3.1. XRD spectroscopy
The glasses of low Nd2O3 (x =0 and 0.5 mol %). were colorless, transparent and amorphous in nature. The addition of Nd2O3 at expense of B2O3 has been shown to be very effective in enhancing the crystallization of the glasses. Precipitation of the small sized crystals in the main glass network lowers the sample transparency and the glass in such a case is partially devitrified.
The XRD spectra revealed that an amorphous structure is the characteristic feature of the samples containing ≤ 0.5 mol% Nd2O3. Transformation into a crystalline structure occurred after introducing higher Nd2O3 concentrations. It is shown from figure 1(a, b) that the XRD spectra contains two broad diffraction bands located between 20-30° and 40-55°. Presence of these broaden bands reflects the amorphous structure of the glasses of low Nd2O3 concentration (0 and 0.5 mol%). On the other hand, the glasses of higher Nd2O3 concentrations become relatively opaque due to formation or precipitation of some crystalline species. Presence of sharp and intense diffraction line plectra in glasses of more Nd2O3 content figure 1(c, d, e) can be correlated to formation of the more ordered crystalline phases in the matrix of the investigated materials.
Comparisons between X-ray sharp diffraction line spectra of the studied materials with that of Ca3(PO4)2, (CaB4O7) and (NdBO3)) crystals were made. It is concluded from the comparison that calcium phosphate, calcium borate and Nd-borate crystalline species are the most formed species. It was found that metaborate (CaB2O4) nanoparticles and tetraborate (CaB4O7) nanoparticles are the main crystalline phases. In addition, crystalline apatite (Ca3(PO4)2) structure is well formed also by the effect of Nd2O3 addition.
The diffraction peaks observed at 2 theta values of 23°, 26°, 31.8°, 41.2°, 47°, 48° are belong to calcium tetraborate [ ICDD PDF 83-2025]. The XRD peaks at 2 theta values of 23.3°, 24.8°, 28.3°, 32°,47° matching with the (111), (210), (220) and (022) the rhombic structure of calcium meta-borate [ICDD PDF 32-0155].
Figure 2 represents the change of the determined crystallinity with increasing Nd2O3 concentration. Then from the figures 1 and 2 one may expect that the crystallization process is offered mainly by effect of Nd2O3, since the material containing even limited addition of Nd2O3 (> 0.5 mol%) is crystallized. The number of diffraction lines in glasses of 1, 3 and 4 mol% Nd2O3 is the same but change in intensities is the most observed parameter. This means that the types of the well-formed crystalline phases are the same but the content of the separated phases increases with increasing Nd2O3 concentrations. These modifications are summarized in figure 2, which reflects a change in the structure throughout changes of crystallinity in glass. It can be seen from this figure that with increasing Nd2O3 concentration, the crystallinity increases to reach its saturated values in the region between 3 and 4 mol %. This interpretation may account on the presence of several diffraction lines in spectra of glasses modified by more than 0.5 mol% Nd2O3 (spectra c, d, e). Some of the well-formed crystalline phases, such as crystalline apatite (calcium phosphate crystals), are categorized as bioactive phases that are useful for the material to be used in the field of biodental and bioactive use [19-26].
There are two parameters that can play a role in improving the process of crystallization in the glasses being tested. The first is the replacement of B2O3 with Nd2O3, as mentioned above. Secondly, the thermal heat treatment process (THT) is alternatively applied also to improve the crystallization behavior. The latter can be applied on sample containing 4 mol% Nd2O3 which characterized with its higher crystallinity in comparison with composition of lower Nd2O3. The temperature at which THT process can be considered can be extracted from DSC curves.
3.2 DSC, thermal treatments, glass transition temperature and Vicker hardness
The maximum crystallinity is found in composition of 4 mol% Nd2O3 under the effect of glass composition. To assure the stability of the well-formed crystals, the sample of 4 mol% Nd2O3 is also investigated under the effect of thermal treated at a specific temperature based on differential scanning ceilometer (DSC) data. As can be shown from figure 3, the DSC curve clearly shows one endothermic peak and one exothermic peak. The endothermic peak corresponds to the glass transition (Tg) while the exothermic peak indicates the crystallization point of the glass (Tc). The glass transition temperature (Tg) as well as crystallization temperature (Tc) are estimated by the slope intercept method. The nature of the DSC curves is typical for other glass compositions. The crystallization temperature was found to be around 800°C. The glass is therefore thermally treated at this temperature. It can be shown from figure 4 that the state of crystallization didn't changes with thermal heat treatment, since XRD spectra of the as prepared and treated samples are nearly not differed. This means that the glass ceramic of 4 mol% Nd2O3 is the most recommended composition containing the maximum concentration of crystals.
The thermal analysis of the glasses was carried out because any change in the coordinating number of atom-forming networks or in the formation of non-bridging oxygen (NBO) or bridging bonds (BB) can simply be expressed by the change of Tg with the composition. The variation of Tg with compositions is shown in figure 5. It can be seen that with the rise of Nd2O3 content, which is the network intermediate here, Tg increases monotonically. It is documented that with the increase of bridging bonds in the main borate glass network, Tg and crystallization temperature Tc are generally increased [20, 27-30]. It is believed that Tg depends on the strength of chemical bonds in the structure. Nd2O3 in general, plays the role of a network intermediate which has been consumed to increase the bonds between different structural units with the increase of its content in the glass system. Increase of bridging oxygen indicates the increase in the strength of chemical bonds, which in turn increases both Tg as shown in figure 5.
3.3 Density, molar volume, free volume and packing density
It was found that the density of glass samples increases with increasing Nd2O3 concentration as shown in figure 6. This is due to the higher molecular weight of Nd2O3 (336.4822 g/mol) than the host structures of the glass samples (B2O3 is 69.6202 g/mol) [19-22]. Therefore, the molar volume (Mv) shows a reverse behavior to density as shown in figure 6. The Mv is the parameter that describes the volume occupied by the unit mass of a glass plus the free volume (Vf) surrounded the structural unit forming the network of the glass. In general, the unit mass is increased upon increasing Nd2O3 at the expense of B2O3. In addition, the free spaces (Vf) associated with borate or NdO4 units is decreased as a result of its occupation with Nd3+ ions which is of larger size than that of B3+. Then, substitution of B2O3 with Nd2O3 is therefore decreases the free volume with a manner which depends on the ionic radius of the glass modifier oxide [23]. As a result, increase of density and decrease of free volumes (Vf) are the two factors played the role of decreasing (Mv) of the investigated glasses. The decrease of the molar volume is due to adding Nd3+ of larger ionic radius (1.123 Å) into interstitial of host structure as the ionic radius of B3 is (0.400 Å) lead to reducing the free spaces formed around the structural units. Therefore, substitution of B2O3 with Nd2O3 decreases the molar volume via reducing the concentration of free spaces which have been filled with Nd3+ ions of larges sizes. Decreasing of open spaces with increasing Nd2O3 means that the packing density should be increased with decreasing the total molar volume of the glass samples [22-24]. Then increasing Pd (density) and decreasing void spaces in the glass network are considered as the main causes in increasing Tg and Hv of the investigating samples.