Dense alumina

Dense alumina AI2O3 Cutting taals, dies wear-resistant surfaces, bearings ... 

High -performance engineering ceramics Diamond Dense alumina Silicon carbide Silicon nitride Zirconia Sialons... 

Frit may be milled dry or wet. The long established dry process is used for cast iron baths and for chemical plant. Vitreous enamel application by a dry electrostatic method is being used on an increasing scale. In these cases, the frit is milled alone, or with inorganic colouring or refractory additives. This is achieved in cylinders using balls of porcelain, steatite or more dense alumina, or with pebbles of flint, to produce a fine powder of predetermined size. 

Roeder et al. [14] used dilute mixtures (8 vol-% solids) for the preparation of dense alumina composites with CeC>2-Zr02 and AhCb-platelets. As main alumina powder (the matrix), AKP-30 was used with an average particle diameter of 0.4 pm and a surface area of 6.5 m /g. The resulting specimens were sintered at 1600°C. No further densification behaviour was mentioned. 

Until now, flat silica membranes were only tested at relatively low temperatures (up to 300°C) because of limitations in thermal stability of the polymer sealing rings [2], However, with the use of dense alumina rings together with carbon sealing it is possible now to measure membrane properties (e.g. permeance) at much higher temperatures (up to 600°C), which will be described below. 

Eigure 31 shows a schematic diagram of a lab-made PEVD reactor for depositing yttria-doped zirconia. The sample tube was placed at the center of a large fully dense alumina reactor tube in an electric furnace. Thus, the sample tube separated the reactor tube into two chambers the air chamber inside the sample tube and the chloride chamber between the sample and reactor tubes. Both chambers were evacuated separately. 

The Youngs moduli of the support has been measured with four point bending tests, Poisson s ratio is taken to be equal to that of dense alumina. Values of Eg and Vg are 65 4 GPa and 0.23 respectively. To obtain measurable values of 5, dg is taken about 0.4 mm, while df is in the range of 1-10 pm. The values of 5 range from 0.05-0.15 mm. 

Dense alumina can also be made by melting, but the high-melting temperature of 2,054°C makes this process expensive and difficult to control. High-value materials such as gem stones and laser hosts can be made by adding various colorants such as chromium, titanium, iron, cobalt, and vanadium to the melt. 

FIGURE 39.16 Flexural strength of dense alumina rods after aging under stress in Ringer s solution. Lot 1 and 2 are from different batches of production. (From Krainess F.E. and Knapp W.J. 1978.. Biomed. Mater. Res. 12 245. With... 

D. C. C. Lam, F. F. Lange and A. G. Evans, Mechanical properties of partially dense alumina produced from powder compacts, / Am. Ceram. Soc., 77(1994)2113-17. 

RBAO involves a similar production route, but in this case the aluminum metal powder is a minor component in a powder mix that contains a substantial fraction of alumina. Oxidation of the aluminum metal by gas percolation yields a metastable oxide phase, which is replaced at higher temperatures by the stable corundum phase. Again, the large specific volume of the oxide product reduces the volume fraction of porosity, but in this case the thermal cycle actually sinters the product, and for the optimized powder formulation, expansion during oxidation of the metal at a moderate temperature is followed by the sintering contraction at a higher temperature to yield a dense alumina product whose final shape and volume is still very close to that of the green powder preform. 

Figure 16.14 Schematic drawing of the reactor set-up and an incorporated BCFZ hollow fiber for OCM. At both ends the 30 long fiber was coated with gold to obtain a 3 cm long gold-free isothermal oxygen permeation zone. The active surface area for the oxygen permeating BCFZ hollow fiber is 0.78 cm. The OCM catalyst was dipersed between the outer dense alumina tube and the fiber, which was inserted into a porous alumina tube.

Figure 5.1 SEM micrograph of dense alumina, etched in boiling H3PO4 for 6 minutes to show the microstructure.

One of the first studies on SWCNT/ceramic composites was reported by Zhan et al. [39] who used the SPS technique to produce fully dense alumina composites containing 10 vol% SWCNTs. By using the SPS method, they achieved almost 100% relative density as well as incorporation of undamaged CNTs at the alumina grain boundaries, resulting in an impressive improvement of the fracture toughness as discussed in the next paragraph. 

Although beryllia usually is selected for a desirable combination of properties, key to most applications is the material s comparatively high thermal conductivity. Even at the highest temperatures, its thermal conductivity is four times that of dense alumina and from room temperature to 5(X)°C, seven to eight times greater. BeO s thermal conductivity is quite dependent on purity. For example, increasing purity from 99% to 99.8% results in a 10-15% rise in conductivity. Physical Properties... 

Certain requirements concerning mechanical performance and rehabihty must be fulfilled in all apphcations, e.g. thermally or electrically insulating materials (dense alumina for electronic circuit supports, thermally insulating closed-pore alumina or zirconia), catalyst supports, filter membranes and membrane supports (open-pore alumina), ionic conductors, oxygen sensors (dense zirconia) and oxygen-ion-conducting membranes in sohd oxide fuel cells, oxygen separators and catalytic membrane reactors (open-pore zirconia) [Atkinson ... 

This is possible because the elastieitj standard relations must hold for all isotropic continua, whether heterogeneous on the mieroscale or not [Torquato 2002]. It will be shown below, that in the case of the dense alumina-zirconia composite ceramics (Poisson ratios 0.23 and 0.31 for alumina and zirconia, respectively) the deviation of the Voigt bound Ey... 

Effective Elastic Moduli of Dense Alumina-Zirconia Composite Ceramics... 

Figure 3. Effective tensile modulus of dense alumina-zirconia composite ceramics Voigt bound (crosses slightly above the upper solid line calculated from the Voigt values of the effective shear and bulk moduli), approximate Voigt bound (according to the mixture rule, upper solid line), Reuss boimd (results of both calculations identical, crosses and lower solid curve), upper and lower Hashin-Shtrikman bounds (dashed curves) and values measured by the resonant frequency method for dense (porosity < 3 %) alumina-zirconia composite ceramics prepared by slip casting.

All the above formulae allow the direct and easy calculation of the effective elastic moduli and Poisson ratios of dense alumina-zirconia composites for a given zirconia volume fraction (f). Since usually the composition of alumina-zirconia composites (in the sequel abbreviated as AZ composites ) is given in weight percent (weight fractions ) the volume fraetions usually to be ealculated via the (theoretical) density. In the... 

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