Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system
Trang 1B Morel
AREVA Comurhex, Pierrelatte, France
S Chatain
Commissariat a` l’E´nergie Atomique et aux E´nergies Alternatives, Gif-sur-Yvette, France
ß 2012 Elsevier Ltd All rights reserved.
2.06.3.3.2 Intermediate fluorides U4F17(UF4.25) and U2F9(UF4.5) 206
Symbols
C 0
p Standard heat capacity at constant
pressure (J K1mol1)
HTH298 Enthalpy increment (kJ mol1)
S0 Standard entropy (J K1mol1)
Tfus Temperature of melting (K)
DfH0 Standard enthalpy of formation
(kJ mol1)
DfusH0 Standard enthalpy of fusion (kJ mol1)
DfusS0 Standard entropy of fusion (kJ mol1)
DsubH0 Standard enthalpy of sublimation
(kJ mol1)
197
Trang 2e Dielectric constant (F m1)
l Thermal conductivity (W m1 C1)
r Density (kg m3or g cm3)
Among the numerous compounds in the U–F system
(UF3, UF4, U4F17, U2F9, UF5, and UF6as condensed
phases, and UF, UF2, UF3, UF4, UF5, U2F10, and UF6as
gaseous species), UF6 is certainly the most known
because of the wide use of this gas to enrich the235U
fraction in uranium Indeed UF6has a vapor pressure of
1500 mbar (1.5 105
Pa) at 337 K that appears as a striking contrast with the refractory UO2, which melts
at 3120 K.1,2 This difference is typical of fluoride/
oxide difference, and also VI/IV oxidation state
UF6was first prepared by Ruff in 19113 through
reaction of F2on U metal or carbide The chemistry
of UF6was then more completely investigated in the
1940s due to the development of nuclear technology
By the end of 1950, Agron had published a phase
diagram including the intermediate fluorides U4F17,
U2F9, and UF5 Further research continued at a
slower pace in the 1960s on these intermediate
fluor-ides The scientific interest later decreased with the
rise of AVLIS laser-based enrichment technology of
U metal that did not need UF6to enrich in235U In
this period, some R&D was also performed on UF6to
define a dry reprocessing route using the fluoride
volatility technique, such as the Fluorex process, to
extract U from less-volatile fluorides such as fission
products
On the other end, UF4had been known for a long
time as a green solid used for the preparation of UF6
and uranium metal It was first prepared by the
reac-tion of aqueous HF on U3O8 by Hermann in 1861
More recently UF4is now considered for molten salt
reactor technology
Finally, the UF3–UF4 system was then studied
more recently from an academic point of view, but
UF3today does not present any industrial application
Except for UF4 that only yields a hydrate when
exposed to air, all these compounds are unstable
when exposed to the humidity of air yielding
UO2F2 and/or UF4 UF6 is also very corrosive and
can act as a strong fluorinating reagent Hence, the
characterization of these intermediate fluorides has
always been quite limited For example, the
descrip-tion of the UF liquid phase is not well known UF
may melt congruently at 621 K or undergo decompo-sition The eutectic compositions between UF4–UF5 and UF5–UF6are unknown
Agron has published a phase diagram (Figure 1) for the intermediate fluorides4 based on the three following reactions:
2U4F17ðsÞ⇄7UF4ðsÞ þ UF6ðgÞ
7
2U2F9ðsÞ⇄3=2U4F17ðsÞ þ UF6ðgÞ 3UF5ðsÞ⇄U2F9ðsÞ þ UF6ðgÞ From the equilibrium constant of these reactions
K ¼ K0eDG0=RT¼ PðUF6Þ, the experimental results can be expressed as logPðUF6Þ¼ log K0 ðDG0=RTÞ, whereK0and DG0=R are constants
Plotting log PðUF6Þ versus 1/T gives the stability domain of these compounds
10 000 5000 2000 1000 500 200 100
Triple point
Transition point
10 3 /T (⬚K)
50 20 10 5 2 1
−5
−2
−1
Figure 1 The equilibrium pressures of the various uranium fluorides in the composition range 4 < F/U < 5 (Agron diagram) From Agron, P., 1948, AECD-1878, Courtesy of Oak Ridge National Laboratory, U.S Department of Energy.
Trang 3The UF3–UF4system has been studied by Khripin
et al.5
and Slovianskikhet al.6
by differential thermal analysis; UF3being obtained through the reduction of
UF4with H2 In the two cases, they found a eutectic
transition at, respectively, (1152 7) K and 1143 K,
which is slightly lower than that at the temperature
found by Thomaet al.7
and selected by Knackeet al.8
The eutectic composition is quite different between
the two authors with 0.7835 at F (atomic fraction
of F) found by Khripinet al.5
(value extrapolated from
the liquidus and solidus data) and 0.788 at F by Slovianskikhet al.6
In 1969 Knackeet al published the most complete phase diagram (Figure 2) to date8with three eutectics at 1165, 621, and 328 K and three con-gruently melting compounds UF3, UF4, and UF6at, respectively, 1700, 1309, and 337 K
2.06.3.1 UF6: Uranium Hexafluoride 2.06.3.1.1 Properties
UF6 is solid at room temperature with a signifi-cant vapor pressure (P ¼ 105 mbar (1.05 104
Pa)
at 298 K) The triple point is 337 K forp ¼ 1.5 bar, as shown on thePðUF6Þ¼ f (T) diagram (Figure 3) The vapor pressure equations are detailed in Section 2.06.4.1.2
The critical temperature was found between 513 and 518 K.9
Many other physical, thermodynamic, and crys-tallographic properties can be found, respectively, by Llewellyn,9Settleet al.,10
and Hoard and Stroupe.11 What can be noted about UF6 is the large dif-ference between the density of the liquid and that
of the solid at the triple point (4830 kg m3 vs
3630 kg m3) If liquid UF6 solidifies in a process pipe, care must be taken during heating because of the swelling The recommended equations for the density of solid and liquid UF6are12
rS ¼ 5200 5:77ðT 273Þ
rL¼ 3946 4:0628ðT 273Þ 1:36102ðT 273Þ2 wherer is in kilogram per cubic meter
1500
1100
700
(1688)
1403
(703) (663) (608)
300
(328) 337
1309
1165
(1700)
UF3
UF4
UF6
UF4.25
621
Atomic ratio F/U
//
//
//
//
//
Figure 2 The U–F system Reproduced from Knacke, V O.;
Lossmann, G.; Mu¨ller, F Z Anorg Allg Chem 1969, 370,
91–103.
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Solid
Gas Liquid
Figure 3 The UF6 phase diagram.
Trang 4The viscosity of liquid UF6is close to that of water
(0.8 cps at 90C): 0.91, 0.85, 0.80, and 0.75 cps at,
respectively, 70, 80, 90, and 100C.13 Liquid UF6
usually flows by gravity to fill the 48Y containers
A 48Y is a container that contains approximately 12.5
tonnes UF6
Liquid UF6 has a dielectric constant e ¼ 2.18 at
65C typical of a nonpolar solvent The solubility of
ionic compounds is low.14
A review of thermal conductivity for UF6in the
solid and liquid forms can be found in Lewiset al.15
Our recommended values are:
k ¼ 1:1Wm1C1at 55C
k ¼ 0:16Wm1C1 at 90C
UF6is thermally stable up to 1000 K However, it is
very difficult to study the stability above 700 K due to
rapid corrosion of the metal reactors Also, UF6 can
be easily dissociated under UV source.16
UF6!hv UF5þ 0:5 F2
The average U–F binding energy in UF6 is 515 kJ
mol1, lower than H–F or Si–F but higher then C–F
or As–F.17
However, the first dissociation energy of UF6is as
low as 286 kJ mol1, yielding UF5 When the
poly-merization energy of UF5 (153 kJ mol1) is
sub-tracted, one obtains an energy of 134 kJ mol1, close
to the dissociation energy of fluorine (153 kJ mol1)
All these properties show that UF6 will act as an
oxidative/fluorinating agent It will not act as a
reduc-ing agent due to its high ionization potential of 14 eV
UF6 is therefore a stronger oxidizing agent than
MoF6 or WF6 A lot of publications have been
devoted to various reactions of UF6 in organic chemistry18or mineral chemistry.19
Two reactions are of industrial importance because of the need to convert the depleted UF6 back into U3O8
Hydrolysis of UF6with water:
UF6þ 2H2O! UO2F2þ 4HF
In the presence of a large excess of UF6, other oxi-fluorides can be formed such as UOF4or U3O5F8.20
Reduction with H2:
UF6þ H2! UF4þ 2HF Apart from oxidative properties, UF6is also consid-ered a Lewis acid and will react with Lewis bases such as KF, which is usually present in the F2used to produce UF6 NaF also forms Na2UF8 complexes that have been used to purify reprocessed UF6from fission products
2.06.3.1.1.1 Crystal structure The structure of solid UF6 was determined by Hoard et al.21
from X-ray single-crystal data UF6
is orthorhombic, space group Pnma (D162h) with
a ¼ 9.00(2) A˚, b ¼ 8.962(2) A˚, and c ¼ 5.207(2) A˚ (Figure 4) This structure was confirmed by single-crystal neutron diffraction at 293 K by Tayloret al.22
witha ¼ 9.924(10) A˚, b ¼ 8.954(9) A˚, and c ¼ 5.198(5) A˚ and by Levyet al.23
with a ¼ 9.92(5) A˚, b ¼ 8.97(5) A˚, andc ¼ 5.22 A˚
2.06.3.1.2 Thermodynamic properties The thermodynamic properties for the crystalline uranium hexafluoride are summarized inTable 1
Figure 4 The crystal structure of UF6.
Trang 52.06.3.1.2.1 Heat capacity
Heat capacity of solid and liquid uranium
hexafluor-ide has been measured between 14 and 370 K by
Brickwedde et al.25
Some experimental difficulties were encountered due to calorimeter deformation,
vapor presence, and lack of reproducibility The
heat capacity for the solid determined by Llewellyn9
is in good agreement with Brickwedde et al.25
but the discrepancy increased near the fusion point
(Figure 5) The heat capacity and entropy at room
temperature have been derived from this work
2.06.3.1.2.2 Enthalpy of formation
The enthalpy of formation of crystalline uranium
hexafluoride is a key value for the U–F
thermochem-istry The value was established by fluorine
combus-tion calorimetry by Seattleet al in 196310
and then
by Johnson et al.26
with a difference of 11 kJ mol1
between the two results The value selected by Grentheet al.24
is from Johnsonet al.26
2.06.3.1.3 Preparation
UF6is usually obtained through the direct fluorina-tion of UF4above 600 K for kinetic reasons:
UF4þ F2! UF6 The reaction mechanism is usually considered as
‘shrinking core,’ the UF4particles reduce in size when burning in F2and gaseous UF6evolves.27The reaction rate is proportional to the square of the particles’ mean radius In case of highly porous particles, a homoge-neous reaction mechanism has also been proposed.28
UF6will be industrially obtained in a flame reac-tor17or in a fluidized bed reactor.29In the latter case,
a fluidizing agent such as CaF2 or Al2O3 is used Construction materials will depend on the tempera-ture of the wall For high temperatempera-ture, Monel or nickel are preferred, although there is corrosion occurring even on passivated NiF2surfaces
Alternatively, UF6can be obtained through direct fluorination of oxides, for example20:
UO2þ 3F2! UF6þ O2 Such a process requires more F2 and yields more energy to evacuate Also other oxidants such as XeF2 which will be reactive even at room temperature can
be used, but the high cost of Xe limits their use for R&D purposes
BrF3 or ClF3 can also be used but this is not economically attractive
At high temperatureT > 873 K, oxygen should be minimized because of potential reaction between
UF4and O2:
2UF4þ O2! UF6þ UO2F2
UO2F2 can also be converted by F2to UF6 but the kinetics of the reaction are 100 times slower than that of UF4 Activation can be provided by catalysts such as CaF2.30
2.06.3.1.4 Uses Because of its high volatility, UF6is widely used in the 235U enrichment processes either the gas diffu-sion through ceramic membranes, centrifugation or SILEX process recently developed in Australia (Sep-aration of Isotope by Laser EXcitation based on 16 mm laser exciting of gaseous235UF6) Moreover, F is the only halogen that presents only one isotope This is needed for an enrichment process based on mass (gas
0
50
100
150
200
Csat
Temperature (K)
Solid
Liquid 337.212 K
Figure 5 Experimental heat capacity measurements of
the solid and liquid uranium hexafluoride.
Table 1 Thermodynamic properties of the crystalline
uranium hexafluoride
DfH 0 (UF6, cr, 298.15 K) (kJ mol1) (2197.7 1.8) 24
S 0 (UF6, cr, 298.15 K)
(J K1mol1)
227.6 1.3 24
C 0 (UF6, cr, 298.15) (J K1mol1) 166.8 0.2 24
C 0 (UF6, cr, T) (J K1mol1) 52.318 þ 0.383798T
(298–337)24
DHfus 0
(UF6, cr, 337.2 K) (kJ mol1) 19.1961
2.876460 10 6 T2
þ 1.9962 10 3 T1 (298–337) is the temperature range for which the Cp(T) function is
valid.
Trang 6diffusion or centrifugation) The reconversion of UF6
through the reductive pyrohydrolysis to UO2yields a
sinterable powder that can be readily used to prepare
fuel pellets
Reaction to form UF6 is also a way to separate
uranium from less-volatile compounds It has been
considered to separate reprocessed uranium from
nonvolatile fission product and less-stable other
hexafluorides such as PuF6and NpF6.31
2.06.3.2 UF4: Uranium Tetrafluoride
2.06.3.2.1 Properties
Although UIVis not stable in aqueous solution, UF4
is a very stable ionic solid that melts at 1309 K
when many covalent tetrafluorides melt below 573 K
and polymeric tetrafluorides (ZrF4, HfF4, SnF4,
PbF4, .) melt around 1100 K This is due to its high
lattice energy of 12 970 kJ mol1 UF4is a green solid
with monoclinicC6
2hsymmetry,a ¼ 12.73 A˚,b ¼ 10.75 A˚,
c ¼ 8.43 A˚, and b ¼ 126200(32) (Figure 6) and a
den-sity of 6700 kg m3 UF4 has a thermal conductivity
of 1.96 W m1C1 In the granular form, the
effec-tive conductivity can be as low as 0.2 W mC1 It
has a small solubility in water (0.1 g l1at 298 K)
When exposed to air, it will form an hydrate
UF42.5H2O after several days
UF4can be pyrohydrolyzed with water The
equi-librated reaction
UO2þ 4HF ⇄ UF4þ 2H2O
has been studied33 and is very significant above
673 K (Figure 7)
A compromise must be found for the synthesis
of UF4 from UO2 with respect to the temperature
To obtain good kinetics, the temperature must be
increased but to avoid pyrohydrolysis and UF4
sintering, it must be lowered
2.06.3.2.1.1 Density The density of liquid UF4 has been measured by Kirshenbaum and Cahill34 from 1309 to 1445 K The results can be represented by:
rðkg m3Þ ¼ 7784 0:92T TðKÞ 2.06.3.2.1.2 Viscosity
The viscosity of pure liquid UF4was measured from
1323 to 1428 K by Desyatniket al.35
and from 1138 to
1618 K by Kulifeev and Panchishnyi.36The results are scattered.35They can be presented by the equation:
Figure 6 The crystal structure of UF4.
100
90
80
70
60
Kuhlman
Johns and Walsh
Legassie et al.
Petretic and Bertram Briggs and Bonfer Domange and Wohlhuter
50
40
30
20
10
0
Temperature (⬚C)
Figure 7 The UO2–UF4 equilibrium Reproduced from Harrington, C.; Ruenhle, A Uranium Production Technology; Van Nostrand; p 69.
Trang 7ðmPasÞ ¼ 0:0159 exp 8183T
TðKÞ
2.06.3.2.2 Thermodynamic properties
The thermodynamic properties of crystalline UF4
have been analyzed in detail by Fuger et al.,37
Grentheet al.,24
and updated by Guillaumontet al.39
The recommended values are reported inTable 2
2.06.3.2.2.1 Enthalpy of formation
UF4has been studied extensively The heat of
forma-tion of UF4has been measured using fluorine bomb
calorimetry by Hayman40 at 315 K (without
tak-ing into account the impurities in the samples), by
Wijbenga,41and by Johnson.42The value obtained by
Hayman was recalculated by Wijbenga considering
the impurities and the value of the heat of formation
of UF6(s) measured by Johnson.26
Mal’tsev et al.43
determined the heat of forma-tion of the highest crystal hydrate of uranium
tetrafluoride, UF42.5H2O, as the sum of the heats
of nine reactions Using the heat of hydration of
UF42.5H2O given by Popovet al.,44
Mal’tesevet al
deduced the heat of formation of the anhydrous
tet-rafluoride Several works reported the enthalpy of
formation by solution calorimetry:
At first, Khanaev et al.45
used various (HClþ
H3BO3þ FeCl3) aqueous solutions at 323 K
A solution of hydrofluoric acid, hydrochloric acid,
and aqueous aluminum chlorate AlCl36H2O was
used by Huet al.46
Finally, Cordfunke et al.47
have chosen a mixed aqueous solvent containing sulfuric acid, boric
acid, and ceric sulfate
The results obtained are widely scattered from
(1884.9 2.9) to (1921.3 4.2) kJ mol1
The selected value came from the IAEA review,37
which is a weighted average of the values published
by Johnson42 using fluorine combustion calorimetry (with the selected enthalpy of formation of UF6(s)) and Cordfunke et al.47
using solution calorimetry (with the selected enthalpies of formation of HF (aq), U3O8(s), and g-UO3).24 The values differ by 10.7 kJ mol1, therefore the NEA-TDB24 recom-mended further measurements to resolve the discre-pancies in the experimental values
2.06.3.2.2.2 Heat capacity The low-temperature heat capacity of crystalline uranium tetrafluoride was measured by:
adiabatic calorimetry by Brickwedde et al.25
from
20 to 350 K;
adiabatic calorimetry by Osborne et al.48
between
5 and 300 K;
and isothermal calorimetry by Burns et al.49
in the 1.3–20 K temperature range
The values are very close except for the values of Brickweddeet al.25
extrapolated atT < 15 K (Figure 8) 2.06.3.2.2.3 Other properties
Dworkin50measured the enthalpy increment of UF4 from room temperature up into the liquid phase These results are in very good agreement with unpub-lished results obtained by Cordfunke and reported by Fuger37(Figure 9)
Based on the results from both studies, the enthalpy is expressed as:
HT H298ðkJ mol1Þ ¼ 35:058 þ 0:1145T
þ 10:27745 106T2þ 4:13159 103T1
298 1309 K
Table 2 Thermodynamic properties of the crystalline
uranium tetrafluoride
DfH 0
(UF4, cr, 298.15 K) (kJ mol1) (1914.2 4.2) 24
S0(UF4, cr, 298.15 K) (J K1mol1) 151.7 0.2 24
C 0 (UF4, cr, 298.15) (J K1mol1) 116.0 0.1 24
C 0 (UF4, cr, T) (J K1mol1) 114.5194þ 20.5549
10 3 T413159T 2 (298–1309)38
DH 0
fus (UF4, cr, 1305 K) (kJ mol1) 47 5 39
(298–1309) is the temperature range for which the Cp(T) function is
valid.
0 20 40 60 80 100 120 140
0 2 4 6 8 10 12 14
Osborne et al.47
Burns et al.48
Brickwedde et al.25
T (K)
Osborne et al.47
Burns et al.48
Brickwedde et al.25
Cp
–1 )
Cp
–1 )
Figure 8 Comparison of the heat capacity measurements
of the crystalline uranium tetrafluoride.
Trang 8HT H298ðkJ mol1Þ ¼ 39:413 þ 0:166T
ð1309 1400 KÞ in liquid state with TðKÞ
Dworkin deduced from his measurements the entropy
and enthalpy of fusion at the fusion temperature
Tfus¼ 1309 K
DfusH¼ 46986 J mol1
DSfus¼ 35:98 JK1mol1
This enthalpy of fusion is higher than the approximate
value calculated by Khripin5from the differential
ther-mal analysis in the UF3–UF4 binary system
(DfusH0
(UF4,s, 1309 K¼ 43 514 2 kJ mol1)
The experimental data of Dworkin for the
enthalpy of fusion has been recommended by
Guillaumont et al.39
in their review, although the vapor pressure data for UF4(cr, l) are more consistent
with a smaller value (36 kJ mol1).39
2.06.3.2.3 Preparation
Two ways are industrially used to obtain UF4 from
UO2powder:
the precipitation of UF4in HF aqueous solution,
the reaction of UO2with gaseous HF at 573–773 K:
UO2þ 4HF ! UF4þ 2H2O
As a consequence, the impurities in UF4are usually
oxygen based, such as UO2F2, UO2, or H2O
The large density variation between the fluoride
and the oxide tends to hinder the reaction because
swelling occurs and the porosity is easily blocked
dur-ing the conversion The kinetics of hydrofluorination
will then depend much on the specific surface of the
UO2 A model has been developed that takes into account the grain and pellet sizes.51 Impurities such
as sodium can also be detrimental because they may favor the sintering The NaF–UF4eutectic melts close
to 893 K
UF4 can be obtained in a rotating furnace, in a fluidized bed, or in a moving bed reactor where the
UO2 has been pelletized as described in Harrington and Ruenhle.33If the hydrofluorination temperature is maintained below 750 K, the effect of sodium is light 2.06.3.2.4 Uses
UF4can be used to produce UF6(see previous section)
or U metal using magnesium reduction in a bomb:
2Mgþ UF4! U þ 2MgF2
In all cases, the oxygen residual level in UF4 will be very important It will affect the conversion rate to
U metal or UF6 Mixtures of fluoride salts with UF4are candidates
as fuel carrier of molten salt reactors52(seeChapter 3.13, Molten Salt Reactor Fuel and Coolant)
2.06.3.3 UFx(4 < X < 6) – Intermediate Fluorides
2.06.3.3.1 UF5: Uranium pentafluoride 2.06.3.3.1.1 Properties and preparation of UF5
UF5was initially prepared by Ruff through the reac-tion of UCl5with anhydrous HF More recent studies have been made on the reaction of gaseous UF6on
UF4as a powder:
UF4þ UF6⇄ 2UF5 However, a low temperature (T 453 K for pUF6< 1bar (105Pa)) is required to avoid the decomposition
to U2F9 This often means a very slow transformation (several days depending on the specific surface) UF5 has a white color but with this technique a gray
or black color powder is often obtained because of presence of U2F9
Asada has more recently prepared UF5 at
643 K but under 3 bars (3 105
Pa) UF6.53However, corrosion problems have been experienced
Probably the best way to obtain pure UF5 from UF4and UF6is to allow evaporation of UF5at 573–673 K and condensation at 346–408 K Blue
UF5needles are then obtained
UF5is often considered a key product because it may be involved in clogging reactors (flame reactor
or fluidized bed) when no excess F2 is maintained
UF5has a tendency to polymerize This very impor-tant property tends to explain why the reactivity of
20
40
60
80
100
120
140
160
180
200
Dworkin 49
Cordfunke et al.46
HT
–1 )
T (K)
Figure 9 Comparison of the increment enthalpy of
crystalline UF4 experimental results.
Trang 9U(V) fluorides is lower than that of U(IV) Also UF5
melts at a low temperature compared to UF4
UF5has two crystalline forms that are both
tetrag-onal (Figure 10):
High-temperature a-UF5 that has an I41m
sym-metry with a ¼ 6.5259(3) A˚ and c ¼ 4.4717(2) A˚,
which was confirmed by high-resolution neutron
powder diffraction data of Howardet al.54
Low-temperature b-UF5that has an I42d symmetry
witha ¼ 11.469(5) A˚ and c ¼ 5.215(2) A˚ (a ¼ 11.456
(2) A˚ , c ¼ 5.195(1) A˚, Z ¼ 8).55
The transition temperature is 398 K, independent of
the UF6pressure The a-phase is obtained from the b
form when slowly raising the temperature (18 h at
458 K) The reverse transformation form a to b has
not yet been achieved
UF5 can be separated from U(VI) compounds
using anhydrous acetonitrile dissolution.56 Again this
property is linked to the polymerization trend of UF5
2.06.3.3.1.2 Thermodynamic properties
The reviewed thermodynamic properties by Fuger
et al.,37
Grenthe et al.,24
and Guillaurmontet al.39
of both a- and b-uranium pentafluoride are presented
inTable 3
Only a single reliable experimental study is reported for the uranium pentafluoride The enthal-pies of formation of both allotropic forms have been measured by solution calorimetric experiments in Ce(SO4)2þ H2SO4by O’Hareet al.57
They found:
DfH0ða-UF5; s; 298:15 KÞ¼ð2075:56:7ÞðkJ mol1Þ and DfH0ðb-UF5; s; 298:15 KÞ
¼ð2083:06:3ÞðkJ mol1Þ:
This value is consistent with the earlier results of Agron when recalculated using the values of DfH0 (UF4, s, 298.15 K) and DfH0
(UF6, g, 298.15 K) recom-mended by Grentheet al.24
Katz and Rabinowitch58 reported unpublished low-temperature heat capacity measurements on a-UF5 performed by Brickwedde et al Because of the presence of 17 wt% UF4 and UO2F2 in the a-UF5samples, Fugeret al.37
adjusted the result to be consistent with Agron’s dissociation measurements.59 The heat capacity variation with temperature
is from Knacke et al.8
who did not distinguish between a- and b-phases The value at 298.15 K is interpolated
The UF5stability domain was revisited recently60
at higher pressure than Agron’s initial study The results were found in good agreement (Figure 11)
Figure 10 The crystal structure of (a) a-UF5 and (b) b-UF5.
Table 3 Thermodynamic properties of the crystalline a and buranium pentafluoride
DfH 0 (298.15 K) (kJ mol1) (2075.3 5.9) 24 (2083.2 4.2) 24
S 0 (298.15 K) (J K1mol1) 199.6 12.6 24 179.5 12.6 24
C 0 (T) (J K1mol1) 125.159 þ 3.0208 10 2 T 192 500 T 2
(298–1000)39
125.159 þ 3.0208 10 2 T 192 500 T 2 (298–1000)39
Trang 102.06.3.3.1.3 Properties of liquid uranium
pentafluoride
Experimental studies on liquid UF5 are difficult
due to its volatility, corrosiveness, tendency to
disproportionate, and sensitivity to moisture Wolf
et al.61
measured the melting point of a-UF5 under
various UF6pressures in excess (from 7.1 104
Pa to 4.7 105
Pa) to avoid the disproportionation They
foundTfus(UF5, cr)¼ 621 K Asada et al.53
have deter-mined some physical properties over the temperature
range 638–693 K under UF6pressure of 3 105
Pa:
Density: rL¼ 4:5g cm3
Electric conductivity: ðO1cm1Þ
¼ 2:63 expð2:84=RTÞ
ViscosityðcpÞ ¼ 1:6 102expð7:03=RTÞ
2.06.3.3.2 Intermediate fluorides U4F17
(UF4.25) and U2F9(UF4.5)
2.06.3.3.2.1 Preparation
U2F9 (which is called uranium enneafluoride) is
prepared by treating UF4 with UF6 The kinetics
are faster (about 1 day) when the process is
per-formed under higher pressure of UF6 and high
temperature Another way is to start from the
penta-fluoride and decompose it into U2F9 It is less
conve-nient because it includes the synthesis of pure
pentafluoride which is less stable U2F9 is the more
stable intermediate fluoride under argon atmosphere
without moisture It forms black needles
U4F17 was obtained by maintaining UF4 at
593 K and introducing UF at 2.3 103
Pa pressure
during 2 days.59The diffraction pattern of this material resembling that of UF4but containing uniquely differ-ent features is described as ‘distorted UF4.’ It is a black powder This synthesis is to be avoided due to the corrosion problem at high temperature under UF6
It is easier to decompose higher intermediate fluorides such as UF5or U2F9than synthesize directly
We have found no physical properties for these compounds in the literature Mixtures of UF5–U2F9 have a tendency to melt at lower temperature than pure UF5
2.06.3.3.2.2 Crystal structure
U2F9as a body-centered cubic structure, space group I43m T3
d with a ¼ 8.4716(5) A˚ (Figure 12) and a theoretical densityrth ¼ 7.06 g cm3
.62,63This sym-metry was confirmed by high-resolution neutron powder diffraction data.54
U4F17has a deformed UF4lattice The symmetry is monoclinic, space group C62h, C2/c, with a ¼ 12.09 (0.08) A˚ , b ¼ 10.81(2) A˚, c ¼ 8.29(4) A˚, and b ¼ 128.0(8).12
2.06.3.3.2.3 Thermodynamic properties The thermodynamic data at room temperature on solid U4F17 and U2F9 recommended by Grenthe
et al.24
are presented inTable 4
With second-law analysis of disproportionation pressure measurements performed by Agron,59 the enthalpy and entropy of each reaction at the mean temperature can be calculated Assuming that
DCp
U4F17and U2F9can be calculated using the thermo-dynamic properties of UF (s) and UF(g) at 298.15 K
Figure 12 The crystal structure of U2F9.
1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7
–2
–1
0
1
2
3
4
5
6
7
8
α-UF 5
β-UF 5
U2F9
U4F17
exp LMI exp LMI fit LMI fit LMI exp Agron exp Agron fit Agron fit Agron
1000/T (K)
Figure 11 Phase stability domain for the intermediate
fluorides after Agron59and Moch et al.60