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Influence of alumina addition and gamma irradiation on the lithium borosilicate glasses
Author’s Accepted Manuscript Influence of alumina addition and gamma irradiation on the lithium borosilicate glasses A.P. Raut, V.K. Deshpande
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S0969-806X(17)30895-2 https://doi.org/10.1016/j.radphyschem.2018.04.011 RPC7818
To appear in: Radiation Physics and Chemistry Received date: 17 August 2017 Revised date: 29 March 2018 Accepted date: 8 April 2018 Cite this article as: A.P. Raut and V.K. Deshpande, Influence of alumina addition and gamma irradiation on the lithium borosilicate glasses, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.04.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Title Influence of alumina addition and gamma irradiation on the lithium borosilicate glasses. Authors and affiliations A.P. Raut and V.K. Deshpande Department of Physics, Visvesvaraya National Institute of Technology, Nagpur-440 010, India.
Corresponding author V.K. Deshpande Tel:+91-712-2801254 E-mail: [email protected]
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Abstract The physical properties like density, glass transition temperature (Tg) and ionic conductivity of Lithium Borosilicate (LBS) Glasses are influenced by Al2O3 addition and gamma irradiation. The addition of Al2O3 increases the Tg and density but decreases the ionic conductivity of LBS glasses when Al2O3 is added at the cost of glass formers. An impedance analysis, modulus study and scaling behavior revealed that the conduction and relaxation mechanism is temperature independent, and depends upon glass composition. The density and Tg of LBS glasses increased and ionic conductivity decreased with 1 kGy gamma irradiation. With increase in gamma irradiation to 2 kGy, the density and Tg were observed to decrease however, ionic conductivity was observed to increase. The comparison of present and earlier reported work [14] reveals that the effect of Al2O3 addition and gamma irradiation on LBS glasses depends on the way in which Al2O3 is added in the glass matrix i.e. whether, it is added at the cost of glass former or glass modifier. Keywords: Lithium aluminoborosilicate glasses; gamma irradiation; ion transport; Impedance spectroscopy; Tg. Highlights The density, Tg and ionic conductivity are modified by Al2O3 addition. Modification in properties depends on the way Al2O3 is added in glass. Gamma irradiation affects the properties of LBS glasses. After 1 kGy, ionic conductivity of glasses decreases with alumina addition. With 2kGy irradiation, the ionic conductivity increases.
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1. Introduction Lithium ion conducting glasses have enormous potential for utilization in electrochemical devices; hence extensive work has been done to develop these glasses with suitable properties [1–4]. It has been established that [1] the ion conduction in oxide glasses containing lithium oxide is due to the motion of the Li+ ions. The mobility of Li+ ions in the glass depends upon the various factors like chemical composition, concentration of mobile ions, internal glass network, and the number of Non-Bridging Oxygens (NBOs). The borosilicate glasses containing Al2O3 can be used for application in nuclear waste immobilization and thermal shock resistant glasses. This is due to formation of highly cross linked network.
In addition, the mixing of two glass
formers has been reported to yield glasses with better thermal stability compared with the single glass former [5-8]. Hence the present work deals with the study of lithium borosilicate glasses with Al2O3 addition. Any change in ion dynamics in ion conducting glasses is well studied through impedance spectroscopy. The material is characterized by complex spectra of conductivity and electric modulus. It may also be used to study the dynamics of bound or mobile charges in the glasses and possible applications of the materials. Thus, the impedance spectroscopy (IS) is a powerful technique to characterize the electrical properties of materials [9, 10]. The interaction of gamma ray with glass samples alters the physical properties based upon glass composition [11]. The change in glass network caused by gamma irradiation strongly depends upon the glass composition, irradiation dose and energy of gamma irradiation [12]. The LBS glass with alumina addition has been investigated with spectroscopic method like FTIR before and after gamma irradiation [11].
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It has been reported that, the physical properties of the LBS glasses are modified by gamma irradiation [13]. The effect of Al2O3 addition and gamma irradiation has been earlier studied in (30-x) Li2O: 70{6/7 B2O3: 1/7 SiO2}: x Al2O3 glasses. In this work, the physical properties like density, Tg and ionic conductivity have been studied [14]. It has been reported that the addition of alumina enhances the ionic conductivity of lithium borosilicate glasses and causes decrease in their density and Tg. After gamma irradiation it was observed that the ionic conductivity decreases where as the density and Tg increase. In order to develop further understanding regarding the effect of alumina addition and gamma irradiation on the densification, Tg and ionic conductivity of LBS glasses, the present work has been undertaken. In this study Al2O3 was added at the cost of two glass formers SiO2 and B2O3 keeping Li2O content fixed. Unlike the previous work where in Al2O3 was added at the cost of glass modifier Li2O keeping glass former content fixed.
2. Material and methods 2.1. Synthesis of glass sample The glass samples of composition 30Li2O: (70-x) :{ 6/7B2O3:1/7SiO2}:xAl2O3, x= (0, 2.5, 5, 7.5 and10) were synthesized by conventional melt quenching technique. A fixed quantity of Li2CO3, SiO2, B2O3 and Al2O3 (Merck) were taken and then mixed thoroughly in acetone. The dried mixture was taken in platinum crucible and kept in a furnace. The melting point of a glass varied according to glass composition from 1148K to 1173K. The molten glass was kept 30K above the melting point for an hour and stirred intermittently to obtain homogeneous melt. It was then poured into mould made of aluminium at room temperature to obtain cylinder shaped glass
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samples which were subsequently annealed at 573K. The cylinder shaped glass samples were cut to get disc shaped glass samples of thickness 2.3 mm. The thermal analysis was done by using DTA (DTG-60 SHIMADZU).
The
impedance measurements were carried out in the temperature range from 423K to 573K with the help of “High Resolution Dielectric Analyzer” (Alpha Analyzer) using silver electrodes within the frequency range 100 Hz - 20 MHz. Glass samples of disc shaped were coated with silver paint on both sides to make electrical contact with electrodes. The ionic conductivity was calculated from the impedance plots. The disc shaped glass samples of each composition were taken to measure the density by Archimedes principal with toluene as an immersion liquid. The estimated error was about ≤ 0.4% based upon repetitive measurement. The IR spectroscopy study was done with the help of IR Affinity-1 maker SHIMADZU. The spectra were recorded in the range 500–2000 cm−1 at room temperature.
2.2. Irradiation facility The glass samples were irradiated by gamma rays at “Bhabha Atomic Research Centre Mumbai, India (BARC)”. All the glass samples were exposed to doses 1kGy and 2kGy of gamma irradiation with a dose rate of 0.2 Gy s−1 at room temperature (300 K). The energy of gamma ray is 1.33 MeV. The samples are placed in the dosimeter for 1 hour 38 minutes to achieve 1 kGy gamma irradiation and 3 hours 17 minutes to achieve 2 kGy gamma irradiation.
3. Results and discussion 3.1Density The variation in density as a function of Al2O3 addition in LBS glasses is depicted in fig. 1. It can be observed that the density of glass samples increases with the addition of Al2O3. The
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increase in density of glass samples may be understood as follows. The alumina addition introduces AlO4 tetrahedra in the glass matrix. Further, there is a change in bond angles of AlO4 tetrahedra (as evident from the FTIR results) to fit in the glass network to form more compact structure which explains increase in density. As reported earlier [14], the addition of Al2O3 at the cost of glass modifier Li2O by keeping SiO2 and B2O3 content fixed decreases the density which was attributed to the formation of AlO4 tetrahedra. The AlO4 tetrahedra being larger in size than BO4 tetrahedra due to which expansion of glass network occur, and hence decrease in density was observed. In this study the FTIR did not show change in bond angle. Thus, the impact of alumina addition on density in both the series is different. The figure also reveals the effect of gamma irradiation on the density of glass samples. The effect of gamma irradiation (1kGy) on the density can be explained on the basis that the gamma collisions with the glass may result in atomic displacements. The atomic displacements due to gamma irradiation lead to filling of interstices, which increases the compactness of glass network. The gamma irradiation mostly affects the peripheral part of glass samples; as a result it becomes highly dense due to atomic displacement [15]. Similar results have been published by the Wagh et al. [16] and N.A. El-Alaily et al. [15] which support this work. The alumina added borosilicate glasses have ability to dissolve the full spectrum of nuclear waste due to low melting point. They can dissolve nuclear waste at temperatures hundreds of degrees below the traditionally used silicate based glasses. The 1 kGy gamma irradiation also causes densification of glass samples by the conversion of BO3 to BO4 and hence it can be used as a nuclear waste immobilization [17].
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Fig. 1: Effect of alumina addition on density of glasses before and after 1 and 2kGy irradiation Ruller and Friebele [18] have suggested that, gamma irradiation causes the breaking of higher membered rings of SiO4 tetrahedra and gives rise to more compact glass network. When lower members of the high-membered rings rebind, they form smaller membered ring configuration hence the density increases. Ezz al din [19] has suggested that due to gamma irradiation, bond angles are changed to fit in the interstices present in the glass network which lead to the compaction of glasses after gamma irradiation. The gamma irradiation causes the breaking of the bonds of BO3 to form BO4 tetrahedra, which lead to densification of glasses [20]. The formed BO4 tetrahedra form 3-dimentional networking throughout the glass network due to which compact structure is formed. Similar enhancement in the density with gamma irradiation was observed in the earlier work [14]. In both the cases, increase in density is attributed to formation of BO4 tetrahedra. The difference in the magnitude of density in two results is due to the fact that the effect of gamma irradiation on physical properties is composition sensitive, and hence the numbers of tetrahedral groups formed in the glass network are also different. Thus, the restoring force exerted by BO4
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also different in both the series according to glass composition. Similar decrease in density of borate glasses when exposed to higher dose of gamma irradiation has been reported by N.A. ElAlaily et.al. [15] Fig. 1 also shows the variation of density after 2kGy of gamma irradiation. The decrease in density is observed when irradiated dose was increased to 2kGy. The density was lesser than the corresponding unirradiated glass samples. The decrease in density after higher dose of gamma irradiation may be attributed to radiation induced defects due to breaking of bonds and conversion of BO4 to BO3 which leads to the disordering of the glass network and lead to decreases in the density. The anomalous behavior of density after 1kGy and 2 kGy gamma irradiation is attributed to change in coordination number of structural units. As discussed earlier, after 1 kGy gamma irradiation causes filling of interstices and conversion of BO3 into BO4 which lead to densification of glasses. The higher gamma irradiation of 2 kGy causes conversion of BO4 to BO3 due to rupturing of bonds and hence decrease in density is observed.
3.2 Glass transition temperature (Tg) The fig. 2 depicts the variation of Tg of lithium borosilicate glass with Al2O3 addition. It is observed that the Tg of the glass samples increases with alumina addition. The increase in Tg is due to increase in density of cross linking with alumina addition. However, it is accepted that when Al forms fourfold coordinated groups, it reduces the number of nonbridging oxygen (NBO) [11]. The addition of alumina as a glass former creates more bridging oxygen. Thus, Al2O3 and SiO2 gave stronger network to these glasses hence Tg increases [21]. Similar results have been reported by the Martin et al [22]. The rise in glass transition temperature of the glass sample is also in agreement with densification of the glass samples with Al2O3 addition. The 8
formation of bridging oxygen (BOs) is responsible for the decrease of CTE as they bring better network connectivity. In the present glass system there is formation of AlO4 tetrahedra which removes NBOs, due to which density of cross linking increases and hence CTE decreases and in turn this glasses can be used as a thermal shock resistant glasses. The Tg results with Al2O3 addition in LBS glasses in the present and earlier reported work [14] exhibit opposite trend. The decrease in Tg in reported work [14] is attributed to weakening of glass structure due to formation of AlO4 tetrahedra, in which Al-O bond length is higher than B-O bond length in BO4 tetrahedra.
Fig. 2: Effect of alumina addition on Tg of glasses before and after 1 and 2 kGy dose The figure shows, how gamma irradiation modifies Tg of LBS glass with Al2O3 addition. It is evident from the figure that, with gamma irradiation of 1kGy, the Tg of glass samples increases. The glass transition temperature (Tg) depends on the fraction of bridging oxygen (BO) linked to SiO4 and AlO4 tetrahedra of glass network. The increase in number of bridging oxygens (BOs) enhances the connectivity of glass network which increases the Tg of glass samples [21]. The rise in Tg after gamma irradiation is attributed to the breaking and bonding of glass network
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formed due to B2O3 and SiO2. The density of newly formed bonds after gamma irradiation increases due to which rise in Tg of glass sample is observed. The similar results are reported by Akshartha et al. [16] The 1 kGy gamma irradiation causes increase in Tg in present and earlier work [14]. Shelby [23] has suggested that irradiation causes breaking of B2O3 to form BO4 tetrahedra, which increases density of cross linking and hence increase in Tg is observed. Thus, increase in Tg is attributed to formed BO4 and SiO4 tetrahedra. The difference in Tg value after gamma irradiation in present and earlier work attributed to varying concentration of BO4 tetrahedra which are responsible for the density of cross linking. The effect of increase in irradiation dose (2kGy) on Tg of LBS glasses is also evident from this figure. The Tg of glass samples decreases after 2 kGy. The gamma irradiation of higher dose is assumed to break the network bonds. Thus, gamma irradiation induces structural changes in the glass matrix along with ionization [24]. This may also be responsible to alter the concentration of bridging oxygen (BO) and non bridging oxygens (NBO) due to which the density of cross linking decreases. This in turn decreases the glass transition temperature. The density results discussed earlier supports the Tg results. The increase and decrease in Tg after 1 and 2 kGy gamma irradiation respectively are attributed to change in the coordination number of boron atoms. The conversion of BO3 to BO4 causes increase in density of cross linking and hence increase in Tg is observed. The 2 kGy gamma irradiation have sufficient energy to break the BO4 tetrahedra and causes lowering of density of cross linking and as a result Tg decreases.
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3.3 Ionic conductivity Figure.3 shows the Arrhenius plot of conductivity of all the prepared glass samples before irradiation. The fig. 4 depicts the variation in ionic conductivity at 448K and activation energy as a function of Al2O3 content. This figure shows that the ionic conductivity increases and activation energy decreases with Al2O3 addition. In this glass series, lithia concentration is fixed and alumina is added at the cost of glass formers SiO2 and B2O3. The addition of Al2O3 may decrease the concentration of NBO which are responsible for the decrease in ionic conductivity. In addition to it, aluminium and silicon possess 3+ and 4+ oxidation state. In order to satisfy the necessary valency, bridging with the oxygen takes place. It removes non bridging oxygen [25]. Thus, increase in bridging oxygens provides less hopping sites to the Li+ ions and hence decrease in conductivity is observed with alumina addition. Figure 5 shows the response of real part of electric modulus (M′) with frequency for 5 mol% Al2O3 containing glass sample as a representative. There exist certain minimum cut off frequency for each temperature at which sudden increase in M′ occurs as shown in the figure. A sudden increase in M′ is because of variation in restoring forces of moving Li+ with increase in frequency [22]. Similar results were obtained for all the glass samples studied in this work. The effect of frequency on imaginary part of electric modulus M″ at different temperatures for 2.5 mol % Al2O3 glass is shown in Fig. 6. The peak value of M″ is shifting towards higher frequency with increase in temperature. The frequency corresponding to the peak value of M″ is called as relaxation frequency (fp). The peak value of M″ divides the plot into the two parts. The region below the relaxation frequency suggests the long range motion of Li + ions and region above the relaxation frequency suggests the carriers are spatially confined to potential wells and short range motion of Li+ ions.
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Fig.3: Variation of ionic conductivity with temperature, of unirradiated LBS glasses containing Al2O3.
Fig 6 depicts that the shifting of M″Max values towards higher frequency with increase in temperature. The relaxation time (τ) corresponding to relaxation frequency (fp ) can be given as : τ = 1/2πfp. The activation energy values for the imaginary part of electric modulus (M″) and dc conductivity are tabulated in Table.1. These two values are in close agreement with each other. Thus, during relaxation and conduction mechanism lithium ion have to cross over the same barrier of activation energy [26].
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Fig.4: Response of alumina addition on activation energy and ionic conductivity.
Fig. 5. Effect of frequency and temperature on real part of electric modulus (M′) for 5 mol% Al2O3 containing LBS glass sample.
Table.1. Variation of Ea(τ) and Ea (dc) with mol% Al2O3 for LBS glasses
mol % Al2O3
Ea(τ) in eV
Ea (dc) in eV
0
0.641
0.632
13
2.5
0.643
0.655
5
0.702
0.692
7.5
0.742
0.754
10
0.775
0.761
Fig.6. Effect of frequency and temperature on M″ for 2.5 mol% Al2O3 containing LBS glass sample. Scaling behavior In order to get better insight in to the conduction and relaxation mechanism, scaling model was put forward by Roling [27]. In order to study temperature dependence of conduction mechanism scaling of conductivity (σ) with respect to frequency for 2.5 mol% Al2O3 as a representative has been done. The graph of σ/ σdc versus f/ σdcT is plotted, for all glasses. The Fig. 8 shows grouping of each isotherm into single master curve. Thus, the superposition of each isotherm into single master curve indicates that the conduction mechanism is independent of temperature and lithium ions have to cross over the same activation energy barrier.
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Fig.7. Variation of relaxation time with temperature In order to explain the temperature dependence of relaxation mechanism scaling of imaginary part of electric modulus M″ with respect to frequency for 7.5 mol% Al2O3 as representative has been done. The graph of M″/M″max versus f/fmax is plotted, for all glass samples. The scaled isotherm overlap to the single master curve, as shown in Fig. 9. Thus, the overlapping of each isotherm to single master curve indicates that the relaxation mechanism is independent of temperature [28]. As discussed earlier the conduction mechanism is composition dependent hence compositional scaling has been done for each composition. The normalized conductivity log(σ/σdc) vs log(fx/σdcT) graph is plotted and shown in Fig. 10. Each composition retains their unique behavior for different mol% of Al2O3. Thus, the conduction mechanism is determined by glass composition.
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Fig.8. Single master curve of overlapped isotherm of different temperatures of conductivity for a 2.5 mol% Al2O3 glass sample.
Fig.9. Single master curve of overlapped isotherm of different temperatures of imaginary part of the electric modulus M″ for a 2.5 mol% Al2O3 glass sample.
Fig.10. Scaling data for different mol% Al2O3 containing glasses at 448 K. 16
Fig.a
Fig.c
Fig.b
Fig.d
Fig.11. (a-e): Comparison of ionic conductivity of LBS glasses before and after gamma irradiation with alumina addition.
Fig.e
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Fig.11. (a-e) shows the Arrhenius plots of unirrdiated, 1kGy and 2kGy irradiated glass samples. Fig.12 depicts the variation of ionic conductivity of glasses as a function of Al2O3 contents before and after irradiation. Figure reveals that the ionic conductivity of glasses decreases with alumina addition. As mentioned earlier, the AlO4 tetrahedra formed in glass network lead to the formation of dense structure. It results in decrease in ionic conductivity with alumina addition. The, trend of ionic conductivity in the present and earlier reported [14] work is opposite. As mentioned earlier Al2O3 in the given composition plays the role of glass former which removes non bridging oxygens hence doorways available to lithium ions are not sufficient to tunnel due to which ionic conductivity decreases. In reported work, an increase in ionic conductivity with Al2O3 addition was attributed to modifier role of Al2O3 which increases the mobility of Li+ ions through the glass network. Thus, the role of Al2O3 is different when it is added at the cost of glass former and glass modifier. The ionic conductivity of alumina added LBS glasses decreases after gamma irradiation of 1kGy. The decrease in ionic conductivity is attributed to decrease in mobility. The variation in ionic conductivity after gamma irradiation depends upon the following factors: 1. The number of NBOs. 2. The number of defects 3. Availability of window to tunnel through the glass network to Li+ ions. The 1kGy gamma irradiation may reduce the concentration of defects and change in bond angles, so that available interstices in the glass network get filled. This in turn reduces the mobility of Li+ ions. This is supported by observed increase in density of these samples after irradiation 1 kGy. The glass network become so dense, Li+ ions are arrested. In other words
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gamma irradiation causes the structural modification which includes conversion of NBO into BO in the glass network. The complex network formed in the glass matrix may trap Li+ due to which mobility of lithium decreases which lead to the decrease in ionic conductivity [29]. Thus, decrease in ionic conductivity in the present work is attributed to the trapping of Li+ ions. The ionic conductivity in present and earlier reported work [14] decreases with Al2O3 addition after 1 kGy gamma irradiation. The decrease in ionic conductivity in both cases is attributed to increase in density due to formation of compact glass network, which reduces the mobility of Li+ ions. The difference in the magnitude of ionic conductivity after 1kGy gamma irradiation in both series can be understood on the basis of varying density of cross linking due BOs associated with BO4 and SiO4 tetrahedra and the number of induced defects after gamma irradiation. Further it can be seen that, after gamma irradiation of 2 kGy the ionic conductivity increases. The increase in ionic conductivity of the glasses with higher dose of gamma irradiation may be attributed to the formation of NBOs. An irradiation breaks the glass network and thus the new network formed in glass, lead to the conversion of bridging oxygen (BOs) to non bridging oxygen (NBOs) [30]. The NBOs provide hopping site to the lithium ion for mobility and hence increases the electrical conductivity. The gamma irradiation of 2kGy has sufficient energy to cause breaking of glass network. The newly formed glass network after 2kGy gamma irradiation induces Si–O− with two NBO and trigonal BO3 with one NBO. The 2kGy gamma irradiation may also cause atomic displacement of atoms from regular site causing defects. Thus, these induced defects and NBOs provide easier pathway to the lithium ions mobility.
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In present work, the ionic conductivity of LBS glasses with Al2O3 addition is gamma irradiation dose dependent. The structural modification of glass network caused by the gamma irradiation depends upon dose. The ionic conductivity of LBS glasses with Al2O3 addition after 2kGy is higher than 1 kGy gamma irradiation. The large difference in values of ionic conductivity of LBS glasses with Al2O3 after 1and 2 kGy gamma irradiation is observed. The decrease in ionic conductivity after 1kGy gamma irradiation is attributed to structural modification due to breaking of bonds which induces BOs and bleaching of defects. The defects may be vacancy or interstitial defects. The bleaching of defects closes the doorways available to the lithium ions due to which decrease in ionic conductivity is observed. In addition, the simultaneous rupturing and bonding of bonds formed by boron, silicon and aluminium units upon gamma irradiation may cause formation of new three dimensional networking by removing NBOs. The higher ionic conductivity of LBS glasses with Al2O3 addition after 2 kGy may be due to the defects with ion trapping ability. The ion trapping defects provides hopping site to the Li+ ions for mobility. In addition, the decrease in density indicates lose glass structure having voids which favors the motion of Li+ ions. Thus, the induced defects are responsible for increase in ionic conductivity after gamma irradiation.
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Fig.12: Variation of ionic conductivity of glasses with Al2O3 addition before and after1 and 2kGy dose at 523K.
3.4 FTIR FTIR spectra of the investigated glasses with different Al2O3 content are shown in Fig.13.a. The effect of 1kGy and 2kGy gamma irradiation on IR spectra of LBS glass series is shown in Fig.13.b and Fig.13.c respectively. Table 2: The IR absorption peaks obtained for various glasses and their assignment.
Peak position
Assignment
References
(cm−1) 727
Asymmetric stretching O-Si or OAlO-Si or Al-O-Al
[31]
linkage 900
Si–O− stretching with two NBO
[32]
949
Si–O–Al linkages in ≡Si(OAl) and possibly in =Si(OAl)2
[32] 21
units 1158
Symmetric stretching of bridging oxygen of BO3 and
[33]
vibration of Si–O–Si links.
1187
Si–O–Si bending vibration with bridging oxygen.
[33]
1213
Stretching vibrations of NBOs of trigonal BO3 units.
[34]
1507
B–O bonds Stretching vibrations of BO3 groups with NBO. [35]
The peak located at 727 cm-1 indicates the presence of Al-O-Al linkage. In case of unirrdiated glass samples the peak centered at 727 cm-1 shows broadening while in irradiated glass samples of 1kGy and 2 kGy the peak become sharp indicating alteration in bond angles after exposure of gamma irradiation. This supports the density results discussed earlier. The shoulder at 900 cm-1 for glass samples irradiated with dose of 2 kGy indicates the presence of Si-O- stretching with two NBOs due to which electrical conductivity increases with Al2O3 addition in glass samples irradiated by 2 kGy. The shoulder at 1181 cm-1 is present in the samples containing 5, 7.5 and 10 mol% of Al2O3 of unirradited glasses disappear after gamma irradiation of dose 2 kGy. It suggests the absence of stretching vibration of Si-O-Si with bridging oxygen in 2kGy irradiated glass samples which may be responsible for decrease in Tg. The broad spectrum present at 1213 cm -1 in unirrdiated glass sample disappear after gamma irradiation of higher dose of 2kGy indicates the
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increase of BO3 with NBOs due to which electrical conductivity increases. The important peaks and their assignments are given in Table1.
Fig. 13.a. Effect of alumina addition on infrared spectrum for lithium borosilicate glasses.
Fig.13.b. Effect of 1kGy gamma irradiation on infrared spectrum for LBS glasses with alumina addition.
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Fig. 13.c. Effect of 2kGy gamma irradiation on infrared spectrum for LBS glasses with alumina addition. The peak at 1507 cm-1 present in unirrdiated and irradiated (2kGy) glass sample is absent in glass samples irradiated by 1 kGy, which indicates the decrease of BO3 with NBO due to which electrical conductivity decreases. Thus the result of IR spectra support the observed conductivity results discussed earlier. The intensity of band lying at 949 cm-1 increase sharply for unirradiated glass samples with Al2O3 addition which explain the increase in bridging oxygen and hence ionic conductivity decreases. After gamma irradiation of 1kGy the intensity of peak centered at 1158 cm-1 increase sharply indicating the growth of symmetric stretching of bridging oxygen of BO3 group in glass samples containing Al2O3 which implies that the structure become compact. This support he density and Tg results discussed earlier. After gamma irradiation of 1kGy, the peak centered at 949 cm-1 is retained in glass sample containing Al2O3 which correspond to SiO4 units with bridging oxygen which indicates the weakening of glass structure and decrease in electrical conductivity. The broadening of peak centered at 727 cm-1 indicates the change of bond angles due to formation of AlO4 tetrahedra with addition of alumina in unirradiated glass samples. Thus it is evident that the IR results 24
support the effect of Al2O3 addition and gamma irradiation on LBS glasses observed in this work.
4. Conclusion It can be concluded from the results obtained that the density and Tg of unirradiated lithium borosilicate glass samples increase with Al2O3 addition while the ionic conductivity decreases. The scaling of imaginary part of electric modulus and conductivity depicts collapse of isotherm into single master curve, which implies that the conduction and relaxation mechanism are temperature independent in these glasses but composition dependent. The irradiation of glasses by gamma rays of 1 kGy dose increases the density and Tg, and decreases the ionic conductivity. With increase in the gamma irradiation dose up to 2kGy, the density and Tg values decrease while the ionic conductivity increases. The result have been explained in the light of formation of AlO4, the number of NBOs and the defects caused by the gamma irradiation and the consequent modification in the glass network, supported by the IR results. The comparison of the results of present study with those of earlier work [14] reveals interesting findings. The density of lithium borosilicate glasses increases when Al2O3 is added at the cost of glass former (present study) however, when it is added at the cost of glass modifier , Li2O (earlier reported work) the density and Tg decreased. The ionic conductivity on the other hand side decrease with Al2O3 addition but increase in earlier work. The gamma irradiation of 1 kGy, the density and Tg of increased and ionic conductivity decreased in both the series. Thus, it can be concluded that the effect of Al2O3 and gamma irradiation on density, Tg and ionic conductivity depends on the way in which Al2O3 is added in the glass matrix.
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5. Acknowledgement Authors wishes to thank the Board of Research in Nuclear Sciences (BRNS), India for providing financial assistance and of Dr M.S. Kulkarni and S.G.Mhatre for providing gamma irradiation facility for the completion of this research project.
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S0969-806X(17)30895-2 https://doi.org/10.1016/j.radphyschem.2018.04.011 RPC7818
To appear in: Radiation Physics and Chemistry Received date: 17 August 2017 Revised date: 29 March 2018 Accepted date: 8 April 2018 Cite this article as: A.P. Raut and V.K. Deshpande, Influence of alumina addition and gamma irradiation on the lithium borosilicate glasses, Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2018.04.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Title Influence of alumina addition and gamma irradiation on the lithium borosilicate glasses. Authors and affiliations A.P. Raut and V.K. Deshpande Department of Physics, Visvesvaraya National Institute of Technology, Nagpur-440 010, India.
Corresponding author V.K. Deshpande Tel:+91-712-2801254 E-mail: [email protected]
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Abstract The physical properties like density, glass transition temperature (Tg) and ionic conductivity of Lithium Borosilicate (LBS) Glasses are influenced by Al2O3 addition and gamma irradiation. The addition of Al2O3 increases the Tg and density but decreases the ionic conductivity of LBS glasses when Al2O3 is added at the cost of glass formers. An impedance analysis, modulus study and scaling behavior revealed that the conduction and relaxation mechanism is temperature independent, and depends upon glass composition. The density and Tg of LBS glasses increased and ionic conductivity decreased with 1 kGy gamma irradiation. With increase in gamma irradiation to 2 kGy, the density and Tg were observed to decrease however, ionic conductivity was observed to increase. The comparison of present and earlier reported work [14] reveals that the effect of Al2O3 addition and gamma irradiation on LBS glasses depends on the way in which Al2O3 is added in the glass matrix i.e. whether, it is added at the cost of glass former or glass modifier. Keywords: Lithium aluminoborosilicate glasses; gamma irradiation; ion transport; Impedance spectroscopy; Tg. Highlights The density, Tg and ionic conductivity are modified by Al2O3 addition. Modification in properties depends on the way Al2O3 is added in glass. Gamma irradiation affects the properties of LBS glasses. After 1 kGy, ionic conductivity of glasses decreases with alumina addition. With 2kGy irradiation, the ionic conductivity increases.
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1. Introduction Lithium ion conducting glasses have enormous potential for utilization in electrochemical devices; hence extensive work has been done to develop these glasses with suitable properties [1–4]. It has been established that [1] the ion conduction in oxide glasses containing lithium oxide is due to the motion of the Li+ ions. The mobility of Li+ ions in the glass depends upon the various factors like chemical composition, concentration of mobile ions, internal glass network, and the number of Non-Bridging Oxygens (NBOs). The borosilicate glasses containing Al2O3 can be used for application in nuclear waste immobilization and thermal shock resistant glasses. This is due to formation of highly cross linked network.
In addition, the mixing of two glass
formers has been reported to yield glasses with better thermal stability compared with the single glass former [5-8]. Hence the present work deals with the study of lithium borosilicate glasses with Al2O3 addition. Any change in ion dynamics in ion conducting glasses is well studied through impedance spectroscopy. The material is characterized by complex spectra of conductivity and electric modulus. It may also be used to study the dynamics of bound or mobile charges in the glasses and possible applications of the materials. Thus, the impedance spectroscopy (IS) is a powerful technique to characterize the electrical properties of materials [9, 10]. The interaction of gamma ray with glass samples alters the physical properties based upon glass composition [11]. The change in glass network caused by gamma irradiation strongly depends upon the glass composition, irradiation dose and energy of gamma irradiation [12]. The LBS glass with alumina addition has been investigated with spectroscopic method like FTIR before and after gamma irradiation [11].
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It has been reported that, the physical properties of the LBS glasses are modified by gamma irradiation [13]. The effect of Al2O3 addition and gamma irradiation has been earlier studied in (30-x) Li2O: 70{6/7 B2O3: 1/7 SiO2}: x Al2O3 glasses. In this work, the physical properties like density, Tg and ionic conductivity have been studied [14]. It has been reported that the addition of alumina enhances the ionic conductivity of lithium borosilicate glasses and causes decrease in their density and Tg. After gamma irradiation it was observed that the ionic conductivity decreases where as the density and Tg increase. In order to develop further understanding regarding the effect of alumina addition and gamma irradiation on the densification, Tg and ionic conductivity of LBS glasses, the present work has been undertaken. In this study Al2O3 was added at the cost of two glass formers SiO2 and B2O3 keeping Li2O content fixed. Unlike the previous work where in Al2O3 was added at the cost of glass modifier Li2O keeping glass former content fixed.
2. Material and methods 2.1. Synthesis of glass sample The glass samples of composition 30Li2O: (70-x) :{ 6/7B2O3:1/7SiO2}:xAl2O3, x= (0, 2.5, 5, 7.5 and10) were synthesized by conventional melt quenching technique. A fixed quantity of Li2CO3, SiO2, B2O3 and Al2O3 (Merck) were taken and then mixed thoroughly in acetone. The dried mixture was taken in platinum crucible and kept in a furnace. The melting point of a glass varied according to glass composition from 1148K to 1173K. The molten glass was kept 30K above the melting point for an hour and stirred intermittently to obtain homogeneous melt. It was then poured into mould made of aluminium at room temperature to obtain cylinder shaped glass
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samples which were subsequently annealed at 573K. The cylinder shaped glass samples were cut to get disc shaped glass samples of thickness 2.3 mm. The thermal analysis was done by using DTA (DTG-60 SHIMADZU).
The
impedance measurements were carried out in the temperature range from 423K to 573K with the help of “High Resolution Dielectric Analyzer” (Alpha Analyzer) using silver electrodes within the frequency range 100 Hz - 20 MHz. Glass samples of disc shaped were coated with silver paint on both sides to make electrical contact with electrodes. The ionic conductivity was calculated from the impedance plots. The disc shaped glass samples of each composition were taken to measure the density by Archimedes principal with toluene as an immersion liquid. The estimated error was about ≤ 0.4% based upon repetitive measurement. The IR spectroscopy study was done with the help of IR Affinity-1 maker SHIMADZU. The spectra were recorded in the range 500–2000 cm−1 at room temperature.
2.2. Irradiation facility The glass samples were irradiated by gamma rays at “Bhabha Atomic Research Centre Mumbai, India (BARC)”. All the glass samples were exposed to doses 1kGy and 2kGy of gamma irradiation with a dose rate of 0.2 Gy s−1 at room temperature (300 K). The energy of gamma ray is 1.33 MeV. The samples are placed in the dosimeter for 1 hour 38 minutes to achieve 1 kGy gamma irradiation and 3 hours 17 minutes to achieve 2 kGy gamma irradiation.
3. Results and discussion 3.1Density The variation in density as a function of Al2O3 addition in LBS glasses is depicted in fig. 1. It can be observed that the density of glass samples increases with the addition of Al2O3. The
5
increase in density of glass samples may be understood as follows. The alumina addition introduces AlO4 tetrahedra in the glass matrix. Further, there is a change in bond angles of AlO4 tetrahedra (as evident from the FTIR results) to fit in the glass network to form more compact structure which explains increase in density. As reported earlier [14], the addition of Al2O3 at the cost of glass modifier Li2O by keeping SiO2 and B2O3 content fixed decreases the density which was attributed to the formation of AlO4 tetrahedra. The AlO4 tetrahedra being larger in size than BO4 tetrahedra due to which expansion of glass network occur, and hence decrease in density was observed. In this study the FTIR did not show change in bond angle. Thus, the impact of alumina addition on density in both the series is different. The figure also reveals the effect of gamma irradiation on the density of glass samples. The effect of gamma irradiation (1kGy) on the density can be explained on the basis that the gamma collisions with the glass may result in atomic displacements. The atomic displacements due to gamma irradiation lead to filling of interstices, which increases the compactness of glass network. The gamma irradiation mostly affects the peripheral part of glass samples; as a result it becomes highly dense due to atomic displacement [15]. Similar results have been published by the Wagh et al. [16] and N.A. El-Alaily et al. [15] which support this work. The alumina added borosilicate glasses have ability to dissolve the full spectrum of nuclear waste due to low melting point. They can dissolve nuclear waste at temperatures hundreds of degrees below the traditionally used silicate based glasses. The 1 kGy gamma irradiation also causes densification of glass samples by the conversion of BO3 to BO4 and hence it can be used as a nuclear waste immobilization [17].
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Fig. 1: Effect of alumina addition on density of glasses before and after 1 and 2kGy irradiation Ruller and Friebele [18] have suggested that, gamma irradiation causes the breaking of higher membered rings of SiO4 tetrahedra and gives rise to more compact glass network. When lower members of the high-membered rings rebind, they form smaller membered ring configuration hence the density increases. Ezz al din [19] has suggested that due to gamma irradiation, bond angles are changed to fit in the interstices present in the glass network which lead to the compaction of glasses after gamma irradiation. The gamma irradiation causes the breaking of the bonds of BO3 to form BO4 tetrahedra, which lead to densification of glasses [20]. The formed BO4 tetrahedra form 3-dimentional networking throughout the glass network due to which compact structure is formed. Similar enhancement in the density with gamma irradiation was observed in the earlier work [14]. In both the cases, increase in density is attributed to formation of BO4 tetrahedra. The difference in the magnitude of density in two results is due to the fact that the effect of gamma irradiation on physical properties is composition sensitive, and hence the numbers of tetrahedral groups formed in the glass network are also different. Thus, the restoring force exerted by BO4
7
also different in both the series according to glass composition. Similar decrease in density of borate glasses when exposed to higher dose of gamma irradiation has been reported by N.A. ElAlaily et.al. [15] Fig. 1 also shows the variation of density after 2kGy of gamma irradiation. The decrease in density is observed when irradiated dose was increased to 2kGy. The density was lesser than the corresponding unirradiated glass samples. The decrease in density after higher dose of gamma irradiation may be attributed to radiation induced defects due to breaking of bonds and conversion of BO4 to BO3 which leads to the disordering of the glass network and lead to decreases in the density. The anomalous behavior of density after 1kGy and 2 kGy gamma irradiation is attributed to change in coordination number of structural units. As discussed earlier, after 1 kGy gamma irradiation causes filling of interstices and conversion of BO3 into BO4 which lead to densification of glasses. The higher gamma irradiation of 2 kGy causes conversion of BO4 to BO3 due to rupturing of bonds and hence decrease in density is observed.
3.2 Glass transition temperature (Tg) The fig. 2 depicts the variation of Tg of lithium borosilicate glass with Al2O3 addition. It is observed that the Tg of the glass samples increases with alumina addition. The increase in Tg is due to increase in density of cross linking with alumina addition. However, it is accepted that when Al forms fourfold coordinated groups, it reduces the number of nonbridging oxygen (NBO) [11]. The addition of alumina as a glass former creates more bridging oxygen. Thus, Al2O3 and SiO2 gave stronger network to these glasses hence Tg increases [21]. Similar results have been reported by the Martin et al [22]. The rise in glass transition temperature of the glass sample is also in agreement with densification of the glass samples with Al2O3 addition. The 8
formation of bridging oxygen (BOs) is responsible for the decrease of CTE as they bring better network connectivity. In the present glass system there is formation of AlO4 tetrahedra which removes NBOs, due to which density of cross linking increases and hence CTE decreases and in turn this glasses can be used as a thermal shock resistant glasses. The Tg results with Al2O3 addition in LBS glasses in the present and earlier reported work [14] exhibit opposite trend. The decrease in Tg in reported work [14] is attributed to weakening of glass structure due to formation of AlO4 tetrahedra, in which Al-O bond length is higher than B-O bond length in BO4 tetrahedra.
Fig. 2: Effect of alumina addition on Tg of glasses before and after 1 and 2 kGy dose The figure shows, how gamma irradiation modifies Tg of LBS glass with Al2O3 addition. It is evident from the figure that, with gamma irradiation of 1kGy, the Tg of glass samples increases. The glass transition temperature (Tg) depends on the fraction of bridging oxygen (BO) linked to SiO4 and AlO4 tetrahedra of glass network. The increase in number of bridging oxygens (BOs) enhances the connectivity of glass network which increases the Tg of glass samples [21]. The rise in Tg after gamma irradiation is attributed to the breaking and bonding of glass network
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formed due to B2O3 and SiO2. The density of newly formed bonds after gamma irradiation increases due to which rise in Tg of glass sample is observed. The similar results are reported by Akshartha et al. [16] The 1 kGy gamma irradiation causes increase in Tg in present and earlier work [14]. Shelby [23] has suggested that irradiation causes breaking of B2O3 to form BO4 tetrahedra, which increases density of cross linking and hence increase in Tg is observed. Thus, increase in Tg is attributed to formed BO4 and SiO4 tetrahedra. The difference in Tg value after gamma irradiation in present and earlier work attributed to varying concentration of BO4 tetrahedra which are responsible for the density of cross linking. The effect of increase in irradiation dose (2kGy) on Tg of LBS glasses is also evident from this figure. The Tg of glass samples decreases after 2 kGy. The gamma irradiation of higher dose is assumed to break the network bonds. Thus, gamma irradiation induces structural changes in the glass matrix along with ionization [24]. This may also be responsible to alter the concentration of bridging oxygen (BO) and non bridging oxygens (NBO) due to which the density of cross linking decreases. This in turn decreases the glass transition temperature. The density results discussed earlier supports the Tg results. The increase and decrease in Tg after 1 and 2 kGy gamma irradiation respectively are attributed to change in the coordination number of boron atoms. The conversion of BO3 to BO4 causes increase in density of cross linking and hence increase in Tg is observed. The 2 kGy gamma irradiation have sufficient energy to break the BO4 tetrahedra and causes lowering of density of cross linking and as a result Tg decreases.
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3.3 Ionic conductivity Figure.3 shows the Arrhenius plot of conductivity of all the prepared glass samples before irradiation. The fig. 4 depicts the variation in ionic conductivity at 448K and activation energy as a function of Al2O3 content. This figure shows that the ionic conductivity increases and activation energy decreases with Al2O3 addition. In this glass series, lithia concentration is fixed and alumina is added at the cost of glass formers SiO2 and B2O3. The addition of Al2O3 may decrease the concentration of NBO which are responsible for the decrease in ionic conductivity. In addition to it, aluminium and silicon possess 3+ and 4+ oxidation state. In order to satisfy the necessary valency, bridging with the oxygen takes place. It removes non bridging oxygen [25]. Thus, increase in bridging oxygens provides less hopping sites to the Li+ ions and hence decrease in conductivity is observed with alumina addition. Figure 5 shows the response of real part of electric modulus (M′) with frequency for 5 mol% Al2O3 containing glass sample as a representative. There exist certain minimum cut off frequency for each temperature at which sudden increase in M′ occurs as shown in the figure. A sudden increase in M′ is because of variation in restoring forces of moving Li+ with increase in frequency [22]. Similar results were obtained for all the glass samples studied in this work. The effect of frequency on imaginary part of electric modulus M″ at different temperatures for 2.5 mol % Al2O3 glass is shown in Fig. 6. The peak value of M″ is shifting towards higher frequency with increase in temperature. The frequency corresponding to the peak value of M″ is called as relaxation frequency (fp). The peak value of M″ divides the plot into the two parts. The region below the relaxation frequency suggests the long range motion of Li + ions and region above the relaxation frequency suggests the carriers are spatially confined to potential wells and short range motion of Li+ ions.
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Fig.3: Variation of ionic conductivity with temperature, of unirradiated LBS glasses containing Al2O3.
Fig 6 depicts that the shifting of M″Max values towards higher frequency with increase in temperature. The relaxation time (τ) corresponding to relaxation frequency (fp ) can be given as : τ = 1/2πfp. The activation energy values for the imaginary part of electric modulus (M″) and dc conductivity are tabulated in Table.1. These two values are in close agreement with each other. Thus, during relaxation and conduction mechanism lithium ion have to cross over the same barrier of activation energy [26].
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Fig.4: Response of alumina addition on activation energy and ionic conductivity.
Fig. 5. Effect of frequency and temperature on real part of electric modulus (M′) for 5 mol% Al2O3 containing LBS glass sample.
Table.1. Variation of Ea(τ) and Ea (dc) with mol% Al2O3 for LBS glasses
mol % Al2O3
Ea(τ) in eV
Ea (dc) in eV
0
0.641
0.632
13
2.5
0.643
0.655
5
0.702
0.692
7.5
0.742
0.754
10
0.775
0.761
Fig.6. Effect of frequency and temperature on M″ for 2.5 mol% Al2O3 containing LBS glass sample. Scaling behavior In order to get better insight in to the conduction and relaxation mechanism, scaling model was put forward by Roling [27]. In order to study temperature dependence of conduction mechanism scaling of conductivity (σ) with respect to frequency for 2.5 mol% Al2O3 as a representative has been done. The graph of σ/ σdc versus f/ σdcT is plotted, for all glasses. The Fig. 8 shows grouping of each isotherm into single master curve. Thus, the superposition of each isotherm into single master curve indicates that the conduction mechanism is independent of temperature and lithium ions have to cross over the same activation energy barrier.
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Fig.7. Variation of relaxation time with temperature In order to explain the temperature dependence of relaxation mechanism scaling of imaginary part of electric modulus M″ with respect to frequency for 7.5 mol% Al2O3 as representative has been done. The graph of M″/M″max versus f/fmax is plotted, for all glass samples. The scaled isotherm overlap to the single master curve, as shown in Fig. 9. Thus, the overlapping of each isotherm to single master curve indicates that the relaxation mechanism is independent of temperature [28]. As discussed earlier the conduction mechanism is composition dependent hence compositional scaling has been done for each composition. The normalized conductivity log(σ/σdc) vs log(fx/σdcT) graph is plotted and shown in Fig. 10. Each composition retains their unique behavior for different mol% of Al2O3. Thus, the conduction mechanism is determined by glass composition.
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Fig.8. Single master curve of overlapped isotherm of different temperatures of conductivity for a 2.5 mol% Al2O3 glass sample.
Fig.9. Single master curve of overlapped isotherm of different temperatures of imaginary part of the electric modulus M″ for a 2.5 mol% Al2O3 glass sample.
Fig.10. Scaling data for different mol% Al2O3 containing glasses at 448 K. 16
Fig.a
Fig.c
Fig.b
Fig.d
Fig.11. (a-e): Comparison of ionic conductivity of LBS glasses before and after gamma irradiation with alumina addition.
Fig.e
17
Fig.11. (a-e) shows the Arrhenius plots of unirrdiated, 1kGy and 2kGy irradiated glass samples. Fig.12 depicts the variation of ionic conductivity of glasses as a function of Al2O3 contents before and after irradiation. Figure reveals that the ionic conductivity of glasses decreases with alumina addition. As mentioned earlier, the AlO4 tetrahedra formed in glass network lead to the formation of dense structure. It results in decrease in ionic conductivity with alumina addition. The, trend of ionic conductivity in the present and earlier reported [14] work is opposite. As mentioned earlier Al2O3 in the given composition plays the role of glass former which removes non bridging oxygens hence doorways available to lithium ions are not sufficient to tunnel due to which ionic conductivity decreases. In reported work, an increase in ionic conductivity with Al2O3 addition was attributed to modifier role of Al2O3 which increases the mobility of Li+ ions through the glass network. Thus, the role of Al2O3 is different when it is added at the cost of glass former and glass modifier. The ionic conductivity of alumina added LBS glasses decreases after gamma irradiation of 1kGy. The decrease in ionic conductivity is attributed to decrease in mobility. The variation in ionic conductivity after gamma irradiation depends upon the following factors: 1. The number of NBOs. 2. The number of defects 3. Availability of window to tunnel through the glass network to Li+ ions. The 1kGy gamma irradiation may reduce the concentration of defects and change in bond angles, so that available interstices in the glass network get filled. This in turn reduces the mobility of Li+ ions. This is supported by observed increase in density of these samples after irradiation 1 kGy. The glass network become so dense, Li+ ions are arrested. In other words
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gamma irradiation causes the structural modification which includes conversion of NBO into BO in the glass network. The complex network formed in the glass matrix may trap Li+ due to which mobility of lithium decreases which lead to the decrease in ionic conductivity [29]. Thus, decrease in ionic conductivity in the present work is attributed to the trapping of Li+ ions. The ionic conductivity in present and earlier reported work [14] decreases with Al2O3 addition after 1 kGy gamma irradiation. The decrease in ionic conductivity in both cases is attributed to increase in density due to formation of compact glass network, which reduces the mobility of Li+ ions. The difference in the magnitude of ionic conductivity after 1kGy gamma irradiation in both series can be understood on the basis of varying density of cross linking due BOs associated with BO4 and SiO4 tetrahedra and the number of induced defects after gamma irradiation. Further it can be seen that, after gamma irradiation of 2 kGy the ionic conductivity increases. The increase in ionic conductivity of the glasses with higher dose of gamma irradiation may be attributed to the formation of NBOs. An irradiation breaks the glass network and thus the new network formed in glass, lead to the conversion of bridging oxygen (BOs) to non bridging oxygen (NBOs) [30]. The NBOs provide hopping site to the lithium ion for mobility and hence increases the electrical conductivity. The gamma irradiation of 2kGy has sufficient energy to cause breaking of glass network. The newly formed glass network after 2kGy gamma irradiation induces Si–O− with two NBO and trigonal BO3 with one NBO. The 2kGy gamma irradiation may also cause atomic displacement of atoms from regular site causing defects. Thus, these induced defects and NBOs provide easier pathway to the lithium ions mobility.
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In present work, the ionic conductivity of LBS glasses with Al2O3 addition is gamma irradiation dose dependent. The structural modification of glass network caused by the gamma irradiation depends upon dose. The ionic conductivity of LBS glasses with Al2O3 addition after 2kGy is higher than 1 kGy gamma irradiation. The large difference in values of ionic conductivity of LBS glasses with Al2O3 after 1and 2 kGy gamma irradiation is observed. The decrease in ionic conductivity after 1kGy gamma irradiation is attributed to structural modification due to breaking of bonds which induces BOs and bleaching of defects. The defects may be vacancy or interstitial defects. The bleaching of defects closes the doorways available to the lithium ions due to which decrease in ionic conductivity is observed. In addition, the simultaneous rupturing and bonding of bonds formed by boron, silicon and aluminium units upon gamma irradiation may cause formation of new three dimensional networking by removing NBOs. The higher ionic conductivity of LBS glasses with Al2O3 addition after 2 kGy may be due to the defects with ion trapping ability. The ion trapping defects provides hopping site to the Li+ ions for mobility. In addition, the decrease in density indicates lose glass structure having voids which favors the motion of Li+ ions. Thus, the induced defects are responsible for increase in ionic conductivity after gamma irradiation.
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Fig.12: Variation of ionic conductivity of glasses with Al2O3 addition before and after1 and 2kGy dose at 523K.
3.4 FTIR FTIR spectra of the investigated glasses with different Al2O3 content are shown in Fig.13.a. The effect of 1kGy and 2kGy gamma irradiation on IR spectra of LBS glass series is shown in Fig.13.b and Fig.13.c respectively. Table 2: The IR absorption peaks obtained for various glasses and their assignment.
Peak position
Assignment
References
(cm−1) 727
Asymmetric stretching O-Si or OAlO-Si or Al-O-Al
[31]
linkage 900
Si–O− stretching with two NBO
[32]
949
Si–O–Al linkages in ≡Si(OAl) and possibly in =Si(OAl)2
[32] 21
units 1158
Symmetric stretching of bridging oxygen of BO3 and
[33]
vibration of Si–O–Si links.
1187
Si–O–Si bending vibration with bridging oxygen.
[33]
1213
Stretching vibrations of NBOs of trigonal BO3 units.
[34]
1507
B–O bonds Stretching vibrations of BO3 groups with NBO. [35]
The peak located at 727 cm-1 indicates the presence of Al-O-Al linkage. In case of unirrdiated glass samples the peak centered at 727 cm-1 shows broadening while in irradiated glass samples of 1kGy and 2 kGy the peak become sharp indicating alteration in bond angles after exposure of gamma irradiation. This supports the density results discussed earlier. The shoulder at 900 cm-1 for glass samples irradiated with dose of 2 kGy indicates the presence of Si-O- stretching with two NBOs due to which electrical conductivity increases with Al2O3 addition in glass samples irradiated by 2 kGy. The shoulder at 1181 cm-1 is present in the samples containing 5, 7.5 and 10 mol% of Al2O3 of unirradited glasses disappear after gamma irradiation of dose 2 kGy. It suggests the absence of stretching vibration of Si-O-Si with bridging oxygen in 2kGy irradiated glass samples which may be responsible for decrease in Tg. The broad spectrum present at 1213 cm -1 in unirrdiated glass sample disappear after gamma irradiation of higher dose of 2kGy indicates the
22
increase of BO3 with NBOs due to which electrical conductivity increases. The important peaks and their assignments are given in Table1.
Fig. 13.a. Effect of alumina addition on infrared spectrum for lithium borosilicate glasses.
Fig.13.b. Effect of 1kGy gamma irradiation on infrared spectrum for LBS glasses with alumina addition.
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Fig. 13.c. Effect of 2kGy gamma irradiation on infrared spectrum for LBS glasses with alumina addition. The peak at 1507 cm-1 present in unirrdiated and irradiated (2kGy) glass sample is absent in glass samples irradiated by 1 kGy, which indicates the decrease of BO3 with NBO due to which electrical conductivity decreases. Thus the result of IR spectra support the observed conductivity results discussed earlier. The intensity of band lying at 949 cm-1 increase sharply for unirradiated glass samples with Al2O3 addition which explain the increase in bridging oxygen and hence ionic conductivity decreases. After gamma irradiation of 1kGy the intensity of peak centered at 1158 cm-1 increase sharply indicating the growth of symmetric stretching of bridging oxygen of BO3 group in glass samples containing Al2O3 which implies that the structure become compact. This support he density and Tg results discussed earlier. After gamma irradiation of 1kGy, the peak centered at 949 cm-1 is retained in glass sample containing Al2O3 which correspond to SiO4 units with bridging oxygen which indicates the weakening of glass structure and decrease in electrical conductivity. The broadening of peak centered at 727 cm-1 indicates the change of bond angles due to formation of AlO4 tetrahedra with addition of alumina in unirradiated glass samples. Thus it is evident that the IR results 24
support the effect of Al2O3 addition and gamma irradiation on LBS glasses observed in this work.
4. Conclusion It can be concluded from the results obtained that the density and Tg of unirradiated lithium borosilicate glass samples increase with Al2O3 addition while the ionic conductivity decreases. The scaling of imaginary part of electric modulus and conductivity depicts collapse of isotherm into single master curve, which implies that the conduction and relaxation mechanism are temperature independent in these glasses but composition dependent. The irradiation of glasses by gamma rays of 1 kGy dose increases the density and Tg, and decreases the ionic conductivity. With increase in the gamma irradiation dose up to 2kGy, the density and Tg values decrease while the ionic conductivity increases. The result have been explained in the light of formation of AlO4, the number of NBOs and the defects caused by the gamma irradiation and the consequent modification in the glass network, supported by the IR results. The comparison of the results of present study with those of earlier work [14] reveals interesting findings. The density of lithium borosilicate glasses increases when Al2O3 is added at the cost of glass former (present study) however, when it is added at the cost of glass modifier , Li2O (earlier reported work) the density and Tg decreased. The ionic conductivity on the other hand side decrease with Al2O3 addition but increase in earlier work. The gamma irradiation of 1 kGy, the density and Tg of increased and ionic conductivity decreased in both the series. Thus, it can be concluded that the effect of Al2O3 and gamma irradiation on density, Tg and ionic conductivity depends on the way in which Al2O3 is added in the glass matrix.
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5. Acknowledgement Authors wishes to thank the Board of Research in Nuclear Sciences (BRNS), India for providing financial assistance and of Dr M.S. Kulkarni and S.G.Mhatre for providing gamma irradiation facility for the completion of this research project.
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