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Abstract
The effects of doping B2O3 on the defects and their induced anti-radiation performance change of the multicomponent phosphate glasses were studied in this work. The introduction of B2O3 reduces the connectivity of phosphate chains and thus increases the concentration of PO3-EC and PO4-EC defects in the phosphate glass network that have large absorption in the high-energy region. Meanwhile, B2O3 can improve the oxidizability of those glasses at the same melting temperature under which Fe2+ ions will be more easily oxidized to Fe3+ ions. However, the addition of B2O3 in terms of H3BO3, as it reaches up to 7.5 wt%, could enhance the gamma radiation resistance of the phosphate glasses, in this case B2O3, and enter the phosphate glass network in the form of B5O8 units. The units enhanced the connectivity of the long phosphate chains, and thus reduced the concentration of PO3-EC and PO4-EC defects in phosphate glasses.
© 2017 Optical Society of America
1. Introduction
Phosphate based glass as one of the high-performance optical materials [1–3] for the promising applications in the fields of high power laser system, lens system in UV microlithography equipment and other special UV optics, have received more and more attentions due to their excellent properties, such as high transparency in the range of UV to NIR, high glass forming ability and good host materials for dopant ions, as compared with some other glass systems [4]. Besides, the use of high-performance UV transmitting materials in high power UV lasers, require not only high transparency but high pulse laser-induced damage thresholds (LIDTs) specially. Unfortunately, various defects will form in the phosphate based glasses during the glass preparation process and post processing that are harmful for their UV transmittance and will significantly decrease their LIDTs particular at the short wavelength [5]. The defects in phosphate based glasses include intrinsic defects that arise from the raw materials and glass matrix, and extrinsic ones that are caused by introduced dopants or impurities [6]. Addition of modifying oxides may cause the breakage of the phosphate chains made of P-tetrahedra in the phosphate based glasses, which subsequently result in formation of a series of intrinsic defects, such as phosphate-related oxygen hole center (POHC), and phosphate-related electron centers (PEC) including PO3-EC and PO4-EC defects [7], etc. On the other hand, some unavoidable trace impurities such as iron or other intentionally doped transition metals, could produce numerous extrinsic defects [8].
Many studies have been carried out on these defects associated with UV transparency for developing more efficient and high transmittance materials to improve the performance of the corresponding devices. Unfortunately, the nature of the defects in the specific glasses is very complex and still not explicit. In that case, it is worthwhile to study the defect-state both in these as-produced glasses and the in-use ones for enhancing their UV performance. It is known that B2O3 can decrease crystallization temperature and the melting point of glass, depressing the volatilization of fluorine and phosphorus, and decreasing the difference between the final glass composition and the original one, but also improving the glass’s thermal stability and chemical durability [9–11]. Therefore, to explore the effects of doping B2O3 on the defects in phosphate based glasses is critical for understanding change of the property for the produced glasses, and thus favors improving the performance of the phosphate based glasses.
In this work, we investigated the types of existing defects and microstructural change of the phosphate based glasses with the doping of B2O3 through analysis of the absorption, Raman and X-ray photoelectron (XPS) spectra. Besides, to expand the application of these phosphate based glass and fiber in space exploring, the radiation resistance experiments were carried out with gamma ray as the radiation source.
2. Experimental details
A series of phosphate glasses were all made from the base glass that has a weight composition (wt%) of Li2O (0.5-2), K2O (3-5), MgO (3-5), BaO (7-10), Al2O3 (8-11), P2O5 (59-64), and SiO2 (0-2). Different amount of H3BO3 (0-7.5 wt%) were introduced into the base glass to obtain verified weight ratios (0:2, 1.5:2, 4.5:2 and 7.5:2) of H3BO3 versus SiO2. The high purity raw materials (≥ 99.99%, Sigma Aldrich Inc.) were weighted, thoroughly mixed and then melted in a 1 L quartz crucible in an electric furnace at 1200 °C under a reducing atmosphere, with use of a 2 h mechanical stirring process for further homogeneity [5]. Thereafter, the glass melts were cast into a copper mold preheated at 300°C. The molded glass samples were then annealed at 400 °C (near the glass transition temperature) through a precision annealing process [12]. After annealing, all the glass samples were cut and precisely polished into the size of 30 mm × 25 mm × 2 mm. In the following, a series of experiments were done to characterize the synthesized glasses. For radiation resistance characterization, all these glass samples were exposed to gamma radiation using a 60Co source at 44.05 rad (Si)/s to accumulate absorbed doses of 20k, 100k, 250k, 500k and 1000k rad(Si), respectively.
The transmission spectra were recorded with a UV-VIS-NIR spectrophotometer (Shimadzu UV-3101) in the range of 200-800 nm. Raman spectra were collected with a Jobin-Yvonne LabRam microscope with a 532 nm laser excitation in the range of 100-1500 cm−1. The XPS measurements were conducted on a Thermo Advantage X-ray photoelectron spectrometer at room temperature using Al K (1486.6 eV) as the radiation source.
3. Results
Figure 1 shows the contrast of the optical transmission spectra between the base glass sample and those B2O3 doped glass samples with different H3BO3/SiO2 ratio (1.5, 4.5 and 7.5). It can be found that all the samples show high transparency in the range of 1-4 eV. Compared with the base glass sample (H3BO3/SiO2 = 0:2), with increasing addition of B2O3, the sample’s UV edges gradually red-shifted until the H3BO3/SiO2 ratio reaches 4.5:2, thereafter the UV edge show a blue-shift at the ratio of 7.5:2. This phenomenon can also be witnessed from the samples’ corresponding absorption spectra, which suggests the addition of B2O3 to some extent could depress the formation of defects that contributing to absorption in the high energy region in these phosphate glasses.
Fig. 1 Transmission and absorption spectra of the series of phosphate based glasses with different H3BO3: SiO2 ratios (0:2, 1.5:2, 4.5:2 and 7.5:2, respectively).
In Fig. 2, the absorption spectra with constituted absorption bands separated through Gaussian peak fitting method demonstrate that several types of defects were generated in these series of phosphate based glasses. The separated absorption bands identified through the Gaussian peak fitting that corresponds to various defects are highlighted with different colors in Fig. 2. It’s known that several bands connected with electron centers lie in the high energy region for phosphate glasses [13]. Here, it can be found that the absorption peaks at around 5.19 eV and 5.8 eV, which is attributed to PO4-EC and PO3-EC defects, respectively. It is obvious that the absorption peak’s area corresponding to PO3-EC defects increases with the increasing H3BO3: SiO2 ratio firstly, whereas it decreases when the ratio reaches 7.5:2. That means excessive introduction of B2O3 could decrease the concentration of PO3-EC defects in those glasses.
Fig. 2 The absorption spectra of a series of phosphate based glasses with different H3BO3:SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2, respectively).
Figure 3 presents the Raman spectra of the series of phosphate glass samples. The strongest peaks located at around 1202 cm−1 arise from the symmetric resonance (O-P-O) of the non-bridging oxygens on the PO2 of phosphate chain. The other dominant peaks at about 706 cm−1 are associated with a symmetric stretching (P-O-P) mode of bridging oxygens of Q2 units [14]. Apart from these pronounced peaks, there are several other ones in these spectra. The bands covering a broad region (200-600 cm−1) originate from the internal deformation bending modes of PO2 and O-P-O chain. Besides, a small band at 1262 cm−1, at the right shoulder of the pronounced peak of 1202 cm−1, is assigned to the asymmetric stretching of non-bridging oxygens [15]. It can be found that those peaks at around 1262 cm−1 decrease with the increase of B2O3 content, more obviously, the small peak gets almost disappeared when the H3BO3:SiO2 ratio in those phosphate glasses reaches 7.5:2. Meanwhile, the intensity of the peaks located at about 666 cm−1 increase, which is related to the increase of B5O8 units [16]. Whereas, the intensity of bands at around 590 cm−1 decrease as the H3BO3:SiO2 ratio became larger, which is associated with the increase of B2O3 and corresponding decrease of SiO2 concentration in the final glasses. The existence of bands involving B5O8, together with the Rajbhandari’s results [17] indicate that B5O8 units do not form mechanically isolated network units but rather that they are bonded to each other to form vitreous networks.
Fig. 3 The Raman spectra of a series of phosphate based glasses with different H3BO3: SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2, respectively).
The O1s XPS collected from these four glass samples are given in Fig. 4. The XPS spectra were decomposed into a sum of Gaussian components to evaluate the bridging-to-terminal oxygen (BO/NBO) ratio in these glasses. The O1s spectra are best fitted with two Voigt peaks. The peaks near the lower binding energy at around 529.5 eV are assigned to the non-bridging oxygen (NBO) bonding to the glass modifier ions, and the peaks close to the higher binding energy at about 531.5 eV are related to bridging oxygen (BO) bonding to the glass former ions in the glass network [18]. With the increasing introduction of B2O3 in those phosphate glasses, the non-bridging oxygens decrease, whereas the bridging oxygens increase. This is in accord with the changes of Raman spectra in Fig. 3.
Fig. 4 XPS spectra of a series of phosphate based glasses with different H3BO3: SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2, respectively).
Figures 5 and 6 show the photographs and optical transmission spectra of the series of phosphate based glass samples after gamma irradiation (20k, 100k, 250k, 500k and 1000k rad(Si), respectively). Gamma irradiation leads to an obvious decline of their transmittance from the UV and the visible spectrum range (less than 650 nm), indicating that the color center was generated in these glasses. With the increase of total radiation dose, the samples show a gradually deepened color of maroon as shown in Fig. 5, and the transmittance decreases in further, especially for the absorption bands at around 385 nm and 525 nm which is related to the POHC defects and electron centers trapped on the central phosphorus atom in PO3 group (PO3-EC) defects [19] respectively. This suggests that more PO3-EC and POHC defects were generated during the irradiation process with increasing total dose. Besides, it can be found that the UV absorption edges of these glass samples gradually red-shifted with the increase of irradiation dose, which is associated with the increase of both Fe3+ and PO3-EC defects. To further characterize the arising source of these absorption in these transmission spectra, their corresponding absorption spectra were fitted and separated into multi Gaussian peaks, as shown by the curves with several different colors in Fig. 7. Here, we only illustrate the absorption spectra of a series of glasses with the irradiation dose of 250k, 500k and 1000k rad(Si) due to the large amounts of data and limited length. Obviously, with the increase of irradiation dose, the absorption of POHC (as shown by the green area) and PO3-EC defects (blue and yellow area) increase. The same changes can also be observed when increasing the H3BO3:SiO2 ratio. It should be mentioned that the absorption area corresponding to POHC and PO3-EC defects in the sample with a H3BO3: SiO2 ratio of 7.5:2 and under the gamma irradiation of 1000k rad(Si) is less than that of the one with a H3BO3:SiO2 ratio of 4.5:2. This is associated with the formation of B5O8 units caused by introduction B2O3 in this kind of glass.
Fig. 5 Photographs of a series of phosphate based glasses with different H3BO3: SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2, respectively) at different radiation doses (20k, 100k, 250k, 500k and 1000k rad (Si)).
Fig. 6 Transmission spectra of a series of phosphate based glasses with different H3BO3: SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2, respectively) at different radiation doses (20k, 100k, 250k, 500k and 1000k rad (Si)).
Fig. 7 Absorption spectra of a series of glasses with different ratios of H3BO3 to SiO2 (0:2, 1.5:2, 4.5:2 and 7.5:2, respectively) at different radiation doses (250k, 500k and 1000k rad (Si)) with Gaussian peak fittings.
4. Discussion
It is known, the structure of phosphate glasses is networked with a series of long phosphate chains made of phosphate tetrahedra as schematically depicted in Fig. 8(a). With regards to the investigated multicomponent phosphate based glasses in this work, the introduction of alkali (R+) and alkaline earth metal cations (R2+), i.e. Li+, K+, and Mg2+, Ba2+ lead to the breakage of pure phosphate chains and promote formation of large numbers of bridging oxygen atoms connected with P, thus the resultant glass networks are consist of different polyhedron forms, such as Q0, Q2 and Q3 units (Fig. 8(b)) [20,21]. These modifying metal cations in the phosphate glass network make the connectivity of these units reduced and thus such defects like PO3-EC and PO4-EC with isolated phosphate anions are formed. Those defects have large absorption in the short wavelength region as shown in Figs. 1 and 2.
Fig. 8 Schematic representations the changes of phosphate glasses network caused by alkali (R+), alkaline earth metal cations (R2+) and B2O3 as well as gamma irradiation (oxygen atoms (red), phosphorus atoms (pink), boron atoms (blue)).
The introduction of B2O3 into the phosphate base glass will cause the breakage (Fig. 8(d)) of phosphate tetrahedra, thus NBO ratio in the glass network decrease, as shown by the changes of the Raman peaks at around 1262 cm−1 and the XPS peaks at about 529.5 eV. And with the increase of B2O3 content added into those phosphate glasses, its bond breaking effect will become more obvious. Meanwhile, the concentration of defects like PO3-EC and PO4-EC are increasing, which have large absorption in the high-energy region, as shown by the red and gray separated peaks in Fig. 2. Besides, the distinct increase of the area corresponding to B5O8 [16] can also be found in the Raman spectra in Fig. 3. It is evidenced that the band at around 590 cm−1 decreases with increasing of B2O3 in those phosphate glasses, as we expected. This is related to the fact that proportion of SiO2 in the total raw materials gradually decrease with the increase of B2O3 content.
It is interesting that the absorption of Fe3+ increased when the B2O3 was doped. And with further increase of B2O3 content in those glasses, the absorption band area of Fe3+ is nearly invariable. As we all know, iron in most times is an unavoidable trace impurity in all multicomponent glasses produced by melt-quenching techniques, which is mainly introduced by the raw materials. Herein, the total content of iron in those four types of glasses is almost same due to the same raw materials and preparation process. The introduced B2O3 is one of the fluxing agents in those glasses that can decrease the melting point of glass, in other words, the B2O3 doped glass will have much more oxidizability at the same glass melting temperature as compared with the B2O3-free glass. So, Fe2+ ions are easy to be oxidized to form Fe3+ ions during the preparation process, and result in the increasing absorption of Fe3+ in the UV range as shown in Fig. 2. The unobvious changes of absorption of Fe3+ is due to the overlap by the absorption of PO4-EC defects in the high-energy region. As we above mentioned that the modified ions contribute to the breakage (Fig. 8(d)) of pure phosphate chains and formation of PO3-EC defects that result in the increase of absorption at the high- energy region, however, with a certain amount of B2O3 is introduced into the phosphate base glass, B2O3 could enter the glass network structure (Fig. 8e) and form B5O8 units that will enhance the connectivity of these long phosphate chains. That is why the absorption of PO3-EC defects decrease when the H3BO3:SiO2 ratio reaches 7.5:2 as shown in Fig. 2.
To expand the application of phosphate glasses in space exploring, the irradiation experiments were also carried out with gamma ray radiation. The distinctly induced optical losses peaking at around 385 nm and 525 nm (Fig. 6) are associated with PO3-EC and POHC defects in the glass matrix caused by the breakage of pure phosphate chains (Fig. 8(c)), respectively. With the increase of doped B2O3 in those phosphate glasses, the decrease of the transmittance in the UV and visible spectrum range is becoming obvious gradually till a critical H3BO3:SiO2 ratio of 4.5:2 when the samples are exposed to gamma radiation, especially for the high irradiation dose, whereas the decline trend of the transmittance weakens when the ratio is 7.5:2. Those results suggested that the formed B5O8 can suppress the formation of PO3-EC and POHC defects in those phosphate glasses. The results provide evidence for enhanced gamma radiation resistances in those multicomponent phosphate glasses. Therefore, B2O3 doped phosphate glasses could be used as a new type of host materials for applications in space exploration and radioactive wastes treatments.
5. Conclusion
We have studied the effects of doping B2O3 on the defects in phosphate based glasses by analysis on their transmittance and absorption, XPS and Raman spectra systematically. The results indicate that introduction of B2O3 causes the increase of PO3-EC defects in phosphate glasses by reducing the connectivity of phosphate chains, but also improves the oxidizability of those glasses that Fe2+ ions can be more easily oxidized to Fe3+ ions. Besides, excess B2O3 can enter the glass skeleton structure, resulting in the formation of B5O8 units that may enhance the gamma radiation resistances of those doped multicomponent phosphate glasses, which shows the potential use of these glasses for space exploring optics.
Funding
National Natural Science Foundation of China (NSFC No.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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