Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
The preparation of a yttria ceramic, beginning with the continuous precipitation of a yttrium hydroxynitrate precursor by the reaction between yttrium nitrate and ammonium hydroxide, was studied to find the interrelationships among the various processing steps. Emphasis was placed on the effect of precipitation conditions on the characteristics of the precursor powder; the effect of these initial powder properties on the product from the subsequent calcination, pressing, and sintering operations was then examined;The precipitation process was modeled using a population balance approach which included the competitive kinetic rate effects of nucleation, crystal growth, agglomeration, and agglomerate break-up. Physically reasonable values for the rate constants were found when the model was fit to experimentally determined population density distributions. The model described a three-particle system, as found experimentally, in which the precipitate consisted of primary crystallites (less than 0.1 (mu)m in size), which were nucleated from solution, tightly bonded aggregates ((TURN)1 (mu)m in size) of crystallites, and less tightly bonded flocs (1 to 30 (mu)m in size), which were agglomerates of crystallites and aggregates;The crystallites formed at low pH were platelet-shaped and formed weak, porous flocs. As the pH was increased, the crystallites became more equiaxed and the flocs became stronger. Agglomerate properties were thus a strong function of precipitation pH. The method by which the precipitates were dewatered and dried had little effect on the low pH derived powders, but had a very significant effect on the high pH derived powders;The nucleation and crystal growth kinetics for the precipitation of yttrium hydroxynitrate showed a marked discontinuity at a precipitation pH of (TURN)8.9, indicating a probable change in nucleation and growth mechanisms at that point. Anomalous zeta-potential values were also found when crystal growth was taking place on the particle surfaces. This latter finding shows that equilibrium zeta-potential data, as usually reported in the literature, is of little value in determining electrostatic particle interaction when crystal growth is occurring.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
James Alan Voigt
Voigt, James Alan, "An integrated study of the ceramic processing of yttria " (1986). Retrospective Theses and Dissertations. 8045.