Processing of Fine Ceramic Powders
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gel amorphous powders. The latter were allowed to settle under the force of gravity into an ordered configuration and were then dried and sintered. Sintering took place at much lower températures and at a faster rate than traditional ceramic processing, e.g., when crystalline powders of broad size distributions were employed. Improvements in strength and toughness of the sintered body were never demonstrated by Bowen's group for thèse novel ceramics. Ordered particles hâve packing faults which lead to domains similar to grains in polycrystalline materials. When sintered, the ordered domains shrink separately and pull away from each other. Consequently, defects in the sintered body occur in a size similar to that of the ordered domains, which can encompass as many as 10,000 particles. Thèse ordered domains lead to weakness in the sintered body according to Griffith's fracture theory. The long-range ordering of particles must be prevented in order to improve the microstructure; random close packing is therefore desired. By analogy with Lindemann's melting rule, such packing can be obtained by using particle size distributions where the standard déviation is greater than 10%. The primary reason for the faster sintering at lower températures observed in Bowen's work is the use of narrowsize distribution sol-gel dispersions. Thèse particles are produced by the aggregation of small, 0.01 /un, particles which are in turn composed of aggregates of yet smaller units as shown in Figure 1. Onoda3 has demonstrated that thèse particles hâve a classic fractal geometry where the fractal dimension, D, is defined as D = 4.0 + In P/lw S P being the packing fraction of each aggregate and S the generation-to-
generation size ratio (i.e., S > 1.0). Since each particle is composed of much smaller subunits, the sintering of sol-gel solids occurs at much lower températures or shorter times than crystalline particles of the same size as the largest agglomerate. Figure 2 shows the faster sintering kinetics4 of sol-gel aggregates as a function of the number of générations of aggregates. This is the same phenomenon as observed with other low density "reactive calcines" produced by aqueous sol-gel methods. To take advantage of the fast sintering kinetics of sol-gel powders with their fractal geometry in the production of ceramic bodies, it is necessary to hâve agglomerate size distributions with standard déviations greater than 10%. This prevents ordered packing in the green body and its deleterious effects on strength. Broad size distributions, however, hâve the disadvantage of large particles being présent, which produce large grains in the sintered ceramic and can also act as Griffith flaws. Suzuki et al.5 hâve shown both theoretically and experimentally that a particle size distribution with a standard déviation between 10 and 30% can be packed to a relatively high green density of 60%. Saks6 demonstrated that thèse higher green densities lead to less shrinkage during faster sintering. Also, broader size distributions lead to higher pore co
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