Jochen Schilm

New sodium-based battery concepts require solid electrolytes as ion conducting separators. Besides NaSICON and β-Al₂O₃ in the Na₂O-R₂O₃-SiO₂ system (R = rare earth), a rarely noticed glass-ceramic solid electrolyte with the composition Na₅RSi₄O₁₂ (N5-type) exists. The present study addresses the investigation of the ionic conductivity of Na₅RSi₄O₁₂ solid electrolytes sintered from pre-crystallized glass-ceramic powders. The sintering behavior (optical dilatometry), the microstructure (SEM/EDX), and phase composition (XRD), as well as electrochemical properties (impedance spectroscopy), were investigated. To evaluate the effect of the ionic radii, Y, Sm and Gd rare elements were chosen. All compositions were successfully synthesized to fully densified compacts having the corresponding conducting N5-type phase as the main component. The densification behavior was in agreement with the melting point, which decreased with increasing ionic radii and specific cell volume. Alternatively, the ionic conductivities of N5-phases decreased from Y to Gd and Sm containing samples. The highest ionic conductivity of 1.82 × 10⁻³ S cm⁻¹ at 20 °C was obtained for Na₅YSi₄O₁₂ composition. The impact of grain boundaries and bulk conductivity on measured values is discussed. A powder-based synthesis method of this glass-ceramic solid electrolyte using different rare earth elements opens possibilities for optimizing ionic conductivity and scalable technological processing by tape casting.

The compound [Ca₂₄Al₂₈O₆₄]⁴⁺(4e‐) named as C12A7:e‐ has outstanding properties because of a low work function and high electron conductivity due to a cage‐like crystal structure. For the application of the material as hollow cathodes in small satellite propulsion systems, its preparation as sinterable glass‐ceramics via the powder route is purposeful. However, the mechanical and thermal properties of the C12A7:e‐ glass‐ceramics have only been insufficiently measured so far. In this study, the measured Vickers hardness (HV0.5) is 6.7 ± 0.2 GPa. The ceramic has a fracture toughness of 1.6 MPa m^0.5 (calculated by Shetty model) and a 4‐pt. bending strength of σ = 75 ± 12 MPa (σ₀ = 90 MPa). The coefficient of thermal expansion CTE is between 4.2 and 6.0 × 10⁻⁶ K⁻¹ (RT‐1000°C), and the thermal conductivity ranges between 2.3 W m⁻¹ K⁻¹ (20°C) and 1.7 W m⁻¹ K⁻¹ (1000°C). For calculating the thermal conductivity, the heat capacity and the thermal diffusivity are measured.

The glass crystallization route for Na‐ion conducting glass ceramics with conductive N5 modification of Na₅YSi₄O₁₂ has been explored. P₂O₅ is used to modify the glass frit for optimization of the sintering and crystallization process. Mass spectroscopy reveals that the evaporation of CO₂ in the sintered microstructure is responsible for enhanced porosity. The glass is not fully transformed to the highly conductive phase (N5) by crystallization and thereby lower conductivities in sintered samples is observed. A pre‐crystallization step of the initial glass powder is evaluated as suitable to eliminate the gas evolution and subsequent sintering of material crystallized in desired N5 modification is successfully performed. With the powder of the pre‐crystallized material, homogenous sintered microstructures consisting of crystalline and glassy phases are obtained. Despite the higher temperature needed for sintering, low porosity and a mechanical strength up to 60 N mm⁻² are achieved for samples from pre‐crystallized powder. The analysis of impedance spectra of sintered samples reveal high impact of grain boundaries on total ionic conductivity. Samples made from pre‐crystallized powder show higher overall ionic conductivity of 8.3 × 10⁻⁶ S cm⁻¹ at room temperature which is lower than the conductivity of crystallized N5 phase grains (1.4 × 10⁻⁴ S cm⁻¹).

The operation of solid oxide fuel cells requires gas tight and stable sealing materials. Glass ceramic materials have been established to seal different stack designs and can be optimized to match the requirements of components and joining conditions. It is known that glass‐based sealants are sensitive to the formation of undesired chromia species in contact with ferritic steels used as interconnects. Their formation depends on the earth alkaline type and content in the glass as well as the chromium content of the interconnect. A robust interconnect material is the chromium‐based sinter alloy CFY with >93 wt.% chromium what in turn enforces the formation of chromia phases. SOFC stacks with chromium‐based interconnects and electrolyte supported cells with 10SCSZ electrolyte are candidates for SOFC systems with high efficiency in a wide power range. This study presents results on the development of sealing glasses adapted for the CFY by optimizing the glass components BaO, CaO, SiO _2, and Al ₂ O ₃ . Depending properties like crystallization behavior, thermal expansion, adhesion and reactivity in contact with CFY have been regarded. The glass has been tested under operating conditions of SOFC (dual atmosphere, electric potentials) in samples as well as in real stacks. It has been shown that optimized glass compositions, especially regarding a limited BaO content combine a good joining behavior and minimized BaCrO ₄ formation in contact with the CFY alloy.

Besides the well-known application as circuit boards and housings, multilayer low-temperature co-fired ceramics (LTCC) offer a flexible and temperature-stable platform for the development of complex sensor elements. Commercial LTCC qualities are usually available with a matching set of metallization pastes which allow the integration of various electrical functions. However, for the integration of ceramic sensor elements based on LTCC into standardized steel housings it is necessary to compensate the mismatching thermal expansion behaviour. Therefore balancing elements made of Kovar^® (Fe–29 wt% Ni–17 wt% Co) and alumina ceramic (Al₂O₃) can be used. These components have to be joined hermetically to each other and to the LTCC sensors. In this study, brazing experiments were performed for combinations of Kovar–Al₂O₃ and Kovar–LTCC with Ag–Cu–Ti- and Ag–Cu–In–Ti-based commercial braze filler metals, Cusil-ABA^® and Incusil^®-ABA, respectively. For both active braze filler metals, optimized processing parameters were investigated to realize hermetic Kovar–Al₂O₃ and Kovar–LTCC joints.