In situ Determination of Phase Equilibria in the Quaternary Hafnia-Tantala-Titania-Tungstate System

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: This project addresses the need for a new, more efficient way to elucidate and measure very high temperature (HT) diagrams of phases or different forms existing under specific conditions of temperature and composition called "phase diagrams" in oxide ceramics. Current methods are slow and based on heating ceramics at high temperatures, cooling them and measuring them at room temperature (RT). However, we do not know that the phase or form observed at RT is the same as what existed at HT. Within this project, new experimental tools have been developed enabling measurements to be made at temperatures up to 3400 C, thereby providing correct and accurate basic scientific knowledge. It is transformational to understand that such ceramic phase diagrams could be determined expediently in a short timeframe. The same techniques can be widely used to make significant and rapid advances in the underlying solid state physics and chemistry knowledge, so as to identify the next generation of complex materials for devices. Both undergraduate and graduate students trained in this research may be subsequently employed in the aircraft design and manufacturing sector, or as basic science researchers, particularly at high intensity, synchrotron X-ray laboratories. Outreach to high school students and teachers is also carried out. This research is relevant to identifying ceramic materials for space exploration and next generation electronic devices. TECHNICAL DETAILS: This research demonstrates a method by which the quaternary, tantala-hafnia-titantia-tungstate (Ta2O5 - HfO2 - TiO2 -WO3) phase diagram can be measured in situ, at temperatures up to 3400 C, in air or under controlled atmospheres. This work combines experimental techniques of (i) homogeneous powder synthesis by the organic steric entrapment method (ii) the quadrupole lamp furnace capable of 2000 C in air and (iii) the conical nozzle levitator capable of 3400 C in controlled atmospheres. Techniques (i) and (ii) were invented at the University of Illinois Urbana- Champaign and (iii) was developed and refined by two small companies. In addition, this research has resulted in theoretical modeling techniques to explore mathematical lattice correspondences for orientation relationships between crystal structures as they change with temperature (thermal expansions, phase transformations, invariant reactions) as well as across compositional ranges. These approaches will be applied to the four-component phase diagram constituting the quaternary system. Alongside the development of experimental and theoretical techniques that are being developed, graduate students are being fully trained, i.e. the scientists of the future. In turn, these graduates will use these new techniques to increase the basic science knowledge necessary to invent new, extremely high temperature ceramics and composites. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.