Graphical Abstracts of Published Research

​Critical mineral source potential from oil & gas produced waters in the United States
Science of The Total Environment (2024)
The volume of produced water, a by-product of oil & gas operations and other energy processes, has been growing across the United States (U.S.) along with the need to manage or recycle this wastewater. Produced water contains many naturally occurring elements of varying concentrations, including critical minerals which are essential to the clean energy transition. However, the current understanding of critical mineral concentrations in produced water and the associated volumes across the U.S. is limited. This study has assessed available databases and literature to gain insight into the presence and concentration of five high priority critical minerals, namely cobalt, lithium, magnesium, manganese, and nickel. The U.S. Geological Survey's National Produced Waters Geochemical Database was the main data source used for determining average critical mineral concentrations in produced water from the major oil and gas reservoirs in the U.S. The volumes of produced water for these major reservoirs were coupled with these concentrations to provide insights into where critical minerals are likely to have high abundance and therefore more recovery options. The analysis indicated the highest recovery potential for lithium and magnesium from produced water in the Permian basin and the Marcellus shale region. However, these assessments should be considered conservative due to the limited availability of reliable concentration data. It is expected more critical mineral recovery options could emerge with comprehensive characterization data from more recent and representative sources of produced water.
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Low-cost Thermal/Environmental Barrier Coatings: A First-principles Study
Computational Materials Science (2023)
Development of low-cost advanced thermal/environmental barrier coating (T/EBC) materials with acceptable thermal and mechanical properties is essential for safeguarding ceramic composites substrate against thermal and chemical degradation, thereby enhancing the efficiency of components in the high-temperature section of gas turbine engines. To this end, we employed density functional theory-based approaches to predict the thermodynamic, mechanical, and thermal properties of rare earth disilicates based on abundant rare earth elements, namely La2Si2O7 and Ce2Si2O7, as potential alternatives to the current-state-of-the-art ytterbium disilicate EBCs that uses expensive and scarce element Yb. The present study predicts that G-phase Ce2Si2O7 has an ultralow thermal conductivity (0.26 W/m/K at 1500 K) and the apparent bulk coefficient of thermal expansion (ABCTE) (≈6.9 × 10-6 K−1) slightly higher than SiC, demonstrating great potential as low-cost high-performance T/EBC. However, La2Si2O7 and Ce2Si2O7 undergo an A- to G-phase polymorphic transition at around 1470 K, resulting in significant changes to crystal structure and lattice parameters, and accordingly CTE and lattice thermal conductivity.
Computational Design of High-Entropy Rare Earth Disilicates as Next-Generation Thermal/Environmental Barrier Coatings
​Acta Materialia (2023)
Development of advanced thermal/environmental barrier coating (T/EBC) materials possessing balanced thermal-mechanical properties is crucial to protect SiC-based ceramic composites from chemical and thermal attack for better performance of components in the hot section of gas turbine engines. In this work we utilize density functional theory (DFT) together with combinatorial chemistry methodology to design high-performance high-entropy rare earth disilicates (β-RE2Si2O7 (RE=Yb, Y, Er, Lu, La, Ce,)) for enhanced phase stability, desired coefficient of thermal expansion (CTE), low lattice thermal conductivity and good mechanical properties. The CTE are determined by phonon calculations at different volumes within the quasi-harmonic approximation. The lattice thermal conductivities are evaluated by the Debye-Callaway model considering three phonon processes. We show that the solid solution of YYbSi2O7 exhibits lowered lattice thermal conductivity than pure cases and a good range of CTE. We also present good EBC candidate materials of Er1/4Lu1/4Y3/4Yb3/4Si2O7 and Er1/2Lu1/2Y1/2Yb1/2Si2O7 with very low lattice thermal conductivity < 0.23 W/m/K at 1500 K and a good match of average CTE (5.1 - 5.2×10−6 K−1) with SiC without compromising mechanical properties. These new EBC with outstanding multi-functional properties are believed to enable significant increases in the performance of protective components in gas turbines. This work also illustrates an efficient and reliable computational framework in accelerating the design of next-generation T/EBC.
Phase stability and phase transformation related to Nb/Ta additions in a Ni-based superalloy
Journal of Alloys and Compounds (2023)
Alloys based on Inconel 725, and with elevated levels of Nb and Ta, have been designed for application in environments requiring
high strength and corrosion resistance at elevated temperatures. The present study further examines the phase stability and subsequent phase transformation in those alloys following aging and long-term exposure of up to 10,000 h at 700 °C. Changes in the microstructure were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two alloy bases with different Ti/Al ratio were considered. Following aging, the Nb/Ta additions favor the development of γ′/γ″ co-precipitates in the low Ti/Al ratio alloy, whereas the high Ti/Al ratio counterpart only contained γ″ precipitates. Upon 10,000 h exposure, complex geometrical closed-packed (GCP) phases formed from the γ′/γ″ co-precipitates in a layer-by-layer manner, i.e., the plate-like δ phase precipitates became decorated by blocky α-Cr and enveloped by a wavy γ′ film. Increasing the concentration of Nb and/or Ta did not change the basic characteristic of this phase transformation; however, the precipitate number density increased in the grain interior. On the other hand, the Ta addition in the alloy with low Ti/Al ratio promoted the formation of Ta-rich η precipitates at the grain boundaries while Nb promoted δ precipitates. The underlying phase transformation behavior of the layer-by-layer structure is likely initiating from the γ″-δ phase transition that ejects Cr and Ti into the neighboring phases, thus resulting in local phase separation into α-Cr and γ′ thin film. This phase transformation process had a significant impact on the phase morphologies in the high Nb/Ta alloys exposed at 700 °C for 10,000 h, with experimental results contradicting those from computational prediction.
Oxidation of nickel in solid oxide cells during electrochemical operation: Experimental evidence, theoretical analysis, and an alternative hypothesis on the nickel migration
Journal of Power Sources (2023)
Solid oxide cells with nickel/yttrium-stabilized zirconia (Ni/YSZ) cermet electrodes exhibit Ni migration which can cause severe cell performance degradation. The experimentally reported migration behavior of Ni is complicated, and the mechanisms remain under debate. This work discusses the possible mechanism of Ni migration related to the oxidation of Ni at the Ni-electrolyte interfaces under polarization via combined experimental study and theoretical analysis. In the experiments, NiO is found at the Ni-YSZ interfaces in the active layer in both tested fuel cells and electrolyzer cells, despite that the nominal oxygen partial pressure at the hydrogen electrode is well below the thermodynamic threshold for Ni oxidation. Due to the volume expansion during Ni oxidation and the outward diffusion nature of NiO growth, Ni oxidation and reduction of NiO back to Ni can cause Ni relocation. Thermodynamic analysis shows that the oxygen partial pressure near the Ni-electrolyte interface can be significantly higher than the hydrogen electrode under polarization, which can cause Ni oxidation and concentration increase of the gaseous Ni(OH)2, and the latter accelerates the transport of Ni. As such, a new hypothesis for Ni migration in solid oxide cells is proposed in which the interfacial oxidation of Ni plays an essential role.
Data-driven discovery of a formation prediction rule on high-entropy ceramics
Acta Materialia (2023)
The interest in high entropy ceramics (HECs) has increased steadily due to their superior properties. However, the prediction of their formation still poses challenges for the discovery of new systems. Here, we discover a rational rule for designing single-phase high entropy transition metal diborides (HEBs) using data-driven approach. The machine learning (ML) model is trained on data collected via high-throughput experiments (HTEs). K nearest neighbors (KNN) model shows an experimental validation accuracy of 93.75%. By implementing interpretable ML method, we demonstrate that a mismatch of the bonds between boron and transition metals (δ_B-TM) dominates the formation of HEBs. We propose an empirical rule that HEBs favor forming a single phase when δ_B-TM < 3.66; otherwise, multiphase. The rule has a high accuracy of 93.33% for new HEBs predictions. In addition, we contribute 165 high quality HEBs data in total, which can promote the development of materials informatics in HEBs. Moreover, this data-driven strategy can be expanded to accelerate the search for new HECs, paving a pathway to design novel HECs with superior properties rapidly.
Phase-field modeling of alloy oxidation at high temperatures
Acta Materialia (2023)
Oxide growth is a complex process involving transport of reactive species, heterogeneous reactions, and microstructure evolution. Predicting  oxidation kinetics, especially the oxide morphological change has been a longstanding challenge. Here we develop a phase-field model for predicting the oxide growth kinetics of a multicomponent alloy during high temperature oxidation, focusing on internal oxidation (non-protective) and its transition to external oxidation (protective). The predicted kinetics and oxide morphology are analyzed and compared to the classical Wagner's theory and an existing analytical model by Zhao and Gleeson. Some assumptions used in the analytical models and the limitation are discussed. In addition, it is demonstrated that the morphology and distribution of the initial oxide nuclei play an important role in the later stage oxide connectivity and thus the transition to external oxidation.
Carburization susceptibility of chromia-forming alloys in high-temperature CO2
Corrosion Science (2022)
It is generally believed that when a chromia scale forms in high-temperature CO2 environments, carbon uptake of alloys is nearly eliminated, thereby preventing carburization and associated degradation of properties. Herein we quantitively assess this notion through careful examination of several chromia-forming Ni-based and Fe-based commercial alloys after long-term (10,000-h) exposures to CO2 and air at 700 °C. Small but measurable carbon uptake ensued independent of alloy base metal (Ni/Fe), alloy crystal structure (FCC/BCC), and chromia growth rate. The rate for Ni-based alloys was strongly dependent on Si and Mn content, highlighting the need to understand possible long-term effects on mechanical properties.
Applied machine learning model comparison: Predicting offshore platform integrity with gradient boosting algorithms and neural networks
Marine Structures (2022)
Offshore oil and gas platforms operating past their design life can pose significant risk to operators and the surrounding environment, as the integrity of these structures decreases over time due to a variety of stressors. This has important implications for industry and government, which are seeking to safely extend the life of platforms for continued use or reuse for alternative offshore energy applications. As a result, there is a need to quantify the remaining useful life (RUL) of operating platforms by analyzing the effects that stressors may have on structural integrity. This study provides a platform risk assessment by employing two machine learning models to forecast the removal age of existing platforms in the U.S. federal waters of the Gulf of Mexico (GoM): a gradient boosted regression tree (GBRT) and an artificial neural network (ANN). These data-driven models were applied to a large, extensive dataset representing the natural and engineered offshore system. Both models were found to provide promising predictions, with 95–97% accuracy and predictions within 1.42–2.04 years on average of the observed removal age during validation. These results can be applied to inform life extension opportunities for fixed and mobile offshore platforms, as well as localized maintenance strategies aiming to prevent operational and environmental risk while maintaining energy production.
The detrimental effect of elemental contaminants when using B additions to improve the creep properties of a Ni-based superalloy
Scripta Metarialia (2021)
The effect of Si contamination when using B to improve the creep properties of a Ni-based superalloy was investigated using advanced characterization techniques and first-principles simulations on alloys with high and low B levels with varying Si contents. The positive effect of B segregation along grain boundaries on the creep properties was mitigated by the presence of Si which showed a similar segregation preference. Density functional theory calculations were used for validation by calculating grain boundary cleavage energies. Silicon was shown to decrease grain boundary cohesion which offsets the positive effect of B.
Data Science Techniques, Assumptions, and Challenges in Alloy Clustering and Property Prediction
Journal of Materials Engineering and Performance (2021)
Data analytics methods have been increasingly applied to understanding materials chemistry, processing due to the manufacturing approach, and uni-axial and cyclic property relationships in the highly complex space of alloy design. There are several benefits to applying data analytics to this space, including the ability to manage non-linearities in the responses of the alloy attributes and the resulting mechanical properties. However, key difficulties in applying and understanding the results of data analytics include the often lack of reported assumptions and data processing steps necessary to improve interpretation and reproducibility in derived results. In this work, the methods used to generate clustering and correlation analyses for experimental 9% Cr ferritic-martensitic steel data were investigated and the resulting implications for mechanical property predictions were assessed. This work uses principal component analysis, partitioning around medoids, t-SNE, and k-means clustering to investigate trends in composition, processing and microstructure information with creep and tensile properties, building on work done previously using a smaller version of the same dataset. The initial assumptions, preprocessing steps and methods are investigated and outlined in order to depict the fine level of detail required to convey the steps taken to process data and produce analytical results. The variations in the resulting analyses are explored due to the influence of new and more varied data.
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​Phase field simulation of anode microstructure evolution of solid oxide fuel cell through Ni(OH)2 diffusion
​Journal of Power Sources (2021)
Microstructure evolution in a solid oxide fuel cell anode is strongly affected by operating condition. The detailed mesoscale evolution mechanism under operating condition is still under debate. Here we develop a phase-field model to simulate microstructure evolution through formation and diffusion of gaseous Ni(OH)2. We studied the coarsening kinetics and redistribution of Ni under different steam distributions and compare it to that under pure hydrogen condition. The results suggest that although the presence of a steady gradient of steam leads to redistribution of Ni, Ni(OH)2 formation and diffusion do not significantly change the Ni coarsening rate under strictly humid conditions, contrary to commonly reported hypotheses. It is concluded that competing mechanisms other than Ni(OH)2 diffusion must be responsible for the experimentally observed Ni redistribution and enhanced coarsening under humid conditions.
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Partitioning of tramp elements Cu and Si in a Ni-based superalloy and their effect on creep properties
Materialia (2020)
An alloy's processing history, including melting, remelting or the choice of stock material, affects its purity and eventually involves tramp element pickup or retention. In this investigation, variants of a novel Ni-based superalloy were manufactured with different levels of purity. The so-called low purity alloys contained 0.138 wt. % Cu and 0.019 wt. % Si while the Cu and Si levels were below x-ray fluorescence (XRF) detection limits of 0.003 and 0.010 wt. %, respectively, in the high purity ingots. Atom-probe tomography (APT) was carried out and revealed Si partitioning at the following interfaces: grain boundaries, MC carbide/γ, M3B2 boride/γ and M3B2 boride/γ′. Copper was found to primarily segregate to the γ′ precipitates. An average of 2.4 × decrease in creep life and 4.3 × decrease in creep ductility was measured in the low purity alloys, which was attributed to the embrittlement caused by Si segregation to grain boundaries. Furthermore, the positive effect of B on the creep properties was mitigated by the presence of Si. Thermodynamic predictions for the matrix and γ′ precipitate compositions represented the trends observed experimentally although the extent of preferential partitioning lacks accuracy. Monte Carlo simulations were performed to describe the partitioning of Cu and Si atoms to either γ or γ′ phases.
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High-temperature oxidation of transient-liquid phase bonded Ni-based alloys in 1 bar and 250 bar carbon dioxide
Materials at High Temperatures (2020)
ompact microchannel heat exchanger designs for supercritical CO2 power cycles are valued for equipment size reduction and increased heat transfer. They can be fabricated via transient-liquid phase (TLP) bonding, which utilises a lower melting point interlayer for bonding facilitation of etched thin plates. Haynes 230 and 282 TLP bonded stacks were exposed to high-purity CO2 at 1 bar/700 °C to 4000 h and 250 bar/720 °C (supercritical) to 1500 h. All samples obeyed parabolic kinetics after an oxide build-up stage of 500–1000 h. Mass gains were similar for the base and TLP bonded alloys, and oxide thicknesses and scales did not significantly differ around the TLP bond regions. There were some slight oxide compositional differences for the H282 TLP samples at 250 bar. There was a slight pressure effect on mass gain, as samples exposed at the higher pressure experienced higher initial mass gains, but then slower continued oxide growth.
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Volatilization behavior of supported Au nanoparticle arrays under H2 at high temperature
The Journal of Physical Chemistry C (2020)
The fabrication and experimental study of model Au nanoparticle arrays (nanodots) were used to allow a direct comparison of reactive evaporation at elevated temperatures and nonambient H2-containing gas atmospheres with theoretical models. Strong temperature and H2-concentration dependencies were confirmed as expected, and the observed dependence in particle diameter with time was effectively described using a reactive evaporation and gas phase diffusion based quantitative model. Deviations from model expectations for a monolithic film of Au due to nanoparticulate structure were effectively captured by accounting for both (1) the reduced effective surface area of Au on the substrate surface and (2) the enhanced partial vapor pressure of Au-containing species in the vapor phase due to surface energy effects. The approach described here is broadly applicable to the quantitative study of stability in noble and precious metal nanoparticles and thin films for sensing and catalytic applications in harsh environmental conditions.
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Reduced-order model for microstructure evolution prediction in the electrodes of solid oxide fuel cell with dynamic discrepancy reduced modeling
Journal of Power Sources (2019)
Microstructure evolution in the electrodes of solid oxide fuel cell is an important degradation mechanism which reduces active sites for redox reaction and the electric conductivity. Phase field models for microstructure evolution simulation are usually expensive for large scale simulations. In this work, a reduced-order coarsening model is developed using dynamic discrepancy reduced modeling, which reduces the model order by inserting Gaussian process stochastic functions into the dynamic equations of Ostwald ripening. The reduced order model has been calibrated on a dataset generated by a phase field model that has been well validated to experiments. A validating dataset has also been generated with which the model prediction show good agreement. This model is further applied to predict long term microstructure evolution in different SOFC electrodes. This work is the first attempt of building a degradation model of SOFC using data science techniques.
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Effect of O2 on the long-term operation and corrosion of steel X65 in CO2-H2O environments for direct supercritical CO2 power cycle applications
​The Journal of Supercritical Fluids (2019)
The corrosion resistance of high-strength low-alloy carbon steel was investigated in simulated conditions that exist in direct supercritical CO2 power cycle heat exchangers. The experiments were performed at 80 bar for 500 h at either 50 °C with samples exposed to H2O-saturated CO2 and CO2-saturated H2O or 250 °C undersaturated CO2. Exposures were conducted with and without O2.
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Corrosion rates were determined via mass loss measurements. Corrosion products were analyzed via X-ray diffraction. Morphology of the corroded surfaces were determined with scanning electron microscopy. Localized corrosion, pitting rate was determined using a optical profilometry. The results indicate that oxygen has a large effect on the corrosion rate of carbon steel in these environments while the effect of temperature is less significant. Pitting, in the absence of oxygen, was minimal and primarily located on polishing scratches. At 50 °C, higher general corrosion and pitting rates were detected in CO2-saturated H2O than in H2O-saturated CO2.
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Design and thermomechanical properties of a γʹ precipitate-strengthened Ni-based superalloy with high entropy γ matrix
Journal of Alloys and Compounds (2019)
​A novel Ni-based superalloy was designed with γʹ precipitate strengthening, controlled γ/γʹ lattice misfit and high configurational entropy of the γ matrix for improved high temperature performance up to 800 °C. The alloy contains nano-sized γʹ precipitates, MC and other grain boundary carbides. Three variants of the alloy were fabricated using vacuum induction melting, a computational-based homogenization cycle for reducing solidification segregation, and thermomechanical processing steps of forging followed by hot rolling to produce plates from which ASTM standard test specimens were extracted. Tensile testing at room temperature and elevated temperatures up to 800 °C revealed superior yield stress when compared to Nimonic 105 with good tensile strength values. Furthermore, the three alloys are machinable with maximum stresses comparable to standard practices as determined using deformation mechanisms maps obtained from Gleeble testing and EBSD analysis. Due to the composition of the experimental alloys falling outside the typical range used to populate thermodynamic databases, differences in phase predictions and related temperatures were observed between the experiment and Thermo-Calc predictions. The γʹ forming elements Ti and Nb had a similar effect on the γʹ precipitates and indirectly contributed to changing the entropy of the γ matrix. Based on the results of this study, these alloys have the potential for use at 800 °C in energy structural applications. Definitions related to this novel class of alloys are discussed.
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Structural evolution of a Ni alloy surface during high-temperature oxidation
Oxidation of Metals ​(2018)
We show that considerable structural transformations occur at a Ni alloy surface during the transient stages of high-temperature oxidation. This was demonstrated by exposing the alloy to high-temperature CO2 for short times at both atmospheric and supercritical pressures. A protective Cr-rich oxide layer formed after only 5 min at 700 °C and persisted for longer exposures up to 500 h. Voids formed and grew over time by the condensation of metal vacancies generated during oxidation, while the alloy surface recrystallized after sufficient oxidation had occurred. The oxygen potential established at the oxide/alloy interface led to oxidation along the newly formed grain boundaries as well as adjacent to and inside of the voids. Al, the most stable oxide-former and present at low concentration in the alloy, was preferentially oxidized in these regions. The results provide an improved understanding of the internal oxidation of Al and its role in enhancing scale adhesion for this class of Ni alloys.
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High-entropy functional materials
Journal of Materials Research (2018)
​While most papers on high-entropy alloys (HEAs) focus on the microstructure and mechanical properties for structural materials applications, there has been growing interest in developing high-entropy functional materials. The objective of this paper is to provide a brief, timely review on select functional properties of HEAs, including soft magnetic, magnetocaloric, physical, thermoelectric, superconducting, and hydrogen storage. Comparisons of functional properties between HEAs and conventional low- and medium-entropy materials are provided, and examples are illustrated using computational modeling and tuning the composition of existing functional materials through substitutional or interstitial mixing. Extending the concept of high configurational entropy to a wide range of materials such as intermetallics, ceramics, and semiconductors through the isostructural design approach is discussed. Perspectives are offered in designing future high-performance functional materials utilizing the high-entropy concepts and high-throughput predictive computational modeling.
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Thermodynamics of concentrated solid solution alloys
​Current Opinion in Solid State and Materials Science (2017)
This paper reviews the three main approaches for predicting the formation of concentrated solid solution alloys (CSSA) and for modeling their thermodynamic properties, in particular, utilizing the methodologies of empirical thermo-physical parameters, CALPHAD method, and first-principles calculations combined with hybrid Monte Carlo/Molecular Dynamics (MC/MD) simulations. In order to speed up CSSA development, a variety of empirical parameters based on Hume-Rothery rules have been developed. Herein, these parameters have been systematically and critically evaluated for their efficiency in predicting solid solution formation. The phase stability of representative CSSA systems is then illustrated from the perspectives of phase diagrams and nucleation driving force plots of the σ phase using CALPHAD method. The temperature-dependent total entropies of the FCC, BCC, HCP, and σ phases in equimolar compositions of various systems are presented next, followed by the thermodynamic properties of mixing of the BCC phase in Al-containing and Ti-containing refractory metal systems. First-principles calculations on model FCC, BCC and HCP CSSA reveal the presence of both positive and negative vibrational entropies of mixing, while the calculated electronic entropies of mixing are negligible. Temperature dependent configurational entropy is determined from the atomic structures obtained from MC/MD simulations. Current status and challenges in using these methodologies as they pertain to thermodynamic property analysis and CSSA design are discussed.
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Phase field modeling of microstructure evolution and concomitant effective conductivity change in solid oxide fuel cell electrodes
Journal of Power Sources (2017)
Microstructure evolution plays an important role in the performance degradation of SOFC electrodes. In this work, we propose a much improved phase field model to simulate the microstructure evolution in the electrodes of solid oxide fuel cell. We demonstrate that the tunability of the interfacial energy in this model has been significantly enhanced. Parameters are set to fit for the interfacial energies of a typical Ni-YSZ anode, an LSM-YSZ cathode and an artificial reference electrode, respectively. The contact angles at various triple junctions and the microstructure evolutions in two dimensions are calibrated to verify the model. As a demonstration of the capabilities of the model, three dimensional microstructure evolutions are simulated applying the model to the three different electrodes. The time evolutions of grain size and triple phase boundary density are analyzed. In addition, a recently proposed bound charge successive approximation algorithm is employed to calculate the effective conductivity of the electrodes during microstructure evolution. The effective conductivity of all electrodes are found to decrease during the microstructure evolution, which is attributed to the increased tortuosity and the loss of percolated volume fraction of the electrode phase.
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Computational modeling of high-entropy alloys: Structures, thermodynamics and elasticity
Journal of Materials Research (2017)
This article provides a short review on computational modeling on the formation, thermodynamics, and elasticity of single-phase high-entropy alloys (HEAs). Hundreds of predicted single-phase HEAs were re-examined using various empirical thermo-physical parameters. Potential BCC HEAs (CrMoNbTaTiVW, CrMoNbReTaTiVW, and CrFeMoNbReRuTaVW) were suggested based on CALPHAD modeling. The calculated vibrational entropies of mixing are positive for FCC CoCrFeNi, negative for BCC MoNbTaW, and near-zero for HCP CoOsReRu. The total entropies of mixing were observed to trend in descending order: CoCrFeNi > CoOsReRu > MoNbTaW. Calculated lattice parameters agree extremely well with averaged values estimated from the rule of mixtures (ROM) if the same crystal structure is used for the elements and the alloy. The deviation in the calculated elastic properties from ROM for select alloys is small but is susceptible to the choice used for the structures of pure components.
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​Thermodynamic effects of calcium and iron oxides on crystal phase formation in synthetic gasifier slags containing from 0 to 27 wt.% V2O3
Fuel (2015)
Thermodynamic phase equilibria in synthetic slags (Al2O3–CaO–FeO–SiO2–V2O3) were investigated with 0–27 wt.% vanadium oxide corresponding to industrial coal–petroleum coke (petcoke) feedstock blends in a simulated gasifier environment. Samples encompassing coal–petcoke mixed slag compositions were equilibrated at 1500 °C in a 64 vol.% CO/36 vol.% CO2 atmosphere (Po2 ≈ 10−8 atm at 1500 °C) for 72 h, followed by rapid water quench, then analyzed by inductively coupled plasma optical emission spectrometry, X-ray diffractometry, and scanning electron microscopy with wavelength dispersive spectroscopy. With increasing CaO content, FeO content, or both; the slag homogeneity region expanded and a composition range exhibiting crystals was reduced. The mullite (Al6Si2O13) crystalline phase was not present in the slags above 9 wt.% FeO while the karelianite (V2O3) crystalline phase was always present in compositions studied if a sufficient amount of vanadium existed in the slag. Based on the present experimental equilibrium evaluation, a set of isothermal phase diagrams showing effects of CaO and FeO on thermodynamic phase stabilities in the vanadium-bearing slags is proposed. Some uses of the diagrams for potential industrial practice are discussed.
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Pinning force from multiple second-phase particles in grain growth
Computational Materials Science (2014)
A factor that can reduce particle pinning force significantly in grain growth is found when the grain-boundary is pinned by multiple particles. The pinning force, in this case, is a function of particle radius over inter-particle distance. A previously proposed phase-field model for particle pinning is used to validate this predicted pinning force reduction in two and three dimensions. When applied to coherent pinning particles, the same effect is observed in simulations. It is shown that, at application relevant high particle volume fraction, the average grain size is affected by this reduction of pinning force.
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 First principles screening of B2 stabilizers in CuPd-based hydrogen separation membranes: (1) Substitution for Pd
Journal of Alloys and Compounds (2013)
We report here a screening study using first-principles method in an attempt to identify ternary elements that can extend the CuPd B2 phase field at reduced Pd contents and thus lower cost. A total of 37 alloying elements are included for unbiased screening. The results show that addition of Mg, Al, Sc, Ti, Y, Hf, Zr, Ga, La, and Zn lowers the enthalpy of formation of the B2 phase noticeably. The atomic size, electronic density of states, charge transfer, and electronegativity are analyzed to interpret the results. Compromise between enthalpy and solubility suggests additional potential alloying elements: V, Fe, Cr, Nb, Ta, and Mn. To assess the effects of alloying on mechanical properties, we calculated the equation of states and elastic constants of 10 example alloys at 6.25 at% solute contents.
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