Real-time monitoring of proton exchange membrane fuel cell stack failure

Uneven pressure drops in a 75-cell 9.5-kWe proton exchange membrane fuel cell stack with a U-shaped flow configuration have been shown to cause localised flooding. Condensed water then leads to localised cell heating, resulting in reduced membrane durability. Upon purging of the anode manifold, the resulting mechanical strain on the membrane can lead to the formation of a pin-hole/membrane crack and a rapid decrease in open circuit voltage due to gas crossover. This failure has the potential to cascade to neighbouring cells due to the bipolar plate coupling and the current density heterogeneities arising from the pin-hole/membrane crack. Reintroduction of hydrogen after failure results in cell voltage loss propagating from the pin-hole/membrane crack location due to reactant crossover from the anode to the cathode, given that the anode pressure is higher than the cathode pressure. Through these observations, it is recommended that purging is avoided when the onset of flooding is observed to prevent irreparable damage to the stack

Excited states of a phosphorus pair in silicon: Combining valley-orbital interaction and electron-electron interactions

Excitations of impurity complexes in semiconductors cannot only provide a route to fill the terahertz gap in optical technologies but can also play a role in connecting local quantum bits efficiently to scale up solid-state quantum-computing devices. However, taking into account both the interactions among electrons/holes bound at the impurities and the host band structures is challenging. Here we combine first-principles band-structure calculations with quantum-chemistry methodology to evaluate the ground and excited states of a pair of phosphorous (shallow donors) impurities in silicon within a single framework. We account for the electron-electron interaction within a broken-symmetry Hartree-Fock approach, followed by a time-dependent Hartree-Fock method to compute the excited states. We adopt a Hamiltonian for each conduction-band valley including an anisotropic kinetic energy term, which splits the 2 p 0 and 2 p ± transitions of isolated donors by ∼ 4 meV, in good agreement with experiments. Our single-valley calculations show the optical response is a strong function of the optical polarization and suggest the use of valley polarization to control optics and reduce oscillations in exchange interactions. When taking into account all valleys, we have included valley-orbital interactions that split the energy levels further. We find a gap opens between the 1 s → 2 p transition and the low-energy charge-transfer states within 1 s manifolds (which become optically allowed because of interdonor interactions). In contrast to the single-valley case, we also find charge-transfer excited states in the triplet sector, thanks to the extra valley degrees of freedom. Our computed charge-transfer excited states have a qualitatively correct energy as compared with previous experimental findings; additionally, we predict a set of excitations below 20 meV. Calculations based on a statistical average of nearest-neighbor pairs at different separations suggest that THz radiation could be used to excite the donor pairs spin-selectively. Our approach can readily be extended to other types of donors such as arsenic, and more widely to other semiconducting host materials such as germanium, zinc oxides, and gallium nitride, etc

Multihole models for deterministically placed acceptor arrays in silicon

In this paper, we compute the electronic structure of acceptor clusters in silicon by using three different methods to take into account electron correlations: the full configuration interaction (full CI calculation), the Heitler-London approximation (HL approximation), and the unrestricted Hartree-Fock method (UHF method). We show that both the HL approach and the UHF method are good approximations to the ground state of the full CI calculation for a pair of acceptors and for finite linear chains along [001], [110], and [111]. The total energies for finite linear chains show the formation of a fourfold-degenerate ground state (lying highest in energy), below which there are characteristic low-lying eightfold and fourfold degeneracies, when there is a long (weak) bond at the end of the chain. We present evidence that this is a manifold of topological edge states. We identify a change in the angular momentum composition of the ground state at a critical pattern of bond lengths, and show that it is related to a crossing in the Fock matrix eigenvalues. We also test the symmetry of the self-consistent mean-field UHF solution and compare it to the full CI; the symmetry is broken under almost all the arrangements by the formation of a magnetic state in UHF, and we find further broken symmetries for some particular arrangements related to crossings (or potential crossings) between the Fock-matrix eigenvalues in the [001] direction. We also compute the charge distributions across the acceptors obtained from the eigenvectors of the Fock matrix; we find that, with weak bonds at the chain ends, two holes are localized at either end of the chain while the others have a nearly uniform distribution over the middle; this also implies the existence of the nontrivial edge states. We also apply the UHF method to treat an infinite linear chain with periodic boundary conditions, where the full CI calculation and the HL approximation cannot easily be used. We find the band structures in the UHF approximation, and compute the Zak phases for the occupied Fock-matrix eigenvalues; however, we find they do not correctly predict the topological edge states formed in this interacting system. On the other hand, we find that direct study of the quantum numbers characterizing the edge states, introduced by Turner et al. [Phys. Rev. B 83, 075102 (2011)], provides a better insight into their topological nature

Ultra high molecular weight polyethylene with incorporated crystal violet and gold nanoclusters is antimicrobial in low intensity light and in the dark

Antibiotics lose their effectiveness over time due to antimicrobial resistance. The increasing risk of hospital-acquired infections from contaminated surfaces and medical interventions requires the development of new antimicrobial materials. We report the first example of a modified ultra high molecular weight polyethylene that showed good antibacterial properties on light activation. Its efficacy was due to the production of reactive oxygen species under low-intensity white light sources (ca. 375 lux). Crystal violet and cysteine capped gold nanoclusters were successfully incorporated into the polymer using a readily available solvent as a dispersing agent followed by the process of compression moulding at 200 °C, 4.5 MPa for 1 min. This modified ultra-high molecular weight polyethylene demonstrates excellent robustness with regards to dye and metal leaching as well as photostability. Despite incorporating antimicrobial agents, the modified ultra-high molecular weight polyethylene retained its mechanical properties and showed >99% reduction in bacterial numbers against Escherichia coli and. To our knowledge, this paper reports the first use of compression moulding to create a light-activated antimicrobial surface which has distinct processing advantages over the widely used “swell-encapsulation-shrink” method and is potentially scalable

Developmental pathways from toddler difficult temperament to child generalized psychopathology and adult functioning

BACKGROUND: Early difficult temperament and child mental health problems are consistently associated with impaired functioning in adulthood. We examined three potential pathways between difficult temperament in toddlerhood (age 2) and depressive symptoms (ages 21-23) and well-being (age 23): i) direct - early difficult temperament directly associates with these outcomes, ii) mediated - these direct effects are also mediated by a general psychopathology factor in late childhood/early adolescence (GPF; ages 7, 10,and 13), and iii) moderated-mediated - these mediated effects are also moderated by negative (age 42 months) and positive (age 33 months) parenting behaviors. METHODS: The analytic sample included 1892 mother-child dyads (33.4% male children) from the Avon Longitudinal Study of Parents and Children (ALSPAC). Mothers reported on their child's difficult temperament, negative parenting, positive parenting, and child's mental health symptoms. In adulthood, participants reported their own depressive symptoms and well-being (i.e. mental well-being, life satisfaction, happiness). RESULTS: First, early difficult temperament associated directly and positively with depressive symptoms, but negatively with well-being in adulthood. Second, the GPF in late childhood/early adolescence mediated these direct associations. Third, the mediated pathways were not moderated by negative or positive parenting. LIMITATIONS: i) low risk community sample, ii) early risks are based on maternal reports. CONCLUSIONS: Temperament is a risk factor for impaired psychosocial functioning in adulthood, manifested through increased susceptibility to psychopathology in childhood/adolescence. Although more research is needed to test their generalizability, these findings suggest that targeting early difficult temperament may alleviate the risk for later mental health difficulties and may increase general well-being

The s ---> d gamma decay in and beyond the Standard Model

The New Physics sensitivity of the s ---> d gamma transition and its accessibility through hadronic processes are thoroughly investigated. Firstly, the Standard Model predictions for the direct CP-violating observables in radiative K decays are systematically improved. Besides, the magnetic contribution to epsilon prime is estimated and found subleading, even in the presence of New Physics, and a new strategy to resolve its electroweak versus QCD penguin fraction is identified. Secondly, the signatures of a series of New Physics scenarios, characterized as model-independently as possible in terms of their underlying dynamics, are investigated by combining the information from all the FCNC transitions in the s ---> d sector.Comment: 54 pages, 14 eps figure

Mixed integer programming in production planning with backlogging and setup carryover : modeling and algorithms

This paper proposes a mixed integer programming formulation for modeling the capacitated multi-level lot sizing problem with both backlogging and setup carryover. Based on the model formulation, a progressive time-oriented decomposition heuristic framework is then proposed, where improvement and construction heuristics are effectively combined, therefore efficiently avoiding the weaknesses associated with the one-time decisions made by other classical time-oriented decomposition algorithms. Computational results show that the proposed optimization framework provides competitive solutions within a reasonable time

Effects of general anesthetics on visceral pain transmission in the spinal cord

Current evidence suggests an analgesic role for the spinal cord action of general anesthetics; however, the cellular population and intracellular mechanisms underlying anti-visceral pain by general anesthetics still remain unclear. It is known that visceral nociceptive signals are transmited via post-synaptic dorsal column (PSDC) and spinothalamic tract (STT) neuronal pathways and that the PSDC pathway plays a major role in visceral nociception. Animal studies report that persistent changes including nociception-associated molecular expression (e.g. neurokinin-1 (NK-1) receptors) and activation of signal transduction cascades (such as the protein kinase A [PKA]-c-AMP-responsive element binding [CREB] cascade)-in spinal PSDC neurons are observed following visceral pain stimulation. The clinical practice of interruption of the spinal PSDC pathway in patients with cancer pain further supports a role of this group of neurons in the development and maintenance of visceral pain. We propose the hypothesis that general anesthetics might affect critical molecular targets such as NK-1 and glutamate receptors, as well as intracellular signaling by CaM kinase II, protein kinase C (PKC), PKA, and MAP kinase cascades in PSDC neurons, which contribute to the neurotransmission of visceral pain signaling. This would help elucidate the mechanism of antivisceral nociception by general anesthetics at the cellular and molecular levels and aid in development of novel therapeutic strategies to improve clinical management of visceral pain