Alumina compaction

Fig. 6.8 Positions of measuring temperature for the punch-die-titanium and punch-die-alumina compacts in the system, with the values within the parenthesis for those in the punch-die-alumina compact and unit in mm. Reproduced with permission from [31]. Copyright 2003, Elsevier...

Rgure 5.4 Green density of a 0.35- im median particle size alumina compact as a function of uniaxial die pressing pressure. At comparable pressing pressures, higher green densities are achieved with 2 vol % of 20 M PEG wax binder as compared with Rhoplex B60-A acrylic wax emulsion. Green density increases proportionally with the log of the applied pressure for both systems. 

Experimental work is reported on the rate of sintering of alumina compacts sintered in a microwave-induced plasma. It is shown that the sintering rate is very much greater for plasma-sintered specimens than for specimens sintered by conventional means. Evidence indicates the generation of plasma inside the pores of the compact, and the increased rate of densification is related to an increased rate of diffusion. 

Examples are provided by the work of Carman and Raal with CF2CI2 on silica powder, of Zwietering" with nitrogen on silica spherules and of Kiselev" with hexane on carbon black and more recently of Gregg and Langford with nitrogen on alumina spherules compacted at a series of pressures. In all cases, a well defined Type II isotherm obtained with the loose powder became an equally well defined Type IV isotherm with the compact moreover both branches of the hysteresis loop were situated (drove the isotherm for the uncompacted powder, but the pre-hysteresis region was scarcely affected (cf. Fig. 3.4). The results of all these and similar... 

Fig. 3.4 Compaction of alumina powder. Isotherms of nitrogen at 77 K, on (A) the uncompacted powder, and (B) on the powder compacted at a pressure of 1480 GN (96 ton in" ). Open symbols, adsorption solid symbols, desorption.

Fig. 3.28 The Kiselev method for calculation of specific surface from the Type IV isotherm of a compact of alumina powder prepared at 64 ton in". (a) Plot of log, (p7p) against n (showing the upper (n,) and lower (n,) limits of the hysteresis loop) for (i) the desorption branch, and (ii) the adsorption branch of the loop. Values of. 4(des) and /4(ads) are obtained from the area under curves (i) or (ii) respectively, between the limits II, and n,. (6) The relevant part of the isotherm.

The discrepancy between the pore area or the core area on the one hand and the BET area on the other is proportionately larger with silica than with alumina, particularly at the higher degrees of compaction. The fact that silica is a softer material than alumina, and the marked reduction In the BET area of the compact as compared with that of the loose material, indicates a considerable distortion of the particles, with consequent departure of the pore shape from the ideal of interstices between spheres. The factor R for cylinders (p. 171), used in the conversion to pore area in the absence of a better alternative, is therefore at best a crude approximation. 

The oxidation of most modem alloys is dependent on the formation of a compact protective film of a slow growing chemically stable oxide such as chromium (ITT) oxide [1308-38-9], alumina [1344-28-1], AI2O2, or siUca [7631-86-9], Si02. These oxides grow much more slowly than do the oxides... 

Silica and Alumina. The manufacture of Pordand cement is predicated on the reaction of lime with siUca and alumina to form tricalcium sihcate [12168-85-3] and aluminate. However, under certain ambient conditions of compaction with sustained optimum moisture content, lime reacts very slowly to form complex mono- and dicalcium siUcates, ie, cementitious compounds (9,10). If such a moist, compact mixture of lime and siUca is subjected to steam and pressure in an autoclave, the lime—silica reaction is greatiy accelerated, and when sand and aggregate is added, materials of concrete-like hardness are produced. Limestone does not react with siUca and alumina under any circumstances, unless it is first calcined to lime, as in the case of hydrauhc lime or cement manufacture. 

Infiltration (67) provides a unique means of fabricating ceramic composites. A ceramic compact is partially sintered to produce a porous body that is subsequently infiltrated with a low viscosity ceramic precursor solution. Advanced ceramic matrix composites such as alumina dispersed in zirconia [1314-23-4] Zr02, can be fabricated using this technique. Complete infiltration produces a homogeneous composite partial infiltration produces a surface modified ceramic composite. 

The compact, nonporous anodic alumina film is the most suitable for fundamental investigations. It is grown by anodization, mostly under constant-current (galvanostatic) conditions, in neutral solutions of borates, tartrates, citrates, and phosphates, all of which possess significant buffering capacity and hence do not allow significant dissolution of the oxide. 

U size into rats produced reticulin nodules that later developed into areas of dense collagenous fibrosis." The latter alumina by the same route in mice and guinea pigs caused development of a reticulin network with occasional collagen, whereas in rabbits only a slight reticulin network was observed. Intratracheal administration of another form of alumina in rats, corundum of particle size less than Ip, caused the development of compact nodules of reticulin. 

Figure 7.26 Compaction behavior of KBr powder and spray-dried granules of alumina and clay tile as a function of punch pressure. From J. S. Reed, Principles of Ceramics Processing, 2nd ed. Copyright 1995 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Refer to Figure 7.26. (a) Calculate the compact ratio for the granulated tile composition, (b) Estimate the PF of the alumina granules in the compact. Assume that the granules pack with PF = 0.50 and the particle density is 3.98 Mg/m ... 

When alumina is combined with the silica, forming a natural clay, a much more compact and fusible compound is formed with the lime than when the silica is alone. Indeed, it has been observed as a general principle, that tire point of fusion is materially affected by the relation and number of bases the whole materials contain thus, a more liquid scoria is obtsined by the addition of a limestone containing magnesia than with a pure limestone. But experience is against the use of a magnesieu limestone, because it deteriorates the iron produced, while the purity of the metal iB the primary consideration. That which contains much silica should also be used sparingly, as silica combines with the iron and injures its quality, -The purest limestones are the most suitable for flux. Common marble is nearly a pnre carbonate of lime but is too rare and expensive to be used as a flux. 

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