Molten Oxide Research Articles

Research on the oxidation process of micro-boron below the melting point temperature

The ignition and combustion characteristics of boron and the energy release rate of boron-based fuel are closely related to the surface oxide layer of boron. To understand the oxidation process of boron below its melting point, the slow oxidation process of boron under different temperature and humidity storage conditions and the fast oxidation process under high-temperature conditions were investigated. The results indicate that the surface oxide layer mainly comprises boron (B), boron suboxides (BxOy), and boron trioxide (B2O3). When the degree of oxidation is low, BxOy and B are the main components. After deep oxidation, BxOy almost disappeared. The thickness and composition of the surface oxide layer vary with changes in environmental temperature, humidity, and aging time. The effect of temperature increase on the growth rate of the oxide layer thickness is significantly greater than that of moisture. With the prolongation of boron aging treatment time, the onset, end, and maximum weight gain rate temperatures of the boron thermal oxidation process all slightly decrease. As the ambient temperature increases, the formation of the oxide layer transitions from a unidirectional diffusion mechanism of oxygen into the interior of boron particles to a bidirectional diffusion mechanism of molten oxide and oxygen.

Key phase diagram experiments and thermodynamic modeling of the ZnO‒B2O3, ZnO‒SiO2, and ZnO‒B2O3‒SiO2 systems

AbstractThermodynamic modeling of binary ZnO‒B2O3 and ZnO‒SiO2 systems and ternary ZnO‒B2O3‒SiO2 system were carried out based on the critical evaluation and optimization of all available phase diagram and thermodynamic property data using the CALculation of PHAse Diagram (CALPHAD) method. To resolve the inconsistency among the available phase diagram data in ternary ZnO‒B2O3‒SiO2 system, key phase diagram experiment was carried out using classical quenching method followed by electron probe micro‐analysis and X‐ray diffraction phase determination. The liquid oxide solution was described using the modified quasichemical model accounting for a short‐range ordering behavior in molten oxide solution. Thermodynamic models with optimized model parameters were used to predict the phase diagram and thermodynamic properties of the ternary systems.

A key feedback loop: building electricity infrastructure and electrifying metals production.

Energy infrastructure requires metals, and metals production requires energy. A transparent, physical model of the metals-energy system is presented to explore under what conditions this dependence constrains or accelerates the transition to a net-zero economy. While the mineral (as high as 340 Mt yr-1 iron ore, 210 Mt yr-1 limestone, 250 Mt yr-1 bauxite and 5.5 Gt yr-1 copper ore in the 2040-2050 decade, assuming no improvements) and total energy (up to 22 EJ yr-1) requirements for building low-carbon energy infrastructure are significant, it compares favourably with the current extraction and energy use supporting the fossil fuel system (15 Gt yr-1 fossil minerals and ~38 EJ yr-1). There are levers to significantly reduce material use and associated impacts over time. The metals industry can play a key reinforcing role in the transition by adapting to the increasing supply of renewable electricity. Specifically, direct electrolysis can extract metal from ore close to the thermodynamic limit, to make efficient use of low-C electricity. The unique features of emerging technologies for iron extraction, molten oxide electrolysis and molten sulphide electrolysis are considered in this evolving system. Electrification enables elegant separations and provides a pathway to build out infrastructure while reducing environmental impacts, though material efficiency measures will still be crucial to meet 2050 carbon budgets.This article is part of the discussion meeting issue 'Sustainable metals: science and systems'.

Three-dimensional PDC-SiBCN network in MA-SiBCN ceramics: Toughening-reinforcing effect and oxidation barrier

A three-dimensional (3D) precursor-derived (PDC) SiBCN network was incorporated into mechanical alloying-derived (MA) SiBCN (MA@PDC-SiBCN) ceramic through a novel hybrid organic-inorganic method, enhancing densification while providing significant reinforcement and toughening effects. The PDC-SiBCN network's submicron continuity and uniformity enable efficient load transfer between under-sintered MA-SiBCN particles, increasing strength by 130%, compared to MA-SiBCN ceramics of the same density. Additionally, as cracks penetrate the PDC-SiBCN network, crack tip deflection and bifurcation result in 57% and 352% increases in fracture toughness respectively, compared to fully sintered and under-sintered MA-SiBCN ceramics of the same density. Besides, due to synergistic oxidation characteristics, the three-dimensional PDC-SiBCN network acts as an oxidation barrier, inhibiting the heterogeneous oxidation of the encapsulated MA-SiBCN component, especially limiting the consumption of BN(C) phase at lower temperatures. The residual PDC-SiBCN network in the oxide layer serves as a rigid framework, preventing bubble growth in the molten oxide layer thereby forming a dense protective oxide layer.

Contactless surface tension measurement of molten oxides using oscillating drop method in an aerodynamic levitator

Aerodynamic levitation stands as a contactless method for thermophysical property measurement of molten oxides at high temperatures, offering the advantage of eliminating sample contamination and enabling surface tension measurement through the oscillating drop method. However, the presence of oscillation mode splitting results in an underestimation of surface tension. By improving levitation stability and optimizing the acoustic excitation method for the levitated drops, this study successfully eliminates oscillation mode splitting for the first time. The l = 2 resonance frequency is directly measured, which is used to calculate surface tension according to the Rayleigh equation. Taking Al2O3 as a representative of oxides, this study demonstrates the accurate surface tension measurement during the frequency scan process when oscillation mode splitting is absent. Additionally, combining damped oscillation analysis with the frequency scan reduces measurement uncertainty from 2.8 % to 1.5 %. Within the temperature range of 2336K–2923K, the surface tension of molten Al2O3 is determined to be γ = −(4.071 ± 2.688) × 10−5 (T-2327)+(0.7465 ± 0.074) N/m. Underestimation of surface tension is prevented when the l = 2 resonance frequency is used for surface tension calculations. The frequency crossover method which is used for surface tension measurement is also found to be affected by oscillation mode splitting.

Economics of Electrowinning Iron from Ore for Green Steel Production

The transition to green steel production is pivotal for reducing global carbon emissions. This study presents a comprehensive techno-economic analysis of various green steel production methods, including hydrogen reduction and three different electrolysis techniques: aqueous hydroxide electrolysis (AHE), molten salt electrolysis, and molten oxide electrolysis (MOE). By comparing process flow diagrams, capital and operational expenditures, specific energy consumption, and production footprint, this work provides a high-level assessment of the economic viability of these processes as they mature. The analysis reveals that MOE, despite its ongoing development, offers a promising route for iron production given its ability to process a wide range of ore qualities and the potential to sell electrolyte as a cement product. However, the best balance between deployment ready technology and economic benefit is AHE. Operational challenges are also discussed, such as electrolyte loss and slag handling. We suggest that the sale of by-products like oxygen may not significantly impact the economics due to market saturation. The findings underscore the importance of continued research and development in process optimization to realize the full potential of green steel technologies. All the calculations have been released as supplementary electronic material (MS Excel workbook). The format has been inspired by the techno-economic assessment template (TECHTEST) distributed by the US Dept. of Energy.Graphical

Electrochemical Extraction of Fe–Si Alloy Form SiO2–CaO–CaF2–FeOx Slags by Molten Oxide Electrolysis

Electrochemical Extraction of Fe–Si Alloy Form SiO2–CaO–CaF2–FeOx Slags by Molten Oxide Electrolysis

Growth mechanism of microarc oxide membrane under the distribution law of Si and Zr elements

This study investigated the growth mechanism of microarc oxidation ceramic coatings on AZ91D magnesium alloy by sequentially treating it in silicate and zirconate electrolyte systems. The distribution patterns of Si and Zr elements were examined to understand the growth mechanism. The results showed that the distribution of Si and Zr elements in the SZ ceramic coatings changed with increasing termination voltage of microarc oxidation. The growth mechanism of microarc oxidation ceramic coatings on magnesium alloys involved the reaction between Si and Zr elements from the electrolyte and Mg elements from the substrate to form molten oxides under the high-temperature and high-pressure conditions of microarc oxidation discharge. At lower termination voltages, some of the molten oxides remained near the discharge channels or the film/substrate interface, resulting in inward growth of the ceramic coating. As the termination voltage increased, some of the molten oxides were ejected outward through the discharge channels, facilitating outward growth of the ceramic coating. The inclusion of silicon elements compared to zirconium elements will significantly increase the concentration of oxygen vacancies in the ceramic membrane, enhancing the electrical conductivity of the ceramic membrane while reducing its corrosion resistance.

Evaluating Phase Relations in Molten Oxides for Electrochemical Operating Windows for Selective Metal Reduction

AbstractMolten oxide electrolysis is a promising pathway to decarbonize primary metal production. The oxide electrolyte is less hazardous and eco‐toxic than halides (molten salt electrolysis) and, with the use of renewably generated electricity and an inert anode, oxygen is produced as a by‐product instead of CO2. Building fundamental understanding of electrolytic operating windows requires starting with simple chemistries. Here we investigate a simplified binary oxide system as an electrolyte for titanium oxide reduction to metal. The TiO2−Na2O binary system forms a eutectic, reducing the liquidus temperature over a wide composition range. FactSage 8.1 predictions suggested Ti reduction would become favorable over Na reduction for compositions greater than 0.49 mole fraction TiO2. The prediction was validated by the detection of metallic Ti after electrolysis experiments. However, the reduction efficiency was too low (0.24±0.08 % at −0.1 V vs Ti reference electrode) for the TiO2−Na2O system to be a viable industrial electrolyte for Ti production. Scoping for other binary oxide systems was performed for electrolytic production of two critical metals, tantalum and neodymium. Based on TiO2−Na2O system's predicted behavior, candidate binary oxide systems were identified that contained congruently melting line compounds flanked by eutectic reactions from the ACerS‐NIST Phase Equilibria diagrams database.

Enhancing high‐temperature mechanics and degradation for phthalonitrile resin via constructing rare earth oxide supported low melt‐point molten oxide

AbstractIn order to improve the comprehensive performance of thermosetting phthalonitrile resin (PN) composites, a novel molten hybrid (Nd2O3@DM35) was successfully prepared by loading Neodymium Oxide (Nd2O3) on the surface of low melt‐point molten oxide (DM35). In thermal gravimetric analysis, the addition of 30 wt% Nd2O3@DM35 postponed the temperature of 5% mass loss (T5%) of PN hybrid (PN‐NdD30) from 485 to 521°C. A series of thermal aging experiments were designed to prove the thermal stability of modified PN. The surface morphology of PN‐NdD30 was mostly maintained after thermal aging. Compared with pure PN, the flexural strength (FS) and interlaminar shear strength (ILSS) of PN‐NdD30 composites at room temperature increased by 31.4% and 32.8%, respectively. After aging at 400°C for 20 min, the FS and ILSS retention rates of PN‐NdD30 composites were 78.4% and 91.5%, respectively. In this work, the introduction of novel hybrid improved the thermal stability of PN composites, while enhancing their mechanical properties at different temperatures. Based on all the results, the heat resistance and mechanical mechanism of PN‐NdD30 composites were carefully analyzed.Highlights A molten hybrid was prepared by Nd2O3 and low melt‐point molten oxide. The novel hybrid can form a continuous protective layer at high temperatures. In the aging test, the mass loss of PN‐NdD30 was significantly reduced. Mechanical properties of PN composites at different temperatures were improved. A novel molten concept was introduced in the heat resistance modification of PN.

Thermochemical interactions between yttria‐stabilized zirconia and molten lunar regolith simulants

AbstractOxygen produced through in‐situ resource utilization (ISRU) is critical to maintaining a permanent human presence on the lunar surface. Molten regolith electrolysis and carbothermal reduction are two promising ISRU techniques for generating oxygen directly from lunar regolith, which is primarily a mixture of oxide minerals; however, both processes require operating temperatures of 1600°C to melt lunar regolith and dissociate the molten oxides. These conditions limit the use of many oxide refractory materials, such as Al2O3 and MgO, due to rapid degradation resulting from reactions between the refractory materials and molten lunar regolith. Yttria‐stabilized zirconia (YSZ) is shown here to be a promising refractory oxide to provide containment of molten regolith while demonstrating limited reactivity. This work focuses on corrosion studies of YSZ powders and dense YSZ crucibles in contact with molten lunar maria and highlands regolith simulants at 1600°C. The interactions between YSZ and molten regolith were characterized using scanning electron microscopy/energy dispersive spectroscopy, X‐ray diffraction, and electron backscatter diffraction. A FactSage thermochemical model was created for comparison with the experimental results. These combined analyses suggest that lunar maria regolith will degrade the YSZ faster than the lunar highlands regolith due to the lower viscosity of the maria regolith. The feasibility of long‐term molten regolith containment with YSZ is discussed based on the YSZ powder and crucible results.

Surface ionic coordination of Al2O3–CaO–based molten slag induced by structural relaxation

AbstractThe surface tensions of molten oxides depend strongly on the structural relaxation of the surface region. The mechanism of surface structural relaxation for molten oxide slags is complex. The surface tension of calcium aluminate slag is minimal at an intermediate composition, although the critical reason has not been identified. Here, two novel approaches were used to evaluate the features of surface ionic structures in the molten state for the range of 25–50 mol% Al2O3: (1) X‐ray absorption analysis of oxygen and cationic elements in glass samples after surface relaxation treatment, and (2) molecular dynamics simulations, based on a polarizable‐ion model, of the ionic distribution in a molten slag with vacuum/melt interfaces. The results indicate that bridging oxygen (BO) ions are preferred to non‐BOs in the surface region. In calcium aluminate slag, BOs are formed by connecting two AlO4 tetrahedrons with charge compensation of two Al3+ ions with one Ca2+ ion. Additionally, the above approaches were used to qualify the effect on the surface ionic structure of adding 1–20 mol% SiO2 to the calcium aluminate slag. The results indicated that the SiO4 tetrahedrons incorporate the vertex connection with AlO4 tetrahedrons to form BOs in the surface region.

Experimental Validation is Always Required for Molten Oxide Electrolysis Laboratory Crucibles

Molten oxide electrolysis (MOE) is a promising electrochemical route to de-carbonize primary and secondary metal production. Developing MOE processes starts with laboratory experiments at temperatures > 1000∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1000\\,^\\circ{\\rm C}$$\\end{document} lasting ~10 hours and requiring long heating/cooling times to protect furnace hardware. Before investigating MOE processes, crucibles must be selected that tolerate the required temperatures while minimizing chemical interactions with the oxide to control melt contamination and contain the melt. Unfortunately no general procedure guiding MOE crucible selection is documented. Here we focus on laboratory crucibles in air for two MOE melts: titania-sodia at 1100∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1100\\,^\\circ{\\rm C}$$\\end{document} and neodymia-boria at 1300∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1300\\,^\\circ{\\rm C}$$\\end{document}. After shortlisting generic crucible materials using Ashby’s method, thermodynamic predictions were made for all-oxide titania-sodia charges using FactSage and cup test experiments were conducted on (i) all-oxide and carbonate charges for titania-sodia and (ii) neodymia-boria charges. While magnesia was predicted to be the best crucible for the titania-sodia melt, alumina was the best choice for both oxide and carbonate charges. The grain boundary networks of both magnesia and YSZ were infiltrated by the oxide and carbonate charges. Platinum was the best crucible for neodymia-boria melts. We show that compositional control during long, high-temperature MOE experiments requires experimental validation for specific chemistries every time.

Using optical emission spectroscopy in atmospheric conditions to track the inflight reduction of plasma sprayed TiO2−x feedstock for thermoelectric applications

Thermal spray deposition (specifically Atmospheric Plasma Spraying, APS) is a well-established surface coating technology with a broad scope of applications (i.e., insulative coatings, tribological coatings, anti-corrosion coatings, etc.). In addition, there is a constant drive to introduce the APS process into new and emerging fields. One such niche application for APS would be sub-stoichiometric TiO2−x coatings with enhanced thermoelectric performance (compared to the bulk material). The APS process in this context has a unique ability—given the use of hydrogen as a plasma gas—to reduce TiO2−x material during processing. However, to this point, there is neither a reliable nor self-consistent method to assess (nor control by parametric optimization) the inflight reduction of molten oxide particles during processing. This study shows that using Optical Emission Spectroscopy (OES), it can be possible—even in atmospheric conditions—to identify characteristic emission peaks associated with the inflight reduction of TiO2 during APS. Using this OES data, the input spray processing parameters and their influence on coating microstructure and the degree of inflight reduction of the material will be shown. Results suggest under equilibrium conditions only a minimal amount of hydrogen gas is needed in the plasma to fulfill the TiO2 reduction.

Discharge channel structure revealed by plasma electrolytic oxidation of AZ31Mg alloy with magnetron sputtering Al layer and corrosion behaviors of treated alloy

Plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy with a thin magnetron sputtering Al layer was carried out in a silicate–hexametaphosphate electrolyte to investigate the PEO mechanism and improve the corrosion resistance of the alloy. SEM and EDS were employed to examine the coating morphology and trace the Mg and Al elements in the coatings. It is found that new coating is mainly formed at the lower part of the discharge channels close to the interface of coating/metal substrate. The previously formed oxides at the upper part of discharge channels are largely kept in their original solid forms within the discharge channels. Only a small fraction of the molten oxide flows out through the micropores of the coating, reaching the top layer. The anionic species can freely access the innermost of the coatings, which are also transported through the electrolyte-filled pores. The corrosion behavior of the coatings was examined by open circuit potentials, polarization curves and EIS tests in 3.5 wt.% NaCl. Owing to its compact nature, the magnetron sputtered Al layer can significantly improve the corrosion resistance of the AZ31 Mg alloy, which is even better than that of the samples after PEO treatment.

Initial microstructure and composition evolution of Al2O3 coatings fabricated by cathode plasma electrolytic deposition under constant voltage

At present, the research on cathode plasma electrolytic deposition (CPED) mainly focuses on improving the coating properties, while the initial structure and formation mechanism of CPED coating are less studied. This work aims to study the initial deposition behavior of Al2O3 coating by CPED at different time scales systematically. The results show that the coating consists of lamellar Al(OH)3 in the pre-plasma discharge period. After reaching the breakdown potential, the preparation process of Al2O3 can be considered as a cyclic process: plasma discharge, injection of molten oxide, sintering and solidification, and stacking. Such an iterative process results in a gradual thickening of the coating. Later in the deposition process, the raised structure of the coating surface flaps off and coating thickness decreases. In addition, the high temperature during plasma discharge results in local melting of the substrate surface, which is co-deposited with Al2O3.

Wetting of Graphite and Platinum Substrate by Oxide System with Graded B2O3 Content

This work focuses on wetting two types of substrates (a platinum substrate and a polished graphite substrate) by molten polycomponent oxide system CaO–MgO–SiO2–Al2O3–B2O3 to test the level of interaction at high temperatures. The tested systems were subjected to high-temperature wetting tests in the temperature range from liquidus temperature to 1550 °C using the sessile drop method. A total of four oxide systems were tested with graded boron oxide contents ranging from 0 to 30 wt%. The experiments were conducted in a CLASIC high-temperature resistance observation furnace and an inert atmosphere of high-purity argon. Droplet silhouettes were obtained with a CANON EOS 550D high-resolution camera during heat treatment, with reactive and non-reactive wetting occurring depending on the substrate type. The dependence of the average wetting angles on temperature and time was evaluated, and it was found that boron oxide decreased the average wetting angles of molten oxide droplets. The analyses were accompanied by the SEM/EDX analysis of the substrate and FTIR analysis of the droplets after high-temperature experiments. The phase composition of the oxide systems was evaluated by XRD analysis.

Regulation of dimensional errors and surface quality of thin-walled components fabricated by blue laser directed energy deposition

The utilization of components fabricated by directed energy deposition (DED) faces certain limitations that hinder its widespread application. These critical challenges primarily include significant errors in dimension accuracy caused by the unreliable build-up of flowing powder and low surface quality arising from the interlayer cumulative effect and powder adhesion. The present study aims to comprehensively investigate the impact of process parameters and deposition height on the geometrical errors observed in the thin-walled structures fabricated by DED. By establishing a rational relationship between the dimensional error and the sample height, the mechanism underlying the influence of deposition height on printing stability and sustainability is elucidated. Furthermore, an innovative building strategy incorporating a variable speed is proposed to eliminate protrusions at the end of the components. The study also examines the surface roughness of inclined thin walls, revealing a correlation between roughness and both position and angle. The presence of adhering powder on the front side of the parts and the formation of a molten oxide layer on the backside result in a diminished impact of angle on roughness. Ultimately, the findings of this study are validated through the fabrication of a thin-walled axisymmetric curved structure, providing a guide for the printing of large special-shaped parts.

(Keynote) Experimental Insights for Extra-Terrestrial Application of Molten Oxide Electrolysis

Molten oxide electrolysis (MOE) has been considered for several decades for oxygen generation from extra-terrestrial oxides. It consists in the electrolytic decomposition of the rocks or soils, which are mixture of the common earth oxides (silica, iron oxide(s), alumina, magnesia...). Therein the oxides mixture is acting as the electrolyte, in the molten state. The target anodic product is oxygen gas, and the cathodic product is a metal. In one version of MOE, the temperature is high enough to sustain the production of a liquid metal, offering the opportunity to conceive a fully continuous process thanks to periodic tapping of the cathodic product. A key attribute of such approach is the high current density (productivity) that can be anticipated thanks to the high concentration of oxygen and metal present in the electrolyte. This leads to electrochemical engineering questions that are not commonly discussed in other electrolysis methods. This presentation offers to review some of the prior findings related to the underlying thermodynamic of the process, as well as the oxygen evolution and transport phenomena that support the development of MOE. Recent findings combining a container-less method and advanced electrochemical techniques will be presented.

(Invited) Zirconia-Based Hollow Anode Development for Molten Regolith Electrolysis

Molten regolith electrolysis (MRE) is a promising in-situ resource utilization technique for producing oxygen and molten metal alloys from lunar regolith. However, MRE requires operating temperatures at or above 1600°C to melt lunar regolith and dissociate the molten oxides to enable electrolysis. Anode degradation in this corrosive and oxidizing environment is a major concern for long-term operation on the lunar surface and can be mitigated by a novel hollow anode design with a solid electrolyte shell and porous metallic core as proposed here. The metallic anode will be shielded from the regolith melt by an oxygen ion-conducting zirconia-based ceramic, allowing electron transfer to occur in the interior of the shell. Additionally, bubble formation at a traditional anode surface is avoided, thereby eliminating the concerns of molten regolith splatter and increased ohmic resistance. This work focuses on corrosion studies of dense stabilized zirconia in contact with molten lunar mare and highlands regolith simulants at 1600°C to support hollow anode development. The interactions between zirconia and molten regolith are characterized using SEM/EDS, with an emphasis on elemental analysis to assess reactivity and degradation of zirconia.