Browsing by Author "Misture, Scott"

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Open Access
Aurivillius Phase Oxides for Photocatalytic Applications
(New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering., 2010-09) Nichols, Eric; Misture, Scott
The systematic design of materials for energy production and remediation of environmental concerns, based on direct knowledge of the structural and optical absorption characteristics of layered and traditional perovskites, is desirable for use in rational photocatalyst development. Aurivillius phase ceramic oxides have been suggested as potential candidates for photocatalytic water splitting applications based on the structural similarities between this class of materials and titanium dioxide, namely, transition metals in octahedral coordination with oxygen. Structural characterization of the three-layer lanthanide titanate Aurivillius oxides Bi2A2Ti3O12 (A2 = La2, Pr2, Nd2, LaPr, LaNd, PrNd) via combined Rietveld refinements of x-ray and neutron powder diffraction data has revealed that these materials reside in the orthorhombic space group B2cb. We have demonstrated that the optical band gaps for the three-layer lanthanide titanates can be red shifted by as much as 0.3 eV via combined structural and electronegative (bismuth replacement) means and exhibit a Vegard type relationship between band gap and chemical composition over the range of compositions tested (A2 = La2, Pr2, Nd2, LaPr, LaNd, PrNd, LaBi, PrBi, NdBi, La0.5Nd0.5Bi, Pr0.5Nd0.5Bi, Bi2). Additionally, the band gaps for these materials can be correlated with a single TiO6 octahedron and the manipulation of its Ti-O bond lengths. A method of quantification of BX6 octahedral anion distortions in traditional ferroic perovskites and layered perovskites has been developed based on Rietveld refinement of neutron diffraction data and vectorial analysis. Quantification of the octahedral distortion characteristics of the archetype perovskites via a survey of data contained within the Inorganic Crystal Structure Database (ICSD) has been completed. A preliminary link between antiferroelectric double hysteresis behavior and octahedral distortions in traditional perovskites has been established for the compounds PbZrO3, Pb2WMgO6, PbHfO3 and NaNbO3. Similar to traditional perovskites, octahedral distortions in the layered perovskites have been demonstrated, quantified and shown to trend with an average A-site cations’ ionic radius below 1.4 Å. Anatase, the photocatalytic polymorph of titanium dioxide, exhibits a similar distorted TiO6 octahedral environment and its structure was analyzed and compared with that of the Aurivillius oxides.
Open Access
Characterizing the Reduction of NixMg1-xAl2O4
(New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering., 2012-04) Hill, Brenden; Misture, Scott
The reduction of NixMg1-xAl2O4 in H2 to form nickel metal and a remnant oxide was characterized by XRD, HTXRD, TGA, pycnometry, TEM, and SEM. The aim of the work was to investigate the dynamics of the system to better understand its capabilities and limitations for catalysis applications. ZrO2 was added to the majority of samples to discourage the transformation of metastable spinel phases to Ɵ or α-Al2O3. As the reduction progresses, one O2- is lost for each Ni2+ which reduces to Ni metal. The temperature of the onset of reduction was shown to vary by composition in flowing 4% H2/Ar via TGA, with NiAl2O4 beginning to reduce at ~ 780 °C. The onset temperature of lower nickel compositions were quite close to each other, starting at ~ 900 °C for Ni0.75Mg0.25Al2O4. Ni0.25Mg0.75Al2O4 and Ni0.5Mg0.5Al2O4 were shown to form Ni metal and a nonstoichiometric spinel of the same Mg-Al ratio as the starting composition. NiAl2O4 and Ni0.75Mg0.25Al2O4 were found to become unstable as full reduction was approached, and metastable spinel, Ɵ-Al2O3, and α-Al2O3 formed sequentially given sufficient time at temperature. A phase diagram was constructed in a previously uninvestigated region of the NiAl2O4 – MgAl2O4-Al2O3 ternary phase diagram using the phase stability of the remnant spinel as indication of the edge of the spinel stability phase field. Rietveld refinements were performed on all compositions reduced at temperatures from 650 to 1100 °C to quantify structural changes in the spinel and phase fraction, crystallite size and microstrain in all phases. The formation of non-stoichiometric spinel upon reduction was confirmed by density measurements of the reduced specimens using helium pycnometry. Significant progress was made towards understanding the dynamics of an important catalyst system. The majority of nickel metal was present as faceted crystallites on the surface, explaining previously observed high catalytic activities. Subsequent studies can use the phase stability and kinetic results of this work to identify additives to stabilize the metastable spinel structures. Good candidates were identified in ZrO2 and Nb2O5, and TiO2 was found to promote the formation of corundum.
Open Access
Charge Storage in Defect Engineered Oxide Nanosheets
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2022-02) Flint, Madeleine N.; Misture, Scott; Pilgrim, Steven; Tidrow, Steven; Shulman, Holly
Layered δ-MnO2 (birnessite) exhibits a unique response to the reduction of Mn4+ to Mn3+, where the reduced Mn3+ Jahn-Teller distorted octahedron creates a 'surface Frenkel defect.' The reduced Mn3+O6 octahedron migrates out of the the plane of the sheet and creates a vacancy within the sheet. The charged defect content may be controlled during flocculation of exfoliated nanosheets by equilibrating the nanosheet suspension in a pH regulated environment. It has been shown earlier that charged defects in δ-MnO2 improve electrochemical properties by increasing capacitance beyond 300 F/g, decreasing charge transfer resistance by a factor of 10, and improving life cycle stability. Building on earlier work, the present study aims to understand the synthesis of layered MnO2 and Mn-containing layered perovskites and their chemomechanical response during charge and discharge. X-ray total scattering, X-ray absorption spectroscopy, and Raman spectroscopy were employed to probe the chemomechanical response during electrochemical cycling of MnO2 electrodes in a custom-built electrochemical cell. While surface Frenkel defect content remained constant throughout electrochemical cycling, reversible reduction of Mn4+ to Mn3+ occurred, presumably within the sheet. MnO2 floccules equilibrated at a pH of 2 and 4 underwent 17% and 13% reduction from Mn4+ to Mn3+ upon K+ intercalation, respectively. Such reductions caused mechanical deformation of porous MnO2 electrodes and was found to be dependent on defect content and degree of restacking of the nanosheets. Floccules with a low degree of nanosheet restacking experienced lateral expansion upon K+ insertion of 0.7% and 0.5% for equilibration at a pH of 2 and 4, respectively, with no change in the interlayer (basal) direction spacing. However, MnO2 nanosheet floccules with a high degree of restacking experienced a 1.1% and 1.2% lateral expansion upon charging and a contraction in their basal direction of 1.7% and 0.7% for pH 2 and 4 samples, respectively. The interlayer contraction has been noted earlier, but largely eliminating the sheet-to-sheet stacking provides direct access to knowledge of the in-sheet chemomechanical response. Microstructures of air-dried and freeze-dried floccules as well as floccules with varying degrees of surface Frenkel defect content were similar. Surface Frenkel defect content remained constant across processing variations such as drying conditions and degree of restacking when equilibrated to the same pH.
Open Access
Cold Sintering Dwell Time Effects on Gadolinium Doped Ceria
(2022-05) Voss, Maria Giovanna; Misture, Scott; Tidrow, Steven; Wu, Yiquan
Gadolinium doped nanocrystalline cerium dioxide (GDC) powder samples were effectively densified using the developing process known as “cold sintering” with adjustments to dwell time investigated. Ceria was chosen due to its application as an electrolyte in solid oxide fuel cells. Trials of 3, 6, and 12 hours were completed with the peak dwell temperature of 350˚C through die jacket heaters within a Carver press applying 500 MPa. The mixed NaOH-KOH molten hydroxide flux facilitated densification to approximately 61, 76, and 79% relative density for sintering durations 3, 6, and 12 hours, respectively. Although density increased, grain growth was not directly affected by sintering time. Measurements found the average grain size in the 3- and 12-hour samples to be greater than that of the 6-hour, with large standard deviations in all samples. Following cold sintering, samples were annealed at 800˚C for 6 hours, in order to burn off the remaining hydroxides. Traces of flux were found between layers of delamination, and striations heavier in sodium were identified on a 12-hour sample. Chemical analysis of 3 different duration samples showed that the plateau of flux removal is reached after 6 hours of sintering. The sodium and potassium concentrations were higher in the 3-hour sample, as compared to the similar values of the 6- and 12-hour samples. Significant data was collected which contributes to the progress of cold sintering development.
Open Access
Colloidal Processing and Photocatalytic Properties of Titanate-Niobate Nanosheets
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2016-09) Gubb, Tyler A.; Misture, Scott; Pilgrim, Steven; Giesche, Herbert
Nanosheets are a class of materials defined by extreme anisotropy, typically with widths and thicknesses on the order of micrometers and nanometers, respectively. These materials are often synthesized from a bulk ceramic using soft-chemical processing to exfoliate individual sheets or clusters of sheets. Nanosheets are advantageous to applications dominated by surface chemistry, for example photocatalysis and electrochemistry. Titanate and niobate nanosheets have been extensively studied for these applications. The layered titanate-niobate nanosheets defined by the chemistries TiNbO5, H3Ti5NbO14 and HTi2NbO7 demonstrate photocatalytic behavior and interesting colloidal properties due to the liquid crystalline nature of nanosheet colloids. In this work, colloidal suspensions of single-layer titanate-niobate nanosheets were synthesized by exfoliation of the parent oxides. AFM showed single layer sheets to be between 1 and 2 nm thick, while XRD measurements showed a mean restacking distance between 2 and 2.5 nm. This restacking distance was shown to vary approximately 6 Å, presumably due to displacement of H2O as suggested by FTIR. Nanosheet colloids displayed liquid crystalline behavior which could be eliminated in favor of new mesophases by reducing pH or by adding various salts such as Na2SO4 in a process called reassembly. Adding acid resulted in gelled colloids defined by a sharp increase in viscosity below a critical pH of approximately 6. Rheological and diffraction measurements support a gelation mechanism resulting from the excluded volume effect and electrostatic repulsion between sheets. Adding a salt, however, resulted in flocculated nanosheets without an accompanying viscosity increase. Flocculation proceeds by some face-to-face restacking of neighboring nanosheets into tactoids which are themselves assembled into an edge-to-face 3D network. XRD patterns suggest tactoid thicknesses of 4-6 sheets. Powders prepared from assembled gels or floccules possessed unique mesostructures dependent on their colloidal processing as shown by BET and SEM. Assembled gels were found to be mostly mesoporous and dominated by small pores formed between restacked sheets, as indicated by average pore widths between 6-15 nm and large N2 adsorption hysteresis. Assembled floccules were significantly less mesoporous with essentially no adsorption hysteresis and larger pores averaging between 20-27 nm formed between linked tactoids. UV-visible adsorption spectra showed band gaps to be reduced from 4.1 eV for the parent phase to 3.6 eV for the exfoliated nanosheets. Reassembly with various cations raised the band gaps to 3.9-4.1 eV. Each chemistry demonstrated photocatalytic hydrogen production with a co-catalyst when exfoliated or reassembled. Exfoliated H3Ti5NbO14 producing the most hydrogen at a rate of 426 μmol H2/hr/g, whereas HTiNbO5 produced the least hydrogen. Assemblies of commercial TiO2 photocatalyst and exfoliated HTiNbO5 nanosheets were found to be highly photochemically active without an additional co-catalyst, producing 412 μmol H2/hr/g. Exfoliated nanosheets were also found to photocatalytically decompose methylene blue with exfoliated H3Ti5NbO14 demonstrating the highest decomposition rate. Photocatalytic behavior was therefore found to vary with nanosheet chemistry but not crystallographic motif.
Open Access
Crystal Structure Studies, Electrical and Magnetic Properties of 2, 3, 4, 5-Layer Aurivillius Oxides
(New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering., 2015-02) Shi, Jiawanjun; Misture, Scott
The crystal structures of assorted Aurivillius compositions have been indexed and refined from combined Neutron and x-ray powder diffraction data. The crystal structure distortions were discussed quantitatively from mainly three aspects: octahedron tilt, perovskite in-plane distortion and interlayer adjustment between perovskite and Bi-O layer. Both A-site cation ionic radius and number of perovskite layers influence the structure distortion details which is decisive for the spontaneous polarization. Dielectric, ferroelectric and conductivity measurements were used to link the structure features with electrical properties. Besides the macroscopic structure information, local nano-region disorder would affect electrical properties. A series of rare earth Aurivillius oxides with Mn-doping was studied for possible multiferroic behavior. All the studied compositions turned out to be paramagnetic with (anti)ferromagnetic interactions at very low temperature and magnetoresistance was observed.
Open Access
Data-driven discovery of a formation prediction rule on high-entropy ceramics
(Elsevier, 2023-07) Wang, Kun; Yan, Yonggang; Pei, Zongrui; Gao, Michael; Misture, Scott
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 (δB−TM) dominates the formation of HEBs. We propose an empirical rule that HEBs favor forming a single phase when δB−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.
Open Access
Deactivation and Spinel Phase Regeneration of Reduced Nickel Aluminate
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2016-09) Ladonis, Alec C.; Misture, Scott; Payton, Eric; Pilgrim, Steven; Lipke, David
Nickel aluminate (NiAl2O4) is an attractive material for dry reforming of methane due to selective reduction yielding catalytically active Ni metal particles. However, during this process, the catalyst is subject to mechanisms that ultimately degrade the catalytic activity. In the present work, characterization of reduced NiAl2O4 + 2.5 wt. % ZrO2 (Ni + defect spinel) via novel in-situ low voltage high resolution high temperature scanning electron microscopy (HTSEM) coupled with in-situ high temperature X-ray diffraction (HTXRD) is investigated to address: 1. The feasibility of observing surface behavior during deactivation and regeneration of the catalyst by reduction and reoxidation; 2. Surface and bulk microstructural evolution during thermal exposure; and 3. Stability of surface and bulk morphology over time at operating temperatures. This work reveals surface microstructural and topographical features as well as their effects on functional behavior that can be observed via in-situ HTSEM. However, challenges in the analysis include local control of pO2, damage to the specimen surface, and the inability to replicate exact service conditions in terms of flowing gasses. HTXRD performed in air demonstrates that oxidation of Ni occurs between 400-500°C, and concludes by 900°C. Spinel XRD peaks grow upon oxidation, indicating phase regeneration. HTXRD under industrial grade N2 with small amounts of oxygen exhibit spinel, Ni metal, and ZrO2 peaks until 1100°C is reached, no NiO formation is exhibited, even upon cooling. An increase in spinel peak intensity is also observed under N2 with 5-20 ppm pO2. HTSEM demonstrates that Ni metal surface particles migrate and coalesce upon the support surface between 900-1000°C revealing pit structures upon the surface. The composition of the primary particles is unknown due to differences in pO2 between HTSEM and HTXRD, but is expected to be Ni metal. Mechanisms for particle movement are also unclear; movement is not gravity driven and Ni is a solid at this temperature. Additional reduction of the matrix is observed between 900-1000°C, which is likely due to low pO2 within the SEM chamber. Above 1000°C, all observed particles reincorporate into the matrix, exposing pit like structures on the surface, similar to ones observed post migration. These pits do not heal with time at temperature; instead the pits and crevices mature with time as evidenced by sharpening of edges and corners of crystalline facets. The overall reduction and reoxidation process is expressed as: NiAl2O4 [H2 + heat] → Ni1-xAl2O4-x + xNi + x/2O2 [O2 + heat] → xNiO + Ni1-xAl2O4-x [time + heat] → NiAl2O4. HTSEM provides direct evidence that particles migrate and coalesce during the initial steps of catalyst regeneration, revealing a roughened surface with pits and crevasses. This surface rearrangement may provide longer distances for particle migration to occur, slowing the coalescence and regeneration of the original spinel to improve the catalyst service time.
Open Access
Defective Manganese and Vanadium Oxide Nanosheets for Electrochemical Supercapacitors
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2018-08) Gao, Peng; Misture, Scott; Edwards, Doreen; Sundaram, S.K.; Liu, Dawei
Electrochemical supercapacitors, which can provide higher power density than batteries and higher energy density than electrostatic capacitors, have received great attention in recent years as promising alternative energy storage devices. δ-MnO2 and VO2 nanosheets are considered as most promising electrode materials, due to their low cost, environmental friendliness, and large capacitance. Especially, their layered structures can provide high-speed pathways for the intercalation of protons or alkali cations during electrochemical cycling, which leads to excellent charge storage capability. While a lot of effort has been devoted to improve the electrode performance by microstructure control or load the samples on conducting materials, new intrinsic approaches are urgently needed to realize more and faster charge storage. in this study, a new strategy has been proposed to increase the capacitance of δ-MnO2 and VO2 nanosheets by intentionally creating defects (such as cation vacancies) in their lattice structures. δ-MnO2 nanosheets are prepared by exfoliating the parent crystals, and then flocculated under carefully controlled experimental conditions. The obtained 3-D porous assemblies with 150 m2/g specific surface area are equilibrated in varied pH, in order to create charged defect pairs we term 'surface Frenkel Defects.' The XANES data demonstrates an increase of the Mn3+/Mn4+ ration with decreasing pH equilibration values. The X-ray scattering and PDF analysis shows that the Mn surface Frenkel defect content reaches 26.5% for the nanosheet assemblies equilibrated at pH = 2 and 19.9% for the pH = 4 sample, indicating that equilibration at lower pH leads to the formation of more Mn vacancies in the reassembled δ-MnO2 nanostructures. The electrochemical results show that the specific capacitance increased from about 200 F/g (pH = 4) to over 300 F/g (pH = 2) by intentional introduction of ~30% surface Frenkel defects, while at the same time the charge transfer resistance decreased from ~15 Ω to ~3 Ω, indicating direct correlation of Mn cation defects with specific capacitance. The alkali cation intercalation mechanism has also been investigated through in-situ X-ray PDF and XANES measurements. The in-situ XRD and PDF data show reversible expansion/contraction of the nanosheet layers upon charge/discharge, as well as unchanged interlayer spacing during cycling. The in-situ XANES data exhibits a reversible shift of the absorption edge to lower energies for different pH equilibrated samples when decreasing the applied potentials, indicating the reduction of Mn4+ to Mn3+ and confirms that the Faradaic redox reaction is the main charge storage mechanism in the defective MnO2 nanosheet system. A slower reduction of Mn for the MnO2 nanosheets with higher defect content has been observed when comparing the oxidation state derived from XANES and that calculated from cyclic voltammetry, emphasizing the important role of defects in the charge storage process without affecting the Mn oxidation states. The pure and Mn-doped VO2(B) nanosheets have been prepared by hydorthermal methods. Mn incorporation leads to porous and more open structures, which facilitates sodium ion intercalation and thereby greatly improves its charge storage performance. In general, this work provides a new way to the design next generation electrochemical supercapacitors through controlling the defect structures of layered transition metal oxide nanosheets.
Open Access
Doping KxMNO2 Nanosheets With Copper, Nickel, and Cobalt Cations to Maximize Capacitance
(2019-12) Wallisch, Abigail E.; Misture, Scott; Stohr, Darren; Eklund, Andrew
Copper, nickel, and cobalt-doped δ-MnO2 nanosheets were synthesized using self-propagating flame synthesis and ion exchange. They were analyzed after both steps using X-Ray diffraction, scanning electron microscopy, and electron dispersive spectroscopy. The samples were painted onto nickel foil to make electrodes to test for the specific capacitance. The copper doped sample had an approximate 1:33 Cu:Mn ratio and a specific capacitance of 176 F/g, which was a 22% increase from undoped samples. The nickel doped sample had an estimated 1:35 Ni:Mn ratio and had a specific capacitance of 194 F/g, which was a 29% increase from undoped samples. The cobalt sample had a Co:Mn ratio of about 1:28 and a specific capacitance of 119 F/g, which was a 14% decrease from the undoped sample. It was concluded that the number of vacancies left behind in the sample after ion exchange increased the capacitance of the sample.
Open Access
Durable Cr-substituted (Ba,Cs)1.33(Cr,Ti)8O16 hollandite waste forms with high Cs loading
(Wiley, 2022-02) Misture, Scott; Zhao, Mingyang; Birkner, Nancy; Schaeperkoetter, Joseph; Koch, Robert; Russell, Patrick; Besmann, Theodore; Amoroso, Jake; Brinkman, Kyle
A series of Cr-substituted hollandite solid solution BaxCsyCr2x+yTi8−2x−yO16 over a broad range of Cs content (x + y = 1.33, 0 ≤ x and y ≤ 1.33) were systematically investigated. A monoclinic-to-tetragonal phase transition was induced by increasing Cs content in the tunnel sites of the hollandite structure, and all members of the series show structure modulations related to the ordering of the Ba/Cs and vacancies along the tunnels. The thermodynamic stability of the Cr-substituted hollandite samples was measured via high-temperature oxide melt solution calorimetry, which included making the first measurements of the enthalpies of drop solution for Cs2O and BaO in sodium molybdate solvent at 800°C. Thermodynamic stability increased with increasing Cs content for the series of Cr-substituted hollandite, which also exhibited a greater thermodynamic stability compared to other substituted hollandite analogs including Zn, Ga, Fe, and Al variants. The leaching performance, also known as aqueous durability, demonstrated that the fractional Cs release in the Cr hollandite samples is much lower than in other hollandite systems. After 7 days of leaching at 90°C, the lowest Cs release was observed in the sample with the highest Cs content, approximately 22 wt.% Cs. The Cs release could be further suppressed, by approximately 3× if the sample was further densified and sintered. The Cs release results correlated inversely to the thermodynamic stability, suggesting that the thermodynamic stability may be used in future materials design for nuclear waste immobilization.
Open Access
Improving Solid Oxide Fuel Cell Cathode Stability with Chemical Substitutions
(New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering., 2015-02) McDevitt, Kyle; Misture, Scott
The chemical flexibility of the perovskite structure allows enormous opportunity to modify lanthanum strontium manganite (LSM), a material that has been demonstrated to be a practical and efficient solid oxide fuel cell cathode. Calcium and nickel substitutions on the A and B sites, respectively, have been identified as potentially stabilizing substitutions on the basis of thermodynamic stability in the perovskite structure and compatibility with other materials in the fuel cell. In this study, the phase stability of YSZ-LSM composites is evaluated experimentally. LSM materials with chemistries La0.8Sr0.2MnO3-δ (LSM-20), La0.8Sr0.1Ca0.1MnO3-δ (LSM+Ca), La0.8Sr0.2Mn0.7Ni0.3O3-δ (LSM+Ni), and La0.8Sr0.1Ca0.1Mn0.7Ni0.3O3-δ (LSM+Ca+Ni), were annealed at 850°C and 1350°C in a compact with 8 mol% yttria stabilized zirconia (YSZ) to evaluate potential reactions between the two materials. Substituting calcium for A-site strontium transformed more than half of the zirconia to a cubic calcia stabilized phase. Nickel substitutions to B-site manganese stopped this transformation, but encouraged precipitation of up to 12 weight percent lanthanum zirconate, versus about 5 weight percent for calcium modified LSM and 3 percent for the base chemistry. SEM analysis shows that the zirconate crystallizes with a fine grained microstructure, and is especially prominent in samples intentionally depleted of manganese. Formation of the zirconate phase trends closely with an expansion of the LSM unit cell, suggesting the material becomes lanthanum deficient as the reaction takes place. CO2 enriched air was shown to be the most reactive atmosphere, and incorporating 3% H2O into the atmosphere limited zirconate formation, especially for the nickel substituted sample. All LSM materials assumed R-3c symmetry after synthesis, however, calcium containing samples tended to form a Pnma phase as the material was annealed. Similarly to the observations on zirconate formation, CO2 enriched air encouraged this development and humid air slowed transformation. The samples annealed in humidified atmospheric air all adopted and remained in the R-3C phase, with no other reactions taking place. The space group symmetry is closely related to the Goldschmidt Tolerance Factor, which changes with chemical substitutions, including hydroxide from humid air, and the oxidation and spin state of transition metals in the structure.
Open Access
Investigations of Phase Transformations in AISI 5160 Steel and Ceria Partially Stabilized Zirconia via Electron Backscatter Diffraction Based Techniques
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2017-12) Tan, Wenxia; Lipke, David; Payton, Eric; Misture, Scott; Pilgrim, Steven; Sundaram, S.K.
In this dissertation, electron backscatter diffraction (EBSD) is applied to study the phase transformation in AISI 5160 steel and 10 mol% Ce doped ZrO2. For AISI 5160 steel, quantitative characterization of the volume fractions and spatial arrangements of the major constituents of microstructure is needed for continued progress toward the next generation of advanced steels. EBSD data can be used to simultaneously characterize orientation relationships as well as determine the volume fraction and size of grains of retained austerite and ferrite due to the difference in diffraction patterns caused by the different crystal structures of the two phases; however, distinguishing among martensite, ferrite, pearlite, and bainite using EBSD remains a challenge. A detailed analysis of the capabilities of EBSD-based methods for separation of the individual microstructural constituents is performed on samples of a commercial steel - AISI 5160. It is demonstrated that kernel average misorientation (KAM) data can be used to distinguish and quantify the constituents. Zirconia is one of many materials that experiences a displacive phase transformation. In zirconia, the tetragonal to monoclinic martensitic transformation can be induced either thermally or by applied stress. Since transformation from tetragonal to monoclinic phase is accompanied by a volume increase, it may be engineered to utilize the martensitic transformation as a toughening mechanism. In the present work, EBSD was used to observe the morphology and orientation relationships simultaneously. According to the EBSD study, all six orientation relationships were present. The theoretical predictions based on the phenomenological theory of martensitic transformations were also calculated for 10 mol% ceria doped zirconia and compared with experimental findings. The calculation of the strains showed that the correspondence B has the lowest lattice invariant strain among the three correspondences, and for the shape strain, the three correspondences are nearly equivalent.
Open Access
Luminescence and Scintillation in the Niobium Doped Oxyfluoride Rb4Ge5O9F6:Nb
(MDPI, 2022-06) Misture, Scott; Carone, Darren; Klepov, Vladislav; Schaeperkoetter, Joseph; Jacobsohn, Luiz; Aziziha, Mina; Schorne-Pinto, Juliano; Thomson, Stuart; Hines, Adrian; Besmann, Theodore; zur Loye, Hans-Conrad
A new niobium-doped inorganic scintillating oxyfluoride, Rb4Ge5O9F6:Nb, was synthesized in single crystal form by high-temperature flux growth. The host structure, Rb4Ge5O9F6, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge3O9 units connected by GeO3F3 octahedra. In its pure form, Rb4Ge5O9F6 shows neither luminescence nor scintillation but when doped with niobium, Rb4Ge5O9F6:Nb exhibits bright blue luminescence and scintillation. The isostructural doped structure, Rb4Ge5O9F6:Nb, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.9960(3) Å, b = 11.7464(6) Å, and c = 19.3341(9) Å. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggest that the niobium is located in an octahedral coordination environment. Optical measurements inform us that the niobium dopant acts as the activator. The synthesis, structure, and optical properties are reported, including radioluminescence (RL) measurements under X-ray irradiation.
Open Access
Mechanochemical Synthesis of Sustainable Ternary and Quaternary Nanostructured Cu2SnS3, Cu2ZnSnS4, and Cu2ZnSnSe4 Chalcogenides for Thermoelectric Applications
(MDPI, 2023-01) Misture, Scott; Nautiyal, Himanshu; Lohani, Ketan; Mukherjee, Binayak; Isotta, Eleonora; Malagutti, Marcelo Augusto; Ataollahi, Narges; Pallecchi, Ilaria; Putti, Marina; Rebuffi, Luca; Scardi, Paolo
Copper-based chalcogenides have emerged as promising thermoelectric materials due to their high thermoelectric performance, tunable transport properties, earth abundance and low toxicity. We have presented an overview of experimental results and first-principal calculations investigating the thermoelectric properties of various polymorphs of Cu2SnS3 (CTS), Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe) synthesized by high-energy reactive mechanical alloying (ball milling). Of particular interest are the disordered polymorphs of these materials, which exhibit phonon-glass–electron-crystal behavior—a decoupling of electron and phonon transport properties. The interplay of cationic disorder and nanostructuring leads to ultra-low thermal conductivities while enhancing electronic transport. These beneficial transport properties are the consequence of a plethora of features, including trap states, anharmonicity, rattling, and conductive surface states, both topologically trivial and non-trivial. Based on experimental results and computational methods, this report aims to elucidate the details of the electronic and lattice transport properties, thereby confirming that the higher thermoelectric (TE) performance of disordered polymorphs is essentially due to their complex crystallographic structures. In addition, we have presented synchrotron X-ray diffraction (SR-XRD) measurements and ab initio molecular dynamics (AIMD) simulations of the root-mean-square displacement (RMSD) in these materials, confirming anharmonicity and bond inhomogeneity for disordered polymorphs.
Open Access
Multiphase Titanate-Based Ceramic Waste Forms: Cerium Incorporation in Zirconolite and Pyrochlore and Radiation Stability
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2018-08) Clark, Braeden M.; Sundaram, S.K.; Misture, Scott; Wu, Yiquan; Cormack, Alastair
This thesis aims at developing ceramic waste forms as a potential replacement for the conventional glass waste forms for safe immobilization and disposal of nuclear wastes from legacy weapons programs as well as commercial power production. The body of work consists of two parts. The first part focused on fabrication and characterization of multiphase waste forms containing hollandite as the major phase with perovskite, pyrochlore, and zirconolite as secondary phases. The second part focuses on single phase pyrochlores and zirconolites for cerium (Ce) incorporation. Part I: Multiphase waste forms: Compositions of simulated multiphase waste forms were developed at Savannah River National Laboratory (SRNL) and processed at Alfred University using melt-processing and spark plasma sintering (SPS). The resulting multiphase waste forms contained a majority of hollandite along with perovskite, pyrochlore, and zirconolite phases as determined by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Cesium (Cs) incorporation into the hollandite phase is more prevalent in melt-processed sample, requiring further optimization of the SPS process. Radiation damage resistance is a vital performance requirement for waste forms. This was simulated in these multiphase ceramics by bombarding the samples with Au3+ and He+ ions. Heavy ions (Au3+) initiated ballistic processes that caused all phases in the affected volumes to amorphize. Regardless of the processing method, the amorphization behavior remained the same in all samples bombarded with Au3+ ions. The ion penetration depth was reduced in SPS-processed samples, attributed to the smaller grain sizes of these materials. Light ion (He+) irradiation caused the breakdown of the hollandite phase, while the remaining phases appeared to be unaffected. This behavior was confirmed in single phase hollandite samples. Part II: Single phase waste forms: High amounts of Ce can be incorporated into zirconolite and pyrochlore structures, up to 50 mol% for Zr in the case of zirconolite and 25 mol% for Nd in Nd2Ti2O7 pyrochlore, via solid state reaction. Perovskite is observed in small amounts in zirconolite materials (up to 7 wt%). With increasing Ce content, a transition from the 2M to 4M-zirconolite polymorph has been reported. The valence state of Ce will affect which atomic site that the Ce substitutes on. 2M-zirconolites contain trivalent Ce, which supports Ce substitution on both Ca and Zr sites. Both trivalent and tetravalent Ce are present in 4M-zirconolites, indicating greater partitioning onto the Zr site. All Ce substituted into pyrochlore materials is converted from tetravalent to trivalent during the reaction process to maintain charge neutrality of the structure. Ce-substituted zirconolite and pyrochlore materials were consolidated using SPS. The reducing environment of the SPS causes 4M-zirconolite to convert to perovskite and 2M-zirconolite due to the reduction of Ce4+ to Ce3+ causing a redistribution of Ce onto the Ca and Zr sites of zirconolite and stabilizing perovskite. The original phase assemblage of the materials can be restored by a heat treatment in air post-sintering. CaCeTi2O7 forms as an intermediate phase up until 1300°C, and 4M-zirconolite begins at 1350°C. The transformation to 4M-zirconolite is slow, but complete conversion to the original phase assemblage is achieved with a 24h heat treatment in air. The sintering behavior of Ce-substituted pyrochlore is unaffected by Ce content in the material. The integral waste form performance properties of radiation damage stability and chemical durability of Ce-substituted zirconolite and pyrochlore were simulated by implanting samples with He+ or Kr3+ with ion fluences equating to 0.5 dpa and Product Consistency Testing (PCT) and Materials Characterization Center (MCC-1) leach tests, respectively. Gracing-incidence x-ray diffraction (GIXRD) of the samples revealed that light ion (He+) irradiation had very little effect on the materials, whereas heavy ion (Kr3+) irradiation caused near complete amorphization of all materials tested, which corroborated well with the results from multiphase samples. No effect of Ce content on radiation damage behavior was seen. For zirconolite materials, Ce was only released during the PCT of CaZn0.9Ce0.1Ti2O7 indicating that the 4M-zirconolite polymorph is more chemically durable than 2M-zirconolite. Ce was released from Nd1.5Ce0.5Ti2O7 during both the PCT and MCC-1. During MCC-1 tests longer than 7 days, no Ce above the detectable limit of the ICP-AES was released. The Ce release values in these studies were comparable to those in similar tests performed previously in the literature.
Open Access
Nanoporous Glass-Ceramics for Gas Separation
(New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering., 2008-12) Miller, Michelene; Misture, Scott
The development of nanoporous gas separation membranes to extract H2 from a mixed fuel stream of CH4/H2O/CO2/CO is desirable for use in methane steam reforming reactions. Ni-containing cordierite glass-ceramics are candidate materials for this application. The final microstructure of these glass-ceramics includes two sieving mechanisms: (1) a 5 Å channel inherent to the cordierite crystal structure which grows normal to the surface of the glass, and (2) a nanoporous non-stoichiometric spinel in the bulk with theoretical pore diameters ranging from 4-8 Å. Through reaction with hydrogen we demonstrate the ability to form nanopores in NixMg1-xAl2O4 spinel. The stability limit for the non-stoichiometric spinel formed during the H2 reaction lies between 0.5_x_0.75. The formation of an equal number of nickel and oxygen vacancies in spinel is determined using Rietveld refinement of X-ray diffraction data and thermogravimetric analysis. These vacant sites form the nanopores in the defect spinel structure.
Open Access
(Ni0.375Cu0.375Mg0.25)Al2O4 Internal Reforming Catalyst for Solid Oxide Fuel Cell Anodes
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2016-09) Sowinski, Peter J.; Misture, Scott; Pilgrim, Steven; Giesche, Herbert
Nickel-based anodes are the most commonly used anodes for solid oxide fuel cell (SOFC). Unfortunately, with nickel based anodes, internal reforming of hydrocarbon fuels is often accompanied by carbon deposition, which covers the active nickel sites, resulting in degradation of cell performance. Ni0.375Cu0.375Mg0.25Al2O4 spinel catalyst was investigated for its use as a potential internal reforming layer for SOFC anodes. To determine if any reactions with NiO, Y0.08Zr0.92O2 (YSZ), or Gd0.2Ce0.8O2 (GDC) occur during fuel cell fabrication, Ni0.375Cu0.375Mg0.25Al2O4 was thoroughly mixed with NiO-YSZ and NiO-GDC and fired at temperatures ranging from 1300°C - 1500°C. Room temperature X-ray powder diffraction of the samples revealed spinel peak shifts, indicating compositional changes towards a higher nickel concentration spinel. Rietveld refinements of high temperature X-ray diffraction patterns were used to calculate the coefficient of thermal expansion of the spinel. With a calculated CTE of 10 x 10⁶ K⁻¹ from 25°C to 850°C, the CTE of the oxide spinel is at least 5% lower than Ni-YSZ, YSZ, and La0.8Sr0.2MnO3 (LSM). When 0.015g of spinel was added to the fuel cell anode, no change in the peak power density was observed while operating under hydrogen. However, 0.015g of spinel addition while operating under a non-coking methane fuel gas mixture caused an increase in peak power density from 74 mW⋅cm⁻² to 98 mW⋅cm⁻² (32% increase) at 850°C and 40 mW⋅cm⁻² to 52 mW⋅cm⁻² (31% increase) at 750°C. After 50 hours operating on a coking methane fuel gas mixture at 850°C, a fuel cell with no spinel additions had a peak power density drop of approximately 16%. With 0.015g of added spinel, the peak power density degraded by a lesser amount, 10%. Finally, with 0.05g of spinel added, the peak power density decreased by only 6%. After endurance testing, no carbon fibers were observed on the anode during SEM imaging. Raman shift shows the fingerprint graphitic carbon bands on the anode with no spinel additions, but not on the fuel cells with spinel additions. Due to the coefficient of thermal expansion mismatch, Ni0.375Cu0.375Mg0.25Al2O4 is not well suited for use as a reforming catalyst for high temperature SOFCs. However, it can be used in intermediate temperature fuel cells (IT-SOFCs). In addition, with optimization of the processing and application to the anode, Ni0.375Cu0.375Mg0.25Al2O4 could eliminate the need for upstream fuel reforming in IT-SOFCs.
Open Access
Observation of Surface and Mass Transport on Selectively Reducible Spinel Oxides
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2020-12) Ladonis, Alec C.; Misture, Scott; Cormack, Alastair; Tidrow, Steven; Ding, Junjun
While many investigations rely on macro- or nanoscale techniques to determine the deactivation mechanisms on selectively reducible oxide catalysts, little attention is paid to the surfaces and near-surface effects. This study addresses the literature gap by investigation of a model NiAl2O4 catalyst by novel low voltage in-situ high temperature scanning electron microscopy and other complementary techniques. Extensions to other compositions of Ni bearing aluminate spinel are presented. Expedited mass transport was identified on a bulk scale, on the surface, and around modifying oxide particles by in-situ techniques at relevant service ρO2 and temperatures. The regeneration pathway was found to depend on system ρO2. Oxidative atmosphere favors NiO formation and reaction with the remnant spinel whereas vacuum favors Ni reincorporation via either reaction at the triple phase boundary or by formation of a core-shell structure. Structural relaxation at the surface is evidenced by Raman spectroscopy, where vacuum reoxidized samples show band sharpening and splitting as compared to the reduced analogue, however, some surface defects are retained. The unusual nature of the surface of the selectively reduced spinel is highlighted by enhanced surface diffusion in the form of metal particle migration and coalescence, which was observed on the spinel above the reduction temperature. Comparison of Ni migration distances on two supports with varied defect content exhibit diffusional distances up to 350 times the predicted value. Additional surface pits are revealed during migration and coalescence, which are crystallographically registered to the spinel. These features do not heal; however, they sharpen with extended time at high temperature. Extensions to technologically relevant Ni-Mg-Co catalysts show that phase assemblages during redox cycling are indeed reversible. morphological changes during reduction and oxidation were proven to be irreversible, creating new features on each reduction. The catalyst upon the second reduction exhibits 50% more exsolved metal, a 4x increase in particle number density, and over a 50% decrease in particle size as compared to the initial reduction. Irreversible changes in the surface metal dispersion and surface area are attributed to microstructural maturation, where surface features promote better metal dispersion and surface area for catalytic reactions. A unique and unusual tunneling phenomenon was identified at the interface of the spinel and modifying oxide at elevated temperatures, implying the process is thermally activated. Changing the modifying oxide from ZrO2 to HfO2 increased the onset temperature by 200°C but had similar rates. Further investigation of the interface revealed non-homogeneously distributed voids form beneath the tunneled ZrO2, where the interface between the spinel and ZrO2 is Ni deficient, reminiscent of a defective spinel structure. A model is proposed based on enhanced interfacial diffusion, where voids occur due to variations in the magnitude of diffusional fluxes arising from the modifying oxide additions.
Open Access
On the Structure of Lithium and Strontium Borate Glasses Modified with Yttrium and Rare-Earth Cations Investigated by Vibrational Spectroscopy
(New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2020-11) Topper, Brian; Möncke, Doris; Clare, Alexis; Misture, Scott; Kamitsos, Efstratios
The thesis begins with a comprehensive review of the structure and properties of borate glasses. This is followed by a predominantly qualitative assessment of highly modified lithium and strontium borates containing yttrium and rare-earth oxide additions that have been prepared by the traditional melt-quenching technique. To the author's knowledge, reports on the ternary glasses studied here are not available in the literature. The feasibility of glass formation for these new compositions is discussed and the structures of the resulting materials have been studied, primarily, with vibrational spectroscopy as well as selectively with differential scanning calorimetry, X-Ray diffraction, and time-resolved fluorescence spectroscopy. Raman and Infrared spectroscopies suggest the glasses formed in the vicinity of the orthoborate stoichiometry are structurally similar to the high temperature phase of the related yttrium or rare-earth orthoborate crystals, regardless of whether the glass transition temperature lies above or below the corresponding phase transformation temperature. A relationship between the crystal phase transformation temperature and the glass transition temperature has been observed through the partial devitrification of borate melts containing relatively high yttrium content. That is, for glasses where the glass transition temperature is above the phase transformation temperature, only short-range order structural units akin to high temperature phase will be present in both the glass and any crystallization products. For glasses where the glass transition temperature is below the phase transformation temperature, the bulk glass is also made up of the short-range order structural units found in the high temperature phase; however, low temperature phase crystallites are detected with XRD. These crystallites, comprised only of tetrahedral orthoborate units arranged n B3O9^9- rings, are seen also as well-defined structures approximately 30 microns in diameter using confocal Raman microscopy. The addition of oxides with the high field strength trivalent yttrium ion to strontium borate glasses was found to depolymerize the borate network into ionic species while simultaneously increasing the glass transition temperature. In this series, the increase of the cation motion band frequency from 180 cm^-1 to 323 cm^-1 indicates the trivalent yttrium ions form stronger bonds with network oxygen than the divalent strontium ions. This correlates with the onset of the glass transition temperature increasing non-linearly from 630.7 °C to 652.2 °C for glasses containing 5 mol% and 25 mol% Y2O3, respectively. Lithium yttrium/rare-earth orthoborate glasses were found to consist solely of isolated trigonal borate units and to be insensitive to the use of either yttrium or other rare-earth elements, all of which have a well-known tetrahedral orthoborate crystalline phase. The absence of isomerization or disproportionation at the orthoborate stoichiometry implies the driving mechanism for glass formation in these glasses can be viewed as having a physical rather than chemical origin. This is to say, vitrification is dependent on the freezing-in of highly distorted, isolated trigonal borate structures - which are favored at high temperatures and in the melt compared to their tetrahedral counterpart. Several of the lithium yttrium/rare-earth orthoborate glasses were doped with Tb^3+ and the measured lifetime of the 543 nm emission using 375 nm excitation was ~2.2 ms. Measured lifetime decays, in conjunction with the relevant literature, suggest that even if trivalent rare-earth cations are present in small quantities, in a partially or fully depolymerized borate glass, the Tb^3+ ions will seek out local sites whose short range order corresponds to that of the high temperature crystalline phase. Indeed, this observation is in excellent agreement with the qualitative Raman interpretation of bulk strontium yttrium borate glasses where the introduction of the small quantities of Y2O3 is seen to induce the formation of trigonal orthoborate units below the pyroborate stoichiometry. Overall, the results obtained from the Tb^3+ doped orthoborate glasses here permit interpretation of previously unexplained published results. This is framed in the context of experimental and computational results pertaining to alkali and alkaline-earth borates that show that metal cations with high field strength will determine their local environment to a greater degree than metal cations present whose field strength is lower.