Crystallization and absorption properties of novel photo-thermal refractive glasses with the addition of B2O3

Journal of Non-Crystalline Solids 368 (2013) 55–62

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Crystallization and absorption properties of novel photo-thermal refractive glasses with the addition of B2O3 Pengfei Wang, Min Lu ⁎, Weinan Li, Fei Gao, Bo Peng ⁎ State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences (CAS), Xi'an 710119, PR China

a r t i c l e

i n f o

Article history: Received 17 January 2013 Received in revised form 25 February 2013 Available online 29 March 2013 Keywords: Photo-thermal-refractive glass; UV exposure; Thermal development; Crystallization; Absorption

a b s t r a c t A new boron-containing photo-thermal-refractive (boron-PTR) glass was prepared by two-step melt-quenching techniques at relative lower glass melting temperature. The influence of Ag+ doping concentration, UV exposure dosage and thermal development parameters on the boron-PTR glass' spectroscopic absorption and crystallization property were investigated and compared with that of the known boron-free PTR glass explored by L. B. Glebov et al. The introduction of B2O3, to some extent, can increase the strength of the silicate bonding thus preventing the liquid–liquid phase separation in this new PTR system when compared to the boron-free PTR system. The developed boron-PTR glass with the processing of 15 s-UV exposure and appropriate thermal development (480 °C/3 h, 500 °C/2 h) exhibits good photo-induced crystallization performance, high VIS-near IR transmittance (670 to 2600 nm) and moderate photo-induced refractive index change (1.1 × 10−3), which at present status enables this new PTR glass basic ability to record holographic elements. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Volume diffractive (or Bragg) gratings made of photo-thermalrefractive (PTR) glasses are very promising elements in optical systems utilizing the narrow-band lasers, which can improve the spectral sensitivity for hyperspectral imaging as narrow-band filter, they have wide ranges of applications in the fields of spatial spectral analysis, spectral beam combining, gradual beam attenuation [1], and especially the fields of space terahertz communications, steering of agile beams in target recognition system [2], and high-power laser beam controls [3,4]. Almost 60 years ago, the photosensitive glass was firstly invented and its corresponding mechanism, i.e. the photo-induced, thermally activated crystallization of this type of glass was initially proposed by Stooky [3–7], which includes photo-oxidation of Ce 3 + and formation of atomic silver during exposure to UV, atomic silver clustering during a first thermal treatment at 450°–500 °C, and heterogeneous nucleation and growth of NaF nano-crystals on further heating, resulting in permanent refractive index change (Δn). Δn depends greatly on the amount and size of nano-crystals and further influences the transmittance of the thermal-prepared glasses. As the research on the photosensitive glasses continued in depth, L. B. Glebov et al. [8] developed the concept of PTR glass at late 1980s. Since then, they have been focused on the research of the photo-thermorefractive properties of some glass ceramics and explored the appropriate glass media, i.e. the known PTR glass, for the recording of volume ⁎ Corresponding authors. E-mail address: [email protected] (B. Peng). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.03.002

diffractive gratings [8–10]. Till now, plants of researches have been carried out including exploration of the present extensively studied Na2O–ZnO–Al2O3–SiO2 based boron-free PTR glasses and investigations on their crystallization kinetics, interrelation between UV exposure and thermal development processing, specific absorption and corresponding ions transformation mechanism during thermal development process [11–18]. With all these efforts, high efficiency volume diffractive gratings have been prepared by L. B. Glebov et al. in the known PTR glass [4,11,19]. However, the currently accepted crystallization mechanism model initially proposed by Stooky has been shown to be incomplete, as later comprehensive study by E. D. Zanotto et al. on the physical–chemical evolution of PTR glass on heating to nucleation and crystallization has demonstrated that there's an interplay between NaF crystallization [20], NaF solubility [21], and liquid–liquid phase separation (LLPS) [22]. All these will directly affect the amount and size of NaF nanocrystals responsible for refractive index changes in the glass that lead to photonics applications, and to the limitations caused by unwanted growth above a certain range and also the development of second phase glass within the parent glass, causing optical losses. In further, it should be noted that present extensively studied PTR glasses (with a basic composition of 15Na2O–5ZnO–4Al2O3–70SiO2– 5NaF–1KBr (in mol%)) were melted in air using Pt crucibles at much higher temperature all in the range from 1460 °C [14,15,19,22–26] to 1500 °C [11,17] for 3–5 h, but the high melting temperature not only accelerates the volatilization of bromine, sodium, fluorine and silver components, leading to the difference of the final glass composition from the original one [27], but also increases the corrosion of glass melts against the Pt crucible, and the Pt inclusions would greatly

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reduce the laser-induced damage threshold of the PTR glasses. Moreover, change of the glass composition will produce further more complicated issues, for example difference in crystallization kinetic of NaF crystals, change of transition temperature (Tg) of the matrix glass and also formation of permanent surface residual stress, etc. Since solubility and composition of the parent glass determine the supersaturation and hence the thermodynamic driving force for NaF crystallization, the solubility of NaF in PTR glass is affected by the presence of bromine which decreases the solubility of NaF in PTR glass, thus increasing the thermodynamic driving force for crystallization of NaF [20]. The discovery of this effect was a key to the development of new compositions and thermal treatments for best optical properties of PTR glass. It is also clear that the depletion of volatile components such as bromine and fluorine in the surface layers of the glass sample in further leads to the presence of two different glasses, producing permanent surface residual stresses on the PTR glasses [28,29]. In very recent years, E. D. Zanotto et al. [29] have examined the effects of cooling rate and final cooling temperature on crystallization of UV exposed PTR glass as well as their spectro-photometric properties using DSC and optical microscopy methods, and verified that use of multi-step heat-treatment (including some cooling steps) induces smaller crystals and a finer structure. Thus, to decrease the glass melting temperature by composition adjustment will facilitate the compositional control as exploring a new type of PTR glass, but for which the UV exposure, thermal treatment program, corresponding crystallization mechanism and refractive index change ability are expected to vary from the known PTR glass. In this work, we developed new PTR parent glasses at lowered melting temperature by introducing low melting component of B2O3, and a two-step melting technique was specially adopted to suppress the volatilization of bromine, sodium, fluorine and silver components. At present, we focused more on the study of basic photo-induced properties of the newly explored PTR glass, including the change of the spectroscopic absorption property and crystallization morphology with Ag + doping concentration, UV exposure dosage and thermal development program. The difference in refractive index between the microcrystals and parent glass was finally characterized to evaluate this new glass' ability to record holographic elements.

partial volatility of these minor components and accelerate the glass homogenization process using larger amount of fining agent (SnO2 and Sb2O3). Firstly, glass frits made of Na2O–ZnO–B2O3–Al2O3–SiO2–AgNO3– CeO2–SnO2–Sb2O3 were prepared in silica crucible and then re-melted again together with the more volatile raw material batch of NaF (corrosive to silica crucible) and KBr at 1280 °C in platinum crucible with a homogenization process at 1400 °C for 2 h. Finally, the homogenized glass melts were cast into heated brass mold and a precision annealing was carried out for each parent glass at a temperature near the transition temperature (Tg ~ 468 °C), with a much slower cooling rate of 1 °C/h and 2 °C/h for each decrease of 100 °C from their annealing temperature and a following natural cooling down to room temperature. The parent glass samples doped with a different amount of AgNO3 were prepared for optimization of their photosensitive performance. The as-annealed boron-PTR parent glass samples were cut into slices with the thickness of 5 mm and polished for ultraviolet exposure and thermal development investigations. UV irradiation exposure was performed using a Green Spot UV light source (American Ultraviolet Co.) with band width of 300–480 nm and a peaking wavelength of 365 nm. The exposure dosage was controlled by varying the exposure time at a constant power. Tg of the parent glass as well as thermal-developed boron-PTR glass samples were measured using a Netzsch 404 differential scanning calorimeter (Netzsch, Germany) in a platinum crucible at 10 °C/min from room temperature until 800 °C. After UV irradiation exposure, the PTR glasses were annealed in the temperature range of 460–520 °C for nucleation and crystallization processing, and the developed boron-PTR glass samples with different processing parameters were labeled respectively in Table 2, together with Tg of each developed sample. The optical transmittance spectra were measured by a UV–VIS NIR spectrophotometer (Shimadzu UV-3101). A decimal absorption coefficient was calculated as A = (Dm − DF) / t where Dm is the measured optical density, DF is the calculated optical density resulting from Fresnel reflection losses and t is the thickness of the sample in centimeter, referring to [11]. The crystallized phase in the developed PTR glasses was analyzed by the X-ray diffractometer (Rigaku D/Max 2500 V/PC), Cu Kα radiation (λ = 1.54056 Å) with a scanning rate of 0.02°/s in 2θ range of 20°–90°. 3. Results

2. Experimental In this study, boron-PTR parent glasses with certain amount of B2O3 and different AgNO3 doping levels were developed from the known PTR glasses explored by Glebov el al. [12], which have the basic composition of 15Na2O–5ZnO–4Al2O3–70SiO2–5NaF–1KBr–0.01Ag2O–0.01CeO2– 0.01–SnO2–0.03Sb2O3 (in mol%) using a two-step melting method. Parent glasses were denoted as B-A2, B-A3 and B-A4 according to the AgNO3 content (see Table 1). The concentrations of CeO2, SnO2 and Sb2O3 in boron-PTR glasses were set to be 0.05, 0.1 and 0.1 wt.% basing on the total main components amount, respectively. We have increased the addition of volatile components (Ag, Ce, Sn and Sb elements) to fulfill two main purposes, one is to increase the crystallization efficiency (through Ag ions) by means of improving the glass' absorption (through Ce ions) of the UV irradiation, the other is to compensate inevitable Table 1 Boron-PTR parent glass samples with different original glass compositions. Parent glass

Glass composition (wt.%)

B-A2

15.7Na2O–5.2ZnO–5.0B2O3–6.5Al2O3–62.4SiO2–3.3NaF–1.9KBr −0.02AgNO3–0.05CeO2–0.1SnO2–0.1Sb2O3 15.7Na2O–5.2ZnO–5.0B2O3–6.5Al2O3–62.4SiO2–3.3NaF–1.9KBr −0.03AgNO3–0.05CeO2–0.1SnO2–0.1Sb2O3 15.7Na2O–5.2ZnO–5.0B2O3–6.5Al2O3–62.4SiO2–3.3NaF–1.9KBr −0.04AgNO3–0.05CeO2–0.1SnO2–0.1Sb2O3

B-A3 B-A4

As an example, Fig. 1 shows the DSC curves of the PTR parent glass (B-A2) and the developed glass samples with different AgNO3 concentration (PTR-3, 4, 5). The PTR parent glass (B-A2) has a Tg value of 468 °C, and its broad crystallization trace consists in two nucleation induced crystallization peaks, and the second peak can be associated with some kind of surface crystallization [28,29]. It is found that Tg is almost the same for different PTR parent samples, and the developed glass samples have a slightly lowered Tg as compared with that of its parent glass, which is 461 °C, 464 °C and 467 °C for PTR-3, 4 and 5 samples, Table 2 Heat-treated boron-PTR glass samples with different UV exposure times and thermal treatment program and their Tg values. The uncertainty of transition temperature (Tg) is ±2–3 °C. Sample ID

Parent glass

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B-A2

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472

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B-A2 B-A2 B-A3 B-A4 B-A2 B-A2 B-A2 B-A2 B-A2

20 s 20 s 20 s 20 s 5 min 30 s 5s 10 s 15 s

460 520 480 480 480 480 480 480 480 480 480

°C/3 h, 490 °C/2 h, °C/3 h °C/3 h, 520 °C/2 h °C/3 h, 500 °C/2 h °C/3 h, 500 °C/2 h °C/3 h, 500 °C/2 h °C/10 h, 500 °C/2 h °C/3 h, 500 °C/2 h °C/3 h, 500 °C/2 h °C/3 h, 500 °C/2 h °C/3 h, 500 °C/2 h

470 461 464 467 480 468 463 465 462

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Fig. 1. DSC curves of the parent PTR glass (B-A2) and the thermally developed glass samples (PTR-3, 4, 5).

respectively. As shown in Fig. 1, the heat treated sample with high Ag concentration also shows a higher crystallization temperature that ranges from about 702 °C to more than 720 °C for the residual glass matrix. However, for UV exposed and thermally developed samples, crystallization peak of NaF is observed to be much more obvious, and it is same as 563 °C for PTR-3, and 4 and 569 °C for PTR-5. Fig. 2 presents the photo-induced crystallization morphology of 0.02 wt.% AgNO3 doped boron-PTR glass (B-A2) samples with same 20 s-UV exposure but different thermal processing. The PTR parent samples were developed with three programs: (a) 460 °C/3 h, 490 °C/2 h, 520 °C/3 h; (b) 480 °C/3 h, 520 °C/2 h; and (c) 480 °C/3 h, 500 °C/2 h. As shown in Fig. 2(a), the sample annealed with program (a) (PTR-1) over-crystallizes with a dark brown color in the exposed area, around which it is opalescent. In Fig. 2(b), a less-transparent whitish edge can also be found in PTR-2 sample, which suggests the crystallization temperature (520 °C) is a bit high. As the nucleation temperature increases from 460 °C to 480 °C and the same time crystallization temperature decreases from 520 °C to 500 °C, the developed PTR-3 sample takes on adequately transparent both in the exposed and unexposed areas

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(see Fig. 2(c)). Thus, the thermal development program (c) is more appropriate for the crystallization of the B-A2 series of boron-PTR parent glass, and it was adopted for further crystallization comparison. Under the constant thermal treatment program (c), the effect of Ag + composition on the photo-induced crystallization morphology of the boron-PTR glass samples was studied, as illustrated in Fig. 3. As AgNO3 concentration decreases from 0.03 wt.% to 0.02 wt.%, the effect of absorption of Ag colloids staining and silver bromide particles on the coloration of annealed PTR samples weakened gradually, so the exposed area of in Fig. 3(a) looks more transparent than that seen in Fig. 3(b). The staining effect rarely changes if AgNO3 concentration is increased to 0.04 wt.%. However, it becomes opaque outward the exposed area and an opalescent ring surrounds the exposed brown area (Fig. 3(c)). It is suggested that much higher Ag + concentration might cause change of the crystallization kinetic of the PTR glass sample, for instance, spontaneous crystallization might have happened during the annealing process. Fig. 3 indicates that at the above constant thermal treatment procedure, the boron-PTR glass with addition of 0.02 wt.% AgNO3 displays better photo-thermal induced crystallization property over the ones with larger AgNO3 content. In order to demonstrate the ultimate level of spontaneous crystalline phase precipitation in the PTR glass, the boron-PTR parent glass sample was intensively exposed for 5 min and annealed at 480 °C for 10 h to get enough nuclei and then annealed at 500 °C for 2 h for almost complete crystallization (opaque). The crystallization morphology of the obtained PTR glass sample (PTR-6) and its XRD pattern is given in Fig. 4. It can be seen the sample was completely devitrified. The crystallized part was selected and grounded for powder XRD analysis. As can be seen in Fig. 4(b), the broad peak in the XRD pattern ranging from 15° to 30° is assigned to the unique diffraction peak of silicate containing glass (most of the time it corresponds to SiO2). While the other two sharp peaks at 39° and 56° are assigned to the diffraction peak of NaF crystal [15]. It indicates that NaF crystals as well as some silicate containing phase precipitate in the crystallized PTR glass sample. With the introduction of B2O3, the glass melting temperature is lowered by 50–100 °C than that of the known boron-free PTR glasses.

Fig. 2. Photo-induced crystallization morphology of UV exposed (20 s), 0.02 wt.% AgNO3 doped boron-PTR glass samples with different thermal development systems: (a) PTR-1 (460 °C/3 h, 490 °C/2 h, 520 °C/3 h), (b) PTR-2 (480 °C/3 h, 520 °C/2 h) and (c) PTR-3 (480 °C/3 h, 500 °C/2 h).

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Fig. 3. Photo-induced crystallization morphology of the UV exposed (20 s) and thermally developed (480 °C/3 h, 500 °C/2 h) boron-PTR glass samples with different AgNO3 concentrations: (a) 0.02 wt.% (PTR-3), (b) 0.03 wt.% (PTR-4) and (c) 0.04 wt.% (PTR-5).

(a)

400

Intensity/a.u.

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200

100 10

20

30

40

50

60

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2 /degree Fig. 4. Morphology of the tensely exposed (5 min), thermally developed (480 °C/10 h, 500 °C/2 h) boron-PTR glass sample (a): PTR-6 and (b): its measured XRD pattern.

This decrease in glass melting temperature looks not very high, but it would inhibit the volatility of these minor components during glass melting to a certain extent. We have tried measuring their elemental content difference between the parent and developed boron-PTR glasses using EDS measurement to investigate the effect of lowering glass melting temperature on their compositional change. The EDS analysis proved that Ag, Ce, Sn and Sb elements exist in the developed samples, but their composition are very near or even below the detectability limit for the technique. The compositional changes have been well evidenced by means of secondary ion mass spectrometry (SIMS) method [27], indicating the volatilization of minor components that the detected content level of Ag, Ce, Sn and Sb elements is three to four orders of magnitude lower than that of Si, K and F, but five to six orders lower than that of Al and Na. Fig. 5 exhibits both the absorption spectra of the annealed (480 °C/3 h, 500 °C/2 h) PTR parent glass without exposure and the PTR-7 sample with 30 s-UV exposure. Their absorption spectra were de-convoluted into a sum of Gaussian components. It can be seen that both exposed and unexposed samples have very low absorbance at 1053 nm, and correspondingly high transmittance is maintained in the range of 670–2600 nm for both of them. The increase of absorption at wavelength > 2600 nm is produced by hydroxyl groups [30]. Further analysis indicates that UV-exposure increases the absorption near 1053 nm by about 20% as compared with that of the unexposed PTR parent, and this effect of UV dosage on 1 μm absorption agrees well with that in Ref. [23]. With the same method in Ref. [31], the de-convolution of the absorption spectra in Fig. 5(a) was extended in Fig. 5(b) and (c) in order to study the change of cerium absorption in details. A relative weak absorption band of Ce 3+ centered around 299 nm is clearly resolved in Fig. 5(b). After UV exposure and thermal development, another absorption band at a value of about 450 nm which corresponds to silver bromide particles [14,32] becomes more pronounced.

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reaches about 1.1 × 10−3, which enables the boron-PTR glass as a medium for volume holographic optical elements, although the present Δn value still needs to continue to further improve. To explore the mechanism of the photo-induced coloration, the absorbance of the PTR samples (PTR-8, 9, 10, 3) with different exposure times but constant thermal development system was investigated, as shown in Fig. 7. The same technique of spectral analysis was used for each absorption spectrum with a separate de-convolution into sum of several Gaussian functions as presented in Fig. 7(b)–(e), respectively. The de-convoluted Gaussian components for each one are listed in Table 3. As we know, the decomposition of the absorption spectra into elementary bands requires an extensive work, for example, two or three bands are needed to describe each of absorption bands in the spectral range of 200–350 nm for Ce 3+ or Ce 4+ in PTR glasses [17]. While this work will be more complicated as the absorption of Ag 0 colloid and silver bromide particles in the spectral range of 400–450 nm [12,14] are taken into account together with cerium absorption in one spectrum. Therefore, we just associated one specific Gaussian band for each assign of Ce 3+, Ce 4+, Ag 0 colloid, AgBr, etc. As can be seen both in Fig. 7 and Table 3, three absorption bands dominate in the spectral range of 200–350 nm, i.e. 250–255 nm, 298–303 nm and 271–278 nm, and their peak wavelength changes little with increasing UV exposure time from 5 s to 20 s. The absorption peak at 250–255 nm 298–303 nm and 271–278 nm can be attributed to the absorption of Ce 4+ [17], the one Ce 3 + and hole centers (Ce 3++) and electrons formed during the ionization process, respectively [17]. While in the spectral range of 400–450 nm, there is an obvious red-shift of the absorption band with increasing exposure time. The absorption at the wavelength of 404.8 nm indicates the appearance of elemental silver containing colloidal particles [12,14] in the developed PTR glass with a rather smaller exposure dosage. As the exposure dosage becomes larger, the absorption near 450 nm corresponding to silver bromide particles [12] becomes more notable. 4. Discussion

10

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Wavelength(nm) Fig. 5. Full absorption spectra (a) and the de-convoluted absorbance spectra of the boron-PTR parent glass: (b) without exposure and thermal development, (c) PTR-7: with exposure (30 s) and thermal development (480 °C/3 h, 500 °C/2 h).

Furthermore, the influence of UV exposure time on the photoinduced crystallization morphology was studied with regard to the 0.02 wt.% AgNO3 doped boron-PTR glass (PTR-8, 9, 10, 3) samples. The B-A2 parent samples were exposed for 5, 10, 15 and 20 s, respectively, and then thermally developed with a constant thermal program (480 °C/3 h, 500 °C/2 h). The resulting crystallization morphology is compared in Fig. 6. The unexposed area is transparent for all these samples, and the sample color becomes darker with an increase of UV exposure time. The shadow images of these samples justified the refractive index change present in the exposed area for all these ones. Meanwhile, we measured the refractive index of the PTR parent glass and its crystallized PTR-10 sample (15 s-UV exposure) with same thermal development process (480 °C/3 h, 500 °C/2 h) using the Abbe refractometer. The refractive index at the wavelength of 589.3 nm (nD) decreases from 1.5162 for the parent glass to 1.5151 for the crystallized one, i.e. after thermal processing the refractive index difference (Δn)

As seen from Table 2, for these samples developed from B-A2 with a certain silver content and same thermal history (480 °C/3 h, 500 °C/2 h), their Tg changes very little with UV exposure dosage, and even decreases weakly with short exposure time. But for those with same UV exposure time (20 s), the thermal processing with a lower nucleation but higher crystallization temperature shows more direct influence on the increase of the Tg for the developed samples. However, this increase is still very slight, and even some of the developed boron-PTR glass samples' Tg is lower than that of their unexposed parent glass. It is same for the sample with different Ag contents. Whereas, this is different from the notion that the formation of NaF crystals and also the volatilization of minor component elements during melting process result in the depletion in fluorine and sodium in the residual glassy matrix, thus an increase in the glass transition temperature Tg with increasing crystallized fraction is expected [21]. It is reasonable that a larger amount of Ag element in the boron-PTR glass samples will trigger a much more dense crystallization of NaF crystals through nucleation and heat treatment, and the higher crystallization volume will be achieved, thus the corresponding sample should have a higher Tg value. Because precipitation of NaF crystals results in the depletion in fluorine and sodium in the residual glassy matrix [21], as a result an increase in Tg with increasing Ag fraction is expected. The substantial change in the original glass composition resulting from volatilization during glass melting process and liquid–liquid phase separation during heat treatment should play an important role on NaF crystallization kinetics, and therefore must be considered for an overall understanding of the crystallization mechanism underpinning the refractive index change in PTR glass [22]. The slight increase or rather decline of transition temperature of the developed

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Fig. 6. Photo-induced crystallization morphology of thermally developed (480 °C/3 h, 500 °C/2 h) 0.02 wt.% AgNO3 doped boron-PTR glass samples with different UV exposure times: (a) PTR-8: 5 s, (b) PTR-9: 10 s, (c) PTR-10: 15 s and (d) PTR-3: 20 s.

boron-PTR glasses compared with that of the boron-PTR parent glass, to some extent, suggests that the liquid–liquid phase separation in the Na2O–ZnO–B2O3–Al2O3–SiO2 based boron-PTR glass and crystallization driving force of NaF crystals might differ from that in the known Na2O–ZnO–Al2O3–SiO2 based boron-free PTR glass. It is a fact that NaF nucleation kinetics is expected to vary if the glass composition is changed, which can be achieved not only via changing the glass batch composition but also via adjusting the extent of liquid immiscibility produced through controlled thermal-treatment parameters [22]. First of all, the tendency of the glass towards phase separation is greatly suppressed in the Na2O–B2O3–SiO2–Al2O3 system [33,34], in which the \B\O\Al\ bond with the three-coordinated boron formed in Na2O–B2O3–SiO2–Al2O3 glasses is difficult to be broken due to the high bond energy, in addition, the silicon network in Na2O–B2O3– SiO2–Al2O3 glasses is also strengthened by the addition of B2O3. Secondly, the molar ratio of SiO2, as substituted partially by B2O3, in the boron-PTR glass is smaller when compared to that of the boron-free PTR system, thus the liquid immiscibility, i.e. LLPS which enhances NaF crystallization kinetics, is to be suppressed and as a result the saturation of NaF in the matrix glass would also be reduced or be relatively lower. Therefore, it is reasonable to deduce that boron can prevent, to some extent, the liquid–liquid phase separation in this new PTR system when compared to the non-boron PTR system. On the other hand, it is also important to take into account that higher crystallized fraction of NaF could be achieved by heat treating the glass at a lower temperature [22], but one should notice that the nucleation temperature (just a little higher than Tg) and NaF crystallization temperature of boron-PTR glass are relatively lower, as compared with the heat treatment temperature for the boron-free PTR glass in [22]. Thus the two aspects, i.e. the kinetics of LLPS at lower nucleation temperature and the crystallization kinetics of NaF upon heat treating, competes against each other, and the difference between the kinetics of the two processes would

decrease at the lower heat treatment temperature [22]. Therefore, a low volume fraction of NaF precipitates in the continuous matrix glass, which greatly contributed to the relatively smaller refractive index difference for the new boron-PTR glass. Even the physical–chemical transformations involved are somewhat complex and not very clear till now, these findings in this work would help us to tailor the optical properties of the new PTR glass through further adjustment of the composition and change of the miscibility gap of this boron-PTR glass system. The opalescent in Fig. 2(a) is because of insufficient exposure around the edge of the exposed area and relatively low nucleation temperature (460 °C), therefore, the crystal nucleuses are less in this area. The less the nucleation, the larger the NaF crystal size grows [23]. Thus, the crystals overgrow results in the local opaque area. The unexposed area is found to be yellowish, which indicates spontaneous crystallization as a result of parasitic exposure happened in the glass itself and also due to long time annealing at a higher temperature (490 °C/2 h, 520 °C/3 h). This agrees well with that in reference [23]. PTR glass develops liquid–liquid phase separation, nucleation and crystallization, and they compete against each other and this leads to local changes in glass composition which are related to the local availability of certain ions/atoms. The onset temperatures involved also depend on the local glass chemistry, and in the case of nucleation and liquid–liquid phase separation, and only passing through a given temperature range (e.g. on heating/cooling, for instance this can be observed when we have the addition of a third leg at 460 °C) can trigger the reaction. As shown in Fig. 1, the sample with high Ag concentration also has a higher crystallization temperature. And the surrounding glass (i.e. unexposed glass) has a much lower crystallization temperature. Thus, when these samples experienced same thermal history, the sample with higher Ag concentration will suffer serious crystallization, as shown in Fig. 3.

P. Wang et al. / Journal of Non-Crystalline Solids 368 (2013) 55–62

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Wavelength(nm) Fig. 7. Full absorption spectra (a) and the de-convoluted absorbance spectra of the thermally developed (480 °C/3 h, 500 °C/2 h) 0.02 wt.% AgNO3 doped boron-PTR glass samples with different UV exposure times: (b) PTR-8: 5 s, (c) PTR-9: 10 s, (d) PTR-10:15 s and (e) PTR-3:20 s.

For the PTR parent glass, there is no obvious absorption from 400 nm to 2600 nm in Fig. 5. While the rapid increase of its absorbance from about 350 nm to the short wavelength the red-shift of ultraviolet absorption edge is due to the absorption of Ce 3++ [12,14,17]. As observed for the exposed and thermally developed

Table 3 Deconvolution into Gaussian fitting peaks of the induced absorption bands in Fig. 7(b)–(e). UV exp. time

Peak 1(nm)

Peak 2(nm)

Peak 3(nm)

Peak 4(nm)

5s 10 s 15 s 20 s

252.3 250.8 252.3 255.5

298.1 302.8 302.8 299.3

278.1 275.1 277.8 271.1

404.8 426.3 438.1 450.5

sample, the absorbance increases from about 1000 nm to VIS range and an obvious absorption near 438 nm is clearly observed. This absorption could be attributed to two aspects from both UV absorption and VIS absorption. Firstly, the UV absorption was caused by the photochemical reactions produced Ce 3++ ions in the glass [12,14,17], and the VIS absorption at 438 nm corresponded to silver bromide particles [14,32]. Secondly, it can be deemed that the serious scattering of passed light at VIS range on the other side results from the growth of Ag 0 colloid particles with nucleation temperature and duration [28] or to some extent the contribution of LLPS [22] given the fact that NaF crystal is colorless and has no absorption of visible light. Correlating the crystallization morphology (Fig. 6) of thermally developed (480 °C/3 h, 500 °C/2 h) 0.02 wt.% AgNO3 doped boron-PTR glass samples with their absorption spectra (Fig. 7), one may find out that increasing UV exposure time would promote formation of larger

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amount of Ag0/AgBr particles in the PTR glasses, the crystallization and growth of NaF crystals and AgBr particles. The absorption maximum of silver bromide particles shifting to longer wavelengths indicates that the silver bromide particles become larger as the Ag0 colloid particles diminish gradually. 5. Summary A new boron-PTR glass based on Na2O–ZnO–Al2O3–SiO2 system was prepared with the introduction of B2O3 by two step melt-quenching technique at a lowered melting temperature. For the boron-PTR glass, doping AgNO3 was optimized to be 0.02 wt.% in terms of scattering losses and sample coloration against different heat treatments, from which a 15 s-UV exposure processing and a thermal development program (480 °C/3 h, 500 °C/2 h) were developed. The produced boron-PTR glass exhibited characteristic absorption of Ce3+, Ce3++, Ce4+ and AgBr within the wavelength b600 nm, and high transmittance from 670 nm to 2600 nm, which together with the property of moderate photo-induced refractive index change in the glass endows itself use of recording holographic elements. Further research work on the boron-PTR glass, including liquid–liquid phase separation, crystallization kinetics, production of gratings in the glass, as well as its laser induced damage property, etc., will be carried out later. Acknowledgments We greatly acknowledge the financial support of the West Light Foundation of the Chinese Academy of Sciences (no. Y129261213) and the National Nature Science Foundation of China (no. 51002181). Special thanks would be given to Prof. Leonid B. Glebov (CREOL, The College of Optics and Photonics, University of Central Florida) for his kind discussion on the problems of parasitic exposure, spontaneous crystallization and photo-induced refractive index change in these glasses. References [1] L.B. Glebov, V.I. Smirnov, C.M. Stickley, I.V. Ciapurin, Proc. SPIE 4724 (2002) 101–109. [2] O.M. Efimov, L.B. Glebov, L.N. Glebova, V.I. Smirnov, US Pat. (2003) 6586141. [3] O.M. Efimov, L.B. Glebov, V.I. Smirnov, US Pat. (2004) 6673497. [4] L.B. Glebov, Volume Bragg gratings in PTR glass — new optical elements for laser design, 3rd Advanced Solid-State Photonics (ASSP) Topical Meeting, ASSP Technical Digest, Paper Code MD1, Nara, Japan, January 2008.

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