Taghi Darroudi

Role of phonon scattering by elastic strain field in thermoelectric Sr1−xYxTiO3−δ

Perovskite-type SrTiO3−δ ceramics are multifunctional materials with significant potential as n-type thermoelectric (TE) materials. The electronic and thermal transport properties of spark plasma sintered polycrystalline Sr1−xYxTiO3−δ (x = 0.05, 0.075, 0.1) ceramics are systematically investigated from (15–800) K. The Sr0.9Y0.1TiO3−δ simultaneously exhibits a large Seebeck coefficient, α > −80 μV/K and moderately high electrical resistivity, ρ ∼ 0.8 mΩ-cm at a carrier concentration of ∼1021 cm−3 at 300 K resulting in a high TE power factor defined herein as (α2σT) ∼ 0.84 W/m-K at 760 K. Despite the similar atomic masses of Sr (87.6 g/mol) and Y (88.9 g/mol), the lattice thermal conductivity (κL) of Sr1−xYxTiO3−δ is significantly reduced with increased Y-doping, owing to the smaller ionic radii of Y3+ (∼1.23 A, coordination number 12) compared to Sr2+ (∼1.44 A, coordination number 12) ions. In order to understand the thermal conductivity reduction mechanism, the κL in the Sr1−xYxTiO3−δ series are phenomeno...

New insights on the synthesis and electronic transport in bulk polycrystalline Pr-doped SrTiO3−δ

Recently, we have reported a significant enhancement in the electronic and thermoelectric properties of bulk polycrystalline SrTiO3 ceramics via praseodymium doping. This improvement was originated from the simultaneous enhancement in the thermoelectric power factor and reduction in thermal conductivity, which was contributed to the non-uniform distribution of Pr dopants. In order to further understand the underlying mechanism, we herein investigate the role of praseodymium doping source (Pr2O3 versus Pr6O11) on the synthesis and electronic transport in Pr-doped SrTiO3 ceramics. It was observed that the high-temperature electronic transport properties are independent of the choice of praseodymium doping source for samples prepared following our synthesis strategy. Theoretical calculations were also performed in order to estimate the maximum achievable power factor and the corresponding optimal carrier concentration. The result suggests the possibility of further improvement of the power factor. This study...

Synthesis of Non-uniformly Pr-doped SrTiO3 Ceramics and Their Thermoelectric Properties

We demonstrate a novel synthesis strategy for the preparation of Pr-doped SrTiO3 ceramics via a combination of solid state reaction and spark plasma sintering techniques. Polycrystalline ceramics possessing a unique morphology can be achieved by optimizing the process parameters, particularly spark plasma sintering heating rate. The phase and morphology of the synthesized ceramics were investigated in detail using X-ray diffraction, scanning electron microcopy and energy-dispersive X-ray spectroscopy. It was observed that the grains of these bulk Pr-doped SrTiO3 ceramics were enhanced with Pr-rich grain boundaries. Electronic and thermal transport properties were also investigated as a function of temperature and doping concentration. Such a microstructure was found to give rise to improved thermoelectric properties. Specifically, it resulted in a significant improvement in carrier mobility and the thermoelectric power factor. Simultaneously, it also led to a marked reduction in the thermal conductivity. As a result, a significant improvement (> 30%) in the thermoelectric figure of merit was achieved for the whole temperature range over all previously reported maximum values for SrTiO3-based ceramics. This synthesis demonstrates the steps for the preparation of bulk polycrystalline ceramics of non-uniformly Pr-doped SrTiO3.

Grain Boundary Engineering at the Interface of Ceramic and Composite Materials used in Alternative Energy Technologies

Complex ceramic and composite materials used in alternative energy technologies often present challenges during electron microscopy analysis. Bulk electrical properties such as transport and mobility are crucial factors in carbon based or nanomaterial composites often analyzed at the atomic or molecular level despite the presence of localized chemical composition and resultant phase transformation along its interface. Such materials often undergo transformations in dynamic environments such as a combination of high temperature and reducing gases. Data derived from multiple electron microscopy analytical techniques are presented as a focus for grain boundary engineering in ceramic and composite materials. The materials are used in solid oxide fuel cells and Li-ion batteries for applications related to the development of catalysts and novel ion conducting materials for alternative energy technologies.

Microwave measurement of giant unilamellar vesicles in aqueous solution

A microwave technique is demonstrated to measure floating giant unilamellar vesicle (GUV) membranes in a 25 μm wide and 18.8 μm high microfluidic channel. The measurement is conducted at 2.7 and 7.9 GHz, at which a split-ring resonator (SRR) operates at odd modes. A 500 nm wide and 100 μm long SRR split gap is used to scan GUVs that are slightly larger than 25 μm in diameter. The smaller fluidic channel induces flattened GUV membrane sections, which make close contact with the SRR gap surface. The used GUVs are synthesized with POPC (16:0–18:1 PC 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), SM (16:0 Egg Sphingomyelin) and cholesterol at different molecular compositions. It is shown that SM and POPC bilayers have different dielectric permittivity values, which also change with measurement frequencies. The obtained membrane permittivity values, e.g. 73.64-j6.13 for POPC at 2.7 GHz, are more than 10 times larger than previously reported results. The discrepancy is likely due to the measurement of dielectric polarization parallel with, other than perpendicular to, the membrane surface. POPC and SM-rich GUV surface sections are also clearly identified. Further work is needed to verify the obtained large permittivity values and enable accurate analysis of membrane composition.

Practical Utilization of Uranium-Containing Particulate Test Samples for SEM/EDS and SIMS Automated Particle Analysis Method Validation

The detection and characterization of uranium particulates from swipe samples collected by nuclear facilities safeguards inspectors often relies on automated search algorithms and instrument software for analysis via scanning electron microscopy (SEM) and/or secondary ionization mass spectrometry (SIMS). Because safeguards samples are inherently cluttered with background environmental material automated particle measurement (APM) methods with correctly tuned instrument parameters are required for robust identification of uranium containing material. Unfortunately, no standard or reference specimens are commercially available for uranium APM method validation or instrument proficiency testing. To meet such challenges the safeguards community has generated and characterized select test specimens over the past decade, but these efforts have been hindered by limited production and challenging characterization [1-3].

3D Reconstruction and Separation of Nickel and Zirconia Based Phases from Solid Oxide Fuel Cell Anode Using Backscatter Electron Imaging

Solid Oxide Fuel Cells (SOFC) are often viewed as one of the best electrochemically generated sources of electricity for high power applications. In all oxide SOFC, optimum ion transport is achieved though usage of complex oxides at the anode, electrolyte and cathode. Electron microscopy analysis of complex oxides often presents challenges due to the varying nature of dopants in these oxides. This often results in complications as a result of high temperature material sintering, where the possibility of phase transformations and diffusion strongly exist. In this study, our goal is to identify and separate complex oxide phases through advanced electron microscopy (EM) analysis in the bulk part of anode material.