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J. Rad. Res. Appl. Sci., Vol. 4, No. 4(A), pp. 1107 - 1120 (2011)

Effect of Gamma-Irradiation on the Optical Properties of Silicate Glass Doped Nd2O3 prepared by Sol-Gel Technique Hanaa H.Mahmoud Radiation Chemistry Department, National Center for Radiation Research & Technology, Nasr-City, Cairo, Egypt E-mail; [email protected] Received: 20/07/2011. Accepted: 27/09/2011.

ABSTRACT

In this work, new results are presented concerning the characterization of silica based Nd doped glass obtained by a modified sol-gel technique, where the Nd ions are in a high local symmetry center, in contrast to the low symmetry found in glasses produced by conventional melting methods and technologies.

The behavior of optical absorption spectra of the irradiated samples indicates a strong dependence with gamma radiation doses, where the magnetic dipole transition 5D0-7F1 of the Nd3+ ions presents huge defects for irradiation doses up to 18 kGy. It was observed that the prepared sample at higher sintered heat treatment temperature (1150oC) exhibit high radiation resistance than that prepared at low sintered temperature (500oC).

The bleaching of irradiated glass was found to permit the reduction of the larger part of Nd4+ ions in the glass. The optical energy gap Eg was found to decrease with the increase of the irradiation doses, and it is suggested that the mechanism of optical transition is forbidden by indirect transition. This study also shows the usefulness of these glasses as yes or no dischargeable irradiation detectors, due to the remarkable color change after irradiation, which persists for a long time (up to 7 days).

Key words: Sol-gel, Nd2O3, Gamma-irradiation, Optical absorption

INTRODUCTION

Sol–gel glasses doped with rare earth ions are an important kind of materials which may lead to applications involving solid state lasers, fiber

Mahmoud, J. Rad. Res. Appl. Sci., Vol. 4, No. 4(A)(2011)

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amplifiers, and optical waveguides. Optical spectroscopy of rare earth ions is an excellent indicator of site symmetry and chemical bonding in glasses (1,2) and biological molecules (3), for that reason rare earth ions has been often used as an optical probe because of its particularly informative luminescence spectrum(4).

Such advantages are important to prepare monolithic products, thin films and optical fibers(5). Understanding the chemical reactions in the various steps in the sol–gel process leads to a better control of the process and therefore to an improvement of reproducibility of the final product(6-8). The sol–gel technique is also an excellent method to prepare hybrid material, and the low temperature synthesis enables organic or inorganic species to be incorporated into rigid silicon oxide matrices without degradation. The resulting composite combines the chemical and the physical properties of the guest with excellent optical, thermal, and chemical stability of the host silicon oxide matrices.

Interest in rare–earth doped glasses has increased over recent years a trend encourages , at least in part ,by their central role in the development of fiber lasers; optical amplifiers , optical switches, nonlinear devices and intrinsic fiber sensors(9). Considerable contemporary interest has focused on the attribute of the Nd doped alkali borate glasses, which are of the concern here, apart from the use of neodymium as a colorant for the study of the glass structure(9).

The hypersensitive transitions such as 5D0-7F2 and 5F0-5D2 are especially sensitive to the chemical bonding formed between rare earth ions and the surrounding ligands(10).

Hirayama and coworkers (11,12) studied the absorption of Nd3+ ions in different glass compositions Although the position of the bands remained identical in all glasses, yet an increase in splitting of the bands at 5750Ao -5850Ao and 7400Ao was noted with increasing alkali oxide content in the alkali borate glasses . It was probable that the Nd3+ ions occupied more than one single site and its large ionic size allowed a coordination number of 6 or 12. Because of its triple charge and large ionic size, the Nd3+ ion occupied a network modifier site.

Radiation damage by radiolytic or photo-chemical mechanism has been well established in alkali –halide crystals, but only recently has it been recognized in oxide glasses(13). In both cases, the ionizing radiation creates bound electron-hole pairs excitons. Radiative recombination of the excition results in luminescence, but if a nonradiative path is followed the deexcitation energy can


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some-times result in displacement of atoms thought to be oxygen in oxide glasses Thus, exposure either ionizing or particle radiation results in atomic displacements or broken bonds by radiolytic or knock-on displacements. Color centers result from trapping of the electrons and holes at these damage sites(14).

In this work the synthesis of a neodymium doped silica glass using the sol–gel technique is described. This study aims at developing glass easily affected by different gamma –irradiation doses by adding neodymium oxide as activator, and to study the role of silica on the glass structure, and, also present preliminarily results of gamma-radiation detection using the optical absorption enhancements of the neodymium excited states. The resistance of bands intensity during moderate environmental conditions was also studied.

2. MATERIALS AND METHODS

2.1. Sol-gel preparation

Monolithic silica glasses were prepared by acid catalyzed hydrolysis and polycondensation of tetraethyl orthosilicate (98% TEOS, Aldrich) with deionized water. Ethanol (spectroscopic grade, Aldrich) was added to produce a homogeneous solution. The molar ratio of TEOS : water : ethanol was 1 : 4 : 4, and a small amount (about 2 drops) of 0.04M HCl was added as a catalyst. Nd3+ 1% mole was introduced during the initial mixing stage by dissolving neodymium chloride (99.9%, Aldrich) in the sol prepared above. The resulting solution mixture was sonicated and then transferred to Teflon moulds and covered with paraffin. Gelation occurred in 24 h, and the gels were allowed to develop and dry at room temperature until stabilization of the weight had been completed. Densification of the prepared glass samples was obtained by annealing in air at 500oC (G1) and at 1150oC (G2) in a muffle furnace type (Lento) for two hours at each annealing temperatures with heating rate 1.5oC/min.

A Canadian gamma cell 60Co (2000 Ci) was used as a source of gamma-irradiation with a dose rate 1.0 G/s at room temperature. The absorption spectra of the glasses studied were recorded on a Perkin-Elmer Lambda 6 UV/VIS spectrometer, 101C/min.covering the wavelength range 200 to 900 nm. The glass specimens were measured before and immediately after irradiation. Fading measurements were carried out for irradiated glass (18kGy) at room temperature (~25oC) after different intervals of time up to 7 days.


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3. RESULTS AND DISCUSSION

The alkoxide sol–gel process is an efficient method to prepare silicate matrix by the hydrolysis of alkoxide precursors followed by condensation, to yield a polymeric oxo-bridged SiO2 network (6,7). The advantages of this technique are the homogeneity and the purity of the gels associated to a relatively low sintering temperature.

The variation in the intensity of some of the spectral bands: the so-called hyper sensitivity transition, e.g. 4I9/2 4G5/2, could be possibly due to the change in the covalency of the metal ligand bonds(15). The hypersensitivity might also be attributed to the enhancement of quadruple transition by the dielectric inhomogeneity in the media, or in other words of change in the symmetry of the field around the lanthanide ion. It was assumed(15) that the symmetry changes about the lanthanide ion could cause hypersensitivity.

The effect of different sintered heat-treatment temperatures on the structure of the prepared pure silica gel and doped samples with Nd3+ (G1 & G2) is shown in figure (1). This figure gives the absorption spectra of the glass studied after been heat-treated at 500oC (G1) and 1150oC (G2). It can be seen that there are a strong sharp absorption band at ~ 280 nm for glass (G1), while this band was shifted to shorter wavelength at ~ 190 nm for glass G2. It is obvious that, there is an observed decrease in the intensity of the optical absorption for sample heat-treated at 1150oC (G2) than the sample heat-treated at 550oC, and this may be due to the completely hydrolyzed and no reesterification of the methoxy groups occurs during drying. On the other hand the optical absorption efficiency of rare earth in sol-gel host material is compromised due to the tendency of rare earth ions to form clusters and by the presence of hydroxyl ions and remnant organic solvent(16). Clustering, results in concentration quenching due to non-radiative energy transfer between the Nd3+

ions within the clusters, while hydroxyl quenching, on the other hand, is caused by residual water, solvent, and silanol groups present in the sol-gel glasses and leads to an enhancement of non-radiative decay pathway of Nd3+ ions.

State of neodymium in glass

The necessary condition for increase of intensity in the lanthanide f-f transitions was that odd parity electric field component be mixed into the excited state wave functions of the f orbital energy levels involved in the transition. Such mixing would, therefore, make the Laporte (parity) forbidden f-f transitions more allowed and their


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intensities should correspondingly increase. Symmetry changes in the lanthanide ion environment could accomplish this mixing if spherical harmonics were present in the crystal field around the metal ion.

Fig. (1): Effect of different sintered

heat-treatment on the glass studied

Fig. (2): Effect of different irradiation doses on the optical absorption for glass G4

It is well known that(17), the increase in intensity of shorter wave-length band components with large radius modifier ions indicated that the weakened ligand field increased the probability of transitions to lower sub-levels, and also, were proved that multiple sites for neodymium ions were probably present. The observed bands which showed marked splitting in their peaks could be assigned to the following transition from 4I9/2 ground state(18) [Table 1.].

Table (1): The transition (ground state) corresponding to peak position

Band Peak position Transition (ground state)

1- 340-360nm 4I9/2 4G5/2

2- 460-480nm 4I9/2 4G11/2 ,2(P,D3/2)

3- 520-540nm 4I9/2 2K1/2,2K1/2,2G9/2

4- 560-600nm 4I9/2 4G5/2 ,2G7/2

5- 730-760nm 4I9/2 4S3/22, 4F7/2

6- 860-880nm 4I9/2 4F3/2

Details of the nature of the sites occupied by the Nd+3 ions could not be deduced directly from spectroscopic data, since a more enumeration of the


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number of coordinating oxygen ions did not define clearly the important structural features in particular the reason for the two coordination states. For example, it would be expected that an oxygen ion associated with an alkali ion would present a different effective charge to the Nd3+ ion than an oxygen ion that is part of a covalentlv bonded silicate tetrahedron. It could be assumed that it was primarily the geometric restrictions involved in packing silicate tetrahedra and alkaline-earth-oxygen polyhedra around the Nd3+ ion that limited the site for Nd3+ to a large six-coordinated (octahedra) or a large nine-coordinated ones. The later site appeared to be preferred when one or more of the larger alkaline-earths were accommodated in the first cation sphere around the Nd3+ site.

It might be assumed that the Nd+3 ion occupied a trigonally octahedral site like that proposed by Robinson and Fournier(19) for Yb3+ in various glasses.

Interaction of Radiation

Irradiation of glass generally leads to the formation of numerous defects which can be characterized by electron spin resonance (ESR). When alkali-borate, silicate and phosphate glasses are irradiated, it is believed that the most prominent induced ESR signal is due to hole trapped on bridging or non-bridging oxygen(14).

One of the principal means of identifying the radiation induced absorption bands in glass as either hole or electron traps has been the introduction of multivalent ions into the glass at the dopant level. Moreover, rare-earth ions when added to glass as impurities compete with the intrinsic defects in the glass to trap the electrons and holes produced by irradiation. The effect of these ions on the electron or hole trapping depends strongly on their chemical nature, valence state and their concentration, as well as glass composition. Rare-earth oxides in glasses are essential components, which act not only as luminescent centers but also as heavy elements to increase absorption of incident rays. Figs. 2 and 3 show the absorption spectra of the studied glasses after different doses of gamma irradiation (3, 8 and 18 kGy). In comparison with unirradiated samples, the irradiated glass samples including the low heat-treated sample (Fig. 2), show intense absorption of both Nd3+ and Nd4+. This observation indicates that part of the Nd3+ ions in the materials were converted to the Nd4+ during the gamma irradiation, i.e. the Nd3+ acts as hole trap in this matrix. The intense absorption of Nd3+ in the gamma-irradiated samples agrees with the assumption that the majority of the neodymium ions in the materials are in trivalent state(20). The


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observed decrease and increase in the broad band intensities at ~ 420-500 nm for glass G5 (Fig. 3), lead to the postulation that it is the Nd3+ ions that are responsible for the suppression of the absorption band induced in the visible at ~ 200 nm. If this band is attributed to positive hole centers formed by loss of electrons from oxygen during the process of irradiation, then the electrons resulting from the reaction: Nd3+ + hv → Nd4+ + e- could annihilate the positive hole centers. Similar discussion resulted in the postulation that the 200 and 290 nm bands are also attributed to hole centers which may have different properties from those associated with 420-500 nm band(21). The center associated with this band was tentatively designated as Nd4+ + e- in which the number of Nd3+ ions and the number of electrons involved in the formation of the center are unknown. Moreover, irradiation for the glasses prepared by sol-gel technique causes dissociation of water leading to an increases in [OH], and causes a transient changes in the anharmonicity of the well(14). Besides that, irradiation causes removal of a proton (protonation) from alkyl radicals (GH3, C2H5,..) dissolved in the glass. In conclusion, the enhancement presented by the ions can be explained as a decrease of non-radiative pathway, where these pathways, mainly water molecule or O-H chemical bonds, are nearly eliminated by the irradiation(22).

Fig. (3): Effect of different irradiation

doses on the optical absorption for glass G5

0

0.5

1

1.5

2

2.5

0 5 10 15 20

Doses (kGy)

Inte

nsity

Fig. (4): Growth curve for the glass

G4 at wavelength ~450nm

The obvious decrease in intensity at dose 8 kGy (Fig. 3) may be due to severe disruption of bonds in the glass matrix that could result in defects rebonding in ‘‘normal’’ configurations. Because of the lack of spectral data, it is impossible to determine if this effect is due to one type of defect center or the


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interference of the tails of several absorption bands(14). Another postulation can be introduced, that the observed decrease in intensity of the absorption spectra for some bands at low irradiation doses can be discussed as follows: the origin of this negative induced absorption may be attributed to the destruction of NBO`s by irradiation, which would cause the absorption edge to shift to shorter wavelength(23). These postulations are in good agreement with previous works(24,25). Another suggestion may be advanced and is simply based on assumption that there are several color centers with different rates contributing to the absorption at a given wavelength. Another postulate may recognize that there are several rate-determining processes operating concurrently during irradiation. Several authors had similar conclusions (27).

As may be seen in Fig. 4, the glass studied indicated an initial fast growth of the absorption band at 450 nm then followed by a slower growth, and then increased as the dose is increased. These results can be understood when it is considered that the optical absorption in glasses of different chemical compositions is the result of equilibrium between the formation and annihilation of color centers. This equilibrium is influenced by the polarizing power of the cations, the concentration of modifying ions, and the abundance of non-bridging oxygens(28).

Post-irradiation stability

In order to explain the fading process (Figs. 5 and 6) it may be assumed that the damage process in glass consists of primary formation of highly mobile electrons and holes caused by irradiation. These mobile electrons then either produce an electron scavenger or become localized over several groups. These localized or solvated electrons are responsible for the observed early decay after irradiation, which is reasonably well correlated with holes trapped on NBO

Fig. (5): Fading curves for glass G4

0

0.5

1

1.5

2

2.5

3

3.5

150 650

Wavelength (nm)

Opt

ical

Abs

orpt

ion

18 kGy

Fading 4D

Fading 7D

Fig. (6): Fading curves for glass G5


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atoms. Trapping of holes means that the neodymium ions are no longer bound by Columbic forces to oxygen atoms and are diffused away and trap electrons.

In addition, radiation annealing effect, which has been observed, consists of an actual annealing and removal of defect flaws rather than just a bleaching and depopulation of the trapped charge. As shown in Fig. 6 the bleaching of the irradiated glass causes slight fading at the beginning, but some times a limited increase in glass absorbance is noticed with further increase in the fading time. This may be due to the photo-oxidized (Nd3+)+ defects in the glass investigated, which can also be formed with the irradiation of glasses and no recovery being detected, in contrast to the photoreduced (Nd4+) defects, which are annealed to Nd4+ by thermal treatment below the Tg temperature. Another postulation of these bleaching experiments assumes that the 450 nm band is an electron trap since it bleaches out whenever one of the hole traps is bleached.

Energy gap

It is observed that, there is an exponential rise in the absorption towards the edge and in all the cases the edge is not sharply defined, signifying the glassy nature of the samples. The values of optical energy gap Eopt are obtained by extrapolation of the line or region of the plots of (αhv)0.5 against hv in Fig. 7 (taking the example of G1 to clarify). The results listed in Table 2 show that the values of Eopt decrease with an increase of gamma-irradiation dose. This in turn allows us to conclude that the absorption mechanism involves indirect optical transition, or it may be due to the fact that the increase of gamma-ray dose increases the spin density and the density of the unpaired electron in the unfilled bands, also the band tailing might be so pronounced as to result in the decrease in the forbidden energy gap. It is also interesting to observe that (Table 2) the Urbach energy increases with increasing doses of gamma-irradiation.

Table (2): Effect of different irradiation doses on the Eg and ∆E for the glass studied.

Eg (eV) ∆E (eV) SAMPLE

0kGy 3kGy 38kGy 18kGy 0kGy 3kGy 8kGy 18kGy

G4 3.623 3.553 3.380 2.898 0.573 0.525 0.540 0.224

G5 4.912 4.173 4.229 4.284 0.239 0.162 0.282 0.337


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Fig. (7): Dependence of (αhv)1/2 on

photon energy (hv) for glass G4.

Fig. (8): Dependence of (αhv)1/2 on

photon energy (hv) for glass G5.

After irradiation, the defect centers formed by charge trapping of the radiolytic electrons or holes often have electronic states in the gap between the valence and conduction bands. Hence optical photons may induce transition between the valence band and the defect levels, or from the defect levels to excited states or the conduction band. In principle, the optical band of a defect center could be extremely narrow and well defined, but site-to-site variation arising from the random nature of the glass invariably causes a distortion in the energy-level positions and a broadening of the band. In addition, the measured spectrum usually consists of one or more broad bands due to different color centers, and the bands often overlap. The shift of the absorption edge to a longer wavelength and the decrease of Eopt to lower energies with an increase of irradiation dose are related to the progressive increase in the concentration of NBOs. This increase gives rise to a possible decrease of BOs. It is probable that for the same reason of the increase in NBOs, the Eopt value shows a tendency to decrease with irradiation doses(20). The real UV absorption is limited by extrinsic charge transfer absorption bands due to neodimium metal ions consisting of Nd3+ & Nd4+. This absorption is shifted to higher energy due to the charge transfer bands of both Nd3+ and Nd4+, which absorb UV photon and lose an electron to form stabilized (Nd3+)+ defect hole centers. Then the electron is trapped, forming silicon related induced electron centers(27).


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CONCLUSION

The effect of gamma irradiation on the optical absorption of Nd2O3 doped SiO2 using sol-gel technique, where the Nd+3 ions are housed in a high local symmetry center was successfully synthesized and studied. Thus with the observed changes in the spectra it can be concluded that the long bonded oxygen atoms actually come from the neodymium, a somewhat wider range than is found in the crystal. It is also reasonable to conclude that all the oxygen neighbors of Nd in the glass will be nonbridging.

Neodymium cations present a large diversity of environments in glasses, which depend on their concentration and valency state. The octahedral (more or less) coordination appears to be frequent for the neodymium in silicate glasses.

Interpretation of the growth rate relationship between induced band growth and applied dose includes several possibilities in the formation and annihilation of induced color centers. The enhancement presented by the ions can be explained as to be due to a decreases of non-radiative pathway, where these pathways, mainly water molecule or O-H chemical bonds, are eliminated by the radiation. The large absorption enhancement presented by these ions (Nd3+) when exposed to γ- radiation, strongly suggests that these materials are of great potential for use as radiation detectors.

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19. Robinson, C. C. and Fournier, J. T. (1970): Coordination of Yb3+ in phosphate, silicate and germanate glasses, J. Phys. Chem. Solids, 32,895-902.

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شعاعیةشعاعیةالإالإبحوث بحوث مجلة المجلة ال والعلوم التطبیقیةوالعلوم التطبیقیة

)٢٠١١( ١١٢٠ –١١٠٧ صص )أ(٤ عدد ٤ مجلد

تاثیر اشعة جاما على الخواص الضوئیة لزجاج السیلیكات المحتوى على اكسید النیودیمیوم و المحضر بطریقة السائل الجیلاتینى

ھناء حنفى محمود

الإشعاعالقومى لبحوث و تكنولوجیا المركز ـ الإشعاعمیاء یقسم ك

أكسیدمن % ٩ھذا البحث یتناول تحضیر عینات من زجاج السیلیكات المحتوى على

1150النیودمیوم بطریقة السائل الجیلاتینى وتم معالجة العینات المحضرة حراریا عند درجة حرارة . درجة مئویة500,

لھذه جاما على شدة الامتصاص الضوئى أشعة الجرعات المختلفة من تأثیروقد تم دراسة العینات قبل التشعیع ظھور قمة فى المدى من لھذهالعینات ولقد وجد انھ عند قیاس امتصاص الضوئى

نانومتر ٩٠٠ الى ٢٠٠

شدة امتصاص ھذه القمة تزداد بزیادة جرعات التشعیع وقد عزى أنولكن بعد التشعیع لوحظ .٤ الى ٣ للنیودمیوم من التكافؤھذا التغیر الى زیادة عدد

مئویا لدیھا مقاومة اشد لتاثیر درجة 1150 العینة التى تم معالجتھا حراریا عند أنوجد وقد الى ٣ النیودمیوم من أكسدةجة تغیر عدد ی كذلك تم دراسة اضمحلال المراكز اللونیة المتكونة نتالإشعاع

أیامن بعد سبعة شدید لكالأولیة الاضمحلال یكون فى الفترات أن عند درجة حرارة الغرفة وقد لوحظ ٤یكون ھناك شبھ عینات فى شدة امتصاص القمة الضوئیة من ھذه الدراسة السابقة یمكن القول ان ھذا النوع من الزجاج والمحضر بطریقة السائل الجیلاتینى یمكن استخدامھ ككاشف اشعاعى وذلك راجع الى

.أیامبعة الصغیرة مع ثباتھ فى الاضمحلال عند سالإشعاعیة بالجرعات تأثیره

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