Defect-induced magnetism in graphite through neutron irradiation

We have investigated the variation in the magnetization of highly ordered pyrolytic graphite (HOPG) after neutron irradiation, which introduces defects in the bulk sample and consequently gives rise to a large magnetic signal. We observe strong paramagnetism in HOPG, increasing with the neutron fluence. We correlate the induced paramagnetism with structural defects by comparison with density-functional theory calculations. In addition to the in-plane vacancies, the trans-planar defects also contribute to the magnetization. The lack of any magnetic order between the local moments is possibly due to the absence of hydrogen/nitrogen chemisorption, or the magnetic order cannot be established at all in the bulk form.Comment: 10 pages, 8 figure

Pulse-field actuation of collinear magnetic single crystals

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.Includes bibliographical references (p. 33-34).Ferromagnetic shape memory alloys (FSMAs) are a class of alloys that exhibits the shape memory effect, as in the alloy nickel-titanium, sometimes known as Nitinol. In FSMAs, though, the shape changes are not brought on just by changes in temperature or mechanical stresses, but can also be driven by the application of a relatively small magnetic field. The large strains exhibited by such materials are a result of the coexistence of several features, including a thermoelastic martensitic transition, and a ferromagnetic martensite (non-equilibrium, low-temperature) phase. The magnetocrystalline anisotropy must also be large, as seen in similar alloys such as iron-palladium (Fe₇₀Pd₃₀) [1]. Nickel-manganese-gallium is an FSMA that has shown up to 10% strain in certain orientations as an effect of unconstrained magnetic actuation [4]. To achieve cyclic actuation in FSMAs, the field-induced extension has conventionally been reversed by a compressive mechanical stress from a spring or field orthogonal to the actuating field. The use of a second FSMA crystal to provide the reset force was unreported. Collinear single crystals are shown here to be able to induce a 2.8% reset strain against one another when subjected alternately to individual pulsed magnetic fields in a custom designed and constructed apparatus. A setup of this type could be used in a bistable microswitch, linear motion actuator, or shutter controller where a low actuation stress is sufficient or the electrical contacts required to activate a piezoelectric device are undesirable.by Catherine A. Jenkins.S.B

The effect of powder re-use on the coalescence behaviour and isothermal crystallisation kinetics of polyamide 12 within powder bed fusion

Polymer powder bed fusion (PBF) is becoming increasingly popular for the fabrication of lightweight, high-performance parts, particularly for medical and aerospace applications. This study investigates the effect of powder re-use and material aging on the coalescence behaviour, melt flowability, and isothermal crystallisation kinetics of polyamide-12 (PA-12) powder. With increased powder re-use, a progressive reduction in melt flowability and material coalescence is observed; at 200 °C, the particle consolidation time increases from 15 s in virgin powder to 180 s in powder recovered from build 6. The observed changes in the behaviour of PA-12 were attributed to polycondensation and cross-linking; these aging phenomena also create structural defects, which hinder the rate and extent of primary crystallisation. At an isothermal crystallisation temperature of 165 °C, the crystallisation half-time increased from 12.78 min in virgin powder to 23.95 min in powder re-used across six build cycles. As a result, the commonly used Avrami model was found to be unsuitable for modelling the crystallisation behaviour of aged PA-12 powder, with the coefficient of determination (R2) reducing from >0.995 for virgin powder to as low as 0.795 for reused powder. On the other hand, an alternative method, the Hay model, is able to successfully track full phase transformation within re-used powder (R2 >0.99). These results highlight the importance of selecting the most appropriate model for analysing the crystallisation kinetics of PA-12 powder re-used across multiple build cycles. This understanding is crucial for obtaining the strong mechanical properties and dimensional precision required for the fabrication of functional, end-use parts within PBF

The Braincase of Eocaecilia micropodia (Lissamphibia, Gymnophiona) and the Origin of Caecilians

The scant fossil record of caecilians has obscured the origin and evolution of this lissamphibian group. Eocaecilia micropodia from the Lower Jurassic of North America remains the only stem-group caecilian with an almost complete skull preserved. However, this taxon has been controversial, engendering re-evaluation of traits considered to be plesiomorphic for extant caecilians. Both the validity of the placement of E. micropodia as a stem caecilian and estimates of the plesiomorphic condition of extant caecilians have been questioned. In order to address these issues, the braincase of E. micropodia was examined via micro-computed tomography. The braincase is considered to be a more reliable phylogenetic indicator than peripheral regions of the skull. These data reveal significant new information, including the possession of an ossified nasal septum, ossified anterior wall of the sphenethmoid, long anterolateral processes on the sphenethmoid, and paired olfactory nerve foramina, which are known only to occur in extant caecilians; the latter are possibly related to the evolution of the tentacle, a caecilian autapomorphy. A phylogenetic analysis that included 64 non-amniote taxa and 308 characters represents the first extensive test of the phylogenetic affinities of E. micropodia. The results place E. micropodia securely on the stem of extant caecilians, representing a clade within Temnospondyli that is the sister taxon to batrachians plus Gerobatrachus. Ancestral character state reconstruction confirms the braincase of E. micropodia to be largely representative of the plesiomorphic condition of extant caecilians. Additionally, the results refine the context within which the evolution of the caecilian form can be evaluated. The robust construction and pattern of the dermal skull of E. micropodia is interpreted as symplesiomorphic with advanced dissorophoid temnospondyls, rather than being autapomorphic in its robust construction. Together these data increase confidence in incorporating E. micropodia into discussions of caecilian evolution.Organismic and Evolutionary Biolog

Enhanced heterogeneity of rpoB in Mycobacterium tuberculosis found at low pH.

OBJECTIVES: The aim of this study was to gain an insight into the molecular mechanisms of the evolution of rifampicin resistance in response to controlled changes in the environment. METHODS: We determined the proportion of rpoB mutants in the chemostat culture and characterized the sequence of mutations found in the rifampicin resistance-determining region of rpoB in a steady-state chemostat at pH 7.0 and 6.2. RESULTS: The overall proportion of rpoB mutants of strain H37Rv remained constant for 37 days at pH 7.0, ranging between 3.6 x 10(-8) and 8.9 x 10(-8); however, the spectrum of mutations varied. The most commonly detected mutation, serine to leucine mutation at codon 531 (S531L), increased from 40% to 89%, while other mutations (S531W, H526Y, H526D, H526R, S522L and D516V) decreased over the 37 day sampling period. Changing the pH from 7.0 to 6.2 did not significantly alter the overall proportion of mutants, but resulted in a decrease in the percentage of strains harbouring S531L (from 89% to 50%) accompanied by an increase in the range of different mutations from 4 to 12. CONCLUSIONS: The data confirm that the fitness of strains with the S531L mutation is greater than that of strains containing other mutations. We also conclude that at low pH the environment is permissive for a wider spectrum of mutations, which may provide opportunities for a successful mutant to survive

Differentiation of ventricular tachycardia from ventricular fibrillation using intraventricular electrogram morphology

Implantable antitachycardia devices are now capable of providing "ramp" therapy including antitachycardia pacing and low- and high-energy cardioversion for sustained ventricular tachycardia (VT) in addition to defibrillation of ventricular fibrillation (VF).1,2 Appropriate therapy, however, is dependent on correct identification of VT and VF. Because of variations in ventricular electrographic amplitude during VF, currently implanted devices are unable to consistently detect consecutive ventricular depolarizations during VF.3 As a result, the rate criteria for VF detection may sometimes overlap with the rate criteria used for VT detection. Morphologic analysis of intracavitary ventricular electrograms using correlation waveform analysis has been demonstrated in previous studies to be effective in discriminating normal sinus rhythm from VT, paroxysmal bundle branch block of supraventricular origin, and retrograde atrial activation. 4,7 Whether VF is distinguishable from sinus rhythm and VT with a similar method has not been determined. This study examined whether VT and VF can be distinguishable from sinus rhythm and from each other by correlation waveform analysis of intracavitary ventricular electrograms.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29843/1/0000190.pd

A Colorimetric Chemosensor Based on a Nozoe Azulene That Detects Fluoride in Aqueous/Alcoholic Media

Colorimetry is an advantageous method for detecting fluoride in drinking water in a resource-limited context, e. g., in parts of the developing world where excess fluoride intake leads to harmful health effects. Here we report a selective colorimetric chemosensor for fluoride that employs an azulene as the reporter motif and a pinacolborane as the receptor motif. The chemosensor, NAz-6-Bpin, is prepared using the Nozoe azulene synthesis, which allows for its rapid and low-cost synthesis. The chemosensor gives a visually observable response to fluoride both in pure organic solvent and also in water/alcohol binary solvent mixtures

Turbulence in the ice shelf–ocean boundary current and its sensitivity to model resolution

The ice shelf-ocean boundary current has an important control on heat delivery to the base of an ice shelf. Climate and regional models that include a representation of ice shelf cavities often use a coarse grid and results have a strong dependence on resolution near the ice shelf-ocean interface. This study models the ice shelf-ocean boundary current with a non-hydrostatic z-level configuration at turbulence-permitting resolution (1 m). The z-level model performs well when compared against state-of-the-art Large Eddy Simulations showing its capability in representing the correct physics. We showthat theoretical results from a one-dimensional model with parameterised turbulence reproduce the z-level model results to a good degree, indicating possible utility as a turbulence closure. The one-dimensional model evolves to a state of marginal instability and we use the z-level model to demonstrate how this is represented in three-dimensions. Instabilities emerge that regulate the strength of the pycnocline and coexist with persistent Ekman rolls, which are identified prior to the flow becoming intermittently unstable. When resolution of the z-level model is degraded to understand the grid-scale dependencies, the degradation is dominated by the established problem of excessive numerical diffusion. We show that at intermediate resolutions (2-4 m), the boundary layer structure can be partially recovered by tuning diffusivities. Lastly, we compare replacing prescribed melting with interactive melting that is dependent on the local ocean conditions. Interactive melting results in a feedback such that the system evolves more slowly, which is exaggerated at lower resolution