Hot Pressing of CeO2 Ceramic Pellets

D.P. Kramer, T.M. Pierson, C.O. SjÖblom, D.W. Grant, S.M. Goodrich, C.D. Barklay, and C.E. Whiting

University of Dayton Research Institute

Nuclear and Emerging Technologies for Space - NETS

February 2015

University of Dayton Research Institute Shaping the technology of tomorrow

Since the development of space nuclear power ~fifty years ago several different 238Pu fuel forms have been employed In general, the U.S. fuel form has

progressed over the years from; - Use of 238Pu metal - 238Pu metal-ceramic cermet - Current 238PuO2 ceramic pellet fuel Supplied all of the heat in U.S.

launched radioisotope powered spacecraft over the last ~20 years Spacecraft/missions include:

Cassini/Saturn, New Horizons/Pluto, and Curiosity/MSL

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Ceramics technology forms the basis of 238PuO2 full pellet fabrication

The fabrication of the 238PuO2 fuel pellets for radioisotope space power systems utilize a number of classical ceramic processing unit operations including; - Ball milling - Sieving - Hot pressing

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Hot pressing utilizes both Temperature + Pressure as driving forces to enhance densification/sintering

During hot pressing powder is first loaded into a die which is then placed within the hot press

During processing a specific time-temperature-pressure (t-T-P) profile is performed which results in the consolidation of the fuel powder into a ceramic pellet

Since 238Pu is radioactive, care needs to be taken in the performance of any processing operation to ensure personnel safety

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Advantages of a non-radioactive surrogate for performing various support endeavors

Reduced personnel exposure Significant reduction in cost

CeO2 is one candidate surrogate material for 238PuO2 Similar chemical properties i.e. Oxygen reduction

Similar crystalline characteristics i.e. Face-Centered Cubic

Ceramic properties?

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Two important ceramic properties – Melting/Sintering Temp and Thermal Expansion

Sintering Temp ~0.7 Tm - PuO2 – Tm ~2387oC - CeO2 – Tm ~2341oC

Thermal Expansion

- CeO2 ~15% greater compared to PuO2 (~1500K)

- CeO2 may be more susceptible to thermal cracking/shock

Touloukian, Y.S., Thermophysical Properties of Matter, v.13, IFI/Plenum, NY, 1977

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Experiments are being performed in a hot press which provides flexibility in developing t-T-P parameters

Induction furnace - Motor generator - ~2000o+C - Variable soaks & ramps

Vacuum - Mechanical/Diffusion

pumps - ~10-5 torr to 1 atm

Load - Manual hydraulics - 10,000+#

Not the newest hot press!

CeO2 powder obtained from Sigma-Aldrich (particle size

CeO2 powder is loaded within graphite dies which are assembled and positioned within the hot press Graphite die prior to assembly

- Various spacers - Ceramic Insulation

An assembled ~1” diameter hot press die

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Die assembly loaded into the hot press prior to the initiation of an experimental run

Die positioned within the induction coil

Before pre-loading of the ram and the closing of the door

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Pictures of an assembled hot press die during an experiment

View through sight glass on hot press door

Load ram during a run

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Temperature during the t-T-P hot press cycle is monitored employing an optical pyrometer

Obtaining good consistent temperature readings is a semi-art form

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Example of a possible hot press t-T-P process cycle exhibiting variations in temp/pressure over time

Temp

Pressure

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After the completion of a hot press experiment the die is removed and the pellet is ejected from the die

Hot press die assembly Ejected hot pressed pellet

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Microstructural examination of hot pressed CeO2 pellets Microstructural examination is an important tool for

understanding how changes in hot press processing parameters directly impact various properties of the ceramic pellet

Sectioning of the pellets was performed using a slow speed diamond saw - The pellets were cut dry with no application of any cutting

fluids which could tend to infiltrate into pores or any cracks in the structure of the pellet making further examination more difficult

After sectioning, a pellet was vacuum impregnated cold mounted using a two part epoxy

Standard grinding and polishing techniques were employed to obtain polished mounted specimens for optical/SEM

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Example of a hot pressed CeO2 pellet (#1) before and after sectioning

Side view before sectioning Sectioned surface

Cut

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Three distinct phases have been observed in hot pressed CeO2 pellet microstructures

“Black” phase - Intra- and inter-granular

porosity regions “Dark” phase

- Columnar regions “Light” phase

- Nodular regions Phase ratio is dependent on

region across the mounted specimen

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Determination of the difference between the “light” and “dark” phases observed in the microstructure

Some variations in the amount of each phase as a function of position across the specimen

Specimen was positioned within an SEM (Zeiss EVO 50) with an EDS

It was not necessary to coat the ceramic specimen to reduce charging as the microscope was operated at a pressure of 10 Pa

At several “light” and “dark” phase regions of the specimen elemental spectra were obtained to determine the relative concentrations of cerium and oxygen within the two distinct phase regions

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SEM/EDS analysis suggests a difference in oxygen (at. %) content of the two main observed phases

Phase “Type” Oxygen (at. %)

Cerium (at. %)

Ce/O ratio

Light #1 63.63 36.37 1.75 Light #2 62.28 37.72 1.65 Dark #1 66.27 33.73 1.96 Dark #2 65.95 34.05 1.94

The “light” phase Ce/O ratio is close to Ce4O7 while the “dark” phase is closer to a slightly sub-stoichiometric form of CeO2 (i.e. CeO2-x)

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Another sectioned CeO2 pellet (#2) microstructure which was hot pressed at a lower temperature

Left – Some regions of Pellet #2’s microstructure was very similar to Pellet #1’s with “dark” and “light” phases and “black” inter- and intra-granular regions

Right - However, some areas of the mounted specimen did not exhibit any “dark” phase within that particular section of the microstructure

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Lessons Learned/Summary

Employing commercially produced CeO2 powder a series of hot press experiments have been performed which determined the feasibility of using this ceramic processing technique in the fabrication of surrogate fuel pellets

Pellets with aspect ratios of up to ~1 have been produced for further evaluation via a number of techniques including optical microscopy, SEM, and EDS

Examination of the microstructures of hot pressed pellets produced tend to show the presence of two crystallite phases which have been determined to differ in their relative atomic percent Ce/O ratios

Generation of these two phases is believed to be the result of exposure of the material to the process parameters of high temperature, low oxygen pressure, and the presence of graphite

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Acknowledgements

This research was conducted under U.S. Department of Energy contract DE-NE0000422

The technical support of Mr. Dirk Cairns-Gallimore (U.S. DOE Office of Space & Defense Power Systems) is greatly appreciated

� Hot Pressing of CeO2 Ceramic Pellets�� Since the development of space nuclear power ~fifty years ago several different 238Pu fuel forms have been employed�Ceramics technology forms the basis of 238PuO2 full pellet fabricationHot pressing utilizes both Temperature + Pressure as driving forces to enhance densification/sinteringAdvantages of a non-radioactive surrogate for performing various support endeavorsTwo important ceramic properties – Melting/Sintering Temp and Thermal ExpansionExperiments are being performed in a hot press which provides flexibility in developing t-T-P parameters CeO2 powder is loaded within graphite dies which are assembled and positioned within the hot pressDie assembly loaded into the hot press prior�to the initiation of an experimental runPictures of an assembled hot press die during an experiment Temperature during the t-T-P hot press cycle is monitored employing an optical pyrometerExample of a possible hot press t-T-P process cycle exhibiting variations in temp/pressure over timeAfter the completion of a hot press experiment the die is removed and the pellet is ejected from the dieMicrostructural examination of hot pressed CeO2 pelletsExample of a hot pressed CeO2 pellet (#1) before and after sectioningThree distinct phases have been observed in hot pressed CeO2 pellet microstructuresDetermination of the difference between the “light” and “dark” phases observed in the microstructureSEM/EDS analysis suggests a difference in oxygen (at. %) content of the two main observed phasesAnother sectioned CeO2 pellet (#2) microstructure which was hot pressed at a lower temperatureLessons Learned/SummaryAcknowledgements