Chapter at ~ 950oC in muffle furnace for 1

Chapter 4Alumino-TelluriteGlasses 4.1 Introduction Thischapter describes the results on the glass formation range of alumino-telluritesystem and the Te-O and Al-O coordinations in the samples using Raman and 27AlMAS-NMR techniques respectively. The thermal properties are studied bydifferential scanning calorimetry.4.2 Glass preparation            Aluminum tellurite glasses of thefollowing composition:xAl2O3- (100-x) TeO2; (x = 1, 2, 3, 5, 7 and 20-mol%) weresynthesized by splat and ice quenching. Appropriate amounts of the startingmaterials Al2O3 (Aldrich India, 99.

9%) and TeO2(Aldrich India, 99%) were mixed and melted in a Pt crucible at ~ 950oCin muffle furnace for 1 h. Samples containing 1 and 2-mol% of Al2O3were prepared by ice quenching due to the higher content of TeO2which required high quenching rate. In ice quenching the bottom of the platinumcrucible containing the melt was immersed in an ice-water bath and transparentthin flakes of the sample were formed. Although other samples (5-20-mol% Al2O3)were prepared by splat quenching in which the small quantity of the melt waspressed between two massive steel plates.

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Transparent and bubble free aluminumtellurite samples were formed. Multiple attempts were carried out to synthesizesufficient amounts of samples for analysis. At higher concentration of Al2O3(20-mol%), semi-transparent sample was obtained. 4.3 XRDXRDpatterns of all alumino tellurite samples exhibit broad hump in the range: 20oto 35o without any sharp peaks, which confirms the amorphousstructure of these samples (Fig. 4.1).

While all samples were clear and transparent, the sample: 20AlTe wassemi-transparent although it was found to be X-ray amorphous. This samplecontained highest concentration of Al2O3 (20-mol%) andits semi-transparency is due to amorphous-amorphous phase separation. Asreported in an earlier studies by Kaur is found that even in Al2O3-B2O3-TeO2glass system, higher concentration of Al2O3 (30-mol%)shows the phase separation and formed inclusions of ?-Al2O3 crystals (Kaur & Khanna, 2014). XRD pattern of sample 1AlTeconfirms that it is possible to prepare TeO2 glass containing verysmall amount of alumina (1-mol%) content at normal quenching rate (~103Ks?1). The short-range structure and thermal properties of 1AlTeglass are expected to be quite similar to that of pure TeO2 glass (Barney et al.

, 2013; Tagiaraet al., 2017).  Fig 4.1: XRD patterns of aluminum tellurite glasses.           4.4 Thermal analysisDSCthermographs of Al2O3-TeO2 glasses are shownin Fig. 4.

2 and their thermalproperties are summarized in Table 4.1.The Tg value increasessteadily from 311oC to 392oC on increasing Al2O3concentration from 1 to 20-mol%. Tgfor alumino-tellurite glasses are in excellent agreement with the results fromthe earlier study, for example Tgfor 1AlTe and 3AlTe glasses prepared in the present work are 311oCand 323oC respectively and Taigara et al. (Tagiara, et al., 2017) reported Tg of 311.9oCand 322.

7oC  for glasses ofthe same compositions. Taigara et. al.and Lombson also reported thatthe pure TeO2 glass prepared in alumina crucible shows Tg at 385oC and380oC respectively while TeO2 glass prepared in Platinum(Pt) crucible shows Tg at303oC (Tagiara, et al., 2017; Lambson, 1984#385).

However in the present work, theaddition of 1-mol% Al2O3 in TeO2 glassprepared in Pt crucible shows Tgat 311oC, which is lower than the pure tellurite glass formed byusing alumina crucible. This indicates that the pure tellurite glass formed inalumina crucible has high content of alumina impurity as compare to telluriteglass containing 1-mol% of Al2O3. The high content of Al2O3forms stronger Al-O-Te linkages over Te-O-Te linkages because of the higherbond dissociation energy of Al-O (512 kJ mol-1) bonds over Te-O (391kJ mol-1) bonds and results the high in Tg values. The increase in Tg of the alumino-tellurite glasses directly correlatewith the bond dissociation energy of the glasses which increases from 392 kJmol-1 to 415 kJ mol-1 with increasing Al2O3 Fig 4.2: DSC patterns of aluminum tellurite glasses.            concentration.The average single bond enthalpy, EBis calculated by the following formula (Kaur et al., 2015):                                                    (4.

1)where,ETe-O and EAl-O are the single bonddissociation energies of Te-O (391 kJ mol-1) and  Al-O bonds (512 kJ mol-1)respectively (Cottrell, 1958; Darwent,1970; Dean,1999; Kaur,et al., 2015). The increase in EB corresponds to thereplacement of weaker Te-O-Te linkages over stronger Al-O-Te/Al-O-Al linkages. Fig. 4.

3 shows the linear variation of Tg with EB according to the following linear equation: Fig 4.3: Variation of Tg with bond dissociation energy of aluminum tellurite glasses.                                                (4.2)          Table 4.1:  Compositional and DSC data of alumino-tellurite glasses.

Sample Code Composition mol% Tg ±1oC Tc ±1 oC Tm ±1 oC ?T = EB kJ mol-1 Al2O3 TeO2 Tc1 Tc2 Tc3 Tm1 Tm2 1AlTe 1 99 311 347 – – 738 – 36 392.30 2AlTe 2 98 321 380 427 453 714 – 59 394.41 3AlTe 3 97 323 384 430   728 – 61 394.63 5AlTe 5 95 332 392 439 478 700 – 60 397.05 7AlTe 7 93 340 408 434 477 695 – 68 399.47 20AlTe 20 80 392 530 615 – 647 680 138 415.

20         The addition of Al2O3beyond 1-mol% in tellurite glasses shows number of exothermic peaks(crystallization peak) and single endothermic peak (melting peak). Thesemultiple crystallization peaks reveals the existence of several crystallinephases of Al2O3-TeO2 system which can beformed by devitrification of the samples after annealing them at certaintemperature (> Tg).Incorporation of Al2O3 in Al2O3-TeO2glasses shifts the Tc1from 347 oC to higher temperature, at 530 oC. Thecrystallization temperatures Tc2and Tc3 also get prominentwith increasing Al2O3 concentration from 1 to 20-mol%.

This increase in crystallization temperature reveals that the tendency of Al2O3-TeO2glasses towards devitrification decreases with increasing Al2O3content. Aluminumtellurite glass with 20-mol% of Al2O3 shows two glasstransitions phenomena, first a strong transition at 393 oC and thesecond weak glass transition at 440 oC. This multiple transitionsphenomena indicate the presence of amorphous-amorphous phase separation inalumino-tellurite glasses, which was also found by Kaur et. al. in case of alumino-borotellurite glasses containing 20-mol%of Al2O3 (Kaur & Khanna, 2014).

This implies that the higherconcentration of Al2O3 in tellurite and as well as inborotellurite leads to the phase separation. The thermal stability  of Al2O3-TeO2 glasses increases from 36oC to 138 oC on increasing Al2O3concentration from 1 to 20-mol% (Table4.1). 4.5 Raman SpectroscopyRamanspectra of alumino-tellurite glasses are shown in Fig 4.4. Raman patterns show a strong peak at ~59 cm-1,a shoulder at 106 cm-1 and two broad bands in the range:  300 to 550 cm-1 and 550 to 970 cm-1.The spectra are baseline corrected and deconvoluted with peaks centered at 619,662, 719, 779 and 821 cm-1.

The peak at 59 cm-1 is theboson peak which is the characteristic peak of vitreous solids in low frequencyregion. The intensity of this peak depends upon the chemical composition andcorrelation functions of the sample and it is growing with increasing the Al2O3content from 1-20-mol% (Malinovsky & Sokolov, 1986). The shoulder at 106 cm-1is due to the longitudinal optical mode vibrations of TeO4 unitsaround the bridging oxygens (Pine & Dresselhaus, 1972). The broad band in the Raman shiftrange: 300 to 550 cm-1 is due to symmetric stretching vibrations ofTe-O-Te and O-Te-O linkages. The intensity of this band increases withincreasing Al2O3 content from 1 to 20-mol %. This is dueto the formation of stronger Al-O-Al/Te-O-Al (512 kJ mol-1) linkagesover Te-O-Te (391 kJ mol-1) linkages on Fig 4.

4: Raman spectra of aluminum tellurite glasses.            addingAl2O3 content. Raman spectra in the range 550 to 970 cm-1corresponds to the tellurite structural units (Table 4.2). Raman spectra of all the alumino-tellurite glasses inthis range were deconvoluted by Gaussian peaks and centered at 619, 662, 724,775 and 806 cm-1 as shown in Fig.4.

5. The peak at 619 corresponds to the antisymmetric vibrations of TeO4subunits. The peak at 662 cm-1 is due to the antisymmetricvibrations of Te-O linkages mainly in TeO4 units Raman bands (cm-1) Assignments 59 Due to boson peak 106 longitudinal optical mode vibrations of TeO4 units around the bridging oxygens 300-550 Stretching vibrations of Te-O-Te, O-Te-O linkages 550-700 Stretching vibrations of TeO4 tetrahedra 700-800 Stretching vibrations of TeO3+1 and TeO3 units 806 Due to the AlO4 units having three bridging and one non bridging oxygen   Table 4.2: Raman bands assignments of alumino-tellurite glasses.       andis also due to small vibrations of Te-O units produced by TeO3+1 andTeO3 subunits.

Peaks at 724 and 775 cm-1 are assigned tostretching vibrations of Te-O- and Te=O bands having NBOs, formed byTeO3+1 and TeO3. (A. Kaur et al.

, 2016; Manning,2011; Seguinet al., 1995). The peak at 806 cm-1is due to the Al-O linkages in AlO4 units having three bridging andone non-bridging oxygen (Yadav & Singh, 2015). The intensity of peaks at 724 and775 cm-1 increases with increasing Al2O3content in Al2O3-TeO2 glasses as shown in Fig. 4.

5, which indicates the increase in concentration of TeO3/TeO3+1units. Even the peak at 806 cm-1 is shifted to the higherwavenumber, 862 cm-1 on increasing Al2O3amount from 1 to 20-mol% which indicates the formation of stronger AlO4units. Fig 4.5: Deconvoluted Raman spectra of aluminum tellurite glasses.

               Theareas under these deconvoluted peaks were used to calculate the coordination ofTe with oxygen, NTe-O from following formula:                                            (4.3) Sample Code Area Ratio NTe-O ±0.01 1AlTe 0.66 3.66 2AlTe 0.64 3.

64 3AlTe 0.61 3.61 5AlTe 0.56 3.56 7AlTe 0.49 3.

49 20AlTe 0.33 3.33   Table 4.3: Te-O speciation calculated from Raman spectroscopy. NTe-O decreases significantly from 3.66to 3.

33 (Table 4.3) with increase inAl2O3 concentration from 1 to 20-mol% which reveals thestructural transformation: TeO4 à TeO3+1àTeO3 in Al2O3-TeO2 glasses. Earlierstudies on tellurite glasses by Barney (Barney, et al., 2013) and Gulenko et. al. (Gulenko et al.

, 2014) it is found that the value of NTe-Ofor pure tellurite glass from neutron diffraction studies to be 3.68 and 3.73respectively. Similarly, Pietrucci (Pietrucci et al.

, 2008) reported from ab initio calculations that NTe-O = 3.73. These valuesare consistent with NTe-O value of 3.

66 for the sample: 1AlTecharacterized in the present work. Hence at low Al2O3content of 1-mol%, the short-range structure of alumino-tellurite glass isquite similar to that of pure tellurite glass.       4.6 27Al MAS-NMRNMRspectra of alumino-tellurite glasses are shown in Fig. 4.6 and it has three resonance peaks at ~ 6 ppm, 31 ppm and 52ppm. The resonance peak at 6 ppm corresponds to hexa-coordinated AlO6(Al6), the peak at 31 ppm is due to the five-fold coordinated AlO5(Al5) and finally the peak at 52 ppm is due to the tetrahedral AlO4(Al4) units (Table 4.3) (Clayden et al.

, 1999). At low concentration of Al2O3(3-mol %) in Al2O3-TeO2 glasses, the amount ofAl6 is higher than that of Al4  which implies that at low Al2O3content mainly hexa-coordinated Al6 exist in Al2O3-TeO2glasses. High concentration of Al2O3 (20-mol%) in Al2O3-TeO2glasses shows the additional peak at 13 ppm which corresponds to Al6units of ?-Al2O3 phase (N. Kaur et al., 2016). On increasing the amount of Al2O3in Al2O3-TeO2 glasses from 3 to 20-mol% theintensity of peak at 52 ppm gets prominent which indicates the formation of AlO4over AlO6 units. Fig 4.

6: Al27MAS-NMR spectra of aluminum tellurite glasses. Table 4.4: Al27 MAS-NMR assignments of Al2O3-TeO2 glasses. Peaks (ppm) Assignments 6, 13 Due to AlO6 units 31 Due to AlO5 units 52 Due to AlO4 units                  Tofind out the concentration of existing A4, A5 and A6units in Al2O3-TeO2 glasses, their NMR spectrawere normalized and fitted with Gaussian peaks centered at 6 ppm, 31 ppm and 52ppm (Fig. 4.7).

The areas (A) under these peaks were used tocalculate the concentration of Al4, Al5 and Al6structural units by using following formulas:                                                           (4.4)                                                          (4.5)                                                          (4.6) The addition of Al2O3upto 20-mol% increases the concentration of Al4 from 16% to 33%while Al6 decreases from 54% to 35%.

The concentration of Al5  increases by a small amount from 30% to 33%.This decrease in Al6 illustrates the structural transformations Al6àAl5 à Al4 on adding Al2O3in Al2O3-TeO2 glasses. These results are alsoconsistent with the DSC and Raman analysis which find an enhancement of Tgand increase in the intensity of Raman peak at ~850 cm-1 due toincrease in the concentration of Al4 with increase in Al2O3mol%. Fig.

4.8 shows the relativeconcentration of Te4, Te3, Al6, Al5and Al4 units with increasing Al2O3 contentin Al2O3-TeO2 glasses. Fig 4.7: One of the deconvoluted Al27MAS-NMR spectra of aluminum tellurite glasses containing 3-mol% of Al2O3.              Table 4.5: Al-O speciation calculated from Al27 MAS-NMR spectroscopy.

Sample Code Relative concentration Al6 Al5 Al4 3AlTe 0.54 0.30 0.16 5AlTe 0.51 0.29 0.20 7AlTe 0.

46 0.32 0.22 20AlTe 0.35 0.33 0.33        Fig.

4.8: Variation of Te4, Te3, Al6, Al5 and Al4 units with Al2O3 mol% in Al2O3-TeO2 glasses.   SummaryAlumino-telluriteglasses containing varying concentration of Al2O3 wereprepared and characterized by XRD, DSC, Raman and Al27 MAS-NMRstudies. The absence of sharp peaks in XRD patterns confirms the amorphousstructure of all samples and transparent glasses can be formed with Al2O3concentration in the range: 1 to 7-mol% while higher concentration of Al2O3(20-mol%) in tellurite glasses form phase separated samples. DSCanalysis found that the Tg of alumino-tellurite glasses increasesfrom a value of 311oC to 393oC on increasing Al2O3concentration from 3 to 20-mol%.

The Tg values show linearcorrelation with the average bond dissociation energy of the glasses, thelatter increases with Al2O3 concentration. The averagebond dissociation energy of glasses increase due to the higher bond enthalpy ofAl-O bonds (512 kJ mol-1) than that of Te-O bonds (391 kJ mol-1).The addition of Al2O3 in tellurite glasses form strongerTe-O-Al linkages over weaker Te-O-Te linkages and enhance the thermal stabilityof the alumino-tellurite glasses. Aluminum tellurite glass containing 20-mol%of Al2O3 shows amorphous-amorphous phase separation.Ramanstudy shows that on increasing Al2O3 content from 1 to20-mol%, the concentration of TeO4 units decreases drastically.

Thedecrease in the concentration of TeO4 units confirms the structuraltransformation: TeO4àTeO3. Raman studies alsofound that the Te-O co-ordination in alumino-tellurite glass containing 1-mol%Al2O3 (NTe-O = 3.64) is close to that of puretellurite glass (NTe-O = 3.68) as reported by neutron diffractionstudies. 27AlMAS-NMR spectra of aluminum tellurite glasses show that at low concentration ofAl2O3 in tellurite glasses, mostly hexa-coordinated, AlO6units exist along with small amount of AlO5 and AlO4units.

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