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Abstract
Obvious healing behavior of gamma radiation induced defects in multicomponent phosphate glass was observed at room temperature. The recovery of the defects depends on the ratio of H3BO3/SiO2 in the investigated glasses, the total gamma radiation dose, and the time of ageing at room temperature. Meanwhile, the synchronous decreases of PO3-EC and POHC defects contribute to the corresponding recovery of the transmittance change at 385 nm and 525 nm, which could be described by the charge transfer. Besides, a general model of the healing mechanism associated with the release and capture of the electrons between PO3-EC and POHC defects in these phosphate glass was proposed.
© 2017 Optical Society of America
1. Introduction
Nuclear fusion is a special case of inexhaustible clean energy source, where, as in the US, the National Ignition Facility (NIF) provides a powerful scientific tool and strict conditions with extreme energy densities and pressures [1] for the inertial confinement fusion (ICF) study related with nuclear fusion and ion physics. Since nonlinear frequency conversion is employed to convert the 1ω laser light to 2ω and 3ω lights in laser-driven controlled ICF experiment [2], many studies have been carried in order to break the related engineering and technology bottlenecks, like use of fused silica based color-separation gratings to filter the unwanted 1ω and 2ω lights. With the present color separation techniques, the filtered 1ω and 2ω lights are still leaved to mixed with the 3ω laser light in the target chamber, which is harmful to ICF experiments [3]. Nevertheless, the endeavor of improving the conversion efficiency by increasing specific absorption towards the unwanted laser lights using present color separation techniques is not full satisfactory [4]. And further research is required to develop some innovative color separation optics to solve these critical obstacles.
Fortunately, the recent development of serial of fluoride-containing phosphate based glasses have thrown lights on the design of novel color separation technique [5], which based on the multi-purpose use of these glasses’ distinctive spectroscopic characteristics of both high transparence to the 3ω laser light and both intense and selective absorption towards the 1ω and 2ω laser lights, as well as the high resistance to nanosecond pulse laser -induced damage which is much higher than that of fused silica. Moreover, phosphate based glass as a type of soft glasses that have higher (ten times or more) rare earth solubility than silica, and easily adjusted refractive indices, have also been researched for long to explore their applications in fibers [6,7]. These researches indicate the phosphate based glass as one of the high-performance optical materials for the promising applications in the fields of both but not limited to high power ns-laser systems and optical fibers, have received enough attention due to their excellent general properties, such as high transparency in the range of UV to NIR, high glass forming ability and good host materials for rare earth dopant ions, as compared with other glass systems [8]. However, microstructural defects will firstly form in the phosphate based glasses during the glass preparation process and then may transform into others types of defects during the usage process with laser radiation or heat treatment, etc., which are detrimental to the degradation of the concerned UV optical performance and thus will significantly decrease their laser induced damage threshold at the short wavelength [5]. Therefore, it is worthwhile to study the defect-states in the as-produced glasses and their evolution behaviors for guidance of developing phosphate based optical components with improved properties.
Basically, the various defects formed in phosphate based glasses can be classified into two types, i.e. the intrinsic defects that arise from the raw materials and glass matrix [9], and the extrinsic ones caused by introduction of dopants or impurities [10]. It is well known that phosphate based glasses typically consist of the phosphate chains made from different P-tetrahedra [11]. And the introduction of modifier cations, the melting of glass, as well as energetic radiations may decrease the connection of P-O bonds in various degree, and thus increase the formation probability of multiple intrinsic defects, e.g. the phosphate-related oxygen hole center (POHC), oxygen-related hole center (OHC), and phosphate-related electron centers POn-EC, where n = 2, 3 and 4 [12], etc. To give an example, the introduction of B2O3 into the phosphate glasses can decrease the melting point and most importantly the crystallization tendency, and resultantly depress the volatilization of phosphorus, and make the final glass compositions more close to their original ones, but also can to some extent improve the glasses’ chemical durability and thermal stability [13,14] by strengthening the glass network, for which the latter effect is also helpful to decrease the corresponding defects in the phosphate glass and thus to improve their laser induced damage thresholds.
Previously, we have studied the generation of the defects in the phosphate based glass induced by gamma radiation and their healing behavior at ambient conditions [15]. To explore the defects generation and recovery is very important for evaluating the corresponding transmission losses in the cases of simulating the total dose effects in the space irradiation environment. In this work, we investigated the natural healing behavior of gamma radiation induced defects in the B2O3 doped multicomponent phosphate glass at room temperature. A model of the natural recovery of the gamma radiation induced defects with ageing time was proposed based on the systematic analysis on the variations of their transmission and absorption spectra.
2. Experimental details
A series of phosphate glasses made from the base glass having a weight composition (wt%) of (0.5-2)Li2O-(3-5)K2O-(3-5)MgO-(7-10)BaO-(8-11)Al2O3-(59-64)P2O5-(2)SiO2-(x)H3BO3, with x = 0, 1.5, 4.5 and 7.5 have been prepared from high purity raw materials (≥ 99.99%, Sigma Aldrich Inc.). The raw glass materials were weighted, mixed and then melted in a 1 L quartz crucible in an electric furnace at 1200 °C under a reducing atmosphere, with a 2 h mechanical stirring process for further homogeneity [5]. The obtained glass melts were poured into a copper mold preheated at 300 °C. The molded samples were annealed at 400 °C (near the glass transition temperature) through a precision annealing process [15]. Afterwards, all the glass samples were cut and precisely polished into the size of 30 mm × 25 mm × 2 mm. After annealing and optical polishing, a series of experiments were done to characterize the synthesized glasses. 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) (The SI unit of an absorbed dose is 1 Gray = 100 rad(Si).), respectively. The irradiated samples were set at room temperature for several months, to investigate the natural healing behavior of the gamma radiation induced defects in multicomponent phosphate glasses at room temperature through characterizing the changes in the transmission and the corresponding absorption for these glass samples.
The transmission spectra were recorded with a UV-VIS-NIR spectrophotometer (Shimadzu UV-3101) in the range of 200-800 nm for all the samples with a thickness of 2 mm. The separation of the absorption spectra was carried out using Gaussian peak fitting method [16].
3. Results
The optical transmission spectra of the series of multicomponent phosphate glass samples with different ratio of H3BO3 to SiO2 (0:2, 1.5:2, 4.5:2 and 7.5:2) and varied gamma radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)) are compared in Fig. 1. As for the low radiation dose case, i.e. a total radiation dose of 20k rad(Si) (Fig. 1(a)), doping of H3BO3 causes the red-shift of UV edges till the H3BO3 content reaches 4.5 wt%, whereas the UV edge show an opposite blue-shift with further increase of H3BO3 content in the phosphate glass. This reverse phenomenon is associated with the formation of B5O8 units evidenced by the Raman spectra in our previous work [17]. It can be found that with the increase of H3BO3 content in these glasses the transmittance in the range of 300-800 nm decrease monotonically even in a tiny degree. With further increment of exposure doses, the transmission spectrum of the 7.5 wt% H3BO3 doped sample is getting closer to that of the one doped with 4.5 wt% H3BO3. When the total radiation dose reaches 500k rad(Si), the former one’s transmittance is higher than that of 4.5 wt% H3BO3 doped one, and this diversity in short wavelength transmittance is more apparent in the case of a total radiation dose of 1000k rad(Si).
Fig. 1 Transmission spectra of a series of glasses with different ratio of H3BO3 to SiO2 (0:2, 1.5:2, 4.5:2 and 7.5:2) with different total dose of gamma radiation (20k, 100k, 250k, 500k and 1000k rad(Si)).
These photographs of the series of multicomponent phosphate glass samples with different H3BO3:SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2) via different doses of gamma radiation (20k, 100k, 250k, 500k and 1000k rad(Si)) are shown in Fig. 2(a). The above-mentioned changes of transmission spectra in the range of 300-800 nm can also be found in the series of glasses with gamma radiation dose of 500k rad(Si) after being aged at ambient condition for 165 h, as given in Fig. 2(b). And the largest recovery was observed in 7.5 wt% H3BO3 doped sample, which maintains the highest transmittance compared with others. As the total gamma radiation dose increased, the transmittance in the UV and visible spectrum range declines obviously and gradually, especially for the characteristic change in the transmittance of the two noticeable absorption bands peaking at 385 nm and 525 nm (Fig. 2(c)), which are ascribed to electron centers trapped on the central phosphorus atom in PO3 group (PO3-EC) and POHC defects [17], respectively. It indicates that a relatively large proportion of PO3-EC and POHC defects was generated in these glasses as exposed to high dose radiation. An interesting phenomenon of a natural healing behavior of the gamma radiation induced defects at room temperature after gamma radiation was found, corresponding to the gradual recovery of transmission in the UV and visible spectrum range with increasing ageing time, as displayed by Fig. 2(d).
Fig. 2 (a) 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) after exposed to different total radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)). (b) Transmission spectra of the series of glasses with different ratio of H3BO3 to SiO2 (0:2, 1.5:2, 4.5:2 and 7.5:2) with a gamma radiation of 500k rad(Si) after being aged for 165 h. (c) Transmission spectra of the irradiated (20k, 100k, 250k, 500k and 1000k rad(Si)) sample with the H3BO3/SiO2 ratio of 0:2 with an ageing time of 165 h. (d) Transmission spectra of the sample (H3BO3/SiO2 = 0:2) with gamma radiation of 500k rad(Si) after different ageing time (0h, 15h, 65h, 165h, 365h, 765h, 1500h) at room temperature.
Figure 3(a) demonstrates the obvious total radiation dose effects on the short wavelength transmittance decline of these irradiated samples with different H3BO3:SiO2 ratio, and the tendency can be evidenced by the transmittance decrease at 385 nm for a detail example. It can be seen that with the increase of H3BO3 content in the glass, the transmittance at 385 nm decrease, and 7.5 wt% H3BO3 doped sample shows the lowest transmittance at a total radiation dose of 20k rad(Si). As the radiation dose increases, the transmittance of the 7.5 wt% H3BO3 doped sample at 385 nm is higher than that of 4.5 wt% H3BO3 doped one, and this phenomenon is becoming more obvious in the case of higher radiation dose, which is in accordance with the changes in transmission spectra of these irradiated samples as observed in Fig. 1. Figure 3(b) shows a cause and effect of both different radiation doses and ageing time on the recovery of the transmittance at 385 nm for the irradiated H3BO3-free sample (H3BO3/SiO2 = 0:2) at room temperature. In the case of higher radiation dose, the sample shows a greater degree of recovery effect at the very beginning of the ageing period, although their recovery effect is less than that in the case of low radiation dose due to more stable defects were generated during the radiation process. The similar phenomenon can also be found in the transmittance decline at 525 nm that corresponds to POHC defects, as shown in Fig. 4.
Fig. 3 Change of the measured transmittance at 385 nm for: (a) the series of glasses with different H3BO3/SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2) under different gamma radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)) and (b) the sample (H3BO3/SiO2 = 0:2) with different gamma radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)) and increasing ageing time (0h, 15h, 65h, 165h, 365h, 765h, 1500h) at room temperature.
Fig. 4 Change of the measured transmittance at 525 nm for: (a) the series of glasses with different H3BO3/SiO2 ratio (0:2, 1.5:2, 4.5:2 and 7.5:2) under different gamma radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)) and (b) the sample (H3BO3/SiO2 = 0:2) with different gamma radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)) and increasing ageing time (0h, 15h, 65h, 165h, 365h, 765h, 1500h) at room temperature.
It is well established that the changes of transmission including decline and recovery are closely related to the absorption of various defects with different concentration in these glass samples. Herein, the absorption spectra were fitted and separated into multiple Gaussian peaks. And the absorption spectra including band separation demonstrates the presence of several types of defects as shown in Fig. 5. It can be found that the band of POHC (2.36 eV) and OHC (4.28 eV) defects lie in the low energy region, and several bands connected with PO3-EC (5.94 eV) and PO4-EC (5.17 eV) are positioned near the high energy region [18]. The band located at around 3.22 eV are also related to the PO3-EC, which shows the lower transmission spectra at 385 nm.
Fig. 5 Separation of radiation (500k rad(Si)) induced absorption band for the sample with the H3BO3/SiO2 ratio of 0:2 after being aged for 165 h.
Obviously, the transmission spectra were dominated by several defects in these glasses, and the transmittance was affected by several factors, including the H3BO3/SiO2 ratio, gamma radiation dose, and ageing time of the irradiated samples. So, the absorption spectra transformed from their transmission spectra were analyzed similarly using Gaussian peak fitting method as mentioned in Fig. 5, in order to get the separated absorption bands corresponding to different defects, and then the defects’ concentration in the series of glasses through computing each separated absorption band’s area ratio.
Figures 6 and 7 present the variety of the absorption-peak’s area of the PO3-EC (here, we only investigate the absorption band peaking at 3.23 eV due to their recovery with ageing time is more obvious than that of the one peaking at 5.94 eV) and POHC defects in the series of glasses with different H3BO3/SiO2 ratio ((a) 0:2, (b) 1.5:2, (c) 4.5:2 and (d) 7.5:2) irradiated with different radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)), respectively, in order to illustrate the changes in the concentration of radiation induced PO3-EC and POHC defects in these glass samples.
Fig. 6 Line chart displaying the change in absorption-peak’s area of PO3-EC defect with ageing time (0h, 15h, 65h, 165h, 365h, 765h, 1500h) in the series of irradiated glasses with different H3BO3/SiO2 ratio (0:2 (a), 1.5:2 (b), 4.5:2 (c) and 7.5:2 (d)) and various gamma radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)).
Fig. 7 Line chart displaying the change in absorption-peak’s area of POHC defect with ageing time (0h, 15h, 65h, 165h, 365h, 765h, 1500h) in the series of irradiated glasses with different H3BO3/SiO2 ratio (0:2 (a), 1.5:2 (b), 4.5:2 (c) and 7.5:2 (d)) and various gamma radiation dose (20k, 100k, 250k, 500k and 1000k rad(Si)).
The concentration of PO3-EC defects in these irradiated samples reduces significantly with the extension of ageing time and accumulative exposure dose. The PO3-EC defect’s concentration of irradiated samples induced by low radiation doses decayed exponentially in the first few hours of the ageing process, whereas, that of sample exposed to much higher radiation doses (1000k rad(Si)) decreased almost linearly. The different healing behavior of the PO3-EC defects distinguished by the high or low radiation dose indicates that the concentration of stable defects in the irradiated glasses varies with the radiation doses. As for POHC defects, the decrease tendency of their concentration is linear for the high dose irradiated sample, and the corresponding slope is larger than that of PO3-EC defects. This suggests that the recovery speed of POHC defects is relatively quicker than that of PO3-EC defects in the initial stage, this is also associated with the deepness of defects energy level, and the structure stability of defects as discussed later.
4. Discussion
Gamma radiation causes the transmittance decline in the UV and visible spectrum range for these multicomponent phosphate glasses. With the increase of gamma radiation doses, the descending range of the transmission spectrum becomes more and more obvious, indicating that increasing amount of related defects were generated during the high radiation doses radiation process, especially for the PO3-EC and POHC defects, which results in the falls of transmittance at 385 nm and 525 nm, respectively. Unexpectedly, the healing behavior of these defects characterized by changes of the transmission spectra with various ageing time at room temperature was detected. The transmission spectra of H3BO3-free and 7.5 wt% H3BO3 doped samples show a rather more significant recovery than that of other samples at the condition of ageing for 165 h at room temperature for the case of a total radiation dose of 500k rad(Si) (Fig. 2(a)). Meanwhile, the healing behavior of samples with different radiation doses (20k, 100k, 250k, 500k and 1000k rad(Si)) also various from each other as evidenced by the difference in the recovering of their transmittance at different ageing times. All of these indicate that the glass sample’s components and the total dose of gamma radiation, as well as the ageing time at room temperature affect the healing behavior of these phosphate based glasses.
As for the change tendency in the transmittance loss at 385 nm and 525 nm, the influence of doping H3BO3 are almost the same. And the descending range of the transmittance at 385 nm is also similar to that at 525 nm when the gamma radiation dose parameter is considered separately. Besides, the healing behavior is surprisingly consistent, considering whether its initial transmittance or the recovery range. The same phenomenon can also be observed from the line charts (Figs. 6 and 7) displaying the corresponding absorption-peak’s area of PO3-EC and POHC defects upon the ageing effect at room temperature. The concentration of PO3-EC and POHC defects first decrease more rapidly and then change rather slowly over the later ageing process.
As we all know, the POHC defect is characterized by an unpaired electron shared between two orbitals of two non-bridging oxygens bound to a phosphorus atom. And the characterization that electron centers are trapped on the central phosphorus atom in PO3 group is closely related to the PO3-EC defect. The POHC and PO3-EC defects both are originated from the Q3 units which have three bridging oxygens that link the phosphate tetrahedral in the phosphate glass network. The Q3 units release and capture electrons to form POHC and PO3-EC defects[12], respectively. The concentration variations of these two types of defects is in good accordance with the change tendency of their transmission and corresponding absorption spectra, as shown by Figs. 4-7.
To completely understand the natural healing behavior of the gamma radiation induced defects in these investigated multicomponent glasses at room temperature, we proposed a general model of the recombination of unpaired electrons between some defects to explain the recovery of transmission, as shown in Fig. 8. Firstly, Q3 units were formed during the glass preparation process. And then, a part of Q3 units release electrons to form POHC defects, which contributed to the significant absorption at 525 nm. On the other hand, parts of conduction band electrons were subsequently captured by Q3 units and transformed into PO3-EC defects that related to the absorption band peaking at around 3.23 eV (385 nm). In the case of ageing at room temperature after gamma radiation process, thermal energy may induce the release of trapped electrons in PO3-EC defects through the conduction band (PO3-EC – e- → Q3), and then recombine with POHC defects to form Q3 units (POHC + e- → Q3), which dominates the recovery of the transmittance. (Unfortunately, we did not observe the changes of Q3 in Raman spectra due to the recoverable of Q3 is less than the main Q3 in these multicomponent phosphate glasses). The recombination process resulted in the synchronous decrease of POHC and PO3-EC defects, which lead to the recovery of the corresponding transmittance at 525 nm and 385 nm, respectively. As for the 7.5 wt% H3BO3 doped sample, excessive B2O3 can enter the glass skeleton structure, resulting in the formation of B5O8 units [17] that may enhance the gamma radiation resistances of the H3BO3 doped multicomponent phosphate glasses, and directly contribute to a higher transmittance as compared with others samples during the defects healing process. After the first few hours of the ageing process, the release of electrons through the conduction band became rather slow, which can explain the slower recovery of the transmittance both at 385 nm and 525 nm with the ageing time longer than 165 h.
Fig. 8 Schematic of the formation and healing mechanism of the PO3-EC and POHC defects during the gamma radiation and ageing process.
5. Conclusion
We have studied the natural healing behavior of the main defects in the multicomponent phosphate glasses after being irradiated by gamma ray through analyzing the changes of their transmission and absorption spectra with different ageing time at room temperature. The changes of transmission spectra depend on the ratio of H3BO3 to SiO2, the total gamma radiation dose, and the time of ageing. The healing recovery of transmittance and corresponding absorption bands of POHC and PO3-EC defects is associated with the charge transfer, which is very important for the prediction of general transmission loss by considering both the total dose effect in in-use space irradiation conditions and the spontaneous natural healing behavior. For further clarification of the transmittance recovery both at 385 nm and 525 nm with ageing time, a general model of the healing mechanism for the PO3-EC and POHC defects was proposed, which related to the process of release and capture of the electrons by these defects in the multicomponent glasses.
Funding
National Natural Science Foundation of China (NSFC No.61307046); Natural Science Basic Research Project in Shaanxi Province (2015JM6315); State Key Research and Development Plan (2016YFB0303804); West Young Scholars Program of the Chinese Academy of Sciences (XAB2016A08); and Youth Innovation Promotion Association of the Chinese Academy of Sciences (2017446), China.
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