Processing, microstructural evolution and electrochemical performance relationships in solid oxide fuel cells

The relationships between the processing parameters, microstructures and electrochemical performance of solid oxide fuel cell (SOFC) components were investigated. The operating regimes (i.e., reducing vs. oxidizing) as well as the elevated temperatures (e.g. 800°C) for their operation introduce several material challenges. Therefore, composite materials are employed to withstand operating conditions while providing sufficient electrochemical performance for fuel cell operation. Analyses on lanthanum-strontium manganite (LSM) - yttria stabilized zirconia (YSZ) compositions (45 vol%-55 vol%) by impedance spectroscopy demonstrated that two competing polarization mechanisms (i.e. charge-exchange and surface adsorption-diffusion of oxygen) limit performance. Optimization of microstructures resulted in total resistances as low as 0.040 Ohm cm². Studies on Ag composites revealed that incorporation of up to 25 vol% oxide particles (LSM and YSZ) with sizes comparable to the Ag grains (~0.5 μm) can minimize the densification and coarsening of the Ag matrix. While the powder based oxide additions increased the stability limit of porous Ag composites from \u3c550°C to 800°C, the use of nanostructured coatings increased the stability limit to 900°C for cathodes and current collectors. Investigations of Ni-YSZ anode microstructures demonstrated that uniform distribution of percolating isometric pores (\u3e5 μm) allows forming desired continuous percolation of all phases (Ni, YSZ and pores) lowering activation polarization below 0.100 Ohm cm² and maintaining significant electrical conductivity (\u3e1000 S/cm). Identification of polarization mechanisms by deconvolution of impedance spectra and tailoring the corresponding microstructures was demonstrated as an effective method for optimization of SOFC components --Abstract, page v

Characterization of Thermo-Mechanical Damage in Tin and Sintered Nano-Silver Solders

abstract: Increasing density of microelectronic packages, results in an increase in thermal and mechanical stresses within the various layers of the package. To accommodate the high-performance demands, the materials used in the electronic package would also require improvement. Specifically, the damage that often occurs in solders that function as die-attachment and thermal interfaces need to be addressed. This work evaluates and characterizes thermo-mechanical damage in two material systems – Electroplated Tin and Sintered Nano-Silver solder. Tin plated electrical contacts are prone to formation of single crystalline tin whiskers which can cause short circuiting. A mechanistic model of their formation, evolution and microstructural influence is still not fully understood. In this work, growth of mechanically induced tin whiskers/hillocks is studied using in situ Nano-indentation and Electron Backscatter Diffraction (EBSD). Electroplated tin was indented and monitored in vacuum to study growth of hillocks without the influence of atmosphere. Thermal aging was done to study the effect of intermetallic compounds. Grain orientation of the hillocks and the plastically deformed region surrounding the indent was studied using Focused Ion Beam (FIB) lift-out technique. In addition, micropillars were milled on the surface of electroplated Sn using FIB to evaluate the yield strength and its relation to Sn grain size. High operating temperature power electronics use wide band-gap semiconductor devices (Silicon Carbide/Gallium Nitride). The operating temperature of these devices can exceed 250oC, preventing use of traditional Sn-solders as Thermal Interface materials (TIM). At high temperature, the thermomechanical stresses can severely degrade the reliability and life of the device. In this light, new non-destructive approach is needed to understand the damage mechanism when subjected to reliability tests such as thermal cycling. In this work, sintered nano-Silver was identified as a promising high temperature TIM. Sintered nano-Silver samples were fabricated and their shear strength was evaluated. Thermal cycling tests were conducted and damage evolution was characterized using a lab scale 3D X-ray system to periodically assess changes in the microstructure such as cracks, voids, and porosity in the TIM layer. The evolution of microstructure and the effect of cycling temperature during thermal cycling are discussed.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201

Ultra-Low Temperature Coefficient of Capacitance (Tcc) of the SrSnO

The perovskite-structured SrSnO3 possessing steady capacitance over the temperature range between 27°C and 300°C in a frequency domain spanning nearly four decades has been evaluated. The samples investigated in this study were synthesized by using solid-state reaction (SSR) and self-heat-sustained (SHS) techniques. These samples were sintered at a temperature (T ) ranging between 1200°C and 1600°C with a soak-time (t) ranging between 2 h and 60 h. The ac immittance (impedance or admittance) measurements were conducted on these sintered bodies in the frequency range 5Hz to 13 MHz. These ac electrical data were found to exhibit relaxation in more than one complex plane formalisms in a simultaneous manner. The magnitude of the terminal capacitance was found to be in a narrow window of 3 pF to 6 pF possessing very weak temperature dependence. Further analysis also revealed that this material system possessed low dielectric constant and ultra-low temperature coefficient of capacitance (TCC) or dielectric constant (TCK). The electrical behavior of these sintered bodies has been systematically correlated with the evolved microstructures. Plausible equivalent circuit elements were extracted using the lumped parameter/complex plane analysis (LP/CPA) and evaluated at various situations

Evolution of nano-pores during annealing of technically pure molybdenum sheet produced from different sintered formats

Molybdenum is a refractory metal with no phase transformation in the solid state and a high melting point. It is therefore an excellent structural material for various high temperature applications. Especially in this field of operation, significant creep resistance is essential. To achieve this, a microstructure with grains in the range of millimeters is desired. However, as demonstrated in the present study, the onset temperature for secondary recrystallization, which would lead to a beneficial grain size, is among other things dependent on the initial dimensions of the sintered part. One possible reason for the different microstructural evolutions is the influence of residual pores in sub-micrometer size. Sheets were thus fabricated via three different production routes employing the same initial Mo powder to exclude chemical variation as an influencing factor. The samples were investigated by in-situ small-angle X-ray scattering at a synchrotron radiation source with two different heating rates. Additionally, selected annealed samples were studied ex-situ with high energy X-rays. The apparent volume fraction of pores is compared to a volatilization model for the vaporization of typical accompanying elements and the induced thermal expansion

Nonlinear Thermo‐Electro‐Mechanical Behaviors of Ag/BaTiO3 Composites

The focus of this study is to understand the influences of blending silver (Ag) phase into barium titanate (BaTiO3) ceramic on its thermal, mechanical and dielectric properties. Silver-barium titanate (Ag/BaTiO3) active composites with varying silver composition were fabricated using powder metallurgy method. Coefficient of thermal expansion (CTE) and heat capacity were measured by thermal mechanical analyzer (TMA) and differential scanning calorimetry (DSC), respectively. Hot disk technique was employed to determine the thermal conductivity. Addition of silver did not change the phase transformation temperatures. CTE stays constant at each crystalline phase, but increases as BaTiO3’s crystal structure changes from orthorhombic to tetragonal phase and further to cubic. Increase of silver content significantly enhances the thermal conductivity. Elastic and dielectric constants were determined using resonant ultrasound spectroscopy (RUS) and dielectric (impedance) spectroscopy, respectively. Young’s modulus decreases as the increase of silver composition, while the dielectric constant was significantly improved by blending silver. Two peaks were observed on dielectric constant around the transformation temperatures, with a larger magnitude at the Curie point. Micromechanics models based on detailed microstructures, either generated randomly by computer algorithm or created by converting scanning electron microscope (SEM) images, were created to numerically study the effects of microstructures on the effective properties of Ag/BaTiO3 composite. Numerical results showed that microstructure induced anisotropy is negligible and the effective properties are insensitive to loading directions. Effective CTE is insensitive to the yielding of silver particles, porosity, and the elastic modulus of BaTiO3. The predictions of CTE and elastic constants were pretty close to the experiment results, while the effective thermal conductivity and dielectric constant predictions underestimated the measured values. The hysteretic mechanical behavior of Ag/BaTiO3 composite was measured under cyclic uniaxial compressive loading using materials test system (MTS). Specimens with 5 vol% and 13 vol% silver composition were broken before the maximum stress was reached. The fractured specimens showed a fracture angle of approximate 45⁰C. Furthermore, a one-dimensional constitutive model based on the thermodynamics of irreversible process was presented to model the hysteretic response from experiment

Ceramic dielectrics for high energy density capacity application

The objective of this dissertation is to investigate the relationship between the processing parameters, microstructural development, defect chemistry and electrical properties of titanium oxide (TiO₂) dielectrics for high energy density capacitor applications. The effects of aliovalent dopants on the dielectric properties of TiO₂ ceramics were investigated, aiming to further improve the desired dielectric properties especially at elevated temperatures (up to 200°C). Due to the segregation of acceptor type impurities in the starting powders, space charge polarization took place in TiO₂ ceramics with relative large grain size (\u3e̲500nm), leading to high dielectric loss and low energy storage efficiency. Increased ratio of grain boundary resistivity to bulk grain resistivity resulted in lower breakdown strength, as larger electric field was applied on the grain boundaries as they became the most resistive part. Donor doping (e.g phosphorus or vanadium) can effectively remove the space charge layer due to charge neutralization of positively charged defects created by donors and negatively charged defects created by acceptors. Large area, crack free tapes were fabricated by tape-casting method using nano-sized (~40nm) TiO₂ powders. An energy density of ~14 J/cm³ was demonstrated by testing of TiO₂ thick films (~100µm). Studies on dielectric materials were extended to BaTiO₃/SrTiO₃ (BST) ceramics which were processed by lamination of BaTiO₃ and SrTiO₃ green tapes with a 2-2 spatial configuration. Preliminary results showed that BST ceramics are promising dielectrics for energy storage applications and offer compositional flexibility to achieve maximum energy density under specified electric fields --Abstract, page iv

Spark-Plasma Sintering and Related Field-Assisted Powder Consolidation Technologies

Electromagnetic field-assisted sintering techniques have increasingly attracted attention of scientists and technologists. Spark-plasma sintering (SPS) and other field-assisted powder consolidation approaches provide remarkable capabilities to the processing of materials into configurations previously unattainable. Of particular significance is the possibility of using very fast heating rates, which, coupled with the field-assisted mass transport, stand behind the purported ability to achieve high densities during consolidation and to maintain the nanostructure of consolidated materials via these techniques. Potentially, SPS and related technologies have many significant advantages over the conventional powder processing methods, including the lower process temperature, the shorter holding time, dramatically improved properties of sintered products, low manufacturing costs, and environmental friendliness