Chemical Durability of Silicon Oxycarbide Glasses

Silicon oxycarbide (SiOC) glasses with controlled amounts of Si—C bonds and free carbon have been produced via the pyrolysis of suitable preceramic networks. Their chemical durability in alkaline and hydrofluoric solutions has been studied and related to the network structure and microstructure of the glasses. SiOC glasses, because of the character of the Si—C bonds, exhibit greater chemical durability in both environments, compared with silica glass. Microphase separation into silicon carbide (SiC), silica (SiO₂), and carbon, which usually occurs in this system at pyrolysis temperatures of >1000°–1200°C, exerts great influence on the durability of these glasses. The chemical durability decreases as the amount of phase separation increases, because the silica/silicate species (without any carbon substituents) are interconnected and can be easily leached out, in comparison with the SiOC phase, which is resistant to attack by OH⁻ or F⁻ ions.     

The power of knowledge sharing

One commonly cited antidote for UK manufacturers against the effects of outsourcing is to innovate and increase the rate of technology transfer. Government policy supports this approach through the creation of knowledge economy - designed to promote the generation of high value-add knowledge, technology and associated skills. Attention is focused on the generation side of the knowledge economy, but without associated swift application, these policies are most likely to fail. This paper discusses the barriers which have to be broached by the UK manufacturing sector in order to gain the benefits of technology transfer.     

Maximizing accuracy in large data conversions

Data conversion looks easy to a lot of people. It's not-particularly if it's a large project. There are just too many ways things can go wrong. You can lose data or mess it up. And you can stall the conversion if you don't have enough computing resources. Project size really makes a difference. Some type of mapping tool can improve your chances for success because the data in the specification is the same data that drives the program that loads the target tables. Automation eliminated the mistakes that cause inconsistency. The tools and processes we developed on a large project worked well on a smaller project, but the reverse certainly would not have been true. A systematic process is required with some way to validate each step     

Analytical and numerical modeling of squeeze-film damping in perforated microstructures

The literature includes a variety of analytical and semi-analytical models to describe squeeze-film damping in MEMS perforated structures. Even if many of them have been validated by means of numerical simulations, nobody seems to have discussed about the accuracy of numerical approaches in this field. In the present paper, we apply both the main analytical models and a commercial finite element software, COMSOL Multiphysics, to solve a good number of squeeze-film problems. They refer to some cases, which were experimentally investigated during the past by different authors. The tested structures are rigid rectangular plates fabricated with different material, different perforation ratio (i.e., the ratio of the hole side to the holes pitch) and different number of perforations. We compare both the analytical and the numerical results with the available experimental data, in order to have an overview about their effectiveness. Numerical simulations offer in all the considered cases valuable agreement with experiments.     

Towards the use of the conformal-evaporated-film-by-rotation technique in fabricating microelectronic circuits and microsystems

We used the conformal-evaporated-film-by-rotation technique to deposit a conformal coating of chalcogenide glass on the external surface of a microsystem—specifically, the outermost surface of a microelectromechanical system comb resonator. We concluded that this technique could complement the multi-step, time-consuming and expensive traditional lithographic processes, resulting in a reduction of the number of steps needed for the fabrication of microsystems and nanosystems.     

Determining the percentages of semi-conducting and metallic single-walled carbon nanotubes in bulk soot

A procedure is presented to determine the percentages of metallic and semi-conducting single-walled carbon nanotubes (SWCNTs) in as-produced soot synthesized by a cobalt/molybdenum-catalyzed chemical vapor deposition (CVD) process. The procedure involves the acquisition of UV–Vis–NIR absorption spectra of solvent-suspended soot samples, and the key aspect is the use of a physically-based subtraction of the π-plasmon absorbance from the overall absorbance to ensure that the percentage of semi-conducting SWCNTs is not overstated. The improvement in accuracy using the procedure is demonstrated using two lots of soot with different amounts of non-tubular carbon (NTC) contaminants. The semi-conducting SWCNT percentage of the soot containing low NTC levels was 82 ± 3%, and this mean was overestimated by 4% when the π-plasmon absorbance was not subtracted. The semi-conducting SWCNT percentage of the soot containing higher NTC levels was 76 ± 9%, and this mean was overestimated by 6% without subtraction of the π-plasmon absorbance.     

Coatings on Glass–Ceramic Granules for Dental Restorative Biomaterials

Two methods for coating glass–ceramic granules have been tested: self-assembly in aqueous solutions and the fluidized-bed technique. Both methods have been successfully used to produce nanocoatings on leucite–fluoroapatite glass–ceramic granules. Two chemical substances were developed for the experiments: (a) a combination of inorganic and organic compounds, (b) organic compounds alone, preferably comb polymers. Both groups of materials positively influenced the sintering properties of the glass–ceramics, because they suppressed tearing. The glass–ceramics were used to produce powder compacts for veneering materials, which would be used on metal frameworks (primarily multiunit dental bridges). With regard to the processing of the aqueous slurry, coatings with inorganic and organic substances are preferred.     

Corrosion of ISG fibers in alkaline solutions

The corrosion behavior of glass fibers synthesized from the International Simple Glass (ISG) reference ingot, an international High‐Level Waste (HLW) borosilicate glass standard, is reported. Bundles of glass fibers were submerged in 120 mL of four different solutions of initial pH values (pH_i) of 9.5, 10.5, 11.5, and 12.5 for static corrosion testing. While all the experiments reached a residual corrosion rate after ~50 days, which remained approximately constant for the duration of the pH_i 9.5‐11.5 experiments, the pH_i 12.5 experiment underwent a Stage II→III dissolution transition after 57 days. This transition was preceded by a decrease in the Al concentration in solution followed by an increase in B and Si concentration in the leachate. Zeolite NaP2 was observed to form on these fibers via scanning electron microscopy and X‐ray diffraction—the crystallinity of the fibers was estimated to be ~40%‐45% (relative to amorphous component) after the Stage II→III transition. Transmission electron microscopy cross‐sectional imaging of sampled fibers revealed several porous layers on the pH_i 9.5‐11.5 samples, and a more aggressive alteration mechanism in the pH_i 12.5 fibers. Potential markers that indicate a transition from Stage II→Stage III corrosion are shown to occur based on experimental observations.     

Transformation Range Viscosity of Fluorozirconate Glasses

Transformation range viscosity was measured for multicomponent fluorozirconate glasses in the system 0.56ZrF₄·(0.34-x)BaF₂·0.06LaF₃·0.04AlF₃·xRF, where R =Li, Na, K, or Cs . The results indicate that the viscosity of these glasses can be described by an Arrhenius equation over the viscosity range from 10⁷ to 10¹² Pa·s with an activation energy of the order of 650 to 850 kJ. In general, the effects of alkali fluoride additions on the viscosity of fluorozirconate glasses are comparable to those in silicate systems.     

Hydroxylation and Dehydroxylation Behavior of Silica Glass Fracture Surfaces

The hydroxylation and dehydroxylation behavior of amorphous silica fracture surfaces was studied using temperature-programmed static SIMS. The results show that vacuum heat treatments result in more extensive condensation of silanol groups on the silica glass fracture surface as compared to fumed silica (Cabosil). This is attributed to differences in the distribution of silanol groups on the two silica surfaces. The rehydration kinetics of the dehydroxylated silica fracture surfaces showed two distinct reaction rates—an initial rapid increase in the silanol concentration, followed by a slower rehydration for longer dosing times. The slower rehydration reaction was shown to follow first-order reaction kinetics with the reaction rate constant, suggesting hydrolysis of strained siloxane bonds on three-membered silicate ring structures. The much faster initial rehydration is attributed to the hydrolysis of extremely strained siloxane bonds in two-membered, edge-shared tetrahedral rings. The effect of the dehydration time and temperature (i.e., thermal history of the surface) on the rehydration kinetics is also discussed.