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In the last 3 years, we have worked on both the nanoscale and microscale understanding of TMPT, defect formation and evolution, and plasticity in SMFOs, specifically in YSTZ. The validated computational investigations have provided unmatched insights into the deformation and shape memory behaviors of SMFOs that could not be obtained from experiments.
The formation of precipitated phases of less reactive metals within an oxide scale during alloy oxidation can disrupt its structural integrity, significantly compromising the protective function of the oxide. To better understand this phenomenon, it is crucial to investigate the atomic-level diffusion mechanisms that drive precipitate formation. In this study, we employ first-principles calculations to examine the diffusion and aggregation of Ni atoms across the NiCr/Cr 2 O 3 interface during the high-temperature oxidation of NiCr alloys. Our results reveal that, in the absence of Cr vacancies, Ni atoms predominantly remain within the NiCr substrate, with minimal migration toward the Cr 2 O 3 layer. However, when Cr vacancies are present at both the NiCr/Cr 2 O 3 interface and within the Cr 2 O 3 bulk, Ni atoms exhibit a strong propensity to migrate from the NiCr substrate into the Cr 2 O 3 bulk, where they aggregate into Ni-rich regions with a corundum structure. This study provides valuable atomic-level insights into the diffusion and aggregation of less-reactive metals at the alloy/oxide interface, enhancing our understanding of solute capture and its contribution to the degradation of oxidation resistance in alloys.
Here, we present an investigation of Fe-14Cr-3W-0.4Y-0.4Zr-0.18Ti (wt.%) as a reduced-activation oxide-dispersion strengthened (ODS) ferritic steel, an alternative to the “14YWT” structural alloys designed for nuclear energy applications. Gas atomization reaction synthesis (GARS) was used to produce these Zr-modified powders with a non-equilibrium (metastable) phase that enable a heat treatment (delayed) route for producing nanoscale oxide-dispersion strengthening. In situ synchrotron X-ray diffraction and transmission electron microscopy were used to analyze phase evolution as a function of temperature and time, revealing formation of highly dispersed oxide nanoprecipitate phases at temperatures above those needed for powder consolidation and shaping of components. This would allow fabrication of net-shape parts using conventional powder-processing methods prior to thermal activation of a reaction producing nanocrystalline Y-(Ti, Zr)-O particles, with a size of 20 ± 7 nm. These results can inform processing of tubes, cladding, sheets, and plates for use in advanced fission and fusion reactors.
The capacity and voltage behavior of electrochemically impregnated sintered nickel positive plates was examined by galavanostatic charging; and discharging in a flooded electrolyte cell. Three different concentrations of KOH (40 percent, 31 percent, and 26 percent) and 31 percent KOH containing dissolved nitrate, sulfate or silicate were investigated. The end of charge voltage at C/10 charge and at 10 C showed the following order: 40 percent KOH is greater than 31 percent KOH alone and in the presence of the anions greater than 26 percent KOE. The middischarge voltage at C/2 discharge was higher in 26 percent KOH, almost the same for 31 percent KOH with and without the added contaminants and much lower for 40 percent KOH. The plate capacity was marginally affected by cycling in all cases except for 40 percent KOH where the capacity declined after 1000 cycles at 80 percent DOD. At the end of cycling the plate tested in the presence of sulfate and silicate experienced measurable weight loss as a result of active material extrusion. Cyclic voltammetry of miniature electrodes in 31 percent KOH showed that the second oxidation peak that corresponds to the formation of a different phase of oxidized Ni has a lower peak current at -5 C compared to 25 C and oxygen evolution occurs a higher potential at -5 C. The reduction peak (discharge reaction) is more polarized at 25 C compared to -5 C. The presence of silicate alters the potentials only marginally. The implications of these results in plate treatment and low temperature operation are discussed.
Three-dimensional flower-like SnO{sub 2}-based structures have been produced by the alkaline hydrothermal treatment of t-SnO{sub 2} powder with no additive and in the presence of aminoterephthalic acid (ATPA). The synthesis products have been characterized by a variety of physicochemical techniques (scanning electron microscopy, transmission electron microscopy, Raman and IR spectroscopies, X-ray diffraction, and others). The results demonstrate that raising the ATPA concentration in the reaction mixture changes the morphology of the materials and leads to the SnO{sub 2} → SnO{sub 2}/Sn{sub 3}O{sub 4} → Sn{sub 3}O{sub 4} phase transformation in the structures through the formation of SnO{sub x} nonstoichiometric tin oxide phases with 1 < x < 2. Hydrothermal treatment of the starting reagents in the presence of ≤75 wt % ATPA leads to the formation of hierarchical structures dominated by a nonstoichiometric tin oxide, which is thermally unstable at t ≥ 500°C. The morphology and phase composition of the synthesized structures have been shown to have a significant effect on the electronic conductivity of the material.
Iron oxide nanoparticles are present throughout the Earth and undergo mineral phase transformations that affect their stability and reactivity. The formation of the iron oxide hematite from a 2-line ferrihydrite (2lnFh) precursor requires both phase transformation and growth. Whether phase transformation occurs before or after substantial particle growth, and by which mechanisms particles grow, remain unclear. We conducted time-resolved studies employing X-ray diffraction, room temperature and cryogenic transmission electron microscopy, preferential dissolution by oxalate buffer, and low temperature SQuID magnetometry to investigate the kinetics and mechanism of hematite formation from 2lnFh. A novel form of magnetic measurement was found to be exquisitely sensitive to the presence of hematite, detecting its formation at far lower concentrations than possible with X-ray diffraction. These results indicate that small hematite domains were present even in as-prepared 2lnFh suspensions as a side-product of 2lnFh synthesis and that the hematite domains increased in size and crystallinity with aging time at elevated temperatures. In conclusion, second-order kinetics reveals that the hematite growth is consistent with a particle-mediated growth mechanism, possibly oriented attachment.
Solid-oxide electrolyzer cells (SOECs) based on a yttria-stabilized zirconia (YSZ) oxide electrolyte produce hydrogen from water with the assistance of excess thermal energy; however, Sr diffusion within the Gd-doped CeO 2 (GDC) barrier layer during processing or operation can lead to the formation of unwanted secondary phases such as SrO and SrZrO 3 . Here, to establish and compare the degree of impact of these phases on SOEC performance, we conduct first-principles calculations to study their bulk oxide conductivities and compare them to that of the YSZ electrolyte. We find that SrO has a low conductivity arising from the poor mobility and low concentration of mobile oxygen vacancies, and its presence in SOECs should therefore be avoided. SrZrO 3 also has a lower oxide conductivity than YSZ; however, this discrepancy is primarily due to lower vacancy concentrations rather than low mobility. We find that sufficient levels of Y-doping on the Zr site can increase oxygen vacancy concentrations in SrZrO 3 to achieve an oxide ionic conductivity on par with that of YSZ, thereby mitigating any potential deleterious effect on transport performance. Energy-dispersive X-ray spectroscopy confirms that Y is the most common minority element present in SrZrO 3 forming near the GDC–YSZ interface, alleviating concerns regarding the impact of SrZrO 3 on device performance. These results from our combined computational–experimental analysis can inform future engineering strategies designed to limit the detrimental effects of Sr-induced secondary phase formation on SOEC performance.
Developing stable cathode materials represents a crucial step toward long-life sodium-ion batteries. P2-type layered oxides are important as cathodes for their reversibility, but their long-term performance in full cells remains a key challenge. Herein, we report Na 0.75 Co 0.125 Cu 0.125 Fe 0.125 Ni 0.125 Mn 0.5 O 2 with an intergrowth of ordered P2 and P3 phases, studied by neutron diffraction and Rietveld refinement. A stable electrochemical performance is achieved in Na half cells with 100% capacity retention at a rate of C/10 after 100 cycles (initial capacity of 90 mAh/g), 96% capacity retention at a rate of 1 C after 500 cycles (initial capacity of 70 mAh/g), and 85% capacity retention at a rate of 5 C after 1000 cycles (initial capacity of 55 mAh/g). Stable full cell performance is achieved with 84.2% capacity retention after 1000 cycles at a rate of 1 C. Synchrotron X-ray diffraction, spectroscopy, and imaging are applied to elucidate the relationship between chemical/structural evolution and battery performance. A reversible local and global structural evolution is observed during initial cycles. Meanwhile, the challenges with enabling prolonged cycling (beyond 1000 cycles) may be associated with Fe dissolution and formation of a copper oxide phase. This study implies that cathodes with complex chemical and structural formations may stabilize electrochemical performance and highlights the importance of decoupling the contribution of each transition metal to performance degradation.
The recently developed solid oxide metal–air redox battery (SOMARB) operating on oxide-ion chemistry represents an emerging and promising energy storage technology, which is well suited for managing grid stability and efficiently harvesting renewable energy. Compared to widely reported metal–air batteries operating on liquid-phase alkali-ion or alkaline chemistries, the SOMARB uniquely features a direct reduction of the high concentration of O 2 molecules in the gas phase without invoking the detrimental formation of a diffusion-blocking oxide phase on the surface of the oxygen electrode. A typical SOMARB is composed of a reversible solid oxide cell (RSOC) and an energy storage unit (ESU), which is capable of storing a high capacity of energy at high power without safety concerns. In this article, the SOMARB concept birth and development are reviewed and discussed with the fundamental differences separating it from other types of metal–air batteries, along with the challenges and opportunities around commercialization. Particular focus is placed on the functional materials tailored for ESU and RSOC applications, including recent experimental and computational efforts to boost the redox activity of Fe-based ESU materials and the performance of intermediate temperature RSOCs. Lastly, candid opinions are offered on the challenges and opportunities facing the future development of SOMARBs toward large-scale energy storage applications.
The development of a new class of multicomponent ultra-high temperature ceramics (MC-UHTCs), often referred to as high-entropy UHTCs, has gained increased interest due to the possibility of improved thermomechanical and oxidation properties. In this study, a systematic approach by gradual addition in the UHTC components ranging from a binary to a dense quaternary (Ta,Nb,Hf,Ti)C is synthesized using spark plasma sintering (SPS). The solid solutioning was the critical factor in homogenizing the composition in the multicomponent system. The segregation of NbC and HfC was seen in binary and ternary UHTC systems, while a single-phase homogeneity was observed in the quaternary UHTC improving its hardness up to 34.8 GPa. The presence of closely spaced slip lines in the MC-UHTCs enhances resistance to indentation damage up to 72% at an applied load of 200 N. The formation of complex mixed oxide phase of Hf6Ta2O17 ensued in the lower to negligible oxidation even up to 3 min of plasma exposure with temperature exceeding 2800°C. In sum, though the entropy remains medium (0.96R) for the selected system, the quaternary UHTC system undoubtedly has significantly better thermomechanical performance when compared to established baseline UHTCs. Furthermore, this raises the debate on the justification for calling a multicomponent system a “high entropy” to be seen in a new light. The developed MC-UHTCs elicits the paradigm of this new class of UHTCs expanding their potential in thermal protection systems for hypersonic applications.
Isoprene has the largest global non-methane hydrocarbon emission, and the oxidation of isoprene plays a crucial role in the formation of secondary organic aerosol (SOA). Two primary processes are known to contribute to SOA formation from isoprene oxidation: (1) the reactive uptake of isoprene-derived epoxides on acidic or aqueous particle surfaces and (2) the absorptive gas–particle partitioning of low-volatility oxidation products. In this study, we developed a new multiphase condensed isoprene oxidation mechanism that includes these processes with key molecular intermediates and products. The new mechanism was applied to simulate isoprene gas-phase oxidation products and SOA formation from previously published chamber experiments under a variety of conditions and atmospheric observations during the Southern Oxidant and Aerosol Studies (SOAS) field campaign. Our results show that SOA formation from most of the chamber experiments is reasonably reproduced using our mechanism, except when the concentration ratios of initial nitric oxide to isoprene exceed ~ 2, the formed SOA is significantly underpredicted. The SOAS simulations also reasonably agree with the measurements regarding the diurnal pattern and concentrations of different product categories, while the total isoprene SOA remains underestimated. The molecular compositions of the modeled SOA indicate that multifunctional low-volatility products contribute to isoprene SOA more significantly than previously thought, with a median mass contribution of ~ 57 % to the total modeled isoprene SOA. However, this contribution is intricately intertwined with IEPOX-derived SOA (IEPOX: isoprene-derived epoxydiols), posing challenges for their differentiation using bulk aerosol composition analysis (e.g., the aerosol mass spectrometer with positive matrix factorization). Furthermore, the SOA from these pathways may vary greatly, mainly dependent on the volatility estimation and treatment of particle-phase processes (i.e., photolysis and hydrolysis). Our findings emphasize that the various pathways to produce these low-volatility species should be considered in models to more accurately predict isoprene SOA formation. The new condensed isoprene chemical mechanism can be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess isoprene SOA formation on a larger scale.
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Gas-phase product detection studies of o-hydroxyphenyl radical and O 2 are reported at 373, 500, and 600 K, at 4 Torr (533.3 Pa), using VUV time-resolved synchrotron photoionisation mass spectrometry. The dominant products are assigned as o-benzoquinone (C 6 H 4 O 2 , m/z 108) and cyclopentadienone (C 5 H 4 O, m/z 80). It is concluded that cyclopentadienone forms as a secondary product from prompt decomposition of o-benzoquinone (and dissociative ionization of o-benzoquinone may contribute to the m/z 80 signal at photon energies ≳9.8 eV). Ion-trap reactions of the distonic o-hydroxyphenyl analogue, the 5-ammonium-2-hydroxyphenyl radical cation, with O 2 are also reported and concur with the assignment of o-benzoquinone as the dominant product. In addition, the ion-trap study also provides support for a mechanism where cyclopentadienone is produced by decarbonylation of o-benzoquinone. Kinetic studies compare oxidation of the ammonium-tagged o-hydroxyphenyl and o-methylphenyl radical cations along with trimethylammonium-tagged analogues. Reaction efficiencies are found to be ca. 5% for both charge-tagged o-hydroxyphenyl and o-methylphenyl radicals irrespective of the charged substituent. G3X-K quantum chemical calculations are deployed to rationalise experimental results for o-hydroxyphenyl + O 2 and its charge-tagged counterpart. The prevailing reaction mechanism, after O 2 addition, involves a facile 1,5-H shift in the peroxyl radical and subsequent elimination of OH to yield o-benzoquinone that is reminiscent of the Waddington mechanism for β-hydroxyperoxyl radicals. These results suggest o-hydroxyphenyl + O 2 and decarbonylation of o-benzoquinone serve as plausible OH and CO sources in combustion.
Nanoscale zero-valent iron (nZVI) is a potential remediation agent for uranium-contaminated groundwaters, however, a complete mechanistic understanding of the processes that lead to uranium immobilization has yet to be achieved. In this study, the short-term anoxic reaction of U(VI) with fresh, (anoxic) aged and corroded nZVI particles was investigated under aqueous conditions conducive to the formation of thermodynamically stable U(VI)-Ca-CO3 ternary aqueous complexes. The first stage of the reaction between U(VI) and nZVI was assigned to sorption processes with the formation of surface U(VI)-carbonate complexes. Aged nZVI removed U(VI) faster than either fresh or corroded nZVI and it is hypothesized that U reduction initially occurs through the transfer of one electron from Fe(II) in the nZVI surface oxide layer. Evidence for reduction to U(V) was obtained through X-ray photoelectron spectroscopy and by determination of U-O bond distances of ~2.05 Å and 2.27 Å by U LIII-edge X-ray absorption spectroscopy detection of U-O bond distances at ~2.05 Å and 2.27 Å with these distances , similar to thoseat observed for the U(V) site in the mixed U(V)/U(VI) carbonate mineral wyartite. Scanning transmission electron microscopy also demonstrated that U was present as a nanoparticulate phase after one day of reaction, rather than a surface complex. Further reduction to U(IV), as observed in previous studies, would appear to be rate-limiting and coincident with the transformation of this meta-stable U-carbonate phase to uraninite (UO2).
We have demonstrated a low temperature formation (300 °C) of higher-k HfO{sub 2} using atomic layer deposition (ALD) on an in-situ thermal oxidation GeO{sub x} interfacial layer. It is found that the cubic phase is dominant in the HfO{sub 2} film with an epitaxial-like growth behavior. The maximum permittivity of 42 is obtained for an ALD HfO{sub 2} film on a 1-nm-thick GeO{sub x} form by the in-situ thermal oxidation. It is suggested from physical analyses that the crystallization of cubic phase HfO{sub 2} can be induced by the formation of six-fold crystalline GeO{sub x} structures in the underlying GeO{sub x} interfacial layer.
The presence of the top electrode on hafnium oxide-based thin films during processing has been shown to drive an increase in the amount of metastable ferroelectric orthorhombic phase and polarization performance. This “Clamping Effect,” also referred to as the Capping or Confinement Effect, is attributed to the mechanical stress and confinement from the top electrode layer. However, other contributions to orthorhombic phase stabilization have been experimentally reported, which may also be affected by the presence of a top electrode. In this study, it is shown that the presence of the top electrode during thermal processing results in larger tensile biaxial stress magnitudes and concomitant increases in ferroelectric phase fraction and polarization response, whereas film chemistry, microstructure, and crystallization temperature are not affected. Through etching experiments and measurement of stress evolution for each processing step, it is shown that the top electrode locally inhibits out-of-plane expansion in the HZO during crystallization, which prevents equilibrium monoclinic phase formation and stabilizes the orthorhombic phase. This study provides a mechanistic understanding of the clamping effect and orthorhombic phase formation in ferroelectric hafnium oxide-based thin films, which informs the future design of these materials to maximize ferroelectric phase purity and corresponding polarization behavior.
An unconventional reaction mechanism in an In 2 O 3 /Pd(1 1 1) inverse model catalyst for the CO 2 hydrogenation reaction has been uncovered: In 2 O 3 is partially reduced at room temperature in a reaction atmosphere as a result of its direct contact with Pd(1 1 1), which is an efficient H 2 splitter. The reduction induces changes in surface free energy, leading to a dynamical restructuring at the In 2 O 3 /Pd(1 1 1) interface via formation of InO x and outward diffusion of Pd, as revealed by ambient pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and density functional theory simulations. This dynamical restructuring eventually promotes the growth of 2D InPd y O x nanodomains as the catalytically active phase and the exclusive formation of methanol upon hydrogenation of CO 2 at room temperature. A comparable high selectivity toward CH 3 OH was found in more realistic bulk catalytic systems (2 wt% Pd/In 2 O 3 catalyst and commercial CZA catalyst). Scanning tunneling microscopy under ultrahigh vacuum and ambient pressure reaction atmospheres further reveals the structural dynamics at the InO x /Pd(1 1 1) interface, where we follow in situ the evolution of the InO x particles on Pd(1 1 1) and the mobility of the InPd y O x nanodomains in a CO 2 + H 2 environment. The present findings of the formation of a mixed oxide phase in a dynamically restructuring metal/reducible-oxide interface indicate further implications for other heterogeneous catalytic systems beyond the present CO 2 hydrogenation example and highlight the importance of in situ investigations.
Fundamental observations of particle size distributions are needed to develop models that predict the fate and transport of radioactive materials in the atmosphere following a nuclear incident. The extent of material transport is influenced by the time scales of particle formation processes (e.g., condensation, coagulation). In this study, we investigated the influence of cooling time scales on size distributions of uranium, aluminum, and iron oxide particles that are synthesized separately under identical run conditions inside the controlled environment of an argon plasma flow reactor. Two distinct temperature distributions are imposed along the flow reactor by varying the argon flow rate downstream of the plasma torch. The vaporized reactants of uranium, aluminum, and iron are cooled from about 5000K to 1000K before they are collected on silicon wafers for ex situ scanning electron microscope analysis. The microscope images show that the sizes of the largest aluminum and iron oxide particles heavily depend on the cooling time scales, whereas significant size variation with cooling rate is not observed for uranium oxide particles. In addition, the size distribution of aluminum oxide particles exhibits the broadest range among all three metal oxides studied. We performed simulations of particle size distributions using a kinetic model that couples gas phase oxidation chemistry with particle formation processes, including nucleation, condensation, and coagulation. The model results demonstrate the strong sensitivity of particle size distribution to different cooling histories (i.e., temperature vs residence time) along the flow reactor. In conclusion, the kinetic model also helps identify directions for future research to improve the predictions.