Comparison of ceramic waste forms produced by hot uniaxial pressing and by cold pressing and sintering Page: 5 of 11
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for 2 hours using a graphite susceptor and an induction furnace. Approximately 50
grams of each composition was also similarly prepared and pressed in bellows at
1170 and 12500C to allow comparison of phase formation and densification.
Samples were prepared for sintering by adding the appropriate amount of nitrate
solutions to small batches of the undoped precursor material described above. The
doped materials were dried and calcined under conditions identical to those used for
the hot-pressed samples. At LLNL, the samples were mixed with carbowax to act as a
binder, pressed into pellets, and placed in a 3" tube furnace. A flow of Ar + 3.5% H2
atmosphere was started through the furnace and then a programmed heating cycle
was started. The temperature was ramped by 10 Co/min from ambient to 2000C and
then held for 30 minutes to burn out the binder. Then the temperature was increased
by 20 Co/min from 200 to 1200 oC, where it was held for 1 hour. The material doped
with 25% U was pressed at 15,000 psi (103 MPa) prior to sintering. The material
doped with Gd and U could not be formed into coherent pellets at low pressure.
Pressing was eventually done at 40,000 psi (276 MPa) to produce acceptable pellets.
Pressing at ANSTO was done using a hand press; no binder was used. Sintering
was done at 12000C in three different atmospheres - ambient air, nitrogen, and N2 +
3.5% H2 without a low temperature hold step.
RESULTS AND DISCUSSION
The undoped powders were pressed in a graphite die at 11500C/20MPa and
yielded an Archimedes density of 3.76 g/cm3. which is 96% of the theoretical density
calculated based on the X-ray lattice parameters given in the JCPDS file. The 400 g
samples pressed at 11200C had Archimedes densities of 4.26 g/cm3 for the Gd + U
sample and 4.06 for the U doped sample, with open porosities of 0.1 and 2.2%
respectively. Both of these densities were lower than values expected based on the
processing parameters normally used for preparation of Synroc samples; however,
the x-ray diffraction data for these samples showed the expected phase assemblages,
except that an additional U02-rich fluorite structured phase appeared in the U-doped
sample. The lower than expected densities were no doubt due to the use of an
insufficiently high pressing temperature. To investigate this possibility, a smaller
sample of each composition (about 4 cm in diameter) was pressed at higher
temperatures. The 11700C samples gave densities of 4.42 g/cm3 for the Gd + U
sample and 4.67 g/cm3 for the U-doped sample. The phases obtained by x-ray
diffraction for these samples were the same as those found for the lower pressing
temperature. The samples pressed at 12500C had Archimedes densities of 4.52
g/cm3 for the Gd + U sample and 4.66 g/cm3 for the 25% U sample.
Back-scattered SEM images indicated the presence of approximately 10 to 20
micrometer-sized regions that had essentially uniform back-scattering intensity within
themselves, apart from micrometer-sized intergrowths of the minor low-Z spinel and
nepheline phases, and were bounded by thin, irregularly shaped regions (see Fig.1).
Some porosity was clearly present also. The irregular boundaries of the 10-20 um-
sized zones suggest that they are not fully-developed zirconolite grains containing
intergrowths of other phases, but rather that they are relict structures of agglomerates,
which were originally present in the calcine. It is believed that hot pressing has
induced a small amount of melting of a metastable glassy phase and that this has
flowed into the spaces between the agglomerates to yield the observed dark
boundaries between the zirconolite-rich regions. The intensity contrast between the
different zirconolite-rich zones is probably due to some variation in the concentrations
of U and Gd , which may have occurred during the late stages of evaporative drying in
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Oversby, V.M. & Vance, E.R. Comparison of ceramic waste forms produced by hot uniaxial pressing and by cold pressing and sintering, article, September 1, 1994; California. (https://digital.library.unt.edu/ark:/67531/metadc620607/m1/5/: accessed May 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.