Carbonate-substituted hydroxyapatites containing up to 9 wt % of carbonate groups were synthesized and fabricated in the form of porous granules with a view to developing materials for use in bone tissue repairs. The use of sintering additives forming a liquid phase allowed the granule sintering temperature to be reduced by 400–450°C. It was found that the carbonate groups enter into the structure of the ceramic by a mixed AB-type substitution; the microstructure of the granules depends substantially on the concentration of the carbonate groups; introduction of 6 wt % of carbonate groups into apatite ensures high biological properties of the granules in experiments in vivo .

Incorporation of carbonate ions to the crystal structure of carbonated hydroxyapatite (CHAp) leads to the formation of point defects (vacancies) in Ca- and OH-sublattices as well as to microstrains revealed in CHAp nanocrystals. Various techniques, such as XRD, FTIR, TEM, FESEM/EDX, TG/DTA, AES (ICP), wet chemical analysis, Ca-ionometry, microvolumetric analysis of evolved CO,, BET adsorption, were applied to determine an efficiency of carbonate substitution, and to quantify the elemental composition, as well as to characterize the structure of the carbonated hydroxyapatite and the site(s) of carbonate substitution. It was shown that there is insignificant incorporation of Na into the crystal structure of HAp. Over the range of 0-4% wt. (x < 0.25), the substitution of OH- by CO32- takes place leading to A-Type of CHAp, further increase of CO32- -content enhances PO43- -substitution giving AB-type of CHAp. According to in vitro test, the bioactivity of the samples is increasing with the growth of carbonate content due to accumulation of the defects in CHAp nanocrystals.

Bioactive materials are characterized by a high rate of formation of a carbonate-containing hydroxyapatite contact layer at the implant-bone interface and by a moderate resorption capacity in weakly acidic media. In vitro tests simulating the precipitation of hydroxyapatite from interstitial tissue fluid at the surface of a material are performed to evaluate the bioactivity of a broad range of materials. The data indicate that the silicon-containing material is characterized by the highest bioactivity in the series of calcium hydroxyapatites.

Conditions for hydroxyapatite (HAP) synthesis in aqueous solutions by hydrolysis of α-Ca 3 (PO 4 ) 2 were studied. Temperature exerts a substantial effect on the rate of α-Ca 3 (PO 4 ) 2 hydrolysis and also changes the morphology of the reaction products. At 40 °C, the plate-like intersecting (perpendicular to the surface of the initial particles) crystals of HAP grow. Their maximum size after the 24-h hydrolysis is 1–2 µm. Needle like HAP crystals are formed upon boiling of the suspension. The morphology observed for the HAP particles agrees well with the conclusions obtained by analysis of the kinetics of tricalcium phosphate hydrolysis.

Summary 1.Data have been obtained which supplement material already published [1–3] and give more precise information on the phase diagram of the Na2SO4−Na2CO3 system in the region of the γ′-phase.2.It has been established that, in the absence of moisture at ordinary temperatures, the phase diagram of the Na2SO4−Na2CO3 system is characterized by the presence of seven solid phases (Fig. 4,a): α-Na2SO4, present in solid melts up to ∼5 mole% Na2CO3 [1–3]; β-phase—a solid solution based on β-Na2SO4 [1–3]; ε-phase—a double salt of composition 9Na2SO4·Na2CO3, formed at about 125° from the β-phase (tetragonal crystals with a=7.87, c=11.41 kX; c/a-1.45; the calculation for the powder diagram was given in [1]); γ′-phase (14–50 mole % Na2CO3)—a bilateral hexagonal burkeite solid solution based on 2Na2SO4·Na2CO3; ω-phase (60–64 mole % Na2CO3)—a solid solution based on the compound 2Na2SO4·3Na2CO3; η-phase—a chemical compound of composition Na2SO4·3Na2CO3 [1]; α′-phase — solid solution with the α-Na2CO3 lattice [1, 10].3.It has been shown that the hexagonal burkeitesolid solution (the α′-phase) is a Daltonide compound of the substitutional solid solution type.4.It has been established that the minimum at about 33 mole % Na2SO4 on the curve for the breakdown of the γ-solid solution corresponds to the eutectic breakdown of the γ-phase to an ω-phase of variable composition from 60–64 mole % Na2CO3 and the compound Na2SO4·3Na2CO3 (η-phase).5.It has been found that solid sodium sulfate-sodium carbonate melts in the presence of water vapor under atmospheric conditions are converted from the two-component to a three-component state and the equilibrium phase diagram of the binary Na2SO4−Na2CO3 system (Fig. 4) is regenerated in one of the sections of the equilibrium phase diagram for the ternary Na2SO4−Na2CO3−H2O system (Fig. 5).

Data have been obtained which supplement material already published [1–3] and give more precise information on the phase diagram of the Na2SO4−Na2CO3 system in the region of the γ′-phase.