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# Waltraud M. Kriven <kriven@uiuc.edu>
# requesting 6 days, at minimum=4 days
# beamline 33BM, High temperature phase transformations in the hafnia-tantala system
# instrument 33BM-C fourc
####################################################
# top:/home/www/beamtime-requests/req00579.txt
# UNICAT Member Beam Time Request  #579
# created Fri Apr 16 14:56:09 CDT 2004
####################################################

beamline:
	33BM

collaboration:
	No

collaborator_Jenia:
	ON

collaborator_Paul:
	ON

contact:
	kriven@uiuc.edu

days:
	6

description:
	Phase transformations have widespread applications in ceramics. For example, transformations with positive volume change (e.g. in zirconia) provide toughening mechanisms, while negative volume change on transformation resulting in debonding of interphases is a transformation weakening phenomenon. Other applications for transformations in ceramics include multifunctional high temperature actuators in "smart" or intelligent" systems. Materials such as tantala (Ta2O5), hafnia-tantala (HfO2-Ta2O5), metal-ion (Ti-, Ga-, and Cr-) doped mullites, calcium phosphates, and rare earth niobates or titanates are examples of model systems for academic understanding as well as future engineering applications. 

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.

Stabilization against the tetragonal to monoclinic phase transformation can be achieved by adding ~4mol% of Ta2O5 to HfO2, which can dissolve up to 5mol%Ta2O5. 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 studies of hafnia showed that it is isostructural to zirconia at room temperature and starts to transform to a tetragonal phase above 1500 deg C. More extensive studies of tantala have been carried out during previous experiments, where we have made considerable advances in understanding the transformation behavior. Interesting results have been seen in particular for the transformations observed around 360 deg C and 960 deg C. Complementary experiments (in-situ high temperature (up to 1600C) optical microscopic investigations) will be carried out to clarify the nature of these transformations.

Although both pure components have been studied by other groups, the results are somewhat contradictory  and virtually nothing is known about the HfO2-Ta2O5 phase diagram. Especially the crystallographic nature and the existence ranges for the metastable high temperature phases of tantala and the temperature ranges found for the transformations in hafnia need further clarification. We therefore propose to study the phase transformations of the pure components hafnia and tantala (above 1360 deg C) as well as possible intermediate compositions in order to provide a phase diagram as the basis for understanding the stabilization mechanism and to explore the engineering application of tantala stabilized hafnia.

equipment_required:
	None

experiment:
	High temperature phase transformations in the hafnia-tantala system

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

hazards:
	The experiment uses a small, water-cooled, four-lamp furnace capable of 2000 deg C. There are no apparent hazards other than those normally associated with using high energy synchrotron radiation. The samples are small sintered rods (300-400 micronns in diameter and < 2.5 cm in length) or pellets (3mm (W) X 5mm (L) X 1 mm (H)) or small quantities of ceramic oxide powders (<0.1gm). While the rod samples are studied in transmission mode, with the x-rays passing through the sample, the pellets and powders are used in reflection geometry. All the samples are non-toxic.

instrument:
	33BM-C fourc

instrument_other:
	

minimumdays:
	4

name:
	Waltraud M. Kriven

nonmembers:
	None

unacceptable_dates:
	June 4th to 6th, 2004
July 20th to August 1st, 2004
August 12th to 14th, 2004 (if possible)

z34ID_change_undulator:
	no

z34ID_details:
	

z34ID_on_axis:
	no

z34ID_parasitic:
	no

z34ID_taper:
	no


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