Certain glass compositions can bonds to bone and they are classified as bioactive glasses [1]. The main characteristic is that on the surface a biologically active hydroxyapatite is developed [2] which interacts with bone tissues. Generally, the compositions include P₂O₅, Na₂O, CaO and SiO₂ components. Nevertheless, the specific proportion of the components is highly important in order to have an adequate application as bioglass or as bioctive glass-ceramics. The first bioglass was obtained by Hench in 1977 and it is known as 45S5 Bioglass, where 45 is the percentage in wt of SiO₂ and 5 is the molar ratio of Ca to P. Such bioglass had higher bioactivity than metals and dense ceramics [3,4]. The proportions of Na₂O, CaO, SiO₂ for obtaining bioglass with biocompatibility/bioactive behaviour (bone-bonding) is circumscriptive/restrictive to a central zone of the corresponding ternary diagram, including or not 6% P₂O₅ [1,5]. The devitrification [6] which can be done afterwards for obtaining bioactive glass-ceramics is also very important because it increases the mechanical resistance of the solid [7,8]. There are many studies about devitrification of glass, nevertheless it will be interesting to analyze the development of phases with temperature before bioglass formation. The aim of this work was to evaluate structural changes of the bioglass different compound mixes by thermal treatment previous to glass formation.

Powders of technical grade silicon oxide (SiO₂), and reactive grade calcium carbonate (CaCO₃), sodium carbonate (Na₂CO₃), and ammonium monobasic phosphate (NH₄H₂PO₄) were used. The percentages of Ca, Na, Si and P expressed as oxides are shown in Table 1 and correspond to three prepared compositions for bioglass type. The composition of the sample named B6 was the same as the well known commercial bioglass B4505 [1]. The sample, B6N, was prepared taking into account the same amount of P₂O₅ and SiO₂, with respect to B6, but changing the amount of Na₂O y CaO, in higher and smaller of 6%, respectively. Finally, in the sample B0NC, the amount of CaO and SiO₂ were the same as B6, the Na₂O was 6% higher and P₂O₅ was not added. The mixtures

Table 1. Samples compositions^*.

^*wt%.

were treated at different temperatures in air atmosphere by using Pt crucibles.

The mixtures were heated at 500˚C, 800˚C, 900˚C, 1000˚C, 1100˚C and 1250˚C during one hour and then quenched. The heated samples were grinded M:100 (<74 μm) before to test.

The crystalline phases were identified by X-ray diffraction by using a Philips 3020 Goniometer with PW 3710 Controller, CuKα radiation (λ = 1.5405 Å) at 40 kV and 20 mA, and Ni filter and powder diffraction files were used for the identification of crystalline phases [9].The relative quantities of crystalline phases were obtained by measuring the highest intensity diffraction peaks of each compound and comparing each other.

X-ray diffractograms of three samples (B6, B6N, B0NC) after thermal treatment (500˚C, 800˚C, 900˚C, 1000˚C, 1100˚C) are shown in Figures 1-3. Modifications on the diffracttograms with an increase of temperature can be appreciated. The treatment up to 1250˚C caused development of visible amorphous phase (Figure 4).

Sample B6 treated up to 500˚C (Figure 1) showed the original carbonated and quartz reactives [10], whereas the initial ammonium dibasic phosphate component was not observed as a crystal phase and sodium phosphate (Na₃PO₄) was formed. Besides, the appearance of a double carbonate of sodium and calcium (NyereriteNa₂Ca(CO₃)₂) took place due to the partial disappearance of the carbonates [11]. After 800˚C treatment, the sodium phosphate amount slightly increased, with an almost total reduction of initial sodium and calcium carbonates [12] and a reduction of quartz too, whose con-

tributed for sodium and calcium silicate

(Na₁₅Ca_3.84Si₁₂O₃₆) and calcium silicate-phosphate

(Ca₅(PO₄)₂(SiO₄)) appearance. Up to 900˚C, quartz suffered almost a total reduction, and sodium phosphate disappeared, both probably responsible for sodium and silicon contribution to the new phase named low combeite (Na_4.2Ca_2.8(Si₆O₁₈)). The Na₁₅Ca_3.84Si₁₂O₃₆ apparition was also more notable and calcium silicatephosphate phase was still present. The last three mentioned compounds were observed at 1000˚C. The disappearance of calcium silicate-phosphate, Ca₅(PO₄)₂(SiO₄), and Na₁₅Ca_3.84Si₁₂O₃₆ was produced by a subsequent increase in temperature up to 1100˚C, and then two phases were

only shown, both belonging to the family of combeites (as it will be later discussed).

According to B6 and B6N composition, the main difference was Na₂O/CaO ratio (Table 1). This way, B6N has a higher Na₂O amount, and a corresponding CaO decrease of equivalent content in the starting mix. B6N treatment up to 500˚C (Figure 2) produced a similar behavior to the one corresponding to B6 at the same temperature. Up to 800˚C, the sodium phosphate presence, Na₃PO₄, and Na₁₅Ca_3.84Si₁₂O₃₆ were like B6 under the same heat treatment, while the quartz amount was lower, and carbonates were not seen.

Another important difference between B6 and B6N was due to the apparition of CaO and sodium-calcium silicate (Na₄Ca₄Si₆O₁₈). This developed phase has the same elements proportions than that found by Clupper and Hench [13] (Na₂Ca₂Si₃O₉) which, according to mentioned authors, presented bioactive behavior.

After 900˚C treatment, quartz, calcium oxide and Na₄Ca₄Si₆O₁₈ disappeared, while sodium phosphate was reduced and Na₁₅Ca_3.84Si₁₂O₃₆ was still present. Low combeite, Na_4.2Ca_2.8Si₆O₁₈, was the only new phase which presents a similar molecular formula to the phase which was gone, Na₄Ca₄Si₆O₁₈. Up to 1000˚C, sodium phosphate disappeared, the relative low combeite amount increased and high combeite appeared at the expense of Na₁₅Ca_3.84Si₁₂O₃₆ similar amount disapperance, as both molecular compositions are similar. After 1100˚C treatment, the still existing phases were the same as such presented in B6.

A detailed analysis showed that B6-1100 and B6N- 1100 samples had different amounts of two types of combeite: high and low. Differences between both phases in studied samples were determined by comparison of the peaks placed on 23.8˚, 33.8˚ and 34.1˚ (2 theta) whose relative intensities are unlike between both samples. This way, the corresponding composition of B6 sample was about 60% of low combeite and 40% of high combeite, and 30% and 70%, respectively for B6N sample.

B0NC thermal treatment up to 500˚C had a similar behavior about phase transformations as previous samples (B6, B6N), (Figure 3), except for sodium phosphate formation, which was not present because of the phosphorous absence in original composition.

At 800˚C, raw material presence was not detected, and new phases were originated: CaO, Na₄Ca₄Si₆O₁₈, Na₁₅Ca_3.84Si₁₂O₃₆, Na₂SiO₃ and Na₂CaSiO₄. The last two phases were absent in samples B6 and B6N treated up to 500˚C. Treatment at 900˚C leaded to almost CaO extinction and sodium-calcium double silicate reduction, with sodium silicate, Na₄Ca₄Si₆O₁₈ and Na₁₅Ca_3.84Si₁₂O₃₆ increase and calcium silicate (Ca₂SiO₄) appearance (probably induced by calcium supply because of CaO disappearance). After rise in temperature up to 1000˚C, still existing phases were the same, except CaO, which was gone. At 1100˚C, Na₄Ca₄Si₆O₁₈, Na₁₅Ca_3.84Si₁₂O₃₆ and Ca₂SiO₄ were the majority phases. The most outstanding thing of this composition treated at the temperature previously mentioned, was that highand low-combeites were not formed. However, Na₄Ca₄Si₆O₁₈ and Na₁₅Ca_3.84Si₁₂O₃₆ phases, which we could call respectively lowand high-combeite forerunners, were present. This gives us an idea that formation processes of combeites are delayed due to phosphorous absence in the mix.

Figures 5-7 show the presence of phases between 800 and 1100˚C heat treatment in B6, B6N and B0NC, respectively, in arbitrary units. Comparative analyses of the figures allowed to highlight the following:

• High-combeite formation was observed at lower temperatures (1000˚C for B6N and between 1000 and 1100˚C for B6) when phosphorous/sodium ratio was higher (0.245 for B6 and 0.200 for B6N). This might suggest that, for a zero relation, as B0NC case, the probability of high combeite appearance would increase at temperatures higher than 1100˚C, without passing liquid temperature of the mix.

• Highand low-combeites would be formed by similar formulation sodium and calcium silicate, which could be called forerunners.

• Higher sodium content in the initial composition with phosphorous (B6 and B6N) originated a higher relative amount of Na₁₅Ca_3.84Si₁₂O₃₆ or high-combeite at high temperature (eg. 1000˚C - 1100˚C).

• As it was mentioned before, both samples (B6 and B6N) showed combeite phases.

• In general, disappearance or reduction of Na₄Ca₄Si₆O₁₈ or Na₁₅Ca_3.84Si₁₂O₃₆ causes the lowor high-combeite appearance, respectively.

• Na₄Ca₄Si₆O₁₈ phase was developed, which has the same minimal molecular formula than that found by Clupper and Hench, and could have bioactive behaviour.

• Phosphorous absence on B0NC originated, after high temperature treatments, two phases that were not present in the other three samples (B6 and B6N): Sodium silicate Na₂SiO₃, calcium silicate Ca₂SiO₄ and sodium-calcium silicate Na₂CaSiO₄. However, it seemed clear that any type of combeite could be formed.

Developed phases along the applied thermal treatments on both B6 and B6N samples were different, probably due to different Na₂O/CaO ratio contents with 1 and 1.62 values for B6 and B6N samples, respectively, still holding the same values of initial phosphorous and silicon amount. The highest Na₂O/CaO ratio value, 1.62, originnated intermediate CaO and Na₄Ca₄Si₆O₁₈, and also high-combeite at lower temperature and with higher final content respect to the sample with ratio equal to 1 (B6 sample). For the sample ratio value 1, phosphorous takes part of Ca₅(PO₄)₂(SiO₄) compound. This phase bioactiveity was shown as be good [14].

The Na₄Ca₄Si₆O₁₈ and Na₁₅Ca_3.84Si₁₂O₃₆ phases can be considered as forerunner phases for low- (Na_4.2Ca_2.8(Si₆O₁₈) and highcombeite (Na_15.78Ca₃(Si₆O₁₂)), respectively.

Na₄Ca₄Si₆O₁₈ could have bioactivity because its similarity to the phase Na₂Ca₂Si₃O₉ found by Clupper and Hench, which showed amorphous calcium phosphate formation on its surface when it was contacted with simulated body fluid.

The other relevant result was correlated to the P₂O₅/ Na₂O ratio value of the samples. The combeite phases were formed at lower temperatures while such ratio increases. This way, combeite in B6 was found at a lower temperature than B6N, whose P₂O₅/Na₂O ratio values were 0.245 and 0.200, respectively, and for zero ratio as shown B0NC sample, the combeite was not formed up to least 1100˚C. However, on B0NC, sodium and calcium silicates, which were previously called combeite forerunners, were present. This indicated that if it is possible to increase P₂O₅/Na₂O ratio, the processes of combeites formation can be accelerated. Besides, it is possible to obtain these included phases at higher temperatures with phosphorous absence, always without passing liquid temperature.

Another thing is that this study can help to elucidate developed crystalline phases by devitrification of glass formulation of biomaterials, by using the found out ten-

dencies.

Finally, future works will be done for evaluate the bioactive behavior highand low-combeite, the last which has the more similar structure as Na₄Ca₄Si₆O₁₈.