A theoretical model for the tilt of the GaAs/Si epilayers

The crystallographic tilt of the epilayers with respect to their substrates has been observed in many heteroepitaxial systems. Many models have been proposed to explain this phenomenon, but none of them is suitable for the large mismatched system, such as GaAs/Si. Here a new model is proposed for GaAs/Si epilayers, which can also be used in other large mismatched systems. The magnitude of the tilt calculated from this model coincide well with the experimental results. Especially, this model can correctly predict the tilt direction of the GaAs/Si epilayers.

Reduction in crystallographic tilt of lateral epitaxial overgrown GaN by using new patterned shape mask

We proposed a new method to suppress the crystallographic tilt in the lateral epitaxial overgrowth of GaN by using an oxide mask with a newly designed pattern. A rhombus mask with edges oriented in the direction of <10 - 10>(GaN) was used instead of the traditional stripe mask. The morphology evolution during the LEO GaN with the rhombus mask was investigated by SEM, and the crystallographic tilt in the LEO GaN was measured by DC-XRD. It is found that using the new rhombus mask can decrease the crystallographic tilt in the LEO GaN. In addition, this method makes the ELO GaN stripes easy to coalesce. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Sensitivity of the Australian summer monsoon to tilt and precession forcing

IEECAS SKLLQG

Temperature phase tilt in unstable baroclinic waves

A general property on the phase relation in linear baroclinic instability is proved analytically: in a potential vorticity homogenization regime, the complex geometry of the quasigeostrophic equations determines that the phase lines of temperature and pressure disturbances tilt with height in opposite directions.

Fast Head Tilt Detection for Human-Computer Interaction

Accurate head tilt detection has a large potential to aid people with disabilities in the use of human-computer interfaces and provide universal access to communication software. We show how it can be utilized to tab through links on a web page or control a video game with head motions. It may also be useful as a correction method for currently available video-based assistive technology that requires upright facial poses. Few of the existing computer vision methods that detect head rotations in and out of the image plane with reasonable accuracy can operate within the context of a real-time communication interface because the computational expense that they incur is too great. Our method uses a variety of metrics to obtain a robust head tilt estimate without incurring the computational cost of previous methods. Our system runs in real time on a computer with a 2.53 GHz processor, 256 MB of RAM and an inexpensive webcam, using only 55% of the processor cycles.

Investigation and Development of a Fully 3D Tilt Capable Hybrid SPECT - CT System for Dedicated Breast Imaging

X-ray mammography has been the gold standard for breast imaging for decades, despite the significant limitations posed by the two dimensional (2D) image acquisitions. Difficulty in diagnosing lesions close to the chest wall and axilla, high amount of structural overlap and patient discomfort due to compression are only some of these limitations. To overcome these drawbacks, three dimensional (3D) breast imaging modalities have been developed including dual modality single photon emission computed tomography (SPECT) and computed tomography (CT) systems. This thesis focuses on the development and integration of the next generation of such a device for dedicated breast imaging. The goals of this dissertation work are to: [1] understand and characterize any effects of fully 3-D trajectories on reconstructed image scatter correction, absorbed dose and Hounsifeld Unit accuracy, and [2] design, develop and implement the fully flexible, third generation hybrid SPECT-CT system capable of traversing complex 3D orbits about a pendant breast volume, without interference from the other. Such a system would overcome artifacts resulting from incompletely sampled divergent cone beam imaging schemes and allow imaging closer to the chest wall, which other systems currently under research and development elsewhere cannot achieve.

The dependence of x-ray scatter radiation on object shape, size, material composition and the CT acquisition trajectory, was investigated with a well-established beam stop array (BSA) scatter correction method. While the 2D scatter to primary ratio (SPR) was the main metric used to characterize total system scatter, a new metric called ‘normalized scatter contribution’ was developed to compare the results of scatter correction on 3D reconstructed volumes. Scatter estimation studies were undertaken with a sinusoidal saddle (±15° polar tilt) orbit and a traditional circular (AZOR) orbit. Clinical studies to acquire data for scatter correction were used to evaluate the 2D SPR on a small set of patients scanned with the AZOR orbit. Clinical SPR results showed clear dependence of scatter on breast composition and glandular tissue distribution, otherwise consistent with the overall phantom-based size and density measurements. Additionally, SPR dependence was also observed on the acquisition trajectory where 2D scatter increased with an increase in the polar tilt angle of the system.

The dose delivered by any imaging system is of primary importance from the patient’s point of view, and therefore trajectory related differences in the dose distribution in a target volume were evaluated. Monte Carlo simulations as well as physical measurements using radiochromic film were undertaken using saddle and AZOR orbits. Results illustrated that both orbits deliver comparable dose to the target volume, and only slightly differ in distribution within the volume. Simulations and measurements showed similar results, and all measured dose values were within the standard screening mammography-specific, 6 mGy dose limit, which is used as a benchmark for dose comparisons.

Hounsfield Units (HU) are used clinically in differentiating tissue types in a reconstructed CT image, and therefore the HU accuracy of a system is very important, especially when using non-traditional trajectories. Uniform phantoms filled with various uniform density fluids were used to investigate differences in HU accuracy between saddle and AZOR orbits. Results illustrate the considerably better performance of the saddle orbit, especially close to the chest and nipple region of what would clinically be a pedant breast volume. The AZOR orbit causes shading artifacts near the nipple, due to insufficient sampling, rendering a major portion of the scanned phantom unusable, whereas the saddle orbit performs exceptionally well and provides a tighter distribution of HU values in reconstructed volumes.

Finally, the third generation, fully-suspended SPECT-CT system was designed in and developed in our lab. A novel mechanical method using a linear motor was developed for tilting the CT system. A new x-ray source and a custom made 40 x 30 cm2 detector were integrated on to this system. The SPECT system was nested, in the center of the gantry, orthogonal to the CT source-detector pair. The SPECT system tilts on a goniometer, and the newly developed CT tilting mechanism allows ±15° maximum polar tilting of the CT system. The entire gantry is mounted on a rotation stage, allowing complex arbitrary trajectories for each system, without interference from the other, while having a common field of view. This hybrid system shows potential to be used clinically as a diagnostic tool for dedicated breast imaging.

The tilt casting of titanium aluminides

TiAl castings are prone to various defects including bubbles entrained during the turbulent filling of moulds. The present research has exploited the principles of the Durville tilt casting technique to develop a novel process in which the Induction Skull Melting (ISM) of TiAl alloys in a vacuum chamber has been combined with controlled tilt pouring to achieve the tranquil transfer of the metal into a hot ceramic shell mould. Practical casting equipment has been developed to evaluate the feasibility of this process in parallel with the development of novel software to simulate and optimize it. The PHYSICA CFD code was used to simulate the filling, heat transfer and solidification during tilt pouring using a number of free surface modelling techniques, including the novel Counter Diffusion Method (CDM). In view of the limited superheat, particular attention was paid to the mould design to minimize heat loss and gas entrainment caused by interaction between the counter-flowing metal and gas streams. The model has been validated against real-time X-ray movies of the tilt casting of aluminium and against TiAl blade castings. Modelling has contributed to designing a mould to promote progressive filling of the casting and has led to the use of a parabolic tilting cycle to balance the competing requirements for rapid filling to minimize the loss of superheat and slow filling minimize the turbulence-induced defects.

Modelling the tilt-casting process for the tranquil filling of titanium alloy turbine blades

The tilt-casting method is used to achieve tranquil filling of gamma-TiAl turbine blades. The reactive alloy is melted in a cold crucible using an induction coil and then the complete crucible-mould- running system assembly is rotated through 180degrees to transfer the metal into the mould. The induction current is ramped down gradually as the rotation starts and the mould is preheated to maintain superheat. The liquid metal then enters the mould and the gas within it (argon) escapes through the inlet aperture and through auxiliary vents. Solidification starts as soon the metal enters the mould and it is important to account for this effect to predict and prevent misruns. The rotation rate has to be controlled carefully to allow sufficient time for gas evacuation, but at the same time preserve superheat. This 3-phase system is modelled using the FV method, with a fast implicit numerical scheme used to capture the transient liquid free surface. The enthalpy method is used to model solidification and predict defects such as trapped bubbles, macro-porosity or surface connected porosity. Modeling is used to support an experimental program for the development of a production method for gamma-TiAl blades, with a target length of 40cm. The experiments provide validation for the model and the model in turn optimizes the tilt-casting process. The work is part of the EU project IMPRESS.

Patch antenna with linear polarisation tilt control

The design of a linearly-polarised agile antenna is presented. The antenna is fed by a quasi-lumped coupler which has the ability to tune the magnitude ratio between its two outputs from -30 dB to 15 dB by modifying the bias of two varactor diodes. In this way the relative power fed to each orthogonal port of a patch antenna can be varied. Consequently, tilt control of the radiated linearly-polarised waves is achieved over a range of 90 degrees.

Padrão de ativação de músculos do core abdominal e tilt pélvico no teste de extensão da anca, em indivíduos com e sem dor lombo- pélvica

Introdução: Os padrões de movimento podem sofrer alterações por atraso no timing de ativação e/ou modificações na sequência de recrutamento muscular, predispondo o indivíduo a disfunções, nomeadamente a dor lombo-pélvica. Objetivo: Investigar o timing e o padrão de ativação de músculos do core abdominal, durante o movimento de extensão da anca, do membro dominante, em indivíduos com e sem dor lombo- pélvica crónica inespecífica. Pretende-se, também, pesquisar a existência do padrão de ativação considerado “normal“ e verificar a relação entre o padrão de ativação e o tilt pélvico, em ambos os indivíduos. Métodos: Estudo transversal, com 64 estudantes universitários, divididos em dois grupos: 31 sem e 33 com dor lombo-pélvica. Através de eletromiografia de superfície foi recolhida a atividade muscular dos Eretores da Espinha ipsilateral e contralateral, Glúteo Máximo e Bicípite Femoral ipsilaterais. Foi analisado o timing de ativação muscular e as respetivas ordens de ativação. Adicionalmente foi medido o tilt pélvico. Resultados: O grupo com dor lombo-pélvica apresentou um atraso significativo no timing de ativação dos músculos Glúteo Máximo ipsilateral (t=-3,171;p=0,002) e Bicípite Femoral ipsilateral (t=-2,092;p=0,041), em comparação com o grupo sem dor. Verificou-se uma associação significativa entre as 5 ordens de ativação mais frequentes e a presença de dor lombo-pélvica (xf2=11,54;p=0,015). A ordem de ativação "normal" – Glúteo Máximo ipsilateral>Bicípite Femoral ipsilateral>Eretor da Espinha contralateral>Eretor da Espinha ipsilateral – não foi utilizada. Verificou-se que o Bicípite Femoral ipsilateral foi maioritariamente o primeiro a ativar-se e o Glúteo Máximo ipsilateral o último em ambos os grupos. Verificou-se um tilt pélvico significativamente superior nos indivíduos que ativam primeiro o Bicípite Femoral ipsilateral nos grupos com (U=51;p=0,001) e sem dor (U=41p=0,001). Conclusão: Os indivíduos com dor lombo-pélvica apresentaram um atraso no timing de ativação dos músculos do core abdominal. Os resultados parecem refutar a ordem de ativação "normal" que tem sido proposta. Não foi possível apoiar nem contestar a teoria de que um atraso na ativação do Glúteo Máximo está associado com dor lombo-pélvica.