This project will study the mechanism of crystal structure changes in hafnia (HfO2)-tantala (Ta2O5)-titania (TiO2) based ceramics by in situ high temperature X-ray synchrotron diffraction measurements up to 2000 degrees C in air. It will also investigate how to stabilize and control the structure changes through chemical additives. Tantala is a promising candidate to replace silica in the microelectronic and integrated microtechnology industries. One of its crystal structures has the potential to increase its dielectric properties by five orders of magnitude. Hafnia is an ultra high temperature material which melts at ~2850 degrees C and can be used as a coating on leading edges of re-entry vehicles from space. It has higher temperature, crystal structure changes analogous to zirconia (ZrO2), which are able to toughen zirconia-based ceramics and make them less brittle, but at higher temperatures than in zirconia. Minority (particularly women) undergraduate students will be encouraged and mentored while assisting with the experiments.
TECHNICAL DETAILS: This project will explore the ternary hafnia-tantala-titania ceramic system to elucidate mechanisms of known and new phase transformations. Newly developed, ultra high temperature, synchrotron and neutron diffraction techniques, in conjunction with dilatometry to 2800 degrees C will be used to gain quantitative crystallographic information in situ, in air. This will lead to an understanding of the role of chemical dopants, which lead to oxygen as vacancy or interstitial defects in solid solution, resulting in stabilization or destabilization of phase transformations. The next generation of American Ph.D. students will be trained in such highly-sophisticated, forefront research techniques at high energy, synchrotron (Advanced Photon Source) and neutron diffraction (Spallation Neutron Source) national facilities.
During the past five years, we have continued to bring our research tools to their fully operational states. This tools are the Quadrupole Lamp Furnace (QLF) (Figs. 1and 2) and a curved image plate synchrotron X-ray detector (CIPD) (Fig. 3). The QLF which can heat up a 4 mm diameter hot spot to 2,000°C in air by means of four halogen infrared reflector lamps. The CIPD can collect a powder diffractometry spectrum in 20 seconds instead of 2 hours. We have streamlined the computer-based commmunications between the programmable QLF, CIPD and synchrotron beamline controls, so as to be able to take data more quickly, continuously and efficiently with automatic, programmable ramp rates, hold times and data acquisition procedures. The aim of our research was to explore phase transformations (i.e. crystal structure changes when heated or cooled) and what phases exist in the hafnia (HfO2) - tantala (Ta2O5) - titania (TiO2) system. Using the above equipment that we developed, we collected high intensity synchrotron X-ray data at temperatures up to 1850 °C. We analyzed this data by advanced X-ray profile fitting computational analyses (Rietveld analysis) so as to determine crystallographic data for all the planes as a function of temperature. From this data we assembled thermal expansion strains in 3D, and elucidated a cooperative movement of groups of cations as the crystal structure expands with increasing temperature (Fig. 4 and 5). For low symmetry, monoclinic hafnia, it was observed that the principal thermal expansion strains do not necessarily occur in the axial directions of the crystallographic unit cell. Thus instead of being 17 x10-6 deg C-1 in the [c] monoclinic axis direction, strains of 34 x10-6 deg C-1 were aboserved at about 45 degrees away. The conventional reports along the axial crystallographic directions would then be seriously wrong by a factor of 2. Using hafnia as an example, this is an important message to the high temperature ceramics community, namely to use accurate data in the design and manufacture of high temperature ceramic components, particularly as thermal barrier coatings in airplane engines and aerospace components. We successfully observed the reversible monoclinic to tetragonal transformation in hafna at 1750 °C and measured the accompanying crystallographic unit cell volume changes. On heating the volume change was -2.74 ± 0.04% and on cooling it was +3.07% ± 0.06 %. We also found a similar transformation in tantala (Ta2O5) at 1360 °C, and discovered a new phase, tantalum hexa-hafnate (Ta2O5•6HfO2) and measured its thermal expansion. This phase has potential application as a nuclear reactor ceramic material. Concerning our outreach programs, our instrumentational capabilities are unique in the world and have attracted collaborators from Wright Patterson Air Force Base who has requested our help to make in situ crystallographic measurements on their intermetallic materials of interest. Our outreach activities include hosting visits to our labs from high school and middle school teachers of science and engineering, for two weeks during the summer, under the "Project Lead the Way" national teacher training organization. The principle investigator (PI) mentored organizations such as the Soceity of Women Engineergs (SWE), as well as minority groups such as for incoming Latino graduate students and GIRRRLS Summer Fun Camp held at the University of Illinois at Urbana-Champaign. Every semester the PI gives review classes in Chemistry and Materials Science to practicing engineers and undergraduate engineers-in-training (EIT) students, both on campus as well as in downtown Chicago and the Underwriters Laboratory in Northbrook, Chicago. The lectures for practising engineers are given under the auspices for the Illinois Society of Professional Engineers (ISPE). The PI is an active member of the American Ceramic Society and serves on the International Editorial Board for the international review journal, Materials Reviews.
