Alumina microstructure

Microstructured surfaces, as well as micromachined substrates and devices discussed in Sects. II, III, and iy are suitable for a number of applications. They include reflective and absorbing surfaces, wavelength-sensitive filters, multiaperture lens arrays and Fresnel microoptics, field emitter arrays, precision apertures, or molds for microstructured surfaces of other materials. Microstructured alumina ceramics can also be used for tuned broadband infrared emitters. In addition, due to the robustness at high temperatures and well-developed and controlled porosity, the freestanding, heat-treated micromachined anodic alumina substrates can be used for the fabrication of sensors that incorporate a high temperature microheater with low power consumption. 

In this example, a microstructured alumina hollow-fiber membrane with Morphology B (Table 10.1) is used, and its morphology and microstructure after normal sintering is shown in Figure 10.7. Because the actual bore liquid for the membranes of this type is a mixture of internal coagulant (distilled water) and solvent, the polymer precipitation is slower than the exterior surface, leading to a more porous interior surface, which is retained after normal sintering. 

Figure 9.11. Microstructures of porous sintered alumina prepared undoped (right) and when doped with magnesia (left). Optical micrographs, originally 250x (after Burke 1996).

The difficulty of obtaining pure / "-material for the electrolyte has been tackled in many production processes worked out in the past. Unless precautions are taken, sintering of a -alumina-derived / "-alumina compositions invariably results in the duplex microstructure and a low-strength ceramic. Therefore a balance has to be struck between conductivity and strength. The problem arises because the conversion from —alumina to / " -alumina is slow... 

The results of development work on processes indicate that the two main methods of preventing the duplex microstructure from forming appear to be fast-firing, or increasing the amount of / "-alumina at low temperatures. Based on these results, Duncan et al. [20] and Zyl et al. [21] have described production processes starting from aluminum oxy-hydroxides or aluminum hydroxides as precursors for the synthesis of the solid electrolyte "-alumina. Duncan et al. described an alumina precursor which substitutes in part or wholly for or-alumina in an established... 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

A Pt catalyst was applied by dry and wet techniques. By means of sputtering using a mask process protecting parts of the microstructure, the micro channel bottom was coated selectively. In addition, an y-alumina layer was applied by the sol-gel technique. Initially, the whole micro structure was covered by such a layer. Then, photoresist was applied and patterned so that only the channel part remained covered. After removal of the exposed photoresist and unprotected y-alumina, only the channel bottom was coated with y-alumina. 

Leenars, A.F.M., K. Keizer, and A.J. Burggraaf, The preparation and characterization of alumina membranes with ultra-fine pores, Part 1. Microstructural investigations on non-supported membranes, /. Mater Sci., 19, 1077-1088, 1984. 

Table 90 Performance data of microstructured Pt/ceria-alumina catalysts versus powdered...

The phase structure of glasses has a significant effect on their physical properties, which is discussed below with reference to chemical durability. The magnitude of the phase separation can be altered by heat treatment, and enhanced or reduced by the addition of various oxides to the melt. In particular, the addition of alumina to commercial soda-lime-silica glasses reduces the tendency to phase separation, improving chemical resistance (Doremus, 1973). A detailed study of the microstructure of soda-lime-silica glasses has been published by Burnett and Douglas (1970). The control of phase separation in the melt is now commercially important for processes such as the... 

M. -Trung Tran, N. S. Gnep, G. Szabo, and M. Guisnet, Influence of the calcination temperature on the acidic and catalytic properties of sulphated zirconia, Appl. Catal. A 171, 207-217 (1998). P. Canton, R. Olindo, F. Pinna, G. Strukul, P. Rieflo, M. Meneghetti, G. Cerrato, C. Morterra, and A. Benedetti, Alumina-promoted sulfated zirconia system Structure and microstructure characterization, Chem. Mater. 13, 1634-1641 (2001). 

Table 2.4. Microstructural Characteristics of Alumina Membranes as a Function of Calcination Time and Temperature (Leenaars et al. 1984, Burggraaf et al. 1989,...

One step closer to up-scaling to industrial environments is the multiple-bead reactor shown in Fig. 4.9. Here pellet-type catalyst carriers, so-called beads, are positioned in square containers. The beads are made of alumina and are 1 mm in diameter. Gases are passed over these beads through microstructured pore membranes in the cover and the base plate of the containers. 

Numerical simulations have been conveniently used to describe complex fluid dynamic behavior in microstructures [21, 86]. Van der Linde et al. [87] solved the coupled diffusion equation for reacting species and compared the results with data from the oxidation of CO on alumina-supported Cr using the step-response method. Transient periodical concentration changes in microchannels have been numerically calculated by various authors [19, 58, 88]. 

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