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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963
ORNIA_3338
Conf-670533-07.
CRITI PROTO
---
4411
HC $23.00 MN_65
on, mi...
MASTER
"...
13
METALLOGRAPHIC PREPARATION OF CERIUM CARBIDES*
-
RECEIVED BY DTIE OCT 2 1967
...
T. M Kegley, Jr. ...
Metals and Ceramics Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee
....
.
:
W.5
ABSTRACT
-
Special procedures were required to prepare cerium carbides metallo-
graphically because of their strong reactivity with moisture. A chemical,
an electrolytic, and a protective-oil method were developed for preparing
these carbides for examination. In both the chemical and electrolytic
methods a stainless steel screw served as a contact with the CeCa.
.
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.
Cerium dicarbide is ej. cher chemically polished or passivated by im-
mersion in equal parts acetic, lactic, and nitric acids depending upon
whether the mount is partially or totally immersed. If the Ceca specimen
is partially immersed so that the stainless steel screw does not contact
the solution, only whemical polishing takes place. If the mounted speci-
men is totally immersed, & galvanic cell is created such that Ceca is the
anode and stainless steel is the cathode. The emf developed by this cell
provides the necessary potential for passivation of the Ceca.
.
;
In the electrolytic method the CeCa was anodized at 5 v for 10 sec in
equal parts acetic, lactic, and sulfuric acids.
In the protective-oil method CeC. was chemically polished in acetic-
lactic-nitric acids and then washed with ethyl alcohol. Silicone oil was
added to displace the ethyl alcohol, and a cover glass was placed on the
CeC specimen. For two-phase specimens containing Ceca and CeaCs, the
protective-oil procedure was followed but with the exception that chemical
polishing was not employed.
1
.
N
#
INTRODUCTION
1
Cerium carbides are of general interest to the nuclear energy program
because of the chemical similarity of lanthanide and actinide compounds.
- -
-
-
-
*Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation..
.
IN LATINA
. :
DISTRIBUTION OF THIS DOCUMENI IS UNLIMITED,
- 2 -
An investigation of the chemical properties of cerium carbides mey give us
an insight to the chemical properties of plutonium carbides. Metallography
serves as one means of characterizing the purity of arc-melted cerium car.
bides used in such an investigation.
In only one instance have photomicrographs of the microstructure of
cerium carbides been found in the scientific literature. This may result
from a lack of awareness of the value of metallography as in investigatory
adjunct or it may result from the difficulties encountered in the metallo-
graphic preparation of cerium carbides, Cerium carbides are difficult to
prepare metallographically, mainly because they react strongly with mois-
ture either in the room air or in the polishing media.
At Oak Ridge National Laboratory (ORNL) we have had considerable suc-
cess in the metallographic preparation of thorium-uranium carbides and
thorium carbides, which are also moisture reactive. The method devised
for these carbides is to immerse the specimens in nitric acid solutions to
give the polished surface sufficient passivity so they do not react with
moisture contained in the room air. Simple immersion in & nitric acid
solution was not effective for cerium carbides since the surface was etched .
rather than rendered passive, We, therefore, resorted to other methods for
preparing these carbides metallographically. We developed what might be
termed an electrolytic method, a chemical method, and a protective-oil
method.
METALLOGRAPHIC PROCEDURES
The arc-melted cerium carbide specimens were mounted in epoxy resin,
ground successively on 320, 400, and 600 grit silicon carbides papers using
ethyl alcohol as the lubricant, and then polished 5 min on a Metcloth-Cuv-
ered wheel using Linde A abrasive and ethly alcohol. Following this polish-
ing, one of three methods, to be described, was employed so the specimens
could be prepared, observed, and photographed properly under the microscope. .
The chemical and electrolytic methods were effective only with single phase
Cecy, while the protective-oil method could be used with all compositions.
In the chemical and electrolytic methods & stainless steel screw was driven
into the epoxy mount to make electrical contact with the Cece specimen.
Chemical Method
The adjective "chemical" as used to describe this method is employed
only in the sense than an externally applied emf is not used. As explained
later, the method actually involves an electrolytic process.
After a mechanical polish, the Cec, specimen was chemically polished
by partially immersing, as indicated in Fig. 1, 15 sec in equal parts acetic,
lactic, and nitric acids.* The dark brown film formed on the polished
*Strengths of concentrated reagents employed were 100% glacial acetic
acid, 85% lactic acid, and 70% nitric acid.
•
3
•
surface during the chemical polish was then removed by washing with ethyl
alcohol. After washing in ethyl alcohol the specimen was totally immersed,
as indicated in Fig. 1, 30 sec in equal parts Acetic, lactic, and nitric
acids. Totally immersing the specimen mount 80 that the stainless steel
screw contacts the acetic-lactic-ritric acid solution resulted in passi-
vation of the specimen surface rather than chemical polishing. During
passivation a golden brown transparent film formed on the polished surface,
This film rendered the surface sufficiently passive 80 the specimen could
be observed in open air for the period required for microscopic observation
and photomicrography.
Totally immersing the specimen mount so the stainless steel screw
contacts the electrolyte in effect creates an electrolytic cell. As Cec,
dissolves in the electrolyte cations go into solution and electrons are
left behind on the CeCa. Since stainless steel dissolves in the electro-
Lyte to a much lesser extent than does the Ceca, it becomes the positive
pole while the Cec, becomes the negative role of an external circus
in the electrolyte cations are attracted to the stainless steel and anions
are attracted to the Ceca, the stainless steel becomes the cathode and the
Ceca the anode of the internal circuit.
We performed a simple experiment to establish the polarity of the cell
just described. We immersed the mounted Ceca specimen in the acetic-lactic-
nitric acid electrolyte so the stainless steel screw contact remained out of
the electrolyte. We then measured the emf existing between the CeC and a
second stainless steel screw placed in the electrolyte and found the em to
be about 1.5 v, the stainless steel being the positive pole, and the Ceca
the negative pole. The resistance of the voltreter kept the current so low
that the Cece was chemically polished rather than anodized. We then con-
nected an ammeter between the Cec, and the stainless steel. Initially, the
current measured 1.6 ms but fell to a value of 0.02 ma after about 1 min.
While the ammeter was connected a golder brown film formed on the CeC and
the Ceca became passivated. At this time the ammeter was replaced with a
voltmeter and the emf then existing between the Ceca and the stainless steel
was found to be 0.10 V. The voltage and current behavior point to the for-
mation of an anodic film on the Ceca and an indication of the passivating
efficiency of the filu. Koch and Picklesimer* have reproted forming similar
protective films on lanthanum and cerium metallographic specimens by using an
impressed emi.
Thus, the passivation of Ceca by acetic-lactic-nitric acids is an anodic
process. Evidently the emf developed between Ceca and stainless steel when
both are iramersed in the acid electrolyte provides the electrical potential
required for passivation of the CeCa.
Electrolytic Method
After a mechanical polish, the Ceca specimen was chemically polished in
equal parts acetic, lactic, and nitric acids just as in the chemical method.
LEGAL NOTICE
Tato report mo popared a la account of Coverament sponsored work. Nolther the Vallad
Balas, nor the Commission, nor any pornoa acting on behalf of the Commiuloa!
A. Makao may warranty or representation, expressed or implicd, with respect to the accu-
rrey, completeness, or unofalness of the laformation contained in wide report, or that the une
of sy laformation, apparatus, method, or procesi di cloned in the report may not latring
printely owned rigua; or
B. Ammo may ilabillues with respect to the wee of, or for damages retung from the
un of way lalormation, appunto, method, or process dixcloud la o raport.
As und in the above, pornoa acting on behall of the Commiulon" includes way toº
ployee or contractor of the Commision, or employee of such contractor, to the extent that
lucha enfloyw or contractor of the Commission, or employee of mich contractor preparat,
donnina , or provides accento, Any Information pursuant to Wo employment or coatract
. with the Cownlaola, or Mo employant with such contractor.
.4.
After the chemical polishing, the specimen was washed with ethyl alcohol and
then anodized 20 sec at 5v in a solution of equal parts acetic, lactic, and
sulfuric acids, * which is the same solution Chamberlain and Paige employed
in the metallographic preparation of uranium and plutonium carbides.S. This
electrolytic anodization gives the surface a golden tint and leaves the sur-·
face passive so the specimen can be observed in room air.
TA
Protective-011 Method
In the protective-oil method, single-phase Cecy and two-phase specimers
of Ceca and Ceaca were treated somewhat differently since single-phase Ceca
could be chemically polished, while two-phase cerium carbide specimens could
not. The acetic-lactic-nitric chemical polish etched and pitted the two-
phase specimens ,
Cerium dicarbide was first chemically polished by immersion in equal
parts acetic, lactic, and nitric acids. This was best done by immersion
since swabbing tended to remove material unevenly, leaving swab traces on.
the surface. A cover glass was placed on the specimen after first washing
with ethyl alcohol; silicone oil was then added to the polished surface to
displace the alcohol. This must be done without allowing the polished sur-
face to come directly in contact with the room air. Any excess oil emerging
from the edge of the cover glass was removed by blotter paper. Fortunately,
the refractive index (1. 517) of silicone oil is near that of the cover glass
80 there was essentially only one additional reflective surface to reduce
the brilliance of the microscope image. Care should be taken that the cover
glass is free of dust and lint.
wo-phase cerium carbide specimens could not be prepared by
the electrolytic or chemical methods, the protective wil method was em-
ployed. After mechanical.ly polishing with Linde A abrasive, the two-phase
specimens were mechanically polished an additional 30 sec with Magomet Mgo
abrasive on a Microcloth-covered wheel using ethyl alcohcl as the lubricant.
After washing with ethyl alcohol, silicone oil plus & cover glass were
placed on the polished surface, as was done with the dicarbide specimen.
RESULTS
The first attempt at preparing Cec, consisted simply of polishing the
specimen with Linde A abrasive on a Metcloth-colvered wheel using ethyl
alcohol as the lubricant. After polishing was completed, silicone pil was
added to the surface and a cover glass placed on the specimen. Although
the Ceca structure was revealed witih polarized light, the results from this
procedure, as given in Fig. 2, left much to be desired.
RP
1-
*Strengths of concentrated reagents employed were 100% glacial acetic
acid, 85% lactic acid, and 95-98% sulfuric acid.
• 5 -
The next development was to chemically polish whe Cec, specimen with
acetic-Lactic-nitric acids and then anodize the specimen in acetic-lactic.
sulfuric acids. The resulting bright field and polarized light microstruc-
tures are given in Fig. 3. The twins presumably are due to a cubic-tetragonal
transition which occurs on cooling. Krikorian et al., have reported that a
tetragonal-to-cubic transition occurs in Cec, on beating at 1090c.
We next tried to find a chemical method for passivating the polished
surface of Ceca. At first we erroneously attributed the good results shown
la Fig. 4 to the use of a procedure in which the Cecwas first swabbed with
the acetic-lastic-nitric acid chemical polish and then swabbed with the
acetic-lactic-sulfuric acid solution. The passive film, which formed on the
polished surface, was actually due to a stainless steel screw which was in
contact with the CeC, when the specimen was swabbed in the acetic-lactic-
lon. Since a passive film was already on the palished
surface, swabbing in the acetic-nitric-sulfuric acid solution really had no
effect. Figure 5 shows the results obtained using the chemical method in
which Ceca is first chemically polisked by partial immersion in acetic-
lactic-aitric acid and ther passivated by total immersion in the same 80-
lution.
The third method developed for preparing Ceca metallographically was
the protective-oil method. In this method, the Cec, was first chemically
polished by immersion in the acetic-lactic-nitric acid solution, silicone
oll added to the surface, and a cover glass placed on thae specimen. Figure
6 shows the resulting microstructure as viewed in bright field, polarized
light, and sensitive tint plate (full wave retardation) rotated to give
maximum contrast.
Since the two-phase corium carbide specimens were not amenable to the
electrolytic and chemical procedures used for Ceca, the protective-oil
method was used. Figures 7 and 8 show the microstructures for carbide
specimens of Cecy.a, and CeC2.48 combined carbon com ositions. The Cenca
appears gray and the Cecy appears white under bright field illumination.
Polarized light is necessary to show the twinned structure of the Ceca
phase.
Reasonably satisfactory methods have been developed for metallographi-
cally preparing the Ceca specimens, but two-phase specimens containing Cec,
and Ceacz could only be prepared for microscope observation through the use
of a protective oil (silicone oil) and & cover glass to prevent reaction
with moisture. The sesquicarbide phase always exhibited fractures, the
origin of which may be due to an improper po'.isbiny procedure, As yet, we
have found no method which prevents these fractures.
.6.
REFERENCES
1.
P. Stecher, A. Neckel, F. Benesovsky, and E. Notwotny, Planseeberichte
f. Pulvermetallurgie 12, 281 (2964).
T. M. Kegley, Jr. and B. C. Leslie, J. Mucl. Mat. 13, 283 (1964).
T. M. Kegley, Jr. and B. C. Leslie, Metallographic Preparation cf Di..
Carbides of Thorium and Thorium-Uranium, ORNL-TM-949 (Nov. 1964).
4.
5.
C. C. Kocia and M. L. Picklesimer, Trans. Met. Soc. AIME 339, 759 (1967).
6. E. Chamberlain and E. L. Paige, A Metallographic Technique for Dis-
tinguishing Between Metal and Sesquicarbide Phases in the U-C and
Das Puis. -C Systems, AERE-R 4334 (June 1963).
N. R. Krikorian, T. C. Wallace and M. G. Bowman, Phase Relationships of
the High Carbon Portion of the Lanthanide Ricarbides, LA-DC-681 (1904).

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FIGURE CAPTIONS
h
.
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Fig. 1.
W
ORNL-DWG
67-3430
Total and Pertial Immersion Methods for Treating Cerium Dicarbide
in Equal Parts Acetic, Lactic and Nitric Acids.
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Fig. 2.
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Y-63483
Y-63462
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Cerium Dicarbide Mechanically Polished on Metcloth Using Lincie A
Abrasive and Ethyl Alcohol. Silicone oil and a cover glass were
placed on specimen to prevent reaction with moisture. ) Bright
field. (b) Polarized light.
250 X
.
.
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Y-77414
Y-77413
Fig. 3.
Cerium Dicarbide Anodized Electrolytically at 5 v for 10 sec in
Equal Parts Acetic, Lactic, and Sulfuric Acids. Specimen was
chemically polished, before anodizing, by partial immersion for 15
sec in equal parts acetic, lactic, and lactic acids. (a) Bright
field. (b) Polarized light.
250 X
Fig. 4.
Y-63501
Y-63500
Cerita Dicarbide After Swabbing with Equal Parts Acetic, Lactic,
and Nitric Acids and Then Swabbing with Equal Parts Acetic, Lactic,
and Sulfuric Acids. (a) Bright field. (b) Polarized light. 750 X
Y-77422
Y-77420
Fig. 5.
Cerium Dicarbide Chemically Passivated by Total Immersion for 30
Bec in Equal Parts Acetic, Lactic, and Nitric Acids. Before pas-
sivating, specimen was chemically polished by partial immersion
for 15 sec in same equal parts solution. (a) Bright field.
(b) Polarized light.
250 X
Fig. 6.
Y-67733
Y-67734
1 Y-67735
Cerium Dicarbide Polished Chemically by Immersion in Equal Parts
Acetic-Lactic-Nitric Acids. Silicone oil and a cover glass were
placed on surface to prevent reaction with moisture. (a) Bright
field. (b) Polarized light. (c) Sensitive tint with half-wave
plate rotated to give maximum contrast.
250 X
Y-67367
Y-67366
Fig. 7.
Two-Phase Cerium Carbide Specimen of Cece, Combined Carbon
Composition Mechanically Polished on Metcloth Using Linde A
Abrasive and Ethyl Alcohol. Silicone oil and cover glass were
placed on surface to prevent reaction with moisture. ( Bright
field. Gray phase is Cezca, white matrix phase is Сec (6) Pol.
arizod light.
250 X
Y-63605
Y-63604
Fig. 8.
Two-Phase Cerium Carbide Specimen of cec, .48 Combined Carbon
Composition Mechanically Polished on Metcloth Using Linde A
Abrasive and Ethyl Alochol. Silicone oil and a cover glass were
placed on specimen to prevent reaction with moisture. (a) Bright
field. Gray matrix 18 Ce, Ca, white phase 18 Ceca. (b) Polarized
light.
250 X
ORNL-DWG 67-3430
STAINLESS STEEL
SCREW
SOLUTION
LEVEL-
INI
LUMINIO
L
- CERIUM
DICARBIDE
1IIIIIIIIII
OTTITUTE
MUNTII
PASSIVATING (TOTAL IM-
MERSION IN EQUAL PARTS
ACETIC, LACTIC AND
NITRIC ACIDS)
CHEMICAL POLISHING
(PARTIAL IMMERSION IN
EQUAL PARTS ACETIC,
· LACTIC AND NITRIC ACIDS).


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