Optical and infrared properties of lithium diborate glasses doped with copper oxide: Effect of gamma irradiation

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 50, June 2012, pp Optical and infrared properties of lithium diborate glasses doped with copper oxide: Effect of gamma irradiation Hatem A El-Batal a *, Zeinab S El-Mandouh b, Hamdia A Zayed c, Samir Y Marzouk d, Gihan M El-Komy b & Ahmed Hosny b a Glass Research Department, National Research Center, Dokki, Cairo, Egypt b Thin Films and Electron Microscope Department, National Research Center, Dokki, Cairo, Egypt c Physics Department, Faculty of Girls, Ain Shams University, Cairo, Egypt d Basic Applied Science Department, Faculty of Engineering, Arab Academy of Science and Technology, P.O. Box 2033, Al-Horria, Heliopolis, Cairo, Egypt * h_elbatal@yahoo.com Received 3 November 2011; revised 20 January 2012; accepted 15 March 2012 A series of lithium diborate glasses containing successive CuO contents have been prepared by conventional melt annealing technique. Optical and infrared spectral properties have been measured before and after successive gamma irradiation to study the effect of gamma irradiation on the prepared glasses. Cu-doped samples show the same strong UV absorption as the undoped sample beside a characteristic broad visible band around 772 nm with marked splitting into four peaks at 614, 656, 774 and 1064 nm in higher CuO contents. Gamma irradiation of undoped lithium borate glass causes the generation of induced absorption bands but the introduction of CuO content causes some shielding behaviour towards successive gamma irradiation. Infrared absorption spectra of the undoped and CuO-doped glasses show characteristic vibrational absorption bands related to triangular and tetrahedral borate units. The addition of CuO in lithium borate glasses causes minor changes on the intensities of the IR bands but their numbers and positions are retained. The same limited effect is observed with gamma irradiation on the measured IR bands. Keywords: Lithium borate glass, Copper ions, UV-visible, IR spectra, Gamma rays 1 Introduction Lithium borate glasses are of great technological interest and several studies on the preparation of Li + - conducting glassy fibers have been published 1,2.The structural evolution and characterization of such glassy lithium borate have been extensively investigated by infrared and Raman spectroscopic measurements 3-8. The importance of borate glasses comes from the ability of boron atom to change its coordination with oxygen between three and four providing a range of anionic environment which are coordinated with the modifying metal cations. CuO-containing glasses are scientifically important because of their interesting semiconducting properties Copper can exist as monovalent Cu +, divalent Cu 2+ ions and metallic copper (Cu 0 ), although most glasses melted under ordinary atmospheric conditions are usually assumed not to contain metallic copper 11,12. The ratio of each type of copper state in glasses, namely Cu + and Cu 2+ depends on the type and the composition of glass. It is accepted that transition metal ions when acted as dopants in glass can influence the formation of induced color centers generated by irradiation 13,14. The transition metal ions can compete with the intrinsic defects which are presented in the glass to trap electrons and positive holes produced by irradiation. The capturing process is strongly dependent on the nature of the transition metal ions and their concentrations as well as on the type of the host glass. The UV-visible and infrared absorption spectra of undoped lithium diborate glass and samples with the same composition but doped with CuO varying content before and after successive gamma irradiation, have been investigated. This study will justify the application of such glasses as radiation dosimeters or as radiation-shielding materials. 2 Experimental Details 2.1 Preparation of Glasses Glasses of the basic composition lithium diborate and other similar samples with added CuO contents as presented in Table 1 have been prepared from chemically pure orthoboric acid (H 3 BO 3 ) and anhydrous lithium carbonate (Li 2 CO 3 ). The addition of Al 2 O 3 (0.33 mol%) to the batches was to prevent moister attack and accidental devitrification. The batches were melted in platinum crucibles in an

2 EL-BATAL et al.: OPTICAL AND INFRARED PROPERTIES OF LITHIUM BORATE GLASSES 399 electric furnace at 1100 C±10 C for 90 min with rotating the melts at intervals to achieve homogeneity. The melts were casted into preheated stainless steel molds and the prepared samples were immediately transferred to a muffle furnace regulated at 350 C for annealing. After 1 hour, the muffle was switched off and the muffle was left to cool to room temperature at a rate of 25 C/h. 2.2 UV-visible absorption measurements The optical absorption of highly polished samples with the dimensions of ( cm 3 ) was recorded at room temperature before and after successive gamma irradiation using a recording spectrophotometer type (JASCO, Corp., V-570, Rel-00, Japan) in the range nm. 2.3 Infrared absorption measurements The infrared absorption spectra of the prepared samples were measured at room temperature in the wavenumber range cm 1 by a Fourier Transform computerized infrared spectrometer type JASCO Corp., FT/IR-430, Japan. The samples were investigated as fine powders which were mixed with KBr in the ratio (2:200 mg glass powder to KBr, respectively). The weighed mixtures were then subjected to a pressure 5 tons/cm 2 to produce clear homogeneous discs. The IR spectra were measured immediately after preparing the required discs. 2.4 Gamma irradiation A 60 Co gamma cell (2000 Ci) was used as a -ray source with a dose rate of 1.5 Gy/sec (150 rads/s) at a temperature of 30 C. The investigated glass samples were subjected to the same gamma dose every time. By using a Fricke dosimeter, the absorbed dose in water was utilized in terms of dose in glass. No cavity theory corrections were made. Each glass sample was subjected to a total dose of Gy (6 M rad). 3 Results Table 1 Glass composition in mol% Sample Symbol Li 2 O B 2 O 3 Al 2 O 3 CuO LB LB LB LB LB UV-visible absorption spectrum of the base undoped lithium borate glass Figure 1 shows the optical absorption spectrum of the base lithium diborate glass (LB1 sample) exhibiting a strong sharp charge transfer ultraviolet Fig. 1 Absorption spectra of base lithium diborate glass before and after gamma irradiation band at 290 nm and no visible bands could be observed. When the base glass was subjected to a dose of Gy of gamma rays, the intensity of the UV band was increased and the band was shifted to 312 nm with shoulder in the visible region at 560 nm. With further increasing the irradiation dose to Gy, the spectral UV-visible curve shows a similar behaviour, but the observed UV peak splits into two induced bands at 234 and 304 nm. 3.2 UV-visible absorption of CuO-doped lithium borate glasses The absorption spectra of the CuO-doped lithium diborate glasses are shown in Fig. 2. LB2 sample (CuO = 0.04 mol%) reveals the same strong sharp UV band as the undoped sample beside the appearance of an extra broad unsymmetrical visible band centered at 766 nm. By subjecting the LB2 sample to a gamma dose of Gy, the UV band intensity is observed to increase [Fig. 2(a)] and the peak shifts to 304 nm and the center of the broad visible band shifts to 756 nm. With a further dose of Gy, the UV-visible spectrum is slightly increased and appears very close to the spectral curve of the first dose. Glass doped with 0.1 mol % of CuO (LB3 sample), the optical spectrum [Fig. 2(b)] is observed to consist of a strong sharp UV band at 300 nm and a distinct broad visible band centered at 772 nm. By subjecting this sample to a gamma dose of Gy, the whole UV-visible absorption spectra are observed to be the same as before irradiation. After the sample is irradiated with Gy, the UV band is split into two peaks at 204 and 342 nm, and in the visible region, the band centered at 770 nm is observed to split into two peaks at 702 and 880 nm.

3 400 INDIAN J PURE & APPL PHYS, VOL 50, JUNE 2012 Fig. 2 Absorption spectra of lithium diborate glass before and after gamma irradiation with varying CuO content It is also observed that the UV spectra of sample LB4 are the same as that before irradiation and shift to longer wavelengths with progressive irradiation. The UV peak is observed to shift to 358 nm in the case of LB4 sample (0.2 mol% CuO), and the spectral curves are almost very close with a slight shift to longer wavelengths [Fig.2(c)]. The highest CuO content sample (LB5= 0.6 mol% of CuO) reveals [Fig. 2(d)] wider UV absorption spectra consisting of three small peaks at about 212, 290 and 382 nm. The visible spectra show two very broad connected bands; the first consists of two peaks at about 636 and 772 nm, and the second band exhibits a peak at about 1064 nm. The spectral curves after irradiation are observed to be close to each other which reveal a shielding behaviour towards continuous irradiation. 3.3 Infrared absorption spectra of lithium diborate glass Figure 3 shows the infrared absorption spectra of the base undoepd lithium diborate glass before and after successive gamma irradiation. The IR spectrum of the base glass shows an extended complex structure. It appears that the vibrational modes of the borate network are mainly active in three regions: Fig. 3 Infrared absorption spectra of the base lithium diborate glass before and after gamma irradiation cm 1 (comprising B-O stretching of trigonal BO 3 units), cm 1 (comprising B-O stretching of tetrahedral BO 4 units), and cm 1 (comprising bending vibrations of various borate groups 1,2,15,16. It is known that boron atom has the smallest mass compared to other familiar network forming cations and thus the main vibrational modes

4 EL-BATAL et al.: OPTICAL AND INFRARED PROPERTIES OF LITHIUM BORATE GLASSES 401 associated with borate glass network appear above 500 cm -1 in the mid-infrared region 3-6. The experimental IR spectra reveal some of the vibrational sites active in the infrared region cm 1 (including Li + cations and Al-O groups), and the nearinfrared vibrational modes which include the OH, water and B-OH and related vibrations in the region cm 1. The distinct IR mid-region spectrum of the base lithium diborate glass consists of three main bands, a sharp medium one at 690 cm 1, a broad band at about 967 cm 1, and a broad band at about 1379 cm 1 together with connected less intense bands or kinks in the ascending or descending lobes of these prominent peaks at about 756, 1040, 1296 and 1498 cm 1. The spectrum shows in the far-infrared region small peaks at 452, 513 and 537 cm 1. It is also evident in the near-infrared region, two small kinks at 2846 and 2919 cm 1 followed by a small broad band centered at 3556 cm 1 and two other subsidiary kinks at 3675 and 3744 cm 1. For CuO-doped glasses the IR absorption spectra are shown in Fig. 4. The spectral curves are likely similar to the IR spectrum of the base undoped sample before irradiation and the CuO is observed to cause no changes within the IR spectra with the doping level added. It is evident that the main IR characteristic bands for triangular and tetrahedral borate groups retain their positions and numbers. It is also obvious that gamma irradiation causes no damages to the structural units or their entities, including the persistence of the main bands in their characteristic positions. 4 Discussion 4.1 Origin of ultraviolet absorption in the lithium diborate glasses Several researchers have identified that most of commercial glasses exhibit prominent ultraviolet absorption bands and they have assumed that such charge transfer absorption bands are originated from unavoidable trace iron impurities within the raw materials used for the preparation of such glasses 17,18. Recently, El-Batal and coworkers have identified experimentally strong charge transfer UV absorption bands in undoped phosphate, borate, silicate, borosilicate, and bioglass and they have assumed that such charge transfer UV bands originated from trace iron impurities within the raw materials used for the preparation of such various glasses Based on previous considerations, it is assumed that the strong UV absorption bands observed in the lithium diborate glasses are correlated to trace iron impurities within the raw materials used for the preparation of the studied glasses. Fig. 4 Infrared absorption spectra of CuO-doped lithium diborate glass before and after gamma irradiation

5 402 INDIAN J PURE & APPL PHYS, VOL 50, JUNE Interpretation of the effect of gamma irradiation on lithium diborate glasses Experimental UV-visible spectra indicate that gamma irradiation of the base lithium diborate glass generates three induced bands; two sharp UV bands at about 290, 234, 304 nm and one visible band at about 560 nm. Bishay 23 has assumed that the visible induced bands are due to positive hole traps while the induced UV bands are related to electron traps. Other scientists 24,25 have agreed to these assumptions of the three induced bands in alkali borate glasses at 2.2 ev ( 550 nm), 3.0 ev ( 400 nm) and 5.1 ev ( 235 nm). In a recent work 8, gamma irradiation studies on lithium diborate glasses have reached to the same previous deductions of Bishay 23 and Friebele 14 which are taken into consideration. The visible induced bands are related to positive boron oxygen hole centers induced from the borate network (BOHc) itself and the UV induced centers are considered to be due to electron centers (ECs) with the sharing of the trace iron impurities. 4.3 Effect of CuO on the induced defects The transition metal ions are accepted to compete with the intrinsic defects present already in the glass to trap the formed pairs of electrons and positive holes produced by irradiation 12-14,23. Moncke and Ehrt 26 have postulated that the formed induced defects are due to photochemical (photooxidation or photo-reduction) of the transition metal ions by the effect of gamma irradiation. Some researchers have suggested the following four photochemical reactions in CuO-doped glasses during the irradiation process 12,25,27-28 : Defect + h trapped electron + free hole Cu + + free hole Cu 2+ Cu 2+ + free electron Cu + Cu + + free electron Cu 0 (1) (2) (3) (4) The extended induced visible bands can be related to the distortion of Cu 2+ ions by Jahn Teller effect which promotes the appearance of the observed extended split broad visible peaks 12. It is believed that irradiation initiates more distortion of the Cu 2+ ions and consequently leading to the split of the visible bands. On the other hand, CuO-doped glasses (LB4 & LB5) reveal shielding behaviour towards successive gamma irradiation and the same result has been recently observed with CuO-doped phosphate glasses and also with V 2 O 5 -doped phosphate glasses It is observed that the spectral curves with progressive irradiation remain very close or parallel. 4.4 Interpretation of the infrared absorption spectra The realization and interpretation of the IR spectra have been carried out in arrangement with the free independent vibrations concept adopted by Tarte 29 and Condrate 30. From the detailed published infrared reflectance 3-6 and IR absorption 7 of lithium diborate glasses, from which it is obvious, that: i. The asymmetric stretching vibrations of tetrahedral borate (BO 4 ) units are active in the cm 1 range by the appearance of the strong band at about 1032 cm 1 and a small peak at 850 cm 1. ii. The modes of boron-oxygen triangular units (BO 3 and BO 2 O - ) are absorbing at ( cm 1 ) and specifically the bands at 1250, 1400, 1460, 1520, and 1634 cm 1. iii. The low-frequency region of the mid-infrared ( cm 1 ) is dominated by the bending vibrations or deformation modes of various borate units and the medium band at 697 cm 1 represents this mode. iv. The far-infrared bands in the range cm 1 are correlated to the vibrations of lithium ions in residing sites 6-8. v. The far-infrared band around 537 cm 1 can be correlated with Al-O unit vibrations as referred to by Khalifa and El-Batal 7. vi. The rest of the absorption bands extending from 2400 to 4000 cm 1 include the characteristic nearinfrared bands due to water, OH and B-OH vibrations as pointed out by several researchers 7, A summary of the previous interpretation is presented in Table 2, which depicts the observed IR absorption bands and their assignments. 4.5 Interpretation of the effect of gamma irradiation on the IR spectra Experimental IR results are shown in Fig. 4 indicating that gamma irradiation produces only limited changes in the intensity of the main IR absorption bands but the number and position of these bands remain unchanged. Earlier, Primak 34 explained that the effect of irradiation on vitreous silica causes compaction which originated from the decrease of the bond angle of Si-O-Si.

6 EL-BATAL et al.: OPTICAL AND INFRARED PROPERTIES OF LITHIUM BORATE GLASSES 403 Table 2 Assignments of the IR bands obtained from the base and the CuO-doped lithium borate glasses Peak position (cm 1 ) BO 3 stretching 1634, 1520 cm , 1400 cm cm -1 BO 4 stretching cm 1 Assignment B-O bands vibrations B-O vibrations of various borate rings Tri-, Penta-, Tetraborate or boroxol group Tri-, Penta-, Tetraborate, Diborate, Triborate Tri-, Penta-, and Diborate groups 850 cm -1 B-O-B bending cm 1 Oxygen bridges between one tetraborate and one trigonal boron atom Metal ions cm 1 Vibrations of lithium cations 570 cm 1 Vibrations of Al-O units Other ions or groups 2919 cm cm , 3744, 3858 cm 1 Hydrogen bonding Molecular water B-OH Hobbs et al 35, described the change in the geometry upon the formation of radiation-induced defects in glass. They assumed that generation of the Frenkel pair effectively breaking the connectivity of the network by irradiation and that adjacent SiO 4 unit may relax into a triangular rather than a tetrahedral structure. Accumulation of broken linkages evidently results in local structure collapse and rebinding. Piao et al 36, have described the mechanism of radiationinduced compaction by assuming that during irradiation process electron-hole pairs are produced, providing paths for bond rearrangements reducing the constraints on structural relaxation. The previous postulations can be applied to explain the observed changes in the intensities of the IR bands. The same achievements have been previously applied on irradiation studies on various borate glasses 8,22. The extent of changes in bond lengths and/or bond angles upon irradiation on various glasses needs further work by combined techniques. 5 Conclusions Ultraviolet, visible, infrared absorption spectra of undoped lithium diborate glass and other similar samples containing CuO with increasing contents have been measured before and after gamma irradiation. The optical absorption spectrum of the undoped lithium diborate glass before irradiation reveals charge transfer UV band at about 290 nm which is related to the presence of unavoidable trace iron (Fe 3+ ) impurities within the raw materials used for the preparation of this glass. CuO-doped samples show beside the same previously mentioned UV band, an extra broad visible band around 770 nm which at high CuO contents exhibits splitting to several component peaks. Such unsymmetrical broad band is accepted to be characteristic for Cu 2+ ions in octahedral coordination showing distinct distortion. Gamma irradiation of the base lithium diborate glass produces radiation-induced defects in the UV and visible regions originating from the sharing of trace iron impurities and the borate network itself. The presence of high CuO contents causes noticeable shielding behaviour towards successive gamma irradiation. Infrared absorption spectra of the studied samples show IR bands which are characteristics of triangular and tetrahedral borate units together with few small bands due to water, OH, BOH, lithium (Li + ) cations and Al-O units. Gamma irradiation is observed to cause some changes in the intensity of some bands while the characteristic network forming units remain unchanged. The shielding behaviour results of the CuO-doped glasses initiate the search for the application of such glasses in nuclear applications as shielding materials and further work is needed to justify the role of copper ions and other TM ions in this field. Also, optical absorption measurements indicate the sensitivity of CuO-glasses to gamma irradiation and their suitability also as glass dosimeters. References 1 Levasseur A, Kbala M, Hagenmuller P, Gouturier G & Dunto Y, Solid State Ionics, 9&10 (1983) Julien C, Massot M, Dzwonkourski P, Emery J Y& Balkanski M, Infrared Phys, 29 (1989) Krogh-Moe J, Phys Chem Glasses, 6 (1965) Kamitsos E I, Patsis A P, Karakassides M A & Chryssikos C D, J Non-Cryst Solids, 126 (1990) Kamitsos E I, Patsis A P & Chryssikos C D, J Non-Cryst Solids, 152 (1993) Kamitsos E I, Phys Chem Glasses, 44 (2003) Khalifa F H, ElBatal H A, in: Wright A C, Feller S A, Hannon A C,Eds, Proc Second Intern Conf Borate Glasses, Crystals and Melts, Society of Glass Technology, Sheffield, England, (1997) p El-Batal F H, El-kheshen A A, Azooz M A & Abo-Naf S M, Opt Mater, 30 (2008) Duran A, Jurado J R & Navarro J M F, J Non-Cryst Solids, 79 (1986) Znášik P & Jamnický M, J Mater Sci Lett, 14 (1995) Metwalli E, J Non-Cryst Solids, 317 (2003) 221.

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