Alumina particle sizes

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. 

Of particular interest was the fact that the bimetallic cluster catalysts, i.e., RuNiY and RuCuY, had considerably better metal dispersions than the pure NiY and CuY catalysts. Further, the zeolite-supported bimetallic catalysts were more resistant to sintering during methanation than those supported on alumina. Particle-size measurement indicated, however, that most of the bimetallic clusters were too large to be located inside the zeolite pores. 

The impact of particle size also follows similar principles in metal CMP. Lu and coworkers [87] confirmed the contact-area mechanism for material removal in copper CMP with silica abrasives. It was observed that the material removal rate increased with the increase in the specific surface area of the abrasives in the slurry. Bielman and coworkers [88] conducted a similar investigation and found that the removal rate of tungsten decreased with the increase in alumina particle size from 0.29 to 2 pm at 4.5% slurry concentration. 

Table I. MSA-Coated Submicrometer a-Alumina Particle-Size Analysis...

The hydrogenolysis of methylcyclopentane has also been studied by Kramer and Zuegg (296). The catalysts were prepared by the evaporation of Pt and its condensation onto amorphous alumina. Particle sizes (2.5 nm) were measured by electron microscopy. In accord with Fig. 17, the nonselective production of hexane was enhanced for small particles. When additional A1203 was then deposited on top of the Pt particles, more hexane was favored. Since the Pt crystallite size was not altered, a mechanism involving catalysis by the phase boundary, Pt-Al203, was proposed (296). However, the decoration of the Pt surface by A1203 could... 

In a second catalyst series, the samples were prepared starting from different alumina particle sizes. The support was groimd and sieved to obtain particle sizes with the following distribution < 20 pm, 20-70 pm and > 70 pm. These particles were subsequently impregnated with a solution containing a Co/Co+Fe atomic ratio of 0.35 and a total metal loading of about 3 wt%. 

D. Y. Lee, D.-J. Kim, B.-Y. Kim and Y.-S. Song, Effect of Alumina Particle Size and Distribution on Infiltration Rate and F racture Tou ness of Alumina-Glass Composites Prepared by Melt Infiltration, Mat. Sci. Eng. A341, 98-105 (2003). 

Further limitations to Lange s model are revealed when the effect of filler particle size on G (c) is considered. Lange himself [53] measured GJc) against Vf for three series of sodium borosilicate glass/alumina composites in which the average alumina particle size was 3.5 Xm, 11 pm and 44 pm, respectively. For any given value of Vf, GJc) increased... 

The sol—gel technique has been used mosdy to prepare alumina membranes. Figure 18 shows a cross section of a composite alumina membrane made by sHp coating successive sols with different particle sizes onto a porous ceramic support. SiUca or titanium membranes could also be made by the same principles. Unsupported titanium dioxide membranes with pore sizes of 5 nm or less have been made by the sol—gel process (57). 

Catalytic properties are dependent on physical form, principally the exposed surface area which is a function of particle size. Industrial PGM catalysts are in the form of finely divided powder, wine, or gauze, or supported on substrates such as carbon or alumina (see Catalysis Catalysts, supported). 

The large majority of activated alumina products are derived from activation of aluminum hydroxide, rehydrated alumina, or pseudoboehmite gel. Other commerical methods to produce specialty activated aluminas are roasting of aluminum chloride [7446-70-0], AIQ calcination of precursors such as ammonium alum [7784-25-0], AlH2NOgS2. Processing is tailored to optimize one or more of the product properties such as surface area, purity, pore size distribution, particle size, shape, or strength. 

Specialty Aluminas. Process control (qv) teclmiques permit production of calcined specialty aluminas ha nng controlled median particle sizes differentiated by about 0.5 ]lm. Tliis broad selection enables closer shrinkage control of high tech ceramic parts. Production of pure 99.99% -AI2O2 powder from alkoxide precursors (see Alkoxides, metal), apparently in spherical form, offers the potential of satisfying the most advanced appUcations for calcined aluminas requiring tolerances of 0.1% shrinkage. 

Fig. 3. Sedigraph particle size distribution for superground submicrometer alumina, (a) Partially dispersed (b) fully dispersed.

Refractories. Calcined alumina is used in the bond matrix to improve the refractoriness, high temperature strength/creep resistance, and abrasion/corrosion resistance of refractories (1,2,4,7). The normal, coarse (2 to 5 )J.m median) crystalline, nominally 100% a-Al202, calcined aluminas ground to 95% —325 mesh mesh are used to extend the particle size distribution of refractory mixes, for alumina enrichment, and for reaction with... 

An abrasive is usually chemically inert, neither interacting with other dentifrice ingredients nor dissolving in the paste or the mouth. Substances used as dentifrice abrasives include amorphous hydrated silica, dicalcium phosphate dihydrate [7789-77-7] anhydrous dicalcium phosphate [7757-93-9] insoluble sodium metaphosphate [10361-03-2], calcium pyrophosphate [35405-51-7], a-alumina trihydrate, and calcium carbonate [471-34-1]. These materials are usually synthesized to specifications for purity, particle size, and other characteristics naturally occurring minerals are used infrequently. Sodium bicarbonate [144-55-8] and sodium chloride [7647-14-5] have also been employed as dentifrice abrasives. 

The surface area of fine powders is enormous. A cupful of alumina powder with a particle size of 1 jUm has a surface area of about 10 m. If the surface energy of alumina is 1 J the surface energy of the cupful of powder is 1 kj. 

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

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