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Low Temperature and Pressure Synthesis of Lithium Nitride Compound with H2O Addition on Lithium Target for BNCT

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Low Temperature and Pressure Synthesis of Lithium

­

Nitride Compound

with H

2

O Addition on Lithium Target for BNCT

Shintaro Ishiyama

1,+

, Yuji Baba

1

, Ryo Fujii

2

, Masaru Nakamura

2

and Yoshio Imahori

2

1Quantum Beam Science Directorate, Japan Atomic Energy Agency, Naka-gun, Ibaraki 319-1195, Japan 2Cancer Intelligence Care Systems, Inc., Tokyo 135-0063, Japan

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 N2gas squirt at room temperature and in the ultra high vacuum chamber under the pressure of 1©10¹8Pa. (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 byin-situLi 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 N2gas with the presence of H2O and O2is also one of very attractive nitridation techniques6­8)in practical use for BNCT target production.

However, very high level of oxygen and carbon contam-inations 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

photoelectron spectroscopy (XPS) using X-rays from syn-chrotron 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

Fig. 1 Procedure of nitridation of Li target surface on Cu; (a) Li deposition process on Cu target, (b) N2gas squirt with H2O and (c) Li­N compound

formation on Li surface.

[image:1.595.323.527.322.531.2]
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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©10¹8Pa. The preparation chamber consisted of a vacuum evaporator and a sample transfer system. The base pressure of the preparation chamber was 1©10¹6Pa. 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 1slines. The binding energy was normalized by C 1sof adventitious organic carbons adsorbed on the samples at 284.8 eV.

2.3 Lithium deposition

The evaporator consisted of a tantalum crucible sur-rounded 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 thefilm 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©10¹4Pa, and the deposition time of lithium was 50 min. Figure 2 shows experimental situation of lithium deposition on Cu specimen in the main chamber.

2.4 Nitridation procedure with H2O at room

temper-ature

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.

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©10¹4Pa 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.

[image:2.595.311.542.384.519.2] [image:2.595.154.442.575.767.2]
(3)

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 thickfilm 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 N2gas 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.

Figure 5 shows the XPS semi-wide scan spectra of the Li/ Cu target after 101.3 Pa N2gas 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 thefigure. 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 contami-nation 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.

(a)

(b)

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.

Fig. 5 XPS semi wide-scan spectra for the Li/Cu target exposed to N2gas;

[image:3.595.310.540.68.248.2]

(a) 0, (b) 5 and (c) 60 min and was categorized as pattern B.

Table 1 Testing conditions of nitridation synthesis on lithium target.

N2pressure

(Pa)

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

27 4.7 5 D ³0

13.3 4.0 5 D ³0

0.027 0.005 5 A 0

0.013 0.004 5 A 0

[image:3.595.56.284.70.407.2] [image:3.595.305.549.319.452.2]
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[image:4.595.133.460.66.215.2] [image:4.595.311.541.267.425.2]

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.

Atfirst, N2gas causes a chemical reaction with lithium as;

6Liþ3N2)2Li3N ð1Þ

After then, Li3N was decomposed in the presence of carbonates above-mentioned in Section 3.1.

2Li3Nþ3CO2þ3H2O)3Li2CO3þ2NH3 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).

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.

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. 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.

[image:4.595.55.284.269.457.2] [image:4.595.304.548.483.688.2]
(5)

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 N2gas squirt at room temperature and 1©10¹8Pa 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 combi-nation 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

to be assistant agent in nitridation chemical reaction between Li and N2gas 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 Commit-tee (PF-PAC 2012G175).

REFERENCES

1) B. Bayanov, V. Belov, V. Kindyuk, E. Oparin and S. Taskaev:Appl. Radiat. Isot.61(2004) 817­821.

2) C. Willis, J. Lenz and D. Swenson: Proc. LINAC08, Victoria, BC, Canada, MOP063, (2008) pp. 223­225.

3) S. Halfon, M. Paul, A. Arenshtam, D. Berkovits, M. Bisyakoev, I. Eliyahu, G. Feinberg, N. Hazenshprung, D. Kijel, A. Nagler and I. Silverman:Appl. Radiat. Isot.69(2011) 1654­1656.

4) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori: Nucl. Instrum. Meth. Phys. Res. B288(2012) 18­22.

5) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori: Nucl. Instrum. Meth. Phys. Res. B293(2012) 42­47.

6) T. Yamamoto, S. Yoshikawa and M. Koizumi: Yogyo-Kyokai-Shi 93

[11] (1985) 68­71 (in Japanese).

7) M. Hasegawa, T. Sekine and N. Nakayama: Report of National Industrial Research Institute of Nagoya, Vol. 28, No. 12 (1979) pp. 388­393. 8) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori:Mater.

Trans.54(2013) 1765­1769.

Figure

Fig. 1Procedure of nitridation of Li target surface on Cu; (a) Li depositionprocess on Cu target, (b) N2 gas squirt with H2O and (c) Li­N compoundformation on Li surface.
Fig. 2Lithium deposition situation in preparation chamber.
Fig. 4(a) XPS wide-scan spectra for copper surface, (b) XPS wide-scanspectra for copper surface after the Li deposition and was categorized aspattern A.
Fig. 7Li/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.

References

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