Thermal Decomposition and Crystallization of Aqueous Sol-Gel Derived Zirconium Acetate Gels: Effects of the Additive Anions

Zirconia powders were prepared by forming gels by desiccation of aqueous precursor solutions of zirconium acetate containing nitric or sulfuric acid at pH 2.4 and 1.4 and pyrolyzing the gels to temperatures up to 825°C. The structure development in the gels and solid pyrolysis products was investigated. The crystalline zirconia structures produced monoclinic (m), metastable cubic (c) and tetragonal (t) polymorphs. The structure transition temperatures were strongly dependent on the pH, the anions and the stoichiometry of the zirconium complex in the precursor solution. The monoclinic polymorph fraction in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing sulfuric acid at pH 2.4 to 750°C approaches zero while this ratio in the zirconia formed by pyrolyzing the gel formed from the precursor solution containing nitric acid at pH 1.4 to 825°C is 0.7.     

Transport properties of solid polymer electrolytes prepared from oligomeric fluorosulfonimide lithium salts dissolved in high molecular weight poly(ethylene oxide)

Transport properties such as ionic conductivity, lithium transference number, and apparent salt diffusion coefficient are reported for solid polymer electrolytes (SPEs) prepared using several oligomeric bis[(perfluoroalkyl)sulfonyl]imide (fluorosulfonimide) lithium salts dissolved in high molecular weight poly(ethylene oxide) (PEO). The salt series consists of polyanions in which two discrete fluorosulfonimide anions are linked together by [(perfluorobutylene)disulfonyl]imide linker chains. The restricted diffusion technique was used to measure the apparent salt diffusion coefficients in SPEs, and cationic transference numbers were determined using both potentiostatic polarization and electrochemical impedance spectroscopy methods. A general trend of diminished salt diffusion coefficient with increasing anion size was observed and is opposite to the trend observed in ionic conductivity. This unexpected finding is rationalized in terms of the cumulative effects of charge carrier concentration, anion mobility, ion pairing, host plasticization by the anions, and salt phase segregation on the conductivity.     

Effect of Oxygen Environment on the Decomposition and Crystallization of an Aqueous Sol-Gel Derived Zirconium Acetate Gel

A precursor gel was prepared by desiccation of an aqueous solution of zirconium acetate and pyrolyzed to form crystalline powders in selected gas environments. Thermal analysis, X-ray powder diffraction, mass spectrometry and Fourier transform infrared spectroscopy were used to investigate the decomposition of the gel and the transformations occurring during pyrolysis to 800°C in nitrogen, air and oxygen. Three transition temperatures were detected: the amorphous-to-cubic crystal transformation, cubic-to-tetragonal transformation, and the initiation of tetragonal-to-monoclinic transition. The presence of molecular oxygen decreased the crystallization temperature, increased the grain size of the tetragonal crystals formed and decreased the temperature at which the tetragonal-to-monoclinic transformation was initiated. The decrease in the crystallization temperature in the presence of oxygen occurred despite an increase in the temperature corresponding to the loss of the major portion of the organic component of the gel.     

Thermal Decomposition and Crystallization of Aqueous Sol-Gel Derived Zirconium Acetate Gels: Effects of the Precursor Solution pH

Crystalline zirconia has previously been formed by desiccation and pyrolysis of a pH 3.4 zirconium acetate precursor solution. In this study, amorphous gels were formed by desiccation of zirconium acetate precursor solutions at pH 0.9 to 3.4 obtained by adding hydrochloric acid and at pH 8.0 by adding ammonium hydroxide to the available pH 3.4 precursor solution. The concentrations of the uncomplexed zirconium ion and the 1 : 1 and 1 : 2 zirconium acetate complexes varied with pH. Decomposition of the amorphous gel and its structure evolution to form crystalline zirconia by pyrolysis were studied. Several decomposition and structure transition temperatures occurring in the pyrolysis and the polymorph distribution in the pyrolysis products were dependent on the pH of the precursor solution. These variations of structure evolution with pH were also related to the relative concentrations of specific zirconium complexes in the precursor solutions. Removal of carbon by pyrolysis was improved by adding hydrochloric acid or ammonium hydroxide to the pH 3.4 precursor solution.     

Mesoporous carbon/zirconia composites: a potential route to chemically functionalized electrically-conductive mesoporous materials

Mesoporous nanocomposite materials in which nanoscale zirconia (ZrO(2)) particles are embedded in the carbon skeleton of a templated mesoporous carbon matrix were prepared, and the embedded zirconia sites were used to accomplish chemical functionalization of the interior surfaces of mesopores. These nanocomposite materials offer a unique combination of high porosity (e.g., ～84% void space), electrical conductivity, and surface tailorability. The ZrO(2)/carbon nanocomposites were characterized by thermogravimetric analysis, nitrogen-adsorption porosimetry, helium pychnometry, powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Comparison was made with templated mesoporous carbon samples prepared without addition of ZrO(2). Treatment of the nanocomposites with phenylphosphonic acid was undertaken and shown to result in robust binding of the phosphonic acid to the surface of ZrO(2) particles. Incorporation of nanoscale ZrO(2) surfaces in the mesoporous composite skeleton offers unique promise as a means for anchoring organophosphonates inside of pores through formation of robust covalent Zr-O-P bonds.     

Dilithium bis[(perfluoroalkyl)sulfonyl]diimide salts as electrolytes for rechargeable lithium batteries

A series of four different dilithium salts of structure F₃CSO₂N(Li)SO₂-(CF₂)_x-SO₂N(Li)SO₂CF₃, with x = 2, 4, 6, 8 were synthesized and characterized in polyethylene-oxide-based solid polymer electrolytes. Each salt may be thought of as two bis[(perfluoroalkyl)sulfonyl]imide anions linked together by a perfluoroalkyl chain of a particular length. Taken together, this homologous series provides an opportunity to study the effects of linker chain length and degree of fluorination in dianionic (and ultimately polyanionic) salts on the properties, particularly the conductivity, of the salts in various solvating media. SPEs in polyethylene oxide were characterized using scanning calorimetry, X-ray diffraction, ¹H and ¹⁹F NMR, and electrochemical impedance spectroscopy for SPEs prepared using ethylene-oxide-oxygen-to-lithium (EO:Li) ratios of 10:1 and 30:1. Trends in SPE ionic conductivity with anion structure revealed an unexpected trend whereby ionic conductivity is generally rising with increased length of the perfluoroalkylene linking group in the dianions. This trend could be the result of a decrease in dianion basicity that results in diminished ion pairing and an enhancement in the number of charge carriers with increasing anion fluorine content, thereby increasing ionic conductivity.     

Atomic-Scale Structure of Water-Based Zirconia Xerogels by X-Ray Diffraction

The structural evolution of zirconia thin films and gel powders has been evaluated by X-ray diffraction. Maxima (r ₁ and r ₂) of the experimental radial distribution function RDF and the bond angles were determined and correlated with TGA (thermogravimetric analysis), DTA (differential thermal analysis) and MS (mass spectrometry). The results indicate that the topological short-range structure (<5 Å) of amorphous zirconia thin films, independent of drying temperature, resembles that of crystalline tetragonal ZrO₂. In contrast, amorphous zirconia powder gels dried at temperatures below 120°C show atomic arrangements similar to that of tetragonal ZrO₂. The structure of these gels annealed at temperatures between 165–340°C resembles a distorted tetragonal ZrO₂, monoclinic-like structure. Zirconia powders and films contain crystalline tetragonal ZrO₂ at 400°C.     

Li+ transport in lithium sulfonylimide-oligo(ethylene oxide) ionic liquids and oligo(ethylene oxide) doped with LiTFSI

The Li+ environment and transport in an ionic liquid (IL) comprised of Li+ and an anion of bis(trifluoromethanesulfonyl)imide anion (TFSI-) tethered to oligoethylene oxide (EO) (EO(12)TFSI-/Li+) were determined and compared to those in a binary solution of the oligoethylene oxide with LiTFSI salt (EO(12)/LiTFSI) by using molecular dynamics (MD) simulations and AC conductivity measurements. The latter revealed that the AC conductivity is 1 to 2 orders of magnitude less in the IL compared to the oligoether/salt binary electrolyte with greater differences being observed at lower temperatures. The conductivity of these electrolytes was accurately predicted by MD simulations, which were used in conjunction with a microscopic model to determine mechanisms of Li+ transport. It was discerned that structure-diffusion of the Li+ cation in the binary electrolyte (EO(12)/LiTFSI-) was similar to that in EO(12)TFSI-/Li+ IL at high temperature (>363 K), thus, one can estimate conductivity of IL at this temperature range if one knows the structure-diffusion of Li+ in the binary electrolyte. However, the rate of structure-diffusion of Li+ in IL was found to slow more dramatically with decreasing temperature than in the binary electrolyte. Lithium motion together with EO(12) solvent accounted for 90% of Li+ transport in EO(12)/LiTFSI-, while the Li+ motion together with the EO(12)TFSI- anion contributed approximately half to the total Li+ transport but did not contribute to the charge transport in IL.