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The latest information from Hiroshima and Nagasaki on radiation-induced cancer in man includes new DS86 dose assignments and extension of the Life-Span Study Sample cohort through 1985. The implications of these new doses and updated data for the assessment of cancer risk at low to moderate doses have been evaluated. Results from the fitting of three dose-response models (linear, quadratic, linear-quadratic) to the data at doses less than 1.5 Gy are reported. Based on statistical analyses of the authors' results, comments are made concerning the possible shapes of dose-response curves for human cancer.

The latest information from Hiroshima and Nagasaki on radiation-induced cancer in man includes new DS86 dose assignments and extension of the Life-Span Study Sample cohort through 1985. The implications of these new doses and updated data for the assessment of cancer risk at low to moderate doses have been evaluated. Results from the fitting of three dose-response models (linear, quadratic, linear-quadratic) to the data at doses less than 1.5 Gy are reported. Based on statistical analyses of the authors' results, comments are made concerning the possible shapes of dose-response curves for human cancer.

Abstract In this study, a multi-phase, two-dimensional model that integrates the bipolar plate (BP) and gas diffusion layer (GDL) interfacial morphology was developed to understand the effects of this interface on mass, charge and heat transport and performance of polymer electrolyte fuel cells (PEFCs). Two different case studies were performed. The first case assumes a perfect contact interface between the BP and GDL, whereas in the second case, the BP|GDL interfacial layer was incorporated as a separate domain based on the measured BP|GDL morphology. In the BP|GDL interface case, the interfacial voids were assumed to be filled with liquid water to investigate the role of the interfacial voids. For both cases, the effects of different current densities on the in-plane temperature, saturation, and oxygen concentration distribution in the GDL were investigated. Simulations indicate that the Ohmic and concentration losses are increased due to the inclusion of the realistic BP|GDL interface. The electrical contact resistance contribution of the BP|GDL interface was predicted to be 3.8 mΩcm 2 . The saturation in the GDL was found to be higher for the BP|GDL interface case, which results in higher concentration losses. The temperature was predicted to be slightly higher for the BP|GDL interface case, which could be attributed to the higher thermal contact resistance due to the fewer contact regions at the interface.

Abstract In this study, a multi-phase, two-dimensional model that integrates the bipolar plate (BP) and gas diffusion layer (GDL) interfacial morphology was developed to understand the effects of this interface on mass, charge and heat transport and performance of polymer electrolyte fuel cells (PEFCs). Two different case studies were performed. The first case assumes a perfect contact interface between the BP and GDL, whereas in the second case, the BP|GDL interfacial layer was incorporated as a separate domain based on the measured BP|GDL morphology. In the BP|GDL interface case, the interfacial voids were assumed to be filled with liquid water to investigate the role of the interfacial voids. For both cases, the effects of different current densities on the in-plane temperature, saturation, and oxygen concentration distribution in the GDL were investigated. Simulations indicate that the Ohmic and concentration losses are increased due to the inclusion of the realistic BP|GDL interface. The electrical contact resistance contribution of the BP|GDL interface was predicted to be 3.8 mΩcm 2 . The saturation in the GDL was found to be higher for the BP|GDL interface case, which results in higher concentration losses. The temperature was predicted to be slightly higher for the BP|GDL interface case, which could be attributed to the higher thermal contact resistance due to the fewer contact regions at the interface.

Abstract When a lithium ion polymer battery (LiPB) is being cycled, one major cause for degradations is the irreversible side reactions between ions and solvent of electrolyte taking place at the surface of anode particles. SEM analysis of cycled battery cells has revealed that the deposits from the side reactions are dispersed not only on particles, but also between the composite anode and the separator. Thus, the solid electrolyte interface (SEI) becomes thicker and extra deposit layers are formed between composite anode and separator. Also, XPS analysis showed that the deposits are composed of Li 2 CO 3 , which is ionic conductive and electronic nonconductive. Based on the mechanisms and findings, we identified four degradation parameters, including volume fraction of accessible active anode, SEI resistance, resistance of deposit layer and diffusion coefficient of electrolyte, to describe capacity and power fade caused by the side reactions. These degradation parameters have been incorporated into an electrochemical thermal model that has been previously developed. The terminal voltage and capacity of the integrated model are compared with experimental data obtained for up to 300 cycles. Finally, the resistance of the deposit layer calculated by the model is validated against the thickness of the deposit layer measured by SEM.

Abstract When a lithium ion polymer battery (LiPB) is being cycled, one major cause for degradations is the irreversible side reactions between ions and solvent of electrolyte taking place at the surface of anode particles. SEM analysis of cycled battery cells has revealed that the deposits from the side reactions are dispersed not only on particles, but also between the composite anode and the separator. Thus, the solid electrolyte interface (SEI) becomes thicker and extra deposit layers are formed between composite anode and separator. Also, XPS analysis showed that the deposits are composed of Li 2 CO 3 , which is ionic conductive and electronic nonconductive. Based on the mechanisms and findings, we identified four degradation parameters, including volume fraction of accessible active anode, SEI resistance, resistance of deposit layer and diffusion coefficient of electrolyte, to describe capacity and power fade caused by the side reactions. These degradation parameters have been incorporated into an electrochemical thermal model that has been previously developed. The terminal voltage and capacity of the integrated model are compared with experimental data obtained for up to 300 cycles. Finally, the resistance of the deposit layer calculated by the model is validated against the thickness of the deposit layer measured by SEM.

Many of the premature failures in the PEM fuel cells are attributed to crossover of the reactant gas from pinholes or through-the-thickness flaws in the membranes. The formation of these pinholes is not fully understood, although mechanical stress is often considered one of the major factors in their initiation and/or propagation. This paper reports evidence of pinhole failure from mechanical stress by cycling between wet and dry conditions in a normally built single 50cm2 fuel cell. In an effort to understand the source of the mechanical stress, to quantify the magnitude, and to correlate its role in membrane failure, a membrane stress model based on linear viscoelastic theory was developed. The effects of temperature, water content, and time are accounted for in the membrane stress model. To satisfy the inputs for the membrane model and to characterize the mechanical behavior of the polymer electrolyte membrane, a series of experiments was performed. Using commercially available Nafion® NR111 membrane as a model material, swelling of 15% and shrinkage of 4% were found from a hydration and de-hydration cycle. Data on elastic moduli versus relative humidity showed discontinuity at the vapor and liquid water transition. We also found that creep compliance master curves can be obtained by double-shifting the compliance curves according to the time-temperature-moisture superposition principle, which significantly simplifies the modeling effort. Combining data on hygro-expansion, elastic moduli, and creep compliance data through the membrane stress model, it was found that the de-hydration process induces significant stress in the membrane. Due to fluctuations in fuel cell operating conditions, the membrane and the associated components are subject to mechanical fatigue which may mechanically degrade the membrane of PEM fuel cells and eventually lead to pinhole formation.

Many of the premature failures in the PEM fuel cells are attributed to crossover of the reactant gas from pinholes or through-the-thickness flaws in the membranes. The formation of these pinholes is not fully understood, although mechanical stress is often considered one of the major factors in their initiation and/or propagation. This paper reports evidence of pinhole failure from mechanical stress by cycling between wet and dry conditions in a normally built single 50cm2 fuel cell. In an effort to understand the source of the mechanical stress, to quantify the magnitude, and to correlate its role in membrane failure, a membrane stress model based on linear viscoelastic theory was developed. The effects of temperature, water content, and time are accounted for in the membrane stress model. To satisfy the inputs for the membrane model and to characterize the mechanical behavior of the polymer electrolyte membrane, a series of experiments was performed. Using commercially available Nafion® NR111 membrane as a model material, swelling of 15% and shrinkage of 4% were found from a hydration and de-hydration cycle. Data on elastic moduli versus relative humidity showed discontinuity at the vapor and liquid water transition. We also found that creep compliance master curves can be obtained by double-shifting the compliance curves according to the time-temperature-moisture superposition principle, which significantly simplifies the modeling effort. Combining data on hygro-expansion, elastic moduli, and creep compliance data through the membrane stress model, it was found that the de-hydration process induces significant stress in the membrane. Due to fluctuations in fuel cell operating conditions, the membrane and the associated components are subject to mechanical fatigue which may mechanically degrade the membrane of PEM fuel cells and eventually lead to pinhole formation.

A tri-layered solid electrolyte and an oxygen permeable solid air cathode for lithium-air battery cells were synthesized in this investigation. Detailed fabrication procedures for solid electrolyte, air cathode and the assembly of real-world lithium-air battery cell are described. Fabrication of real-world lithium-air button cells was performed using the synthesized tri-layered solid electrolyte, an oxygen permeable air cathode, and a metallic lithium anode. The lithium-air button cells were tested under dry air with 0.1mA∼0.2mA discharge/charge current at different temperatures. It was found that interfacial contact resistances play an important role in Li-air battery cell performance. Experimental results suggested that the lack of robust interfacial contact among solid electrolyte, air cathode and lithium metal anode were the primary factors for the cell’s high internal resistances. It was also found that once the cell internal resistance issues were resolved, the discharge curve of the battery cell was much smoother and the cell was able to discharge at above 2.0V for up to 40 hours. It indicated that in order to have better performing lithium-air battery cell, interfacial contact resistances issue must be resolved very efficiently.

A tri-layered solid electrolyte and an oxygen permeable solid air cathode for lithium-air battery cells were synthesized in this investigation. Detailed fabrication procedures for solid electrolyte, air cathode and the assembly of real-world lithium-air battery cell are described. Fabrication of real-world lithium-air button cells was performed using the synthesized tri-layered solid electrolyte, an oxygen permeable air cathode, and a metallic lithium anode. The lithium-air button cells were tested under dry air with 0.1mA∼0.2mA discharge/charge current at different temperatures. It was found that interfacial contact resistances play an important role in Li-air battery cell performance. Experimental results suggested that the lack of robust interfacial contact among solid electrolyte, air cathode and lithium metal anode were the primary factors for the cell’s high internal resistances. It was also found that once the cell internal resistance issues were resolved, the discharge curve of the battery cell was much smoother and the cell was able to discharge at above 2.0V for up to 40 hours. It indicated that in order to have better performing lithium-air battery cell, interfacial contact resistances issue must be resolved very efficiently.