Analysis of types of questions asked by sixth grade children concerning social studies content.

Thesis (M.A.)--Boston Universit

Gravity in a Box

We consider a brane-world construction which incorporates a finite region of flat space, ``the box,'' surrounded by a region of anti-de Sitter space. This hybrid construction provides a framework which interpolates between the scenario proposed by Arkani-Hamed, Dimopoulos and Dvali, and that proposed by Randall and Sundrum. Within this composite framework, we investigate the effects of resonant modes on four-dimensional gravity. We also show that, on a probe brane in the anti-de Sitter region, there is enhanced production of on-shell nonresonant modes. We compare our model to some recent attempts to incorporate the Randall-Sundrum scenario into superstring theory.Comment: 15 pages, 1 figur

Three axis pulsed plasma thruster with angled cathode and anode strip lines

A spacecraft attitude and altitude control system utilizes sets of three pulsed plasma thrusters connected to a single controller. The single controller controls the operation of each thruster in the set. The control of a set of three thrusters in the set makes it possible to provide a component of thrust along any one of three desired axes. This configuration reduces the total weight of a spacecraft since only one controller and its associated electronics is required for each set of thrusters rather than a controller for each thruster. The thrusters are positioned about the spacecraft such that the effect of the thrusters is balanced

Modelling and Fabrication of Organic Semiconductor Devices for RFID Tags

As organic semiconductor materials advance in both performance and stability, opportunities to integrate them into commercial applications increase considerably. The main benefits of organic semiconductor-related technology are the realisations of large area, flexible, cheap electronics. Already, organic light emitting diodes (OLEDs) are being integrated into small screens for mobiles phones, MP3 players, digital cameras, and high-resolution micro-displays. Such portable devices favour the high light output of OLEDs for easier readability in sunlight as well as their low power drain. However, at this time, the issue of low field effect mobilities still haunt these materials, limiting their application into low speed circuits. More recently, the introduction of the next generation of small molecule-based organic materials has increased the possibility of attaining high field effect mobilities. The majority of the focus is on pentacene as this has been shown to demonstrate effective mobilities of up to 1.5 cm2y-1s-l. Unfortunately, this type of pentacene was evaporated which does not seem feasible for large scale manufacturing. More recently, modified pentacene materials have surfaced allowing them to be dissolved in common solvents, one such example being triisopropylsilyl-pentacene (TIPSpentacene). Thin film transistors (TFTs) made with this material have been reported to have mobilities as high as 1.2 cm2y-ls-1 and onloffratios of 108. Materials such as TIPS-pentacene are now preferred as they can be integrated into low-cost manufacturing techniques such as inkjet printing and roll-to-roll processing. One of the major prospective applications for these organic materials is the integration into radio frequency identification tags (RFID); these operate at 13.56 MHz. This is a great challenge as the rectification stage will require devices with high mobilities to enable carriers to follow the signal, thus gaining the maximum amount of energy from an inductively coupled magnetic field. It is not clear as to whether Schottky diodes or gated-transistors will be required here. The advantage of gated-transistors is the simple incorporation into the fabrication process. Schottky diodes with these materials require thicker films which are incompatible with spin coated thin film transistors. This thesis focuses on three of the main components for a potential organic RFID tag: the tag antenna, the schottky diode and the organic thin film transistor. These are all vital components in the successful operation of an RFIO tag. The antenna is imperative for the power supply as it absorbs energy due to inductive coupling with the RFID reader antenna. The Schottky diode is important for the frontend/rectification stage, converting AC power to a DC supply voltage for an organic chip. The thin film transistor is hugely important as it is the backbone for logic and memory. The fundamental background into inductive coupling based RFID systems is explored and discussed. Major components such as the reader, tag and control system are introduced, while their role and importance are also looked into. Operational principles such as near field inductive coupling for systems functioning at 13.56 MHz are featured, involving the issues of data transfer and power supply. From here, the concept of mutual inductance is explored in detail, as well as highlighting the fact that most RFID systems of this nature comprise lower coupling, as low as I %. The core theory behind predicting the chip voltage on the tag is also explained, to illustrate just how many design parameters are involved and how they affect the performance of an RFID system. The challenges presented to organicbased RFID tags are also summarised and discussed. The numerous charge transport models proposed so far to represent conduction in organic semiconductors are assessed. These models include variable range Miller Abraham hopping and Poole-Frenkel mechanisms. Currently, an outright universal understanding of carrier transport is yet to be widely agreed. An analytical model is developed to demonstrate the carrier density dependence of mobility that is generally observed in organic semiconductors. An empirical relationship between mobility and carrier density, known as the Universal Mobility Law (UML) is recognised. The polycrystalline-based theory which consists of deriving expressions for quasi-drift and quasi-diffusion regions of operation is explained. TIPS-pentacene is utilised here for the first time to test the model. The fabrication procedure for creating bottom-gate bottom-contact organic thin film transistors is covered, with aluminium as the gate material, a high-K alumina gate dielectric and gold for the source and drain contacts. The transistors were fabricated and characterised in a clean non-vacuum environment. The effect of solvent choice is also investigated. comparing tetralin and toluene solvents. The field effect mobility of the charge carriers calculated were approximately 0.02 cm2Ns with threshold voltages ranging from -1V to +1V depending on the chosen solvent. The on/off current ratio estimated from the transfer characteristics were found to be six order of magnitude. Schottky diodes made with TIPS-pentacene show onloff current ratios three to four order magnitude higher than the P3HT devices, suggesting they are much more suitable for rectification circuits. The introduction of mixing the TIPSpentacene material with binders such as poly( a-methyl styrene ) (PAMS) and polystyrene (PS) produced some intriguing results. Both types of binders produced much smoother drop cast films than TIPS films without any binder applied, indicating that the binders were definitely improving the film morphology. Different surface treatments were also employed to help further increase the performance of the devices. It appeared that applying oxygen plasma to the bottom contact definitely helped adhesion. However, chemical treatments such as pentafluorobenzenethiol (PFBT) and I-hexadecanethiol (HDT) either did not affect performance or severely inhibited it

Dissecting the Gaseous Halos of z~2 Damped Lyα\alpha Systems with Close Quasar Pairs

We use spectroscopy of close pairs of quasars to study diffuse gas in the circumgalactic medium (CGM) surrounding a sample of 40 Damped Lya systems (DLAs). The primary sightline in each quasar pair probes an intervening DLA in the redshift range 1.6 < z_DLA < 3.6, such that the second quasar sightline then probes Lya, CII, SiII, and CIV absorption in the CGM transverse to the DLA to projected distances $R_{\perp} < 300$ kpc. Analysis of the Lya profiles in these CGM sightlines constrains the covering fraction (f_C) of optically thick HI (having column density N_HI > 10^17.2 cm^-2) to be greater than ~30% within $R_{\perp} < 200$ kpc of DLAs. Strong SiII 1526 absorption with equivalent width W_1526 > 0.2 Ang occurs with an incidence f_C (W_1526 > 0.2 Ang) = 20(+12/-8)% within $R_{\perp}<100$ kpc, indicating that low-ionization metal absorption associated with DLAs probes material at a physical distance R_3D < 30 kpc. However, we find that strong CIV 1548 absorption is ubiquitous in these environments (f_C (W_1548 > 0.2 Ang) = 57(+12/-13)% within $R_{\perp} < 100$ kpc), and in addition exhibits a high degree of kinematic coherence on scales up to ~175 kpc. We infer that this high-ionization material arises predominantly in large, quiescent structures extending beyond the scale of the DLA host dark matter halos rather than in ongoing galactic winds. The Lya equivalent width in the DLA-CGM is anticorrelated with $R_{\perp}$ at >98% confidence, suggesting that DLAs arise close to the centers of their host halos rather than on their outskirts. Finally, the average Lya, CII and CIV equivalent widths are consistent with those measured around z~2 Lyman Break Galaxies. Assuming that DLAs trace a galaxy population with lower masses and luminosities, this finding implies that the absorption strength of cool circumgalactic material has a weak dependence on dark matter halo mass for M_h < 10^12 M_sun.Comment: Submitted to ApJ. 30 pages, 13 figures, 3 tables, 1 appendix. Uses emulateapj forma

Combustor with fuel preparation chambers

An annular combustor having fuel preparation chambers mounted in the dome of the combustor. The fuel preparation chamber comprises an annular wall extending axially from an inlet to an exit that defines a mixing chamber. Mounted to the inlet are an air swirler and a fuel atomizer. The air swirler provides swirled air to the mixing chamber while the atomizer provides a fuel spray. On the downstream side of the exit, the fuel preparation chamber has an inwardly extending conical wall that compresses the swirling mixture of fuel and air exiting the mixing chamber

An Opportunity Cost Model of Subjective Effort and Task Performance

Why does performing certain tasks cause the aversive experience of mental effort and concomitant deterioration in task performance? One explanation posits a physical resource that is depleted over time. We propose an alternative explanation that centers on mental representations of the costs and benefits associated with task performance. Specifically, certain computational mechanisms, especially those associated with executive function, can be deployed for only a limited number of simultaneous tasks at any given moment. Consequently, the deployment of these computational mechanisms carries an opportunity cost – that is, the next-best use to which these systems might be put. We argue that the phenomenology of effort can be understood as the felt output of these cost/benefit computations. In turn, the subjective experience of effort motivates reduced deployment of these computational mechanisms in the service of the present task. These opportunity cost representations, then, together with other cost/benefit calculations, determine effort expended and, everything else equal, result in performance reductions. In making our case for this position, we review alternative explanations for both the phenomenology of effort associated with these tasks and for performance reductions over time. Likewise, we review the broad range of relevant empirical results from across sub-disciplines, especially psychology and neuroscience. We hope that our proposal will help to build links among the diverse fields that have been addressing similar questions from different perspectives, and we emphasize ways in which alternative models might be empirically distinguished

Empowering local communities to make lifestyle changes: is the Health Mela a potential solution?

Background: Health Melas are community-led public health events held in the North West of England that provide health information and free health checks. This descriptive observational study evaluates whether Health Melas are able to identify undiagnosed cardiovascular disease (CVD) risk factors in hard-to-reach communities and encourage individuals to make lifestyle changes. Methods: Attendees ≥18 years at three separate Health Melas in 2016–2017 were invited to participate in screening and counselling for CVD risk factors as part of a Health MOT. Information was collected about demographics, CVD risk factors, blood pressure, total cholesterol, blood sugar and attendees’ feedback. QRISK2 scoring system was used to estimate CVD risk. Results: 375 attendees completed a questionnaire. The highest proportion (36.9%) of attendees were from areas of the lowest Index of Health Deprivation and Disability quintile; 38.8% were of South Asian ethnicity. Of the attendees who were eligible for a free National Health Service Health Check, 9.1% had received one. Overall, 57.5% of all attendees had a QRISK2 score ≥10% (of whom 56.9% were not on statins), 92.2% of attendees believed the Health Mela will help them to make lifestyle changes, 98.2% said they had improved their understanding of their health, and 99.6% thought the Health Mela was useful. 73.6% of those who had received a previous Health MOT reported making lifestyle changes. There was a positive correlation between South Asian ethnicity and QRISK2 score. Conclusion: This study suggests the Health Melas successfully involve South Asian populations and people from a lower Index of Health Deprivation and Disability. Attendees felt the events were useful, improved understanding of their health needs and encouraged them to make lifestyle changes. High rates of modifiable CVD risk factors were newly identified and a high proportion of attendees were found to be at intermediate to high risk of CVD