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Thesis (M.S.)--Boston Universit

Simulating the symmetron: domain walls and symmetry-restoring impurities

In this paper we study the dynamics of relativistic domain walls in the presence of static symmetry-restoring impurities. The field theory is precisely the same as what is known to cosmologists as the "symmetron model", whereby the usual $\mathbb{Z}_2$ symmetry breaking potential is appended with a space-varying mass-term (the space-variation is set by the profile of the impurity, which we take to be a "tanh"-function). After presenting the outcomes of a suite of different numerical experiments we have three main results: (1) domain walls pin to impurities, (2) domain wall necklaces can be energetically preferred configurations, and (3) impurities significantly modifies the usual ${N}_{\rm dw}\propto t^{-1}$ scaling law for random networks of domain walls.Comment: 24 pages; 14 figure

Documents of the JPL Photovoltaics Program Analysis and Integration Center: An annotated bibliography

A bibliography of internal and external documents produced by the Jet Propulsion Laboratory, based on the work performed by the Photovoltaics Program Analysis and Integration Center, is presented with annotations. As shown in the Table of Contents, the bibliography is divided into three subject areas: (1) Assessments, (2) Methdological Studies, and (3) Supporting Studies. Annotated abstracts are presented for 20 papers

Effective action approach to cosmological perturbations in dark energy and modified gravity

In light of upcoming observations modelling perturbations in dark energy and modified gravity models has become an important topic of research. We develop an effective action to construct the components of the perturbed dark energy momentum tensor which appears in the perturbed generalized gravitational field equations, {\delta}G_{\mu\nu} = 8{\pi}G{\delta}T_{\mu\nu} + {\delta}U_{\mu\nu} for linearized perturbations. Our method does not require knowledge of the Lagrangian density of the dark sector to be provided, only its field content. The method is based on the fact that it is only necessary to specify the perturbed Lagrangian to quadratic order and couples this with the assumption of global statistical isotropy of spatial sections to show that the model can be specified completely in terms of a finite number of background dependent functions. We present our formalism in a coordinate independent fashion and provide explicit formulae for the perturbed conservation equation and the components of {\delta}U_{\mu\nu} for two explicit generic examples: (i) the dark sector does not contain extra fields, L = L(g_{\mu\nu}) and (ii) the dark sector contains a scalar field and its first derivative L = L(g_{\mu\nu}, {\phi}, \nabla_{\mu}{\phi}). We discuss how the formalism can be applied to modified gravity models containing derivatives of the metric, curvature tensors, higher derivatives of the scalar fields and vector fields.Comment: Version accepted by JCAP. Typos corrected. Covariant decoupling conditions adde

The 5C 6 and 5C 7 surveys of radio sources

5C 6 and 5C 7 continue the series of deep surveys made at 408 and 1407 MHz with the One-Mile telescope at Cambridge. They were intended (1) to provide a sample of faint radio sources suitable for further study; (2) to improve the statistics of source counts N(S) and spectral-index distributions at low flux densities; (3) to study the isotropy of the distribution of faint sources. Each observed field is about 4° in diameter at 408 MHz and 1° in diameter at 1407 MHz, and the field-centres are a α = 02ʰ14ᵐ, δ = 32° (5C 6) and α = 08ᵏ17ᵐ, δ = 27° (5C 7). The synthesized beamwidths (FWHM) are 80 arcsec (408 MHz) and 23 arcsec (1407 MHz). The techniques of observation and data-analysis followed closely those used for 5C 5 (Pearson, T. J., 1975. Mon. Not. R. astr. Soc., 171, 475), with some minor variations which are noted in Section 2

Erosion-corrosion behaviour of Zirconia WC-6Co, WC-6Ni and SS316

The current study investigates a ceramic, two cermets and a metal under solid-liquid impingement with 3.5% NaCl and 150mg/l hydraulic fracturing sand at two extreme angles of impact, 90° and 20°. The materials tested were Zirconia, sintered WC-6Co, sintered WC-6Ni and SS316. Each material was exposed to a testing regime using re-circulating impinging jet apparatus with a velocity of 19m/s and one hour duration. The electrochemical properties of the materials were investigated in-situ through anodic and cathodic polarisation and application of cathodic protection. Post experimental analysis of the degraded surface was completed using Scanning Electron Microscopy (SEM) and Optical 3D Imaging. Zirconia exhibited a brittle response to erosion-corrosion testing with the mass loss at 90° being fifty times greater than the negligible mass loss at 20°. WC-6Co and WC-6Ni both outperformed SS316 under all solid-liquid impingement erosion-corrosion testing regimes. WC-6Ni exhibited slightly better erosion-corrosion resistance over WC-6Co at both 90° and 20°. SS316 had the best corrosion resistance and showed passivation during anodic polarisations in solid-liquid impingement conditions. The nickel binder increased the corrosion resistance of WC-6Ni over WC-6Co. Cathodic protection was successfully applied on sintered WC-6Co and SS316 isolating the key components of erosion-corrosion

Toughening mechanism in elastometer-modified epoxy resins: Part 1

Several plaques of Epon 828, cured with piperidine, modified with hycar(r) CTBN 1300X8, Hycar(R) CTBN 1300X13, and Hycar(R) CTBN 1300x15, and in some cases modified with biphenol A (BPA), yielded properly toughened epoxies with rubber particle diameters ranging from 0.1 to 10 microns. Fracture toughness experiments indicate that toughness was more a function of rubber content than the rubber particle size. Tensile volumetric behavior of the near resin exhibits two regions: an initial region where the increase in volume strain was due to the Poisson's effect, and a second region where a slower rate of increase in volume strain was due to shear deformation. Tensile volumetric deformation of an elastomer-modified epoxy exhibits the same type of behavior to that of the neat resin at low rates ( 3.2x0.01 sec(-1)). But at very high strain rates, which correspond more closely to the strain rates at the crack tip, there exists an increase in volume strain beyond the Poisson's effect. TEM, SEM and OM studies indicate that the rubber particles had voided. When a thin section from the deformed region is viewed under crossed-polarized light, shear bands are seen connecting voided rubber particles. From this information cavitation and enhanced shear band formation is proposed as the toughening mechanism