Introduction

TeO2-based glasses have a technical and scientific interest because of their low melting temperature, no hygroscopic property, good infrared transmission, electrical properties, optical properties and thermopower features [1–13]. Study in structural characteristics of glasses by spectral analyzing, DSC curve and elastic modulus can be a suitable way to understand the behavior of glasses as a function composition [14]. The role of changing in glass composition on the polarization power of network formers, modifiers, their coordination numbers, the concentration of non-bridging oxygen, rigidity, and packing of glass can help us to reach to optimized combination satisfying high thermal stability against thermal shocks for technological applications.

Nickel oxide (NiO) is an interesting material due to its useful electronic, magnetic, and catalytic properties [15]. NiO has also become very important for it can be used as electrode material in battery systems [16]. To the best of our knowledge, there are some papers on the calorimetric, structural, and physical properties of TeO2-based glasses and then, tricomponent glass systems of the form AmOn–TeO2–V2O5 (AmOn is an another oxide) have been studied [3, 4, 17–19]; in this work, due to the importance of the binary TeO2–V2O5 glass, the structural and calorimetric properties of V2O5–NiO–TeO2 glasses are studied, searching for glasses having high thermal stability; thus, in this work, we are going to follow two aims, first is the study of FTIR spectra and DSC curves of the ternary tellurite-vanadate glasses containing nickel oxide, to introduce samples with higher thermal stability, and second is finding logical correlation between thermal and structural characters and also application of Makishima and Mackenzie’s theory.

Experimental procedure

The ternary 40TeO2–(60−x)V2O5–xNiO glass systems with 0  ≤  x  ≤  30 (in mol%), hereafter, termed as TVNx, were prepared by rapid melt quenching method. The details of these sample preparation and their XRD patterns can be found in our previous work [4]. Also, the glass transition temperature (T g) of these samples were obtained using differential scanning calorimetry (DSC: Pyris1, USA) which increase with increasing of NiO content and was in the range 249.3–356.3 °C. The FTIR absorption spectra of the produced glass samples were recorded at room temperature on a Perkin Elmer Spectrum RX/I FT-IR System(USA) over a spectral range of 400–4,000 cm−1. First, disks of KBr, 1 cm in diameter, were prepared by pressing, and their infrared absorption spectra measured, showed transparency to light. Then, potassium bromide (KBr)-glass pressed powder pellets were used at 1:30 volume ratio of glass to KBr (preparation of pellets with high IR transmission) to collect the IR spectra and using these results, we are able to determine structural units.

Results and discussion

Differential scanning calorimetry (DSC)

Figure 1 shows the DSC charts obtained for the TVNx glasses; for better clarity of the plots, DSC charts have been plotted separately.

Fig. 1
figure 1

DSC curves of a TVN0, b TVN5, c TVN10, d TVN20, and e TVN30 (for better clarity are shown separately)

Full size image

The DSC curves for the glasses show a glass transition correspond to temperature T g that we usually just take the middle of the incline to be the T g. This transition is followed by one exothermic peak corresponding to crystallization temperature T cr; in other word, DSC measurements show a pronounced crystallization exotherm after the glass transition temperature indicating the glassy nature of the materials at temperature below the glass transition. No indication of T cr was observed in DSC spectrum of TVN30 in the studied temperature range (0–600 °C). The data of T g, T cr, glass transition width (ΔT g = T g.end−T g.onset, where Tg.end and Tg.onset are the end point and start point of endothermic part of DSC curve corresponding to glass transition phenomenon) and heat capacity change at glass transition (ΔC P) are presented in Table 1; ΔC P values of these glasses have been reported only at transition temperature during the DSC experiment. Figure 2 shows the variation of the glass transition temperature with NiO content in the TVNx samples. From this figure, one sees that the glass transition temperature is very sensitive to the NiO concentration. The addition of NiO to vitreous TeO2–V2O5 from 0 to 30 mol% results in a regular increase of T g from 249.32 to 365.30 °C. Such a result was reported also for vanadium telluride blown film glasses [20]. Increasing of T g can be interpreted as increasing of the rigidity of the glass; in other word, the change in T g for different samples is related to the structure of each glass, arising from the interaction between the three oxides; this statement has been justified upon the FTIR results, presented in “Fourier transform infra-red analysis”. Thermal stability depends on \( \Updelta T = T_{\text{Cr}} - T_{\text{g}} , \) where T Cr is crystallization temperature [18, 19, 21–24]. Upon the data of T g, T Cr, and ΔT (i.e., Table 1), it is obvious that ΔT increase with increasing NiO content; hence, it can be predicted that the thermal stability of these glasses will be >80 and 67 K, which were the highest thermal stability measured for CoO–V2O5–TeO2 [18] and V2O5–Fe2O3–TeO2 [19], respectively; since T g and T Cr are the characteristic temperatures of any glass and T Cr can not be spotted within the studied temperature range for TVN30, it can be therefore expected that the T Cr value of this glass must be above 600 °C (higher ΔT between other TVNx glasses), which make it strong against thermal shocks. The variation of the excess heat capacity, ΔC P, in the present system at the glass transition temperature as a function of composition is shown in Fig. 3. It can be seen in Fig. 3 that ΔC P decreases with increasing NiO concentration. In general, a smaller ΔC P is a characteristic feature of a fragile glass which has larger glass-forming tendency and a higher ΔC P corresponds to a strong glass which has a smallest glass-forming tendency [25–28]. The thermodynamic fragility, F, can be calculated by using the relation:

$$ F = \,\frac{0.151 - \,\chi }{0.151 + \,\chi } $$
(1)

where \( \chi = \,{{\Updelta T_{\text{g}} } \mathord{\left/ {\vphantom {{\Updelta T_{\text{g}} } {T_{\text{g}} }}} \right. \kern-\nulldelimiterspace} {T_{\text{g}} }} \) [25, 29]; ΔT g data have been listed in Table 1. The variation of thermodynamic fragility with NiO content is shown in Fig. 3 for these glasses. Thermodynamic fragility is found to increase with increasing NiO content. This can be attributed to the glass network getting loosely packed with increasing of NiO content.

Table 1 Values of density ρ, excess heat capacity at transition temperature ΔC P, glass transition temperature T g, crystallization temperature T Cr, Fragilty F, molar volume V m, Poisson’s ratio μ cal, Young’s modulus E, bulk modulus K, shear modulus S, and other needed parameters for TVNx glasses
Full size table
Fig. 2
figure 2

Variations of glass transition temperature and Poisson’s ratio for TVNx glasses

Full size image
Fig. 3
figure 3

Variations of excess heat capacity and fragility of TVNx glasses

Full size image

A similar conclusion has been drawn in the case of CoO–V2O5–TeO2 [18]. However, the glass-forming tendency increases with increasing NiO content.

Fourier transform infrared analysis (FTIR)

The structure of the glasses was also studied using FTIR analysis in the wave number range 400–4,000 cm−1. There are no characteristic absorption bands in the range 1,150–4,000 cm−1, only shallow oscillations due to interference effects in the glass; bands at 2,356 and 2,900 cm−1 are associated to the aparatus’s background absorption. Figure 4 shows FTIR spectra of TVNx glasses in the range 400–1,200 cm−1. For comparison, the absorption bands of KBr pellets of individual pure powders are also presented in Table 2. Comparison of the absorption bands of all the ternary glass systems and individual oxide powders reveal that, the FTIR spectra of the glass systems are not characteristics of a mixture of the three oxides. We suggest that, this property of the spectra indicates a chemical interaction between the three oxides [29–32]. The similarity between the FTIR spectra of all glasses indicates a similarity in their networks. In compare with the results obtained previously [14, 20, 32, 33], we suggest that the glass structure is continuous tellurite network with vanadate and nickel discontinuous. The absorption band in pure tellurium oxide powder is at 677 cm−1. During the addition of other oxides and making glasses, this band changes to a shoulder (655–682 cm−1). Also pure tellurite glasses have an IR absorption band at 640 cm−1 which is attributed to TeO4 tetragonal pyramids [20, 22, 34]. The major observed absorption bands in the ternary TeO2–V2O5–NiO glass system are summarized in Table 2. It is clear from Fig. 4 and Table 2 that the 640 cm−1 which is characteristics of pure TeO2 glass, changes to a broad band and shifts from 640 to 665–678 cm−1 on addition of V2O5 and NiO. Dimitriev et al. [20] attributed this absorption band, 665–680 cm−1, to TeO3 trigonal pyramids. The weakness of this band may be due to decrease of the amount of 4-coordinate tellurium in the studied glasses (increasing of non-bridging oxygens). It is suggested that the 3-coordinated Te may become significant with increasing the concentration of V2O5. In the TeO2 glasses containing 60 mol% V2O5, a new band is observed at about 1,158 cm−1, which shifts to lower wave numbers with decreasing the concentration of V2O5. This peak is probably attributed to V=O band of VO5 group [14, 33, 35]. So, in contrast to Dimitriev observation, its intensity does not significant frequency and is due to the presence of V5+ ions; these ions have coordinated by five oxygen. The VO5 fragments may be attached to other metal ions via V=O double bonds or V–O single bonds.

Fig. 4
figure 4

FTIR spectra of a TVN0, b TVN5, c TVN10, d TVN20, and e TVN30 (for better clarity are shown separately); see Table 2

Full size image
Table 2 Observed FTIR absorption bands in TVNx glasses
Full size table

Chopra et al. [33] reported that the intensity of the band decreases with decreasing the V5+ ions and increasing V4+ ions (increasing of V4+/V tot fraction). Also, the peak shifts to lower wave number with decreasing the content of V2O5. Such lowering in stretching frequency may be the second reason of the observation of the new band at about 1,158 cm−1, which shifts to lower wave numbers.

Addition of NiO besides V2O5 in TeO2 glass, cause another complex situation in the spectra. Oxygen deficiency and the presence of V4+ in the mixed oxides lead to the simultaneous formation of VO6 and VO5 polyhedra with several short V–O bonds. This is probably, the reason for a new shoulder observed at about 1,120 cm−1 for TVNx glasses with x ≥ 10 mol%. The absorption band at 830–988 cm−1 arises due to Ni–O bonds [36]. Therefore, it is suggested that the similarity between the IR spectra of all glasses indicates a similarity in the network of these samples, namely continuous tellurite network with vanadate and nickle discontinuous.

Elastic modulus and Poisson’s ratio

There is a famous model to determine elastic properties of glasses: Makashima–Mackenzie’s model.

It is known that the young’s modulus of crystalline oxides is given by:

$$ E = \frac{ 2\alpha U}{{r_{\text{o}}^{ 3} }} $$
(2)

where α is the made lung constant, U is electrostatic energy and r o is the interatomic distance. In the vitreous materials, as a consequence of disorder nature, we cannot define madelung constant as for crystalline oxides.

Based on Makashima–Meckenzie’s model, first we introduce the packing factor of oxide M x O y (V i ) and the packing density of glass (V t) as:

$$ V_{\text{i}} = \frac{ 4}{ 3}\pi \,N_{\text{A}} (x.R_{\text{M}}^{ 3} + y.R_{\text{O}}^{ 3} ) $$
(3)
$$ V_{\text{t}} = V_{\text{M}}^{ - 1} \sum {(x_{\text{i}} .V_{\text{i}} )} $$
(4)

where R M and R O are the respective Pauling’s ionic radius of metal M and oxygen O, N A is Avogadro’s number, V M the molar volume of glass, xi the mole fraction of oxide component i; the values of V M have been taken from our previous paper [4]. So Young’s modulus E of oxide glasses can be revealed in term of the packing density of glass V t and the dissociation energy per unit volume G t, as [37]:

$$ E = 2\,V_{\text{t}} .\,G_{\text{t}} $$
(5)

In above equation, G t is determined by:

$$ G_{\text{t}} = \sum {(x_{\text{i}} .G_{\text{i}} )} $$
(6)

where G i is dissociation energy per unit volume of the ith oxide.

Thus, we can obtain some of the elastic parameters such as bulk modulus K and shear modulus S as:

$$ K = 1.2V_{\text{t}} E = 2.4V_{\text{t}}^{2} G_{\text{t}} $$
(7)
$$ S = \frac{ 3EK}{( 9K - E)} $$
(8)

The calculated values of Young’s modulus, Shear modulus, bulk modulus and molar volume are presented in Table 1. In these evaluations, the values of dissociation energy of oxides have been taken from [38, 39], which are 54, 69.5, and 82.21 J m−3 for TeO2, V2O5, and NiO, correspondingly. It is known that Poisson’s ratio is defined by the ratio of the transverse (lateral) to the longitudinal (axial) strain, and the lateral strain would be smaller in loosly packed glasses because there is more space for atoms to move in [34]. This means that Poisson’s ratio is small if atoms are loosely packed in the oxide glass whereas tightly packed glass has a higher Piosson’s ratio. Theoretically, Poisson’s ratio calculated from the expression given by Makishima and Makenzie [17, 37] as

$$ \mu_{\text{cal}} = 0.5 - \frac{1}{{7.2V_{\text{t}} }} $$
(9)

Results show the decrease in Poisson’s ratio from 0.270 to 0.255 as shown in Fig. 2. This result justifies the results of ΔC P and fragility.

As seen from Table 1 and Figs. 5 and 6, bulk modulus is higher than that of SiO2–Na2O–B2O3 glasses [23]. Insertion of the modifier NiO with low content 0–10 mol% will cause a rapid decrease in elastic moduli suggesting the decrease in the average number of bonds per unit volume. The addition of NiO content up to 30 mol%, will result the increase in E and S. Figures 5 and 6 show the variation of shear and Young’s moduli with NiO content. The behavior of both shear and Young’s moduli show rapid decrease from 30.144 and 76.593 GPa to 29.919 and 75.550 Gpa, respectively with the increase of NiO content from 0 to 10 mol%. Upon the increase of NiO concentration to 30 mol%, both shear and Young’s moduli show increasing to 76.113 and 30.322 Gpa, respectively. It is clear from the above results that the type of bonding in the network structure plays a dominant role in deciding the rigidity of these glass structures. It is believed that the behavior of both shear and Young’s moduli are associated with the change in cross-linkage and coordination of the glass network [23]. Bulk modulus and Poisson’s ratio decreased with the increase of NiO content.

Fig. 5
figure 5

Variations of Young’s modulus E and bulk modulus K for TVNx glasses

Full size image
Fig. 6
figure 6

Variation of shear modulus for TVNx glasses

Full size image

In general, results of this work show that glasses with x = 0 and 30 have the highest shear and young’s modulus which make them as suitable candidate for the manufacture of strong glass fibers in technological applications; but it should be mentioned that glass with x = 30 has probably higher handling temperature and super resistance against thermal shock, because of (a): it has the highest glass transition temperature, which makes it stronger against structural changes [18, 19, 39] and (b): glasses with higher thermal stability have higher resistance against heat shocks; on the other hand, thermal stability depends on \( {{\Updelta}}T = T_{\text{Cr}} - T_{\text{g}} \) [18, 19, 21–24]; thus, as presented in Fig. 1 and Table 1, ΔT increase with increasing of NiO content (For TVN30, T Cr has not shown because it is larger than 600 °C and therefore, its ΔT is larger than that of other TVNx samples). Finally, TVN30 has the highest thermal stability, and (c): a higher value of elastic moduli is somewhat in accordance with lower thermal expansion [39], which imply to higher resistance against thermal shocks.

Conclusions

The density, elastic properties, FTIR studies, and thermal properties on the network structure of TVNX glasses reveal the following conclusions:

  1. 1.

    The density of the glass system studied increases with an increase in mol percentage of NiO.

  2. 2.

    Elastic moduli of these glasses have minima at NiO content about 10 mol%.

  3. 3.

    Analysis of the FTIR spectra shows an increase in the fraction of non-bridging oxygens (NBO’s) with an increase of NiO content.

  4. 4.

    Bulk modulus and Poisson’s ratio decreased with the increase of NiO content.

  5. 5.

    Results of this work show that glasses with x = 0 and 30 have the highest shear and young’s modulus which make them as suitable candidate for the manufacture of strong glass fibers in technological applications; but it should be mentioned that glass with x = 30 has higher handling temperature and super resistance against thermal shock.