bluuebunny/crossref_metadata_2025 huggingface dataset:
Dinoflagellate cyst record of the Eocene-Oligocene boundary succession in flysch deposits at Leluchów, Carpathian Mountains, Poland
Authors: Gedl Przemysław
Published: September 2007

Organic-walled dinoflagellate cysts from Eocene-Oligocene transitional deposits have been studied in a section at Leluchów, Flysch Carpathians, Poland. The Eocene-Oligocene boundary, as based on dinoflagellate cyst distribution, is placed in the upper part of the Leluchów Marl Member. The main biostratigraphic events associated with this boundary interval are the highest occurrence of
Areosphaeridium michoudii
and
Areosphaeridium diktyoplokum
, and the lowest occurrence of
Wetzeliella gochtii
. Distinct changes in dinoflagellate cyst assemblages and palynofacies across the Eocene-Oligocene boundary at Leluchów imply a drop in relative sea-level within the Carpathian flysch basin that might correlate with a major eustatic fall during the earliest Oligocene. A drop in sea surface temperature is recognized prior to the Eocene-Oligocene boundary, and evidence is presented for an increase in nutrient level and decrease in salinity within the photic zone during the earliest Oligocene.
Polynostratigraphy and palaeoenvironments across the Oligocene-Miocene boundary within the Centinela Formation, southwestern Argentina
Authors: Guerstein G. R. , Guler M. V. , Casadío S.
Published: September 2007

Palynological analysis of the Centinela Formation, exposed in the foothills of the Patagonian Andes, has revealed the presence of pollen, dinoflagellate cysts, and chlorococcalean and prasinophycean algae. These groups are here reported from the Centinela Formation for the first time. Sporomorph and dinoflagellate cyst assemblages suggest a Late Oligocene and Early Miocene age. These results coincide with a
87
Sr/
86
Sr age close to the age of the Oligocene-Miocene boundary obtained from the lower part of the section. Palynological information from the Centinela Formation permits correlation with Upper Oligocene and Lower Miocene units cropping out along the Atlantic Patagonian coast. Assemblages from the lower part of the section suggest that the beds were deposited under marine, near-shore palaeoenvironmental conditions with a strong continental influence. In the middle part of the section, high dinoflagellate cyst ratios coincide with a maximum flooding surface recorded in the Centinela Formation. Towards the top of the Centinela Formation, the sporomorph assemblages reflect the development of vegetation adapted to coastal environments, which agrees with the sparse occurrence of marine palynomorphs. A new dinoflagellate species,
Hystrichostrogylon sulcatum
, is proposed. This species appears to range across the Oligocene-Miocene boundary and is particularly abundant in the lowest Miocene.
Early-Middle Pleistocene transitions: an overview and recommendation for the defining boundary
Authors: Head Martin J. , Gibbard Philip L.
Published: September 2007

The Early-Middle Pleistocene transition (
c.
1.2–0.5 Ma), sometimes known as the ‘mid-Pleistocene revolution’, represents a major episode in Earth history. Low-amplitude 41-ka obliquity-forced climate cycles of the earlier Pleistocene were replaced progressively in the later Pleistocene by high-amplitude 100-ka cycles. These later cycles are indicative of slow ice build-up and subsequent rapid melting, and imply a transition to a strongly non-linear forced climate system. Changes were accompanied by substantially increased global ice volume at 940 ka. These climate transformations, particularly the increasing severity and duration of cold stages, have had a profound effect on the biota and the physical landscape, especially in the northern hemisphere. This review assesses and integrates the marine and terrestrial evidence for change across this transition, based on the literature and especially the following 17 chapters in the present volume. Orbital and non-orbital climate forcing, palaeoceanography, stable isotopes, organic geochemistry, marine micropalaeontology, glacial history, loess-palaeosol sequences, pollen analysis, large and small mammal palaeoecology and stratigraphy, and human evolution and dispersal are all considered, and a series of discrete events is identified from Marine Isotope Stage (MIS) 36 (
c.
1.2 Ma) to MIS 13 (
c.
540–460 Ma). Of these, the cold MIS 22 (
c.
880–870 ka) is perhaps the most profound. However, we here endorse earlier views that on practical grounds the Matuyama-Brunhes palaeomagnetic Chron boundary (mid-point at 773 ka, with an estimated duration of 7 ka) would serve as the best overall guide for establishing the Early-Middle Pleistocene Subseries boundary.
Large mammal turnover in Africa and the Levant between 1.0 and 0.5 Ma
Authors: O’Regan H. J. , Bishop L. C. , Lamb A. , Elton S. , Turner A.
Published: September 2007

Faunal change at the Early-Middle Pleistocene boundary in Europe has long been a topic for discussion. However, analyses of large mammal turnover at this time in Africa have been lacking, largely because of the low number of sites dated to this interval. Recent work, particularly in the last 10 years, has resulted in a much larger published sample of sites and we synthesize these data in this paper. In our multivariate (TWINSPAN) analyses of African and Levantine large mammal faunas we found that localities were subdivided by geographic regions, not by age. There were some small-scale changes with the appearance or extinction of particular taxa, but there was no large-scale turnover such as that seen in Europe. The Levant was included as a possible route for faunal interchange with east Africa, but no similarities were found between these areas. It therefore appears that the modern zoogeographic separation of the Levant and north Africa into the Palaearctic region and sub-Saharan Africa into the African region can be traced back to at least the Early-Middle Pleistocene boundary.
Comparative Studies in Conversation Analysis
Authors: Sidnell Jack
Published: September 2007

Conversation analysis initially drew its empirical materials from recordings of English conversation. However, over the past 20 years conversation analysts have begun to examine talk-in-interaction in an increasingly broad range of languages and communities. These studies allow for a new comparative perspective, which attends to the consequences of linguistic and social differences for the organization of social interaction. A framework for such a comparative analysis focusing on a series of generic interactional issues or “problems” (e.g., how turns are to be distributed among participants) and the way they are solved through the mobilization of local resources (grammar, social categories, etc.) is sketched. Comparative studies in conversation analysis encourage us to think of interaction in terms of generic organizations of interaction, which are inflected or torqued by the local circumstances within which they operate ( Schegloff 2006 ).
A Review of Different and Promising Defect Etching Techniques: from Si to Ge
Authors: Abbadie Alexandra , Hartmann Jean-Michel , Brunier F
Published: September 2007

A review of defect selective etchings used for process monitoring of Si, SiGe and Ge is presented. Cr-based and Cr-free solutions as well a gaseous HCl etch technique are described. Some applications of these techniques on Si and SiGe relaxed substrates as well their etching mechanisms are highlighted and discussed.
Methods for the Controlled Reduction of Carrier Lifetime in Power Devices
Authors: Hille Frank , Niedernostheide Franz-Josef , Schulze Hans-Joachim
Published: September 2007

Power devices require a thick lightly doped drift layer to support the required blocking capability. In order to obtain also an optimized on-state and dynamic switching behavior, the shape of excess charge carrier profile in the drift region has to be tailored. Often, and especially for freewheeling diodes, this can only be achieved by a precise adjustment of the carrier lifetime. In this work, we use the free carrier absorption technique to determine the excess charge carrier profile in power diodes. Methods to extract the carrier lifetime from the carrier profile of platinum diffused diodes are demonstrated. The injection level and temperature dependence of the carrier lifetime are determined. The origin of the specific shape of the excess charge carrier profile is analyzed by complementary measurements of the deep level profile by DLTS.
Photoluminescence-Based Metrology for Si and SiGe Materials
Authors: Buczkowski Andrzej
Published: September 2007

Application of room temperature, edge-band, carrier-lifetime-reliant photoluminescence metrology for characterization of contamination and defectivity in Si and SiGe materials is discussed in the paper. The physics of the technique is illustrated using 2D-Axisymmetric PL intensity modeling examples. PL signals are simulated for a variety of ion-implanted Ultra Shallow Junction (USJ) silicon and epitaxial Si/SiGe/Si structures. High PLi sensitivity to defect presence, ion implantation dose, energy, specie and implantation conditions (implanter tool signatures) is demonstrated and ultimately applied to detect process variations at sub-1% levels. Examples of quantifiable doping activation effects, defect re-growth kinetics, contamination presence, annealing quality, and formation and relaxation of extended defects are also provided.
Novel Ultrasonic Tool for Vacancy Observation in Crystalline Silicon
Authors: Goto Terutaka , Yamada-Kaneta Hiroshi , Sato Koji , Hikin Masatoshi , Nemoto Yuichi , Nakamura Shintaro
Published: September 2007

Low-temperature ultrasonic measurements were employed for observation of isolated vacancies in crystalline silicon. The elastic constants of FZ crystalline silicon revealed low-temperature softening below about 20 K down to base temperature 20 mK in present experiments. The softening of non-doped FZ silicon being free from applied filed indicates a non-magnetic charge state V0 of vacancy. The disappearance of softening in boron-doped FZ silicon under applied magnetic fields up 2T means magnetic charge state V+. This novel ultrasonic tool succeeded in observation of low-temperature softening in perfect crystal region of non-doped CZ silicon ingot indicating distribution of isolated vacancies.
Ab Initio Analysis of Point Defects in Plane-Stressed Si or Ge Crystals
Authors: Sueoka Koji , Spiewak Piotr , Vanhellemont Jan
Published: September 2007

The effect of compressive and tensile plane-stress on the formation energy of vacancies (V) and self-interstitials (I) in Si or Ge crystals was studied with an ab initio approach for in-plane strains up to 5.0%. In both Si and Ge crystals, (i) generally, the formation energy of I decreased under tensile in-plane strain, while the formation energy of V decreased under compressive in-plane strain, (ii) compressive strain increases the stability region of V -2 in the band gap, while the strain does not shift significantly the stability region of I 0 and I +2 at the [110] dumbbell site. The dependence of the formation energy of I at the tetrahedral site on in-plane strain was different between Si and Ge. The other calculations showed that Young's modulus and Poisson's ratio of Si or Ge crystals are in good agreement with experimental results. Mechanical strength of a thin Si film, which is important for device applications, is also evaluated with an ab initio approach.
Particle Removal from Micrometer-Sized Trenches Using High-Velocity-Aerosol Cleaning and Comparison with Megasonic Tank Cleaning
Authors: Wostyn Kurt L. , Sano Ken-Ichi , Eitoku Atsuro , Janssens Tom , Bearda Twan , Leunissen Leonardus H.A. , Mertens P.
Published: September 2007

Cleaning technologies employing physical forces (such as megasonic and high-velocity-aerosol cleaning) are usually evaluated by comparing the particle removal efficiency on blanket wafers with damage formation on fragile structures, typically gate stack type structures. However patterned wafers provide a much more realistic cleaning challenge. Particle removal efficiencies from trenches in micrometer dimension have been evaluated using high-velocity-aerosol cleaning. When using short cleaning times, a large decrease in cleaning efficiency is found for even modest pattern dimension. By extending the cleaning time, cleaning efficiencies comparable to blanket-wafer tests can be reached. Moreover, batch megasonic cleaning shows a similar decrease in performance as high-velocity-aerosol cleaning.
Etching of Thermal SiO2 in Supercritical CO2
Authors: Malhouitre Stephane , Van Hoeymissen Jan , Case Carlye , Granger Pascal
Published: September 2007

Etching of thermal SiO2 using the HF/pyridine complex in supercritical CO2 (scCO2) was studied. Etch rate increases when HF concentration, processing temperature and fluid flow rates are increased. The decrease of the instantaneous etch rate as the processing time increases is attributed to the formation of etching residues on the film surface that act as a diffusion barrier. These residues are soluble in water and are identified as (Pyr)2SiF4 by FTIR analysis. For similar nominal HF concentrations, the SiO2 etch rate in anhydrous pyridine/scCO2 is much higher than in H2O.
Automating Metal Lift-Off for Mass MEMS Production
Authors: Naughton John J. , Franklin Patrick
Published: September 2007

AMI Semiconductor and the Indigo Operations of FLIR were challenged to transfer a microbolometer manufacturing process from a class 1000 Fab to a class 1 facility. Since this micro-electro-mechanical systems (MEMS) process includes several a-typical metals for which there is no established plasma etch chemistry, it requires lift-off processing for metal patterning. To facilitate metal stringer removal, the original process included a manual swab of the wafers. Using a solvent jet and dry CO2 particulate surface treatment in tandem proved an automated method of performing this lift-off for mass MEMS production.
Study of SiO2 Modifications Induced by Oxygen Plasmas and Their Effect on Wet Processes
Authors: Votta Annamaria , Bellandi Enrico , Alessandri Mauro , Vedda Anna , Ferretti Anna , Fasoli Mauro , Moretti Federico
Published: September 2007

Oxygen plasmas, commonly used in typical C-MOS process flow, cause silicon oxide structure to be significantly modified: as a consequence wet processes, that are sensitive to oxide defectivity, need to be retargeted. In this paper structure modifications, induced in boron implanted SiO2 by RF and TCP plasma have been studied by thermally stimulated luminescence, charging measurements and Electron Paramagnetic Resonance (EPR). Different kind and quantity of point defects have been identified by thermally stimulated luminescence technique as a function of plasma generator. These results match with EPR analysis, charging measurements and HF etches rate profile.
Gate Oxide Cleans on Single Wafer Tool
Authors: Garnier Philippe , Vessa Tony , Larrea Anne Sophie , Pernet B. , Gomez Y. , Barge D. , Torres A. , Tavel B. , Henderson C. , Levy D.
Published: September 2007

Pre gate oxide and especially "Double Gate Oxide" cleans are the most critical surface preparation steps in the semiconductor industry. This paper describes the key parameters to perform this kind of operations on a fully integrated single wafer cleaning tool. This switch from conventional batch cleaning tool enables significant defect density reduction and therefore yield enhancement. Nonetheless the main challenge of the double gate oxide clean is the prevention and the control of photo lithography resist lift off during the wet etch. This clean is also composed of two other steps, less critical. Nonetheless special care has been considered in the surface preparation in order to avoid any electrical mobility degradation.
Wet Etching Step Evolutions for Selective Removal on Silicide or Germanide Applications
Authors: Carron Veronique , Besson P. , Pierre François
Published: September 2007

The paper deals with the nickel selective etching chemistries with respect to NiGe and high Ge content NiSiGe layers. Chemistries depicted in the literature are discussed. A focus is put on H2SO4 96% ("water free" approach), for which the etching mechanism is investigated.
Post Salicidation Clean: Selective Removal of Un-Reacted NiPt Towards NiPtSi(Ge)
Authors: Xu Kaidong , Lauwers Anne , Vos Rita , Archer Leo , Kraus Harald , Demeurisse Caroline , Mertens Sofie , Henry Sally-Ann , Gale G. , Mertens P. , Kovacs Frederic , Dalmer Michael , Gaulhofer Ernst
Published: September 2007

Ni(alloy) silicides have recently gained much attention due to their improved properties. Ni(Pt)Si has indeed become one of most promising candidates to replace NiSi as contact electrodes. To avoid bridging at the spacers and the isolation areas, after self-aligned silicidation (salicidation), un-reacted NiPt needs to be removed. This post salicidation clean process has to be selective towards all possibly exposed materials such as Si3N4 (spacers), SiO2 (field oxide), NiPtSi (contact and gate electrodes), and NiPtSiGe (contact electrodes of source/drain for strain applications). In this work, it is found that dilute HCl/HNO3 mixtures can well be used for NiPt post salicidation clean. It is shown that this process has a high etch rate for NiPt and a high selectivity towards other possibly exposed materials. Furthermore, the sheet resistance and the thickness of the silicides after this selective etch are well within specification providing a clean, environmentally-friendly process with low cost-of-ownership.
Local Removal Frequency of Nano Particles in Megasonic Cleaning
Authors: Janssens Tom , Wostyn Kurt L. , Arnauts Sophia , De Geyter Annelies , Bearda Twan , Mertens P.
Published: September 2007

In single wafer megasonic cleaning non homogeneous cleaning efficiencies across the wafer are often observed. By studying the local cleaning dynamics, which is quantified by the particle removal frequency fR, it will be shown that the most important factor generating these non homogeneities is a difference in dwell time of the sound field above different spots of the wafer. The impact of the dwell time is studied for 2 different tool designs: a megasonic nozzle and a rod system. For the megasonic nozzle system, adapting the scan speed of the nozzle above the wafer can eliminate most of the variation in removal frequency across the wafer. For the megasonic rod system the attenuation of the sound field in the rod results in a linear dependence of the removal frequency on the wafer radius, thereby cleaning fastest at the spots with the shortest dwell time.
Parametric Nano Particle Cleaning
Authors: Ligai Slav , Pravdin Sergey , Ligai Maria
Published: September 2007

Under certain conditions, with specified pressure and temperature profiles, bubbles of nanoscale sizes can be grown under or in close proximity to the nanoparticles being removed. This bubble growth can create a lifting force that exceeds the adhesive force between the particles and the surface. The basic cleaning cycle would consist of several stages. Once the pressure in the cleaning chamber drops below the saturated vapor pressure, the liquid becomes superheated, i.e. it starts to boil. Small (nanoscale) bubbles will be generated on particles and impurities, both on the surface of the wafer and inside the liquid. By operating with just a single parameter - pressure - we can make the bubbles grow, coalesce, and then extract particles from the wafer. The particle-removing forces to be discussed are expansion, resonant, drag, and flotation forces.
Modifications of Porous Low-k by Plasma Treatments and Wet Cleans
Authors: Xu Kaidong , Vereecke Guy , Kesters Els , Le QuocToan , Lux Marcel , Henry Sally-Ann , Kraus Harald , Archer Leo , Mertens P. , Kovacs Frederic , Dalmer Michael , Gaulhofer Ernst , Luo Shijian , Han Qingyuan , Berry I.
Published: September 2007

With the implementation of Cu and low-k dielectrics in back-end-of-line (BEOL), issues arise in every process step that must be addressed. One of the challenges is the susceptibility of low-k materials to attack during residue removal (polymer) after low-k etch and resist strip. This work focuses on the influence of the plasma etch and ash processes on compatibility of porous low-k dielectrics with a selection of wet chemistries that are used for BEOL polymer removal. The low-k material investigated was porous nanoclustered silica. For blanket films, it is shown that the chemical and structural changes to ultra low-k(ULK) by the plasma treatments are the main contributors to the plasma damage. Two wet chemistries including dHF are found to be compatible with most plasma treatments used in this study. It is also demonstrated that chemistry selection for wet cleaning is linked with the plasma treatments used for etch and ash.
Post Cu-CMP Engineering Challenges for the 65 nm Technology Nodes and Beyond
Authors: Petitdidier Sebastien , Mellier Maxime , Guiheux Denis , Juhel Marc
Published: September 2007

In this paper, the methodology to improve the robustness and the efficacy of the overall post Cu-CMP sequence for 65 nm node technology is presented. The critical challenges to answer is to improve, in a manufacturing context, the slurry particles removal, the prevention of corrosion issue and organic residues generation, taking into account the change arising from new barrier slurry and the exposure of hydrophobic low-k film. The change caused by the exposure of hydrophobic low-k on slurry particle contamination and watermark is first discussed. The control of the Cu passivation and organic contamination on the polishing pad is shown to be a critical step for particle like defects and corrosion susceptibility. Finally, some clarifications on the relationship between the organic contamination of the Cu surface from the platen and the generation of organic residues are presented by studying the kinetic of its removal by scrub chemistries.
Amorphous Lanthanum Lutetium Oxide Thin Films as an Alternative High-k Material
Authors: Lopes Joao M. , Roeckerath Martin , Heeg Tassilo , Schubert Jurgen , Littmark Uffe , Mantl Siegfried , Besmehn Astrid , Myllymaki Pia , Niinistö Lauri , Adamo Carolina , Schlom Darrel G.
Published: September 2007

Lanthanum lutetium oxide thin films have been grown on Si substrates by three different techniques: pulsed-laser deposition, molecular beam deposition, and atomic layer deposition. Post-deposition annealing in O2 was performed to improve their electrical properties. Capacitance-voltage and current-voltage measurements reveal well-behaving C-V curves and low leakage current density levels. Most strikingly, a dielectric constant around 30 could be achieved for amorphous thin films of this material. These results are discussed in terms of the microstructural characteristics of the films before and after thermal treatment.
A Proton Conducting Electrolyte Based on Imidazole Supported Inorganic Porous Matrix for the Unhumidified Intermediate Temperature Fuel Cell
Authors: Hojo Nobuhiko , Yamamoto Taisuke , Okada Yukihiro , Niikura Junji
Published: September 2007

A proton conducting electrolyte based on imidazole supported inorganic porous matrix was examined to obtain the proton conductor that can operate without humidification at intermediate temperature. The proton conductivity of composite electrolytes with large pore size(0.1μm, 10μm) showed high at higher temperature than 363K, and dropped at lower temperature than 363K as well as imidazole(bulk). However the proton conductivity of composite electrolytes with small pore size(4nm) did not drop even at lower than 363K, that is, showed good temperature dependency. In addition coupling acid functional groups on the pore surfaces contribute to the raise of proton conductivity. These results suggest that high proton conductivity at wide range of temperature without humidification could be achieved by controlling pore size and the acid functional groups coupled with the pore surfaces.
Study on Degradation of the Pt-Ru Anode of PEFC for Residential Application (3): Interpretation of AC Impedance Analyses of MEAs and the Nature of CO Poisoning on Anode of PEFC
Authors: Tabata Takeshi , Yamazaki Osamu , Oomori Yasuharu , Shintaku Hideki
Published: September 2007

An interpretation of AC impedance concerning anode and a quantitative model of CO poisoning characteristics are proposed to explain the voltage in the presence of CO. It was found that low frequency impedance can be understood by modulation in Uf induced by electric modulation at a constant flow rate of fed fuel. The "limiting current" characteristics of CO poisoning can be explained by H2 adsorption rate limited by the collision of H2 onto the empty active sites that are extremely reduced by CO poisoning. The limiting current according to the model is proportional to pH2, 1/pCO, and the electrochemical surface area of the anode under isothermal condition.
The Effect of Electrode Ink Processing and Composition on Catalyst Utilization
Authors: Xu Hui , Brosha Eric L. , Garzon F. H. , Uribe Francisco , Wilson Mahlon , Pivovar Bryan
Published: September 2007

Platinum surface area was chemically and electrochemically characterized using X-ray diffraction (XRD), H2S adsorption, and electrochemical techniques including half-cells and fuel cells. Electrode ink processing and composition were found to have a strong impact on observed surface area with increased processing leading to a reduction in platinum surface area. Fuel cell electrode measurements show a significant portion of the platinum may not be available. This unutilized Pt may be due to either ionic or electronic connectivity as evidence exists to support both ionic and electronic isolation.
Artifacts in Measuring Electrode Catalyst Area of Fuel Cells through Cyclic Voltammetry
Authors: Carter Robert N. , Kocha Shyam S. , Wagner Frederick , Fay Matthew , Gasteiger Hubert A.
Published: September 2007

Cyclic Voltammetry is a well-established technique to measure the electrochemically active surface area (ECA) of a catalyst in an electrode through hydrogen adsorption/desorption (HAD). In the field of proton exchange membrane fuel cells this method can be applied to scales ranging from rotating disc electrodes to single-cell and even multiple-cell stacks where the working electrode (WE) is part of a membrane-electrode assembly (MEA). It is shown herein for MEA tests that molecular hydrogen evolution can create significant artifacts in the shape of the HAD curve, which impacts the calculated ECA. This artifact is related to the rate of mass transfer of molecular hydrogen away from the WE and depends on the nitrogen purge flow rate used for the WE during the voltage sweep. Discussion of spatially resolved CV measurements is also provided showing that this same artifact can create the appearance of spatial variation in a cell's ECA.
Investigation of Hydrogen Cross-leak Behaviors in a Polymer Electrolyte Membrane by Direct Gas Mass Spectroscopy and Magnetic Resonance Imaging
Authors: Tsushima Shohji , Nanjo Takanori , Hirai Shuichiro
Published: September 2007

Hydrogen permeability of polymer electrolyte membranes is of great importance since hydrogen cross-leak plays a major role in degradation of the membrane and/or catalyst layers in polymer electrolyte membrane fuel cells (PEMFCs). In this study, the hydrogen cross-leak mechanism was investigated using direct gas mass spectroscopy (DGMS) to monitor hydrogen in the cathode exhaust and magnetic resonance imaging (MRI) technique to monitor membrane water content. The DGMS measurement revealed that hydrogen permeability increased with humidity, temperature and hydrogen partial pressure in the anode. MRI visualization verified that hydrogen permeability was strongly affected by membrane hydration. Influence of hydrogen partial pressure was a key factor influencing permeation rates over a wide range of relative humidity. Reduced hydrogen permeation was observed under fuel cell operating conditions presumably due to hydrogen consumption in the anode. These observations suggest that dissolution and diffusion of hydrogen in the membrane play an important roll in hydrogen cross-leak in the PEMFC.
Visualization of Membrane Hydration Path in an Operating PEMFC by Nuclei-Labeling MRI
Authors: Kotaka Toshikazu , Tsushima Shohji , Hirai Shuichiro
Published: September 2007

A nuclei-labeling magnetic resonance imaging (NL-MRI) technique was used to investigate the effect of water vapor and generated water on hydration of polymer electrolyte membrane (PEM) separately in polymer electrolyte membrane fuel cell (PEMFC) during operation. It was successfully shown that the membrane hydration path (i.e. anode humidification, cathode humidification or generated water) could be distinguished in an operating PEMFC and the validity and utility of this approach is confirmed by comparing the summation of separation analysis results with conventional MRI result under ordinary fuel cell operation. From visualization results, it was found that water vapor from the anode and the cathode has little effect on the membrane hydration. This indicates that most membrane hydration came from water generated by the electrochemical reaction.
Cold Start Analysis of a Polymer Electrolyte Fuel Cell
Authors: Khandelwal Manish , Ko JaeJun , Mench Matthew M.
Published: September 2007

Study of cold start focusing on the stack as well as single cell is being performed at FCDDL to predict the cell/stack freeze-out, performance limitations and to reduce the start-up time. In the present work, a transient one-dimensional thermal model for a generic PEFC stack was developed to predict the temperature distribution and the start-up time. For a 20 cell size generic stack, the model predicted almost 12-15 ºC temperature difference between the end cell and center cell. A fundamental thermodynamic analysis was also performed to estimate the supersaturation required for the ice formation in the fuel cell components. The development of a coupled thermal and water transport model for the cold start of a single fuel cell is ongoing at the FCDDL. The developed single cell model will help to investigate the individual cell characteristic in the PEFC stack.
Oxygen Reduction Reaction Kinetics and Electrode Mass Transport in Sub-Freezing PEM Fuel Cells
Authors: Thompson Eric L. , Jorne Jacob , Gasteiger Hubert A.
Published: September 2007

A procedure involving isothermal, galvanostatic operation was developed to measure the charge (or ice) accumulation in frozen PEM fuel cells. Charge storage capacity in the membrane and in the electrode was estimated, then compared to measured charge accumulation. Cryo-SEM imaging of electrodes following voltage failure was used to assess pore volume filling in the electrode. At very low currents, in which adequate time allowed for water diffusion, the maximum storage capacity was found in the membrane and electrode. At higher currents, less charge was passed before failure occurred, and significantly less ice had accumulated in the electrode. Lower initial water content corresponded to more available charge storage capacity within the membrane. Operation at high initial water content prevented water uptake by the membrane and allowed isolation of electrode storage capacity, and subsequent estimation of the partition between charge storage in the electrode and membrane at intermediate initial water contents.
Water Sorption, Desorption and Transport in Nafion Membranes
Authors: Majsztrik Paul , Satterfield Barclay , Bocarsly Andrew , Benziger Jay B.
Published: September 2007

Water sorption, desorption, and permeation in and through Nafion membranes 51-606 μm thick were measured as functions of temperature between 30 and 90ºC. Water permeation rates are weakly dependent on membrane thickness. Water permeation from liquid water was faster than water permeation from 100% relative humidity gas. These results indicated that interfacial mass transport at the membrane/gas interface was the limiting resistance to transport and not diffusion. Water sorption is initially limited by interfacial mass transport; above a critical water content water sorption is limited by polymer relaxation dynamics associated with swelling. Water desorption from a water saturated film was an order of magnitude faster than water sorption. Different regimes of water transport and sorption in Nafion membranes corresponding to diffusion, interfacial transport and polymer swelling depend on membrane thickness, temperature and the contacting phase.
Effects of Microstructure of Gas Diffusion Layer on Two-Phase Flow Transport Properties
Authors: Yoneda Masakazu , Takimoto Masato , Koshizuka Seiichi
Published: September 2007

Two-phase flow transport properties inside the gas diffusion layer of Proton Exchange Membrane (PEM) fuel cells is investigated by direct numerical simulation. This work presents the effects of the microscopic structure such as basic frame of carbon fibers, mean porosity, alignment of fibers and partial degradation of PTFEs on the distribution of liquid water and effective gas diffusivity inside the GDL.
Drops, Slugs and Flooding in PEM Fuel Cells
Authors: Kimball Erin , Whitaker Tamara , Kevrekidis Yannis , Benziger Jay B.
Published: September 2007

Local current density measurements have been correlated with flooding in a single channel Polymer Electrolyte Membrane (PEM) fuel cell in horizontal and vertical orientations. Water formed at the cathode/membrane interface flows through the largest pores in the GDL when driven by a hydraulic pressure sufficient to overcome the hydrophobic surface energy of the GDL's pores. Liquid water drops grow on the surface of the GDL until gravity or shear forces overcome the interfacial tension between the water in pore and the drop on the surface. After drops detach from the pores they aggregate and form slugs. Local current densities drop when liquid water accumulates in the gas flow channel inhibiting mass transport between the gas flow channel and the membrane/electrode interface. Flooding in PEM fuel cells is gravity dependent and the local current densities depend on dynamics of liquid slugs moving through the flow channels.
A Half Cell Model to Study Performance Degradation of a PEMFC due to Cationic Contamination
Authors: Kienitz Brian , Baskaran Harihara , Zawodzinski T. , Pivovar Bryan
Published: September 2007

When cationic impurities, such as Na+, Ca2+, ammonium or metal cations, are present in a polymer electrolyte membrane fuel cell (PEMFC) the performance of the PEMFC can be significantly reduced. A half cell model was developed to explain the most relevant processes causing performance degradation in fuel cells under cationic contamination. In fuel cells contaminated with foreign cations the exchange current density and the limiting current decreases. These effects can be explained by accounting for the concentration of available protons in the CCL and by the transport of cations by diffusion and migration.
The Effects of Multiple Contaminants on Polymer Electrolyte Fuel Cells
Authors: Rockward Tommy , Urdampilleta Idoia , Uribe Francisco , Brosha Eric L. , Pivovar Bryan , Garzon F. H.
Published: September 2007

Operating polymer electrolyte fuel cells (PEFC) on ultra high purity hydrogen (UHP) and clean air would be ideal. However, producing UHP H2 may be cost prohibitive and therefore tradeoffs between hydrogen clean up and fuel cell cost and durability need to be explored. Hydrogen for wide spread fuel cell commercialization will likely primarily come from reforming of fossil fuels, at least initially. The reformation of hydrocarbon fuels such as methane or coal produces impurities. The concentration and type depend on the fossil fuel and the reformation process. We have tested a fuel specification, listed in Table 1, provided by the US FreedomCar Tech Team as a function of different Pt anode loadings. These loadings were in line with the Department of Energy's 2010 and 2015 technical targets. Our findings indicate that NH3 and H2S are responsible for the majority of performance losses. We observed an increase in the high frequency resistance suggesting ammonium poisoning of the membrane, and cyclic voltammetry showed a large oxidation peak (>0.8V), which suggests sulfur poisoning of the anode. CO was not found on the surface at long exposure times.
Impacts of Carbon Corrosion on Cell Performance Decay
Authors: Chizawa Hiroshi , Ogami Yasuji , Naka Hiroshi , Matsunaga Atsushi , Aoki Nobuo , Aoki Tsutomu , Tanaka Kazuhisa
Published: September 2007

In a previous work, authors demonstrated a new carbon corrosion mode caused by rapid voltage rises in PEFC cathode. In this study detailed investigations for the carbon corrosion are conducted with full-scale cells. Test results suggest that higher potential sweep rate and larger potential gain which relates to rapid voltage raises during start-up could accelerate carbon corrosion significantly, without local or global fuel starvation. Impacts of carbon corrosion on material degradation and cell performance decay are also evaluated. The results indicate that voltage cycles between start-up and shutdown, which advances carbon corrosion on the cathode, accelerates cell performance decay with electrochemical surface area reduction and mass transport loss on the cathode. The voltage cycle testing focusing on carbon corrosion is expected to be one of the most promising test protocols for accelerated durability testing.
Modeling of Transient State Carbon Corrosion for PEMFC Electrodes
Authors: Takeuchi Norimitsu , Fuller T.
Published: September 2007

Models of carbon corrosion induced by oxygen and hydrogen coexistence on anode electrode are presented. In polymer exchange membrane fuel cells (PEMFCs), these phenomena occur during several situations, such as startup, shutdown and hydrogen starvation. The oxygen reduction reaction on the negative electrode decreases the solution potential and induces a large enough overpotential to cause carbon corrosion on cathode electrode. Not only in the case of high partial pressure, as observed during startup, but also for low partial pressure as a result of gas permeation through the membrane, the carbon corrosion current is large enough to influence PEMFC life time.
Mitigation of Carbon Corrosion in Microporous Layers in PEM Fuel Cells
Authors: Owejan Jeanette E. , Yu Paul T. , Makharia Rohit
Published: September 2007

The corrosion of carbon in microporous layers (MPL) has been identified and characterized through the use of potential hold at 1.2V vs RHE and accelerated startup/shutdown (SUSD) testing. Carbon oxidation in the MPL at the MPL/electrode interface leads to increased mass transport limitation of the gas diffusion layer (GDL), as shown by subsequent fuel cell testing. Mitigation of such corrosion in the MPL is achieved through employing graphitized carbon material. A 25% improvement in startup/shutdown degradation rate at 1.2A/cm2 has been observed with implementation of a graphitized carbon in the MPL.
In-Situ Reference Electrode Testing for Cathode Carbon Corrosion
Authors: Dross Robert , Maynard Bruce
Published: September 2007

Carbon corrosion of the cathode electrode is a significant decay mechanism in PEM fuel cells that must be mitigated in order to meet start/stop cycle requirements. A measurement technique which uses an in-situ reference electrode was developed to measure the interfacial potentials which cause carbon corrosion. Start/stop cycles are studied using this diagnostic tool in real time where both inlet and outlet interfacial potentials are measured. Rate of carbon loss per cycle is estimated using the measured interfacial potentials. An improved shutdown method was studied using the reference electrode and was demonstrated to be effective. Throughout the testing the reference electrode measurements verified the predicted interfacial potential trends, and proved to be reliable.
Micro And Nano X-Ray Tomography Of PEM Fuel Cell Membranes After Transient Operation
Authors: Garzon F. H. , Lau Siew Hock , Davey John R. , Borup Rod
Published: September 2007

In this study we report the use of a novel high resolution and high contrast X-ray Computed Tomography to image changes in the internal morphology, Pt distribution and carbon content of the fuel cell membranes subjected to the drive cycle testing with and without stop/start (depolarization/polarization) cycles. Membrane electrode assembly samples were imaged in air with minimal sample preparation and morphological disturbance. X-ray Computed Tomography revealed a redistribution of Pt within the cathode, migration of Pt into the electrolyte membrane and significant amounts of carbon corrosion
Mechanisms of Activity Loss in PtCo Alloy Systems
Authors: Ball Sarah C. , Hudson Sarah L. , Leung Joe Hei , Russell Andrea E. , Thompsett David , Theobald Brian R.
Published: September 2007

The mechanism of activity loss in Pt and PtCo alloy systems has been investigated during 0.6 - 1.0 V and 0.6 - 1.2 V vs RHE cycling regimes using an electrochemical cell that allows electrochemical metal area (ECA) loss measurement and electrolyte sampling. Dissolution of platinum and base metal into the electrolyte was quantified using ICP-MS and TEM used to look for changes in catalyst morphology with cycles. ECA loss and base metal removal were found to be potential dependent. PtCo alloys were found to be more stable to ECA loss by Ostwald-ripening and dissolution than Pt. In terms of base metal loss the PtCo alloy was unstable and cycling was found to remove similar amounts of Co by dissolution as chemical acid leaching. Loss of Co from the surface layers of the catalyst was found to remove the enhanced stability and activity benefits of this catalyst making it more platinum like.
Development of Stable Pt-Co Cathode Catalysts for PEM Fuel Cells
Authors: Li Xuguang , Colon-Mercado Hector , Wu Gang , Lee Jong-Won , Popov Branko
Published: September 2007

A new procedure was developed to synthesize a platinum-cobalt (Pt-Co) alloy catalyst for oxygen reduction using Co/C composite as a support. The Co/C composite was synthesized using a chelation approach and treated in an acidic solution to remove excess of Co. Nano-sized Pt was deposited onto the Co/C support, followed by a heat-treatment to form Pt-Co alloy. The Pt-Co/C catalyst showed improved activity and long-term stability in PEM fuel cells when compared with a conventional Pt-Co/C. Durability studies and electron probe microanalysis indicated that the Co content in the alloy remains stable without poisoning the electrolyte membrane during long-term operation.
Durability and Activity Study of Single-Walled, Double-Walled and Multi-Walled Carbon Nanotubes Supported Pt Catalyst for PEMFCs
Authors: Chen Zhongwei , Deng Weiqiao , Wang Xin , Yan Yushan
Published: September 2007

Pt nanoparticles (2-3 nm) supported on carbon black (C), single-walled carbon nanotubes (SWNT), double-walled carbon nanotubes (DWNT) and multi-walled carbon nanotubes (MWNT) are prepared by an ethylene glycol reduction method. The catalytic activity towards the oxygen reduction reaction (ORR) and the durability of the catalysts are evaluated under condition simulating the environment of proton exchange membrane fuel cell (PEMFCs). The Pt electrochemical surface area (ECSA) of Pt/DWNT and Pt/MWNT degrade 32% after accelerated durability test (ADT), compared with 38% for Pt/SWNT and 48% for Pt/C. The degradation rate of Pt/C is about 1.5 times higher than that of Pt/DWNT. The typical ORR polarization curves of Pt/C, Pt/SWNT, Pt/DWNT and Pt/MWNT obtained before and after ADT are also examined. Before ADT, Pt/DWNT shows the highest catalytic activity towards ORR, followed by Pt/MWNT, Pt/C, and Pt/SWNT in that order. After ADT, the sequence of ORR activity of Pt/C and Pt/SWNT has changed and the overall order is: Pt/DWNT > Pt/MWNT > Pt/SWNT > Pt/C. The Pt/DWNT catalyst shows the highest catalytic activity and the best durability for ORR among the C and CNT supported Pt catalysts.
Migration of Platinum Under Open Cell Voltage: Effect of the Type of Ionomer Membrane
Authors: Peron Jennifer , Jones Deborah , Roziere Jacques
Published: September 2007

Membrane-electrode assemblies (MEAs) comprising perfluorosulfonic acid (PFSA)-type and sulfonated polyaromatic membranes have been submitted to accelerated ageing under open circuit, and characterised during ageing runs and at end of life to better understand the origins of membrane failure under these conditions. Membrane permeability, reactant gas consumption and water production, as well as the decline in cell voltage, have been monitored with time. Aged MEAs have been characterised using scanning and transmission electron microscopy. We surmise that the origin of failure of PFSA-based MEAs at high cell potential is due to electrochemically-induced platinum dissolution, migration and subsequent progressive in situ reduction of oxidised platinum species to give platinum nanoparticles within the membrane. These dispersed particles, comprising a metallic core and ionic shell, could also act as centres for the generation of free radicals that chemically degrade the membrane, leading to increased porosity, gas crossover and ultimately membrane and MEA failure.
An Electrodeposited Pd-Co Cathode Catalyst for a Microfabricated Direct Methanol Fuel Cell
Authors: Tominaka Satoshi , Ohta Sousuke , Momma Toshiyuki , Osaka Tetsuya
Published: September 2007

Pd-Co was electrodeposited as a rough cathode catalyst for a direct methanol fuel cell fabricated using micro-electromechanical systems technology. Pd-Co was electrodeposited at -10 mA/cm2 for 60 s or -200 mA/cm2 for 5 s. The deposits were evaluated with respect to microstructures, electrochemical characteristics, and fuel cell performances. As a result, oxygen reduction activity of the sample prepared at -200 mA/cm2 was higher than that prepared at -10 mA/cm2, while the electrochemically active surface areas were almost the same. Microstructures of the former and the latter were observed dendritic and flat, respectively. We conclude that the difference of oxygen reduction activity is attributed to the difference of the microstructure. In addition, we modified the dendritic sample by depositing Pd-Co at -0.75 V vs. Ag/AgCl. The modification increased open circuit potential for oxygen reduction by 30 mV from 0.70 to 0.73 V.
Structure and Performance of Pd-Nafion® Composite Membranes for DMFC
Authors: Zhang Lu , Cheng Xuan , Zhang Ying , Fan Qinbai
Published: September 2007

The Nafion membrane was modified using plasma activation and Palladium-sputtering in an effort to reduce methanol crossover in DMFC. The structure, morphology and chemical composition of Pd thin layer were characterizied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectrometer (EDX). It was shown that pure Pd metal adhered to the substract very well. The preliminary results in simulative DMFC test revealed that the Nafion112/Pd/Nafion112 composite membrane exhibited better performance in methanol inhibition than the Nafion112/Nafion112 composite membrane.
Electronic Effects in Pt-Sn Alloy as Anode in Direct Ethanol Fuel Cell
Authors: Colmati Flavio , Gonzalez Ernesto
Published: September 2007

Carbon supported Pt-Sn alloy catalysts were prepared by reduction of Pt and Sn precursors with formic acid and characterised by X-ray diffraction, high resolution transmission electron microscopy and X-ray absorption near-edge structure analyses. The electrocatalytic activity for ethanol oxidation was compared with that of a commercial Pt3Sn/C electrocatalysts from E-TEK. The synthetized Pt3Sn catalysts presents a lower degree of alloy that increases when the catalysts is submitted to a thermal treatment. The prepared Pt3Sn catalyst, submitted to a thermal treatment, showed a better performance as anode material in a direct ethanol fuel cell (DEFC) operating at 90 oC.