Materials Science Research Lecture

Abstract:

The McCormack lab has been focusing on understanding thermochemical and thermophysical properties of refractory materials that can operate in extreme environments. From this, I would like to present two key stories: (i) Enthalpy of mixing in entropy stabilized oxides and (ii) the thermal expansion of metal diborides up to 3050 ˚C.

Enthalpy of mixing in entropy stabilized oxides

William Rosenberg1, Stuart Ness1, and Scott J. McCormack*2

There has been a lot of interest in "High" Entropy and/or entropy stabilized materials and their properties. However, there has been little studies that dig deep into the thermodynamics and quantify thermodyamic parameters like the excess mixing enthalpy and mixing volume. Here, we measure the enthalpy of mixing between two entropy stabilized oxides (HfTiO4-ZrTiO4) to quantify the enthalpic effects in these disordered materials. These measurements are compared with the anisotropic molar volume of mixing. The enthalpy of mixing in HfTiO4-ZrTiO4 was found to be negative with respect to HfTiO4 and ZrTiO4 end members. This suggests that there is local ordering when comparing the HfTiO4-ZrTiO4 solid solution to HfTiO4 and ZrTiO4 entropy stabilized end members. This non-ideal mixing was also observed in the anisotropic molar volume of mixing, where the excess strain was confined predominantly to the b-direction. More local structure studies are required to understand the degree of local ordering in these entropy stabilized oxides.

Thermal expansion of metal diborides (MB2 | M = Ti, Zr, Nb, Hf, Ta) up to 3050 ˚C

Elizabeth Sobalvarro3, Fox Thorpe1, Jesus Rivera3, Harry Charalambous3, Gabriella King3, James Cahill3, Wyatt L. Du Frane3, Joshua D. Kuntz3 and Scott J. McCormack2*

Metal diborides (MB2 | M = Ti, Zr, Nb, Hf, Ta) are considered an ultra-high temperature refractory ceramic due to their relatively low reactivity and high melting point. While these properties are appealing, they also make it difficult to fully characterize their thermochemical and thermophysical properties up to melting. This work will discuss in-situ high temperature X-ray diffraction measurements up to ~3050 ˚C using a conical nozzle levitator system equipped with a 400 W CO2 laser. The high temperature X-ray diffraction data was used to calculate anisotropic coefficients of thermal expansion. The coefficients were compared amongst the five diborides (MB2 | M = Ti, Zr, Nb, Hf, Ta). It was found that the anisotropy could be related to the atomic displacement parameters of the metal cations. These thermophysical measurements will be critical in developing ultra-high temperature material systems for applications in hypersonic vehicles, nuclear fission/fusion reactors, and spacecraft.

1 Department of Chemical Engineering, University of California, Davis

2 Department of Materials Science and Engineering, University of California, Davis

3 Materials Science Division, Physical Life Sciences Directorate, Lawrence Livermore National Laboratory

More about the Speaker:

Scott J. 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 Engineering at the University of Wollongong, NSW, Australia in 2013. He then completed his Ph.D. in Materials Science and Engineering from the University of Illinois at Urbana-Champaign, IL, USA in 2019. He is now an Assistant Professor of Materials Science and Engineering at the University of California, Davis, USA. He was a recipient of National Science Foundation (NSF) Early Career Award in 2021. His research focuses on the interplay of crystal symmetry and energetic stability of materials in extreme environments (ultra-high temperature) for applications in hypersonic platforms, nuclear fission/fusion and space exploration. More information can be found at: https://mccormacklab.engineering.ucdavis.edu