High Yield Synthesis and Processing of Nanoscale YTZP Ceramics
Open Access
- Author:
- Szepesi, Christopher Joseph
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 18, 2010
- Committee Members:
- James Hansell Adair, Committee Chair/Co-Chair
David John Green, Committee Member
Christopher L Muhlstein, Committee Member
Darrell Velegol, Committee Member - Keywords:
- nano-grain bulk ceramics
yttria-doped zirconia
YTZP
synthesis
green forming
sintering - Abstract:
- Nanomaterials are the subject of increasing interest. The expectation of new and enhanced mechanical, optical, magnetic, and electronic properties, in part due to the high concentration of interfaces and grain boundaries within the nanoscale microstructure, has initiated many studies on how to synthesize materials and process components with a final grain size below 100 nm. Several obstacles have hindered the practical application of such materials. Issues related to the production of sufficient quantities of powders composed of nanoparticles, the tendency of the nanoparticles to aggregate, the production of homogeneous green bodies, and avoiding grain growth during sintering have all been addressed with varying degrees of success. The focus of this thesis research is to address several issues inherent to the processing of nanoscale particulates, specifically in yttria tetragonal stabilized zirconia polycrystalline (YTZP) materials, for the purpose of fabricating dense, bulk components with a nano-scale microstructure. These issues include the synthesis of sufficient quantities of an appropriate material in a dispersed state, the formation of homogeneous green bodies of high green density, and design of sintering conditions that retain the fine-grain microstructure while allowing densification to near theoretical density. A recently-developed hydrothermal precipitation procedure was chosen for the production of nano-YTZP because crystalline, 8-10 nm particles of zirconia or YTZP can be produced with a flexible composition and dispersed for further processing. A wide range of characterization techniques are employed to verify particle size, phase, composition, and impurity content. Material yields are increased to 100 g of zirconia or YTZP per liter of stock solution by increasing in the reagent concentrations. Despite the increased ionic strength of the as-synthesized suspension, a laundering and dispersion procedure is described in which well-dispersed suspensions of up to 20wt% nano-YTZP are recovered. DLVO theory (named for authors Derjagiun and Landau, Verwey and Overbeek) is applied to the zirconia-water system to characterize the dispersion behavior. Two green forming procedures are compared: pressure filtration and dry powder compaction. Homogeneous green bodies can be formed by pressure filtration; however, due to strong capillary forces that act within the nano-scale pore structure during drying, green bodies could not be recovered without cracks. Two methods for the recovery of dry powder suitable for compaction are compared: pan-drying and freeze-drying. During pan-drying, primary particles tend to agglomerate; agglomerates are compressed into high-strength, low-density aggregates that negatively affect densification and final component properties. During freeze-drying, the crystallization of water breaks apart agglomerates, primary particles are immobilized, and capillary forces are avoided. Compaction behavior of pan- and freeze-dried powders is compared using a model that describes powder consolidation as a combination of activated and saturation processes. Formation of a nanograin bulk component requires that grain growth during densification be minimized. A two-step sintering procedure that accomplishes this is discussed, along with constant heating rate and (CHR) master sintering curve (MSC) models. Sintering activation energies are calculated with these models and found to be slightly lower than previously reported values. With the aid of the MSC, a two-step sintering procedure is presented that produces crack-free, bulk YTZP components of >95% theoretical density and grain sizes <100nm.