#################################################### # Waltraud M. Kriven # requesting 6 days, at minimum=6 days # beamline 33BM, High temperature phase transformations in the rare earth tantalates, vanadates and phosphates # instrument 33BM-C fourc #################################################### # top:/home/www/beamtime-requests/req00661.txt # UNICAT Member Beam Time Request #661 # created Fri Aug 20 14:36:19 CDT 2004 #################################################### beamline: 33BM collaboration: Yes contact: kriven@uiuc.edu days: 6 description: We propose to undertake in-situ high-temperature x-ray diffraction (XRD) studies of phase transformations in oxide ceramics exhibiting polymorphic phase transformations at elevated temperatures. The experiment involves heating of the samples in air, using a thermal-image furnace, and the crystal structural changes are simultaneously recorded as x-ray diffraction patterns using synchrotron radiation. The importance of in-situ studies is well recognized, as materials can be studied in their anticipated working environment. Moreover, for systems exhibiting ferroelastic phase transformations, which are displacive in nature, the energy barrier for transformations is low compared with the level of available thermal energy. Therefore, the high temperature form is unquenchable on cooling. This further underscores the importance of in-situ studies for studying these types of phase transformations. Materials such as tantala (Ta2O5), hafnia (HfO2), and rare earth tantalates, vanadates and phosphates are model systems for academic understanding as well as future engineering applications, where very little is known about phases, and their transitions. The system hafnia (HfO2) - tantala (Ta2O5) is of special interest here due to the chemical and physical similarities between hafnia and zirconia. Although the melting temperatures are close to each other, the phase transformations (monoclinic->tetragonal->cubic), which have been extensively studied in zirconia, appear at significantly higher temperatures in hafnia (>1500 deg C for monoclinic to tetragonal and ~2500 deg C for tetragonal to cubic). In addition, hafnia has a low rate of volatilization compared to either zirconia or yttria. This potentially allows extending the temperature range for possible applications. It was proposed that stabilization against the tetragonal to monoclinic phase transformation could be achieved by adding ~4mol% of Ta2O5 to HfO2. However, it is not clear whether the stabilization works via the microstructure route and uses the matrix constraint mechanism as in stabilized ZrO2 or whether it is caused by a solid solution effect, which reduces the volume change during the phase transformation as seen for example in PZT (lead zirconate titanate). Preliminary investigations on a series of HfO2-Ta2O5 samples showed presence of an additional phase with increased concentrations of Ta2O5. This phase could either be similar to the high temperature tetragonal phase with modified lattice parameters or it could be described as an orthorhombic phase similar to the Hf6Ta2O17 phase described by Spiridonov et al. The transformation to this phase occurs at lower temperature for increasing Ta2O5 contents of the powder. Additional high resolution studies to clarify the crystallographic character of this phase as well as its region of existence would be desirable in order to understand the stabilization mechanism and to explore the engineering application of tantala stabilized hafnia. Preliminary experiments to explore the the transformation behavior of CePO4 and DyVO4 are planned in addition. equipment_required: experiment: High temperature phase transformations in the rare earth tantalates, vanadates and phosphates foreign_nationals: Kerstin Jurkschat Pankaj Sarin hazards: High temperature furnace as used in previous experiments. Small amounts (<1g)of HfO2, Ta2O5, CePO4, DyVO4 and smal amounts (<0.5g) of Pt powder and MgO for temperature calibration instrument: 33BM-C fourc instrument_other: minimumdays: 6 name: Waltraud M. Kriven nonmembers: unacceptable_dates: z34ID_details: #REMOTE_HOST: mach-37.mse.uiuc.edu #REMOTE_ADDR: 128.174.228.37 #CONTENT_LENGTH: 3804 #HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm #HTTP_USER_AGENT: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)