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Abstract
The introduction of SiO2 affects the glass’s micro-structure and micro-defects as well as the corresponding optical and physical properties of the B2O3-containing multi-component phosphate glasses. The addition of SiO2 increases the transition temperature and causes shifts of the UV cut-off wavelength (λcut-off). The changes of λcut-off are closely related to the PO3-EC and PO4-EC defect centers that are associated with the Q2 tetrahedral in phosphate chains. The corresponding variations of Q2 units can also be found in the Raman, XPS, and 31P MAS-NMR spectra. With increasing the content of SiO2, the ratio of non-bridging oxygen (associated with Q2 units) is gradually decreased down to a critical level when SiO2/B2O3 ratio is 4/1.5, whereas the ratio of non-bridging oxygen increases when SiO2/B2O3 ratio is larger than 4/1.5, which suggests that the doping SiO2 can influence the concentration of PO3-EC and PO4-EC defects. Those results can be better obtained when the sample is exposed to gamma irradiation. As the SiO2 content increased gradually up to 4/1.5, the concentration of the PO3-EC defects declined, while the concentration of PO4-EC and the POHC defects increased when considering the irradiation dose, indicating that the addition of SiO2 can regulate the structure-related defects in phosphate based glasses.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Phosphate based glasses have attractive much attention due to their excellent properties, such as good glass forming ability and fiber drawing stability [1], large transmission range from deep ultraviolet to the near infrared [2-3], which endow them with applications as gain medium for high power NIR laser systems [4], lens optics in UV lasers for microlithography [5] and potential novel UV optics with high laser induced damage threshold for high energy UV laser facilities (e.g., NIF in USA and SG III in China) [6]. In this case, the irradiation resistance of these phosphate glasses should be paid considerable attentions. Nevertheless, against high energy ions/rays (e.g. neutron, X-ray, gamma-ray) or high-power laser irradiation, phosphate glasses will confront an obvious decline of transmission, especially in the short range from UV to visible, due to the formation of various defects, which limits the use of these optics in the irradiation-related environments. In this case, investigation on the change of defects’ state and concentration in the phosphate based glasses after irradiation is of great necessity to guide the improvement of their performance.
The defects formed in phosphate based glasses can be divided into two types, including extrinsic defects and intrinsic defects [7]. For the former, trace impurity of transition metal as the main precursor of extrinsic defects in these phosphate glasses may affect the UV and visible transmittance. Especially, iron is the unavoidable one mostly introduced by the raw materials, whose trivalent ions have strong absorption in the UV region, even with tens of ppm level of contents [8–10]. For the latter, the intrinsic defects are associated with the glass micro glass structure of the phosphate glasses, in which phosphate chains made of phosphate tetrahedral constituent the glass network. As the phosphate glass networks are broken by introduction of modifier cations in the glass preparation process or by the high energy radiations, varies of intrinsic defects including hole trapped center, phosphate-related oxygen hole center (POHC) [8–10], oxygen-related hole center (OHC) and electronic trapped center (PO3-EC, and PO4-EC) will form, which have been generally investigated in our previous work [11–13].
Compared with phosphate based glasses, silica glasses have better mechanical and chemical stability, and higher UV transmittance mostly due to its strengthening glass network. Hence, introduction of some SiO2 into the phosphate glasses to modify the phosphate glass network through the comprehensive network forming effect of silicate and phosphate polyhedrons can adjust the phosphate glasses’ properties. Many studies have been carried out to investigate the thermal property and bioactivity of SiO2 doped phosphate glasses [14–16]. But little attention has been paid to explore the effects of adding SiO2 on the micro-defects that associate closely with the optical transparency and irradiation resistance of the modified phosphate based glasses, which may contribute to developing high performance phosphate based optical glasses for high energy UV laser applications.
The aim of this work is to study changes in the micro-defects and related glass structure of B2O3-containing phosphate glasses doped with different SiO2 content via systematic analysis on the samples’ transmission, absorption, Raman, X-ray photoelectron (XPS) and 31P Magic Angle Spinning-Nuclear magnetic resonance (MAS-NMR) spectra. The total irradiation dose effect of gamma irradiation was taken as the main experimental method to reveal the underlying defects sates, relative concentration and their revolution for the B2O3-containing multi-component phosphate glasses after little amount of SiO2 was doped.
2. Experimental
Multi-component B2O3-containing phosphate glasses composed of Li2O (0.5-2), K2O (3-5), MgO (3-5), BaO (7-10), Al2O3 (8-11), P2O5 (59-64), and B2O3 (2) in weight percent (wt%) doped with different content of SiO2 (0-6 wt%) were prepared by the melt-quenching technique [13]. The weight ratio of SiO2/B2O3 was set to be 1:1.5, 3:1.5, 4:1.5 and 6:1.5 respectively. High purity raw materials (≥ 99.99%, Sigma Aldrich Inc.) were weighted and sufficiently mixed. Then the raw material mixtures were melted at 1200 °C under a reducing atmosphere in a quartz crucible, with use of a 2 h mechanical stirring process for achieving a high level of homogeneity. Subsequently the glass melts were poured into a brass mold preheated at 300 °C and then annealed at 405 °C (a little lower than the glasses’ transition temperature). After that, these glasses were cut and polished precisely to prepare the samples with a size of 30 mm × 25 mm × 2 mm. To study the effects of doping SiO2 on the irradiation resistance and defects’ changes in these multi-component phosphate based glasses, all these glass samples were exposed to gamma radiation using a 60Co source at an irradiation dose rate of 44.05 rad(Si)/s to accumulate absorbed doses of 20k, 100k, 250k, 500k and 1000k rad(Si) (The SI unit of an absorbed dose is 1 Gray = 100 rad).
The glass transition temperature (Tg) was measured by differential scanning calorimetry (DSC) with an SDT Q600 V20.9 (TA Instruments) in the temperature range 30-800 °C with a 10 °C/min heating rate. The transmission spectra were obtained using a UV-VIS-NIR spectrophotometer (Shimadzu UV-3101) in the range of 220-800 nm. Raman spectra measurement in a range of 100-1500 cm−1 was taken using a Jobin-Yvonne LabRam microscope with a 532 nm laser excitation source. XPS spectra were measured on a Thermo Advantage X-ray photoelectron spectrometer at room temperature using Al K (1486.6 eV) as the radiation source. Solid state 31P MAS-NMR were performed at 162.0 MHz on Bruker DSX-400 instrument, using a 4 mm probe at spinning rate of 12.0 kHz. The spectra were recorded with a 4.3 μs pulse length (90°angle) and relaxation time was 120 s. And the chemical shifts are referred to 85% H3PO4.
3. Results
The evolution of glass’ Tg changed with the doping content of SiO2 is depicted in Fig. 1. The addition of SiO2 increases Tg value gradually, which changes from 407.2 °C for the sample with a SiO2/B2O3 ratio of 1/1.5 to 420.5 °C for the one with a SiO2/B2O3 ratio of 6/1.5, suggesting a strengthening effect of doping SiO2 on the glass network. The internal transmission (without Fresnel reflection) spectra of these glasses have also been examined and are shown in Fig. 2. All these samples have general high transmission in the wavelength region of 325-800 nm, while as for the wavelength shorter than 300 nm, the transmission of the glasses decreased more rapidly. Their UV cut-off wavelength (λcut-off) varies with the introduced SiO2 content.
Fig. 1 Differential scanning calorimetry curves of SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively).
Fig. 2 Internal transmission (without Fresnel reflection) spectra of SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively).
The λcut-off is closely related to PO4-EC and PO3-EC defects with absorption band maximum at around 5.19 eV and 5.8 eV [17], respectively, as shown in Fig. 3. Regarding the prominent absorption band corresponding to PO3-EC defects, its relative absorption peak’s area is 69.4%, 64.8%, 67.9% and 68.2% for the samples with SiO2:B2O3 ratio of 1/1.5, 3/1.5, 4/1.5 and 6/1.5, respectively. It suggests that the relative concentration of PO3-EC defects decreases as the SiO2/B2O3 ratio increases from 1/1.5 to 3/1.5, then its relative concentration increases with increase of SiO2 content.
Fig. 3 Absorption spectra of SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively) fitted with Gaussian components.
The Raman spectra of these samples between 100 cm−1 and 1500 cm−1 are compared in Fig. 4. The weak bands in the range of 200-500 cm−1 are due to the bending vibration of phosphate polyhedral [18]. The bands at a broad region (620-820 cm−1) are assigned to the symmetric stretching modes of bridging oxygen (BO). The main peaks located at around 708 cm−1 are related to the P-O-P symmetric stretching of bridging oxygen [13]. The high frequency bands in the range of 1000-1400 cm−1 are associated to the symmetric and asymmetric stretching modes of P-nonbridging oxygen (NBO) (or terminal oxygen). And the peaks at around 1210 cm−1 are due to the symmetric stretching vibration on Q2,P tetrahedral [13]. The obvious variations at around 1280 cm−1 are assigned to asymmetric stretching of non-bridging oxygen on Q2,P units. It can be found that the intensity of this band that is associated with NBO decreases with the increase of SiO2/B2O3 ratio in these phosphate glasses.
Fig. 4 Raman spectra of SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively).
The O1s XPS spectra for the investigated phosphate based glasses were recorded and the O1s spectra were well fitted with two types Voigt peaks as shown in Fig. 5. The peaks located at the lower and higher binding energy are assigned to the NBO and BO contributions, respectively [19]. The detailed fitting peak parameters, including peak position and NBO/(BO + NBO) ratio, are concluded in Table 1. As the SiO2 content increases, the ratio of NBO firstly decreases from 55.20% and reaches a minimum of 44.43% when SiO2/B2O3 ratio is 4:1.5, and at last it abruptly increases to 54.99% for the sample with a SiO2/B2O3 ratio of 6:1.5.
Fig. 5 O1s XPS spectra of SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively) were fitted with a Voigt function. The detail fitting parameters are provided in Table 1.
Table 1. Parameters of the fitted BO and NBO peaks from the O1s XPS spectra of SiO2 doped B2O3-containing phosphate based glasses (The errors on the measurement are ± 0.3 eV for peak position and ± 2% for BO/(BO + NBO) ratio).
Figure 6 shows the 31P MAS-NMR spectra of the B2O3-containing phosphate based glasses doped with different SiO2 content. A dominant resonance near −35 ppm is observed for all the investigated glasses, which is the characteristic of Q2 tetrahedral. And a weak peak centered at around −11 ppm on the left shoulder of the above main peak is assigned to Q1 tetrahedral [20,21]. To quantify the concentration of Qn tetrahedral, the 31P MAS-NMR spectra were fitted with three Gaussian peaks, and the fitted parameter are given in Table 2. The fitting results indicate that there exist two species of Q2 tetrahedral: Q2,Si and Q2,P, peaking at around −27 ppm and −37 ppm, respectively. It is supposed that the existence of Q2,Si species is due to the incorporation of Si-tetrahedral into the phosphate chains. With the increase of SiO2 content in the phosphate glasses, the percentage of Q1 and Q2,P phosphate tetrahedral both shows a decrease at first stage and their relative ratio reaches a minimum of 0.41% and 84.41% respectively as SiO2/B2O3 ratio is 4:1.5, after that their content increases oppositely. Meanwhile, the relative ratio of Si-related tetrahedral (Q2,Si) keeps increasing to 15.18% and at last it drops to 10.89% for the sample with a SiO2/B2O3 ratio of 6:1.5. The change of evaluated relative ratio of Q1 and Q2,P tetrahedral is falling in accordance with the change of NBO ratio as shown in Fig. 5 and Table 1, as well as the variation of PO3-EC defects at 5.8 eV to some extent (Fig. 3).
Fig. 6 31P MAS-NMR spectra and deconvolution model for SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively).
Table 2. 31P isotropic chemical shifts and relative area percentage of the individual Q(n) phosphate species, obtained from the deconvolution of the 31P MAS NMR spectra in Fig. 6 (The errors on the measurement are ± 0.1 ppm for δiso and ± 0.1% for ratio).
Figure 7 presents UV-visible transmission spectra for the samples before and after being exposed to different total doses (20k, 100k, 250k, 500k and 1000k rad(Si)) of gamma irradiation with a same irradiation dose rate of 44.05 rad(Si)/s. Before the irradiation, all samples have similar transmittance in the range of 325-800 nm but their λcut-off vary a little as shown in Fig. 2. Gamma irradiation leads to an obvious decline of their visible transmittance in the spectrum range of less than 700 nm. After irradiation, two prominent absorption bands are observed, peaking at around 525 nm (2.36 eV) and 385 nm (3.22 eV). They are related to the POHC defects [22], and its absorption intensity increased rapidly with the increase of irradiation doses, especially when the irradiation dose is higher than 100k rad(Si). The increasingly dense absorption of the samples with much higher irradiation dose (≥250k rad(Si)) makes the as-irradiated samples’ color become red-brown gradually, as depicted by the samples’ photographs in above part of Fig. 7, together with the red-shift of their λcut-off.
Fig. 7 UV-visible transmission spectra and the corresponding photographs of SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively) at different total irradiation doses (0k, 20k, 100k, 250k, 500k, 1000k rad (Si)).
To identify the types of induced defects and reveal their evolutionary mechanism under gamma irradiation, in Fig. 8, the corresponding absorption spectra of these samples were fitted using Gaussian peak fitting method. There are several types of defects existed in the phosphate glasses, including POHC1 (around 2.31 eV), POHC2 (around 3.13 eV), OHC (around 4.24 eV), PO4-EC (around 5.12 eV) and PO3-EC (around 5.90 eV) [7–9,11,13]. Obviously, with the increase of irradiation dose, the absorption of these defects increased, among which electron captured defects (PO4-EC and PO3-EC defects) that have large absorption in the high-energy region are the dominant defects.
Fig. 8 Absorption spectra of SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively) at different total irradiation doses (20k, 100k, 250k, 500k, 1000k rad(Si)).
The variation of the absorption-peak’s area of the corresponding defects was used for qualitative evaluation of the changing concentration of various defects, and the results are shown in Fig. 9. It can be observed that the concentration of all these defects increases with increasing total irradiation dose. In addition, the electron captured defects (PO3-EC and PO4-EC) show a more rapid increase than these oxygen hole center defects (POHC1, POHC2 and OHC) when the total irradiation dose is lower than 250k rad(Si). As the irradiation dose becomes much higher, their concentration changes rather slowly, but the concentration difference between the electron captured defects and these oxygen hole center defects continues becoming larger.
Fig. 9 Absorption peak’s area of several types of defects (POHC1, POHC2, OHC, PO4-EC, PO3-EC) in SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively) at different total irradiation doses (20k, 100k, 250k, 500k, 1000k rad(Si)).
The defects’ concentration was compared in the terms of the ratio of separated absorption peak’s area of corresponding defect to the total absorption spectra area in Fig. 10. Increasing SiO2 content does not change the relative concentration of POHC1 and POHC2 defects obviously. For PO3-EC and PO4-EC defects, their relative concentration is affected by SiO2 content to a larger extent, especially when the irradiation dose is lower than 250k rad(Si). Compared with the defects concentration of the sample with lowest SiO2 content (SiO2/B2O3 = 1:1.5), the samples with the SiO2/B2O3 ratio higher than 3:1.5 show a rather distinctive characteristic that the concentration of PO3-EC increase and meanwhile that of the PO4-EC defects decrease. Under higher irradiation doses (500k and 1000k rad(Si)), the influence of doping SiO2 on the relative concentration of PO3-EC and PO4-EC defects is relatively weak, which may because that the defects’ concentration tend to be saturated.
Fig. 10 Gamma irradiation induced change of relative concentration for several types of defects (POHC1, POHC2, OHC, PO4-EC, PO3-EC) in SiO2-doped B2O3-containing phosphate based glasses with different SiO2:B2O3 ratios (1:1.5, 3:1.5, 4:1.5 and 6:1.5, respectively) at different total irradiation doses (20k, 100k, 250k, 500k, 1000k rad(Si)).
4. Discussion
It is essential to understand the glass microstructure of investigated multi-component phosphate based glasses for exploring the correlation between changes of defect states and doping content of SiO2. It is known that the structure of phosphate glasses is build up by a series of long phosphate chains made of phosphate tetrahedral. When alkali and alkaline earth metal cations are introduced to phosphate glasses, phosphate chains would be shortened by forming various phosphate tetrahedral units, such as Q0, Q2 and Q3 [13]. The depolymerization of phosphate glass network by introducing modifying metal cations will contribute to the formation of intrinsic defect, including PO3-EC and PO4-EC defect centers, which largely affect the UV transmission and λcut-off of the investigated phosphate glasses, as indicated by Figs. 2-3.
The main glass microstructural characteristic of the investigated phosphate based glasses is evidenced by the dominant vibration of Q2 tetrahedral including Q2,Si and Q2,P verified by the 31P MAS-NMR, which take a proportion of larger than 98%. Besides Q2 tetrahedral, there also exists a very small amount of Q1 tetrahedral with a ratio of less than 2%, which is either terminal of phosphate chains or phosphate dimmers (two Q1 tetrahedral linked by bridging oxygen). When low content of SiO2 (SiO2/B2O3 ratio≤4:1.5) was introduced into the phosphate based glasses, the relative concentration of Q2,Si increased, whereas the Q1 ratio decreased as shown by the 31P NMR spectra in Fig. 6 and the evaluated results in Table 2, which indicates the polymerization of phosphate glass network was enhanced due to the incorporation of Si atoms into the phosphate chains, acting as a bridging medium. The strengthening of phosphate glass network can also be demonstrated by the corresponding decrease of NBO/(BO + NBO) ratio as depicted by the XPS spectra. On the other hand, the strengthening of the phosphate glass network led to the increase of the Tg (Fig. 1).
Nevertheless, when the SiO2/B2O3 ratio is further increased up to 6/1.5, the sample’s corresponding NBO/(BO + NBO) ratio drops abruptly down to 54.99%, reaching almost the same level as that of the sample (55.20%) with a SiO2/B2O3 ratio of 1/1.5. This suggests that the strengthening effect on the phosphate glass network by introducing SiO2 is limited. For the case of a rather high content of SiO2 (SiO2/B2O3 = 6/1.5) is introduced, it is supposed that only small part of SiO2 entered the phosphate glass network in term of Q2,Si tetrahedral to enhance the glass network’s polymerization, and the rest part of SiO2 might form separated Si-O-Si linkages [16] (that are over hidden by the relative higher intensity of P-O bands in Raman spectra in the range of 600-1100 cm−1) and stay among the phosphate chains, resulting in the decreased strengthening effect on phosphate related network. But due to the intrinsic higher glass’ Tg of silica than the phosphate glass phase, the Tg value for the glass sample with a SiO2/B2O3 ratio of 6:1.5 would continue to increase.
The slight shift of λcut-off is observed for these phosphate based glasses, but there is no clear dependence of λcut-off on the variation of the SiO2/B2O3 ratio, and the change of λcut-off did not show a similar tendency with the change of NBO/(BO + NBO) ratio (Table 1) or the evaluated relative ratio of Q1 and Q2,Si tetrahedral (Table 2). For these SiO2 doped samples, their UV edge is determined by the concentration of PO3-EC and PO4-EC defects (Figs. 2-3). Judging from Fig. 9, as the total irradiation dose became larger, the relative concentration of PO3-EC and PO4-EC defects increased rapidly. Thus, their comprehensive effects resulted in the red-shift of UV absorption edge and fast reduce of UV transmission for these phosphate based glasses. From Figs. 3, 7-9, it can be seen that the absorption of POHC1 and POHC2 defects determines the absorptive characteristic in the visible spectrum range, while the absorption of PO3-EC and PO4-EC defects mainly dominate the UV absorption, thus, as the irradiation dose increased, the samples’ UV (especially less than 300 nm) transmittance showed a more pronounced decline compared with the visible range.
For the investigated phosphate based glasses, Q2 phosphate tetrahedral construct their main glass network. Before gamma irradiation, PO3-EC defects are the main defects in the phosphate glasses, which is twice of the PO4-EC defects (see Fig. 3). As we know, the generation process of defects is companied by the release and recapture of the trapped electrons. Under the effect of gamma irradiation, the ground-state electrons of Q2 tetrahedral would be photo-ionized to the conduction band and then free electrons will be released off, which is accompanied by the generation of POHC defects. The accumulation effect was viewed by the rapid increase of POHC defects’ absorption intensity and denser coloring of the as-irradiated samples. Meanwhile, as the released electrons were subsequently captured by some Q2,P tetrahedral, PO4-EC defects can be formed. Under the accumulated irradiation of gamma rays, which is more obvious at relative lower irradiation dose (≤250k rad(Si)), the increasing Q2,Si tetrahedral facilitated the transformation of some PO3-EC defects to PO4-EC defects most possibly due to the formation of Si-O-P bonds. During the irradiation process, the formation of PO3-EC defects might in two steps, first an electron was trapped on four coordinated phosphates [22], and then the linkage to one of bridging oxygen was broken. It indicated that the formation of PO3-EC defects might be slower than that of PO4-EC defects, cause PO4-EC defects may be generated from two sources, both from Q2,P tetrahedral and PO3-EC defects, and thus the decline of PO3-EC was accompanied by the increase of PO4-EC defects in Fig. 10. As the irradiation dose became much higher, the transformation tended to reach a relative equilibrium, and the defects’ concentration got saturated gradually.
5. Conclusions
We have studied the changes of defect states, relative concentration and glass network induced by doping of SiO2 through gamma irradiation experiments for the B2O3-containing phosphate based glasses. With the increase of doping SiO2 content, the glass’ Tg continued increasing from 407.2 °C to 420.5 °C, and the ratio of non-bridging oxygen decreased gradually when the SiO2/B2O3 ratio is no more than 4/1.5 and finally increased to its original level, indicating a different strengthening effect of SiO2 on the phosphate glass network. The relative ratio of Q1 tetrahedral in the glass network showed a similar changing tendency with that of NBO ratio. Through gamma irradiation experiments, P-related defects (POHC1 (2.31 eV), POHC2 (3.13 eV), PO4-EC (5.12 eV), PO3-EC (5.90 eV)) and oxygen-related defects (OHC (4.24 eV)) were clarified. The PO4-EC and PO3-EC defects together determined the UV absorption, while the POHC1 and POHC2 defect resulted in the prominent absorption band peaking at 525 nm and 385 nm respectively and the continuing decline of visible transmittance as the irradiation dose increased. Their increasing defects concentration made the glass samples turned to be dark red-brown gradually. The electron captured defects showed a more rapid increase than these oxygen hole center defects when the total irradiation dose was lower than 250k rad(Si), and the influence of doping SiO2 on the transformation of PO3-EC to PO4-EC defects was relative obvious. As the irradiation dose became much higher, the PO4-EC defects’ concentration overtook the PO3-EC defects and the influence of doping SiO2 on these defects became unobvious.
Funding
National Natural Science Foundation of China (NSFC No. 61775235, 61307046); Natural Science Basic Research Project in Shaanxi Province (2015JM6315); West Young Scholars Program of the Chinese Academy of Sciences (XAB2016A08), and Youth Innovation Promotion Association CAS (2017446), China.
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