J Therm Anal Calorim (2014) 115:367–374 DOI 10.1007/s10973-013-3229-6
Comparison of electrical conductivity and thermal properties of borosilicate glass with and without simulated radioactive waste Hrudananda Jena • R. Asuvathraman • K. V. Govindan Kutty • P. R. Vasudeva Rao
Received: 9 January 2013 / Accepted: 26 April 2013 / Published online: 21 May 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013
Abstract Electrical conductivity and percentage linear thermal expansion of the borosilicate glass (BSG) and simulated waste-loaded borosilicate glass (BSGW) were measured in the temperature range of 300–780 K and compared. Pronounced increase in electrical conductivity was observed around glass transition temperature (Tg) of BSG and BSGW. The activation energy (Ea) of electrical conduction determined from the measured data for BSG and BSGW is 0.961 ± 0.005 and 0.960 ± 0.005 eV, respectively. The % average linear thermal expansion of BSGW showed a slight decreasing trend compared with pristine BSG. The average coefficient of thermal expansion determined from dilatometry data is 12.87 ± 0.24 9 10-6 and 11.94 ± 0.23 9 10-6 K-1 for BSG and BSGW, respectively. The Tg measured by dilatometry is 806 ± 24 K for BSG and 790 ± 23 K for BSGW, respectively. The Tg measured by DTA was found to be 820 ± 7 and 805 ± 5 K for BSG and BSGW, respectively, for heating cycle. The Tg values obtained from DSC measurements are 805 ± 5 and 803 ± 5 K for BSG and BSGW, respectively. The Tg of BSGW showed a slight decrease compared with that of BSG. The values obtained by DSC examination also showed the lowering of Tg values for the waste-loaded composition. The lowering of Tg may be attributed to the interaction of glassforming agents and simulated waste elements. Keywords Borosilicate glass Electrical conductivity of BSG Thermal expansion Glass transition temperature of waste-loaded glass
H. Jena (&) R. Asuvathraman K. V. G. Kutty P. R. V. Rao Materials Chemistry Division, Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India e-mail:
[email protected];
[email protected]
Introduction Borosilicate glass (BSG) is the widely accepted matrix for immobilizing radioactive high-level waste (HLW) [1–3]. The motivation for incorporating radioactive wastes into a BSG matrix is to transform the liquid waste into a durable solid which can prevent the release of radioactivity into the environment during its storage and disposal [4]. Immobilization of HLW is performed by using various vitrification processes [5–7]; the Joule heated ceramic melting (JHCM) is one of them. In this process, the HLW slurry and glassforming chemicals are melted by electric melting [8]. In Joule heated melting process, electric current flows through the material, because of the internal resistance of the material, the current loses power and transfers heat energy to the material. The dissipated power is predicted by Joule’s law as expressed in Eq. 1. P ¼ I2R
ð1Þ
where P is the power, I is the current, R is the resistance [9]. The resistivity of the melt varies with concentration and nature of the components like fission product elements and transition metals like corrosion products or other sources of Fe, Ni, etc. present in the glass melt. Further, when the platinum group metals (PGMs) are removed from the molten BSG using solvent metals (Cu, Sn, etc) by alloying [10, 11] before the final vitrification using induction melting; a small percentage of solvent metals (Cu, Sn, etc.) remain dissolved in the BSG matrix. The dissolved solvent metals remained in the BSG will influence the resistivity of the BSG which is meant for final vitrification by JHCM technique, therefore, the effect of these elements on resistivity of the BSG should also be considered in addition to fission products. Hence, electrical resistivity of the glass melt and waste-loaded glass melt as
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a function of temperature is a very important property for JHCM of glass waste forms. Several other thermo-physical properties (thermal expansion, glass transition temperature, viscosity of the melt, etc.) of the BSG melts are also essential for a comprehensive understanding of the vitrification process. The effect of composition on electrical conductivity of sodium BSG without any waste loading was studied by Grandjean et al. [12, 13]. Various glass compositions have been studied for the suitability of the matrices for nuclear waste [14–19]. In this study, a simplified SON68 glass composition [14, 16] with the major glass-forming constituents (SiO2, B2O3 and Na2O) was taken as the base BSG without waste loading and its electrical conductivity, thermal expansion and glass transition properties were measured and compared with the simulated waste-loaded borosilicate glass (BSGW). The BSG and waste-loaded BSGW compositions investigated in this study are different from the compositions reported in the literature [12–19]. Electrochemical impedance spectroscopy (EIS) and DC techniques were used to measure the electrical resistivity of the samples. Thermo-mechanical analysis (TMA), differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TG)–differential thermal analysis (DTA) techniques were used to evaluate the thermo-physical properties of the samples. The results are discussed based on the experimental observations.
Experimental Preparation of borosilicate glass (BSG) and waste-loaded BSG (BSGW) The glass-forming reagents (SiO2: 57 mass%/59.36 mol%, B2O3: 25 mass%/22.46 mol%, Na2O: 18 mass%/18.18 mol%) with ratio of SiO2/B2O3 = 2.64, Na2O/B2O3 = 0.81 [10] were taken and mixed, ground well to ensure homogenization. The mixture was melted at 1,473 K/2 h by resistive heating in alumina crucible in air to prepare the BSG. The melt was transferred or poured into a graphite crucible of diameter 13.0 mm and 15 mm height to fabricate a BSG pellet of required dimension for thermal expansion measurement by dilatometry. The pellet was removed from the graphite crucible and annealed at 800 K in a platinum crucible for 15 h in air. BSGW was prepared by mixing 19.4 mass%/8.72 mol% simulated waste and glass-forming reagents as given in Table 1 [10]. The mixture was ground well to ensure homogenization and transferred to a graphite crucible and melted at 1,473 K for 1 h under argon cover. The graphite crucible was cut opened to remove the pellet. The BSGW was annealed in a platinum crucible at 800 K for 15 h in air. The powders and pellets were characterized by XRD to confirm the absence of any crystalline phases in the final product.
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H. Jena et al. Table 1 Typical simplified composition of the simulated HLW waste-loaded BSG (abbreviated as BSGW) Component SiO2
mass% 45.94
mol% 54.29
B2O3
20.14
20.54
Na2O
14.51
16.62
Nd2O3
4.6
0.97
SnO2
1.0
0.47
SrO
2.2
1.51
Fe2O3 ZrO2
3.5 2.7
1.56 1.56
NiO
0.4
0.38
Pd
1.0
0.67
MoO3
1.8
0.89
Cs2O Total
2.2 100
0.55 100
SiO2/B2O3 = 2.64, Na2O/B2O3 = 0.81
Thermal characterization by TG/DTA and DSC The measurement of glass transition temperature of the powders and pellets was carried out by DSC and further examined by DTA. A heat flux type DSC (model number DSC821e/700 of M/s. Mettler Toledo GmbH, Switzerland) was used in the present study. The measurements were carried out at 298–873 K. The annealed samples were hermetically sealed in 40 lL Al-pans. High purity argon was used as the purge gas at a flow rate of 50 mL min-1. A three segment-heating program was used. The first segment lasting for 5 min was an isothermal one at the initial temperature; the second segment was a dynamic one with a heating rate of 10 K min-1 and the final segment lasting for 5 min was another isothermal one at the final temperature. For TG/DTA analysis, M/s. SETARAM-SETSYSEvolution model was used for recording the TG and DTA data. The heating and cooling rate used in the heating or cooling cycle were 5 K min-1. Pt sample holder was used for these experiments. The experiments were carried out in air at 298–1,173 K. Thermal expansion measurements by dilatometry The linear thermal expansion measurements on BSG (10.76 mm height and 12.95 mm diameter) and wasteloaded BSG pellet (12.20 mm height and 12.95 mm diameter) were carried out by using a home-built dilatometer [20], in the temperature range of 300–800 K. Linear variable differential transformer (LVDT) was used as the displacement sensor (M/s. Syscon Pvt. Ltd., Bangalore, India). The accuracy of displacement measurement by LVDT was ±1 lm. Chromel–alumel (type-K) thermocouple was used to measure the temperature of the sample
Electrical conductivity and thermal properties of borosilicate glass
Intensity/a.u.
50 0 150
Borosilicate glass (57 wt.% SiO2 + 25 wt. % B2O3 + 18 wt. % Na2O)
100 50 0 20
40
60
80
2θ /°
Fig. 1 XRD patterns of BSG and 19.4 mass% waste-loaded BSG showing the absence of crystalline phases in these matrices
(a) 0.0028 0.0024
σ ,BSGW heating σ ,BSGW cooling σ ,BSG heating σ ,BSG cooling
0.0020 0.0016 0.0012 0.0008 0.0004 0.0000
Tg= 820 400
500
600
700
800
900
1000
Temperature/K
(b)
Tg = 820 K
1
BSG BSGW
0
Log(σ T )/S K Cm
The electrical resistivity measurements of solid glass pellets (10 mm diameter and 5 mm thickness) below the glass transition temperature were carried out by DC and AC impedance techniques. The pellets used for electrical resistivity measurements were annealed in air at 800 K for 10 h. The bottom and top flat surfaces of pellets were metalized using Ag-paste. The metalized pellets were loaded in the high temperature conductivity cell, and the cell was put inside the furnace well. The details of the high temperature electrical conductivity cell are reported elsewhere in those of the earlier studies [23, 24]. The temperature of the furnace was controlled by a programmable PID temperature controller with ±1 K accuracy. The sample temperature was measured with a K-type (Chromel–alumel) thermocouple placed at about 2 mm from the sample in the conductivity measurement cell. Resistances of the sample were measured at each 20 K interval. The impedance (Z) measurements were carried out using an Autolab frequency response analyser (FRA) in the frequency range of 100 Hz–1 MHz. The resistance of the samples at various temperatures was determined by fitting
BSG + 19.4 wt. % waste loaded
100
Conductivity (σ )/SCm–1
Electrical conductivity measurements of BSG and BSGW pellets
150
–1
and was placed very close to the sample. Programmed heating, cooling and data acquisition of the sample were performed through a computer program and necessary electronics required for data processing and control. The heating and cooling rates used for these measurements were 2 and 5 K min-1, respectively. The instrument was calibrated by using standard single crystal pellet of MgO, ThO2 (procured from Nuclear Fuel Complex, Hyderabad, India) and standard Stainless Steel sample obtained from National Institute of Standards and Technology (NIST), USA. The % linear expansion of MgO pellet was measured and compared with the values reported in the literature [21]. Similarly, the percentage thermal expansion of the ThO2 pellet was measured and compared with the values reported by Belle and Berman [22]. The values measured were reproducible and agreeing well with the literature data. The difference in fitted value and experimentally measured value of standard MgO single crystal are taken as the correction factor for the samples studied. The propagation of error in the measurements was estimated taking into account the sources of errors involved in the measurement of temperature (±1 K), displacement (±1 lm) and sample length (±0.001 mm). The estimated error was *3 %. The density and dimension of the pellets were once again measured after the thermal expansion measurements were over, and were found to be the same. The coefficient of thermal expansion (CTE) of the glasses was calculated below the glass transition temperature of the glass samples.
369
–1 –2 –3
Ea = 0.961(5) eV
E a= 0.960(5) eV BSGW
–4
BSG
–5 –6 –7
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
1000/T K –1
Fig. 2 a Electrical conductivity of BSG and BSGW measured by DC and AC technique at 300–950 K in air. b Log (rT) versus 1,000 T-1 to find out the activation energy of conduction
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the data of Z00 versus Z0 Nyquist plot using fit and simulates functions available in the Autolab FRA system. The real part of the semicircle (Z00 vs. Z0 ) is taken as the resistance of the sample at a particular temperature. One such fitted semicircle for BSGW sample at 590 K is delineated in Fig. 3 as an example of the procedure. The fitting of the semicircle was performed by trial and error method on assigning various equivalent circuit models available with the software provided by Autolab. The equivalent circuit that fits all the points on the semicircle with minimum error is taken as the accepted model and the values of R and C calculated by the model for a particular temperature were taken as the accepted value. The DC resistance measurements were performed by Agilant 34401A model. The conductivity of samples was calculated using the formula as given in Eq. 2. r¼
1 L R A
ð2Þ
where r is the conductivity in X-1 cm-1 or Siemens cm-1 (abbreviated to S cm-1), R is the resistance in ohms, L is the height or thickness of the pellet and A is the cross sectional area of the pellet.
Results and discussion Electrical conductivity of BSG and BSGW The powder XRD patterns (Fig. 1) show the formation of the glass and the absence of any crystalline phases corresponding to the constituent elements of the BSG or simulated waste elements added to the glass matrix. The electrical conductivities measured on the BSG and simulated waste-loaded BSG are shown in Fig. 2a, b. In Fig. 2a, conductivity (r in S cm-1) versus T (K) show an increase in conductivity around the glass transition temperature. In Fig. 2b, the plots of log (rT) versus 1,000 T-1 show the variation of electrical conductivity with temperature. The electrical conductivity of BSG is mainly due to the transport of the Na? ions in the glass network and can be
Ci is the concentration of species i, Zi and li are the charge and mobility of the species i, respectively. Various studies reported in the literature discuss about the ion transport mechanisms in silicate oxide glasses [12, 13, 25, 26]; many of the authors accepted the cation migration through interstitial pairs in the glass network [25, 26]. The interstitial pair or interstitial defect is defined as the combination of two cations surrounding non-bridging oxygen (NBO) in the glass network [25]. The electrical conductivity of BSGW was found to be slightly lower than that of the BSG as shown in Fig. 2b. In this case, the major contributing current carrier is the Na?; however, when waste elements are added to the BSG, Na2O concentration is reduced to 16.62 mol% Na2O (9.67 at.% Na? in BSGW) from 18.18 mol% Na2O (10.54 at.% Na? in BSG) (Table 1). The presence of Cs? in the BSG can replace equivalent concentrations of Na? in the glass network that will affect the conductivity of BSG. Further, the presence of Fe and Sn in the BSG can also introduce some electronic conduction in the matrix. However, the concentration of Cs, Fe, Sn, etc. (Table 1 is very low compared to Na? ions; hence, their contribution to conductivity can be expected to be less dominant. Log (rT) versus 1,000 T-1 followed an Arrhenius behaviour below Tg. The activation energy of electrical conduction was calculated using the Arrhenius equation as shown in the Eq. 4. rT ¼ A expðEa =kTÞ
ð4Þ
where r is the conductivity, Ea is the activation energy of electrical conduction, k is the Boltzmann constant, T is the temperature, A is the pre-exponential factor. The activation energy (Ea) of electrical conduction below Tg is 0.961 ± 0.005 eV for BSG and 0.960 ± 0.005 eV for BSGW. Electrical conductivity of BSG and BSGW was found to increase above Tg as shown in Fig. 2a, b. In Fig. 2b, the slope of the conductivity curve changes
50 x 104
C BSGW 590 K
Equivalent circuit
40 x 104
–Z ''/ohm
Fig. 3 Fitting of Z00 versus Z0 for BSGW at 590 K using software supplied by Autolab is shown as an example. The resistance (R) and the capacitance (C) of the circuit are calculated after fitting the semicircle by using the software
expressed as the product of three terms; carrier concentration, mobility and charge of the species as expressed in Eq. 3. X ð3Þ r¼ Ci Zi li
R 30 x 104 20 x 104 10 x 104 0 0
10 x 104
20 x 104
30 x 104
Z '/ohm
123
40 x 104
50 x 104
Electrical conductivity and thermal properties of borosilicate glass
371
9.0 x 105
(a)
0.6
Linear thermal expansion/%
BSG 483 K BSG 523 K BSG 613 K
Im Z /ohm
6.0 x 105
3.0 x 105
1 MHz 100 Hz 613 K
0.4 0.3 0.2 0.1 0.0
0.0 0.0
6.0 x 105
3.0 x 105
9.0 x 105
1.2 x 106
300
Re Z / Ohm
(b)
BSG without waste loading BSG with waste loading (BSGW)
0.5
400
500
600
700
800
Temperature/K
4 x 104
BSG 613 K BSG 713 K BSG 858 K
Fig. 5 Percentage thermal expansion of pristine BSG and simulated waste-loaded BSGW by dilatometry measurement
110 2 x 104
1 x 104
1 MHz
100 Hz
0 0
1 x 104
2 x 104
3 x 104
4 x 104
5 x 104
Re Z /Ohm
(c)
4 x 106
BSGW 491 K BSGW 526 K BSGW 598 K
3 x 106
Thermal expansion/micrometers
Im Z /Ohm
3 x 104
BSG BSGW
100
790
90 80 70 60 50 40 30 300
Im Z/Ohm
806
400
500
600
700
800
900
Temperature/K 2 x 106
1 MHz
Fig. 6 Tg determined from linear thermal expansion versus T (K) of BSG and BSGW 1 x 106
100 Hz 0 0
1 x 106
2 x 106
3 x 106
4 x106
5 x 106
Re Z/Ohm
Fig. 4 a Nyquist plots of BSG at various temperatures (Re Z = Z0 and ImZ = Z00 ). b Nyquist plots of BSG at indicated temperatures (Re Z = Z0 and ImZ = Z00 ). c Nyquist plots of BSGW at indicated temperatures (Re Z = Z0 and ImZ = Z00 )
upwards at about 820 K for BSG coinciding with Tg determined by other techniques. The Ea measured in this study for BSG and BSGW are well within the range reported by various investigators for BSG [12, 13]. However, the glass composition and the ratio of SiO2/B2O3
(2.64) and Na2O/B2O3 (0.81) are slightly different from the studies reported in the literature. The marginal decrease in the conductivity shown in Fig. 2b may be attributed to the decrease or dilution of Na? concentration in the BSGW compared with BSG. The BSG composition used in this study is different from the compositions reported in the literature but within the vitrification range of the SiO2– Na2O–B2O3 phase diagram [27, 28]. In Fig. 3, Z00 versus Z0 plot of BSGW at 590 K is shown as a typical example, to show how the fittings were carried out to calculate R and C values using equivalent circuits. All the semicircles could be fitted to RC circuits with R and C in parallel combination. The value of capacitance (C) was mostly in the pico-Faraday (pF) range and the variation of capacitances was small on moving from one
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H. Jena et al.
BSG–cool BSG heat
ΔT/ μV
0
–4
compositions reported by Ehrt et al.; however, the activation energy reported by them is comparable with the systems investigated in this study. The decrease in activation energy above Tg can be attributed to the thermally activated mobility of the current carriers and decrease in viscosity of the glass melt.
BSG heating BSG cooling BSGW heating BSGW cooling
BSGW cool
4
801 817
BSGW heat Exo
Thermal expansion measurements on BSG and BSGW
805
–8
820
The percentage (%) linear thermal expansion of the BSG and BSGW pellets was calculated by using the formula given in Eq. 5.
Endo –12
300
400
500
600
700
800
900
1000
Percentage linear thermal expansion ¼
Temperature/ K
Fig. 7 Tg determined from DTA plots of BSG and BSGW
Heat flow/mW
0
where LT is length of the sample at temperature T and LRT is the length of the sample at room temperature. The average thermal expansion coefficient or average CTE was estimated by using the formula,
805 BSG BSGW
–5
CTE ¼ 803
–10
Exo
–15
Endo
–20 300
400
500
600
700
800
900
Temperature/K
Fig. 8 Tg determined by DSC measurements on BSG and BSGW
temperature to the other. The resistance component of the equivalent circuit was found to decrease on increasing temperature; however, the capacitance component was observed to increase with temperature for BSG and BSGW samples. In Fig. 4a, b, the Nyquist plots of Z00 versus Z0 show the decrease of resistance with temperature. The Nyquist plots in Fig. 4c show the decrease of resistance of the BSGW sample with increasing temperature. The prominent semicircles appeared below Tg indicating resistance (R) and capacitance (C) circuit elements are in parallel combination. Above Tg, points or lines are appeared in the Nyquist plot instead of semicircles, indicating very low contribution from capacitive circuit elements. The variation of activation energy of electrical conduction with temperature and composition of the sodium silicate and BSGs is reported by Ehrt and Keding [29]. The compositions investigated in this study are different from the
123
LT LRT 100 LRT ð5Þ
DL 1 LRT DT
ð6Þ
where DL is the change in length (LT - LRT) in lm, LRT is the length of the sample at room temperature (RT) before measurement, DT is the difference of temperature in K (i.e. highest temperature minus room temperature). The average CTE of BSG (12.87 ± 0.24 9 10-6 K-1) is slightly higher than that of the waste-loaded BSGW (11.94 ± 0.23 9 10-6 K-1) in the same temperature range of 325–763 K. In Fig. 5, the % thermal expansion versus T (K) is shown. The % linear thermal expansion of wasteloaded BSGW below Tg was found to decrease [600 K. The average linear thermal expansion coefficient measured in this study falls in the same range as that of compositions reported in the literature [17]. The slight lowering of % linear thermal expansion of the waste-loaded BSG is ascribed to the weakening of the glass network [30–34]. The weakening of rigidity may be due to the interaction of waste elements and the BSG network forming ions (Si, B). The introduction of Cs?, Nd3? ions into the glass matrix facilitates the formation of NBO in the BSG network [34]. One oxygen atom bound with two network formers (Si, B) is said to be ‘bridging’, whereas one oxygen atom bound with a single network former is said to be ‘non-bridging’. NBO may decrease the strength of the network and decrease the viscosity of the material, which is more pronounced at higher temperatures. Glass transition temperature (Tg) determined by thermomechanical measurements (TMA) of the BSGW is 790 ± 21 K which is lower than that of the pristine BSG (806 ± 21 K) (Fig. 6). The estimated error in these measurements by thermo-mechanical method is around 3 %.
Electrical conductivity and thermal properties of borosilicate glass
The Tg measured by DTA (Fig. 7) on pristine BSG is 820 ± 7 K for heating cycle and 817 ± 7 K for cooling cycle. The Tg of BSGW measured by DTA is 805 ± 7 K for heating cycle and 801 ± 7 K for cooling cycle (Fig. 7). Tg measured by DSC is 805 ± 5 K (Fig. 8) for pristine BSG (SiO2/B2O3 = 2.64 and Na2O/B2O3 = 0.81) and for waste-loaded composition is 803 ± 5 K. Tg obtained in this study is quite close to the Tg reported by Grandjean et al. [12, 13]. They had made various compositions of BSG with varying ratio of SiO2/B2O3 ranging from 1 to 3.76, and ratio of Na2O/B2O3 ranging from 0.35 to 2.04. Tg obtained in this study for BSGW (SiO2/B2O3 = 2.64 and Na2O/B2O3 = 0.81) is slightly lower than that of the BSG; however, Tg of BSGW measured in this study is *34 K higher than the Tg measured by Rose et al. [17] for the composition with waste-loaded BSGW having SiO2/ B2O3 = 55.90/17.79 = 3.14 and Na2O/B2O3 = 0.56 with more number of fission elements added to it. The higher Tg obtained in the present study for BSGW may be attributed to higher ratio of Na2O/B2O3 = 0.81 compared with Rose et al. and less number of fission elements added to the BSGW. The more the number of fission elements added to the BSG, more will be the formation of NBOs sites in the glass network and will lead to the lower rigidity. There are differences in the Tg measured by three techniques but are within the experimental error limits. However, in all the three it was observed that the waste-loaded BSGW showed slightly lower Tg value compared with BSG without waste loading. This further confirms the interaction of glass network formers and modifiers with the waste elements and 19.4 mass% waste loading is tolerable by the glass matrix.
Conclusions The glass transition temperature of the waste-loaded BSGW is slightly lower than the pristine BSG confirms the interaction of waste elements with the glass-forming agents. The interaction is more pronounced above Tg. The change in the electrical conductivity, thermal expansion and glass transition temperature are the indicators of the interaction of waste elements with the glass network formers and modifiers. Further studies are required for the comprehensive understanding of the interaction of individual waste elements with the glass network; and the thermo-physical properties of the waste-loaded glass compositions which are vital to the vitrification technology of radioactive waste for safe storage and disposal. Acknowledgments The authors gratefully thank Mr. Abhiram Senapathy, Dr. R. Venkat Krishnan for recording the Tg data by DSC and Mr. Sajal Ghosh for recording the DTA data of the samples.
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