Induced crystallization and physical properties of Li2O–CaF2–P2O5:TiO2 glass system

Journal of Alloys and Compounds 450 (2008) 477–485

Induced crystallization and physical properties of Li2O–CaF2–P2O5:TiO2 glass system Part I. Characterization, spectroscopic and elastic properties G. Murali Krishna a , N. Veeraiah a,∗ , N. Venkatramaiah b , R. Venkatesan b a

Department of Physics, Acharya Nagarjuna University PG Centre, Nuzvid 521201, India b Department of Chemistry, Pondicherry University, Pondicherry, India

Received 5 October 2006; received in revised form 31 October 2006; accepted 1 November 2006 Available online 1 December 2006

Abstract Li2 O–CaF2 –P2 O5 glasses mixed with different concentrations of TiO2 (ranging from 0 to 0.8 mol%) were crystallized at 500 ◦ C. The samples are characterized by X-ray diffraction, scanning electron microscopy and differential thermal analysis techniques. Spectroscopic properties (IR and Raman) and elastic properties (viz., Young’s modulus E, shear modulus G and micro-hardness H) at room temperature are studied. The X-ray diffraction and the scanning electron microscopic studies revealed the presence of lithium phosphate, lithium titanium phosphate and titanium phosphate crystal phases. The differential thermal analysis traces of these samples exhibit three crystalline temperatures. The IR and Raman spectra of these samples have exhibited bands due to TiO4 and TiO6 structural units in addition to the conventional bands due to various phosphate structural groups. The analysis of these results indicated that the sample crystallized with 0.6 mol% of TiO2 possesses the highest density, high mechanical strength and more compact network. © 2006 Elsevier B.V. All rights reserved. Keywords: Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics; X-ray diffraction; Differential thermal analysis; IR and Raman spectra; Elastic properties

1. Introduction Crystalline glass materials also known as pyroceramics or glass-ceramics have appeared recently with very interesting peculiarities and properties. Intensive investigation on synthesis and characterization of these materials is being carried out by a number of researchers [1–5]. The crystalline glass-ceramic materials have a fine-grained uniform structure consisting of small crystals of irregular, distorted form often aggregated into sphenulites with residual glass interlayers cementing the crystalline glass-ceramic concretion. As a result these materials are expected to have good mechanical, electrical and thermal properties, high chemical durability and very low coefficient of thermal expansion. The crack growth in these materials is also hindered due to the presence of interwined crystals. Hence, a considerable interest is attached in this connection to the studies on crys-

∗

Corresponding author. Tel.: +91 8656 235551; fax: +91 8656 235200. E-mail address: [email protected] (N. Veeraiah).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.005

tallization and its bearing on the physical properties. Catalysts generally used for controlled crystallization processes, giving rise to enormous numbers of nucleation centers in the original glass are, gold, silver, platinum or the oxides of Ti, Cr, Mn, Ce, V, Ni and Zr or certain sulfides or fluorides of heavy or transition metals. Among various nucleating agents, the titanium oxide is observed to be easily miscible in the glassy matrix at high temperatures. The presence of small quantities of TiO2 in the glass matrices is observed to enhance the glass forming ability and chemical durability of the glasses [6–8]. Earlier, this oxide was considered as a nucleating agent for crystallization in silicate glasses. However, there are certain recent reports, where titanium ions were used as nucleating agents and observed to induce phase separation during the cooling of the melt in other glass systems like P2 O5 , GeO2 [9,10]. Lithium phosphate glasses are well-known due to their variety of technological applications like solid electrolytes in electrochemical devices such as high energy density batteries [11]. These materials possess superior physical properties

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such as low melting and softening temperatures and high ultraviolet and far-infrared transmission [12,13]. However, the poor chemical durability, high hygroscopic and volatile nature of phosphate glasses, have prevented them from replacing the conventional glasses in a wide range of technological applications. In recent years, there has been an enormous amount of research on improving the physical properties and the chemical durability of alkali phosphate glasses by adding different metal oxides. Addition of titanium oxide, for example, is reported to increase the chemical durability of these glasses to a large extent and may make them suitable for non-linear optical devices that can be operated at smaller input power [14–16]. The crystallization with TiO2 as stimulator, may further improve the physical properties of these glasses. Addition of CaF2 to alkali phosphate glass matrices is anticipated to lower the viscosity and to decrease the liquidus temperature to a substantial extent and further it also acts as an effective mineralizer [17] similar to TiO2 . In addition to the conventional X-ray/electron microscopic methods, it is also quite possible to understand the structure of the glass-ceramics to a substantial extent by investigating various physical properties, viz., mechanical, optical, magnetic, electrical properties. Though few recent studies are available on some lithium phosphate glass-ceramics [12], devoted studies on the above physical properties are not available. The current investigation is aimed at an understanding the catalyst action of the titanium ion on the crystallization of Li2 O–CaF2 –P2 O5 glass system by means of above mentioned analytical techniques. For the sake of convenience, the paper is split in to two parts. Part I deals with characterization, mechanical and spectroscopic properties (IR and Raman), whereas in part 2 it is planned to report the studies on optical absorption, magnetic and electrical properties. 2. Experimental For the present study, a particular composition (30 − x) Li2 O–10 CaF2 –60 P2 O5 :x TiO2 with five values of x ranging from 0 to 0.8 is chosen and the samples are labeled as T0 (x = 0), T2 (x = 0.2), T4 (x = 0.4), T6 (x = 0.6), T8 (x = 0.8). Analytical grade reagents of NH4 H2 PO4 , Li2 CO3 , CaF2 and TiO2 powders in appropriate amounts (all in mol%) were thoroughly mixed in an agate mortar and melted using a thick walled platinum crucible at 1000 ± 10 ◦ C in a PID temperature controlled furnace for about 2 h. The resultant bubble free melt was then poured in a brass mould and subsequently annealed at 250 ◦ C. The specimens with various concentrations of TiO2 were heat treated in a furnace at 500 ◦ C for 4 h. Automatic controlling furnace was used to keep the temperature at the required level. After the heat treatment in the furnace at specified temperature, the samples were quenched in air to room temperature. The crystalline phases in the glass-ceramic samples were identified using Xpert PRO’panalytical X-ray diffractometer fitted with copper target and nickel filter operated at 40 kV, 25 mA. Scanning electron microscopy studies were also

carried out on these glasses to observe the crystallinity in the samples using HITACHI S-3400N Scanning Electron Microscope. Differential thermal analysis was carried out using NETZSCH STA 409 C/CD instrument with a programmed heating rate of 10 ◦ C/min, in the temperature range 30–1000 ◦ C, to determine the glass transition temperature and crystalline peaks. The density d of the glasses was determined to an accuracy of (±0.001) by the standard principle of Archimedes’ using o-xylene (99.99% pure) as the buoyant liquid. The mass of the samples was measured to an accuracy of 0.1 mg using Denver balance, model APX-200. For evaluating the chemical durability, the bulk glasses were suspended by a weightless strand in about 100 ml of water of pH 6 for about 4 h at 90 ◦ C. The weight loss (W) is measured and the average dissociation rate is evaluated using, DR = W/(S × t) g/cm2 /min, where S is the surface area of the sample and t is the time of immersion. Infrared transmission spectra were recorded on a JASCO-FT/IR–5300 spectrophotometer with a resolution of 0.1 cm−1 in the range 400–2000 cm−1 using potassium bromide pellets (300 mg) containing pulverized sample (1.5 mg). These pellets were pressed in a vacuum die at ∼680 MPa. The Raman spectra were recorded on Raman Spectrometer (Bruker FRA 106/RFS) using 514 nm exciting light of argon laser. The dimensions of the samples used for longitudinal (1.5 cm × 1.5 cm × 0.25 cm) and shear (1.5 cm long and 0.25 cm in diameter) velocity measurements were almost identical to those of X-cut 0.13 MHz quartz transducers used. The ultrasonic velocities were obtained from a piezoelectric composite oscillator. If fq and fc are the resonance frequencies of the transducers and the composite bar, respectively, the resonance frequency fs of the glass-ceramic samples were determined using the standard equation [18]. From these resonance frequencies longitudinal and shear velocities were evaluated using V = 2Lfs , where L is the length of the specimen.

3. Results The scanning electron microscopic (SEM) pictures of the preheated samples containing different concentration of TiO2 have been presented in Fig. 1a. The pictures do not show any significant crystallinity. Fig. 1b shows SEM pictures of the crystallized (at 500 ◦ C) Li2 O–CaF2 –P2 O5 glasses doped with different concentrations of TiO2 ; the pictures show an increasing crystallinity with increasing concentration of TiO2 up to 0.6 mol% and there after a sort of coagulation seems to be developed. The density of the TiO2 free glass-ceramic material is measured to be 2.4495 g/cm3 . The introduction of TiO2 (up to 0.6 mol%) caused an increase in the density of these ceramic materials. When the concentration of TiO2 is raised from 0.6 to 0.8 mol% a slight decrease in the density could be detected (Table 1). From the measured values of the density and average molecular weight M of the samples, various other physical parameters such as titanium ion concentration Ni , mean titanium ion separation ri , polaran radius rp in Li2 O–CaF2 –P2 O5 :TiO2 ceramic samples are computed and presented in Table 1. The dissociation rate (DR) measured as per the procedure described

Table 1 Physical parameters of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics Glass

Density (g/cm3 )

Titanium ion concentration Ni (1021 ions/cm3 )

Mean titanium ion ˚ separation ri (A)

Polaron ˚ radius rp (A)

Dissociation factor (×10−6 ) g/cm2 /min)

30 Li2 O–10 CaF2 –60 P2 O5 29.8 Li2 O–10 CaF2 –60 P2 O5 :0.2 TiO2 29.6 Li2 O–10 CaF2 –60 P2 O5 :0.4 TiO2 29.4 Li2 O–10 CaF2 –60 P2 O5 :0.6 TiO2 29.2 Li2 O–10 CaF2 –60 P2 O5 :0.8 TiO2

2.4495 2.4512 2.4628 2.4731 2.4498

– 2.894 5.809 8.742 11.534

– 7.02 5.56 4.85 4.43

– 2.83 2.24 1.96 1.78

5.37 2.27 2.11 0.81 1.72

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Fig. 1. (a) SEM pictures of pre-heated Li2 O–CaF2 –P2 O5 :TiO2 samples. (b) SEM pictures of crystallized Li2 O–CaF2 –P2 O5 :TiO2 samples.

in Section 2 also presented in the same table; the dissociation factor is observed to be the minimum for the sample T6 (Table 1). Fig. 2 shows the differential thermal analysis traces of Li2 O–CaF2 –P2 O5 :TiO2 samples. For the pre-crystallized

Fig. 2. DTA Traces of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics.

sample an endothermic change due to the glass transition temperature Tg is observed. At still high temperature Tc an exothermic peak due to the crystal growth followed by another endothermic peak due to re-melting of the glass are also observed. The appearance of only one crystallization peak in the DTA traces of pre-crystallized samples indicates that the dopant, i.e., TiO2 , is homogeneously distributed in the material. DTA traces of all the crystallized samples exhibited an endothermic change due to the glass transition between 370 and 400 ◦ C. In addition, three clear exothermic peaks Tc1 , Tc2 and Tc3 due to the crystal growth followed by an endothermic peak due to re-melting of the samples have been observed. As the concentration of TiO2 is increased (Tc2 − Tg ), a parameter proportional to the glass forming ability is found to increase up to 0.6 mol%. The pertinent data related to DTA studies have been presented in Table 2. The X-ray diffraction analysis of the pre-heated Li2 O– CaF2 –P2 O5 :TiO2 sample shows that the samples prepared are of amorphous in nature. After the samples are crystallized at 500 ◦ C the diffraction pattern exhibit peaks due to lithium phos-

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Table 2 Summary of data on differential thermal analysis studies of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics Glass composition

T0 T2 T4 T6 T8

Glass transition temperature, Tg (◦ C)

374 379 387 396 392

Crystallization temperatures (◦ C) Tc1

Tc2

Tc3

Tc2 − Tg

– 490 509 514 511

503 510 521 534 527

– 542 556 566 561

129 131 134 138 135

Fig. 3. XRD patterns of TiO2 doped Li2 O–CaF2 –P2 O5 glasses crystallized at 500 ◦ C.

phate, lithium titanium phosphate and titanium phosphate crystal phases. With increase in the content of crystallizing agent, i.e., TiO2 , in the glass matrix, the structure of the diffraction pattern becomes more complex and a clear variation in the intensity and the pattern of diffraction peaks can be visualized (Fig. 3). From the values of resonance frequency fs , the corresponding longitudinal and shear velocities (Vl and Vs ) of ultrasonic waves in the glasses were evaluated and presented in Table 3. With these ultrasonic velocities, using the standard equations [18], Young’s modulus E, shear modulus G, Poisson’s ratio σ

and micro-hardness H of these glasses were determined. All these parameters show an increasing trend with TiO2 concentration up to 0.6 mol% and beyond that these parameters are found to decrease (Fig. 4a and Table 3). The mechanical loss factor Q−1 that gives the information on the mechanical strength is evaluated using standard formula [18] and its variation with the concentration of TiO2 is shown as the inset of Fig. 4b. The infrared transmission spectrum of TiO2 free Li2 O– CaF2 –P2 O5 glass-ceramics (Fig. 5a) exhibit vibrational bands at 1300 cm−1 (identified due to asymmetrical stretching vibrations of PO2 − groups, this region may also consists of bands due to P O stretching vibrations), 1050 cm−1 (symmetrical stretching vibrations of PO2 group and normal vibrational mode in PO3− 4 group arising out of ν3 -symmetric stretching), at 907 cm−1 (due to P–O–P asymmetric stretching vibrations) and another band at 793 cm−1 due to P–O–P symmetric stretching vibrations [19–21]. With the introduction of TiO2 (up to 0.6 mol%) and heat treatment, the intensity of the bands symmetrical stretching vibrations of PO2 group and P–O–P symmetric stretching vibrations, is observed to increase with a shift in the band position towards slightly lower wavenumber; the intensity and the position of P–O–P asymmetric bending vibrational band, exhibited a decreasing trend with in this concentration range of TiO2 (<0.6 mol%). However, for the sample T8 , the intensity associated with the symmetrical bands is observed to decrease while that of the asymmetrical band is observed to increase when com-

Fig. 4. (a) Variation of elasitic coefficients of Li2 O–CaF2 –P2 O5 glass-ceramic with the concentration of TiO2 . (b) Gives the variation of internal friction with the concentration of TiO2 .

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Table 3 The data on various elastic properties of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics Sample

Longitudinal velocity, Vl (m/s)

Shear velocity, Vs (m/s)

Young’s modulus, E (GPa)

Shear modulus, G (GPa)

Poisson’s ratio, σ

Micro-hardness, H (GPa)

T0 T2 T4 T6 T8

4823 4872 4890 5031 4832

3236 3267 3275 3371 3239

58.00 58.20 58.90 62.60 57.20

26.10 26.16 26.42 28.11 25.70

0.111 0.112 0.115 0.114 0.113

6.65 6.76 6.78 7.24 6.63

pared with that of the bands observed in the spectrum of the sample T6 . It may be noted here that the band positions of various phosphate structural groups observed for the present samples are found to be well within the ranges reported in the literature [19–21]. In general, TiO4 units exhibit four normal vibrational bands at about 750 cm−1 (νs (A1 )), 306 cm−1 (δd (A)), 770 cm−1 (νdas (F)) and at 370 cm−1 (δd (F)) [22]; all of them are active in the Raman spectrum but only νdas (F) and δd (F) were found to be active in the IR spectrum. On the other hand, TiO6 units are anticipated to exhibit six normal vibrational bands, but only νdas (F) and δd (F) modes were reported to be active in the IR spectrum. In the IR spectrum of the sample T2 , two additional prominent bands were also observed at 740 and 652 cm−1 . Further, it was also established that in the IR spectra of glasses and their crystalline products, the bands due to TiO6 units lie around 650 cm−1 , whereas the bands due to TiO4 units lie in the high frequency region [23–28]. Based up on the literature survey, the first band is identified due to Ti–O–Ti symmetric stretching vibrations of TiO4 units. Though there are certain reports sug-

gesting that this band is due to the vibrations of isolated TiO4 units but some recent reports suggest that this band is due to TiO4 structural units that take part network forming positions. The band at 650 cm−1 is attributed to the vibrations of Ti–O bonds in the deformed octahedra [29–32]. With increase in the concentration of TiO2 up to 0.6 mol%, the intensity of the band due to TiO4 structural units is observed to increase and that of TiO6 structural units is observed to decrease. However, a reversal trend in the intensity of these two bands has been observed, with further raise in the concentration of TiO2 . It may be worth mentioning here that the band due to TiO4 structural units is observed to shift towards lower wavenumber with increase in the concentration of TiO2 up to 0.6 mol%. In the spectrum of the sample T6 this band is located at about 700 cm−1 at where the vibrational band due to pyrophosphate groups (P2 O7 )4− is also expected [30]; as a result we may attribute the band at about 700 cm−1 to the common vibrations of TiP2 O7 structural units. The detailed variation of the intensity of different structural groups with the concentration of TiO2 of

Fig. 5. (a) IR spectra of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics. (b) Relative variation of the intensity of bands due to various structural groups in the IR spectra of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics with the concentration of TiO2 .

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Table 4 Summary of the data on band positions (cm−1 ) of various absorption bands in the IR spectra of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics Structural unit −

PO2 asymmetric groups/P O PO2 − symmetric groups P–O–P asymmetric stretching P–O–P symmetric stretching TiO4 TiO6

Glass T0

Glass T2

Glass T4

Glass T6

Glass T8

1300 1051 907 794 – –

1294 1041 911 788 740 652

1287 1038 915 787 725 654

1274 1031 918 782 705 658

1282 1035 916 784 730 655

of the type P–O–Ti, in the network. With increase in the concentration of TiO2 from 0.6 to 0.8 mol%, bending and torsional vibrations of PO4 structural units seem to be dominant over the vibrations of symmetrical structural units; further, a clear resolution between the bands due to TiO6 structural units and P–O–P symmetric vibrations could also be detected in the spectrum of the sample T8 . The summary of the data on band positions of various structural groups is presented in Table 5. 4. Discussion P2 O5 is a well-known strong glass forming oxide, participates in the glass network with PO4 structural units. One of the four oxygen atoms in PO4 tetrahedron is doubly bonded to the phosphorus atom with the substantial ␲-bond character to account for pentavalency of phosphorous. The PO4 tetrahedrons are linked together with covalent bonding in chains or rings by bridging oxygens. Neighbouring phosphate chains are linked together by cross-bonding between the metal cation and two non-bridging oxygen atoms of each PO4 tetrahedron; in general, the P–O–P bond between PO4 tetrahedra is much stronger than the crossbond between chains via the metal cations [35]. The presence of such PO4 units in the present glass-ceramic samples is evident from the IR and Raman spectral studies. Similar to fluoro borate glass network, the phosphate glass network containing alkaliearth fluorides like CaF2 may consist of P (O, F)4 or PO3 F or PO2 F2 units [36,37]. Li2 O is a conventional modifier oxide and enter the glass network, either by rupturing or by breaking up the P–O–P bonds (normally the oxygens of Li2 O break the local symmetry while Li+ ions occupy interstitial positions) and introduce coordinated defects known as dangling bonds along with non-bridging oxygen ions as shown in Fig. 7. TiO2 is the most common nucleating agent; in many glassceramics it was found to form compounds with the other constituents of the glass in the form of high density fine crystals. These crystals act as heterogeneous nuclei for crystallization of

Fig. 6. Raman spectra of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics.

Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics is presented in Fig. 5b. The band positions of vibrations due to various structural groups are presented in Table 4. Fig. 6 shows the Raman spectra of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramic samples. The spectrum of TiO2 free glass-ceramic sample has exhibited three conventional bands at 1068, 725 and 400 cm−1 due to νs -PO2 mode, νs -POP mode and due to bending and torsional vibrations of PO4 structural units, respectively [33]. The spectrum of TiO2 mixed glass-ceramic sample (T2 ) exhibits two additional Raman bands at 730 and 900 cm−1 assigned to TiO6 and isolated TiO4 units, respectively [34]. With increase in the concentration of TiO2 up to 0.6 mol%, the intensity of the bands due to symmetrical phosphate structural units and TiO4 units is observed to increase, while that of TiO6 structural units and bending mode of PO4 polyhedra showed a decreasing trend. In the spectrum of the sample T6 , the band due to TiO6 structural units seems to be merged with symmetric stretching vibration of P–O–P band indicating possible linkages

Table 5 Band positions (in cm−1 ) in the Raman spectra of Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics Glass

T0 T2 T4 T6 T8

Phosphate groups

Titanium groups

PO2 mode

POP mode

Bending vibrations of PO4 units

TiO4

TiO6

1068 1056 1053 1046 1051

725 713 710 707 708

400 403 410 417 400

– 902 896 893 895

– 722 728 – 730

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Fig. 7. A schematic illustration of fluoro phosphate glasses with Li2 O as modifier.

the remaining glass. In certain cases, TiO2 is also found to act as surface active agent and to increase the nucleation rate [38]. A slight compositional dependence of density of Li2 O– CaF2 –P2 O5 :TiO2 glass-ceramics has been observed (Table 1). In general, the degree of structural compactness, the modification of the geometrical configuration of the glassy network, the change in the coordination of the glass forming ions and the fluctuations in the dimensions of the interstitial holes are the factors that influence the density of the glass-ceramic material. The progressive introduction of TiO2 (up to 0.6 mol%) causes a slight increase in the density; this is an indicative of growing structural compactness of the material. It is also an evocative of the presence of titanium ions largely in Ti4+ state (in the sample T6 ) whose field strength is very high making the sample more compact when compared with that of Ti3+ state [39]. The decrease in the average dissociation rate (DR) of the glasses with the increase in the concentration of TiO2 up to 0.6 mol% indicates that the concentration of undisturbed P–O–P, P–O–Ti bonds is higher in the sample T6 . The presence of such bonds, in higher concentrations, makes the samples more hydration resistant. Additionally, the length and the orientation distribution of PO4 chains also play a major role in deciding the chemical durability of these samples. Previous empirical studies showed that the shortened PO4 chains in the glass network are responsible to some extent for the high corrosion resistance of the glasses [40]. This conclusion obviously suggests that up to 0.6 mol% concentration, the titanium ions prefer to go in to tetrahedral positions, make more and more P–O–Ti bonds rather than acting as modifiers which create more dangling bonds in the glass network. X-ray diffraction pattern exhibits microstructural changes. The crystalline lithium phosphate (orthophosphate, meta phosphate and pyrophosphate), titanium phosphates that are kinetically and thermodynamically feasible seemed to be the main products in the present samples. In titanium rich glasses, the presence of LiTi2 (PO4 )3 crystalline phases are also detected. These metastable crystals are usually in solid-solution phases that can incorporate the major constituents of the glass composition in the approximately same proportion as they were present in the original glass matrix. This type of separation of various phases may lead to a Ti fortification of the droplet, leaving the glass matrix with a very low content of titanium. The Ti rich areas in the glass may enhance the reactivity of Ti with the other oxides that precipitate as a high density fine Ti rich crystals. These tiny crystals act as heterogeneous nuclei for the crystallization of the remaining glass. The scanning electron microscopic pictures of TiO2 doped glass-ceramics exhibit larger crystals than those visible in the TiO2 free samples. At least up to 0.6 mol% of tita-

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nium mixed glasses, a reasonably homogeneous distribution of the crystals can be seen. The residual glass phase may act as interconnecting zones among the crystallized areas making the samples free of voids and cracks. With increase in the concentration of TiO2 up to 0.6 mol%, the glass transition temperature Tg and the parameter Tc2 − Tg have been observed to increase. The augmented cross-link density of various structural groups and closeness of packing are responsible for such an increase of these parameters. The appearance of different crystallization temperatures in the DTA pattern obviously suggests the presence of different phases of crystallization in the samples. The crystallization in the samples may take place based on the surface and bulk nucleations. The non-isothermal devitrification process is represented by the equation [41]  ln

1 1−α

 =

AN −(nE/RT ) e βn

(1)

where α is the fraction of the volume crystallized at temperature T, E the activation energy, β the DTA heating rate and A is the constant. The nuclei centers, N, are the sum of surface and bulk nuclei. The higher the value of N, the lower is the crystallization temperature Tc . In the surface nucleation mechanism, the nucleating centers are formed only on the surface and the crystals start developing from the surface to the inside of the glass one dimensionally (in this case n = 1). On the other hand, in the bulk nucleation the crystals grow three dimensionally (in this case n = 3). The general shape of the crystallization peaks is strongly dependent with the value of n. The higher the value of n the narrower is the width of the crystallization peak [42]. For the surface crystallization we may therefore expect relatively wider peaks when compared with those of bulk crystallization peaks. The pattern of the DTA peaks suggests that the crystallization is predominantly due to the surface crystallization in the sample T6 , whereas in the sample T8 , the bulk crystallization seems to prevail. In general, in a less disordered glass framework, the energy introduced by the vibrator is distributed more rapidly among the vibrational degrees of freedom of the glass framework. The time required for the establishment of equilibrium distribution of energy goes on increasing in comparison with the period of oscillation of the vibrator and hence a decrease in the mechanical loss factor or coefficient of internal friction, this leads to an increase in the elastic coefficients and micro-hardness of the glasses. The highest values of elastic coefficients (Table 3) and the lowest value of mechanical loss factor for the sample T6 suggest the lowest degree of disorder in the network. In other words, these results indicate that the sample T6 is more rigid among all other glass-ceramics. Titanium ions are expected to exist mainly in Ti4+ state in the present Li2 O–CaF2 –P2 O5 :TiO2 glass network. However, the reduction of Ti4+ to Ti3+ appears to be viable during melting, annealing and crystallization processes of the glasses. Earlier reports on some other glass systems containing TiO2 suggested that upon heating at about 700 ◦ C, there is a possibility for the reduction of Ti4+ ions to Ti3+ ions [11]. Further, the reduction, Ti4+ + e = Ti3+ , takes place only with E0 = 0.2 V. The Ti4+ ions

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study of DTA suggests the crystallization is predominantly due to the surface crystallization in the sample T6 , where as in the sample T8 , the bulk crystallization seems to prevail; the study on elastic properties indicates the highest hardness for the same sample. The IR and Raman spectral studies indicate the titanium ions occupy tetragonal positions in larger concentrations in the sample T6 and increase the compact ness of the sample.

References

Fig. 8. The local structure of TiO6 octahedron entwined with PO4 units.

are in general largely in six-fold coordination as corner-sharing [TiO6 ]2− units in the glass network. The band observed in the IR spectra between 640 and 650 cm−1 is due to Ti–O–Ti symmetric stretching vibrations of such TiO6 units [23–28]. The TiO6 octahedrons may be viewed as entwined with PO4 structural units forming TiPO9 where all the oxygens are bridging (Fig. 8). Nevertheless, these ions also present in the glass network with TiO4 structural units; the band observed in the IR spectra between 730 and 740 cm−1 , in fact represents vibrations due to such tetragonal units [22]. Though it is not quite clear from the present studies that this band is whether due to network forming structural units or due to isolated TiO4 units, the Raman spectral studies clearly indicated that there are TiO4 structural units that take part network forming positions in the present glass system. Tetragonally positioned Ti4+ ions do not induce the formation of any non-bridging oxygen ions but octahedrally positioned ions may do so [22] similar to Ti3+ ions. The highest intensity of the band due to TiO4 structural units observed in the Raman spectrum of the sample T6 indicates, the presence of titanium ions largely in tetragonal positions, occupy substitutional positions and hence, we may attribute the highest rigidity to this sample among all other glass-ceramics. When the concentration of TiO2 is raised from 0.6 to 0.8 mol% in the glass network, the intensity of the band due to TiO6 structural units is observed to increase at the expense of the band due to TiO4 structural units, indicating an increasing degree of disorder in the glass network. Similarly, in the Raman spectra, the strength of the bands due to conventional symmetric vibrations, viz., PO2 and POP modes is observed to increase while that of the torsional bending vibrational mode is observed to decrease with the increase in the concentration of TiO2 up to 0.6 mol% in the glass matrix. Thus, the analysis of the results of IR and Raman spectral studies points out that there is a low degree of disorder in the network of glass-ceramic when the concentration of TiO2 is ∼0.6 mol%. 5. Conclusions The X-ray diffraction and scanning electron microscopic studies on Li2 O–CaF2 –P2 O5 :TiO2 glass-ceramics indicate that these ceramic samples contain (orthophosphate, meta phosphate and pyrophosphate), titanium phosphate crystal phases. The

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