Alumina particles

Figure C2.11.3. A scanning electron micrograph of tire spherical alumina granules produced by spray drying a ceramic slurry. The granules are comprised of individual alumina particles, sintering additives, and an organic binder.

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). 

An important appHcation of MMCs in the automotive area is in diesel piston crowns (53). This appHcation involves incorporation of short fibers of alumina or alumina—siHca in the crown of the piston. The conventional diesel engine piston has an Al—Si casting alloy with a crown made of a nickel cast iron. The replacement of the nickel cast iron by aluminum matrix composite results in a lighter, more abrasion resistant, and cheaper product. Another appHcation in the automotive sector involves the use of carbon fiber and alumina particles in an aluminum matrix for use as cylinder liners in the Prelude model of Honda Motor Co. 

The hot (400—500°C) inlet gases warm the alumina particles and the mildly exothermic reaction serves to maintain the heat of the furnace between 500 and 600°C. The alumina particle size is critical for maintaining a good reaction rate and a fluidized bed. 

FIGURE 3 Scanning electron micrograph (1200x magnification) of the surface of a porous alumina particle coated with poly(diphenoxy-phosphazene). Surface nitration, reduction, and glutaric dialdehyde coupling immobilized enzyme molecules to the surface. (From Ref. 23.)... 

As illustrated in Figure 10.6, the high para-selectivity in the toluene disproportionation is caused by the selective removal of p-xylene from the silica-alumina particles, which leads to an apparent equilibrium shift between the xylene isomers. 

The catalyst was prepared by impregnating porous alumina particles with a solution of nickel and lanthanum nitrates. The metal loading was 20 w1% for nickel and 10 wt% for lanthanum oxide. The catalyst particles were A group particles [8], whereas they were not classified as the AA oup [9]. The average particle diameter was 120 pm, and the bed density was 1.09 kg m . The minimum fluidization velocity was 9.6 mm s. The settled bed height was around 400 mm. The superficial gas velocity was 40-60 mm s. The reaction rate was controlled by changing the reaction temperature. 

GP 9] [R 16] The reaction rate and activation energy of metal catalysts (Rh, Pt or Pd) supported on alumina particles ( 3 mg 53-71 pm) were determined for conversions of 10% or less at steady state (1% carbon monoxide 1% oxygen, balance helium 20-60 seem up to 260 °C) [7, 78]. The catalyst particles were inserted into a meso-channel as a mini fixed bed, fed by a bifurcation cascade of micro-channels. For 0.3% Pd/Al203 (35% dispersion), TOF (about 0.5-5 molecules per site... 

The new process avoids these problems by gas-phase treatment using a fluidized bed of alumina particles (Figure 1.2). A mixture of air, ammonia, nitrogen, natural gas, pg (liquefied petroleum gas), and other gases are used as the fluidizing gas to carry out the heat treatment. The bed is heated by electricity or gas and quenching is also carried out in a fluidized bed.14... 

To better address the role of Pt in the reduction mechanism, a physical mixture of the binary Pt/y-Al203 and BaAy-A O, samples was also prepared and tested. This catalytic system is constituted by all the components of the reference ternary system, even if in the physical mixture Pt and Ba are deposited on different alumina particles [44],... 

Third, a poly[bis(phenoxy)phosphazene] has been coated on porous alumina particles, surface nitrated, reduced to the amino-derivative, and then coupled to the enzyme glucose-6-phosphate dehydrogenase or trypsin by means of glutaric dialdehyde. The immobilized enzymes were more stable than their counterparts in solution, and they could be used in continuous flow enzyme reactor equipment (25). 

This paper reviews the recent experimental results and mathematical modeling from the literature on electrocodeposition of particles in a metallic matrix. Although many different particle/metal systems are discussed, the electrocodeposition of alumina particles in copper matrix is highlighted as this widely-studied system illustrates the various effects of process variables and it exemplifies the types of contradictions concerning these results in the literature. 

From RCE experiments at 1500 rpm and 158 g/1 loading of particles in suspension, 4.4 wt% of gamma alumina particle was incorporated, which is more than three times the previously reported maximum amount of gamma alumina incorporation of 1.45 wt% [37], These results will be discussed further in terms of the process variables. 

Fig. 2. Influence of the RDE rotational speed on the codeposition of alumina particles in gold (adapted from ref. 30).

Perhaps even more noteworthy is the effect of crystallographic phase. While one phase of a specific composition may readily incorporate from a particular bath composition, another phase of the same composition may incorporate to a much lower extent or not at all. For instance, in the alumina particle system, the alpha phase has been found to readily incorporate from an acidic copper bath while the gamma phase incorporates at less than one tenth the amount of alpha, if at all, as shown in Table 1 [2, 11, 27, 31, 33],... 

Upon dissolving Al into liquid Ga, the alumina layer that instantly forms from exposure to air or water at the surface is either discontinuous or porous. In either case the surface of the Ga-Al liquid is not passivated. As a result, when water contacts the surface of the liquid, Al atoms at the surface split the water, liberating hydrogen and heat with the formation of alumina. Since the liquid is fluid, the alumina cannot form a bonded layer at the liquid surface that would passivate pure, solid Al. Instead, the alumina is swept away by convection or agitation as a suspension of alumina particles in the water. The surface of the liquid alloy is now depleted of Al. This depleted region at the surface is replenished via diffusion or convection of Al from the bulk to the surface where it continues to split water. This process continues until all of the Al in the liquid alloy is converted to alumina. To summarize, liquid Al-Ga alloys rich in Ga split water because the Al component is not passivated as it is in solid pure Al. 

S. Desset, O. Spalla, and B. Cabane, Redispersion of alumina particles in water, Langmuir 16, 10495-10508 (2000). 

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