Enhancing Transparency in Spark Plasma Sintered YAG Ceramics: The Role of Ta and Mo Diffusion Barriers

Transparent Yttrium Aluminum Garnet (Y₃Al₅O₁₂, commonly called YAG) ceramics are used in various industrial applications due to their superior physical and optical properties. For instance, they are used in solid-state laser materials due to their good optical properties, and capability to be doped with various rare earth ions. They are also critical in creating high-power lasers for industrial cutting, welding, and military applications. Moreover, YAG ceramics are used in optoelectronics and photonics for fabricating optical components such as lenses, waveguides, and substrates that require materials with high optical transparency and thermal stability. Furthermore, they are used in scintillators for detecting ionizing radiation in medical imaging, high-energy physics experiments, and security systems. However, the presence of carbon contamination poses significant challenges and can degrade the optical transparency of YAG ceramics, which is detrimental to their performance in laser and optical applications. Carbon particles can scatter light, reducing the material’s transparency. Carbon contaminants can also react with YAG during the sintering process, potentially forming unwanted carbides or affecting the stoichiometry of the material which can lead to phase instability, and affects the material’s physical properties. Current method to remove carbon contamination requires strict control over the manufacturing environment, including the use of high-purity raw materials and controlled atmosphere conditions. This can increase the complexity and cost of producing transparent YAG ceramics.

To this account, a new study published in ACS Applied Materials & Interfaces and led by Professor Gideon Grader from the Wolfson Department of Chemical Engineering at Technion-Israel Institute of Technology and conducted by Michal Sakajio, Gennady Shter, Meirav Mann-Lahav, Vadim Beilin, and Shai Zamir, the authors investigated the effectiveness of molybdenum (Mo) and tantalum (Ta) foils as barriers against carbon contamination during spark plasma sintering (SPS) of YAG ceramics. The authors investigated how these metal foils, used separately and in tandem, influence the transparency and microstructure of the sintered ceramics, and tried to understand the underlying mechanisms that contribute to their effectiveness. They chose the SPS technique for its rapid heating and cooling rates, which allows for the densification of materials at relatively low temperatures, preserving fine microstructures critical for high optical quality. The researchers created three main barrier configurations to study the effects of Mo and Ta foils, the reference sample (C-YAG-C) used carbon foil as a baseline for comparison. Secondly, the Mo Foil configuration (C-Mo-YAG-Mo-C) which employed Mo foils as barriers on either side of the YAG powder, within the carbon foil. Thirdly, Ta foil configuration (C-Ta-YAG-Ta-C) which acted similar to the Mo configuration but with Ta foils. Additionally, a multilayered Ta-Mo configuration was tested to assess the combined effects of both barriers.

The team used advanced analytical techniques including high-resolution scanning electron microscopy and energy-dispersive X-ray spectroscopy to examine the microstructure and composition of the sintered samples. Moreover, optical transparency of the sintered samples was measured to assess the effectiveness of the barrier materials in preventing carbon contamination. The researchers found the Ta foil configuration to be significantly more effective than Mo in preventing carbon contamination. A reaction layer formed at the Ta-YAG interface, creating a YTaO₄-Al₂O₃ eutectic composition that acted as a barrier against carbon penetration, thereby enhancing the optical transparency of the ceramics. Moreover, Mo foils, while preventing direct contact between carbon and YAG, allowed carbon diffusion into the ceramic, resulting in the formation of non-uniform microstructural features. However, Mo did not form a reactive layer and was easily removed from the YAG surface. Furthermore, the authors showed that the multilayered Ta-Mo configuration improved outcomes in preventing carbon contamination when the Ta layer was above a certain thickness (∼100 μm). This improvement was attributed to Ta’s interior diffusion-blocking mechanism. According to the authors, both the single-material and multilayered barrier approaches demonstrated similar improvements in optical performance, highlighting the effectiveness of these strategies in enhancing the transparency of YAG ceramics. The study also explored a novel strategy for bonding oxide ceramics to metals by adding a Ta layer at the joint interface, expanding the potential applications of the findings beyond improving the transparency of sintered ceramics. Overall, the work of Professor Gideon Grader and his team provided significant insights into the mechanisms through which Mo and Ta foils serve as effective barriers against carbon contamination in the SPS process. The findings highlight the superior performance of Ta foils in enhancing the optical quality of YAG ceramics, which offers a promising approach for the production of transparent ceramics and for bonding oxide ceramics to metals.