Generation, properties, and order packing of monodispersed spherical colloid particles of yttrium hydroxy-carbonate: A colloidal route to minimizing voids in ceramics
Monosized spherical colloid particles of yttrium hydroxy-carbonate were prepared by in situ decomposition of urea at elevated temperatures in yttrium nitrate solutions. The nucleation and growth phenomena generating these particles were studied through measurement of induction periods, particle sizes and growth rates in silica seeded and unseeded systems. It was found that precipitation took place only under neutral conditions. In unseeded systems, homogeneous nucleation yielded about 3 x 10[superscript]10 nuclei/cm[superscript]3 while in seeded precipitation enough nuclei were produced to bring the total number of particles to 4 x 10[superscript]10 seeds/cm[superscript]3. Systems containing at least 4 x 10[superscript]10 seeds/cm[superscript]3 underwent pure heterogeneous precipitation on to the existing seeds. Two diffusion-controlled growth models and the diffusion chronomal analysis were applied to the data of growth rate. These analyses and the narrowing of particle size distribution with growth time confirmed the diffusion-controlled particle growth mechanism;Electron diffraction analysis showed the particles to be crystalline. The surface charge on the particles was characterized by electrophoretic studies. There is a charge reversal from positive to negative as the pH is increased, with an isoelectric point (IEP) of about 7.4. The chemical composition of the powders was determined by thermal, x-ray photoelectron spectroscopic, and chemical analyses to be Y(OH)CO[subscript]3·H[subscript]2O;The stability of aqueous suspensions of particles with a radius of 0.21[mu] was studied as a function of pH (in the range of 7-12.3) using a turbidimetric method. The suspensions are stable in the pH range 8.4 through 11.6, with maximum stability being achieved at pH 10.8. Maximum flocculation appears around the isoelectric point. Ordered particles were prepared from suspensions with particle volume fractions of 0.2. Particle ordering was attributed to flocculation in the secondary minimum in some cases and to ordering of the Kirkwood-Alder character in others. Particles ordered in the secondary minimum were arranged in a close-packed structure of high coordination number. The IEP was exploited, in the former case to rapidly "freeze" the ordered structure into the primary minimum. ftn*DOE Report IS-T-1302. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.