Sulphur Induced Degradation of Nickel-Based Solid Oxide Fuel Cell Anodes

Solid Oxide Fuel Cells (SOFCs) are high temperature solid-state electrochemical devices that convert fuel into electricity and heat with high efficiency. Many fuels suitable for SOFCs derive from hydrocarbon sources, such as natural gas or biogas; however, these contain significant impurities, most notably compounds containing sulphur, which can poison the nickel electrocatalyst in the anode of the fuel cell. Sulphur removal is usually carried out but it is complicated and expensive to achieve levels below 1 part per million (ppm). An enhanced scientific understanding of chemical interactions on the surface of SOFC electrodes is critical to the development of robust nickel-based anodes, but the mechanisms and effects of sulphur poisoning are not fully understood. The scope of this thesis is to advance the field of sulphur-poisoning research by studying the effect of current density on the sulphur-induced degradation, and focuses on intermediate-temperature (IT) conditions with nickel-gadolinia doped ceria (Ni-CGO), which is the most promising anode material for IT-SOFCs, operating between 600–800 °C. The work of this thesis is aimed at (i) investigating the kinetics of sulphur poisoning, and the effect of current density, by use of a specially built three-electrode electrochemical test rig; (ii) analysis of structural modifications to the anode microstructure as a result of exposure to fuel cell conditions and (iii) development and prototype testing of a miniature SOFC test rig with optical access for in situ Raman spectroscopy. Fuel cell operation at higher current density was found to partially mitigate the sulphur poisoning of up to 3 ppm H2S in H2 fuel, while microstructural analysis found that the presence of as little as 0.5 ppm H2S accelerated restructuring of nickel grain surfaces. Finally, preliminary proof-of-concept results were obtained for the in situ Raman rig, and suggestions for a future design are discussed

Trends in Modern War Gaming: The Art of Conversation

Lieutenant William McCarty Little—a war-gaming visionary—was truly a man ahead of his time.1 Although physically sight impaired and medically retired from active naval service, he opted to use his ideational vision and keen mind to support the Naval War College, in Newport, Rhode Island, during its first few years of operation after its founding in 1884. Initially an unpaid volunteer, he was appointed in 1887 as a member of the faculty, where he developed two-sided war gaming at the College—a construct that is still in use at the state-of-the-art facility that today bears his name

Searching for the Donor Stars of ULX Pulsars

We report on our search for the optical counterparts of two ultraluminous X-ray pulsars with known orbital periods, M82 X-2 and NGC 5907 X-1, in new and archival HST observations, in an effort to characterize the donor stars in these systems. We detect five near-infrared sources consistent with the position of M82 X-2 that are too bright to be single stars. We also detect seven sources in the WFC3/UVIS F336W image whose photometry matches that of 10-15 M$_\odot$ stars turning off the main sequence. Such stars have densities consistent with the properties of the donor star of M82 X-2 as inferred from X-ray timing analysis, although it is also possible that the donor is a lower mass star below our detection limit or that there is a significant contribution from the accretion disc to the optical emission. We detect three candidate counterparts to NGC 5907 X-1 in the near-infrared. All of these are too bright to be the donor star of the ULX, which based on its orbital period is a red giant. The high background at the location of NGC 5907 X-1 precludes us from detecting this expected donor star. The recently discovered NGC 5907 ULX-2 also falls within the field of view of the near-infrared imaging; we detect four sources in the error circle, with photometry that matches AGB stars. The star suggested to be the counterpart of NGC 5907 ULX-2 by Pintore et al. (2018) falls outside our 2-$\sigma$ error circle.Comment: 10 pages, 3 figures, accepted for publication in Ap

X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue

AGN are known to have complex X-ray spectra that depend on both the properties of the accreting SMBH (e.g. mass, accretion rate) and the distribution of obscuring material in its vicinity ("torus"). Often however, simple and even unphysical models are adopted to represent the X-ray spectra of AGN. In the case of blank field surveys in particular, this should have an impact on e.g. the determination of the AGN luminosity function, the inferred accretion history of the Universe and also on our understanding of the relation between AGN and their host galaxies. We develop a Bayesian framework for model comparison and parameter estimation of X-ray spectra. We take into account uncertainties associated with X-ray data and photometric redshifts. We also demonstrate how Bayesian model comparison can be used to select among ten different physically motivated X-ray spectral models the one that provides a better representation of the observations. Despite the use of low-count spectra, our methodology is able to draw strong inferences on the geometry of the torus. For a sample of 350 AGN in the 4 Ms Chandra Deep Field South field, our analysis identifies four components needed to represent the diversity of the observed X-ray spectra: (abridged). Simpler models are ruled out with decisive evidence in favour of a geometrically extended structure with significant Compton scattering. Regarding the geometry of the obscurer, there is strong evidence against both a completely closed or entirely open toroidal geometry, in favour of an intermediate case. The additional Compton reflection required by data over that predicted by toroidal geometry models, may be a sign of a density gradient in the torus or reflection off the accretion disk. Finally, we release a catalogue with estimated parameters such as the accretion luminosity in the 2-10 keV band and the column density, $N_{H}$, of the obscurer.Comment: 28 pages, 18 figures, catalogue available from https://www.mpe.mpg.de/~jbuchner/agn_torus/analysis/cdfs4Ms_cat/, software available from https://github.com/JohannesBuchner/BX

The Broadband X-Ray Spectrum of the X-Ray-obscured Type 1 AGN 2MASX J193013.80+341049.5

We present results from modeling the broadband X-ray spectrum of the Type 1 active galactic nucleus (AGN) 2MASX J193013.80+341049.5 using NuSTAR, Swift, and archival XMM-Newton observations. We find this source to be highly X-ray obscured, with column densities exceeding 10²³ cm⁻² across all epochs of X-ray observations, spanning an 8 yr period. However, the source exhibits prominent broad optical emission lines, consistent with an unobscured Type 1 AGN classification. We fit the X-ray spectra with both phenomenological reflection models and physically motivated torus models to model the X-ray absorption. We examine the spectral energy distribution of this source and investigate some possible scenarios to explain the mismatch between X-ray and optical classifications. We compare the ratio of reddening to X-ray absorbing column density (E_(B−V)/N_H) and find that 2MASX J193013.80+341049.5 likely has a much lower dust-to-gas ratio relative to the Galactic interstellar medium, suggesting that the broad line region itself could provide the source of extra X-ray obscuration, being composed of low-ionization, dust-free gas

Evidence for a Variable Ultrafast Outflow in the Newly Discovered Ultraluminous Pulsar NGC 300 ULX-1

Ultraluminous pulsars are a definite proof that persistent super-Eddington accretion occurs in nature. They support the scenario according to which most Ultraluminous X-ray Sources (ULXs) are super-Eddington accretors of stellar mass rather than sub-Eddington intermediate mass black holes. An important prediction of theories of supercritical accretion is the existence of powerful outflows of moderately ionized gas at mildly relativistic speeds. In practice, the spectral resolution of X-ray gratings such as RGS onboard XMM-Newton is required to resolve their observational signatures in ULXs. Using RGS, outflows have been discovered in the spectra of 3 ULXs (none of which are currently known to be pulsars). Most recently, the fourth ultraluminous pulsar was discovered in NGC 300. Here we report detection of an ultrafast outflow (UFO) in the X-ray spectrum of the object, with a significance of more than 3{\sigma}, during one of the two simultaneous observations of the source by XMM-Newton and NuSTAR in December 2016. The outflow has a projected velocity of 65000 km/s (0.22c) and a high ionisation factor with a log value of 3.9. This is the first direct evidence for a UFO in a neutron star ULX and also the first time that this its evidence in a ULX spectrum is seen in both soft and hard X-ray data simultaneously. We find no evidence of the UFO during the other observation of the object, which could be explained by either clumpy nature of the absorber or a slight change in our viewing angle of the accretion flow.Comment: 10 pages, 4 figures. Accepted to MNRA

Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase

We present broad-band, multi-epoch X-ray spectroscopy of the pulsating ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and NuSTAR data from 2014 are best described by a multi-color black-body model with a temperature gradient as a function of accretion disk radius significantly flatter than expected for a standard thin accretion disk (T(r) ~ r^{-p}, with p=0.608^{+0.014}_{-0.012}). Additionally, we detect a hard power-law tail at energies above 10 keV, which we interpret as being due to Comptonization. We compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from 2003, 2012, and 2013, and investigate possible spectral changes as a function of phase over the 78d super-orbital period of this source. We find that observations taken around phases 0.3-0.4 show very similar temperature profiles, even though the observed flux varies significantly, while one observation taken around phase 0 has a significantly steeper profile. We discuss these findings in light of the recent discovery that the compact object is a neutron star and show that precession of the accretion disk or the neutron star can self-consistently explain most observed phenomena.Comment: 7 pages, 5 figures, submitted to ApJ; comments welcom

NuSTAR Observations of WISE J1036+0449, a Galaxy at z ~ 1 Obscured by Hot Dust

Hot dust-obscured galaxies (hot DOGs), selected from Wide-Field Infrared Survey Explorer's all-sky infrared survey, host some of the most powerful active galactic nuclei known and may represent an important stage in the evolution of galaxies. Most known hot DOGs are located at z > 1.5, due in part to a strong bias against identifying them at lower redshift related to the selection criteria. We present a new selection method that identifies 153 hot DOG candidates at z ~ 1, where they are significantly brighter and easier to study. We validate this approach by measuring a redshift z = 1.009 and finding a spectral energy distribution similar to that of higher-redshift hot DOGs for one of these objects, WISE J1036+0449 (L_(Bol ≃ 8 x 10^(46) erg s^(-1)). We find evidence of a broadened component in Mg II, which would imply a black hole mass of M_(BH) ≃ 2 x 10^8 M⊙ and an Eddington ratio of λ_(Edd) ≃ 2.7. WISE J1036+0449 is the first hot DOG detected by the Nuclear Spectroscopic Telescope Array, and observations show that the source is heavily obscured, with a column density of N_H ≃ (2-15) x 10^(23) cm^(-2). The source has an intrinsic 2–10 keV luminosity of ~6 x 10^(44) erg s^(-1), a value significantly lower than that expected from the mid-infrared/X-ray correlation. We also find that other hot DOGs observed by X-ray facilities show a similar deficiency of X-ray flux. We discuss the origin of the X-ray weakness and the absorption properties of hot DOGs. Hot DOGs at z ≾ 1 could be excellent laboratories to probe the characteristics of the accretion flow and of the X-ray emitting plasma at extreme values of the Eddington ratio

Spectral and temporal properties of the ultra-luminous X-ray pulsar in M82 from 15 years of Chandra observations and analysis of the pulsed emission using NuSTAR

The recent discovery by Bachetti et al. (2014) of a pulsar in M82 that can reach luminosities of up to 10^40 ergs s^-1, a factor of ~100 the Eddington luminosity for a 1.4 Msol compact object, poses a challenge for accretion physics. In order to better understand the nature of this source and its duty cycle, and in the light of several physical models that have been subsequently published, we conduct a spectral and temporal analysis of the 0.5-8 keV X-ray emission from this source from 15 years of Chandra observations. We fit the Chandra spectra of the pulsar with a power-law model and a disk black body model, subjected to interstellar absorption in M82. We carefully assess for the effect of pile-up in our observations, where 4/19 observations have a pile-up fraction >10%, which we account for during spectral modeling with a convolution model. When fitted with a power-law model, the average photon index when the source is at high luminosity (L_X>10^39 ergs s^-1) is Gamma=1.33+/-0.15. For the disk black body model, the average temperature is T=3.24+/-0.65 keV, consistent with other luminous X-ray pulsars. We also investigated the inclusion of a soft excess component and spectral break, finding that the spectra are also consistent with these features common to luminous X-ray pulsars. In addition, we present spectral analysis from NuSTAR over the 3-50 keV range where we have isolated the pulsed component. We find that the pulsed emission in this band is best fit by a power-law with a high-energy cut-off, where Gamma=0.6+/-0.3 and E_C=14^{+5}_{-3} keV. While the pulsar has previously been identified as a transient, we find from our longer-baseline study that it has been remarkably active over the 15-year period, where for 9/19 (47%) observations that we analyzed, the pulsar appears to be emitting at a luminosity in excess of 10^39 ergs s^-1, greater than 10 times its Eddington limit.Comment: Accepted for publication by Ap