Irene Plitz

A comparative study of Li-ion battery, supercapacitor and nonaqueous asymmetric hybrid devices for automotive applications

Abstract The specific energy, specific power, fast-charge capability, low temperature operation, cycle-life and self-discharge of five energy storage devices was compared. The group included a conventional carbon–carbon supercapacitor, Li-ion battery and three types of asymmetric hybrid supercapacitors. Asymmetric hybrid supercapacitors use a nanostructured Li 4 Ti 5 O 12 anode, and an acetonitrile electrolyte containing a lithium salt. Their cathode was activated carbon, LiCoO 2, or LiMn 2 O 4 . All devices were built using common plastic Li-ion technology developed by Telcordia Technologies.

Enhancement of the electrochemical properties of Li1Mn2O4 through chemical substitution

The link between room temperature (RT) cycling failure for Li1Mn2O4-type spinels and elevated temperature (ET) failure of Li1.05Mn1.95O4 materials was investigated by physical and electrochemical characterization. Failure for both ET and RT cycling occurred at the end of discharge. Substantial evidence suggesting a link based on the cooperative Jahn–Teller distortion was found. Based on this knowledge, LiAlxMn2−xO4−δFz materials were fabricated. These novel compounds were found to offer much improved capacity and ET performance than present generation materials. Three hundred cycles at 55°C resulted in 15% capacity loss. Storage in charged and discharged state for 4 days at 70°C revealed less than 1.6% irreversible capacity loss.

In situ technique for monitoring the gelation of UV curable polymers

Ultraviolet-radiation-cured cross-linked systems are used extensively in optoelectronic applications. We describe a new in situ technique for doing controlled curing of photosensitive materials and monitoring the gelation through dynamic theological measurements. In this method, a sample placed between quartz windows of a modified parallel plate fixture of a Rheometrics Mechanical Spectrometer is radiated from a collimated UV source using a liquid light guide and a mirror. By varying the sample exposure to UV radiation, the degree of cross-linking can be precisely controlled to observe the material behavior in the pre, post, and critical gel state. Time-dependent measurements on urethane based materials show the gelation behavior to have an “induction” period with no change in dynamic moduli (G′, G″), followed by their sharp increase as the materials develop a network structure. The critical gel point is characterized by a power-law dependence of the dynamic moduli on frequency. The gelation kinetics scales with sample thickness and radiation intensity, the scaling factor in both cases being the critical gelation time. The rheological measurements correlate with differential photocalorimetric studies.

Self-stripping of optical fiber coatings in hydrocarbon liquids and cable filling compounds

Optical fiber coatings are in continuous contact with filling cornpounds in optical cables. They are also exposed to hydrocarbon liquids (either as cleaning or lubricating fluids) during splicing operations. We observed that hydrocarbon solvents such as toluene, acetone, ethyl alcohol,isopropyl alcohol, and light rnineral oils (in cable filling cornpounds)cause swelling and self-stripping in some dual-coated fibers. A numerical stress analysis for a swollen dual-coated fiber revealed that swelling induces a compressive radial stress and a tensile tangential stress in the secondary coating; both stress components attain their maximum values at the primary/secondary coating interface. The self-stripping process occurs when the energy stored (due to increasing tensile tangential stress at the interface) exceeds the fracture toughness of the secondary coating. This analysis has provided quantitative measures of coating hydrocarbon solvent/filling compound compatibility and will establish a rational basis for compatibility in optical cable filling compound design.

Reliability characterization of UV-curable adhesives used in optical devices

UV (ultra-violet)-curable adhesives were identified as the underlying cause for failure of devices subjected to accelerated aging conditions. These adhesives must be resistant to degradation and dimensional/mechanical instabilities such as creep. We examined two UV-curable adhesives and found that thermal post-curing caused some shrinkage and degradation. However, post-curing also raised the adhesive glass transition temperature, thereby reducing the reliability risk associated with mechanical instability. We investigated the dimensional/mechanical stability of UV adhesives by measuring thermal expansion/contraction and creep compliance. We found that the adhesive thermal expansion and creep compliance are large enough to pose device reliability risk. Raising the glass transition temperature of UV-curable adhesives by thermal post-cure can improve optical device reliability by lowering the creep compliance.

Reliability characterization of adhesives used in passive optical components

The number of passive optical devices, connectors, and splices used in optical fiber systems to the home and work place is increasing at an accelerating rate. The extent to which the manufacturing and packaging of these components relies on heat-curable epoxy-based and UV- curable (epoxy acrylate or urethane acrylate-based) adhesives is unprecedented in the telecommunications industry. The use of these materials introduces new reliability issues which must be resolved; the most important of these is the ability of the components to function satisfactorily for long periods of time under adverse environmental conditions.

Effects of fluid immersion on splice index-matching gel and splice performance

Although optical splices are protected from fluid immersion during their service life, unanticipated exposures have occurred. For this reason, Bellcore generic requirements include a water immersion test to determine whether splices can tolerate a short-term water exposure and still perform adequately. Mechanical splice failures have been observed in the field and in laboratory experiments. Almost all mechanical splices use index-matching gel, and failures were believed to result from fluid migration along the fiber/gel interface, or fluid diffusion through the gel. We used a photographic analysis to provide direct evidence for these failure modes. We immersed simulated splices in a variety of contaminated fluids and observed that the most rapid mechanism, and the one we consider most likely to result in splice failure, was fluid migration. In addition, we measured loss and reflectance performance for actual splices immersed in contaminated fluids, correlating performance to the results of our photographic analysis. We show that immersion in contaminated water representative of the outdoor plant environment accelerates splice failures.

Fiber coating interactions with buffer tube gels

Significant degradation of the fiber coatings occurred in an optical fiber cable that was aged for five days at 85°C. Incompatibility between the coating and the surrounding buffer tube gel was the suspected cause. To investigate this problem, fiber samples were aged in several gels, the compositions of which were determined using a variety of analytical techniques. Coating degradation occurred as cracking or discoloration in all but one of the gels. These degradations were found to have a strong temperature dependence and to be closely related to the molecular weight distributions of the gel components. Compatibility screening was identified as a necessary procedure to avoid future recurrences of this problem