Fabrication and Analysis of Thermally Invariant Smart Composites via Ultrasonic Additive Manufacturing

Poster Division: Engineering, Math, and Physical Sciences: 3rd Place (The Ohio State University Edward F. Hayes Graduate Research Forum)Ultrasonic additive manufacturing (UAM), a form of 3D printing based on ultrasonic metal welding, is a fabrication technique that is rapidly altering the development of new components within the research and commercial industries. Through the use of piezoelectric boosters, vibrating at 20 kHz, and the application of normal forces in excess of 5000 Newtons, thin metal foils can be welded in a fusionless, low-temperature process to produce bulk structures. Because of its low-temperature, UAM provides the opportunity to embed thermally sensitive materials, such as nickel-titanium (NiTi), a shape memory alloy. NiTi exhibits a shape change as it undergoes thermally-induced crystallographic phase transformation between martensite, the low-temperature phase, and austenite, the high-temperature phase. During phase transformations, NiTi can recover up to 8% elastic strain and have a change in elastic modulus of 100%. When embedded, the strain recovery of NiTi can be used to counteract the thermal expansion of the matrix material—specifically aluminum in this study—for the purpose of producing components with low coefficients of thermal expansion (CTE) while keeping the weight at a minimal level. The work herein covers the design, fabrication, and characterization of Al-NiTi composites to aid in the development of a composite that has a coefficient of thermal expansion at, or below, 5 µє/°C. A composite is produced that has a CTE of 13.83 µє/°C; a 40.4% decrease as compared to Al alone. In addition, electrical resistivity measurements in the longitudinal direction and thermal diffusivity measurements in the out-of-plane directions are presented.A one-year embargo was granted for this item

Race, Identity, and Belonging in Early Zimbabwean Nationalism(s), 1957-1965

This thesis interrogates traditional understandings of race within Zimbabwean nationalism. It explores the interactions between socio-cultural identities and belonging in black African nationalist thinking and politics, and focuses on the formative decade between the emergence of mass African nationalist political parties in 1957 and the widespread adoption of an anti-white violent struggle in 1966. It reassesses the place of non-black individuals within African anti-settler movements. Using the chronological narrative provided by the experiences of marginal non-black supporters (including white, Asian, coloured, and Indian individuals), it argues that anti-colonial nationalist organisations during the pre-Liberation War period were heavily influenced by the competing racial theories and politics espoused by their elite leadership. It further argues that the imagined future Zimbabwean nations had a fluid and reflexive positioning of citizens based on racial identities that changed continuously. Finally, this thesis examines the construction of racial identities through the discourse used by black Zimbabweans and non-black migrants and citizens, and the relationships between these groups, to contend that race was an inexorable factor in determining belonging. Drawing upon archival sources created by non-black 'radical' participants and Zimbabwean nationalists, and oral interviews conducted during fieldwork in South Africa and Zimbabwe in 2015, the research is a revisionist approach to existing academic literature on Zimbabwean nationalism: in the words of Terence Ranger, it is not a nationalist history but a history of nationalism. It situates itself within multiple bodies of study, including conceptual nationalist and racial theory, the histories of marginal groups within African nationalist movements, and studies of citizenship and belonging. It seeks to critically approach the ideologies and practices of Zimbabwean nationalism, and to interrogate the role race played in defining the imagined Zimbabwean nation, her citizens, and her politics. It also provides much-needed detail into the under-examined histories of minority racial groups and their relationships to early Zimbabwean nationalist parties. The conclusions drawn demonstrate that identities and participation within Zimbabwean nationalism were inherently affected by overarching concepts of biological race and skin colour, and that Zimbabwean nationalism was reciprocally shaped by these factors as well.Cambridge Home and European Scholarship Scheme (CHESS) Smuts Memorial Fun

Solar Power Concentrators for Space Applications

Space based solar power is an attractive solution to growing energy needs, overcoming the operational downtime and atmospheric losses inherent to terrestrial solar technologies. Ideal solar satellites include concentrator systems in order to produce a high specific power output, keeping mass low for economic launch. A meta analysis of photovoltaic concentrator research is presented to determine viability in space applications, considering contributions to cell heating, subcell current matching and efficiency of the InGaP/GaAs/Ge multi-junction cell. We find that the CaF2 Fresnel lens is an optimal choice with an efficiency increase of 4.9% at 17 suns. The large range of concentration ratio, low mass, compact design and minimal effect on spectral irradiance allow a wide space for optimisation in temperature control, whilst the current matching conditions under concentration can be regulated by appropriate tunnel junction width and bandgap

Searches for Stellar Radio Activity in Circular Polarisation

Transient radio bursts produced in the coronae of stellar atmospheres and within planetary magnetospheres often feature a high degree of circular polarisation. These events reveal the presence of strong magnetic fields, and trace particle accelera- tion driven by magnetic reconnection, space weather, and auroral current systems. The detection of stellar radio emission has historically been challenged by the over- whelmingly higher surface density of extragalactic sources that dominate the radio sky, which produce a large number of false positive associations to optically identi- fied stars. Our knowledge of the radio star population has therefore been primarily driven by targeted study of the most active, nearby systems, impacting the inference of population statistics and discovery of new regimes of stellar radio activity. In this thesis I present a body of work that exploits the circular polarisation of magnetically driven stellar radio emission to identify large samples of radio stars in widefield surveys. I demonstrate this technique as part of the first all-sky survey with the Australian Square Kilometre Array Pathfinder (ASKAP)— the Rapid ASKAP Continuum Survey (RACS). I further extend this search technique to the multi-epoch Variables and Slow Transients (VAST) pilot survey resulting in 196 detections of a sample of 46 radio stars, and use the repeat sampling of this group to present constraints on the statistical fraction of the M-dwarf population that produce radio bursts. Finally, I describe the use of this technique to discover auroral radio pulses from the M9.5 ultracool dwarf SCR J1845−6357—the oldest and slowest rotator among radio detected ultracool dwarfs to date. I conclude by outlining the application of circular polarisation searches to future surveys with ASKAP and next generation instruments such as the Square Kilometre Array, which will result in tens of thousands of new radio star detections and provide a new window into stellar magnetism

Knowledge-how, understanding-why and epistemic luck:An experimental study

Reductive intellectualists about knowledge-how (e.g., Stanley & Williamson Journal of Philosophy 98, 411–44, 2001; Stanley Noûs 45, 207–38, 2011a, 2011b; Brogaard Philosophy Compass 3, 93–118, 2008a, Grazer Philosophische Studien 77, 147–90 2008b, Philosophy and Phenomenological Research 78, 439–67 2009, 2011) hold, contra Ryle (1946, 1949), that knowing how to do something is just a kind of propositional knowledge. In a similar vein, traditional reductivists about understanding-why (e.g., Salmon 1984; Lipton 2004; Woodward 2003; Grimm The British Journal for the Philosophy of Science 57, 515–35, 2006; Greco 2009; Kelp 2014) insist, in accordance with a tradition beginning with Aristotle, that the epistemic standing one attains when one understands why something is so is itself just a kind of propositional knowledge—viz., propositional knowledge of causes. A point that has been granted on both sides of these debates is that if these reductive proposals are right, then knowledge-how and understanding-why should be susceptible to the same extent as knowledge-that is to being undermined by epistemic luck. This paper reports experimental results that test these luck-based predictions. Interestingly, these results suggest a striking (albeit, imperfect) positive correlation between self-reported philosophical expertise and attributions of knowledge-how, understanding-why and knowledge-that which run contrary to reductive proposals. We contextualize these results by showing how they align very well with a particular kind of overarching non-reductive proposal, one that two of the authors have defended elsewhere (e.g., Carter and Pritchard Philosophy and Phenomenological Research 91, 181–99, 2015a, Noûs 49, 440–53, 2015b, Australasian Philosophical Quarterly 93, 799–816, 2015c) according to which knowledge-how and understanding-why, but not knowledge-that, essentially involve cognitive achievement (i.e., cognitive success that is primarily creditable to cognitive ability). We conclude by situating the interpretive narrative advanced within contemporary discussions about the role of expertise in philosophical judgment

Periodic Radio Emission from the T8 Dwarf WISE J062309.94-045624.6

We present the detection of rotationally modulated, circularly polarized radio emission from the T8 brown dwarf WISE J062309.94-045624.6 between 0.9 and 2.0 GHz. We detected this high proper motion ultracool dwarf with the Australian SKA Pathfinder in $1.36$ GHz imaging data from the Rapid ASKAP Continuum Survey. We observed WISE J062309.94-045624.6 to have a time and frequency averaged Stokes I flux density of $4.17\pm0.41$ mJy beam$^{-1}$, with an absolute circular polarization fraction of $66.3\pm9.0\%$, and calculated a specific radio luminosity of $L_{\nu}\sim10^{14.8}$ erg s$^{-1}$ Hz$^{-1}$. In follow-up observations with the Australian Telescope Compact Array and MeerKAT we identified a multi-peaked pulse structure, used dynamic spectra to place a lower limit of $B>0.71$ kG on the dwarf's magnetic field, and measured a $P=1.912\pm0.005$ h periodicity which we concluded to be due to rotational modulation. The luminosity and period we measured are comparable to those of other ultracool dwarfs observed at radio wavelengths. This implies that future megahertz to gigahertz surveys, with increased cadence and improved sensitivity, are likely to detect similar or later-type dwarfs. Our detection of WISE J062309.94-045624.6 makes this dwarf the coolest and latest-type star observed to produce radio emission.Comment: Accepted for publication in ApJ Letters; 11 pages, 3 figures and 2 table

Classical Novae in the ASKAP Pilot Surveys

We present a systematic search for radio counterparts of novae using the Australian Square Kilometer Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, which covered the entire sky south of declination $+41^{\circ}$ ($\sim34,000$ square degrees) at a central frequency of 887.5 MHz, the Variables and Slow Transients Pilot Survey, which covered $\sim5,000$ square degrees per epoch (887.5 MHz), and other ASKAP pilot surveys, which covered $\sim200-2000$ square degrees with 2-12 hour integration times. We crossmatched radio sources found in these surveys over a two-year period, from April 2019 to August 2021, with 440 previously identified optical novae, and found radio counterparts for four novae: V5668 Sgr, V1369 Cen, YZ Ret, and RR Tel. Follow-up observations with the Australian Telescope Compact Array confirm the ejecta thinning across all observed bands with spectral analysis indicative of synchrotron emission in V1369 Cen and YZ Ret. Our light-curve fit with the Hubble Flow model yields a value of $1.65\pm 0.17 \times 10^{-4} \rm \:M_\odot$ for the mass ejected in V1369 Cen. We also derive a peak surface brightness temperature of $250\pm80$ K for YZ Ret. Using Hubble Flow model simulated radio lightcurves for novae, we demonstrate that with a 5$\sigma$ sensitivity limit of 1.5 mJy in 15-min survey observations, we can detect radio emission up to a distance of 4 kpc if ejecta mass is in the range $10^{-3}\rm \:M_\odot$, and upto 1 kpc if ejecta mass is in the range $10^{-5}-10^{-3}\rm \:M_\odot$. Our study highlights ASKAP's ability to contribute to future radio observations for novae within a distance of 1 kpc hosted on white dwarfs with masses $0.4-1.25\:\rm M_\odot$ , and within a distance of 4 kpc hosted on white dwarfs with masses $0.4-1.0\:\rm M_\odot$.Comment: This paper has been accepted for publication in PASA. It consists of 13 pages, 5 figures and 4 table

A pilot ASKAP survey for radio transients towards the Galactic Centre

We present the results of a radio transient and polarisation survey towards the Galactic Centre, conducted as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients pilot survey. The survey region consisted of five fields covering $\sim265\,{\rm deg}^2$ ($350^\circ\lesssim l\lesssim10^\circ$, $\vert b\vert \lesssim 10^\circ$). Each field was observed for 12\,minutes, with between 7 and 9 repeats on cadences of between one day and four months. We detected eight highly variable sources and seven highly circularly-polarised sources (14 unique sources in total). Seven of these sources are known pulsars including the rotating radio transient PSR~J1739--2521 and the eclipsing pulsar PSR~J1723--2837. One of them is a low mass X-ray binary, 4U 1758--25. Three of them are coincident with optical or infrared sources and are likely to be stars. The remaining three may be related to the class of Galactic Centre Radio Transients (including a highly likely one, VAST~J173608.2--321634, that has been reported previously), although this class is not yet understood. In the coming years, we expect to detect $\sim$40 bursts from this kind of source with the proposed four-year VAST survey if the distribution of the source is isotropic over the Galactic fields.Comment: 17 pages, 13 figures, Accepted for publication in MNRA