A. Rafferty

The influence of calcium to phosphate ratio on the nucleation and crystallization of apatite glass-ceramics.

The nucleation and crystallization behavior of a series of glasses based on 4.5SiO2- 3Al2O3-YP2O5-3CaO-1.51CaF2 was studied. The parameter Y was varied to give calcium to phosphate ratios between one and two. All of the glasses studied crystallized firstly to fluorapatite (Ca5(PO4)3F). The glass with a calcium to phosphate ratio of 1.67, corresponding to apatite, bulk nucleated to give fluorapatite (FAP). The glasses with calcium : phosphate ratios either less than that of apatite, or greater than that of apatite all exhibited surface nucleation of FAP. However, following a nucleation hold of one hour at approximately 50 K above the glass transition temperature these glasses exhibited bulk nucleation of FAP.

Carbothermal reduction vapor phase transport growth of ZnO nanostructures: Effects of various carbon sources

ZnO nanostructures were grown via carbothermal reduction vapor phase transport with carbon black, activated carbon, and graphite powders. Nanostructures can be grown at significantly lower temperatures with carbon black and activated carbon, although with different morphologies compared to graphite. The surface areas of the carbon black and activated carbon are higher than those of graphite; this has been used previously to explain the origin of such growth and morphology differences. We use different ZnO∕graphite ratios to equalize surface areas compared to carbon black and eliminate this effect, but differences in nanostructure growth and morphology remain. We discuss the effects of thermodynamics and carbon purity and conclude that the high surface activities of the carbon black and activated carbon are the reason for our results.

Preparation, characterisation and modification of carbon-based monolithic rods for chromatographic applications

A range of porous carbon-based monolithic (PCM) rods with flow-through pore sizes of 1, 2, 5 and 10 mum, were produced using a silica particle template method. The rods were characterised using SEM and energy-dispersive X-ray spectroscopy, BET surface area and porous structure analysis, dilatometry and thermal gravimetry. SEM evaluation of the carbon monolithic structures revealed an interconnected rigid bimodal porous structure and energy-dispersive X-ray spectroscopy analysis verified the quantitative removal of the embedded silica beads. The specific surface areas of the 1, 2, 5 and 10 mum rods were 178, 154, 84 and 125 m(2)/g after pyrolysis and silica removal, respectively. Shrinkage of the monolithic rods during pyrolysis is proportional to the particle size of the silica used and ranged from 9 to 12%. Mercury porosimetry showed a narrow distribution of pore sizes, with an average of approximately 700 nm for the 1 mum carbon monolith. The suitability of bare and surface oxidised PCM rods for the use as a stationary phase for reversed and normal phase LC was explored. The additional modification of PCM rods with gold micro-particles followed by 6-mercaptohexanoic acid was performed and ion-exchange properties were evaluated.

Properties of zirconia-toughened-alumina prepared via powder processing and colloidal processing routes

Alumina-zirconia composites were prepared by two routes: powder processing, and colloidal processing. Unstabilised zirconia powder was added to alumina in 5 wt%, 10 wt% and 20 wt% quantities. For the colloidal method, zirconium(IV) propoxide solution was added to alumina powder, also in 5 wt%, 10 wt% and 20 wt% quantities. Additions of glacial acetic acid were needed to form stable suspensions. Suspension stability was verified by pH measurements and sedimentation testing. For the powder processed samples Vickers hardness decreased indefinitely with increasing ZrO(2) additions, but for colloidal samples the hardness at first decreased but then increased again above >10 wt% ZrO(2). Elastic modulus (E) values decreased with ZrO(2) additions. However, samples containing 20 wt% zirconia prepared via a colloidal method exhibited a much higher modulus than the powder processed equivalent. This was due to the homogeneous dispersion of zirconia yielding a sample which was less prone to microcracking.

An investigation of co-fired varistor-ferrite materials

The purpose of this work was to co-fire crack-free varistor-ferrite ceramic multilayers fabricated via a dry pressing route. Multilayers were sintered using a standard industrial grade varistor sintering regime. Sinter shrinkages of both varistor and ferrite materials were measured using dilatometry and showed that the varistor shrunk significantly more than the ferrite material. X-ray diffraction analysis indicated that no significant phase changes occurred in the materials under investigation as a result of the sintering process. Scanning electron microscopy observations of the dry-pressed co-fired varistor-ferrite revealed vertical cracking in the ferrite due to thermal expansion mismatch between the materials. By pressing a mixed composition interlayer in the ratio 50:50, between the varistor and ferrite materials, a crack-free multilayer structure could be obtained. Energy dispersive X-ray analysis of the co-fired ferrite and varistor confirmed diffusion of Fe and Ni components from the ferrite into the varistor material. The degree of diffusion was reduced by using 50:50 ratio mixed composition interlayers.

Amorphous phase separation of ionomer glasses

Ionomer glasses of generic composition SiO2-Al2O3-P2O5-CaO-CaF2 were studied using differential scanning calorimetry (DSC) and high temperature dynamic-mechanical thermal analysis (DMTA). High temperature DMTA was used to measure the glass transition temperatures (Tg) of the original starting glass compositions, as well as being able to follow amorphous phase separation (APS) within the glass. High temperature DMTA traces of all the glasses studied exhibited two maxima in tan δ. These maxima correspond to two glass transition temperatures and demonstrate that amorphous phase separation of the parent glass into two glass phases had occurred. A DMTA study of a Sodium-Boro-Silicate glass, which is known to undergo amorphous phase separation yielded similar results. DSC studies showed that the ionomer glasses underwent a nucleation process at temperatures just above the glass transition temperature which is probably associated with APS. The glasses exhibited optimum nucleation temperatures which moved to lower temperatures with longer hold times indicating the time dependency of the APS process.

An investigation into the amorphous phase separation characteristics of an ionomer glass series and a sodium-boro-silicate glass system

Amorphous phase separation of ionomer glasses, also known as fluoro-phospho-alumino-silicate glasses, of generic composition SiO2-Al2O3-P2O5-CaO-CaF2 were investigated. A sodium-boro-silicate glass system, which is known to undergo amorphous phase separation was also investigated. High Temperature Dynamic-Mechanical Thermal Analysis (DMTA), combined Differential Thermal Analysis/Thermo Gravimetric Analysis (DTA/TGA) and Scanning Electron Microscopy (SEM) were the principal analytical techniques used in this study. High temperature DMTA was used to measure the glass transition temperatures (Tg) of the original starting glass compositions, as well as being able to follow amorphous phase separation (APS) within the glass. High temperature DMTA traces of both the ionomer glasses and the sodium-boro-silicate glasses exhibited two maxima in tan δ, corresponding to two glass transition temperatures and demonstrating that amorphous phase separation of the parent glass into two glass phases had occurred. DTA/TGA of the ionomer glasses detected a glass transition and two crystallisation peaks for apatite and mullite, accompanied by a gradual weight loss of 1–3% on passing through the crystallisation region. The sodium-boro-silicate base glass showed no evidence of a glass transition, but a prominent glass transition was detected for a second sample which had undergone a heat-treatment of 240 min at 580°C. SEM analysis of the ionomer glass compositions revealed smooth spherical droplets of 2–15 nm while the background morphology appeared rough and speckled. A classic interconnected structure was observed for the sodium-boro-silicate glass.

Sintering and characterisation of nano-sized yttria-stabilised zirconia

Two types of commercially available nano-sized, ZrO2?3mol%Y2O3 (Y-Tetragonal Zirconia Polycrystals (TZP)) powders were investigated. BET surface area analysis showed the powders to be nano-sized as determined using Equivalent Spherical Diameter (ESDBET) theory. Dilatometry analysis showed that one of the powders underwent complete shrinkage during isothermal sintering; the other did not. The point of maximum densification rate for the powders differed by approximately 100?C. A dramatic expansion associated with the tetragonal to monoclinic transformation was observed by dilatometry for unstabilised zirconia, but not for TZP samples. X-ray diffraction showed that the starting TZP powders contained some monoclinic zirconia, but confirmed that discs sintered from this powder were wholly tetragonal. For one powder type, the fracture toughness of discs decreased almost linearly with increasing sintering temperature, whereas for the other the fracture toughness was relatively stable across the temperature range...

Production and Treatment of Porous Glass Materials for Advanced Usage

Porous glass is derived from glass that is heat treated to form two interconnecting phases: a silica-rich phase, and an alkali-rich borate phase. The heat-treated glass is then leached selectively to remove one of the phases. The heat-treatment step and leaching conditions can be adjusted to achieve the desired pore size, pore volume, and surface area. This glass has an interconnected pore structure with a uniform pore distribution. Therefore, porous glass based on the sodium borosilicate system which is widely known to undergo amorphous phase separation can be tailor-made to required specifications, thus offering flexibility in terms of end applications.

Approach for sintering nano-sized yttria-stabilised zirconia

Commercially available nano-sized ZrO2-3mol%Y2O3 (TZ-3Y-EB.) was investigated. Particle size of powder was found to be 1.29 ?m, suggesting that the particles agglomerate in water. BET analysis yielded a specific surface area of 14.52 m?/g. The powder had a true density of 5.98 g/cm? when analysed using helium gas pycnometry. DTA/TGA analysis indicated that binder burnout occurred on heating in the range 300-400?C. An expansion associated with the tetragonal to monoclinic transformation on cooling from 1400?C was observed by dilatometry for unstabilised zirconia but not for TZ-3Y-EB. The density of discs which were sintered across the temperature range 1350?C to 1500?C varied between 6.02 g/cm? and 6.05 g/cm?, corresponding to 99.12% and 99.49% of theoretical density. Dilatometry showed that a maximum shrinkage rate occurred at 1285?C. X-ray diffraction data revealed that the starting TZ-3Y-EB powder contained some monoclinic zirconia, but that discs sintered from this powder were wholly tetragonal after sintering.