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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963
90
Orne-p_1252
MASTER
-
--
JUN 24.96!
CFSTI PRICES
17191130 - 337 1:139
H.C. $ 3.00; MN-50
CORRELATION OF COMPOSITION WITH THE MICROSTRUCTURES OF
ARC-CAST THORIUM CARBIDES*, **
By
Mildred J. Bradley and T. M. Kegley, Jr.
Oak Ridge National Laboratory, Oak Ridge, Tennessee
-LEGAL NOTICE -

THI raport me propers au mocant of Goveransat sponsored work, Nollber the United
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NOT FOR PUBLIC RELEASEDENCIALE
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RELEASED FOR ANNOUNCEMENT
nus
IN NUCLEAR SCIENCE ABSTRACTS
"esearch sponsored by the U.S. Atomic Energy Commission under contract with the
Jaion Carbide Corporation.
**For review by the Journal of the Electrochemical Society.
b..
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ORNL - KEC - OFTICIAL
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diidiiii.
CORRELATION OF COMPOSITION WITH THE MICROSTRUCTURES OF
ARC-CAST THORIUM CARBIDES
Tiigriidedo -530
Mildred J. Bradley and T. M. Kegley, Jr.
Oak Ridge National Laboratory, Oak Ridge, Tennessee
*Research sponsored by the U.S. Atomic Energy Commission under contract with the
Union Carbide Corporation.
ABSTRACT
Arc-cast thorium carbide specimens with total-C/Th etom ratios varying from
0.8 to 2.1 (4 to 10 wt % carbon) were prepared for metallographic examination, and
the microstructures correlated with the chemical analyses, the X-ray diffraction
analyses, and the hydrolysis products. The thorium monocarbide phase was found
over a range of composition from about Thco.4 (4.6% C) to at least Thco.37 (4.0%.
c) and probably lower. The maximum combined-c/Th atom ratio obtained by arc-melt-
ing with graphite electrodes was 1.95 (9.2% c) rather than the theoretical 2.00
(9.4% C). Specimens with compositions from Thco.99 to ThС.88 (4.9 to 8.9% C)
were two-phase mixtures of the mono- and dicarbides. There was no evidence for
any significant range of composition for the dicarbide.
A new metallographic procedure contributed significantly to the study.' A
solution of equal parts of nitric acid (70%) and water was used to passivate and
etch thorium dicarbide and two-phase thorium dicarbide-monocarbide specimens. Ad-
vantages of this technique over air etching are: (1) clearer microstructures;
(2) specimens can be observed directly in air (no protective oil film is neces-
sary); and (3) specimens can be examined more deliberately than air-etched speci-
mens which continue to react rapidly with moisture even under an wil film. Single-
phase thorium monocarbide specimens were prepared for metallographic examination
by an attack-polish procedure, using dilute nitric acid and 0.3 4 alumina, followed
by etching in a solution of 50 parts 70% HNO2, 50 parts water, and 2 parts con-
17101310- 334 - 1;1330
centrated HCL.
INTRODUCTION
Previous investigators of the thorium-carbon system have generally used
"air-etching" (exposure of the polished suriace to the moisture in ambient air)
to reveal the microstructures (1-5). While air-etching does reveal some micro-
structural features, the results are not as distinctive as in the chemical etch-
ing of metals. In addition the specimen must be hurriedly examined immediately
after preparation since the moisture from the air reacts continually with the
specimen. A new procedure using nitric acid to passivate and etch thorium di-
carbide gives much clearer microstructures than those developed by air-etching (6,7).
This paper describes the application of the new nitric acid procedure to the
metallography of arc-cast specimens with total-C/Th atom ratios varying from 1.0
to 2.1 (4.9 to 20 wt% carbon). An attack-polish procedure and HNO, -H,0-HCI
etcluant were developed for single-phase thorium monocarbide specimens with com-
positions of ThCo.8 and Thco.9 (4.0 and 4.6% carbon). Samples of these same
carbide specimens were used in extensive chemical studies of their hydrolysis
properties (8), and were also carefully characterized by chemical and X-ray dir-
fraction analyses.
THORIUM CARBIDE SPECIMENS
Carbide preparation.--Most, specimens were prepared by arc-melting high-purity
thorium metal and spectroscopic grade carbon in a 50 vol % argon--50 vol % helium
atmosphere using graphite electrodes to avoid tungsten contamination.“ Specimen
08!:i-.8.CC - OFFICIAL
“Attempts to prepare thorium dicarbide with tungsten electrodes resulted in speci-
mens containing as much as 6 wt % tungsten.
+
THC -1A was arc-melted with a tungsten electrode in an argon atmosphere. Buttons
were melted until visual examination showed that all the carbon had dissolved in
the melt, and then were melted an additional four times (2-min melts) to ensure
1:13idi) - OD1
50
complete reaction. Mixtures containing less than 8.5 wt% carbon picked up about
0.1 wt aso carbon from the graphite electrodes during melting, while those initially
containing 9.4% carbon or higher usually lost carbon (Table I). Specimen THC-3H
was heat-treated in a graphite crucible for 65 hr at 1600°C using a Brew High
Vacuum Furnace, No. 424B.
Chemical and X-ray diffraction analysis.--Analyses for thorium, total carbon,
free carbon, oxygen (<0.07 wt %), and nitrogen (40 to 172 ppm), and the major car-
bide constituents identified by X-ray analysis are given in Table II. The maximum
combined-C/Th atom ratio was 1.95, rather than the theoretical 2.00. Increasing
the carbon concentration above Thc2.95 resulted in a mixture of ThC,.95 and free
carbon. Specimen ThС,-1A also contained 6.1% tungsten as an impurity from the
tungsten electrode. The specimens prepared with graphite electrodes contained 10
to 100 ppm tungsten. By spectrographic analyses these spécimens contained about
6 ppm calcium, 3 ppm iron, 2 ppm aluminum, 2 ppm magnesium, and 1 ppm copper.
All other metallic impurities were less than 1 ppm. The monocarbide and di-
carbide (ThC, 05) X-ray powder patterns were the same as those reported by
Kempter and Krikorian (9). In the X-ray diffraction analysis of nixtures, there
was no detectable change in the monocarbide portion of the pattern as the total-
c/Th atom ratio increased from 1.0 to 1.5 or in the dicarbide portion as the
ratio increased from 1.5 to 2.1. The lattice parameter of thorium monocarbide
vias 5.346 Å in the Thc2.01 specimen, 5.345 Å in ThC..92, and 5.340 Å in ThCo.81 (10).
No thorium metal was detected by X-ray analysis of the Thco.81 or the ThC., 92 speci-
mens. Thorium mcaocarbide was detected in the Thc, .38 specimen, but not in specimens
with combined-c/th atom ratios of 1.91 or higher. The 6 wt % tungsten impurity in
0:11!! -;.!C - OFICIA!
Table 1. Carbon Contamination of Thorium Carbides from the Graphite Electrodes
971313:0 - 334 - 15:30
Carbon (wt %)
In Original
Th-C
Mixture
In
Arc-Cast
Carbide
Specimen
Tac-4A
3.98
4.00
Thc-10A
4.61
4.50
4.90
Tac-7A
4.86
THC-2A
4.
Tac-8D
5.00
5.21
6.15
6.88
Tac-5A
Thc-3A
4.92
5.85
6.76
7.65
8.52
8.76
8.96
THC-6A
Thc2-58
Thc2-4
Thcz-3
ThC2-6
7.88
8.40
8.78
8.91
9.23
9.38
9.38
Thc2-8B
9.28
9.98
Thcz-?
10.22
specimen ThС-1A could not be detected by X-ray analyses of the original specimen;
however, after hydrolyzing the specimen and dissolving the resultant thorium
oxide in 6 M HCl, half of the tungsten was recovered ai unreacted w,c. One
diffraction line which could have been attributed to WC was also observed in
the X-ray powder pattern of this residue. It is not known which chemical species
of tungsten (W, W,C, or wc) hydrolyzed.
The following is a synopsis of the analytical procedures used in the present
Investigation. X-ray powder patterns were determined with a Debye-Scherrer
0:21!-7.EC - OFFICIAL
ATAU
Table II. Composition of Thorium Carbides
Elemental Analysis
wt%
Total
Free
C C
Carbined-
c/Ta
Atom
Ratio
ORI!!MEC - OFFICIAL
ppm
X-Ray
Powder Pattern
Thc Tac, os
Specimen
Th
ThC-4A
4.00
125
0.81
So
Thc-10A
95.74
95.14
94.90
4.61
0.01
0.07
0.02
272
0.92
THC-7A
4.86
126
0.99
TAC-2A
94.80
0.03
0.06
0.01
0.01
0.06
0.01
0.03
0.03
1.01
ThC-8D
94.67
93.67
Inc-5A
5.00
5.21
6.15
6.88
6.92
7.88
8.40
S
S
S
W
M
s
93.06
92
0.02
0.02
0.02
0.03
0.06
0.02
0.10
1.06
1.27
1.42
1.42
1.64
1.76
93.07
92.10
91.60
0.02
0.01
57
M
S
91.26
8.78
1.84
Thc-3A
ThC-3
THC-6A
ThC2-5B
ThC7-4
ThC23
Thc2-6
Thc2-8B
TC -7
THC,-34.
91.02
1.88
1.95
8.91
9.23
9.28
9.98
9.07
70.76
90.66
89.97
86.0
0.07
0.02
0.01
0.03
0.01
0.02
0.05.
0.09
0.06
0.33
0.90
0.908
1.92
2.95
-
44
45
-1.88
as, strong; M, moderate; W, weak; and T, trace.
"Lattice parameter, 5.340 Å.
"Lattice parameter, 5.345 A.
"Lattice parameter, 5.346 Å.
especimen ThC-3A after heat-treating 65 hr at 1600°C.
Specimen contains 6.1% tungsten.
Samount of carbon combined with the tungsten is not precisely known.
"Plus a few, faint, unidentified, extra lines.
ORN-KEC - OFFICIAL
t
.
0316 -1.50 - 0781018L
114.59-mm-diameter powder camera using Cu K-a radiation. Thorium was determined
by burning 1 g (Thc) to 2 g (Thco) of carbide, which had been crushed to -10 -50
mesh particles, and determining the weight of Tho, at 950°C on a thermobalance.
The weight of Tho, must be measured at high temperatures since the finely divided
powder, even after heating at 1250°C, gained weight upon cooling through surface
sorption resulting in a significant error (W2%) in the thorium anwlysis. The
method for analyzing specimen Incg-lA which contained 6% tungsten was different:
the specimen was hydrolyzed, the resulting thorium dioxide dissolved in 6 N HCI,
the residue removed by filtration, and the filtrate analyzed gravimetrically for
thorium by precipitation as the oxalate. Total carbon in all samples was deter-
mined by burning 0.78 (Thcz) to 1 & (THC) of carbide, which had been crushed to
-10 +50 mesh particles, in pure oxygen at 1000°C for 2 hr (11). The effluent
gases were first passed through copper oxide (650°C) to ensure complete combus-
tion to Con, then through an Anhydrone trap to remove the water, and finally an
Ascarite and Anhydrone trap to sorb the cog. Carbon was determined by weighing
as cog. Complete combustion was not routinely achieved with larger particles
because the Tho, product formed a protective layer over the carbide. The comous-
tion tube must be cooled below 450°C before introducing the sample, since the
thorium carbides ignite in oxygen at ~550°C. Free carbon was determined after
dissolving 1.5 to 3.0 g of caroide as -10 +100 mesh particles in refluxing 6 M
HC1 and filtering through a Leco disposable filtering crucible (No. 528-30,
Laboratory Equipment Corp., St. Joseph, Michigan) (11). The residue was washed
with acetone and ether to disso).ve any wax, dried, and the carbon determined as
co, in the Leco apparatus. A fluorination procedure was used for the oxygen
analysis (12). In the analysis of thorium carbides for nitrogen, 200 to 250 mg
of crushed carbide was dissolved in 9 M H2PO2--2.5 M H2SO4 (11). The solution
was then made basic, the resulting ammonia steam-distilled into a receiver con-
.-.
S
0!!:L-DEC-OFFICIAL
taining dilute sulluric acid, and the ammonia determined spectrophotometricaliy
.
with sodium phenate (13). Trungsten was determined by neutron activation after
first dissolving the carbide in nitric acid containing a trace of hydrofluoric
acid (14). While this was the procedure actually used, for future analyses of
specimens containing >3% tungsten we recommend the gravimetric procedure used
for uranium-tungsten-carbide (15). Neutron activation is still the best method
2:1!1 - 160 - 0TICIAL
for low concentrations of tungsten.
Hydrolysis of thorium carbide --The two thorium carbides yielded distinctly
different gaseous products when reacted with water at 25 to 99°c (8). Thorium
monocarbide produced mostly methane, and also two moles of hydrogen for each
Thorium dicarbide (ThC7.95) produced Ca- to Cg-hydrocarbons, wax, and hydrogen.
The preponderance of hydrocarbons with an even numoer of carbon atoms in the gas
from the hydrolysis of the dicarbide is to be expected because of the presence of
discrete Cg units in the dicarbide crystal lattice (16). Carbon spacing in the
thorium monocarbide lattice has not been studied, but the carbon is probably
present as single C units as in uranium monocarbide (17), which also yields
methane when hydrolyzed (18). The total volume of gas evolved decreased from
110 ml/g to 49 ml/g as the c/m atom ratio increased from 0.81 to 1.95 (Fig. 1);
there was no further change in volume as the total-c/mn atom ratio increased
above 1.95. Specimens with compositions between Thc and Thc, os gave the pro-
ducts expected for ThC-Thc2.95 binaries, showing a regular decrease in the amount
of methane and increase in the amount of Co- to Cg-hydrocarbons as the c/ma atoui
ratio increased (Fig. 2). Within experimental error, specimen ThC2 -7 with a
total-c/Th atom ratio of 2.14 (combined-c/Th, 1.95) zave the same products as
specimen Thc2-6 (ThC2.95). Fifteen percent of the carbon in Thc,..2 (specimen
C-2A) was found as Cz- to Cg-hydrocarbons (8), which are 'dicarbide products,
vs. only 5% from ThCo.92 (8) and 8% from UC..97 (18). This indicates that Thcy.cz
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Thorium Carbides.
drolysis at 80°C OI AS-Cast
probably contains a little thorium dicarbide. A complete discussion of the
1:131:29 no 31*7;100
hydrolysis properties of the specimens used in this study is given in Ref. 8.
METALLOGRAPHIC PROCEDURES
Specialized procedures were required for the metallographic preparation of
thorium carbides because of their strong tendency to hydrolyze.
Metallographic preparation of thorium dicarbide and two-phase_monocarbide-
dicarbide specimens.--The procedures used in the metallographic preparation of
thorium dicarbide and monocarbide-dicarbide mixtures were similar to procedures
previously developed for (m2, U)c(6,7). Fragments of the arc-melted specimens
were mounted in epoxy resin and then rough-ground on 320-, 400-, and 600-grit
silicon carbide papers using either absolute ethyl alcohol or silicone oil as
the lubricant. The specimens were then either vibratorily polished 6 to 12 hr
within a "dry" box using a silicone oil-0.3 w alumina slurry on a nylon cloth,
or were mechanically polished 3 min outside the dry box using an absolute ethyl
alcohol.-0.3 u alumina suspension on a wheel covered with a nylon cloth and
operated at 100 to 200 rpm. Direct exposure of the active polished surface to
the moisture contained in room air was avoided by immediately washing the surface
with absolute ethyl alcohol and then immersing it in a solution of equal parts of
nitric acid (70%) and water without allowing the alcohol to evaporate completely.
After immersion in the nitric acid solution for 1 min, specimens were washed with
ethyl alcohol and dried with an air dryer.
Inomersion in the nitric acid, solution produces a passive film on the thorium
carbides which retards reaction with moisture. While this treatment gives only
limited protection, it does protect the surface from moisture for about an hour
which is sufficient for microscopic observation.
07.-lic - OFFICIEL
TIT::0 - 331 - 1:930
The nitric acid solution also served as an etchant for specimens which con-
sisted of mixtures of thorium mono- and dicarbides. Although the dicarbide was
not appreciably etched by the nitric acid solution when viewed under bright field
illumination, its microstructure was satisfactorily revealed under polarized light.
Single-phase thorium monocarbide was not etched by the nitric acid.
The nitric acid technique just described possesses several advantages over
the air etching technique customarily used (1-5): (1) clearer microstructures
are obtained; (2) specimens can be observed directly in air without immersing
them in oil to minimize the reaction with moisture; and (3) specimens can be
examined more deliberately than air etched specimens which continue to react
rapidly with the moisture in the air even under an oil film.
Metallographic preparation of single-phase thorium monocarbide specimens.--
Thorium monocarbide was more difficult to prepare for metallography than either
the dicarbide or monocarbide-dicarbide mixtures. During polishing thorium mono-
carbide behaved rather toughly and it was necessary to use an attack-polish
procedure to remove grinding scratches. Specimens were mounted in epoxy resin,
ground on 320-, 400-, and 600-grit silicon carbide papers using ethyl alcohol as
-
the lubricant, rough polished 3 to 5 min using 0.5 w diamond compound and fairly
-.
-.
-
..
heavy pressure on a Metcloth wheel, and finally attack-polished 3 min using 0.34
alumina and dilute nitric acid (9 parts water to 1 part 70% HNOZ) on a Microcloth .
wheni operated at 100 to 200 rpm. The specimen was then etched by immersing for
5 min in a solution of 50 parts HNO, (70%), 50 parts water, and 2 parts concentrated
HC1. One problem in using this etchant is that the polished surface must be
fairly free of pits and crevices since the HINO, -H, 0-HCl etchant attacks thorium
monocarbide along the edges and at pits and crevices. During the etching a dark.
blue film forms which must be removed by rubbing the specimen with a cotton swab
in running distilled water. After washing, the specimen was dried with an air
ORNL - REC - OFFICIAL
dryer.
This attack-polish procedure yielded a polished surface which appeared to be
free of scratches. Yet, etching this surface with most et chants always uncovered
polishing scratches which could not be removed even by repeated cycles of polish-
ing and etching. These scratches were quite fine, and were undoubtedly produced
in the last polishing stage. The HNO2-H20-HCl etchant, which acted somewhat as
a chemical polish, removed most of the polishing scratches and also satisfactorily
delineated the grain boundaries. Therefore, this etchant was employed even thrugha
crevices, which were always present in the single-phase thorium monocarbide.
MICROSTRUCTURES OF THE THORIUM CARBIDES
Photomicrographs at 1000x of arc-cast thorium carbides with total-c/Th atom
ratios varying from 0.8 to 2.1 (4 to 10 wt % c) are shown in Figs. 3 to 17.
Specimens with compositions of ThCo.81 and ThCo.92 were single-phase, carbon-
deficient thorium monocarbide (Figs. 3 and 4). These specimens were difficult
polishing. The Thco.99 and ThC7.01 specimens (4.9 and 5.0% c) contained signi-
ficant amounts of thorium dicarbide which was not detected by X-ray diffraction
analysis, in addition to the thorium monocarbide matrix (Fig. 5). Specimens with
compositions from Thco.99 to Thc,.88 (4.9 to 8.9% c) were two-phase mixtures of
the mono- and dicarbides (Figs. 5 to 13). Bright field illumination was slightly.
better for detecting small amounts of thorium dicarbide in the monocarbide matrix,
while near the dicarbide end of the range small amounts of monocarbide could be
detected only under polarized light as a general cloudiness of the microstructure.
Specimen Thc2-6 (total-c/Th, 1.96; combined 1.95) was single-phase, high-purity
thorium dicarbide (Fig. 14), while specimen Thc2-8B (total-c/Th, 1.98; combined,
1.91) which had a similar total-carbon content was a mixture of dicarbide and
a trace or graphite (Fig. 15). Twins were observed throughout the arc-cast
Oni!l-!EC - OFFICIAL
Y-S7A6417
OS::L*1.1.C - O:FIC1..1

NCHES
100x
GU
Fig. 3. As-cast Thco.81 (4.00% C; specimen THC-44)
showing single-phase thorium monocarbide. Free metal was
not observed. Bright field illumination, nitric-hydro-
chloric etch.
Y-59746
INCHES
1.001

1000X
1.002
Fig. 4. As-cast Taco.92 (4.61% C; specimen TC-10A)
showing single-phase thorium monocarbide. Free metal was
not observed. Bright field illumination, nitric-hydro-
chloric etch.
OCIL - AEC - OFFICIAL
15
1:131150-
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Y
INCHES
7:15)
.
.001
1000 x
Fig. 5. As-cast Thc1.01 (5.00% C; specimen ThC-2A)
showing thorium monocarbide matrix with dicarbide platelets
under bright field illumination. The specimen contaias a
significant amount of thorium dicarbide impurity which was
not detected by X-ray dilfraction analysis. The micro-
structure of as-cast Taco.99 (4.86% C; specimen THC-7A) was
virtually identical with this. Nitric etch.
i
'
in
1: 7.56
Y-56164
00
...
....
INCHES
.001
1000X
Fig. 6. As-cast ThC1.06 (5.21% C; specimen ThC-8D)
showing thorium monocarbide matrix with dicarbide platelets
under bright Iield illumination. Thorium dicarbide was
detected by X-ray diffraction analysis of this specimen.
Note the dicarbide at the grain boundaries which could be
caused by nonequilibrium freezing since there is a minimum
in the liquidus at ~6% carbon (1). Nitric etch.
09:1-8.50 - 0:FICIEL
16
on L-MEC - OFFICIAL
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it.
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Fig. 7. As-cast Thc1.27 (6.15% C; specimen THC-5A)
showing thorium monocarbide (dark) and dicarbide (light)
under bright field illumination. This composition is
close to the minimum in the liquidus (1) and the specimen
has almost no material in the grain boundary. Nitric etch.

.
INCHES
1.001
.
"
1000X
á''
Fig. 8. As-cast Thcz.42 (6.88% C; specimen THC-3A)
showing thorium monocarbide (dark) and dicarbide (light)
under bright field 11lumination. Note the monocarbide at
the grain boundaries which could be caused by nonequilibrium
freezing since there is a minimum in the liquidus at 26%
carbon (1). Nitric etch.
ORI:L - AEC - OFFICIAL
.
tay."
Y-56714
697
17131110 - 33; -1;730
INCHES
1.001
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Fig. 9. Inca no heat-treated 65 hr at 1600°C and
rapidly furnace-cooled (6.92% C; specimen ThC-3H) showing
thorium monocarbide (dark) and dicarbide (light) under brigat
field illumination. There is no indication of any sesqui-
carbide as is formed in the uranium-carbon system under these
conditions (20). Nitric etch.
INCHES
.
.
1.001
1000 x
.
C
Fig. 20. As-cast ThС2.64 (7.88% C; specimen ThC-6A)
showing thorium dicarbide "light) and monocarbide (dark) under
bright field illumination. Note the twinned structure of the
room temperature stable, C-centered monoclinic dicarbide within
the equiaxed grain structure of the high-temperature face-
centered cubic form. The thorium monocarbide platelets do not
cross the dicarbide-twinning boundary indicating that they did
not precipitate from the solid solution until after the di-
carbide transformation had occurred; 1.e. that the cubic
ThC2.64 solid solution 18 stable at least to the transiorma-
tion point. Nitric etan.
021!L-210-02Fictiil
18
Y53531)
Qalib mAEC - OFFICIAL
SI
ordinario
c
INCHES
1.003
odinome siis
..........--------............
1000 X
-002
.
nien
Fig. 11. As-cast ThC:.76 (8.40% C; specimen Thc2-5B)
showing thorium dicarbide matrix with monocarbide under
polarized Light. The monocarbide is only faintly visible
under bright field illumination. Nitric etch.
:
1
.
.
..

Y-53535
-,. ..
-,
---
---
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i e
or
---
1
1.001
I
imamen
1000x

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von
-002
imamente
Fig. 12. As-cast THC7.84 (8.78% C; specimen THC2-4)
showing thorium dicarbide matrix with monocarbide. The mono-
carbide can barely be detected under bright field illumina-
tion, but shows up as a cloudiness under polarized light at
1000X when compared with Fig. 14. Nitric etch.
ORNI-KEC - OFFICIAL
1/101330-
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Fig. 13. As-cast ThC, 88 (8.91% C; specimen ThС,-3)
showing thorium dicarbide matrix with a trace of monocarbide.
The monocarbide is not visible under bright field illumina-
tion but shows up as a cloudiness under polarized light at
1000X when compared with Fig. 14. Thorium monocarbide was
detected by X-ray diffraction analysis of this specimen.
Nitric etch.
-
-
-
-
-
-
-
--
minim

.
.
INCHES
U
X0001

1.002
nominato de ser
Fig. 14. As-cast Thc2.95 (9.23% C; specimen ThC2-6)
showing high purity, single-phase thorium dicarbide under
polarized light. The twinned structure disappeared when
the specimen was heat-treated at 2350°C (21). No thorium
monocarbide was detected by X-ray diffraction analysis of
this specimen. Nitric etch.
ORK-7.0C-OFFICI:..
071:1-6EC - OFFICIAL
Y-56172
001
.002
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.001
1000X
.002
(b)
Fig. 15. As-cast ThC1.98 (9.28% C; specimen Thcg-8B)
showing thorium dicarbide with a combined-C/Th atom ratio of
1.91 and a trace of graphite under bright field illumina-
tion (a) and under polarized light (b). The graphite shows
up more clearly under bright field illumination. No thorium
monocarbide was detected by X-ray diffraction analysis.
Nitric etch.
Onil-LEC - OFFICIEL
.;.:. T.:.
1.1.1..
thorium dicarbide phase.
They probably were formed when the high-temperature,
1:101:) -33%-15:00
iace-centered cubic structure transformed to the room temperature C-centered
monoclinic structure as the specimens cooled from the melt (4, 19). The thorium
monocarbide platelets do not cross the dicarbide twin boundary, indicating that
the cubic Thc, 6L to Thc, .89 solid solutions are stable to the dicarbide trans-
formation point (Figs. 20-13). Large amounts of graphite as a thorium dicarbide-
graphite eutectic were observed in the specimen with a total-C/Th atom ratio of
2.14 (9.98% C; combined-c/Th, 1.95) (Fig. 16). Bright field illumination revealed
the graphite in thorium dicarbide better than polarized light. Specimen Thcg-IA,
which contained 6% tungsten, consisted of grains of relati.vely high purity thorium
dicarbide with a eutectic structure at the grain boundaries which had precipitated
into graphite and the tungsten-containing phase as it cooled from the melt (Fig. 17).
.
..
...
.
..
..
.
-
----
-----
--...
...
...
PHASES PRESENT IN ARC-CAST THORIUM CARBIDES
The metallographic and X-ray diffraction studies showed that the thorius
monocarbide phase exists over a range of composition from ThС:0.94 to ThCo.8; and
probably lower. Specimens with c/Th atom ratios lower than 0.81 were not examined
in this study; however, other investigators have reported that the monocarbide
phase extends to ThCo.7 (1, 22, 23) and possibly as low as ThC0.56 (24). The
break in the lattice parameter vs. composition curve places the upper limit for
the monocarbide phase at about Thcool (Fig. 18) rather than the theoretica..
1.00. This agrees well with the metallographic studies in which 5 to 8% thoriwn
.
åicarbide impurity was found in specimens with the Thc, 90+0.01 composition.
While thorium dicarbide was not detected by X-ray diffraction analysis of these
specimens, afte: hydrolysis 15% of the carbon was found as Co- to Cg-hydrocarbons,
which are dicarbide products, compared with only 5% Prom Thco.87 (8).
*Note that in the hydrolysis of uranium carbide mixtures a few of the C units from
the monocarbide reacted with other carbon units to form Ca - to Cg-hydrocarbozs (18).
Therefore, one cannot conclude that Thco.37 contains 2% dicarbide because it selds
5% Cg- to cg-hydrocarbons, however, one can conclude that ThС2.00 probably ccrtuins
5% more dicarbide than does ThC, 8, because it yields 10% more Co- to Cg-hydrocarbons.
ORI:L-BEC - OFFICIEL
22
C:FICI.it
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: Y-567101
SONI
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.....
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1000X
:.
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rodinama
t
brachiisen
6.002
m.
c aron
wemaseminte
voimassamittaristot
de income
(b)
Fig. 16. As-cast ThС2.14 (9.98% C; specimen Thc2-7)
showing primary thorium dicarbide with a combined-cm atom
ratio of 1.95 and a thorium dicarbide-graphite eutectic under
bright field illumination (a) and under polarized light (b).
Nitric etch.
1:17!!:)-1.
1:101::0 - 03 V31130

: Y-56631:
INCHES
.coi
1000X
1.002
Fig. 17. As-cast Th.000.00C2.04 (9.1% C, 6.1% W;
specimen Thcg-1A) showing grains of thorium dicarbide with
E eutectic structure at the grain boundaries which preci-
pitated into graphite and the tungsten-containing phase as
the specimen cooled from the melt. Chemical and X-ray dif-
Traction analysis indicated that at least hall OL the tungs-
ten was present as W2C. Bright field illumination, nitric
etch.
7:!311:0-07-1:30
ORK. TO CE2853
(
@pi.
.....
.
.
.
.
..
.
..
-----
..., I
n i .
-----
----
-
wym.
- 5.340
C
'
C
C
0
---C-C-- . --- ---- -------
5.34.4
LATTICE PARAMETER
5.342
_IL
5.340
:
5.336.
7
-
0e
--
0.
9 4 0
COMUNID-C/Th /
1.2
-
1.3
N
RATIO
.
Fig. 18. Variation of the Lattice Parameter of Arc-Case Thorium Mono-
carbide with Composition.
:::L-KEC - OFFICIEL
ORTL - AEC - :FICI/L

25
.
Wilhelm and Chiotti (1) and Street and Waters (22) have reported that Thó 95
is about the upper limit for the monocarbide phase. The monocarbide lattice
17111:0 - OIV w 1.710
parameter was 5.346 Å for arc-cast Thc,00+0.07 which is exactly the same value
as found by Kempter and Krikorian (9) and Nevitt (24). Lower values have been
reported by Wilhelm and Chiotti (5.34 Å) (1), Street and Waters (5.335 Å for
ThС.05) (22) an . Henny, Jones, and Hill (5.344 Å) (23); however, all these
specimens were prepared by sintering thorium metal and graphite powders. The
lattice parameter of 5.340 Å for the arc-cast ThCo.81 specimen prepared for this
study is somewhat larger than the 5.33 Å extrapolated from the data of Nevitt (24)
for arc-cast specimens which had been annealed at 950°C, and considerably larger
than the 5.31 Å extrapolated from specimens which were not arc-melted (1, 22, 23).
Additional studies of the variation of the thorium monocarbide lattice para-
meter with composition and heat-treatment are needed to clarify this region of
the phase diagram.
The maximum combined-C/Th atom ratio obtained by arc-melting with graphite
electrodes was 1.95 rather than the theoretical 2.00, indicating that thorium di-
carbide is slightly carbon deficient. Material of this composition has been
prepared which was single-phase by both metallographic and X-ray diffraction
examination. Specimens with total-c/Th atom ratios greater than 1.95 always
contained free carbon by metallographic and chemical analysis. Scaife and Wylie (25)
reported a maximum combined-c/Th ratio of 1.95 for specimens prepared by heating
thorium oxide and sugar charcoal at 2170°C. One may calculate from Kempter and
Krikorian's (9) data that the combined c/Th atom ratio of their material was 1.95
iż one assumes that the missing 0.4% in their analytical summation was oxygen
present as oxide, or 1.88 1f the missing 0.4% was an error in the thorium analysis.
In some studies of thorium dicarbide, the specimens contained so much free carbon
(4, 19, 26) that it is doubtful if the analyses would be sufficiently accurate
to differentiate between a combined-c/Th atom ratio of 1.95 and 2.00, since the
Ori!!-HEC - OFFICI:!
26
amount of combined carbon is determined by the difference between the total and
Tree carbon. As the amount of free carbon increases (and necessarily also the
total carbon), the combined carbon becomes a small difference between two large
ORAL-AEC - OFICIAL
numbers instead of a large difference between a large and a small number. Dupli-
cate free carbon analyses show much greater variation when specimens contain
2 wt or more free carbon than when they contain less than 1% (20).
Specimens with compositions from Thce through ThC, .88 were two-phase mix-
tures of thorium monocarbide and thorium dicarbide by metallographic, X-ray dif-
fraction, and hydrolysis examination. There was no detectable shift in any of
the X-ray diffraction lines in the dicarbide powder pattern as the composition
of the mixture changed. We found no evidence to support the belief of Langer,
et al (4) in a wide range of composition for thorium dicarbide. In general
our results were consistent with the phase diagram proposed by Wilhelm and Chiotti (1)
except that we found 1.95 as the maximum combined-C/Th atom ratio.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the advice and support of L. M. Ferris
and R. J. Gray during the course of this work. We wish to thank members of the
ORNL Metals and Ceramics Division for their aid in this study: B. C. Leslie for
the metallographic preparation and photomicrographs of many of the specimens,
R. E. McDonald and L. Queener for arc-melting the carbides, and J. P. Hammond
aza C. Hamby for heat-treating specimen THC-3H. The ORNL Analytical Chemistry
Division provided analyses: X-ray analyses by R. L. Sherman; and most of the
chemical analyses by the group of W. R. Laing. We are also indebted to J. F.
Land, RNL Chemical Technology Division, for the thermogravimetric analyses ozi
the carbides for thorium.
O??!! - 1.FC-OFFICI!
REFERENCES
05!L - AEC ** 0710146
1. H. A. Wilhelm and P. Chiotti, Trans Am. Soc. Metals 42, 1295 (1950).
2. N. Brett, D. Law, and D. T. Livey, J. Inorg. Nucl. Chem. 13, 44 (1960).
3. Ĝ. B. Engle, F. D. Carpenter, W. V. Goeddel, and D. L. Menken, Meta lography
. 01 Carbide Fuel Compounds, GA-2067 (1961).
4. S. Lenger, N. Baldwin, P. Gantzel, F. Kester, and C. Hancock, Studies in the
Thoriun-Carbon Binary System in J. T. Waber, P. Chiotti, and W. N. Miner, Ed.,
Nuclear Metallurgy, Volume X, International Symposium on Compounds of Interest
in Nuclear Reactor Technology, IMD Special Report No. 13, The Metallurgical
Society of the American Institute of Mining, Metallurgical, and Petroleum
Engineers, 1964, pp. 359-386.
5. N. M. Griesnauer, M. S. Farkas, and F. A. Rough, Thorium and Thorium-Uranium
Compounds as Potential Thermal Breeder Fuels, BMI-1690 (1964).
6. T. M. Kegley, Jr. and B. C. Leslie, Metallographic Preparation of Dicarbides
of Thorium and Thorium-Uranium, to be published in J. Nuclear Materials.
7. T. M. Kegley, Jr. and B. C. Leslie, Metallographic Preparation of Dicarbides
of Thorium and Thorium-Uranium, paper presented at 18th AEC Metallography
Group Meeting, Atomics International, Canoga Park, California, June 22-24,
1964; paper will be available in the published proceedings; issued also as
ORNL-TM-949.
8. M. J. Bradley and L. M. Ferris, Hydrolysis of Thorium Carbides Between 25 and
99°C, to be published in J. Inorg. Nucl. Chem.
9. C. P. Kempter and N. H. Krikorian, J. Less-Cormon Metals 4, 419 (1962).
10. R. L. Sherman, ORNL Analytical Chemistry Division, private communication to
M. J. Bradley.
11. W. R. Laing, ORNL Analytical Chemistry Division, personal communication to
M. J. Bradley..
12. G. Goldberg, A. S. Meyer, Jr., and J. C. White, Anal. Chem. 32, 314 (1960).
13. ^. B. Crowther and R. S. Large, Analyst 81, 64 (1956).
14. R. R. Rickard and E. I. Wyati, ORNL Analytical Chemistry Division, personal
communication to M. J. Bradley.
Ori!!-4.50 - OFFICIAL
....
.... ------...--.--. ....-
28
a
971310- 33V - 11:30
15. M. J. Bradley and L. M. Ferris, The Effect of Tungsten on the Hydrolysis of
Uranium Dicarbide, to be published in Inorg. Chem. 4, May, 1965.
16. E. B. Hunt and R. E. Rundle, J. Am. Chem. Soc. 73, 4777 (1951).
17. A. E. Austin, Acta. Cryst. 12, 159 (1959).
M. J. Bradley and L. M. Ferris, Inorg. Chem. 3, 189 (1964).
N. A. Hill and O. B. Cavin, J. Am. Ceram. Soc. 47, 360 (1964).
20. M. J. Bradley, unpublished data.
21. J. L. Cook, ORNL Metals and Ceramics Division, private communication to
T. M. Kegley, Jr. and M. J. Bradley.
22. R. S. Street and T. N. Waters, The Variation vith Carbon Content of the
Lattice Parameter of Thorium Monocarbide, AERE-M 1114 (1962).
23. J. Henny, J. W. S. Jones, and N. A. Hill, Cell Size Variations in Thoriwa
. Monocarbide, AERE-R 4456 (1963).
24. M. V. Nevitt, Argonne National Laboratory, personal communication to
M. J. Bradley, October 21, 1964.
25. D. E. Scaife and A. W. Wylie, The Preparation of Thorium Carbide and Som.
Aspects of the High Temperature Decontamination of Irradiated Carbide s'uels,
Australian Atomic Energy Symposium, Melbourne University Press 1958, p. 175.
26. P. K. Gantzel and N. L. Baldwin, Acta Cryst. 17, 772 (1964).
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