ABSTRACT

Glasses and glass ceramics in the system xCeO₂∙(50 − x)PbO∙50B₂O₃ (0 ≤ x ≤ 50) have been studied, for the first time, by NMR and FTIR techniques. Effect of CeO₂ substitution with PbO on NMR parameters has been discussed in terms of changing both boron and cerium coordination. The quantitative fraction of four coordinated boron (N₄) has been simply determined from ¹¹B NMR spectroscopy. On the other hand, the fraction of total tetrahedral structural units B₄ (BO₄ + PbO₄ + CeO₄) is obtained from FTIR spectral analysis. It is not possible to get the fraction of cerium oxide directly from the applied spectroscopic tools. Therefore, a simple approach is applied, for the first time, to determine CeO₄ fraction by using the different criteria of both ¹¹B NMR and FTIR spectroscopy. The fraction of B₄ species is equal to N₄, within the experimental error, of the same glasses in the composition region of up to 10 mol% CeO₂. On the other hand, there is a clear difference between both N₄ and B₄ values in glasses of higher CeO₂ content (>10 mol%). The related difference showed a linear increasing trend with increasing the content of CeO₂ in the glass. This was discussed on the bases of structural role of CeO₂ which acts as a glass former in the region >10 mol%, while, at lower concentration, it consumed as a glass modifier.

Keywords:

New approach, Cerium Oxide, Boron Fraction

1. Introduction

Borate glasses containing CeO₂ and PbO are used in wide field of applications. This may be due to their distinction features including wide glass forming region and low thermal expansion [1] - [4] . Several investigations have been reported on the structure of lead containing glasses [2] - [6] . It was concluded that glass and glass ceramics containing high concentration from PbO are of great importance from the viewpoint of glass formation. This may because PbO in such a case inters the network of the glass as s former.

In the low PbO content (up to 50 mol%), majority of PbO act as a network modifier. The reverse behavior is found in binary CeO₂-B₂O₃ glasses, since cerium oxide acts as a glass former [3] [4] . Each atom of PbO added is used to convert two BO₃ species into two BO₄ units. The fraction of BO₄ groups increases with increasing PbO contents reaching maximum value ~0.50 - 0.53 [7] - [10] at equal amount of PbO and B₂O₃ oxides. Further increase of PbO content decreases the number of BO₄ units by forming nonbridging oxygen atoms in glass network. In such situation Pb ions are mainly surrounded by two BO₄ tetrahedral units and in part by BO₃ units with NBOs [11] . But major portion of CeO₂ inters the network of the glass as a modifier at extremely high contents (40 - 60 mol %) [3] .

In this paper, the structure of CeO₂-PbO-B₂O₃ glasses is investigated by means of FTIR absorbance and NMR techniques with an aim to determine the structure role of cerium in ternary cerium lead borate glasses. In this regard, there is a remarkable lack and shortage information about the role of ceria in glass ceramics.

2. Experimental details

2.1. Glasses preparation

The glass samples were prepared by mixing and fusing the desired amount of CeO₂, PbO, and H₃BO₃ compounds in alumina crucibles. The melting process was carried out at different temperatures depending on the glass compositions. The glasses were prepared with a wide variety of the cerium oxide concentration which is varied from 2.5 to 50% CeO₂. The melt was swirled frequently and then poured on stainless steel plate and pressed by another plate to get disc like shape.

2.2. ¹¹B NMR measurements

All samples were measured with JEOL GSX-500 high-resolution solid-state MAS NMR spectrometer in a magnetic field of 11.4 T. ¹¹B MAS NMR spectra were recorded at a frequency of 160.4 MHz and spinning rate of 15 KHz. The glass samples were measured with a single pulse length of 0.5 - 1.0 ms and a pulse delay of 2.5 s, and an accumulation of 200 - 300 scans is obtained.

3. Results and discussion

3.1. Binary Borate glasses

NMR spectra of both 50CeO₂-50B₂O₃ and 50PbO-50B₂O₃ binary glasses are presented in figure 1. It is clear from this figure that there is a great difference between the features of the two spectra. In case of PbO-B₂O₃ glass (free from cerium), well resolved rsonance peaks characterizing BO₃ (both in ring and nonring) and BO₄ groups are

clearly distinguished. The fraction of boron tetrahedral units is ~0.53 which means that 53% from the total boron is found in its tetrahedral coordination with oxygen atoms. This suggests that the majority of PbO is consumed as a glass modifier. It converts two BO₃ units to two BO₄ groups. On the other hand, broader and anti-symmetric NMR band of CeO₂-B₂O₃ glass are clearly evidenced by the effect of CeO₂. It can be seen from the ¹¹B spectrum of the glass that the bands characterizing BO₃ is diluted by CeO₂. As a result, a wide spectrum containing overlapped peaks characterizing BO₃ and BO₄ is obtained, see figure 1. The fraction of tetrahedral boron is much lower than that of lead borate glass, since the determined value of B₄ = 0.34.

Similar observation can be also found in FTIR absorbance spectra of the two glasses, see figure 2. It can be seen from the spectrum of cerium free glass that individual resonances representing BO₃ and BO₄ are markedly distinguished. Higher CeO₂ concentration (50 mol%) leads to appearing of broader and anti-symmetric FTIR resonance band. This wide band is assigned to mixed Ce₄-O, B₄-O and B₃-O stretching vibration mode.

3.2. Ternary cerium Lead borate glasses

There are two main spectral regions which characterize the NMR resonance modes of the ternary glass network. The first is dominant in glass of 0 ≤ CeO₂ ≤ 10 mol%, since both resonance spectra characterizing separated BO₃ and BO₄ groups are clearly appeared [12] - [15] (figure 3). The wide spectral band of chemical shift exists between 11 - 20 ppm is attributed to BO₃ (both in ring and nonring) groups [12] [13] . The second

appears at 0 ppm which is due to tetrahedral BO₄ units. An extra increase in CeO₂ at expense of PbO concentration will result in overlapping resonance spectra characterizing both BO₃ and BO₄ groups. This may reflect change in structure role of cerium oxide in this composition region, since it acts as a glass former [1] [13] [16] [17] . As a direct result, tetrahedral CeO₄ species are suggested to be formed upon a frequent replacement of CeO₂ with PbO. Increasing concentration of CeO₄ groups is accompanied with a decrease in the fraction of BO₄ units. This is because part of modifier oxide is consumed to modify CeO₂ network. The same feature is reported in detail in our previous work [18] on the same glasses investigated by FTIR spectroscopy.

Figure 4 represents the changes of B₄ and N₄ fractions with CeO₂ concentration. The B₄ can be showed to change with different rates upon addition of CeO₂ concentration.

First, both B₄ and N4 value in both cases changes slightly around fixed value with introducing CeO₂ up to ~10 mol%. This little difference between the two fraction (B₄ and N₄) may be considered due to the same role of both lead and cerium as glass modifier, since substitution of PbO by CeO₂ hasn’t remarkable effect on both values. Further substitution of PbO by CeO₂ (in the region >10 mol %) will result in decreasing in both B₄ and N₄ with different rates. Moreover, the differences between them increase with increasing CeO₂ contents. Increasing differences between B₄ and N₄ may lead to conclusion that the ability of CeO₂ to act as a glass former is increased with its content. Thus the formation of CeO₄ groups as the most dominate species in this region is considered as the main reason of reduction in the tetrahedral (BO₄) groups in the glass network. As a direct effect, B₄ is abruptly decreased upon more addition of CeO₂ (see figure 4). The difference between B₄ and N₄ for each composition gives a quantitative concentration of CeO₄ fraction as a glass former. Figure 5 presents the change of CeO₄.

fraction with CeO₂ contents. Ce₄ is observed to increase linearly with increasing CeO₂ concentration. The linear correlation between Ce₄ and CeO₂ concentration leads to confirm that most of all CeO₂ inters the glass network as a strong glass former.

Based upon the above considerations, we suggest that Ce (in high CeO₂) glasses has strong ability to form its own structural units and preferentially bridge to BO₃ rather than increasing number of tetrahedral BO₄ groups. For this reason both B₄ and N₄ values are continuously decreased with increasing CeO₂ contents.

4. Conclusion

Glasses in system of xCeO₂∙(50 − x)PbO∙50B₂O₃ with 0 ≤ x ≤ 50 mol% have been investigated, for the first time, by ¹¹B NMR structural technique. It is evidenced that cerium and lead ions play a dual role in the studied system. In low CeO₂ content, ≤10 mol, CeO₂ plays a modifier role. The structure role of CeO₂ is changed from modifier to a glass former at higher content. The fraction of the tetrahedral cerium (Ce₄) as a former species is determined from a suggested approach which is based on correlation between structural feature obtained from both NMR and FTIR analysis. Accordingly, Ce₄ fraction is determined from the differences between values of B₄ and N₄.

Cite this paper

El-Damrawi, G., Gharghar, F., Ramadan, R. and Aboelez, M. (2016) ¹¹B NMR Spectroscopy of Lead Borate Glasses: Additive Effect of Cerium Oxide. New Journal of Glass and Ceramics, 6, 57-63. http://dx.doi.org/10.4236/njgc.2016.64007

References