Alumina based composites

Dogan, C.P. and Hawk, J.A. Influence of whisker reinforcement on the abrasive wear behavior of silicon nitride- and alumina-based composites , Wear, 203-204 (1997) 267-277. 

Sekino, T., Niihara, K., Fabrication and mechanical properties of fine-tungsten-dispersed alumina-based composites, J. Mater. Sci., 1997, 32(15) 3943. 

At the DLR Institute of Materials Research an all-oxide CMC (WHIPOX = wound highly porous oxide) consisting of alumina fibres Nextel 610 or mullite based fibre (Nextel 720, both 3M) and an alumina or mullite matrix, respectively, has been developed in recent years [1,2]. Mullite-based CMCs typically offer higher creep stability than alumina-based composites but display lower thermal conductivity. Lower thermal stability of alumina-based materials, however, can be accepted, since service temperature is significantly lower for these materials as a result of the better cooling efficiency. Therefore material development was focused on alumina... 

Alumina-based composites These comprise especially zirconia-toughened alumina (ZTA), and alumina-based nanocomposites with non-oxide second phases such as SiCorTiC. However, thelattertwo, in particular, are b ond the scopeofthis chapter. 

The modulus or stiffness of a fiber reflects its structural or internal order [3]. Figure 5 compares the fiber modulus of silica-alumina and calcia-alumina based composition ranging from 100% silica (or calcia) to 100% alumina. The fiber modulus increases with increasing structural order, I.e., from 41 to 125 GPa for amorphous glass fibers to 125 to 250 GPa for nanocrystalline glass fibers, 250 to 400 GPa for polycrystalline ceramic fibers, and 405 to 410... 

Silica and, to a less extent, alumina aerogels are detailed below because most of the catalytic aerogels are silica- or alumina-based composites. As mentioned in the Abstract section, the synthesis of an aerogel results from a two-step process First, a solution-to-sol-to-gel is prepared, and second, this gel is supercritically dried in an autoclave. 

T. Sekino and K. Niihara, Fabrication and Mechanical Properties of Fine Tungsten Dispersed Alumina Based Composites, J. Mater. Sci., 32, 3943 49 (1997). 

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally platinum or platinum—rhenium on an alumina base. The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions. Nonplatinum catalysts are used in regenerative processes for feedstocks containing sulfur, although pretreatment (hydrodesulfurization) may permit platinum catalysts to be employed. 

Nickel—beryllium casting alloys are readily air melted, in electric or induction furnaces. Melt surface protection is suppHed by a blanket of argon gas or an alumina-base slag cover. Furnace linings or cmcibles of magnesia are preferred, with zirconium siUcate or mullite also adequate. Sand, investment, ceramic, and permanent mold materials are appropriate for these alloys. Beryllium ia the composition is an effective deoxidizer and scavenger of sulfur and nitrogen. 

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

Glass in the form of fibres is relatively inexpensive and is the principal form of reinforcement used in plastics. The fibres are produced by drawing off continuous strands of glass from an orifice in the base of an electrically heated platinum crucible which contains the molten glass. The earliest successful glass reinforcement had a calcium-alumina borosilicate composition developed... 

For the methanation reaction in the process of converting coal to a high Btu gas, various catalyst compositions were evaluated in order to determine the optimum type catalyst. From this study, a series of catalysts were developed for studying the effect of nickel content on catalyst activity. This series included both silica- and alumina-based catalysts, and the nickel content was varied (Table I). 

An alumina-based catalyst will be bound, for the purpose of mechanical strength, with carbon. The alumina-carbon mixture is essentially a composite support for adsorbing the Pt precursor. If it is desired that all metal go onto the alumina phase, which type of carbon (oxidized or unoxidized) and what type of Pt complex should be used and why A sketch of the surface potential vs. pH for alumina and the carbon binder will help. 

The next series of experiments were run varying the base composition (Table VI). With only a slight excess of base, the yield of byproduct ether was much lower using potassium methoxide, benzyl bromide, and 1 atm CO. The alkoxide on alumina reagents again gave higher conversions and selectivities to ester. 

Although detectable concentrations for several elements could be found after fusion, it is felt that the volatility of mercury and possibly lead and tin would make their determination by lithium tetraborate fusion questionable. Table I shows the elements selected for analysis and the accuracy and precision data for the standards used to check the fusion method. Each standard in Table I was of known composition and siliceous in nature. The standards were separately prepared 10 times so that a statistical evaluation of the results could be made. The standards used were USGS Standards G-2, W-l, BCR-1, commercially prepared silica-alumina based standards, and unfused synthetic standards prepared by the Coal Research Bureau (9, 10, 11, 12). The synthetic standards were used because no commercially prepared standard having... 

Chan, H., An, L., Padture, N. Lawn, B.R. (1996) Damage resistant alumina-based layer composites. J. Mater Res. 11, 204-210. 

Singh, M. Low, I.M. (2002b) Layered and graded alumina-based hybrid composites by infiltration processing. Key Engineering Materials 224-226, 493 4-98. 

SEM image of the fracture surface of 5.7 vol% SWCNT-Fe-Al203 composite densified by spark plasma sintering (SPS) of a mixture of nanometric alumina and ropes of SWCNTs. Reprint from Nature Materials, No. 2, 2002, pp. 38-42, Zhan G.-D., Kuntz J.D., Wan J. and Mukherjee A.K., Single-wall carbon nanotubes as attractive toughening agents in alumina-based nanocomposites, with the permission of Nature Materials and of the authors (http // www.nature.com/nmat/index.html). 

Goeuriot-Launay, D., Brayet, G., and Thevenot, F., Boron nitride effect on the thermal shock resistance of an alumina based ceramic composite, J. Mater. Sci. Lett., 5 940-942 (1986). 

Chemical composition silica-based and silica-alumina-based materials, chrome, magnesia, chrome-magnesia, spinel, SiC, materials containing carbon (more than 1% carbon or graphite), and special materials (containing other oxides or materials such as zircon, zirconia, Si3N4, etc.)... 

Figure 5. Pore size distributions of alumina for compositions with base to alkoxide ratios (a) 0.3 (b) 0.8(c) 1.1...

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