Crystallographic relationships among the thermal expansions and peritectic reaction of Hf6Ta2O17

Significance 

Recent technological advances, particularly in the energy world, have spurred significant interest in energy generation and efficiency, thereby pushing for better fabrication of novel and improved high temperature materials. For optimum operations, materials that can withstand harsh environments (high temperature and stresses) of energy generations are often required. Currently, thermal barrier coatings (TBCs) are a major component of such systems due to the fact that they enable high temperature capabilities; a characteristic attributable to a number of excellent properties they possess.

Presently, researchers assessing various attributes, such as: linear thermal expansion, thermal conductivity and chemical stability of hafnium tantalate (Hf6Ta2O17)- a prime candidate for applications as a TBC- have been published. In addition, the crystal structure of hafnium tantalate has recently been elucidated. This study enabled to fill a missing link that had hindered detailed information to be retrieved for materials such as hafnium tantalate. Therefore, now that the crystal structural information is available, it is imperative that scientists start to conduct more detailed analyses, based on atomic mechanisms regarding thermal expansion, thermal conductivity and chemical reactivity.

Recently, Professor Waltraud Kriven and her PhD candidate Scott J. McCormack from the Department of Materials Science and Engineering at University of Illinois at Urbana-Champaign, in collaboration with Dr. Richard Weber at Materials Development, Inc. conducted thorough studies to measure the anisotropic coefficients of thermal expansions for hafnium tantalate from room temperature up to its complete melting point. Additionally, they demonstrated how in-situ diffraction could be used to determine the orientation relationship and lattice variant deformation for the topotactic, peritectic transformation. Their work is currently published in the research journal, Acta Materialia.

The research team synthesized hafnium tantalate as a powder via the organic steric entrapment of cations, followed by preliminary characterization. The researchers then carried out high temperature X-ray diffraction where they used the quadrupole lamp furnace and conical nozzle levitator equipped with a CO2 laser system. These procedures demonstrated that the orthorhombic-hafnium tantalate and the tetragonal-HfO2 phases were related to each other via simple polyhedral rotations of a common crystallographic subcell.

The authors observed that the topotactic, peritectic transformation could be fully described by extracting the orientation relationship, lattice variant deformation and a motif (grouping) of cations that relates the two structures at the transformation temperature. Interestingly, the three factors were seen to allow for symmetry decomposition of the structure which in turn revealed that the orthorhombic-hafnium tantalate and the tetragonal-HfO2 structure were simply related by polyhedral rotations and loss of 1 mol of oxygen.

In summary, Professor Waltraud Kriven and her colleagues successfully measured the anisotropic coefficients of thermal expansion and the peritectic transformation of orthorhombic hafnium tantalate to tetragonal-HfO2 plus liquid, via in-situ, in air, X-ray powder diffraction from room temperature to complete melting in air, using a quadrupole lamp furnace and a conical nozzle levitator equipped with a CO2 laser. Altogether, a detailed analysis of atomic mechanisms focusing on the desired characteristics was presented and will provide a foundation upon which further work could be built upon.

Crystallographic relationships among the thermal expansions and peritectic reaction of Hf6Ta2O17 - Advances in Engineering

Crystallographic relationships among the thermal expansions and peritectic reaction of Hf6Ta2O17 - Advances in Engineering

About the author

Waltraud Kriven received a Ph. D in 1976 in Solid State Chemistry from the University of Adelaide in South Australia. The B.Sc. (Hons) and Baccalaureate degrees were in Physical and Inorganic Chemistry, and Biochemistry, also in Adelaide. Dr. Kriven spent one year as a Post Doctoral Teaching and Research Fellow in the Chemistry Dept. at the University of Western Ontario in Canada. She then spent three years (1977-1980) jointly at the University of California at Berkeley, and at the Lawrence Berkeley Laboratory. There, Dr. Kriven conducted post-doctoral research in transmission electron microscopy of ceramics and was a Lecturer, teaching Phase Equilibria in the senior undergraduate Ceramics Program of the Dept. of Materials Science and Mineral Engineering. For almost four years (1980-1983) Dr. Kriven was a Visiting Scientist at the Max-Planck-Institute in Stuttgart, Germany. There she studied the mechanism of transformation toughening of composite ceramics by 1 MeV HVEM, while working in the electron microscopy group headed by Dr. M. Rühle.

Since 1984, Professor Kriven has been at the University of Illinois at Urbana-Champaign where she is also an Affiliate Professor in the Mechanical Science and Engineering Dept. of UIUC. Kriven has internationally recognized expertise and has made major contributions to the areas of in situ high temperature (≤ 3000°C) synchrotron studies of ceramics to study phase transformations, thermal expansions, in situ chemical reactions and phase diagrams; geopolymers and geopolymer composites; low temperature syntheses of oxide ceramic powders; microstructure characterization by electron microscopy techniques. Kriven has co-authored 300 research publications, 56 conference proceedings, edited or co-edited 27 books, 38 keynote and plenary talks, 245 invited talks, 460 conference talks, 6 patents.

For the past 35 years Kriven has been an active member of ACERS, particularly in the Engineering Ceramics Division (2000-2006) where she has served as Chair (2005). Kriven has organized or co-organized 20 annual symposia in Geopolymers as well as 5 symposia on Phase Transformations in Ceramics: Science and Applications. She is also active in the World Academy of Ceramics since 2004, as well as organizing conferences for Engineering Conferences International (ECI). She continues to co-organize 3-4 international conferences per year.

About the author

Scott McCormack grew up in the small fishing village of Eden on the Far South Coast of Australia. He completed a Bachelor of Engineering with First Class Honors (H1), majoring in Materials Science and Engineering at the University of Wollongong, Australia (Graduated: 2013). He is currently a Ph.D. student at the University of Illinois at Urbana-Champaign in the Department of Materials Science and Engineering (Graduating: 2019).

His research interests focus on the interplay between crystallographic symmetries and energetics that can extend our understanding of thermophysical and thermochemical properties of next generation materials. Engineering applications include: (i) high temperature materials, (ii) energy storage materials and (iii) structural materials (transformation toughening and shape memory). His research is a multidisciplinary endeavor, aimed at merging ideas from materials synthesis, crystallography, calorimetry and computation.

ORCID

Reference

Scott J. McCormack, Richard J. Weber, Waltraud M. Kriven. In-situ investigation of Hf6Ta2O17 anisotropic thermal expansion and topotactic, peritectic transformation. Acta Materialia, volume 161 (2018) page 127-137.

Go To Acta Materialia

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