A Proposal for a Composite with Temperature-Independent Thermophysical Properties: HfV₂–HfV₂O₇

• Main Authors: Philipp Keuter, Anna L. Ravensburg, Marcus Hans, Soheil Karimi Aghda, Damian M. Holzapfel, Daniel Primetzhofer, Jochen M. Schneider
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: Materials
• Subjects: thermoelasticity; negative thermal expansion; composites; magnetron sputtering
• Online Access: https://www.mdpi.com/1996-1944/13/21/5021
• ISSN: 1996-1944

The HfV₂–HfV₂O₇ composite is proposed as a material with potentially temperature-independent thermophysical properties due to the combination of anomalously increasing thermoelastic constants of HfV₂ with the negative thermal expansion of HfV₂O₇. Based on literature data, the coexistence of both a near-zero temperature coefficient of elasticity and a coefficient of thermal expansion is suggested for a composite with a phase fraction of approximately 30 vol.% HfV₂ and 70 vol.% HfV₂O₇. To produce HfV₂–HfV₂O₇ composites, two synthesis pathways were investigated: (1) annealing of sputtered HfV₂ films in air to form HfV₂O₇ oxide on the surface and (2) sputtering of HfV₂O₇/HfV₂ bilayers. The high oxygen mobility in HfV₂ is suggested to inhibit the formation of crystalline HfV₂–HfV₂O₇ composites by annealing HfV₂ in air due to oxygen-incorporation-induced amorphization of HfV₂. Reducing the formation temperature of crystalline HfV₂O₇ from 550 °C, as obtained upon annealing, to 300 °C using reactive sputtering enables the synthesis of crystalline bilayered HfV₂–HfV₂O₇.