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Composition-dependent mechanical property changes in Au-ion-irradiated borosilicate glasses
Accepted Manuscript Composition-dependent mechanical property changes in Au-ion-irradiated borosilicate glasses P. Lv, L. Chen, B.T. Zhang, W. Yuan, B.H. Duan, Y.D. Guan, Y. Zhao, X.Y. Zhang, L.M. Zhang, T.S. Wang PII:
S0022-3115(18)31453-3
DOI:
https://doi.org/10.1016/j.jnucmat.2019.04.025
Reference:
NUMA 51573
To appear in:
Journal of Nuclear Materials
Received Date: 25 October 2018 Revised Date:
18 April 2019
Accepted Date: 18 April 2019
Please cite this article as: P. Lv, L. Chen, B.T. Zhang, W. Yuan, B.H. Duan, Y.D. Guan, Y. Zhao, X.Y. Zhang, L.M. Zhang, T.S. Wang, Composition-dependent mechanical property changes in Auion-irradiated borosilicate glasses, Journal of Nuclear Materials (2019), doi: https://doi.org/10.1016/ j.jnucmat.2019.04.025. 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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT Composition-dependent mechanical property changes in Au-ion-irradiated borosilicate glasses P. Lv a, L. Chen a, b, *, B.T. Zhang a, W. Yuan a, B. H. Duan a, Y. D. Guan a, Y. Zhao a, X.Y. Zhang a, L.M. Zhang a, b, T.S. Wang a, b, * a
School of Nuclear Science and Technology, Lanzhou University, Lanzhou, 730000,
b
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China
Key Laboratory of Special Function Materials and Structure Design Ministry of Education, Lanzhou University, 730000, China
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Abstract
The mechanical property changes in three kinds of ternary borosilicate glasses (named
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NBS glasses) induced by 8.0-MeV Au3+ were studied by nanoindentation measurements. For these borosilicate glasses, the molar ratio R=[Na2O]/[B2O3] is varied (1.34, 0.75, and 0.40 for NBS1, NBS2, and NBS3, respectively), while their molar ratio K=[SiO2]/[B2O3] is fixed at 4.04. The nanoindentation results indicate that both the hardness and reduced Young modulus were decreased after irradiation. No obvious difference in both hardness and modulus variations of
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these three NBS glasses was observed. Based on the main structures of their pristine samples, the reedmergnerite groups containing Si-O-BIV may be the structural origin of Au-ion-irradiated mechanical property changes in the homogeneous glasses with R>0.5.
mechanical
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However, the modification of the silica network and phase separation may dominate the property
changes
for
glasses
with
R≤0.5.
The
ion-
and
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electron-irradiation-induced mechanical property changes of these NBS glasses were also compared in this study. The extended incubation dose and the slower decreasing rate can be observed in electron-irradiation-induced hardness and modulus variations, which may be caused by the different mechanisms between both irradiation scenarios. Keywords Sodium borosilicate glass, Ion irradiation, Mechanical property, Composition dependence
* corresponding authors E-mail：[email protected] (L. Chen), [email protected] (T.S. Wang)
ACCEPTED MANUSCRIPT 1. Introduction As the candidate with the most potential for underground disposal, borosilicate glass has aroused much concern owing to its radiation durability [1–4]. External ion irradiation has been proved to be a powerful tool to simulate the radiation field caused by the decay of radionuclides.
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The irradiation-induced mechanical property changes are of importance for the glass waste form that serves as the first barrier for high-level waste (HLW). The degraded mechanical property of this waste form is riskier than the more released radionuclides, and finally, do harm to the biosphere. on
the
mechanical
property
changes
mainly
focus
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Studies
on
the
charged-particle-irradiation-induced hardness and modulus variation [5–10], discrepancy
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between different ions [5,11,12], and corresponding structural evolutions in these irradiated borosilicate glasses [7,9,13,14]. The mechanical property changes of borosilicate glass after irradiation have been claimed to be controlled by Na2O, B2O3, and SiO2 [15]. Moreover, the structure of this series of ternary glass was governed by two composition-related parameters,
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i.e., the molar ratio K (defined as [SiO2]/[B2O3]) and R (defined as [Na2O]/[B2O3]) [16–18]. Several researchers have covered the composition-dependent property and structure changes in particle-irradiated borosilicate glasses. Boffy et al. performed a series of neutron
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irradiations on four kinds of borosilicate glass and reported several structural changes and modifications in macroscopic behavior [19–21]. However, the K and R values were not
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considered in their works on complex glasses. Moreover, the hardness and modulus variations induced by neutron irradiation were not covered in their study. Kilymis et al. [22–25] studied the irradiation effects of three kinds of ternary sodium borosilicate glasses using molecular dynamics (MD). Their results have provided some insights into the structural origins and composition dependence of the mechanical property changes. Nevertheless, both the K and R values of their samples increased. In addition, experimental data of these ternary sodium borosilicate glasses have not yet been reported. Wang et al. [6] reported a hardness decrease of approximately 24% in 5-MeV Xe-ion-irradiated borosilicate glass (70 wt% SiO2+1 wt% Na2O+5 wt% Al2O3+4 wt% B2O3+4 wt% Al2O3), and the modulus variation was
ACCEPTED MANUSCRIPT approximately −7.4% below the initial value. Chen et al. [5] also investigated the irradiation damage of borosilicate glass (80.6 wt% SiO2+12.8 wt% B2O3+4.1 wt% Na2O+2.4 wt% Al2O3 and minor amounts of other constituents) induced by 5-MeV Xe ions. The nanoindentation results indicate that the hardness decreased by 12.8%, while the modulus increased by 3.5%.
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Based on the results of MD studies on the ternary sodium borosilicate glasses and experimental studies on the complex borosilicate glasses, composition-dependent mechanical property changes may exist in ion-irradiated borosilicate glasses. Our recent studies on electron-irradiated ternary borosilicate glass illustrate the influence of ratio R on their
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mechanical property changes [26]. However, the detailed role of the composition in ion-irradiation-induced mechanical property changes is unclear, and a systematic study should
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be carried out to clarify the detailed mechanism.
For this purpose, the ternary Na2O-B2O3-SiO2 (named NBS) glasses with different R values and constant K were investigated through external irradiation with 8.0-MeV Au3+ ions. Nanoindentation measurements were performed to investigate the evolution of hardness and
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reduced Young modulus before and after irradiation. The chemical compositions of the NBS glasses in this paper are the same as in Ref. [26], and a direct comparison can be made to further understand the mechanism of the irradiation-induced mechanical property changes from the
2. Experiment
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aspect of deposited energy.
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2.1. Chemical compositions and irradiation conditions The NBS glass samples were prepared by melting a mixture of Na2CO3, H3BO3, and
SiO2. Table 1 shows the chemical composition of these samples and the corresponding molar ratios with a maximum uncertainty given by weighting of 0.05%. The samples were then cut into squares of 10 mm size with a thickness of 1 mm. All the samples were polished to a surface finish of 0.5 µm, and the surface roughness was less than 5 nm. Before irradiation, all the samples were ultrasonically cleaned. Room-temperature irradiation with 8.0-MeV Au3+ generated by the 3-MV Tandetron accelerator was conducted at the Key Laboratory of Radiation Physics and Technology of the Ministry of Education, Institute of Nuclear Science
ACCEPTED MANUSCRIPT and Technology, Sichuan University, China [27]. Various fluences from 1.12×1012 to 1.20×1015 ions/cm2 were used. The beam flux during irradiation was approximately 2.36×1011 (ions/cm2)/s. The stopping power vs the projected range of incident Au ions in NBS1 glass was determined by SRIM2010 code [28], which is shown in Fig. 1. The curves of NBS2 and NBS3
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glasses were not plotted as their tendencies are very similar to NBS1 glass. It can be seen from this figure that the electronic process dominates in the front of the particle track, while the nuclear process is focused around the projectile range. Using this code, the deposited electronic energy and nuclear energy were also calculated. Their ratios, along with other information, are
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given in Table 2. 2.2. Characterization methods
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To characterize the mechanical property changes before and after radiation, an MTS G200 nanoindenter equipped with a Berkovich indenter was used. During the measurement, the continuous stiffness mode (CSM) was used under normal temperature and pressure (NTP). The maximum load was 500 mN, and the corresponding maximum penetration depth was
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approximately 1900 nm. This depth is close to the maximum range of the 8.0-MeV Au ion when we take longitudinal straggling into consideration (see Table 2). At least five indentations were carried out on each glass sample. The fused silica as reference material was
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also measured for calibration.
To verify the phase-separated nature of NBS3 glass, the Raman spectroscopy
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measurements (LabRAM HR 800) were made in confocal mode. A 532-nm Ar-ion laser with an output power of 100 mW was used. The spectra were obtained with a 100× objective lens under NTP. 3. Results
3.1. Hardness and reduced Young modulus curves of pristine and irradiated glasses Figure 2 displays the typical hardness and reduced Young modulus curves vs penetration depth for the three kinds of pristine glass samples and corresponding irradiated samples at an ion fluence of 1.20×1015 ions/cm2. Compared with the pristine glasses, an obvious decrease in hardness can be observed in the irradiated glasses, as shown in Fig. 2(a). Following a rapid
ACCEPTED MANUSCRIPT increase, the hardness of the pristine and irradiated glasses tends to be constant beneath the surface influence zone area. A slight increase is shown at the end of the hardness curves of irradiated glasses, which reflects the property of a deep layer without irradiation. Owing to the tip effect, the data of the first hundreds of nanometers were ignored [5,9,10,29]. The plateau
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region is not obvious in these curves meanwhile its choice has no significant influence on evaluating the mechanical property changes. Therefore, the hardness value of the samples was extracted from 300 to 920 nm which is based on the plastic interaction zone usually extends to approximately two times the plastic penetration depth [9,10,30]. As shown in Fig. 2(b), the
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modulus curves of both pristine and irradiated glasses show a similar tendency compared with the hardness curves. However, no plateau region in the modulus curves of irradiated glasses can
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be identified. This can be attributed to the fact that the elastically affected region is far larger than the plastically affected region [5]. To quantify the modulus variation of the ion-irradiated glasses, the measured values with penetration depths between 300 and 920 nm were also used to describe the modulus of irradiated glasses.
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3.2. Mechanical property changes in Au-ion-irradiated glasses
The hardness and reduced Young modulus variations of the NBS glasses vs ion fluence are shown in Figs. 3 and 4, respectively. The error bars shown in both figures represent the
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statistical error derived from multiple measurements. As shown in Fig. 3, the hardness of three NBS glasses is significantly decreased with increasing ion fluence. It should also be noted that
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a slightly increasing tendency does exist for all the hardness curves exceeding a fluence of 1014 ions/cm2. The possible mechanism is discussed in Section 4.1. As shown in Fig. 4, the modulus is decreased with increasing fluence, and the decreases are smaller than those of the hardness. Moreover, the modifications of modulus for the NBS glasses reach a saturation when the ion fluence is approximately 3×1013 ions/cm2, which is equal to the same magnitude of deposited nuclear energy (1020 keV/cm3). The mechanical property changes were associated with the R values to reveal the composition dependence in heavy-ion-irradiated borosilicate glasses. Figure 5 shows the hardness and modulus variations of irradiated glasses at 1.20×1015 ions/cm2 as a function of R.
ACCEPTED MANUSCRIPT From this figure, it can be seen that both hardness and modulus show no significant difference among these three NBS glasses. The hardness variations for NBS1, NBS2, and NBS3 glass are −23%±2.9%, −27%±3.0%, and −19%±3.1%, respectively. It is worth mentioning that the value of NBS3 is slightly smaller than that of NBS1 even if we take the error into account.
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However, the modulus variations are −10%±2.3%, −9%±2.2%, and −7%±1.6% for NBS1, NBS2, and NBS3 glasses, respectively. 3.3. Comparision of ion and electron irradiation
The hardness and modulus variations in electron-irradiated NBS glasses were compared
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with the modifications induced by Au-ion irradiation. The data of electron-irradiated NBS glasses were taken from our previous paper [26]. The hardness and modulus variations as a
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function of deposited electronic energy are plotted in Figs. 6 and 7, respectively. As shown in both sets of figures, the incubation dose of hardness and modulus variations induced by electron irradiation are different from those of the ion-irradiation scenario. The ion-irradiation-induced hardness and modulus variations already tend to stabilize or even
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slightly increase in hardness when the electron-irradiated ones are about to decrease. This means the incubation dose of the ion-irradiation scenario is smaller than that of the electron-irradiation ion. Moreover, the decreasing rate of hardness and modulus variations in
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ion-irradiated-glass samples is faster than that of electron-irradiated ones. In addition, it is also worth mentioning that the mechanical properties can be significantly
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affected by electron irradiation, and their variations may not always be smaller than those induced by heavy-ion irradiation. Compared with Au-ion irradiation, there is no difference in electron-irradiation-induced hardness and modulus variations in NBS2 and NBS3 glasses within the range of uncertainty. 4. Discussion 4.1. Effect of deposited energy on mechanical property changes Although the incident Au ions lose their energy in two ways, i.e., electronic and nuclear interactions, the hardness and modulus variations of ion-irradiated borosilicate glasses have been mainly correlated with the deposited nuclear energy. The variations have been clarified
ACCEPTED MANUSCRIPT to be saturated when the deposited nuclear energy reached approximately 1021 keVnucl./cm3 [5–8,10,12]. However, both the hardness and modulus variations in Au-ion-irradiated NBS glasses tend to be saturated when the deposited nuclear energy is approximately 3×1020 keVnucl./cm3. The smaller saturation point may be caused by chemical composition or the
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contribution of deposited electronic energy. The electron-irradiation-induced hardness and modulus variations are about to decrease when the ion-induced ones already tend to stabilize or even slightly increase the hardness variations (shown in Figs. 6 and 7), which illustrates that the deposited nuclear energy plays an important role in ion-irradiation-induced mechanical
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property changes. Peuget et al. also suggested that the nuclear-interaction-induced totally damaged layer in the glass volume is responsible for such saturation [32].
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In addition, it also should be noted that a slight increase in the hardness variations can be observed after the ion fluence reached approximately 1014 ions/cm2, as shown in Fig. 3. This phenomenon has not been reported in the previous studies on ion-irradiated borosilicate glasses and requires further study. Nevertheless, considering the electronic energy deposition
hardness
variations
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shown in Fig. 6, the start point of the slight increase in the Au-ion-irradiation-induced is
similar
to
that
of
the
significant
decrease
in
the
electron-irradiation-induced hardness variations. Therefore, we can speculate that there may
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exist a deposited electronic energy threshold (~9×1021 keVelec./cm3) for the hardness recovery of ion-irradiated NBS glasses. Indeed, the hardness of heavy-ion-irradiated borosilicate glass
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can be partially recovered by He-ion irradiation, in which the deposited electronic energy accounts for 99.6% of the total energy deposition [12,33]. Mir et al. suggested that the electronic-interaction-induced connectivity enhancement of damaged networks is responsible for the hardness recovery [33]. As a consequence, both kinds of deposited energy play important roles in mechanical property changes in several-MeV-heavy-ion-irradiated sodium borosilicate glasses. The deposited nuclear energy plays an important role in the initial decrease in both ion-irradiation-induced hardness and modulus, while the slight increase in hardness variations may be attributed to the damage recovery induced by the deposited electronic energy
ACCEPTED MANUSCRIPT deposition. Such a recovery, only observed in our ion-irradiated NBS glasses, appears to be caused by their sensitivity to the deposited electronic energy. Our previous study has already proved that the NBS glasses are more sensitive to the deposited electronic energy, and the hardness of the electron-irradiated samples at an electron dose of 1×109 Gy was decreased by
borosilicate glasses at the same dose [5,10,31].
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−11% to −27% [26]. These values are larger than the reported variations of electron-irradiated
Compared with the results of Au-ion irradiation, extended incubation dose and slower decreasing rate have been observed in electron-irradiation-induced hardness and modulus
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variations, which could be related to the physical processes of irradiation. For the ion-irradiation scenario, atomic displacements are the dominant mechanism. The mechanical
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property changes can be attributed to an increasingly disordered structure caused by collision cascade [34]. The cascade events among the irradiation volumes can be induced by a single heavy ion of several MeV. For the electron-irradiation scenario, ionization and electronic excitation are the dominant mechanisms. The point defects are the most important defects in
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this case, which could account for the mechanical property changes in electron-irradiated borosilicate glasses [5,10,26]. The extended incubation dose and slower decreasing rate in electron-irradiation-induced hardness and modulus variations may be related to the survival
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and accumulation of point defects.
4.2. Main structures of the pristine glasses
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To discuss the possible structural origins of the mechanical property changes in ion-irradiated NBS glasses, the main structures among the networks of these three sodium borosilicate glasses have been calculated according to the Yun, Bray and Dell (YBD) model [16–18,35]. In short, the sodium in NBS3 glass (R≤0.5) is consumed to transformation of [BO4] to [BO3] units, and the structure can be regarded as diborate (Na2O·2B2O3, containing two [BO4] and two [BO3]) diluted by SiO2. In this case, the borate (mainly trigonal) and silicate units cannot mix perfectly and may induce the phase separation of the NBS3 glass. The phase-separated nature of the NBS3 glass can be verified by the Raman spectrum shown in Fig. 8, which suggests that the NBS3 glass has a clustered structure with silica- and borate-rich units
ACCEPTED MANUSCRIPT [5,20,36]. For NBS2 glass (0.5
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For NBS1 glass (0.5+K/16
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structures in NBS1 and NBS2 glasses both have diborate and reedmergnerite groups, while the network of NBS3 glass is composed of [SiO4] tetrahedra, [BO3] trihedral, and diborate group.
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Furthermore, the only discrepancy between NBS2 and NBS3 glasses is the relative concentration of B species, while the NBOs concentration is the only discrepancy between NBS1 and NBS2. The concentrations of both parameters are different for NBS1 and NBS3 glasses:
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2 B2 O3 + Na2 O → ( Na2 O i2 B2 O3 ) , ( R ≤ 0.5)...............................(1) 1 1 K [( Na2 Oi 2 B2 O3 )] + 4SiO2 + Na2 O → [0.5( Na2 Oi B2 O3 i8SiO2）], (0.5 < R ≤ + 0.5)..............(2) 4 16 4 3 K K [0.5( Na2 Oi B2 O3 i8SiO2） ] + Na2 O → [0.5(2.5 Na2 O i B2 O3 i8SiO2 )], ( + 0.5 < R ≤ + 0.5).......(3) 16 4 4
4.3 Effect of glass composition on mechanical property changes
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As many authors have suggested, ion-irradiation-induced mechanical property changes of the borosilicate glass should be caused by the damage of the glass network [8,12,37]. There is no doubt that ion irradiation can alter the short- and/or medium-range order in the glass, which facilitates the disorder in the glass network, along with the formation of point defects. A more disordered structure has been confirmed by nuclear magnetic resonance (NMR) spectroscopy and MD simulations [14,22,38]. A mechanism based on the damage of the glass network has been proposed to interpret the mechanical property changes in ion-irradiated borosilicate glass [8,15,39–41]. In the ternary Na2O-B2O3-SiO2 glass system, the coordination number of boron
ACCEPTED MANUSCRIPT atoms can be decreased by the migration of charge-compensation sodium from a local position to the interstitial sites of the Si network. These sodium ions that have migrated to the interstitial sites play a role in modifying the Si network and produce the non-bridging oxygen atoms [7,8]. The results of our recent MD simulation on a homogeneous Na2O-B2O3-SiO2 glass (R>0.5)
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suggested that the damage of the mixed Si-O-BIV network plays an important role in the processes mentioned above and in the mechanical property changes induced by ion irradiation [40].
A lower boron coordination number has been confirmed in both ion-irradiation
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experiments [5,7,14,15,32,42] and MD studies [22,24,25,30,41]. Mendoza et al. [14] investigated heavy-ion-irradiation-induced structural modifications by NMR spectroscopy.
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Their results indicated that the transformation of [BO4] to [BO3] units depends on the glass composition. The net increment of the [BO3] concentration in BS3 (R>0.5+K/16) is slightly higher than that in the BS6 (R≈0.5+K/16) glass. Furthermore, the three coordinated boron atoms of the irradiated SBN12 (R<0.5), SBN14 (0.50.5+K/16)
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glasses increased by 3%, 10%, and 11%, respectively [25]. Given the population of boron species in the pristine samples, the transformation of [BO4] to [BO3] units in our NBS glasses may follow the order NBS1>NBS2>NBS3.
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However, an increasing concentration of NBOs after ion irradiation has also been widely reported [7–9,14,15,23,24,43]. The broken Si-O bonds induced by ion irradiation are
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considered another important source that produces NBOs [8,15,43]. Peuget et al. [8,14] investigated the ion-irradiation-induced structure evolutions in some borosilicate glasses by NMR spectroscopy. All their 29Si and 23Na spectra indicate the depolymerization of the silicate network and the shortening of the average Na-O distance after irradiation. In addition, Kilymis’s studies on three different ternary sodium borosilicate glasses indicated that ion-irradiation-induced NBOs variations also depend on glass composition [23]. The concentrations of NBOs in these disordered glasses increased by 1.26%, 2.36%, and 1.87% for SBN12 (R<0.5), SBN14 (0.5K/16+0.5) glasses, respectively.
ACCEPTED MANUSCRIPT Therefore, an increase of NBOs concentration can be speculated in our Au-ion-irradiated NBS glasses, and the relative variations may follow this tendency, i.e., NBS2>NBS1>NBS3. As mentioned above, the reedmergnerite groups (composed of the mixed Si-O-BIV network) dominate the main structures among NBS1 and NBS2 glasses, while the [SiO4]
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tetrahedra dominate the main structures of NBS3 glass. Thus, ion-irradiation-induced mechanical property changes in NBS3 glass should be reflected more on the structural change of the SiO2 network. Mendoza et al. [14] also claimed that the mechanical property changes in krypton-irradiated SiO2 glass should be more likely related to the Si-O− bond. Boffy et al. [20]
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studied the structural evolutions of neutron-irradiated borosilicate glasses, and their results show that the glass with smaller R value had a notable blue shift, indicating significant
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modification of the SiO2 network in NBS3 glass after ion irradiation.
In summary, the modifications in the network structures of ion-irradiated borosilicate glass depend on glass composition. However, the ion-irradiation-induced mechanical property changes should be related to the combined effect of many factors, such as boron coordination in
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parallel with NBO formation and point defects. Our results on Au-ion-irradiated NBS glasses illustrate that there is no composition-dependent modulus variation within the error range for these three NBS glasses. Compared to NBS2 glass, the slightly smaller hardness variation of
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NBS3 glass may be attributed to its phase-separated nature [5,24]. In contrast, no composition-dependent hardness variations are observed between NBS1 and NBS2 glass,
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which means that the NBOs might make a very small contribution to the hardness change of homogeneous glasses. The similar hardness changes in NBS1 and NBS3 glasses may be caused by a synergy process in both NBOs and phase separation. 5. Conclusions
The mechanical property changes in three kinds of Au-ion-irradiated NBS glasses were investigated by means of nanoindentation measurements. Results indicate that both the hardness and reduced Young modulus decrease after Au-ion irradiation. In addition to the deposited nuclear energy, the deposited electronic energy could also induce the mechanical property changes in ion-irradiated NBS glasses. In this case, the slight increase in the hardness
ACCEPTED MANUSCRIPT variations can be observed after the ion fluence reached approximately 1014 ions/cm2, which may due to the hardness recovery induced by the deposited electronic energy. Both the hardness and modulus variations have no strong compositional dependence in these three NBS glasses. The only discrepancy observed in the hardness variations between
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NBS2 and NBS3 glass may be attributed to the phase separation, while NBOs might make a very small contribution to the hardness change in homogeneous glasses (NBS1 and NBS2 glasses). Furthermore, the reedmergnerite groups containing the Si-O-BIV network may be the structural origin of ion-irradiation-induced mechanical property changes for the NBS glasses
mechanical property changes in the glass with R≤0.5.
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with R>0.5. The modifications of the silica network and phase separation may dominate the
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By comparing the results of Au-ion and electron irradiation, different decreasing tendencies are observed in the hardness/modulus variations. The extended incubation dose and the slower decreasing rate in electron-irradiation-induced hardness and modulus variations may be attributed to the different mechanisms for both irradiation scenarios. In addition, the
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mechanical property changes induced by electron irradiation may not always be smaller than those induced by heavy-ion irradiation. Acknowledge
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This work was supported by the National Nature Science Foundation of China [NSFC, No. 11505084, No. U1867207 and No. 11675068]; and the Fundamental Research Funds for the
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Central University of China [No. lzujbky-2016-37]. Key Laboratory of Radiation Physics and Technology of the Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University is acknowledged for providing technical support. The authors are also grateful to the Public Center for Characterization and Test, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences.
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Zhang, L. Chen, T.S. Wang, Structural origin of hardness decrease in irradiated sodium
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166-167(2000) 445-450.
ACCEPTED MANUSCRIPT Figure/Table Captions Table 1 Composition of NBS glasses in mol%. Fig. 1 Stopping power vs projected range calculated with SRIM2010 code. Table 2 Other detailed information for NBS glasses. Eelec. and Enucl. are the deposited electronic
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energy and nuclear energy, respectively. Fig. 2 Typical (a) hardness and (b) reduced Young modulus curves of pristine and irradiated NBS glasses.
Fig. 3 Evolution of mean hardness variations vs fluence for NBS glasses. Solid lines are guides
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for the eye.
guides for the eye.
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Fig. 4 Evolution of mean modulus variations vs fluence for NBS glasses. Solid lines are
Fig. 5 Hardness and modulus variation vs R after Au-ion irradiation at 1.20×1015 ions/cm2. Fig. 6 Hardness variations of irradiated NBS glasses vs electronic energy deposition. Solid lines are guides for the eye.
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Fig. 7 Hardness variations of irradiated NBS glasses vs electronic energy deposition. Solid lines are guides for the eye.
Fig. 8 Raman spectrum of pristine NBS3 glass.
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model.
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Table 3 Main structures in NBS glasses derived from the direct calculation of the YBD
ACCEPTED MANUSCRIPT Table 1 Compositions of NBS glasses in mol%.
NBS1
NBS2
NBS3
SiO2
63.38
69.64
74.31
B2O3
15.68
17.21
18.40
Na2O
20.94
13.15
7.29
K=[SiO2]/[B2O3]
4.04
4.04
R=[Na2O]/[B2O3]
1.34
0.75
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Samples
4.04
0.40
ACCEPTED MANUSCRIPT Table 2 Other detailed information for NBS glasses. Eelec. and Enucl. are the deposited electronic energy and nuclear energy, respectively. Density (g/cm3)
Range (µm)
Eelec./Enucl.
NBS1
2.52
1.71±0.14
1.85
NBS2
2.45
1.76±0.15
1.82
NBS3
2.34
1.84±0.15
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Samples
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1.82
ACCEPTED MANUSCRIPT Table 3 Main structures in NBS glasses derived from the direct calculation of the YBD model. NBS2
NBS3
SiO2, [SiO4]
0
0
87.0%
B2O3, [BO3]
0
0
4.3%
Na2O·2B2O3
20.0%
20.0%
8.7%
0.5(Na2O·B2O3·8SiO2)
16.0%
80.0%
−
0.5(2.5Na2O·B2O3·8SiO2)
64.0%
−
−
Bridging oxygen atoms, 70% BOs
Atomic
NBOs
fraction
3 coordinated boron atoms, [3]
B
100%
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Non-bridging oxygen atoms,
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Units
NBS1
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Main structure
100%
30%
0%
0%
25%
25%
60%
75%
75%
40%
4 coordinated boron atoms, B
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[4]
ACCEPTED MANUSCRIPT Highlights
Mechanical property changes of three sodium borosilicate glasses irradiated with 8-MeV Au3+ ions were investigated. No significant composition dependence of mechanical property changes was observed in
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ion-irradiated borosilicate glasses.
Both the deposited nuclear and electronic energy are responsible for the variations in
The reedmergnerite groups containing Si-O-BIV network may be the structural origin of
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Au-ion-irradiated glasses with R>0.5; while modification of the silica network, as well as
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phase separation, may dominate the glasses with R≤0.5.
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mean hardness and reduced Young modulus.
S0022-3115(18)31453-3
DOI:
https://doi.org/10.1016/j.jnucmat.2019.04.025
Reference:
NUMA 51573
To appear in:
Journal of Nuclear Materials
Received Date: 25 October 2018 Revised Date:
18 April 2019
Accepted Date: 18 April 2019
Please cite this article as: P. Lv, L. Chen, B.T. Zhang, W. Yuan, B.H. Duan, Y.D. Guan, Y. Zhao, X.Y. Zhang, L.M. Zhang, T.S. Wang, Composition-dependent mechanical property changes in Auion-irradiated borosilicate glasses, Journal of Nuclear Materials (2019), doi: https://doi.org/10.1016/ j.jnucmat.2019.04.025. 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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT Composition-dependent mechanical property changes in Au-ion-irradiated borosilicate glasses P. Lv a, L. Chen a, b, *, B.T. Zhang a, W. Yuan a, B. H. Duan a, Y. D. Guan a, Y. Zhao a, X.Y. Zhang a, L.M. Zhang a, b, T.S. Wang a, b, * a
School of Nuclear Science and Technology, Lanzhou University, Lanzhou, 730000,
b
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China
Key Laboratory of Special Function Materials and Structure Design Ministry of Education, Lanzhou University, 730000, China
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Abstract
The mechanical property changes in three kinds of ternary borosilicate glasses (named
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NBS glasses) induced by 8.0-MeV Au3+ were studied by nanoindentation measurements. For these borosilicate glasses, the molar ratio R=[Na2O]/[B2O3] is varied (1.34, 0.75, and 0.40 for NBS1, NBS2, and NBS3, respectively), while their molar ratio K=[SiO2]/[B2O3] is fixed at 4.04. The nanoindentation results indicate that both the hardness and reduced Young modulus were decreased after irradiation. No obvious difference in both hardness and modulus variations of
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these three NBS glasses was observed. Based on the main structures of their pristine samples, the reedmergnerite groups containing Si-O-BIV may be the structural origin of Au-ion-irradiated mechanical property changes in the homogeneous glasses with R>0.5.
mechanical
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However, the modification of the silica network and phase separation may dominate the property
changes
for
glasses
with
R≤0.5.
The
ion-
and
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electron-irradiation-induced mechanical property changes of these NBS glasses were also compared in this study. The extended incubation dose and the slower decreasing rate can be observed in electron-irradiation-induced hardness and modulus variations, which may be caused by the different mechanisms between both irradiation scenarios. Keywords Sodium borosilicate glass, Ion irradiation, Mechanical property, Composition dependence
* corresponding authors E-mail：[email protected] (L. Chen), [email protected] (T.S. Wang)
ACCEPTED MANUSCRIPT 1. Introduction As the candidate with the most potential for underground disposal, borosilicate glass has aroused much concern owing to its radiation durability [1–4]. External ion irradiation has been proved to be a powerful tool to simulate the radiation field caused by the decay of radionuclides.
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The irradiation-induced mechanical property changes are of importance for the glass waste form that serves as the first barrier for high-level waste (HLW). The degraded mechanical property of this waste form is riskier than the more released radionuclides, and finally, do harm to the biosphere. on
the
mechanical
property
changes
mainly
focus
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Studies
on
the
charged-particle-irradiation-induced hardness and modulus variation [5–10], discrepancy
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between different ions [5,11,12], and corresponding structural evolutions in these irradiated borosilicate glasses [7,9,13,14]. The mechanical property changes of borosilicate glass after irradiation have been claimed to be controlled by Na2O, B2O3, and SiO2 [15]. Moreover, the structure of this series of ternary glass was governed by two composition-related parameters,
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i.e., the molar ratio K (defined as [SiO2]/[B2O3]) and R (defined as [Na2O]/[B2O3]) [16–18]. Several researchers have covered the composition-dependent property and structure changes in particle-irradiated borosilicate glasses. Boffy et al. performed a series of neutron
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irradiations on four kinds of borosilicate glass and reported several structural changes and modifications in macroscopic behavior [19–21]. However, the K and R values were not
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considered in their works on complex glasses. Moreover, the hardness and modulus variations induced by neutron irradiation were not covered in their study. Kilymis et al. [22–25] studied the irradiation effects of three kinds of ternary sodium borosilicate glasses using molecular dynamics (MD). Their results have provided some insights into the structural origins and composition dependence of the mechanical property changes. Nevertheless, both the K and R values of their samples increased. In addition, experimental data of these ternary sodium borosilicate glasses have not yet been reported. Wang et al. [6] reported a hardness decrease of approximately 24% in 5-MeV Xe-ion-irradiated borosilicate glass (70 wt% SiO2+1 wt% Na2O+5 wt% Al2O3+4 wt% B2O3+4 wt% Al2O3), and the modulus variation was
ACCEPTED MANUSCRIPT approximately −7.4% below the initial value. Chen et al. [5] also investigated the irradiation damage of borosilicate glass (80.6 wt% SiO2+12.8 wt% B2O3+4.1 wt% Na2O+2.4 wt% Al2O3 and minor amounts of other constituents) induced by 5-MeV Xe ions. The nanoindentation results indicate that the hardness decreased by 12.8%, while the modulus increased by 3.5%.
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Based on the results of MD studies on the ternary sodium borosilicate glasses and experimental studies on the complex borosilicate glasses, composition-dependent mechanical property changes may exist in ion-irradiated borosilicate glasses. Our recent studies on electron-irradiated ternary borosilicate glass illustrate the influence of ratio R on their
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mechanical property changes [26]. However, the detailed role of the composition in ion-irradiation-induced mechanical property changes is unclear, and a systematic study should
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be carried out to clarify the detailed mechanism.
For this purpose, the ternary Na2O-B2O3-SiO2 (named NBS) glasses with different R values and constant K were investigated through external irradiation with 8.0-MeV Au3+ ions. Nanoindentation measurements were performed to investigate the evolution of hardness and
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reduced Young modulus before and after irradiation. The chemical compositions of the NBS glasses in this paper are the same as in Ref. [26], and a direct comparison can be made to further understand the mechanism of the irradiation-induced mechanical property changes from the
2. Experiment
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aspect of deposited energy.
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2.1. Chemical compositions and irradiation conditions The NBS glass samples were prepared by melting a mixture of Na2CO3, H3BO3, and
SiO2. Table 1 shows the chemical composition of these samples and the corresponding molar ratios with a maximum uncertainty given by weighting of 0.05%. The samples were then cut into squares of 10 mm size with a thickness of 1 mm. All the samples were polished to a surface finish of 0.5 µm, and the surface roughness was less than 5 nm. Before irradiation, all the samples were ultrasonically cleaned. Room-temperature irradiation with 8.0-MeV Au3+ generated by the 3-MV Tandetron accelerator was conducted at the Key Laboratory of Radiation Physics and Technology of the Ministry of Education, Institute of Nuclear Science
ACCEPTED MANUSCRIPT and Technology, Sichuan University, China [27]. Various fluences from 1.12×1012 to 1.20×1015 ions/cm2 were used. The beam flux during irradiation was approximately 2.36×1011 (ions/cm2)/s. The stopping power vs the projected range of incident Au ions in NBS1 glass was determined by SRIM2010 code [28], which is shown in Fig. 1. The curves of NBS2 and NBS3
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glasses were not plotted as their tendencies are very similar to NBS1 glass. It can be seen from this figure that the electronic process dominates in the front of the particle track, while the nuclear process is focused around the projectile range. Using this code, the deposited electronic energy and nuclear energy were also calculated. Their ratios, along with other information, are
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given in Table 2. 2.2. Characterization methods
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To characterize the mechanical property changes before and after radiation, an MTS G200 nanoindenter equipped with a Berkovich indenter was used. During the measurement, the continuous stiffness mode (CSM) was used under normal temperature and pressure (NTP). The maximum load was 500 mN, and the corresponding maximum penetration depth was
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approximately 1900 nm. This depth is close to the maximum range of the 8.0-MeV Au ion when we take longitudinal straggling into consideration (see Table 2). At least five indentations were carried out on each glass sample. The fused silica as reference material was
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also measured for calibration.
To verify the phase-separated nature of NBS3 glass, the Raman spectroscopy
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measurements (LabRAM HR 800) were made in confocal mode. A 532-nm Ar-ion laser with an output power of 100 mW was used. The spectra were obtained with a 100× objective lens under NTP. 3. Results
3.1. Hardness and reduced Young modulus curves of pristine and irradiated glasses Figure 2 displays the typical hardness and reduced Young modulus curves vs penetration depth for the three kinds of pristine glass samples and corresponding irradiated samples at an ion fluence of 1.20×1015 ions/cm2. Compared with the pristine glasses, an obvious decrease in hardness can be observed in the irradiated glasses, as shown in Fig. 2(a). Following a rapid
ACCEPTED MANUSCRIPT increase, the hardness of the pristine and irradiated glasses tends to be constant beneath the surface influence zone area. A slight increase is shown at the end of the hardness curves of irradiated glasses, which reflects the property of a deep layer without irradiation. Owing to the tip effect, the data of the first hundreds of nanometers were ignored [5,9,10,29]. The plateau
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region is not obvious in these curves meanwhile its choice has no significant influence on evaluating the mechanical property changes. Therefore, the hardness value of the samples was extracted from 300 to 920 nm which is based on the plastic interaction zone usually extends to approximately two times the plastic penetration depth [9,10,30]. As shown in Fig. 2(b), the
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modulus curves of both pristine and irradiated glasses show a similar tendency compared with the hardness curves. However, no plateau region in the modulus curves of irradiated glasses can
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be identified. This can be attributed to the fact that the elastically affected region is far larger than the plastically affected region [5]. To quantify the modulus variation of the ion-irradiated glasses, the measured values with penetration depths between 300 and 920 nm were also used to describe the modulus of irradiated glasses.
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3.2. Mechanical property changes in Au-ion-irradiated glasses
The hardness and reduced Young modulus variations of the NBS glasses vs ion fluence are shown in Figs. 3 and 4, respectively. The error bars shown in both figures represent the
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statistical error derived from multiple measurements. As shown in Fig. 3, the hardness of three NBS glasses is significantly decreased with increasing ion fluence. It should also be noted that
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a slightly increasing tendency does exist for all the hardness curves exceeding a fluence of 1014 ions/cm2. The possible mechanism is discussed in Section 4.1. As shown in Fig. 4, the modulus is decreased with increasing fluence, and the decreases are smaller than those of the hardness. Moreover, the modifications of modulus for the NBS glasses reach a saturation when the ion fluence is approximately 3×1013 ions/cm2, which is equal to the same magnitude of deposited nuclear energy (1020 keV/cm3). The mechanical property changes were associated with the R values to reveal the composition dependence in heavy-ion-irradiated borosilicate glasses. Figure 5 shows the hardness and modulus variations of irradiated glasses at 1.20×1015 ions/cm2 as a function of R.
ACCEPTED MANUSCRIPT From this figure, it can be seen that both hardness and modulus show no significant difference among these three NBS glasses. The hardness variations for NBS1, NBS2, and NBS3 glass are −23%±2.9%, −27%±3.0%, and −19%±3.1%, respectively. It is worth mentioning that the value of NBS3 is slightly smaller than that of NBS1 even if we take the error into account.
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However, the modulus variations are −10%±2.3%, −9%±2.2%, and −7%±1.6% for NBS1, NBS2, and NBS3 glasses, respectively. 3.3. Comparision of ion and electron irradiation
The hardness and modulus variations in electron-irradiated NBS glasses were compared
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with the modifications induced by Au-ion irradiation. The data of electron-irradiated NBS glasses were taken from our previous paper [26]. The hardness and modulus variations as a
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function of deposited electronic energy are plotted in Figs. 6 and 7, respectively. As shown in both sets of figures, the incubation dose of hardness and modulus variations induced by electron irradiation are different from those of the ion-irradiation scenario. The ion-irradiation-induced hardness and modulus variations already tend to stabilize or even
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slightly increase in hardness when the electron-irradiated ones are about to decrease. This means the incubation dose of the ion-irradiation scenario is smaller than that of the electron-irradiation ion. Moreover, the decreasing rate of hardness and modulus variations in
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ion-irradiated-glass samples is faster than that of electron-irradiated ones. In addition, it is also worth mentioning that the mechanical properties can be significantly
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affected by electron irradiation, and their variations may not always be smaller than those induced by heavy-ion irradiation. Compared with Au-ion irradiation, there is no difference in electron-irradiation-induced hardness and modulus variations in NBS2 and NBS3 glasses within the range of uncertainty. 4. Discussion 4.1. Effect of deposited energy on mechanical property changes Although the incident Au ions lose their energy in two ways, i.e., electronic and nuclear interactions, the hardness and modulus variations of ion-irradiated borosilicate glasses have been mainly correlated with the deposited nuclear energy. The variations have been clarified
ACCEPTED MANUSCRIPT to be saturated when the deposited nuclear energy reached approximately 1021 keVnucl./cm3 [5–8,10,12]. However, both the hardness and modulus variations in Au-ion-irradiated NBS glasses tend to be saturated when the deposited nuclear energy is approximately 3×1020 keVnucl./cm3. The smaller saturation point may be caused by chemical composition or the
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contribution of deposited electronic energy. The electron-irradiation-induced hardness and modulus variations are about to decrease when the ion-induced ones already tend to stabilize or even slightly increase the hardness variations (shown in Figs. 6 and 7), which illustrates that the deposited nuclear energy plays an important role in ion-irradiation-induced mechanical
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property changes. Peuget et al. also suggested that the nuclear-interaction-induced totally damaged layer in the glass volume is responsible for such saturation [32].
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In addition, it also should be noted that a slight increase in the hardness variations can be observed after the ion fluence reached approximately 1014 ions/cm2, as shown in Fig. 3. This phenomenon has not been reported in the previous studies on ion-irradiated borosilicate glasses and requires further study. Nevertheless, considering the electronic energy deposition
hardness
variations
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shown in Fig. 6, the start point of the slight increase in the Au-ion-irradiation-induced is
similar
to
that
of
the
significant
decrease
in
the
electron-irradiation-induced hardness variations. Therefore, we can speculate that there may
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exist a deposited electronic energy threshold (~9×1021 keVelec./cm3) for the hardness recovery of ion-irradiated NBS glasses. Indeed, the hardness of heavy-ion-irradiated borosilicate glass
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can be partially recovered by He-ion irradiation, in which the deposited electronic energy accounts for 99.6% of the total energy deposition [12,33]. Mir et al. suggested that the electronic-interaction-induced connectivity enhancement of damaged networks is responsible for the hardness recovery [33]. As a consequence, both kinds of deposited energy play important roles in mechanical property changes in several-MeV-heavy-ion-irradiated sodium borosilicate glasses. The deposited nuclear energy plays an important role in the initial decrease in both ion-irradiation-induced hardness and modulus, while the slight increase in hardness variations may be attributed to the damage recovery induced by the deposited electronic energy
ACCEPTED MANUSCRIPT deposition. Such a recovery, only observed in our ion-irradiated NBS glasses, appears to be caused by their sensitivity to the deposited electronic energy. Our previous study has already proved that the NBS glasses are more sensitive to the deposited electronic energy, and the hardness of the electron-irradiated samples at an electron dose of 1×109 Gy was decreased by
borosilicate glasses at the same dose [5,10,31].
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−11% to −27% [26]. These values are larger than the reported variations of electron-irradiated
Compared with the results of Au-ion irradiation, extended incubation dose and slower decreasing rate have been observed in electron-irradiation-induced hardness and modulus
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variations, which could be related to the physical processes of irradiation. For the ion-irradiation scenario, atomic displacements are the dominant mechanism. The mechanical
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property changes can be attributed to an increasingly disordered structure caused by collision cascade [34]. The cascade events among the irradiation volumes can be induced by a single heavy ion of several MeV. For the electron-irradiation scenario, ionization and electronic excitation are the dominant mechanisms. The point defects are the most important defects in
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this case, which could account for the mechanical property changes in electron-irradiated borosilicate glasses [5,10,26]. The extended incubation dose and slower decreasing rate in electron-irradiation-induced hardness and modulus variations may be related to the survival
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and accumulation of point defects.
4.2. Main structures of the pristine glasses
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To discuss the possible structural origins of the mechanical property changes in ion-irradiated NBS glasses, the main structures among the networks of these three sodium borosilicate glasses have been calculated according to the Yun, Bray and Dell (YBD) model [16–18,35]. In short, the sodium in NBS3 glass (R≤0.5) is consumed to transformation of [BO4] to [BO3] units, and the structure can be regarded as diborate (Na2O·2B2O3, containing two [BO4] and two [BO3]) diluted by SiO2. In this case, the borate (mainly trigonal) and silicate units cannot mix perfectly and may induce the phase separation of the NBS3 glass. The phase-separated nature of the NBS3 glass can be verified by the Raman spectrum shown in Fig. 8, which suggests that the NBS3 glass has a clustered structure with silica- and borate-rich units
ACCEPTED MANUSCRIPT [5,20,36]. For NBS2 glass (0.5
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For NBS1 glass (0.5+K/16
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structures in NBS1 and NBS2 glasses both have diborate and reedmergnerite groups, while the network of NBS3 glass is composed of [SiO4] tetrahedra, [BO3] trihedral, and diborate group.
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Furthermore, the only discrepancy between NBS2 and NBS3 glasses is the relative concentration of B species, while the NBOs concentration is the only discrepancy between NBS1 and NBS2. The concentrations of both parameters are different for NBS1 and NBS3 glasses:
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2 B2 O3 + Na2 O → ( Na2 O i2 B2 O3 ) , ( R ≤ 0.5)...............................(1) 1 1 K [( Na2 Oi 2 B2 O3 )] + 4SiO2 + Na2 O → [0.5( Na2 Oi B2 O3 i8SiO2）], (0.5 < R ≤ + 0.5)..............(2) 4 16 4 3 K K [0.5( Na2 Oi B2 O3 i8SiO2） ] + Na2 O → [0.5(2.5 Na2 O i B2 O3 i8SiO2 )], ( + 0.5 < R ≤ + 0.5).......(3) 16 4 4
4.3 Effect of glass composition on mechanical property changes
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As many authors have suggested, ion-irradiation-induced mechanical property changes of the borosilicate glass should be caused by the damage of the glass network [8,12,37]. There is no doubt that ion irradiation can alter the short- and/or medium-range order in the glass, which facilitates the disorder in the glass network, along with the formation of point defects. A more disordered structure has been confirmed by nuclear magnetic resonance (NMR) spectroscopy and MD simulations [14,22,38]. A mechanism based on the damage of the glass network has been proposed to interpret the mechanical property changes in ion-irradiated borosilicate glass [8,15,39–41]. In the ternary Na2O-B2O3-SiO2 glass system, the coordination number of boron
ACCEPTED MANUSCRIPT atoms can be decreased by the migration of charge-compensation sodium from a local position to the interstitial sites of the Si network. These sodium ions that have migrated to the interstitial sites play a role in modifying the Si network and produce the non-bridging oxygen atoms [7,8]. The results of our recent MD simulation on a homogeneous Na2O-B2O3-SiO2 glass (R>0.5)
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suggested that the damage of the mixed Si-O-BIV network plays an important role in the processes mentioned above and in the mechanical property changes induced by ion irradiation [40].
A lower boron coordination number has been confirmed in both ion-irradiation
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experiments [5,7,14,15,32,42] and MD studies [22,24,25,30,41]. Mendoza et al. [14] investigated heavy-ion-irradiation-induced structural modifications by NMR spectroscopy.
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Their results indicated that the transformation of [BO4] to [BO3] units depends on the glass composition. The net increment of the [BO3] concentration in BS3 (R>0.5+K/16) is slightly higher than that in the BS6 (R≈0.5+K/16) glass. Furthermore, the three coordinated boron atoms of the irradiated SBN12 (R<0.5), SBN14 (0.5
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glasses increased by 3%, 10%, and 11%, respectively [25]. Given the population of boron species in the pristine samples, the transformation of [BO4] to [BO3] units in our NBS glasses may follow the order NBS1>NBS2>NBS3.
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However, an increasing concentration of NBOs after ion irradiation has also been widely reported [7–9,14,15,23,24,43]. The broken Si-O bonds induced by ion irradiation are
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considered another important source that produces NBOs [8,15,43]. Peuget et al. [8,14] investigated the ion-irradiation-induced structure evolutions in some borosilicate glasses by NMR spectroscopy. All their 29Si and 23Na spectra indicate the depolymerization of the silicate network and the shortening of the average Na-O distance after irradiation. In addition, Kilymis’s studies on three different ternary sodium borosilicate glasses indicated that ion-irradiation-induced NBOs variations also depend on glass composition [23]. The concentrations of NBOs in these disordered glasses increased by 1.26%, 2.36%, and 1.87% for SBN12 (R<0.5), SBN14 (0.5
ACCEPTED MANUSCRIPT Therefore, an increase of NBOs concentration can be speculated in our Au-ion-irradiated NBS glasses, and the relative variations may follow this tendency, i.e., NBS2>NBS1>NBS3. As mentioned above, the reedmergnerite groups (composed of the mixed Si-O-BIV network) dominate the main structures among NBS1 and NBS2 glasses, while the [SiO4]
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tetrahedra dominate the main structures of NBS3 glass. Thus, ion-irradiation-induced mechanical property changes in NBS3 glass should be reflected more on the structural change of the SiO2 network. Mendoza et al. [14] also claimed that the mechanical property changes in krypton-irradiated SiO2 glass should be more likely related to the Si-O− bond. Boffy et al. [20]
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studied the structural evolutions of neutron-irradiated borosilicate glasses, and their results show that the glass with smaller R value had a notable blue shift, indicating significant
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modification of the SiO2 network in NBS3 glass after ion irradiation.
In summary, the modifications in the network structures of ion-irradiated borosilicate glass depend on glass composition. However, the ion-irradiation-induced mechanical property changes should be related to the combined effect of many factors, such as boron coordination in
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parallel with NBO formation and point defects. Our results on Au-ion-irradiated NBS glasses illustrate that there is no composition-dependent modulus variation within the error range for these three NBS glasses. Compared to NBS2 glass, the slightly smaller hardness variation of
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NBS3 glass may be attributed to its phase-separated nature [5,24]. In contrast, no composition-dependent hardness variations are observed between NBS1 and NBS2 glass,
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which means that the NBOs might make a very small contribution to the hardness change of homogeneous glasses. The similar hardness changes in NBS1 and NBS3 glasses may be caused by a synergy process in both NBOs and phase separation. 5. Conclusions
The mechanical property changes in three kinds of Au-ion-irradiated NBS glasses were investigated by means of nanoindentation measurements. Results indicate that both the hardness and reduced Young modulus decrease after Au-ion irradiation. In addition to the deposited nuclear energy, the deposited electronic energy could also induce the mechanical property changes in ion-irradiated NBS glasses. In this case, the slight increase in the hardness
ACCEPTED MANUSCRIPT variations can be observed after the ion fluence reached approximately 1014 ions/cm2, which may due to the hardness recovery induced by the deposited electronic energy. Both the hardness and modulus variations have no strong compositional dependence in these three NBS glasses. The only discrepancy observed in the hardness variations between
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NBS2 and NBS3 glass may be attributed to the phase separation, while NBOs might make a very small contribution to the hardness change in homogeneous glasses (NBS1 and NBS2 glasses). Furthermore, the reedmergnerite groups containing the Si-O-BIV network may be the structural origin of ion-irradiation-induced mechanical property changes for the NBS glasses
mechanical property changes in the glass with R≤0.5.
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with R>0.5. The modifications of the silica network and phase separation may dominate the
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By comparing the results of Au-ion and electron irradiation, different decreasing tendencies are observed in the hardness/modulus variations. The extended incubation dose and the slower decreasing rate in electron-irradiation-induced hardness and modulus variations may be attributed to the different mechanisms for both irradiation scenarios. In addition, the
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mechanical property changes induced by electron irradiation may not always be smaller than those induced by heavy-ion irradiation. Acknowledge
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This work was supported by the National Nature Science Foundation of China [NSFC, No. 11505084, No. U1867207 and No. 11675068]; and the Fundamental Research Funds for the
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Central University of China [No. lzujbky-2016-37]. Key Laboratory of Radiation Physics and Technology of the Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University is acknowledged for providing technical support. The authors are also grateful to the Public Center for Characterization and Test, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences.
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ACCEPTED MANUSCRIPT Figure/Table Captions Table 1 Composition of NBS glasses in mol%. Fig. 1 Stopping power vs projected range calculated with SRIM2010 code. Table 2 Other detailed information for NBS glasses. Eelec. and Enucl. are the deposited electronic
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energy and nuclear energy, respectively. Fig. 2 Typical (a) hardness and (b) reduced Young modulus curves of pristine and irradiated NBS glasses.
Fig. 3 Evolution of mean hardness variations vs fluence for NBS glasses. Solid lines are guides
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for the eye.
guides for the eye.
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Fig. 4 Evolution of mean modulus variations vs fluence for NBS glasses. Solid lines are
Fig. 5 Hardness and modulus variation vs R after Au-ion irradiation at 1.20×1015 ions/cm2. Fig. 6 Hardness variations of irradiated NBS glasses vs electronic energy deposition. Solid lines are guides for the eye.
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Fig. 7 Hardness variations of irradiated NBS glasses vs electronic energy deposition. Solid lines are guides for the eye.
Fig. 8 Raman spectrum of pristine NBS3 glass.
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model.
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Table 3 Main structures in NBS glasses derived from the direct calculation of the YBD
ACCEPTED MANUSCRIPT Table 1 Compositions of NBS glasses in mol%.
NBS1
NBS2
NBS3
SiO2
63.38
69.64
74.31
B2O3
15.68
17.21
18.40
Na2O
20.94
13.15
7.29
K=[SiO2]/[B2O3]
4.04
4.04
R=[Na2O]/[B2O3]
1.34
0.75
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Samples
4.04
0.40
ACCEPTED MANUSCRIPT Table 2 Other detailed information for NBS glasses. Eelec. and Enucl. are the deposited electronic energy and nuclear energy, respectively. Density (g/cm3)
Range (µm)
Eelec./Enucl.
NBS1
2.52
1.71±0.14
1.85
NBS2
2.45
1.76±0.15
1.82
NBS3
2.34
1.84±0.15
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Samples
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1.82
ACCEPTED MANUSCRIPT Table 3 Main structures in NBS glasses derived from the direct calculation of the YBD model. NBS2
NBS3
SiO2, [SiO4]
0
0
87.0%
B2O3, [BO3]
0
0
4.3%
Na2O·2B2O3
20.0%
20.0%
8.7%
0.5(Na2O·B2O3·8SiO2)
16.0%
80.0%
−
0.5(2.5Na2O·B2O3·8SiO2)
64.0%
−
−
Bridging oxygen atoms, 70% BOs
Atomic
NBOs
fraction
3 coordinated boron atoms, [3]
B
100%
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Non-bridging oxygen atoms,
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Units
NBS1
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Main structure
100%
30%
0%
0%
25%
25%
60%
75%
75%
40%
4 coordinated boron atoms, B
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[4]
ACCEPTED MANUSCRIPT Highlights
Mechanical property changes of three sodium borosilicate glasses irradiated with 8-MeV Au3+ ions were investigated. No significant composition dependence of mechanical property changes was observed in
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ion-irradiated borosilicate glasses.
Both the deposited nuclear and electronic energy are responsible for the variations in
The reedmergnerite groups containing Si-O-BIV network may be the structural origin of
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Au-ion-irradiated glasses with R>0.5; while modification of the silica network, as well as
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phase separation, may dominate the glasses with R≤0.5.
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mean hardness and reduced Young modulus.