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Chap 13 Solns-6E


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CHAPTER 13 APPLICATIONS AND PROCESSING OF CERAMICS PROBLEM SOLUTIONS 13.1 The two desirable characteristics of glasses are optical transparency and ease of fabrication. 13.2 (a) Devitrification is the process whereby a glass material is caused to transform to a crystalline solid, usually by a heat treatment. (b) Two properties that may be improved by devitrification are 1) a lower coefficient of thermal expansion, and 2) a higher thermal conductivity. Two properties that may be impaired are 1) a loss of optical transparency, and 2) a lowering of mechanical strength when stresses are introduced from volume changes that attend the transformation. In some cases, however, strength may actually be improved. 13.3 Glass-ceramics may not be transparent because they are polycrystalline. Light will be scattered at grain boundaries in polycrystalline materials if the index of refraction is anisotropic, and when those grains adjacent to the boundary have different crystallographic orientations. This phenomenon is discussed in Section 21.10. 13.4 For refractory ceramic materials, three characteristics that improve with increasing porosity are decreased thermal expansion and contraction upon thermal cycling, improved thermal insulation, and resistance to thermal shock. Two characteristics that are adversely affected are load-bearing capacity and resistance to attack by corrosive materials. 13.5 (a) From Figure 12.25, the maximum temperature without a liquid phase corresponds to the temperature at the MgO(ss)-[MgO(ss) + Liquid] boundary at this composition, which is approximately 2240 ° C (4060 ° F). (b) This maximum temperature lies at the phase boundary between MgAl 2 O 4 (ss)-(MgAl 2 O 4 + Liquid) phase fields (just slightly to the right of the congruent melting point at which the two phase boundaries become tangent); this temperature is approximately 2090 ° C (3790 ° F). 98
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13.6 For each section of this problem two SiO 2 -Al 2 O 3 compositions are given; we are to decide, on the basis of the SiO 2 -Al 2 O 3 phase diagram (Figure 12.27), which is the more desirable as a refractory and then justify the choice. (a) The 99.8 wt% SiO 2 -0.2 wt% Al 2 O 3 will be more desirable because the liquidus temperature will be greater for this composition; therefore, at any temperature within the cristobalite + liquid region on the phase diagram, there will be a lower fraction of the liquid phase present than for the 99.0 wt% SiO 2 -1.0 wt% Al 2 O 3 composition, and, thus, the mechanical integrity will be greater. (b) The 74 wt% Al 2 O 3 -26 wt% SiO 2 composition will be more desirable because, for this composition, a liquid phase does not form until about 1750 ° C [i.e., the temperature at which a vertical line at 74 wt% Al 2 O 3 crosses the boundary between the mullite and (mullite + liquid) phase regions]; for the 70 wt% Al 2 O 3 -30 wt% SiO 2 material, a liquid phase forms at a much lower temperature-- 1587 ° C.
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