High alumina coatings

A higher density sol—gel abrasive, produced by the introduction of seed crystaUites formed by wet-milling with high alumina media or by introduction of submicrometer a-alumina particles, was patented (28) and designated Norton SG. The microstmcture of this abrasive consists of submicrometer a-alumina crystals (Fig. 1) and its bulk density approaches that of fused alumina. Norton SG has proven to be an exceptional performer in coated and bonded abrasive products it was awarded the 1989 ASM Engineering Materials Achievement Award (29). 

Subsequent reaction of the monosulfate with gypsum produces acicular crystals of ettringite. Monosulfate apparently does not contribute to expansion, whereas the formation of ettringite involves expansion. A recently issued patent [75] covers the use of prehydrated high-alumina cement (H-HAC), lime and gypsum mixtures. Particle type, size, thickness of protective coating, and presence of moisture determine the rate and extent of expansion [Fig. 6.11]. 

M.R. Hendrick, The Effects of Combustion CVD-Applied Alumina Coatings on the High Temperature Oxidation of a Ni-Cr Alloy, Thesis, Georgia Institute of Technology, Atlanta, 1996. 

Zeolite monoliths have been useful for such apphcations as rotatory adsorbers for use as dehumidifiers and desiccant cooling processes [253] or in VOC treatment systems [269]. Alumina-coated sUicon carbide monoliths have also been employed as supports for B-ZSM-5 membranes [270] providing a larger surface area per unit volume, compared to traditional membrane supports. With these membranes, these authors have reported n/f-butane and H2/f-butane separation selectivities of 35 and 77, respectively [85]. Also, silicalite-1 membranes supported on stainless steel grids (Figure 10.29) have shown a good performance in the separation of n/f-butane mixtures, with separation factors as high as 53 at 63°C [255]. 

Ellis, H. Fire barrier coating composition containing magnesium oxychlorides and high alumina calcium aluminate cements or magnesium oxysulphate, US Patent 4,572,862. 

A second catalyst support type is commonly called a monolith which is a thin walled multi channeled honeycomb. The ceramic walls between the channels are the base support surfaces for the catalyst. (FIGURE 3) Although they are porous, they are not the direct surface for the precious metal. An intermediate alumina coating called "washcoat provides an ultra high surface for the catalyst sights (FIGURE 4 is an illustration of the washcoat precious metal relationship). The catalyzed monolith is likewise assembled into the metal container using a compressible interface material. 

Industrial membrane separators must have high surface area (SA) to volume (V) ratios. Conventional flat and tubular configurations have SA/V ratios of 30 and 250 m2 m-3, respectively [4, 216]. Therefore, membranes are being developed that address the requirement of assembly into compact, high-throughput modules. Spin-extruded ceramic hoUow fibers (diameter < 1 mm) consisting of a-alumina coated with y-alumina were coated with 2 to 3 gm thick layers of palladium [184]. 

The results shown in Table 2 indicate that the relative amounts of the P and P" phases are relatively constant during the exposures at both sodium concentrations, and at 1130 and 1205°C. This indicates that the p/P composition is relatively stable under the conditions tested. However, the apparent spalling observed on the high-sodium, high-temperature a-alumina sample indicates that a p/p"-alumina coating may still fail. A serious concern is that the rapid diffusion of sodium through a p/P"-alumina coating will lead to a subsurface corrosion problem. 

Various DHBCs have successfully been used as a stabilizer for the purely steric or combined steric/electrosteric stabilization of ceramic precursor particles like AI2O3 [308,309], BaTiOs [310-314] colloidal silica [315-320] alumina coated Ti02 [321], Ce02 [322], o -Fe203 (hematite) [281,323-325], and various pigments [327] in aqueous solution. The solid amoimt can be stabilized to extremely high concentrations (up to 80 wt % [327]) showing the stabilization capabilities of optimized DHBCs. 

Lahiri 1 et al (2011) Ultrathin alumina-coated carbon nanotubes as an anode for high capacity Li-ion batteries. J Mater Chem 21 13621-13626... 

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