X-ray diffraction (XRD)
The X-ray diffraction (XRD) analysis was used to determine whether the material was in amorphous or crystalline state. The X-ray diffraction patterns of the prepared borotellurite glasses doped with manganese dioxide and strontium oxide were recorded. The following figures showed the XRD diffraction patterns of the two glass series.
Both Figures 1 and 2 showed broad hump patterns in the XRD spectra. Broad humps for both glass series indicated the presence of long range structural disorder [4]. Meanwhile, Rosmawati et al. (2008) [8] stated that the absence of sharp and strongly diffracted beams in the glass X-ray diffraction pattern indicated that there were no well-defined planes in the structure. Hence, the broad hump pattern in the XRD analysis confirmed that the prepared glass samples were in amorphous state.
Fourier Transform Infrared (FTIR)
FTIR spectroscopy measurement was a technique that was mainly used to obtain and explain the internal structural unit and the fundamental groups in the studied glass systems. The infrared spectra of absorption and transmission in the glass samples explained the characteristic of every molecule. The FTIR spectra of the glass samples were recorded in the range of 280-1680 cm-1 and shown in Figure 3 and 4. The observed broad bands could be associated with a combination of higher degeneracy of vibrational states, thermal broadening of the lattice dispersion, mechanical scattering of the powdered samples and the corresponding band assignments [9].
The FTIR spectra had several peaks specifying its local structure. For every structure, specific frequencies had been assigned. Table 1 showed the peak positions and its assignments from the FTIR result.
Table 1: Assignment of infrared transmission bands of the prepared glass samples with different concentration of manganese dioxide and strontium oxide.
No
|
0.01
|
0.02
|
0.03
|
0.04
|
0.05
|
Assignment
|
1
|
654.21
|
653.81
|
656.28
|
656.11
|
655.17
|
TeO4 group existed in all tellurite containing glass ranging from 600-665 cm-1 [10].
|
2
|
1216.85
|
1215.45
|
1216.73
|
1218.56
|
1217.28
|
Trigonal B-O bond stretching vibration of BO3 units from boroxyl groups ranging from 1200-1500 cm-1 [10].
|
3
|
1350.98
|
1364.74
|
1346.40
|
1341.81
|
1355.57
|
The stretching vibration of the B-O of trigonal (BO3) units were assigned at 1350 cm-1 [11].
|
Tellurite oxide consists of two types of structural configuration units which are trigonal bipyramid TeO4 and trigonal pyramid, TeO3. Pure TeO2 was characterized around 640 cm-1. Meanwhile, the absorption band at 600 – 700 cm-1 was assigned as the stretching vibration of Te-O bonds. The stretching vibration of TeO3 posed higher frequency (665-700 cm-1) rather than TeO4 stretching vibration (600-665 cm-1) [12]. In this work, TeO4 group was found to exist in the range of 654 – 656 cm-1 which was in the normal range of TeO4. The absorption band of pure borate glass, B2O3 was centered at 806 cm-1, indicating characteristics of boroxyl ring [9]. As the absorption spectra indicated the absence of boroxyl ring, BO3 and BO4 appeared as the substitution to the boroxyl ring [10]. In this work, the absorption band positioned at 1215 – 1218 cm-1 was assigned to the trigonal B-O bond stretching vibration of BO3 units from boroxyl groups. The absorption band at 1350 – 1364 cm-1 was assigned to the stretching vibration of the B-O of trigonal (BO3) units [13]. Meanwhile, the manganese oxide and strontium dioxide could not be detected in the FTIR spectra due to the small amount of dopants’ concentration.
Physical analysis
Density and molar volume
The calculated density and molar volume of the prepared glass samples were recorded in Table 2.
Table 2: Density and molar volume of borotellurite glass doped with manganese oxide and strontium dioxide
Molar fraction, x
|
Density Mn (g/cm3)
|
Molar volume Mn (cm3/mol)
|
Density Sr (g/cm3)
|
Molar volume Sr (cm3/mol)
|
0.01
|
3.505
|
37.700
|
2.8280
|
46.7877
|
0.02
|
3.323
|
39.649
|
3.2976
|
40.0350
|
0.03
|
3.113
|
42.149
|
3.6462
|
36.1295
|
0.04
|
3.108
|
42.069
|
3.7501
|
35.0512
|
0.05
|
3.066
|
42.503
|
3.8053
|
34.4663
|
From the table above, it could be observed that as the concentration of manganese dioxide increased, the density of the prepared glass sample decreased. The increase of manganese cation concentration made the structure of the glass to become more spaced out, producing higher numbers of non-bridging oxygen (NBO) [15]. The introduction of manganese dioxide ruptured some of the BO4 and TeO4 bonds transforming them into BO3 and TeO3 which created more NBO. The decrease in density of the glass samples was also due to the addition and substitution of lighter and manganese dioxide as compared to tellurite. The atomic mass of manganese dioxide is 86.9368 g/mol while the atomic mass of borate is 69.6182 g/mol which are both lighter compared to tellurium (127.60 g/mol). Hence, this caused the density to decrease as the molar fraction increased. This result also agreed with Doweider and Saddek (2009) [16] which stated that density will change if there was replacement of atom in the glass network.
According to Krogh Moe. (1962) [17], the conversion from the trigonal boron atom (BO3) into four-fold (BO4) coordinated boron atom could occur upon modification by an alkali oxide through the creation of bridging oxygen network between each BO4 negatively charged structural group and four oxygen. This phenomenon led to the changes in composition that were proven by the density result which reflected the underlying atomic arrangements in a quantitative manner. The increment in the density was attributed by the factors of conversion BO3 triangles into BO4 tetrahedral which was due to the insertion of the higher atomic weight of strontium (87.62 amu). In addition, the increment of the oxygen-boron ratio could also influence the increment of density as the concentration of SrO increased [18]. The smaller ionic radius of strontium ion, Sr2+ (11810-12 m) compared to tellurium ion, Te4+ (22110-12 m) forces the strontium ions to fit into the interstitial spaces of the glass network in order to form the BO4 groups which then led to the increment of density, resulting in improvement of the glass system compactness.
Molar volume was inversely proportional to the density of the glass sample. This explaines that as the density decreased, the molar volume increased. As displayed in Figure 6, the increment of molar volume was directly proportional to the molar fraction of manganese dioxide in the glass sample. Based on Azianty et al., (2012) [19], the increment of NBO in the glass network had been proposed as the reason behind the increment in molar volume values. The experimental result adhered to the theoretical result which stated that density was inversely proportional to molar volume. The increase in the formation of NBO in the glass structure encouraged the rise in molar volume due to the production of more interatomic space between the molecules. Therefore, the glass structure became less dense. Besides that, the increment of molar volume may be attributed by the increase in bond length and inter atomic spacing between the atoms [20]. The introduction of manganese dioxide ion affected the bond length between the atoms. This was because manganese dioxide had ~ 1.900 Å bond length compared to tellurium that had 2.684 Å bond length. This imbalance in bond length between manganese and tellurium also caused more interspatial space to be created in the glass structure thus, causing an increase in the molar volume.
The calculated molar volume of the prepared glass samples was also recorded in Table 4.2. Based on Figure 8, the molar volume graph showed a decrement behavior from the density graph as the molar fraction of strontium oxide increased. This might be due to the fact that there was an incorporation of strontium ion of radii 1.12 Å to replace the tellurium ion of radius 0.97Å which altered the stretching of the glass network. The current trend of molar volume was also similar to the result obtained by Norihan et al. (2018) [21].
Elastic properties
Elastic properties provides information about the internal arrangement of constituent elements and the mechanical strength of the glass [22,23,24]. In this work, the elastic constants and the other physical parameters were evaluated from the experimental measurements obtained by using ultrasonic velocities in the glass system. From this technique, shear and longitudinal velocities could be obtained as they were vital parameters in order to identify the other elastic parameters. Glasses were considered as elastic substance which could be characterized by using ultrasonic non-destructive technique or pulse-echo technique. According to Halimah et al. (2010) [14], glasses were isotropic and had only two independent elastic constants which were longitudinal and shear elastic moduli. Both of those parameters were obtained from the ultrasonic velocities and density of the glass.
Table 3: Longitudinal and shear velocities of Mn and Sr-doped borotellurite glass
Molar fraction
|
Mn-doped glass
|
Sr-doped glass
|
vL (m/s)
|
vS (m/s)
|
vL (m/s)
|
vS (m/s)
|
0.01
|
3790
|
2200
|
8334
|
4401
|
0.02
|
3760
|
2190
|
7427
|
4317
|
0.03
|
3820
|
2230
|
7582
|
4344
|
0.04
|
3860
|
2240
|
7227
|
4185
|
0.05
|
3900
|
2250
|
7849
|
4482
|
As observed in manganese oxide glass, for longitudinal velocity, the value dropped at x=0.01 until x=0.02 from 3790 m/s to 3760 m/s, then it increased gradually from x=0.03 molar fraction onward. The same trend followed in shear velocity as it dropped at x=0.01 until x=0.02 from 2200 m/s to 2190 m/s, then the value went up from 2230 m/s to 2250 m/s ahead. For strontium glass, it could be seen that both ultrasonic velocities, vL and vS showed the same decreasing trend for molar fraction of x=0.01 to x=0.02. The vL values decreased from 8334 m/s to 7427 m/s while vS values decreased from 4406 m/s to 4317 m/s respectively with the addition of SrO. However, vL and vS increased from 7427 m/s at x=0.02 to 7582 m/s at x=0.03 and 4317 m/s at x=0.02 to 4344 m/s at x=0.03 respectively. Meanwhile, for the molar fraction of x=0.04, a decreasing trend was observed for both ultrasonic velocities. The vL values dropped from 7582 m/s to 7227 m/s and vS values dropped from 4344 m/s to 4185 m/s before both parameters went up to 7849 m/s and 4482 m/s respectively.
The ultrasonic velocities were strongly related to the density parameter. Both of the parameters were directly proportional to each other. The decrement of velocities at a certain concentration could be associated with the open structure of the glass samples, producing numbers of non-bridging oxygen within the glass system. The bridging oxygen (BO) was said to turn to NBO in the glass samples. This conversion made the glass samples became less rigid. The reduction in ultrasonic velocities could be attributed by the inferior packing density which was possibly caused by the increase in molar volume [25]. Besides that, this declining trend could be better explained by the action of manganese oxide and strontium oxide that acted as a modifier to produce cleavages in the glass network [26]. This was also agreed by a study from Halimah et al. (2010) [14]; which stated that the decreasing trend could be associated with the splitting of Te-O-Te and O-B-O bonds due to the conversion of non-bridging oxygen (NBO) from bridging oxygen (BO).
Some parts involving an increase in ultrasonic velocity values could also be observed. According to Hasnimulyati et al. (2016) [27], the rising trend of these parameters indicated the strengthening of the glass network and the increase in the rigidity of the glass samples. The transformation of BO3 triangular boron into BO4 tetrahedral boron coordination and the formation of TeO4 trigonal bipyramid were the factors that described the increase in the glass rigidity. Besides, the increase of packing density in the glass network could also enhance the increment of both longitudinal and shear velocities [28].
Table 4: Elastic moduli of Mn and Sr-doped borotellurite glasses
Molar fraction, x
|
Mn-doped glass
|
Sr-doped glass
|
L (GPa)
|
G (GPa)
|
K (GPa)
|
Y (GPa)
|
L (GPa)
|
G (GPa)
|
K
(GPa)
|
Y (GPa)
|
0.01
|
50.40
|
16.77
|
28.04
|
41.95
|
196.40
|
54.89
|
143.38
|
123.21
|
0.02
|
46.95
|
16.09
|
25.50
|
39.87
|
181.91
|
61.47
|
153.04
|
99.95
|
0.03
|
45.43
|
15.42
|
24.87
|
38.34
|
209.58
|
68.80
|
172.78
|
117.85
|
0.04
|
45.26
|
15.35
|
25.39
|
38.95
|
195.86
|
65.68
|
163.91
|
108.28
|
0.05
|
45.14
|
15.49
|
25.89
|
38.74
|
234.43
|
76.46
|
192.37
|
132.48
|
The trends were discussed according to their respective dopants which were manganese oxide and strontium oxide. The shear modulus and longitudinal modulus decreased as the molar fraction of MnO2 increased. Generally, both moduli depended on the trend of density as they were directly proportional to each other. The decrement of both moduli could be associated with the increment in number of NBO, which intruded the glass network and discouraged the rigidity and connectivity of the glass samples. The manganese ion filled the interstitial position and produced Mn-O bonds which then decreased its rigidity as it produced more free spaces within the glass structure.
For strontium oxide, the decrement in the elastic moduli was significant to the increment in number of non-bridging oxygen (NBO) group with more open structure as the network modifier (strontium ions) broke the network structure, similar to the ability of manganese as stated before. However, when molar fraction of strontium oxide was x = 0.01, an opposite trend for both longitudinal and shear moduli was observed. Longitudinal modulus (L) showed high decrement from 196.40 GPa to 181.91 GPa and shear modulus (G) showed high increment from 54.89 GPa to 61.47 GPa. These differences might be caused by the drastic structural modification against the respective forces imposed on the shear and longitudinal directions. The same reason also contribute to the reduction of bulk modulus (K), from 123.21 GPa to 99.95 GPa. Nazrin et al. (2018) proposed that the reduction in both bulk and Young’s moduli encouraged the number of formation in non-bridging oxygen (NBO) group that will eventually encourage more open structure. The declination in the rigidity of the glasses had been proved by the FTIR result, displayed by the shifting of the TeO4 (~605.91 cm-1) peaks to higher wavenumber. This indicated the structural distortion due to the transformation of TeO4 trigonal bipyramid to TeO3 trigonal pyramid structures which contributed to the increase in non-bridging oxygen (NBO) [19].
Meantime, the increment of bulk modulus from 0.02 to 0.05 molar fraction of Mn suggested the increment in bridging oxygen (BO) [19]. Specifically, for Young’s modulus, the fluctuating trend could be associated with the competition of NBO and BO. This indirectly agreed that the increment and decrement trends promoted the presence of BO and NBO. According to Pavai and Indhira (2015) [29], the inclination in the elastic constant was also attributed by the large packing density that enhanced the rigidity of the glass network and by the increment of bridging oxygen (BO) ions in the glass network that was more abundant as indicated by the relative intensity of TeO4 trigonal bipyramid.
Apart from the creation of non-crystalline phase induced by the addition of the strontium oxide, the oxygen from the dopant could break the local symmetry of tellurite bond and fill into interstitial position, therefore producing a strong covalent bond that increased the rigidity of the glass network [12]. It could be clearly seen that there was a huge difference of values where longitudinal modulus (L) ranged around 200 GPa while shear modulus ranged around 70 GPa. According to Pavai and Indhira (2015) [29], the effect of volume in glass could be associated with the presence of compression and expansion, provided the major impact was on longitudinal strain rather than on the shear strain. According to Afifi and Marzouk (2003) [30], the high rigidity of the glass might also be attributed by the role of strontium ions that are encompassed within the interstices of the glass. Therefore, the rigidity of the glasses increased.
Table 5: Poisson’s ratio, σ of Mn and Sr-doped borotellurite glasses
Molar fraction,
x
|
Mn-doped glass
|
Sr-doped glass
|
0.01
|
0.251
|
0.3060
|
0.02
|
0.239
|
0.2448
|
0.03
|
0.243
|
0.2556
|
0.04
|
0.244
|
0.2477
|
0.05
|
0.251
|
0.2580
|
For the trend of Poisson’s ratio, most of the values ranged from 0.2 to 0.3. Poisson’s ratio was defined as the ratio of lateral to longitudinal strain when tensile forces were exerted [12]. Tensile strain created was not affected by the cross link but the lateral strain will be affected by the existence of cross link density. This was because, a high cross-link generated a strong covalent force to resist lateral contraction and thus, resulting in a decrease of Poisson’s ratio value. The addition of manganese dioxide into the glass interstices led to more ions to open up within the glass network. Hence, the glass structure will be weak and loosely packed, causing the splitting of TeO2 and B2O3 from bridging oxygen (BO) to non-bridging oxygen (NBO). This eventually made the glass less rigid. A decrease in rigidity of glass caused the decrease in ultrasonic velocities and and thus, an increase in the Poisson’s ratio [19]. The glass was also considered as high cross link density due to its low range of order from 0.2 to 0.3.
In Figure 15, the decrement of Poisson’s ratio values was observed from 0.3060 to 0.2448 at x = 0.01 molar fraction and from 0.2556 to 0.2477 at x = 0.03 molar fraction of SrO. This condition might be referred to the increase in the covalent cross-linking as a result of the transformation of B-O-B into B-O-Sr bonds. Meanwhile, the increment of Poisson’s ratio values from 0.2448 to 0.2556 at x = 0.02 molar fraction and from 0.2477 to 0.2580 at x = 0.04 molar fraction was mainly due to the change in the type of bonding from covalent bond to ionic bond as the glass network started to change from the continuous borate structure to the strontia structure. In addition, a similar trend in the respective regions had been reported by Bridge and Higazy (1986) [31], in the study of ZnO-P2O5 and CoO-P2O5 glasses. According to Gaafar et al. (2009) [32], the decreasing trend of Poisson’s ratio in the first region was associated with the increase in cross-link density, while the increase in the second region was associated with the fact that there was a presence of Zn2+ and Co2+ ions in octahedral coordination. The Poisson’s ratio was directionally weak and produced a low ratio of bending force constant with respect to the stretching force constant. This was similar in the present study referring to the declining trend of Poisson’s ratio in the first region that was due to the increase in cross-link density. In the second region, the presence of SrO2- ions produced a low ratio of bending force constant that was fully linked to the stretching force constant [33].