Synthesis of hydroxyapatite using phosphate-rich glasses in the system CaO-P2O5-H2O and acoustic waves

Phosphate-rich glass powders in the CaO-P₂O₅ system were prepared by the sol-gel method using aqueous solutions of Ca(NO₃)₂·4H₂O and H₃PO₄. Glass powders were subjected to reaction with Ca(OH)₂ and water under varied experimental conditions, such as temperature (25–90 °C), particle size (1–5 and 40 m), and ultrasound waves (20 kHz). Parallel experiments were also conducted without ultrasound for comparison. The reaction products were composed of Ca(H₂PO₄)₂·H₂O (monocalcium phosphate monohydrate), CaHPO₄ (monatite), CaHPO₄ · 2H₂O (brushite), Ca₅(PO₄)₃OH (hydroxyapatite) and amorphous calcium phosphate. It is interesting to note that hydroxyapatite and brushite were obtained at a low temperature of 60 °C in a very short period of time (30 min). All samples were characterized by X-ray diffraction and scanning electron microscopy.     

Thermoanalytical characteristics of powders in dental cast investment

The present study examined the thermal properties of phosphate-bonded investments, a gypsum-bonded investment and an experimental investment powder when the basic powders were heated to high temperatures by simultaneous differential thermal analysis (DTA) and thermogravimetry (TG). The phosphate-bonded investments showed values of about 59 kcal mol^–1 (247 kJ mol^–1) (thermal decomposition of NH₄H₂PO₄) and about 11 kcal mol^–1 (46 kJ mol^–1) (formation of NH₄MgPO₄). Thermal reactions occurred clearly on the DTA-TG curves for the investment powders, using powders of NH₄H₂PO₄, and MgO with NH₄H₂PO₄/MgO = 1 as main components in the investment.     

Preparation and characterization of selenite substituted hydroxyapatite

Selenite-substituted hydroxyapatite (Se-HA) with different Se/P ratios was synthesized by a co-precipitation method, using sodium selenite (Na₂SeO₃) as a Se source. Selenium has been incorporated into the hydroxyapatite lattice by partially replacing phosphate (PO₄^3 ?) groups with selenite (SeO₃^2 ?) groups. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) techniques reveal that substitutions of phosphate groups by selenite groups cause lower carbonate groups occupying at phosphate sites and change the lattice parameters of hydroxyapatite. The powders obtained are nano-crystalline hydroxyapatite when the Se/P ratios are not more than 0.1. The particle shape of Se-HA has not been altered compared with selenite-free hydroxyapatite but Se-incorporation reduces the crystallite size. The crystallinity was reduced as the Se/P ratios increased until amorphous phase (Se/P = 0.3) appeared in the Se-HA powder obtained, and then another crystal phase presented as calcium selenite hydrate (Se/P = 10). In addition, the sintering tests show that the Se-HA powders with the Se/P ratio of 0.1 have thermal stability at 900 °C for 2 h; hence they have great potential in the fabrication of bone repair scaffolds.     

Preparation and characteristics of nanosized carbonated apatite by urea addition with coprecipitation method

Nanosized carbonated apatites (CAp) were prepared by coprecipitation method with urea addition at pH 8 and pH 12 with 12 hours aging, which resulted in needle-like and rod-like structures, respectively. From examining the XRD patterns, it was determined that the as-prepared powders contained no crystalline phase other than CAp. However, the powder prepared at pH 8 partially transforms to β-tricalcium phosphate (β-TCP) after heat treatment at 900 °C. Under heat treatment, powder prepared at pH 12 is more stable than powder prepared at pH 8. In addition, the as-prepared powders at pH 8 with 12 hours aging yield needle-like calcium deficient apatite (CDA) with substitution of CO₃^2? for PO₄^3? in low content. Their particle size is about 20 nm in width and 200–300 nm in length. The as-prepared powders at pH 12 for 12 hours aging result in rod-like nanostructures about 50 nm in diameter and 300 nm in length with 8.2 wt.% CO₃^2?, which is similar to human bone composition. Various characteristic analyses of CAp have been employed including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and transmission electron microscopy (TEM).     

Novel synthesis and characterization of bismuth nano/microcrystals with sodium hypophosphite as reductant

In this paper, we report on the low temperature solution reduction method employed in the synthesis of large quantities of nano/micro-sized bismuth (Bi) crystals with sodium hypophosphite (NaH₂PO₂·H₂O) as reductant in acidic solutions. The achieved Bi crystals exhibited plate-like (100 nm in size and few nanometers in thickness) or polyhedral (500 nm in size) shapes. Bi nanocrystals transformed to octahedron-like Bi microcrystals only by prolonging the reaction time. The assembly and oriented growth should be the reason. To understand the growth mechanism, we also discussed the possible growth of the Bi nanocrystals exhibits influence of experimental parameters such as reaction time, NaH₂PO₂·H₂O concentration, and pH value. The resulting Bi crystals were characterized by using scanning electron microscopy, X-ray powder diffraction and differential thermal analysis and thermogravimetry. Optical properties of the samples were studied by ultraviolet–visible spectroscopy.     

Si-substituted hydroxyapatite nanopowders: Synthesis, thermal stability and sinterability

Synthetic hydroxyapatites incorporating small amounts of Si have shown improved biological performances in terms of enhanced bone apposition, bone in-growth and cell-mediated degradation.This paper reports a systematic investigation on Si-substituted hydroxyapatite (Si 1.40 wt%) nanopowders produced following two different conventional wet methodologies: (a) precipitation of Ca(NO₃)₂·4H₂O and (b) titration of Ca(OH)₂. The influence of the synthesis process on composition, thermal behaviour and sinterability of the resulting nanopowders is studied.Samples were characterised by electron microscopy, induced coupled plasma atomic emission spectroscopy, thermal analysis, infrared spectroscopy, N₂ adsorption measurements, X-ray diffraction and dilatometry. Semicrystalline Si-substituted hydroxyapatite powders made up of needle-like nanoparticles were obtained, the specific surface area ranged between 84 and 110 m²/g. Pure and Si-substituted hydroxyapatite nanopowders derived from Ca(NO₃)₂·4H₂O decomposed around 1000 °C. Si-substituted hydroxyapatite nanopowders obtained from Ca(OH)₂ were thermally stable up to 1200 °C and showed a distinct decreased thermal stability with respect to the homologous pure sample. Si-substituted hydroxyapatites exhibited higher sintering temperature and increased total shrinkage with respect to pure powders. Nanostructured dense ceramics were obtained by sintering at 1100 °C Si-substituted hydroxyapatites derived from Ca(OH)₂.     

Synthesis and characterization of P-doped TiO2 nanotubes

Titanium dioxide (TiO₂) doped with phosphorus (P) was synthesized by anodization of Ti in the mixed acid electrolyte of H₃PO₄ and HF and characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis spectrum. The morphology greatly depends on the applied voltage. The as-formed nanotubes under the optimized condition, at 20 V for 2 h, are highly ordered with ~ 200 nm in length and the average tube diameter is about 100 nm. By annealing the initial samples at different temperatures, the importance of the crystalline nature is confirmed. Significantly, the peak positions of anatase in XRD patterns shifts to lower diffraction angles with an increase in the amount of H₃PO₄ ion. A remarkable red shift of the absorption edge has been observed for the sample formed in the electrolyte of HF and H₃PO₄, which is related to the introduction of P⁵⁺ into TiO₂ crystallization and the possible impurity energy level formed in the TiO₂ band gap. The presence of P 2p state in XPS spectrum can further confirm the P⁵⁺ which can replace a part of Ti⁴⁺ has been introduced into TiO₂ crystallization. The present convenient synthesis technique can be considered to the composition of other doped oxide materials.     

Preparation of double-doped BaCeO3 and its application in the synthesis of ammonia at atmospheric pressure

Perovskite-type oxides BaCe_0.90Sm_0.10O_3−δ (BCS) and BaCe_0.80Gd_0.10Sm_0.10O_3−δ (BCGS) were synthesized by the sol–gel method and characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Using the sintered samples as solid electrolytes and silver–palladium alloy as electrodes, ammonia was synthesized from nitrogen and hydrogen at atmospheric pressure in a solid-state proton-conducting cell reactor. The maximum rate of production of ammonia was 5.82×10⁻⁹ mol s⁻¹ cm⁻².     

Direct nanocrystalline hydroxyapatite formation on titanium from ultrasonated electrochemical bath at physiological pH

An electrochemical method of producing nanocrystalline hydroxyapatite coatings on titanium surface is reported. The bath contained Ca(NO₃)₂ and NH₄H₂PO₄ in the molar ratio 1.67:1. The electrolyte was maintained at physiological pH and was ultrasonically agitated throughout the time of electrolysis. Coatings were deposited for 30 min at 10 and 15 mA/cm² and contained mono hydroxyapatite phase whose crystal sizes were lower than 30 nm. These sizes are comparable to the size of the bone hydroxyapatite crystals. Small globules of hydroxyapatite covered the coating surface completely. Fourier transformed infra-red spectroscopy (FT-IR) studies showed that the coatings contained large amounts of hydroxide and phosphate groups to enable the formation of hydroxyapatite. The coatings had a roughness (R_a) of about 0.3 μm and water contact angles of about 49°. Ultrasonic agitation promoted the formation of nanocrystalline structure which will help in better attachment of bone tissues to the implant surface.     

Nanocrystalline hydroxyapatite doped with magnesium and zinc: Synthesis and characterization

During recent years, there have been efforts in developing nanocrystalline bioceramics, to enhance their mechanical and biological properties for use in tissue engineering applications. In this research, we made an attempt to synthesize nanocrystalline bioactive hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) ceramic powder in the lower-end of nano-range (2–10 nm), using a simple low-temperature sol–gel technique and studied its densification behavior. We further studied the effects of metal ion dopants during synthesis on powder morphology, and the properties of the sintered structures. Calcium nitrate and triethyl phosphite were used as precursors for calcium and phosphorous, respectively, for sol–gel synthesis. Calculated quantities of magnesium oxide and zinc oxide were incorporated as dopants into amorphous dried powder, prior to calcination at 250–550 °C. The synthesized powders were analyzed for their phases using X-ray diffraction technique and characterized for powder morphology and particle size using transmission electron microscopy (TEM). TEM analysis showed that the average particle size of the synthesized powders were in the range of 2–10 nm. The synthesized nano-powders were uniaxially compacted and then sintered at 1250 °C and 1300 °C for 6 h, separately, in air. A maximum average sintered density of 3.29 g/cm³ was achieved in structures sintered at 1300 °C, developed from nano-powder doped with magnesium. Vickers hardness testing was performed to determine the hardness of the sintered structures. Uniaxial compression tests were performed to evaluate the mechanical properties. Bioactivity and biodegradation behavior of the sintered structures were assessed in simulated body fluid (SBF) and maintained in a dynamic state.     

Synthesis and characterization of nanoporous hydroxyapatite using cationic surfactants as templates

Nanoporous hydroxyapatite was synthesized utilizing cationic surfactants as templates. The effects of cetyltrimethylammonium bromide and reaction temperatures on the phase and morphology of HA were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The thermal stability of nanoporous structures was studied by XRD and thermal analyzers (TGA/DTA), while the pore structure of HA was observed using high resolution TEM. It was found that the pore size was about 1 nm, and the pore structure of HA was thermally stable up to 700 °C and the pore size did not change with reaction temperature and CTAB:PO₄^3? ratio. The possible formation mechanism of nanoporous structure was proposed.     

Preparation and pseudo-capacitance of birnessite-type MnO2 nanostructures via microwave-assisted emulsion method

A microwave-assisted emulsion process has been developed to synthesize birnessite-type MnO₂ one-dimensional (1D) nanostructures. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). TEM images confirmed that the particles were composed of nanowires and nanobelts. As a consequence of the small size, such MnO₂ nanostructures exhibit a high specific capacitance of 277 F g⁻¹ at the current density of 0.2 mA cm⁻². Furthermore, the simple synthetic approach may provide a convenient route for the preparation of birnessite-type MnO₂ nanowires and other 1D nanostructured materials on a large scale.     

The comparison of powder characteristics and physicochemical,mechanical and biological properties between nanostructure ceramics of hydroxyapatite and fluoridated hydroxyapatite

In this study, several fluorine-substituted hydroxyapatite ceramics with the general chemical formula Ca₅(PO₄)₃(OH)_1 ? xF_x (0 ≤ x ≤ 1), where x = 0.0 (hydroxyapatite; HA), x = 0.68 (fluorhydroxyapatite; FHA) and x = 0.97 (fluorapatite; FA) were prepared. The powders were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infra-red (FTIR), X-ray diffraction (XRD), F-selective electrode, atomic absorption spectroscopy (AAS) and EDTA titration analyses. The powders were uniaxially pressed and were formed as a disc shape. Subsequently, sinterability and thermal stability of synthesized powders were compared together. Also the effect simultaneously of fluoride content and temperature were examined on the lattice parameters and crystallites size of the obtained powders. Mechanical properties including hardness, elastic modulus and fracture toughness were measured using indentation. The in vitro dissolution studies of the samples were carried out at osteoclastic resorption conditions. Finally, the biocompatibility and cytotoxicity of the samples were carried out using osteoblast-like cells and L929 cell line, respectively. The obtained results showed that the thermal stability substantially is increased with increase incorporated fluoride into HA structure. Also it was found that the fluoride reduced the lattice parameters and crystallites size of HA. Finally, the in vitro dissolution studies results suggest that the fluoride substitutions in HA offer the ability to prepare HAs with different degrees of solubility.     

Hydrometallurgical route to recover molybdenum, nickel, cobalt and aluminum from spent hydrotreating catalysts in sulphuric acid medium

This work describes a hydrometallurgical route for processing spent commercial catalysts (CoMo and NiMo/Al₂O₃), for recovering the active phase and support components. They were initially pre-oxidized (500 °C, 5 h) in order to eliminate coke and other volatile species present. Pre-oxidized catalysts were dissolved in H₂SO₄ (9 mol L⁻¹) at 90 °C, and the remaining residues separated from the solution. Molybdenum was recovered by solvent extraction using tertiary amines. Alamine 304 presented the best performance at pH around 1.8. After this step, cobalt (or nickel) was separated by adding aqueous ammonium oxalate in the above pH. Before aluminum recovery, by adding NaOH to the acid solution, phosphorus (H₂PO₄⁻) was removed by passing the liquid through a strong anion exchange column. Final wastes occur as neutral and colorless sodium sulphate solutions and the insoluble solid in the acid leachant. The hydrometallurgical route presented in this work generates less final aqueous wastes, as it is not necessary to use alkaline medium during the metal recovery steps. The metals were isolated in very high yields (>98 wt.%).     

Synthesis and characterization of LiCo0.3−xGaxNi0.7O2 (x = 0, 0.05) as a cathode material for lithium ion battery

The single phase of LiCo_0.3−xGa_xNi_0.7O₂ (x = 0, 0.05) was synthesized by a sol–gel method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical performance. The powders are homogeneous and have a good-layered structure. The synthesized LiCo_0.25Ga_0.05Ni_0.7O₂ exhibits better electrochemical performance with an initial discharge capacity of 180.0 mAh g⁻¹ and a capacity retention of 95.2% after 50 cycles between 2.8 and 4.4 V at 0.2C rate. The study on the structural evolution of the material during the cycling shows that Ga-doping improves the structure stability of LiCo_0.3Ni_0.7O₂ at ambient temperature and 55 °C. Meanwhile, Ga-doping not only suppresses the alternating current (AC) impedance of LiCo_0.3Ni_0.7O₂ but also promotes the Li⁺ diffusion in LiCo_0.3Ni_0.7O₂. Furthermore, thermal stability of the charged LiCo_0.25Ga_0.05Ni_0.7O₂ is improved, which may be attributed to the retard of O₂ evolution in LiCo_0.3Ni_0.7O₂ and the suppression of electrolyte oxidation during cycling by Ga-doping_.     

Synthesis and characterization of tungsten trioxide powders prepared from tungstic acids

WO₃ powders were prepared by the thermal decomposition of tungstic acids (WO₃·nH₂O, n=1/3, 1, 2). The tungstic acids were synthesized from WO₄²⁻ aqueous solutions under a variety of conditions of pH, temperature and W(VI) concentrations. The thermal decomposition of the tungstic acids into WO₃ was analysed by TG and DSC methods. Nano-sized WO₃ powders with different morphological characteristics were obtained by thermal treatment of the tungstic acids at 500 °C in air atmosphere. The morphologies of WO₃ powders were characterised by scanning electron microscopy and infrared absorption spectroscopy. Patterns of infrared spectra were related with distinct powder morphologies.     

Influence of the polysaccharide galactomannan on the dielectrical characterization of hydroxyapatite ceramic

Hydroxyapatite (HAP), Ca₁₀(PO₄)₆(OH)₂, has a wide range of biomedical applications because it is excellent biocompatibility and similar to natural bone tissue. The synthesis of the nanocrystalline powders of hydroxyapatite (HAP) was developed by the high energy method of dry mechanical alloying. The Ca(OH)₂ and CaHPO₄ were used in the preparation. Galactomannans are polysaccharides that occur in the endosperm of the seeds of leguminous plants as cell wall storage components was obtained from seeds of Adenanthera pavonina L. family Leguminosae (Fabaceae) and subfamily Mimosoideae. The nanocrystalline powders of hydroxyapatite were mixed with 10% (Gal 90), 20% (Gal 80) and 30% (Gal 70) of galactomannan. The aim of the study was to investigate influence of the galactomannan on the electrical characterization of bioceramic materials based on hydroxyapatite. The samples were studied by X-ray diffraction (XRD) was recorded to confirm the formation of a single phase solid solution and comportment this phase through the Rietveld analysis, scanning electron microscopy (SEM) and Dielectric measurements in the 10 Hz–100 MHz frequency range, at room temperature, have been performed. The presence of nanocrystals was confirmed by X-ray diffraction of HAP with size crystallite of 110 nm.     

Sol-gel-derived hydroxyapatite powders and coatings

Hydroxyapatite (HAP) and tri-calcium phosphate (TCP) powders and coatings with a Ca/P molar ratio from 1.56 to 1.77 were prepared by the sol-gel technique using calcium 2-ethylhexanoate (Ca(O₂C₈H₁₅)₂) and 2-ethyl-hexyl-phosphate as calcium and phosphorus precursors, respectively. The structural evolution and phase formation mechanisms of HAP and tri-calcium phosphate in calcined powders and coatings on Si wafer and Ti-alloy substrates (Ti-30Nb-3Al and Ti-5Al-2.5Fe) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The elimination of organics was studied by differential thermal analysis (DTA) and thermogravimetry (TGA). Two different formation mechanisms of crystallization are proposed. In sols with Ca/P 1.67, -tricalcium phosphate is formed as the major phase and hydroxyapatite as a minor phase by calcination at 700°C. At 900°C these phases react to form AB-type carbonated hydroxyapatite (Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ][(OH)_2–x/3–2y (CO₃) _y ]). A release of CO₂ substituting PO₄ ^3– occurs between 900°C and 1100°C yielding carbonate apatite, Ca₁₀(PO₄)₆[(OH)_2–2y (CO₃) _y ], whereas CO₂ substituting OH^– groups in the apatite structure is released above 1200°C. In sols with Ca/P 1.70, rather than carbonate apatite, B-carbonated hydroxyapatite Ca_10–2x/3[(PO₄)_6–x (CO₃) _x ](OH)₂ is formed, which subsequently decomposes into HAP and CaO above 1200°C. The optimum sintering conditions for coatings on Ti-alloys are found to be 600°C for 10 minutes, since, at higher temperature, oxidation of titanium and the formation of rutile (TiO₂) occur. Dip coating and sintering in two cycles yielded a homogeneous and dense coated film with a thickness of 250 nm.     

Separative recovery with lime of phosphate and fluoride from an acidic effluent containing H3PO4, HF and/or H2SiF6

Fluoride content and flow-rate of fertilizer plant wastewater from phosphoric acid and/or triple superphosphate (TSP) production lead to the discharge of several thousand tons of fluoride (F⁻) per year and even more for phosphate (PO₄³⁻). Since sustainability is an important environmental concern, the removal methods should allow phosphorus and fluoride to be recycled as a sustainable products for use as raw materials either in agricultural or industrial applications. In the present work, separative recovery with lime of these two target species was investigated. A preliminary speciation study, carried out on the crude effluent, showed that two forms of fluoride: HF and H₂SiF₆ are present in a highly acidic medium (pH  2). Evidence that fluoride is present under both free (HF) and combined (H₂SiF₆) forms, in the phosphate-containing effluent, was provided by comparing potentiometric titration curves of a crude wastewater sample and synthetic acid mixtures containing H₃PO₄, HF and H₂SiF₆. In a second step synthetic effluent containing mixtures of the following acids: HF, H₂SiF₆ and H₃PO₄, were treated with lime. The behaviour of these compounds under lime treatment was analysed. The data showed that fluoride has a beneficial effect on phosphate removal. Moreover, by acting on the precipitation pH, a “selective” recovery of fluoride and phosphate ions was possible either from phosphoric acid/hydrofluoric acid or phosphoric acid/hexafluorosilicic acid mixtures. Indeed, the first stage of the separative recovery, led to a fluoride removal efficiency of 97–98% from phosphoric acid/hydrofluoric acid mixture. It was of 93–95% from phosphoric acid/hexafluorosilicic acid mixture. During the second stage, the phosphate precipitation reached 99.8% from both acidic mixtures whereas it did not exceed 82% from a solution containing H₃PO₄ alone. The XRD and IR analyses showed that during lime treatment, a H₂SiF₆ hydrolysis occurred, instead of CaSiF₆ solid formation, leading to CaF₂ precipitate. Calcium fluoride and calcium phosphate based-by-products resulting from the two-step treatment process can be used as raw materials in several industrial sectors, such as ceramic and phosphate fertilizer industries.     

Preparation and Comprehensive Characterization of a Calcium Hydroxyapatite Reference Material

Numerous biological and chemical studies involve the use of calcium hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂. In this study detailed physicochemical characterization of HA, prepared from an aqueous solution, was carried out employing different methods and techniques: chemical and thermal analyses, x-ray diffraction, infrared and Raman spectroscopies, scanning and transmission microscopies, and Brunauer, Emmett, and Teller (BET) surface-area method. The contents of calcium (Ca²⁺), phosphate (PO₄³⁻), hydroxide (OH⁻), hydrogenphosphate (HPO₄²⁻), water (H₂O), carbonate (CO₃²⁻), and trace constituents, the Ca/P molar ratio, crystal size and morphology, surface area, unit-cell parameters, crystallinity, and solubility of this HA were determined. This highly pure, homogeneous, and highly crystalline HA is certified as a National Institute of Standards and Technology (NIST) standard reference material, SRM 2910.