Synthesis and characterization of chloroborosilicate glasses in the K2O–BaO–Al2O3–B2O3–SiO2–BaCl2 system

Journal of Non-Crystalline Solids 398–399 (2014) 32–41

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Synthesis and characterization of chloroborosilicate glasses in the K2O–BaO–Al2O3–B2O3–SiO2–BaCl2 system Nilanjana Shasmal, Atiar Rahaman Molla, Basudeb Karmakar ⁎ Glass Science and Technology Section, Glass Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700 032, India

a r t i c l e

i n f o

Article history: Received 26 February 2014 Received in revised form 11 April 2014 Available online xxxx Keywords: Choloroborosilicate glass; X-ray diffraction; Transmission/absorption spectra; BaCl2 nanocrystal

a b s t r a c t Generally borosilicate glasses are rarely used in photonic applications. Here we modified borosilicate glass by incorporating BaCl2 with a view to their application in the area of photonics. Chloroborosilicate glasses in the system with general composition (mol%) (100 − x)(42SiO2–30B2O3–20BaO–4K2O–4Al2O3)–xBaCl2 (where x = 0–30) were prepared by the melt quench technique. All the glasses were found to be X-ray amorphous and transparent. They were characterized by X-ray diffraction, infrared and UV–vis-NIR spectral analyses, differential scanning calorimetry, dilatometry, refractive index, dielectric constant measurements, etc. Their glass transition temperature (Tg), coefficient of thermal expansion (CTE), refractive index and dielectric constant have been found to vary in the ranges 614–641 °C, 73–91 × 10−7 K−1, 1.58–1.63 and 9.79–11.73 respectively. All the properties are found to be controlled by BaCl2 content. Their covalent character, metallization criterion and Abbe number were found to decrease with increasing BaCl2 content whereas the molar volume, average molar refraction and electronic polarizability increase with increasing BaCl2 content. The devitrified materials have been characterized by XRD which manifests the presence of BaCl2 nanocrystals of sizes 15–30 nm. Thus these glasses have the potential to develop BaCl2 nanocrystal containing low phonon energy (~350 cm−1) glass-ceramics and are promising for different photonic applications. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Now-a-days an increasing interest is being observed to halide glasses, especially fluoride glasses, in both scientific and technological aspects. The scientific interests are mainly due to the different kinds of structure of glasses as compared to conventional oxide glasses. Technical interests are because of their potential use for making infra-red optical components and ultra-low-loss optical fibers. In contrast to the great deal of recent works on fluoride glasses, there is relatively little information on glasses based on the other halides (chlorides, bromides, iodides and mixtures of them) [1–5]. The history and present developments of glass chemistry are largely dominated by oxide systems. In order to form stable glasses, strong three dimensional covalent bonds are required to be present, which are found to exist in small highly charged cations like Si4+, B3+, and P5+. The situation is quite different for halides (X = F, Cl, Br, I), which are being the most electronegative elements, when associated with a metal, show the tendency to form a purely ionic bond and as a consequence crystalline phases form. So the challenge involved in obtaining a glassy state is to suppress the crystallization tendency so that the glass network formation can take place. This challenge is very difficult and a few metal halides have overcome it giving a 3D framework ⁎ Corresponding author. E-mail address: [email protected] (B. Karmakar).

http://dx.doi.org/10.1016/j.jnoncrysol.2014.04.017 0022-3093/© 2014 Elsevier B.V. All rights reserved.

stabilized by strong covalent bonds. This kind of difficulty is responsible for their recent discovery and absence in naturally occurring materials [6–9]. Although glasses based on ZnCl2 were first observed over 80 years ago, the scope of halide glass science was narrowly restricted to those materials for several decades. Vitreous ZnCl2 is one of the best known chlorides. With some mixed halide systems like ZnCl2–KI glass formations have been observed [6,10]. Practical application of ZnCl2 glass is difficult due to its highly hygroscopic nature. CdCl2 has also been reported as a glass-forming material (e.g. in CdCl2–BaCl2–NaCl or CdCl2–CdF2–BaF2 systems). But only thin hygroscopic samples have been obtained as the system requires very high cooling rate to avoid the fast crystallization [11]. The glass forming ability of BiCl3 and ThCl4 has been reported in systems like BiCl3–KCl and ThCl4–NaCl–KCl etc. [12]. In some systems, chlorides are combined with modifiers for synthesis of glasses. But they also exhibit very poor chemical durability in the presence of moisture. The most stable composition includes both chlorides and fluorides in ternary or more complex combinations like CdCl2–BaCl2 and CdCl2–BaF2 systems [1,6]. ZnBr2 and CdI2 have been described to act as glass former and they form far-IR transmitting glasses. Addition of PbCl2, PbI2, TlI, and CsI increases their chemical durability. But as their glass transition temperature is very low, they exhibit very poor stability [1,13]. Therefore it is evident that the biggest challenge for halide glasses is their poor chemical resistance to moisture. This is partially overcome in the oxychloride system Sb2O3–PbCl2–ZnCl2. For this system the glass

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degrades in the presence of moisture but in a very small amount as compared to the halide glasses [14]. Oxychloride glasses have been investigated in the Sb2O3–CdCl2–SrCl2 system. The addition of ZnCl2 enlarges the vitreous domain. This is a fairly stable system having an average Tg of around 300 °C [15]. Recently the properties of 70TeO2–10Sb2O3– 20PbCl2 composition are reported and discussed by Bošák et al. It was observed that the lead–antimony–tellurium oxychloride system forms stable glasses in a broad range of compositions [16]. Ternary glasses have been synthesized and studied in the Sb2O3–PbCl2–MoO3 system. Compositional limits of glass formation are reported. High Sb2O3 containing glasses show Tg close to 290 °C and decreases when it is substituted by MoO3 or PbCl2 [17]. The glass-forming regions in the PbCl2–PbBr2–P2O5 system were explored and glasses were prepared [18]. These results indicate that in a stable glass system, if halides are incorporated, the problem of its instability can be resolved to some extent. In such systems chlorides may work as a glass former without affecting the stability of the glass. Hoell et al. reported that a small quantity of chloride (0.47% NaCl) in 13Na2O–11CaO–76SiO2 (mol%) glass system causes a dramatic change of kinetics and equilibrium conditions for the phase separation process in the glass in comparison with a pure soda lime silica glass. The phase separation process is considerably accelerated and as a result volume fraction of the silica droplet phase is increased to the maximum. The effect is interpreted as a shift of the miscibility gap position by 45 K up to higher temperatures. The effect of chloride on the phase separation is mainly attributed to a rise of the miscibility gap, which increases the thermodynamic driving force [19]. Oxychloride glasses based on the TeO2–ZnO–ZnC12 system have also been prepared. This system provides a wide and stable glass formation range in which a Tc–Tg gap beyond 140 °C can be achieved [20]. Bulk oxyhalide glasses have been prepared in the TeO2–ZnO–ZnX2 and TeO2–PbO–PbX2 systems (X = Cl, Br, I). The glass-forming regions increased in the direction I b Br b Cl. These systems, in particular for X = Cl, yield stable glass of good optical quality [21]. Barium chloride–oxide tellurite glasses in the system BaCl2–BaO–TeO2 are studied by Sokolov et al. Continuous network of barium chloride–oxide tellurite glass has been proved to be formed [22]. Yb3+/Tm3+-co-doped oxychloride germanate glasses for developing potential upconversion lasers have been fabricated and characterized. Structural properties were obtained indicating that PbCl2 plays an important role in the formation of glass network and has an important influence on the maximum phonon energies of host glasses [23]. In this paper, glass forming ability of BaCl 2 is studied in a borosilicate glass SiO 2 –B 2 O 3 –BaO–K 2 O–Al 2 O 3 (mol%) system. A series of transparent and stable glass is obtained in the composition (in mol%) (100 − x)(42SiO2–30B2O3–20BaO–4K2O–4Al2O3)–xBaCl2 (where x = 0–25). They were characterized by X-ray diffraction, infrared transmission and UV–vis-NIR spectral analyses, differential scanning calorimetry, dilatometry, refractive index and dielectric constant measurements. Some other properties like molar volume, covalent character, electronic polarizability, Abbe number, dispersive power, and third order susceptibility have also been calculated using standard theoretical equations. 2. Experimental procedure 2.1. Glass preparation The raw materials were quartz, SiO2 (GR, Bremthaler, Quarzitwerk, Usinger. Germany), boric acid, H3BO3 (GR, 99%, Loba Chemie, Mumbai, India), barium carbonate, BaCO3 (GR, 99%, Fluka Chemie GmbH, Buchs, Switzerland), potassium carbonate, K2CO3 (GR, 99%, Loba Chemie, Mumbai, India), aluminum oxide, Al2O3 (GR, Aldrich Chemical Company Inc, Milwaukee 53233, USA) and barium chloride, BaCl2·2H2O (GR, Dehydrated extra pure Loba Chemie, Mumbai, India). They were used directly without any further purification. 60 g of glasses was prepared by melting the well-mixed batches of calculated composition in a

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high-purity platinum crucible at 1400 °C for 1.5 h with intermittent stirring for 0.5 min with a silica rod in air in a raising hearth electric furnace. All the molten samples were cast into an iron plate in air and annealed at 550 °C for 1 h in order to remove the residual thermal stresses, followed by a slow cooling down to room temperature. The monolithic glasses thus obtained were cut and polished into the desired shapes and sizes required for the different characterizations, as described below. 2.2. Characterization The densities (ρ) of the glass samples were determined by the standard Archimedes principle. The measurements were done using single pan balance and distilled water as an immersion liquid. The density was obtained from the relation. ρ ¼ aρx =ða−bÞ

ð1Þ

where a is the weight of the glass sample in air, b is the weight of the glass sample when suspended in distilled water (density of water, ρx = 0.997604 g·cm−3 at 25 °C). The X-ray diffraction (XRD) patterns of the bulk samples were recorded in an X'pert Pro MPD diffractometer (PANalytical, Almelo, the Netherlands), operating at 40 kV and 30 mA, using Ni-filtered CuKα radiation with the X'celerator, with a step size of 0.05°(2θ) and a step time of 0.5 s, from 10° to 80°. The coefficient of thermal expansion (CTE, α), glass transition temperature (Tg), and dilatometric deformation temperature (Td) was measured using a horizontal vitreous silica dilatometer (DIL 402C, Netzch-Gerä tebau GmbH, Bavaria, Germany) with a heating rate of 4 K/min taking a cylindrical sample of approximately 25 mm length and 5 mm diameter and heating it at a rate of 4 K/min up to the temperature where the glass softens, after calibration with a standard alumina supplied with the instrument by the manufacturer. The CTE values reported here are in the temperature range 50–150 °C. The CTE, Tg and Td values were reproducible with ± 1 ºC for all samples. The softening point (Ts) of each of the bulk samples was measured five times by a glass softening point system (Harrop/ Labino, Model SP-3A) and the average values are reported. They are accurate to ± 2 °C. The instrument was previously calibrated with a NBS (National Bureau of Standards, USA) standard glass of known softening point. It was measured by taking a glass block of approximately 5 mm length, 5 mm width and 3 mm thickness and heating it at a rate of 25 K/min up to the temperature where the glass softens. Differential scanning calorimetric experiment was performed by a Differential Scanning Calorimeter (NETZSCH Model STA 449 Jupiter F3, NETZSCH-Gerätebau GmbH, Selb, Germany) taking powdered sample within the temperature range of 30–900 °C in Nitrogen atmosphere at the heating rate of 10 K/min. The instrument was calibrated previously by a Suprasil-W silica glass (ε = 3.8). All the measurements were carried out at room temperature. The infrared transmission spectra (FT-IRTS) of the powdered samples were recorded with a Fourier transform infrared (FTIR) spectrometer (FTIR 1615, Perkin-Elmer Corporation, Norwalk, CT) by KBr pellet method, in the range of 400–4000 cm− 1 and at an incident angle of 15° and a resolution of ±1 cm−1 by taking 256 scans. The UV–Vis absorption spectra were obtained with a double-beam spectrophotometer (Lambda 950, Perkin-Elmer Corporation, Norwalk, CT). The uncertainty of the band position is ±0.1 nm. The refractive index was measured by prism coupler (Model 2010/M, Metricon Corporation, USA) using wavelengths 473, 532, 633, 1064 and 1552 nm. The dielectric constant was measured with an accuracy of ±0.5% at a frequency of 1 MHz using a LCR meter (Model 3532-50 LCR Hitester, Hioki, Japan) at 25 °C. All the measurements were carried out at room temperature. 3. Results and discussion The composition and some physical properties of the glasses in the series having composition (mol%) (100 − x)(42SiO2–30B2O3–20BaO–

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4K2O–4Al2O3)–xBaCl2 (where x = 0–30) are listed in Table 1. The glasses containing BaCl2 are transparent up to N7. N8 is faintly translucent while N9 and N10 are opaque crystalline material. The photographs of all the samples prepared are shown in Fig. 1(a). N9 and N10 could not be shaped due to their very brittle nature. The positions of the elaborately studied samples are clearly depicted in the ternary diagram, as shown in Fig. 1(b). Fig. 2 shows the EDX spectrum of sample N6 showing the inclusion of BaCl2 into the glass.

Fig. 3. The average crystallite diameter (d) was calculated using Scherrer's formula [24]. d ¼ 0:9 λ=FWHM cosθ ðpeakÞ

ð2Þ

where λ is the wavelength of X-ray radiation (CuKα = 1.5406 Å), FWHM is the full width at half maximum at 2θ. The average diameter of the crystallites present in N8–N10 varies in the of range 15–30 nm.

3.1. Density 3.3. Dilatometry and softening point measurement The densities of the glasses are in the range 3.16–3.74 g·cm−3. The density (ρ) values are monotonously increased with an increase in BaCl2 content. The values have been enlisted in Table 3. The increase in density is the effect of increasing BaCl2 content. The density of N1, which contains no BaCl2, is 3.16 g·cm− 3 and the density of BaCl2 is 3.86 g·cm−3. As we go from N1 to N10, a fraction of the lower density glass is being replaced by a higher density species. Thus the increase in density is due to the increasing mole fraction of BaCl2. As we go from N1 to N10, mole fraction of BaCl2 increases from 0 to 30. In coherence with the additive rule, the density increases with increasing amount of BaCl2 [8,9]. The densities were measured at different time intervals after long exposure to moisture and found to be unaltered even after immersing into water. This ensures their resistance to moisture as well as to water. 3.2. X-ray diffraction The formation of glasses and their devitrification are explicitly determined by the XRD analysis. Fig. 3 shows the XRD spectra of the samples N1–N10. The hump between 2θ = 20°–35° signifies the amorphicity of the glasses. The presence of such a hump and the absence of any peaks in samples N1–N7 indicate that they are amorphous in character and may be considered as glasses. The presence of sharp peaks, in addition to the hump in sample N10, indicates its devitrified nature. The XRD spectra of samples N8–N10 show prominent peaks at 2θ = 20.6362°, 23.8971°, 24.9087°, 27.6630°, 29.3410°, 33.9044°, 34.6379°, 35.3657°, 37.6002°, 39.4937°, 40.1628°, 44.8147°, 57.6878°, and 61.6117°. Peaks at 2θ = 20.6362°, 23.8971° and 33.9044° are due to diffractions from (111), (200) and (220) planes respectively of cubic crystalline phases of BaCl2 (JCPDS 70–1076). Peaks at 2θ = 24.9087°, 27.6630°, 37.6002°, 39.4937°, 44.8147° and 61.6117° are due to diffractions from (220), (211), (321), (002), (430) and (602) planes respectively of monoclinic crystalline phase of BaCl2 (JCPDS 83–0786). Peaks at 2θ = 34.6379° is due to diffraction from (220) plane of cubic crystalline phases of BaCl2 (JCPDS 24–0095). Peaks at 2θ = 35.3657° and 40.1628° are due to diffractions from (121) and (003) planes respectively of orthorhombic crystalline phase of BaCl2 (JCPDS 02–0794). Peaks at 2θ = 29.3410° and 57.6878° are due to diffractions from (202) and (413) planes respectively of tetragonal crystalline phase of Ba3OSiO4 (JCPDS 70–0667). All the peaks have been identified as shown in

Fig. 4(a) shows the dilatometric curves with respect to temperature for glasses with different BaCl2 contents. It must be mentioned that the thermal expansion is a complex property connected with the magnitude and distribution of forces acting in the system, and it reflects any change in the distribution of forces with increasing thermal vibrations [25]. It is due, basically, to the presence of anharmonic vibrations of atoms about their equilibrium position in the solid lattice. The dilatometric curve of N8 is clearly distinguished from the other glasses. All the glasses (N1 to N7) show similar nature of the curve, while N8 shows a very slow rate of expansion from its Tg to Td. The temperature difference between its Tg and Td is about 148 °C while for other glasses it varies between 30 and 40 °C. This happens because N8 is a devitrified glass, as indicated by the XRD results. The XRD pattern of N8 shows sharp crystallization peaks which are due to the formation of BaCl2 and Ba3OSiO4 nano crystals. At temperature higher than Tg, those nano crystals begin to grow and those crystallized parts cause hindrance to its expansion, therefore shifting its Td to a much higher temperature (~760 °C) [26,27]. The values of glass transition temperature (Tg), dilatometric softening point (Td) and softening point (Ts) are listed in Table 2. All the three characteristic temperatures show the same trend of variation, a gradual increase up to N4 followed by a gradual decrease and then a sharp increase for N8 which is a devitrified glass. This happens due to the Cl atoms which form a bridging bond between the glass forming cations, therefore increasing the network connectivity of the glass system. This strengthening effect is observed up to 10 mol% BaCl2 inclusion. Beyond this limit, further addition of BaCl2 exhibits the opposite effect because the system gets compelled to expand itself to accommodate the increased number of comparatively large Cl− ions and the huge Ba2 + ions. Therefore the intercalation of structure results looser network and the Tg, Td and Ts values gradually go down [15]. Fig. 4(b) shows the variation of CTE value with BaCl2 content at different temperatures. The value of CTE gradually increases with increasing BaCl2 content. Up to 5 mol% BaCl2 content the change in CTE value is negligible. After that the increase in CTE is much more profound. Here, these phenomena can be attributed to the loosening of the network structure due to the inclusion of large Cl− and huge Ba2+ ions [28]. Beyond 22.5 mol% the glass starts to exhibit some tendency towards crystallization. The formation of a crystalline phase suppresses the tendency

Table 1 Composition and some physical properties of the investigated samples. Glass ID

Composition (mol %) SiO2

B2O3

BaO

K2O

Al2O3

BaCl2

N1 N2 N3 N4 N5 N6 N7 N8 N9 N10

42.00 39.90 38.85 37.80 35.70 33.60 32.55 31.50 30.45 29.40

30.00 28.50 27.75 27.00 25.50 24.00 23.25 22.50 21.75 21.00

20.00 19.00 18.50 18.00 17.00 16.00 15.50 15.00 14.50 14.00

4.0 3.8 3.7 3.6 3.4 3.2 3.1 3.0 2.9 2.8

4.0 3.8 3.7 3.6 3.4 3.2 3.1 3.0 2.9 2.8

0 5.0 7.5 10.0 15.0 20.0 22.5 25.0 27.5 30.0

Color

Form

Nature

Colorless Colorless Colorless Colorless Colorless Colorless Colorless Whitish White White

Transparent monolith Transparent monolith Transparent monolith Transparent monolith Transparent monolith Transparent monolith Transparent monolith Translucent monolith Opaque broken pieces Opaque broken pieces

Glass Glass Glass Glass Glass Glass Glass Glass Crystallized solid Crystallized solid

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35

(a) N1

N2

N3

N4

N7

N8

N10

N9

(b)

N6

N5

BaCl2

B2O3 +SiO2

K2O+BaO +Al2O3

Fig. 1. (a) Photographs of the prepared glasses, (b) Ternary diagram showing the position and composition of the investigated glasses in the K2O–BaO–Al2O3–B2O3–SiO2–BaCl2 system. The unfilled circles represent glass, gray and dark circles represent devitrified glass and crystallized material respectively (for composition see Table 1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

to increase the CTE. That is why the devitrified glass (N8) shows a decrease in CTE [26]. 3.4. Differential scanning calorimetry DSC study was performed for all the samples taking finely ground powders and heating them at the rate of 10 K/min from 30 to 900 °C. Fig. 5 shows that N1–N4, N9 and N10 show a flat DSC curve throughout

Fig. 2. EDX spectrum of sample N6 showing the inclusion of BaCl2 into the glass matrix (for composition see Table 1).

Fig. 3. X-ray diffractograms of samples (a) N1, (b) N2, (c) N3, (d) N4, (e) N5, (f) N6, (g) N7, (h) N8, (i) N9, (j) N10 showing amorphous glass formations up to N7. Devitrification takes place for N8–N10. Peak identifications for devitrified samples are shown with corresponding JCPDS file numbers (For composition see Table 1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 4. (a) Representative dilatometric thermogram of samples N1, N4, N6 and N8 showing their nature of thermal expansion and corresponding Tg and Td values and (b) Variation of CTE values of the samples at different temperature intervals as a function of BaCl2 content (for composition see Table 1, lines are drawn to guide the eye).

the range indicating that there are no chances for any type of endothermic or exothermic reaction to be taken place within that temperature range. N5, N6, N7 and N8 behave quite differently exhibiting a sharp exothermic peak which indicates that those glasses have tendency to crystallize if they are heated at that particular peak temperature. To confirm that, they were heat treated at their respective peak temperature for 1 h and then undergone X-ray diffraction. The XRD patterns show the presence of the same crystalline phases as found in the devitrified glasses (i.e. N8, N9 and N10) which have been formed during heat treatment. The Tg values can also be obtained from the DSC study. They are in the range 600–630 °C and comparable to the Tg values obtained from the dilatometric study. The trend of variation is found to be same but the values are lower than that of the dilatometric study. This is due to the difference in physical state of the samples. In DSC, powdered samples are used whereas in dilatometric study, bulk glass is used. It requires more energy to mobilize a long chain of molecular segment in a three dimensional rigid network structure of a bulk solid than that of a comparatively smaller molecular unit of a powder which is not confined like the former one. Thus the Tg for bulk samples is slightly higher than that of the powdered samples.

molecular water [29]. The band around 1390 is due to the B\O vibration related to [BO4] groups. The band having highest intensity is around 1010–968 cm−1, is due to the anti-symmetric stretching vibration of Si\O\Si bond of [SiO4] units [8,30], gradually shifting to lower Wavenumber region from N1 to N8. The fundamental vibration frequency, ν (in cm−1) of a linear harmonic oscillator can be expressed by the Szigeti relation as follows [31]
0:5 2 2 ν ¼ k=4π c μ

ð3Þ

where (k) is the force constant, (c) is the speed of light and (μ) is the reduced mass. The μ can be defined as: −1

−1

μ ¼ m1 þ m2

ð4Þ

where m1 and m2 are the masses of the two bond forming atoms or ions (e.g. m1 = Si4+ and m2 = O2−). Thus the IR absorption edge shifts to longer wavelengths for heavier atomic masses or due to reduced force constant (k) of the bond by the weak Columbic interaction. As chlorides are incorporated into the system, some of them replace the O atom from

3.5. Fourier transformed infrared transmission spectroscopy The infrared transmission spectra were recorded for N1–N10 using KBr-pellet method shown in Fig. 6. Sharp bands were found in the regions 1390, 1010–968, 907, 840, 710, 618 and 456 cm−1. Some weak bands are found near 3450 and 1624 cm−1. The band around 3450 cm−1 is due to the stretching vibration of OH groups which are not hydrogen bonded [29]. The band around 1624 cm−1 is due to the bending vibration of

Table 2 Thermal properties of the investigated samples. Sample ID

Glass transition temperature, Tg (°C)

Dilatometric softening point, Td (°C)

Softening temperature, Ts (°C)

N1 N2 N3 N4 N5 N6 N7 N8

621 630 634 642 635 612 613 632

664 665 676 678 665 647 650 761

711 715 718 721 708 703 ND ND

ND = Not determined due to limitation of the instrument up to 800 °C

Fig. 5. DSC thermograms of samples (a) N1, (b) N2, (c) N3, (d) N4, (e) N5, (f) N6, (g) N7, (h) N8, (i) N9 and N10 (for composition see Table 1).

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the samples in a long exposure to moisture and the nature of the spectra was found to be unchanged. This ensures the chemical stability of the glasses as well as their resistance to moisture. 3.6. UV–vis-NIR spectroscopy

Fig. 6. Infrared transmission spectra (FT-IRTS) of N1–N10 taken by the KBr pellet method (* peaks are due to atmospheric hydrocarbon and carbon dioxide; for composition see Table 1, dotted lines are drawn to guide the eye).

the Si\O\Si and Si\O− linkages. The atomic radius of Cl is higher than that of O. As result the inter-atomic distance increases, which reduces the force required to stretch the bond, which means the force constant decreases. On the other hand, the replacement of O atom in the Si\O\Si bond by Cl atom and thereby forming Si\Cl\Si bond increases the reduced mass. Thus, the decrease in k and, at the same time, increase in μ result in lowering the vibration frequency of the bond that leads to shifting the band to a lower wavenumber region, from 1010 to 968 cm−1. The bands around 907 cm− 1 is due to the −O\Si\O− stretching vibration with two non-bridging oxygens [32]. These bands are prominent for N8–N10, which are devitrified materials. This is because the crystallization of BaCl2 results in the formation of nonbridging oxygens (NBOs). Bands around 840 and 710 cm− 1 are due to the stretching vibrations of B\O of [BO4] units and B\O bending vibration modes of [BO 3] units, respectively [33–35]. The bands around 618 and 456 cm− 1 are due to the O\Si\O symmetric stretching vibration and Si\O\Si bending modes of [SiO4], respectively [30,32]. The spectra have been recorded once again keeping

The ultraviolet–visible-near infrared absorption spectra were measured for bulk samples in the range of 250–3300 nm. The glasses are transparent within the whole visible region. Two absorption bands were observed in the range 250–350 and 2700–3200 nm. The spectra (Fig. 7(a)) show that the position of the cut off wavelength or fundamental absorption edge at 0.75 absorption unit shifts towards lower wavelength from 292 nm to 273 nm with an increase in BaCl2 content up to N7. Fig. 7(b) shows the variation of UV-cut off values of N1, N2, N4, N5, N6 and N8 at 0.75 absorption unit with BaCl2 content (mol%). This is due to the decrease in the number of non-bridging oxygen (NBO) atoms, which is the effect of increasing amount of BaCl2 incorporation. As more and more amount of BaCl2 is added to the system, the chloride ion behaves like a bridging element by linking two glass former cations (Si and B). Some of the chloride ions replace some of the bridging oxygen atoms between the glass former cations. Some of them form bridging bonds replacing some NBOs therefore reducing the number of NBOs. As a result, some of the Si\O\Si linkages get transformed into Si\Cl\Si linkage. Also, some glass forming cations which were not connected before get connected through some Cl atoms. On the other hand, with increasing amount of BaCl2 addition, the number of Ba+2 ions also increases. Each Ba2+ ion creates 2 NBOs by breaking a bridging bond between two glass former cations. Thus, the number of NBOs gets increased. So for each BaCl2 molecule, we are getting 2 NBOs from the Ba2 + part and 4 bridging bonds (2 for each Cl−) from the Cl− part. Clearly the bridging effect overweighs the effect of the formation of NBOs, resulting in the shifting of UV-cut off wavelength to a lower value. For N8, which is a devitrified glass, the cut-off wavelength is shifted to higher wavelength 310 nm. This may happen due to the Rayleigh and Mie scattering effect manifested by the tiny crystals present within it [36,37], as well as the increase in NBOs due to the phase separation of nanocrystalline BaCl2 from the glass network. From the absorption bands in the NIR region, as shown in Fig. 8(a), we can calculate the OH content by using the following equation [38] α OH ¼ ða−bÞ=d

ð5Þ

Fig. 7. (a) Representative UV–vis absorption spectra of the polished glasses N1, N2, N4, N5, N6 and N8 showing the UV-Cut off wavelength at 0.75 absorption unit (dotted lines are drawn to guide the eye) and (b) variation of UV-cut off wavelength of N1, N2, N4, N5, N6 and N8 at 0.75 absorption unit as a function of BaCl2 content (sample thickness = 2 mm; for composition see Table 1, lines are drawn to guide the eye). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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N. Shasmal et al. / Journal of Non-Crystalline Solids 398–399 (2014) 32–41

Fig. 8. (a) Absorption spectra of glasses N1, N2, N4, N5 and N8 in the NIR region showing the absorption band of OH group and (b) variation of OH-content of glasses N1, N2, N4, N5 and N8 as a function of BaCl2 content (for composition see Table 1, lines are drawn to guide the eye).

where a = absorbance at 3000 cm−1 (3333 nm), b = absorbance at 7142.85 cm−1 (1400 nm) and d = thickness of sample. The OH content can be calculated using above relation and according to the following equation [38]
 −1 OH content ðppmÞ ¼ 30  α OH cm :

ð6Þ

The variation of OH content gradually decreases from 134 ppm to 27 ppm with increasing BaCl2 content as shown in Fig. 8(b). This is due to the increasing dehydration rate associated with increasing BaCl2 content. The UV–Visible spectra can also be used to evaluate the optical band gap Eopt of the materials. According to the Davis and Mott theory, the relationship between absorption coefficient α(ν) and Eopt is given by [39]
n α ðν Þ ¼ B hν−Eopt =hν
n α ðν Þhν ¼ B hν−Eopt

ð7Þ

where a(v) = (1/d) ln(I0/It). Here ln(I0/It) is the absorbance (A), where I0 and It are the intensities of incident and transmitted light, respectively and d is the thickness of the sample. B is the proportionality constant and hν is the photon energy of incident radiation. The nature of the transition is determined by the superscript letter ‘n’ which takes the values 2, 3, 1/2 and 1/3 corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden optical transitions that are involved in the position of the UV absorption edge when electromagnetic radiation interacts with the electron in the valence band [33]. Fig. 9(a) represents the Tauc's plot [(αhν)2 vs. hν] for different (N1–N8) samples. The optical band gap energies can be determined from the Tauc's plot by extrapolating the linear portion of the curve to intersect the photon energy axis at zero absorption. The intersection of the slope of the linear part of the plot (αhν)2 vs. hν gives the values of the Eopt (in eV). It is seen that the Eopt value increases from 4.15 eV to 4.54 eV with increasing concentration of BaCl2. Fig. 9(b) shows the variation of Eopt value with respect to BaCl2 content. This happens because of the bridging action of Cl atoms which increases the network connectivity. UV–vis-NIR spectra, alike the FTIR spectra, have also been found to be unaffected with time.

Fig. 9. (a) (αhν)2 vs. hν plot of samples N1, N2, N4, N5, N6 and N8 (inset) for evaluation of optical band gap, Eopt (dotted lines are drawn to guide the eye) and (b) variation of Eopt values as a function of BaCl2 content (for composition see Table 1, lines are drawn to guide the eye).

N. Shasmal et al. / Journal of Non-Crystalline Solids 398–399 (2014) 32–41

39

12.0

Dielectric Constant

11.5

11.0

10.5

10.0

9.5 0

5

10

15

20

25

BaCl2 Content (mol%) Fig. 10. Refractive index of samples (a) N1, (b) N2, (c) N3, (d) N4, (e) N5, (f) N7 and (g) N8 as a function of wavelength (for compositions see Table 1, lines are drawn to guide the eye).

Fig. 11. Variation of dielectric constant of the samples as a function of BaCl2 content (for composition see Table 1, lines are drawn to guide the eye).

3.7. Refractive Index

ε¼n

Refractive index (ni) at different wavelengths was measured for all the transparent glasses by Prism coupler using lasers of five different wavelengths. Fig. 10 shows the variation of refractive index of different glasses with respect to the wavelength. Refractive indices are in the range 1.58–1.63 for 532 nm wavelength. The ni is found to increase gradually with increasing BaCl2 content. This is due to the increased density and electronic polarizability (discussed later) of glasses with increasing BaCl2 content. As the glass becomes denser the velocity of light through it decreases. The ni of a particular medium is the ratio of velocity of light through the vacuum (c) to the velocity of light through the medium (v).

where N A is the Avogadro's number, and M av and ρ are the average molecular weight and density of the glass respectively. However, Eq. (11) does not correlate well as could be observed from the experimental results.

ni ¼ c=v

V m ¼ M=ρ

2

ð8Þ

ð11Þ

3.9. Some calculated properties 3.9.1. Molar volume The molar volume (Vm) of the glass samples was calculated using the molecular weight (M) and density (ρ) of the glasses with the following relation and these values are included in Table 3. ð12Þ

Hence, the higher the density of the glass, the lower will be the denominator. As result, refractive index increases. The refractive index values were found to be constant at different time intervals after a long exposure to open air.

Molar volumes of the glasses increase with increase in BaCl2 content. This is the effect of the increasing inclusion of huge Ba atoms which is responsible for the expansion of glass volume.

3.8. Dielectric constant

3.9.2. Covalent character The extent of covalent bonding character of the samples (glasses and glass-ceramics) can be calculated approximately using the formula [43]

Dielectric constant (ε) of the glasses has been calculated by using the following formula [40] ε ¼ cd=ð0:0885AÞ

ð9Þ

where c, d and A are the capacitance in pico Farad (pF), thickness of glass (in cm) and area of the dielectric (in cm2) respectively. It is seen that the dielectric constant of the glasses gradually increases with increase in BaCl2 content which is shown in Fig. 11. The dielectric constant is directly correlated with the polarizability of the samples. It has already been reported that Ba2+ ions are highly polarizable due to their large ionic radii and small cation unit field strength [41,42]. Due to the increasing amount of Ba-containing component, the dielectric constant also increases. This correlates well with the calculated electronic polarizability as will be discussed later. Dielectric constant (ε) is associated with polarizability (a p ) and refractive index (n) by the following Lorentz–Lorenz (Eq. (10)) and Maxwell equations (Eq. (11)) [8]

 2 2 α p ¼ 3M av n −1 =4 n þ 2 πN A ρ

ð10Þ


 2 Covalent character ð%Þ ¼ exp −0:25Δχ  100

ð13Þ

where Δχ is the electronegativity of the composite; that is, the electronegativity difference (χA − χC) of the anions and the cations. The average electronegativity of the anions (χA) or cations (χC) can be evaluated by the following relation [44] χA

or

X Ni χ i χC ¼ X Ni

ð14Þ

where Ni and χi are the number of individual constituent atom per mole and its electronegativity respectively. Here, we have considered the electronegativity in Pauling's scale as χO = 3.44, χCl = 3.16, χB = 2.04, χSi = 1.9, χAl = 1.61 χBa = 0.89 and χK = 0.82. The calculated covalent character is shown in Table 3. It is found to vary in the range 43.53–48.34% (±0.05%) and decreased with increase in BaCl2 content. As the content of BaCl2, which is an ionic compound, increases the

40

N. Shasmal et al. / Journal of Non-Crystalline Solids 398–399 (2014) 32–41

Table 3 Some measured and calculated properties of the investigated samples. Properties

Glass ID/Property values N1

Measured properties Density, ρ (g·cm−3) Refractive index, ni (at 532 nm) Calculated properties Average molecular weight, M (g·mol−1) Molar volume, Vm (cm3·mol−1) Covalent character % Average molar refraction, Rm (cm3·mol−1) Electronic polarizability, αm (Å3) Metallization criterion, M Abbe number, υD Dispersive power, 1/υD

N2

N3

N4

N8

N9

N10

3.411 1.6016

3.496 1.6064

3.5652 1.6159

3.6181 1.6255

3.6633 1.6291

3.7151 1.6322

3.7259 –

3.7394 –

84.65 26.78 48.34 8.8776 3.52 0.6684 62.27 0.01605

92.61 27.82 47.62 9.4087 3.73 0.6618 61.24 0.01632

96.61 28.32 47.25 9.7093 3.85 0.6572 60.39 0.01655

100.60 28.78 46.87 9.9319 3.94 0.6549 59.11 0.01691

108.58 30.46 46.07 10.6427 4.22 0.6506 57.82 0.01729

116.62 32.23 45.26 11.4029 4.52 0.6462 55.59 0.01798

120.56 32.91 44.83 11.6958 4.64 0.6446 55.40 0.01804

124.55 33.53 44.41 11.9630 4.74 0.6432 55.20 0.01811

128.54 34.50 43.97 – – – – –

132.53 35.44 43.53 – – – – –

ð15Þ

where Vm is the molar volume (Vm = Mav/ρ), Mav is the average molecular weight, ρ is the density, and n is the refractive index obtained from the measurement by Prism coupler. The molar polarizability (αm) is calculated as [45] ð16Þ

where NA is the Avogadro's number. The calculated values are shown in Table 3. 3.9.4. Metallization criterion According to the theory on metallization of the condensed matter proposed by Herzfeld the refractive index becomes infinite for the condition Rm/Vm = 1 in the Lorentz–Lorenz equation [46]. In materials with such a condition, the system acquires metallic status. The necessary and sufficient condition for predicting the non-metallic nature of solids is Rm/Vm b 1. The difference from Rm/Vm = 1, M, is so-called metallization criterion. Optical non-linearity is caused by electronic polarization of a material upon exposure to intense radiation. So, electronic polarizability is one of the most important properties that determine the non-linear response of the material. The metallization criterion (M) can be calculated as [31,45] M ¼ 1−Rm =V m :

N7

3.3286 1.5916

3.9.3. Electronic polarizability of ions The average molar refraction (Rm) was calculated following the Lorentz–Lorenz equation [8,9]

α m ¼ 3Rm =4πNA

N6

3.1615 1.5772

ionic character of the glass system increases. Hence the covalent character gradually decreases.


 n2 −1   Vm Rm ¼  2 n þ2

N5

ð17Þ

The calculated M is listed in Table 3. It is seen that the metallization criterion decreases with increasing BaCl2 content, meaning that the tendency for metallization in the electronic structure is high in the glasses with high BaCl2 contents.

where nD, nF and nC are the refractive indices of the material at the wavelengths of the Fraunhofer D-, F- and C-spectral lines (589.3 nm, 486.1 nm and 656.3 nm respectively) [8,9]. Abbe number of the samples ranges in between 55.2 and 62.2, showing gradual decrease with increasing BaCl2 content. The values are presented in Table 3. Dispersive power is the power of a transparent medium to separate different colors of light by refraction and it is expressed as 1/υD. Dispersive power ranges between 0.016 and 0.018 showing gradual increase with increasing BaCl2 content. The values are presented in Table 3. 3.9.6. Third order susceptibility (χ3) The materials exhibit nonlinear properties due to the distortion of electronic structure of any atom, complex, or molecule by the application of an electric field to produce net dipole moment that is linearly or nonlinearly proportional to that field. The third order susceptibility, χ3, has been calculated using an indirect method. According to Boling's semi empirical equation [47], χ3 of materials is strongly dependent on both the nonlinear refractive index (n2) and linear refractive index (n). χ3 is expressed by the following relation, according to Vogel et al. [48]
2 2 ð n−1 Þ n þ 2 n 17 3 χ ¼ n ¼      12π 2 3π υd 1:52 þ n2 þ 2 ðn þ 1Þ υd 0:5 6n

ð19Þ

where n is the refractive index of glass at 587.6 nm and n2 is the nonlinear refractive index estimated from an empirical relation derived by Vogel et al. [48]. The χ3 values range between 0.385 and 0.538 × 10− 14 esu. The values thus obtained were higher than the value reported for fluoride glasses [49] and lower than tellurite glasses [50] and much lower than the chalcogenide and semiconductor doped glasses [51–53]. In silicate, phosphate, tellurite and halide glass, different amounts and types of network modifier ions are used to increase or decrease optical nonlinearities. Chalcogenide glasses and inorganic glasses containing organic dye molecules and semiconductor microcrystals exhibit much larger nonlinearities. Table 4 represents calculated third-order nonlinear susceptibilities (χ3) of chloroborosilicate glass and other glasses reported in the literature [54]. 4. Conclusions

3.9.5. Abbe number (υD) and dispersive power (1/υD) Abbe numbers were calculated from the well-known empirical relation using the refractive index of light at different wavelengths

Glasses having composition (100 − x)(42SiO2–30B2O3–20BaO– 4K2O–4Al2O3)–xBaCl2 (mol%),where x = 0–30 were prepared by simple melt-quench technique and characterized. The following conclusions may be drawn from their detailed study.

n −1 υD ¼ D n F −nC

• Transparent monolithic glasses were obtained up to x = 22.5. After further addition of BaCl2 the glass gets devitrified. The properties are found to be dependent on BaCl2 content.

ð18Þ

N. Shasmal et al. / Journal of Non-Crystalline Solids 398–399 (2014) 32–41 Table 4 Comparison of the calculated third-order nonlinear susceptibilities (χ3) of chloroborosilicate glass with different glasses reported in the literature. Glass

Third order susceptibility, χ3 (10−14 esu)

Chloroborosilicate glasses [present study] N1 N2 N3 N4 N5 N6 N7 N8

0.385 0.412 0.432 0.453 0.480 0.523 0.531 0.538

Fluoride glasses [49] BeF2 BeF2–KF–CaF2–AlF3

0.078 0.110

Tellurite glasses [50] 10 Na2O–xNb2O5–(90 − x) TeO2 x=0 x=5 x = 10 x = 20

2.8 2.3 2.7 4.9

Oxide glasses [50,51] Schott SF6 Schott SF59 PbO–SiO2

4.5 7.5 8.0

Chalcogenide glasses [52] As2S3 Ge–S As–S–Ge

720 200 1410

refractive index measurement respectively. NS would like to express her sincere gratitude for her fellowship sponsored by the Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research, and CSIR-Central Glass and Ceramic Research Institute.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

Semiconductor doped glasses [53,54] CdSSe doped silicate Corning CS3-68 Corning CS2-73 Schott RG 695

100 ~1,300,000 ~500,000 ~300,000

• Glass transition and deformation temperatures are in the ranges 614– 641 °C and 648–780 °C respectively. The glasses are stable and chemically durable. They do not get affected by moisture. • Inclusion of BaCl2 in borosilicate glass network causes a structural alteration by replacing some of the NBOs with Cl atoms. This change is reflected in the increase in UV-cut off values and shifting IR absorption peaks of the glasses. • The OH content varies in between 27 and 134 ppm. • Glasses having x ≥ 15 show crystallization peaks in the range 630– 770 °C in DSC study. Heat treatment at their respective crystallization temperatures gives nanocrystals of BaCl2 and Ba3OSiO4. The size of the crystals varies in the range 15–30 nm. • Refractive indices, CTEs and dielectric constants are in the range 1.58– 1.63, 73–91 × 10−7 K−1 and 9.79–11.73 respectively. All of them increase with increasing BaCl2 content. Thus it may be concluded that chloroborosilicate glasses in this investigated system are a stable, chemically durable glass system whose properties are controlled by BaCl2 content. These glasses have the potential to develop BaCl2 nanocrystal containing low phonon (~350 cm−1) glass-ceramics promising for photonic applications. Further work in this respect is now under progress. Acknowledgments The authors are thankful to Mr. Kamal Dasgupta, Acting Director of the institute and Dr. Ranjan Sen, Head, Glass Division for their encouragement and support. The authors acknowledge the technical supports provided by the X-ray section of the institute and also Dr. Kalyandurg Annapurna and Dr. Koushik Biswas for recording the FTIR spectra and

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