Effects of electron- and/or gamma-irradiation upon theoptical behavior of transparent MgAl2O4 ceramics:

Different color centers induced by electron-beam and c-ray q

Jie He, Li-bin Lin *, Tie-cheng Lu, Pen WangDepartment of Physics, Key Laboratory for Radiation Physics and Technology of Ministry of Education,

Sichuan University, Chengdu 610064, China

Abstract

Samples of transparent MgAl2O4 ceramics were irradiated by electron beams with energy of 1.7 MeV and fluence

from 1� 1013 to 6� 1016 cm�2 and c-ray with dose from 0.1 to 3500 kGy, respectively. After irradiation, the samples

were annealed at different temperatures from 250 to 550 �C. Transmittance spectroscopy and positron annihilation

technique has been used to examine the behavior and the nature of defects in the spinel produced by irradiation

and annealing. The results of our study indicate that two absorption bands are produced after 1016 cm�2 electron-

irradiated sample and one absorption band is produced in c-irradiated ones. The absorption bands produced after

electron-irradiation are destroyed by isochronal annealing. F centers aggregates during annealing. The F center ab-

sorption band centered at 273 nm shifts to 210 nm. After c-irradiation there are few displacement of O2�, the voids exist

in grain boundary split into monovacancies. Isochronal annealing destroys V type center absorption band in the

sample. With increase of annealing temperature, cation vacancies aggregate. � 2002 Elsevier Science B.V. All rights

reserved.

Keywords: The transparent MgAl2O4 ceramics; c-irradiation; Electron-radiation; Color center

1. Introduction

The HIP (hot isostatic pressing) transparentMgAl2O4 ceramics have excellent transmittance(75–85%) from ultraviolet to infrared. Under-standing mechanisms of defects production inspinel ceramics can give us valuable informationfor producing new materials with even better anti-

irradiation properties. Over the last twenty years,the study of color center produced by variousirradiation treatments in spinel has led to anincreased understanding of color center in thismaterial [1–5]. But the relative complex nature ofthe structure has made it difficult to interpretthe details of microscopic models for various de-fects. The positron annihilation technique (PAT) isa defect-sensitive technique [6]. In this study,transmittance spectroscopy and PAT has beenused to examine the behavior and the nature ofdefects in the spinel produced under irradiationand annealing.

Nuclear Instruments and Methods in Physics Research B 191 (2002) 596–599

www.elsevier.com/locate/nimb

qProject 19928510 supported by NSFC.*Corresponding author. Tel.: +86-28-5412031; fax: +86-28-

541-7106.

E-mail address: libinlin@hotmail.com (L.-b. Lin).

0168-583X/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0168 -583X(02)00617 -1

2. Experimental methods

The samples (1:7 mm� 10 mm� 10 mm) werecut from a disc which was obtained from ZhongfeiResearch Instituted of Synthetic Crystals. A 60Cosource was used for c-radiation at 20 kGyh�1 withthe dose ranging from 0.1 to 3500 kGy. The Vande Graff electrons were used for electron-radiationwith the energy of 1.7 MeV and fluence from 1 �1013 to 6� 1016 cm�2. After irradiation the sam-ples were annealed at different temperature from250 to 550 �C in the air and the isochronal an-nealing time was 30 min. Optical spectra mea-surements were performed at room temperature bymeans of a SHIMADZU UV-2100 spectropho-tometer. The measured wavelength range was 200–900 nm.

Positron lifetime measurements were carriedout at two locations: the electron-irradiation andannealing samples were measured at Wuhan Uni-versity; the lifetime spectrometer is an ORTECfast–fast coincidence system, which has a resolu-tion of 235 ps. The positron source is 22Na withintensity 25 lCi sandwiched between the sam-ples. The c-irradiation and annealing sampleswere measured at Sichuang University. The life-time spectrometer is the pool style, the resolutionis about 260 ps and the intensity of positron sourceis about 10 lCi. Each lifetime spectrum contains atleast 1� 106 total counts. The lifetime spectrawere analyzed by PATFIT.

3. Experiment results

There is no absorption band before irradiation.After samples irradiated by electron beams withenergy of 1.7 MeV produce an optical-absorptionband at 370 nm. With the fluence up to 1016 cm�2.There is another optical-absorption band appear-ing at 273 nm. The two bands are attributed toabsorption by the V-type center and F center re-spectively [2]. The transmittance spectra of sam-ples are shown in Fig. 1. The absorption band at370 nm is destroyed at lower temperature than theband at 273 nm. With the increase of the temper-ature, the position of the 273 nm-absorption bandshifts to 210 nm. When the temperature of isoch-

ronal annealing gets up to 450 �C, there is no ab-sorption peak in transmittance curve. There is anoptical absorption band at 370 nm after c-irradi-ation. It can be seen from Fig. 2 that the trans-mittance at 370 nm of MgAl2O4 does not changewith increasing dose when D > 2 kGy. Positronannihilation lifetime spectra have been analyzed bythree lifetime components. The size of defects be-comes big in the case of s2 increasing in the sam-ple. I2 is positron annihilation intensity in thedefects, The value of s2=sb represents the nature of

Fig. 2. Transmittance at 370 nm of MgAl2O4 changes with the

irradiation dose for c-irradiated sample.

Fig. 1. Transmittance spectra of MgAl2O4, before and after

electron irradiation and isochronal annealing. Curve 1, before

irradiation; curve 2, annealing at 550 �C; curve 3, annealing at

500 �C; curve 4, annealing at 400 �C; curve 5, annealing at 200

�C; curve 6, annealing at 100 �C; curve 7, after electron irra-

diation (6� 1016 electrons cm�2).

J. He et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 596–599 597

defects [7]. Ii ði ¼ 1,2Þ have been normalized to thevalues of I1 þ I2. The results of positron lifetimeanalysis are shown in Tables 1 and 2.

4. Discussions

Although transparent MgAl2O4 ceramics aremade in the condition of high temperature andhigh pressure, it cannot be avoided that the grainboundary exists in the ceramics, and it will lead tovarious defects exit in grain boundary. In our ex-periment, s2=sb ¼ 1:68 and I2 ¼ 4:00% show thatthere exist a small amount of voids in the ceramicsbefore irradiation [7].

It is very easy to produce V-type center inMgAl2O4 after electron-irradiation. When thefluence gets up to 1016 cm�2, F center absorptionband appears in the transmittance curve. There-fore it seems that F center absorption band re-quires the concentration of F center up to a certaindegree. Bunch [8] has reported that F centers arecreated by a displacement process. MgAl2O4 isirradiated by electron beam with energy of 1.7MeV and results in isolated point defects. BecauseV-type centers are due to trapped holes at cationvacancies, which have already presented in thesamples, the 370 nm band does not change whenthe fluence increases. The defects produced byelectron-irradiation are interstitial-vacancy pairsof O2�. Table 1 shows that s2=sb ¼ 1:32 andI2 ¼ 18:61% after 6� 1016 cm�2 electron-irradia-tion. s2 can be seen the weighted average of dif-ferent types defect-state lifetime [9]. We can see s2is the weighted average of lifetime positron anni-hilated in monovacancies and voids. Therefore6� 1016 cm�2 electron-irradiation results in anincrease of s2 and a decrease of I2.

99% of V-type centers are destroyed when theannealing temperature is at 300 �C. The defectsthat produce V-type center restore to the originalconditions and they are just the same as pre-irra-diation in the sample. We leave these defects out ofconsideration. The change of s2 and I2 representchange of F center. A small part of F center hasalready obtained enough energy and moved inlattice in this condition. When F center meets an-other F center, they aggregate and convert into F2

center, the position of the 273 nm-absorption bandshifts to 220 nm (Fig. 1) and s2 increases and I2decreases at the same time. Since s2 and I2 did notchange significantly, taking voids into consider-ation monovacancy is the major defect in sample.237 nm-band is destroyed about 50% when thetemperature is at 350 �C. s2=sb ¼ 1:35 and I2 is halfof irradiated sample’s, F center is consumed toconvert into F2 center, the concentration of Fcenter decreases. I2 decreases and transmittance at237 nm increases. Therefore F center is almostconverted into F2 center. With the increase of an-nealing temperature, s2 becomes long and I2 be-comes small continuously. It shows that vacanciesstill aggregate. The aggregated defects decrease the

Table 2

The PAT parameters of c-irradiation and annealing (3500 kGy)

Sample s1 (ps) I1 (%) s2 (ps) I2 (%) s2=sb

Un-irradiation 201 87.97 399 12.03 1.77

2 kGy 197 82.66 358 17.34 1.52

35 kGy 198 81.92 368 18.08 1.60

100 kGy 196 79.30 340 20.70 1.52

350 kGy 200 83.64 390 16.36 1.68

3500 kGy 204 82.66 363 17.34 1.57

Annealing at

250 �C194 71.32 321 28.68 1.40

Annealing at

300 �C195 81.11 353 18.89 1.58

Annealing at

400 �C207 89.67 397 10.33 1.75

Annealing at

500 �C206 88.22 398 11.78 1.75

Table 1

The PAT parameters of electron-irradiation (6� 1016 elec-

trons cm�2) and annealing

Sample s1 (ps) I1 (%) s2 (ps) I2 (%) s2=sb

Un-irradiation 199 96.00 341 4.00 1.68

Electron

rradiation

189 81.39 265 18.61 1.32

Annealing at

300 �C190 82.10 272 17.90 1.35

Annealing at

350 �C195 90.28 280 9.72 1.39

Annealing at

400 �C198 94.64 319 5.36 1.57

Annealing at

450 �C201 97.48 352 2.52 1.73

598 J. He et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 596–599

surface energy of defects; therefore the aggregateddefects make the structure stable. The monova-cancies are converted into divacancies, trivacanciesand tetravacancies. Because F centers change intoFagg centers, the wavelength of F center absorptionband changing with temperature is shown in Fig.1. In the process of annealing some O2� ions gainenough energy and recombine with vacancies. Atlast the residual vacancies convert into voids andmove to the grain boundary.

There is an absorption band at 370 nm afterc-irradiation. The ionization effects are the majoreffect in MgAl2O4 after c-irradiation. ThereforeF-center absorption band dose not appear whenthe dose reaches 3500 kGy (1016 electrons cm�2 canbe converted into about 400 kGy).The decrease ofs2 and increase of I2 after 2 kGy c-irradiation in-dicates that new vacancies produce in the sample.There are few displacement ions in lattice. There-fore only small parts of voids existing in grainboundary are split into vacancies. s2, I2 and s2=sbdo not change significantly with the increase ofthe dose and the transmittance at 370 nm doesnot change after D > 2 kGy. Excitons can becaused by c-irradiation in samples; the excitedelectrons delivered energy to lattice before it hasbeen trapped. The energy is consumed by the wayof thermal vibration of lattice, which results inpartial annealing in MgAl2O4. Because of thepartial annealing effect, the holes trapped at somesite deficient in positive charge already present andsplit from voids in the sample can obtain energyand release from shallow traps. The transmittancedoes not change when the rate of hole release fromshallow traps equals to the rate of holes trapped.

s2 ¼ 321 ps and s2=sb ¼ 1:4 upon annealingtemperature at 250 �C. 370 nm-band has alreadydestroyed about 95%. The cation vacancies thatthe holes release from appear deficient in positivecharge. Because of coulombic forces, positrons canbe trapped in these defects. The change of s2 and I2

in this condition is attributed to the vacanciessplit from voids. With the increase of annealingtemperature, the change of s2 and I2 are the sameas isochronal annealing the electron-irradiationsample. But the vacancies are not F center. It iscation vacancy, which split from voids.

5. Conclusion

We have studied the behavior of transparentMgAl2O4 ceramics after electron and c-irradia-tion using UV–VIS and PAT. We concludedthat electron-irradiation produces two absorptionbands. Isochronal annealing destroys F and Vabsorption bands. F centers aggregate during an-nealing. c-irradiation produces one absorptionband that is attributed to absorption by V typecenters; there are few displacement of O2�.However voids exist at grain boundary split intomonovacancies after c-irradiation. Isochronalannealing destroys the V type center absorptionband. Cation vacancies aggregate with increase ofannealing temperature.

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