INVESTIGATIONS OF THE CHEMICAL COMPATIBILITY OF RHENIUM WITH
URANIUM DIOXIDE AT ELEVATED TEMPERATURES
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
To Ourdia Alouache
I would like to thank Dr. Samim Anghaie for supporting me during these two
years at INSPI. His trust and encouragement were very much appreciated. I would like
also to thank Dr. Edward T. Dugan for his support and for being so available as a teacher
throughout my graduate work at the University of Florida. I am grateful to Dr. Rajiv K.
Singh from the Materials Sciences and Engineering department for his support as a
teacher and for taking time to be part of my committee. I would also like to thank
everyone at INSPI (Lynne, Travis, Blair, Gary, Vadi) for their valuable help and kindness
and for the agreeable ambiance. I would also like to thank my mother and family for
their continuous support, and my friends whose support was very encouraging.
TABLE OF CONTENTS
A C K N O W L E D G M EN T S ..................................................................... ...................iv
L IS T O F T A B L E S ................................................................................................... v ii
L IS T O F F IG U R E S ..................................... .......................................................... v iii
1 INTRODUCTION AND BACKGROUND..............................................................
The 710-R actor Program ..................... ......... ........................ ...................... 2
Latta's Study on Solidus Liquidus Temperatures of UO2+x................... ..............6
2 RHENIUM .................................... ................................... .......... 8
Rhenium 's Physical Properties............................................................. .............. 9
Ductility of Rhenium ................. .................................................... 10
W orkability of R henium ....... .. .......... .. ......... ............................ .............. 11
Fabrication of Rhenium Parts............................... .................. 12
Physicomechanical Properties of Rhenium.............................................................. 13
R henium as an A lloying Elem ent.................................................................... ...... 15
Interactions of Rhenium with Other Elements......................................................... 16
Chemical Properties.................................... ........................... ............ 17
3 THEORETICAL AND EXPERIMENTAL INVESTIGATIONS ON THE
COMPATIBILITY BETWEEN URANIUM DIOXIDE AND RHENIUM ................21
Thermodynamic Considerations: Predictions Based on F*A*C*T code ....................21
E xperim mental W ork ................................................................. 27
P principle of the M ethod.......................................................................... ........27
M aterials/Protocol .................................................................. .. ......... 28
R e su lts .............................................................................................................. 3 0
S a m p le 1 ..................................................................3 0
Sam ple 2 ................................................................ 37
C o n clu sio n s .............................................................................4 2
A: X-RAY POWDER DIFFRACTION ANALYSIS ............................ ...................44
B: ENERGY-DISPERSIVE X-RAY SPECTROMETRY (EDS) .............. ............ ..46
C : S A M P L E 3 ...............................................................................................................5 0
LIST O F R EFEREN CE S ............ ...................................................... .............. 53
BIO GRAPH ICAL SK ETCH ................................................. .............................. 55
LIST OF TABLES
1: Results obtained with the F*A*C*T code for the Re/02 system at different high
tem peratures.................................... ................... ................ .......... 24
2: Results obtained with the F*A*C*T code for the rhenium-oxygen system with excess
oxygen, at different high tem peratures ..................................................................... 25
3: Results obtained with the F*A*C*T code for the Re/UO2 system at different high
tem peratures.................................... ................... ................ .......... 25
LIST OF FIGURES
1: Ductile-to-brittle transition behavior of refractory metal versus temperature ................ 11
2: Ultimate Tensile Strength (UTS) of refractory metals (left). Creep-Rupture Strength of
refractory metals (right) (from Bryskin B. D. 1992).......................................................14
3: Rate of corrosion of refractory metals in air. ........................................ .............. 20
4: Rotary tumbler used to mix the Re powder with the U02 powder .................................28
5: Induction-furnace chamber used for sintering the samples. .....................................29
6: Inductive coil and heated susceptor radiating inside the furnace chamber....................30
7: Tungsten susceptor after sintering Sample 1 at 2500 K for 10 hours..............................31
8: X-ray powder diffraction pattern of the pre-sintered of Re and U02 powders ...............33
9: X-ray powder diffraction pattern of the sintered mixture of Re and U02 powders left
inside the crack...................................................................... ................... 33
10: Growth outside of the crack. ...................................................................... 34
11: X-ray powder diffraction pattern of the growth outside the crucible. ..........................34
12: X-ray powder diffraction pattern of the top region of sample 1. ..................................35
13: Dark deposit on the coil, after sintering the tungsten crucible the first time ................36
14. XRD pattern of the deposit found on the furnace chamber walls. ............. .............36
15: Tungsten susceptor after the sintering of sample 2 ................................................37
16: X-ray powder diffraction pattern of the pre-sintered mixture of UO2 and Re powders..39
17: X-ray powder diffraction pattern of the bottom region of the sample .........................39
18: X-ray powder diffraction pattern of the middle region of the sample............................40
19: Back-scattered scanning electron micrograph showing compositional contrast of Re-
UO2 powders sintered for 10 hr. at 2500 K ............... ........... .............. ............. 41
20: Back-scattered scanning electron micrograph showing compositional contrast of Re-
UO2 powders sintered for 10 hr. at 2500 K....................................... .............. 41
21: Line scan operated across the region showed on Figure 19........................................ 42
22: Crucible cap surrounded by oxide deposits. ............................. ...........51
23: Coil after removal; a blue-black deposit on the walls was observed similar to the
dep o sit ................................................. .... ....... . ...... 5 1
24: X-ray analysis pattern of the top part of the failed sample. ........................................52
25: X-ray analysis pattern of the bottom region of the sample ...................... ...............52
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Master of Science
INVESTIGATION OF THE CHEMICAL COMPATIBILITY OF RHENIUM WITH
URANIUM DIOXIDE AT VERY HIGH TEMPERATURES
Chairman: Samim Anghaie
Major Department: Nuclear and Radiological Engineering Department
Because of its high melting point, outstanding ductility, good strength and creep
behavior, rhenium has been a very good candidate for high-temperature applications in
the field of aerospace. It is particularly suitable for application as structural material in
space nuclear reactor cores. Rhenium is also proposed as fuel cladding material for high
temperature space reactors. It has also been proposed as a liner for the fuel pin in a space
power reactor design (SP-100 program) because of its high neutron absorption cross
section that could make the reactor subcritical under a hypothetical water submersion
accident. Despite all positive high temperature properties, Rhenium lacks corrosion
resistance in the oxidation environment. As a cladding material for uranium dioxide fuel
pellets, no chemical interaction between the two materials is acceptable. The purpose of
this study is to investigate chemical compatibility between urania and rhenium at elevated
The "F*A*C*T" code was used to determine from a thermodynamic point of
view the chemical equilibrium composition of a U02/Re system in the range of
temperatures [2000 K 3000 K]. The code results, which were based on calculations of
Gibbs free energies, have shown no chemical interaction between the two species in the
temperature range of interest. This is evident because uranium dioxide is much more
stable than any of the possible chemical products involving rhenium.
An induction-heating furnace was used to sinter mixtures of uranium dioxide and
rhenium powders for 10 hours, at temperatures as high as 2500 K. This temperature is
much higher than the intended design temperature for which rhenium is a candidate. The
x-ray powder analysis technique was used to analyze the samples prior to and after the
sintering process. Back-scattered scanning electron imaging was used to complement the
x-ray analysis when relevant. The results led to the conclusion that rhenium and uranium
dioxide do not chemically interact at temperatures as high as 2500 K.
INTRODUCTION AND BACKGROUND
Rhenium (Re) was discovered in June 1925 by Walter and Ida Noddack (husband
and wife). Since then, the interest in this metal has never slowed down. However, its
scarcity and high cost prevented it from being used extensively in large structural
components (especially in its pure form). The rapid development of this recently
discovered element is actually due to its very unique combination of physical and
mechanical properties. However, rhenium has been used as an alloying element for a long
time now because of its positive effects on other refractory metals. Those refractory metal
alloys have been used as heating elements, high-temperature thermocouples, anti-friction
and low wear parts, compact electromagnet coils, and high-temperature elastic elements.
Because of their unique properties, rhenium and its alloys have been considered for many
applications in the field of aerospace.
The SP-100 Program is one of them . This program was an aerospace project
of high-temperature liquid-metal-cooled nuclear power plant; it was designed to provide
tens to hundreds of kWe power for 7 years at full power and 10 years of overall
operation. The clad provided structural strength for the fuel pin, which was the heart of
the reactor design. The layer of rhenium was designed to prevent interaction with fuel. It
also allowed thinner cladding, and would act as a thermal poison if the reactor was
immersed in water because of its high neutron capture cross-section in certain conditions.
As for the fuel pin cladding material, it was niobium alloy PWC-11. These structural
materials were needed to meet important requirements involving their physical,
mechanical, thermal, and chemical properties. For the SP-100 program in particular, the
structural elements had to show good resistance to corrosion by liquid alkali metal
coolants and good compatibility with candidate fuel materials such as uranium dioxide
(U02). To date, little work has been done in this area. The chemical compatibility of
rhenium (Re) with U02 at very high temperatures has never really been studied for itself,
and was the object of this investigation. However in the past, some experiments involved
rhenium and UO2 as part of their materials; they are briefly reported below.
The 710-Reactor Program
During the 710-space-nuclear-reactor program initiated in the 1960s, UO2 was
chosen as the most probable reactor fuel because it had the highest melting point of all
potential fuel candidates at that time and because it had the lowest vapor pressure at high
temperature. Moreover, the technology for processing U02 had benefited from years of
experience in the nuclear industry, which made it readily available commercially. During
further development of the fuel program, it was established that UO2 could be combined
with W to form a cermet (ceramic-metal) material: a W matrix encasing fuel particles. A
U02-(40 vol %)W cermet system was achieved and showed high strength, adequate
ductility, and thermal conductivity one order of magnitude higher than that of U02 alone.
Later rhenium was added to the matrix to improve the ductility of the fuel. The final
matrix needed to be contained in a cladding. And of course the choice of the cladding
material was the first question to be solved.
The structural material supporting and containing the fuel matrix needed special
properties such as a high melting point for structural integrity, low vapor pressure,
adequate ductility, a fabricability to close-to-dimensional tolerances, good strength, and
of course compatibility with the nuclear fuel and the coolant. Based on these
requirements, refractory metals appeared to be the most advantageous materials for
structural applications because they combined the above properties and were most likely
to be compatible with UO2. Some of these refractory metals were also compatible with
H2 and liquid metals, which made them very good candidates for the 710-fuel program.
Many possibilities were considered before actually choosing the final cladding materials
that would be tested. Tungsten had the highest melting point (3410 C) and the lowest
vapor pressure of all the possible refractory structural materials but it had low ductility
and poor grain boundary strength, thus enhancing grain boundary diffusion, which
constituted a problem for high temperature and partial pressure U02 in a thin tungsten
A tungsten-based alloy such as W-Re was then considered to be the best cladding
candidate from the very beginning but because it was not commercially available at that
time (1961) in the required sizes and shapes, other materials were selected for the initial
development work. Nevertheless, research work on W-Re alloy was initiated in parallel
so that in 1964 it resulted in the production of two W-Re-Mo alloys. Further studies in
the 710-reactor program on systems using those alloys as the cladding material were
achieved. Tantalum was also considered for cladding purposes because it had the highest
melting point of the refractory metals commercially available at that time (1961), and it
could be fabricated in the wanted shapes and sizes, and it also had good high-temperature
material properties. A Ta-(10 vol %)W alloy was also available commercially in
fabricated shapes and dimensions required for the 710 program; it had mainly the same
properties as Ta with better stress-rupture properties. Molybdenum and Niobium were
also available in the desired shapes but their relatively low melting point constituted a
Henceforward, early work on the 710-reactor program concentrated on Ta and Ta-
10W cladding materials. Work was also conducted on two W-Mo-Re alloys: (W-30Re-
30Mo, at%), and (W-25Re-30Mo, at%). In 1962, a better tantalum alloy called T-11
(Ta-8W-2.4Hf, wt%) became available commercially and was tested as well. Testing all
these different materials actually allowed the scientists to compare them and see how all
of them behave in contact with the W-U02 matrix. The results and conclusions of
relevance to this thesis are presented below.
For the Ta-clad/W-UO2 system with hyperstochiometric U02 fuel, post-test
metallographic and microbeam analyses showed the formation of TaO5 in the cladding.
As for the studies involving T-11 alloy, they showed that W-U02 cores with O/U
ratios not less than 2.00 before cladding in the T- 11 alloy had substochiometric O/U
ratios after hot-gas pressure bonding for 3 hours at 3180 F in a 10000-psig helium
atmosphere. Further metallographic, microprobe, and chemical analyses of the cladding
showed that this reduction in stochiometry was due to oxygen gettering by hafnium
contained in the T-11 alloy since HfO2 was detected in the grain boundaries and within
grain bodies of the cladding.
The W-Re-Mo alloy was first used in a study on the dimensional stability of the
fuel where W-Re-Mo capsules enclosing W-U02 with different initial O/U ratios were
cycled from 500 F to 3000 F. Two of the supposedly initially hyperstochiometric
capsules were submitted to metallographic and chemical analysis. The analysis showed
no presence of free uranium and a stochiometric O/U ratio. But microprobe analysis
detected grain boundaries stringers of W02 in the W matrix. These WO2 stringers seemed
to alter the dimensional stability of the cermet by raising the ductile-to-brittle transition
temperature of the W matrix by dissociation to gaseous WO3 and by forming UO2-
UxWO3 eutectic phases. The discovery of WOx phases within the cores clad in W-Re-Mo
led to an actual study of the compatibility of W-Re-Mo cladding with U02 and UO2+x at
3000 F. Two capsules were prepared: one contained a core sintered in conditions leading
to an O/U ratio of 2.003+0.004, the other was spiked with U308, which resulted in an
O/U ratio of 2.026. Both capsules were electron-beam welded in vacuum and heated in
argon at 3000 F for 70 hours. In both cases, the clad disclosed oxide precipitates of
various colors: gold, blue, violet, brown; however, the clad of the hyperstochiometric
core showed much greater amounts of oxide deposits. Its X-ray diffraction examination
indicated the presence of W02 and MoO2, a perovskite-type bronze (possibly UxWO3),
UO2, and W metal. Electron probe analysis of metallographic sections detected mixed W-
Mo oxides on the cladding of the stochiometric capsule, and a mixture of three oxide
phases, which was a liquidus grain boundary phase containing W and U with lower mass
density than U02. It was presumed to be W-U oxide. Analysis showed an increase of the
level of oxygen in both claddings. Based on these results it could be concluded that
reactivity can occur between W-Re-Mo clad and UO2.026 and that some interaction is
even possible between the W-Re-Mo cladding and UO2.003.
These tests actually showed that all of the refractory metals of interest (W, Ta, Hf)
chemically interacted with U02 (even though the temperatures were not ultra-high),
except Re. It is important to note that absolutely no interaction between Re and UO2 was
mentioned in the reports. However it is also important to note that in all the experiments,
Re was always present in minor quantities, as an alloying element in materials involving
other refractory metals, never as a pure cladding element .
Latta's Study on Solidus Liquidus Temperatures of UO2x
In 1969, R. E. Latta and R. E. Fryxell worked on the "determination of solidus-
liquidus temperatures in the UO2+x system (-0.50
capsules to contain the urania . But they also used rhenium capsules since this metal
was known to be quite inert to uranium and had a very low oxygen permeation rate. They
mentioned that Re dissolved in stochiometric U02 and that for oxygen-excess
compositions, i.e. hyperstochiometric urania, the interaction was much more extensive.
It is important to note that Weidenbaum and Haussner had previously raised the problem
of tungsten dissolving in stochiometric urania; they had reported that tungsten dissolved
in melt up of urania to a few tenths of a weight percentage and equilibrated within a few
minutes. They observed that the hyperstochiometric urania which melted in tungsten
presented rather large amounts of metallic tungsten and a violet or blue precipitate which
was identified as a ternary oxide with perovskite structure UxWO3 (x having a value near
0.10). This was of course accompanied with a change in the urania composition, which
finally got stochiometric. As for the rhenium capsules, they showed that molten
hyperstochiometric urania dissolved rhenium which precipitates during cooling as pure
metal in which no uranium was detected, leaving the urania with its initial O/U ratio. It
goes without saying that the dissolution of the structural elements is to be avoided, as is
the melting of the fuel that this implies .
Based on these studies, it could be stated that problems of chemical compatibility
with uranium dioxide do occur for all other refractory metals hafniumm, molybdenum, and
the widely utilized tungsten) except for rhenium: no chemical interaction was noted
between rhenium and uranium dioxide. However, it must be noted that none of the
studies quoted above were focused on the particular issue of the chemical compatibility
of rhenium with urania. It was not the purpose of the study done by Latta, and even
during the 710-reactor program when studies on the chemical compatibility between the
cladding material and the UO2 fuel were done; rhenium was only used in minor
quantities, as an alloying element in the cladding material, never in its pure form. Hence,
investigations needed to be done on this issue; especially since pure rhenium is seriously
considered as necessary for lining the fuel pin in space power reactor programs.
Therefore the purpose of this work was to investigate the compatibility of pure rhenium
with urania at very high temperatures. The next chapter of this thesis addresses the
physical, material, and chemical properties of rhenium, and describes its special features
for high-temperature applications, such as in space power; it shows what makes rhenium
such a unique material. The following chapter presents the theoretical and experimental
work achieved, as well as the results, conclusions, and discussions.
Rhenium is a unique metal. Rhenium was discovered in 1925 in Germany but its
production began only in 1948 in the former Soviet Union, and in East Germany and
USA during the 1950s. Research in the area of treatment, properties, phase diagrams, and
the design of rhenium alloys was initiated in the 1960s in the USA, Soviet Union, United
Kingdom, Germany, France, and Poland. Rhenium belongs to the family of the refractory
metals. This term refers to metals with a very high melting point, but it is somewhat used
arbitrarily in the metals industry. For instance, the Refractory Metals Committee,
organized under the Metallurgical Society of the American Institute of Mining,
Metallurgical and Petroleum Engineers, uses the term refractory to cover only metals
with a melting temperature above about 1900 C. However, in the Metals Handbook
published by the American Society for Metals, the refractory metals are those "metals
having melting points above the range of iron, cobalt and nickel". The main refractory
metals of interest for space nuclear power are those with the highest melting point:
tungsten, rhenium, molybdenum, tantalum, niobium, hafnium, iridium... The particular
combination of desirable properties of rhenium for high-temperature industrial
applications makes it special. Because of its very high melting point, its high neutron
capture cross section, and its numerous very good mechanical properties, rhenium was
early considered as a very good structural material candidate in the aerospace industry for
complex nuclear reactor core designs (SP-100 program, 710-high-temperature-gas-
reactor program). However, because of its scarcity and its cost, pure rhenium had barely
been used in experimental work so far. Nowadays, tremendous advancements have been
achieved due to the never-ending interest the scientific community has had in this
material. This led to an increased commercial availability and a better knowledge of its
unique combination of properties, which are described in this chapter [4, 5].
Rhenium's Physical Properties
Rhenium has an hexagonal close-packed lattice whose lattice parameters depend
on the purity of the material: a = 0.2753-0.2765 nm, c = 0.4445-0.4480 nm, and c/a =
1.6147-1.6160. It retains its structure to its melting point (3180C), which is the second
highest melting temperature of all refractory metals, after tungsten (3380C). This
parameter decreases with addition of any impurity including tungsten.
Rhenium has a low thermal expansion coefficient (6.7x10-6 K-1), and a low vapor
pressure at its melting point. As a dense metal (21.0 g/cm3), it is only surpassed by
osmium (22.6 g/cm3), iridium (22.56 g/cm3), and platinum (21.4g/cm3). The boiling
temperature of rhenium is 6173 K, which is equal to that of tungsten and exceeds those of
other metals. The vaporization rate of rhenium is 8.41x10-6 g m-2 c1. The maximum
operating temperature at which the weight loss caused by vaporization does not exceed
1% after 10 h is 19100C for molybdenum, 23800C for rhenium, and 25600C for tungsten.
The thermal conductivity of 71.2 W/m K is approximately 2, 3, and 5.5 times smaller
than that of Mo, W, and Cu, respectively. The linear thermal expansion coefficient
(LTEC) of polycrystalline rhenium is 6.7x10-6 K-1, which is somewhat low but still larger
than that of Mo and W.
The high specific electrical resistance of rhenium (p) depends on the purity and
on the degree of deformation of the material; p is 17.2 [t2 m for pure rhenium processed
by zone electron-beam melting and containing (in wt %) 0.0021 02, 0.0005 H2, and
0.0006 N. For rhenium processed by vacuum arc melting and containing an increased
amount of 02 (0.0081-0.022 wt %), p is 18.75-22.85 p1? m. For recrystallized rhenium
processed by traditional methods of powder metallurgy, this parameter is 19.4 p1? m. An
addition of group-IV-VI transition metals into rhenium increases its specific electrical
With a value of~10-16 10-11 at temperatures in the range of 1800 2400 C, the
self-diffusion coefficient of rhenium is close to that of W and Ta and is lower than that of
other metals and alloys.
Ductility of Rhenium
Refractory metals are usually divided into two groups: the Group VA elements
include Ta, Nb, and V, whereas the Group-VIA elements include Mo, W, and Cr. All
elements of the Groups VA and VIA having a bcc lattice structure exhibit a ductile-to-
brittle fracture transition behavior temperature. This temperature is higher for the Group
VIA elements Cr, W, Mo than for the Group VA elements that exhibit a ductile-to-brittle
transition temperature well below room temperature. Pure tantalum is ductile in tensile
testing at 4.2 K. Figure 1 shows this ductile-to-brittle transition behavior for the
refractory metals. Rhenium does not appear on the figure because it is the only refractory
metal that is ductile in the entire range of temperatures, which makes it a very
recommended material for cladding purposes. Research found that the addition of
rhenium to the Group VIA elements lowers their ductile-to-brittle transition temperature
to below room temperature, which is known as the rheniumm ductillizing effect".
--- ---""-- ---- --------
I I J ...
SO w | /
0 2M 3.;,0 Aa X Soo OPO 700 am 9W
T4inrpe raLuiae KeM ri
Figure 1: Ductile-to-brittle transition behavior of refractory metal versus temperature
Workability of Rhenium
Rhenium can be cold worked or warm worked under vacuum. The deformation
resistance of rhenium sharply increases after small deformation (; 5%). The strain
hardening rate estimated from the slopes of the "true stress strain" curves of rhenium is
3.5 times higher than those of W and Mo. Rhenium has a strong strain hardening even
when working at 13500C. The observed hardening effect is caused by limited plastic
deformation of the grain core associated with heavy twinning. Under industrial
conditions, rhenium may be subjected to cold rolling and/or drawing to s=10-15%
between intermediate annealings without cracking.
Alloying with molybdenum and tungsten decreases the ability of rhenium to strain
harden and actually improves its workability: rhenium containing 5-10 atomic percent of
Mo may be cold-worked without cracking to the degree of reduction of 30-40%. The
workability of rhenium is unaffected by tantalum addition but deteriorates with the
additions of zirconium and hafnium. An addition of ThO2 to rhenium increases its strain-
hardening rate .
Fabrication of Rhenium Parts
Since rhenium has great ductility, it is generally fabricated by cold-working and
can be rolled into thin sheet, swaged and drawn into wire or tubing. The problem is that
rhenium also has an important resistance to deformation and becomes very work-
hardened even after cold-deformation of only a few percent. Its work hardening rate is
higher than any other pure metal known. The tensile strength and ductility of processed
rhenium parts vary as a function of the degree of cold-working applied to the parts; this
phenomenon is more obvious than for any other pure metal. Cold-worked rhenium is also
very tough and abrasion-resistant and can be used as a wear resistant material. In order to
solve the problem of work-hardening, recrystallization or stress relief annealings are
necessary at temperatures of 1500-1900 K in a vacuum or in an atmosphere of dry
hydrogen or a hydrogen-nitrogen mixture follow each operation of cold-working. Besides
eliminating the work-hardening, these operations also decrease the resistance to
deformation. And unlike tungsten, for which recrystallisation results in extreme
brittleness at room temperature, rhenium has a good ductility in both recrystallized and
cast conditions .
Physicomechanical Properties of Rhenium
At room temperature, the elastic modulus of rhenium is as high as 460-510.3 GPa.
This value is somewhat lower than those of osmium and iridium (570 and 538.3 GPa,
respectively) but exceeds those of tungsten (420 GPa) and other metals. This provides
high rigidity and dimensional stability of sheet, foil, strip, and wire... However, with
increasing temperature, the elastic modulus of rhenium decreases, and at temperatures
above 10000C, rhenium has no longer an advantage on tungsten.
At room temperature, rhenium combines high strength and ductility: ultimate
tensile strength (UTS) of wire 1.27-1.6 mm diameter is about 1200 MPa with relative
elongation (EL) = 25%. Single-crystal rhenium produced by ZEBM has UTS = 500 MPa
and EL = 100%. An addition of 10 atomic percent Mo increases UTS to 850 MPa and
decreases EL to 60%. The larger the atomic percentage of Mo and W, the more x phase is
formed, which decreases both strength and ductility of rhenium. Figure 2 presents the
UTS of rhenium compared with other refractory metals.
At high temperatures (up to 2400-2700 C), rhenium surpasses both bcc
refractory metals from group VA and group VIA and the fcc and hcp refractory noble
metals Ir, Rh, Os, and Ru in short term strength. Thus, at 1600 and 2000 K, rhenium has
UTS of 360 and 200 MPa, respectively. The UTS values of tungsten, molybdenum,
tantalum, and niobium at 16000C do not exceed 200, 180, 100, and 90 MPa, respectively.
The long-term (10 h) strength is also substantially higher than those of unalloyed W, Mo,
Nb, and Ta at least up to 3000 K.
In strength, pure rhenium ranks above pure forms of other refractory metals (cf.
Figure 2) but below modem bcc refractory alloys including the W and Mo alloys
containing rhenium, carbides, or oxides at temperatures up to 2000-2400 K. However at
temperatures above 0.7Tm, the contribution of the solid-solution strengthening both in
rhenium alloys and in the alternative and competitive tungsten alloys is virtually absent
and the contribution of dispersion hardening abruptly decreases. Rhenium exhibits a
sharp increase in high-temperature ductility at about 20000C, which is the temperature at
the completion of recrystallization, but not at the starting recrystallization temperature as
the majority of bcc refractory metals exhibit. A high ductility is maintained on the entire
range of temperature as seen previously. This causes rhenium to be insensitive to thermal
shock and to thermal cycling. Thruster nozzles produced from rhenium have undergone
100,000 thermal fatigue cycles from room temperature to over 2500 K without failing.
Rhenium with high concentrations of interstitial impurities retains the ductility reserve
upon operating in nitrogen-, oxygen-, and carbon-containing environments .
.II j 1. tl [ 1*1.1 U F, I I
1 1 F i I_
0 400 00 1,200 1,600 2.00 2,400 10 15 20 25
TefperaIur (K) T (15 + log )x 10 (K]
Figure 2: Ultimate Tensile Strength (UTS) of refractory metals (left). Creep-Rupture
Strength of refractory metals (right) (from Bryskin B. D. 1992)
Rhenium as an Alloying Element
The effect of adding rhenium to other refractory metals on their high-temperature
properties has been studied. Rhenium decreases melting temperatures of the more
refractory transition metals of groups III and IV (Sc, Y, Nb, Ta, W, and Mo), and
increases the melting point of the more fusible metals of the period I in the Periodic table
(Ti, V, Cr, Mn, Fe, Co, and Ni) and of all VIII-group noble metals including the most
refractory ones (Rh, Ru, Os, Ir, Pd, Pt).
Diffusion of rhenium in refractory metals occurs at rates as low as the rate of
rhenium self-diffusion. Alloying of bcc refractory metals W and Mo with rhenium up to
20-30 atomic percent increases the diffusion rate by one to two orders of magnitude,
which correlates to the melting temperature decreasing in this alloy due to rhenium
additions. The inverse is to be expected for more fusible metals whose melting point is
increased due to alloying with rhenium.
The effect of rhenium on the ductility of cold-brittle bcc metals of group VIA (W,
Mo, and Cr) is unique. As mentioned previously, the rheniumm effect" consists of
lowering the ductile-to-brittle transition temperature to below room temperature, which
allows better ductilitity and workability. It also gives better allied characteristics such as
an improvement of the weldability, and a decrease in the tendency of splitting and
cracking in longitudinal and sections. It also increases strength at low and high
temperatures. The explanations for this rheniumm effect" have been (and still are) the
subject of several research projects. These include the increase in the state density of
Fermi surface, the increase in the solubility of interstitial impurities in solid solution and
the decrease in their tendency to segregate at lattice defects. As for the elements of group
VA, low levels of rhenium additions to niobium and tantalum result in a dramatic loss of
low temperature ductility, which is opposite to what is observed when rhenium is added
to metals of group VIA, in other words the so called "Rhenium Ductilizing Effect" is not
observed with elements of group VA.
The effect of rhenium on strain hardening of metals is known as the "Rhenium
effect-2". Bcc metals (Nb, Mo, Ta, W) or fcc metals (Ni) are usually strengthened upon
cold working but only weakly. However, addition of rhenium increases their ability to
strain-harden. This effect of rhenium on strain hardening was used for the design of
elastic elements with the ultimate tensile strength up to 5000-7000 MPa .
Interactions of Rhenium with Other Elements
To date, the interaction of rhenium with 64 elements is known. More than 100
complete or partial binary and ternary phase diagrams have been constructed that were
used to establish some features of the interaction of rhenium with other elements.
The maximum solubility of metals in rhenium is determined by their crystal
lattice, their difference in atomic radii (AR) and electronegativities (AE). The solubility of
nonisomorphic metals in hcp rhenium is generally low, since the AE and AR values are
higher. For the same values of AE and AR, solubility of fcc metals in rhenium is
significantly higher than that of bcc metals. The solubilities of fcc and bcc metals in
rhenium sharply decrease as AR increases above 5% and AE increases above 0.1.
The intermetallic compounds of rhenium with transition metals are mainly of
three types: k, o, and x phases. The type of intermetallic compound depends on
dimensional (Rmetal/RRe), the electrochemical factors determined by the position in the
periodic table, and by the electronic structure of the metal. The stability of the Laves
phases is maximum in systems involving electropositive metal of group-III characterized
by Rmetal/RRe = 1.25-1.37; typical values of this ratio for X phases are in the range: 1.1-
1.5. As the Rmetal/RRe ratio decreases, x phase (involving group-III metals such as Sc) and
a phase (involving group-IV metals such as Zr and Hf) appear in the systems in addition
to the X phase. The stability of the Laves phase is less for Rmetal/RRe ratios less than 1.1;
the x phase looses stability as Rmetal/RRe decreases to about 1.0. This phase is most stable
when the rhenium intermetallic system involves metallic elements from group IV and
group V, and the c phase is most stable in systems with the group-VI elements. The a
phases form at temperatures above 2350 C and exist in a wide range of concentrations.
The c phases are less stable when they involve elements of groups V, VII, and VIII,
which they actually decompose by eutectoid reactions. All intermetallic compounds in
the systems of rhenium form by peritectic reactions, except the x phases in the systems
involving metals of group VIA. In the systems with W or Mo, the x phases form by
peritectoid reactions, and the x phase stability is lost when the other metal has a different
electronic structure or that the Rmetal/RRe is not adequate.
Chemically, rhenium behaves as a half-noble metal, but its behavior depends on
its state of division. The massive metal remains unchanged in the air, while a fine powder
may be pyrophoric. When heated in oxygen, Re207 forms; dry chlorine acts on rhenium
below 100 C to give volatile chlorides; bromine does so less readily, and iodine not at
all. Sulphur converts it into the disulphide ReS2; nitrogen has no action at any
temperature up to 2000 C. Metallic rhenium has a good resistance to sulfuric acid and a
very high corrosion resistance to resistance to hydrochloric acid. Actually, air-free water,
either alone, or when it contains HF, HC1, HBr, H2S04, KOH, or ammonia, has
practically no effect on the metal. But rhenium metal is readily attacked by oxygen,
especially in the presence of concentrated acids, and oxidizing chemical substances like
HN03, H202, or chlorine water.
Under conventional conditions, rhenium does not form stable carbides and
nitrides. If rhenium is put in a heated methane atmosphere, it begins to decompose at 800
C, and the separated carbon dissolves in the metal up to 0.9 %, but no carbides form at
temperatures up to 2000 C. Metallic rhenium powder is however said to form a carbide
when heated in carbon monoxide above 500 C (Re4C3). Although carbon diffusion into
rhenium causes some solid solution hardening, no catastrophic embrittlement occurs as
when carbide forms in tungsten, molybdenum and other refractory metals. Therefore
rhenium could be used in contact with graphite and carbon composites. However, the
solubility of carbon in rhenium results in a eutectic melting point of 2773 K with
approximately 0.85 wt % of carbon.
The rhenium halides are volatile and their compounds decompose easily, which
makes their use for the production of rhenium and its alloys with W and Mo by CVD
method possible. Rhenium also interacts with alkalis (especially, in the presence of
Rhenium can have valences from -1 to +7 depending on the compound it is in.
The most stable rhenium oxide Re207 forms at temperature above 160 C, melts at 279
C and sublimates without decomposition at temperatures above 400 C. Re207 has a
good electrical conductivity. This provides good contact resistance stability under
oxidizing environments. Rhenium is oxidized in steam forming Re207, but its
evaporation and transfer of Re upon "water cycle" in electronic devices are substantially
smaller than those of tungsten. At higher temperatures in oxygen-containing atmospheres,
the Ir coating helps protect it from oxidation.
The main rhenium oxides are rhenium heptoxide Re207, rhenium trioxide ReO3,
rhenium dioxide ReO2, and rhenium sesquioxide Re203. The greater the valence of the
metallic rhenium involved, the more stable the oxide is.
Re207 is obtained when the metal is heated in oxygen to a temperature about 150
C; it is also formed when an aqueous solution of perrhenic acid HreO4 is evaporated to
dryness, and it is always produced whenever rhenium compounds are ignited in oxygen.
Re207 exists in two forms white and yellow. The white form is not very well understood.
It is formed by the direct oxidation of rhenium or its dioxide, when a cold stream of
oxygen gas gets in contact with the hot metal or oxide. At 150 C, the white form
changes to the yellow form without any loss of oxygen, which implies the two forms
must be isomeric. The yellow form of Re207 is the usual form; it darkens when heated,
however at -80 C it is colorless. Solid Re207 melts at 304 C and boils at 350 C. Its
vapor is colorless, stable, and monomeric at least up to 520 C. It is easily soluble in
water in which it is reversibly hydrated. Hydrogen reduces Re207 to its blue-black ReO2
at 300 C, and to rhenium metal at 500 OC.
ReO3 is obtained by heating rhenium with the heptoxide at temperatures about
200-250 C, or by burning the metal in a slight atmosphere containing oxygen. The
powder formed can have any color from dark-blue to copper color according to its state
of division. Its crystalline structure is similar to that of WO3; it is not attacked by water,
or by hydrochloric acid (even hot). If it is heated alone in vacuo to 400 OC it is converted
to rhenium dioxide and heptoxide.
ReO2 is produced by heating rhenium metal in a low supply of oxygen or by
reducing the higher rhenium oxides with hydrogen. Anhydrous rhenium dioxide is a
black powder that absorbs greatly solutes and gases. Although it is very stable in oxygen-
free conditions, if it is heated to high temperatures it is converted in vacuo into a mixture
of metallic rhenium and rhenium heptoxide. If heated in oxygen, rhenium dioxide is
converted to heptoxide.
Rhenium sesquioxide Re203 has only been prepared in an hydrate form that
appears as a black precipitate when a solution of trivalent rhenium (like ReC13) is treated
with alkali hydroxide. It is very easily oxidized by oxygen contact [13, 14].
Figure 3 shows the corrosion rate for some refractory metals, in air. It appears
clearly that Re has very poor resistance to oxidation compared to other refractory metals,
4oo u8 IH
Figure 3: Rate of corrosion of refractory metals in air.
THEORETICAL AND EXPERIMENTAL INVESTIGATIONS ON THE
COMPATIBILITY BETWEEN URANIUM DIOXIDE AND RHENIUM.
Thermodynamic Considerations: Predictions Based on F*A*C*T code
When chemically reactive species are put together under certain conditions of
pressure and temperature, chemical reactions can take place, and several different
compounds may be formed, but out of all the possibilities, only the stable compounds
will be part of the final composition, once chemical equilibrium is reached. Of all
possible products, the most stable ones are those resulting from spontaneous reactions
since the more spontaneous the reaction, the more likely the corresponding products are
to form. Some reactions are spontaneous because they give off energy in the form of heat
(AH < 0). Others are spontaneous because they lead to an increase in the disorder of the
system (AS> 0). AH is the change in enthalpy (heat content of a substance) due to the
reaction, and AS the change in entropy (the degree of disorder in a system). Therefore,
calculations of AH and AS can be used to probe the driving force behind a particular
reaction. However, it can happen that one of the potential driving forces behind a
chemical reaction is favorable and the other is not. In that case, whether the reaction is
likely to happen or not cannot be answered obviously. The Gibbs free energy function
was thus created in order to solve the problem. It actually reflects the balance between the
two driving forces. The Gibbs free energy of a system at any moment in time is defined
as the enthalpy of the system minus the product of the temperature times the entropy of
The change in the Gibbs free energy of the system that occurs during a reaction is
therefore equal to the change in the enthalpy of the system minus the change in the
product of the temperature times the entropy of the system.
AG= AH A(TS),
AG = AH TAS,
when the reaction is run at constant temperature. When AG is negative the
reaction is spontaneous process and is giving off energy. In other words, the lower AG,
the higher the probability of occurrence of the reaction, and the more stable the chemical
compound produced is. When AG is positive the reaction is not spontaneous since the
reaction would need energy from a catalyst for it to take place, therefore the
corresponding products of the reaction are unlikely to be formed; they are not considered
as stable. The values for G, S, and H (which are all state functions) may be found in data
tables at any set of temperature and pressure values for any element or compound for
which data is available. Thus, the value of AG for a reaction, under specific temperature
and pressure values can be calculated. And the probability of formation of a specific
compound can be evaluated. At the specified temperature and pressure, the most stable
product will be the one resulting from the reaction with the lowest change in Gibbs free
Based on this principle, the numerical code F*A*C*T (and more particularly
"EQUILIB", a program that is part of the code) is used to determine the equilibrium
composition of a system given the reactants, the final conditions (temperature and
pressure). The program actually proceeds through a search of all possible product
compounds found in an internal database. And after calculation of the corresponding G
values, the output reprints the reactants as well as the predicted most stable quantities of
the possible products initially selected in the database mentioned previously. Therefore, it
can be seen whether a product is likely to form or not, at the specified final temperature
and pressure, given the input reactants.
The code was used in order to determine the expected composition of a system
initially composed of rhenium and uranium dioxide. This actually gives the same results
as those that one would obtain by inputting uranium, dioxygen, and rhenium as the
reactants, since the list of possible products that the program selects in its database is the
same in both cases, which means that the code selects the possible products according to
the elemental initial composition. The runs were done for different temperatures ranging
from 2000 K to 3000K, for a final pressure of 1 atmosphere. The results for the runs of
interest are given below (Tables 1, 2 and 3).
Before making any conclusions based on these runs, it is important to remind the
limitations of the code that supposes ideal conditions and whose database is certainly not
exhaustive; for instance, rhenium sesquioxide Re203 was not included in the internal
library of the code. Another limitation of the EQUILIB code is that it ignores the kinetics
of the reaction. It actually gives the final composition of the system, assuming the
equilibrium is reached and does not take into account the fact that the equilibrium may be
long to reach; and it can happen that the kinetics associated with the formation of a
compound that appears to be stable (from a thermodynamic point of view) is so slow that
it may not be observed during the real experiment. Those are the two main limitations of
the code that one should keep in mind before interpreting any result.
Table 1: Results obtained with the F*A*C*T code for the Re/02 system at different high
Reactants and their initial quantities Re (1 mole), 02 (1 mole)
Possible products looked at by F*A*C*T O, 02, 03, Re, (gas and solid), Re207 (gas
and solid), ReO2, ReO3
Final pressure and 1 atm, 2400 K 1 atm, 2600K 1 atm, 3000K
Product formed and 0.28977 mol of 0.30079 mol of 0.44308 mol of
their equilibrium (0.98064 Re207 + (0.93099 Re207 + (0.51719 Re207 +
(the activity of the 0.18197E-01 02 + 0.63196E-01 02 + 0.41070E-01 02 +
phase formed is
given) 0.11624E-02 O + 0.58142E-02 O + 0.72108E-02 O +
0.56857E-07 Re + 0.10113E-07 Re + 0.98402E-07 Re +
0.54939E-08 03) 0.62505E-08 03) 0.25548E-08 03)
(ideal gas) (ideal gas) (ideal gas)
+ + +
0.43168 mol Re 0.43993 mol Re 0.54169 mol Re
(solid) (solid) (solid)
Table 2: Results obtained with the F*A*C*T code for the rhenium-oxygen system with
excess oxygen, at different high temperatures.
Reactants and their initial quantities Re (1 mole), O (1 mole), 02 (1 mole)
Possible products looked at by O, 02, 03, Re, (gas and solid), Re207 (gas and
F*A*C*T solid), ReO2, ReO3
500 K, 1000 K, 1.0000 mol ReO3
1800 K 0.42860 mol of (0.99991 Re207+ 0.84269E-04 02 + 0.11097E-05 O +
0.15140E-13 03 + 0.19531E-14 Re (ideal gas) + 0.14288 mol Re (solid)
2000 K 0.42880 mol of (0.99927 Re207 + 0.71649E-03 02 + 0.17792E-04 O +
0.99303E-12 03 + 0.30445E-12 Re) (ideal gas) + 0.14303 mol Re (solid)
2500 K, 2800 K, 0.42857 mol Re207 (liquid) + 0.14286 mol Re (solid)
3500 K 1.8431 mol (0.61362 02 + 0.38395 O + 0.23620E-02 Re207 + 0.64235E-
04 Re + 0.10778E-05 03) (ideal gas) + 0.99118 mol Re (liquid)
Table 3: Results obtained with the F*A*C*T code for the Re/UO2 system at different
Reactants and their initial quantities Re (1 mol), U02
Possible products looked at by F*A*C*T O, 02, 03, Re, ReO3 (solid), ReO2, Re207, U,
UO, U02, U03, U409, U308
Final pressure and 1 atm, 2000 K, 2200 K, 2400 1 atm, 4000 K
temperature K, 2600 K, 2800 K, 3000 K
Product formed and 1.00000 mol Re 1.0369 mol of (0.80387 UO2
their equilibrium (a=1.00000) + 0.96479E-01 UO
concentration 1.00000 mol U02 (a=1.00000) + 0.63288E-01 UO3
+ 0.33783E-01 O
(the activity of the + 0.12402E-02 Re
phase formed is + 0.81685E-03 U
given) + 0.52077E-03 02
+ 0.49788E-10 03
+ 0.17468E-15 Re207) (ideal gas)
+ 0.99871 mol Re (liquid)
In the case where the system is initially composed of 1 mole of rhenium and 1
mole of uranium dioxide, the only rhenium oxide that forms in the temperature range of
interest is rhenium heptoxide (Table 1). The code was also run for an initial composition
of 1 mole of Re with 1 mole of 02 and 1 mole of O in order to simulate the case where
rhenium is in excess of oxygen. The results are given in Table 2. The oxidation of
rhenium is quantitatively more important since less rhenium is left unreacted in a solid
phase. For the lowest temperatures, ReO3 is the only stable compound appearing in the
equilibrium composition. This is because rhenium trioxide has a lower enthalpy of
formation than that of rhenium heptoxide at low temperatures. Actually, the use of
REACTION (an other code included in F*A*C*T) showed that even rhenium dioxide is
more stable than Re207 at low temperature from a thermodynamic standpoint (? G
values), but it is less stable than ReO3, which explains why it does not appear in the final
composition. Between 1600 K and 1800 K, a change occurs: the enthalpy of formation
for high temperatures is less for Re207; therefore, rhenium heptoxide becomes the most
stable rhenium oxide and thus it is the only oxide that appears in the equilibrium
composition. For temperatures higher than the melting point of rhenium, 3500k for
instance, very little rhenium is found in the form of oxide; 99.8% of the initial rhenium is
found in a pure liquid phase (less reactive).
Therefore, the results show that in the range of temperatures of interest (2000 K-
3000 K), rhenium in presence of oxygen does form rhenium oxides in a gas phase
(Tablel and Table 2). However, when uranium is present in the system, rhenium does not
form any stable oxide because uranium dioxide has a much lower Gibbs free energy than
any of the possible rhenium oxides (Table 3). Therefore UO2 is the most stable phase; It
is not expected to interact with Re. At 3000 K, the code predicts that uranium dioxide
remains totally stable, which is doubtful, since it should start dissociate as the
sublimation temperature is approached. For an ultrahigh temperature such as 4000K,
most of the uranium dioxide is supposed to volatilize, some UO and UO3 are also
supposed to form; rhenium is mainly unreacted (in a liquid form), the rest of it is found in
a vapor phase; extremely little quantity of Re207 is found in the final atmosphere.
Rhenium appears to be less oxidable in a liquid form. However, the most important part
of these runs is that for the temperature range [2000 K-3000 K], rhenium and uranium
dioxide are expected to remain unreacted. Those runs are to be taken as a complement to
a necessary experimental investigation that needs to be done. This experimental
investigation is described below.
Principle of the Method
Experimentally, the compatibility of rhenium (Re) with uranium dioxide (U02)
was determined mainly by comparing the spectrums obtained from the X-Ray powder
Diffraction (XRD) analysis of a mixture of Re and U02 powders before and after it was
sintered for ten hours at 2500 K. The XRD pattern of a sample gives its phase/elemental
composition. Therefore, by comparing the XRD spectra of the initial mixture and the
sintered mixture, it was possible to determine whether a new compound or more
generally a new phase were formed during the sintering. The compatibility of Re with
UO2 could be assessed only if the spectra of the pre-sintering and post-sintering samples
have similar peak characteristics. A visual inspection of the samples using Back-scattered
Electron Microscopy (BSE) was also done to complement the conclusions that could be
derived from the XRD analysis.
The rhenium powder was 99.999% pure and the urania powder, 99.98% pure. The
average sizes of the powders were respectively -50 mesh (<300 [tm) and -22 mesh (<800
[tm). Since the size of the rhenium powder particles was larger than the size of the urania
particles, the proportion of urania particles had to be larger in order to enhance the
surface contact of the U02 grains with the Re ones. For each sample, about 9 grams of
urania were mixed to about 5 grams of rhenium. Each mixture was mixed for one day on
a rotary tumbler in order to obtain a very homogeneous mixture. A picture of the tumbler
is shown in Figure 4.
Figure 4: Rotary tumbler used to mix the Re powder with the U02 powder.
A small sample of this mixture was submitted to x-ray powder diffraction
analysis. The rest of the mixture was pressed in a tungsten crucible with special care
because of the extreme brittleness of tungsten. Indeed, high-pressure pressing would
automatically result in cracks in the crucible wall, enhancing the possibilities of leakage.
The samples were sintered in a 20 kW, 450 kHz induction heating furnace. A picture of
the furnace chamber as well as a picture of the inside coil are shown in figures 5 and 6.
Figure 5: Induction-furnace chamber used for sintering the samples.
After removal from the crucible, each sample was sectioned using a slow-speed
watering saw. Part of the sample was mounted in epoxy resin and polished through 0.5
|tm using diamond paste giving an individual section that could be examined using a
Back-scattered electron microscope (BSE) equipped with energy dispersive x-ray
spectroscopy (EDS). The remaining of the sample was crushed into a fine powder that
was subjected to phase analysis with an x-ray powder diffraction (XRD) device using
CuKa radiation. This analytical work was done using the available equipment of the
Major Analytical Instrumentation Center (MAIC), one of the Materials Science and
Engineering centers of the University of Florida. The principles of the analytical methods
used are developed in the appendix attached to this work.
Figure 6: Inductive coil and heated susceptor radiating inside the furnace chamber.
The sample was sintered for a total cumulative time often hours at 2500 K. The
sintering process was then stopped due to an abnormal increase of the pressure inside the
furnace chamber, signaling a leak in the system. Once taken out of the furnace, the
susceptor showed a long crack in the middle part, where the sample was located. This
crack had have been initiated when the powder mixture was cold-pressed, due to the
extreme brittleness of the tungsten crucible. Figure 7 shows the cracked susceptor with a
protuberance built up on the outside part of the crack.
Figure 7: Tungsten susceptor after sintering Sample 1 at 2500 K for 10 hours.
The initial crack must have acted as a concentrator for magnetic lines generated
by the inductive coil and therefore the induced heat and therefore temperature at the crack
must have been much higher then in the rest of the crucible, higher then the tungsten
melting point, since the crucible obviously melted in this very precise region, increasing
the width of the crack.
The crack actually allowed the mixture to migrate outside of the crucible. Little
material was left inside the crucible in the region corresponding to the crack, which
indicated that part of the initial mixture went through the crack. The sintered mixture left
inside of the crack was x-ray analyzed and the pattern obtained is presented in Figure 9.
This XRD pattern was compared to the XRD spectrum obtained from the x-ray powder
analysis of the U02-Re mixture prior to sintering (Figure 8).
Two main comments must be made. First, none of the main peaks corresponding
to any of the possible rhenium oxides or rhenium-uranium phases were observed. The
only peaks present on the pattern were attributed to either rhenium or uranium dioxide.
Second, the pattern was however different from that of the initial mixture prior to
sintering: the relative heights of the rhenium and uranium dioxide peaks were not
conserved. The intensity of the uranium dioxide peaks was somehow less important
relatively to the intensity of the rhenium peaks, which was not the case prior to sintering.
This shows that uranium dioxide migrated out through the crack more readily than
rhenium that remained in larger quantities inside the cracked crucible.
The migration of the inside material through the crack gave rise to a growth on
the outside wall. This protuberance of grayish color is shown in Figure 10. This
protuberance was analyzed using x-ray powder diffraction analysis. The obtained pattern
is shown in Figure 11.
0. -------- ------------
0.9 ------------ -------L__ -U --- --------- ------ ------ ------
i i Re
S0.1 --- -U---^
0.7 ------------ ------- -- --------- ------------ ------- -- ----
I 0.6 i-- - - - -. - - - - u--
0.5 ------- --------------- ------.------------ .
0.5 - -- ----- -------------- -- --- -----
S0.4 -- ----- -- -- -
0.1 - .... T. _... -
10 20 30 40 50 60 70 80 90 100
Figure 8: X-ray powder diffraction pattern of the pre-sintered of Re and U02 powders.
o r, I , , I , , I. i I
10 20 30 40 50 60 70 80 90
Figure 9: X-ray powder diffraction pattern of the sintered mixture of Re and U02
powders left inside the crack.
Note: All -ray powder diffraction patterns are given versus (2 theta).
Figure 10: Growth outside of the crack.
0.9 -- L
0.8 ------- ------- --------- --..-----------
0. ------- ------ -------- ---- --- -- ---- ----------
0.7 -------- ------ --------- -------- ---- --- ----------- -----------------------
UO2 U 02
0.6 -------------- ------------- ------- --- ----- ----- --w ---
0.1 --- ------! -.---- ---.-- ---- --- --I--
0 .3 .......-... ...
0.1 ----- -- -- -------- -------- -------- ------------------------ -
10 20 30 40 50 60 70 80 90 100
Figure 11: X-ray powder diffraction pattern of the growth outside the crucible.
Apparently, the outside growth was free of rhenium, but did contain tungsten
(from the susceptor); if it contained some rhenium it must have been in very little
quantities since none of characteristic peak of rhenium was observed in the pattern, which
actually is consistent with the conclusions previously derived.
In the top region of the crucible, the inside mixture was clearly not altered like in
the crack region. The x-ray analysis pattern obtained for that part of the sample is given
in Figure 12. It is similar to the x-ray analysis pattern of the pre-sintered mixture. None of
the main peaks corresponding to any of the possible rhenium oxides or rhenium-uranium
phases were observed. However, some of the peaks did not have the expected height
relatively to the other peaks of the series they belonged to. For instance, the peak
referenced as 1, 2, and 3 were larger then expected. After investigation, they were
identified as being also characteristic of tungsten. This showed that tungsten deposited on
the top surface of the sample, which was open to the furnace chamber atmosphere.
0.9 --------- I-------------------------- :To- -- ----- --------- -- --------------^
0.7 --------. --------. --- ----- --- .....
0.6 ------- ----- --------------------
0. -------------- --- ------ ------- ------- -- --------------
S0.4 ............ ......... .....
10 20 30 40 50 60 70 80 90 100
Figure 12: X-ray powder diffraction pattern of the top region of sample 1.
As a matter of fact, a dark-blue-black deposit was observed on the walls of the
furnace, as well as on the copper coil (Figure 13). The deposit on the walls was x-ray
analyzed in order to determine the compounds formed. Figure 14 gives the XRD
Figure 13: Dark deposit on the coil, after sintering the tungsten crucible the first time.
10 20 30 40 50 60 70 80 90 100
Figure 14. XRD pattern of the deposit found on the furnace chamber walls.
The spectrum clearly shows that this deposit on the walls of the chamber was
tungsten and tungsten oxide. Special care was therefore further provided in order to keep
the atmosphere in the chamber inert during the following runs; regular purges with argon
would be operated.
Sample 2 was prepared using the same weight proportions as for sample 1. In
order to avoid fissuring the tungsten crucible (like in the first experiment), the mixture of
Re and U02 powders was not pressed; it was tamped. The sample was sintered for a total
cumulative time of 10 hours at 2500 K. A picture of the baked crucible is given in Figure
figure 13: tungsten susceptor arter tne sintering or sample z.
After removal, the sample which was in a good condition in spite of the fact that
there had been no cold-pressing of the initial mixture in order to avoid crack formation on
the brittle tungsten crucible. The sample was sectioned using the watering saw (as
previously described in the protocol) and was submitted to x-ray analysis. The XRD
pattern obtained for the bottom region is given below (Figure 17) as well as the pattern
obtained from the pre-sintered initial mixture (Figure 16) for comparison.
The XRD pattern corresponding to the sintered sample was very similar to that of
the pre-sintered mixture. Only peaks corresponding to Re and U02 were observed. None
of the main peaks characteristic of pure uranium, Re-U phase, or any of the possible
rhenium oxides were observed, which consolidated the conclusion that could already
been derived from the results obtained for sample 1: rhenium did not chemically interact
with uranium dioxide during the sintering process. Moreover the results were consistent
for different sections of the sample analyzed. The XRD pattern corresponding to the
middle part is shown in Figure 18.
Again, for this part, the sample was clearly composed only of rhenium and
uranium dioxide, which is consistent with the conclusion previously derived. However, a
series of new peaks appeared on this pattern, which was not present on the XRD pattern
of the bottom region of the sample. These peaks actually were attributed to another phase
of pure rhenium, which means that rhenium may give a new phase during sintering
although it does not chemically react with uranium dioxide.
0.9 -------- ------- -------- ---- ------------H------ ----- ----------
0.8 -------r---------------------- ----------
0.9 .7. . .
0.7 -------- ------ ------r--Re ----- ------------- -- ------- -------- -----
0.6 -------- ------ -r------- --- ---- -- ----------------- --------- -- --
0. --- - --------. .. -
0.3 -------- -- . . ..- ---........ J-......... ..... ........ ...
SRe D2 Re
0.2 ------ -- .- M ,--- I1 -,- -----
_.... .. .
0.2 F-------:---------- ---- - ------- ------- --- ---- --- ---
01~ t - - _--- -- -- ---A--iU I-l-- -
0 .. I.... ... ...... ..... ....., .. ..
10 20 30 40 50 60 70 80 90 100
Figure 16: X-ray powder diffraction pattern of the pre-sintered mixture of UO2 and Re
0.7 -------- -------- --------- ------------- ---------------------------
0.4 ---------- ---- ------- ---- ---- -------------------
0.3 . -.-.U ----
0.3 ............. :-..... ........... .... .. ..................
-0.2 ----- - - l -E- ------ l -l- -
10 20 30 40 50 60 70 80
Figure 17: X-ray powder diffraction pattern of the bottom region of the sample.
0.6 -------- --------- --------- -------- ---- --- -------- -------- -------- --------
0.4 -------- ------ ---- ---- ------ --
S0.4 --------r-- ------r -------- -------- L---- --- L-------- L-------- ----- ---.--------
0.3 --------- r------ -r--------r--------r---- --- -------- --------r----- --r--------
10 20 30 40 50 60 70 80 90 100
Figure 18: X-ray powder diffraction pattern of the middle region of the sample.
Furthermore, Backscattered Scanning Electron (BSE) images of the samples
whose spectrums are shown above are represented in Figures 19 and 20. These images
provide a qualitative map of the sample's chemical composition. This is due to
proportionality between the average atomic number of the elements in an area scanned
for the image and the yield ofbackscattered electrons. Lighter colored parts of the image
correspond to areas of higher average atomic number (Z). Although uranium (Z=92) is
greater than rhenium (Z=75), the lighter colored areas in Figures 19 and 20 correspond to
Re since the average atomic number is lower for U02 containing oxygen atoms (Z=8).
Re since the average atomic number is lower for U02 containing oxygen atoms (Z=8).
Figure 19: Back-scattered scanning electron micrograph showing compositional contrast
of Re-U02 powders sintered for 10 hr. at 2500 K.
Figure 20: Back-scattered scanning electron micrograph showing compositional contrast
of Re-U02 powders sintered for 10 hr. at 2500 K.
The interface between the Re and UO2 regions is clear, sharp and distinct. EDS of
the different regions confirmed that the lighter colored regions are unreacted rhenium.
Line scans across these regions and their interface show sharp discontinuities at the
interface indicating practically no interdiffusion of the elements, and no mixed phase.
The region of the line scan is indicated on Figure 20, which is a picture of the middle part
of the sample. It was operated from left to right on the sample. The corresponding line-
scan graph itself is given in Figure 21. Once again, it appears clearly that the light phase
is the rhenium phase and the darker one is the uranium dioxide.
IMAGE 2122 670
* BSE 179217
Figure 21: Line scan operated across the region showed on Figure 19.
Attention should be brought on the intrinsic difficulty to carry out high
temperature corrosion and materials compatibility. Due to the high probability of reaction
of chamber wall, heating elements, inductive coils, and residual gases, chemical isolation
R e I I I I | I
S I I I I28
of the test chamber is a challenging task. Although special efforts were made to keep an
inert atmosphere in the furnace chamber by purging the system with helium gas,
preventing oxygen ingression proved to be a daunting task. The presence of even low
concentration of oxygen in the system could potentially make the chemical compatibility
results useless. Another complication is the possible vaporization of U02 at the top
surface of crucible. The electron-beam-welding technique that could have been well for
such experiments was not available. Electron-beam welding of crucibles required pre-
sintering and melting of the samples before the crucibles could actually be sealed. This
would have required reaching extremely high temperatures with the same equipment
available (with the same problems of potential external leaks.) This obviously could have
defeated the purpose of the experiment.
As a conclusion, it should be reminded that the thermodynamic analysis suggests
that, in the temperature range of interest [2000 K-3000 K], oxygen reacts with rhenium
and forms volatile Re20O. However, when uranium is present in the system, rhenium does
not form any stable oxide because uranium dioxide has a much lower Gibbs free energy
of formation than any of rhenium oxides. Therefore UO2 is the most stable phase; it is not
expected to react with Re at temperatures up to 3000 K. Experiment at 2500 K for ten
hours confirms the thermodynamic analysis results that suggest full chemical
compatibility of rhenium with UO2 at temperatures up to 3000 K.
X-RAY POWDER DIFFRACTION ANALYSIS
The three-dimensional structure of non-amorphous materials is defined by
regular, repeating planes of atoms that form a crystal lattice. When a focused X-ray beam
interacts with these planes of atoms, part of the beam is transmitted, part is absorbed by
the sample, part is refracted and scattered, and part is diffracted. Diffraction of an X-ray
beam by a crystalline solid is analogous to diffraction of light by droplets of water,
producing the familiar rainbow. X-rays are diffracted by each material differently,
depending on what atoms make up the crystal lattice and how these atoms are arranged.
In X-ray powder diffractometry, X-rays are generated within a sealed tube that is
under vacuum. A current is applied that heats a filament within the tube; the higher the
current the greater the number of electrons emitted from the filament. This generation of
electrons is analogous to the production of electrons in a television picture tube. A high
voltage, typically 15-60 kilovolts, is applied within the tube. This high voltage
accelerates the electrons, which then hit a target, commonly made of copper. When these
electrons hit the target, X-rays are produced. The wavelength of these X-rays is
characteristic of that target. These X-rays are collimated and directed onto the sample,
which has been ground to a fine powder (typically to produce particle sizes of less than
10 microns). A detector detects the X-ray signal; the signal is then processed either by a
microprocessor or electronically, converting the signal to a count rate. Changing the
angle between the X-ray source, the sample, and the detector at a controlled rate between
preset limits is an X-ray scan.
When an X-ray beam hits a sample and is diffracted, we can measure the
distances between the planes of the atoms that constitute the sample by applying Bragg's
Law. Bragg's Law is:
n = 2dinO
where the integer n is the order of the diffracted beam, Xis the wavelength of the incident
X-ray beam, d is the distance between adjacent planes of atoms (the d-spacings), and Ois
the angle of incidence of the X-ray beam. Since we know Xand we can measure 0, we can
calculate the d-spacings. The geometry of an XRD unit is designed to accommodate this
measurement (fig. 1). The characteristic set of d-spacings generated in a typical X-ray
scan provides a unique "fingerprint" of the mineral or minerals present in the sample.
When properly interpreted, by comparison with standard reference patterns and
measurements, this "fingerprint" allows for identification of the material.
ENERGY-DISPERSIVE X-RAY SPECTROMETRY (EDS)
In a SEM, when the incoming electron beam strikes the sample being analyzed,
not only does it give secondary electrons and backscattered electrons, (which are used for
imaging), it also generates characteristic x-rays. Indeed, those x-rays result from the
incoming electrons knocking inner shell electrons out of atoms in the sample. As outer
electrons drop into the vacancy, they are obliged to dispose of the excess energy, often as
an x-ray photon. Since each element has its own set of energy levels, the emitted photons
are indicative of the element that produced them. Analyzers are then used to characterize
the x-ray photons for their energy (or wavelength) and abundance to determine the
chemistry of sample.
EDS operates by using a crystal of silicon or germanium to detect the x-rays. Each
photon generates multiple electron-hole pairs equal in total energy to the energy of the
photon (each pair has a fixed energy determined by the crystal). A voltage is applied to
the crystal to separate the electrons and holes so that the charges appear as a small step-
change in voltage. Pre-amplifiers and amplifiers process the signal and pass it to a
multichannel analyzer (i.e., analog-to-digital converter) so that the x-ray spectrum can
eventually be displayed as a histogram of x-ray intensity as a function of energy.
The detector crystal is kept under vacuum at liquid nitrogen temperatures. It thus requires
a window of some kind to isolate it from the SEM chamber. Early windows were made
out of beryllium, but severely attenuated x-rays from elements lighter than sodium.
Improvements in materials has led to a generation of "thin-window" detectors which can
pass x-rays down to and including boron. This is a great help when analyzing minerals
(0) and organic compounds (C and 0).
Types of X-ray Analyses
X-ray analyses may be broadly divided in to qualitative and quantitative analyses.
Qualitative analyses are those that are concerned with determining the elements in a
sample and perhaps a rough measures of their abundance, especially as trends across a
sample. Qualitative analyses would thus include survey analyses, line-scan profiles, and
x-ray maps. Quantitative analyses have as their goal the determination of the elemental
composition at one or more points. There are a number of requirements that must be met
for quantitative analyses to succeed. These analyses are discussed below.
These analyses answer the general question "What is it?" They are useful if there
is little hint of the composition of an unknown sample. These analyses seek to determine
the elements present and their distribution. Spectra are collected for a number of points
and the elements present are identified. Digital beam control allows users to probe around
areas in a digital image to identify the various features and to check for consistency.
Mapping generates a two-dimensional image using the abundance of an element
as the intensity of the image. It easily shows where an element is and is not. Mapping
only works for elements that have been specified. Therefore, it is first necessary to
determine which elements are present by the use of a survey analysis. Mapping has come
a long way in recent years with digital beam control and faster electronics. Now, maps
for multiple elements can be collected rapidly so that users get an idea of the elemental
distribution in just a few minutes instead of the hours that used to be required. It is also
often possible to combine x-ray maps to get even better ideas of the relative distribution
(complementary or correlating) of elements.
Similar to x-ray maps, this technique plots the abundance of an element with
distance along a line rather than as intensity over a two-dimensional image. It is
especially helpful for examining trends at interfaces and concentration gradients within a
This involves careful determination of the identification and abundances of the
elements present in a sample. Accuracy is often possible to tenths of a percent when
standards are used. Standardless analyses can generally provide a compositional estimate,
but with less accuracy. Several requirements must be met to ensure the success of a
quantitative analysis. Samples must generally be flat, homogenous, and thick to satisfy
assumptions in the analysis software. Standards are advisable to account for particular
characteristics of the microscope. And all elements should be measurable.
Relative x-ray intensity varies with the angle of the surface of the sample.
Therefore, the surface must at least be at some known, consistent angle if not altogether
Correction algorithms assume a uniform distribution of elements on an atomic
scale. Therefore, it is not possible to guarantee results when the excited volume is not
uniform as in the case of dendritic structures or mixtures of particles.
Algorithms assume that electrons penetrate and are completely absorbed in the
sample (i.e., not escaping from the underside). Likewise, samples must also be
homogenous with depth so that the beam is entirely contained in a single phase.
Sample 3 was a sample processed in a similar way to samples 1 and 2. It was
meant to be run at 2600 K for one hundred hours. However, after 35 hours of experiment,
the furnace arced; and the run was stopped. After analysis, it was found that the tungsten
crucible had melt at some point and a protuberance built up outside. This actually
diminished the distance separating the coil from the conductive crucible. At one point,
this distance was short enough to trigger an arc between the coil and the crucible, which
resulted in piercing the coil. Water came into the system, which obviously constituted a
source of oxidation.
A blue deposit was observed on the walls of the furnace chamber, as well as some
multicolor precipitate around the coil. Pictures of the system are provided in this
appendix (Figures 22 and 23). The sample was submitted to x-ray powder diffraction
analysis, and the obtained patterns are given below (Figures 24 and 25).
figure 22: crucible cap surrounded Dy oxiae deposits.
I~ ~ ~ ~ r .jul*-iKif*
Figure 23: Coil after removal; a blue-black deposit on the walls was observed similar to
0.9 ------ ------ -- ---- ------ ------ ------ ------ ------
0.8 ------ ------- ------ ------ ------ ------ ------
0.7 ~----- ------ ------ ----------;--
w3 Re Pe
0.5 ------------- 2--- ---------------- -----------------
-- - ---- R------
10 20 30 40 50 60 70 80 90 100
Figure 24: X-ray analysis pattern of the top part of the failed sample.
0 9 .. .... .... ........ .. .. ....
0.8 ------ ----- - ---- -------- --.---
0.6 ------ ------ ------ ---------------.-----._----------
0.5 ------ rv -- -. _- _- 2
S0.1 ------ .---- -- --. -.. -- .P-- --i--
10 20 30 40 50 60 70 80 90 100
Figure 24: X-ray analysis pattern of the btom region of the failed sample.
0.9 ----- ------ --- ---- ------ ------ ------ ------ ------
0 .7- - - -- - - - - -
0.5 ----- ----. .-- --- --- --
0 .2 ..... -'- -'- -- -- e --" -
10 20 30 40 50 60 70 80 90 100
Figure 25: X-ray analysis pattern of the bottom region of the sample.
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Gas Reactor, Program Summary Report, Volume Ill-Fuel Element Development,"
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UO2+x System (-0.50
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Alloys," (paper presented at the International Symposium on Rhenium and Rhenium
Alloys, Orlando, 1997), in Rhenium and Rhenium alloys edited by B.D. Bryskin,
TMS, Warrendale, PA, 1996.
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15. T. W. Knight, D. Kaoumi, S. Anghaie, "Study of the Chemical Compatibility of
Uranium Dioxide with Rhenium," STAIF-2001, Abuquerque, edited by Mohamed S.
Djamel Kaoumi was born on March 28, 1977 in Marseille (France). He went to
the Marcel Pagnol high-school in Marseille, where he graduated with his French
"baccalaureat" in sciences with honors. In 1997, he entered the French engineering
school "Ecole Nationale Superieure de Physique de Grenoble" (ENSPG), part of the
"Institut National Polytechnique de Grenoble" (INPG). In 1999, he decided to seize the
opportunity to come to the United States as part of an exchange program with the
University of Florida. In 2000, he graduated with his French engineering diploma in the
field of physics sciences with a specialty in nuclear engineering (with honors). He has
been in the United States to complete his degree of Master of Science in nuclear
engineering sciences with a minor in materials science and engineering. He has also been
working at the Innovative Nuclear Space Power Institute (INSPI) since 1999 as a
graduate research assistant.