Oxide Melt Research Articles

Carbon monoxide and hydrogen reduction of metals from the B2O3 – CaO – FeO – PbО melts: thermodynamic modelling

A thermodynamic modelling procedure has been developed that enables to predict the results of metal reduction from oxide melt in bubbling units by building an almost-true-to-real-process model. The procedure involves determining an equilibrium for each introduced portion of gas while taking the concentrations of target metal oxides in every modelling cycle from the previous data. The resultant metal phases and gases are excluded from the following calculations. This approach helps make the models most true to real processes and analyze how complete the reactions are that take place in pyrometallurgical units. The authors use the above procedure to describe the processes of combined iron and lead reduction from the B2O3 – CaO – FeO – PbO melt by carbon monoxide and hydrogen. This paper includes a comparative analysis of how the temperature T and the amount of V СО or VН2 gas introduced can impact the reduction of lead and iron from an oxide melt. The B2O3/CaO ratio taken for the modelling purposes is equal to 3, which corresponds to eutectic composition. The initial concentration of components, wt. %: 58.5 В2О3; 19.5 СаО; 20.0 FeO; 2 PbО; the temperature range: 1273–1773 К. The calculations were carried out based on the disproportionation of FeO into Fe and Fe3O4. The interaction of FeO and Fe with PbO ensures a partial transition of lead in metallic state. Depending on the temperature, the degree of lead metallization can vary from 76.8 (at 1,273 К) to 19.4 % (at 1,773 К). As the temperature rises, the concentration of FeO and PbO in the melt tends to increase whereas that of Fe3O4 tends to decrease. The authors analyzed how the concentration of lead and iron oxides in the melt and their reduction degree changed depending on the amount of reducing agent introduced. In the case of lead and iron reduction, no difference in the resulting products was found when using hydrogen versus carbon monoxide as the reducing agent. At the same time, hydrogen reduction results in a more complete reaction compared with carbon monoxide reduction. Thus, a smaller amount of gas is required to achieve a close-to-100% reduction of lead: СО — 10÷13 dm3/kg, Н2 — 5.8÷10.1 dm3/kg. Higher temperatures lead to the transition of some of the lead in gas phase. The obtained data indicate that one should account for the disproportionation of FeO into Fe3O4 and Fe when analyzing the interaction between the reducing agent (carbon monoxide or hydrogen) and the B2O3 – CaO – FeO – PbО oxide melt in the bubbled layer, which is associated with the changing composition of the melt.The work was conducted according to the State assignment of the Institute of Metallurgy of the RAS Ural branch (State registration of the theme No. 122020100404-2).

Thermochemistry of 3D and 2D Rare Earth Oxychlorides (REOCls).

The thermodynamic stability of rare earth (RE) materials plays a key role in the design of separation and recycling processes for RE elements. Thermodynamic stability is fundamentally influenced by the lanthanide contraction, as observed in the systematic reduction of unit cell volumes with increasing atomic number. RE materials are found in the form of solids having primary bonds in three dimensions (3D materials) as well as ones with primary bonds in two dimensions (2D materials) whose layers are held together by weak van der Waals (vdW) forces. While studies of synthesis, structure, and physical properties of 2D RE materials are numerous, no systematic research has compared their thermodynamic stability to that of 3D materials. In the present work, RE oxychlorides (REOCls), which display a structural transition from a 3D-polyhedral network (PbFCl-type) to a vdW-bonded layered one (SmSI-type) as the RE size decreases, were all synthesized by the flux method. High-temperature oxide melt solution calorimetry was used to determine their formation enthalpies to enable Born-Haber cycles to calculate lattice energies. Our results indicate that REOCl compounds are thermodynamically stable when compared to their binary oxides and chlorides. The lattice energies of 3D REOCls increase with decreasing RE size yet are insensitive to unit cell volumes for 2D REOCls. This is caused by interatomic interactions parallel and perpendicular to layers in the SmSI-type REOCls, causing a different structure response to the lanthanide contraction than 3D RE materials.

Energetics of hydroxylbastnäsite solid solutions, La1−xNdxCO3OH

Bastnäsites (LnCO3(F,OH)) are a group of common rare earth elements (REE)-bearing minerals and are one of the primary global sources of REE. Due to the chemical similarities among REE, bastnӓsites tend to occur as solid solutions instead of end members in REE containing ores. To better understand the processes and the mechanisms of formation of such deposits, it is essential to determine the thermodynamic properties of bastnӓsites, including hydroxylbastnӓsite (LnCO3OH) solid solutions. In this work, we performed detailed structural and calorimetric investigations on synthetic hexagonal La–Nd hydroxylbastnӓsite (La1–xNdxCO3OH, x = 0, 0.25, 0.5, 0.75, 1) solid solutions. X-ray diffraction confirms the crystal structure of the solid solution series in the P6¯ space group, and pair distribution function (PDF) analysis reveals local bonding environments characterized by three different types of 9-coordinated metal-oxygen polyhedra. Unit cell parameters of La1–xNdxCO3OH exhibit a nearly linear relation with the Nd content x, suggesting a random distribution of La and Nd in the structure. Their standard enthalpies of formation (ΔH°f) were determined by high temperature oxide melt drop solution calorimetry, from which the enthalpies of mixing (ΔHmix) were derived. The ΔHmix can be fitted by a regular solution model with an interaction parameter of 12.58 ± 0.16 kJ/mol, suggesting enthalpic metastability of La1–xNdxCO3OH relative to the two endmembers. Combining entropy and enthalpy, we further estimated the Gibbs free energies of mixing (ΔGmix) at relevant temperatures, revealing favorable temperatures under which the intermediate La1–xNdxCO3OH phases can be stabilized. Such entropy-driven stabilization, as is consistent with our geochemical modeling results, may explain the enhancement of thermal stability of the solid solutions in nature. Additionally, the temperature range constrained from this study may be used to estimate the thermal history of REE bastnӓsite deposit.

Distribution of noble metals between slag and matte phases during smelting of copper-sulphide and copper-nickel concentrates

In this work, we analyse and structure the information on the distribution of noble metals during pyrometallurgical processing of copper-sulphide and copper-nickel concentrates, available in domestic and foreign scientific sources. The data on the influence of oxygen partial pressure in the system and, consequently, the matte composition, process temperature, and the phase composition of slag on the distribution of platinum group metals, gold, and silver between the smelting products were analysed. Broad-ranging information on the distribution of the examined metals between matte and slag in the processing of copper-sulphide and copper-nickel concentrates was studied. It was established that the distribution of noble metals between the products of smelting using modern analytical methods is still in its early stages. Contradictory information on the behaviour of gold, silver, and platinum group metals during the pyrometallurgical processing of copper-sulphide and copper-nickel concentrates is presented in literature due to the variations in the experimental procedures, preparation, and analysis of test samples, and, as a result, further interpretation of the obtained results. Furthermore, no data on the influence of such technological parameters as magnetite content in the slag phase, the relationship between copper and nickel in the initial furnace charge and obtained mattes on the distribution of gold, silver, and platinum group metals between the products of smelting are available. In addition, no information on the distribution of noble metals during the pyrometallurgical slags depletion obtained in oxidative smelting of copper-nickel production has been published. Thus, it was decided to perform further research on establishing optimal technological conditions for oxidising smelting of copper-sulphide and copper-nickel concentrates, as well as pyrometallurgical depletion of slags, allowing for the highest extraction rates of noble metals into the target product.

Investigation of mitigation of nitric oxide emission characteristics and slagging properties from biomass combustion by the additive of coal gangue

The co-combustion of coal gangue (CG) and biomass was expected to increase the utilization rate of solid waste and compensating for individual feedstock disadvantages. However, the emission of nitric oxide (NO) and ash melting characteristics during this process was of concern, but no relevant study had focused on this. In this work, the combustion behavior of Salix limb(SL)/Wheat straw(WS), CG and biomass-CG blends was studied by non-isothermal thermogravimetry and tube oven method. The results indicated that when CG was mixed into biomass fuel as an additive, the burn-out effect of CG was improved. The NO emissions from the experimental value of 80% SL/WS blend with 20% CG were reduced by 8.9%/10.1% compared to the theoretical value. Moreover, the potassium aluminum silicates with high melting temperatures were formed in the ash of the composite fuel, which reduced the serious slagging phenomenon that has been observed in the biomass combustion process. The composite fuel of 80% biomass (SL/WS) + 20% CG could promote combustion and reduce NO emissions, which was a meaningful method to treat CG to the maximum benefit and reduced the phenomenon of biomass slagging.

A Microstructural and Compositional Study of ε-Fe2O3 Crystals in the Hare’s Fur Jian Ware

The Jian kilns in the present-day Jianyang County of Fujian Province are well known for their thick and lustrous black-glazed porcelain production. The hare’s fur (HF) glazed Jian wares characterized by radial fur-like strips, as one of the most typical representatives of black-glazed tea bowls, originate from phase separation of glaze melt and crystallization of iron oxides. In this study, various techniques were performed on the yellowish-brown HF samples, including portable energy-dispersive X-ray fluorescence (PXRF), synchrotron X-ray absorption near-edge spectroscopy (XANES), optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy (RS). The objective of this study was to well understand the microstructure characteristics and chemical compositions of glaze patterns. Results showed that the main constituents of the ceramic glaze were alumina (10.61–16.43 wt.%), silica (62.20–77.07 wt.%), calcium (3.85–6.97 wt.%), and iron oxide (4.10–8.35 wt.%). The studies provided evidence that metastable epsilon-iron oxide crystals (ε-Fe2O3) formed on the brownish-yellow glazed surface. Microstructural analysis revealed that there were three types of crystal structures in the glaze surface: One consisted of well-grown leaf-like or dendritic-like structure with highly ordered branches at micrometers scales; another comprised flower-like clusters accompanied by branches radiating from the center, petals growing along the branches, and needles on both sides of the petals; the last type involved a honeycomb structure tightly packed with plentiful spherical or irregular-shaped particles. In addition, ε-Fe2O3 crystals in the cross-section of the glaze showed a gradient distribution.

Durable Cr‐substituted (Ba,Cs)1.33(Cr,Ti)8O16 hollandite waste forms with high Cs loading

AbstractA 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.

A core‐shell structured diffusion‐bubbling membrane for efficient oxygen separation: Formation and transport properties

AbstractThe newly developed core‐shell structured molten oxide membranes with fast combined diffusion‐bubbling oxygen mass transfer and theoretically infinite selectivity are of technological interest because of their high separation efficiency. In this article, a core‐shell structured molten V2O5–Cu2O‐ based diffusion‐bubbling membrane was prepared by one‐step thermal treatment of initial CuO–25 wt.% Cu5V2O10 ceramic composite in a chemical field (under an oxygen partial pressure difference across the composite) above copper vanadate peritectic transformation temperature (816°C). Oxygen fluxes through the membrane were measured at 830°C, using either gas mixtures (O2 + N2) with different oxygen concentrations or air as feed gas at the shell of the membrane and helium (He) as sweep gas. Oxygen flux through the membrane with a shell thickness of 0.15–0.61 mm was 3.8·10–8–1.4·10–7 mol/cm2/s under an oxygen partial pressure difference of 0.1 –0.75 atm, respectively. The effect of oxygen partial pressure on the thickness of the membrane shell is found. The relationship between membrane shell thickness, oxygen partial pressure difference across the membrane, and oxygen permeation flux through the membrane is established. Oxygen permeation flux through the dual‐phase MIEC membrane shell is described in terms of the diffusion model. Oxygen permeation flux through the membrane core is described both within the framework of the stationary model and nonstationary model for uniform (the membrane thickness is much larger than the characteristic distance of bubble dynamic relaxation) and accelerated (the membrane thickness is comparable to the characteristic distance of bubble dynamic relaxation) bubble motion in a viscous oxide melt, respectively.

Genesis of magmatic ilmenite ores associated with the Mazua ultramafic intrusion, NE Mozambique

The Mazua ultramafic intrusion is among the most promising intrusions hosting Fe-Ti oxide ores in Mozambique. Fe-Ti oxide ores are mainly composed of ilmenite occurring as lenses, veins and layers, either massive or disseminated in hornblende pyroxenite. This study discusses the nature and origin of the intrusion and related ilmenite ore body on the basis of whole-rock and mineral chemistry. Textural and structural evidences support a cumulate origin for these rocks, including the occurrence of relict cumulate textures, layering of alternating oxides and silicate-rich rocks, and size-grading of silicate-rich layers. Primary magmatic features have, however, largely been overprinted by subsolidus processes. The occurrence of the cumulate assemblage (Fe-Ti oxides and clinopyroxene), without plagioclase and olivine, suggests that the magma underwent differentiation before its emplacement. The evolved magmatic nature of the intrusion is consistent with the calculated Mg-number of approximately 50 for the melt in equilibrium with clinopyroxene crystals. The high TiO2 concentrations (>4 wt%) of the evolved magma, by differentiation, are interpreted as the main factors controlling the crystallization of Fe-Ti oxides. We propose that the ores formed from crystallization and settling of Fe-Ti oxide minerals from an evolved basaltic magma. This model is consistent with (1) the occurrence of Fe-Ti oxides and clinopyroxene as cumulates in relation to the coexisting amphibole, (2) the layering structures of alternating oxides and silicate matrix, (3) the scarcity of apatite supported by the extremely low whole-rock P2O5 concentrations and (4) the restricted occurrence of ore veins, excluding the formation from immiscible oxide melts.

Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies

The technology of laser fusion on a powder bed is modified for the manufacture in air, of multi-material parts formed in the three dimensions of space, with zones which can be either a dielectric ceramic, based on alumina, or an aluminum - silicon metallic conductor. This additive manufacturing method uses only one powder. This powder is an AlSi12 alloy previously activated by a chemical treatment, which gives it, increased reactivity to oxygen. Therefore, the only adjustment of the manufacturing conditions, through the power and the scanning speed of the laser, as well as the hatch spacing, makes it possible to choose the construction of ceramic or metallic type zones. The modification of AlSi12 powders by chemical activation is the subject of a detailed characterization, allowing the selection of treatment conditions adapted to the multi-material manufacturing process. The ceramic or metallic areas that are produced, are also characterized for subsequent use of this process, for the manufacturing of microwave devices dedicated to the space industry. • Chemical etching of AlSi12 powder to increase its reactivity toward oxygen. • Parts combining in 3D space, dielectrics and electrical conductors, manufactured by laser fusion on powder bed. • Multi-materials obtained from an activated single AlSi12 powder by selective laser oxidation and selective laser melting. • Selective laser oxidation tuned by energy density. • Potential applications to microwave devices for space industry.

Calcination Condition of Dolomite‐Based Materials Influencing Static Dissolution in Synthetic Electric Arc Furnace Slag

The addition of basic additives in steelmaking processes is essential because these lead to slag adaptation and enable refining reactions. In calcination treatment, the process parameters temperature and dwell time influence the decomposition of dolomite as a magnesium‐containing lime substitute. It results in specific properties such as varying residual fractions of carbonates CaCO3·MgCO3 and porosity. The properties affect the dissolution kinetics, and the fast and complete dissolution is of interest. Hence, the static high‐temperature dissolution tests using dolomite‐based samples in diverse conditions (raw, modified, soft‐burnt, and hard‐burnt) are executed to determine the impact of the calcination state. The specimens are immersed in synthetic electric arc furnace slag, including 10 wt% Al2O3, 25 wt% SiO2, 25 wt% CaO, 8 wt% MnO, and 32 wt% FeO. The dolomite or dolime dissolves in the stagnant oxidic melt within the reaction time of 10 min at 1450 °C. The resulting chemistries of quenched slags, representing the current dissolution status, are examined by scanning electron microscopy using energy‐dispersive X‐ray spectroscopy analysis on metallographically prepared cross‐sections and via X‐ray fluorescence analysis of mechanically extracted slag fractions. The dissolution performance is characterized and compared for the different additive materials.

Influence of Different Carbon Content on Reduction of Zinc Oxide via Metal Bath

Electric arc furnace dust (EAFD) is an important secondary resource for the zinc industry. The most common process for its recycling is the pyro-metallurgical treatment in the Waelz process. However, this process focuses on the recycling of the zinc, whereas the recovery of other metals from the EAFD—such as iron and other alloying elements—is neglected. An up-to-date version of reprocessing can involve multi-metal recycling by means of a metal bath containing carbon. The use of a liquid iron alloy requires a higher processing temperature, which enables the reduction and melting of iron oxides as well as other compounds occurring in the dust. Furthermore, the Zn yield is higher and the reduction kinetics are faster than in the Waelz process. This paper is only focused on the zinc reduction in such a metal bath. In order to determine the influence of the carbon content in the molten metal on the reduction rate, experiments were carried out on the reduction behavior of zinc oxide using a synthetic slag. This slag, with a basicity B2 = 1, was applied to an iron bath with varying carbon contents. (0.85%, 2.16%, 2.89%, and 4.15%) The decrease in the zinc oxide concentration was monitored, along with the reaction rates calculated from these data. It was found that the reaction rate increases with rising carbon content in the melt.

Numerical model of species transport and melt stoichiometry in β-Ga2O3 crystal growth

In current work, we present a computer model, which describes the processes responsible for the loss of melt stoichiometry: dissociation of gallium oxide melt into gallium and oxygen ions, independent evaporation of the melt components from the free surface into the gas atmosphere, and loss of the melt stoichiometry. The mass transport model is coupled with heat transfer and calculation of the melt and gas flows. It also incorporates the effect of CO2 added to the growth atmosphere.Model operation was demonstrated for 2-inch β-Ga2O3 crystal growth process by Czochralski method with various amounts of added CO2 and predicted reduction of melt non-stoichiometry with addition of carbon dioxide to the gas mixture. Several modifications of the hot zone design have been considered in terms of their effect on the melt stoichiometry. Area of the melt free surface was found to have the most pronounced and non-linear effect on the amount of CO2 required to compensate the loss of oxygen.

Cluster structure of complex oxide melts

Structural cluster approach to non-crystalline condensed systems of oxide materials has been introduced as a result of many years’ experience of authors in the complex oxide melts research field. Nanoclusters distributed in a quasi-gas matrix form the structure of an oxide melt, possess the oxygen atoms’ close packing of regular polyhedrons where cations occupy its cavities. Nanoclusters and quasi-gas matrix are in the state of diffuse equilibrium at continuous interaction with each other.

Experimental and first-principles study on the effect of oxygen vacancy on infrared emissivity of CeO2

Ceria is considered a potential candidate for high-temperature infrared stealth materials due to its high melting point, oxidation resistance and low infrared emissivity. As reported, the infrared emissivity of ceria can be reduced after high temperature treatment. However, the available researchers have only analyzed the reasons for this change from the aspect of microstructure morphology. In this study, ceria with different oxygen vacancy concentrations were obtained by heat treatment of ceria powder at different temperatures and their infrared emissivities in 2.5–14 μm were characterized. The experimental results show that with the increase of heat treatment temperature, the oxygen concentration increases and the emissivity decreases. The electronic structure and reflectivity of ceria at different oxygen vacancy concentrations were obtained by first-principles calculations. The calculated results show that the reflectivity of ceria and the DOS peak area near the Fermi level increase with increasing oxygen vacancy concentration. Through this study, we demonstrate that the emissivity of ceria decreases with increasing oxygen vacancy concentrations, which is due to the increase in free carrier concentration.

Insight into low-temperature sintering of samarium-doped ceria mixed with scavenging lithium

The refractory nature of oxide-ion conducting Sm-doped CeO2 (SDC) has hindered the development of SDC devices. Lithium has been found to be an excellent sintering aid that enables sintering at temperatures as low as 900 °C and also increases grain boundary conductivity. Even though Li–Si oxides melt only above the sintering temperature in reality, it is generally believed that these benefits are given by virtue of a formation of liquid phases with silicon contaminants. In this study, the key element for low-temperature sintering was revealed to be aluminum, which is spontaneously introduced into SDC by a covert but intense chemical reaction between the mixed lithium and the alumina crucible. The results show that 1 wt% lithium addition is responsible for the unintended introduction of 32 mol% aluminum. Computational thermodynamic calculations for a Li–Si–Al oxide system clarified that aluminum introduction is essential to reduce the melting point below the sintering temperature. A nano-scale analysis clearly indicates the segregation of aluminum at SDC grain-boundary triple junctions, like LiAlO2, while silicon appears to migrate towards the SDC-SDC grain boundaries as amorphous SiO2. This suggests that the decomposition of the Li–Si–Al liquid oxide filling the grain boundaries is associated with the reflow of silicon and the loss of lithium as sintering proceeds. It was shown that zirconium is also introduced into SDC due to ball-mill processing, and that it potentially forms Li–Si–Zr oxides which remain in their solid state during sintering. The results of this study highlight the importance of taking the impact of unintended aluminum introduction into consideration for a variety of ceramics mixed with even minute quantities of lithium.

Thermodynamic Studies on Ba3SrNb2O9 Employing Calorimeter

In this study, Ba3SrNb2O9(s) was synthesized using conventional solid-state reaction route, characterized by powder XRD and SEM-EDAX. The standard molar enthalpy of formation of Ba3SrNb2O9(s) was determined measuring enthalpies of solution of Ba3SrNb2O9(s) in {PbO + B2O3} solvent (in 2:1 molar ratio) at T = 966 K using an oxide melt solution high temperature calorimeter. The enthalpy increment of Ba3SrNb2O9(s) was measured with same calorimeter. The heat capacity of the Ba3SrNb2O9(s) was also measured employing DSC. Based on the smoothed values of heat capacity, a table of thermodynamic data from 298 to 1000 K for Ba3SrNb2O9(s) was also constructed.

Thermodynamic data of belite polymorphs

One major component of Portland cement clinker is Ca2SiO4 (C2S, belite). Synthetic samples of three polymorphs, β-C2S (larnite), γ-C2S (calcio olivine), and x-C2S were investigated by high-temperature oxide melt solution calorimetry at 700 °C to obtain their enthalpies of formation. A natural sample of wollastonite (CaSiO3) was measured for comparison. Low-temperature heat capacity data of the C2S phases were recorded by relaxation calorimetry using a PPMS. Supplementary DSC data of β- and γ-C2S were obtained to verify their heat capacity evolution to 400 °C.The recommended values for β-C2S and γ-C2S are:ΔfH0298γ−C2S = −2316.0 ± 4.5 kJ mol−1, S0298γ−C2S = 119.7 ± 0.9 J mol−1 K−1,CP(γ-C2S) = (249.098 − 1949.5 T–0.5 – 1.60822 ∙ 106 T−2 + 2.77532 ∙ 108 T−3) J K−1 mol−1, [T] = K,ΔfH0298β−C2S = −2307.6 ± 4.5 kJ mol−1, S0298β−C2S = 122.2 ± 0.9 J mol−1 K−1, andCP(β-C2S) = (259.721 − 2466.09 T–0.5 + 2.62687 ∙ 106 T−2 – 4.77891 ∙ 108 T−3) J K−1 mol−1, [T] = K.

Synthesis and thermodynamics of uranium-incorporated α-Fe2O3 nanoparticles

• Uranium-incorporated hematite is thermodynamically metastable. • Increasing uranium incorporation within hematite increases its formation enthalpy. • Hydrothermal treatment can increase uranium concentrations in hematite. • U-Fe 2 O 3 coordination in radioactive waste environments may be process-dependent. Hematite nanoparticles were synthesized with U(VI) in circumneutral water through a coprecipitation and hydrothermal treatment process. XRD, TEM, and EXAFS analyses reveal that uranium may aggregate along grain boundaries and occupy Fe sites within hematite. The described synthesis method produces crystalline, single-phase iron oxide nanoparticles absent of surface-bound uranyl complexes. EXAFS data were comparable to spectra from existing studies whose syntheses were more representative of naturally occurring, extended aging processes. This work provides and validates an accelerated method of synthesizing uranium-immobilized iron oxide nanoparticles for further mechanistic studies. High temperature oxide melt solution calorimetry measurements were performed to calculate the thermodynamic stability of uranium-incorporated iron oxide nanoparticles. Increasing uranium content within hematite resulted in more positive formation enthalpies. Standard formation enthalpies of U x Fe 2–2x O 3 were as high as 76.88 ± 2.83 kJ/mol relative to their binary oxides, or -764.04 ± 3.74 kJ/mol relative to their constituent elements, at x = 0.037. Data on the thermodynamic stability of uranium retention pathways may assist in predicting waste uranyl remobilization, as well as in developing more effective methods to retain uranium captured from aqueous environments.

Combustion Characteristics and Mechanisms of Two Gun Barrel Steels by Promoted Ignition Combustion

ABSTRACT The combustion characteristics of 25Cr3Mo2NiWVNb (MPS700V) and 30SiMn2MoV gun barrel steels are investigated in oxygen-enriched atmosphere. For MPS700V, threshold pressure of combustion increases from 0.26 to 0.46 MPa with diameter increases from 2.4 to 4.0 mm, where are higher than that of 30SiMn2MoV in the same size. Three different zones including oxide zone, melting zone, and matrix zone are observed in both 30SiMn2MoV and MPS700V steels. Elements Mo, C in 30SiMn2MoV and Mo, Ni, C in MPS700V are enriched in the melting zone. When pressure is lower than threshold pressure, samples do not entirely burn but numerous ablation pits form on the surface. The average depth and area of ablation pits on MPS700V are less than that on 30SiMn2MoV under the same pressure. Dendritic microstructure is observed around ablation pits in both MPS700V and 30SiMn2MoV. For MPS700V, dendritic microstructure is composed by Mo2C and γ-Fe phases, where Mo, C are enriched in the dendrite and Ni is enriched in interdendrite. For 30SiMn2MoV, dendritic microstructure is composed by Mo2C and Fe3C carbides and ferrite, where Mo, C are enriched in the dendrite. Based on above, the difference of combustion mechanism for 30SiMn2MoV and MPS700V is discussed.