Mechanism of Cluster Formation on Cerium Borosilicate Glasses Based on TEM-EDP and SEM-EDEX Investigations ()
1. Introduction
The changes in both structure and properties of borosilicate glasses [1] [2] [3] with addition of different types of glass constituents have been shown to depend on the field strength (i.e. charge/radius) of the cation introduced. In borosilicate glasses containing alumina or ceria or both, an increased bridging oxygen (BO) fraction is induced with the increase in CeO2/Na2O molar ratio. The increase in BO bonds is correlated to the higher field strength of the Ce2+ as compared to Na+ cation. As a result, some of Na2O as a modifier can be preferentially associated with the higher field strength cation Ce2+ and as a result CeO4 as a glass former unit is the product which in turn increases the number of birding bond, BO [1] [2] [3] [4] [5].
Understanding and interpreting of data based on structural changes upon addition of cerium oxide in silicate [3], borate [6] [7] [8] and borosilicate glasses [1] [2] are of major interest. This is maybe because many commercial borosilicate glasses that contain some amount of such oxides are suitable in the field of applications. Similarly both boron and silicon can also be affected by Al2O3 and CeO2 which undergo a considerable change in their coordination [1] [3]. The simultaneous introduction of CeO2 within glass network brings even more complexity when defining the structure of such glasses [5] [6] [7] [8] [9].
The structure changes of the borosilicate glasses were severally reported to depend on structural factors called R and K (R = modifier/B2O3 and K = SiO2/B2O3) [1] [2] [3]. The condition of R > k forces Na2O to participate between both the borate and the silicate glass networks [1] [2] [3]. As a result, the degree of mixing between the silica-rich and boron-rich network is strongly enhanced and homogeneous borosilicate network is produced. The structure of glass and therefore its properties are deeply influenced by changing both R and K.
Studies based on CeO2 in borate and silicate glasses have been recently reported [1] [2] [3] [4]. On the other hand, role of cerium oxide in complicated borosilicate glasses, to our knowledge, is not explored. Therefore the present work is devoted to determining the structural role of cerium oxide in the borosilicate glasses. More clarification will be considered by using the advantage of additional, XRD, EDP, SEM and TEM techniques.
2. Experimental
2.1. Preparation of Glass
The chemically pure SiO2, H3BO3, Al2O3, CeO2 and Na2CO3 were used to prepare all glass samples from the starting materials. Stoichiometric powders were carefully mixed and melted for 50 minutes in a platinum crucible at 1340˚C - 1450˚C. Subsequently, melts were quenched on a metal plate that was pre-heated to 350˚C to avoid cracking progress.
2.2. Experimental Techniques
A Shimadzu X-ray type Dx-30 diffractometer is used for X-ray diffraction measurements. The values of the maximum peak and intensity are used to determine the material type that is compared to patterns in the joint committee for powder diffraction 108 standards’ international powder diffraction file (PDF) database (JCPDS). A transmission electron microscope (TEM) of type JEOL-JEM-1011 was used for the determination of the size and shape of the studied samples. Microstructural data was tested using the JSM-7500F field emission scanning electron microscope. The machine operated with an accelerated voltage of 25 kV. All samples were sputter-coated with a thin layer of gold (3 - 4 nm) to avoid sample charging and increase the signal-to-noise ratio.
3. Results and Discussion
3.1. XRD Spectroscopy
Figure 1 shows XRD patterns of the glass samples containing different concentration from CeO2. There is no evidence for crystal formation in the sample of CeO2 > 1 mol%. At lower concentration, there are weak diffraction peaks which may be situated at 26˚, 32˚ and 34˚. This weak diffraction peaks were identified as the stable sodium silicate phase enriched with NBO atoms [JCPDS 40-0376] [10] [11]. After further increasing of CeO2, the intensity of these peaks disappeared and started to emerge in the amorphous background in glasses of higher CeO2 concentration. Based on the present results, the mechanism proposed is that both metasilicate and disilicate phases nucleate concurrently at the very early stages of nucleation [8]. As the content of cerium oxide increases, nucleation and growth of metasilcate becomes restricted. As a results, the relative number of metasilicate (Na2SiO3) crystals decreases, and they are no longer observed after addition of one mol% CeO2, since significant amount of glassy phase is the dominant. We can thus safely state that equilibrium state has not been reached and that metasilicate is a metastable phase and its constituent may be changed with more cerium oxide addition. As a result, another structural species containing CeO4 are evidenced to be formed. Therefore, disappearance of diffraction peaks in spectra of glasses may be considered as a result of decreasing
![]()
Figure 1. XRD patterns of borosilicate glasses containing different concentrations from CeO2.
concentration of Q2 (silicate units of 2NBO) species which perform metasilcate Na2SiO3 phases. As a result, phases containing less NBO atoms (disilcate) are the dominant upon CeO2 addition. Decreasing Q2 and increasing Q3 upon increasing CeO2 concentration might be considered the main reasons for increasing the amorphous nature of the glasses.
It has been found that more addition of CeO2 to the glass network resulted in spreading of CeO2 over silicate structural groups to form two-dimensional amorphous phase (silicate precursor) containing nanocrystalline cerium silicate of undefined structure [8] [9] [10].
At extremely high cerium oxide concentration, the structural units of the tetragonal Ce2Si2O7 disilicate began to form at expense of Na2SiO3 that resulted in increase of disorder in a glassy phase [12].
3.2. Morphology and Phase Analysis, TEM and EDP
Evidences based on XRD data are in a good agreement with that obtained from TEM and EDP, Figure 2 and Figure 3. Both confirmed that well-formed structural species are constructed in its ordered state in glasses of low CeO2 contents.
![]()
![]()
Figure 2. TEM and EDP of glass containing 1 mol% CeO2.
![]()
![]()
Figure 3. TEM and EDP of glass containing 20 mol% CeO2.
Samples of 1 & 20 mol% CeO2 are presented as examples in Figure 2 and Figure 3. It can be observed from these figures that at lower CeO2 contents (1 mol%, for example), sub crystallized species are still formed within the glassy state. The crystal size and morphology of the crystalline phases is shown to depend on the glass composition [8] [9], particularly on CeO2 concentration. The size of the well-formed ordered species in glass of 1 mol% CeO2 is higher than that of glass containing higher CeO2 concentration, since it lies in the region of 70 - 110 nm in low ceria glass, since it lies in the region of 70 - 110 nm in low ceria glass.
Cerium has a great capacity to remove NBO from the silicate and serves the role of a glass former, as seen by the reduction in cluster size as CeO2 level increases. This would be accompanied by sodium silicate clustered phases decreasing in size and content. Furthermore, some of the modified cerium species are detached from the original glass phases to produce smaller species of sphere-like structures that characterize the CeO2 phases [6] [12], as seen in Figure 3.
Since cerium and sodium oxide can act as modifiers, the micro-crystallized clustered species are clearly visible in glasses containing 1 mol % CeO2. Ordered species are always represented by such diffractions [6] [12]. The size of the aggregation clustered species, on the other hand, appears to diminish with increasing CeO2 level (see Figure 3). CeO2 has the potential to remove more and more NBO and Na ions from the silicate and borate networks in this circumstance, reducing the content and size of the well-formed clusters.
The linkage between distinct species weakens and almost completely separates when CeO2 concentration increases, as seen by TEM and EDP, forming a sphere-like structure representing changed cerium units. These species, which contain around 20 mol% cerium oxide, may be easily detected with a size less than that of glasses containing 1 mol% cerium oxide (see Figure 3).
Features observed by Means TEM (Figure 3) is supported to a great extent with that detected by atomic force microscope Figure 4 which clarified that irregular amplitude of images of cerium occurs in microparticles size (MPs; a, b) and spherical like shape of cerium oxide is found in nanoparticles (NPs; c, d).
Generally increasing CeO2 content at the expense of B2O3 in borosilicate glasses will lower the size of Na2SiO3 units through withdrawing of Na2O which is taken to form Na2CeO4 units of lower size. Formation of the latter affects the nature of glass matrix, since less ordered network is the main product. Macro size species are found in nonspecific shape as presented from TEM micrograph and AFM (Figure 2 and Figure 4(a), Figure 4(b)) [12]. On the other hand, nonosized species are found in spherical like shapes (Figure 3 and Figure 4(c), Figure 4(d)).
3.3. EDS-Spectroscopy
In order to extract the structural role of CeO2, it is important in this study to compare EDS spectra of pure CeO2 with that of cerium containing glass. Figure 5 represents TEM and EDS spectra of pure cerium oxide. It is shown that most
![]()
Figure 4. Typical atomic force microscopy (AFM) amplitude images of cerium microparticles (MPs; (a), (b)) and cerium oxide nanoparticles (NPs; (c), (d)).
![]()
Figure 5. TEM and ESD spectra of pure CeO2.
of line spectra are concentrated around 4 - 6 KeV (Ce bonded to oxygen with covalent bond). Little of Ce are bonded with ionic bond (spectra at about 1 KeV). Coordination of cerium with oxygen atoms gives uniform units of spherical morphology [5]. As seen from TEM micrograph representing pure CeO2, spherical shaped units are seen to be greatly distributed in a layers manner, Figure 5.
Different observation is observed from EDS spectra of cerium containing glasses, since intensity of Ce line spectra at about 1 KeV is more higher in sample containing 1 mol% CeO2 (Figure 6) when it compared with sample of both pure CeO2 (Figure 5) and one containing 20 mol%, Figure 7. This observation supports that both concentration of NBO and the modifier in the silicate network are relatively high. Formation of cerium as former is not evidenced, since there is no resolved Ce peak at higher energy (4 - 6 KeV). Reverse behavior is observed in cerium rich glasses, since the reduction in peak intensity at 1 KeV is shown (for both pure and high CeO2 glass) and new different cerium spectral peaks (3 lines) are clearly formed in the region of (4 - 6 KeV). This leads that CeO2 changes its role from modifier to former to form CeO4 groups. As a result the size and concentration of polycrystalline species (Na2SiO3) [12] [13] should be changed.
It is shown from this figure that most of cerium is a modifier. Since intense peak at about 1 KeV is only resolved. There are no peaks that are evidenced between 4 - 6 KeV. In addition, it found that the concentration of Na atoms (18%) is higher
![]()
Figure 6. EDS spectra of borosilicate glass containing 1 mol% CeO2.
![]()
Figure 7. EDS spectra of borosilicate glass containing 30 mol% CeO2.
![]()
Figure 8. Fraction of boron tetrahedral as a function of CeO2 concentration.
than that of glasses contains higher cerium oxide. The concentration of Na decreases reaches 8% and Ce increase (from 0.48% to 5%) with increasing CeO2 contents.
In such a case CeO2 plays the role of glass former, since three peaks are evidenced between 4 - 6 KeV. It can be noted from EDS spectra of the investigated glasses that, the total Na2O concentration in the glassy phase is highly reduced upon CeO2 addition. The corresponding Na concentration (16 by atom) of glass of 1 mol% CeO2 is lowered to 8 for glass of 20 mol% CeO2. Reverse behaviour is noticed for silicon, its atomic content in low CeO2 concentration was 9.8 and it increased to reach 16 upon more addition of CeO2. Increasing Si and decreasing Na concentration is interpreted on basses of formation of more shielded silicate units upon increasing CeO2 content. In addition some of Na2O should be with drowning from the original phase and consumed to build CeO4 groups. This in turn results in reduction of its (Na2O) content in the original glass phase. These considerations are found to correlate with that obtained by NMR spectroscopy Figure 8. The decreasing trend of the fraction of tetrahedral boron (N4) detected by 11B NMR spectroscopy (Figure 8) may be referred to most of Na2O as a modifier is with drowning by CeO2 to form tetrahedral cerium species. Presence of the latter was highly evidenced from EDS spectroscopy, since new spectral lines at about 5 Kev are grown at expense of EDS lines at 1 KeV.
4. Conclusions
Cerium oxide has the effect of removing nonbridging oxygen atoms (NBO) from the glass network and serves. CeO2 entered the glass network as a strong glass former.
The well formed CeO4 units cause a reduction in the concentration of tetrahedral boron groups (N4). At lower CeO2 (1 mol%) the latter has a good ability to form crystalline clusters in the glass. Higher CeO2 concentration leads to formation of amorphous glass network as documented by XRD and TEM-EDP spectra. Clustered species has a great benefit in the field of application, used as shielding material for ionized radiations.
Data Availability as Authors
We increasingly make our research data available and Data will be made available on request.
Consent to Participate
All of the listed authors have approved the manuscript before submission, including the names and order of authors.
Consent for Publication
All of the listed authors agree to publish this work in this Journal and agree to all of the publishing rights.