New Researchers -Previous years

Irradiated and contaminated reactor graphite is a growing problem in the UK and around the world. Graphite moderation blocks were used in uranium-graphite reactors including many Generation-I designs, such as MAGNOX, which are now being decommissioned. Approximately 230,000 tonnes of graphite waste now exists worldwide. The bulk of the graphite waste could be reused in applications where its contamination is not significant. The waste graphite could be immobilised via sintering with glass to form a glass composite material, with the aim that this material could be used in some waste storage/repository structural or shielding applications. This study focused on sintering of silicate glass-graphite composites. Three candidate waste glasses (Soda-lime silica, Lead Glass and Borosilicate glass) were sintered in a range of atmospheres, over a range of temperatures, and with different loadings of graphite (wt.%: 10, 30 and 50). It was found that graphite would not be retained in the composite material unless an inert atmosphere was used during sintering above 600 oC. Durable glass-graphite composites could be made if temperatures 150 oC above their glass transition temperatures were used. Lead and SLS glasses were seen to readily sinter with few problems. However the borosilicate glass composite material suffered a range of problems in sintering, from glass blistering at high temperatures too increased porosity and a lack of densification, which could be due to a high wetting angle at the borosilicate glass / graphite interface. The paper outlines the most important structural features of glass-graphite composites produced.

Vitrification of high-level nuclear waste (HLW) has been a major focus internationally for decades. The UK glass is an alkali borosilicate with 25 wt% waste loading.  Much of the initial dissolution behaviour of alkali borosilicate glass in aqueous environments is well understood and agreed upon; however, the consensus on the role of the gel layer as a diffusion barrier in slowing the dissolution rate is controversial.  Even with the controversy over the role of the gel layer, the alteration products in the gel layer are clearly important to understanding the role that the gel layer plays in the dissolution rate with respect to the radionuclide source term. 
This presentation discusses our study of alteration products with respect to the gel layer and their role in the dissolution behaviour of UK HLW glass.  Alteration and modelling studies of full scale stimulant UK HLW glasses are underway.  Here we report our studies on monolithic samples of UK HLW glass and Ca-Zn doped HLW glass after exposure to water vapour in order to study the alteration products formed in the gel layer.  These samples include a baseline glass without the simulated waste and the baseline glass with a 20 wt% simulated waste loaded glass.  X-ray Absorption Near Edge Spectroscopy (XANES) will be used to determine the structure and role of the Zn in the alteration phases. 

It has been reported there is a “cliff” in glass stability with respect to the dissolution rate (D.M. Strachan, et al., J. Nucl. Mater. 298 (2001) 69).  The role of magnesium aluminosilcate alteration phases in this “cliff” will be studied using monolithic samples treated in water vapour and the alteration phases studied to determine the dissolution rate with respect to the percent of Al2O3 and MgO in the glass.  Other experiments which can further the discussion on the dissolution rate and the role of the gel layer will also be discussed.

Among the various biomaterials used as bone substitutes or as prosthetic coatings in orthopedic or dental surgery, bioactive glasses have attracted considerable interest. When these materials are in contact with living tissues, several physical and chemical reactions (dissolution, precipitation) take place at the material / bone interface, and lead to the formation of a phosphocalcic layer (apatite). This apatite layer is used as a mineralization site for bone cells and allows finally an intimate chemical bond between the bioactive glass and the bone. The composition in major and trace elements and the textural properties can modify the physico-chemical reactions occurring at the glass periphery. In this work sol-gel process has been used to dope a ternary glass with one and five weight percent of magnesium oxide. The samples were synthesized under powder form and immersed in an acellular biological medium for delays from 30 minutes to 4 days. At each time period, quantitative chemical maps of powders were recorded with a micrometer resolution using Particle Induced X-ray Emission coupled to Rutherford Backscattering spectroscopy (PIXE-RBS). This technique allows monitoring the physico-chemical reactions at the bioactive glass / biological environment interface and the evolution of the traces elements by concentration measurements with an excellent sensitivity (10-6 g / g).
Results show the formation of a bone-like phosphocalcic layer containing traces of magnesium, after 12 hours of interaction. Then this layer grows in thickness, and the grains are totally transformed in calcium phosphate after 3 days. The magnesium concentration influences the kinetics of these physico-chemical reactions, as evidenced by the kinetics of evolution of the Si, Ca, P, Mg, concentrations. This result gives hopes for the development of applications based on Mg-doped glasses in treatment of osteoporosis and tissue engineering.

 

A bioactive glass is one which can elicit a biological response when in the presence of a biological medium. In the present case, this response leads to the formation of hydroxyapatite at the interface, and gives the material an ability to bond to bone and to stimulate new bone growth. Bioglass® is a commercially available melt-derived bioactive glass which is routinely used in clinical practice as a treatment for periodontal disease (Perioglas®) and as a bone filling material (Novabone®). In recent years, strontium has become a source of interest, firstly due to it’s chemical similarity to Ca secondly because of it’s greater radio-opacity compared to that of Ca and most importantly strontium is known to stimulate the bone forming osteoblasts and inhibit the bone resorbing osteoclasts, making it particularly attractive for use in osteoporotic patients. Here, the influence of substituting Sr for Ca in the glass series 49.46 SiO2 26.38Na2O1.07P2O5(23.08-X)CaOXSrO (where x = 0, 11.54, 23.08) has been studied in terms of its physical and structural characteristics using neutron diffraction. This new data is interpreted in light of existing information from a number of techniques including FTIR and NMR spectroscopy, thermal analysis and in vitro bioactivity.

 

Glass strength can be increased by applying epoxy surface coatings. Much work has been done to observe and analyze glass strengthening with these coatings. A number of models have been considered to explain the strengthening but until now there is no clear evidence to definitively support one model over another. Glass slides with controlled defects (Vickers indentation cracks) were used to study the strengthening achieved after coating with water based epoxy. Different crack dimensions were used for further validation for the finite element model. The mechanical properties of the glass specimens and coating were measured. Finite element models (FEM) of four bending test specimens were created and used to study the strength of cracked uncoated and surface coated specimens in order to identify the strengthening mechanism. The FEM results showed that the epoxy coating is sufficient to heal the crack if the coating inside the crack is ideally glued to the crack surfaces. Conversely, the four point bending experimental results showed that, in most of the failed samples, the crack was the origin of the fracture. Hence, cohesive zone model (CZM) was used to model the coating inside the crack. The interfacial energy release rate for glass/epoxy was measured using double cantilever bending DCB delamination test and then supplied to the model. The FEM model indicated an increase of the strength in good agreement with the average of the experimental results.

 

Barium diborate, BaO
2B2O3, was prepared in the vitreous and crystalline states. Initially 0.1mol% Fe2O3 was added to the melt for the express purpose of reducing nuclear magnetic resonance (NMR) relaxation times. It was found by dierential thermal analysis that the kinetics of devitrication are sensitive to this addition, as well as to the presence of water in the glass network. Furthermore, signicant eects arising from the existence of both surface and bulk nucleating phases were revealed. Several dierent crystalline phases were formed via dierent devitrication routes and the existence of three previously undiscovered phases were identied via x-ray powder diraction and conrmed by NMR. Volatilisation of the melt meant that not all crystal phases were of the diborate stoichiometry.

 

The structure and optical properties of ternary tellurium oxide -based glasses co-doped with Er3+/Yb3+/Ce3+ are investigated.  The melting conditions for these glasses have been analyzed together the structural analysis of glass samples using Raman spectroscopy. The Raman spectra reveal that the un-doped glass consists of TeO3, TeO4 and WO6 polyhedra. The emission cross sections of the doped glasses were calculated using the McCumber theory and measured absorption cross-section. The refractive index was measured over a range of wavelengths, and the wavelength dependence of refractive index was calculated using Sellmeier’s dispersion formula. The FWHM of Er3+-ion fluorescence and lifetimes of the emission at 1535nm were analyzed with dopant concentrations for optimizing the Er3+ ions concentration for device engineering. The results indicate that the glasses have potential for fabricating lasers and amplifiers operating in the wavelength region of 1530-1610nm under 980nm laser diode pumping.

 

Signal amplification is the most significant feature of optical communication systems which is becoming ever so important in the internet age. The demand for data transmission has reached Tb/s, and there is pressure on industry to increase the bandwidth even more at a lesser cost. Optical integration for signal processing is therefore an important means for reducing the overall cost. The research looks at the potential of tellurite and phosphate glasses in combination, for engineering a highly stable rare-earth doped glass for waveguide optical amplifier engineering with rare earth ions such as Er3+, Yb3+ and Ce3+. Phospho-tellurite, which is a homogeneous mixture of TeO2 and P2O5 glass-forming network oxides, has intermediate phonon energy than pure phosphate glass. Unlike pure phosphate the new compositions also depict improved thermal stability and resistance to moisture attack. These properties are characterized by analyzing the processing conditions of glass, FTIR and Raman spectroscopic data together with the results of thermal analysis. We report these properties to point out to the desired structural features for engineering rare-earth dopants for amplifier design and engineering.

 

The green lasers developed until now are based either on semiconductor technologies or on neodymium frequency-doubled scheme; these two options yield sources with little tuneability.  Our aim in this research is to develop a green fibre laser that is cheap, tuneable (520 nm - 560 nm), with an output power of several Watts. The up-conversion mechanism, via two or three photons, is being pursued for a novel semiconductor pumping scheme. A choice of rare earth doping schemes    (Dy3+, Tm3+-Tb3+, Nd3+-Tm3+-Tb3+, Yb3+-Tm3+-Tb3+), has been chosen for detailed spectroscopic analysis for fibre laser engineering, which is carried out by selecting a range of  bulk glass hosts  based on germanate, tellurite and fluorine modified silicate. We report detailed spectroscopic analysis for the chosen dopants using the near-IR pumping scheme and compare the up-conversion efficiencies.

 

A molecular dynamics study of magnesium silicate glass was performed in order to study the local atomic environment of Mg in MgSiO3 with different composition. Molecular dynamics has been used to obtain models of glasses with = 50,54,58,62 and 67, ~ 1000 atoms, and size~ 24 Å. As expected, the models in the glass-forming regain have a corner-shared tetrahedral silica network, , and Mg acts as a network modifier. The average coordination number of magnesium by oxygen is increased from 4.6±0.1 to 5.1±0.1 by increasing x. The MD models have been compared directly with the complement x ray and neutron diffraction data to enable detailed comparison beyond the total structure level.

 

Based on recent scientific results the decorative effect of stains composed of mixtures of copper salts with ingrediants such as ochre, clay, copper oxide and silver stain has been investigated. Utilising both tin and non tin sides of soda lime glass the stains were aplied using varied parameters such as temperature, time and kiln atmosphere where investigated. Colours that were achieved included yellows, oranges, red, ruby and several greens along with a faint sky blue.The effects were reproducible with often several colours appearing with the inclusion of a "halo" of different colour around the original brush stroke. The paper will endevour to explain and show the effects of applying silver nitrate and to what extent various parameters can effect and lead to exciting colours results within the glass that could not be achieved previously.

 

hallium-containing glasses give some of the highest nonlinear optical responses measured in heavy-metal oxide containing glasses but few structural studies have been published on them. Thallium, in its +1 oxidation state contains a lone pair of electrons and it is reported that this may be stereochemically active or inert, depending on glass composition. In the thallium germanate system, there is the added complication of the possibility of germanium being both 4- and 6- coordinated by oxygen. As part of a collaboration, using multiple techniques to investigate the structure of thallium germanate glasses, we have employed 203Tl and 205Tl NMR to examine the thallium sites in the glasses and in the thermodynamically stable and metastable forms of the crystal phase Tl2Ge4O9. The 203Tl and 205Tl isotopes are both abundant (29.5% and 70.5% respectively), both I=1/2 and both have large magnetic moments. Signals are easy therefore easy to obtain and, in principle, lineshapes should be simple.  However, spectra are considerably broadened by various Tl-Tl exchange interactions. The thallium spectra for the glasses show a gradual shift to more positive values of chemical shift and the changes in lineshape indicate the successive formation and disappearance of at least 3 sites. In the case of the metastable crystal phase, at least three sites are present, one of which has a very short T2 which may indicate pairing or motion.