This work systematically investigates structure-stability relations in various polymer derived ceramic (PDC) systems and metal organic frameworks (MOFs), both of which are hybrid materials. The investigation of silicon carbides (SiC) confirms thermodynamic stabilization of PDCs with increasing mixed bonding (Si…
This work systematically investigates structure-stability relations in various polymer derived ceramic (PDC) systems and metal organic frameworks (MOFs), both of which are hybrid materials. The investigation of silicon carbides (SiC) confirms thermodynamic stabilization of PDCs with increasing mixed bonding (Si bonded to both C, O and/or N). The study of more complex silicon oxycarbide (SiOC) structures shows stabilization of SiOCs with increasing pyrolysis temperature (between 1200 and1500 oC), and points to dissimilarities in the stabilizing effect of different mixed bonding environments (SiO3C, SiO2C2, or SiOC3) and their relative amounts. Analyses of quaternary silicon oxycarbonitride (SiC(N)(O)) materials suggests increased stabilization with increasing N content, and superior stabilization due to SiNxC4-x compared to SiOxC4-x mixed bonds. Investigation of the energetics of metal filler (Nb, Hf, Ta) incorporation in SiOCs shows that choice of metal filler influences the composition, structural evolution, and thermodynamic stability in PDCs. Ta fillers can stabilize otherwise unstable SiO3C mixed bonds. Independent of metal incorporation or lack thereof, in SiOC systems, higher pyrolysis temperature (1200-1500 oC) forms more stable ceramics. The stabilizing effect of order/disorder of the free carbon phase is system-dependent. The work on (MOFs) highlights stabilization trends obtained from the investigation of zeolitic imidazolate frameworks (ZIFs) and boron imidazolate frameworks (BIFs) based on azolate linkers. Study of the energetics of metal (Co(II), Cu (II), and Zn (II) ) substitution in isostructural ZIFs shows that in MOFs the stabilizing effect of metal is dependent on both framework topology (diamondoid (dia) > sodalite (SOD)) and dimensionality (2D > 3D). Thermodynamic analyses of metal substitution (Ag(I), Cu(I), and Li (I)) in isostructural ii SOD and dia BIF systems confirm increase in density as a general descriptor for increased stabilization in MOFs. The study of energetics of guest-host interactions during CO2 incorporation in azolate frameworks (i.e., ZIF-8) shows strong dependence of energetics of adsorption on choice of linker and metal. Additionally, several energetically favorable reaction pathways for the formation of CO3-ZIF-8 have been identified. Both PDCs and MOFs show a complex energetic landscape, with identifiable system dependent and general structural descriptors for increased thermodynamic stabilization and tunability of the energetics of guest-host interactions.
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Lithium nickel manganese cobalt oxides (NMCs) are layered oxide cathode materials which are becoming increasingly popular as the demand for lithium-ion batteries increases. Lithium-ion batteries are used to power modern vehicles and for other battery applications. To better understand the…
Lithium nickel manganese cobalt oxides (NMCs) are layered oxide cathode materials which are becoming increasingly popular as the demand for lithium-ion batteries increases. Lithium-ion batteries are used to power modern vehicles and for other battery applications. To better understand the structure and energetics of NMCs, various molar ratios of these compounds were synthesized via a sol-gel method and characterized with powder X-ray diffraction profile fitting. Lattice constants for the nickel, manganese, and cobalt solid solutions were determined. High temperature oxide melt solution calorimetry was used to determine the enthalpies of formation and mixing. All but Li2MnO3 had the same space group as LiCoO2 (R-3m). The lattice constants approximately followed a linear fit with cobalt mole fraction (R2average= 0.973) for the cobalt series. As the molar ratio of cobalt increased the lattice constants decreased. The nickel series was less linear (R2average=0.733) and had an opposite lattice constant trend to cobalt. The manganese series possessed a roughly linear trend when excluding the outlier Li2MnO3 (R2average=0.282). The formation enthalpy of the cobalt series becomes more negative as more cobalt is added. A second order polynomial fit could be used to model the enthalpies of mixing for the series. NMC2.5,2.5,5 exhibited the most stable energetics. A third order polynomial fit could be used to model the enthalpy of mixing for the nickel and manganese series with NMC811 and NMC181 exhibiting the most stable energetics.
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There has been a recent push to examine the materials that nature is able to synthesize and consider whether the materials that humans have invented are geomimetic in nature, and whether designing nature-inspired materials is economically and environmentally beneficial. Mesoporous…
There has been a recent push to examine the materials that nature is able to synthesize and consider whether the materials that humans have invented are geomimetic in nature, and whether designing nature-inspired materials is economically and environmentally beneficial. Mesoporous silica represents a class of materials with pore sizes of 2-50 nm and has been studied in catalysis, separations, and drug delivery. It has generally been made using organosilicon precursors, but in this work, we demonstrate for the first time the successful synthesis of mesoporous silica with uniform mesoporosity of 10 nm using the mineral forsterite (Mg2SiO4) as a silica source, providing a potentially cheaper and more Earth-friendly route to making this technologically important material. Forsterite was synthesized by a solid-state chemistry route and underwent dissolution-reprecipitation in an aqueous acid solution containing the soft template surfactant, Pluronic P123. The formation of forsterite was confirmed with X-ray diffraction (XRD), the successful templating of surfactant was demonstrated with thermogravimetric analysis (TGA) and Fourier-transform infrared spectroscopy (FTIR), the surface area was determined through Brunauer-Emmett-Teller (BET) analysis, and pore size and distribution were demonstrated with Barrett-Joyner-Halenda (BJH) analysis. The synthesized mesoporous silica at optimal conditions has surface area of 740 m2/g and pore volume of 1.4 mL/g.
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Ammonia is one of the most important chemicals for modern civilization as well as a potentially invaluable intermediary component of a future sustainable H2 economy, yet its current production is decidedly unsustainable. Accordingly, researchers are attempting to devise new paradigms…
Ammonia is one of the most important chemicals for modern civilization as well as a potentially invaluable intermediary component of a future sustainable H2 economy, yet its current production is decidedly unsustainable. Accordingly, researchers are attempting to devise new paradigms for ammonia production, one of which would involve the cyclical reaction of H2 with a nitride compound and the renitridation of that compound with N2 - a thermochemical loop that would allow for ammonia production with renewable inputs and at relatively low pressures. In this paper, researchers identified several ternary and quaternary metal nitrides with the potential to exhibit relatively favorable thermodynamics for both the reduction and nitridation steps of that reaction cycle. These compounds were synthesized via co-precipitation and Pechini synthesis and several were tested under gas flows of 75% H2/Ar at 100-700 C and 75% H2/N2 at 700 C to determine their behavior under these conditions. As suggested by the available literature, Co3Mo3N was found to be a far better candidate for thermochemical looping than Fe3Mo3N or Ni2Mo3N - with higher mass loss and mass regain. Interestingly, quaternary nitrides containing Fe and Co in addition to Mo also demonstrated remarkable reduction and nitridation capability under ambient pressures. Ultimately, this paper demonstrates the feasibility of synthesizing a variety of single phase ternary and quaternary nitrides and the potential that several of these nitrides hold for producing ammonia sustainably via cyclic thermochemistry.
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