Thermal Behavior of the Hafnia-Tantala-Titania SystemWaltraud M. Kriven, University of Illinois at Urbana-Champaign, DMR 0706606 Outcome:Researchers at the University of Illinois are able to observe crystals expand and change crystal structure upon heating to 1850 °C, using high intensity synchrotron X-rays. Impact: The new knowledge will enable the design of the next generation very high dielectric ceramics for computers, electronic, optical and nuclear applications. Explanation: The temperature ranges and mechanisms of structure change for tantala(Ta2O5) andhafnia (HfO2) are being worked out. • A new phase of tantalum hafnate (Ta2Hf6O17) or (Ta2O5•6HfO2) has been discovered, which is useful in nuclear shielding applications. • Tantalahas applications as: • - Dynamic random access memory (DRAM) e.g. in computers, that stores bits of data in a separate capacitor within an integrated circuit. When a capacitor has a very high dielectric constant, it is much more powerful. • - Electroluminescent devices • - Biological and chemical sensors • - Anti-reflective coatings in solar cells • - Optical wave guides • • Hafnia is a very high temperature material, with good nuclear properties Prof. Kriven (right) with research team at the Advanced Photon X-ray synchrotron Source (APS) at Argonne National Lab
Mechanism of Thermal Expansion in HafniaWaltraud M. Kriven, University of Illinois at Urbana-Champaign, DMR 0706606 • In studying crystal structures on heating to high temperatures such as 1700 °C, it is useful to identify groups of atoms so that it is easier to see why the crystal structure expands the way that it does. The figures on the right demonstrate imaginary shapes joining similar types of atoms which move cooperatively in groups as the whole crystal structure expands on heating (top picture) and contracts on cooling (bottom picture). Such a knowledge is useful in engineering applications for these materials. • The thermal expansions have been measured for hafnia, tantala and hafnium tantalate. • Several crystal structure changes exist in tantala, but because they are complex and not understood, currently industry avoids using crystalline tantala, and keeps it doped it in the less powerful, amorphous phase.
OUTREACH ACTIVITIESWaltraud M. Kriven, University of Illinois at Urbana-Champaign, DMR 0706606 The PI served as a Guest Lecturer and lab tour guide for groups of high school teachers as part of the national Project Lead the Way® program. Engineering faculty and organizers run intensive 2-week summer courses for high school science and engineering teachers, helping them to design a year-long curriculum for introducing high school students to engineering. Two courses are held at the University of Illinois campus during summers. The PI served as an Instructor in Chemistry and in Materials Science for the ASME and ASCE review classes for the Fundamentals of Engineering Program, as part of the comprehensive 3-day exams for the Professional Engineering (PE) qualification. These courses are taught to approximately 100 student engineers on the campus of the University of Illinois at Urbana-Champaign. For the ISPE, the review classes for engineers who have been practicing for over 5 years are held at the Underwriters Laboratory in Northbrook, in the Chicago, IL area, as well as at the Sargent and Lundy Architectural Company in Chicago.