Crystallization Behavior of New Transparent Glass-Ceramics Based on Barium Borate Glasses

Journal of the Ceramic Society of Japan 116 [5] 624-631 2008 Paper

Crystallization behavior of new transparent glass-ceramics based on barium borate glasses

Fatma Hassan MARGHA,*,** Salwa Abdel-Hameed Mohamed ABDEL-HAMEED,* Nagwa Abd El-Shafy GHONIM,* Shigeo SATOKAWA**,† and Toshinori KOJIMA**

*Glass Research Department, National Research Center, Dokki, Cairo 12622, Egypt **Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, Tokyo 180-8633, Japan

This paper describes the preparation of several new transparent and very fine crystal glass-ceramics from the BaO–B2O3 system utilizing an appropriate additive of fluorides, partial replacement of B2O3 by SiO2, and introducing nucleating agents, such as TiO2. The physical properties of the prepared materials and the changes with varying base glass compositions and heat treatment programs were investigated. The thermal behavior and microstructure of the developed phases were characterized using DTA, XRD, and SEM. Glass-ceramics with marked transparency were prepared. These transparent derivatives owe their transparency to the distinctive properties of the nano-crystalline samples. The dielectric constant of transparent glass- ceramics samples at 100 kHZ were between 14–20, which is very suitable for a wide range of applications, such as the high- – speed switching of large-scale integrators. It was found that the addition of F and SiO2 greatly influenced the transparency of the produced glass-ceramics. Also, the addition of TiO2 greatly enhanced transparency, in spite of increasing cutoff in the UV region to a higher wavelength. ©2008 The Ceramic Society of Japan. All rights reserved.

Key-words : Glass-ceramics, Transparent, Barium borate, Dielectric

[Received December 8, 2007; Accepted March 21, 2008]

ride crystal phase, offer an economical alternative with substan- 1. Introduction tial performance improvements over fluoride glasses.4) Oxyfluo- Transparent glass-ceramics have emerged as important mate- ride glasses have created much interest as host materials for rials for electro-optics, solid-state lasers, and zero coefficient of active optical devices, such as lasers and optical amplifiers. thermal expansion glass-ceramics used in gas laser systems as The field of non-linear optics is attracting growing attention laser envelopes and in telescope mirrors.1) Glass-ceramic mate- around the world because of its application to telecommunica- rials may transmit visible light if either of the following condi- tions and possibilities for optical information storage and com- tions is operative: (i) the crystallites of all types are much smaller puting. The low-temperature β-phase of BaB2O4 (BBO) plays an than the wavelength of visible light; or (ii) the optical anisotropy important role in non-linear optical devices. Single crystals of (birefringence) within the crystals and refractive index difference BBO were discovered by Chen et al.5),6) This is an interesting between crystals and glass are very small. So transparent glass- material characterized by a wide range of transparency in both ceramics generally have two distinctive properties: they are the ultraviolet and the near infrared regions. Its large birefrin- nanocrystalline, and they have greater thermal stability than their gence allows phase matching for many harmonic generation pro- parent glasses.2) The advantage of transparent and translucent cesses, and it has been used in particular to generate the second glass-ceramics over polycrystalline light-transmitting substances harmonic of Nd:YAG.7) These properties indicate that BBO is formed by conventional ceramic processes lies in the capacity potentially useful in many applications, especially in the ultravi- and diversity of the glass-forming processes. It is also easier to olet region. However, single crystals are costly. achieve transparency in a polycrystalline article through con- Barium metaborate shows good promise for frequency conver- trolled nucleation and by growing the nanocrystals in glass than sion in optical waveguides and fibers. Transparent BaTiO3 glass- by any other ceramic process. ceramics have very important potential application as ferroelec- Oxyfluoride glass-ceramics are another of the new hybrid tric materials in optics and electronics, especially in capacitor 8) optical materials that provide the optical properties (low phonon and sensor technologies. Also Ba2TiSi2O8 (BTS) is a polar energy) of a fluoride crystal, and the durability, and mechanical glass-ceramic, or polar-oriented glass-ceramic, which has been and physical properties of an oxide glass. They are actually well investigated for its piezoelectric, pyroelectric, and non-lin- 10,000 times more durable than fluoride glass. Therefore, the ear optical properties.9) properties of oxyfluoride glass-ceramics offer great advantages In this study, we investigated the synthesis methods for trans- over fluoride glass-ceramics, which are characterized by being parent glass-ceramics based on barium borate glasses, offering very expensive, toxic, corrosive, unstable, and must be processed optical and electrical properties for technical applications. in a dry oxygen-free atmosphere.3) Further, oxyfluoride glass- 2. Experimental ceramics, in which the optically active ions partition into a fluo- Reagent grade H3BO3, BaCO3, TiO2, SiO2, and/or BaF2 were † Corresponding author: S. Satokawa; E-mail: [email protected]. well mixed as powder batches. Table 1 shows the batch compo- jp sitions tested in the present work. This mixture was melted at

Table 1. Chemical Composition of Glass Samples in mass % used to measure the dielectric constant (έ) at room temperature.

Samples BaO B2O3 BaF2 TiO2 SiO2 3. Results and discussion B1 60.29 39.71 — — — 3.1 Crystallization of glass ceramics BT1 56.36 37.11 — 6.53 — 3.1.1 Thermal properties of mixed glasses BTF1 43.06 36.46 14.07 6.41 — Figures 1 and 2 show the DTA curves of all the glasses. The BT1S1 57.67 20.99 — 6.68 14.66 glass transition temperatures (Tg) of B1, BT1 and BTF1 were ° ° ° BT1S3 58.8 7.13 — 6.81 27.26 588 C, 572 C, and 551 C, respectively (Fig. 1). It was observed that Tg of BT1 was shifted to a lower temperature by adding 6.5% of TiO2 in the base glass (B1). This result shows that TiO2 acts as a nucleating agent and decreases the viscosity of the glass. Table 2. Heat-treatment Schedule of the Prepared Samples The Tg of BTF1 was also shifted to a lower temperature by add- – Heat-treatment ing F in the BaF2 with TiO2. The low temperature shift of Tg Sample Temperature/°CTime through the addition of BaF2 and TiO2 in the base glass may be caused by the decrease in the viscosity of the glass. The exother- 550 30 h mic peak of B1 was also shifted to a lower temperature in the B1 595 30 h DTA curve of BTF1. This suggests that the addition of BaF2 and 715 5 min TiO2 decreases the crystallization temperature of BaO–B2O3 550 30 h glass-ceramics. 595 30 h The glass transition temperatures (Tg) of BT1, BT1S1, and BT1 BT S were 572°C, 605°C, and 638°C, respectively (Fig. 2). It 740 5 min 1 3 was observed that Tg of BT1 was shifted to a higher temperature 740 then 765 5 min and 5 min by partial replacement of B2O3 by SiO2. The exothermic peaks 550 30 h were also shifted to higher temperatures in BT1S1 and BT1S3. 570 30 h This suggests that the replacement of SiO2 increases the viscosity BTF1 705 5 min of the base glass due to an increase in the crystallization temper- 705 1 h ature of BaO–B2O3 glass-ceramics. 550 30 h 595 30 h BT1S1 795 5 min 795 4 h 550 30 h 645 30 h BT1S3 810 5 min 810 2 h

1100°C for 2 h in air in a Pt crucible in an electric furnace. The as-cast glass was immediately annealed at 400–500°C and slowly cooled to room temperature. The annealed glass was col- orless and transparent. Table 2 shows the heat-treatment schedules of the prepared samples. Many different heat-treatment schedules were performed with various temperatures, rates, and Fig. 1. DTA traces of B1, BT1 and BTF1. times until transparent glass ceramics were obtained. DTA curves were recorded on powdered specimens of about –1 150–250 mg under N2 gas at a heating rate of 10 Kmin , with Al2O3 as a reference material, using a 7 series Perkin–Elmer, and the glass transition (Tg) and crystallization temperatures were extracted using DTA instrument software. Identification of crystalline phases was carried out by X-ray diffraction using a Bruker D8 Advance diffractometer (Cu Kα radiation) through examina- tion of fine powders of the glasses before and after heat-treatment. The surfaces of glass-ceramic samples were observed using a scanning electron microscope (SEM) and a JEOL–840A electron probe microanalyzer after coating with gold. Optical transmission in the UV–vis region was analyzed using a JASCO V350 spectrophotometer with samples in a thickness range of 0.245–0.29 cm. The dielectric constant of the samples was mea- sured at 100 kHz. The samples were prepared in the form of discs 10 mm in diameter and 1–2.4 mm in thickness. A Philips R.L.C bridge type PM 6304 programmable automatic RLC meter was Fig. 2. DTA traces of BT1, BT1S1 and BT1S3.

625 JCS-Japan Margha et al.: Crystallization behavior of new transparent glass-ceramics based on barium borate glasses

3.1.2 Crystallization behavior of glass-ceramics XRD patterns of heat-treated B1 glass samples are shown in Fig. 3. The heat-treated B1 at 595°C for 30 h (Fig. 3b) was clearly different from the B1 heat-treated at 550°C for 30 h (Fig. 3a) and revealed slight crystallization of β-BaB2O4 and Ba4B2O7. Increasing the temperature to 715°C for 5 min revealed the crystallization of the above-mentioned phases with high intensities (Fig. 3c). Scanning electron microscope (SEM) observation of the glass heat-treated at 595°C/30 h showed volume crystallization of uniform microcrystalline structure of very fine, anhedral crystals (~250 nm) as shown in Fig. 4a. Figure 4b shows spherical crystallites of β-BaB2O4 in between fiber bundles of Ba4B2O7 crystals. XRD patterns of heat-treated BT1 glass samples are shown in Fig. 5. XRD pattern of heat-treated BT1 at 595°C for 30 h (Fig. 5b) showed slight crystallization in some phases. Heat-treatment at 740°C (i.e. at the 1st exothermic peak for 5 min) did not promote the crystallization of these crystalline phases (Fig. 5c). Thermal treatment of this glass at 740°C for 5 min followed by 765°C for 5 min resulted in the crystallization of BaB2O4 as a major phase, followed by BaTi(BO3)2 and Ba4B2O7 phases, as shown in Fig. 5d. SEM observation revealed that the texture of the transparent sample heat-treated at 595°C for 30 h showed very fine spherical crystal dispersed in glassy matrix (Fig. 6a). The sample heat- treated at 740°C for 5 min showed interlocked needle-like crystals of BaTi(BO3)2 embedded in pyramids-like crystals of barium borate (Fig. 6b). Double stage heat-treatment at (740°C for 5 min + 765°C for 5 min) produced fine grains of rounded crystals of barium borate and needle-like crystals of barium titanate (Fig. Fig. 4. SEM of B1 heat treated at (a) 595°C for 30 h and (b) 715°C for 5 min.

Fig. 3. XRD patterns of B1 glass heat-treated at (a) 550°C for 30 h, (b) Fig. 5. XRD patterns of BT1 glass heat-treated at (a) 550°C for 30 h, 595°C for 30 h and (c) 715°C for 5 min. (b) 595°C for 30 h, (c) 740°C for 5 min and (d) (740 + 765)°C for 5 min.

626 Journal of the Ceramic Society of Japan 116 [5] 624-631 2008 JCS-Japan

Fig. 7. XRD patterns of BTF1 glass heat-treated at (a) 550°C for 30 h, (b) 570°C for 30 h, (c) 705°C for 5 min and (d) 705°C for 1 h.

Fig. 6. SEM of BT1 heat treated at (a) 595°C for 30 h, (b) 740°C for 5 min and (c) 740°C for 5 min + 765°C for 5 min.

6c). Figure 7 shows the XRD patterns of heat-treated BTF1 glass samples. The crystallization of BTF1 glass sample started just after heat treatment at 570°C for 30 h. The XRD pattern of heat- treated BTF1 glass sample at 705°C for 1 h revealed the fully crystallization of BaF2, Ba4B2O7, and BaTi2O5. SEM observation revealed that the texture of the transparent sample heat-treated at 570°C for 30 h showed ultra-fine grains (~0.4–0.9) μm of the crystallized phases embedded in the glassy matrix (Fig. 8a). The above unshaped crystals were expected to grow into star-like ° crystals when the sample was heat-treated at 705 C for 5 min Fig. 8. SEM of BTF1 heat treated at (a) 550°C for 30 h and (b) 705°C (Fig. 8b). This sample was still transparent after being heat- for 5 min.

627 JCS-Japan Margha et al.: Crystallization behavior of new transparent glass-ceramics based on barium borate glasses

Fig. 9. XRD patterns of BT1S1 glass heat-treated at (a) 550°C for 30 h, Fig. 11. XRD patterns of BT1S3 glass heat-treated at (a) 550°C for (b) 595°C for 30 h, (c) 795°C for 5 min and (d) 795°C for 4 h. 30 h, (b) 645°C for 30 h, (c) 810°C for 5 min and (d) 810°C for 2 h.

° treated at 705 C for 5 min. The SiO2 content in BT1S3 glass sample increased to ~27%. The crystallization of BT1S1 glass sample in which ~15% of The XRD pattern of BT1S3 glass sample heat-treated at 645°C B2O3 was replaced by SiO2 started just after heat treatment at for 30 h shows the start of crystallization. Heat-treatment at ° 595 C for 30 h (Fig. 9b). XRD patterns of the transparent sam- 810°C for 5 min led to the crystallization of Ba4B2O7 and ° ples heat-treated at 795 C for 5 h reveal the fully crystallization Ba2TiSi2O8. Increasing the soaking time to 2 h led to the crystal- of Ba2TiSi2O8 and Ba4B2O7, as shown in Fig. 9d. lization of Ba2TiSi2O8 and Ba4B2O7, with more pronounced SEM observation revealed the texture of sample heat-treated peaks (Fig. 11). at 595°C for 30 h exhibited volume crystallization of feather-like Figure 12 shows the SEM-observed texture of a transparent crystals of barium titanium silicate in-between minute granules sample heat-treated at 645°C for 30 h, with the formation of only of barium borate (~0.5 μm). The crystalline phases are embed- minute crystals of barium borate embedded in the glassy phase. ded in glassy phase (Fig. 10). Thermal-treatment of sample The transparent sample heated at 810°C for 5 min exhibited BT1S1 at 795°C for 5 min led to uniform, volume crystallization largely crystalline material of uniform ultra-fine grained micro- of ultra-fine grained crystals of the above said phases. structure of Ba2TiSi2O8 and Ba4B2O7. 3.1.3 Effect of fluoride ion addition In general, after thermal treatment, it is possible to obtain a glass-ceramic, in which fluoride nanocrystals are embedded in a primarily oxide glass-matrix. Moreover, such glass-ceramics remain highly transparent in spite of the high volume fraction of the nanocrystal phase (20–30%).10) – In this study, the incorporation of BaF2, or more precisely, F ions, into the glass, significantly reduced the viscosity of the melts and glasses, as well as both the melting and crystallization onset temperatures. These results may be explained on the following basis: 1) The reduction in the viscosity of the polymerized melts occurs due to the substitution of fluorine for bridging oxygen with consequent de-polymerization of these melts.11) In accord with the above mechanisms, the fluoride ions may act as network breakers in the glass anionic structure. The bridging oxygen can be replaced by fluoride ions due to their radius sim- Fig. 10. SEM of BT1S1 heat treated at 595°C for 30 h. ilarity, which leads to the weakening of the glass network struc-

628 Journal of the Ceramic Society of Japan 116 [5] 624-631 2008 JCS-Japan

gives a more polymerized structure, increasing the glass stability with respect to devitrification.15) On the other hand, the positive action of TiO2, as a nucleating agent in promoting the crystallization process is as follows: Pernice et al.16) noticed that the thermal treatment of barium borate glasses containing TiO2 leads in the first stage to formation of two micro-regions: one enriched in polymerized units, and the other enriched in depolymerized units. Also, it can promote a metastable glass-in-glass phase separation with a following formation of crystalline nuclei, which may be the starting point for the internal crystallization of the entire volume. The amorphous microphases resulting from such chemical compositions approach the future crystalline phases.17) In such unmixed regions, crystal nucleation, as a precursor for crystallization, is much higher than in a homogeneously struc- tured monolithic base glass. This is because in such unmixed regions, the elements of nucleation are already present at the site of development. (The activation energy for nucleation, which entails the diffusion of ions, is not necessary in this case, and thus the nucleation speed increases). In this work, it appears that TiO2 performed its role as a nucleating agent where the presence of TiO2 led to a decrease in Tg, as shown in Fig. 1, and thus the nucleation temperature, and also enhanced the bulk crystallization (Figs. 6c, 8b, 10, 12) (sample 18) without TiO2 crystallized through surface crystallization). Nucleation may proceed via glass-in-glass phase separation, as in the above-mentioned studies, where no separation of TiO2 crystals occurs at low temperature. From the above, the presence – of both TiO2 and F (as in the sample BTF1) shows the double effect of these nucleating agents on lowering the viscosity of the – glass. Thus, in samples containing F , as in BTF1, the high activation energy phase (barium titanate) is precipitated more easily than in BT1, in which low activation energy (barium titanium Fig. 12. SEM of BT S heat treated at (a) 645°C for 30 h and (b) 1 3 borate) is precipitated. 810°C for 5 min. 3.1.5 Effect of adding SiO2 in place of B2O3 The partial replacement of B2O3 by SiO2 in this glass was expected to affect the structure, and hence the property of the ture. This is also reflected in facilitating the melting process at obtained glasses, where silica introduces stronger SiO4 groups, lower temperatures and in reducing the viscosity of the resultant which lead to more compactness of the internal structure. Thus melts and glasses. As a result of the decrease in viscosity, the dif- an improvement in the chemical and mechanical properties was fusion and mobility of the different ions and structural groups in anticipated. The introduction of silica led to a rise in the temper- the glass during the crystallization process is markedly increased. ature at which thermal treatment should convert glass to their 2) Fluorides, in large amounts, are also known to be immiscible corresponding glass-ceramics, as shown in Fig. 2. Ba2TiSi2O8 in melts leading to glass-in glass phase separation, wherein was precipitated at higher temperatures in the samples containing numerous droplets of one glass phase dispersed in another phase more SiO2. Introducing silica did not affect the production of are formed. The formation of such phase separation decreases nano-sized microcrystals obtained from the base borate glass. the energy barriers that are necessary for crystallization. How- The uniform microstructure of SiO2 containing glass-ceramics ever, phase separation alone may not be sufficient to induce vol- seems to be the most suitable for the production of transparent ume (internal) crystallization in the glass. In some cases, glasses glass-ceramics. that have a tendency for phase separation, exhibit opalescence during heat-treatment, and do not show volume crystallization.12) 3.2 Properties Due to the weakening of the glass structure, the fluoride also 3.2.1 Transmission measurements modified the kinetic barriers for nucleation and growth of crys- The UV-Visible transmission curves of glasses and their cor- tals, which can take place at lower temperatures. The data responding transparent glass-ceramics heat-treated at the nucle- –1 obtained from the present glasses showed that a BaF2 phase (flu- ation temperature for 30 h by rate 3 Kmin are presented in Fig. orite) was always separated out at the initial stages of crystalli- 13. These samples were heat-treated at the nucleation tempera- zation. The crystallization of this phase is verified by the minor ture, and the UV-Visible measurements were again carried out low-temperature DTA exotherm and XRD analysis. This is in without further polishing. These procedures were applied agreement with the above-mentioned studies, whereby a simple because in almost all glasses, surface crystallization is expected binary fluoride phase was at first separated out from the glass to take place. Since transparency is an important requirement for which then heterogeneously nucleated the main crystallized specified glass-ceramics in optical applications, only the trans- phase(s).13),14) mission curves for the transparent glass-ceramics were investi- 3.1.4 Effect of TiO2 addition on crystallization behavior gated. It is generally accepted that TiO2, acting as a glass former, Figure 13a shows the UV-Visible spectrum of base glass com-

629 JCS-Japan Margha et al.: Crystallization behavior of new transparent glass-ceramics based on barium borate glasses

Fig. 13. UV-Visible transmission curves of (a) B1, (b) BT1, (c) BTF1 and (d) BT1S3 samples.

3– pared with the spectrum of the corresponding transparent glass- crystalline BBO, the (B3O6) boroxyl ring has been identified as ceramics. The transmission curves of the heat-treated glass- the microscopic unit giving the material non-linearity. The high ceramics samples are slightly shifted towards higher wave- wavelength shift of the UV cutoff through adding TiO2 is thought lengths. Fifty percent transmission occurs at approximately 315 to result from charge transfer in the Ti–O bond, responsible for nm, and more than 90% transmission occurs for wavelengths the yellow-brown coloration of Ti-bearing glasses. In some sam- higher than approximately 600 nm. Both glass and glass-ceram- ples, the transparency of the glass-ceramics is higher than that of ics samples show ultraviolet cutoffs at 250 nm. This cutoff is glass. The reason for this disparity is that the Rayleigh scattering somewhat longer than that of single crystal BBO at 190 nm, law applies only to systems in which the scattering centers are comparable to that of urea at 210 nm and shorter than that of noninteracting and interference between scattered photons is not KTP at 350 nm.1) significant. This is not the case for most transparent glass ceram- The transmission curves for both glass and glass-ceramics of ics, or phase-separated glasses in which the scattering centers are BT1 seems identical, as shown in Fig. 13b, where 50% transmis- spaced in the order of 10–50 nm, or 10 times smaller than the sion occurs at 350 nm for glass and glass-ceramics and more than wavelength of visible light. Tick et al.,19) demonstrated that the 90% transmission occurs at wavelengths higher than 520 nm, angular dependence of scattering in these materials is not Ray- which is lower than that of B1. Cutoff for glass in the UV region leigh in nature. Only the BTF1 and BT1S3 samples showed better is at 325 nm and this value is shifted to 225 nm for glass-ceramic transparency than the corresponding glass samples. Comparing derivatives. Thus, the presence of TiO2 seems to slightly influ- these two samples, it could be concluded that BT1S3 is the best ence the transparency of glass-ceramics. sample as 50% transmission was obtained at a lower wavelength. In BTF1 glass composition ~14% BaF2 replaced BaO in BT1 3.2.2 Dielectric constatnt – to study the effect of both TiO2 and F . Fig. 13c shows that the The dielectric constant of selected samples is illustrated in transparency of glass-ceramics is superior to that of the base Table 3. In general the dielectric constant of all samples at 100 glass. The transmission curve of BTF1 glass-ceramics is quite kHz is small, ranging from 15.78–18.72. It was noticed that add- – similar to that of BT1 glass-ceramics, where 50% transmission ing F in the form of BaF2 led to an increase in έ, which reflected occurs at wavelengths higher than 406 nm. Ultraviolet cutoffs for the effect of F– in weakening the network structure as described both glass and glass-ceramics occur at 326 nm as for the BT1 earlier. Samples containing TiO2 had the same effect of increas- composition. The composition of the glass sample BT1S3 is similar to BT1 Table 3. Dielectric Results (έ) of Some Transparent Glass-ceramics at but 27% of SiO2 was added at the expense of B2O3. In general, the transmission curves of this composition were shifted to Different Frequencies slightly higher wavelengths than the BT1 sample. Ultraviolet cut- F(frequency) B1 BT1 BT1F1 BT1S3 offs for both glass and glass-ceramics occur at 331 nm, which is 100 Hz 25.59 30.99 27.94 31.05 slightly higher than that of BT1, and 50% transmission is 120 Hz 25.09 30.07 26.94 29.72 obtained at a wavelength of 360 nm, while more than 90% transmission occurs at wavelengths higher than 530 nm (Fig. 13d). 1 kHz 19.74 23.35 21.19 23.42 The transparency for crystalline BBO in the UV regime is due 10 kHz 16.75 19.63 17.99 19.86 to the large electronegativity difference in B–O bonds1) where, in 100 kHz 15.78 18.43 16.94 18.72

630 Journal of the Ceramic Society of Japan 116 [5] 624-631 2008 JCS-Japan

ing έ but this effect was lower than that of F–. Paper, XIII IQEC, paper MCC5 (1984). The combination of transparency and low dielectric in glass 6) C. Chen, B. Wu, A. Jiang and G. You, Sci. Sinica B, 28, 235– ceramics is considered to have many applications, such as a fer- 243 (1985). roelectric material in optics and electronics, especially in capac- 7) K. Kato, IEEE J. Quantum Electron, 22, 1013–1014 (1986). itor and sensor technology. 8) M. Todorovic, L. J. Radonjic and J. Dumic, Key Eng. Mater., 132–136, 193–196 (1997). 4. Conclusion 9) M. Zhu, B. Wang, R.i Wang, S. Sun, H. Yan and Z. Ding, J. Non-Cryst. Solids, 324, 172–176 (2003). Transparent glass ceramics based on barium borate glasses 10) J. Mendez-Ramos, V. Lavin, I. R. Martin, V. R. Rodriguez– were prepared. The transparency was due to the fine crystals Mendoza, J. A. Gonzalez–Almeida, V. D. Rodiguez, A. D. – formed. It was found that the addition of F and SiO2 greatly Lozano–Gorrin and P. Nunez, J. Alloys and Comp., 323–324, influenced the transparency of the glass-ceramics produced. 753–758 (2001). Also, the addition of TiO2 greatly enhanced transparency, in spite 11) D. B. Dingwell, C. M. Scarfe and D. Cronin, Am. Mineral., 70, of increasing cutoff in the UV region to higher wavelengths. 80–87 (1985). The dielectric constant of the transparent glass-ceramic sam- 12) Z. Strand, “Glass-Ceramics Materials,” Glass Science Tech- ples at 100 kH was between 15–18, the best sample being B nology, 8, Elsevier, Amsterdam (1986). Z 1 13) P. W. McMillan, “Glass-ceramics 2nd ed.,” Academic press, because it had the lowest value, making it very suitable to many London (1979). applications, such as the high-speed switching of large-scale 14) D. R. Uhlmann, “Nucleation and Crystallization in Glasses,” integrators. Am. Ceram. Soc. Columbus, Ohio (1982). 15) W. A. Weyl and E. C. Marboe, “The Constitution of Glasses II,” John Wiley and Sons. Inc., New York (1962). References 16) P. Pernice, S. Esposito, A. Aronne and V. N. Sigaev, J. Non- 1) Y-H. Kao, Y. Hu, H. Zheng, J. D. Mackenzie., K. Perry, G. Cryst. Solids, 258, 1–10 (1999). Bourhill and J. W. Perry, J. Non-Cryst. Solids, 167, 247–254 17) A. W. A. El-Shennawi, A. A. Omar and A. M. Morsy, Ther- (1994). mochimica Acta, 58, 125–153 (1982). 2) G. H. Beall and L. R. Pinckney, J. Am. Ceram. Soc., 82, 5–16 18) S. A. M. Abdel-Hameed, N. A. Ghoniem, E. A. Saad and F. (1999). H. Margha, Ceram. Inte., 31, 499–505 (2005). 3) M. J. Dejneka, J. Non-Cryst. Solids, 239, 149–155 (1998). 19) P. A. Tick, N. F. Borrelli, L. K. Cornelius and M. A. Appl. Phys. Lett. 63 4) Y. Wang and J. Ohwaki, , , 3268–3270 Newhouse, J. Appl. Phys., 78, 6367–6374 (1995). (1993). 5) C. Chen, B. Wu, G. You, A. Jiang and Y. Huang, Dig. Tech.

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