Abbie Jones

The impact of inert gases on the structure, properties and growth of nanocrystalline diamond

For biomedical and electronic applications, it is highly desirable to deposit smooth diamond films with crystal sizes in the nanoscale range. We present experimental results of chemical vapour deposition diamond growth from CH4 with incremental substitution of H2 with He or Ar gases; the concentrations of the inert gases were varied between 0 and 98 vol%. Results show that initially the addition of either argon or helium increases the growth rate and significantly alters the film structure and crystallinity up to 60 vol%. With additions of argon or helium greater than 60 vol% in the gas phase the growth decreases and there is degradation of the crystal structure. In general, nanocrystalline diamond has been deposited at dilutions in excess of 90 vol% helium or argon.

Chemical vapour deposition diamond coating on tungsten carbide dental cutting tools

Diamond coatings on Co cemented tungsten carbide (WC-Co) hard metal tools are widely used for cutting non-ferrous metals. It is difficult to deposit diamond onto cutting tools, which generally have a complex geometry, using a single step growth process. This paper focuses on the deposition of polycrystalline diamond films onto dental tools, which possess 3D complex or cylindrical shape, employing a novel single step chemical vapour deposition (CVD) growth process. The diamond deposition is carried out in a hot filament chemical vapour deposition (HFCVD) reactor with a modified filament arrangement. The filament is mounted vertically with the drill held concentrically in between the filament coils, as opposed to the commonly used horizontal arrangement. This is a simple and inexpensive filament arrangement. In addition, the problems associated with adhesion of diamond films on WC-Co substrates are amplified in dental tools due to the very sharp edges and unpredictable cutting forces. The presence of Co, used as a binder in hard metals, generally causes poor adhesion. The amount of metallic Co on the surface can be reduced using a two step pre-treatment employing Murakami etching followed by an acid treatment. Diamond films are examined in terms of their growth rate, morphology, adhesion and cutting efficiency. We found that in the diamond coated dental tool the wear rate was reduced by a factor of three as compared to the uncoated tool.

Dimensional change, irradiation creep and thermal/mechanical property changes in nuclear graphite

Since the start of the ‘nuclear age’ graphite has been employed as a moderator in around 100 nuclear reactors, and today there are still some 30 graphite-moderated reactors operating and there are plans for new Generation IV high-temperature reactors. Many of the graphite moderator reactors now producing power are operating beyond their original design life. Therefore in some cases, to aid the reactor operators and designers, the existing graphite irradiation databases need to be extended either to a higher temperature or higher neutron fluence. Furthermore, data are needed for the different grades of graphite that are available at present. This can either be achieved by expensive, time consuming irradiation programmes or by improving the understanding of the mechanisms and processes which lead to irradiation-induced dimensional and property changes in the graphite core components. This review looks at three of the most important graphite properties which change with exposure to irradiation, namely dimensional change, irradiation creep and thermal expansion. The behaviour of UK AGR, Magnox and an experimental grade of German reactor graphite are explored in some detail. First graphite reactor core design is briefly discussed, giving examples of typical graphite components and core arrangements. Issues related to aging graphite component and core behaviour are illustrated through examples of component internal and thermal stress generation, and issues related to whole core behaviour are also outlined. Second the manufacture and microstructure of different nuclear graphite grades are discussed, highlighting how the choice of raw materials and manufacturing technique influences the graphite properties. Third the coefficient of thermal expansion, dimensional change and irradiation creep are analysed using microstructural and averaging methods which are used to relate crystal to bulk properties by accounting for graphite crystal orientation and porosity. These techniques, which were first applied to nuclear graphite in the 1960s, are extended and discussed with the aim of trying to lend some understanding to the role the microstructural crystallite and porosity distributions play in defining the dimensional stability and properties of virgin graphite, irradiated graphite and stressed graphite.

Transmission Electron Microscopy Study of Graphite under in situ Ion Irradiation

Graphite is employed as a moderator and structural component in 18 of the United Kingdoms fleet of Magnox and Advanced Gas-cooled Reactors (AGRs). During the operational lifetime of a reactor, graphite undergoes complex physical and mechanical property changes including dimensional modification, owing to the effects of temperature, oxidation and irradiation-induced atomic displacements. In order to safely extend the lifetime of the current fleet of AGRs, and also to develop materials for GenIV concepts such as the Very-High- Temperature Reactor (VHTR), it is important to gain a better understanding of the fundamental atomic processes which underpin the behaviour of graphite under current and future operational conditions. This study has focused on the effects of temperature and displacing radiation on the evolution of Mrozowski cracks in highly-orientated pyrolytic graphite (HOPG) using the new Microscope and Ion Accelerator for Materials Investigations (MIAMI) facility. This instrument allows transmission electron microscopy to be performed in situ whilst simultaneously ion irradiating to radiation damage levels typically reached in a reactor. By using this technique, it is possible to explore the development of radiation damage under a range of different conditions continuously from start-to-finish rather than just observing the end-states accessible in ex situ studies.

Structure of different grades of nuclear graphite

Owing to its low neutron absorption cross-section, large scattering cross section and thermal and chemical stability, graphite is a key component of operational nuclear reactors where it is used as a moderator, reflector and as major structural component for 90% of current UK nuclear plants. It is also of interest for use in developing the future high temperature gas-cooled reactors. The properties of the nuclear graphite are influenced by its structural characteristics, which change as a function of neutron irradiation, temperature and oxidation. The principal structural changes during neutron irradiation that affect the integrity and dimensions of nuclear graphite components, thereby affecting service lifetime, are that the a-axis contracts and the c-axis expands in the crystallites. Characterization of virgin graphite structure and of the damage evolution after irradiation of nuclear graphite has an important role to play in the understanding and development of materials used in current and future nuclear reactors, respectively.

Time-modulated chemical vapor deposition of diamond films

This article investigates the role of substrate temperature in the deposition of diamond films using a newly developed time-modulated chemical vapor deposition (TMCVD) process. TMCVD was used to deposit polycrystalline diamond coatings onto silicon substrates using hot-filament chemical vapor deposition system. In this investigation, the effect of (a) substrate temperature and (b) methane (CH4) content in the reactor on diamond film deposition was studied. The distinctive feature of the TMCVD process is that it time-modulates CH4 flow into the reactor during the complete growth process. It was noted that the substrate temperature fluctuated during the CH4 modulations, and this significantly affected some key properties of the deposited films. Two sets of samples have been prepared, in each of which there was one sample that was prepared while the substrate temperature fluctuated and the other sample, which was deposited while maintaining the substrate temperature, was fixed. To keep the substrate temperature constant, the filament power was varied accordingly. In this article, the findings are discussed in terms of the CH4 content in the reactor and the substrate temperature. It was found that secondary nucleation occurred during the high timed CH4 modulations. The as-deposited films were characterized for morphology, diamond-C phase purity, hardness, and surface roughness using scanning electron microscopy, Raman spectroscopy, Vickers hardness testing, and surface profilometry, respectively.

Experimental and gas phase modeling of nanocrystalline diamond films grown on titanium alloys for biomedical applications

For biomedical applications, it is highly desirable to be able to deposit smooth adherent diamond films on various complex-shaped substrates using the hot filament chemical vapor deposition technique (HFCVD). The properties of these films are affected profoundly by process parameters such as filament temperature, gas composition, and pressure. In this study, we present an insight into the gas phase chemistry involved in HFCVD of smooth nanocrystalline diamond films using Ar/CH4/H2 precursor mixtures. Experimental results on the growth, surface morphology, and crystalline structure are also presented. It is evident that the addition of a noble gas such as argon has a considerable effect on the gas surface chemistry. Notably at high concentrations of inert gas dilution (>90 vol.% argon) there are significant changes in diamond crystallinity ranging from polycrystalline through microcrystalline, and at argon concentrations >98 vol.%, nanocrystalline facets are observed. Modeling of the gas phase chemistry showed that the relative concentrations of CH3 and C2H alter significantly in this region, and these in turn influence surface morphology and crystallinity of the deposited films.

Thermal Oxidation of Nuclear Graphite: A Large Scale Waste Treatment Option

This study has investigated the laboratory scale thermal oxidation of nuclear graphite, as a proof-of-concept for the treatment and decommissioning of reactor cores on a larger industrial scale. If showed to be effective, this technology could have promising international significance with a considerable impact on the nuclear waste management problem currently facing many countries worldwide. The use of thermal treatment of such graphite waste is seen as advantageous since it will decouple the need for an operational Geological Disposal Facility (GDF). Particulate samples of Magnox Reactor Pile Grade-A (PGA) graphite, were oxidised in both air and 60% O2, over the temperature range 400–1200°C. Oxidation rates were found to increase with temperature, with a particular rise between 700–800°C, suggesting a change in oxidation mechanism. A second increase in oxidation rate was observed between 1000–1200°C and was found to correspond to a large increase in the CO/CO2 ratio, as confirmed through gas analysis. Increasing the oxidant flow rate gave a linear increase in oxidation rate, up to a certain point, and maximum rates of 23.3 and 69.6 mg / min for air and 60% O2 respectively were achieved at a flow of 250 ml / min and temperature of 1000°C. These promising results show that large-scale thermal treatment could be a potential option for the decommissioning of graphite cores, although the design of the plant would need careful consideration in order to achieve optimum efficiency and throughput.

Pulsed biased growth of nanocrystalline diamond by hot filament chemical vapour deposition

Abstract For many industrial applications such as biomedical instruments, optical devices and microelectromechanical systems, the control of the film structure, crystallinity and morphology is of critical importance. The crystallite size, orientation and surface roughness have a profound effect on the mechanical, optical and electrical properties of the films and therefore the final product performance. In order to reduce the crystallite size and surface roughness, inert gases were added to the methane and hydrogen mixture during chemical vapour deposition of nanocrystalline diamond films. In addition, the results on the influence of pulsed biasing on the morphology of these films are reported. Bias voltages in the range –300-0 V were investigated. Increasing the bias voltage significantly alters the crystallite size and morphology of the deposited films. Raman spectroscopy, SEM and atomic force microscopy were used to characterise the nanocrystalline diamond films. SE/501

Electrochemical behaviour of graphite- and molybdenum electrodes modified with thin-film diamond

Abstract Films of undoped polycrystalline diamond were deposited on molybdenum or graphite substrates by hot filament chemical vapour deposition (HFCVD), using a precursor gas mixture of methane in excess hydrogen. The morphology and quality of the as-deposited films were monitored by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction, and the electrochemical activity monitored with cyclic voltammetry. The results suggest a direct correlation between electrochemical activity, film thickness, and quality. Analysis of the ferrocyanide–ferricyanide couple at a diamond-modified molybdenum or graphite electrode suggest some extent of electrochemical reversibility, but the rates of charge transfer across the diamond–substrate interface varied with the methane concentration and substrate type. The ratio of the anodic and cathodic peak currents was always close to unity. Ex situ studies with energy dispersive spectroscopy show that some Prussian Blue was deposited on the graphite-modified electrode, grown using 0.5% CH4 in H2.