568 Journal of Nuclear Matcrialx 155- 157 (IYXXj 5~~~571

North-Holland. Am\tcrd;~m

IRRADIATION DAMAGE IN SOLID BREEDER MATERIALS

K. NODA I, Y. ISHII I, H. MATSUI *. H. OHNO ‘, S. HIRANO ’ and H. WATANABE ’

’ Japun Atomic Energv Research Institute, Tokai-muru, Naka-gun, Iburaki-km. 319 - Ii. Jup~n .’ Facult)* of Engineering, Nagciya University, Furo-cho. Chlkusa-ku, Nago.pcr. 464, Japun

The effect of irradiation on ion conductivity of lithium oxide (Li,O) was studied using oxygen ion (120 MeV) irradiation.

The conductivity at 489 K decreased with the oxygen ion fluence. while that at 443 K increased. From the thermal recovery

behavior of conductivity, the measured decrease at 489 K due to the irradiation was attributed to the Ft centers. On the other

hand. the measured increase of conductivity at 443 K during the irradiation was considered to arise from unspecified defects

which enhanced the conductivity. Also. a preliminary optical absorption study of lithium aluminate (y-LiAlO,) irradiated by

oxygen ions was done. No prominent absorption band due to the irradiation was observed for the specimens irradiated to

1.4~10~’ and 6.6X10’” ions/m2 by oxygen ions with energies of 1 and 120 MeV. respectively. The energy of the

fundamental optical absorption edge (band gap energy) for y-LiAlO, was found to he about 4.8 eV.

1. Introduction

In solid breeder materials of D-T fusion reactors a

large number of irradiation defects will be introduced

by the high energetic neutrons (up to 14 MeV), tritons (2.7 MeV) and helium ions (2.1 MeV) produced from

‘Li(n, a)3H reactions. These defects will induce swell- ing, cracking and mechanical property changes. Fui-

thermore, they may have a large influence on migration of tritium in the materials as well as effects on compati- bility with the structural materials, through diffusivity

changes in the composing atoms of the materials. Li,O and y-LiAlO, are attractive candidates for

solid breeder materials. Defects in Li,O irradiated with thermal neutrons and oxygen ions have been investi-

gated by spectroscopic methods including electron spin resonance (ESR) and optical absorption, to understand fundamental properties [I-S]. In addition. a damage study of y-LiAlO, irradiated with electrons has been done by ESR and electron microscopy 161.

In the present study, measurements of ion conductiv-

ity and optical absorption spectra of Li,O and y-LiAlO, irradiated with oxygen ions were conducted to get infor- mation which complemented the previous works.

2. Experimental

2. I. Ion conductivity measurements of Li,O

The specimens used were thin plates (7 to 9 mm in length, 8 to 9 mm in width, 0.3 to 0.4 mm in thickness) of Li,O single crystals [7]. The specimens were annealed at 1270 K for 10 h in a vacuum better than 1 X 10e3 Pa to eliminate OH- ions or LiOH in or on the specimens. After annealing, the specimens were quickly mounted on the stage for ion conductivity measurements in an irradiation vacuum chamber which was attached to a tandem accelerator at JAERI.

The specimens were irradiated by oxygen ions with an energy of 120 MeV. Penetration depth of the oxygen ions for Li,O was about 0.1 mm. Consequently, a third

0022-3115/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics ~blishing Division)

to a quarter of the specimens by depth was irradiated. Measurements of the ion conductivity along the direc- tion perpendicular to the thickness were conducted “in-situ” in the irradiation chamber with a two-terminal AC method using a HP Model 4192 A or 4194 A impedance analyzer after interrupting the irradiation. The conductivity was determined by measuring the

complex impedance in the range from 0.2 to 200 kHz.

2.2. Optical absorption measurements of y-LiAlO,

The specimens were thin disks (0.2 mm in thickness) of y-LiAlO, pellets (density: about 100% TD) which were made from high purity fine powder prepared by

hydrolysis of metal alkoxides [8]. The specimens were irradiated by oxygen ions with an energy of 1 or 120

MeV using a 2 MV Van de Graaff accelerator or the tandem accelerator at JAERI. Before and after the irradiation, optical absorption measurements were car- ried out at room temperature with a Cary Model 14 R spectrometer.

3. Results and discussion

3. I. Efjects of irradiation on ion conductivity of Li,O

Prior to irradiation, the conductivity of specimens was measured in the temperature range from 411 to 519 K in the irradiation chamber. The temperature depen- dence of the conductivity was similar to that extrapo- lated from the data of uni~adiated Liz0 single crystals in the study by Ohno et al. [9], although the values measured in the present study were slightly smaller.

The ion conductivity at a measurement temperature of 443 or 489 K was measured “in-situ” in the irradia- tion chamber at various ion fluences using one speci- men for each measurement temperature. Fig. 1 shows typical examples of relationships between the ion con- ductivity and measurement time at 443 K for various ion fluences. In each run the specimen was heated to the measurement temperature after interrupting the irradia-

K. Noda et al. / Irradiation damage in solid breeder materials 569

Lip0 Single Crystal Oxygen ion brad. (120MeV 1

Ian Conductivity Temp. ; 443 K

Fig. 1. Typical examples of relationships between the ion conductivity of Liz0 and measurement time at 443 K for

various oxygen ion fluences.

tion, since the specimen temperature during the irradia- tion was below 400 K. Each run was started when the

specimen temperature reached the measurement tem- perature. The conductivity decreased with the measure- ment time in each run, reaching constant values after 5000 to 10000 s. Such relationships between the conduc- tivity and the measurement time were also observed at 489 K. The decrease of conductivity with measurement time was assumed to be due to desorption of water from the surface of the specimen, which arose from adsorp- tion of water vapor in the irradiation chamber at tem- peratures lower than the measurement temperatures. This possibility was described in a preli~na~ study of ion conductivity changes due to irradiation [lo]. Conse- quently, the constant values of conductivity after 5000 to 10000 s can be regarded as the ion conductivity of the specimen,

Further evidence for the above-mentioned adsorp- tion-deso~tion behavior of water on the surface of Li,O in vacuum was provided by an experiment in which the behavior of water or OH- ions near the

surface of Liz0 single crystals in vacuum at various temperatures was studied with elastic recoil detection (ERD) using 2.0 MeV helium ions [II]. In that study, water or OH- ions retained on the surface of Li,O were released in the temperature range from 300 to 500 K by heating in-situ in a vacuum chamber for the ERD measurements. Adsorption occurred during subsequent cooling.

Fig. 2 shows the relationships between the ion con- ductivity and the oxygen ion fluence at 443 and 489 K.

The conductivity at 443 K increased with the fluence to

attain a constant value above a fluence of about 6 x lOI9

ions/m2. The conductivity at 489 K decreased continu- ously with fluence in the examined range (up to 3.45 x

lOI ions/m2). After irradiation, the recovery of ion conductivity

was investigated by isochronal annealing experiments. The conductivity was measured at 443 and 489 K after each annealing for 30 or 90 min. The recovery behavior

of the conductivity is shown in fig. 3. For measurements

at 489 K (each annealing period: 90 mm), the conduc- tivity increased in the annealing temperature range from

489 to 570 K, to be completely recovered to the level before the irradiation. On the other hand, for the mea- surement at 443 K (each annealing period: 30 min) the conductivity decreased once with the annealing temper- ature in the temperature range from 443 to 498 K, and then it increased with the annealing temperature in the range from 498 to 548 K.

The temperature range in which the conductivity at 443 and 489 K increased was almost the same as that for the recovery of the Ft center (an oxygen ion vacancy

trapping an electron) in Li,O [2-41. Accordingly, the increase of the conductivity at 443 and 489 K in fig. 3

can be attributed to the recovery of F+ centers. The decrease of the conductivity at 443 K during the anneal- ing may be due to unspecified irradiation defects which enhanced the conductivity in contrast with the Ff

centers and were recovered in the range 443 to 498 K.

Both the F+ centers and the above-mentioned un- specified defects were introduced during the irradiation when the temperatures of the specimens were lower than about 400 K. At 489 K, the unspecified defects were recovered while the conductivity measurements were made, and only the F+ centers remained. Although both the F+ centers and the unspecified defects were

-L ii20 Single Crystal

F. Oxygen Ion hod i 120 MeV I

2

‘E

07 ’ ’ 5 ’ 3 fi 1 1 ’ 1 0 t 2 3 4 5 6 7 8 9 too

Oxygen Ion Fluence f l~‘io”~/m*J

Fig. 2. Relationships between ion conductivity of Liz0 and oxygen ion fluence at 443 and 489 K.

Ion Conductwry

Temp.. 443 K 35

30

- YE

-2

2 5 Tz

b

20

Annealing Temperature ( K 1

Fig. 3. The recovery behavior of ion conductivity of Li,Q at

443 and 489 K during the isochronal annealing experiments.

contained in the specimens for the m~urements at 443 K, only the effects of the unspecified defects appeared in the conductivity measurements. So, we consider that

the number of the unspecified defects which were intro- duced by the irradiation and remained at 443 K was larger than that of the Ft centers, or that the efficiency of the unspecified defects for increasing the conductiv-

ity was larger than that of the F+ centers for decreasing

it. It is the migration of lithium ions. rather than oxygen

ions, in Li,O that leads to the ion conductivity, since lithium ions diffuse much more rapidly [12]. Since lithium diffuses by a vacancy mechanism, the ion con-

ductivity can be expressed by

0 = NevvL,t+,.,y (1)

where N is the number of lithium ions, 9v,i the mol fraction of lithium ion vacancies, and pv,, the mobility of lithium ion vacancies. The lithium ion vacancies have a negative charge, while the F’ centers have a positive charge. Therefore, the lithium ion vacancies are electro- statically attracted by the F+ centers. Accordingly, the decrease of ion conductivity at 489 K due to the irradia- tion can be attributed to the decrease of mobility of lithium ion vacancies due to retardation by the F’ centers, or to the decrease in number of mobile lithium ion vacancies due to trapping by the F’ centers.

In addition, the unspecified irradiation defects en- hancing the ion conductivity at 443 K are considered to

be defects which increase the number of mobile lithium

ion vacancies.

Fig. 4 shows optical absorption spectra of y-LiAlO,

pellets, unirradiated and irradiated to 1.4 x 10” ions/m’ by oxygen ions with an energy of 1 MeV. In

both spectra a strong absorption band, which seemed to be the fundamental optical abso~tion edge, was observed below a wave length of 260 nm (4.8 eV). No

prominent absorption band other than the fundamental optical absorption edge was observed for the specimens irradiated to 1.4 x lo*’ and 6.6 x 10’” ions/m’ by oxygen ions with energies of 1 and 120 MeV. respec- tively.

In a study j6] of electron irradiated y-LiAfO,, un- specified paramagnetic centers, which were recovered

below room temperature, were introduced at 20 K. In the present study the irradiation defects in y-LiAlO, were investigated by oxygen ion irradiation simulating the fusion irradiation conditions. However, no irradia- tion defect was observed in the examined fluence range. It is probable that the defects introduced by the oxygen ion irradiation were also defects being recovered below room temperature, since the irradiation and measure- ments in the present study were carried out at room temperature.

From the energy of the fundamental optical absorp- tion edge (band gap energy), the threshold energy (EC) for electron excitation in y-LiAlO, by tritons and helium ions produced from 6Li(n, a) 3H reactions can be

evaluated by

EC = l/8( M/m)Z, (2)

where M is the mass of a triton or a helium ion, rn is the mass of an electron, and I is the energy of the

Photon Energy (eVI

4- 654 3 2 ,(/ ! I

Y- LiAI02

: ~ Non hod. 3-

_--- Oxygen Ion hod

0 200 400 600 i

Wove Length I nmi

Fig. 4. Optical absorption spectra of y-LiAlO, pellets unirradi- ated and irradiated to 1.4 x 102’ ions/m2 by oxygen ions with

an energy of 1 MeV.

K. Noda et al. / Irradration damage in solid breeder materials 571

fundamental optical absorption edge [13]. Using the

value of 4.8 eV for I, the values of EC were calculated to be about 3.3 and 4.4 keV for the triton and the helium

ion, respectively. From these values of EC, the number of displacements due to elastic collisions by a “Li(n, a) 3H reaction, is roughly calculated to be 154, on the assumption that the threshold energy of displacement, Ed, is 25 eV, and that only tritons and helium ions with

the kinetic energies lower than E, contribute to dis- placements due to elastic collision.

For Li,O, the energy of the fundamental optical

absorption edge is more than 6 eV [2] and the number of displacements due to elastic collisions per ‘Li(n, CY)

3H reaction is more than 192, on the same assumption. The number of displacements due to elastic colli-

sions per 6Li(n, a) 3H reaction for y-LiAlO, is smaller than that of Li,O. Furthermore, we think the electron excitation process contributes to defect production in Li,O [4.5], but not for y-LiAlO, [6]. So, possibly the

dpa (displacements per atom) introduced by “Li(n, (Y) ‘H reactions in y-LiAlO, is considerably smaller than for Li,O under the same irradiation conditions.

The authors wish to express their thanks to Dr. T. Kondo, Dr. K. Shiba and Dr. N. Shikazono for their interest in this work.

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