Structural Role of Strontium Oxide in Modified Silicate Glasses

Glasses in the system 24.5Na2O.24.5CaO.6P2O5.xSrO.(45-x)SiO2 have been studied in the composition region of x = 0–15 mol%. The as-prepared glasses are transparent and have an amorphous network structure. On the other hand, heat-treated glasses are transformed to opaque white glass-ceramic characterized by their highly crystalline network structure. Crystalline apatite (calcium phosphate, Ca3(PO4)2), wollastonite (calcium silicate, CaSiO3), and strontium calcium phosphate [Ca2Sr (PO4)2) are the main well-formed crystalline species that played the major role in material bioactivity. Increasing SrO leads to enhancing material crystallite and enhances the hardness of the host glass matrix. The change of XRD spectra,31P NMR chemical shift, and hardness number upon increasing SrO are considered due to modification of the apatite Ca(PO3)2 to involve Sr ions inducing Ca2Sr (PO4)2 apatite one. Such species play the role in enhancing material properties and hardness.


Introduction
Porous inorganic materials have unique properties which are useful for the development of biomaterials to become eligible for controlled loading stresses and/or release of biologically active substances [1,2]. Specifically, the term bioactive glasses or glass-ceramics is only applied on any compatible material that can form a calcium phosphate interfacial layer that resembles the biological apatite present in bones [3,4]. In addition, bioactive glasses can actively stimulate bone growth through the release of critical concentrations of ionic dissolution products that cause rapid expression of genes regulating osteogenesis and the production of growth factors [1,5]. In this regard, strontium-based bioactive glasses have the great ability to inhibit bone resorption by osteoclasts [6,7]. It was recently demonstrated that SrO enhances the surface adhesion properties for strontium-containing glasses leading to an increasing in material hardness. The enhancement of surface adhesion is considered to be due to the lower electronegativity of Sr compared with that of Ca 2+ ions in the glassy network [8,9]. The low electronegativity of Sr leads also to the formation of a more stable Si O Sr bond due to a more balanced distribution of electronic charges. The well-formed Ca-P species would be more quickly changed into an apatite layer and more Ca 2+ substitutions are made possible within the newly formed apatite layer for Sr-doped glasses [10].
The strontium oxide-doped glasses in the SiO 2 -CaO-SrO and SiO 2 -CaO-P 2 O 5 -SrO systems have been studied [11,12]. The interaction processes of the bioactive glasses with biological fluids were determined and it was found that the strontium-containing glasses have a good potential for the formation of bone-like apatite [13][14][15]. Bioactivity of glasses is usually correlated to the exchange of alkali (Na) or alkaline (Ca 2+ ) earth ions with H + of the solution. This process leads to the condensation of silanols groups (≡ Si-OH) which is an important function that readily reacts with hydroxyl groups, carboxylic acids, and oxides present on inorganic compounds. In such a case, the surface is characterized by its high area. This silica gel layer offers or provides a large number of sites required for the formation and growth of hydroxycarbonate apatite species equivalent to the mineral phase of bone [16].
The transition from the highly bioactive glass to biocompatible compositions is characterized by a marked increase in the connectivity of the silicate network through bonding with Sr cations and by increasing the fraction of phosphate groups involved as P − O−Si or P-O-Sr cross-links. Our analysis also highlights a possible correlation between the enhancement of crystalline apatite through aggregation between Ca 2+ and PO 4 3ions to form an apatite crystals structure which may be precipitated on the CaSiO 3 as a wallsotinte crystalline phases. In this situation, the process of thermal heat treatment is applied as an effective route for crystallization enhancement [17,18]. The nucleation and crystallization of glasses in the system makes them important in the field of orthopedic and orthodontic application.this is considered as a novel consideration for strontium silicate glasses.

Sample Preparation
An ordinary dissolve quenching technique was used to produce amorphous glasses within the system xSrO-(45-x)SiO 2 -24.5CaO-24.5Na 2 O-6P 2 O 5 , (0 to 15 mol %). Samples are obtained from reagent grade mixtures CaCO 3 , Na 2 CO 3 , SiO 2 , SrCO 3 , and (NH 4 ) 2 HPO 4 which have been melted in a Pt-Au crucible. To remove NH 3 and H 2 O, The specimens were heat-treated at a slow rate of 2°/min from room temperature to 600°C, then melted for 20-30 min between 1000 and 1200°C before being quenched by pouring the melt between two metallic plates. To restrict P 2 O 5 volatilization and keep total glass weight losses under 2 %, the time of melting and temperatures were optimized.

Infrared Spectra (IR)
The FTIR absorption spectroscopy for different samples was carried out by means of KBr pellets technique. The spectra are measured in the region of 400-4000 cm -1 with a spectral resolution of 2 cm using a Mattson 5000 FTIR spectrometer. 1 The obtained spectrum was normalized to the spectrum of blank KBr pellet and was corrected to the background and dark currents using two-point baseline correction. The normalization is necessary to eliminate the concentration effect of the powder sample in the KBr disc.

X-ray Diffraction Spectroscopy
Shimadzu X-ray diffract meter is used for X-ray diffraction measurements (the apparatus type Dx-30, Metallurgy institute, El Tebbin-Cairo). The values of the maximum peak and intensity are used to determine the material type that compared to patterns in the joint committee for powder diffraction standards' international powder diffraction file (PDF) database (JCPDS).

Differential Scanning Calorimetry
A NETZSCH STA 409 C/CD instrument was used to perform the DSC analysis. Crushed samples of known mass (30 mg) were put in an aluminum tray, sealed with a crimped lid, and heated at a rate of 5°C/min with argon as the carrier gas at a flow rate of 30 cm 3 /min from 25 to 1000°C.

Heat Treatment (HT)
The samples containing 0, 3, 5, 10, and 15 mol% SrO, were heated in a muffle furnace (Heraeus KR170) controlled within ± 2°C. The samples were heat-treated at temperatures 500 and 650 o C for a treatment time interval of 6 h. After heating, the glasses were then kept in the furnace and held at the temperature of heat treatment for the desired time before cooling normally at room temperature.

Density and Molar Volume
The densities of the prepared samples were calculated using the Archimedes principle and benzene as the immersion solvent. The density was determined using the following formula: where W a is the weight in the air, W b is the weight in benzene, and D b is the density of benzene. The molar volumes (V m ) were numerically determined using the equation that follows: Where V m is the molar volume. M T is the glass sample's molecular weight and D is the sample's density.

Microhardness
The hardness value (H v ) of the prepared samples was determined using the SHIMADZU-HMV-G20S (Shimadzu, Kyoto, Japan) microhardness tester at room temperature Ten indentations have been loaded at different places on the surface. The corresponding length of the indentation imprint diagonals registered by a high-resolution microscope to ensure the accuracy of the measurement. The formula was used to calibrate the microhardness (H v ) values: Where H v is the Vickers hardness in kg/mm 2 , F is the applied force in newtons, and d is the indentation's mean diagonal length in meters.

Magnetic Resonance Measurements
The JEOL GSX-500 high-resolution solid-state MAS NMR spectrometer with a magnetic field of 11.74 T was used to analyze finely powdered samples of various compositions (Mansoura University-EGYPT). 31 P MAS NMR experiments were also conducted at the resonance frequency (202.4 MHz) using a 3.2 mm diameter rotor spinning at 15 kHz. Solid NH4H2PO4 was used as a secondary reference compound and the signal from this set to 0.9 ppm. A pulse length of 2.5 µs and a recycle delay of 5 s were applied.

Results and Discussion
SrO gives the glasses good advantages toward improvements of their properties like extremely high resistance to thermal shock, high mechanical strength, good chemical stability, crystallinity, and bioactivity [19]. It is evidenced from x-ray diffraction (XRD) and FTIR Figs. 1 and 2, of the as-prepared glasses that the well-formed structural species are constructed in their amorphous state [8,20]. The addition of SrO at expense of SiO 2 has no effect on the material structure since the amorphous structure is the most dominant type, see Figs. 1 and 2. From these figures, the spectral features do not change upon increasing SrO contents. Because the glass composition has a limited effect, the thermal heat treatment processes can be applied as an alternative to change the material structure. Figure 3 shows differential scanning calorimetry (DSC) curves from which both glass transition (Tg) and crystallization temperatures (T C ) can be determined. The crystallization temperature was ranged between 650 O C and 700 O C as is shown from Fig. 3. In addition, T g is around 550 O C. According to DSC data, the glasses in the present study were  Comparisons of the studied materials' X-ray sharp diffraction line spectra with that of apatite (Ca 3 (PO 4 ) 2 ) wollastonite (CaSiO 3 ) and [Ca 2 Sr (PO 4 ) 2 ) crystals were considered. The most developed crystalline species are calcium phosphate, calcium silicate, and calcium strontium phosphate, according to the comparison [21]. The crystalline apatite and wollastonite formed species are considered the main units which play the role of biocompatibility and or bioactivity of the studied materials [20,22]. Figures 1, 4, and 5 of the XRD spectra indicate that the crystallization process is only available by thermal treatment. The number of diffraction lines in all investigating glasses is the same, but the most noticeable parameter is the change in intensities. This means that the wellformed crystalline phase types remain the same, but the content of the separated phases increases as SrO concentrations rise. Some well-formed crystalline phases, such as crystalline apatite (calcium phosphate crystals) and strontium calcium phosphate, are classified as bioactive phases that are beneficial to dental materials [23,24].
There is a clear difference between the FTIR spectra of asprepared and treated samples, Figs. 2 and 6. The lowfrequency peaks between 400 and 600 cm − 1 showed splitting in the case of thermal treating cases. Such splitting in the absorption peaks lent support that both apatite and wollastonite are formed in their crystalline phases. This is confirmed by XRD Fig. 5. Then, in general, SrO in the matrix of the heattreated glasses modifies the silicate and phosphate structural units, forming wollastonite -apatite crystal phases and Ca 2 Sr (PO 4 ) 2 apatite microcrystals [card no. 52-0467]. The latter type is more chemically stable against acid and fluid attack when it is applied as a cement for oral applications.
Then from the above discussions, we conclude that increasing SrO leads to enhancing material crystallite of the treated glasses which in most cases enhances the mechanical properties. Figure 7 shows the dependence of the hardness nimbler of the glasses on SrO content. Increasing the amounts of SrO enhances the hardness of the glasses which is changed from 280 kg/mm 2 to 450Kg/mm 2 .
The change of XRD spectra and hardness number upon increasing SrO are considered due to modification of the apatite Ca(PO 3 ) 2 to involve Sr ions inducing Ca 2 Sr (PO 4 ) 2 apatite one. Such species play the role in enhancing material hardness. Figure 8 presents 31 P NMR spectra of different glasses containing 0, 5, and 15 mol% SrO which were all treated thermally at 650 O C for 6 h. The phosphate units of Q 0 species (all oxygens are nonbridging) have been found in the main glass network indicating that the majority of phosphorus exists as orthophosphate species in the glass (Na 3 PO 4 or Ca 3 (PO 4 ) 2 or mixing between them) [25]. The change of the relative area of three spectra with SrO confirms the above consideration since  the relative area under the 31 P NMR spectra is increased by increasing SrO content that means that some of SrO may be forced to enter the apatite phase forming stransium apatie one. Then the apatite crystals should be formed from mixed cation (Ca and Sr) instead of the one type (Ca) presented by the lower area in Fig. 8; Table 1.

NMR Measurements
The clear difference between 23 Na NMR spectra of glasses of 0 and 15 mol% SrO Fig. 9 support the version that Sr can substitute both Na or Ca from the apatite phase and some of Sr can share in performing the apatite crystalline phases. From Fig. 9, the chemical shift of Na nuclei of the glasses containing Sr is lower than that of Sr free glass. This means that bond strength in the network structure of glasses containing Sr is stronger than that of Sr free ones.

Density and Molar Volume Measurements
Changes of both density (D) and molar volume (V m ) with increasing SrO concentration are presented in Fig. 10. The density increased and its molar volume decreased as the content of SrO is increased from 0 to 15 mol%. The observed increase in density values is mainly due to the higher molecular weight of SrO (103.62 g/mol) when it compared with that of SiO 2 (60.09 g/mol). Accordingly, increasing SrO content forces it to enter gradually the network as a glass-forming species which leads to increasing the total bridging bonds at the expense of non-bridging ones (Sr-O-Si). As a consequence, the formation of shortening Sr-O-P linkages is considered the main reason for the well-decreased volume of the network structure. The decrease in NBO is accompanied by decreasing the open volume and void spaces surrounded NBO ions which in all cases results in decreasing the molar volume of the studied glass.

Conclusions
Bioglasses and glass-ceramics containing different SrO concentrations have been studied by different structural techniques. The amorphous nature of the glass free from SrO is confirmed by XRD. Some types of crystalline species are formed in SrO containing glasses after heat treatment. Well-formed apatite (Ca 3 (PO 4 ) 2 ), wollastonite (CaSiO 3 ), and [Ca 2 Sr (PO 4 ) 2 ) phases containing strontium ions are evidenced in SrO containing glasses. More enhancement in crystallinity was confirmed via the thermal heat-treatment process. The presence of Sr ions in both crystalline apatite and wollastonite matrix promotes its biocompatibility, particularly orthopedic bioactivity. Damrawi] and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Data Availability As authors, we are increasingly make our research data available and Data will be made available on request.

Declarations
Conflicts of Interest/Competing Interests Authors declare that we have no conflict of interest. We are agreed upon all the Ethical Rules applicable for this journal.
Consent to Participate We all of listed authors have approved the manuscript before submission, including the names and order of authors.

Consent for Publication
We all of listed authors agree to publish this work in Journal of " SILICON' and agree to all of the publication rights.