Matching Items (5)

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Advanced Synthesis Methods for Lithium Conducting Garnets

Description

Lithium conducting garnets in the family of Li7La3Zr2O12 (LLZO) are promising lithium conductors for solid-state batteries, due to their high ionic conductivity, thermal stability, and electrochemical stability with metallic lithium. Despite these advantages, LLZO requires a large energy input to

Lithium conducting garnets in the family of Li7La3Zr2O12 (LLZO) are promising lithium conductors for solid-state batteries, due to their high ionic conductivity, thermal stability, and electrochemical stability with metallic lithium. Despite these advantages, LLZO requires a large energy input to synthesize and process. Generally, LLZO is synthesized using solid-state reaction (SSR) from oxide precursors, requiring high reaction temperatures (900-1000 °C) and producing powder with large particle sizes, necessitating high energy milling to improve sinterability. In this dissertation, two classes of advanced synthesis methods – sol-gel polymer-combustion and molten salt synthesis (MSS) – are employed to obtain LLZO submicron powders at lower temperatures. In the first case, nanopowders of LLZO are obtained in a few hours at 700 °C via a novel polymer combustion process, which can be sintered to dense electrolytes possessing ionic conductivity up to 0.67 mS cm-1 at room temperature. However, the limited throughput of this combustion process motivated the use of molten salt synthesis, wherein a salt mixture is used as a high temperature solvent, allowing faster interdiffusion of atomic species than solid-state reactions. A eutectic mixture of LiCl-KCl allows formation of submicrometer undoped, Al-doped, Ga-doped, and Ta-doped LLZO at 900 °C in 4 h, with total ionic conductivities between 0.23-0.46 mS cm-1. By using a highly basic molten salt medium, Ta-doped LLZO (LLZTO) can be obtained at temperatures as low as 550 °C, with an ionic conductivity of 0.61 mS cm-1. The formation temperature can be further reduced by using Ta-doped, La-excess pyrochlore-type lanthanum zirconate (La2Zr2O7, LZO) as a quasi-single-source precursor, which convert to LLZTO as low as 400 °C upon addition of a Li-source. Further, doped pyrochlores can be blended with a Li-source and directly sintered to a relative density up to 94.7% with high conductivity (0.53 mS cm-1). Finally, a propensity for compositional variation in LLZTO powders and sintered ceramics was observed and for the first time explored in detail. By comparing LLZTO obtained from combustion, MSS, and SSR, a correlation between increased elemental inhomogeneity and reduced ionic conductivity is observed. Implications for garnet-based solid-state batteries and strategies to mitigate elemental inhomogeneity are discussed.

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2021

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Synthesis and Reactivity of Diiminopyridine and Iminopyridine Cobalt Complexes

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Alkylphosphine- and alkylpyridine-substituted 2,6-bis(imino)pyridines (pyridine diimines, PDI) have recently been used as polydentate, redox non-innocent ligands that support the development of highly active catalysts. The alkyl phosphine-substituted ligand, Ph2PPrPDI, was added to (Ph3P)3CoCl and subsequent reduction using excess sodium amalgam

Alkylphosphine- and alkylpyridine-substituted 2,6-bis(imino)pyridines (pyridine diimines, PDI) have recently been used as polydentate, redox non-innocent ligands that support the development of highly active catalysts. The alkyl phosphine-substituted ligand, Ph2PPrPDI, was added to (Ph3P)3CoCl and subsequent reduction using excess sodium amalgam yielded (Ph2PPrPDI)Co. Electronic structure analysis revealed a cobalt(I) complex that features a singly reduced PDI chelate. Additionally, low valent Ph2PPrPDI complexes of Fe and Ni were synthesized and structurally characterized. Furthermore, a series of Ph2PPrPDI Mn, Fe, Co, and Ni complexes were investigated to evaluate ligand denticity and redox activity. Finally, the catalytic hydrosilylation of carbonyls was investigated to compare the activity of the series and determine whether electron count plays a role in catalysis. An analogous ligand system featuring alkylpyridine substituents, PyEtPDI, was added to CoCl2, affording a cobalt dichloride complex with the formula [(PyEtPDI)CoCl][Cl]. Single crystal X-ray diffraction revealed a high-spin cobalt(II) center that possesses an octahedral geometry and an outer-sphere chloride ion. Further treatment using 2 equivalents of NaEt3BH resulted in the formation of (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co, which has been verified by multinuclear NMR spectroscopy and single crystal X-ray diffraction. (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co was then used in the catalytic hydroboration of nitriles at ambient conditions to yield the corresponding N,N-diborylamines, which were used as precursors for amide formation.

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2021

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Design and Isolation of a Novel Phosphine Functionalized Carbodiimide as a Versatile Precursor to Accessing Hemilabile Amidinate and Guanidinate Ligands

Description

Amidinates and guanidinates are promising supporting ligands in organometallic and coordination chemistry, highly valued for their accessibility, tunability, and comparability with other popular anionic N-chelating hard donor ligands like β-diketiminates. By far the most powerful way to access these ligands

Amidinates and guanidinates are promising supporting ligands in organometallic and coordination chemistry, highly valued for their accessibility, tunability, and comparability with other popular anionic N-chelating hard donor ligands like β-diketiminates. By far the most powerful way to access these ligands involves direct metal-nucleophile insertion into N,N’- substituted carbodiimides. However, the majority of reported examples require the use of commercially accessible carbodiimide peptide coupling reagents with simple alkyl substituents leading to low variation in potential substituents. Presented here is the design, synthesis, and isolation of a novel N,N’-bis[3-(diphenylphosphino)propyl]carbodiimide via an Aza-Wittig reaction between two previously described air stable substrates. At room temperature, 3-(diphenylphosphanyl-borane)-propylisocyanate was added to N-(3-(diphenylphospino)propyl)-triphenylphosphinimine, leading to product formation in minutes. One-pot phosphine-borane deprotection, followed by simple filtration of the crude mixture through a small, basic silica plug using pentane and diethyl ether granted the corresponding carbodiimide in high purity and yield (over 70%), confirmed by 1H, 13C, and 31P NMR spectroscopy. In addition to accessing different central carbon substituents, modification of phosphine substituents should be easily accessible through minor variations in the synthesis. With these precursors, anionic amidinates and guanidinates capable of κ4 -N,N,P,P-coordination may be accessed. The ability of the labile phosphine arms to associate and dissociate may facilitate catalysis. Thus, this carbodiimide provides a tunable, reliable one step precursor to novel substituted amidinates and guanidinates for homogeneous transition metal catalysis.

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2022-05

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Production of Organosiloxane-Modified Hydrophobic Geopolymer Materials

Description

The possibility of creating inorganic/organic hybrid materials has yet to be fully explored within geopolymer research. Using PDMS as an organic precursor, the surface of sodium and potassium geopolymers of varying precursor composition were functionalized with degraded PDMS oligomers.

The possibility of creating inorganic/organic hybrid materials has yet to be fully explored within geopolymer research. Using PDMS as an organic precursor, the surface of sodium and potassium geopolymers of varying precursor composition were functionalized with degraded PDMS oligomers. Both types of geopolymer yielded hydrophobic materials with BET surface area of 0.6475 m2/g and 4.342 m2/g for sodium and potassium geopolymer, respectively. Each respective material also had an oil capacity of 74.75 ± 4.06 weight% and 134.19 ± 4.89 weight%. X-ray diffraction analysis demonstrated that the PDMS functionalized sodium geopolymers had similar crystal structures that matched references for zeolite A and sodalite. The potassium geopolymers were amorphous, but showed consistency in diffraction patterns across different compositions.

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2022-05

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Development of Wet Chemical Synthesis Strategies for the Class of MAX Phases

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MAX phases are an intriguing class of materials with exotic combinations of properties, essentially turning them into metallic ceramics. Despite this unique feature, no commercialization has been accomplished yet. Looking at the state of the art within the MAX phase

MAX phases are an intriguing class of materials with exotic combinations of properties, essentially turning them into metallic ceramics. Despite this unique feature, no commercialization has been accomplished yet. Looking at the state of the art within the MAX phase community, almost all published studies can be summarized using the term “traditional high temperature synthesis”. Contrasting the scientific interest that has been on the rise especially since the discovery of MXenes, the synthetic spectrum has been largely the same as it has been over the past decades.Herein, the newly-emerging sol-gel chemistry is being explored as an alternative non-conventional synthetic approach. Building on the successful sol-gel synthesis of Cr2GaC, this study focuses around the expansion of sol-gel chemistry for MAX phases. Starting with a thorough mechanistic investigation into the reaction pathway of sol-gel synthesized Cr2GaC, the chemical understanding of this system is drastically deepened. It is shown how the preliminary nano-structured metal-oxide species develop into bulk oxides, before the amorphous and disordered
graphite partakes in the reaction and reduces the metals into the MAX phase.
Furthermore, the technique is extended to the two Ge- based MAX phases V2GeC and Cr2GeC, a critical step needed to prove the viability and applicability of the newly developed technique. Additionally, by introducing Mn into the Cr-Ga-C system, a Mn-doping was achieved, and for the first time for (Cr1–xMnx)2GaC, a unit cell increase could be recorded. Based on magnetometry measurements, the currently widely accepted assumption of statistically distributed Mn in the M-layer is challenged.
The versatility of wet chemistry is explored using the model system Cr2GaC. Firstly, the MAX phase can be obtained in a microwire shape leveraging the branched biopolymer dextran, eliminating the need for any post-synthesis machining. Via halide intercalation, the electrical transport properties could be purposefully engineered.
Secondly, leveraging the unique and linear biopolymer chitosan, Cr2GaC was obtained as thick films and dense microspheres, drastically opening potential areas of application for MAX phases. Lastly, hollow microspheres with diameters of tens of μm were synthesized via carboxymethylated dextran. This shape once more opens the door to very specific applications requiring sophisticated structures.

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2022