Barrier alumina

Specific structural features are observed in the formation of composite oxides. Kobayashi, Shimizu, and their co-workers have, in a series of papers, reported studies of the structure of barrier alumina films, anodically formed on aluminum covered by a thin (5 nm) layer of thermal oxide.198,199 Their experiments have shown that the thermally oxidized thin layer generally contains y- alumina crystals of about 0.2 nm size. This layer does not have a pronounced effect on ionic transport in the oxide during anodization. Also, islands of y -alumina are formed around the middle of anodic barrier oxides. They are nucleated and developed from tiny crystals of y -Al203 and grow rapidly in the lateral direction under prolonged anodization.198,199... 

Flame retardants (qv) are incorporated into the formulations in amounts necessary to satisfy existing requirements. Reactive-type diols, such as A/ A/-bis(2-hydroxyethyl)aminomethylphosphonate (Fyrol 6), are preferred, but nonreactive phosphates (Fyrol CEF, Fyrol PCF) are also used. Often, the necessary results are achieved using mineral fillers, such as alumina trihydrate or melamine. Melamine melts away from the flame and forms both a nonflammable gaseous environment and a molten barrier that helps to isolate the combustible polyurethane foam from the flame. Alumina trihydrate releases water of hydration to cool the flame, forming a noncombustible inorganic protective char at the flame front. Flame-resistant upholstery fabric or liners are also used (27). 

Indeed, for operation at temperatures above 1 000°C (now required of advanced aircraft turbine blading materials) it appears that reliance must be placed on alumina as the protective layer, partly because in high velocity oxidising gas streams volatility of CrO, leads to appreciable loss of scale by the oxidation of the CrjO, layer, and partly because alumina is inherently a much better diffusion barrier. 

Other refractory oxides that can be deposited by CVD have excellent thermal stability and oxidation resistance. Some, like alumina and yttria, are also good barriers to oxygen diffusion providing that they are free of pores and cracks. Many however are not, such as zirconia, hafnia, thoria, and ceria. These oxides have a fluorite structure, which is a simple open cubic structure and is particularly susceptible to oxygen diffusion through ionic conductivity. The diffusion rate of oxygen in these materials can be considerable. 

Alumina, present in the gamma modification, is the most suitable high surface area support for noble metals. The y-Al203 in washcoats typically has a surface area of 150-175 m g However, at high temperatures y-alumina transforms into the alpha phase, and stabilization to prevent this is essential. Another concern is the diffusion of rhodium into alumina, which calls for the application of diffusion barriers. 

Lanthanum oxide is valence invariant, and does not exhibit any oxygen storage capacity, but it effectively stabilizes 5/-AI2O3. It spreads over the alumina surface and provides a barrier against dissolution of rhodium in the support. 

The adhesive was developed for application to seal food containers with alumina foils and to seal glass plates. In this case, the vinyl polymerization step can be carried out by UV light radiation through the already mounted glass plates with a photoactive radical initiator to cure the viscous adhesive material. Another application of this material is the development of diffusion barrier containing protective coatings on medieval stained glasses (31). 

When the results for oxide growth and anion incorporation172,160 are compared with the kinetics of space charge accumulation in barrier and porous alumina films [see Section IV(1)], it can be concluded that anion incorporation modifies the electrostatics of the external oxide interface, thus influencing oxide dissolution and pore formation.172... 

Fig. 12. Scanning electron micrographs (6000 x) of residues of air-pyrolyses of halogen-free ethylene-vinyl acetate, containing Zinc Borate (33%) and alumina trihydrate (ATH) (33%). At 500°C (top picture), a powdery residue was obtained no sintering took place between the Zinc Borate (small particles) and ATH (large particles). At 550°C (bottom picture), a hard and porous residue was obtained Zinc Borate acted as a sintering agent. A hard and porous residue is a good thermal barrier.

Zirconium oxide is of interest as a catalyst, as a support for other catalysts and as a diffusion barrier. In the latter application a thin layer of Zr02 prevents the dissolution of rhodium in alumina supports under severe oxidative conditions [32], Here it is essential to apply Zr02 on the support at the highest possible... 

Another study on the preparation of supported oxides illustrates how SIMS can be used to follow the decomposition of catalyst precursors during calcination. We discuss the formation of zirconium dioxide from zirconium ethoxide on a silica support [15], Zr02 is catalytically active for a number of reactions such as isosynthesis, methanol synthesis, and catalytic cracking, but is also of considerable interest as a barrier against diffusion of catalytically active metals such as rhodium or cobalt into alumina supports at elevated temperatures. 

Mitrovic and Knezic (1979) also prepared ultrafiltration and reverse osmosis membranes by this technique. Their membranes were etched in 5% oxalic acid. The membranes had pores of the order of 100 nm, but only about 1.5 nm in the residual barrier layer (layer AB in Figure 2.15). The pores in the barrier layer were unstable in water and the permeability decreased during the experiments. Complete dehydration of alumina or phase transformation to a-alumina was necessary to stabilize the pore structure. The resulting membranes were found unsuitable for reverse osmosis but suitable for ultrafiltration after removing the barrier layer. Beside reverse osmosis and ultrafiltration measurements, some gas permeability data have also been reported on this type of membranes (Itaya et al. 1984). The water flux through a 50/im thick membrane is about 0.2mL/cm -h with a N2 flow about 6cmVcm -min-bar. The gas transport through the membrane was due to Knudsen diffusion mechanism, which is inversely proportional to the square root of molecular mass. 

HF-based chemistry is particularly interesting due to its compatibility with all back-end metals and barriers. Unfortunately as the absolute values of tbe zeta potential in the A area of Fig. 13 are lower than in alkaline media, the removal mechanism is even more difficult. Indeed as seen in Fig. 19, the particle removal efficiency in the HF-HCl mixture is almost zero for actual alumina slurries. Very-high-power megasonics performed in a specific HF-compatible bath are absolutely necessary to obtain the same good residual particle level as with the scrubber. 

The Rb-82 generator permits serial studies in the same patient as often as every 10 minutes with 20-60 mCi of Rb-82 for rapid bolus intravenous infusion. Inherent in the administration of high levels of Rb-82 activity is the need for precise flow control from an automated system to deliver the desired amount of radioactivity. The development of the alumina column parameters and the elution protocol as well as the automated microprocessor system controller are presented here. Some of the details of this system have been discussed in earlier publications (15,21). Generator produced Rb-82 is used as a diffusible flow tracer in myocardial perfusion studies and as a nondiffusible tracer in brain studies to assess blood brain barrier permeability changes in patients with brain tumors or Alzheimer s type dementia. 

Currently, the only biological barrier registered as a biochemical pest control agent is kaolin, a clay mineral [47]. Kaolin is a ubiquitous clay substance found in soils worldwide and consists of a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra [Al2Si205(0H)4]. 

Consider the battery in Fig. 7.18. The sodium beta alumina barrier allows sodium ions formed at the anode to Row across to the sulfur compartment, where, together with the reduction products of the sulfur, U forms a solution of sodium trisulfide in the sulfur. The latter is held at 300 CC to keep it molten. The sodium beta alumina also acts like an electronic insulator to prevent short circuits, and it is inert toward both sodium and sulfur. The reaction is reversible. At the present state of development, when compared with lead storage cells, batteries of this sort develop twice the power on a volume basis or four times the power on a weight basis. 

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