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# Waltraud M. Kriven <kriven@uiuc.edu>
# requesting 6 days, at minimum=4 days
# beamline 33BM, In-situ high temperature phase transformation studies in hafnia-tantala system, and rare earth vanadates and phosphates
# instrument 33BM-C fourc
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# top:/home/www/beamtime-requests/req00698.txt
# UNICAT Member Beam Time Request  #698
# created Sun Dec 05 12:05:30 CST 2004
####################################################

apsrun:
	2005-01

beamline:
	33BM

collaboration:
	Yes

collaborator_Jenia:
	ON

collaborator_Paul:
	ON

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 vanadates and phosphates are model systems for academic understanding as well as for future engineering applications. Due to the chemical and physical similarities between hafnia and zirconia (ZrO2), the HfO2-Ta2O5 is of special interest. 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 has been suggested 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 i.e. by the matrix constraint mechanism, as in stabilized ZrO2, or whether it is caused by a solid solution effect, as seen for example in PZT (lead zirconate titanate), which reduces the volume change during the phase transformation. 

Preliminary investigations on a series of HfO2-Ta2O5 samples showed presence of a new 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 temperature decreased with increasing Ta2O5 content. It is desirable to conduct additional high resolution studies to clarify the crystallographic character of this phase as well as determine its region of existence in order to understand the stabilization mechanism to explore the engineering application of tantala stabilized hafnia.  CePO4 and DyVO4 powders were synthesized by steric entrapment method and studies on the transformation behavior of these two materials are currently underway. We propose to continue our in-situ high temperature XRD investigations to understand the phase transformation behavior of hafnia-tantala system, and the rare earth vanadates and phosphates.

equipment_required:
	None

experiment:
	In-situ high temperature phase transformation studies in hafnia-tantala system, and rare earth vanadates and phosphates

foreign_nationals:
	Professor Waltraud M. Kriven (Australian); Dr. Pankaj Sarin (Indian)

hazards:
	The experiment uses a small, water-cooled, four-lamp furncae capable of 2000 degree C. There are no apparent hazards other than those normally associated with using high energy synchrotron radiation. The specimen are small quantities (< 1 mg) of powder samples. All samples are non-toxic.

instrument:
	33BM-C fourc

instrument_other:
	

minimumdays:
	4

name:
	Waltraud M. Kriven

nonmembers:
	None

unacceptable_dates:
	None

z34ID_change_undulator:
	no

z34ID_details:
	

z34ID_on_axis:
	no

z34ID_parasitic:
	no

z34ID_taper:
	no


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