Sintering behaviour of fluorapatite–silicate composites produced from natural fluorapatite and quartz

In this work, the sintering behaviour of fluorapatite (FAp)–silicate composites prepared by mixing variable amounts of natural quartz (2.5 wt% to 20 wt%) and FAp was studied. The composites were pressureless sintered in air at temperatures from 1000 °C to 1350 °C. The effects of temperatures on the densification, phase formation, chemical bonding and Vickers hardness of the composites were evaluated. All the samples exhibited mixed phase, comprising FAp and francolite as the major constituents along with some minor phases of cristobalite, wollastonite, dicalcium silicate and/or whitlockite dependent on the quartz content and sintering temperature. The composite containing 2.5 wt% quartz exhibited the best sintering properties. The highest bulk density of 3 g/cm³ and a Vickers hardness of >4.2 GPa were obtained for the 2.5 wt% quartz–FAp composite when sintered at 1100 °C. The addition of quartz was found to alter the microstructure of the composites, where it exhibited a rod-like morphology when sintered at 1000 °C and a regular rounded grain structure when sintered at 1350 °C. A wetted grain surface was observed for composites containing high quartz content and was believed to be associated with a transient liquid phase sintering.     

Sol-gel Preparation and Preliminary in vitro Evaluation of Fluorapatite／Hydroxyapatite Solid Solution Films

Fluorapatite/hydroxyapatite solid solution has better biological properties than other apatites, especially used as films or coatings. In this work, sol-gel preparation and in vitro behavior of fluorapatite/hydroxyapatite solid solution films on titanium alloy were investigated. Ca(NO3)2-4H20 and PO(OH)x(OEt)3-x were selected as precursors, and hexafluorophosphoric acid (HPFo) was used as a fluorine containing reagent. The Ca and P precursors were mixed with HPFo to keep the Ca/P molar ratio 1.67. The mixtures refluxed for 12 h were used as dipping sols for the preparation of the films. The phase of the films obtained at 600℃ was apatite. The F contents in the films increased with the concentrations of HPF6 in the dipping sols. The solid solution films were shown to have better stability than hydroxyapatite films, and a reasonably good bioactivity in the in vitro evaluation.     

New approach to environmentally benign epoxidation: the solid-phase-system using urea–H2O2 and recyclable dodecatungstate on apatite

On the basis of new concept using a solid disperse phase we have developed an efficient catalytic solid-phase-system for epoxidations of alkenes using urea–hydrogen peroxide (urea–H₂O₂) complex and cetylpyridinium dodecatungstate ((CetylPy)₁₀[H₂W₁₂O₄₂]) catalyst on fluorapatite (FAp). The recovered solid catalyst phase was reused to keep the catalytic activity after several times. In the conceptual idea it is a key point that in situ solid-phase-activation of the catalyst with urea–H₂O₂ proceeds to form microcrystals of the active species dispersed on the solid phase. The dispersion of the catalyst on FAp in the case of tungstic acid (H₂WO₄) was suggested by EPMA analysis. We proposed the peroxo type of species keeping the parent polyoxometalate framework as novel active species from FT-IR spectroscopic studies. FAp phase plays important roles of dispersing the active species on its surface to have high catalytic activity and of stabilizing the active species to lead to high reusability.     

Synthesis and dissolution behavior of nanosized silicon and magnesium co-doped fluorapatite obtained by high energy ball milling

Nanosized hydroxyapatite (HA) powders exhibit a greater surface area than coarser crystals and are expected to show an improved bioactivity. In addition, properties of HA can be tailored over a wide range by incorporating different ions into HA lattice. The aim of this study was to prepare and characterize silicon and magnesium co-doped fluorapatite (Si–Mg–FA) with a chemical composition of Ca_9.5Mg_0.5 (PO₄)_5.5(SiO₄)_0.5F₂ by the high-energy ball milling method. Characterization techniques such as X-ray diffraction analysis (XRD), Fourier transformed infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM) were utilized to investigate the structural properties of the obtained powders. Dissolution behavior was evaluated in simulated body fluid (SBF) and physiological normal saline solution at 37 °C for up to 28 days. The results of XRD and FTIR showed that nanocrystalline single-phase Si–Mg–FA powders were synthesized after 12 h of milling. In addition, incorporation of magnesium and silicon into fluorapatite lattice decreased the crystallite size from 53 nm to 40 nm and increased the lattice strain from 0.220% to 0.296%. Dissolution studies revealed that Si–Mg–FA in comparison to fluorapatite (FA), releases more Ca, P and Mg ions into SBF during immersion. 175 ppm Ca, 33.5 ppm P and 48 ppm Mg were detected in the SBF containing Si–Mg–FA after 7days of immersion, while for FA, it was 75 ppm Ca, 21.5 ppm P and 29 ppm Mg. Release of these ions could improve the bioactivity of the obtained nanopowder. It could be concluded that the prepared nanopowders have structural properties such as crystallite size (~40 nm), crystallinity degree (~40%) and chemical composition similar to biological apatite. Therefore, prepared Si–Mg–FA nanopowders are expected to be appropriate candidates for bone substitution materials and also as a phase in polymer or ceramic-based composites for bone regeneration in tissue engineering applications.     

Apatite ore mine tailings as an amendment for remediation of a lead-contaminated shooting range soil

This study investigated the use of tailings from apatite ore beneficiation in the remediation of a heavily contaminated shooting range soil. The tailings originating in Siilinjärvi carbonatite complex, Finland, consist of apatite residues accompanied by phlogopite and calcite. In a pot experiment, organic top layer of a boreal forest soil predisposed to pellet-derived lead (Pb) was amended with tailings of various particle-sizes (Ø > 0.2 mm, Ø < 0.2 mm and unsieved material) differing in their mineralogical composition. After 9-, 10-, 14- and 21-month incubation, the samples were monitored for tailings-induced changes in the different Pb pools by means of sequential fractionation. Following the incubation, the samples were extracted with water and the extracts were analyzed for Pb species distribution by means of a cation exchange resin. The results revealed that Pb was continuously released from the shotgun pellet fragments due to weathering. However, the apatite and calcite compartments in the tailings counteracted the mobility of the released Pb through the formation of sparingly soluble fluorpyromorphite and cerussite. Furthermore, the tailings efficiently reduced the bioavailability of Pb by transferring it from the water-soluble and exchangeable pools into the organic one. The material also increased the proportion of the less toxic non-cationic Pb to the total dissolved Pb from the initial level of 5% to 9-12%. The results suggest that the tailings-induced stabilization of Pb may be an environmentally sound remediation technique at polluted sites.     

Bioactive fluorapatite-containing glass ceramics

Fluorapatite-containing glass ceramics were synthesized on the basis of the glass-forming system SiO₂–Al₂O₃–P₂O₅–CaO–CaF₂. The introduction of phosphorus and fluorine containing materials, as well as the specific regime of heat treatment of the glasses gave glass ceramic materials with crystalline phases of the apatite group—fluorapatite (Ca₁₀(PO₄)₆F₂), apatite (Ca₃(PO₄)₂), vitlokite (Ca₉P₆O₂₄), etc. The X-ray phase analysis showed that the main phase in all the glass ceramic samples was fluorapatite. The phase composition, structure and some of the basic properties of the glass ceramic samples were determined.     

Fabrication and characterization of magnesium–fluorapatite nanocomposite for biomedical applications

Recent studies indicate that there is a high demand for magnesium alloys with adjustable corrosion rates, suitable mechanical properties, and the ability for precipitation of a bone-like apatite layer on the surface of magnesium alloys in the body. An approach to this challenge might be the application of metal matrix composites based on magnesium alloys. The aim of this work was to fabricate and characterize a nanocomposite made of AZ91 magnesium alloy as the matrix and fluorapatite nano particles as reinforcement. A magnesium–fluorapatite nanocomposite was made via a blending–pressing–sintering method. Mechanical, metallurgical and in vitro corrosion measurements were performed for characterization of both the initial materials and the composite structure. The results showed that the addition of fluorapatite nano particle reinforcements to magnesium alloys can improve the mechanical properties, reduce the corrosion rate, and accelerate the formation of an apatite layer on the surface, which provides improved protection for the AZ91 matrix. It is suggested that the formation of an apatite layer on the surface of magnesium alloys can contribute to the improved osteoconductivity of magnesium alloys for biomedical applications.     

In-situ transmission electron microscopy observation of the helium bubble evolution in pre-irradiated fluorapatite during annealing

The polycrystalline fluorapatite Ca₁₀(PO₄)₆F₂ ceramic synthesized by a standard solid-state sintering method was pre-irradiated with 80 keV He⁺ ions to a fluence of 5 × 10¹⁶ ions/cm² at room temperature. After that, an in-situ annealing experiment was performed inside a transmission electron microscope to monitor the evolution of helium bubbles during heating to 723 and 823 K. Initially, no helium bubble formation was observed in the damage layers of the pre-irradiated samples. However, as the temperature increased, helium bubbles first became visible and then began coarsening, ultimately reaching an asymptotic radius during annealing. The migration and coalescence of helium bubbles in the fluorapatite matrix was complete at a temperature of 823 K, and its likely mechanism involved the existence of two different types of coalescing bubbles.     

Vanadomolybdophosphoric acid/fluorapatite solid-phase system for aerobic oxidative dehydrogenation

We developed a solid-phase-oxidation-system using FAp disperse phase and vanadomolybdophosphoric acid (H_3+nPV_nMo_12−nO₄₀: PVn) catalysts with molecular oxygen as a new green reaction system. The PV4/FAp system was an efficient and recyclable solid–catalyst system for solvent-free oxidative dehydrogenation of -terpinene to p-cymene under 1 atm of molecular oxygen at 50 °C. The catalytic activity in the solid-phase system was comparable to that in the homogeneous liquid-phase system with acetonitrile. Even if under air conditions, PV4/FAp system was able to promote the catalytic dehydrogenation at room temperature sufficiently.     

Structural,physical and antibacterial properties of pristine and Ag+ doped fluoroapatite nanomaterials

Calcium phosphate ceramics, owing to their resemblance to bone structure, are widely used for different biomedical applications. However, such a specific bone-structure pattern is making those materials susceptible to colonisation by pathogenic bacteria. This work presents results of the study on physical characterisation and microbiological assessment of biocide properties of modified man-made fluor- and hydroxyapatites. The structures and physical properties were characterised by a diffuse reflectance spectra, FTIR, Raman spectroscopy, TEM and Zeta-potential analyses. The bactericidal properties were assessed with Staphylococcus aureus, the main pathogenic species responsible for implant-associated infections. It was found that manufactured materials had structures typical for the biological (bone) apatites. Doping with silver has not changed their morphology. Results from the assays have confirmed that built-in silver substantially increased their biocidal properties, thus Ag-doped fluorapatite is a promising new resistant biomaterial with great bactericidal effect that potentially could be applied in tissue engineering, or dentistry.