This report describes the proposed efforts for a three-year (CY23-25) program to develop refractory metal boride/carbide precursors for metal-organic chemical vapor deposition (MOCVD) applications. Reported are the CY24 results on the thermal processing of bis-cyclopentadienyl dialkyl and tetra-alkyl precursors to obtain metal carbide products. Precursors evaluated are commercially available. Materials were processed within in a custom-built MOCVD system at 1000 ⁰C, as well as in a hot isostatic press (HIP) at temperatures of 1000 ⁰C or 1650 ⁰C at pressures of 5000 psi. The products were identified as metal carbide, metal oxide, or a mixture of carbide and oxide phases depending on the starting material and process used. Density functional theory calculations were performed to determine the decomposition mechanism and to inform how ligand choice led to the products.
The tunability of metal-organic frameworks (MOFs) makes them exceptional materials for the development of highly selective, low-power sensors for toxic gas detection. Herein, we demonstrate enhanced detection of NO2 gas by a MOF-based electrical impedance sensor made using a unique mixed metal MOF-on-MOF synthesis. A combined experimental and computational study was performed using the exemplar NixMg1-x-MOF-74 to understand the fundamental structure-property relationships behind metal mixing and MOF film synthesis methods on sensor performance. Density functional theory results indicated that the presence of Ni in Mg-MOF-74 increased framework stability and increased the electron density of states at lower energies near the HOMO, as well as enhanced the NO2-Mg adsorption interaction. Impedance data of the NixMg1-x-MOF-74 films with larger Ni contents showed greater impedance change after exposure to 1 ppm of NO2 gas. Furthermore, when synthesized through either a drop-cast or direct solvothermal film growth approach, the monometallic Ni-based sensors had the best performance. However, the mixed metal NixMg1-x-MOF-74 sensors synthesized through a MOF-on-MOF approach resulted in the highest impedance change, outperforming all monometallic Ni-based sensors. In particular, the mixed metal Ni-on-Mg-MOF-74 film was the best-performing sensor with an impedance change of 309 upon trace NO2 exposure. Change in impedance response after NO2 exposure was improved by 52% compared to the best monometallic Ni-on-Ni-MOF-74 sensor. Structural analysis of the Ni-on-Mg film showed that the first Mg-MOF-74 layer acts as a structural template controlling the structural features of the final film after metal exchange with Ni. This led to improved film quality, evidenced by the greater crystallinity and larger MOF grain sizes, and resulted in enhanced sensor performance which was not achievable through other metal mixing methods. Altogether, this study identifies structure-property relationships and synthetic templating methods that inform MOF-based sensor design, allowing for improved detection of toxic compounds.
Understanding accident progression and the potential/conditions for fission product release from fuel is necessary to evaluate safety for any nuclear reactor system. Molten Salt Reactors (MSRs) under development need such analysis to support safety evaluations. Fission product chemistry specific to MSR concepts is a critical area that introduces distinct considerations relative to the current state-of-knowledge in reactor safety, primarily developed for water-moderated nuclear reactor systems. In Light Water Reactor (LWR) systems, it is necessary to capture the chemical interaction of fission products with the reactor evironment, containment and confinement systems. The overall effects at this point are relatively well understood for the purposes of performing safety evaluations. A key insight from LWR studies is that fission product chemical behavior can be reasonably captured by modeling approaches where the chemistry is "frozen". These modeling approaches assume that radionuclide reaction and speciation can be represented by chemical classes, each with characteristic transport behavior that is invariant under a broad range of thermochemical conditions. However, radionuclides can exhibit a range of behavior in the liquid salt-melt phase of the coolant used in MSRs. Radionuclides, salt, and the metal containment surfaces (i.e. pipes) can co-exist in dynamic equilibrium that could evolve with small system mass changes. A detailed investigation to the degree the equilibrium state can dynamically evolve with changes in the conditions of the molten salt mixture has not been previously conducted. It is currently not well understood where frozen chemistry assumptions are valid. Expanding the state-of-knowledge in this regard is relevant to better assessing the range of chemical effects that should be incorporated as part of MSR safety assessments. This investigation used the Oak Ridge Isotope GENeration (ORIGEN) module of the Standardized Computer-Analysis for Licensing Evaluation (SCALE) code to generate simulated radionuclide inventories for the MSR Experiment (MSRE) and then modeled reactor chemical speciation using the Molten Salt Thermodynamic Database – Thermochemical (MSTDB-TC) coupled with Thermochimica. The effect of composition variation during decay of fission product inventory in a molten salt over a period of 500 days prolonged post- at multiple temperatures was studied. Mass fractions for fluorine and berilium were varied in order to probe the effects of free fluorine control. Finally, speciation of fluoride reactors were showed by comparing MSRE readionuclide inventories with a FLiBe based molten salt breeder reactor (MSBR). The results showed that fission product mass change has little effect on phase mass changes and vapor pressures for fluoride species, but differ with varying carrier and fuel salt compositions. However, iodine species were found to have a vapor pressure not only dependent on temperature, but also the free fluorine potential, releasing iodine when the free fluorine potential is equal to the iodine inventory. This observation, however, arose under free fluorine potentials that are very unlikely to be realized in typical molten salt mixtures. Despite this observation, temperature was found to be the dominant parameter that drove phase change and fission product species vapor pressure. The results indicate that the current frozen chemistry approach is adequate for MSR analysis.
The differences between molten salt reactors (MSRs) and light water reactors (LWRs) has required modification of previous approaches to model reactor accident progression. Part of this is related to the different chemical phenomenology of these reactors as their fuel is not as contained and can react with their surroundings. MELCOR is a reactor simulation tool that has implemented methods and models to model MSRs. However, additional development of MELCOR for modeling MSRs is required. Although understanding the chemistry surrounding MSRs is important to general MSR licensing and operation, only a subset of reactions and phenomenology are required to model beyond design basis incidents and therefore to be captured by MELCOR. This report is intended to guide the chemical phenomenology to improve MELCOR MSR accident modeling and discusses phenomenology models that are in development.
This report summarizes the FY24 activities to model the Molten Salt Tritium Transport Experiment (MSTTE) using MELCOR. Summarized are the approaches used in construction of the MELCOR methods used for modeling, the construction of the MELCOR deck and the thermohydraulic calculations. The results show good comparison with previously reported calculation data, providing confidence in MELCOR to model experiments in the future.
MELCOR has been used extensively to facilitate virtual investigations into severe nuclear accidents for light-water reactors (LWRs). Non-light water reactors (non-LWRs) render some LWR-centric approaches potentially unsuitable. MELCOR has been instrumental in analyzing source terms for LWRs and has recently expanded its applicability to non-LWRs. To simplify radionuclide (RN) tracking, MELCOR currently groups elements into 17 classes, each containing representative species. This grouping, optimized for LWRs, is not appropriate for non-LWRs due to the different chemistry. This necessitates a reevaluation of radionuclide transport modeling. This report introduces a new class scheme for MELCOR tailored to MSR modeling, expanding the current 17 classes to 32 and are explained in the context of a UF4 fueled FLiBe carrier MSR. It provides a discussion and justification for the new groupings and outlines a methodology for discovering and defining additional classes in MELCOR using a sample calculated RN inventory and a Gibbs energy minimizer (GEM).
Porous liquids (PLs), which are solvent-based systems that contain permanent porosity due to the incorporation of a solid porous host, are of significant interest for the capture of greenhouse gases, including CO2. Type 3 PLs formed by using metal-organic frameworks (MOFs) as the nanoporous host provide a high degree of chemical turnability for gas capture. However, pore aperture fluctuation, such as gate-opening in zeolitic imidazole framework (ZIF) MOFs, complicates the ability to keep the MOF pores available for gas adsorption. Therefore, an understanding of the solvent molecular size required to ensure exclusion from MOFs in ZIF-based Type 3 PLs is needed. Through a combined computational and experimental approach, the solvent-pore accessibility of exemplar MOF ZIF-8 was examined. Density functional theory (DFT) calculations identified that the lowest-energy solvent-ZIF interaction occurred at the pore aperture. Experimental density measurements of ZIF-8 dispersed in various-sized solvents showed that ZIF-8 adsorbed solvent molecules up to 2 Å larger than the crystallographic pore aperture. Density analysis of ZIF dispersions was further applied to a series of possible ZIF-based PLs, including ZIF-67, −69, −71(RHO), and −71(SOD), to examine the structure-property relationships governing solvent exclusion, which identified eight new ZIF-based Type 3 PL compositions. Solvent exclusion was driven by pore aperture expansion across all ZIFs, and the degree of expansion, as well as water exclusion, was influenced by ligand functionalization. Using these results, a design principle was formulated to guide the formation of future ZIF-based Type 3 PLs that ensures solvent-free pores and availability for gas adsorption.
Porous liquids (PLs) are an attractive material for gas separation and carbon sequestration due to their permanent internal porosity and high adsorption capacity. PLs that contain zeolitic imidazole frameworks (ZIFs), such as ZIF-8, form PLs through exclusion of aqueous solvents from the framework pore due to its hydrophobicity. The gas adsorption sites in ZIF-8 based PLs are historically unknown; gas molecules could be captured in the ZIF-8 pore or adsorb at the ZIF-8 interface. To address this question, ab initio molecular dynamics was used to predict CO2 binding sites in a PL composed of a ZIF-8 particle solvated in a water, ethylene glycol, and 2-methylimidazole solvent system. Further, the results show that CO2 energetically prefers to reside inside the ZIF-8 pore aperture due to strong van der Waals interactions with the terminal imidazoles. However, the CO2 binding site can be blocked by larger solvent molecules that have greater adsorption interactions. CO2 molecules were unable to diffuse into the ZIF-8 pore, with CO2 adsorption occurring due to binding with the ZIF-8 surface. Therefore, future design of ZIF-based PLs for enhanced CO2 adsorption should be based on the strength of gas binding at the solvated particle surface.
Porous liquids (PLs) based on the zeolitic imidazole framework ZIF-8 are attractive systems for carbon capture since the hydrophobic ZIF framework can be solvated in aqueous solvent systems without porous host degradation. However, solid ZIF-8 is known to degrade when exposed to CO2 in wet environments, and therefore the long-term stability of ZIF-8-based PLs is unknown. Through aging experiments, the long-term stability of a ZIF-8 PL formed using the water, ethylene glycol, and 2-methylimidazole solvent system was systematically examined, and the mechanisms of degradation were elucidated. The PL was found to be stable for several weeks, with no ZIF framework degradation observed after aging in N2 or air. However, for PLs aged in a CO2 atmosphere, formation of a secondary phase occurred within 1 day from the degradation of the ZIF-8 framework. From the computational and structural evaluation of the effects of CO2 on the PL solvent mixture, it was identified that the basic environment of the PL caused ethylene glycol to react with CO2 forming carbonate species. These carbonate species further react within the PL to degrade ZIF-8. The mechanisms governing this process involves a multistep pathway for PL degradation and lays out a long-term evaluation strategy of PLs for carbon capture. Additionally, it clearly demonstrates the need to examine the reactivity and aging properties of all components in these complex PL systems in order to fully assess their stabilities and lifetimes.
Rare-earth terephthalic acid (BDC)-based metal-organic frameworks (MOFs) are promising candidate materials for acid gas separation and adsorption from flue gas streams. However, previous simulations have shown that acid gases (H2O, NO2, and SO2) react with the hydroxyl on the BDC linkers to form protonated acid gases as a potential degradation mechanism. Herein, gas-phase computational approaches were used to identify the formation energies of these secondary protonated acid gases across multiple BDC linker molecules. Formation energies for secondary protonated acid gases were evaluated using both density functional theory (DFT) and correlated wave function methods for varying BDC-gas reaction mechanisms. Upon validation of DFT to reproduce wave function calculation results, rotated conformational linkers and chemically functionalized BDC linkers with −OH, −NH2, and −SH were investigated. The calculations show that the rotational conformation affects the molecule stability. Double-functionalized BDC linkers, where two functional groups are substituted onto BDC, showed varied reaction energies depending on whether the functional groups donate or withdraw electrons from the aromatic system. Based on these results, BDC linker design must balance adsorption performance with degradation via linker dehydrogenation for the design of stable MOFs for acid gas separations.
Direct air capture (DAC) of CO2 is a negative emission technology under development to limit the impacts of climate change. The dilute concentration of CO2 in the atmosphere (~400 ppm) requires new materials for carbon capture with increased CO2 selectivity that is not met with current carbon capture materials. Porous liquids (PLs) are an emerging candidate for carbon capture and consists of a combination of solvents and porous hosts that creates a liquid with permanent porosity. The fundamental mechanisms of carbon capture in a PL are relatively unknown. To uncover these mechanisms, PLs were synthesized consisting of three different zeolitic-imidazolate framework (ZIF-8, ZIF-67, or ZIF-69) porous host in a water/glycol/2-methylimidazole solvent. The most stable composition was based on ZIF-8 and exhibited carbon capture following exposure to CO2. Density functional theory identified a three-step carbon capture mechanism based on (i) reaction of OH- with ethylene glycol in the solution followed by (ii) formation of 2-hydroxyethyl carbonate, which (iii) further react with OH- to form a carbonate species. This mechanism was validated with experimental nuclear magnetic resonance spectroscopy (NMR) to identify the dissolved carbonate phases and the decrease in the pH during CO2 exposure. Deuterated samples of the ZIF-8 PLs were synthesized and analyzed via neutron diffraction at the Spallation Neutron Sources at Oak Ridge National Laboratory. Results identified differences in diffraction for PLs pre- and post-CO2 exposure that will be combined with ab initio molecular dynamics data of the same PL composition to identify how the presence of a solvent-porous host interfaces results in carbon capture.
