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Structural investigation of bismuth borate glasses under the influence of γ-irradiation through ultrasonic studies
ARTICLE IN PRESS Physica B 404 (2009) 3371–3378
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Structural investigation of bismuth borate glasses under the influence of g-irradiation through ultrasonic studies G. Sharma a,, V. Rajendran b, K.S. Thind c, Gagandeep Singh d, Amarjit Singh c a
Department of Physics, Kanya Maha Vidyalaya, Jalandhar, Punjab, India Research and Development and Centre for Nano Science and Technology, KS Rangasamy College of Technology, KSR Kalvi Nagar, Tiruchengode 637 215, India c Department of Physics, Guru Nanak Dev University, Amritsar 143005, India d Department of Chemistry, Lyallpur Khalsa College, Jalandhar, Punjab, India b
a r t i c l e in fo
abstract
Article history: Received 30 January 2009 Received in revised form 10 May 2009 Accepted 13 May 2009
The ultrasonic velocity and attenuation measurements for different compositions of irradiated heavy metal oxide (HMO) borate glasses xBi2O3 (1x) B2O3 (where x ¼ 0.25, 0.30, 0.35, 0.40, 0.45) has been investigated at room temperature (303 K) using pulse echo overlap method. The elastic moduli, Debye temperature, Poisson’s ratio and other acoustical parameters have been obtained from experimental data. Structural changes after irradiation have been investigated by using FTIR spectroscopy and ultrasonic studies. As the changes are strongly dependent on the internal structure of the absorbing substance, in the present investigation ultrasonic velocities before and after g-irradiation in bismuth borate glasses are measured as a function of composition, from which the structural changes in the network former B2O3 and modifier Bi2O3 due to irradiation are obtained. Crown Copyright & 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Glasses Ultrasonic velocity Irradiation
1. Introduction Glasses containing heavy metal oxide (HMO) have attracted the attention of several workers in recent years for their excellent infrared transmission compared with conventional glasses. Attempts [1–7] have been made to explore the mechanical, thermal, and optical properties of these glasses. Need of high density, radiation hardness and fast inorganic scintillators have put again HMO glasses at the center stage [8,9]. Glasses containing bismuth have been investigated for possible use in scintillation detectors for high energy physics [10–11]. The properties of glasses are closely related to inter-atomic forces and potentials in lattice structure. Thus, any change in lattice due to doping and/or irradiation can be directly noted. The elastic properties, Poisson’s ratio, attenuation or other related parameters are of great interest to investigate the linear and anomalous variation as a function of composition of glass and have been interpreted in terms of a structural softening or transformation of cross-linkages in the glass structure [12]. The addition of B2O3, which forms a network structure related to the silicates, creates a glass with higher melting point and greater ability to withstand temperature changes. To investigate the structure of oxide glasses, the coordination number of network former and the change of oxygen bonds of
Corresponding author.
E-mail address: [email protected] (G. Sharma).
frame work induced by the cation modifiers need to be investigated. Much information on this can be obtained from FTIR spectroscopy. The majority of structural investigations on heavy metal oxide glasses before irradiation in past time have been made using vibrational spectroscopy and ultrasonic study [12–17]. Ultrasonic technique similar to other techniques plays a significant role in understanding the structural characteristics of glass network. Recently El-Batal [18] has studied effect of gamma irradiation on thermal expansion and infrared studies of bismuth borate glasses. An interaction of g-ray with glasses results in change in optical and physical properties [19] by formation of induced defects during progressive irradiation [20–23]. The present study represents change in the structure of bismuth borate glasses after gamma irradiation through ultrasonic studies. In order to explore the validity of the ultrasonic technique elastic constants have been compared before [24] and after irradiation.
2. Experimental details 2.1. Sample preparation Glasses of type xBi2O3 (1x) B2O3 (where x is the mole fraction) were prepared by using the melt-quenching technique. The chemical compositions of all these glasses are shown in Table 1. Commercial grade chemicals of Bi2O3 (Fluka Chemical Company) and B2O3 (Aldrich Chemical Company) having 99.99%
0921-4526/$ - see front matter Crown Copyright & 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.05.018
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Table 1 Sample name, thickness, mole fraction, density and molar volume (before and after irradiation) for present glass system at 303 K. Sample name
BB1 BB2 BB3 BB4 BB5
Thickness t 103 (m)
12.0 11.0 13.0 11.0 12.5
Density (kg/m3)
Mole fractions
Molar volume Vm 106 (m3 mol1)
Bi2O3 (x)
B2O3 (1x)
Before g-fall
After g-fall
Before g-fall
After g-fall
0.25 0.30 0.35 0.40 0.45
0.75 0.70 0.65 0.60 0.55
5201 5361 5691 5796 6211
5328 5442 5476 6023 6207
30.9 32.9 34.6 36.1 37.0
31.7 34.6 38.0 37.9 39.9
purity level were used as starting materials. Appropriate amounts of oxides were weighed by using an electronic balance having an accuracy of 0.001 g. The crucible containing powder was heated at 973 K in muffle furnace for 10 min. The weight loss measurements were taken to insure stoichiometric accuracy. The chemicals were then mixed in a pestle mortar for half an hour. The platinum crucible containing the batch was then placed in an electric furnace capable of reaching a temperature of 1673 K. It was heated at a temperature of 1173 K for half an hour under normal atmospheric conditions and then dry oxygen was bubbled through it using a quartz tube to ensure homogeneity of the glass melt. Heating was performed for a longer period of time. The melt was then poured into a cylindrical graphite mould. The mold was moved into an annealing furnace kept at 623 K [25] for half an hour and then allowed to cool down for 12 h. The samples were grounded with the help of an electric machine using different grades of SiC abrasives and aluminum oxide with machine oil by setting the sample in a specially designed holder to maintain the two faces parallel. The polishing was done with cerium oxide to obtain flatness. Thickness measurement was carried out by micrometer. 2.2. Gamma-ray irradiation All glass samples were irradiated using 60Co radioisotope at Radiology Department, Guru Teg Bahadur Hospital, Amritsar, India. The dose rate was 1.77 Gy min1. The samples were irradiated for necessary time interval to achieve the overall dose of 2.5 kGy.
back wall echoes, (iii) cross-correlation of two desired echoes for finding the approximate time delay, (iv) adopting cubic spline interpolation method to the peak portion of the cross-correlated function to arrive at exact time delay, and (v) calculating ultrasonic velocity from the sample thickness and the measured time delay. Variation in glass sample thickness and plane parallelism between faces was minimized by surface grinding of the samples and obtaining plane parallelism with an accuracy of 75 m s1 which corresponds to a percentage change in velocity of about 70.1%. 2.5. Attenuation measurements Attenuation measurements were carried out using a contact type transducer operated at a frequency of 5 MHz. Care was taken to avoid the problem of near field effects. A constant pressure was maintained between transducer and glass sample during attenuation measurements. Among the various available couplants, filtered machine oil was found to be more suitable to get steady back wall echo train in the oscilloscope screen. Attenuations in all the glass samples were measured both for longitudinal and shear waves by measuring the peak amplitude of the successive back wall echoes of ultrasonic signals from the glass samples using the following relation: 20 Im (1) a¼ log In 2ðm nÞd where Im and In are, respectively, the maximum amplitude (voltage) of mth and nth pulse echoes. The percentage error in the attenuation measurement is 72%.
2.3. Optical measurements Infrared absorption spectra of powdered glass samples were recorded in the range 400–4000 cm1 using KBr technique at room temperature. A recording spectrometer of type Shimadzu FTIR-8700 was used to reveal the absorption spectra. Optical measurements were taken immediately after irradiation. 2.4. Ultrasonic velocity measurements The ultrasonic velocity and the attenuation in all glass samples were measured using a specially designed experimental set-up in one of the author’s laboratory. A high power ultrasonic process control system (Fallon Ultrasonic Inc., Canada), a 100 MHz digital storage oscilloscope (Hewlett Packard, USA) and a PC were used to record the ultrasonic rf signals. Precise transit time measurements were carried out employing cross-correlation technique [26,27] using longitudinal and shear ultrasonic waves generated by PZ transducers operated at 5 MHz frequency. Various steps involved in the precise time measurements were: (i) acquisition, digitization and storage of the rf signals from the transducer, (ii) application of the window technique to select any two successive
2.6. Density and molar volume measurements The density was obtained from Archimedes principle using benzene as buoyant. The density was determined employing the following relation:
r¼
Wa rb Wa Wb
(2)
where Wa is the weight in air, Wb the weight in buoyant and rb the density of buoyant. All the weight measurements have been made using a digital balance (M/s. Sartorius, Model: BP221S, USA). The accuracy in the measurement of weight is 70.1 mg. The experiment was repeated five times to get an accurate value in density. The overall accuracy in the density measurement is 70.5 kg m3 and hence, the percentage error in the measurement of density is 70.05%. The molar volume, Vm, of a glass is given by using molecular weight (M), mole fraction (x) and density (r) of the component i as follows: Pn xM (3) V m ¼ i¼1 i i
r
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Table 1 lists the measured values of density and molar volume for present glass system before and after irradiation.
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Poisson’s ratio for bismuth borate glass system maxima is observed at 0.35 mol fraction. For Debye temperature maxima is observed at 0.30 mol fraction of Bi2O3. Acoustical impedance and microhardness behaviour, with increase in Bi2O3 content is shown in Figs. 9 and 10. In order to explain observed variations in the acoustical parameters of present systems two facts must be considered namely (i) effect of glass composition and hence, structure of glass matrix and (ii) effect of irradiation. It is well known that structure of pure borate glass consists of six membered boroxyl ring constructed from three BO3 units located outside boroxyl rings; the mole fraction of boron atoms in boroxyl ring and outside the boroxyl ring is, respectively, 0.65 and 0.35 [28]. Addition of modifier leads to structural changes by the formation of nonbridging oxygen (NBO) atoms and/or formation of BO4 units. Displacement of atoms from their positions in glass network caused by irradiation cascades can give rise to a number of structural changes [29]. The change in geometrical configuration, co-ordination number, cross-link density and dimensions of
3. Results and discussion Figs. 1 and 2 shows the plots of longitudinal (UL) and shear wave velocities before and after gamma irradiation against Bi2O3 contents. Before irradiation the smooth variation of ultrasonic velocity with increase in Bi2O3 content have been observed. It is found that for bismuth borate glass system variation of ultrasonic velocity (both longitudinal and shear) is not smooth with increase in Bi2O3 contents. Both longitudinal and shear velocities exhibit maxima at 0.30 mol fraction of Bi2O3 and minima at 0.35 mol fraction of Bi2O3. Figs. 3–6 shows variation of longitudinal and shear modulus, Young’s and bulk modulus with Bi2O3 content, respectively. All elastic moduli exhibit maxima for BB2 glass sample and minima for BB3 sample. Figs. 7 and 8 shows variation of Poisson’s ratio and Debye temperature with Bi2O3 content.
4300 UL before irradiation UL after irradiation
6100
UL (m/s) before irradiation
5900
4250 4200
5700 5500
4150
5300
4100
5100 4900
4050
4700
4000
4500 4300
UL (m/s) after irradiation
6300
3950
4100 3900
3900 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 1. Variation of longitudinal (UL) velocity before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
2425
3600 US before irradiation US after irradiation
3500 3400
2375
3200
2325
3100 3000
2275
2900 2800
2225
2700 2600
2175
2500
US (m/s) after irradiation
US (m/s) before irradiation
3300
2400 2125
2300 2200 2100
2075 0.25
0.30
0.35
0.40
0.45
Bi2O3 content (mole fraction) Fig. 2. Variation of shear (US) velocity before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
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224 118
L before irradiation L after irradiation
204
184
108
164
103
144
98
124
93
104
88
84
L (GPa) after irradiation
L (GPa) before irradiation
113
83 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 3. Variation of longitudinal modulus (L) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
40
69 G before irradiation G after irradiation
64
38 36
54
34
49 32 44 30 39
G (GPa) after irradiation
G (GPa) before irradiation
59
28
34
26
29
24
24 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 4. Variation of shear modulus (G) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
interstitial space of glass decide the density and therefore, density is a tool in revealing the degree of change in structure with composition of glasses [30]. In the studied binary system, the replacement of B2O3 by heavy metal oxide causes an increase in density and molar volume (Table 1) both before [24] and after irradiation. This increase in density with HMO content can be attributed to change in crosslink density, the greater molecular weight of bismuth atom and the co-ordination number of Bi3+ ions. Comparison of density data before and after irradiation indicates decrease in density after irradiation for BB3 and BB5 glasses and increase for BB1, BB2 and BB4 glass compositions. Shelby [31] suggested that boron atoms are directly involved in densification process. Damage by irradia-
tion can cause compaction of B2O3 by breaking the bond between trigonal elements, allowing the formation of tetrahedral BO4 [19]. The tetrahedral groups are strongly bonded than the triangular BO3 and a compact structure is expected leading to higher density after irradiation as found for BB1, BB2 and BB4 in present study. The observed decrease in density for BB3 and BB5 compositions can be related to vacancies created in glass structure due to irradiation. These vacancies increase the molar volume and decrease in density after irradiation (Table 1). Prado et al. [29] suggested that density changes during irradiation are associated with the generation of vacancies and also with effective transport of an atom between inhomogeneities. The decrease of density for BB3 and BB5 shows presence of transition metal impurities and
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94
165
E before irradiation E after irradiation
90 88 86 84
155
82 80 150
78 76 74
145
72
E (GPa) after irradiation
E (GPa) before irradiation
160
92
70
140
68 66 64
135
62 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 5. Compositional dependence of Young’s modulus (E) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
70
118 K before irradiation K after irradiation
116
66
112 64
110 108
62
106
60
104
58
102
K (GPa) after irradiation
K (GPa) before irradiation
114
68
56 100 54
98
52
96 0.25
0.30
0.35
0.40
0.45
Bi2O3 content (mole fraction) Fig. 6. Compositional dependence of bulk modulus (K) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
creation of vacancies for BB3 and BB5. This result indicates that irradiation leads to decrease in rigidity for BB3 by formation of vacancies, whereas for BB5 glass composition effect of irradiation is very less in comparison to BB3 as 45 mol% of Bi2O3 for BB5 glass impart radiation hardness in comparison to BB3 (35 mol% of Bi2O3). Any change in the cross-link density and bond stretching force constant consequently changes the bulk modulus of glass network [32]. To interpret our data on irradiation effect on elastic moduli let us first reveal the results obtained by El-Adawy [24] on elastic properties of present system which indicate that there is change in the behaviour of the compositional dependence of all the elastic properties at 25 mol%. The longitudinal and shear wave velocities and elastic moduli increase with increase in Bi2O3 content up to
25 mol% and then decrease as Bi2O3 content increases further. However, measurement of elastic properties after gamma fall indicates that ultrasonic velocity and elastic moduli do not show any smooth variation. This indicates that irradiation effects on acoustical properties strongly depend on composition. The observed variation in elastic moduli show a minimum at 0.35 and maximum at 0.30 mol fraction of Bi2O3 (Figs. 3–6), which further confirms the softening of BB3 glass composition and maximum value of BB2 indicates rigidity of glass after irradiation. According to Bridge et al. [32], Debye temperature describes the temperature at which nearly all modes of vibration in a solid are excited and it decreases when ultrasonic velocity and hence rigidity decreases (Fig. 8). Fig. 7 shows variation of Poisson’s ratio which is a measure of cross-link density and it decreases with increase in
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0.300 σ before irradiation σ after irradiation
σ before irradiation
0.295
0.295
0.290
0.290
0.285
0.285
0.280
0.280
0.275
0.275
0.270
0.270
0.265
0.265
0.260
0.260
0.255
0.255
0.250 0.23
0.28
0.33 0.38 Bi2O3 content (mole fraction)
σ after irradiation
0.300
0.250 0.47
0.42
Fig. 7. Variation of Poisson’s ratio (s) before and after irradiation with different Bi2O3 content.
450 θD before irradiation θD after irradiation
θD (Κ) before irradiation
520
440 430
500
420
480
410 400
460
390 440
380
420
370
400
360
380
350
θD (K) after irradiation
540
340
360
330 340
320
320
310 300
300 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 8. Variation of Debye temperature (yD) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
cross-link density. Therefore, decrease of Debye temperature and increase of Poisson’s ratio at 0.35 mol fraction of Bi2O3 supports our present discussion of softening of BB3 glass compositions In Figs. 9 and 10, acoustical impedance and microhardness are plotted against Bi2O3 content. The decrease in microhardness and acoustical impedance for BB3 glass further indicates that this glass composition is most affected by irradiation. The decrease in hardness after gamma irradiation can be attributed to defects created in the glass. The change of hardness depends on concentration and type of defects [33], depending on the glass matrix and its dopants irradiation can lead to formation of intrinsic [34] and extrinsic [35] defects. Irradiation with relatively high doses of gamma rays produces a similar structural change
with those produced by heat treatment. Heat treatment produces hard particles dispersed in soft matrix, thus the overall hardness increases and on the other hand formation of soft particles in hard matrix causes a decrease in microhardness [36]. Infrared investigations reveal structural changes and observed changes in infrared spectra due to irradiation and can be interpreted by assuming that irradiation of glass leads to formation of induced defects which lead to a decrease of the intensity of the main characteristic mid absorption bands [18]. Vibrational modes of borate network are mainly active in three infrared spectral regions: first region at around 700 cm1 is due to bending of B–O–B linkage, the second at 1200–800 cm1 is due to B–O bond stretching of tetrahedral BO4 units and third at
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2.0x107 2.0x107 1.9x107 1.9x107 1.8x107 1.8x107 1.7x107 1.6x107 1.6x107 1.6x107 1.5x107 1.5x107 1.4x107 1.4x107 1.3x107
before irradiation after irradiation
0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
Z (kg m-2 sec-1) after irradiation
Z (Kg m-2 sec-1) before irradiation
G. Sharma et al. / Physica B 404 (2009) 3371–3378
0.45
Fig. 9. Change in acoustic impedence (z) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
20
8 before irradiation after irradiation
7
6
16
5
14
4 12 3
H (GPa) after irradiation
H (GPa) before irradiation
18
10 2 8 1 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 10. Change in microhardness (H) with content of Bi2O3. The drawn lines are guide to eyes.
1500–1200 cm1 is due to stretching of trigonal BO3 units [37–44]. Bismuth containing glasses have four fundamental vibrations in the IR spectral regions at 830, 620, 450, and 350 cm1 [2,4,17,18]. Fig. 11 reveals the spectra of irradiated glasses. All samples show the presence of broad band at about 900 cm1 and a broad band centred at 1280 cm1 indicates the presence of BO4 tetrahedra. The low frequency band at 460 cm1 is attributed totally symmetric bending vibrations of BiO3 pyramidal units. A sharp peak at 1352 cm1 are attributed to the vibrations of bridging oxygen between BO3 and BO4 groups. An absorption band centred at 530 cm1 is observed for BB3 glass composition which indicates the presence of BiO6 octahedral units and thus, octahedra provides weaker binding in the glass network
in agreement with ultrasonic velocity measurements. The absorption band at 806 cm1 is not observed in present system, which reveals the absence of boroxol groups. The absorption peak around 670 cm1 indicates oxygen bridges between the two trigonal boron atoms.
4. Conclusions The observed changes in ultrasonic velocity and elastic moduli are related to change in structure of glass matrix due to irradiation. The acoustical properties of bismuth borate glasses indicates that BB3 glass is most affected by irradiation and have
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References
BB5
Absorption ( a..u.)
BB4 BB3
BB2
BB1
4000 3500 3000 2500 2000 1750 1500 1250 1000 750 1/cm
500
Fig. 11. Optical spectra of bismuth borate glasses after irradiation.
soft network whereas BB2 had rigid structure in comparison to other samples studied. FTIR spectra also support the structural changes after irradiation by change in the co-ordination number of boron atoms. Thus, it can be concluded that BB2 glass composition is more rigid in comparison to other glass compositions and hence, it can be used as radiation hard material for dose applied in the studied system. This result is in agreement with our previous study on irradiation effect on optical properties of heavy metal oxide borate glasses [22]. The observed results reveal that ultrasonic studies can be used as a tool in exploring the stability of the glass exposed to radiations.
Acknowledgements Authors are grateful to Head, Department of Applied Chemistry, Guru Nanak Dev University, Amritsar, for their constant support to carry out this work and to Thiru C. Subhasingh, Correspondent and The Principal, Mepco Schlenk Engineering College for their constant encouragement to do this collaborative work.
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Structural investigation of bismuth borate glasses under the influence of g-irradiation through ultrasonic studies G. Sharma a,, V. Rajendran b, K.S. Thind c, Gagandeep Singh d, Amarjit Singh c a
Department of Physics, Kanya Maha Vidyalaya, Jalandhar, Punjab, India Research and Development and Centre for Nano Science and Technology, KS Rangasamy College of Technology, KSR Kalvi Nagar, Tiruchengode 637 215, India c Department of Physics, Guru Nanak Dev University, Amritsar 143005, India d Department of Chemistry, Lyallpur Khalsa College, Jalandhar, Punjab, India b
a r t i c l e in fo
abstract
Article history: Received 30 January 2009 Received in revised form 10 May 2009 Accepted 13 May 2009
The ultrasonic velocity and attenuation measurements for different compositions of irradiated heavy metal oxide (HMO) borate glasses xBi2O3 (1x) B2O3 (where x ¼ 0.25, 0.30, 0.35, 0.40, 0.45) has been investigated at room temperature (303 K) using pulse echo overlap method. The elastic moduli, Debye temperature, Poisson’s ratio and other acoustical parameters have been obtained from experimental data. Structural changes after irradiation have been investigated by using FTIR spectroscopy and ultrasonic studies. As the changes are strongly dependent on the internal structure of the absorbing substance, in the present investigation ultrasonic velocities before and after g-irradiation in bismuth borate glasses are measured as a function of composition, from which the structural changes in the network former B2O3 and modifier Bi2O3 due to irradiation are obtained. Crown Copyright & 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Glasses Ultrasonic velocity Irradiation
1. Introduction Glasses containing heavy metal oxide (HMO) have attracted the attention of several workers in recent years for their excellent infrared transmission compared with conventional glasses. Attempts [1–7] have been made to explore the mechanical, thermal, and optical properties of these glasses. Need of high density, radiation hardness and fast inorganic scintillators have put again HMO glasses at the center stage [8,9]. Glasses containing bismuth have been investigated for possible use in scintillation detectors for high energy physics [10–11]. The properties of glasses are closely related to inter-atomic forces and potentials in lattice structure. Thus, any change in lattice due to doping and/or irradiation can be directly noted. The elastic properties, Poisson’s ratio, attenuation or other related parameters are of great interest to investigate the linear and anomalous variation as a function of composition of glass and have been interpreted in terms of a structural softening or transformation of cross-linkages in the glass structure [12]. The addition of B2O3, which forms a network structure related to the silicates, creates a glass with higher melting point and greater ability to withstand temperature changes. To investigate the structure of oxide glasses, the coordination number of network former and the change of oxygen bonds of
Corresponding author.
E-mail address: [email protected] (G. Sharma).
frame work induced by the cation modifiers need to be investigated. Much information on this can be obtained from FTIR spectroscopy. The majority of structural investigations on heavy metal oxide glasses before irradiation in past time have been made using vibrational spectroscopy and ultrasonic study [12–17]. Ultrasonic technique similar to other techniques plays a significant role in understanding the structural characteristics of glass network. Recently El-Batal [18] has studied effect of gamma irradiation on thermal expansion and infrared studies of bismuth borate glasses. An interaction of g-ray with glasses results in change in optical and physical properties [19] by formation of induced defects during progressive irradiation [20–23]. The present study represents change in the structure of bismuth borate glasses after gamma irradiation through ultrasonic studies. In order to explore the validity of the ultrasonic technique elastic constants have been compared before [24] and after irradiation.
2. Experimental details 2.1. Sample preparation Glasses of type xBi2O3 (1x) B2O3 (where x is the mole fraction) were prepared by using the melt-quenching technique. The chemical compositions of all these glasses are shown in Table 1. Commercial grade chemicals of Bi2O3 (Fluka Chemical Company) and B2O3 (Aldrich Chemical Company) having 99.99%
0921-4526/$ - see front matter Crown Copyright & 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.05.018
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Table 1 Sample name, thickness, mole fraction, density and molar volume (before and after irradiation) for present glass system at 303 K. Sample name
BB1 BB2 BB3 BB4 BB5
Thickness t 103 (m)
12.0 11.0 13.0 11.0 12.5
Density (kg/m3)
Mole fractions
Molar volume Vm 106 (m3 mol1)
Bi2O3 (x)
B2O3 (1x)
Before g-fall
After g-fall
Before g-fall
After g-fall
0.25 0.30 0.35 0.40 0.45
0.75 0.70 0.65 0.60 0.55
5201 5361 5691 5796 6211
5328 5442 5476 6023 6207
30.9 32.9 34.6 36.1 37.0
31.7 34.6 38.0 37.9 39.9
purity level were used as starting materials. Appropriate amounts of oxides were weighed by using an electronic balance having an accuracy of 0.001 g. The crucible containing powder was heated at 973 K in muffle furnace for 10 min. The weight loss measurements were taken to insure stoichiometric accuracy. The chemicals were then mixed in a pestle mortar for half an hour. The platinum crucible containing the batch was then placed in an electric furnace capable of reaching a temperature of 1673 K. It was heated at a temperature of 1173 K for half an hour under normal atmospheric conditions and then dry oxygen was bubbled through it using a quartz tube to ensure homogeneity of the glass melt. Heating was performed for a longer period of time. The melt was then poured into a cylindrical graphite mould. The mold was moved into an annealing furnace kept at 623 K [25] for half an hour and then allowed to cool down for 12 h. The samples were grounded with the help of an electric machine using different grades of SiC abrasives and aluminum oxide with machine oil by setting the sample in a specially designed holder to maintain the two faces parallel. The polishing was done with cerium oxide to obtain flatness. Thickness measurement was carried out by micrometer. 2.2. Gamma-ray irradiation All glass samples were irradiated using 60Co radioisotope at Radiology Department, Guru Teg Bahadur Hospital, Amritsar, India. The dose rate was 1.77 Gy min1. The samples were irradiated for necessary time interval to achieve the overall dose of 2.5 kGy.
back wall echoes, (iii) cross-correlation of two desired echoes for finding the approximate time delay, (iv) adopting cubic spline interpolation method to the peak portion of the cross-correlated function to arrive at exact time delay, and (v) calculating ultrasonic velocity from the sample thickness and the measured time delay. Variation in glass sample thickness and plane parallelism between faces was minimized by surface grinding of the samples and obtaining plane parallelism with an accuracy of 75 m s1 which corresponds to a percentage change in velocity of about 70.1%. 2.5. Attenuation measurements Attenuation measurements were carried out using a contact type transducer operated at a frequency of 5 MHz. Care was taken to avoid the problem of near field effects. A constant pressure was maintained between transducer and glass sample during attenuation measurements. Among the various available couplants, filtered machine oil was found to be more suitable to get steady back wall echo train in the oscilloscope screen. Attenuations in all the glass samples were measured both for longitudinal and shear waves by measuring the peak amplitude of the successive back wall echoes of ultrasonic signals from the glass samples using the following relation: 20 Im (1) a¼ log In 2ðm nÞd where Im and In are, respectively, the maximum amplitude (voltage) of mth and nth pulse echoes. The percentage error in the attenuation measurement is 72%.
2.3. Optical measurements Infrared absorption spectra of powdered glass samples were recorded in the range 400–4000 cm1 using KBr technique at room temperature. A recording spectrometer of type Shimadzu FTIR-8700 was used to reveal the absorption spectra. Optical measurements were taken immediately after irradiation. 2.4. Ultrasonic velocity measurements The ultrasonic velocity and the attenuation in all glass samples were measured using a specially designed experimental set-up in one of the author’s laboratory. A high power ultrasonic process control system (Fallon Ultrasonic Inc., Canada), a 100 MHz digital storage oscilloscope (Hewlett Packard, USA) and a PC were used to record the ultrasonic rf signals. Precise transit time measurements were carried out employing cross-correlation technique [26,27] using longitudinal and shear ultrasonic waves generated by PZ transducers operated at 5 MHz frequency. Various steps involved in the precise time measurements were: (i) acquisition, digitization and storage of the rf signals from the transducer, (ii) application of the window technique to select any two successive
2.6. Density and molar volume measurements The density was obtained from Archimedes principle using benzene as buoyant. The density was determined employing the following relation:
r¼
Wa rb Wa Wb
(2)
where Wa is the weight in air, Wb the weight in buoyant and rb the density of buoyant. All the weight measurements have been made using a digital balance (M/s. Sartorius, Model: BP221S, USA). The accuracy in the measurement of weight is 70.1 mg. The experiment was repeated five times to get an accurate value in density. The overall accuracy in the density measurement is 70.5 kg m3 and hence, the percentage error in the measurement of density is 70.05%. The molar volume, Vm, of a glass is given by using molecular weight (M), mole fraction (x) and density (r) of the component i as follows: Pn xM (3) V m ¼ i¼1 i i
r
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Table 1 lists the measured values of density and molar volume for present glass system before and after irradiation.
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Poisson’s ratio for bismuth borate glass system maxima is observed at 0.35 mol fraction. For Debye temperature maxima is observed at 0.30 mol fraction of Bi2O3. Acoustical impedance and microhardness behaviour, with increase in Bi2O3 content is shown in Figs. 9 and 10. In order to explain observed variations in the acoustical parameters of present systems two facts must be considered namely (i) effect of glass composition and hence, structure of glass matrix and (ii) effect of irradiation. It is well known that structure of pure borate glass consists of six membered boroxyl ring constructed from three BO3 units located outside boroxyl rings; the mole fraction of boron atoms in boroxyl ring and outside the boroxyl ring is, respectively, 0.65 and 0.35 [28]. Addition of modifier leads to structural changes by the formation of nonbridging oxygen (NBO) atoms and/or formation of BO4 units. Displacement of atoms from their positions in glass network caused by irradiation cascades can give rise to a number of structural changes [29]. The change in geometrical configuration, co-ordination number, cross-link density and dimensions of
3. Results and discussion Figs. 1 and 2 shows the plots of longitudinal (UL) and shear wave velocities before and after gamma irradiation against Bi2O3 contents. Before irradiation the smooth variation of ultrasonic velocity with increase in Bi2O3 content have been observed. It is found that for bismuth borate glass system variation of ultrasonic velocity (both longitudinal and shear) is not smooth with increase in Bi2O3 contents. Both longitudinal and shear velocities exhibit maxima at 0.30 mol fraction of Bi2O3 and minima at 0.35 mol fraction of Bi2O3. Figs. 3–6 shows variation of longitudinal and shear modulus, Young’s and bulk modulus with Bi2O3 content, respectively. All elastic moduli exhibit maxima for BB2 glass sample and minima for BB3 sample. Figs. 7 and 8 shows variation of Poisson’s ratio and Debye temperature with Bi2O3 content.
4300 UL before irradiation UL after irradiation
6100
UL (m/s) before irradiation
5900
4250 4200
5700 5500
4150
5300
4100
5100 4900
4050
4700
4000
4500 4300
UL (m/s) after irradiation
6300
3950
4100 3900
3900 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 1. Variation of longitudinal (UL) velocity before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
2425
3600 US before irradiation US after irradiation
3500 3400
2375
3200
2325
3100 3000
2275
2900 2800
2225
2700 2600
2175
2500
US (m/s) after irradiation
US (m/s) before irradiation
3300
2400 2125
2300 2200 2100
2075 0.25
0.30
0.35
0.40
0.45
Bi2O3 content (mole fraction) Fig. 2. Variation of shear (US) velocity before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
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224 118
L before irradiation L after irradiation
204
184
108
164
103
144
98
124
93
104
88
84
L (GPa) after irradiation
L (GPa) before irradiation
113
83 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 3. Variation of longitudinal modulus (L) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
40
69 G before irradiation G after irradiation
64
38 36
54
34
49 32 44 30 39
G (GPa) after irradiation
G (GPa) before irradiation
59
28
34
26
29
24
24 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 4. Variation of shear modulus (G) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
interstitial space of glass decide the density and therefore, density is a tool in revealing the degree of change in structure with composition of glasses [30]. In the studied binary system, the replacement of B2O3 by heavy metal oxide causes an increase in density and molar volume (Table 1) both before [24] and after irradiation. This increase in density with HMO content can be attributed to change in crosslink density, the greater molecular weight of bismuth atom and the co-ordination number of Bi3+ ions. Comparison of density data before and after irradiation indicates decrease in density after irradiation for BB3 and BB5 glasses and increase for BB1, BB2 and BB4 glass compositions. Shelby [31] suggested that boron atoms are directly involved in densification process. Damage by irradia-
tion can cause compaction of B2O3 by breaking the bond between trigonal elements, allowing the formation of tetrahedral BO4 [19]. The tetrahedral groups are strongly bonded than the triangular BO3 and a compact structure is expected leading to higher density after irradiation as found for BB1, BB2 and BB4 in present study. The observed decrease in density for BB3 and BB5 compositions can be related to vacancies created in glass structure due to irradiation. These vacancies increase the molar volume and decrease in density after irradiation (Table 1). Prado et al. [29] suggested that density changes during irradiation are associated with the generation of vacancies and also with effective transport of an atom between inhomogeneities. The decrease of density for BB3 and BB5 shows presence of transition metal impurities and
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94
165
E before irradiation E after irradiation
90 88 86 84
155
82 80 150
78 76 74
145
72
E (GPa) after irradiation
E (GPa) before irradiation
160
92
70
140
68 66 64
135
62 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 5. Compositional dependence of Young’s modulus (E) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
70
118 K before irradiation K after irradiation
116
66
112 64
110 108
62
106
60
104
58
102
K (GPa) after irradiation
K (GPa) before irradiation
114
68
56 100 54
98
52
96 0.25
0.30
0.35
0.40
0.45
Bi2O3 content (mole fraction) Fig. 6. Compositional dependence of bulk modulus (K) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
creation of vacancies for BB3 and BB5. This result indicates that irradiation leads to decrease in rigidity for BB3 by formation of vacancies, whereas for BB5 glass composition effect of irradiation is very less in comparison to BB3 as 45 mol% of Bi2O3 for BB5 glass impart radiation hardness in comparison to BB3 (35 mol% of Bi2O3). Any change in the cross-link density and bond stretching force constant consequently changes the bulk modulus of glass network [32]. To interpret our data on irradiation effect on elastic moduli let us first reveal the results obtained by El-Adawy [24] on elastic properties of present system which indicate that there is change in the behaviour of the compositional dependence of all the elastic properties at 25 mol%. The longitudinal and shear wave velocities and elastic moduli increase with increase in Bi2O3 content up to
25 mol% and then decrease as Bi2O3 content increases further. However, measurement of elastic properties after gamma fall indicates that ultrasonic velocity and elastic moduli do not show any smooth variation. This indicates that irradiation effects on acoustical properties strongly depend on composition. The observed variation in elastic moduli show a minimum at 0.35 and maximum at 0.30 mol fraction of Bi2O3 (Figs. 3–6), which further confirms the softening of BB3 glass composition and maximum value of BB2 indicates rigidity of glass after irradiation. According to Bridge et al. [32], Debye temperature describes the temperature at which nearly all modes of vibration in a solid are excited and it decreases when ultrasonic velocity and hence rigidity decreases (Fig. 8). Fig. 7 shows variation of Poisson’s ratio which is a measure of cross-link density and it decreases with increase in
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0.300 σ before irradiation σ after irradiation
σ before irradiation
0.295
0.295
0.290
0.290
0.285
0.285
0.280
0.280
0.275
0.275
0.270
0.270
0.265
0.265
0.260
0.260
0.255
0.255
0.250 0.23
0.28
0.33 0.38 Bi2O3 content (mole fraction)
σ after irradiation
0.300
0.250 0.47
0.42
Fig. 7. Variation of Poisson’s ratio (s) before and after irradiation with different Bi2O3 content.
450 θD before irradiation θD after irradiation
θD (Κ) before irradiation
520
440 430
500
420
480
410 400
460
390 440
380
420
370
400
360
380
350
θD (K) after irradiation
540
340
360
330 340
320
320
310 300
300 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 8. Variation of Debye temperature (yD) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
cross-link density. Therefore, decrease of Debye temperature and increase of Poisson’s ratio at 0.35 mol fraction of Bi2O3 supports our present discussion of softening of BB3 glass compositions In Figs. 9 and 10, acoustical impedance and microhardness are plotted against Bi2O3 content. The decrease in microhardness and acoustical impedance for BB3 glass further indicates that this glass composition is most affected by irradiation. The decrease in hardness after gamma irradiation can be attributed to defects created in the glass. The change of hardness depends on concentration and type of defects [33], depending on the glass matrix and its dopants irradiation can lead to formation of intrinsic [34] and extrinsic [35] defects. Irradiation with relatively high doses of gamma rays produces a similar structural change
with those produced by heat treatment. Heat treatment produces hard particles dispersed in soft matrix, thus the overall hardness increases and on the other hand formation of soft particles in hard matrix causes a decrease in microhardness [36]. Infrared investigations reveal structural changes and observed changes in infrared spectra due to irradiation and can be interpreted by assuming that irradiation of glass leads to formation of induced defects which lead to a decrease of the intensity of the main characteristic mid absorption bands [18]. Vibrational modes of borate network are mainly active in three infrared spectral regions: first region at around 700 cm1 is due to bending of B–O–B linkage, the second at 1200–800 cm1 is due to B–O bond stretching of tetrahedral BO4 units and third at
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2.0x107 2.0x107 1.9x107 1.9x107 1.8x107 1.8x107 1.7x107 1.6x107 1.6x107 1.6x107 1.5x107 1.5x107 1.4x107 1.4x107 1.3x107
before irradiation after irradiation
0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
Z (kg m-2 sec-1) after irradiation
Z (Kg m-2 sec-1) before irradiation
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0.45
Fig. 9. Change in acoustic impedence (z) before and after irradiation with different Bi2O3 content. The drawn lines are guide to eyes.
20
8 before irradiation after irradiation
7
6
16
5
14
4 12 3
H (GPa) after irradiation
H (GPa) before irradiation
18
10 2 8 1 0.25
0.30 0.35 0.40 Bi2O3 content (mole fraction)
0.45
Fig. 10. Change in microhardness (H) with content of Bi2O3. The drawn lines are guide to eyes.
1500–1200 cm1 is due to stretching of trigonal BO3 units [37–44]. Bismuth containing glasses have four fundamental vibrations in the IR spectral regions at 830, 620, 450, and 350 cm1 [2,4,17,18]. Fig. 11 reveals the spectra of irradiated glasses. All samples show the presence of broad band at about 900 cm1 and a broad band centred at 1280 cm1 indicates the presence of BO4 tetrahedra. The low frequency band at 460 cm1 is attributed totally symmetric bending vibrations of BiO3 pyramidal units. A sharp peak at 1352 cm1 are attributed to the vibrations of bridging oxygen between BO3 and BO4 groups. An absorption band centred at 530 cm1 is observed for BB3 glass composition which indicates the presence of BiO6 octahedral units and thus, octahedra provides weaker binding in the glass network
in agreement with ultrasonic velocity measurements. The absorption band at 806 cm1 is not observed in present system, which reveals the absence of boroxol groups. The absorption peak around 670 cm1 indicates oxygen bridges between the two trigonal boron atoms.
4. Conclusions The observed changes in ultrasonic velocity and elastic moduli are related to change in structure of glass matrix due to irradiation. The acoustical properties of bismuth borate glasses indicates that BB3 glass is most affected by irradiation and have
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References
BB5
Absorption ( a..u.)
BB4 BB3
BB2
BB1
4000 3500 3000 2500 2000 1750 1500 1250 1000 750 1/cm
500
Fig. 11. Optical spectra of bismuth borate glasses after irradiation.
soft network whereas BB2 had rigid structure in comparison to other samples studied. FTIR spectra also support the structural changes after irradiation by change in the co-ordination number of boron atoms. Thus, it can be concluded that BB2 glass composition is more rigid in comparison to other glass compositions and hence, it can be used as radiation hard material for dose applied in the studied system. This result is in agreement with our previous study on irradiation effect on optical properties of heavy metal oxide borate glasses [22]. The observed results reveal that ultrasonic studies can be used as a tool in exploring the stability of the glass exposed to radiations.
Acknowledgements Authors are grateful to Head, Department of Applied Chemistry, Guru Nanak Dev University, Amritsar, for their constant support to carry out this work and to Thiru C. Subhasingh, Correspondent and The Principal, Mepco Schlenk Engineering College for their constant encouragement to do this collaborative work.
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