Colloidal Coating: A Novel Ceramic Processing Approach
Aqueous colloidal processing has the advantages of controlling the interactions between particles and environmental friendliness. I have developed a colloidal coating method that modifies the surface of ceramic powders by nanolayering. The nanolayer coating serves multiple functions. It can enhance the chemical and thermal stability of the powders, improve the consolidation and rheological properties of slurries, and lower the sintering temperatures of the green compacts. For example, silicon nitride and silicon carbide powders were coated by boehmite (AlOOH) via a sol-gel process. The coated powder suspensions were shown to have a significantly higher solids loading than the uncoated powder in water. Viscosity measurements and centrifugation showed that the coating changes the long-range interaction between the silicon nitride particles. Rheological studies indicate that the suspensions of coated powders have lower viscosity and wider linear viscoelastic region than that of uncoated powders. Furthermore, as the coating thickness increases, the shear modulus of boehmite-coated SiC gel decreases because the boehmite coating prevents the close contact of the SiC particles thereby reducing the van der Waals attraction interaction between the SiC particles. We have extended this part of research to the biomedical applications of nanometric hydroxyapatite coating for implant surface modifications. Additionally, we have also synthesized microporous and mesoporous nanostructured materials that are suitable for enzyme immobilization and absorbents for environmental wastes.
Low-Temperature Processing of Piezoelectric Ceramics with Enhanced Properties
Based on the nanocoating approach, a low-temperature, direct sintering approach for high performance perovskite lead magnesium niobate-lead titanate (PMN-PT) solid solution ceramics was developed using Mg(OH)2-coated Nb2O5 particles. The mixtures of Mg(OH)2-coated Nb2O5, PbO particles, and lead titanate particles are compacted and sintered to near full density at temperatures lower than 1000ºC with superior dielectric and piezoelectric properties.
More recently, we developed a method to fabricate freestanding piezoelectric films with giant electric-field-enhanced piezoelectric response. The d31 piezoelectric coefficient for PMN-PT layers can be as high as 2000 pm/V, larger than that of commercial single crystalline PMN-PT bulk, at 10 kV/cm (or 20 V over the 20-micron film thickness). In contrast to single crystals, the polycrystalline freestanding films are easy to make and can be made into any size. They are also easy to be miniaturized. The method can be applied to any piezoelectric material. The freestanding films can be easily stacked to form multilayer actuators as well as multilayer capacitors. They are ideal for miniaturized sensors and actuators applications.
Synthesis of Dispersed Hydroxyapatite Particles and Gels
Hydroxyapatite, Ca10(PO4)6(OH)2, was synthesized using a sol-gel method. When sufficient amount of citric acid or sodium citrate was added to the precursor salts, a translucent suspension was formed, in contrast to the opaque suspension obtained without the citrate addition. Particle size analysis found that the size of the HA varied from 2.5 mm to 4 mm when the citrate concentration is below 0.8 M. Above 0.8 M of citrate, the particle size was 0.1 mm. In addition, the Ca/P ratio in the particles made with sodium citrate is higher than that without sodium citrate. The gelation behavior of the submicron HA particles was used for the fabrication of coatings on implant surface for bioactivity study.