Materials Transactions, Vol. 54, No. 12 (2013) pp. 2233 to 2237
© 2013 The Japan Institute of Metals and Materials
Low Temperature and Pressure Synthesis of Lithium­Nitride Compound
with H2O Addition on Lithium Target for BNCT
Shintaro Ishiyama1,+, Yuji Baba1, Ryo Fujii2, Masaru Nakamura2 and Yoshio Imahori2
1
Quantum Beam Science Directorate, Japan Atomic Energy Agency, Naka-gun, Ibaraki 319-1195, Japan
Cancer Intelligence Care Systems, Inc., Tokyo 135-0063, Japan
2
Low temperature synthesis of lithium­nitride compound was conducted on the lithium target for BNCT by N2/H2O mixing gas squirt in
the ultra high vacuum chamber, and the following results were derived. (1) Lithium­nitride compound was synthesized on the lithium target
under 101.3 Pa N2 gas squirt at room temperature and in the ultra high vacuum chamber under the pressure of 1 © 10¹8 Pa. (2) Remarkable
contamination by O and C was observed on the lithium­nitride compound synthesized under the squirt pressure of 13.3­80 Pa/1.33­4.7 Pa N2/
H2O mixing gas. (3) No contamination and synthesis of Li­N compound was observed under the squirt pressure of 0.013­0.027 Pa/0­0.005 Pa
N2/H2O mixing gas. (4) Contamination by O and C was enhanced with excessive addition of H2O at the pressure of over 1.33 Pa.
[doi:10.2320/matertrans.M2013242]
(Received June 26, 2013; Accepted September 25, 2013; Published November 9, 2013)
Keywords: boron neutron capture therapy, neutron source, lithium target, lithium nitride, nitrogen gas, contamination, H2O addition
1.
Introduction
Implemented deployment of accelerator-driven neutron
source for Boron Neutron Capture Therapy (BNCT) is
scheduled in 2013 in National Cancer Center, Japan. This
BNCT system was designed with the production of neutrons
via threshold 7Li (p, n) 7Be reaction at 25 kW proton beam
with energy of 2.5 MeV and starts its installation at middle
of 2013.
Many types of pilot innovative accelerator-based neutron
source for neutron capture therapy with lithium target were
designed1­4) and these designs face serious problems such as
evaporation of lithium with the progressive power run-up.
In the previous paper, we have proposed that the
evaporation can be reduced by synthesis of Li3N on the
surface of Li target exposed to proton beam, because lithium
nitride is thermally very stable up to 1086 K and exhibited
Li3N synthesis on lithium target by in-situ Li deposition and
ion implantation technique.5)
The conceptual lithium target model for BNCT is
illustrated in Fig. 1(c). Heat load receiving area of the target
is consisted of Li target (³100 µ mt) with Li3N thin layer and
copper substrate.
There are many reports4­9) about nitridation techniques of
lithium, and direct synthesis of Li3N in low temperature and
pressure N2 gas with the presence of H2O and O2 is also one
of very attractive nitridation techniques6­8) in practical use for
BNCT target production.
However, very high level of oxygen and carbon contaminations on the lithium­nitride compound layer surface was
reported in previous low temperature direct synthesis study
under the ultra-high vacuum condition.8,9)
Therefore, present paper primarily intends to ascertain the
cause of these contaminations observed in direct synthesis
of the Li­N compounds on lithium in nitrogen gaseous
atmosphere with H2O addition. The surface condition of
the lithium­nitride compounds was characterized by X-ray
+
Corresponding author, E-mail: [email protected]
Fig. 1 Procedure of nitridation of Li target surface on Cu; (a) Li deposition
process on Cu target, (b) N2 gas squirt with H2O and (c) Li­N compound
formation on Li surface.
photoelectron spectroscopy (XPS) using X-rays from synchrotron light source.
2.
Experimental Method
2.1 Specimens
High-purity copper (Cu) plates (5 mm © 5 mm © 1 mmt)
were used as a substrate. As a source material for deposition,
metallic lithium rod (5 mm¤ © 8 mm) purchased from
Kojundo Chemical Laboratory Co. Ltd. was used. Purity
of the lithium was higher than 99.98% and Na(0.004%),
Ca(0.006%),
K(0.001%),
Fe(0.001%),
Si(0.001%),
N(0.006%) and Cl(0.001%) were contained in this pellet.
2.2 Apparatus
Experiments4,5,8,9) were performed at the BL-27A station
of the Photon Factory in the High Energy Accelerator
2234
S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori
Research Organization (KEK-PF). The X-rays were emitted
from the bending magnet, and the photon energy was tuned
by an InSb (111) double crystal monochromator. The energy
resolution of the monochromator was 0.9 eV at 2000 eV.
The analysis chamber consisted of a manipulator, an
electron energy analyzer, and a cold cathode ion gun. The
base pressure of the analysis chamber was 1 © 10¹8 Pa. The
preparation chamber consisted of a vacuum evaporator and a
sample transfer system. The base pressure of the preparation
chamber was 1 © 10¹6 Pa. The sample can be transferred
between two chambers without exposing the sample to air.
XPS spectra were measured with hemispherical electron
energy analyzer (VSW Co. Class-100). The X-rays were
irradiated at 55 degree from surface normal and a take-off
direction of photoelectrons was surface normal. Typical
photon energy used was 2000 eV. An X-ray tube with yttrium
anode (Y M¦ line, h¯ = 132.3 eV) was also used to measure
Li 1s lines. The binding energy was normalized by C 1s of
adventitious organic carbons adsorbed on the samples at
284.8 eV.
2.3 Lithium deposition
The evaporator consisted of a tantalum crucible surrounded by the spiral type tungsten filament. The crucible
was floated at +1.5 kV, while the filament was grounded.
Therefore, the crucible was heated by the bombardment of
1.5 keV electrons. The distance between the crucible and the
substrate was 50 mm. A shutter that is electrically isolated
from the ground was equipped between the crucible and the
substrate in order to precisely control the evaporation rate of
the source material. Since a part of the evaporated lithium
atoms is ionized due to the surface ionization, a positive
current was observed at the shutter. The thickness of the film
was precisely determined by the product of the shutter current
and the evaporation time that was calibrated by XPS
measurements. The vacuum pressure during the lithium
deposition was 1.3 © 10¹4 Pa, and the deposition time of
lithium was 50 min. Figure 2 shows experimental situation of
lithium deposition on Cu specimen in the main chamber.
Fig. 3
Nitridation procedure with H2O at room temperature
Nitridation procedure of Li/Cu target was illustrated in
Figs. 1 and 3 shows the gas mixing apparatus connected in
the main chamber, in which Li/Cu specimen was installed
and N2 gas and H2O was supplied from a bottle of
compressed nitrogen and a glass test tube, respectively.
Partial pressure of N2 gas and H2O was measured by Pirani
gage and N2/H2O mixing gas was squirted out of the nozzle
to the Li/Cu target under the pressure combination of 0.01­
101.3 Pa/0­4 Pa. Exposing time of the Li/Cu was controlled
within 5­60 min.
2.4
3.
Results and Discussion
3.1 Chemical conditions of lithium deposition surface
Figures 4(a) and 4(b) show XPS scan spectra for the
copper surface before and after Li deposition, respectively.
The pressure during the deposition was 3 © 10¹4 Pa and the
deposition time was 20 min. Narrow scan in Li 1s region after
Li-deposition is also shown as small inset in Fig. 4(b). After
the deposition, the intensity of the Cu 2p peak from the
copper substrate decreased, and O 1s, C 1s and Li 1s peaks
were observed. Here, this spectra pattern (Fig. 4(b)) with low
Fig. 2 Lithium deposition situation in preparation chamber.
Main chamber with N2/H2O gas mixing apparatus.
Low Temperature and Pressure Synthesis of Lithium­Nitride Compound with H2O Addition on Lithium Target for BNCT
2235
(a)
(b)
Fig. 5 XPS semi wide-scan spectra for the Li/Cu target exposed to N2 gas;
(a) 0, (b) 5 and (c) 60 min and was categorized as pattern B.
Table 1 Testing conditions of nitridation synthesis on lithium target.
H2O
pressure
(Pa)
Exposing
time
(min)
Categorized
pattern
IN1s/IO1s
101.3
0
5, 60
B
0.3­3
101.2
0.1
5
B
2
80
1.33
5
C
0.03
N2 pressure
(Pa)
Fig. 4 (a) XPS wide-scan spectra for copper surface, (b) XPS wide-scan
spectra for copper surface after the Li deposition and was categorized as
pattern A.
level of O and C contamination without N1s peak is
categorized as pattern A. The higher intensities of the O 1s
and C 1s peaks compared with that of the Li 1s peak is due to
the extremely low photoionization cross sections of Li 1s by
2000 eV photons. In the previous work,4,5) we have shown
that O 1s and C 1s peaks for the Li-deposited sample come
from the water and carbonates adsorbed on Li surface after
the Li-deposition, and the main chemical states of lithium is
not Li2O but metallic lithium.
The considerable decrease of the Cu 2p peak after the
Li-deposition suggests that copper surface was covered with
fairly thick film of lithium. The photon energy used for XPS
measurements was 2000 eV, so the kinetic energy of the Cu
2p photoelectrons was about 1070 eV. Considering that the
inelastic mean free path (IMFP) of 1070 eV electrons in solid
lithium is about 1.3 nm,4,5) it is suggested that the thickness of
the lithium layer was fairly larger than this value.
3.2
Contamination on the lithium­nitride compound
surface
According to the results,6­8) the presence of H2O in N2 gas
is significant to promote nitridation of Li and believed to be
assistant agent in nitridation chemical reaction between Li
and N2 gas. Therefore, to establish a role of H2O addition
to these reactions, N2/H2O mixing gas was squirted to the
Li/Cu target surface in present study.
27
4.7
5
D
³0
13.3
0.027
4.0
0.005
5
5
D
A
³0
0
0.013
0.004
5
A
0
0.013
0
5
A
0
Figure 5 shows the XPS semi-wide scan spectra of the Li/
Cu target after 101.3 Pa N2 gas squirt for 5 and 60 min. O 1s,
N 1s and Li 1s peaks were observed and the intensity of the
O1s peak decreased with nitridation time, whereas N1s peak
increased. This spectra pattern is here categorized as
pattern B.
The binding energy of the peak of N 1s and Li 1s is
identified as 391.3 and 54.5 eV in the figure. From the results
of previous work, the binding energy of the Li 1s peak is
52.8 eV due to the metallic Li and 2.3 eV chemical shift was
observed after nitridation. So, we assigned that the Li 1s peak
observed in Fig. 5 originated from lithium nitride compound.
Figure 6 shows XPS wide scan spectra of Li/Cu target
after 80 Pa/1.3 Pa N2/H2O mixing gas squirt for 5 min and
very higher intensity of the O 1s and C 1s peaks were
observed with very small peak of Li 1s. This spectra pattern
is categorized as pattern C. Remarkable contamination by O
and C was observed in this case, we discuss the contamination level on the nitridation conditions and use the intensity
of the photoelectrons, I and intensity ratio, IN1s/IO1s as
contamination parameter, and are listed in Table 1. From
the table, it is found that the ratio IN1s/IO1s decreases with
increase of H2O pressure above 13.3 Pa N2. These results
mean that there is a progression of contamination by O and C
on Li­N compound surface with H2O addition.
2236
S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori
Fig. 6 XPS spectra of Li specimens after 80 Pa/1.3 Pa N2/H2O squirt for 5 min. N1s peak was clearly observed after nitridation with O
and C contamination and was categorized as pattern C.
Fig. 8 XPS spectra of Li specimens before nitridation. Remarkable
contamination by O and C was observed with Li1s peak and was
categorized as pattern D.
Fig. 7 Li/Cu targets after N2/H2O squirt; (a) after Li deposition, (b) 80 Pa/
1.3 Pa N2/H2O mixing gas squirt and (c) 1­3 min after squirt.
A few 10 s after 80 Pa/1.3 Pa N2/H2O mixing gas squirt,
the colour of Li/Cu surface was immediately changed from
silver to black, then black colour was faded into transparent
colour with white-tinged within few minutes, as shown in
Fig. 7.
In the case of 27 Pa/4.7 Pa N2/H2O mixing gas exposure
for 5 min, the same spectra pattern as pattern C was obtained
except N1s peak shown in Fig. 8 and this pattern with high
level of contamination by O and C without N1s peak is
categorized as pattern D.
These results indicated that there are two types of reaction
processes between lithium, H2O and N2.
At first, N2 gas causes a chemical reaction with lithium as;
6Li þ 3N2 ) 2Li3 N
ð1Þ
After then, Li3N was decomposed in the presence of
carbonates above-mentioned in Section 3.1.
2Li3 N þ 3CO2 þ 3H2 O ) 3Li2 CO3 þ 2NH3
Fig. 9 Contamination process of the lithium­nitride compound surface;
observed by XPS spectra of (a) pattern A, (b) pattern C and (c) pattern D.
ð2Þ
So, we tentatively assign that the O1s, C1s and Li1s peaks
observed in pattern B, C and D originate from Li2CO3.
These contamination processes are illustrated in Fig. 9.
After formation of lithium­nitride compound on the surface of
lithium target (pattern B), the surface of the Li­N compound
is partially over-layered by Li2CO3 with H2O addition
(Pattern C) and these over-layers expand on entire surface
of Li­N compound by excessive addition of H2O (pattern D).
Low Temperature and Pressure Synthesis of Lithium­Nitride Compound with H2O Addition on Lithium Target for BNCT
On the contrary, the same spectra pattern as pattern A with
lower intensity of O 1s and C 1s peaks was obtained in the
case of 0.013­0.027 Pa/0­0.05 Pa N2/H2O. These results
mean that there is no contamination and nitridation reaction
expressed in eqs. (1) and (2) due to inadequate supply of N2
and H2O.
4.
Conclusions
To prevent vaporization damage of BNCT (Boron Neutron
Capture Therapy) lithium target during operation, synthesis
of lithium­nitride compound was conducted on the lithium
target by N2/H2O mixing gas squirt in the ultra high vacuum
chamber and the structures, chemical states of nitridated
zone formed on the lithium surface were characterized by
XPS, and the following results were derived;
(1) Lithium­nitride compound was synthesized on lithium
target under 101.3 Pa N2 gas squirt at room temperature and
1 © 10¹8 Pa in the ultra high vacuum chamber.
(2) Remarkable contamination by O and C was observed
on the lithium­nitride compound over-layer synthesized by
squirt with N2/H2O mixing gas under the pressure combination of 13.3­80 Pa/1.33­4.7 Pa.
(3) No contamination and synthesis of Li­N compound
was observed under 0.013­0.027 Pa/0­0.005 Pa N2/H2O
squirt.
(4) Contamination of O and C was enhanced with
excessive addition of H2O over the pressure of 1.33 Pa.
(5) XPS observation suggested that Li2CO3 over-layered
on the surface of Li­N compound.
From these results, it is concluded that lithium is very
sensitive to the existence of H2O in nitrogen gas, however no
evidence was found to support the fact that H2O is believed
2237
to be assistant agent in nitridation chemical reaction between
Li and N2 gas at room temperature.
Acknowledgement
The authors would like to thank the staff of the KEK-PF
for their assistance throughout the experiments. They also
thank the members of the Surface Reaction Dynamics
Research Group, Quantum Beam Science Directorate, Japan
Atomic Energy Agency for their helpful discussion and
experimental supports. The work has been conducted under
the approval of Photon Factory Program Advisory Committee (PF-PAC 2012G175).
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