_.
.
I OF I
ORNL P.
1446
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PFEFEEE
12
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MICROCOPY RESOLUTION TEST CHART
NATIONAL QURE AU OF STANDARDS - 1963
i
caratter
Paper No. 18 presented at the International Conference on Thermionic
Electrical Power Generation, London, England, September 20-24, 1965.
.. con
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ORNL. P. 1446-1
Collf-650908-9
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man hayatim
-.!
****..
SEP 29 1965
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l....na

- LEGAL NOTICE
TU: raport me prepared um account of Govon wat sponsored work. Nolther the Uniad
Ham., nor the con elsodom, sor way pornon sung ca ball of the Coanelow:
A. Makas sy wuruty or reprowaution, expressed or implied, wil rospect to the accu.
ruey, completo , or woulson of the labor lon contained in this report, or what he wou
of my laboration, apparatus, authod, or procus daclound in de report may not latring.
primis omad rane; or
8. A may labiula. We noepoct to the wool, or for dary.. reowuns Irom the
w alay laformation, appartu, method, or proc. disclosed ua Wo report.
Awoln the word, "porna sulagan bolall of the Cou lon" lecinder my ou.
mogu or intelor of the Cwmlsslon, or employw of much contractor, to the oximat that
nok plogue or contractor of the Commission, or employs of much contractor propery,
daromadla, or provides accen lo, wy hatarmallow purouant to us onployment or contrac:
wiu de Comunaton, or wo employ wat wil much contractor.
THERMOCHEMICAL DETOSITION OF REFRACTORY METALS, ALLOYS,
AND COMPOUNDS FOR APPLICATION IN THERMIONIC DEVICES

RELEASED FOR ANNOUNCEMENT
IN NUCLEAR SCIDNCE ABSTRACTS
by
J. I. Federer, R. L. Heestand, F. H. Patterson, and C. F. Leitten, Jr.
Metals and Ceramics Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee
PIE

Jui
.
..
THERMOCHEMICAL DEPOSITION OF REFRACTORY METALS, ALLOYS, AND COMPOUNDS
FOR APPLICATION IN TERMIONIC DEVICES*
.
..
...
J. I. Federer, R. L. Heestand, F. H. Patterson, and C. F. Leitten, Jr. **
.--..-.-.
Abstract., -Thermionic devices impose rigid requirements on refractory
materials technology in relation to fabrication, purity, and physical
characteristics. Thermochemical deposition of refractory metals,
alloys, and compounds is being investigated as a technique for solving
these materials problems. Tubular and sheet deposits of tungsten,
rhenium, molybdenum, and tungsten-rhenium alloys are being prepared by
hydrogen reduction of the metal hexafluorides in the temperature range
500 to 1000°C at pressures of 5 to 20 torr. Factors affecting deposi-
tion rates, purity, and grain orientation are being studied. High-
purity tungsten sheet of uniform thickness has been prepared at about
570°C. Tungsten deposited below 800°C is highly oriented with (100)
parallel to the deposition surface. Joining of thermochemically
deposited and wrought tungsten by deposition has been demonstrated by
the preparation of joints in sheet and tubing. Tungsten-rhenium alloys
deposited in a static hot zone at temperatures of 450 to 700°C are
inherently nonuniform in composition and deposition rates due to rela-
tive differences in the stabilities of WF6 and ReFo. A moving hot-
zone technique shows promise of alleviating this problem. Uranium
dioxide having oxygen-to-uranium ratios ranging from 2.001 to 2.166
has been deposited by reaction of UF6 with hydrogen and water vapor
or oxygen at 1250 and 1300°C and pressures of 3 to 6 torr. Both bulk
solids and extremely fine powder (60 to 200 A) have been prepared.
Stoichiometric silicon carbide has been deposited at a rate of
10 mils/hr by hydrogen reduction of Sici. in the presence of methane
at 1300°C and a pressure of 100 torr. In general, thermochemical
deposition is a unique method for fabricating many refractory
materials of high density and purity and requiring a minimum of
finishing operations.
.
.
. .
.
. .
.
.o.
.
Introduction. - The high temperatures to which thermionic components are
subjected limit the choice of materials to refractory metals, alloys,
and compounds.' High-temperature stability and compatibility, purity,
and other physical properties further limit the selection. Finally,
fabrication and assembly of components constitute an important aspect
of materials selection because thermionic devices require many elements
fabricated to close tolerances. By analogy with nuclear fuels, fab-
rication costs will surely exceed materials costs in thermionic
elements. Satisfactory fabrication methods are needed, therefore,
Por materials which appear to be suitable for thermionic application.
-
-
-
Refractory materials fabricated by conventional techniques often
po88e88 defects which render them unsuitabie for many applications.
These defects include high-impurity content, low density, and struc-
tural instability. The purpose of this paper is to discuss an alternate
fabrication method for refractory materials termed thermochemical, or
*Research sponsored by the U. 8. Atomic Energy Commission under
contract with the Union Carbide Corporation.
**Metals and Ceramice Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee, USA.
...NL,'
vapor depcaition. The process refers to the chemical interaction of
gaseous compounds at a heated surface to form a solid deposit and a
gaseous by-product. Thermochemical deposition is capable of fabricating
refractory materials of high density and purity to required shapes and
tolerances, thus minimizing finishing operationr. Also, the proce88 is
generally conducted at lower temperatures than other fabriontion methods,
thereby reducing equipment costs and decreasing the possibility for
contamination. Many materials suitable for use in thermionic devices
may be fabricated by this method.
......
iii. ...
...
.
In order to fully exploit the potential of thermochemical deposi-
tion, the ORNL program involves the fabrication of free-standing refrac-
tory bodies such as tubing and sheet, coatings, and joints between
refractory materials. Included in this program are tungsten, rheniun,
.., molybdenum, tungsten alloys, uranium dioxide, and silicon carbide -
..
...
of the physical and mechanical properties of the prorlucts are being
correlated with deposition conditions. Through these studies. a
greater understanding of the deposition process and control of material
characteristics is being achieved. In following sections, the deposi-
tion process and characteristics of each product are discussed.
-
...
.
'--'
,
Tungsten. - A refractory metal of prime interest as a structural material
and/or emitter is tungsten. Usually the metal is deposited as tubes of
round or square cross section, but the process is no less applicable to
a variety of geometrical shapes. The apparatus used for depositing
tungsten has been described elsewhere? and 18 shown schematically in
Fig. 1. Hydrogen and tungsten hexafluoride (WF6) are metered into a
-

ORNL-OWO 63-6121
We
.............
DEPOSITION FURNACE
SCAVENGER FURNACE
BURNOFF .
writisnemo onda otros.......

---
in
------
COLD TRAP
a
VACUUM PUMP
SCRUBBER
DRAIN
Mg. 1. Tungsten Deposition Apparatus.
piny
D
.
..
2 ...
...
.. ...
... .
•
mandri. contained in the main deposition furnace where hydrogen reduce
tion of the halide occurs according to the reaction
WF6 (8) + 3H2 (8) ~ W (8) + 6HF (8).
(1)
The products of this reaction are solid tungsten which forms on
the wall of the mandrel and gaseous HF. Other components of the system
include a scavenger furnace maintained at 900°C for reducing any halide
which passes unreacted through the main deposition furnace, a vacuum
pump for maintaining the desired system pressure, a cold trap for con-
densing pump oil vapor and minimizing carbon contamination of the
deposit, and a water scrubber for HF. Deposition equipment for other
refractory metals, alloys, and compounds are similar to this system.
Deposition conditions can be readily varied and include tempera-
ture, pressure, composition of gases, flow rates, substrate or mandrel
material, and geometry. The deposition conditions used in this study.
are contained within the limits indicated below.
500 to 1000°C
5 to 20 torr
Temperature
Pressure
Hydrogen-to-
WF6 ratios
Flow rates
· 15:1 to 70:1
H2
WT.
Substrate
Geometry
1000 to 3000 cm3/min
15 to 200 cm/min
Copper, molybdenum
3/4-in. OD X 12 in. long or
1 1/2 in. square X 18 in. long
If varied independently, any of these conditions can affect the
character of the deposit. The effect of temperature on deposition
rates 18 shown in the data of Fi&. 2 which enaompass a typical set of
conditions. The deposition profiles - deposition rates vs distance
from the iniet of the reaction furnace - show that WF, is rapidly
reduced by hydrogen at 800 and 1000°C, resulting in significantly
higher deposition rates near the inlet than farther downstream. At
lower temperatures, the gas mixture is not so quickly depleted in WF
and the profiles flatten. Thus, uniformity of deposit thickness is
favored by lower deposition temperatures for tubular geometry. Similar
profiles are obtained for various conditions within the limits previously
indicated except as will be discussed.
The process was scaled up to produce sheet material for a mechan-
ical properties evaluation. The tungsten was deposited on the inner
surfaces of a 1 1/2- x 1 1/2- X 18-in. long mandrel prepared by sinter-
ing packed molybdenum powder. Molybdenum was substituted for copper
to minimize distortion due to a larger difference in thernal expansion
coefficients between copper and tungsten. Subsequently, the molyb-
denum is dissolved from the tungsten in a nitric-sulfuric acid solution.
The deposition conditions were 555°C at the inlet and 585°C at the out-
let of the reaction zone, 200 cm3 of WF. per min, 3000 cm3 of H2 per
min, and 5 torr pressure. The gases were preheated to 350°C before
entering the deposition zone. Under these conditions, the deposition
rate is about 3 mils/hr, and the thickness variation of nominal
1/16-in. thick deposits is 20.004 in. over a 14-in. length.
Etzizkit.com imanendone
:
An initially high nucleation rate results iu numerous small.
grains in the material adjacent to the strate. Preferred growth
directions are quickly established, however, and growtio continues as
UNCLASSIFIED
ORNL-OWG 64-4972

.
.
.
-
-
..
.
.
-.:.......
.. ..
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DEPOSITION RATE (mils/hr)
3000
700°C
6000
500°c
1
2
3 4 5 6 7
DISTANCE FROM INJECTOR (in.)
..
Fig. 2. Deposition Rate of Tungsten vs Distance from the Inlet
of the Reaction Furnace.
"
.
columnar grains having the long axis of each perpendicular to the sub-
strate. Viewed in cros8 section, that is, parallel to the substrate,
the grains have an average diameter of 0.1 mm or less. The deposit
faithfully reproduces the smooth surface of the substrate, which is an
economic advantage in many applications. The last deposited surface :s
less smooth because grain growth is not uniform on a microscale. Trans-
mission electron microscopy reveals that the grain structure of material
deposited below 800°C is highly oriented with (100) parallel to the
substrate. In deposits prepareâ above 800°C, the (100) orientation 18
retained locally, but grains having various high index planes parallel
to the substrate are mostly observed.
-.
.
-
Recent annealing studies by Mills et al. 2 show that the columnar
grain structure 13 thermally stable. If the structure is highly oriented,
little or no growth occurs during heat treatments of 100 hr at 1800°C,
15 hr at 2000°c, 15 hr at 2200°c, and 1 hr at 2500°C. In studies of the
effect -f deposition conditions and heat treatments on orientation,
McMurray et al.3 found that (100) orientations were stable after 2 hr
at 2100°C. Wrought structures, however, recrystallize and undergo
extensive grain growth during similar heat treatments.
Chemical analyses show that the material 18 quite pure. A typical
analysis is presented in Table 1. General vacuum tightness and system
cleanliness are responsible for the low contents of interstitial impuri-
ties, while the low deposition pressures may minimize the fluorine
content.
Table 1.
Typical Analysis of Thermochemically
Deposited Tungsten
Impurity
Concentration
(ppm)
OG RUME
< 5
< 10
1-10
0.1-10
1-10
2-100
In many applications, refractory materials must be joined to com-
plete an assembly. Preliminary results indicate that satisfactory joints
in thermochemically deposited and wrought tungsten can be obtained by
deposition of tungsten if the joints are properly designed. The charac-
teristic columnar growth of deposited tungsten affects the integrity of the
joint. A 45° plane of weakness occurs at the intersection of columnar
grains growing from intersecting surfaces oriented at 90° with each
other. Increasing the included joint angle beyond 90° decreases the
angle of intersection between columnar grains growing from the mating
surfaces and minimizes the tendency for generation of a plane of
weakness. Using this principle, sound joints were prepared in wrought
tungsten sheet Metallographic examination of the bonds showed no
evidence of voids, fre.ctures, or impurity concentrations. Tube joints
were impervious to helium when tested with a helium leak detector set
at a sensitivity of 10-8 cm3/min.
The results of these experiments have demonstrated that sound
joints can be prepared by thermochemical deposition, and that the tech-
nique warrants further investigation.
:
Rhenium. - Rhenium ranks high among the refractory metals considered for
thermionic emitters in an atmosphere of cesium vapor. Deposition studies
are being extended, therefore, to establish procedures for fabricating
rhenium as free-standing shapes, or coatings, on other refractory
materials. The deposition reaction is the hydrogen reduction of rhenium
hexafluoride Equipment similar to that shown in Fig. 1 is used for
these studies. Good quality rhenium deposits are not as easily obtained
as tungsteia, although the deposition processes are similar. The depo-
sition conditions that have been studied are included within the limits
indicated below.
nwiring and Winiaiore"...*-
400 to 800°C
2 to 50 torr
a
10 to 20
Temperature
Pressure
Hydrogen-to-
Ref. ratios
Flow rates
H2 .
Ref.
Substrate
Geometry
250 to 2000 cm?/min
10 to 2 cm?/min
Copper
3/4-in. OD X 12 10. long
ind
Although the optimum conditions for deposition of uniformly thick,
high-density metal have not been determined, ductile coatings of high
density metal, 1 to 2 mile thick and 10 in. long, have been deposited at
600° C with a hydrogen-to-Ref 6 ratio of 12:1 and at a system pressure of
10 torr. The flow rates were 250 cm? of Ha per min and 20 cm of ReFs
per min. The deposition rate was about 0.5 mils/hr, and the reduction
efficiency was 25%.
Preliminary x-ray diffraction data show that the groin structure
of rhenium deposits is randomly oriented. The typical grain structure
18 columnar, although the grains are less needlelike than columnar
tungsten grains,
Molybdenum. - Molybdenum als0 is amenable to thermochemical deposition
by hydrogen reduction of molybdenum hexafluoride (MoF6). Preliminary
results indicete that slightly higher temperatures are required for
this reaction than for either tungsten or rhenium deposition. The grain
structure of saolybdenum deposited at 800°C, hydrogen-to-MOF6 ratio of
40:1, and a system pressure of 10 torr 18 columnar and similar in appear-
ance to vapor-deposited tungsten.
Tungsten Alloys. – Tungsten alloys also are being prepared by thermo-
chemical deposition. Initial efforts are directed towards alloy systems
wherein improved ductility has already been established in material fab-
ricated by conventional techniques. Tungsten-rhenium alloys are being
deposited by the simultaneous hydrogen reduction of WF6 and ReF6.
The specific objective of these studies is to determine the conditions
required to fabricate allows of any specific composition.
These studies show that the relative ease of reduction of ReF
compared to WF. results in nonuniformity of composition and thickness
in alloy deposits. The deposits are richer in rheniun. near the inlet
of the reaction zone. In general, deposition rates are also greater
near the inlet than for unalloyed tungsten, correlating qualitatively
with the higher rhenium content in that region. The axial variation
in rhenium content for several deposition conditions is shown in Fig. 3
wherein the rhenium content of the deposits is plotted vs distance
from the inlet. The recovery values shown were determined by com-
paring the metal content of both metered fluorides with the weight
of deposit obtained in the main reaction zone,
The curve labeled Wre-16 is typical of deposits prepared at
500 to 700°C – exhibiting a high rhenium content near the inlet followed
by a rapid decrease in rhenium with distance from the inlet. The curves
labeled WRe-18 and WRe-19 represent deposits prepared under the same
conditions as WRe-16 except that argon was included with the reacting
gases. Although the mechanism is not understood, argon causes the
rhenium to be distributed differently in the deposits. The rhenium
content in WRe-19 was substantially increased throughout the deposit
and was constant over the distance 5 to 12 in. from the inlet. The
presence of argon, however, lowered the metal recovery from 89 to 54%.
Lowering the temperature to 450°C also caused a significant change in
rhenium distribution as shown by the curve labeled Wre-20. The very
high rhenium content near the inlet reflects the greater ease of
reduction of ReFo compared to WT 6. Analysis of the composition pro-
files for several deposito reveals that a greater-than-proportionate
amount of rhenium was recovered than tungsten.
1
.-------
ORNL-DWG 64-10145
WRe-16 WRe-18 WR6-19 WR.-20
600
600 500 600 4!
10
1500 1500

---
WRO-20
TEMPERATURE (°C)
PRESSURE (mm Hg)
Hz (cc/min)
WF, (cc/min)
RoF, (cc/min)
Ar (cc/min)
RECOVERY (%)
pegael e
--
-
500
500
--...--...---------
RHENIUM CONTENT (%)
WRO-18
WRO-9
WRO-16
1012
DISTANCE FROIA INLET (in.)
Fig. 3. Composition of Tungsten-Rhenium Alloys vs Distance from
the Inlet of the Reaction Furnace.
To eliminate the axial variation of rhenium content and deposi-
tion rates, a moving hot zone has been substituted for the static hot
zone. The desired alloy composition is obtained by control of the gas
composition and the desired thickness is obtained by control of the .
rate of movement of the hot zone, temperature, and feed rate. The
experimental arrangement utilizes an induction coil about 1 in. long,
a gas preheat furnace, and a mechanism for elowly moving the deposition
mandrel through the induction coil and preheat furnace. Preliminary
experiments were conducted under the following conditions.
Deposition temperature: , - 700 to 750°C
Gas preheat temperature 300°C
Hot zone movement
3/4 in. /nr
Flow rates
250 and 500 cm°/min
15 and 30 cm3/min
ReF6
5 and 10 cm3/min
Pressure
10 torr
.
H2
.
WF6
et te verwante Antson ...........
Deposition rates of about 5 mils/hr and 100% efficiency of
reduction are obtained under these conditions. A typical analysis of a
deposit prepared with a moving hot zone is presented in Table 2.
Although the process is not yet optimized, these results indicate the
potential for obtaining uniform compositions by this technique.
h
die einstein i
The interstitial content of alloy deposits is similar to that of
unalloyed tungsten (Table 1). The grain structure is typically columar
with a triency towards more needlelike grains in alloys containing
greater than 25% Re. Hardness values increase from 450 DPR for
Table 2. Typical Analysis of Tungsten-Rhenium Deposit
Prepared with a Moving Hot Zone
Distance Along Length
of Deposit
(in.)
Rhenium
(%)
10.6
21.8
26.8
24.4
22.4
18.7
.
unalloyed tungsten to about 1800 DPH for alloys containing 25 to 30% Re.
X-ray diffraction results indicate that the hardnese increase is assoc-
iated with an increasing amount of a beta-tungsten type structure (cubic)
which coexists with the body-centered cutic solid solution of rhenium
in tungsten. The solid solution is the principal phase in low rhenium
deposits, while the beta-tungsten type structure is favored by hirsh
rhenium contents. The beta-tungsten structure commonly occurs as an
intermediate phase in transition-metal systems, but does not appear
in previously reported tungsten-rhenium phase diagrams. 6,7 These
diagrams, however, have not been established below 1200°C, whereas the
deposits were prepared in the temperature range 500 to 750°C. Neither
sigma phase, which is reported to occur in alloys containing from 26 to
63% rhenium, nor elemental rhenium has been found in any of the as-
deposited alloys.
Fuel Compounds. - In addition to the fabrication of structural components,
thermochemical deposition of refractory fuel compounds is being investi-
gated. The main objective of this program 18 to investigate methods
for the direct fabrication of refractory fuel compounds from gaseous
metal halides. Initial studies have concentrated on the thermochemical
deposition of UO2 by the reaction of hydrogen and oxygen, or steam,
with UF6 as follows:
.
UFO + 2H20 + H2 - UO2 + 6HF
.
. .
-.:.
This reaction has been proven thermodynamically feasible when
accomplished in steps as shown in reaction paths (3) and (4) or (5) and
(6
).
UF6 + H2 - UT. + 2HF
UF4 + 2H20 - UO2 + 4HF
UFO + 2H20 - 102F2 + 4HF
UO2F"2 + H2 - UO2 + 2HF
The overall reaction (2) is believed to proceed simultaneously
by the paths shown in (3) and (4) or (5) and (6). Thermodynamic and
kinetic factors favoring direct reduction are (1) low system pressure,
(2) temperatures sufficiently high to limit formation and condensation
of intermediate compounds, and 13) excess amounts of both hydrogen and
oxygen to drive tibe reactions to completion.
.
.
Experiments are conducted in equipment essentially similar to that
shown in Fig. 1.: A water-cooled injector admits UFG directly into the
hot zone to prevent condensation of intermediate products UF. or VC2F2
in the cold zones. Argon 18 used as a blanket to separate the reactants
at the injector tip and to prevent buildup of VO2 on the injector.
..
.
-
..
The deposition process has not been optimized in regard to all
variables; however, the conditions and results of several deposition
experiments presented in Table 3 indicate certain trends. Under the
first set of conditions presented in Table 3, a dendritic, crystalline
deposit is obtained Deposits of this type have oxygen-to-uranium
ratios of 2.000 to 2.003 and contain less than 10 ppm fluorine. Reducing
the pressure to 2 to 3 torr and increasing the UF, content of the gases
to greater than 3 vol % results in a theoretically dense, uniform deposit.
Tubes of this material having a wall thickness of about 40 mils were
deposited at a rate of 6 mils/hr.
Stoichiometry in bulk Voz can be controlled by adjusting the feed
gas composition for a given temperature and pressure. The effect of gas
composition on the oxygen-to-ure.nium ratio at a deposition temperature
of. 1300°C and pressure of 3 torr is shown in Table 3. The lowest oxygen-
to-uranium ratio was obtained for a gas composition relatively high in
UFand low in oxygen. The deposits having oxygen-to-uranium ratios
of 2.008 and 2.116 contained a second phase identified as V409, whereas
the deposit having an oxygen to uraniun ratio of 2.001 was single phase.
Oxygen-to-uranium ratios are constant along the length of a given deposit.
Table 3. Conditions and Results of UO2 Deposition Experiments
Tempera-
ture H O
UT
(°C) (vol 6) (vọĩ %) (vol 8)
Pres. Oscygen-to-
sure Uranium
(to) Ratio
Description
1250
78
2019
2
6
1300
176
5
3
2.000 Dendritic. Crystallites
2.003 are red and translucent
under intense light.
Readily crushed to pow-
der.
2.019 Dense (10.96 g/cm²),
uniform thickness. De-
position surface and
fractures appear glassy.
2.008 Two-phase structure of
UO2 and 1409.
2.166 Ivo-phase structure of
UO2 and U409.
2.001 Single-phase structure.
2.001 60 to 200 A powder.
1300
3
1300
ecimiento para mainittanniadania niewiw.wo.....
1300
aw w
1300
82
16
2
"As water vapor.
is it mort
Uranium dioxide powder 18 obtained by withdrawing the injector
from the hot zone so that gas mixing occurs at a temperature of 900 to
1000ºC. VO2F2 "anow" is assumed to form on mixing, subsequently passing
into the 1300°C hot zone where reduction to UO2 occurs. Uranium dioxide
powder, 60 to 200 A in diameter, was produced at 6 torr with a gas com-
position of 16 vol % steam, 82 vol % H2, and 2 vol % UF6. The oxygen-
to-uranium ratio of 2.074 was readily reduced to 2.001 by heat treat-
ment in hydrogen at 1000°C for 4 hr.
Within the gas composition limits established for depositing
bulk UO2, less than 1% of the feed material passes through the system
as intermediate or unreacted fuel compounds. Although VO2 is formed
in the hot zone when UFconcentration in the feed material is raised
above 10%, some intermediate uranium compounds pass through the system
1.nto the cold trap. For steam or oxygen concentrations in excess of
40%, trace amounts of intermediate uranium compounds are again found in
the cold trap. Raising the system pressure above 20 torr yields UO2F2
powder under conditions that produce only 102 at lower pressure. Within
the current limits of investigation, temperatures above 1300°C give no
significant deposition advantage.
Silicon Carbide. - Oxidation-resistant coatings are required for protec-
tion of refractory materials in numerous applications, for example, gas-
fired thermionic converters. Thermochemical deposition of Sic has been
investigated in order to fill this need. Deposition of the carbide has
been accomplished by the hydrogen reduction of silicon tetrachloride
(81014) in the presence of methane (CHA) in equipment similar to that
shown in 14.8. 1 and described elsewhere. 10 A typical set of deposition
conditions follows.
1300°c
Temperature
Pressure
Flow Rates
H2
Sim
CHE
100 torr
2200 cm3/min
35 cm3/min
80 cm3/min
Stoichiometric silicon carbide has been deposited under these
conditions at a rate of approximately 10 mils/hr. Small variations in
temperature and hydrogen content of the reacting gases do not affect
composition. Adherent coatings have been deposited on graphite, tungsten,
and alumina.
T:e grain structure of vapor-deposited Sic 18 columnar, resem-
bling pyrolytic graphite. A knoop hardness of about 2750 was measured
for this material.
These experiments have demonstrated the feasibility of preparing
high-density, stoichiometric SiC as coatings or free-standing shapes by
thermochemical deposition. Further studies should include deposition of
uniform coatings, compatibility with various refractory materials, and
oxidation studies.
In suramary, these studies have shown that refractory materials
having thermionic applications can be prepared by thermochemical deposi-
tion. Specific advantages of the process are the lower temperatures that
are required as compared to conventional fabrication processes, the smooth
surface finish that, Jan be obtained, the applicability to a variety of
lo
geometrical shapes, and the oriented structures that are possible. The
relatively inexpensive equipment and simplicity of the process may pro-
vide a further economic advantage in the fabrication of the mumerous
elements requirog in a thormionio device.
11
.
";
,,ir
REFERENCES
. ...
1.
.
.
R. L. Heestand, J. I. Federer, and C. F. Leitten, Jr., Preparation
and Evaluation of Vapor-Deposited Tungsten, USAEC Report ORNL-3662,
Oak Ridge National laboratory, August 1964.
.
4
•
.
2. R. G. Mills, J. R. -Lindgren, and A. F. Weinberg, An Evaluation of
Vapor-Deposited Tungsten Tubing, USAEX Report GA-5721, General
Atomic Division of General Dynamics, October 1965.
>
illili!!'
.....
3.
N. D. McMurray, R. H. Singleton, K. E. Muszar, Jr., and D. R. Zim-
merman, "Improved Tungsten Thermionic Emitter Surfaces by Chemical
Vapor Deposition," Allison Division, General Motors Corporation,
November 1964.
.
4.
....
J. I. Federer and C. F. Leitten, Jr., Wapor Deposition and Charac-
terization of Tungsten-Rhenium Alloys," to be published in Nuclear
Applications.
..
5. M. N. Nevitt, "Atomic Size Efects in Cr30-Type Structures," Frans.
AIME 212, 350–355 (1958).
P. Greenfield and P. A. Beck, "Intermediate Phases in Binary Systems
of Certain Transition mements," Trans. AIME 206, 265–276 (1956).
...
J. J. English, Binary and Ternary Phase Diagrams of Columbium,
Molybdenum, Tantalum, and Tungsten, USAEC Report DMIC 152, p. 92,
Battelle Memorial Institute, April 1961.
.
8. R. L. Heestand and C. F. Leitten, Jr.,, "Thermochemical Reduction of
. Uranium Hexafluoride for the Direct Fabrication of Uranium Dioxide
Ceramic Fuels," to be published in Nuclear Applications.
9.
I. E. Knudsen, H. E. Hootman, and N. M. Levitz, A Fluid Bed Process
for Direct Conversion of Uranium Hexafluoride to Uranium Dioxide,
USAEC Report ANL-6066, Argonne National Laboratory, 1963.
10.
R. L. Heestand, J. I. Federer, and C. F. Leitten, Jr., "Thermochem-
ical Preparation and Properties of Carbides for Nuclear Applications,"
pp. 53945 in International Symposium on Compounds of Interest in
Nuclear Reactor Technology, ed. by J. T. Waber, P. Ghiotti, and
W. N. Miner ("Nuclear Metallurgy," vol X) IMD Spec. Rept. No. 13,
Met. Soc. AIME, 1964.

1
NH
-
-
-
-
END
DATE FILMED
10/25/65






is
.
'1.
.
Table 2
The Final Dose Estimates for the Five
llost Highly Irradiated Persons

Dose Estimates
Group 1
Group 2
Hematologic
Changes
Clinical
Course
Patient | rads
Rank
Orders
rems
Rank
Orders
Rank Orders | Rank Order
276
320
188
256
230
292
239
272
-
163
201
*Rank Order:
1 - most severe; 5 - least severe
.
--
-
-
END
DATE FILMED
18/31 /65







+