Michael P. Manahan

A New Postirradiation Mechanical Behavior Test—The Miniaturized Disk Bend Test

A Miniaturized Disk Bend Test (MDBT) capable of extracting postirradiation mechanical behavior information from disk-shaped specimens no larger than those used for transmission electron microscopy ...

Increased Performance of PEFCs with Engineered Mass-Transport Pathways

A long-standing technical challenge in polymer electrolyte fuel cells (PEFCs) is proper water management. Excessive drying leads to accelerated degradation and poor ionic conductivity of the polymer membrane; excessive liquid water prevents reactant gases from accessing catalyst reaction sites. The diffusion media (DM) has been identified as a key component to proper water management. Under low humidity conditions, the DM must deliver inlet water vapor to the catalyst layer (CL) and membrane to ensure proper hydration of the membrane. Under high humidity conditions, the DM must remove excessive water vapor and condensate to ensure an adequate supply of reactant gas to the CL. Several studies hypothesize an ideal DM to consist of both larger pores for liquid water transport/storage and smaller pores for gas transport (1,2). The present study experimentally investigates the introduction of laser-cut perforations in the DM in attempt to create engineered pathways for improved gas and liquid flow. Conceptually, the perforations allow for increased gas and vapor access to the CL at low current, and at high current they act as water conduits for removing excess liquid water. This effect has been studied in this work using electrochemical impedance spectroscopy (EIS), neutron radiography (NR), and steady state polarization testing. In this study, the cathode-side DM was modified by introducing laser perforations with diameters ranging from 40 μm to 300 μm. Figure 1a shows the Nyquist plot for perforated DM (100 and 300 μm) and unaltered (virgin) DM under 100% inlet relative humidity (RH) at 0.2 A cm. Even at such a low current density, the 300μm DM shows a low-frequency arc (0.3 to 15 Hz) that is approximately 7 times larger diameter than the arc of the virgin DM and 100μm DM. The low frequency arc is traditionally attributed to the mass-transport related processes involved in the fuel cell due to their relatively long timescale compared to faster processes (e.g., charge transfer) in the fuel cell (3). Accompanying polarization curves (not shown) confirm this large arc is indeed attributed to excessive flooding in the 300-μm diameter perforations at the high-humidity conditions. The 100-μm and virgin DM arcs in Figure 1a are nearly identical, indicating that only minor differences in transport characteristics exist at all frequencies ranges. Figure 1b shows EIS spectra for the same cells, except with the inlet RH of the anode and cathode held at 50%. This condition yields minimal liquid water due to the sub-saturated conditions throughout the cell. The plot shows a drastic decrease in arc diameter of the 300-μm DM in the low frequency range, indicating the absence of the mass-transport limitations observed at 100% RH. Furthermore, both the 100-μm and 300-μm perforations have arc diameters smaller than the virgin DM. This suggests that the perforations enhance the mass-transport properties at low humidity conditions by increasing the gas access to the CL and shows promise for the implementation of advanced DM structure modification to improve PEFC performance. Corresponding polarization curves at 50% inlet RH (not shown) indicate that the 100-μm perforations increase the limiting current by 7%, as well as increase the cell voltage by 7% at lower currents compared to the virgin DM. While the 300-μm perforations show an increased voltage at lower currents, mass-transport losses are evident at higher currents. Optimization of perforation diameter for enhanced mass-transport properties under all humidity and current conditions is sought, and further experiments on 40-μm perforations are currently underway. In summary, the data show that well-engineered, tailored DM structural modifications yield desirable improvements in water management for PEFCs.

A Comparison of the BUGLE-80, SAILOR, and ELXSIR Neutron Cross-Section Libraries for PWR Pressure Vessel Surveillance Dosimetry and Shielding Applications

In this paper three multigroup neutron cross-section libraries are used in synthesized three-dimensional discrete ordinates transport analyses to investigate their similarities, differences, and results for pressurized water reactor (PWR) pressure vessel surveillance dosimetry and shielding applications. The calculated-to-experimental (C/E) rations and the calculated reaction rates of several fast reactions are compared for the BUGLE-80, SAILOR, and ELXSIR cross-section libraries at the 97-deg surveillance capsule of the San Onofre Nuclear Generation Station Unit 2 (SONGS-2) and at the 90- and 97-deg (C/E ratios only) cavity dosimetry locations for another PWR (referred to as Reactor X).

A generalized methodology for obtaining quantitative charpy data from test specimens of nonstandard dimensions

Miniaturized specimen technology enables mechanical behavior determination using a minimum volume of material. A method for obtaining the ductile-brittle transition temperature (DBTT) of ferritic steels was developed using a miniaturized notch test (MNT). Comparisons between conventional and miniaturized specimen DBTTs show that the MNT specimens are capable of delivering a 41-J transition temperature shift with the same accuracy as that obtained using conventional specimens. The work reported was performed on an American Society for Testing Materials A508 steel.

Simultaneous detection of multiple environmental contaminants through advanced signal processing of electrochemical sensor signals

The possibility of large-scale attacks using chemical warfare agents (CWAs) has exposed the critical need for fundamental research enabling the reliable, unambiguous, and early detection of trace CWAs and toxic industrial chemicals. This paper presents a unique approach for identification and classification of environmental contaminants by perturbing an electrochemical (EC) sensor with an oscillating potential rather than static voltage levels. The dynamic response, being a function of the degree and mechanism of contamination, is then processed with a symbolic dynamic filter for extraction of representative patterns, which are then classified using a trained neural network. Extraction of statistically rich information from the current response enables identification of characteristics species even when they are mixed with other confounding gases. The approach presented in this paper promises to extend sensing power and sensitivity of these EC sensors by augmenting and complementing the sensor technology with state-of-the-art embedded real time signal processing capabilities.

A signal processing framework for simultaneous detection of multiple environmental contaminants

The possibility of large-scale attacks using chemical warfare agents (CWAs) has exposed the critical need for fundamental research enabling the reliable, unambiguous and early detection of trace CWAs and toxic industrial chemicals. This paper presents a unique approach for the identification and classification of simultaneously present multiple environmental contaminants by perturbing an electrochemical (EC) sensor with an oscillating potential for the extraction of statistically rich information from the current response. The dynamic response, being a function of the degree and mechanism of contamination, is then processed with a symbolic dynamic filter for the extraction of representative patterns, which are then classified using a trained neural network. The approach presented in this paper promises to extend the sensing power and sensitivity of these EC sensors by augmenting and complementing sensor technology with state-of-the-art embedded real-time signal processing capabilities.

Determining the physical properties of steam generator tube scale using miniature specimens

Small flakes that consists primarily of magnetite have been discovered on the secondary side of the steam generator of the Three Mile Island Unit 1 plant. These iron oxide flakes are believed to cause significant increases in flow resistance, which in turn causes abnormal increases in steam generator water level. This article describes the measurement of the physical properties of the tube scale so that the maximum amount of loose flakes can be generated prior to hydrodynamic cleaning (water slap). The study of the flake properties to shed light on the flake formation and transport mechanisms.