Structural Features and Properties of the Vitreous Part of the System 50P 2 O 5 -25CaO-(25 − x)Na 2 O-xCoO (with 0 ≤ x ≤ 25; mol%)

The glass series 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO (with (0 ≤ x ≤ 25; mol%), has been prepared by direct melting at 1080˚C ± 20˚C. The introduction of cobalt in calcium phosphate glasses is used to compare its effect with calcium in inhibition corrosion. The dissolution rate has been investigated. It indicated an improvement of chemical durability when the cobalt oxide increases in the network glass at the expense of Na 2 O content. Both, IR spectroscopy and X-ray diffraction have confirmed the structure changes when the CoO content increases in the glass. This change results in the disappearance of isolated orthophosphate groups followed of a polymerizing of the structure from isolated orthophosphate towards pyrophosphate chains (Q 1 ) by promoting the formation of olygophosphates (mixed Q 1 -Q 2 ) rich in pyrophosphates. Analysis of the density values, showed an increase of density with the increase CoO content. The covalent radius values of oxygen r cal (O 2− ) indicate a signifi-cant decrease and therefore a relatively high reinforcement of the met-al-oxygen-phosphorus (Co-O-P) bonds. SEM micrograph confirms the evolution of the glass structural morphology. The sample having a maximum CoO content confirms a homogeneous glass phase with quite crystalline particles. This property is prerequisite for many interesting industrial applications.


Introduction
Due to their poor chemical durability phosphate glasses have rather limited technological application despite their investigation so far conducted by many researchers [1] [2]. However, several phosphate glasses with high aqueous corrosion resistance have been reported [3] [4] [5] [6] [7]. Their properties (low melting point, high thermal expansion coefficient, bioactivity, optical properties etc.) make these glasses serious potential candidates for many technological applications. It has been found that the introduction of oxides, such as ZnO, Fe 2 O 3 ,  [5] [7] [8] [9] [10] [11]. The synergy of phosphate glasses with some types of nuclear waste has indicated the possibility of a form of waste with a lower corrosion rate than borosilicate glasses [8] [12]. As a result of high chemical durability, iron phosphate glasses have been considered as better candidates for the vitrifying of some type of nuclear wastes when compared with borosilicate glasses [4] [5] [6] [8]. The aim of the present work is to synthesize and select phosphate glasses in the system 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO (with 0 ≤ x ≤ 25; mol%) for two reasons: • the first reason is to analyze glasses, with low cobalt content, by different techniques arranged for further later studies in the biomedical field [13] [14] [15]; • the second reason is to compare the effect of cobalt with that of iron in inhibition of corrosion [3] [6] [16]. The studied series indicated the structural change when cobalt content increases and causes an important tendency polymerization from orthophosphates to pyrophosphate groups which are at the origin of the improvement of chemical durability.

Experimental Section
Phosphate glasses are prepared by direct melting of the (NH 4 )H 2 PO 4 (98,99% pure), CaCO 3 (99.5% pure), Na 2 O (99% pure), CoCO 3 , xH 2 O (Co 43% -47% pure) mixtures with suitable proportions. The reagents are intimately crushed then introduced into a porcelain crucible. They were initially heated at 300˚C for 2 h and then kept at 500˚C for 1 h to complete the decomposition. The reaction mixture was then heated at 850˚C. for 1 h and finally at 1080˚C for 30 minutes. The homogeneous liquid was poured in aluminum plate previously heated to 200˚C to avoid thermal shock. Pellets about 5 to 10 mm in diameter and 1 to 3 mm thick were obtained. The samples were polished with carbon Silica sandpaper (with CSI of sufficiently high level), cleaned with acetone and immersed in pyrex beakers containing 100 ml of distilled water and carried to 90˚C. The sample surface must be constantly submerged in distilled water for 21 consecu-tive days. The dissolution rate was evaluated from the mass loss as a function of time. The IR spectra of the studied phosphate glasses were determined in the frequency range between 400 and 1600 cm −1 with a resolution of 2 cm −1 using a Fourier transform infrared spectrometer (IR AFFINITY-1S). The samples were finally ground and mixed with KBr (potassium bromide), which is transparent in the IR and serves as a template. The ratio of the matter/KBr in the pellets was 10% by weight. The vitreous state was first evidenced from the shiny and transparency aspect, which was confirmed by X-ray diffraction patterns (XRD type BRUKER D8 ADVANCE). The glasses S 0 , S 2 and S 4 were annealed at 540˚C, 551˚C and 660˚C, respectively, for 72 hours. Differential thermal analysis (DTA) was performed using a DTG-60 SUMULTANEOUS DTA-DTG Apparatus, at a heating rate of 10˚C/min in atmospheric air with alumina crucibles. The Archimedes method was used to measure the density of glasses using orthophthalate as a floating medium. The microstructures of the sample glasses were characterized by scanning electron microscopy (SEM), equipped with a full system micro-analyser (EDX-EDAX).

Analysis of Chemical Durability of Series Glasses 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO
The chemical durability (D R ) of the glass series 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO (with 0 ≤ x ≤ 25 mol%) was determined from the dissolution rate (D R ) of the samples immersed in 100 ml of distilled water at 90˚C for 21 consecutive days. The dissolution rate is defined as the weight loss of the glass expressed in g•cm −2 •min −1 . The values of D R and of pH of the leaching aqueous solution are represented respectively, in figures 1 and grouped in Table 1. In Figure 1, the shape of the D R curve indicates a progressive improvement of the chemical durability of the glass from 5.44 × 10 −5 to 8.60 × 10 −7 (g•cm −2 •min −1 ) when the CoO content varies from 0 to 25 mol% [10].

Density and Molar Volumes
Density measurements allowed us to follow the evolution of the molar volume depending on the composition of the system 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO. The density measurements were completed at room temperature. As can be observed from Figure 2, the variation in density versus CoO content (mol%) indicates an increase of density. On the other hand, it was possible to deduce the value of the molar volume and oxygen radius from density measurements, calculated from the approximate hypothesis of the close packing of oxygen anions, O 2− , each having r cal (O 2− ) recapitulated for each composition in Table 2 [4] [10] [16] [17]. The molar volume of oxygen and the radius of anions of oxygen (O 2− ) in the glass have been determined from Equations (1) and (2), respectively.
With M = molar mass, ρ = density, N A = Avogadro number; *N 0 = number of oxygen atoms in the molecular formula. A detailed analysis of the data in Table  2 shows that the molar volume decreases increasing of the CoO content. The covalent radius value of the oxygen atom (O 2− ), calculated by the molar volume using the Equation (2) for each composition, decrease, also, indicating a reinforcement of the metal-oxygen-phosphorus (Co-O-P) bond with increasing of CoO content.

Structural Approach by Infrared Spectroscopy
Infrared spectra of glass series 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO (0 ≤ x ≤ 25; mol%) are shown in Figure 3. The assignments of the vibration bands are given in Table 3. All vibration bands of treated phosphate glasses are shown in the range of frequencies between 400 and 1600 cm −1 . The band at 490 -510 cm −1 is attributed to skeletal deformation δ ske (P-O-P) [ [23]. The band at 1280 cm −1 is attributed to asymmetric stretching of two non-bridging oxygens ν sym PO 2 . Analysis of the IR spectra obtained (Figure 3

X-Ray Diffraction and DTA Analysis
As expected, X-ray diffractions have confirmed the vitreous character of all of the investigated glass samples (see Figure 4). DTA analysis of the phosphate glass 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO (with 0 ≤ x ≤ 25; mol%), shown in Figure  5, indicates both an increase in the glass transition temperature and the crystallisation temperature versus the CoO content. When the CoO content increases from 0 to 25 mol%, the glass transition temperature (Tg) increases in the 399˚C -477˚C range, whereas the crystallisation temperature (Tc) increases in the 502˚C -657˚C range ( Table 1). The Tc-Tg difference is significant, which explains the high thermal stability [13] [28]. The heat treatments of the S 0 , S 2 and S 4 glasses at 540˚C, 551˚C and 660˚C for 72 h, respectively, give the XRD patterns shown in Figure 6. These spectra show a structural evolution from orthophosphate (O/P = 4) and olygophosphate phases ( Figure 7 illustrate the morphology of the glasses considered in this work. The glass form of S 1 shown in Figure 7(a), exhibit the presence crystalline phases with different form and size [5] [9] [10] [23]. When the CoO content increases in the glass, the number of crystallites decreases. Hence, SEM analysis confirms a homogenous vitreous phase with feeble crystalline particles in the S 4 sample (Figure 7(e)) which has the maximum CoO content. Some different crystalline phases were identified by XRD and it seems that a decrease of crystallisation tendency is enhanced and Co(PO 3 ) 2 , Ca 2 P 2 O 7 and Co 2 P 2 O 7 phases are crystallized in the last sample (S 4 ) [13]. This probably explains the structural change towards more short pyrophosphates at the detriment of shorter isolated

Discussion
The glasses series 50P 2 O 5 -25CaO-(25−x)Na 2 O-xCoO (with 0 ≤ x ≤ 25; mol%), were prepared by direct melting at 1080˚C. The structure and the chemical durability of these glasses have been investigated using various techniques such as density, X-Ray, DTA, diffraction, IR and SEM. 2) Glasses with CoO content between 5 and 15 mol% can be tested successfully in the biomedical field because they can increase the rigidity of the glass and participate in the osteoinduction of bone tissue [14] [15] [18] [31].
3) Glasses with content between 20 and 25 mole%, can be used, with some improvement, in the electrical conduction range since cobalt can be found under two degrees of oxidation Co 2+ and Co 3+ which ensures the hopping mechanism of the electrons and therefore oxidation reduction phenomenon [21] [32].
Hence, a better understanding of phosphate glass structure is very relevant to the industry in the development of technical glasses to achieve good performances.

Conclusions
The structure and properties of xCoO-(25−x)Na 2 O-25CaO-50P 2 O 5 phosphate glasses (with 0 ≤ x ≤ 25; mol%) have been investigated in the present paper. Here are some conclusions from this paper: 1) The structure of the Co-Na-Ca-phosphate samples glasses, predominantly, consists of olygophosphate, Q 2 -Q 1 units, and the CoO leads to the conversion of Q 0 units to Q 1 units.
2) The glass transition temperature is improved by increasing CoO content in the glass network and leads to the increase of thermal stability.
3) Increasing the glass transition temperature leads to improved chemical durability.
4) The SEM Micrograph indicates an obvious decrease in crystallites with the increase in CoO, causing a relatively large equilibrium between the glass bath and the crystallites.