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Posted: Jan 09, 2017

Controlling the structure and phase composition of lanthanide-doped alumina for emerging applications

(Nanowerk Spotlight) Lanthanide dopants play an important role in desirable phase transformations of aluminum oxide (alumina) in order to achieve optimized physical and chemical properties. For example, the presence of dopants – such as ytterbium, gadolinium or lanthanum – strengthens the grain boundaries of alumina, largely affecting its mechanical properties.
While alumina usually exists in the most thermodynamically stable α-phase, it can also exist in several other metastable/transitional phases. The transformation from one phase into another can be directly influenced by chemistry, crystallite morphology, and other processing parameters. Doping alumina with lanthanide generally leads to such a phase transformation.
Identification of the mechanisms responsible for thermal stability increase of the alumina phases has been particularly difficult. Understanding the factors that dictate the interactions between the lanthanides (or rare earth elements) and the alumina lattice could provide the necessary insight for enhancing the intrinsic properties of alumina.
Alumina is an excellent candidate for laser gain media, as it has a significantly higher thermal conductivity than typical polycrystalline ceramic gain media (such as Nd:YAG). Higher thermal conductivity is required for sufficient cooling of the material as the power is increased during lasing.
However, there are two major challenges to using alumina in this application, as 1) alumina is anisotropic, which makes it challenging to fabricate as a transparent material; and 2) the large-size lanthanide dopant is difficult to incorporate into the nanoscale alumina matrix.
New research, reported in the January 6, 2017 online edition of Scientific Reports ("Structural Effects of Lanthanide Dopants on Alumina"), indicates that there are significant structural changes to the lattice with addition of the dopant, and that understanding these effects will lead to future research for addressing these challenges.
"In the existing literature, the concentration of lanthanide dopants is significantly high, making it difficult to distinguish the subtle effects that they have on the alumina lattice," Shenqiang Ren, an Associate Professor in the Mechanical Engineering at Temple University, and Raymond Brennan, a Materials Engineer at Army Research Laboratory, explain to Nanowerk. "Our new work represents the first time that the influence of dilute dopants is reported on the structural evolution of alumina at high temperatures. Our findings represent a potential path for advancing the processing and performance of alumina materials."
This multidisciplinary work is the result of a collaboration Ren's group and and the US Army Research Laboratory (Dr. Raymond Brennan and Dr. Victoria Blair).
The thermal stability of lanthanide dopant in the alumina lattice
The thermal stability of lanthanide dopant in the alumina lattice. (Image: Ren Group, Temple University)
In the paper, the team demonstrates that doping lanthanides into the alumina lattice delays phase transformation, making the materials more resilient at higher temperatures.
Specifically, in this study, a very small dopant concentration of lanthanide ions, on the order of 400 ppm, was able to generate a significant shift in the phase transition temperature.
"The lanthanide dopant effect on phase transformation of solution-processed alumina, as well as its high temperature behavior, has been a difficult challenge to address within the scientific community," say Ren and Brennan. "Motivation for our work stems from the significant potential impact of alumina on high temperature applications."
"There are also important implications for the optical community, as rare earth dopants are required for laser gain media," he adds. "A foundational understanding of rare earth effects on alumina is required for furthering research in high thermal conductivity materials for optical applications."
There are numerous applications that can benefit from the results of this study, which has demonstrated stability of alumina phases at 100-300°C higher temperatures.
For example, lanthanide-doped alumina is attractive for laser gain media applications, as it leads to a significant increase in thermal conductivity, enabling rapid dissipation of heat away from the laser host.
Alumina-based lubricants are ideal for applications that require operation at high temperatures. As another example, γ-alumina is commonly used as a catalyst in many applications due to its large surface area.
While the current research has been focused exclusively on the effects of lanthanide dopants in the alumina lattice, the scientists believe it is possible that the temperature effects may also be present in other oxide material systems, which warrants additional investigation. It is also of interest to explore the effects of lanthanide ions on magnetic properties, such as magnetic susceptibility.
Ren and Brennan conclude that future research in this field will be motivated by gaining an understand of the transition metal dopant effect on the structure-processing-property relationship of alumina nanomaterials.
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