Committee Members:

Prof. Robert Speyer, Advisor, MSE

Prof. Naresh Thadhani, MSE

Prof. Rosario Gerhardt, MSE

Prof. Arun Gokhale, MSE

Prof. Wenshan Cai, ECE/MSE

Advances in Processing of Transparent Lutetia-Based Ceramics for High Power Laser Applications

Abstract:

Since the development of the first ceramic Nd:YAG laser by Ikesue in 1995, transparent polycrystalline laser gain media have been of great interest. They are often more cost-effective than single crystals for large-scale manufacturing, allow for the production of near net shape parts, and accommodate greater dopant concentrations.

Rare-earth sesquioxides (RE2O3) are an appealing class of potential laser host materials, in part due to their high thermal conductivity, mitigating some of the deleterious effects from high thermal loads (e.g., birefringence from thermal lensing). To this end, Yb3+-doped Lu2O3 is the best candidate.

Past development of optical-grade, transparent, lutetia-based ceramics relied on the chemical co-precipitation of the doped ceramic powders and/or the use of heat/pressure treatment equipment that may be prohibitively expensive to commercially scale. The primary goal of this research is to develop methods of producing theoretically transparent undoped and Yb3+-doped lutetia ceramics, using standard thermal/pressure processing equipment, and commercially available powders without chemical reprocessing.

In the bulk of the work, processing methods to form highly transparent, undoped Lu2O3 ceramics were developed by optimizing various green body conditions (i.e., binder systems, consolidation methods, and organics removal). Pressureless (vacuum) sintering"with the use of lithium fluoride (LiF) vapor as a sintering aid, coined vapor-phase assisted sintering"was performed and schedules optimized to obtain samples in the closed porosity state with relative densities of 97-98% and micrometer-sized grains. Samples were post-HIPed at or below sintering temperatures to obtain theoretically dense (>99%) ceramics. An important focus of the work was processing steps which mitigated gap retention between spray dried granules in powder compacts, which ultimately negatively affected transparency.

The optimized processing parameters from the undoped work translated well to the development of Yb3+-doped lutetia ceramics. Dopant concentrations of up to 16-mol% Yb3+ were studied. Transmittance and photoluminescence behaviors were measured and evaluated. Transmittances of optimally processed specimens approached theoretical limits. Fully miscible solid-solution of the ytterbia and lutetia were confirmed at all studied dopant concentrations.