Much research has focused on biomedical glasses and glasses as promising materials for diverse applications [1] - [6] . Phosphate glasses have interesting characteristics and properties such as a low melting point, high thermal expansion coefficient, and bioactivity, including the concept of the degradation of biomaterials, which make them useful as biomaterials. Several studies have shown that single phosphate glasses do not have good chemical stability compared to phosphate glasses with several components and have also demonstrated that the macroscopic properties of phosphate bioglasses can be improved, by making small changes in the molar concentration of the modifying oxides and the intermediate network. The latter make it possible to reinforce the structure of the vitreous network while at the same time releasing an amount of calcium and phosphate, during the attack by an SBF solution, necessary to accelerate the regeneration of damaged tissue and improve chemical resistance [7] . Through the use of phosphorus pentoxide (P₂O₅) as the initial network, and sodium oxide (Na₂O) with calcium oxide (CaO) as network modifiers, followed by adding other oxides (Al₂O₃, Fe₂O₃, ZnO and TiO₂), the control of degradation may be achieved. Phosphate bioglasses can react with bone tissue through the formation of a hydroxyapatite layer, which is equivalent to the mineral phase of bone; that will then be involved in the process of bone regeneration via a set of physical and chemical reactions [8] . These surface reactions are also responsible for the degradation of the bioglass after implantation [9] [10] [11] . The aim of the present study was to study the structural changes, chemical resistance modification and properties of bioglasses in the Al₂O₃-CaO-NaO-P₂O₅ system, focusing particularly on the effect of the addition of CaO and Al₂O₃ to phosphate oxide glasses for use in the medical field. So the study of series of glasses of composition xAl₂O₃-(40-x)CaO-10NaO-50P₂O₅ (with 0 ≤ x ≤ 10 moles%) indicates that the substitution of CaO by Al₂O₃ in the glass network entrained a decrease of the chemical durability, and a important change from metaphosphate (Q²) and pyrophosphate structural units (Q¹) toward short isolated orthophosphate units (Q⁰) when the Al₂O₃ content reached 10 mol%, confirmed by IR spectrum and X-Ray diffraction.

Phosphate glasses were produced by the direct melting of a mixture of (NH₄)₂HPO₄, CaCO₃, Na₂O and Al₂O₃ in suitable proportions. The reagents were ground together and then introduced into a porcelain crucible. Then, they were heated initially to a temperature of 300˚C for 2 hours and then 500˚C for 1 h to complete decomposition. The reaction mixture was then heated at 900˚C for 40 min and finally at 1080˚C for 30 minutes to obtain a homogeneous liquid. This was then poured onto an aluminum plate which had a temperature of 200˚C to avoid thermal shock. This procedure provided pellets 5 - 10 mm in diameter and 1 to 3 mm thick. The prepared samples were attacked with distilled water at 90˚C for 20 days to determine the dissolution rate estimated from the mass loss. IR spectroscopy analysis was done in a frequency range between 400 cm⁻¹ and 1300 cm⁻¹ with a resolution of 2 cm⁻¹ using a Fourier transform Vertex 70 spectrometer and recorded on a DTGS detector (deuterium triglycine sulfate). The samples were ground and mixed with KBr, which is transparent to infrared. The ratio of the material to KBr in the pellets was 10% to 90% by weight. The analysis by X-ray diffraction was used to identify the structure of the glasses annealed at 550˚C and 660˚C for 48 hours. The samples were analyzed by an X’Pert Pro MPD Panalytical diffractometer. The microstructure of the glass samples was characterized using a scanning electron microscope (SEM).

As can be seen in the ternary diagram in Figure 1, the location of the studied glasses indicates that these glasses are theoretically formed of metaphosphate and pyrophosphate groups. The composition of each sample is given in Table 1.

The dissolution rate (D_R) calculated for the series of bioglasses xAl₂O₃-(40 - x)CaO-10Na₂O-50P₂O₅ (with 0 ≤ x ≤ 10, mol%) is defined as the loss of

glass mass after immersion in 100 mL of distilled water at 90˚C for 20 days, and expressed as g∙cm⁻²∙min⁻¹. The average dissolution rates, which are shown in Figure 2 and Table 2, were measured with respect to the glass surface and the time of exposure. The results show that the chemical durability was improved after increasing the molar percentage of CaO to the detriment of Al₂O₃ [12] [13] .

The IR spectra of the glasses xAl₂O₃-(40 - x)CaO-10Na₂O-50P₂O₅ (with 0 ≤ x ≤ 10) are shown in Figure 3, and the vibration bands of the assignments are given