Glasses and glass ceramics containing metal oxide have a wide variety in most of industrial and technical applications    . However, a growing development of technical technology requires specific types of glasses. For instance, specific composition from borosilicate glasses has been devoted as special sealants for Molten Carbonate Fuel Cells (MCFC)   . Both academic and technical advantages of borosilicate glasses have been correlated with some specific structural studies which were carried out on these materials. Powerful techniques such as spinning NMR and FTIR spectroscopy     are the most suitable methods applied to get a quantitative analysis of different structural units forming a network structure of the tested material.
The changes in microstructure of borosilicate glasses were found to depend on their own building species such Qn [SiO4], [BO3] and [BO4] units in borosilicate network. In miscibility region of borate and silicate network structure, the oxygen atoms can be bonded to boron and silicon atoms and have a Na+ ion as a charge compensator   . In such situation, phase separation can’t be considered and both the borate and silicate network are both mixed and modified by Na2O. On the other hand, in the immiscibility region, phase separation is the dominant feature and has a significant influence on the structure and properties of the materials     . The properties in phase separating glasses strongly depend on the thermal history and material composition     . In borosilicate glasses, at least two phases are separated in certain composition regions. The original glassy phase tends to separate into a silica-rich phase, and a borate-rich one upon changing glass composition.
The arrangement of the modifier oxide between silicate and borate network depends on two structural factors namely R and K. Generally R is the ratios of Na2O/B2O3 and K is the ratio of SiO2/B2O3   . For example, for small value of R (<0.5) and higher K, separated phases can take place. Otherwise, the structural species of silicate and borate units are mixed.
The present paper aims to make use of our recent published work which based on NMR and FTIR spectroscopy  to shed more light on detailed structure of borosilicate glasses containing iron oxide. In this regard, XRD, ED spectroscopy, SEM and TEM are applied to give information on different range ordered structure which may be affected by changing Fe2O3 concentration in the studied glasses.
2. Experimental Methods
2.1. Sample Preparation
The glasses were prepared from raw materials graded SiO2, H3BO3, Na2CO3, Al2O3 and Fe2O3. The sodium borosilicate glasses were prepared by melting batches in alumna crucibles at a temperature ranging from 1250˚C to 1520˚C depending on composition. Then the melt was frequently swirled to make the material free from air bubbles and to enhance homogeneity of the melt. Then, the melt has been quenched by pouring it over a stainless steel plate.
2.2. XRD Measurement
XRD measurements were carried out using a Brucker Axs-D8 technique. CuKα radiation source (λCuKα = 0.1540600 nm) has been utalized. Data was accumulated steeply with an interval of 0.02˚, over a 2θ range of 4˚ - 65˚ using a dwell time of 0.4 seconds. The obtained experimental patterns were compared to standards compiled by Joint Committee on Powder Diffraction and Standards (JCDPS).
2.3. Transmission Electron Microscopy (TEM)
Transmission Electron Microscopy (TEM) is a common technique used to evaluate the shape, size, and morphology of the bulk of the material. TEM investigations were carried out using a JEOL-JEM-2100, with an acceleration voltage of 200 kV. During this technique, a high energy beam of electrons is transmitted through a very thin specimen, causing interactions between the electrons and the atoms and producing the TEM images.
2.4. SEM Micrographs and EDX Spectra
SEM micrograph were obtained with a JEOL JSM 6400 scanning electron microscope equipped with a Link analytical system. The electron energy used was 20 keV. The SEM and EDX analysis were carried out for some selected samples.
3. Results and Discussion
3.1. XRD Studies
The structure of some of the investigated glasses may offers neither long-range order (like crystals) nor short-range order like glasses. It possess polycrystalline structure with intermediate range order. XRD patterns of some selected glass compositions are presented in Figure 1. Weak diffraction peaks are appeared in
Figure 1. XRD pattern of borosilicate glasses containing different Fe2O3 concentrations.
the patterns of the glass containing 0, 1, 3 and 5 mol% Fe2O3. These diffractions are centered at 2θ = 18.5, 25.2, 30, and 60 degree, revealing precipitation of some ordered Na2SiO3 species in these glasses [Card No. 16-818]. Formation of polycrystalline cluster from Na2SiO3 is accessible in these glasses, since network is enriched with NBO atoms. The total modifier content from both Na2O and Fe2O3 is extremely high in these glasses  . This leads to formation of high concentration from non-bridging bonds (NBO) in the silicate network  . On the other hand, maximum BO4 groups in borate network are also considered. This was evidenced from NMR data of the same selected glasses  .
In initial glasses (0 - 5 mol% Fe2O3), part of modifier oxide (Na2O and Fe2O3) is consumed to produce NBO in silicate, the other portion can be consumed to modify the borate network and the rest is distributed as accumulated polycrystalline clustered species containing Na and Fe ions. The extra modifier oxide are forced to form some nano-size crystallites or clusters from its own. As a direct result, Na2SiO3 are the main species which are simply to be formed. The angular position of the diffraction lines in the XRD peaks closely match the Na2SiO [card nu.78-1713C]. On the other hand, diffraction peaks disappear for all glasses of Fe2O3 > 5 mol%. Only a broad hump arises revealing of less ordered glassy matrix containing poly crystalline species of extra small size. As can be seen from Figure 1 a broad band, free from sharp diffraction, in samples of higher Fe2O3 concentration (6 - 20 mol%) is the dominant. These features reflect less ordered structure of investigated glasses  -  .
Absence of sharp diffraction peaks in Fe2O3 rich glasses reveals that Fe2O3 inter the network as glass former   . As the content of Fe2O3 increases the concentration of total modifier and NBO ion decreases  . As a consequence, the concentration of the well-formed ordered Na2SiO3 should be decreased. The decrease in Na2SiO3 concentration is considered as a result of formation of FeO4 units as a former species. Formation of such units required more modifier which would be withdrawn from both silicate and borate network. As a result the modified silicate units (Na2SiO3) should be reduced.
3.2. Morphology and Phase Analysis
3.2.1. TEM and EDP
Evidences based on XRD data are in a good agreement with that obtained from TEM and EDP, Figures 2-6. Both confirmed that well-formed structural species are constructed in its amorphous state in glasses of high concentration Fe2O3 (≥ 6 mol%). Samples of 10 & 20 mol% Fe2O3 are presented as examples, Figure 5 and Figure 6. On the other hand, at lower Fe2O3 contents, sub crystallized species are formed within the glassy state. The crystal size and morphology of the crystalline phases is shown to depend on the glass composition, particularly on Fe2O3 concentration. The size of well-formed ordered species in poor iron glass (1 mol% Fe2O3) is ranged from 21 to 37 nm as listed in TEM micrograph Figure 1. While the size is decreased upon more addition of Fe2O3, since it lies in the region of 7 - 10 nm. The decrease of cluster size with increasing Fe2O3 content
Figure 2. TEM and EDP of glass containing 1 mol % Fe2O3.
Figure 3. TEM and EDP of glass containing 3 mol Fe2O3.
confirms that iron has high ability to withdraw NBO from the silicate and plays the role of a glass former  . This would accompanied by a decreasing size and the content of sodium silicate clustered phases.
As shown in Figure 2 and Figure 3, micro-crystallized clustered species are clearly evidenced in glasses containing 1 and 3 mol% Fe2O3, since iron and sodium oxide can play a role of modifier  . On the other hand, the concentration of the aggregated clustered species is shown to decrease with increasing Fe2O3 content, see Figures 4-6. In such situation, Fe2O3 has the ability to withdraw more and more NBO and Na ions from the silicate network which in turns
Figure 4. TEM and EDP of glass containing 6 mol Fe2O3.
Figure 5. TEM and EDP of glass containing 10 mol Fe2O3.
reduces the content and the size of the well-formed clusters.
It can be observed from TEM and EDP that the disorder of the aggregated species increases with increasing Fe2O3 concentration. The interconnection between different species drops down on further increase in Fe2O3 concentration and becomes almost indistinguishable from the background at around 20 mol% % Fe2O3, see Figure 6.
3.2.2. SEM and EDEX Analysis
Figure 6. TEM and EDP of glass containing 20 mol %Fe2O3.
Figure 7. SEM and EDEX spectra of glass containing 3 mol% Fe2O3.
The concentration of the aggregated species is shown to be more higher in sample containing 3 mol% Fe2O3 (Figure 7) when it compared with sample of higher iron oxide Figure 8. This observation supports that both concentration of NBO and the modifier in the silicate network are reduced. Fe2O3 changes its role from modifier to former to form FeO4 groups. As a result the size and concentration
Figure 8. SEM and EDEX spectra of glass containing 10 mol% Fe2O3.
of polycrystalline species (Na2SiO3) should be decreased as presented from both SEM and TEM micrographs.
Elemental analysis of samples containing 3 and 10 mol% Fe2O3 has been also done using EDEX spectroscopy. The spectra of glass of low Fe2O3 content (3 mol%) contains only Na, Si and O elements, Figure 7. There is no any evidence for presence of Fe in the crystalized phase. This means that (Na2SiO3) clustered phase is the dominant. On the other hand, glasses of higher Fe2O3 (10 mol%), see Figure 8 presented several Fe EDEX spectral lines which are highly resolved. In addition, spectrum of Na, O and Si are also present. The phase in such case is enriched with FeO4 groups which in turns result in increasing the disorder of the (Na2SiO3) phases. Presence of FeO4 groups in in sodium silicate phase causes mismatching between units which form the glass network. As a result, amorphous structure in such a case is the dominant feature of glass of containing high Fe2O3 concentration. This conclusion is agreed to great extend with that obtained from both XRD and TEM results which discussed above.
X-ray diffraction (XRD), transmission electron microscope (TEM), electron diffraction pattern (EDP) and SEM experiments are applied to analyze the structural changes. Species of polycrystalline structure are formed in low Fe2O3 containing glasses. The obtained size is ranged from 10 to 33 nanometers. An intermediate range ordered structure is dominant in glass ceramics of Fe2O3 < 8 mol%. Less ordered structure is a feature of glasses of higher Fe2O3 concentration. The type of range order is found to be connected with the role of Fe2O3 and Na2O in glasses. Na2O is the strong glass modifier in the studied composition region, while Fe2O3 is consumed also as a modifier in composition of < 8 mol%. Fe2O3 is consumed as a network former in the composition region of higher iron oxide concentration (≥8 mol%).
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