Electro-conducting glass-ceramics produced by ion-exchange and reduction treatments

The crystallization characteristics of some bismuth-containing soda-lime-silica glasses have been studied. The addition of bismuth reduces the glass transition temperature as well as the crystallization temperatures of these glasses. Electrically conducting layers have been induced in such glass-ceramics by subjecting them to a Na⁺ Ag⁺ ionexchange reaction followed by a reduction treatment in hydrogen. Resistances of the surfaces vary from 0.08 /square to 14.76 /square depending on the glass composition as well as the reduction parameters. Induced surface conductance tends to rise with the increase of volume of the crystalline phase in the parent glass. The TCR values range between 400 and 2300 p.p.m. K^–1. The thicknesses of these layers are about 130 m. The high surface conductivity arises from the percolation of the silver metallic phase in the glass-crystal boundary region. The glass-crystal interface is believed to act as heterogeneous nucleation sites.     

Microstructure and optical absorption of silver granules in oxide glasses produced by ion-exchange and reduction treatments

An electron micrographic investigation has been carried out on two oxide glasses containing alkali ions and subjected to a sodiumsilver ion-exchange followed by reduction treatments at various temperatures. The presence of metallic silver in the silver-rich droplet phases has been confirmed by selected-area electron diffraction. The silver-rich phases are found to have diameters ranging from 3 to 50 nm. The nucleus density of these particles is found to have a maximum value at temperatures in the range 250 to 300° C. The optical absorption spectra of the reduced glasses show a maximum around a wavelength of 400 nm. Maxwell-Garnett theory has been used to calculate the optical absorption and the predicted wavelength for maximum absorption is in reasonable agreement with the experimental value.     

Microstructural studies of glass-metal composites produced by ion-exchange and hydrogen treatments

Detailed electron micrographic studies have been carried out on float glass surface subjected to sodium ? copper ion-exchange in molten salts followed by a reduction treatment in hydrogen. Spherical copper particles of diameters ranging from 50 to 300 å are formed in the glass matrix depending on the ion-exchange and reduction temperatures. The nucleus density of these particles is maximum at around 450?C. Phase separation in glasses whether incipient or induced by ion-exchange, increases considerably the nucleus density when such samples are subsequently reduced. This is ascribed to the increase in the number of copper atoms surrounding the nucleation sites in the phase separated glasses. The optical absorption spectra of the glass-metal composites show two bands, namely at 17 400 cm^?1 and 23 000 cm^?1 respectively. The absorption maxima for both the bands occur for specimens reduced at temperatures between 450 and 500? C corresponding to the temperature of maximum nucleus density. The bands at 17 400 cm^?1 and 23 000 cm^?1 are assigned to the conduction resonance and plasma resonance respectively as predicted by Maxwell-Garnet theory on aggregated metal systems dispersed in a dielectric matrix.     

Increasing the strength of glass by etching and ion-exchange

The tensile strength of ordinary soda-lime glass can be increased by an order of magnitude to 3×10⁵ psi^* by etching with hydrofluoric acid, but this high strength is catastrophically reduced by surface damage and by heating. But for the loss in strength on heating it would be possible to protect the etched surface from damage in handling by ion-exchange with the salt of a cation larger than sodium, which produces a compressive stress in the surface. This investigation showed that a major factor responsible for heat-damage in etched glass is water adsorbed on the surface, and that glass that has either been dehydrated after etching, or etched in an anhydrous reagent, retains a high proportion of its strength on heating. This result has made it possible to protect the surface of etched glass against abrasive damage by ion-exchange in molten potassium and silver nitrates. In this way, tensile strengths of the order of 10⁵ psi have been obtained in glass with a deliberately abraded surface.     

Optical channel waveguides by copper ion-exchange in glass

Optical channel waveguides have been obtained by electric?field?assisted diffusion of copper films on glass substrates. The mode indices of these channel waveguides were determined with the prism?coupling technique, and the refractive?index profile of the waveguide was reconstructed from measurements of the near?field intensity distribution.     

Nanoscale ductile grinding of glass by diamond fibres

Chemical vapour-deposited diamond fibres have been used to grind soda glass. The surface topography was studied using scanning electron microscopy and atomic force microscopy techniques. The diamond surface facets and edges led to grinding without surface cracking and to surface roughness, Ra, values in the range 3–50 nm. The grinding mechanism involved the formation by ductile flow of glass ribbons adjacent to the grinding grooves. This grinding mechanism was similar to that reported for single-point diamond machining. The potential for ductile grinding with diamond fibres is discussed.     

Integrated optical ring resonators made by silver ion-exchange in glass

The first (to our knowledge) integrated optical ring resonators to be fabricated using silver ion-exchanged waveguides are reported. Both circular and racetrack shaped resonators have been made, both types being capable of high finesse (>15) and efficiency (>90%). The circular resonators are much more difficult to make, however, requiring a double-diffusion process and precise control of the ion-exchange. For this reason, the racetrack resonators have been the more successful and have behaved exactly as expected from the previous work on losses and directional couplers.     

Silver nanocluster formation in silicate glass by single step ion-exchange

We report the formation of silver nanoclusters of average size ∼ 1.1 nm in a single step ion-exchanged silica glass. Silver clusters are formed in a glass during ion-exchange by fine-tuning the ion-exchange parameters such as salt composition, temperature and time. Such nanocluster formation during ion-exchange preserves the waveguiding properties of the ion-exchanged surface of the glass making them suitable for nonlinear waveguide device fabrication.     

Potassium source for ion-exchange glass waveguide fabrication

We propose and demonstrate the use of a new potassium-ion source for glass waveguide fabrication based on a KI -ZnCl2 mixture. Planar waveguide were made using glasses such as BK7, Pyrex, soda lime, and commercial semiconductor doped glasses. The optical quality of the waveguides prepared compare favorably with the quality obtained using other fabrication techniques and the processing time is considerably reduced.     

Stresses in ion-exchange layers of soda-lime-silicate glass

This paper presents a new method to determine both the magnitude and the sign of the surface stresses that develop as a consequence of sodium/hydrogen ion exchange in soda‐lime‐silicate glass immersed in water. At 90 °C, very thin layers that develop at the surfaces of polished glass specimens are found to have extremely high compressive stresses, ?2.4 GPa. The negative sign of the stress is consistent with earlier findings that the ion‐exchange process involves hydronium ions (H₃O⁺) and not bare protons (H⁺).     

Synthesis of aluminum nitride fibres from aluminum silicate fibres by carbothermal reduction method

Aluminum nitride (AlN) fibres were synthesized by carbothermal reduction of aluminum silicate fibres and carbon black in a flowing nitrogen atmosphere. XRD and SEM were employed to study the phase structure, chemical composition and morphologies of the products. It is suggested that aluminum silicate fibres can be converted into AlN fibres by carbothermal reduction. SEM confirmed that the fibres maintained the morphologies of aluminum silicate fibres during reduction, but their details of surface have been altered. A carbothermal nitriding model of the aluminum silicate fibres has been developed.     

Fracture strength of ion-exchange silicate-containing dental glass ceramics

Dental glass ceramics with the composition of (0.2K, 0.8Na)₂O–xAl₂O₃–ySiO₂ (x = 0.4–0.8, y = 4–6) were studied for their mechanical properties. Different ion-exchange practices were used to modify the sub-surface concentration distributions of K⁺, Na⁺, and H⁺ of these glass ceramics. Specimens were heat-treated in molten KNO₃, and NaNO₃ + KNO₃ salt baths at 350–450 °C for the ion exchanges of K⁺ and Na⁺, or in the 4% acetic aqueous solution at 85 °C for a hydration treatment. Some glass ceramics contained a feldspar crystalline phase, which was not affected by different ion-exchange practices. Specimens with a single ion-exchange process or with the hydration treatment had higher flexural strength than those without either of these two treatments. For double ion-exchange specimens, the flexural strength increased with decreasing ion-exchange temperature. The double ion-exchange specimens had flexural strength up to 280 MPa, which was slightly lower than that of the single ion-exchange specimens, but much higher than that of the as-annealed specimens. However, the Weibull modulus of these double ion-exchange specimens was 5–8 because of the presence of large defects. For further increasing mechanical reliability, silicate-containing dental glass ceramics were required to have appropriate flaw controls and ion-exchange processes.     

Phase transformation of calcia-alumina-magnesia fibres produced by inviscid melt spinning

CaO-Al₂O₃-MgO (CAM) ceramic fibre produced via inviscid melt spinning (IMS) was investigated for phase transformation. Differential thermal analysis (DTA) on the as-spun CAM fibre gave two transformation peaks, one for exothermic peak at around 927°C and the other for endothermic one at around 1100°C. In order to identify each phase transformation x-ray diffraction (XRD) analysis was performed on the CAM fibres heat-treated to each phase transformation completion temperature. The exothermic peak was determined to represent crystallization of remaining amorphous phase in the as-spun CAM fibre. The endothermic peak was determined to correspond to transformation of non-equilibrium CaO · Al₂O₃ phase to equilibrium 3CaO· 5Al₂O₃ phase.     

Properties of glass fibres in cement environment

Fibres produced from a soda-silica-zirconia glass were reacted with Portland cement extracts at 20 and 65° C for various lengths of time and their strength and stiffness determined. The results indicate that these glass fibres resist the attack of cement extracts reasonably well at ambient temperatures. Fibre strengths of the order of 1200 to 1300 N mm^–2 are obtainable after 2 years at 20° C, sufficient to reinforce cement, and there is no change in the Young's modulus of the fibre during this period. At higher temperatures both strength and stiffness are reduced but these temperatures are unlikely to be encountered in practice over extended periods of time. When fibres removed from cement composites containing commercially made alkali-resistant glass fibres are examined, it is found that fibre strengths depend very strongly on the environment in which the composites were kept. For air storage, fibre properties remain relatively unaffected but for composites kept under water continuously, an initial loss in fibre strength is observed. This difference in fibre strength is reflected in the relative strength of the cement composites.