Elena Sherman

Aging Mechanisms of LiFePO4 Batteries Deduced by Electrochemical and Structural Analyses

The performance loss of lithium-ion batteries with lithium iron phosphate positive chemistry was analyzed using electrochemical characterization techniques such as galvanostatic charge-discharge at different rates, ac impedance, and hybrid pulse power characterization measurements. Differentiation analysis of the discharge profiles as well as in situ reference electrode measurement revealed loss of lithium as well as degradation of the carbon negative; the cell capacity, however, was limited by the amount of active lithium. Destructive physical analyses and ex situ electrochemical analyses were performed at test completion on selected cells. While no change in positive morphology and performance was detected, significant cracking and delamination of the carbon negative was observed. In addition, X-ray diffraction analysis confirmed the changes in the crystal structure of the graphite during cycling. The degradation of the carbon negative is consistent with the observations from the electrochemical analysis. Ex situ electrochemical analysis confirmed that active lithium controlled cell capacity and its loss with cycling directly correlated with cell degradation. The relationship between carbon negative degradation and loss of active lithium is discussed in the context of a consistent overall mechanism.

Aqueous room temperature synthesis of cobalt and zinc sodalite zeolitic imidizolate frameworks

Sodalite zeolitic imidazolate frameworks containing Co (ZIF-67) and Zn (ZIF-8) were synthesized at room temperature under aqueous conditions in 10 min. A trialkylamine deprotonated the 2-methylimidazole ligand and nucleated the frameworks. Furthermore, the ligand acted as a structure directing agent in place of an organic solvent.

Reversible ligand exchange in a metal-organic framework (MOF): toward MOF-based dynamic combinatorial chemical systems.

Reversible benzene dicarboxylate/2-bromobenzene dicarboxylate ligand exchange has been studied in the cubic metal-organic framework MOF-5. Significant exchange (up to ∼50%), with continuous compositional variation, was observed using ex-situ (1)H NMR following treatment over ∼6 h at ∼85 °C in 10-40 mM ligand solutions. Exchange occurred without significant structural degradation as characterized by X-ray diffraction, nitrogen adsorption, and scanning electron microscopy. Solid-state (13)C NMR was used to show that exchanged ligands were incorporated into the framework lattice and not simply adsorbed within the pores. Exchange was found to be sensitive to the small free energy changes caused by the ligand concentration in the exchanging solution indicating that exchange is energetically nearly degenerate. This demonstration of reversible, nearly isoenergetic exchange indicates that mixed ligand MOFs could be developed as dynamic combinatorial chemical systems.

Addressing factors for polymer-dispersed liquid-crystal displays

Suitable addressing methods for polymer dispersed liquid crystal (PDLC) displays are dependent upon both the type of display and the PDLC properties. Video displays require fast frame rates, high resolution, and gray scale. The addressing method depends upon PDLC properties such as threshold voltage, operating voltages, response times, resistivity, and hysteresis effects. the properties of PDLC films which are formed by ultraviolet (UV) polymerization of LC/monomer solutions depend upon their formation conditions. The threshold and operating voltages, contrast ratio and response times are affected by formation factors including the UV intensity, temperature, LC concentration, LC composition, field pre- alignment conditions, cell thickness, and droplet shape. The PDLC resistivity and hysteresis properties are more difficult to control, and can strongly affect the light throughput, response times, and gray scale of video displays. Apparent limitations from these properties can be overcome by utilizing pulse mode addressing factors pertinent for active matrix and photoactivated light valve PDLC displays.

A liquid crystal based polymer for applications in MMW modulation devices

Abstract A new liquid crystal monomer, the diacrylate of a bis phenyi diazene, was synthesized. It was polymerized to form both unoriented and oriented polymer solids–the latter in the presence of a magnetic field. The millimeter wave birefringence was measured for a polymeric rod, whose macroscopic orientation was perpendicular to its long axis, using a waveguide version of the Mach-Zehnder interferometer, and the measured δn was 0.075 at 30 GHz. The mechanical strength of oriented and unoriented polymer plates was measured. The tensile modulus in the direction parallel to the oriented axis was 6.8×108 N/m2, which was about 4 times that of the same material without macroscopic orientation. The torsional rigidity modulus of a thin rod (orientation perpendicular to its length) was measured to be of the order 107 N/m4. Both the large dielectric anisotropy and high mechanical strength make the oriented polymer potentially suitable for application in a novel class of millimeter wave modulation devices.

Droplet-size effects and lighting techniques in direct-view PDLC displays

— By varying liquid-crystal droplet sizes in designated regions, direct-view polymer-dispersed liquid-crystal (PDLC) displays with simpler addressing schemes are constructed. A multiple-droplet-size PDLC is used to hide addressing-line voltage effects in a segmented display which requires a clear (non-scattering) region surrounding the segments. Edge lighting improves the contrast ratio of these direct-view displays.

Liquid Crystal Monomer Additives for Improved Polymer Dispersed Liquid Crystals

Abstract The performance of polymer dispersed liquid crystal (PDLC) films was improved markedly by introduction of a monofunctional liquid crystal monomer component into the photopolymerizable solution from which the PDLC was formed by phase separation. Small amounts of an LC-monomer additive enhanced the LC phase separation and slowed the rate of polymerization. Use of 1-10% of LC-monomer in the solution led to the formation of larger LC droplets in a less rigid polymer, resulting in PDLC films with lower threshold and operating voltages and increased the contrast ratio. The effects of LC-monomer additives on PDLC properties were studied as a function of their structure and concentration, particularly in a photopolymerizable solution of Norland NOA65 monomer system with BDH-E9. For example an optimized amount of LC-monomer, such as 4% p-butylphenyl p-acryloyloxybenzoate in a 1:1 solution of NOA65 and BDH-E9, gave 50% lower threshold and operating voltages, together with 9% higher on-state transmission th...

Gas Chromatography Analysis of Diphenyl-Diacetylene Liquid Crystals

Abstract A gas chromatography method is used to investigate the purity of dialkyl-diphenyl-diacetylene liquid crystal components and at the same time identify the structure and concentration of the major impurities.

Insect Abatement on Lubricious, Low Adhesion Polymer Coatings Measured with an Insect Impact Testing System

Insect debris disrupts laminar flow, obstructs operator vision, and degrades vehicle aesthetics. To protect vehicle surfaces, anti-contamination coatings have been under development for 70 years, but no known homogeneous coating both adequately reduces debris and survives on vehicle surfaces. Coatings with synergistic combinations of physical properties and materials, however, may enable improved anti-fouling and maintain long-term durability. Transparent, spray-on coatings were developed that contain a combination of fluorinated and hygroscopic chemistries in which the fluorinated component reduces wetting of insect debris while the hygroscopic component produces a lubricating layer of absorbed water that interferes with debris adhesion. Debris area after insect impact was approximately twice as low on these coatings as compared to homogeneous control materials. Furthermore, the sensitivity of debris accumulation to hygroscopic content, and thus lubricity, was measured. At least 13 wt% hygroscopic content in the developed coatings was required for decreased debris accumulation compared to a pure fluorinated surface. Lastly, resistance to common vehicle fluids and scribe-tape adhesion was measured on fluorinated-hygroscopic coatings as initial demonstrations of durability.