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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963
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photo
ANALYSIS AND SIGNIFICANCE OF RARE-EARTH OXIDES PRODUCED
BY A SOL-GEL METHOD*
LJUN 27 1966
H.C.37.00;
S. R. Buxton, C. J. Hardy, ** and M. H. Lloyd
April 26, 1966
LEGAL NOTICE
RELEASED FOR ANNOUNCEMENT
This report mo prepared as an account of Goverocent sponsored work. Neither the United
Suatos, aor ebe Commission, aor Lay Dorsod acung oa beball of the Commission:
A. Makes way wurrunty or represcataloa, expressed or implied, with respect to the accu.
racy, completeness, or usefulness oi tho iaformation contained lo wla roport, or that the uso
of any information, apparatus, molbod, or process disclosed in this ropor: may not infringe
brinitely owned rights; or
B. Assumes way labilities will respect to the use of, or for damages resulung from the
use of way information, apparatus, qelbod, or process disclosed in this repori.
As used in the above, "pornoo acting oa beball of the Commission" Includes nay em-
ployee or contractor of lbe Commission, or omployee of such contractor, to the extent that
SUCA employee or contractor of the commission, or employee o! such contractor prepe.res,
disseminates, or provides access to, any information pursuant to his employment or contract
with the Commission, or bls employment with such contractor.

IN NUCLEAR SCIEFCB ABSTRACTS
OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
Operated by
UNION CARBIDE NUCLEAR COMPANY
for the
U. S. ATOMIC ENERGY COMMISSION
* Research sponsored by the U, S. Atomic Energy Commission under contract with the
Union Carbide Corporation.
** Guest scientist at the Oak Ridge National Laboratory, 1965-1966. from the Chemistry
Division, Atomic Energy Research Establishment, Harwell, England.
The text of a paper to be presented at the Third International Materials Symposium
on Ceramic Microstructure, Berkeley, California, June 14-16, 1966.
ODN! - Àfr-nrririni
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ANALYSIS AND SIGNIFICANCE OF RARE-EARTH OXIDES PRODUCED
BY A SOL-GEL METHOD
S. R. Buxton. C. J. Hardy, and M. H. Lloyd
ABSTRACT
Rare-earth oxides have been prepared by a sol-gel method and their micro-
structure examined at various stages by metallography, electron microscopy, and
x-ray spectroscopy. A rare-earth hydroxide sol is prepcred from the nitrate, de-
hydrated to a gel, and this is calcined to dense oxide at relatively low temperature
under conditions which depend on the crystal form and properties required, e.g.,
cubic europium oxide at 1000°C, monoclinic europium oxide at 1475°C. Rare-
earth oxides have potential applications in the production of uniform cermers, e.g.,
monoclinic europium oxide in control rods in nuclear reactors, and as stand-ins
for americism and curium oxides which are being producted in the national heavy
element research program.
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ANALYSIS AND SIGNIFICANCE OF RARE-EARTH OXIDES PRODUCED
BY A SOL-GEL METHOD
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S. R. Buxton, C. J. Hardy, and M. H. Lloyd
INTRODUCTION
Rare-earth oxides have been made in the form of microspheres of controlled
size, high density, and high crushing strength, by the use of sols and gels as inter-
mediates. Rare-earth oxides in this form have potential applicarions in nuclear
reactors and in industry in general.
FLOWSHEET FOR THE PREPARATION OF RARE-EARTH OXIDES
A flowsheet is given in Figure 1 for the five stages of precipitation of hydroxide,
washing of the precipitate, formation of the sol, formation of the gel microspheres,
and calcination to oxide. in the washing step an alternative method to repeated
centrifugation and resuspension is the washing of the precipitate on a sintered-glass
filter. The heating step of 1 hour at 80°C is advisable for all the rare-earths and is
essential for the heavier ones in order to convert the viscous paste to a watery liquid
which is then suitable for evaporation to a metal concentration of up to about 2.5 M.
The pH decrea ies from 9 to about 6.5 in the heating step. The mole-ratio of residual
nitrate to metal varies between 0.06 for the heavy rare-earths to 0.26 for the light
rare-earths.
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1
STRUCTURE OF THE SOLS AND GELS
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.
The structure of the particles in the sols and gels has been determined by
electron microscopy and is discussed in detail in a separate paper.' The essential
feature of the sols is that the colloidai particles afier the heat treatment are bundles
of rods, which for praseodymium are up to 1000 Ă long and 200 Ă wide. The rods
consist of aggregates of small crystals which electron diffractior patterns show to be
the hexagonal form of the rare-earth trihydroxides.
FORMATION OF GEL MI CROSPHERES AND CALCINATION TO OXIDE
The method of forming microspheres of gel by dehydration of droplets of an
aqueous sol with a long-chain alcohol in a tapered column has been described in an
ORNL report, and the apparatus is shown schematically in.Fig. 2.
Micrcspheres of praseodymium hydroxide gel and calcined oxide are
illustrated in Fig. 3, and of europium hydroxide gel and calcined oxide in Fig. 4.
The gel microspheres were dried in vacuum for 24 hours at 120°C, heated in
vacuum + - 500°C over a period of about 2 hours and held at that temperature for 3
hours to decompose the residual nitrate. The praseodymium hydroxide-oxide spheres
were then heated in air to 1000°C over a period of 2 hours, held at that temperature
for 3 hours, and slowly cooled to room temperature. The europium hydroxide gel
microspheres were treated similarly except that the firal temperature was 1475°C
in order to obtain the monoclinic form. The cubic form of Eu, Oz is obtained at
1000°C. 3
Polished cross-sections of oxides prepared from dilute (0.5 M) sols of
praseodymium and europium – droxides are shown in Fig. 5. Some internal porosity is
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.t
-
-,
-
PE--
:
visible and this can be reduced by increasing the concentration of the sol to 2-2.5 M
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in the sphere-forming stage. Some physical properties of several batches of the
two oxides are given in Table 1. The ease with which oxide of high density is obtained
at these relatively low temperatures is explained by the small size of the basic
crystallites in the sols and gels which give rise to high sintering pressures.
Table 1. Properties of Pr,Oy and Eu, Oą Microspheres
Maximum
Temp. of
Calcination
(°C)
Crystallite
Size
(x-ray)
Density
(toluene)
(% theory)
Crystal
Form
Surface
Ayea
(
m g)
Crushing
Strength
(156u diam)
(g)
Oxide
1000
cubic
200-300
90-95
---
600
Pro ou
Eu, O2
1475
monoclinic
600-700
90-95
0.01-0.03
300
SIGNIFICANCE AND APPLICATIONS
The two significant features of the method described are that particles
of rare-earth oxides can be prepared (i) with a controlled size without the use of
expensive or wasteful sieving operations (however, if irregularly shaped aggregate
is required, it can be produced by eliminating the sphere-forming stage), and
(ii) with a high density at a relatively low calcination temperature (the density
achieved is higher than that obtained, for example, by pressing as-received europium
oxide into pellets and firing at temperatures up to 1800°c."
Some of the potential applications of rare-earth oxides in the form of
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microspheres are sumn arized in Table 2.
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Table 2. Applications of Rare-Earth Oxide Microspheres
1.
Incorporation of samarium, gadolinium, or europium oxide as a neutron absorber
in cermets in reactor control rods, or as dispersions in fissile oxides (UO)
as burnable poisons or to improve properties.
2. Radioactive isotopes of europium, thulium, ci promethium in thermo-electric
generators or high-level radiation sources.
3.
Storage and dispensing of rare-earth oxides in a form with low surface area and
low rate of absorption of CO2; this may be particularly useful for small
amounts of valuable separated rare-earth isotopes.
4.
Production of metal oxide catalysts containing rare-earth oxides in a very fine
state of divisico by mixing a sol of a rare-earth hydroxide with a suitable
sol of a compound of another element, and calcining the mixture. This
method may also be applicable to making rare-earth oxide phosphors with
a uniform distribution of the activating ions in the host lattice.
5. Application of the methods to the preparation of oxides of trivalent americium
and curium for irradiation in the High Flux I satope Reactor to produce
transcurium isotopes in the national heavy element research program.
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:
ACKNOWLEDGMENTS
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The Analytical Chemistry Division is thanked for measuring densities, surface
areas and crystallite sizes of microspheres, the Metallography Section for polished
cross-sections and photomicrographs, and T. E. Willmarth for electron microscopy
and electron diffraction measurements.
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REFERENCES
1.
S. R. Buxton, C. J. Hardy, and M. H. Lloyd, "The Preparation and Nature of
Rare-Earth Hydroxide Sols and Implications on Transplutonium Element
Oxides." Paper to be presented to ihe 12th Annual Meeting of the American
Nuclear Society, Denver, Colorado, June 19-23rd, 1966.
2.
J. P. McBride, et. ai., ORNL-3874 (1966).
3. C. E. Curtis and A. G. Tharp, J. Amer. Ceramic Soc. 42, 151 (1959).
4.
G. L. Ploetz, C. W. Krystyniak, and H. E. Dumas, J. Amer. Ceramic Soc. 41, 551 (1958).
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-.
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.
LIST OF FIGURES
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Figure 1. Flowsheet for Production of Lanthanide Hydroxide Sols and
Oxide Microspheres.
Figure 2. Schematic Diagram of Apparatus for the Preparation of Gel
Microspheres.
Figure 3. Microspheres of Praseodymium Hydroxide Gel and Calcined Pr,Oy:
Figure 4. Microspheres of Europium Hydroxide Gel and Caicined Eu, Oz.
Figure 5. Polished Cross-Sections of Microspheres of Pr, o, arid Eu, Oą.
ORNI-AFC-OFFICIAL......
ORNL-DWG 66-3235R

PRECIPITATION OF HYDROXIDE
WASHING
ADDITION OF 0.2 M Ln (NOzla
TO 20X EXCESS OF 8 M NH OH
WITH STIRRING, 25°C
CENTRIFUGATION AND 5-6
WASHES WITH CO,-FREE
WATER UNTIL pH = 9
GEL MICROSPHERES
SOL FORMATION
FORMATION AND DRYING OF SPHERES
WITH LONG-CHAIN ALCOHCL IN
TAPERED COLUMN
I HR, 80°C; EVAPORATED
FROM [M] = 0.5-2.5 M
(NOZ/M = 0.06-0.26)
CALCINATION
OXIDE MICROSPHERES
VAC, 120°C, 5-70 HR
VAC, 500°C, 3 HR
AIR 1200-1475°C, 3 HR
50-250 DIAMETER
90-95% DENSITY
0.01-0.05 m?/
FIG.
FLOWSHEET FOR PRODUCTION OF LANTHANIDE HYDROXIDE
SGLS AND OXIDE MICROSPHERES
TVIDI10-33V - INTO
INDISO-336-INDO
uma ASSIFIED
TWO-FLUID NOZZLE
AQUEOUS SOL FROM
SYRINGE PUMP
TO DISTILLATION
SYSTEM +
6-in. DIAM
12 in.
3-in. DIAM -
0-0.2 GPM
0-3 GPM
*
CENTRIFUGAL
PUMP
€
40 in.
FROM DISTILLATION SYSTEM
DIAM
2-in. DIAM-Ho
in.
3-in. DIAM
16
ORGANIC SOLVENT ENTERS
COLUMN TANGENTIALLY
COLLECT GELO
SPHERES
FIG. 2. SCHEMATIC FLOW DIAGRAM OF EQUIPMENT
FOR FORMING GEL SPHERES
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PHOTO 83245
Y-70893
Y-70894
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-0.018 INCHES -
TTTTTTT 200XTTTT
i Euro
hemma
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Polished Cross-Sections of Microspheres of Praonand Eu,Oz.
FIG.5.
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END
DATE FILMED
71 / 29 / 66






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