Chemical Durability and Structural Properties of Al 2 O 3-CaO-Na 2 OP 2 O 5 Glasses Studied by IR Spectroscopy , XRD and SEM

Various characterization techniques were used to study the composition of the glass series xAl2O3-(40 x)CaO-10Na2O-50P2O5 (with 0 ≤ x ≤ 10) in terms of chemical durability, X-ray diffraction, IR spectroscopy and scanning electron microscopy (SEM). The improved chemical durability was attributed to the replacement of easily hydrated P-O-P bonds by covalent and resistant Ca-O-P and Al-O-P bonds. However, the change in the dissolution rate (DR) versus time showed a marked decrease in chemical durability with increasing the Al2O3 content to the detriment of the CaO content. The X-ray diffraction analysis of glasses annealed at 550 ̊C and 660 ̊C for 48 hours indicated the presence of pyrophosphate phases and predominant metaphosphates or cyclic metaphosphate phases when the Al2O3 content was ≤7.5 mol%. Nevertheless, both, X-ray diffraction and IR spectroscopy confirmed the structural tendency change from metaphosphate (Q2) and pyrophosphate structural units (Q1). Toward short isolated orthophosphate units (Q0) when the Al2O3 content above 7.5 mol%. SEM micrographs illustrated that the number of crystallites increased in the glass network when the Al2O3 content increased at the expense of the CaO content. An increase in the Al2O3 content to 10 mol% led to the formation of a larger number of crystallites of different sizes, dominated by small crystallite sizes assigned to short isolated orthophosphate groups. This phenomenon led to a decrease in chemical durability and seems to be a favorable factor for the formation of the apatite layers which enclose the glass, in a SBF solution test, able of regenerating bone tissue in biomedical application. How to cite this paper: Er-rouissi, Y., Chabbou, Z., Beloued, N. and Aqdim, S. (2017) Chemical Durability and Structural Properties of Al2O3-CaO-Na2O-P2O5 Glasses Studied by IR Spectroscopy, XRD and SEM. Advances in Materials Physics and Chemistry, 7, 353-363. https://doi.org/10.4236/ampc.2017.710028 Received: July 7, 2017 Accepted: October 15, 2017 Published: October 18, 2017 Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access Y. Er-rouissi et al. DOI: 10.4236/ampc.2017.710028 354 Advances in Materials Physics and Chemistry


Introduction
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 (P 2 O 5 ) as the initial network, and sodium oxide (Na 2 O) with calcium oxide (CaO) as network modifiers, followed by adding other oxides (Al 2 O 3 , Fe 2 O 3 , ZnO and TiO 2 ), 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 Al 2 O 3 -CaO-NaO-P 2 O 5 system, focusing particularly on the effect of the addition of CaO and Al 2 O 3 to phosphate oxide glasses for use in the medical field.So the study of series of glasses of composition xAl 2 O 3 -(40-x)CaO-10NaO-50P 2 O 5 (with 0 ≤ x ≤ 10 moles%) indicates that the substitution of CaO by Al 2 O 3 in the glass network entrained a decrease of the chemical durability, and a important change from metaphosphate (Q 2 ) and pyrophosphate structural units (Q 1 ) toward short isolated orthophosphate units (Q 0 ) when the Al 2 O 3 content reached 10 mol%, confirmed by IR spectrum and X-Ray diffraction.

Experimental Procedures
Phosphate glasses were produced by the direct melting of a mixture of (NH 4 ) 2 HPO 4 , CaCO 3 , Na 2 O and Al 2 O 3 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).

Ternary Diagram
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.

Chemical Durability
The dissolution rate (D R ) calculated for the series of bioglasses xAl 2 O 3 -(40x)CaO-10Na 2 O-50P 2 O 5 (with 0 ≤ x ≤ 10, mol%) is defined as the loss of  1 gives the corresponding compositions in the quaternary system Al2O3-CaO-Na2O-P2O5.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 Al 2 O 3 [12] [13].

Infrared Spectra
The IR spectra of the glasses xAl 2 O 3 -(40 -x)CaO-10Na 2 O-50P 2 O 5 (with 0 ≤ x ≤ 10) are shown in Figure 3, and the vibration bands of the assignments are given Table 1.Glass composition expressed in terms of quaternary systems.

Glass sample
Chemical compositions Glass compositions inside the ternary diagram Table 2.The composition of the glasses in mol% and some characteristics of the quaternary glasses xAl2O3-(40 -x)CaO-10Na2O-50P2O5.

Glass Sample
The various compositions of glass (mol%)  in Table 3.All vibration bands are located between 400 and 1300 cm −1 .The vibration bands located around 700 -780 cm −1 are attributed to the vibration mode υ sym (P-O-P) of pyrophosphate groups (Q 1 ), while the bands between 900 -924 cm −1 are attributed to the vibration mode υ asym (P-O-P) of pyrophosphate groups (Q 1 ).When the Al 2 O 3 content reached 10 mol%, we observed the appearance of a band at 980 cm −1 , assigned to the vibration mode υ asym (P-O-P) of isolated orthophosphate groups Q 0 , to the detriment of bands at 750 -780 cm −1 and 900 -924 cm −1 attributed to pyrophosphate groups, which became single shoulders.The band in the frequency range between 1225 -1280 cm −1 is assigned to the vibration mode υ asym (PO 2 ) of metaphosphate groups without a bridging oxygen (Q 2 ) [14] [15] [16], which became weaker when the value of x was greater than 7.5 mol%.The vibration bands located around 1070 and 1120 cm −1 , characteristic of the vibration modes υ ym (PO 2 ) and υ asym (PO 3 ) of tetrahedral units, Q 2 + Q 1 , which are a mixture of pyrophosphate and metaphosphate groups, also underwent a decrease in intensity when the Al 2 O 3 content exceeded 5 mol%.

X-Ray Diffraction
X-ray diffraction confirmed the vitreous character of all the samples.The XRD spectra of samples S 0 50 to S 10 50 , annealed at 550˚C and 660˚C for 48 h are shown in Figure 4. Several phases were detected, including calcium metaphosphate or However, for the compound S 10 50 , there is a radical structural change which indicates the predominance of orthophosphates phase type AlPO 4 and eventually Ca 3 (PO 4 ) 2 .groups in the glass network at the expense of cyclical chain metaphosphate groups and pyrophosphate chains when the Al 2 O 3 content exceeded 7.5 mol% can be explained by the fact that we were close to the border area between crystal and glass.The number of crystallites of different sizes generally increased and exceeded the equilibrium that must be established between the glass and the crystallites, which led to a significant decrease in chemical durability [19].Indeed the compound S 10 50 , has an opaque white appearance which explains its frontal position between the glass and the crystal.However, the favorable formation of the isolated orthophosphate groups when the CaO oxide is substituted by Al 2 O 3 oxide, and especially when the Al 2 O 3 level reaches 10 mol%, can be of great utility in the formation of the apatite layers in an organic-substance-free a cellular simulated body fluid (SBF), able of regenerating bone.

Conclusion
The structure and chemical durability of a glass series composed of xAl 2 O 3 -(40 − x)CaO-10Na 2 O-50P 2 O 5 (with 0≤ x ≤10) (mol%) were investigated using various techniques such as IR, X-ray diffraction and SEM.The improved chemical durability in these glasses was attributed to the replacement of the easily hydrated P-O-P bonds by covalent and resistant Ca-O-P bands.However, the substitution of CaO by Al 2 O 3 in the glass network led to a considerable decrease in the chemical durability.The IR spectra indicated a radical structure change from metaphosphate or cyclic metaphosphate and pyrophosphate towards short isolated orthophosphate groups when the Al 2 O 3 content was above 7.5 mol%.Consequently, we can predict the depolymerization of the large phosphate network into isolated short chains of the orthophosphate (Q˚) type.The SEM micrographs of S 0 50 , S 7.5 50 and S 10 50 illustrate that the number of crystallites increased from S 0 50 to S 10 50 when the Al 2 O 3 content increased at the expense of the CaO content.The increase in the number of crystallites that led to the formation of isolated short chains of the orthophosphate (Q˚) type almost certainly caused disproportionality between the glass and the crystallites, which caused a significant decrease in chemical durability.Hence, a better understanding of glass corrosion is very relevant to the industry in the development of technical bioglasses to achieve both good performance and, at the same time, provide calcium ion and Orthophosphate ions, in an aqueous solution or SBF solution, necessary for the formation of effective hydroxyapatite layers in bone regeneration in biomedical applications.

Figure 2 .
Figure 2. The chemical durability of phosphate glasses versus the Al2O3 content.

Figure 4 .
Figure 4. XRD patterns for the glass samples S0 50 , S7.5 50 and S10 50 after heat treatment for 48 h in an air atmosphere at 550˚C and 600˚C.

Figure 5
Figure 5 illustrates the morphology of the glasses, before and after immersion in distillated water for 20 consecutive days, considered in this work.It shows the existence of two phases, one crystalline and the other glass [13] [16] [18] [19].It also indicates the formation of agglomerates of the crystalline phase from one micrometer to a few tens of micrometers in size.The SEM micrograph also indicates that the number of crystallites increased from S 0 50 to S 10 50 when the Al 2 O 3