Undoped and calcium doped borate glass system for ...

Journal of Non-Crystalline Solids 352 (2006) 3608–3612 www.elsevier.com/locate/jnoncrysol

Undoped and calcium doped borate glass system for thermoluminescent dosimeter S.S. Rojas

a,*

, K. Yukimitu b, A.S.S. de Camargo c, L.A.O. Nunes c, A.C. Hernandes

a

a

c

Grupo Crescimento de Cristais e Materiais Ceraˆmicos, Instituto de Fı´sica de Saõ Carlos, Universidade de Saõ Paulo, CP 369, 13560-970 Saõ Carlos, SP, Brazil b Departamento de Fı´sica e Quı´mica, Universidade Estadual Paulista, 15385-000 Ilha Solteira, SP, Brazil Laborato´rio de Laser e Aplicaço˜es, Instituto de Fı´sica de Saõ Carlos, Universidade de Saõ Paulo, CP 369, 13560-970 Saõ Carlos, SP, Brazil Available online 4 August 2006

Abstract Borate glasses present an absorption coefficient very close to that of human tissue. This fact makes some borates ideal materials to develop medical and environmental dosimeters. Glass compositions with calcium tetraborate (CaB4O7) and calcium metaborate (CaB2O4), such as the xCaB4O7 (100x)CaB2O4 system (0 6 x 6 100 wt%) were obtained by the traditional melting/quenching method. A phenomenon widely known as the ‘boron anomaly’ was observed in our thermal analysis measurements, as indicated by the increase of Tg and the appearance of a maximum value in the composition with 40 wt% of CaB2O4. The Dy doped and Li co-doped 80CaB4O7–20CaB2O4 (wt%) glass samples were studied by the thermoluminescence technique. The addition of Dy improved the signal sensitivity in about three times with respect to the undoped glass sample. The addition of Li as a co-dopant in this glass caused a shift to a lower temperature of about 20 C in the main glow peak. The structural analysis of the 80CaB4O7–20CaB2O4 (wt%) undoped and doped samples were studied through infrared absorption. We have noted an increase in the coordination number of the boron atoms from 3 to 4, i.e., the conversion of the BO3 triangular structural units into BO4 tetrahedra. 2006 Elsevier B.V. All rights reserved. PACS: 87.66.Sq; 65.60.+a; 61.43.Fs Keywords: Glasses; Oxide glasses; Borates; Radiation; Thermal properties

1. Introduction Borate compounds have been widely studied due to their features as glass formers and also on account of being very advantageous materials for radiation dosimetry applications [1]. These compounds present an effective atomic number which is very close to that of human tissue (Zeff = 7.42). This fact makes some borates ideal materials to develop medical and environment dosimeters [2]. The increasing use of ultraviolet, and X and c radiations, in processes associated with industrial, medical and agriculture

*

Corresponding author. Tel.: +55 16 3373 9828; fax: +55 16 3373 9824. E-mail addresses: [email protected] (S.S. Rojas), hernandes@ifsc. usp.br (A.C. Hernandes). 0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.128

applications has motivated researches on the search for new host materials with adequate dosimetric properties [3]. Many authors have studied glass and polycrystalline ceramics of alkaline and alkaline earth borate compounds using the thermoluminescence (TL) technique [1–11]. These compounds usually have higher sensitivities when compared to LiF:Mg,Ti (TLD-100) material [1]. Calcium tetraborate CaB4O7, glasses, and calcium metaborate CaB2O4, polycrystalline ceramics, are some of the most attractive systems among the borate compounds as they present a good chemical stability due to their very low hygroscopic nature [1,8–11]. This good stability is an important point to be considered in the development of a TL dosimeter, once the presence of water causes an adverse effect on the TL efficiency of the material, associated with non-radiative relaxations during thermal stimulation [12].

S.S. Rojas et al. / Journal of Non-Crystalline Solids 352 (2006) 3608–3612

Fukuda et al. and other authors [6,10,11] have studied the thermoluminescence of CaB4O7 at great length. Recently, Sangeeta and Sabharwal [8] showed that the polycrystalline CaB2O4 presents a TL intensity 50 times higher than barium metaborate BaB2O4 [8,9]. To date, no report has been published on a TL study of the compounds among the calcium metaborate and tetraborate systems, to the best of our knowledge. Herein, we present the results obtained from xCaB4O7-(100–x)CaB2O4 (0 6 x 6 100 wt%) system, aiming to develop new host materials for thermoluminescence dosimeter (TLD). 2. Experimental The undoped samples were prepared from grade purity reagents B2O3 (Merck 99.8%) and CaCO3 (Merck 99%). Li2CO3 (Alfa Aesar 99.999%) and Dy2O3 (Merck 99+) were used as dopants. Precise amounts of the reagents were weighed (Mettler Toledo AE 163, ±1 mg), mixed and then placed in a pure platinum crucible (99.9% purity, Heraeus Vectra). The powder was melted in an electrically heated furnace in air atmosphere for about 35 min. Vitrification was achieved by rapid cooling the melts on stainless steel plates, at room temperature. The samples were annealed for several hours below glass transition temperature, Tg, before being cooled at room temperature. With less than 40 wt% of CaB4O7 in the composition, non-homogeneous or glass-ceramic samples were consistently obtained under our experimental conditions. The composition (80CaB4O7–20CaB2O4 wt%) was actived with 0.16 wt% of Dy and 1.5 wt% of Li ions. These dopant concentrations were chosen according to the results published by Prokic [6], who showed that the addition of the Li co-dopant resulted in higher luminescence efficiency, either due to better incorporation of activator ions, or to the improvement in energy transference processes. No other samples with different Dy and Li concentrations were prepared. Characteristic temperatures such as glass transition Tg, onset of crystallization Tx, and melting temperature Tm, were determinated (Netzch STA 409 simultaneous Thermal Analyzer, DTA/TG mode, ±2 C) with the 10 C min1 heating rate. Synthetic air atmosphere (20% O2 and 80% N2) and an Al2O3 crucible as a reference, were used in all DTA/TG runs. DT was defined as the thermal stability against devitrification, obtained by DT = Tx Tg.

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The vitreous phase of the samples was checked by X-ray diffraction – XRD (Rigaku Rotaflex – RU 200B X-ray difractometer at 50 kV and 100 mA with Cu Ka1 radiation). The density values were measured by the fluid displacement method (Archimedes method) using distilled water at room temperature (25 ± 1 C). The infrared absorption spectra of the glasses, in the 400–4000 cm1 range, were measured (Nicolet Magna IR 850 spectrophotometer) using the KBr (Merck 99.5%) pellet technique. In order to obtain good quality spectra, the samples were crushed in an agate mortar to obtain particles of micrometer sizes. For the thermoluminescence measurements (Harshaw Model 3500 TL Analyzer), glass powders (3 mg) without previous thermal annealing, were used. Irradiation was performed at room temperature with an UV light (500 W Hg lamp) for about 30 min. The linear heating rate was set at 5 C/s and all measurements were taken in ambient atmosphere. Light emission was integrated in a temperature range between 50 and 400 C. 3. Results 3.1. Glass composition and density The starting composition, melting and annealing temperatures, and final aspects of the undoped samples are summarized in Table 1. The CaB4O7, 80CaB4O7– 20CaB2O4 (80CBO7) and 60CaB4O7–40CaB2O4 (60CBO7) bulk glasses were crystal-free, as observed by XRD, Fig. 1. The CaB4O7 and 80CBO7 glasses presented stability against devitrification (DT 80 C) which enabled us to obtain a large sample (60 · 50 · 5 mm3), Fig. 2. According to XRD data, the glass-ceramic materials present only the CaB2O4 crystalline phase, and their patterns were coincident with the card reported by the joint committee on powder diffraction standards (JCPDS) no: 75-0640. The characteristic temperatures determined by DTA runs are shown in Table 2. The CaB4O7 and 80CBO7 glass samples presented two peaks of crystallization identified as Tx1 and Tx2, respectively, the 60CBO7 and 40CBO7 showed only one peak. A maximum in the Tg values is observed for the (100–x) = 40 wt% of CaB2O4, which corresponds to the 60CBO7 glass (Fig. 3). No effect of the dopant concentrations on the principal temperatures was observed.

Table 1 Experimental information concerning the calcium borate glass system Name

Composition (weight%)

Melting temperature (C)

Annealing temperature (C)

Commentsa

CaB4O7 80CBO7 60CBO7 40CBO7 20CBO7 CaB2O4

CaB4O7 80CaB4O7–20CaB2O4 60CaB4O7–40CaB2O4 40CaB4O7–60CaB2O4 20CaB4O7–80CaB2O4 CaB2O4

1450 1450 1450 1450 1400 1400

500 500 500 500 – –

Colorless glass Colorless glass Colorless glass Non-homogeneous glass-ceramic White glass-ceramic White glass-ceramic

a

Visual inspection.

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646

xCaB4 O7 .(100-x)CaB2 O4

60CBO7

Tg (°C)

Intensity (arb.units)

644

642

80CBO7

640 CaB4O7

638 0

10

20

30

40

50

60

70

10

80

2θ (degrees) Fig. 1. X-ray powder diffraction patterns of CaB4O7, 80CBO7 and 60CBO7 glass samples.

20

30

40

50

60

(100-x) CaB2O4 (wt %) Fig. 3. Glass transition temperature, Tg, of the calcium borate glasses. The maximum Tg value for x = 40 wt% of the CaB2O4 (60CBO7 glass sample) corresponds to the boron anomaly. The dotted line is a guide for the eyes.

The densities of the CaB4O7, 80CBO7 and 60CBO7 glass samples were found to be (2.55 ± 0.02 g cm3), (2.63 ± 0.02 g cm3) and (2.67 ± 0.02 g cm3), respectively. These values are lower than for the CaB4O7 single crystal (2.743 g cm3), as reported by (JCPDS) file no: 31-0253. 3.2. Infrared absorption and thermoluminescence To gain further insight of the structure of the undoped and doped 80CBO7 glass samples, infrared spectra were carried out, Fig. 4. The characteristic vibration modes of

Fig. 2. Photograph of large CaB4O7 glass sample (60 · 50 · 5 mm3).

Table 2 Thermal properties of calcium borate glass system, indicated by the DTA curves Sample

Tg (±2 C)

Tx1 (±2 C)

Tx2 (±2 C)

Tm (±2 C)

DT = Tx1 Tg (±4 C)

CaB4O7 80CBO7 60CBO7 40CBO7

639 641 643 639

720 721 718 712

775 759 – –

900 961 1046 1084

81 80 75 73

907

Glass samples 80CBO7 80CBO7:Dy 80CBO7:Dy,Li

1399

Absorbance (arb.units)

685

2755

500

1000

1500

2000

2500

3000

3500

4000

Wavenumber (cm-1) Fig. 4. Infrared spectra of 80CBO7 glass sample: undoped (solid line), Dy doped (dashed line) and Li co-doped (dotted line).


borate groups and the presence of hydroxyl groups were analyzed in the 400–1600 cm1 and 2000–4000 cm1 spectral range, respectively. The bands and their assignments are summarized in Table 3. The TL glow curves of the CaB4O7, 80CBO7 and 60CBO7 undoped glass samples presented main peak temperatures at 191 C, 250 C and 209 C, respectively, after UV irradiation for 30 min. The maxima in the normalized TL intensity values were 0.7, 1 and 0.5 for the CaB4O7, 80CBO7 and 60CBO7, respectively. The 80CBO7 glass presented a good thermal stability against devitrification and the best thermoluminescent response among the undoped glass samples. Therefore, its composition was doped with Dy and Li ions to study its structural and thermoluminescent properties. The TL glow curve of the undoped 80CBO7 glass showed a peak at 250 C (main peak) accompanied by a shoulder at 163 C, Fig. 5. Doped and co-doped 80CBO7

Table 3 Frequencies and their assignments for IR spectra of undoped and doped 80CBO7 glasses Frequencies (cm1)

Assignment

Ref.

685 700

B–O–B bending vibration B–O–B bending vibration in borate rings B–O stretching vibration of BO4 units B-O stretching vibration of BO4 units in tri, tetra and pentaborate groups B–O stretching vibrations of BO3 units B–O stretching vibrations of BO3 units in varied borate rings O–H stretching vibration

a [17–19]

850–1100 907

1400 1399 2755

[16] a

[16,18] a a

a = to refer to this work.

Intensity x 104 (arb.units)

2.0 Glass samples 80CBO7:Dy,Li 80CBO7:Dy 80CBO7 undoped

1.5

235 258

1.0

253

0.5

163

0.0 50

100

150

200

250

300

350

Temperature (°C) Fig. 5. TL spectra of pure and doped 80CBO7 glass samples, irradiated with UV light for 30 min.

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showed a higher TL intensity and a different glow curve structure. The higher efficiency, due to the introduction of these dopants in borate glasses, was also observed by Prokic [6]. 4. Discussion Colorless calcium borate glasses devoid of bubbles were prepared by the traditional melting/quenching method. CaB4O7, and 80CBO7 glass samples were produced without any visible surface crystallization and were similarly stable against devitrification (DT 80 C, as determined by DTA analysis), allowing the preparation of large samples (60 · 50 · 5 cm3). However, we were unable to obtain the 60CBO7 sample with the same 80CBO7 dimensions, despite only a slight difference at DT between the glass compositions. Our attempts to produce glass samples with the same dimensions were not successful due to devitrification in the compositions containing less than 56.67 mol% of B2O3. According to the CaO–B2O3 phase diagram, the formation of the CaB2O4 crystalline phase is more favorable when the CaO contents in the glass compositions are increased [13]. The increase of the modifier contents in the pure B2O3 results in the initial formation of four coordinated boron atoms where higher modifier contents induce the formation of non-bridging oxygen containing borate triangles [14,15]. This phenomenon, widely known as the ‘boron anomaly’, was observed in our thermal analysis measurements, represented by the increasing of Tg and the appearance of a maximum value in the composition with 40 wt% of CaB2O4, (60CBO7 glass sample). The measured glass density values were similar to those of the CaB4O7 single crystal. We have been using the definition of free volume (V free ¼ 1 VV xg , where Vx is the molar volume of the crystalline form and Vg is the molar volume of the glass) to find some evidence of similarities in densities. The free volume of the CaB4O7 glass sample was 7% when compared to the crystalline phase. This value implies that the glass does not have a large fraction of interstitial space, within the network, for accommodation of other ions, such as the monovalent alkali ions, and the divalent alkaline earth ions. Therefore, by adding a higher quantity of CaB2O4 to the glass compositions, the free volume values of 6% and 1% were obtained for the 80CBO7 and 60CBO7 glass samples, respectively. The IR spectrum of the vitreous B2O3 contains an intense absorption band at 1265 cm1, a lower absorption band at 700 cm1, and also, a band situated at 1400 cm1 [17,19]. In the IR spectra, Fig. 4, the main bands at 700 cm1 and 1400 cm1 of vitreous B2O3 can be found, but the first band was shifted when compared to its initial position. We have also observed that the used Dy and Li dopant concentrations did not modify the vibration modes of the borate groups (Fig. 4). Borate tetrahedra are clearly identified in the spectra and their vibrational frequencies are assigned in Table 3.

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The band centered at 685 cm1 appears in all the IR spectra and it is due to the B–O–B bending vibration. The stronger absorption centered at 907 cm1 was attributed to the B–O stretching vibration of BO4 units in tri, tetra and pentaborate groups, confirming the change in the coordination number of the boron atoms from 3 to 4, i.e., the conversion of the BO3 triangular structural units into BO4 tetrahedra. This behavior was in accordance with the one found in the Tg range of the pure glass samples (DTA analysis). The band at 1399 cm1 was assigned to the B–O stretching vibrations of BO3 units in varied borate rings, and the lower absorption band centered at 2755 cm1 was attributed to the O–H stretching vibration, since the glasses were not prepared in a controlled atmosphere. However, no influence of these groups in the TL measurements was observed. Fig. 5 shows the TL spectra of the 80CBO7 undoped and doped glass samples after irradiation for 30 min with ultraviolet radiation. The curves were obtained by averaging four measurements and the error bars are related to the weight change of the samples. The TL glow curve of the pure 80CBO7 glass sample exhibits a glow peak at a temperature of 250 C (main peak) accompanied by a shoulder at 163 C. An improvement in the TL sensitivity of about 3 times, with respect to the undoped sample, was observed for the Dy doped glass sample, along with an increase of the TL intensity of a main peak. The addition of the Li co-dopant improved the TL sensitivity at about 10% in comparison to the Dy doped glass sample. The maximum of the 80CBO7:Dy,Li glow curve shifted towards a lower temperature ranging from 258 C (Dy doping) to 235 C (Dy:Li co-doped samples). The influence of Li co-doping in some TL phosphors was studied by Prokic [6], who observed that it induces an enhancement of the lower temperature peaks, which causes a shift of the glow curve to lower temperatures. The Li addition in calcium borate materials contributes to the improvement of the TL curve and also to the batch uniformity. In spite of the different compositions, our results are in accordance with those presented by Prokic. The 80CBO7:Dy and 80CBO7:Dy,Li glass samples presented interesting TL characteristics making these glasses potential candidates for the UV radiation dosimeter. A

more detailed study concerning the TL characteristics such as fading, dose response and their corresponding kinetics are in progress and the results will be presented in the future. 5. Conclusions Transparent, colorless and large 80CaB4O7– 20CaB2O4:Dy,Li glass sample were obtained by the traditional melting/quenching method at about 1450 C in air atmosphere. The IR absorption spectra confirmed the change in the coordination number of the boron atoms from 3 to 4, i.e., the conversion of the BO3 triangular structural units into BO4 tetrahedra and the presence of the hydroxyl group. Thermoluminescence measurements of these compounds showed a high intensity and temperature peaks of 235 C for 80CBO7:Dy and 258 C for 80CBO7: Dy,Li, making these glasses interesting materials for application as UV dosimeters. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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