Direct Synthesis of Li3N Thin Layer on Lithium Target Surfacefor BNCT in N2 Gaseous Conditions

Shintaro Ishiyama1,+, Yuji Baba1, Ryo Fujii2, Masaru Nakamura2 and Yoshio Imahori2

1Quantum Beam Science Directorate, Japan Atomic Energy Agency, Naka-gun, Ibaraki 319-1195, Japan2Cancer Intelligence Care Systems, Inc., Tokyo 135-0063, Japan

To prevent vaporization damage of BNCT (Boron Neutron Capture Therapy) lithium target during operation, direct synthesis of Li3N thinlayer on lithium target surface was demonstrated in 0.1MPa N2 gas at temperature below 548K and the following conclusions were derived; (1)Synthesis of Li3N thin layer on lithium surface was confirmed after nitridation at 276­548K with surface contamination by oxygen and carbon.(2) Rapid nitridation over 1­5mass%/min was observed above Li melting temperature, whereas slow reaction under 0.02­0.5mass%/minbelow melting temperature. (3) During nitridation, removal of oxygen contamination on Li3N thin layer is taken place by nitrogen below Limelting temperature. [doi:10.2320/matertrans.M2013212]

(Received June 3, 2013; Accepted June 17, 2013; Published July 26, 2013)

Keywords: boron neutron capture therapy (BNCT), neutron source, lithium target, lithium nitride, nitrogen gas, contamination, argonsputtering

1. Introduction

Implemented deployment of accelerator-driven neutronsource for boron neutron capture therapy (BNCT) isscheduled in 2014 in National Cancer Center, Japan. ThisBNCT system was designed with the production of neutronsvia threshold 7Li (p, n) 7Be reaction at 25 kW proton beamwith energy of 2.5MeV and starts its installation at middleof 2013.

Many types of pilot innovative accelerator-based neutronsource for neutron capture therapy with lithium target weredesigned1­4) and these designs face serious problems such asevaporation of lithium with the progressive power run-up.

In the previous paper, we have proposed that theevaporation can be reduced by synthesis of Li3N on thesurface of Li target exposed to proton beam, because lithiumnitride is thermally very stable up to 1086K and exhibitedLi3N synthesis on lithium target by in-situ Li deposition andion implantation technique.5)

The conceptual lithium target model for BNCT isillustrated in Fig. 1. Heat load receiving area of the targetis consisted of Li target (100­200 µm t) with Li3N thin layer,micro grid mesh and cupper substrate. The lithium target isexposed to 2.5MeVaccelerated proton particles and heat loadis mainly released inside the cupper substrate. In this design,heat transfer with micro grid mesh and convection flow oflithium provides heat removal from lithium target.

There are many nitridation techniques of lithium werereported,6,7) and direct synthesis of Li3N in low temperatureand pressure N2 gas is also one of very attractive nitridationtechniques in practical use for BNCT target production.However, no report about this technique was foundconcerning the precise control conditions for lithium surfaceconversion to Li3N. Also, detailed characterization of theelectronic structure for lithium nitride obtained by thisprocess has not yet been reported.

Present paper demonstrates direct synthesis of Li3N onlithium surface in nitrogen atmosphere at low temperature

and reaction control conditions for this conversion wasinvestigated. The surface condition of the Li3N/Li wascharacterized by X-ray photoelectron spectroscopy (XPS)using X-rays from synchrotron light source.

2. Experimental Method

2.1 SpecimensMetallic lithium pellet (º5mm © 8mmL) purchased from

Honjo Chemical Co. Ltd. was used. Purity of the lithiumwas higher than 99.98% and Na(0.004%), Ca(0.006%),K(0.001%), Fe(0.001%), Si(0.001%), N(0.006%) andCl(0.001%) were contained in this pellet. Lithium specimens(5mm © 8mm © 1mm t) were cut-out from the pellets toensure smooth nitridation reaction on active Li surface.

2.2 Apparatus2.2.1 Globe box and bag

Globe box (GB-JV065; W650 © D500 © H450mm,SUS304 and reinforced glass, Globe-box, Japan) and globebag (S-20-20; W500 © D500 © H300, polyethylene, Gas-Col Co.) were used for nitridation of the Li specimens aboveand below 435K, respectively.2.2.2 Nitridation procedure

Globe box and bag were evacuated to ¹0.1MPa anddisplaced by argon gas up to 0.1MPa before nitridation. Lispecimens were installed in Petri-dish filled with N2 gas toavoid rapid nitridation reaction during operation, and removeto globe box and bag to heated up to 548K in 0.1MPa N2

gas (>99.9%) by digital hot staler (DP-1S, As-one, Co.).P2O5 power was also installed inside these box and bag forwater removal.2.2.3 XPS measurement

Experiments were performed at the BL-27A station of thePhoton Factory in the High Energy Accelerator ResearchOrganization (KEK-PF). The X-rays were emitted from thebending magnet, and the photon energy was tuned by an InSb(111) double crystal monochromator. The energy resolutionof the monochromator was 0.9 eV at 2000 eV.

+Corresponding author, E-mail: Ishiyama.shintaro@jaea.go.jp

Materials Transactions, Vol. 54, No. 9 (2013) pp. 1765 to 1769©2013 The Japan Institute of Metals and Materials

The analysis chamber consisted of a manipulator, anelectron energy analyzer, and a cold cathode ion gun. Thebase pressure of the analysis chamber was 1 © 10¹8 Pa. Theanalysis chamber was connected to the load-lock chamber.After the nitridation, the sample was put into the load-lockchamber, and the sample was transferred into the analysischamber without exposing to air.

XPS spectra were measured with hemispherical electronenergy analyzer (VSW Co. Class-100). The X-rays wereirradiated at 55 degree from surface normal and a take-offdirection of photoelectrons was surface normal. Typicalphoton energy used was 2000 eV. The binding energy wasnormalized by C 1s of adventitious organic carbons adsorbedon the samples at 284.8 eV.2.2.4 Argon sputtering

Argon sputtering to Li specimen surface after nitridation,was done using a cold cathode ion gun. High-purity argongas (>99.9%) was used as an ion source. The energy of theAr+ ions was first set at 0 eV. In this condition, the averagekinetic energy of the ions was about 30 eV due to thedischarge voltage. For second sputtering, the energy ofthe Ar+ ions was 3 keV. The typical ion flux was 1.4 ©1014 atom·cm¹2·s¹1, and the pressure during the argonsputtering was 1.2 ©10¹3 Pa. The time of each sputteringwas 10min. The direction of the ion beam was 45 degree.

3. Results and Discussion

3.1 Nitridation of lithium surfaceFigures 2(a), 2(b), 2(c) and 2(d) shows the photographs

of the Li samples before (Fig. 1(a)) and after nitridationat 423K for 1min (Fig. 1(b)), 75min (Fig. 1(c)) and548K (Fig. 2(d)) for 3 (right) and 15min (left) in N2 gas,respectively. The photograph of the Li sample afternitridation was also taken after exposing to air in a fewminutes and shown in Fig. 2(d). The colour of the specimensurface changed after nitridation from metal silver (Fig. 2(a))to blue (Fig. 2(b)) and a slightly shining, and turned into darkafter 75min (Fig. 2(c)) at 423K. At 548K for 3 and 15min,the specimens were melted to lose original shape with avariety of colours, as shown in Fig. 1(d).

Figure 2(e) shows surface of the specimen after airexposure. Surface colour changed into white after fewminutes air exposure, it indicated that LiOH was formed by

moisture in the atmosphere, resulting with decomposition ofLi3N with H2O in air.

Nitridation of the Li specimens were conducted at 308­548K for 1­600min in N2 gas and these conditions are listedin Table 1.

Figures 3 and 4 show the wide-scan XPS spectra for the Lispecimen before and after nitridation at 423K for 75min.Besides Li 1s, O 1s and C 1s peaks are seen before and afternitridation, and N 1s peak are observed after nitridation(Fig. 4).

From electron kinetic energy at 2000 eV, inelastic meanenergy path (IMFP) is calculated and the relationshipbetween electron energy and mean free path with measureattenuation lengths for various materials was given in Ref. 8).For the Li3N/Li target model in present study, IMFP isestimated in the order of 20¡ from the surface of Li3N layer.

The high intensity of the O 1s and C 1s peaks comparedwith that of the Li 1s and N 1s are due to the highphotoionization cross section of O 1s (1.75 © 104 barn) andC 1s (5.73 © 103 barn) by 2000 eV photons compared withthat of Li 1s (3.04 © 102 barn) and N 1s (1.05 © 104 barn),9)

(a)

(b) (c) (d)

(e)

423K

548K

Fig. 2 Nitridation process of Li specimen at 423K and 548K for 1­75minin N2 gas; (a) before nitridation, after nitridation (b) at 423K for 1min,(c) at 423K for 75min, (d) after exposure in air and (e) at 548K.

Table 1 Nitridation conditions of temperature and time at present experi-ment. Li specimen was installed in glove box and bag filled with 0.1MPaN2 gas.

SampleNo.

Temperature(K)

Reactiontime(min)

Weightchange¦W

(mass%)

BindingenergyLi1s(eV)

nN1s/nLi1s

#1 548 3 45.1 54.9 0.127

#2 548 15 78.2 54.9 0.085

#3 435 60 63.1 ® 0.301

#4 435 60 64.0 54.5 0.297

#5 423 1 2.4 53.8 0.080

#6 423 75 42.2 54.5 0.271

#7 353 60 25.3 58.2 0.171

#8 320 180 54.7 ® 0.635

#9 308 600 10.1 54.6 0.535

Mean(SD)

55.1(«1.33)

0.283(«0.18)

Ds-Cupper substrate

25kW × 2.5MeVproton

Li3N thin layerLi-target

Micro grid mesh

Convection flow

200µm

50µm

Fig. 1 Concept of multi-layered lithium target for BNCT.

S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori1766

and the O 1s peak observed in the spectrum for the Lispecimen seems to come from adsorbed water in the topsurface region of metallic Li.5) It is suggested that the topsurface region of lithium was covered with lithium oxides.

The XPS spectrum in the nitrogen region is shown as smallinset of Fig. 3. The binding energy of the peak of N 1s is391.3 eV, and binding energy of Li 1s is also identified as54.5 eV and the values of binding energy observed in presentexperiment are listed in Table 1.

There have been no reliable data about Li 1s and N 1sbinding energy for stoichiometric lithium nitride (Li3N). Inprevious paper,5) we measured the XPS spectra for N2

+-ionimplanted lithium layer that was deposited in vacuum. In thatwork, it was shown that before the N2

+ bombardment, thebinding energy of the Li 1s peak is 52.8 eV due to themetallic Li,10) and after nitridation, the binding energy shiftedto 55.1 eV, so the chemical shift is 2.3 eV. From Table 1, Li1s binding energies obtained in present study are located at55.1 « 1.33 eV, and it seems that Li­N compounds areformed with ionic bonds due to the high positive nature oflithium compared with nitrogen. So, we assigned that the Li1s peak observed in Fig. 3 originated from lithium nitride.

3.2 Chemical states of the nitridated layerIn order to estimate the stoichiometry of lithium nitride,

we also calculated the atomic ration of nitrogen to lithium,

nN/nLi on the basis of the XPS peak intensities. The value ofnN/nLi is calculated by5)

nN

nLi¼ IN1s=·N1s

ILi1s=·Li1s

ð1Þ

Where I (cps) is the intensity of the photoelectrons and· (barn) is the photoionization cross sections by 2000 eVphotons for the respective core levels and given as·N1s = 1.1 © 103 and ·Li1s = 3.0 © 102,9) respectively. ThenN/nLi ratio obtained by eq. (1) is to be 0.28 « 0.18 inTable 1.

The value is far from the ratio of lithium azide LiN3

(nN/nLi = 3.0),11­14) but rather close to that of lithium nitrideLi3N (nN/nLi = 0.33). The value is also close to that obtainedin the previous works for nitrogen-ion implanted lithium.5)

Although the exact composition is not clear only judgingfrom XPS peak intensities, we consider that the nearlystoichiometric Li3N layer is formed at the lithium surface,considering surface observation above-mentioned.

3.3 Formation of Li3N thin layer on the lithium surfacein N2 gas

The weight change ¦W (mass%) of the Li specimensafter nitridation were measured and listed in Table 1, andnitridation reaction rate, R (mass%/min) was evaluated as thefollowing equation.

�W ¼ Rt ð2ÞWhere t is nitridation time (min) and the reaction rate

evaluated by eq. (2) are listed in Table 2. We plot the ¦W asa function of nitridation temperature, which is displayed inFig. 5. From the figure, it is found that the reaction ratebehaves quite different below and above lithium meltingpoint Tmp (= 453K). The reaction rate increases drasticallywith reaction temperature in the case above Tmp, whereas thatincreases slowly below Tmp and these behaviours can beexpressed as the broken line in the figure. These resultsindicate that reaction between lithium and N2 gas is enhancedwith reaction temperature and facilitated above Tmp.

Here, on the assumption of uniform formation of Li3N thinlayer on the Li surface specimen, the thickness of the layer isestimated by the following eqs. (3) and (2) and the results arelisted in Table 2.

1500 1000 500 0

C 1s

O 1s

Inte

nsit

y (a

rb.u

nits

)

Binding energy, E /eV

Li 1s

b

Fig. 3 XPS spectra of Li specimens before nitridation. Remarkablecontamination by O and C was observed with Li 1s peak.

C 1s

O 1s

Inte

nsit

y (a

rb.u

nits

)

1500 1000 500 0

410 400 390 380

200

400N 1s

Inte

nsit

y (c

ps)

Binding energy, Eb

/eV

Li 1s

Binding energy, E /eVb

Fig. 4 XPS spectra of Li specimens after nitridation at 423K for 75min. N 1s peak was clearly observed after nitridation with O and Ccontamination.

Direct Synthesis of Li3N Thin Layer on Lithium Target Surface for BNCT in N2 Gaseous Conditions 1767

�W ¼ µLi3N

µLi� 1

� � ðt1 � LÞðt2 � LÞðt3 � LÞV0

� �ð3Þ

Where L, t1, t2, t3 and V0 are thickness of Li3N layer, thespecimen dimensions and volume, and µLi3N and µLi aredensity of Li3N and Li, respectively. The thickness, L areevaluated in the ordered of 5­360µm and correspondapproximately to the thickness value observed in Sec. 3.1.

3.4 Contamination on the surface of Li3N thin layerRemarkable contamination by O and C was observed after

nitridation in Sec. 3.1, we discuss the contamination levelon the nitridation conditions and use the intensity of thephotoelectrons, I and intensity ratio, IN1s/IO1s as contami-nation parameter. These data measured in present study islisted in Table 3, and we plot the IN1s/IO1s ratio as a functionof nitridation temperature, which are displayed in Fig. 6.From Fig. 6, it is found that IN1s/IO1s ratio decreases withincrease in nitridation temperature and reach to the bottomvalue above Tmp. These results mean that the replacement ofoxygen on the Li3N layer surface observed in Fig. 5, is takenplace by nitrogen during nitridation reaction and enhancedwith increasing temperature.

To reduce oxygen contamination on Li3N layer, the Li3Nlayer formed at 320K was sputtered by Ar+ ions. Theintensity ratio of IN1s/IO1s was 0.03 before sputtering. Theratio increased to 0.07 after the first sputtering (EK � 30 eV),and the ratio was not changed after second sputtering(EK = 3 keV). Since the sputtering yield of lithium by30 eVAr+ ions is less that 10¹2 atom/ion,15,16) it is suggestedthat the thickness of the surface lithium oxide layer is in theorder of nanometer.

4. Conclusions

Direct nitridation technique to create lithium target forBNCT apparatus was demonstrated in 0.1MPa N2 gas. Aftercreation of Li3N thin layer on lithium surface, the layerwas sputtered by Ar+ ion. The structures, chemical statesof nitridated zone formed on the lithium surface werecharacterized by XPS, and the following results were derived;(1) Uniform nitridation thin layer was formed on lithium

surface by direct nitridation technique.(2) During nitridation process, the colour of nitridation

surface changes from silver, blue and dark as nitridationproceeding.

(3) The chemical state of the nitridated zone was close tothe stoichiometric lithium nitride, Li3N and its surfaceis contaminated by O and C.

Table 2 Reaction rate and as a function of nitridation temperature afternitridation. Thickness of Li3N thin layer formed on Li surface wasestimated by eq. (3).

SampleNo.

Temperature(K)

Reactiontime(min)

Reactionrate R

(mass%/min)

Thickness ofLi3N thin layer

(mm)(Estimated)

#1 548 3 5.58 0.19

#2 548 15 5.58 0.36

#3 435 60 1.06 0.28

#4 435 60 1.06 0.28

#5 423 1 0.56 0.01

#6 423 75 0.56 0.17

#7 353 60 0.42 0.1

#8 320 180 0.03 0.24

#9 308 600 0.02 0.04

2

4

6

8

0300 400 500 600

Nitridation temperature, T/K

Rea

ctio

n r

ate,

R (

wt%

/min

)

Li melting temperature, Tmp

Fig. 5 The relation between the reaction rate, R and nitridation temper-ature. Reaction behavior quite different above and below Li meltingtemperature. Broken line is well expressed as R = 6e¹2T 7.7.

Table 3 The intensity of photoelectrons in nitridation Li samples obtainedin XPS spectra. IN1s/I01s ratio provides contamination level on Li3N thinlayer surface formed on Li specimen.

SampleNo.

Temperature(K)

Reactiontime(min)

IO1sIntensity(cps)

IN1sIntensity(cps)

IN1s/IO1sratio

#1 548 3 25,030 213.5 0.0085

#2 548 15 12,440 77.5 0.0062

#3 435 60 28,900 194.9 0.0067

#4 435 60 28,900 187.1 0.0065

#5 423 1 25,080 159.8 0.0064

#6 423 75 19,820 181.8 0.0092

#7 353 60 10,290 159.6 0.0155

#8 320 600 14,810 223.2 0.0151

#9 308 180 7,160 281.8 0.039

0.01

0.02

0.03

0.04

0.0300 400 500 600

Nitridation temperature,T/K

I N1s

/Io

1sra

tio Li melting point,

Tmp

Fig. 6 The relation between IN1s/IO1s ratio and nitridation temperature. Theratio decreased with increasing nitridation temperature and indicatedoxygen contamination proceeds. Lithium melting temperature is indicatedas broken line.

S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori1768

Acknowledgement

The authors would like to thank the staff of the KEK-PFfor their assistance throughout the experiments. They alsothank the members of the Surface Reaction DynamicsResearch Group, Quantum Beam Science Directorate,Japan Atomic Energy Agency for their helpful discussionand experimental supports. The work has been conductedunder the approval of Photon Factory Program AdvisoryCommittee (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. B 288 (2012) 18­22.

5) S. Ishiyama, Y. Baba, R. Fujii, M. Nakamura and Y. Imahori: Nucl.Instrum. Meth. Phys. Res. B 293 (2012) 42­47.

6) T. Yamamoto, S. Yoshikawa and M. Koizumi: Yogyo-Kyokai-Shi(in Japanese) 93 (1985) 728­731.

7) M. Hasegawa, T. Sekine and N. Nakayama: Rep. National IndustrialRes. Inst. Nagoya Vol. 28, No. 12, (1979) pp. 388­393.

8) M. P. Seah and W. A. Dench: Surf. Interface Anal. 1 (1979) 2­11.9) J. H. Scofield: Theoritical Photoionization Cross Section from 1 to

3000 eV; Lawrence Livemore Laboratory, Livemore UCRL-51326,1973.

10) S. Tanaka, M. Taniguchi and H. Tanigawa: J. Nucl. Mater. 283­287(2000) 1405­1408.

11) P. Marcus and M. E. Bussel: Appl. Surf. Sci. 59 (1992) 7­21.12) P. Y. Jouan, M. C. Peignon, Ch. Cardinaud and G. Lempérière: Appl.

Surf. Sci. 68 (1993) 595­603.13) J. C. Sánches-López, M. D. Alcalá, C. Real and A. Fernández:

Nanostruct. Mater. 11 (1999) 249­257.14) C. D. Wagner, J. F. Moulder, L. E. Davis and W. M. Riggs: IGGS,

Handbook of X-ray Photoelectron Spectroscopy, (Perking-ElmerCorporation, 1979, Eden Prairie, MN, USA).

15) H. H. Andersen: Appl. Phys. 18 (1979) 131­140.16) N. Matsunami, Y. Yamamura, Y. Itikawa, N. Itoh, Y. Kazumata, S.

Miyagawa, K. Morita, R. Shimizu and H. Tawara: At. Data Nucl. DataTables 31 (1984) 1­80.

Direct Synthesis of Li3N Thin Layer on Lithium Target Surface for BNCT in N2 Gaseous Conditions 1769