Nuclear power supplies: Their potential and the practical problems to their achievement for space missions

Various issues associated with getting technology development of nuclear power systems moving at a pace which will support the anticipated need for such systems in later years is discussed. The projected power needs of such advanced space elements as growth space stations and lunar and planetary vehicles and bases are addressed briefly, and the relevance of nuclear power systems is discussed. A brief history and status of the U.S. nuclear reactor systems is provided, and some of the problems (real and/or perceived) are dealt with briefly. Key areas on which development attention should be focused in the near future are identified, and a suggested approach is recommended to help accelerate the process

Wafer scale heteroepitaxy of silicon carbon and silicon carbide thin films and their material properties

For years now, many have believed the solution to reducing the cost of the wide bandgap compound semiconductor silicon carbide (SiC) is to grow its cubic form (3C-SiC) heteroepitaxially on silicon (Si). This has the potential to reduce cost, increase wafer size and integrate SiC with Si technology. After decades of research, 3C-SiC grown on Si is still yet to penetrate the commercial market as the process is plagued with various issues such as very high growth temperatures, thermal stresses, high cost, poor epitaxial material quality and poor scalability to wafer sizes beyond 100 mm diameter. The first section of this thesis starts with a focus on the traditional, high temperature growth of 3C-SiC carried out in the first industrial type SiC based reduced pressure chemical vapour deposition (RP-CVD) reactor installed in a UK University. After the process demonstrated little promise for mass scale implementation into the semiconductor industry, a radical change in strategy was made. The research pivoted away from SiC and instead focussed on silicon carbon alloys (Si1-yCy) with carbon (C) contents in the range of 1-3%. Si1-yCy has a range of applications in strain engineering and reducing contact resistance, differing from 3C- SiC quite significantly. Crystalline alloys with C contents around 1.5% were achieved using an industry standard Si based RP-CVD growth system. Analysis was carried out on the defects that form due to the saturation of C in higher content alloys. The high temperature annealing of Si1-yCy resulted in out diffusion of C and traces of 3C-SiC growth which presented itself as a potential buffer layer for 3C-SiC epitaxy. Through the careful selection of growth precursors and process optimisation, high crystalline quality 3C-SiC was grown heteroepitaxially on Si within the industry standard Si based RP-CVD and in-depth material characterisation has been carried out using a vast range of techniques. High levels of electrically active dopants were incorporated into the 3C-SiC and its electrical properties were investigated. Various investigations were carried out on suspended 3C-SiC and Si1-yCy films including strain and tilt measurements through micro X-ray diffraction and the effect of thickness and doping on their optical properties. The results led to a greater understanding of suspended films and provide a foundation for a number of applications in microelectromechanical systems (MEMS) and optical devices. Further material growth research was carried out on Si1-yCy multilayers, selective epitaxy of 3C-SiC and the growth of 3C-SiC on suspended growth platforms. Each topic presents an interesting area for further research. The research presented demonstrates new, state of the art 3C-SiC heteroepitaxial material and its basic structural, electrical and optical properties. A new low-cost and scalable process has been developed for the heteroepitaxial growth of 3C-SiC on Si substrates up to 100 mm with a clear path to scaling the technology up to 200 mm and beyond. Not only does the developed technology have a high commercial impact, it also paves the way for many interesting future research topics, some of which have been briefly investigated as part of this work

OCT for diagnosis of periodontal disease

We have developed a hand-held in vivo scanning device for use in the oral cavity. We produced, using this scanning device, in vivo OCT images of dental tissues in human volunteers. All the OCT images were analyzed for the presence of clinically relevant anatomical structures. The gingival margin, periodontal sulcus, and dento-enamel junction were visible in all the images. The cemento-enamel junction was discernible in 64% of the images and the alveolar bone presumptively identified for 71% of the images. These images represent, to our knowledge, the first in vivo OCT images of human dental tissue

Ultrasonic inspection and self-healing of Ge and 3C-SiC semiconductor membranes

Knowledge of the mechanical properties and stability of thin film structures is important for device operation. Potential failures related to crack initiation and growth must be identified early, to enable healing through e.g. annealing. Here, three square suspended membranes, formed from a thin layer of cubic silicon carbide (3C-SiC) or germanium (Ge) on a silicon substrate, were characterised by their response to ultrasonic excitation. The resonant frequencies and mode shapes were measured during thermal cycling over a temperature range of 20--100~$^\circ$C. The influence of temperature on the stress was explored by comparison with predictions from a model of thermal expansion of the combined membrane and substrate. For an ideal, non-cracked sample the stress and Q-factor behaved as predicted. In contrast, for a 3C-SiC and a Ge membrane that had undergone vibration and thermal cycling to simulate extended use, measurements of the stress and Q-factor showed the presence of damage, with the 3C-SiC membrane subsequently breaking. However, the damaged Ge sample showed an improvement to the resonant behaviour on subsequent heating. Scanning electron microscopy showed that this was due to a self-healing of sub-micrometer cracks, caused by expansion of the germanium layer to form bridges over the cracked regions, with the effect also observable in the ultrasonic inspection

Single step silicon carbide heteroepitaxy on a silicon wafer at reduced temperature

A single step growth approach for wafer-scale homogeneous cubic silicon carbide (3C-SiC) heteroepitaxy, using chemical vapour deposition (CVD), on a silicon (Si) substrate is demonstrated. Residual biaxial tensile strain causing a wafer bow is eliminated in the 3C-SiC epilayer via in-situ defects engineering and heteroepitaxy at reduced temperature. Thermal mismatch between the 3C-SiC epilayer and substrate is minimised by a substantial reduction of growth temperature, down to ∼1000 °C. Heteroepitaxy of high quality, fully relaxed 3C-SiC epilayers with minimal wafer bow is demonstrated, made possible by careful process optimisation. Unusually very high growth rate of 3C-SiC of > 10 µm/hr is achieved. At the same time the epilayer is free from any other silicon carbide (SiC) polytype inclusions. Moreover, the reduced growth temperature unlocks the ability to deposit high quality 3C-SiC epilayers within traditional Si-based cold walled CVD reactors, enabling the growth of such thin films on unprecedently high volumes and wafer diameters up to 300 mm and above

Determination of Arsenic, Mercury and Barium in herbarium mount paper using dynamic ultrasound-assisted extraction prior to atomic fluorescence and absorption spectrometry

A dynamic ultrasound-assisted extraction method using Atomic Absorption and Atomic Flourescence spectrometers as detectors was developed to analyse mercury, arsenic and barium from herbarium mount paper originating from the herbarium collection of the National Museum of Wales. The variables influencing extraction were optimised by a multivariate approach. The optimal conditions were found to be 1% HNO3 extractant solution used at a flow rate of 1 mL min-1. The duty cycle and amplitude of the ultrasonic probe was found to be 50% in both cases with an ultrasound power of 400 W. The optimal distance between the probe and the top face of the extraction chamber was found to be 0 cm. Under these conditions the time required for complete extraction of the three analytes was 25 min. Cold vapour and hydride generation coupled to atomic fluorescence spectrometry was utilized to determine mercury and arsenic, respectively. The chemical and instrumental conditions were optimized to provide detection limits of 0.01ng g-1 and 1.25 ng g-1 for mercury and arsenic, respectively. Barium was determined by graphite-furnace atomic absorption spectrometry, with a detection limit of 25 ng g-1. By using 0.5 g of sample, the concentrations of the target analytes varied for the different types of paper and ranged between 0.4–2.55 µg g-1 for Ba, 0.035–10.47 µg g-1 for As and 0.0046–2.37 µg g-1 for Hg

A fast approach to measuring the thickness uniformity of a homoepilayer grown on to any type of silicon wafer

Optical reflection spectroscopy techniques offer a non-destructive and fast method of measuring the thickness of silicon (Si) epilayers, enabling very fast thickness uniformity mapping across the full surface of epiwafers up to 450 mm in diameter. However, their use for undoped or low doped epilayers has traditionally been constrained by a dependence on high levels of substitutional doping in the Si wafer, at values of approximately 5 × 1019 cm−3. Whilst the high dopant concentration of this wafer creates the necessary reflectance boundary for optical reflection, their commercial availability is mainly limited to the (001) surface orientation only. Optical reflectance techniques are therefore also limited in use to this orientation. In this article, an approach to measure the thickness of a Si epilayer on any Si wafer, independent of its crystallographic orientation, doping type and value, using Fourier transform infrared reflection spectroscopy is proposed and demonstrated. Because the use of non-destructive optical reflection spectroscopy is already common and well-understood within both industry and academia, this technique could easily be implemented within existing industrial and research fabrication facilities. Furthermore, this approach could be adapted, with further work, to suit other semiconductor materials and other optical reflection techniques

Sensitivity of selected organ dissection to diagnose Taenia solium cysticercosis in pigs from endemic areas

Taenia solium, also known as the pork tapeworm, is a neglected zoonotic parasite which is endemic in many developing countries, including Zambia. The tapeworm causes two disease conditions in humans: (1) taeniosis, which is the intestinal tapeworm infection, obtained after consumtion of raw/undercooked infected pork; and (2) cysticercosis, which is the metacestode larval stage infection, obtained after ingestion of tapeworm eggs. A human tapeworm carrier can excrete high numbers of eggs with the stool (100 000 eggs per day) and is thus an important source of environmental contamination. The transmission of cysticercosis is thus enhanced with poor sanitation and the lack of clean drinking water. After ingestion of the eggs, oncospheres hatch in the intestine and disseminate to several body tissues, including the central nervous system. Infection of the central nervous system with cysticerci is called neurocysticercosis, which is a major cause of acquired epilepsy worldwide

On-Chip Real-Time Single-Copy Polymerase Chain Reaction in Picoliter Droplets

The first lab-on-chip system for picoliter droplet generation and PCR amplification with real-time fluorescence detection has performed PCR in isolated droplets at volumes 10{sup 6} smaller than commercial real-time PCR systems. The system utilized a shearing T-junction in a silicon device to generate a stream of monodisperse picoliter droplets that were isolated from the microfluidic channel walls and each other by the oil phase carrier. An off-chip valving system stopped the droplets on-chip, allowing them to be thermal cycled through the PCR protocol without droplet motion. With this system a 10-pL droplet, encapsulating less than one copy of viral genomic DNA through Poisson statistics, showed real-time PCR amplification curves with a cycle threshold of {approx}18, twenty cycles earlier than commercial instruments. This combination of the established real-time PCR assay with digital microfluidics is ideal for isolating single-copy nucleic acids in a complex environment