J. Plasma Fusion Res. SERIES, Vol. 10 (2013)

Effects of Li4TiO4 Structure on Tritium Release Kinetics from
Lithium-Enriched Li2+xTiO3
Makoto KOBAYASHI1, Kiyotaka KAWASAKI1, Katsuyoshi TATENUMA2, Masanori HARA3,
Masao MATSUYAMA3, Toshiyuki FUJII4,
Hajimu YAMANA4, Yasuhisa OYA1 and Kenji OKUNO1
1

Radioscience Research Laboratory, Faculty of Science, Shizuoka University, Japan
2
Kaken Co. Ltd., 1044, Hori, Mito-city, Ibaraki, 310-0903, Japan
3
Hydrogen Isotope Research Center, University of Toyama, Toyama, Japan
4
Research Reactor Institute: Kyoto University, Kumatori, Japan(Received: 10 May 2012 / Accepted: 3 December 2012)

Keywords : Tritium, Li2TiO3, Li4TiO4, diffusion, de-trapping, TDS
The release kinetics of tritium for Li2+xTiO3 (x = 0, 0.2, 0.4) and Li4TiO4 were studied by means of Thermal Desorption
Spectroscopy (TDS). Tritium-TDS spectrum at the heating rate of 0.5 K / min for Li2+xTiO3 irradiated with thermal
neutron consisted of two release stages at 480 K and 580 K, namely Peaks 1 and 2, respectively. The activation energy ofPeak 1 was estimated to be around 0.37 eV, while that of Peak 2, about 0.63 eV. The latter corresponded to the activationenergy of tritium diffusion in Li2TiO3. For Li4TiO4 exposed to tritium-deuterium mixture gas, two tritium release stages
named as Peaks A and B were also observed at 450 K and 600 K, respectively, in tritium-TDS spectrum at the heating rateof 0.5 K / min. As the release temperature regions of Peak 1 and Peak A were almost the same, tritium releases of thesepeaks were considered to be originated from a same process. O-T bonds formed on the surface of Li4TiO4 were
decomposed as Peak B in TDS spectra.

1. Introduction
For the development of D-T fusion reactors, it isimportant to establish an effective fuel recycling system
and a comprehensive model of tritium migration processes
in solid tritium breeding materials. In the test blanket
module for ITER, lithium titanate (Li2TiO3), one ofternary lithium oxides, is a candidate of tritium breeding
materials because of good tritium release property,
chemical stability, and so on [1]. During the operation of
D-T fusion reactor, energetic tritium is generated by thereactions of 6Li(n, D)T and 7Li(n, nD)T in Li2TiO3 andlithium will be consumed by these reactions, resulting inlithium-depletions in Li2TiO3. As lithium-depletion in
Li2TiO3 induces lower tritium generation rate, thelithium-depleted tritium breeding materials will be
replaced after certain periods of operations.
The use of Li2+xTiO3 (x=0.2, 0.4), which is thelithium-enriched Li2TiO3, is proposed recently to improve
the life cycle of tritium breeding materials with respect totritium generation property [2-4]. Li2+xTiO3 consists of
Li2TiO3 structure and excess lithium: i.e. Hara et al. [5],reported that both of Li2.2TiO3 and Li2.4TiO3 were themixture of Li2TiO3 and Li4TiO4 structures, indicating that
excess lithium induced the formation of Li4TiO4 structure
in Li2TiO3 structure during fabrication.
In our previous study, tritium release behaviors and theirkinetics for Li2+xTiO3 irradiated with thermal neutron were
investigated by means of TDS [6]. Tritium-TDS spectrum
for Li2.0TiO3 at the heating rate of 0.5 K/min consisted ofsingle release stage at around 580 K. An additional releasestage was observed at around 480 K for the tritium-TDS
spectra for Li2.2TiO3 and Li2.4TiO3, although the major
release stage was located at 580 K, which was assigned tothe tritium release controlled by tritium diffusion in
Li2TiO3 structure from kinetic analyses. In addition, theamount of tritium released at 480 K was increased withincreasing lithium concentration. It was considered that
this additional tritium release stage would be associatedwith excess lithium, that is, Li4TiO4 structure. However,
the effects of Li4TiO4 structure on tritium release forLi2+xTiO3 are still unclear because tritium releasemechanisms for Li4TiO4 have not been studied in detail.
Therefore, in this study, tritium release behavior for
Li4TiO4 was studied by means of TDS. Tritium was
generated in Li4TiO4 by thermal neutron irradiation or
introduced by thermal tritium gas absorption. The tritium
release kinetics for Li2+xTiO3 irradiated with thermal
neutron was also evaluated for comparison.

2. Experimental
Powders of Li4TiO4 and Li2+xTiO3 purchased from Kaken
Co. were used as samples. The average grain diameters of
Li4TiO4, Li2.0TiO3, Li2.2TiO3 and Li2.4TiO3 measured by
SEM (Scanning Electron Microscope) were 12, 3.0, 1.0
and 1.0 μm, respectively. For thermal neutron irradiation
experiments, 0.3 g of these samples were introduced into

f5144008@ipc.shizuoka.ac.jp

7©2013 by The Japan Society of Plasma
Science and Nuclear Fusion Research

M. Kobayashi et al., Effects of Li4TiO4 Structure on Tritium Release Kinetics from Lithium-Enriched Li2+xTiO3

He

M.F.

CH4

M.F.

MS-5A

Furnace

Cu

SampleProportional
Counter 1

MS-5A

Stack

Proportional
Counter 2

CuO

Water bubbler 䐠

Water bubbler 䐟

Fig. 1 Schematic drawing of TDS systemof oxidized T and reduced T trapped in WB1 and WB2,
respectively, are quantified by a Liquid Scintillation
Counter (ALOKA LSC-5100). The CH4 gas at the flowrate of 25 sccm was used as a counting gas for the
proportional counters. The CH4 gas was humidified bypassing through another water bubbler for reduction ofmemory effect in proportional counters. The humidified
CH4 gas was converged to He gas flow just before PC1 sothat humidified CH4 is not purged directly to Mo cruciblewith sample during heating. The quartz tube, stainlesstubing of this system and the proportional counters werealso heated at around 373 K to reduce the memory effect.
In the tritium-TDS experiments, the samples were heated
up to 1173 K. Isothermal heating experiments were also
conducted around 600-700 K.
FT-IR (Fourier Transform Infrared Spectroscopy)
measurements were also carried out for Li4TiO4 exposed
to pure D2 gas at Shizuoka University to reveal chemical
states of deuterium in Li4TiO4. Changes of crystal
structure of Li4TiO4 by D2 gas exposure were also
investigated by X-ray diffraction (XRD) technique atUniversity of Toyama.

quartz tubes individually and annealed at 1073 K for
Li4TiO4 and 1173 K for Li2+xTiO3 under He gas with thepressure of less than a few Pa for 3 h. After heating, thesetubes were sealed under vacuum. Thermal neutron
irradiations were conducted at Pneumatic tube 2 (Pn-2) of
Research Reactor Institute, Kyoto University (KURRI).
The thermal neutron flux was 5.5 × 1012 n cm-2 s-1. The
thermal neutron irradiation was carried out for 10 min,corresponding to the thermal neutron fluence of 3.3 × 1015
n cm-2. The temperatures of the samples during irradiation
were estimated to be lower than 350 K.
For hydrogen isotope gas exposure, Li4TiO4 sample washeated at 1073 K for 3 h in vacuum under the pressurearound 10-5 Pa, then sample temperature was kept at 673
K. Thereafter, the tritium-deuterium mixture gas
consisting of D2 and DT (T/D = 0.005) or the pure D2(isotopic purity: 99.4%) gas was introduced into thesample with the total pressure of 200 Pa for 1 day. Afterthe hydrogen isotope gas exposure, the sample wasquickly cooled down by water and liquid nitrogen.
Out-of-pile tritium release experiments were performedin tritium-TDS system at Shizuoka University as shown in
Fig. 1. The sample put in Mo crucible was set in quartz
tube surrounded by a ceramic furnace. The He gas at theflow rate of 15 sccm was introduced into MS-5A and aheated Cu at 623 K to exclude impurities such as waterand oxygen in He gas, and then purged into the system
during experiments. The released tritium is recovered bypurge gas and is carried to the first proportional counter(PC1). The volume of PC is 30 cm3, and the voltage of 2.4
kV was applied to PC during measurement. After PC1,
purge gas with tritium passes through first water bubbler(WB1) to trap the water-form tritium such as HTO and
T2O, named as “oxidized T”. The second proportionalcounter (PC2) set after WB1 measures gas-form tritium
such as HT and T2, named as “reduced T”. Thereafter, thereduced T are oxidized by heated CuO at 623 K after PC2
and trapped by second water bubbler (WB2). The amounts

3. Results and discussion
Fig. 2 shows the tritium-TDS spectra for Li2+xTiO3 at theheating rate of 0.5 K / min measured by PC1, representing
the overall release of tritium. These spectra can beseparated into two release stages, namely Peak 1 at 480 K
and Peak 2 at 580 K. The TDS spectra for Li2.0TiO3
consisted of one release stage as Peak 2, althoughadditional release stage of Peak 1 was also found forLi2.2TiO3 and Li2.4TiO3. The amount of tritium for Peak 1
was increased with increasing lithium concentration. To
understand the tritium release kinetics for these peaks, theheating rate dependence experiments were carried out. In
thisanalysis,
Kissinger-Akahira-Sunose(KAS)
model-free-kinetics method [6] which has been widely
applied to analyze TDS spectra was adopted in the present

8

㻌

20

Li2.2TiO3
Li2.4TiO3

10

2

Peak 250300

Peak 1
Peak 2-15.0

ln (E / Tp )

15

-14.5

Li2.0TiO3

Peak 1㻌-1

Tritium release rate / Bq s g-1

M. Kobayashi et al., Effects of Li4TiO4 Structure on Tritium Release Kinetics from Lithium-Enriched Li2+xTiO3

-15.5
-16.0
-16.5
-17.0
-17.5

400

500 600 700 800
Temperature / K
Fig. 2 Tritium-TDS spectra of Li2+xTiO3
at the heating rate of 0.5 K / min

900

1.4

-1
-1

Tritium release rate / Bq s g

Table 1 Peak temperatures of each peak for Li2.4TiO3
at various heating rates
TP for Peak 2 (K)

0.5

453

591

1

515

603

5

580

705

10

601

718

2.2

1.2 eV [11], indicating that Peak 1 would not be
associated with Li2TiO3.
Fig. 4 shows the tritium-TDS spectrum for Li4TiO4irradiated with thermal neutron measured at the heating
rate of 0.5 K / min. This spectrum was measured by PC1
and no tritium release was found by PC2, indicating that
almost all of tritium was released as water form. The
tritium-TDS spectrum of Li4TiO4 can be separated into
two release stages at 450 K and 600 K, and named as Peak
A and Peak B, respectively. Especially, FWHM of Peak B
was quite narrow compared to the other spectra as shownin Fig. 2. The peak temperature of Peak A was almost
consistent with that of Peak 1.
Isothermal heating experiments were done for Li4TiO4exposed to tritium-deuterium mixture gas to determine thetritium release kinetics for Li4TiO4. Because of the
exposure temperature of 673 K, which is higher than therelease temperature of Peak A, the release kinetics of Peak
B was examined in these experiments especially. Two
probable processes which control the tritium release in
Li4TiO4 are considered as diffusion and de-trapping
processes. In the diffusion limited release process, tritium
would diffuse slowly toward the surface and release on thesurface quickly. In the de-trapping limited one, tritium
would be initially trapped strongly by the trapping sites.
Tritium release would be initiated by de-trapping from the§ E ·§ RA · E a¸(1)
ln¨ 2 ¸ ln¨¨¸¨T ¸
© E a g (D ) ¹ RTP© p ¹
where R is gas constant, A is pre-exponential factor, Ea is
activation energy of reaction. The term of g(D) is amodel-function determined by rate limiting process ofreaction. The linear relation between ln(E/TP2) and 1/TP
was deduced as Eq. 1, and the slope of which isrepresenting -Ea/R. The peak separation analysis was done
for TDS spectra of Li2.2TiO3 and Li2.4TiO3 under various
heating rates, then peak temperatures for Peaks 1 and 2 ineach spectrum were estimated. Table 1 summarizes thepeak temperatures of Peaks 1 and 2 for Li 2.4TiO3 atvarious heating rates. The peak temperatures were shifted
toward higher temperature region with increasing heatingrate. Fig. 3 shows the ln(E/TP2) vs 1/TP plot for Peaks 1
and 2 for Li2.4TiO3. Liner relations between ln(E/TP2) and1/TP were found for each peak. The obtained activationenergies of these release peaks were evaluated to be 0.37
eV and 0.63 eV for Peaks 1 and 2, respectively. The
similar activation energies were also obtained forLi2.2TiO3. Especially, the activation energy of Peak 2 wasconsistent with that of tritium diffusion for Li2TiO3obtained in our previous study and other works [7, 8]. The
activation energy around 0.4 eV has not been reported for
Li2TiO3 with neutron irradiation [8-10]. In addition, theactivation energy for desorption of hydrogen isotopes
adsorbed on the surface of Li 2TiO3 was reported as about

TP for Peak 1 (K)1.8 -1 2.0
1000/Tp / K

Fig. 3 Analytical results of release kinetics
for Peaks 1 and 2 in Li2.4TiO3study. In this method, ln(E/TP2) are plotted against 1/TP for
the determination of activation energy, where E is theheating rate and TP is the peak temperature, based onfollowing equation,

Heating rate (K / min)

1.6

30

Peak A

20

Peak B

10
0
300

400

500 600 700
Temperature / K

800

Fig. 4 Tritium-TDS spectrum of Li4TiO4with thermal neutron irradiation

9900

M. Kobayashi et al., Effects of Li4TiO4 Structure on Tritium Release Kinetics from Lithium-Enriched Li2+xTiO3

trapping sites. The other processes are quite faster than thede-trapping process, so these processes are not apparent on
tritium release kinetics. The tritium release controlled bydiffusion process is expressed by Fick’s diffusion equation[12]. In particular for tritium release via diffusion in
spherical media, tritium release with respect to elapsed
time, t can be expressed in the following equation,
f§  Dn 2S 2 t ·61

¸
¨
D
1 2
exp
2
2
¸
¨
S
n
a

¹
©
n 1
where D is fraction of released tritium to total amount, a is
the radius of media, namely grain radius and D isdiffusivity. On the other hand, the rate of de-trapping
limited release should be controlled only by theconcentration of trapped tritium in the material, indicatingthat de-trapping limited release is the first-order reaction,
which is expressed as follows,k (1  D )

D

(3)

㻌

D

0.2
0

5
3
Time / 10 s

5

103Time / 10 s

15

decomposition energy of hydrogen from O-H bonds [14].
A FT-IR spectrum for Li4TiO4 exposed to pure D2 gas
showed the IR absorption in the wavenumber range of
2450-2750 cm-1 (not shown here). It was reported thatO-D bonds in lithium ceramics showed IR absorption inthis wavenumber region[15], indicating that O-D bonds
were formed in Li4TiO4 by D2 gas exposure. Therefore, it
was considered that tritium trapped as O-T bonds werede-trapped by the decomposition of O-T bonds andreleased as Peak B for Li4TiO4.
However, the activation energy of Peak 2 in Li2+xTiO3
was different from that of the decomposition of O-T bonds
in Li4TiO4. In addition, the tritium retentions as Peak 2 inLi2+xTiO3 were almost similar to that of Li2TiO3, althoughmore than half of tritium was retained as O-T bonds in
Li4TiO4 with neutron irradiation. These facts would
indicate that the tritium releases derived from O-T bonds
should be small in Li2+xTiO3.
The crystal structure of Li4TiO4 before and after D2 gas
exposure were compared by XRD method as shown in Fig.
7. In the measurements, silicon was used for reference.
The diffraction patterns of Li4TiO4 exposed to D2 gas werequite similar to that without D2 gas exposure, indicatingthat the crystal structure was not changed by D2 gas
exposure. These results indicated that LiOD structure(crystal) was not formed in Li4TiO4. It was reported that
Li4TiO4 is reactive with water vapor and forms O-H bondson the surface [4], considering that water vapor as
impurity was adsorbed on the surface of Li4TiO4 during
air exposure or TDS measurements, which would induce
the formation of O-H bonds. Similar results have beenobtained in Li2O [13]. The rate limiting process of tritium
release for Li2O was changed by heating temperature: i.e.
in a lower temperature region, tritium release wascontrolled by decomposition of surface O-T bonds. In ahigher temperature region, tritium release was governedby diffusion process. The rate limiting process waschanged at 570 K, and this temperature was closed to Peak
B appearing in this study, indicating that O-T bonds was
mainly formed on the surface of Li4TiO4 and decomposed

0.4

0.0

0

Fig. 6 Isothermal tritium release for Li4TiO4exposed to tritium-deuterium mixture gas

673 K
623 K
detrapping model
diffusion model

0.6

0.4

0.0

㻌

0.8

0.6

0.2

where k is the rate constant of reaction [13]. Comparing
the theoretical curve derived from Eqs. 1 and 2 withtritium release by isothermal heating, the rate limiting
process on tritium release was determined. Fig. 5 shows
the isothermal tritium release behavior at various
temperatures for Li4TiO4 exposed to tritium-deuterium
mixture gas. The theoretical curves by de-trapping limited
release model and diffusion limited release model werealso shown in this figure. As shown in Fig. 5, theoreticalcurves based on diffusion limited release model could
hardly reproduce isothermal tritium release for Li4TiO4.
The theoretical curves by de-trapping limited releasemodel could reproduce tritium release curve in thetemperature region around 600-700 K, as shown in Fig. 6,
indicating that tritium release for Li4TiO4 was controlled
by de-trapping process.
The activation energy of tritium de-trapping was derived
to be about 1.2 eV from the de-trapping rate constants at
various temperatures, which was close to the

1.0

673 K
648 K
623 K
598 K
detrapping model

0.8

¦

dD
dt

㻌

1.0

10

Fig. 5 Comparison of isothermal tritium release totheoretical curves derived from de-trappingand diffusion processes

10M. Kobayashi et al., Effects of Li4TiO4 Structure on Tritium Release Kinetics from Lithium-Enriched Li2+xTiO3Intensity / arb. unit

200

Before D2 exposure

‫ە‬

300

After D2 exposure

‫ە‬

‫ی‬

‫ی‬
‫ی‬

‫ی‬

‫ە‬

‫ە‬

‫ی‬

100
0
20

30

40

50
2 theta / deg.

60

70

80

Fig.7 X-ray diffraction patterns for Li4TiO4 before and after D2 gas exposure‫ ی‬indicates Li4TiO4 structure and ‫ ە‬indicates reference silicon [5]release stages named as Peaks A and B were also observed
at 450 K and 600 K, respectively, in tritium-TDS spectrumat the heating rate of 0.5 K / min. As the releasetemperature regions of Peak 1 and Peak A were almost thesame, tritium releases of these peaks were considered to
be originated from the same process. The O-T bonds were
easily formed on the surface of Li4TiO4, appearing as Peak
B in TDS spectrum.

as observed as Peak B. The small tritium retention as O-T
bonds in Li2+xTiO3 would imply that the amount of wateron the surface of Li4TiO4 in Li2+xTiO3 was small. Because
the Li2+xTiO3 was the mixtures of Li2TiO3 and Li4TiO4,
the effective surface area of Li4TiO4 in Li2+xTiO3 should
be small, resulting in the small retention of tritium as O-T
bonds.
Peak B appearing in the TDS spectrum of Li 4TiO4 could
be assigned to the release of tritium as O-T bonds in thisstudy. However, the assignment for Peak A was notcompleted although this peak also appeared in TDS
spectra of Li2+xTiO3. The water vapor on the surface of
Li4TiO4 may provide trapping sites for tritium as O-T
bonds. A part of tritium migrated from bulk region of
Li4TiO4 at lower temperatures, corresponding to tritium
release as Peak A, would be trapped on the surface atlower temperature region. Li4TiO4 would easily react withwater vapor in air, therefore, the experiments for Li4TiO4without any air exposure are required to elucidate thetritium retention and release as Peak A, and consequently
the relations of tritium retention as Peak 1 and the amountof excess lithium in Li2+xTiO3. The experiments in vacuum
system would be a promising way to exclude the effects ofsurface O-T bonds. Therefore, it was considered thathydrogen isotope gas exposure and subsequent TDS
experiments in the same vacuum system would reveal thetritium release mechanisms from Li4TiO4 in detail.

Acknowledgements
This work was supported by the Grant-in-Aid for
Scientific Research (A), KAKENHI 20246131.

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4. Conclusions
The release kinetics of tritium for Li2+xTiO3 andLi4TiO4 were studied by means of TDS. Tritium-TDS
spectrum with the heating rate of 0.5 K / min for Li2+xTiO3
irradiated with thermal neutron consisted of two releasestages at 480 K and 580 K, namely Peaks 1 and 2,
respectively. The activation energy of Peak 1 wasestimated to be around 0.37 eV, while that of Peak 2 wasabout 0.63 eV. The latter one corresponded to theactivation energy of tritium diffusion in Li 2TiO3. For
Li4TiO4 irradiated with thermal neutron, two tritium11