Silicate glasses and glass ceramics in the system CeO 2-PbO-SiO 2 have been studied as a function of the structure factors R and K. The latter two factors are defined as: R = (CeO 2 + PbO)/SiO 2 and K = (SiO 2/CeO 2) molar ratios. In this glass, PbO is fixed at 50 mol% and CeO 2 increases at the expense of SiO 2. NMR investigations have revealed that increasing R which is accompanied with decreasing K leads to reasonable decrease in the shielding of silicon atoms. The chemical shift (δ) showed an increasing behavior due to increasing non-bridging oxygen atoms (NBO) in silicate network. It is evidenced that NBO in cerium free glass is much lower than that of glasses containing CeO 2. Increasing R is clearly leading to higher chemical shift and higher NBO. This reflects that CeO 2 has an effective structural role, since it would be consumed in all cases as an intermediate oxide. The main portions from CeO 2 and PbO inter as glass modifiers which are consumed to form NBO atoms. A limited portion of CeO 2 acts as glass former which consumed to form tetrahedral cerium containing NBO due to modification by PbO as a modifier oxide. Increasing R = [(CeO 2 + PbO)/SiO 2] from 1 to 2.34 leads to a frequent increase of NBO in the average glass network. FTIR spectroscopy of the glasses showed a clear shift of the main absorbance peak toward the low wavenumber with increasing R which confirms the increasing silicate units containing NBO atoms. XRD of the investigated materials revealed the presence of some nanostructures from cerium silicate crystalline phases. Formation of separated phases containing micro clusters is found to depend on NBO concentration, since NBO can facilitate process of phase separation. Majority of modifier are consumed to form NBO in the glass network and the rest are aggregated or separated to form silicate phase riches with cerium cations. In such case, some of silicon atoms are electrically compensated with both Pb and Ce cations.
Glass ceramics containing PbO are useful to be studied because of their importance in several fields of applications [
Structure of cerium borate and borosilicate glasses has been recently investigated via FTIR and NMR spectroscopy [
Glass samples containing different concentrations from CeO2, PbO and SiO2 have been prepared by mixing the desired components in silica crucibles. The crucible and its content was transferred into an electric furnace and the temperature is raised gradually to reach the limit suitable for melting. The melting temperature is ranged between 1350˚C -1450˚C, depending on the material composition. The melt was swirled severally to ensure homogeneity and to get bubble free matrix. Finally, the melt was poured between two stainless steel plates. The obtained samples are transferred to another furnace and annealed at 350˚C to reduce internal stress. The samples are obtained in disc like shape of 2 mm thickness and 5 mm radius.
Fine powdered samples of different compositions have been investigated by using JEOL GSX-500 high-resolution solid state MAS NMR spectrometer of magnetic field of 11.74 T (Mansoura University-EGYPT). Spectra of silicon nuclei were recorded at a frequency of 99.3 MHz. A spinning rate of 7 kHz has been applied by using zirconia sample holder. An electric Pulse of 2.62 μs length and of 30 s recycle delay are used. Several scans (10,000 - 12,000) were acquired to get high resolution NMR spectra.
XRD measurements were undertaken using a Bruker D5005 diffractmeter, at 40 kV - 30 mA power. Scans were taken between 10˚ - 70˚ with 0.04˚ increments, 15 - 30 seconds/increment.
The FTIR absorption spectra were obtained, by KBr pellets technique, at room temperature in the range 400 - 4000 cm−1 using Mattson 5000 FTIR spectrometer with a spectral resolution of 2 cm−1. The glass powder of 0.02 g was mixed with a 0.2 g of KBr and pressed to form a thin disc. At least three samples of each glass were analyzed. The spectrum of each sample is obtained due to collected 20 scans. The obtained spectrum was normalized to the spectrum of blank KBr pellet; i.e. a pure KBr spectrum was subtracted from each glass spectrum. In addition, the spectra were corrected to the background and dark currents using two-point baseline correction. Then the spectra were normalized by making the absorption of each spectrum varies between 0 and 1 arbitrary unit. In addition such normalization is necessary to eliminate the concentration effect of the powder sample in the KBr disc.
NMR spectra of glasses having different R values (1, 1.2. 1.86 and 2.34) are shown in Figures 1(a)-(d). As shown from this figure, there is a remarkable shift in center of the main peak position of 29Si NMR spectra with increasing R, i.e. increasing CeO2 concentrations. The NMR resonance peak centered at −87.3 ppm in cerium free glass (spectrum a) is shifted clearly toward much higher value (−75.4 ppm) in glass of 20 mol% CeO2 (R = 2.34). Increasing chemical shift of the silicate nuclei with increasing (PbO + CeO2) concentration is attributed to the modification role of both cerium and lead oxides. In addition, decreasing of SiO2 concentration as a result of increasing CeO2 and decreasing K will result in increasing NBO per SiO2 groups.
In terms of Qn notation, (Q is silicon atom and n is the number of bridging bonds between Si and oxygen atoms), chemical shift value of base glass (−87.3 ppm) is attributed to mixture of Q3 and Q2 (silicate unit containing three and two bridging oxygen atoms as a major portion). In addition, little of Q2[OPb] configurations may also be present [
containing 5 mol% CeO2, Q2 species are the dominant. On the other hand, glasses containing 15 and 20 mol% CeO2 (R = 1.68 and 2.34) contain Q1 and Q0 species respectively. In addition few silicate units containing cerium oxide in the second coordination sphere are suggested to be present.
The concentration of different silicate units (Qn, n = 0 - 4) could be quantitavely obtained by an integration process which is applied to all NMR spectra of silicon nuclei.
of the more stronger Si-O-Si bonds. This assures that cerium oxide inters the glass as an strong glass modifier. In addition, silicate units of the type Ce[2OSi] containing NBO atoms are formed. As a result NBO, Si-O-Ce and Pb-O-Si bonds can deshield silicate units relative to stronger Si-O-Si bond in glasses of higher modification levels. Increasing deshielding upon increasing CeO2 concentration is the main reason of increasing chemical shift with increasing CeO2 concentration as represented in
To determine the structural role of CeO2 as an effective modifier, it is useful to compare FTIR spectra of cerium containing glasses with that of free CeO2. As an example,
contains 5, 15 and 20 mol% CeO2 as an example. The main band at ca. 960 cm−1 in glass free from cerium oxide is shifted progressively towards 860 cm−1, since the activity of Q2 and Q1 species containing Ce linkages are present in glass containing 5 and 15 mol%. In addition, week envelope represents Q0 is appeared at about 770 cm−1 in the spectra of glass containing 20 mol%CeO2. These changes are clearly evidenced in glass of higher CeO2 oxide (15 and 20 mol% CeO2), see
The nature of XRD pattern is known to depend upon the content of NBO in the main glass forming units such as [QnSiO4] and [Qn(PO)3] species [
Glass contains SiO2 and PbO doesn’t greatly affect the process of crystallization or clustring, but the main changes were found to depend on CeO2.
20 mol% CeO2,
NMR investigation has revealed that increasing of CeO2 at the expense of SiO2 at a constant concentration of PbO increases chemical shift (δ) of 29Si nuclei through increasing (NBO) in silicate network. NBO atoms in cerium free glass are much lower than those of glasses containing CeO2. Higher concentration of CeO2 leads to higher chemical shift and higher NBO. The structural role of CeO2 is definitely determined as a modifier oxide in the investigated lead silicate network. FTIR analysis revealed that increasing CeO2 will result in increasing silicate units which are enriched with NBO atoms The major modifiers are consumed to form NBO in silicate network and the few are aggregated to form crystalline silicate phase riches with cerium oxide. XRD patterns of cerium containing glasses reflect the crystalline order of the glasses which is totally differed from that of cerium free glass, since amorphous character is the dominant.
El-Damrawi, G. and Behairy, A. (2018) Structural Role of Cerium Oxide in Lead Silicate Glasses and Glass Ceramics. Journal of Minerals and Materials Characterization and Engineering, 6, 438-447. https://doi.org/10.4236/jmmce.2018.63031