Journal of the Less-Common Metals, 157 (1990) 139 · 146
139
ice of Basic Energy Sciences, Materials
Solid State Chern., 26 (1978) 189.
Chern. Phys., 70 (1979) 4907.
C. R. Acad. Sci. Ser. B, 256 (1963) 1793.
Chern., 3 (1964) 1220.
Chern ., 4 (1965) 970.
. Jacobson and H . F . Franzen, J. Solid State
1iversity, Ames, IA, 1986.
ISOTHERMAL DECOMPOSITION OF URANIUM HYDRIDE
D. L. LINDNER
Sandia National Laboratories, Livermore, CA 00060 (U.S.A.)
Received March 13, 1989; in revised form May 11, 1989)
Abstract
We have investigated two related dynamic properties of the U- H system. In part of the work reported here, we determined dynamic pressurecomposition-temperature (P-C-T) properties of uranium hydride in the
temperature range 375- 525 °C. We also investigated the pressure-dependent
kinetics and mechanism of the decomposition of uranium hydride in the
range 390- 500 °C. From the functional time dependence and the functional
pressure dependence of the decomposition, we conclude that the reaction is
controlled by the rate of advance of the metal- metal hydride phase
boundary. The decomposition has an activation energy of 18.9 ± 1.8 kcal
mol 1 and the reaction has a ln(P 0 /P) pressure dependence, where P 0 is the
corresponding dynamic P-C-T plateau pressure.
1. Introduction
The reaction of uranium metal with hydrogen gas to form uranium
hydride has been the subject of a large number of investigations [1 - 6].
tudy of the decomposition reaction has been more limited [3, 5], however.
Condon and Larson [3] investigated the decomposition of uranium hydride
into high vacuum in the range 70- 200 °C. They found the reaction to have
an activation energy of 17.4 ± 0.8 kcal mol- 1 • Stake bake [ 5] also studied the
kinetics of decomposition into vacuum, but at higher temperature (200300 °C). He found the activation energy to be 9.5 :r 0.6 kca:l moC 1 • Results
of both investigations indicate that the reaction obeys zero-order kinetics
initially.
We routinely use high-temperature uranium hydride beds as a source of
pure hydrogen gas. The rate at which we can deliver hydrogen from such
beds is therefore of some interest. These beds are typically used at higher
temperatures than were investigated in the previous studies, and extrapolation of the results to the higher temperatures of interest is complicated by
the discrepancy in the reported activation energy. Furthermore, in our
application we decompose the hydride into a hydrogen overpressure, not
into a vacuum, so it is also important that we understand the pressure dependence of the decomposition.
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•
140
2. Experimental details
2.1. Dynamic P-C-T properties
Previous work on the decomposition kinetics of metal hydrides [7] has
demonstrated that the "dynamic" plateau pressure of the metal hydride can
be an important parameter in the pressure dependence of the dehydriding
(or hydriding) kinetics. Since we were not aware of any previous measure·
ments of this property for uranium hydride, we began this study with an
examination of the dynamic P-C-T properties of the hydride under condi·
tions similar to those of interest here. In general, the plateau pressure
measured for a metal hydride depends, in part, on the way in which it is
measured [7- 10]. Previous P-C-T work with uranium hydride has used the
standard method of adding and removing hydrogen by aliquots.
Figure 1 is a schematic of the apparatus used in this work. In these
P- C- T measurements, the "transfer controller" was a hydrogen mass flow
controller (Datametrics 1510/822) oriented to control hydrogen flow to the
bed for uptake studies, or to control flow out of the bed for hydride decom·
position measurements. The pressure transducers used were Baratron model
315BH capacitance manometers. The entire apparatus was connected to an
ultrahigh vacuum manifold and was evacuable to a base pressure of less than
1 X 10- 8 Torr. All of the hydrogen used in this work was delivered off the
hydrogen storage bed, a vessel containing uranium hydride powder. Before
delivering hydrogen, this bed was evacuated to less than 1 X 10- 6 Torr, and
gas was then delivered by heating the bed externally. The uranium sample
used in the hydride reactor bed was 9.2 g of what had originally been a
nominal 100 p.m low-oxygen uranium powder sample. This sample was
activated with a high-temperature vacuum bakeout and then hydrided and
dehydrided 20 times prior to beginning the measurements reported here.
This sample powder was held as a "pancake" of thickness about 1 mm
within a large split copper block to minimize bed temperature excursions
during hydriding or dehydriding re
alumel thermocouple was held in tl
that such fluctuations were indeed s.
Immediately prior to each
hydrided and dehydrided. The samJ:
an external heater, which used a se•
input. For hydriding, the calibrated
the storage bed to an appropriate r
ple was hydrided while the volum
initiated by opening the flow contr•
the sample as required. Hydrogen fl
and bed temperature was constant
was followed to determine the dy
hydride, while the pressure over t
hydrogen:metal ratio of the hydridE
gas in the free volume of the reactor
since that volume was held to a mini
2 shows some typical data from t·
temperatures shown, the plateau prE
composition range. The number we
sure is actually the measured pressw
least squares, best fit line through thE
shoot" observed near maximum sto
metal and metal alloy hydrides in si
the kinetics of the {J to a phase char
also been observed in uranium hyd:
dard aliquot method [11, 12].
500 - r - - - - - - - - - - - To Vacuum Manifold
and Gas Manifold
Dynamic PCT Cur ves for
De co mposit ion of Urani um Hyd r id e
400
300
200
Hydrogen
Storage
Bed
IOO-t---,.---r--......,--...,...-0
Heaters
Fig. 1. Experimental apparatus.
Volumes
0.5
1.5
2
H/ U
Fi 2. Typical dynamic P- C- T results.
2.
141
n kinetics of metal hydrides [7] has
u pressure of the metal hydride can
ure dependence of the dehydriding
not aware of any previous measuredride, we began this study with an
perties of the hydride under condi. In general, the plateau pressure
in part, on the way in which it is
k with uranium hydride has used the
hydrogen by aliquots.
)aratus used in this work. In these
1troller" was a hydrogen mass flow
ted to control hydrogen flow to the
v out of the bed for hydride decomnsducers used were Baratron model
tire apparatus was connected to an
:uable to a base pressure of less than
in this work was delivered off the
tg uranium hydride powder. Before
ted to less than 1 X 10- 6 Torr, and
ed externally. The uranium sample
2 g of what had originally been a
powder sample. This sample was
m bakeout and then hydrided and
: the measurements reported here.
mcake" of thickness about 1 mm
timize bed temperature excursions
during hydriding or dehydriding reactions. The bare junction of a chromealumel thermocouple was held in the sample bed and monitored to ascertain
that such fluctuations were indeed small.
Immediately prior to each experiment, the uranium sample was
hydrided and dehydrided. The sample was then brought to temperature with
an external heater, which used a second, external thermocouple as a control
input. For hydriding, the calibrated volumes were filled with hydrogen from
the storage bed to an appropriate pressure; while for dehydriding, the sample was hydrided while the volumes were evacuated. An experiment was
initiated by opening the flow controller and controlling gas flow to or from
the sample as required. Hydrogen flow in this work was 15 standard em 3 s- 1
and bed temperature was constant to ± 0.5 °C. The pressure in the reactor
was followed to determine the dynamic plateau pressure of the uranium
hydride, while the pressure over the volumes was used to determine the
hydrogen:metal ratio of the hydride sample (a correction to account for the
gas in the free volume of the reactor arm was also applied, but this was small
since that volume was held to a minimum in design of the apparatus). Figure
2 shows some typical data from two decomposition experiments. At the
temperatures shown, the plateau pressure varies by about 50 Torr across the
composition range. The number we report here as the dynamic plateau pressure is actually the measured pressure at the composition H:U = 1.5 on the
least squares, best fit line through the data in the plateau region. The "undershoot" observed near maximum stoichiometry has been observed for other
metal and metal alloy hydrides in similar work [7] so it may be a result of
the kinetics of the {3 to a phase change. However, a similar "undershoot' has
also been observed in uranium hydride P- C- T data obtained by the standard aliquot method [11, 12].
500~------------------------------------,
Dynamic PCT Cur ves for
Decomposition of Uranium Hydride
400
";:'
]
Volume
Pressure
Transducer
'-
0
0
~J 300
""
Q)
'-
IL
200
100i-----~----~--~----~-----r----~--~
Calibrated
Volumes
0
0.5
1
1.5
2
H/U
Fig. 2. Typical dynamic P- C- T results.
2.5
3
3.5
.. .
Jit
•
•
11
2.2. Decomposition kinetics
Studies of the decomposition kinetics of uranium hydride were also
conducted using the apparatus of Fig. 1. In this work, the hydrogen
"transfer controller" was a Varian high vacuum leak valve which was
controlled manually so as to maintain constant pressure over the uranium
hydride. Immediately prior to each experiment in this series, the uranium
sample bed was hydrided to full stoichiometry at a pressure well above the
P-C-T plateau pressure. An experiment was initiated by quickly reducing
the overpressure to a nominal value, where it was held throughout the exper·
iment. Pressure was maintained to within ± 3 Torr of nominal. Temperature
of the hydride sample decreased by as much as 3 °C but the typical variation
was less than 1 °C.
The extent of decomposition was determined by an ideal gas law
P- V-T calculation of the gas quantity transferred into the calibrated
volumes. Some typical experimental results are shown in Fig. 3. Experiments
were conducted over the range 390 - 500 °C.
1.2
"2
.......,
c
0.8
'
.2
u0
~
~
Temperoture=449°C
Pressure=840 torr
'
0.6
c
0
130
0.4
Octo
Q)
a::
30 lnterfoce Conlrol
-
0.2
·-
1000
6
Dehydrldlng: 0 .035 H/U-min
v Dehydrlding: Kinetics
~dr!~.!~iz:.~o~ - o Hydriding: 0.035 H/ U-min
IOOt--,---,----r---r---r--..,1.20
1.25
1.30
1.35
1.40
1.45
1.5C
1000/T
Fig. 4. Van't Hoff plots of P- C- T data a
Gibb [14 ].
Figure 4 shows the data (at H
the hydriding and dehydriding plate
also shown in Fig. 4 and calculated
Table 1. The hydrogen flow rate m
rate of 0.035 H:U min- \ which is
quently encountered in decomposit:
those experiments we were able to c;
slower flow rate (see Section 3.2),
equation. Parameters derived from t
data is shown in Fig. 4. Also shown
sure curve obtained by aliquot methc
..--::?"'
#
IOOOO,r - - - - - - - - - - - -
·- - ·-
' Zero Order ' Kinetics
----------------
TABLE 1
0
0
10
5
15
20
Time (minutes)
Experimentally determined values for the p
Fig. 4
Fig . 3. Typical decomposition kinetics experimental results compared with theoretical
curves. The plotted delay in reaction is an artefact resulting from a delay in initiation of
the experiment after initiation of data acquisition .
Hydriding
This work: 0.035 H:U min-
3. Results and discussion
Dehydriding
This work: 0.035 H:U minThis work : kinetic studies
Libowitz and Gibb [ 14]
1
1
3.1. Dynamic P-C-T properties
The plateau pressures of uranium hydride have been found to be well
represented by an equation of the form [13]
A
log 10(P) = - - + B
(1)
T
3.2. Dehydriding kinetics
In general, hydriding or dehydri
general form [7]
where Pis pressure, Tis the temperature in Kelvin, and A and B are constants.
f**(cx)
=
A{(P)f*(T)t
143
IOOOOr-----------------------.
netics of uranium hydride were also
g. 1. In this work, the hydrogen
1igh vacuum leak valve which was
constant pressure over the uranium
periment in this series, the uranium
ometry at a pressure well above the
1t was initiated by quickly reducing
ere it was held throughout the exper·
in ± 3 Torr of nominal. Temperature
nuch as 3 °C but the typical variation
o-........._
" "' ":-.... o ,
~'-...
o'o,
~
1000
o-....z....o
4.
~o......_
~~
~- 0~
~
6 Dehydrld lng: 0 .0 35 H/ U min
v Dehydriding: Kinetics
.
~
~
~dr~:- ~o!!__ , _
.•.
."'-....
•.
:11
o Hydriding: 0.0 35 H/ U- m in
IOO-!--..,--....,....----r----,--"T"'""--r---r--l
ts determined by an ideal gas law
ity transferred into the calibrated
llts are shown in Fig. 3. Experiments
I °C.
1.20
1.25
1. 30
1.35
1.40
1.45
1.50
1.55
1.60
,,
1000/ T
Fig. 4. Van 't Hoff plots of P- C- T data and best fit lines. Aliquot line after Libowitz and
Gibb [14] .
Figure 4 shows the data (at H :U = 1.5) obtained in this work for both
the hydriding and dehydriding plateau pressures. Best fit lines to the data are
also shown in Fig. 4 and calculated parameters for equation (1) are shown in
Table 1. The hydrogen flow rate used in our P- C-T work corresponds to a
rate of 0.035 H:U min- 1, which is somewhat faster than flow rates subsequently encountered in decomposition kinetics experiments. However, from
those experiments we were able to calculate dynamic plateau pressures at the
slower flow rate (see Section 3.2}, and to also fit that data to the above
equation. Parameters derived from those fits are shown in Table 1, and the
data is shown in Fig. 4. Also shown in Fig. 4 is a dehydriding plateau pressure curve obtained by aliquot methods [14] for comparison.
TABLE 1
20
Experimentally det ermined values for the paramet ers of eqn. (1) from the best fit lines o f
Fig. 4
ental results compared with theoretical
tct resulting from a delay in initiation of
Hydriding
This work: 0 .035 H :U minDehydriding
This work : 0 .035 H :U minThis work : kinetic studies
Libowitz and Gibb [14]
A
B
1
3473
8 .0 5
1
4717
4700
4410
9.46
9 .47
9.14
dride have been found to be well
l]
(1)
~elvin, and A and B are constants.
I
3.2. Dehydriding kinetics
In general, hydriding or dehydriding reactions obey an equation of the
general form [7]
f**(a) = Af(P)f*(T}t
(2)
'*'
I'·
/
144
where a is reaction fraction (0.;;;;; a.;;;;; 1), A is a constant, t is time, Pis pressure and Tis temperature.
The function f**(a) can take a number of forms, depending on the
mechanism of the reaction [7, 15 - 17]. In particular, for a zero-order reaction, f**(a) =a, so
0.1 5 - r - - - - - - - - - - Temp erol ure= -47<4 °C
P 0 "" 1440 to rr
<
l
0.10
c0
;;
c
0
a=k'(P,T)t
(3)
where k'(P,T) is the rate constant. If a reaction obeys such zero order
kinetics, then under isothermal, isobaric conditions, a plot of the extent of
reaction should be linear in time. From Fig. 3, we see that our data apparently exhibits such linear behavior over about the first 40% of the course of the
decomposition. Previous studies of the dehydriding [3, 5] have also noted
that the reaction initially exhibits apparent zero order kinetics. However,
many solid reaction mechanisms can appear to give "zero order" kinetics at
low reaction extent [15] so that observation of such linear behavior is not
really an indication that a reaction obeys zero order kinetics. Furthermore,
such kinetics are difficult to explain for reactions in solids. To properly
analyze such data for solid state reactions it is important to examine the
entire data set.
Comparison of our complete data set with many different theoretical
curves for reactions in the solid state [ 15] reveals that a model in which the
reaction is controlled by the advance of the metal-metal hydride phase
boundary gives good agreement with our observed kinetics (see Fig. 3). For
these kinetics, f**(a) = 1- (1- a) 1 13 , and so we get (from equation 2)
a= 1 - {1 - k(P, T) tP
(4)
where k(P,T) is the rate constant (it should be noted that within the context
of these equations, k'"" 3k at short times). Since we expect the reaction to
exhibit Arrhenius temperature dependence, we then have
k(P,T) = Af(P)t; exp(-Ea/RT)
u 0.05
0•
"'
o.oo :!"o.o::-----o..,...
.l - - -o.,.2- - -o.,.3 - - _oj..
ln(P0 / P)
Fig. 5. Functional pressure dependence of
Fig. 6. Arrhenius plot of the decompositi<
in hydriding-dehydriding reaction
ln(Po/P) dependence is characteristi<
such as diffusion, interface moveme1
Diffusion of hydrogen is too fast tc
observed here, and the a us. time c1
characteristic of reactions controlle,
[15]. Therefore, both the function
pressure dependence of the reactic
uranium hydride is controlled by th
in the solid.
In Fig. 6 we present an Arrh€
logarithm of the slopes of the rate c
We get linear behavior, as expected.
E.= 18.9 ± 1.8 kcal mol- 1 , in good a
(5)
From this equation, we observe that at fixed temperature, a plot of k
against f(P) should give a straight lirie of slope Af*(T). We use this to
determine the functional pressure dependence of the reaction. Furthermore
we can use the slopes of these plots to determine the activation energy of the
reaction by plotting the logarithm of the slope against 1/T to get a line of
slope Ea/R .
In Fig. 5 we plot measured rate constants (at fixed temperature) against
ln(P 0 /P) . As can be seen, the data fall on a straight line when so plotted.
Furthermore, since a plot of the measured rate constant us. ln(P0 /P) should
pass through the origin, we were able to determine P 0 under the flow conditions of these experiments by adjusting that parameter to comply with this
constraint. We examined other possibilities that the reaction constant might
depend on different functions of pressure, possibly (P- P 0 ), (../P -yP0) or
(1/P0 - 1/P)". Only the ln(P0 /P) function was consistent with our data.
Flanagan [16] has discussed these functional forms of pressure dependence
4. Conclusions
We have measured the dynamic
over the temperature range 375 - 52f
measured in this work exhibit signific
the usual method of adding or removi
We find that the decomposition
500 oC is well described by the equati<
a= 1-- {1- k(P,T)tP
where
k(P,T) =a In(;) exp(
R;)
145
0.15 - r - - - - - - - - - - - - - - - - - - .
a constant, t is time, Pis pres·
~r
of forms , depending on the
articular, for a zero-order reac·
Temperature= 474oc
P0 z 1440 torr
i
E0 = 1! .9tt.! keel/mole
0.10
'Q)
c0
a.
0
;;
(3\
!action obeys such zero order
ditions, a plot of the extent of
, we see that our data apparent·
te first 40% of the course of the
1driding [ 3, 5] have also noted
zero order kinetics. However.
to give "zero order" kinetics at
1 of such linear behavior is not
co order kinetics. Furthermore,
~actions in solids. To properly
lt is important to examine the
3c
l··j.//
.
T
o.oo +-----r----r----~---~
0.0
0.1
0.2
0.3
0 .4
1.25
1.30
1.35
ln(P 0 /P)
1..(0
1.4-5
1.50
1.55
1000/T
Fig. 5. Functional pressure dependence of the decomposition .
Fig. 6. Arrhenius plot of the decomposition data.
in hydriding-dehydriding reactions. In particular, he points out that a
Jn(P0/P) dependence is characteristic of bulk phase reaction control processes
such as diffusion, interface movement, or nucleation and growth phenomena.
Nith many different theoretical Diffusion of hydrogen is too fast to account for the relatively slow kinetics
observed here, and the a us. time curves do not exhibit the sigmoidal shape
weals that a model in which the
characteristic of reactions controlled by nucleation and growth phenomena
;he metal-metal hydride phase
[15]. Therefore, both the functional time dependence and the functional
;erved kinetics (see Fig. 3). For
pressure dependence of the reaction indicate that the decomposition of
we get (from equation 2)
uranium hydride is controlled by the movement of the {3- a phase boundary
(41 in the solid.
In Fig. 6 we present an Arrhenius plot of the data as a plot of the
e noted that within the context
logarithm of the slopes of the rate constant us. ln(P0 /P) curves against 1/T.
lince we expect the reaction to
We get linear behavior, as expected. From the slope of the line we calculate
•e then have
E.= 18.9 ± 1.8 kcal mol- 1 , in good agreement with Condon and Larson [ 3] .
(5)
fixed temperature, a plot of k
slope Af*(T). We use this to
:e of the reaction . Furthermore
line the activation energy of the
)pe against 1/T to get a line of
;s (at fixed temperature) against
1 straight line when so plotted.
:~.te constant us. ln(P0 /P) should
ermine P 0 under the flow condiparameter to comply with th~
hat the reaction constant might
ossibly (P- P 0 ), h/P -..jP0 ) or
was consistent with our data.
l forms of pressure dependence
4. Conclusions
We have measured the dynamic P- C- T properties of uranium hydride
over the temperature range 375 - 525 °C. We find that the plateau pressures
measured in this work exhibit significant differences from those obtained by
the usual method of adding or removing aliquots of hydrogen.
We find that the decomposition of uranium hydride in the range 390 500 °C is well described by the equation
a=1-· {1- k(P,T)tP
where
k(P,T) =a ln
(-Ea)
p
RT
(P
0 )
exp
.!
'
146
Journal of the Less-Common Met
and
Ea = 18.9 ± 1.8 kcal mol- 1
Pt 5An (An= Am, Cm, Bk, C
4700
log(P0 ) = - - - + 9.47
T
a
= 1860 min- 1
The activation energy measured here is in good agreement with that
measured at much lower temperature (50- 200 °C) [3]. Both the functional
time dependence and the functional pressure dependence of the decomposi·
tion kinetics indicate that the dehydriding reaction of uranium hydride is
controlled by the movement of the metal-metal hydride phase interface in
the solid.
••
'
•••
Acknowledgment
This work was supported by the U.S. Department of Energy under
contract No. DE-AC04-76DP00789.
References
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135.
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V. M. RADCHENKO, V . D. SHU:
L. 8. LEBEDEV A, V. Ya. VASIL.
V.I. Lenin Research Institute of,
(Received March 23, 1989; in revi
Summary
X-ray studies of Pt 5An .
on a platinum substrate have
hexagonal structure of the Cu
been measured. X-ray arnorp
120 h) and Pt 5 249Cf (for 70 <
perature and its recovery afu
been observed in the Pt 5Bk 1<
140-day exposure. It is connE
~-decay and the appropriate i
relation between the Pt 5An •
demonstrated.
1. Introduction
Platinum-base alloys witl
sources. Such alloys consist c
metallics. Since the intermet.
source, a comprehensive stud'
of internal irradiation is essent~
Pt 5Am and Pt 5Cm interr
Erdmann and Keller [ 2) in th,
an orthorhombic structure of t
intermet.allics can exhibit a he
Data on Pt 5Bk and Pt 5Cf struct
2. Experimental details
To prepare intermetallic:
l44Cm , 24~k , 249Cf. Th ese ac t·11
strate of diameter 5 - 8 mm an
placed in a molybdenum heat•
0022-5088/90/$3.50