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Effects of TiO2 and P2O5 on solarization and crystallization of photosensitive lithium silicate glass
Journal of Non-Crystalline Solids 430 (2015) 25–30
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Effects of TiO2 and P2O5 on solarization and crystallization of photosensitive lithium silicate glass Neda Ghaebi Panah ⁎, Bijan Eftekhari Yekta, Vahak Marghussian, Elnaz Mohaghegh Ceramic group, School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Tehran 16846-13114, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 10 July 2015 Received in revised form 13 September 2015 Accepted 14 September 2015 Available online xxxx Keywords: Crystallization; Photosensitive lithium silicate glass; Silver cluster; UV-radiation; UV-shielding effect
In this study, the parent photosensitive glass and glass-ceramic in the system of lithium silicate containing AgCl under radiation of ultraviolet (UV) light were successfully prepared by melting process. Besides, the effects of adding TiO2 and P2O5 as nucleating agents on the solarization and crystallization of lithium silicate photosensitive glass were studied. The characteristics of the samples were identified by ultraviolet-visible absorption spectroscopy (UV-Visible), simultaneous thermal analysis (STA-TG/DTA), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). The results showed that adding P2O5 in the amount of 2 wt.% to this system could be an acceptable nucleating agent, while adding 4 wt.% TiO2 to it resulted in shielding behavior against the UV light. Therefore, this amount of TiO2 is not successful in both solarization improvement and nucleation process. In addition, lithium disilicate crystalline phase formed only in the samples containing P2O5. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
heat‐treatment
xAgo → Ago
Nowadays, applications of glasses in various fields are wide spread. One of the most attractive types is photosensitive glass which possesses the capability of precipitation manipulating in soluble ceramic phase from the base glass. Typically, adding a photoactive compound and metal ions as nucleating agent to the glass matrix can take the control of the local precipitation process. Hence, it brings possibility of microshaping and micro-structuring on photosensitive glass ceramics through optical lithographic patterning and chemical etching processes. The most common dopants such as Sb2O3, Ag2O, SnO2 and CeO2 within the glass matrix are able to emerge photosensitivity feature. Technically, UV radiation can release electrons from Ce3+ and Sn2+ and result in Ce4+ and Sn4+. The Ag+ ions would absorb these electrons and agglomerate to Ag clusters through the heat treatment then. All the mentioned processes are based on succeeding Equations. 3þ
2Ce4þ þ Sb
5þ
↔ 2Ce3þ þ Sb ;
ð1Þ
Ce3þ þ hν → Ce4þ þ e;
ð2Þ
Sn2þ þ hν → Sn4þ þ 2e;
ð3Þ
Agþ þ e → Ago ;
ð4Þ
⁎ Corresponding author. E-mail address: [email protected] (N. Ghaebi Panah).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.09.018 0022-3093/© 2015 Elsevier B.V. All rights reserved.
x
ð5Þ
where x is the number of silver atoms aggregated into clusters. Considering the observations, Ce3+ ions are stabilized in presence of Sb3+ based on Eq. (1) during the photosensitive glasses melting process. Particularly for lithium silicate systems, the lithium-metasilicate crystallizes would be appeared around the Ag clusters; therefore, grain formation and growth will happen [1–5]. On the other side, introducing some suitable nucleating agents to the glasses may help to have an appropriate glass ceramics through heat treatment. Admittedly, precipitated crystals and their microstructures can play an important role in many properties of glass ceramics. Both TiO2 and P2O5 as influential nucleating agents could provide a finegrained interlocking microstructure in lithium disilicate glass ceramics and fortify mechanical properties [6,7]. More specifically, P2O5 in the range of 1.5–2.5 mol% can be a successful nucleating agent for lithium silicate glass ceramics. In practice, it boosts heterogeneous nucleation and forms a fine-grained interlocking morphology after heat treatment. It is noteworthy that manipulation of both composition and heat treatment schedule would be able to bring desirable properties [7–9]. Another nucleating agent in the preparation of glass ceramics can be TiO2 a member of 3d transition metal oxides [10–12]. Titanium can be found in forms of trivalent (Ti3+) and tetravalent valence (Ti4+) in glasses but the proportion of each type might be varying depending on the glass type, composition and condition of melting [11,12]. Although it is known that P2O5 and TiO2 are used as nucleating agents in various glass systems, there is only few publications about
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this glass system in which these oxides used as nucleating agents [13]. Therefore, the role of these nucleating agents in presence of the photosensitive elements Ce, Sb, Sn and Ag under radiation of UV light in solarization, crystallization temperature, and the microstructure for various glasses is worthy of more attention. In this work, the parent photosensitive glass and glass-ceramic in the system of lithium silicate containing AgCl under radiation of UV light were successfully prepared by melting process to reveal the effects of adding P2O5 and TiO2 as nucleating agents on the crystallization and solarization of lithium silicate photosensitive glass.
temperatures for 2 h. After cutting and polishing, rectangular cube samples are formed in dimensions of 10 × 10 × 2 mm. Then the samples got radiated for 2 h by a 1000-W UV-ray lamp. This source, invented in the local lab, includes UV lamp, cooling fans, radiation cylindrical shield, refractory fibers, etc. Here the distance between lamp and sample is 7 cm in this structure. After that, the GB, GP and GT irradiated glasses were named GB(UV), GP(UV), and GT(UV), respectively. Finally, heat treatment of all the samples was carried out at DTA crystallization peak temperature for 2 h at the heating rate of 10 °C/min. 2.2. Characterization methods
2. Experimental procedure 2.1. Sample preparation The study was conducted for parent glass of lithium silicate system (GB) with regard to several publications based on the following chemical composition (wt.%): 75.00% SiO2, 5.00% Al2O3, 12.00% Li2O, 4.00% K2O, 4.00% Na2O, 0.15% AgCl, 0.30% Sb2O3, 0.05% CeO2, and 0.07% SnO2 [1,2,4]. The employed substances here are extra pure supplied by Merck Co. except silica obtained after acid washing of a commercially grade silica powder. The purity of resulted silica powder might go further than 99.8 wt.%. Two other glasses containing all abovementioned materials are called GT and GP when 4.00 wt.% TiO2 and 2.00 wt.% P2O5 are added respectively. The glasses were prepared in an electric furnace at 1450 °C for 1 h. The heating rate was 10 °C/min and the crucible was alumina. It should be noted that the crucible were covered with an alumina lid to minimize the volatilization of volatile components like phosphorous, alkali oxides, and silver. Indeed, covering the melt can increase the partial pressure of the volatile components above the melt and set a dynamic equilibrium between the dissolved and vaporized species to prevent significant losses [14,15]. Besides, in the case of silver, the volatilization loss can be controlled by introducing a small amount of tin to the composition. It is believed that tin ions can participate in the glass as a network former; and at the same time can exert forces on the noble metal by their residual valencies. These forces make a condition under which the noble metal cannot volatilize from the melt [16,17]. On the whole, since photosensitivity is observable in the final glasses, it can be concluded that these two factors have provided a suitable situation to keep sufficient amounts of some volatile components like Ag, Sb, P, and alkali oxides in compositions. It is noteworthy that interactions between glass compositions and the alumina crucible will inevitably happen during melting and increase Al2O3 content of the resulting glass. According to previous studies and our experiments, this reaction is negligible and has no negative effects on glass properties [2,4,18]. The prepared glasses were annealed at 500 °C, close to their glass transition
The UV-Visible absorption spectra of the glasses were measured via UV-VIS double beam spectrophotometer (SHIMADZU UV-2501PC model) in the wavelength range of 300–700 nm. The standard sample was GB silver free. Besides, the simultaneous thermal analysis was performed on a NETZSCH STA 409 PC/PG at the heating rate of 10 °C/min under air atmosphere. The curve was acquired in the temperature range of 30–1100 °C. The reference material in these experiments was α-Al2O3 powder. In addition, the X-ray powder diffraction patterns were examined on a JEOL JDX-8030 with Cu Kα radiation in the 2θ range of 10–80° at 40 kV and 40 mA. Finally, The microstructures of the heat treated samples were observed by scanning electron microscope (Philips XL30) working at 30 kV after polishing and chemical etching in 0.5% HF solution for 3 min. The samples were coated with a thin layer of gold before imaging. 3. Results and discussion Fig. 1 shows UV–Visible absorption spectra of the lithium silicate glasses GB, GP, and GT before and after irradiation. In the case of GB, there is a small peak around 360 nm that can be referred to colloidal silvers which precipitates via redox reactions during melting. It is believed that some ionic silvers can be reduced to their colloidal form due to redox conditions during melting. As a matter of fact, the equilibrium between ionic silver and colloidal one is affected by oxidation-reduction conditions (batch composition, furnace atmosphere). For example, small amount of tin compounds in silicate glasses can be existed in both states of Sn2+ and Sn4+. These stannous and stannic ions can participate in the reactions during the glass melting as oxidizing or reducing agents, respectively [16–18]. After UV radiation, the small peak of GB widened and continued to 600 nm which confirmed that UV radiation can reduce ionic silver to colloidal one. Additionally, the absorption peak height arose after irradiation. This might indicate further ionic silver reduction into colloidal form during irradiation. On the other hand, the result of UV-Visible absorption spectrum of GP indicates that there is no peak around 360 nm before irradiation; thus, ionic silver reduction
Fig. 1. UV–visible absorption spectra of the lithium silicate glasses GB, GP, and GT. (A) before irradiation, (B) after irradiation.
N. Ghaebi Panah et al. / Journal of Non-Crystalline Solids 430 (2015) 25–30
27
ionic silver amount as GT. Considering the absorption peak which arose around wavelength 327 nm in Fig. 2, it can be concluded that the absorption peak refers to tetravalent titanium ions. It is noteworthy that there is no new band in the visible region for GT(UV) after UV radiation that could be caused by the UV-shielding behavior of titanium ions.
Fig. 2. UV–visible absorption spectra of the lithium silicate glass GT with the standard sample of GB.
may not happen or partially occur. While GP(UV) shows a strong peak that continued to 600 nm the same as GB(UV) sample. Admittedly, electron centers can mainly generate induced bands within the UV region whereas positive hole centers are mostly responsible for the induced bands inside the visible region [11,19]. This evidence is strong enough to explain the existence of colloidal silver in GP(UV) based on the Eqs. (1)–(5). Comparison of GB and GP before and after UV radiation indicates a difference between their baselines. Actually, before irradiation, there is a gap between samples of GB and GP while these samples illustrate the similar pattern without any gap after UV radiation. This gap can be attributed to the difference between sample's thickness arised during their surface finishing. Furthermore, in the cases of GT and GT(UV) a strong peak at around 338 nm that extended up to 400 nm can be seen that perhaps is due to both colloidal silver and titanium ions. Actually, the optical properties of silver atoms, ions, and clusters have been well studied and exhibit characteristic UV–Visible absorption bands. The UV-Visible absorption bands of Ag+, Ago, and small Agn clusters are at 190–230 nm, 250–330 nm, and also 330–360 nm and 440–540 nm, respectively [20,21]. As mentioned earlier, there are titanium ions in both states of trivalent and tetravalent in glass. In fact, Ti3+ ions refer to the 3d1 configuration hence appear with a single absorption band at 480–540 nm. Similarly, Ti4+ ions of do configuration do not yield d-d visible bands however they can have UV absorption [11, 22]. Since no visible band was detected in the experimental results of UV-Visible spectrum in the case of GT, the presence of trivalent ions in the studied glass with 4 wt.% TiO2 is not verifiable. However, the peaks of both colloidal silver and tetravalent titanium ions might overlap with each other. In order to determine whether the absorption peak is due to titanium ionic or colloidal silver, another supplementary UVVisible spectrophotometry analysis carried out on GT. Note that the standard sample in the recent analysis was GB containing the same
Fig. 3. The appearance of the bulk annealed glasses before and after irradiation. (1) GP, (2) GT, (3) GB, (4) GP(UV), (5) GT(UV), (6) GB(UV).
Fig. 4. DTA graphs of the lithium silicate glasses. (A) GB and GB(UV), (B) GP and GP(UV), and (C) GT and GT(UV).
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In this condition, titanium ions might impede the generation of induced visible peaks during the irradiation process. Many researches reveal that some glasses containing few metallic ions such as titanium, vanadium, and copper are able to shield UV light after gamma irradiation [11,23,24]. Fig. 3 shows the appearance of the bulk annealed glasses before and after irradiation. It is clear that GP and GB are colorless while both of them turned to light yellow with UV radiation. This visible coloration is a sign that color centers are formed in these samples by irradiation. Indeed, if ionizing radiation strikes glasses, physical properties may alter remarkably. Among the possible changes, color centers have great significance [19,25,26]. However, GT is the sole sample without considerable change in appearance. This finding was predictable regarding UV–Visible absorption spectra results. In fact, since both GT and GT(UV) samples are colorless it can be concluded that titanium ions predominantly are in tetravalent state. Optical absorption studies on TiO2 containing glasses indicate that under ordinary conditions of melting, titanium ions are incorporated into silicate glasses in its higher valence state as Ti4+ ions because it is difficult to obtain Ti3+ ions under such conditions. Indeed, alkali silicate and alkali borate glasses melted under normal atmospheric condition favor the colorless high tetravalent titanium ions while alkali phosphate and lead phosphate glasses advocate the presence of the purple trivalent titanium ions [5,11,23,27]. Additionally, DTA graphs were used to determine the crystallization temperatures of the glasses. Fig. 4 illustrates the DTA results of GB, GP, and GT before and after irradiation. Two exothermic crystallization peaks can be seen for each sample that could be referred to lithium metasilicate (LM) and lithium disilicate (LD) phases. Considering Fig. 4A related to GB and GB(UV) with the glass transition temperatures (Tg) of about 466 °C, there are two crystallization peaks around 740 °C and 840 °C with a small difference in their shapes before and after irradiation. In the cases of GT and GT(UV) with the glass transition temperatures of about 472 °C, the UV radiation does not have any significant influence on the crystallization peaks due to the UV-shielding behavior of titanium ions but the first peak of these samples located about 765 °C is almost 15 °C higher than the parent glass. Besides, the second peak in Fig. 4C is weaker than the Fig. 4A which can be a sign of the difficulty in formation of LD crystalline phase. Indeed, addition of 4 wt.% TiO2 to parent glass caused the first crystalline temperature to increase. This can be explained by the role of Ti4+ ions in elevation of glass viscosity or strengthening of glass network which might lead to a poor crystallization performance. Similarly, increasing the first crystallization temperature in the lithium silicate glass containing ZrO2 is reported previously [28]. Despite of titania role in raising LM crystallization temperature, addition of 2 wt.% P2O5 could reduce both crystallization temperatures from around 740 °C and 840 °C to 717 °C and 817 °C, respectively. It is noteworthy that the glass transition temperatures are around 468 °C
for both samples of GP and GP(UV). Moreover, the DTA graphs of samples containing phosphorous in Fig. 4B illustrate that the first peak of both samples are almost similar while the second one after irradiation has become wider than before. To explain these observations, there can be at least two reasons; nucleating agents of P2O5 and silver clusters. It is possible that P2O5 and Li2O in glass are able to form Li3PO4 crystals which can act as heterogeneous nucleation sites for other crystalline phases. Firstly, LM phase can precipitate on Li3PO4 crystals and after that with increasing temperature the growth of LD phase occurs at the expense of consuming LM phase. This phenomenon was observed in previous researches [9,29,30]. In addition, during the UV radiation, electron transfer can occur in the photosensitive elements of Ce, Sb, Sn, and Ag based on the Eqs. (1)–(5). In fact, within the UV radiation, photoelectrons can eject from Ce3+ and Sn2+ then get absorbed by silver ions and subsequently form silver clusters through the heat treatment process at 500 and 600 °C. Finally, these clusters can act as nucleating agents to form LM phase. These results can be verified by previous studies [1,3,4]. Fig. 5 depicts the XRD patterns of the lithium silicate glass ceramics and the precipitated crystalline phases after heat treatment at crystallization temperatures. The detected crystalline phases were LM—Li2SiO3 (JCPDS Database No. 01-072-1140) and LD—Li2Si2O5 (JCPDS Database No. 00-040-0376). Fig. 5A relating to the heat treatment at the first crystallization temperature illustrates that the major crystallized phase in this step is LM phase. In Fig. 5A, a small difference between GB(UV) and GB can be due to the silver clusters role in nucleation process. Besides, there is no obvious change in the case of GT before and after UV radiation because of the UV-shielding ability of TiO2. Moreover, comparing GB and GT, it seems that the increase of viscosity by Ti4+ might lead to abate the relative content of LM phase. These results based on the DTA observations were expected. Despite of the TiO2-containing glasses, it can be seen that the relative content of LM phase in the sample with the phosphorous is increased after irradiation. Furthermore, in the presence of P2O5 the level of formed LM phase is higher than the parent glass. Therefore, it is probable that P2O5 has played the role of nucleating agent via Li3PO4 phase in this glass system, but maybe the content of 2 wt.% P2O5 is not enough to be detected by XRD and to prove the presence of Li3PO4 phase. Based on Fig. 5B, the second exothermic peak in DTA is attributed to LD phase while this phase is observed only in the cases GP and GP(UV). It can be concluded that the silver clusters in the nucleation process of the parent glass is not singly capable enough; however, when the 2 wt.% P2O5 content is added LD phase can be formed. In addition, the relative content of LM phase in GB(UV) is elevated slightly rather than GB due to the silver clusters role, whereas the relative content of this phase for the sample containing TiO2 remained unchanged. On the other side, according to the increased LM phase in the presence of P2O5 content at the first crystallization temperature, it seems that both nucleating agents of the silver clusters and also Li3PO4 crystals
Fig. 5. The XRD patterns of the lithium silicate glass ceramics after heat treatment at: (A) the first crystallization temperature, (B) the second crystallization temperature.
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29
Fig. 6. SEM images of the lithium silicate glass ceramics at: (A) the first crystallization temperature, (B) the second crystallization temperature. (1) GB, (2) GB(UV), (3) GT, (4) GT(UV), (5) GP, (6) GP(UV).
altogether led to raise the precipitation of LM phase and eventually to grow LD phase at the expense of consuming LM phase. Fig. 6A and B show the microstructures of the lithium silicate glass ceramics at the first and the second crystallization temperatures, respectively. Regarding Fig. 6A, the SEM images illustrate LM main phase in two different morphologies; the needle-like and the dendrite-like crystals. The former is observed at the samples of GB, GB(UV), GT, and GT(UV) and the latter is found in the samples containing P2O5 which is an indicator of difference between the sources of nucleation. As this phase is more readily dissolved in HF in comparison to silica glass matrix, the surface pits resulting from HF etching process reflect the microstructure of LM crystals. After the second crystallization step in Fig. 6B, needle-like LM phase grew up and turned to the rod-like crystals in both the parent and the TiO2-containing glasses. It should be noted that it is not clear whether small or nanometric LD phase is formed or not after UV radiation in these two samples but it can be seen that LM crystals only in the parent glass are greater than before. On the other side, the microstructure of P2O5-containing samples is changed totally and an interlocking microstructure related to LD phase is successfully formed. 4. Conclusions The parent photosensitive glass and glass-ceramic in the system of lithium silicate containing AgCl under radiation of UV light were successfully prepared by melting process to reveal the effects of adding TiO2 and P2O5 as nucleating agents on the solarization and crystallization of lithium silicate glass. The results showed that UV radiation could turn the ionic silvers to the silver clusters in both parent glass
and the glass containing 2 wt.% P2O5. While in presence of 4 wt.% TiO2, the UV-shielding effect happened and led to hamper generation of the induced bands in visible region. Also, despite of adding P2O5 which could reduce the crystallization temperatures, TiO2 additive increased them. In addition, presence of P2O5 content could elevate LM phase at the first crystallization temperature; consequently, it led to raise the precipitation of LM phase and eventually to grow LD phase at the expense of consuming LM phase. However, TiO2 additive causes poor crystallization performance. At the end, only in the samples containing P2O5 an interlocking microstructure related to LD phase is successfully formed.
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Effects of TiO2 and P2O5 on solarization and crystallization of photosensitive lithium silicate glass Neda Ghaebi Panah ⁎, Bijan Eftekhari Yekta, Vahak Marghussian, Elnaz Mohaghegh Ceramic group, School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Tehran 16846-13114, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 10 July 2015 Received in revised form 13 September 2015 Accepted 14 September 2015 Available online xxxx Keywords: Crystallization; Photosensitive lithium silicate glass; Silver cluster; UV-radiation; UV-shielding effect
In this study, the parent photosensitive glass and glass-ceramic in the system of lithium silicate containing AgCl under radiation of ultraviolet (UV) light were successfully prepared by melting process. Besides, the effects of adding TiO2 and P2O5 as nucleating agents on the solarization and crystallization of lithium silicate photosensitive glass were studied. The characteristics of the samples were identified by ultraviolet-visible absorption spectroscopy (UV-Visible), simultaneous thermal analysis (STA-TG/DTA), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). The results showed that adding P2O5 in the amount of 2 wt.% to this system could be an acceptable nucleating agent, while adding 4 wt.% TiO2 to it resulted in shielding behavior against the UV light. Therefore, this amount of TiO2 is not successful in both solarization improvement and nucleation process. In addition, lithium disilicate crystalline phase formed only in the samples containing P2O5. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
heat‐treatment
xAgo → Ago
Nowadays, applications of glasses in various fields are wide spread. One of the most attractive types is photosensitive glass which possesses the capability of precipitation manipulating in soluble ceramic phase from the base glass. Typically, adding a photoactive compound and metal ions as nucleating agent to the glass matrix can take the control of the local precipitation process. Hence, it brings possibility of microshaping and micro-structuring on photosensitive glass ceramics through optical lithographic patterning and chemical etching processes. The most common dopants such as Sb2O3, Ag2O, SnO2 and CeO2 within the glass matrix are able to emerge photosensitivity feature. Technically, UV radiation can release electrons from Ce3+ and Sn2+ and result in Ce4+ and Sn4+. The Ag+ ions would absorb these electrons and agglomerate to Ag clusters through the heat treatment then. All the mentioned processes are based on succeeding Equations. 3þ
2Ce4þ þ Sb
5þ
↔ 2Ce3þ þ Sb ;
ð1Þ
Ce3þ þ hν → Ce4þ þ e;
ð2Þ
Sn2þ þ hν → Sn4þ þ 2e;
ð3Þ
Agþ þ e → Ago ;
ð4Þ
⁎ Corresponding author. E-mail address: [email protected] (N. Ghaebi Panah).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.09.018 0022-3093/© 2015 Elsevier B.V. All rights reserved.
x
ð5Þ
where x is the number of silver atoms aggregated into clusters. Considering the observations, Ce3+ ions are stabilized in presence of Sb3+ based on Eq. (1) during the photosensitive glasses melting process. Particularly for lithium silicate systems, the lithium-metasilicate crystallizes would be appeared around the Ag clusters; therefore, grain formation and growth will happen [1–5]. On the other side, introducing some suitable nucleating agents to the glasses may help to have an appropriate glass ceramics through heat treatment. Admittedly, precipitated crystals and their microstructures can play an important role in many properties of glass ceramics. Both TiO2 and P2O5 as influential nucleating agents could provide a finegrained interlocking microstructure in lithium disilicate glass ceramics and fortify mechanical properties [6,7]. More specifically, P2O5 in the range of 1.5–2.5 mol% can be a successful nucleating agent for lithium silicate glass ceramics. In practice, it boosts heterogeneous nucleation and forms a fine-grained interlocking morphology after heat treatment. It is noteworthy that manipulation of both composition and heat treatment schedule would be able to bring desirable properties [7–9]. Another nucleating agent in the preparation of glass ceramics can be TiO2 a member of 3d transition metal oxides [10–12]. Titanium can be found in forms of trivalent (Ti3+) and tetravalent valence (Ti4+) in glasses but the proportion of each type might be varying depending on the glass type, composition and condition of melting [11,12]. Although it is known that P2O5 and TiO2 are used as nucleating agents in various glass systems, there is only few publications about
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this glass system in which these oxides used as nucleating agents [13]. Therefore, the role of these nucleating agents in presence of the photosensitive elements Ce, Sb, Sn and Ag under radiation of UV light in solarization, crystallization temperature, and the microstructure for various glasses is worthy of more attention. In this work, the parent photosensitive glass and glass-ceramic in the system of lithium silicate containing AgCl under radiation of UV light were successfully prepared by melting process to reveal the effects of adding P2O5 and TiO2 as nucleating agents on the crystallization and solarization of lithium silicate photosensitive glass.
temperatures for 2 h. After cutting and polishing, rectangular cube samples are formed in dimensions of 10 × 10 × 2 mm. Then the samples got radiated for 2 h by a 1000-W UV-ray lamp. This source, invented in the local lab, includes UV lamp, cooling fans, radiation cylindrical shield, refractory fibers, etc. Here the distance between lamp and sample is 7 cm in this structure. After that, the GB, GP and GT irradiated glasses were named GB(UV), GP(UV), and GT(UV), respectively. Finally, heat treatment of all the samples was carried out at DTA crystallization peak temperature for 2 h at the heating rate of 10 °C/min. 2.2. Characterization methods
2. Experimental procedure 2.1. Sample preparation The study was conducted for parent glass of lithium silicate system (GB) with regard to several publications based on the following chemical composition (wt.%): 75.00% SiO2, 5.00% Al2O3, 12.00% Li2O, 4.00% K2O, 4.00% Na2O, 0.15% AgCl, 0.30% Sb2O3, 0.05% CeO2, and 0.07% SnO2 [1,2,4]. The employed substances here are extra pure supplied by Merck Co. except silica obtained after acid washing of a commercially grade silica powder. The purity of resulted silica powder might go further than 99.8 wt.%. Two other glasses containing all abovementioned materials are called GT and GP when 4.00 wt.% TiO2 and 2.00 wt.% P2O5 are added respectively. The glasses were prepared in an electric furnace at 1450 °C for 1 h. The heating rate was 10 °C/min and the crucible was alumina. It should be noted that the crucible were covered with an alumina lid to minimize the volatilization of volatile components like phosphorous, alkali oxides, and silver. Indeed, covering the melt can increase the partial pressure of the volatile components above the melt and set a dynamic equilibrium between the dissolved and vaporized species to prevent significant losses [14,15]. Besides, in the case of silver, the volatilization loss can be controlled by introducing a small amount of tin to the composition. It is believed that tin ions can participate in the glass as a network former; and at the same time can exert forces on the noble metal by their residual valencies. These forces make a condition under which the noble metal cannot volatilize from the melt [16,17]. On the whole, since photosensitivity is observable in the final glasses, it can be concluded that these two factors have provided a suitable situation to keep sufficient amounts of some volatile components like Ag, Sb, P, and alkali oxides in compositions. It is noteworthy that interactions between glass compositions and the alumina crucible will inevitably happen during melting and increase Al2O3 content of the resulting glass. According to previous studies and our experiments, this reaction is negligible and has no negative effects on glass properties [2,4,18]. The prepared glasses were annealed at 500 °C, close to their glass transition
The UV-Visible absorption spectra of the glasses were measured via UV-VIS double beam spectrophotometer (SHIMADZU UV-2501PC model) in the wavelength range of 300–700 nm. The standard sample was GB silver free. Besides, the simultaneous thermal analysis was performed on a NETZSCH STA 409 PC/PG at the heating rate of 10 °C/min under air atmosphere. The curve was acquired in the temperature range of 30–1100 °C. The reference material in these experiments was α-Al2O3 powder. In addition, the X-ray powder diffraction patterns were examined on a JEOL JDX-8030 with Cu Kα radiation in the 2θ range of 10–80° at 40 kV and 40 mA. Finally, The microstructures of the heat treated samples were observed by scanning electron microscope (Philips XL30) working at 30 kV after polishing and chemical etching in 0.5% HF solution for 3 min. The samples were coated with a thin layer of gold before imaging. 3. Results and discussion Fig. 1 shows UV–Visible absorption spectra of the lithium silicate glasses GB, GP, and GT before and after irradiation. In the case of GB, there is a small peak around 360 nm that can be referred to colloidal silvers which precipitates via redox reactions during melting. It is believed that some ionic silvers can be reduced to their colloidal form due to redox conditions during melting. As a matter of fact, the equilibrium between ionic silver and colloidal one is affected by oxidation-reduction conditions (batch composition, furnace atmosphere). For example, small amount of tin compounds in silicate glasses can be existed in both states of Sn2+ and Sn4+. These stannous and stannic ions can participate in the reactions during the glass melting as oxidizing or reducing agents, respectively [16–18]. After UV radiation, the small peak of GB widened and continued to 600 nm which confirmed that UV radiation can reduce ionic silver to colloidal one. Additionally, the absorption peak height arose after irradiation. This might indicate further ionic silver reduction into colloidal form during irradiation. On the other hand, the result of UV-Visible absorption spectrum of GP indicates that there is no peak around 360 nm before irradiation; thus, ionic silver reduction
Fig. 1. UV–visible absorption spectra of the lithium silicate glasses GB, GP, and GT. (A) before irradiation, (B) after irradiation.
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ionic silver amount as GT. Considering the absorption peak which arose around wavelength 327 nm in Fig. 2, it can be concluded that the absorption peak refers to tetravalent titanium ions. It is noteworthy that there is no new band in the visible region for GT(UV) after UV radiation that could be caused by the UV-shielding behavior of titanium ions.
Fig. 2. UV–visible absorption spectra of the lithium silicate glass GT with the standard sample of GB.
may not happen or partially occur. While GP(UV) shows a strong peak that continued to 600 nm the same as GB(UV) sample. Admittedly, electron centers can mainly generate induced bands within the UV region whereas positive hole centers are mostly responsible for the induced bands inside the visible region [11,19]. This evidence is strong enough to explain the existence of colloidal silver in GP(UV) based on the Eqs. (1)–(5). Comparison of GB and GP before and after UV radiation indicates a difference between their baselines. Actually, before irradiation, there is a gap between samples of GB and GP while these samples illustrate the similar pattern without any gap after UV radiation. This gap can be attributed to the difference between sample's thickness arised during their surface finishing. Furthermore, in the cases of GT and GT(UV) a strong peak at around 338 nm that extended up to 400 nm can be seen that perhaps is due to both colloidal silver and titanium ions. Actually, the optical properties of silver atoms, ions, and clusters have been well studied and exhibit characteristic UV–Visible absorption bands. The UV-Visible absorption bands of Ag+, Ago, and small Agn clusters are at 190–230 nm, 250–330 nm, and also 330–360 nm and 440–540 nm, respectively [20,21]. As mentioned earlier, there are titanium ions in both states of trivalent and tetravalent in glass. In fact, Ti3+ ions refer to the 3d1 configuration hence appear with a single absorption band at 480–540 nm. Similarly, Ti4+ ions of do configuration do not yield d-d visible bands however they can have UV absorption [11, 22]. Since no visible band was detected in the experimental results of UV-Visible spectrum in the case of GT, the presence of trivalent ions in the studied glass with 4 wt.% TiO2 is not verifiable. However, the peaks of both colloidal silver and tetravalent titanium ions might overlap with each other. In order to determine whether the absorption peak is due to titanium ionic or colloidal silver, another supplementary UVVisible spectrophotometry analysis carried out on GT. Note that the standard sample in the recent analysis was GB containing the same
Fig. 3. The appearance of the bulk annealed glasses before and after irradiation. (1) GP, (2) GT, (3) GB, (4) GP(UV), (5) GT(UV), (6) GB(UV).
Fig. 4. DTA graphs of the lithium silicate glasses. (A) GB and GB(UV), (B) GP and GP(UV), and (C) GT and GT(UV).
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In this condition, titanium ions might impede the generation of induced visible peaks during the irradiation process. Many researches reveal that some glasses containing few metallic ions such as titanium, vanadium, and copper are able to shield UV light after gamma irradiation [11,23,24]. Fig. 3 shows the appearance of the bulk annealed glasses before and after irradiation. It is clear that GP and GB are colorless while both of them turned to light yellow with UV radiation. This visible coloration is a sign that color centers are formed in these samples by irradiation. Indeed, if ionizing radiation strikes glasses, physical properties may alter remarkably. Among the possible changes, color centers have great significance [19,25,26]. However, GT is the sole sample without considerable change in appearance. This finding was predictable regarding UV–Visible absorption spectra results. In fact, since both GT and GT(UV) samples are colorless it can be concluded that titanium ions predominantly are in tetravalent state. Optical absorption studies on TiO2 containing glasses indicate that under ordinary conditions of melting, titanium ions are incorporated into silicate glasses in its higher valence state as Ti4+ ions because it is difficult to obtain Ti3+ ions under such conditions. Indeed, alkali silicate and alkali borate glasses melted under normal atmospheric condition favor the colorless high tetravalent titanium ions while alkali phosphate and lead phosphate glasses advocate the presence of the purple trivalent titanium ions [5,11,23,27]. Additionally, DTA graphs were used to determine the crystallization temperatures of the glasses. Fig. 4 illustrates the DTA results of GB, GP, and GT before and after irradiation. Two exothermic crystallization peaks can be seen for each sample that could be referred to lithium metasilicate (LM) and lithium disilicate (LD) phases. Considering Fig. 4A related to GB and GB(UV) with the glass transition temperatures (Tg) of about 466 °C, there are two crystallization peaks around 740 °C and 840 °C with a small difference in their shapes before and after irradiation. In the cases of GT and GT(UV) with the glass transition temperatures of about 472 °C, the UV radiation does not have any significant influence on the crystallization peaks due to the UV-shielding behavior of titanium ions but the first peak of these samples located about 765 °C is almost 15 °C higher than the parent glass. Besides, the second peak in Fig. 4C is weaker than the Fig. 4A which can be a sign of the difficulty in formation of LD crystalline phase. Indeed, addition of 4 wt.% TiO2 to parent glass caused the first crystalline temperature to increase. This can be explained by the role of Ti4+ ions in elevation of glass viscosity or strengthening of glass network which might lead to a poor crystallization performance. Similarly, increasing the first crystallization temperature in the lithium silicate glass containing ZrO2 is reported previously [28]. Despite of titania role in raising LM crystallization temperature, addition of 2 wt.% P2O5 could reduce both crystallization temperatures from around 740 °C and 840 °C to 717 °C and 817 °C, respectively. It is noteworthy that the glass transition temperatures are around 468 °C
for both samples of GP and GP(UV). Moreover, the DTA graphs of samples containing phosphorous in Fig. 4B illustrate that the first peak of both samples are almost similar while the second one after irradiation has become wider than before. To explain these observations, there can be at least two reasons; nucleating agents of P2O5 and silver clusters. It is possible that P2O5 and Li2O in glass are able to form Li3PO4 crystals which can act as heterogeneous nucleation sites for other crystalline phases. Firstly, LM phase can precipitate on Li3PO4 crystals and after that with increasing temperature the growth of LD phase occurs at the expense of consuming LM phase. This phenomenon was observed in previous researches [9,29,30]. In addition, during the UV radiation, electron transfer can occur in the photosensitive elements of Ce, Sb, Sn, and Ag based on the Eqs. (1)–(5). In fact, within the UV radiation, photoelectrons can eject from Ce3+ and Sn2+ then get absorbed by silver ions and subsequently form silver clusters through the heat treatment process at 500 and 600 °C. Finally, these clusters can act as nucleating agents to form LM phase. These results can be verified by previous studies [1,3,4]. Fig. 5 depicts the XRD patterns of the lithium silicate glass ceramics and the precipitated crystalline phases after heat treatment at crystallization temperatures. The detected crystalline phases were LM—Li2SiO3 (JCPDS Database No. 01-072-1140) and LD—Li2Si2O5 (JCPDS Database No. 00-040-0376). Fig. 5A relating to the heat treatment at the first crystallization temperature illustrates that the major crystallized phase in this step is LM phase. In Fig. 5A, a small difference between GB(UV) and GB can be due to the silver clusters role in nucleation process. Besides, there is no obvious change in the case of GT before and after UV radiation because of the UV-shielding ability of TiO2. Moreover, comparing GB and GT, it seems that the increase of viscosity by Ti4+ might lead to abate the relative content of LM phase. These results based on the DTA observations were expected. Despite of the TiO2-containing glasses, it can be seen that the relative content of LM phase in the sample with the phosphorous is increased after irradiation. Furthermore, in the presence of P2O5 the level of formed LM phase is higher than the parent glass. Therefore, it is probable that P2O5 has played the role of nucleating agent via Li3PO4 phase in this glass system, but maybe the content of 2 wt.% P2O5 is not enough to be detected by XRD and to prove the presence of Li3PO4 phase. Based on Fig. 5B, the second exothermic peak in DTA is attributed to LD phase while this phase is observed only in the cases GP and GP(UV). It can be concluded that the silver clusters in the nucleation process of the parent glass is not singly capable enough; however, when the 2 wt.% P2O5 content is added LD phase can be formed. In addition, the relative content of LM phase in GB(UV) is elevated slightly rather than GB due to the silver clusters role, whereas the relative content of this phase for the sample containing TiO2 remained unchanged. On the other side, according to the increased LM phase in the presence of P2O5 content at the first crystallization temperature, it seems that both nucleating agents of the silver clusters and also Li3PO4 crystals
Fig. 5. The XRD patterns of the lithium silicate glass ceramics after heat treatment at: (A) the first crystallization temperature, (B) the second crystallization temperature.
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Fig. 6. SEM images of the lithium silicate glass ceramics at: (A) the first crystallization temperature, (B) the second crystallization temperature. (1) GB, (2) GB(UV), (3) GT, (4) GT(UV), (5) GP, (6) GP(UV).
altogether led to raise the precipitation of LM phase and eventually to grow LD phase at the expense of consuming LM phase. Fig. 6A and B show the microstructures of the lithium silicate glass ceramics at the first and the second crystallization temperatures, respectively. Regarding Fig. 6A, the SEM images illustrate LM main phase in two different morphologies; the needle-like and the dendrite-like crystals. The former is observed at the samples of GB, GB(UV), GT, and GT(UV) and the latter is found in the samples containing P2O5 which is an indicator of difference between the sources of nucleation. As this phase is more readily dissolved in HF in comparison to silica glass matrix, the surface pits resulting from HF etching process reflect the microstructure of LM crystals. After the second crystallization step in Fig. 6B, needle-like LM phase grew up and turned to the rod-like crystals in both the parent and the TiO2-containing glasses. It should be noted that it is not clear whether small or nanometric LD phase is formed or not after UV radiation in these two samples but it can be seen that LM crystals only in the parent glass are greater than before. On the other side, the microstructure of P2O5-containing samples is changed totally and an interlocking microstructure related to LD phase is successfully formed. 4. Conclusions The parent photosensitive glass and glass-ceramic in the system of lithium silicate containing AgCl under radiation of UV light were successfully prepared by melting process to reveal the effects of adding TiO2 and P2O5 as nucleating agents on the solarization and crystallization of lithium silicate glass. The results showed that UV radiation could turn the ionic silvers to the silver clusters in both parent glass
and the glass containing 2 wt.% P2O5. While in presence of 4 wt.% TiO2, the UV-shielding effect happened and led to hamper generation of the induced bands in visible region. Also, despite of adding P2O5 which could reduce the crystallization temperatures, TiO2 additive increased them. In addition, presence of P2O5 content could elevate LM phase at the first crystallization temperature; consequently, it led to raise the precipitation of LM phase and eventually to grow LD phase at the expense of consuming LM phase. However, TiO2 additive causes poor crystallization performance. At the end, only in the samples containing P2O5 an interlocking microstructure related to LD phase is successfully formed.
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