Synthesis of tetracalcium phosphate from mechanochemically activated reactants and assessment as a component of bone cements

The aim of this work was to gain a better understanding about the synthesis of tetracalcium phosphate (TTCP, Ca₄(PO₄)₂O) through a solid-state reaction from mechanochemically activated CaCO₃–(NH₄)₂HPO₄ mixtures. The evolution of the reaction was followed by DTA, XRD, FTIR and SEM techniques. An enhanced reactivity of the mixtures was detected as the mechanochemical treatment times increased. This effect was related to both the loss of crystallinity of the reactants and the production of defects on their surfaces. 6 h of mechanochemical processing at 1190 rpm, followed by 3 h of thermal treatment at 1500°C, were enough to obtain pure TTCP. The crystallinity and purity of the obtained TTCP were checked by XRD and FTIR. The morphologic characteristics were analyzed by SEM and BET analysis. The behavior of synthesized TTCP powder in combination with commercial dicalcium phosphate anhydrous (DCPA, CaHPO₄), as the solid phase of bone cements, was tested. Both the combination of different particle sizes of TTCP and DCPA and the effect of different kinds of accelerator agents (disodium hydrogen phosphate, tartaric acid, citric acid and oxalic acid) on setting time and degree of conversion to hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) were evaluated. The combination of TTCP (0.32 m²/g) with DCPA (1.52 m²/g), in a 1/1 molar ratio, showed the shortest setting times and high conversions to HA when an oxalic acid solution (5% volume fraction) was used as the liquid phase of the formulation. Results obtained from this work demonstrated that synthesized TTCP shows promising behavior as a component of bone cements, exhibiting not only a smaller particle size than that usually reported but also a low degree of crystallinity, all of which increases the reactivity of the obtained TTCP. This study provided a very efficient method for synthesizing pure TTCP through a modified solid-state reaction from mechanochemically activated reactants, employing very short times of thermal treatment in comparison with the conventional processes.     

Application of impedance spectroscopy to evaluate the effect of different setting accelerators on the developed microstructures of calcium phosphate cements

The main goal of the present study was to evaluate the effect of different setting accelerator agents on the developed microstructures of calcium phosphate cements (CPCs) by employing the impedance spectroscopy (IS) technique. Six compositions of CPCs were prepared from mixtures of commercial dicalcium phosphate anhydrous (DCPA) and synthesized tetracalcium phosphate (TTCP) as the solid phases. Two TTCP/DCPA molar ratios (1/1 and 1/2) and three liquid phases (aqueous solutions of Na₂HPO₄, tartaric acid (TA) and oxalic acid (OA), 5% volume fraction) were employed. Initial (I) and final (F) setting times of the cement pastes were determined with Gillmore needles (ASTM standard C266-99). The hardened samples were characterized by X-ray powder diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and apparent density measurements. The IS technique was employed as a non-destructive tool to obtain information related to porosity, tortuosity and homogeneity of the cement microstructures. The formulation prepared from a TTCP/DCPA equimolar mixture and OA as the liquid phase presented the shortest I and F (12 and 20 min, respectively) in comparison to the other studied systems. XRD analyses revealed the formation of low-crystallinity hydroxyapatite (HA) (as the main phase) as well as the presence of little amounts of unreacted DCPA and TTCP after 24 h hardening in 100% relative humidity. This was related to the proposed mechanisms of dissolution of the reactants. The bands observed by FTIR allowed identifying the presence of calcium tartrate and calcium oxalate in the samples prepared from TA and OA, in addition to the characteristic bands of HA. High degree of entanglement of the formed crystals was observed by SEM in samples containing OA. SEM images were also correlated to the apparent densities of the hardened cements. Changes in porosity, tortuosity and microstructural homogeneity were determined in all samples, from IS results, when the TTCP/DCPA ratio was changed from 1/1 to 1/2. The cement formulated from an equimolar mixture of TTCP/DCPA and OA as the liquid phase presented setting times, degree of conversion to low-crystallinity HA and microstructural features suitable to be used as potential bone cement in clinical applications. The IS technique was shown to be a very sensitive and non-destructive tool to relate the paste composition to the developed microstructures. This approach could be very useful to develop calcium phosphate bone cements for specific clinical demands.     

Low temperature synthesis of SnO2 nanocrystals from tin, sulfur and ammonium chloride powders

Tetragonal phase SnO₂ nanocrystals were synthesized directly by heating the mixture of tin (Sn), sulfur (S) and ammonium chloride (NH₄Cl) powders in air at 400 °C for 2-5 h. The phase, size and purity of the resultant products were characterized by means of powder X-ray diffraction (XRD), filed emission scanning electronic microscope (FESEM), and energy dispersive X-ray (EDX) spectra. Besides, the effects of heating temperature, duration, and composition of the reactants on the phases of the resultant products were investigated. It was found that both of the S and NH₄Cl additives played important roles in the current low temperature (400 °C) synthesis of pure SnO₂.     

Evaluation and characterization of nanostructure hydroxyapatite powder prepared by simple sol-gel method

Many attempts have been focused on preparing of synthetic hydroxyapatite (HA), which closely resembles bone apatite and exhibits excellent osteoconductivity. Low temperature formation and fusion of the apatite crystals have been the main contributions of the sol-gel process in comparison with conventional methods for HA powder synthesis. This paper describes the synthesis of nano-HA particles via a sol-gel method. Nanocrystalline powder of hydroxyapatite (HA) was prepared using Ca(NO₃)₂·4H₂O and P₂O₅ by a simple sol-gel approach. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for characterization and evaluation of the phase composition, morphology and particle size of products. The presence of amorphous and crystalline phases in the as-dried gel precursor was confirmed by the evaluating technique. Single phase of HA was also identified in the heat treated powder by XRD patterns. SEM and TEM evaluations showed that the obtained powder after heat treatment at 600 °C was agglomerated and composed of nanocrystalline (25-28 nm) HA particles. Increasing the sintering temperature and time could cause decomposition of HA into β-tricalcium phosphate and calcium oxide. The prepared nanocrystalline HA is able to improve the contact reaction and the stability at the artificial/natural bone interface for medical applications.     

Synthesis and characterization of thorium phosphate phases by spray pyrolysis: chemistry of thorium phosphates

This paper describes the synthesis of some thorium phosphate compounds with different Th/P ratio (1/2, 2/3 and 3/4) by a spray pyrolysis technique. The so-prepared rough compounds were annealed at different temperatures for 2 h and then analyzed by mainly X-ray diffraction on powder and infrared spectroscopy. Every rough compound is composed by very badly crystallized ThO₂ phase polluted by carbon residue. An annealing treatment at 800 °C leads to the thorium diphosphate phase, α-ThP₂O₇ in every case. At 900 °C, such a phase is decomposed into a thorium phosphate diphosphate phase (Th₄(PO₄)₄P₂O₇, called TPD). However, a thorium excess in the initial mixture (Th/P = 3/4) leads also to observe the ThO₂ phase. The TPD phase is stable up to 1200 °C and does not react with the ThO₂ compound. Beyond 1200 °C, the TPD phase is slowly decomposed into a thorium phosphate compound which should be a thorium oxide phosphate; this compound does not contain any diphosphate species.     

The cation inversion and magnetization in nanopowder zinc ferrite obtained by soft mechanochemical processing

Two zinc ferrite nanoparticle materials were prepared by the same method – soft mechanochemical synthesis, but starting from different powder mixtures: (1) Zn(OH)₂/α-Fe₂O₃ and (2) Zn(OH)₂/Fe(OH)₃. In both cases a single phase system was obtained after 18 h of milling. The progress of the synthesis was controlled by X-ray diffractometry (XRD), Raman spectroscopy, TEM and magnetic measurements. Analysis of the XRD patterns by Rietveld refinement allowed determination of the cation inversion degree for both obtained single phase ZnFe₂O₄ samples. The sample obtained from mixture (1) has the cation inversion degree 0.3482 and the sample obtained from mixture (2) 0.400. Magnetization measurements were confirmed that the degrees of the inversion were well estimated. Comparison with published data shows that used method of synthesis gives nano powder samples with extremely high values of saturation magnetizations: sample (1) 78.3 emu g⁻¹ and sample (2) 91.5 emu g⁻¹ at T = 4.5 K.     

Synthesis and phase evolution of Si-C-Ti powder derived from poly(methylsilaacetylene) and Ti

Si-C-Ti powder was synthesized by reactive pyrolysis of poly(methylsilaacetylene)(PSCC) precursor mixed with metal Ti powder. Pyrolysis of PSCC/Ti mixture with certain atomic ratio was carried out in argon atmosphere between 1300 °C and 1500 °C. The metal-precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy equipped with EDX. Ti₃SiC₂ phase was obtained from reaction of PSCC and Ti for the first time. Ti₃SiC₂ formation started at 1300 °C and its amount increased significantly at 1400 °C. In addition, liquid formed by additive CaF₂ could promote the formation of Ti₃SiC₂ phase.     

Combustion synthesis of porous titanium microspheres

The synthesis of titanium porous microspheres by a combustion technique was studied under an argon atmosphere by using a TiO₂ − 2.5Mg reactive mixture. The precursor, a fine TiO₂ powder, was thermally treated in the range 600–1300 °C prior to the combustion experiments. TiO₂ microspheres whose diameters were between 10 and 50 μm were obtained from precursor particles annealed in the range 900–1100 °C. A biphase product consisting of Ti and MgO phases was obtained when the TiO₂ microspheres were reduced with Mg. The spherical morphology of the final particles was retained despite the relatively high combustion temperatures (1630–1670 °C) used in this study. Moreover, porous titanium microspheres were obtained when the MgO particles were dissolved using acid leaching. Scanning electron microscopy (SEM) images of the microspheres suggested that the spherical structure contained ∼0.5–2.0-μm-diameter porous windows. The Brunauer–Emmett–Teller (BET) surface area of the Ti microspheres was determined to be 2.8 m² g⁻¹.     

Synthesis and regulation of α-LiZnPO4·H2O via a solid-state reaction at low-heating temperatures

A simple and novel route for the synthesis of a lithium zinc phosphate hydrate, α-LiZnPO₄·H₂O, was studied, and the target product was obtained with LiH₂PO₄·H₂O and ZnCO₃ as raw materials and polyethylene glycol-400 (PEG-400) as a surfactant via a one step solid-state reaction at room temperature (25 °C). The product was characterized with X-ray powder diffraction (XRD), thermogravimetric analysis and the 1st derivativative of thermogravimetric analysis (TG/DTG) and Fourier transform infrared spectroscopy (FTIR). The comparison experimental results suggested that aging temperature controlled the products of the synthesis, that is, the α-LiZnPO₄·H₂O was formed when the reaction mixture was aged at room temperature, and the α-LiZnPO₄ was obtained when the reaction mixture was aged at 80 °C.     

Molten salt synthesis of ZnNb2O6 powder

Pure ZnNb₂O₆ powder was successfully prepared by the molten salt synthesis method using Nb₂O₅ and ZnO as raw materials and a mixture of NaCl and KCl as the solvent. The phase form and morphology of the prepared powder were characterized by X-ray diffraction and scanning electron microscopy. The effect of reacting temperature on phase formation was investigated. The results indicated that the single phase ZnNb₂O₆ powder can be obtained by the molten salt synthesis method at 600 °C, and the SEM photographs show that the grains of the powder are rod-like particles.     

Electrochemical lithium storage of Li-Ti-O compound calcined at different temperatures

The layered nanotubes of Li-Ti-O compound were prepared by ultrasonic treatment of sodium titanate nanotubes in LiOH solution, which was involved in the ion-exchange process. It was found that Li-Ti-O compound maintained layered structure below 400 °C and underwent phase transition to a mixture of Li-poor anatase Li_xTiO₂ and spinel Li₄Ti₅O₁₂ as the main phases at 500 and 600 °C. The lithium titanate nanotubes calcined at 400 °C exhibited the large capacity and good high rate capability.     

Synthesis and photocatalytic activity of nanocrystalline TiO2-SrO composite powders under visible light irradiation

Nano-sized homogeneously distributed TiO₂-20, -40, -60 wt.% SrO composite powders were successfully synthesized by a sol-gel method. The as-received amorphous TiO₂—20 wt.% SrO composite powders were crystallized with anatase TiO₂ at around 750 °C. As calcination temperatures increased, the anatase TiO₂ crystalline phase was transformed to rutile TiO₂ at about 900 °C, whereas nano-sized, squarish SrTiO₃ phase was detected. The peaks obtained after calcining at 1050 °C mainly exhibited the rutile TiO₂ and SrTiO₃ phases. However, a small number of SrO₂ peaks were also detected. For the comparison of photocatalytic activity depending on light sources, TiO₂-SrO composite powders were tested in phenol degradation. TiO₂-60 wt.% SrO composite powder showed good visible light photoactivity for the photo-oxidation of phenol.     

Nanocrystalline Al/Al12(Fe,V)3Si alloy prepared by mechanical alloying: Synthesis and thermodynamic analysis

Al–Al₁₂(Fe,V)₃Si nanocrystalline alloy was fabricated by mechanical alloying (MA) of Al–11.6Fe–1.3V–2.3Si (wt.%) powder mixture followed by a suitable subsequent annealing process. Structural changes of powder particles during the MA were investigated by X-ray diffraction (XRD). Microstructure of powder particles were characterized using scanning electron microscopy (SEM). Differential scanning calorimeter (DSC) was used to study thermal behavior of the as-milled product. A thermodynamic analysis of the process was performed using the extended Miedema model. This analysis showed that in the Al–11.6Fe–1.3V–2.3Si powder mixture, the thermodynamic driving force for solid solution formation is greater than that for amorphous phase formation. XRD results showed that no intermetallic phase is formed by MA alone. Microstructure of the powder after 60 h of MA consisted of a nanostructured Al-based solid solution, with a crystallite size of 19 nm. After annealing of the as-milled powder at 550 °C for 30 min, the Al₁₂(Fe,V)₃Si intermetallic phase precipitated in the Al matrix. The final alloy obtained by MA and subsequent annealing had a crystallite size of 49 nm and showed a high microhardness value of 249 HV which is higher than that reported for similar alloy obtained by melt spinning and subsequent milling.     

Low temperature solid-phase synthesis of tetragonal BaTiO3 powders and its characterization

We report an alternative synthesis route for tetragonal and submicron barium titanate particles through a low temperature isothermal heat treatment of a precursor powder obtained from the BaO₂-TiO₂-C mixture utilizing combustion synthesis technique. Effort has been concentrated on the investigation and optimization of the synthesis process including combustion temperature and velocity, heat treatment procedure, and the characteristics of the powder obtained. It was shown that precursor powder prepared by the combustion method easily transformed to the tetragonal BaTiO₃ starting from 700 °C. Characterization of the products was performed by powder X-ray diffraction, scanning electron microscopy (SEM) and BET. From the results, it has been found that the formation of tetragonal BaTiO₃ powder at low temperature is conditioned by high chemical activity and specific characteristics of combustion products. The BaTiO₃ specimens were found to possess high dielectric constant and low dissipation factor at room temperature.     

Crystallization of alkaline earth zirconates and niobates from compositionally flexible aqueous solution-gel syntheses

Barium and strontium zirconates and niobates were prepared by means of an economic and ecologic aqueous solution-gel route with high compositional flexibility. The synthesis of precursor solutions and gels, and thermal analysis of the gels are presented. The crystallization of the different oxides was studied in situ using high-temperature X-ray diffraction measurements (HT-XRD). Zirconate crystallization was observed between 600 °C and 700 °C. Small amounts of Ba(II)- and Sr(II)-carbonate crystallized before the oxide did, and decomposed between 700 °C and 800 °C. More sensitive Fourier transform infrared spectra, showed that very small traces of carbonate remain at 900 °C. The crystallization behavior of strontium niobates was strongly dependent on the Sr:Nb ratios, as observed in HT-XRD. Phase pure SrNb₂O₆ was obtained at low temperature (600 °C). Sr₂Nb₂O₇ and Sr₅Nb₄O₁₅ were formed at 900 °C from an intermediate phase, with a minor secondary phase. Nb₂Sr₄O₉ crystallized at 600 °C, but transformed to a mixture of phases at high temperature (1000 °C). By off-line furnace annealing, phase pure strontium niobates of all four compositions were obtained. The presented work demonstrates the high compositional flexibility of the aqueous solution-gel route, allowing fast synthesis of different multimetal oxides with high purity.     

Structural study on Al-26 mass% Si-8 mass% Ni powder

The structure of Al-26 mass% Si-8 mass% Ni alloy, a material used as an input product for manufacturing light weight products via powder metallurgy, was investigated by in situ powder diffraction techniques up to 700°C. Thermal expansion of the dominant phases indicates substitutional alloying of silicon by nickel atoms, forming a solid solution phase stable in the temperature region from 330 to 550°C. The presence of Al, Si, Al₃Ni and Si₃Ni was determined by phase analysis from a sample annealed at 700°C (re-melted) and cooled down to room temperature. EXAFS analysis of as prepared and re-melted samples documented similar local atomic structure around the Ni atoms in both stages.     

Comparison between titania thick films obtained through sol-gel and hydrothermal synthetic processes

Titania films were obtained through two synthetic processes, a traditional sol-gel method and a hydrothermal route. In SG synthesis, thermal decomposition of the precursor in oven at 400 °C for 2 h led to pure anatase TiO₂; in HY synthesis, instead, crystalline anatase grains were obtained in autoclave at 200 °C for 1 h. To investigate the microstructural evolution of SG and HY titania with temperature, each powder was annealed at 650, 750, 850 °C for 1 h and subjected to XRD analysis. Surprisingly, HY titania, contrary to SG, maintained the anatase phase, up to 850 °C, without any introduction of foreign elements. The sensing layers, obtained from as grown powders, were fired at 650, 750 or 850 °C and tested vs. methane and carbon monoxide. Both types of films fired at 850 °C yield insignificant responses to both CO and CH4, demonstrating the lack of influence of the crystalline phase on the gas response. Moreover, as regards the films fired at 650 and 750 °C, the gas responses are higher for SG than for HY samples, despite larger particle size.     

Synthesis and sintered properties evaluation of calcium phosphate ceramics

The present paper describes calcination of calcium phosphate ceramics (hydroxyapatite, HA) at 200, 400, 600, 800 and 1000 °C to observe the phase change using X-ray diffraction. HA phase was found to be stable up to 600 °C and later got dissociated into other non-stoichiometric phases like tricalcium phosphate (Ca₃(PO₄)₂ [TCP]), calcium pyrophosphate (Ca₂P₂O₇ [CPP]) and calcium hydrogen phosphate (CaHPO₄ [CHP]). TCP was found to be the major phase above 1000 °C. FTIR spectra showed the presence of various PO₄³⁻ and OH⁻ groups present in the powder. Powders compacted and sintered at 900, 1000, 1100 and 1200 °C showed an increase in density from 2.11 to 2.95 g/cm³ while biaxial flexural strength (BFS) was found to be higher (48.7 MPa) when the samples were sintered at 1100 °C and it decreased with further increase in sintering temperature.     

Effect of high temperature post-oxidizing on tribological and corrosion behavior of plasma nitrided AISI 316 austenitic stainless steel

In this research, the microstructure, tribological and corrosion properties of plasma nitrided-oxidized AISI 316 austenitic stainless steel at high oxidation temperature were studied and compared with conventional plasma nitride. The structural, tribological and corrosion properties were analyzed using XRD, SEM, microhardness testing, pin-on-disk tribotesting and electrochemical polarization. Plasma nitriding was conducted for 5 h at 450 °C with gas mixture of N₂/H₂ = 1/3 to produce the S-phase. The nitrided samples were post-oxidized at 500 °C with gas mixture of O₂/H₂ = 1/5 for 15, 30 and 60 min. X-ray diffraction confirmed the development of CrN, ? and γ′ nitride phases and magnetite (Fe₃O₄) oxide phase under plasma nitriding-oxidizing process. In addition, it was found that oxidation treatment after plasma nitriding provides an important improvement in the friction coefficient and the corrosion resistance. The optimized wear and corrosion resistance of post-oxidized samples were obtained after 15 min of oxidation.     

Hydrothermal precipitation and characterization of nanocrystalline BaTiO3 particles

Crystalline BaTiO₃ powders were precipitated by reacting fine TiO₂ particles with a strongly alkaline solution of Ba(OH)₂ under hydrothermal conditions at 80°C to 240°C. The characteristics of the powders were investigated by X-ray diffraction, transmission electron microscopy, thermal analysis and atomic emission spectroscopy. For a fixed reaction time of 24 hours, the average particle size of BaTiO₃ increased from 50 nm at 90°C to 100 nm at 240°C. At synthesis temperatures below 150°C, the BaTiO₃ particles had a narrow size distribution and were predominantly cubic in structure. Higher synthesis temperatures produced a mixture of the cubic and tetragonal phases in which the concentration of the tetragonal phase increased with increasing temperature. A bimodal distribution of sizes developed for long reaction times (96 h) at the highest synthesis temperature (240°C). Thermal analysis revealed little weight loss on heating the powders to temperatures up to 700°C. The influence of particle size and processing-related hydroxyl defects on the crystal structure of the BaTiO₃ powder is discussed.