A new photo thermal refractive glass system is introduced. Glasses in the system Na2O/K2O/CaO/CaF2/Al2O3/SiO2 were doped with Ag2O, CeO2, KBr, SnO2 and Sb2O3. They were irradiated with UV light and subsequently thermally annealed at 530 °C, a temperature just above Tg. This led to a slightly yellow coloration. A second thermal annealing step at a temperature of 560 °C led to the crystallization of cubic CaF2 as proved by x-ray diffraction. Samples, annealed in a two step process at 530 °C for 1 h and 560 °C for 20 h without previous irradiation did not show crystallization. Furthermore, the effect of irradiation time on crystallization behavior and the role of KBr in the photoinduced crystallization process were also investigated.
© 2014 Optical Society of America
Glasses which can be structured by light and subsequent thermal annealing are denoted as photo thermo refractive glasses (PTR) [1–4]. They are first irradiated by UV light and subsequently thermally treated at one or two temperatures above the glass transition temperature, Tg. This results in the formation of tiny crystals in those parts of the glass which have been irradiated. In the irradiated region of the glass, the refractive index changes during thermal treatment, while in the not irradiated regions, which do not contain crystals, the refractive index remains approximately the same as before irradiation and subsequent annealing [1–3, 5]. Thus, the refractive index of these glasses can locally be altered and hence a structurization using light of appropriate wavelengths is possible [6, 7]. For photonic applications it is essential to avoid light scattering. Therefore the formed crystals should be smaller than one-half of the wavelength λ of the light which is used . It should also be taken into account, that the formed crystals do not possess a unique size, but exhibit a certain size distribution. Using chemical systems which show an interface controlled nucleation, enables to produce fairly narrow crystal size distributions. In these compositions, the growing crystals form a shell around themselves which acts as a diffusion barrier that hinders further crystal growth, leading to crystallite sizes in the range of a few nm [9, 10]. Nevertheless, the crystal sizes should preferably be smaller than λ/4. If the particles are not homogeneously distributed, e.g. agglomerated, scattering may occur at far smaller particle sizes. In the literature predominantly, one system is described which shows a photo thermo refractive effect in the volume . It is a silicate glass which contains Ce2O3 and Sb2O3, fluoride and trace quantities of silver halides. According to the literature, during irradiation of this glass, Ce3+ is oxidized (Ce3+ + hυ → Ce4+ + e-) and the electron formed is trapped by a silver ion which is transformed to a silver atom (Ag+ + e- → Ag0). In a subsequent annealing step at a temperature slightly above Tg, clustering of silver atoms occurs. These clusters act as nuclei during the second annealing step and lead to the crystallization of NaF [5, 12–15].
In order to design a new PTR-glass, numerous crystalline phases which might be precipitated were taken into account. The main prerequisite is that the crystals do not grow to a size which leads to light scattering. Changes in the refractive index should run parallel to any crystallization process . In the past few years, some oxyfluoride glass-ceramics have been described in the literature, which are optically transparent and nevertheless contain notable quantities of halide crystals [17–22], much more than usually present in PTR glasses. From these oxyfluoride glass systems, the crystallization of CaF2 [17, 19, 23, 24], SrF2 [10, 25], BaF2 [26, 27] and LaF3 [18, 20] as well as that of NaF [12, 13] has been reported. During crystal growth the glass near the crystal is depleted in those components which form the crystal [10, 23, 24, 26, 28]. This means for example, the melt near the crystal is depleted in fluoride and e. g. in Ba2+  or Ca2+  and hence also enriched in the other components of the glass. This results in the formation of a diffusion profile around the crystal and more important also in a decrease of the diffusion coefficient over several orders of magnitude . These diffusion layers around the growing crystals hinder further crystal growth and lead to crystallite sizes in the range of a few nm.
Crystalline CaF2 has already been reported to precipitate by simple thermal treatment from melts in the system Na2O/K2O/CaO/CaF2/Al2O3/SiO2 [23, 24]. In this case, a wide variation of the annealing temperatures and annealing times did not result in notably different crystallite sizes. It should further be noted that the refractive index of CaF2 crystals is 1.48. Therefore, calculations show that scattering at 1 micron should be about two orders of magnitude smaller for crystals with the same size as in regular NaF PTR glass. Nevertheless, the used fluoride concentration is much larger than in regular PTR glasses. The chemical composition in this system was varied by reducing the CaF2 concentration in order to avoid spontaneous precipitation of CaF2 . The glass described in the present paper was doped with Ag2O, Ce2O3, KBr, SnO2 and Sb2O3. In the following, the effect of irradiation and subsequent thermal treatment on the crystallization process and especially the role of KBr in the glass composition are described.
2. Experimental procedure
Glasses in the system Na2O/K2O/CaO/CaF2/Al2O3/SiO2 were melted from analytical grade Na2CO3, K2CO3, CaCO3, CaF2, ZnO, Al(OH)3 and SiO2 in batches of 300 g. The glass was additionally doped with Ag2O, CeO2, KBr, SnO2 and Sb2O3. Further a glass without KBr in the batch was melted. The batch compositions are given in Table 1.
The powdered components were accurately weighed and homogenized for 1 h with a ball mill. The material was melted in a covered platinum crucible using an inductive furnace at a temperature of 1400 °C for 3 h. The unavoidable fluorine loss, as determined by energy-dispersive X-ray (EDX) spectrometry using the scanning electron microscope (SEM) Jeol JSM 7001F FEM, amounted on average to about 39% ± 3%. After homogenizing the melt, it was cast in a preheated steel mould and given to a muffle furnace preheated to 530 °C. Then the furnace was switched off and the sample was allowed to cool. Differential thermal analyses (DTA, powdered sample heated at 10 K/ min) and dilatometer (heated at 5 K/ min, Netzsch 402) measurements were carried out to obtain values for the thermal properties. The glass transition temperature was found to be 520 °C and the onset of crystallisation was found to be 580 °C. Polished glass samples with a size of 1 × 20 × 10 mm3 were prepared. Irradiation of the samples was carried out using a sun simulator (Xe/Hg high pressure lamp, LOT Oriel) which emits at wavelengths larger than 195 nm. Irradiation power was 100 W/ cm2. The sample was directly placed below the condenser lens. The samples were thermally treated in a muffle furnace in a one or two step process at a temperature of 530 °C for 1 h and at 560 °C for 20 h with a heating rate of 5 K/ min. Absorption of the samples was measured from 200 to 1200 nm using a double beam spectrophotometer (Shimadzu, 3101PC) by recording absorption of air as reference. Since all samples are of the same thickness, Fresnel reflection were not subtracted and absorption coefficients were not calculated.
From polished samples, X-ray diffraction (XRD) measurements were performed (Siemens D 5000) using a Cu anode (λKα1 = 1.789 Å) with a resolution of 2θ = 0.05°. The diffraction patterns were scanned over the 2θ range 10° - 60° with a 0.02° step width.
In Table 2 conditions of sample preparation are listed including the time of exposure to radiation and the heat treatment steps. Samples with the additional label “Br0” were prepared from the Br-free glass.
All glasses and prepared samples were visually transparent. UV-vis absorption spectra were recorded from non irradiated and irradiated samples in order to evaluate structural changes during irradiation and heat treatment.
Figure 1 shows the absorption spectra of glass sample in the UV-vis range before and after irradiation. The spectrum of the non-irradiated sample shows an absorption peak at around 315 nm and an UV-cut off at around 275 nm which was determined with the tangent method. Exposure to radiation causes an increase of absorption in the short wavelength range.
In Fig. 2, UV-vis absorption spectra of samples after thermal treatment are shown. The black curve (0AB) is attributed to a sample, which was only annealed at 530 °C for 1 h and at 560 °C for another 20 h without prior irradiation. The red and grey curves show spectra of samples which were irradiated (30 min) before thermal treatment. Irradiation and annealing at 530 °C give rise to the formation of a broad absorption peak at 425 nm. Irradiation and annealing in two steps, at 530 °C for 1 h and subsequently at 560 °C for another 20 h leads to a shift of the absorption band to larger wavelengths (440 nm) and to increasing intensity. Furthermore, these samples were transparent but show a yellow color. The peak at 440 nm is not observed in the sample which was annealed without prior irradiation. Moreover, heat treatment causes a decrease of absorption in the short wavelength range and the Ce3+ absorption band is observed again. While the absorption depends on the ratio of Ce3+ / Ce4+ higher temperatures prefer the stabilization of the reduced form.
In Fig. 3, XRD-patterns of the base glass and a sample without previous irradiation, just thermally treated at 530 °C for 1 h and finally annealed at 560 °C for another 20 h are shown. The glass sample which was neither irradiated nor thermally annealed was amorphous. The non-irradiated sample after subsequent thermal treatment at 530 °C for 1 h is also amorphous and does not show any hint at a crystalline phase. The pattern attributed to the sample annealed at 530 °C for 1 h and subsequently at 560 °C for another 20 h shows a peak of low intensity centred at 2θ = 28.4 deg which is attributed to cubic CaF2 (JCPDS file no 35-0816). That means, without pre-irradiation CaF2 crystals were barely developed.
In Fig. 4, an XRD-pattern of a sample which was first irradiated for 30 min and then annealed at 530 °C for 1 h is shown. The XRD-pattern shows two peaks of low intensity at 2θ = 28.4 deg and 2θ = 31.7 deg. The curve of sample 30B shows an irradiated sample which was subsequently thermally treated at 560 °C for 20 h. It shows a small intensity peak at 2θ = 28.4 deg. As curve of sample 30AB, an XRD-pattern of an irradiated samplelater thermally annealed at 530 °C and at 560 °C for another 20 h is shown. This curve shows a high intensity peak at 2θ = 28.4 deg related to a notable volume fraction of CaF2 crystals in the glass matrix.
In Fig. 5 UV-vis spectra of samples irradiated for different periods of time and subsequent annealed at 530 °C for 1 h and subsequently at 560 °C for 20 h are shown. The black curve shows the absorption spectrum after 10 min exposure to UV light and subsequent annealing. The red and the blue line were recorded after irradiation for 30 and 60 min, respectively; both samples were subsequently annealed. All spectra show a broad absorption band at around 440 nm related to silver clusters. The intensities of the peaks are larger if the samples were irradiated for 30 or 60 min. The curves for the 30 and 60 minutes of irradiation are the same within the limits of error.
In Fig. 6, XRD-patterns of samples first irradiated for different periods of time, then annealed at 530 °C for 1 h and finally annealed at 560 °C for another 20 h are shown. Longer irradiation times lead to a notable increase of the peak at 2θ = 28.4 deg. A tiny peak at 2θ = 31.7 deg is also observed. Furthermore after 60 min irradiation and subsequent heat treatment, peaks at 2θ = 31.7 deg and 57 deg occur.
The refractive index of the uncrystallized glass was 1.5485, whereas it was 1.5473 for the irradiated and heat treated sample. This means during the latter procedure the refractive index slightly increases.
In Fig. 7, XRD-patterns of samples which were melted without using KBr as raw material are shown. The samples were exposed to radiation for 30 min or were not irradiated but in any case were thermally treated at 530 °C for 1 h and at 560 °C for 20 h. Pre-irradiated samples show a light yellow coloration after heat-treatment. The peak at 2θ = 28.4 deg occurs with low intensity both in the irradiated and in the non-irradiated samples. The XRD-patterns of both samples show a distinct peak at 2θ = 46 deg. These peaks are attributed to cubic CaF2.
The UV-vis absorption spectrum of the transparent and colorless non irradiated sample (see black curve in Fig. 1) shows a peak at around 315 nm. This peak is due to the presence of Ce3+ (4f-5d bands) [31, 32]. During irradiation at a wavelength above 195 nm, the absorbance of the glass increases especially in the UV-range. The absorption at wavelengths λ< 270 nm is due to the combination of absorption of several components, such as Ce4+, Ag+, Sb3+ and Br–  which overlap the peak due to Ce3+ at 315 nm. The changes in the spectra are caused by the following reactions. The Ce3+ of the glass absorbs the UV light and reacts to Ce4+ and an electron, Eq. (1).Eq. (2).
Initial thermal treatment for 1 h at 530 °C (a temperature close to Tg) results in the formation of nucleation centers indicated by the yellow color of the sample. These nucleation centers are silver clusters which show characteristic absorption lines with a maximum at 425 nm (see Fig. 2). The absorption band is typical for the colloidal silver particles related to the surface plasmon resonance , Eq. (3).
The further growth of the colloidal silver particles Agn is hindered by the high matrix viscosity because the annealing temperature of the first annealing step is just above Tg (520 °C). Subsequent annealing at 560 °C for 20 h leads to the formation of CaF2 crystals, furthermore to a shift of the absorption peak to 440 nm and to a further increase in the absorptivity. The change in the maximum position is caused by the size effects according to Mie theory, i.e. to a coarsening of the silver particles. Hence increasing intensity of the absorption band is also a result of scattering effects.. Samples without prior irradiation which nevertheless were thermally treated did not show a plasmonic resonance caused by clusters of elemental silver. Furthermore, the XRD patterns of samples without prior irradiation did not show distinct lines related to cubic CaF2 as illustrated in Fig. 3. Samples that were thermally treated without previous UV irradiation barely develope CaF2 crystals. By contrast, XRD patterns of samples which were not irradiated before thermal treatment do not show distinct lines as shown in Fig. 4. After irradiation and thermal treatment at 530 °C for 1 h a peak at 2θ = 28.4 deg attributed to cubic CaF2 (JCPDS file no 35-0816) and a peak at 2θ = 31.7 deg appear. After thermal treatment at 530 °C for 1 h and at 560 °C for 20 h, the patterns of the pre-irradiated sample reveals an intense and narrow peak at 2θ = 28.4 deg related to CaF2. This is a significant difference to the non-irradiated sample. Crystallization of CaF2 was not observable after one step thermal treatment at 560 °C. Prior irradiation and a subsequent two step thermal treatment have a distinct effect on crystallization behavior of CaF2 in the glass matrix. It should be noted that also the first step of the thermal treatment is quite essential; obviously silver clusters do not form at 560 °C and then also the crystallization of CaF2 is not triggered.
Further investigations were aimed at the effect of irradiation time on crystal size and size distribution. The samples were irradiated for different periods of time and then the two step thermal treatment was performed. While the intensity of the plasmonic resonance, i.e. the concentration of the silver clusters strongly increased from 10 to 30 min irradiation time, a further extension did not result in an increase of the absorption maximum due to the plasmonic resonance, nor to a shift in the peak position (see Fig. 5). By contrast, the irradiation time has a significant effect on the volume concentration of CaF2 as shown in the XRD patterns in Fig. 6. Longer irradiation times led to a notable increase of the intensity of the peak at 2θ = 28.4 deg attributed to cubic CaF2. Although, the intensity of the plasmonic resonance peak is not changed, the concentration of crystalline fluoride is higher after thermal treatment. Possibly, silver clusters which are too small to cause plasmonic resonance peaks in the UV-vis absorption spectrum also act as nucleating agents. Furthermore after irradiation for 60 min, an additional peak at 2θ = 57 deg occurs which is also related to cubic CaF2. After 30 min and 60 min irradiation time and subsequent two step heat treatment, also a peak at 2θ = 31.7 deg appears. The ratio of the intensity of these two peaks at 2θ = 28.4 deg and 2θ = 31.7 deg increases by a factor of two after using a twice as long exposure time. A peak at the same 2θ value has already been observed in Fig. 1 for a sample irradiated for 30 min and thermally treated at 530 °C for 1 h. In principle, this peak might be due to occurrence of AgBr which (200)-peak according to the JCPDS file no. 06-0438 fits well. Since the occurrence of silver in the glass is only 0.04%, the concentration seems to be very small for a peak of comparably high intensity. However, NaBr and AgBr form solid solutions at high temperatures, which, however, might decompose during cooling . Nevertheless, NaF and also solid solutions with AgBr have lattice constants which are fairly similar to that of AgBr and more probably should be the reason for the occurrence of the peak at 31.7 deg. This peak appears only in KBr containing PTR glass and at a two step heat treatment (see Fig. 4). The same glass also shows the highest CaF2 concentrations. The melting temperatures of sodium bromide and silver bromide are 755 °C and 428 °C, respectively. Hence, it can be assumed that the crystals are below their melting points and hence in the solid state.
In order to analyze how the absence of bromide affects the crystallization and the formation of this phase, a glass without KBr was produced and studied with respect to its crystallization behavior by X-ray diffraction. The formation of Ag0 takes also place as indicated by a light yellow coloration after thermal treatment and pre-irradiation By comparison this CaF2 containing glass without KBr does not show peaks for silver or sodium bromide in the XRD patterns (Fig. 7) after exposing to UV light and thermal treatment. In addition, a peak at 2θ = 46 deg occurs which is also attributable to cubic CaF2 and the peak at 2θ = 28.4 deg has less intensity. This strengthens the assumption that crystal growth follows another mechanism in this case. Especially notable is that XRD pattern of the non-irradiated colorless sample also show peaks of notable intensity which are due to cubic CaF2. This means that spontaneous crystallization occurs and a photo induced crystallization mechanism is not predominant in the studied composition under the supplied conditions. Recent studies on conventional PTR glasses demonstrated that bromide plays a key role also in the crystallization of NaF [12, 36]. It was shown that decreasing the bromide concentration severly hindered NaF crystallization. Hence, it seems that bromide plays an essential part for both, the precipitation of NaF as well as of CaF2 in PTR glasses.
It should be noted that in the studied system, it was not possible to prepare samples which could be studied by TEM because the sample was damaged during irradiation with the electron beam . This was also the case if using an aberration corrected TEM with an acceleration voltage of only 80 kV. By comparison in a similar system, enabling the precipitation of BaF2 , TEM studies were possible even using electron energy loss spectroscopy. Since microscopic studies of the nanosized CaF2 crystals were not successful, small angle X-ray scattering (SAXS) and anomalous small angle X-ray scattering (ASAXS) were used to determine the microstructure  as it has previously be reported for BaF2 . In this base Na2O/K2O/CaO/CaF2/Al2O3/SiO2 system  which does neither contain silver nor ceria and hence is not photosensitive, it has been shown that around the CaF2 crystal, a thin layer enriched in SiO2 exists. This structure has previously been postulated on the basis of the increase of the glass transition temperature during the course of the crystallization of CaF2. The formation of such a layer and its effect on crystal growth has further been explained by theoretical models and numerical simulations [40, 41]. They assumed that the crystals first grow fast and then the chemical composition changes. Near the crystal, the melt is depleted in components which if added to the glass would lead to a decrease in the viscosity. That means during crystallization of alkali or alkaline earth fluorides, the viscosity increases near the crystals until the attributed Tg is larger than the crystallization temperature, then the system can no longer mechanically relax within the time scale of the experiment performed. This all runs parallel to a drastic decrease in the crystal growth velocity [23, 24, 26]. Since in a first approximation, the refractive index should decrease with decreasing density, the photo induced crystallization should run parallel to a decrease in the refractive index . However, as recently reported for a system which chemical composition is somewhat different, the photo refractivity is probably a much more complex effect.
Future work will be focused on the effect of laser irradiation on changes in the refractive index. We expect that due to the larger fluoride concentrations these changes should be larger than in other PTR glasses.
A new photo-thermo refractive glass composition is presented. Glasses in the system Na2O/K2O/CaO/CaF2/Al2O3/SiO2 were doped with Ag2O, CeO2, KBr, SnO2 and Sb2O3. They were irradiated with UV light and subsequently thermally annealed in a two step process at 530 °C and subsequently at 560 °C. The irradiation led to the formation of metallic silver which forms clusters in the first annealing step. During the second annealing step, the crystallization of nano crystalline CaF2 is observed as proved by X-ray diffraction. Furthermore, not irradiated PTR glass does not show CaF2 crystallization supplying the same thermal treatment. The longer irradiation time, the higher is the volume density of CaF2 crystals in the glass matrix. Glass samples from melts without KBr showed the precipitation of CaF2 during a one or a two step thermal treatment without previous irradiation. This means that spontaneous crystallization predominates and no photoinduced mechanism controls the crystallization process. This strengthens the thesis that KBr plays an important role in the photoinduced crystallization process. Nevertheless, it would not be advantageous to replace Ag2O and KBr by silver halides like AgBr or AgCl, which are well-known for their photosensitivity because these halides have low melting and boiling points. They might even be evaporated before the raw materials are completely melted. Finally, the new photo-thermo-refractive glass enables a controlled nanoscale crystallization of CaF2 in limited areas.
This work was funded by the Bundesministerium für Forschung und Bildung, Germany (Wachstumskern Brightlas, 03WKCF3E)
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