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A preliminary study of three cermets based on uranium oxide
J. Nuclear
Energy
I. 1957, Vol.
4. pp. 164 to 178
Pergamon
Press
Ltd.,
London
A PRELIMINARY STUDY OF THREE CERMETS BASED ON URANIUM OXIDE* L. S. WILLIAMs,t D. T. LIVEY, E. BARNES, and P. MURRAY (Received 26 April 19.56) Abstract-The fabrication and some properties of three cermets have been investigated in outline. Fabrication by hot-pressing methods is feasible, provided an argon atmosphere is used. The thermal-shock properties of UOt are not improved by the addition of zirconium; however, the UO,-Th system, at high metal contents, and to a lesser extent the U02-Si system, up to 60 v/o of metal, show improvement in this respect. The poor properties of the zirconium cermets is probably due to oxygen embrittlement of the zirconium as a result of reaction with the oxide constituent. The use of silicon as the metallic constituent at 20 per cent by volume improves the oxidation resistance of UO, in air at temperatures up to 900°C. Addition of silicon beyond 20 v/o has little effect on the rate of oxidation. 1.
INTRODUCTION
ONE of the newer concepts designed to reduce the inherent deficiencies of ceramics is that of the metal ceramic mixture, or cermet. Outside the atomic energy field the major study of such systems at high temperatures has been on the chromium-alumina (MOORE et al., private communication; BLACKBURN and SHEVLIN, 1951) system, although attention is also being directed to carbide-metal systems such as Tic-Co. The latter have, of course, been used for many years in the cutting-tool applications. In the atomic-energy project, from the outset, emphasis has been placed primarily on the use of metallic systems for fuel elements. Only recently has serious consideration been given to the possibilities of refractory uranium compounds, the corrosion resistance of UO, in water systems and the apparently good irradiation behaviour being significant properties. At Harwell some research has been done on the fabrication and properties of UO, and UC. The oxide, however, is brittle and has poor thermal shock resistance and the work reported here was designed as a preliminary investigation of a cermet system of the oxide-metal type. For this, two alternatives are possible: (1) to combine a suitable ceramic with a fissile metal, (2) to combine the oxide of a fissile element with a suitable metal. The second of these has been the main basis of the present study for the following reasons. (a) Uranium-based ceramics are considerably more refractory than the metal, and do not suffer from the anisotropy and other unsatisfactory characteristics of uranium itself. (b) A selection from a wider range of metals is possible where the metal is the nonfissile component. The restrictions then are adequate refractoriness (both high melting-point and strength) combined with some inherent ductility and acceptable nuclear characteristics.
* Communication from t Now at CSIRO,
the Metallurgy Division, Melbourne, Australia.
UKAEA, 164
Research
Group,
Harwell.
A preliminarystudy of three cermets based on uranium oxide
165
Three metals are considered here which, besides bonding effectively to the uranium oxide, are suitable in at least one of the properties indicated in (b). These are zirconium, thorium, and silicon. Emphasis has been placed on the fabrication and properties of the cermets; in particular, on their ability to withstand the severe thermal shock that may be experienced in a fuel element during service. The thermal-shock test consisted of quenching the specimen from high temperatures in uacuo into a water bath. 5.46 ? ?A.E.R.E
check points
5.42
5.38
5.26
5.22
5.18
10
20
30
40
“/o FIG. I.-Effect
2.
50
60
70
80
ZrO2
of solid solution on lattice parameter of UO,.
PRELIMINARY
CONSIDERATIONS
(a) Urania-zirconia system This system has been studied by LAMBERTSON and MUELLER (1953), who concluded that it consists primarily of two solid solutions separated below 1900°C by a two-phase region. Results were obtained by sintering compacted pellets of appropriate composition for 100 hr at 1700°C in hydrogen with intermittent crushing, grinding, and pressing. The main solid-solution phases were studied by X-ray diffraction, this showing that Urania takes up to 52 mol per cent of zirconia into solid solution with a progressive decrease in the parameter of the cubic lattice from 5*46A to 532A (Fig. 1). From 53/100 mol per cent zirconia, the lattice was tetragonal when investigated by quenching techniques. There was also some indication that zirconia was transformed to a polymorphic form other than monoclinic or tetragonal at approximately 1900°C. The affinity of zirconium for oxygen is well known, and FAST(1948) found that 40 at. per cent (15 ‘wt.per cent) of oxygen could be taken into solid solution. However, CUBICCIOTTI (1951) sets the figure at 55 at. per cent (17.6 wt. per cent) and also reported that ZrO, could take up to 15 at. per cent (11.5 wt. per cent) of zirconium into solid solution. NORTON and KINGERY,using stabilized zirconia, confh-med that
166
L. S. WILLIAMS, D. T. LIVEY,E. BARNES, and P. MURRAY
at least 5 wt. per cent of zirconium would go into solid solution in ZrO,, and since they used 5 wt. per cent increments it is possible that the true saturation figure is not far below the value given by CUBICCIOTTI.In any case, the presence in the zirconia lattice of the stabilizing magnesium or calcium atoms would be expected to reduce the extent to which other atoms could enter. (b) Urania-thoria system According to LAMBERTSON et al. (1953), UO, and ThO, form a continuous series of solid solutions with the solidus-liquidus extending from 2875°C for UOz to above 3200°C for ThO,. Unlike zirconium, thorium does not take oxygen into solution. (c) Urania-silica system LANG et al. (1952) established, in a preliminary investigation, that UO, and SiO,, in an oxygen-free atmosphere, form no compounds and show no solid solubility by solid-state reaction from 800°C to lOOO”C,for the range 10 mol. per cent to 90 mol per cent UO,. The eutectic temperature was placed at 1600” f 50°C. Tn view of the difference in size of the U4+ ion and Si4+ ion (I.01 A and 0.41 A respectively) and the different structures of UO, (cubic) and SiO, (quartz), solid solution between the oxides would not be expected. 3. THERMODYNAMIC STABILITY OF THE PHASES INVOLVED The free energies of formation (AG, 298°K) of the oxides UO,, ThO,, ZrO,, and SiO, are -247, -280, -248, and -195 kcals/mol respectively. (a) UO,-ZrO, These oxides are equally stable and reaction would therefore be expected between a UO, phase and a zirconium phase, and the more readily since ZrO, can also enter into solid solution with UO,. Where the phases are in bulk form, i.e. coarse powders, the reaction would probably be interfacial only. However, using fine powders, which may be very active depending on the method of preparation, reaction may be considerable. This is particularly so for nonstoichiometric U02.13, which is highly active and contains interstitial oxygen. The extra oxygen can be removed (e.g. by hydrogen reduction) and may therefore be expected to go into the zirconium, which has been reported to take up to 40 at. per cent of oxygen into solid solution (FAST, 1948). Chemical reaction and the possibility of the formation of a UO,/ZrO, solid solution is expected therefore in UO,/Zr cermets. The formation of a solid solution appears desirable for good bonding behaviour in oxide-metal cermets, since a few per cent of Cr,O, in solid solution in Al,O, seems necessary for successful bonding of Al,O,-Cr cermets. From this point of view, the UO,-ZrO, system should provide good bonding behaviour. However, the possible loss of ductility of the zirconium due to dissolved oxygen may offset any good bonding mechanism, although the results of HAUSNERet al. (1951) on vacuum-sintered compacts indicate that this may not be so. They found that a small increase in the amount of oxygen taken into solution at high temperatures had only a very slight effect on ductility.
A preliminary study of thrtk cermets based on uranium oxide
167
(b) UO,-ThO, Thoria is by far the more stable phase, and hence reaction between UO, and thorium will occur particularly in the powder form. The possible UO,/ThO, solid solution should provide good bonding behaviour and, since thorium has no solid solubility for oxygen, no embrittlement of the thorium would be expected. However, whether the strong reaction will provide good compacts can only be determined by experiment. (c) U02--SO, Thermodynamically, silicon However, vigorous reaction has (MURRAYand LIVEY1955), and silicide. With solid silicon, little
should not react with UO, to produce uranium. been observed between liquid silicon and solid UO, this is probably due to the formation of a uranium reaction would be expected. 4.
MATERIALS
The general properties of the materials are given in Table 1. TABLE
I.-GENERAL
PROPERTIES OF THE MATERIALS USED IN THE CERMETS
T
Material
~
UO,
1 U30s
~ Thorium
ThO,
/ Zirconium
-
~ZrO, ~ ZrH,
Si
_ Crystal structure
Fluoride cubic
Orthorhombic
Facecentred cubic
Fluoride cubic
11.2
10.07
10.9
Monoclinic
Facecentred tetragonal
5.49
6.4
8.3
6.5
,Cubic ._
~--
_-Density
Bodycentred cubic
2.33
.
----~ Meltingpoint (“C) Average particle size 0~)
3030
1845
1700
2700
-
1415
_.
4
2
15
-I
13
12
85
28
30
-
For the actual tests and sample preparation, British uranium oxide of composition U0213 was used, and for later experiments U,O, was prepared by oxidation of this oxide at 500°C. The zirconium-metal powder and hydride were from Murex Ltd.. although for some experiments BDH zirconium powder was used. Thoria was obtained by calcination of thorium oxalate at 1lOO”C, and the silicon metal powder was BDH laboratory-grade material. Particle-size distributions on these powders were determined in a light-extinction sedimentometer and are detailed in Table 1 above. 5.
METHOD
OF
FABRICATION
Cold pressing and sintering Mixed powders of the requisite composition of British UO, and metal powder were milled dry in glass bottles charged with glass beads. They were then coldcompacted in hardened-steel dies at 8000 lb/ in2 giving specimens approximately 8 in. in diameter and 1 in. long. (a)
168
L. S. WILLIAMS, D. T. LIVEY, E. BARNES, and P. MURRAY
Initial experiments on the sintering of these compacts in argon at a temperature of 1400/15OO”C were not promising, presumably due to the temperature being insufficiently high. Sintering was therefore done in vacua at 1800/19OO”C in a molybdenum tube furnace, the specimens being supported on a tantalum stand. The sintering temperature was reached in 30 min and the pressure was never allowed to rise above 1 micron. On reaching temperature the power supply was cut off and the specimen allowed to cool. 2000
lb/in2
1
1 Sighting hole for temperature measurement
0
0 0 High-frequency
0 0 0 0
N
0
s
Compact Insulation
FIG. 2.-Assembly
of
for hot pressing.
This method of fabrication proved possible for the UO,-Zr system for all the metal contents investigated. In the UO,-Th system, however, the excessive chemical reaction between UO, and thorium-producing uranium metal with a melting-point of 1170°C prevented successful fabrication of compacts for thorium contents <60 per cent by volume, since the uranium metal formed drained from the compact. At higher thorium contents, however, this did not occur and the compacts remained intact. Compacts consisting of UO, and silicon could not be fabricated by this method owing to the high vapour pressure of silicon and the consequent excessive loss of silicon from the system. (b) Hot pressing In this method the milled powders are compacted at high temperature under pressure in carbon dies. Two methods were used: 1. Compaction in air. Here the arrangement is as shown in Fig. 2, pressure being applied by a hydraulic press on one end only. The carbon dies used were 4-in. outside diameter, 0.75in. inside diameter, and 4 in. long, heated by a 25kw
A prelimiw
study of three cerrnets based on uranium oxide
169
high-frequency generator. In the normal procedure, ‘1000 lb/in2 was applied while the system was cold and this pressure held constant until 1000°C had been reached. The pressure was then increased to 2000 lb/in.2 and the specimen held for 10 min at the top temperature under this pressure. The pressure was then released and the system allowed to cool, the specimen being removed at room temperature. This method proved suitable for UO,-Si mixtures, high densities being readily obtained. However, with thorium or zirconium,. the nitride of the metal formed during pressing at the high temperature. On removal from the die, the compacts disintegrated to a Gne powder, due to reaction of the nitride with the water vapour of the atmosljhere. This was indicated by the presence of ammonia. 2, Compaction in an argon atmosphere. This was the method employed for UO,-Zr mixtures and UO,-Th mixtures in order to avoid the formation of nitride. In this case the pressing was done in a carbon-tube furnace through which argon could be passed, the graphite die being supported in the hot zone of the furnace. The dies in this arrangement were 3-in. outside diameter, 0.75-m inside diameter, and 6 in. long, with plungers fitted at both ends. This allowed double ended pressing, which has the advantage of reducing density gradients in the compact. Otherwise the procedure was the same as in the above method, 1000 lb/in2 pressure being applied up to lOOO”C, and 2000 lb/in2 for 10 min at the top temperature. Hot pressing has many advantages as a method of fabrication. These are: (a) it is rapid, (b) fabrication is carried out in one stage, (c) a lower temperature than in normal sintering will give a required density, (d) higher ultimate densities can be achieved, usually close to theoretical, (e) highly reactive constituents which might show excessive shrinkage and cracking in normal sintering can be used, *(f) the product can be made close to the required dimensions. Against these advantages is the possible formation of carbide by reaction with carbon at high temperatures. MURRAYet al. (1954) have reported carbon penetration of various oxide compacts hot-pressed in carbon dies. The formation of a metal carbide would affect the mechanical properties of a cermet and also its corrosion resistance to air and to water. In the present experiments all the specimens had a fragile carbonaceous skin about 2 mm thick, and this was removed before measurements or tests were made. 6. (a)
SINTERING
RESULTS
Cold-pressed compacts
The densities of vacuum-sintered UO,-Zr compacts are shown in Table 2, the theoretical densities quoted being calculated on the basis that the compacts contained U02 and zirconium only. The temperature-density curves are shown in Fig. 3, indicating that up to 20 w/o of zirconium metal little sintering has occurred, even at 1900°C. With 30 w/o of metal, sintering takes place above 19OO”C, and with still higher metal contents, 40 w/o and 50 w/o, sintering to high densities will occur at or above 1750°C. A metal content of 15 w/o or less is probably insufficient to wet every particle of UO, and consolidate the compact, and wetting is also probably inhibited by the formation of a high melting phase at the advancing edge of the liquid. At high metal contents
L. S. WILLIAMS,D. T. LIVEY, E. BARNES, and P. MURRAY
170
‘iz
6*o
IE
I
I
I
1700
1800
1900
I
2000 OC
Temperature
FIG. 3.-Variation
TABLE
Composition UO,/Zr
v/o
85/15
775122.5
2.-DENSITIES
OF SINTERED
UO,-Zr
COMPACTS
I ’
Sintering treatment
_~ w/o
of density with temperature for UO,-Zr compacts.
Calculated density (g/c.c.)
Atmosphere
Observed density (g/cc.)
Porosity
,
(%)
I 2 hr
1380°C
Argon Argon Vacuum
~ 9.94
I I - ~_ SO/20
705129.5
to
70130
58.5141.4
to
60/40
47.5152.5
50/50
375162.5
1750°C 1850°C 1950°C
Vacuum
966
1750°C 1890°C 1960°C
Vacuum
9.11
to
1750°C 1850°C
Vacuum
8.62
to
1780°C 1850°C 1850°C
Vacuum
8.17
--,--
I
5.88 5.99 6.35 6.27 6.40 6.48 6.51 6.65 6.60
,--
40.8 39.7 36.9 36.1 35.6 34.8 __32.6 31.2 31.6 ____
6.28 6.57 8@0
I
31.1 27.9 12.2
I1 I
7.13 8.42
17.3 2.3
7.05 8.44 8.75
13.7 nil nil
171
A preliminary study of three cermets based on uranium oxide
(40 and 50 w/o) the consolidation will be more dependent on the sintering properties of the metal, and this can occur below the melting-point of the metal (1850°C & 30°C). It should be noted that maximum bulk density may not be a desirable feature in cermets for fissile applications, since a permeable cermet may show less tendency to disintegrate on irradiation if the fission-product gases can diffuse to the free surface of an open pore. It may be necessary to compromise in this connection, however, since such porosity may nullify, at least theoretically, the ductility conferred a continuous metal phase. by HAUSNER et al. (1951) sintered cold-pressed compacts of zirconium metal and zirconium-hydride powders in vacua with success. The highest densities were obtained with the hydride, and this was attributed to the fact that when the hydride is decomposed the metal produced is highly reactive; this results in rapid sintering. Some compacts were therefore sintered of UO,ZrH, to observe the effect of using the hydride rather than metal powder. The density results from these compacts are shown in Table 3 and the temperature-density relation in Fig. 4. The composition relations strictly refer to UO,-ZrH,, though they are only fractionally inaccurate if rccepted as UO,-Zr. It would have been expected that the highly active zirconium produced by decomposition of the hydride (complete above 800°C in vacua) would provide better sintering properties. Contrary to expectation, however, the relative density is lower than for the UO,-Zr series of compacts. Only with the 50/50 composition has TABLE 3.-DENSITIES OF SINTEREDUO,-ZrH, Composition UO,/ZrH, Max. temp. “C (vacuum) w/o
COMPACTS
Calculated density
Observed density
Porosity
(g/c.c.)
(g/c.c.)
(%)
vlo
1750 1850 1950
9.67
5.68 5.75 5.86
41.3 40.6 39.4
58.5/41.5
1750 1850 1950
9.12
5.76 5.60 7.03
36.8 38.6 22.9
47.51525
1650 1750 1850 1950
80/20
70.5129.5
70130
60/40
1
--
so/so
37.5162.5
1550 1650 1750 1750 1750 1750
8.62
8.19
6.35 6.42 6.45 7.30
26.5 25.7 25.4 15.5
5,57 6.88 7.53 7.49 7.78 7.38
32.0 36.0 8.1 8.5 5.0 9.9
L. S. WILLIAMS,D.
172
BARNES,and
T. LIVEY,E.
P.
MURRAY
maximum density been attained at a lower temperature. The poorer sintering properties here may be due to evolution of hydrogen causing an initial expansion which is not finally recovered. As a matter of interest, compacts of UO,-ZrO, were also sintered to see if the American solid solution data (LAMBERTSON and MUELLER,1953), shown in Fig. 1, could be reproduced with shorter heating times in vacua. Mixes of 90/10, 80/20, and 8.0 “E S
0 80-20 “A x 70 -30 A 60-40 7.0
0 50-
0 ,B’ 0
50
/
z? .? 2
/
/
/
6.0 /
/
/A
_A__--_---4 0”
/ /”
,/
/
/ /
,’
/
: /I /I
/
,
/
/
/
/’
-
P
I
5.01 1500
----
I
1600
I
I
1800
1700
1900 OC
Temperature
FIG. 4.-Variation
I
of density with sintering temperature for UO,-ZrH, compacts.
70130 w/o were milled in the manner already described, cold-pressed at 16,000 lb/in2, and sintered at 1750°C for one hour. The X-ray diffraction results obtained show close agreement with the American data. It has already been indicated that UO,-Th compacts with thorium contents 60 w/o could not be sintered, due to drainage of uranium metal from the compact. The sintering properties of a compact of higher thorium content, in terms of density achieved, is shown in Table 4, where it is compared with ThO,-ZrH, and UO,-ZrH,, all three having the same metal content by volume. (Oxide-metal, 37*5/625 v/o.) TABLE
4.-DENSITY
OF
ThO,-ZrH,,
UO,.,,-Th,
UO,.,,-Zr,
COMPACTS
1
Components and composition w/o
Sintering treatment
Calculated density (g/c.c.)
Observed density (g/c.cJ
Porosity ( %I
.J ThOB-ZrHZ 47 53
3 min at 1730°C
6.81 7.02
13.6 10.9
6.86
39.2 38.9 7.5 12.2
I-
UO,.,,-Th 36 64 UO,.,,-ZrHz 50 50
3 min at 1750°C
11.29
I 3 min at 1750°C
8.19
:
6.90 7.58 7.19
-
173
A preliminary study of three cermets based on uranium oxide
The much higher porosity of the UO,-Th compacts is probably due to the chemical reaction between the UOs and thorium to produce liquid uranium. This is nonwetting to the oxides present and tends to drain from the compact rather than fili the pores and consolidate the material. (b) Hot-pressed compacts The densities of hot-pressed UOs-Zr compacts are given in Table 5 and shown in Fig. 5. TABLE ~.~DENSITES
Composition UO,-Zr
w/o
OF HOT~PRESSED URANIA-ZIRCONIUM
-
-
Calculated density (glc.c.1
Temp. ‘C 10 min
COMPACTS
Observed density (glc.c.1
Porosity (%I
8.25 9.50 9.41 9.26
14.6 1.2 2.6 4.1
VI0 -
T
SO/20
70.5129.5
1300 1400 1500 1600
9.66
70130
58.5141.5
1200 1250 1350 1450 1450 1500 1600
9.11
6.07 7.96 9.11 9.10 9.01 9.11 9.11
60140
47.5152.5
1150 1250 1300 1350 1350 1350 1375 1425 1425 1425 1430 1460 1550
8.62
5.76 7.84 8.63 8.61 8.44 8.63 8.73 8.63 8.73 8.78 8.73 8.65 8.60
l-
_‘_
.-
-
-
-
j_
-!
33.4 12.6 nil llil 1.1 nil nil
-
33.2 9.1 nil 0.1 2.1 nil nil llil nil nil nil nil nil
The results show that high densities can be obtained under pressure at temperatures ranging from 1300°C to 16OO”C, depending on the metal content. For the 80/20,70/30, and 60/40 mixes, compacting was rapid above 123O”C, 12OO”C, and 1150°C respectively and had fallen off by i400°C, 135O”C,and 1300°C respectively. These results are in accordance with normal hot-pressing experience, a given density being achieved at a lower temperature and the end density achieved at a given temperature being higher than for normal sintering.
174
L. S. WILLIAMS,D. T. LIVEY, E. BARNES,and P. MURRAY
In the UOJSi system, hot-pressing at any temperature above compacts of density >95 per cent of theoretical for all the compositions i.e. from zero to 60 v/o silicon metal.
OC Temperature characteristics for UO,-Zr mixtures.
FIG. 5.-Hot-pressing 7.
THERMAL
SHOCK
PROPERTIES
Each compact was taken to 1000°C in vacuum The results of the tests are shown in Table 6. TABLE 6.-THERMAL
SHOCK
and quenched
uo, Zr
UO,/Zr UO,/Zr UO,/Zr UO,/ZrH, UOJTh UO,/Si UO,/Si
60140 w/o so/o2 w/o so/so w/o so/50 w/o 36164 w/o SO/20 v/o 40160 v/o
I
in a water-bath.
PROPERTIES OF CERMETS I
I
Composition
1400°C gave investigated,
Number of cycles to failure at 1000°C
Method of preparation
Hot-pressed Vat.-sintered
1750°C 1600°C
10 min 40 min
Hot-pressed (argon) Hot-pressed (argon) Vat.-sintered Vat.-sintered Vat-sintered Hot-pressed Hot-pressed
1430°C 1460°C 1760°C 1760°C 1730°C 1450°C 1450°C
10 min 10 min 5 min 5 min 5 min 10 min 10 min
1 No failure after 8 at 1000°C 8 at 1200°C 8 at 1450°C 2 2 2 2 8 2 2
The thermal-shock resistance of sintered zirconium is high, while that of hotpressed UO,., is low. On addition of zirconium to UO,.,, the shock resistance remains unaffected even in the SO/SO w/o composition (62.5 v/o zirconium). This could possibly be due to one of the two following causes: the bond between the
A preliminary
study of three cermets based on uranium oxide
175
metal and the ceramic may be poor; or, as a result of the high oxygen solubihty in zirconium, oxygen embrittlement of the metal may have occurred. Of these two explanations, the latter is more likely, since the U02/Th compact containing an 8.0
x uo, 80/20
% UO-/Si
70/30 60/40 50/50 40/60
Tempemture FIG.
6.-Plot
of shock
OC
of average bend-strength in temperature of thermal shock for UO,.l, Si cermets.
equal v/o of metal (62.5 per cent w/o thorium) had a higher thermal shock resistance, and thorium metal is known to have a low solubility for oxygen. The UO,/Si compacts showed no improvement in shock resistance, probably due to the low ductility of silicon itself. However, some further tests were carried out by quenching from a series of temperatures in order to grade- the severity of the test and to determine if silicon had any effect at all on the shock resistance. In these tests the resistance to shock was measured by the change in the bend strength of the material. The results of the tests are shown in Fig. 6. As the silicon content of the compacts is increased, the bend strength of the unshocked material progressively falls. The bend strength of UO, after quenching from any temperature above 200°C is very low. The strength of the silicon compacts is apparently higher after quenching from 200°C and the shock resistance is fair up to 400°C. Beyond this temperature, however, the shock resistance is not improved over pure UO,. The improvement achieved by the silicon content up to 400°C is little affected by increases in the silicon content beyond 20 v/o silicon. 8.
OXIDATION
RESISTANCE
OF
UO,-Si
COMPACTS
In view of the high oxidation resistance of silicon, even in powder form, presumably due to the formation of a protective skin of silica, oxidation tests were carried out on the UO,-Si compacts. The test simply consisted of heating the specimen in dry
176
L. S.
WILLIAMS, D. T. LIVEY, E. BARNES, gnd P. MURRAY
air at various temperatures and noting the change in weight with time. The rate of oxidation of each composition is shown in Table 7. Silicon has a marked effect on the rate at which oxidation occurs. At 800”/900” C, 20 v/o silicon apparently increases the rate of oxidation, and this may be due to the silicon inhibiting the formation of semisintered layers of U,O,. In the absence of silicon this tends to take place and slow down the rate of oxidation. Beyond 20 per cent silicon, however, the rate of oxidation falls off rapidly, particularly in changing from UO,-rich to silicon-rich compositions. TABLE 7.-OxrDaTIoN Composition UO,.,,/Si v/o
uo2.13 80120 70/30 60140 so/so 40/60
RATES FOR UO,/Si Density
I
CERMETSAT .~PPR~xIMATELY900°C Temperature of oxidation (“C)
Rate of oxidation
(g/c.c.)
IO.4 9.3 8.8 7.2 6.3 5.6
880 880 890 890 950 880
55.0 65.0 21.0 9.0 5.0 1.0
(mg/cm%r)
In Fig. 7 the rates of oxidation of UO,.,, alone and of UO&Si, 40/60 v/o are shown as a function of temperature. Taking UO,.,, alone, there is a maximum in the rate of oxidation at 7OO”C,and this is probably due to the fact that above this temperature, formation of U,Os is accompanied by sintering to give partially sintered layers at the surface of the compact, thus reducing the rate of oxidation. Below 7OO”C,the U,Os which formed simply dusted from the compact, and the lower rates of oxidation are then due to the lower temperatures. On addition of silicon, the height of this maximum at 700°C is markedly reduced, this being shown for the 40/60 UO,/Si composition in Fig. 8. The oxidation rates as a function of temperature and composition are shown in Fig. 8; above 400°C there is a marked reduction in the rate of oxidation once 20 per cent silicon has been added. Beyond this composition, although the oxidation resistance improves, the relative effect is not so marked. At least 20 per cent of silicon by volume is therefore required to provide the protection. One important point is that the only cermets which remained intact during these tests were the silicon-rich compositions, 50 and 60 V/O silicon. The other compositions powdered during the course of oxidation. Also, in every case the degree of oxidation corresponded closely to the oxidation of UOP13 to U,Os only, the silicon being almost completely unoxidized. Several points are worth noting from these results: (a) Hot-pressed UO,.,, of high density has a negligible rate of oxidation up to 300°C (0.3 mg/cm2/hr at 300°C). (b) The rate of oxidation of hot-pressed UO,.,, increases above 3OO”C,to reach a maximum of 160 mg/cm2/hr at 700% thereafter the rate of oxidation decreases, probably due to the onset of sintering of UsOs.
A preliminary study of three cermets based on uranium oxide
111
(c) The addition of silicon reduces the rate of oxidation and has greatest effect in the temperature range where the rate of oxidation of UOs.13 is highest. The addition of 20 per cent silicon by volume is sufficient to reduce the rate of oxidation markedly. Beyond this composition the oxidation resistance improves, though less markedly. .13 essed
200
400
600 800 Temperature
1000
1200 OC
1000
1200
g;ymJwJ-j 200’
&I--600
80;
OC
Temperature
FIG. ‘T.--Rate of oxidation of UO,.,o and of U02.,,/Si 40/60 V/O compacts as a function
of temperature.
IO 20 30 40 50 60 70 80 v/o silicon
p
50
F
10 20 30 40
L 4
‘QOO’C 50X-------. ‘.
S E 10 20 30 40 50 60 70 80 y. silicon
FIG. S.-Rate
50 60 70 80
0
X.
*x-wx
IO 2030405&~70-i % silicon
of oxidation of UO,/Si cermets in dry air.
x
178
L. S. WILLIAMS, D. T. LIVEY,E. BARNES,and P. MURRAY 9.
CONCLUSIONS
1. The data of LAMBERTSONand MUELLER on the UO,-ZrO, verified by vacuum sintering of compacts at 1750°C.
system has been
2. Fabrication of UO,-Zr compacts to high density is possible by hot-pressing methods, provided an argon atmosphere is employed. The use of temperatures higher than that required to produce a given density results in brittle compacts, probably due to the formation of metal carbide. 3. Fabrication to high density of UO,-Zr compacts by cold-pressing and sintering is only possible at very high temperatures (19OO’C)unless the metal content is greater than 50 per cent by volume. 4. Fabrication of UO,-Th compacts by cold-pressing and sintering is not possible for low metal contents, due to the formation and drainage from the compact of liquid uranium. Even compacts with 62.5 per cent thorium do not sinter to high density at 175O”C, presumably also due to the chemical reaction between UOg and thorium. 5. The thermal shock properties of UO,.,, are not improved by the addition of zirconium, even where the zirconium is present to the extent of 62.5 v/o. The thermal shock resistance of an equivalent thorium compact (62.5 v/o thorium) is much improved over the UO,,,/Zr, suggesting that the poor properties of the zirconium cermets may be due to oxygen embrittlement of the metal, since zirconium has a high solubility for oxygen and thorium has no solubility for oxygen. 6. Compacts of UO,.,,-Zr formed from UO,.,,-ZrH,, contrary to expectation do not have superior sintering properties to compacts formed from UO,l, and zirconium metal powder. 7. The thermal shock resistance of UO,.l, is improved by the addition of silicon for quenching temperatures up to 400°C. Beyond this temperature the shock resistance is not affected. 8. The oxidation resistance of UO,.,,JSi compacts is greater than for UO,.,, compacts. Beyond 20 v/o silicon the rate of oxidation diminishes but is not markedly affected by the increase in silicon content. However, compacts with silicon contents greater than 50 v/o are superior in that no disintegration takes place during oxidation. REFERENCES
I. BLACKBURN A. R. and SHEVLINP. S. (1951) J. Amer. Cer. Sot. 34 (11) 327-331. 2. CIJBICCIOTTI D. D. (1951) J. Amer. Chem. Sot. 73, 2032. 3. FASTJ. D. (1948) h4etallwirtschnft 17, 641, 4. HAUSNERH. H., KALISCHH. S., and ANGIERR. P. (1951) J. Metals 3 (8) 625. 5. LAMBERTSON W. A. and MUELLERM. H. (1953) J. Amer. Cer. Sot. 36, 365. 6. LAMBERTSON W. A., MUELLERM. H., and GUNZELF. H. (1953) J. Amer. Cer. Sot. 36, 397. 7. LANG S. M., FILLMORE C. L., and ROTH R. S. (1952) Natl. Bureau of Standards, Interim Report to AEC. 8. LIVEY D. T. and MURRAY P. (1955) The Wetting Properties of Solid Oxides and Carbides by Liquid Metals, Powder Metallurgy Seminar, Reutte, June. 9. MOOREN. C., HUFFADINE J. W., and LANGLANDL. Plessey Co. Ltd., private communication. 10. MURRAYP., RODGERS E. P., and WILLIAMSA. E. (1954) Trans. Brit. Cer. Sot. 53, No. 8.
Energy
I. 1957, Vol.
4. pp. 164 to 178
Pergamon
Press
Ltd.,
London
A PRELIMINARY STUDY OF THREE CERMETS BASED ON URANIUM OXIDE* L. S. WILLIAMs,t D. T. LIVEY, E. BARNES, and P. MURRAY (Received 26 April 19.56) Abstract-The fabrication and some properties of three cermets have been investigated in outline. Fabrication by hot-pressing methods is feasible, provided an argon atmosphere is used. The thermal-shock properties of UOt are not improved by the addition of zirconium; however, the UO,-Th system, at high metal contents, and to a lesser extent the U02-Si system, up to 60 v/o of metal, show improvement in this respect. The poor properties of the zirconium cermets is probably due to oxygen embrittlement of the zirconium as a result of reaction with the oxide constituent. The use of silicon as the metallic constituent at 20 per cent by volume improves the oxidation resistance of UO, in air at temperatures up to 900°C. Addition of silicon beyond 20 v/o has little effect on the rate of oxidation. 1.
INTRODUCTION
ONE of the newer concepts designed to reduce the inherent deficiencies of ceramics is that of the metal ceramic mixture, or cermet. Outside the atomic energy field the major study of such systems at high temperatures has been on the chromium-alumina (MOORE et al., private communication; BLACKBURN and SHEVLIN, 1951) system, although attention is also being directed to carbide-metal systems such as Tic-Co. The latter have, of course, been used for many years in the cutting-tool applications. In the atomic-energy project, from the outset, emphasis has been placed primarily on the use of metallic systems for fuel elements. Only recently has serious consideration been given to the possibilities of refractory uranium compounds, the corrosion resistance of UO, in water systems and the apparently good irradiation behaviour being significant properties. At Harwell some research has been done on the fabrication and properties of UO, and UC. The oxide, however, is brittle and has poor thermal shock resistance and the work reported here was designed as a preliminary investigation of a cermet system of the oxide-metal type. For this, two alternatives are possible: (1) to combine a suitable ceramic with a fissile metal, (2) to combine the oxide of a fissile element with a suitable metal. The second of these has been the main basis of the present study for the following reasons. (a) Uranium-based ceramics are considerably more refractory than the metal, and do not suffer from the anisotropy and other unsatisfactory characteristics of uranium itself. (b) A selection from a wider range of metals is possible where the metal is the nonfissile component. The restrictions then are adequate refractoriness (both high melting-point and strength) combined with some inherent ductility and acceptable nuclear characteristics.
* Communication from t Now at CSIRO,
the Metallurgy Division, Melbourne, Australia.
UKAEA, 164
Research
Group,
Harwell.
A preliminarystudy of three cermets based on uranium oxide
165
Three metals are considered here which, besides bonding effectively to the uranium oxide, are suitable in at least one of the properties indicated in (b). These are zirconium, thorium, and silicon. Emphasis has been placed on the fabrication and properties of the cermets; in particular, on their ability to withstand the severe thermal shock that may be experienced in a fuel element during service. The thermal-shock test consisted of quenching the specimen from high temperatures in uacuo into a water bath. 5.46 ? ?A.E.R.E
check points
5.42
5.38
5.26
5.22
5.18
10
20
30
40
“/o FIG. I.-Effect
2.
50
60
70
80
ZrO2
of solid solution on lattice parameter of UO,.
PRELIMINARY
CONSIDERATIONS
(a) Urania-zirconia system This system has been studied by LAMBERTSON and MUELLER (1953), who concluded that it consists primarily of two solid solutions separated below 1900°C by a two-phase region. Results were obtained by sintering compacted pellets of appropriate composition for 100 hr at 1700°C in hydrogen with intermittent crushing, grinding, and pressing. The main solid-solution phases were studied by X-ray diffraction, this showing that Urania takes up to 52 mol per cent of zirconia into solid solution with a progressive decrease in the parameter of the cubic lattice from 5*46A to 532A (Fig. 1). From 53/100 mol per cent zirconia, the lattice was tetragonal when investigated by quenching techniques. There was also some indication that zirconia was transformed to a polymorphic form other than monoclinic or tetragonal at approximately 1900°C. The affinity of zirconium for oxygen is well known, and FAST(1948) found that 40 at. per cent (15 ‘wt.per cent) of oxygen could be taken into solid solution. However, CUBICCIOTTI (1951) sets the figure at 55 at. per cent (17.6 wt. per cent) and also reported that ZrO, could take up to 15 at. per cent (11.5 wt. per cent) of zirconium into solid solution. NORTON and KINGERY,using stabilized zirconia, confh-med that
166
L. S. WILLIAMS, D. T. LIVEY,E. BARNES, and P. MURRAY
at least 5 wt. per cent of zirconium would go into solid solution in ZrO,, and since they used 5 wt. per cent increments it is possible that the true saturation figure is not far below the value given by CUBICCIOTTI.In any case, the presence in the zirconia lattice of the stabilizing magnesium or calcium atoms would be expected to reduce the extent to which other atoms could enter. (b) Urania-thoria system According to LAMBERTSON et al. (1953), UO, and ThO, form a continuous series of solid solutions with the solidus-liquidus extending from 2875°C for UOz to above 3200°C for ThO,. Unlike zirconium, thorium does not take oxygen into solution. (c) Urania-silica system LANG et al. (1952) established, in a preliminary investigation, that UO, and SiO,, in an oxygen-free atmosphere, form no compounds and show no solid solubility by solid-state reaction from 800°C to lOOO”C,for the range 10 mol. per cent to 90 mol per cent UO,. The eutectic temperature was placed at 1600” f 50°C. Tn view of the difference in size of the U4+ ion and Si4+ ion (I.01 A and 0.41 A respectively) and the different structures of UO, (cubic) and SiO, (quartz), solid solution between the oxides would not be expected. 3. THERMODYNAMIC STABILITY OF THE PHASES INVOLVED The free energies of formation (AG, 298°K) of the oxides UO,, ThO,, ZrO,, and SiO, are -247, -280, -248, and -195 kcals/mol respectively. (a) UO,-ZrO, These oxides are equally stable and reaction would therefore be expected between a UO, phase and a zirconium phase, and the more readily since ZrO, can also enter into solid solution with UO,. Where the phases are in bulk form, i.e. coarse powders, the reaction would probably be interfacial only. However, using fine powders, which may be very active depending on the method of preparation, reaction may be considerable. This is particularly so for nonstoichiometric U02.13, which is highly active and contains interstitial oxygen. The extra oxygen can be removed (e.g. by hydrogen reduction) and may therefore be expected to go into the zirconium, which has been reported to take up to 40 at. per cent of oxygen into solid solution (FAST, 1948). Chemical reaction and the possibility of the formation of a UO,/ZrO, solid solution is expected therefore in UO,/Zr cermets. The formation of a solid solution appears desirable for good bonding behaviour in oxide-metal cermets, since a few per cent of Cr,O, in solid solution in Al,O, seems necessary for successful bonding of Al,O,-Cr cermets. From this point of view, the UO,-ZrO, system should provide good bonding behaviour. However, the possible loss of ductility of the zirconium due to dissolved oxygen may offset any good bonding mechanism, although the results of HAUSNERet al. (1951) on vacuum-sintered compacts indicate that this may not be so. They found that a small increase in the amount of oxygen taken into solution at high temperatures had only a very slight effect on ductility.
A preliminary study of thrtk cermets based on uranium oxide
167
(b) UO,-ThO, Thoria is by far the more stable phase, and hence reaction between UO, and thorium will occur particularly in the powder form. The possible UO,/ThO, solid solution should provide good bonding behaviour and, since thorium has no solid solubility for oxygen, no embrittlement of the thorium would be expected. However, whether the strong reaction will provide good compacts can only be determined by experiment. (c) U02--SO, Thermodynamically, silicon However, vigorous reaction has (MURRAYand LIVEY1955), and silicide. With solid silicon, little
should not react with UO, to produce uranium. been observed between liquid silicon and solid UO, this is probably due to the formation of a uranium reaction would be expected. 4.
MATERIALS
The general properties of the materials are given in Table 1. TABLE
I.-GENERAL
PROPERTIES OF THE MATERIALS USED IN THE CERMETS
T
Material
~
UO,
1 U30s
~ Thorium
ThO,
/ Zirconium
-
~ZrO, ~ ZrH,
Si
_ Crystal structure
Fluoride cubic
Orthorhombic
Facecentred cubic
Fluoride cubic
11.2
10.07
10.9
Monoclinic
Facecentred tetragonal
5.49
6.4
8.3
6.5
,Cubic ._
~--
_-Density
Bodycentred cubic
2.33
.
----~ Meltingpoint (“C) Average particle size 0~)
3030
1845
1700
2700
-
1415
_.
4
2
15
-I
13
12
85
28
30
-
For the actual tests and sample preparation, British uranium oxide of composition U0213 was used, and for later experiments U,O, was prepared by oxidation of this oxide at 500°C. The zirconium-metal powder and hydride were from Murex Ltd.. although for some experiments BDH zirconium powder was used. Thoria was obtained by calcination of thorium oxalate at 1lOO”C, and the silicon metal powder was BDH laboratory-grade material. Particle-size distributions on these powders were determined in a light-extinction sedimentometer and are detailed in Table 1 above. 5.
METHOD
OF
FABRICATION
Cold pressing and sintering Mixed powders of the requisite composition of British UO, and metal powder were milled dry in glass bottles charged with glass beads. They were then coldcompacted in hardened-steel dies at 8000 lb/ in2 giving specimens approximately 8 in. in diameter and 1 in. long. (a)
168
L. S. WILLIAMS, D. T. LIVEY, E. BARNES, and P. MURRAY
Initial experiments on the sintering of these compacts in argon at a temperature of 1400/15OO”C were not promising, presumably due to the temperature being insufficiently high. Sintering was therefore done in vacua at 1800/19OO”C in a molybdenum tube furnace, the specimens being supported on a tantalum stand. The sintering temperature was reached in 30 min and the pressure was never allowed to rise above 1 micron. On reaching temperature the power supply was cut off and the specimen allowed to cool. 2000
lb/in2
1
1 Sighting hole for temperature measurement
0
0 0 High-frequency
0 0 0 0
N
0
s
Compact Insulation
FIG. 2.-Assembly
of
for hot pressing.
This method of fabrication proved possible for the UO,-Zr system for all the metal contents investigated. In the UO,-Th system, however, the excessive chemical reaction between UO, and thorium-producing uranium metal with a melting-point of 1170°C prevented successful fabrication of compacts for thorium contents <60 per cent by volume, since the uranium metal formed drained from the compact. At higher thorium contents, however, this did not occur and the compacts remained intact. Compacts consisting of UO, and silicon could not be fabricated by this method owing to the high vapour pressure of silicon and the consequent excessive loss of silicon from the system. (b) Hot pressing In this method the milled powders are compacted at high temperature under pressure in carbon dies. Two methods were used: 1. Compaction in air. Here the arrangement is as shown in Fig. 2, pressure being applied by a hydraulic press on one end only. The carbon dies used were 4-in. outside diameter, 0.75in. inside diameter, and 4 in. long, heated by a 25kw
A prelimiw
study of three cerrnets based on uranium oxide
169
high-frequency generator. In the normal procedure, ‘1000 lb/in2 was applied while the system was cold and this pressure held constant until 1000°C had been reached. The pressure was then increased to 2000 lb/in.2 and the specimen held for 10 min at the top temperature under this pressure. The pressure was then released and the system allowed to cool, the specimen being removed at room temperature. This method proved suitable for UO,-Si mixtures, high densities being readily obtained. However, with thorium or zirconium,. the nitride of the metal formed during pressing at the high temperature. On removal from the die, the compacts disintegrated to a Gne powder, due to reaction of the nitride with the water vapour of the atmosljhere. This was indicated by the presence of ammonia. 2, Compaction in an argon atmosphere. This was the method employed for UO,-Zr mixtures and UO,-Th mixtures in order to avoid the formation of nitride. In this case the pressing was done in a carbon-tube furnace through which argon could be passed, the graphite die being supported in the hot zone of the furnace. The dies in this arrangement were 3-in. outside diameter, 0.75-m inside diameter, and 6 in. long, with plungers fitted at both ends. This allowed double ended pressing, which has the advantage of reducing density gradients in the compact. Otherwise the procedure was the same as in the above method, 1000 lb/in2 pressure being applied up to lOOO”C, and 2000 lb/in2 for 10 min at the top temperature. Hot pressing has many advantages as a method of fabrication. These are: (a) it is rapid, (b) fabrication is carried out in one stage, (c) a lower temperature than in normal sintering will give a required density, (d) higher ultimate densities can be achieved, usually close to theoretical, (e) highly reactive constituents which might show excessive shrinkage and cracking in normal sintering can be used, *(f) the product can be made close to the required dimensions. Against these advantages is the possible formation of carbide by reaction with carbon at high temperatures. MURRAYet al. (1954) have reported carbon penetration of various oxide compacts hot-pressed in carbon dies. The formation of a metal carbide would affect the mechanical properties of a cermet and also its corrosion resistance to air and to water. In the present experiments all the specimens had a fragile carbonaceous skin about 2 mm thick, and this was removed before measurements or tests were made. 6. (a)
SINTERING
RESULTS
Cold-pressed compacts
The densities of vacuum-sintered UO,-Zr compacts are shown in Table 2, the theoretical densities quoted being calculated on the basis that the compacts contained U02 and zirconium only. The temperature-density curves are shown in Fig. 3, indicating that up to 20 w/o of zirconium metal little sintering has occurred, even at 1900°C. With 30 w/o of metal, sintering takes place above 19OO”C, and with still higher metal contents, 40 w/o and 50 w/o, sintering to high densities will occur at or above 1750°C. A metal content of 15 w/o or less is probably insufficient to wet every particle of UO, and consolidate the compact, and wetting is also probably inhibited by the formation of a high melting phase at the advancing edge of the liquid. At high metal contents
L. S. WILLIAMS,D. T. LIVEY, E. BARNES, and P. MURRAY
170
‘iz
6*o
IE
I
I
I
1700
1800
1900
I
2000 OC
Temperature
FIG. 3.-Variation
TABLE
Composition UO,/Zr
v/o
85/15
775122.5
2.-DENSITIES
OF SINTERED
UO,-Zr
COMPACTS
I ’
Sintering treatment
_~ w/o
of density with temperature for UO,-Zr compacts.
Calculated density (g/c.c.)
Atmosphere
Observed density (g/cc.)
Porosity
,
(%)
I 2 hr
1380°C
Argon Argon Vacuum
~ 9.94
I I - ~_ SO/20
705129.5
to
70130
58.5141.4
to
60/40
47.5152.5
50/50
375162.5
1750°C 1850°C 1950°C
Vacuum
966
1750°C 1890°C 1960°C
Vacuum
9.11
to
1750°C 1850°C
Vacuum
8.62
to
1780°C 1850°C 1850°C
Vacuum
8.17
--,--
I
5.88 5.99 6.35 6.27 6.40 6.48 6.51 6.65 6.60
,--
40.8 39.7 36.9 36.1 35.6 34.8 __32.6 31.2 31.6 ____
6.28 6.57 8@0
I
31.1 27.9 12.2
I1 I
7.13 8.42
17.3 2.3
7.05 8.44 8.75
13.7 nil nil
171
A preliminary study of three cermets based on uranium oxide
(40 and 50 w/o) the consolidation will be more dependent on the sintering properties of the metal, and this can occur below the melting-point of the metal (1850°C & 30°C). It should be noted that maximum bulk density may not be a desirable feature in cermets for fissile applications, since a permeable cermet may show less tendency to disintegrate on irradiation if the fission-product gases can diffuse to the free surface of an open pore. It may be necessary to compromise in this connection, however, since such porosity may nullify, at least theoretically, the ductility conferred a continuous metal phase. by HAUSNER et al. (1951) sintered cold-pressed compacts of zirconium metal and zirconium-hydride powders in vacua with success. The highest densities were obtained with the hydride, and this was attributed to the fact that when the hydride is decomposed the metal produced is highly reactive; this results in rapid sintering. Some compacts were therefore sintered of UO,ZrH, to observe the effect of using the hydride rather than metal powder. The density results from these compacts are shown in Table 3 and the temperature-density relation in Fig. 4. The composition relations strictly refer to UO,-ZrH,, though they are only fractionally inaccurate if rccepted as UO,-Zr. It would have been expected that the highly active zirconium produced by decomposition of the hydride (complete above 800°C in vacua) would provide better sintering properties. Contrary to expectation, however, the relative density is lower than for the UO,-Zr series of compacts. Only with the 50/50 composition has TABLE 3.-DENSITIES OF SINTEREDUO,-ZrH, Composition UO,/ZrH, Max. temp. “C (vacuum) w/o
COMPACTS
Calculated density
Observed density
Porosity
(g/c.c.)
(g/c.c.)
(%)
vlo
1750 1850 1950
9.67
5.68 5.75 5.86
41.3 40.6 39.4
58.5/41.5
1750 1850 1950
9.12
5.76 5.60 7.03
36.8 38.6 22.9
47.51525
1650 1750 1850 1950
80/20
70.5129.5
70130
60/40
1
--
so/so
37.5162.5
1550 1650 1750 1750 1750 1750
8.62
8.19
6.35 6.42 6.45 7.30
26.5 25.7 25.4 15.5
5,57 6.88 7.53 7.49 7.78 7.38
32.0 36.0 8.1 8.5 5.0 9.9
L. S. WILLIAMS,D.
172
BARNES,and
T. LIVEY,E.
P.
MURRAY
maximum density been attained at a lower temperature. The poorer sintering properties here may be due to evolution of hydrogen causing an initial expansion which is not finally recovered. As a matter of interest, compacts of UO,-ZrO, were also sintered to see if the American solid solution data (LAMBERTSON and MUELLER,1953), shown in Fig. 1, could be reproduced with shorter heating times in vacua. Mixes of 90/10, 80/20, and 8.0 “E S
0 80-20 “A x 70 -30 A 60-40 7.0
0 50-
0 ,B’ 0
50
/
z? .? 2
/
/
/
6.0 /
/
/A
_A__--_---4 0”
/ /”
,/
/
/ /
,’
/
: /I /I
/
,
/
/
/
/’
-
P
I
5.01 1500
----
I
1600
I
I
1800
1700
1900 OC
Temperature
FIG. 4.-Variation
I
of density with sintering temperature for UO,-ZrH, compacts.
70130 w/o were milled in the manner already described, cold-pressed at 16,000 lb/in2, and sintered at 1750°C for one hour. The X-ray diffraction results obtained show close agreement with the American data. It has already been indicated that UO,-Th compacts with thorium contents 60 w/o could not be sintered, due to drainage of uranium metal from the compact. The sintering properties of a compact of higher thorium content, in terms of density achieved, is shown in Table 4, where it is compared with ThO,-ZrH, and UO,-ZrH,, all three having the same metal content by volume. (Oxide-metal, 37*5/625 v/o.) TABLE
4.-DENSITY
OF
ThO,-ZrH,,
UO,.,,-Th,
UO,.,,-Zr,
COMPACTS
1
Components and composition w/o
Sintering treatment
Calculated density (g/c.c.)
Observed density (g/c.cJ
Porosity ( %I
.J ThOB-ZrHZ 47 53
3 min at 1730°C
6.81 7.02
13.6 10.9
6.86
39.2 38.9 7.5 12.2
I-
UO,.,,-Th 36 64 UO,.,,-ZrHz 50 50
3 min at 1750°C
11.29
I 3 min at 1750°C
8.19
:
6.90 7.58 7.19
-
173
A preliminary study of three cermets based on uranium oxide
The much higher porosity of the UO,-Th compacts is probably due to the chemical reaction between the UOs and thorium to produce liquid uranium. This is nonwetting to the oxides present and tends to drain from the compact rather than fili the pores and consolidate the material. (b) Hot-pressed compacts The densities of hot-pressed UOs-Zr compacts are given in Table 5 and shown in Fig. 5. TABLE ~.~DENSITES
Composition UO,-Zr
w/o
OF HOT~PRESSED URANIA-ZIRCONIUM
-
-
Calculated density (glc.c.1
Temp. ‘C 10 min
COMPACTS
Observed density (glc.c.1
Porosity (%I
8.25 9.50 9.41 9.26
14.6 1.2 2.6 4.1
VI0 -
T
SO/20
70.5129.5
1300 1400 1500 1600
9.66
70130
58.5141.5
1200 1250 1350 1450 1450 1500 1600
9.11
6.07 7.96 9.11 9.10 9.01 9.11 9.11
60140
47.5152.5
1150 1250 1300 1350 1350 1350 1375 1425 1425 1425 1430 1460 1550
8.62
5.76 7.84 8.63 8.61 8.44 8.63 8.73 8.63 8.73 8.78 8.73 8.65 8.60
l-
_‘_
.-
-
-
-
j_
-!
33.4 12.6 nil llil 1.1 nil nil
-
33.2 9.1 nil 0.1 2.1 nil nil llil nil nil nil nil nil
The results show that high densities can be obtained under pressure at temperatures ranging from 1300°C to 16OO”C, depending on the metal content. For the 80/20,70/30, and 60/40 mixes, compacting was rapid above 123O”C, 12OO”C, and 1150°C respectively and had fallen off by i400°C, 135O”C,and 1300°C respectively. These results are in accordance with normal hot-pressing experience, a given density being achieved at a lower temperature and the end density achieved at a given temperature being higher than for normal sintering.
174
L. S. WILLIAMS,D. T. LIVEY, E. BARNES,and P. MURRAY
In the UOJSi system, hot-pressing at any temperature above compacts of density >95 per cent of theoretical for all the compositions i.e. from zero to 60 v/o silicon metal.
OC Temperature characteristics for UO,-Zr mixtures.
FIG. 5.-Hot-pressing 7.
THERMAL
SHOCK
PROPERTIES
Each compact was taken to 1000°C in vacuum The results of the tests are shown in Table 6. TABLE 6.-THERMAL
SHOCK
and quenched
uo, Zr
UO,/Zr UO,/Zr UO,/Zr UO,/ZrH, UOJTh UO,/Si UO,/Si
60140 w/o so/o2 w/o so/so w/o so/50 w/o 36164 w/o SO/20 v/o 40160 v/o
I
in a water-bath.
PROPERTIES OF CERMETS I
I
Composition
1400°C gave investigated,
Number of cycles to failure at 1000°C
Method of preparation
Hot-pressed Vat.-sintered
1750°C 1600°C
10 min 40 min
Hot-pressed (argon) Hot-pressed (argon) Vat.-sintered Vat.-sintered Vat-sintered Hot-pressed Hot-pressed
1430°C 1460°C 1760°C 1760°C 1730°C 1450°C 1450°C
10 min 10 min 5 min 5 min 5 min 10 min 10 min
1 No failure after 8 at 1000°C 8 at 1200°C 8 at 1450°C 2 2 2 2 8 2 2
The thermal-shock resistance of sintered zirconium is high, while that of hotpressed UO,., is low. On addition of zirconium to UO,.,, the shock resistance remains unaffected even in the SO/SO w/o composition (62.5 v/o zirconium). This could possibly be due to one of the two following causes: the bond between the
A preliminary
study of three cermets based on uranium oxide
175
metal and the ceramic may be poor; or, as a result of the high oxygen solubihty in zirconium, oxygen embrittlement of the metal may have occurred. Of these two explanations, the latter is more likely, since the U02/Th compact containing an 8.0
x uo, 80/20
% UO-/Si
70/30 60/40 50/50 40/60
Tempemture FIG.
6.-Plot
of shock
OC
of average bend-strength in temperature of thermal shock for UO,.l, Si cermets.
equal v/o of metal (62.5 per cent w/o thorium) had a higher thermal shock resistance, and thorium metal is known to have a low solubility for oxygen. The UO,/Si compacts showed no improvement in shock resistance, probably due to the low ductility of silicon itself. However, some further tests were carried out by quenching from a series of temperatures in order to grade- the severity of the test and to determine if silicon had any effect at all on the shock resistance. In these tests the resistance to shock was measured by the change in the bend strength of the material. The results of the tests are shown in Fig. 6. As the silicon content of the compacts is increased, the bend strength of the unshocked material progressively falls. The bend strength of UO, after quenching from any temperature above 200°C is very low. The strength of the silicon compacts is apparently higher after quenching from 200°C and the shock resistance is fair up to 400°C. Beyond this temperature, however, the shock resistance is not improved over pure UO,. The improvement achieved by the silicon content up to 400°C is little affected by increases in the silicon content beyond 20 v/o silicon. 8.
OXIDATION
RESISTANCE
OF
UO,-Si
COMPACTS
In view of the high oxidation resistance of silicon, even in powder form, presumably due to the formation of a protective skin of silica, oxidation tests were carried out on the UO,-Si compacts. The test simply consisted of heating the specimen in dry
176
L. S.
WILLIAMS, D. T. LIVEY, E. BARNES, gnd P. MURRAY
air at various temperatures and noting the change in weight with time. The rate of oxidation of each composition is shown in Table 7. Silicon has a marked effect on the rate at which oxidation occurs. At 800”/900” C, 20 v/o silicon apparently increases the rate of oxidation, and this may be due to the silicon inhibiting the formation of semisintered layers of U,O,. In the absence of silicon this tends to take place and slow down the rate of oxidation. Beyond 20 per cent silicon, however, the rate of oxidation falls off rapidly, particularly in changing from UO,-rich to silicon-rich compositions. TABLE 7.-OxrDaTIoN Composition UO,.,,/Si v/o
uo2.13 80120 70/30 60140 so/so 40/60
RATES FOR UO,/Si Density
I
CERMETSAT .~PPR~xIMATELY900°C Temperature of oxidation (“C)
Rate of oxidation
(g/c.c.)
IO.4 9.3 8.8 7.2 6.3 5.6
880 880 890 890 950 880
55.0 65.0 21.0 9.0 5.0 1.0
(mg/cm%r)
In Fig. 7 the rates of oxidation of UO,.,, alone and of UO&Si, 40/60 v/o are shown as a function of temperature. Taking UO,.,, alone, there is a maximum in the rate of oxidation at 7OO”C,and this is probably due to the fact that above this temperature, formation of U,Os is accompanied by sintering to give partially sintered layers at the surface of the compact, thus reducing the rate of oxidation. Below 7OO”C,the U,Os which formed simply dusted from the compact, and the lower rates of oxidation are then due to the lower temperatures. On addition of silicon, the height of this maximum at 700°C is markedly reduced, this being shown for the 40/60 UO,/Si composition in Fig. 8. The oxidation rates as a function of temperature and composition are shown in Fig. 8; above 400°C there is a marked reduction in the rate of oxidation once 20 per cent silicon has been added. Beyond this composition, although the oxidation resistance improves, the relative effect is not so marked. At least 20 per cent of silicon by volume is therefore required to provide the protection. One important point is that the only cermets which remained intact during these tests were the silicon-rich compositions, 50 and 60 V/O silicon. The other compositions powdered during the course of oxidation. Also, in every case the degree of oxidation corresponded closely to the oxidation of UOP13 to U,Os only, the silicon being almost completely unoxidized. Several points are worth noting from these results: (a) Hot-pressed UO,.,, of high density has a negligible rate of oxidation up to 300°C (0.3 mg/cm2/hr at 300°C). (b) The rate of oxidation of hot-pressed UO,.,, increases above 3OO”C,to reach a maximum of 160 mg/cm2/hr at 700% thereafter the rate of oxidation decreases, probably due to the onset of sintering of UsOs.
A preliminary study of three cermets based on uranium oxide
111
(c) The addition of silicon reduces the rate of oxidation and has greatest effect in the temperature range where the rate of oxidation of UOs.13 is highest. The addition of 20 per cent silicon by volume is sufficient to reduce the rate of oxidation markedly. Beyond this composition the oxidation resistance improves, though less markedly. .13 essed
200
400
600 800 Temperature
1000
1200 OC
1000
1200
g;ymJwJ-j 200’
&I--600
80;
OC
Temperature
FIG. ‘T.--Rate of oxidation of UO,.,o and of U02.,,/Si 40/60 V/O compacts as a function
of temperature.
IO 20 30 40 50 60 70 80 v/o silicon
p
50
F
10 20 30 40
L 4
‘QOO’C 50X-------. ‘.
S E 10 20 30 40 50 60 70 80 y. silicon
FIG. S.-Rate
50 60 70 80
0
X.
*x-wx
IO 2030405&~70-i % silicon
of oxidation of UO,/Si cermets in dry air.
x
178
L. S. WILLIAMS, D. T. LIVEY,E. BARNES,and P. MURRAY 9.
CONCLUSIONS
1. The data of LAMBERTSONand MUELLER on the UO,-ZrO, verified by vacuum sintering of compacts at 1750°C.
system has been
2. Fabrication of UO,-Zr compacts to high density is possible by hot-pressing methods, provided an argon atmosphere is employed. The use of temperatures higher than that required to produce a given density results in brittle compacts, probably due to the formation of metal carbide. 3. Fabrication to high density of UO,-Zr compacts by cold-pressing and sintering is only possible at very high temperatures (19OO’C)unless the metal content is greater than 50 per cent by volume. 4. Fabrication of UO,-Th compacts by cold-pressing and sintering is not possible for low metal contents, due to the formation and drainage from the compact of liquid uranium. Even compacts with 62.5 per cent thorium do not sinter to high density at 175O”C, presumably also due to the chemical reaction between UOg and thorium. 5. The thermal shock properties of UO,.,, are not improved by the addition of zirconium, even where the zirconium is present to the extent of 62.5 v/o. The thermal shock resistance of an equivalent thorium compact (62.5 v/o thorium) is much improved over the UO,,,/Zr, suggesting that the poor properties of the zirconium cermets may be due to oxygen embrittlement of the metal, since zirconium has a high solubility for oxygen and thorium has no solubility for oxygen. 6. Compacts of UO,.,,-Zr formed from UO,.,,-ZrH,, contrary to expectation do not have superior sintering properties to compacts formed from UO,l, and zirconium metal powder. 7. The thermal shock resistance of UO,.l, is improved by the addition of silicon for quenching temperatures up to 400°C. Beyond this temperature the shock resistance is not affected. 8. The oxidation resistance of UO,.,,JSi compacts is greater than for UO,.,, compacts. Beyond 20 v/o silicon the rate of oxidation diminishes but is not markedly affected by the increase in silicon content. However, compacts with silicon contents greater than 50 v/o are superior in that no disintegration takes place during oxidation. REFERENCES
I. BLACKBURN A. R. and SHEVLINP. S. (1951) J. Amer. Cer. Sot. 34 (11) 327-331. 2. CIJBICCIOTTI D. D. (1951) J. Amer. Chem. Sot. 73, 2032. 3. FASTJ. D. (1948) h4etallwirtschnft 17, 641, 4. HAUSNERH. H., KALISCHH. S., and ANGIERR. P. (1951) J. Metals 3 (8) 625. 5. LAMBERTSON W. A. and MUELLERM. H. (1953) J. Amer. Cer. Sot. 36, 365. 6. LAMBERTSON W. A., MUELLERM. H., and GUNZELF. H. (1953) J. Amer. Cer. Sot. 36, 397. 7. LANG S. M., FILLMORE C. L., and ROTH R. S. (1952) Natl. Bureau of Standards, Interim Report to AEC. 8. LIVEY D. T. and MURRAY P. (1955) The Wetting Properties of Solid Oxides and Carbides by Liquid Metals, Powder Metallurgy Seminar, Reutte, June. 9. MOOREN. C., HUFFADINE J. W., and LANGLANDL. Plessey Co. Ltd., private communication. 10. MURRAYP., RODGERS E. P., and WILLIAMSA. E. (1954) Trans. Brit. Cer. Sot. 53, No. 8.