Continuous Production of Narrow Size Distribution Sol-Gel Ceramic Powders

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Abstract
In the early 1980s Prof. Bowen at Massachusetts Institute of Technology advanced the concept that sintered ceramic bodies could be improved by decreasing the defects in green bodies. In addition, his group advanced the idea that ideal green bodies should be composed of monodisperse particles packed into an ordered array. They used the hydrolysis of metal alkoxides to produce narrow size distribution sol-gel powders (i.e., amorphous). These powders were allowed to settle under the force of gravity into an ordered array which was dried and sintered. Sintering took place at much lower temperatures and at faster rates than traditional ceramic processing (e.g., broad size distributions of crystalline powders). Improvements in strength and toughness of the sintered body were never demonstrated by Bowen's group for these novel ceramics. Ordered particles have packing faults which lead to ordered domains, as shown in Figure 1, similar to grains in polycrystalline materials. When sintered, the ordered domains shrink separately and pull away from each other as shown in Figure 2. This leads to defects in the sintered body of sizes similar to those of the ordered domains which can encompass as many as 10,000 particles. These ordered domains lead to weakness in the sintered body according to Griffith's theory.