–Zn system

Abstract On the basis of a critical assessment, experimental investigations by EPMA on ternary Al–Mg–Zn alloys were specifically performed to provide missing data of the ternary solubilities of the Al–Mg and Mg–Zn phases as well as to improve the knowledge of the extensions of the homogeneity ranges of the ternary τ- and Φ-phases. A thermodynamic description for the Al–Mg–Zn system was obtained taking into account those experimental data together with constitutional, thermodynamic and crystallographic literature information. The binary intermetallic phases are modelled to have ternary solubilities. The ternary τ-phase is modelled according to its crystal structure with cubic symmetry as (Mg) 26 ( Mg , Al) 6 ( Al , Zn , Mg) 48 (Al) 1 in the compound-energy-formalism. The Φ-phase is described by the sublattice formula Mg 6 ( Al , Zn ) 5 .

PHASE EQUILIBRIA AND CRYSTAL CHEMISTRY IN THE Y2O3-AL2O3-SIO2 SYSTEM

In order to clarify inconsistencies in the literature and to verify assumed ternary solubilities, the phase equilibria in the Y 2 O 3 –Al 2 O 3 –SiO 2 system at 1600, 1400, and 1300 °C were experimentally determined using x-ray diffraction (XRD), scanning electron microscope with attached energy-dispersive detector system (SEM-EDX), and electron probe microanalyzer (EPMA). Six quasibinary phases were observed: Y 4 Al 2 O 9 (YAM), YAlO 3 (YAP), Y 3 Al 5 O 12 (YAG), Y 2 SiO 5 , Y 2 Si 2 O 7 (C and D modifications), and ˜ 3 Al 2 O 3 · 2SiO 2 (mullite). Y 4 Al 2 O 9 forms an extended ternary solid solution with the formula Y 4 Al 2(1- x ) Si 2 x O 9+ x ( x = 0 2 ˜0.31). The lowest ternary eutectic temperature was determined at 1371 ± 5 °C by high-temperature differential scanning calorimetry (DSC). The results were compared with previous data available for the Y 2 O 3 –Al 2 O 3 –SiO 2 system and with data for other RE 2 O 3 –Al 2 O 3 –SiO 2 (RE = rare earth element) systems.

Low temperature synthesis of ultrafine Pb(Zr, Ti)O 3 powder by sol-gel combustion

Lead zirconate titanate powders are derived from a novel aqueous-based citrate-nitrate/oxynitrate sol-gel combustion process. Aqueous solutions of metal nitrates or oxynitrates are transformed into gels with citric acid under heating. The received gels undergo a self-propagating combustion reaction on heating to 180{degree}C and subsequently yield voluminous ashes. These ashes form single phase perovskite Pb(Zr{sub 0.53}Ti{sub 0.47})O{sub 3} powder with a specific surface area of 8m{sup 2}/g upon calcination at 550{degree}C. The ashes show a homogeneous distribution of lead, zirconium, and titanium ions which guarantees short diffusion paths in solid state formation of PZT perovskite. The redox behavior of the gels was studied with the help of DTA experiments. Powders are characterized in terms of XRD, SEM, and EDX analysis. {copyright} {ital 1997 Materials Research Society.}

High temperature mechanical behaviour of liquid phase sintered silicon carbide

Abstract Silicon carbide was liquid phase sintered using Y 2 O 3 and AlN as sintering additives. A globular grained microstructure was obtained from α-SiC powder whereas a mixture of β-SiC, with a small amount of α-SiC seeds, revealed platelet shaped grains with an aspect ratio of eight. Beside this difference in grain morphology of the α-SiC grains, the phase content of the grain boundary phase is different. The influence of both aspects onto the mechanical behaviour, i.e. four-point bending strength between room temperature and 1400°C and fracture toughness between room temperature and 1100°C, are investigated.

Structural investigations of Si/C/N-ceramics from polysilazane precursors by nuclear magnetic resonance

Abstract Polymeric precursors to Si/C/N-ceramics can be obtained by thermal crosslinking of polysilazanes. The precursor is then pyrolysed at a temperature of 1000 °C into the amorphous ceramic. Between 1000 and 2000 °C this amorphous state is transformed into the thermodynamically stable phases. The processes during pyrolysis and crystallization are connected with microstructural conversion. Simultaneously, changes of chemical and physical properties take place. The successive conversion of the atomic coordination during the transformation from the polymer into the amorphous ceramic and finally into the crystalline ceramic was studied using solid-state NMR. 29 Si-, 13 C- and 1 H-spectra were recorded. The result is a detailed report of the reactions and changes in structure occuring during ceramization of a polyhydridomethylsilazane and crystallization of its ceramic residue. Furthermore, the amorphous state of this material was compared with a polyvinylsilazane-derived amorphous ceramic.

Assessment of thermodynamic parameters in the system ZrO2-Y2O3-Al2O3

Abstract The thermodynamic parameters have been assessed for the systems of ZrO2–Y2O3, ZrO2 –Al2O3 and ZrO2–Y2O3–Al2O3 using the Calphad technique. The solid solutions are described by the compound energy formalism. A two-sublattice ionic liquid model is used to describe the liquid phase. The phase diagrams of the ZrO2–Y2O3 and ZrO2–Al2O3 and Y2O3–Al2O3 systems are in fairly good agreement with phase equilibrium data. Isothermal sections of the phase diagram of the ZrO2–Y2O3–Al2O3 system at subsolidus conditions are calculated. Introducing ternary interactions in the liquid phase results in a better fit to experimental data of the liquidus surface and isoplethal sections.

Boron-modified polysilazane as a novel single-source precursor for SiBCN ceramic fibers: synthesis, melt-spinning, curing and ceramic conversion

A novel boron-modified polysilazane functionalised with Si- and N-bonded methyl groups has been synthesised and characterised by means of FT-IR and NMR spectroscopies as well as elemental analysis. Both Si- and N-bonded methyl groups inhibited extensive cross-linking to yield a tractable polymer which was successfully processed into polymer green fibers by a melt-spinning process. After the shaping processing, the N-bonded methyl groups offered the capability of facile polymer cross-linking in an ammonia atmosphere at 200 °C to increase the ceramic yield of the polymer and avoid inter-fiber fusion. The as-obtained cured fibers were subsequently pyrolysed at 1400 °C in a nitrogen atmosphere to provide amorphous and stable Si3.0B1.0C5.0N2.4 ceramic fibers with ca. 1.3 GPa in tensile strength, ca. 170 GPa in Youngs modulus and ca. 23 μm in diameter.