Novel in-situ synthesis and characterization of nanostructured magnesium substituted β-tricalcium phosphate (β-TCMP)

Tricalcium phosphate (TCP, Ca₃(PO₄)₂) in its pure form cannot be synthesized under physiological conditions in normal aqueous solutions due to phase instability, resulting in its transformation to hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) in the presence of water. However, substituting magnesium in lieu of calcium is known to stabilize TCP, preventing its conversion to hydroxyapatite. There are several methods known for synthesizing magnesium substituted tricalcium phosphate (TCMP). In the present study, a novel in-situ method has been developed to synthesize β-TCMP using magnesium substituted brushite as a precursor.Substitution of 50% of calcium by magnesium results in the formation of semi-spherical nanocrystalline particles (～ 100 nm) of brushite. Boiling the nanocrystalline brushite powder in aqueous condition for only 30 min results in the generation of rosette shaped nanocrystals (～ 80 nm) of β-TCMP that emerge from the original brushite spheres. The β-TCMP particles exhibit a specific surface area of ～ 200 m²/g. Details about the synthesis procedure and the possible mechanisms involved in the formation of β-TCMP from Mg-substituted brushite is further discussed.     

Beta tricalcium phosphate ceramics with controlled crystal orientation fabricated by application of external magnetic field during the slip casting process

Beta tricalcium phosphate (β-TCP) is a resorbable bioceramic that has hitherto been utilized in the medical field. Since it crystallizes in the anisotropic hexagonal system, properties such as chemical and physical ones are expected to depend on its crystal axis direction and/or on its crystal plane (anisotropy). Control of crystal orientation is thus important when used in polycrystalline form. Meanwhile, application of a strong magnetic field has been found to be a promising technique to control crystal orientation of anisotropic shape or structured crystals. In this work, we attempted to fabricate β-TCP ceramics with controlled crystal orientation by applying an external magnetic field during the slip casting process and subsequently sintering them at 1050 °C, below the β–α transition temperature. Application of a vertical magnetic field increased intensities of planes perpendicular to c-plane on the top surface, while a horizontal one with simultaneous mechanical mold rotation decreased it. These results indicated that crystal orientation of β-TCP ceramics were successfully controlled by the external magnetic field and together that the magnetic susceptibility of β-TCP is χ_c⊥ > χ_c//.     

Chemical synthesis and characterization of magnesium substituted amorphous calcium phosphate (MG-ACP)

Amorphous calcium phosphate (ACP) was synthesized by a simple aqueous precipitation using CaCl₂ and Na₃PO₄ in the presence of MgCl₂ to ensure the formation of the ACP phase at room temperature. Magnesium substituted ACP phases corresponding to two different compositions representing the two most prominent calcium phosphate phases (hydroxyapatite: Ca + Mg/P = 1.67 and tricalcium phosphate: Ca + Mg/P = 1.5) were synthesized by this simple approach. Both compositions of ACP phases resulted in their transformation into β-tricalcium phosphate upon heat treatment in air at 600 °C. X-ray diffraction (XRD), heat treatment, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Brunauer-Emmett-Teller (BET) analyses were used to characterize the phase, thermal stability, surface area, and morphology of the synthesized ACP powders corresponding to the two different nominal Ca/P compositions. Although it is known that α-TCP is the phase that appears upon heat treatment at 600 °C unsubstituted ACP, substitution of magnesium ion in ACP (both TCP and HA composition) stabilized the structure of β-TCMP phase at 600 °C. Moreover, FT-IR analysis revealed that the ACP phase regardless of the composition, exhibited characteristic bands corresponding to that of HA, with the exception of the ACP corresponding to HA composition which exhibited a prominent OH vibrational mode.     

Fabrication of biodegradable nano-sized β-TCP/Mg composite by a novel melt shearing technology

Biodegradable magnesium-matrix composites have attracted increasing interest for application in implant material fields. In this study, a new type of nano-sized β-tricalcium phosphate (β-TCP)/Mg–3Zn–Ca composite was proposed and produced using a novel melt shearing technology combined with high-pressure die casting (HPDC) process. The effect of the mixing methods on the distribution of β-TCP particles was investigated. Microstructure evolution during solidification process was analysed and the mechanical properties of the composite were also evaluated. Compared with the conventional mechanical stirring, the agglomerate phenomenon of the β-TCP particles in the matrix can be decreased by using the high shear unit and further decreased by melt shearing in the MCAST unit. The results also showed that the main constitutes in the matrix of the β-TCP/Mg–3Zn–1Ca composite are α-Mg and Ca₂Mg₆Zn₃ phase and most of the β-TCP particles are adjacent to the eutectic Ca₂Mg₆Zn₃ phase around the grain boundary. The average Vickers hardness, yield strength (0.2% YS), ultimate tensile strength (UTS), elastic modulus and elongation of as-cast of this composite are 79.0, 125.4 MPa, 150.0 MPa, 45.3 GPa and 2.85%, respectively.     

Coating and spark plasma sintering of nano-sized TiN on Y-α-sialon

TiN coating on Y-α-sialon was accomplished by depositing TiO₂ on their particle surfaces through controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ and subsequent nitridation with NH₃ gas at 1000 °C. TiN particles covering Y-α-sialon were about 20 nm in size. Spark plasma sintering (SPS) of TiN/Y-α-sialon particles produced composite ceramics with continuous TiN networks at 1400 °C, but with TiN grains isolated in elongated β-sialon grains at 1600 °C. The relative density and Vickers hardness of TiN/sialon ceramics SPSed at 1400–1600 °C containing 25 vol.% TiN were measured. The electrical resistivity was in a wide range of 10⁻⁴ to 10⁰ Ω cm for the ceramics sintered at 1400 °C, but lowered to the order of magnitude of 10⁻¹ and 10⁵ Ω cm at higher temperatures ≥1500 °C. It was found that the complete transition to β-sialon increased the resistivity to 10³ to 10⁵ Ω cm, due to breaking up continuous TiN layers by elongated β-sialon grains.     

Phase transition in (1 − x)Bi(Mg1/2Ti1/2)O3-xPbTiO3 ceramics

The phase structure and phase transition of (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ (BMT-PT) ceramics with x = 0.0–0.42 were investigated. It was found that pure perovskite phases were achieved for x ≥ 0.28, while Bi₄Ti₃O₁₂ or Bi₁₂TiO₂₀ phase existed for x ≤ 0.15. The anomaly dielectric peaks were observed around 620 °C for BMT-(0.28–0.38)PT samples, thus phase transition in (1 ? x)Bi(Mg_1/2Ti_1/2)O₃-xPbTiO₃ ceramics was studied using thermal expansion. It was found that dielectric anomalies at ~ 620 °C were resulted from the phase transition of the second phase and defects inside samples.     

Microstructural study of a titanium-based biocomposite produced by the powder metallurgy process with TiH2 and nanometric β-TCP powders

A titanium-based composite with Ca-P phases was prepared in situ by powder metallurgy processing with TiH₂ and nanometric β-TCP powders. Crystal phases of the as-fabricated composite are found to be α-Ti, CaTiO₃ and Ti_xP_y phase(s). The Ti_xP_y and CaTiO₃ phases resulted from the reaction between titanium and β-TCP at about 1135 °C. The composite presented a mean compressive strength of 635 MPa and a lower contact angle than pure titanium.     

Preparation and characterization of novel bioactive α-Tricalcium Phosphate doped with Dicalcium Silicate ceramics

This work describes the evaluation of pure α-Tricalcium Phosphate (TCP) and silica containing TCP ceramics in the system Dicalcium Silicate–Tricalcium Phosphate (C₂S–TCP) as a potential substrate for bone tissue engineering. The ceramics were soaked in dynamic simulated body fluid (SBF) for 2 weeks and characterized by SEM-WDS, XRD, and TEM analysis, and the results indicated that a carbonated hydroxyapatite (CHA) was formed on the surface of the ceramics. In addition, cell attachment assay showed that the ceramics supported the adult mesenchymal stem cells of human origin (hMSCs-A) adhesion and spreading, and the cells established close contacts with the ceramics after 24 h of culture. Also, cellular assays have shown a greater ability of hMSCs-A to express markers of the osteoblast phenotype (ALP, Col I, OC) in the C₂S doped α-TCP ceramics, indicating osteoblastic differentiation as a result of the increased concentration of silicon in solid solution in α-TCP. These findings indicate that the C₂S doped α-TCP ceramics possess good bioactivity, and biocompatibility, and might be promising for bone implant material.     

Microstructure and properties of porous silicon nitride ceramics prepared by gel-casting and gas pressure sintering

Wave-transparent porous Si₃N₄ ceramics were prepared by gel-casting and gas pressure sintering, and the effects of solid loading on microstructure, mechanical and dielectric properties were investigated. Microstructures with interlocked elongated β-Si₃N₄ grains and uniformly distributed pores were observed, while both the β-Si₃N₄ phase content and grain aspect ratio reduced as the solid loading increased due to the restrained anisotropic growth of β-Si₃N₄ grains. As the solid loading increased from 30 to 45 vol.%, the porosity of ceramics declined from 57.6% to 36.4%. The flexural strength increased linearly from 108.3 to 235.1 MPa, and the dielectric constant and loss tangent of ceramics increased from 2.63 and 2.85 × 10⁻³ to 3.68 and 3.56 × 10⁻³ (10 GHz), respectively.     

Microstructure,mechanical property and corrosion behavior of co-continuous β-TCP/MgCa composite manufactured by suction casting

The co-continuous β-TCP/MgCa composite was fabricated by infiltrating MgCa alloy into porous β-TCP using suction casting technique. The microstructure, mechanical property and corrosion behaviors of the composite have been evaluated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), mechanical testing, electrochemical and immersion tests. It was shown that the composite structure was compact and the interfacial combination between MgCa alloy and β-TCP scaffold was very well. The composite had an ultimate compressive strength of (147 ± 13) MPa, which was near with the natural bone (2–180 MPa) and about 1000-fold higher than that of the original porous β-TCP scaffold, but it still retained over half of the strength of the MgCa bulk alloy. The electrochemical and immersion tests indicated that the corrosion resistance of the composite was better than that of the MgCa bulk alloy, and the corrosion rate of the MgCa matrix alloy was quicker than that of the porous scaffold for the composite. The corrosion products of the composite surface were mainly Mg(OH)₂, hydroxyapatite (HA) and Ca₃(PO₄)₂.     

Enhanced sintering ability of biphasic calcium phosphate by polymers used for bone scaffold fabrication

Biphasic calcium phosphate (BCP), which is composed of hydroxyapatite [HAP, Ca₁₀(PO₄)₆(OH)₂] and β-tricalcium phosphate [β-TCP, β-Ca₃(PO₄)₂], is usually difficult to densify into a solid state with selective laser sintering (SLS) due to the short sintering time. In this study, the sintering ability of BCP ceramics was significantly improved by adding a small amount of polymers, by which a liquid phase was introduced during the sintering process. The effects of the polymer content, laser power and HAP/β-TCP ratios on the microstructure, chemical composition and mechanical properties of the BCP scaffolds were investigated. The results showed that the BCP scaffolds became increasingly more compact with the increase of the poly(l-lactic acid) (PLLA) content (0–1 wt.%) and laser power (6–10 W). The fracture toughness and micro-hardness of the sintered scaffolds were also improved. Moreover, PLLA could be gradually decomposed in the late sintering stages and eliminated from the final BCP scaffolds if the PLLA content was below a certain value (approximately 1 wt.% in this case). The added PLLA could not be completely eliminated when its content was further increased to 1.5 wt.% or higher because an unexpected carbon phase was detected in the sintered scaffolds. Furthermore, many pores were observed due to the removal of PLLA. Micro-cracks and micro-pores occurred when the laser power was too high (12 W). These defects resulted in a deterioration of the mechanical properties. The hardness and fracture toughness reached maximum values of 490.3 ± 10 HV and 1.72 ± 0.10 MPa m^1/2, respectively, with a PLLA content of approximately 1 wt.% and laser power of approximately 10 W. Poly(l-lactic-co-glycolic acid) (PLGA) showed similar effects on the sintering process of BCP ceramics. Rectangular, porous BCP scaffolds were fabricated based on the optimum values of the polymer content and laser power. This work may provide an experimental basis for improving the mechanical properties of BCP bone scaffolds fabricated with SLS.     

Biomimetic apatite coating on biomorphous alumina scaffolds

Highly porous Al₂O₃ scaffolds were prepared from natural cellulosic sponges via pyrolysis and Al-vapour phase infiltration. Subsequent oxidation and sintering in air resulted in porous Al₂O₃ ceramics with an open cellular morphology and a total porosity of 95%. The Al₂O₃-sponges were immersed in highly supersaturated simulated body fluid (5 × SBF) solutions with different Mg²⁺ and HCO³⁻ concentrations. After soaking of the porous Al₂O₃ sponges for 4 days a homogeneous calcium phosphate layer with a thickness of approximately 2 μm and a Ca : P ratio of 1.62 (apatite) was found.     

Electrocaloric properties of (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 ferroelectric ceramics near room temperature

Electrocaloric effects of (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ [abbreviated as (1 − x)PMN–xPT] ferroelectric ceramics with x being, 0.08, 0.10, and 0.25, respectively, were measured near room temperature, and the origins of the electrocaloric effects of these ceramics were discussed. It was found that these ceramics possess large electrocaloric effect with ΔT being, more than 1 K under an electric field of 1.5 kV mm⁻¹ in a wide temperature range (more than 10 K) near room temperature, and this effect is due to both of the electrocaloric effect resulting from the electric field induced first-order phase transition and the linear electrocaloric effect. It is expected that these ceramics could be used for multi-stage cascade ferroelectric refrigeration near room temperature.     

Preparation,crystal structure and characterization of α-NaSbP2S6 and β-NaSbP2S6 phases

The new phases α-NaSbP₂S₆ and β-NaSbP₂S₆ were synthesized by ceramic and reactive flux methods at 773 K. The structures of α-NaSbP₂S₆ and β-NaSbP₂S₆ were determined by the single-crystal X-ray diffraction technique. α-NaSbP₂S₆ crystallizes in the monoclinic space group P2₁/c with a = 11.231(2) Å, b = 7.2807(15) Å, c = 11.640(2) Å, β = 108.99(3)°, V = 900.0(3) Å³ and z = 4. β-NaSbP₂S₆ crystallizes in the monoclinic space group P2₁ with a = 6.6167(13) Å, b = 7.3993(15) Å, c = 9.895(2) Å, β = 92.12(3) °, V = 484.10(17) Å³ and z = 2.The α- and β-phases of NaSbP₂S₆ are closely related, the main difference lies in the stacking of the [Sb[P₂S₆]]_nⁿ⁻ layers. The structure of α-NaSbP₂S₆ consists of two condensed layers (MPS₃ type) to give an ABAB… sequence with Na⁺ cations located in the interlayer space. The packing of β-NaSbP₂S₆ is formed by monolayers of [Sb[P₂S₆]]_nⁿ⁻ stacked in an AA… fashion separated by a layer of Na⁺ cations. Both phases are derivates of the M¹⁺M³⁺P₂Q₆ family.The optical band gaps of α-NaSbP₂S₆ and β-NaSbP₂S₆ were determined by UV–vis diffuse reflectance measurements to be 2.17 and 2.25 eV, respectively.     

Processing and electrical properties of (1 − x)[(1 − y)(Pb(Mg1/3Nb2/3)O3)–y(Pb(Yb1/2Nb1/2)O3)]–xPbTiO3 ceramics

Ferroelectric ceramics in the vicinity of morphotropic phase boundary (MPB) with compositions represented as (1 ? x)[(1 ? y)(Pb(Mg_1/3Nb_2/3)O₃)–y(Pb(Yb_1/2Nb_1/2)O₃)]–xPbTiO₃ were prepared by solid state reaction. The addition of PYbN to PMN–PT decreased the sintering temperature from 1200 °C (y = 0.25) to 1000 °C (y = 0.75). The PT content, where the MPB was observed, increased with the PYbN addition. A remanent polarization value of 28.5 µC/cm² and a coercive field value of 11 kV/cm were measured from 0.62[0.25PMN–0.75PYbN]–0.38PT ceramics, which were close to the ones measured from PMN–0.32PT ceramics. In addition, the Curie temperature was found to increase with PYbN additions.     

Magnesium substitution in brushite cements

The use of magnesium-doped ceramics has been described to modify brushite cements and improve their biological behavior. However, few studies have analyzed the efficiency of this approach to induce magnesium substitution in brushite crystals. Mg-doped ceramics composed of Mg-substituted β-TCP, stanfieldite and/or farringtonite were reacted with primary monocalcium phosphate (MCP) in the presence of water. The cement setting reaction has resulted in the formation of brushite and newberyite within the cement matrix. Interestingly, the combination of SAED and EDX analyses of single crystal has indicated the occurrence of magnesium substitution within brushite crystals. Moreover, the effect of magnesium ions on the structure, and mechanical and setting properties of the new cements was characterized as well as the release of Ca^2 + and Mg^2 + ions. Further research would enhance the efficiency of the system to incorporate larger amounts of magnesium ions within brushite crystals.     

Influence of V2O5 additions to Ba(Mg1/3Ta2/3)O3 ceramics on sintering behavior and microwave dielectric properties

The microstructures and the microwave dielectric properties of barium magnesium tantalate ceramics prepared by conventional mixed oxide route have been investigated. The prepared Ba(Mg_1/3Ta_2/3)O₃ exhibited a mixture of cubic perovskite and a hexagonal superstructure with Mg and Ta showing 1:2 order in the B-site. It is found that low level doping of V₂O₅ (up to 0.5 wt.%) can significantly improve densification of the specimens and their microwave dielectric properties. The density of doped Ba(Mg_1/3Ta_2/3)O₃ ceramics can be increased beyond 95% of its theoretical value by 1500 °C-sintering, which is caused by the liquid-phase effect of V₂O₅ addition. The detected second phase Ta₂O₅ was mainly the result of V⁵⁺ substitution in the ceramics. Dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) were not significantly affected, while the unloaded quality factors Q were effectively promoted by V₂O₅ addition due to the increase in B-site ordering. The ε_r value of 24.1, Q×f value of 149,000 (at 10 GHz) and τ_f value of 7.2 ppm/°C were obtained for Ba(Mg_1/3Ta_2/3)O₃ ceramics with 0.25 wt.% V₂O₅ addition sintered at 1500 °C for 3 h.     

Low-firing of BiSbO4 microwave dielectric ceramic with V2O5–CuO addition

Effects of 1.0 wt.% V₂O₅–CuO mixture addition on the sintering behavior, phase composition and microwave dielectric properties of BiSbO₄ ceramics have been investigated. BiSbO₄ ceramics can be well densified below temperature about 930 °C with 1.0 wt.% V₂O₅–CuO mixtures addition with different ratios of CuO to V₂O₅. The formation of BiVO₄ phase and substitution of Cu²⁺ can explain the decrease of sintering temperature. Dense BiSbO₄ ceramics sintered at 930 °C for 2 h exhibited good microwave dielectric properties with permittivity between 19 and 20.5, Qf values between 19,000 and 40,000 GHz and temperature coefficient of resonant frequency shifting between ?71.5 ppm °C^?1 and ?77.8 ppm °C^?1. BiSbO₄ ceramics could be a candidate for microwave application and low temperature co-fired ceramics technology.     

Fabrication and luminescent properties of Ce:LaAlO3 translucent ceramics

Ce³⁺ doped LaAlO₃ translucent ceramics were fabricated with solid-state reaction method and sintered in vacuum condition. LaAlO₃ single phase was formed at 1200 °C. Their microstructures were observed and luminescent properties were investigated. The average grain size increases with the increase of sintering temperature. A dense and pore-free microstructure is displayed at 1700 °C. A band to band absorption of LaAlO₃ host is round 220 nm. Three excitation peaks at 249, 317 and 354 nm were observed in the Ce³⁺:LaAlO₃ ceramics, they were attributed to the 4f–5d transition of Ce³⁺ ions. The scintillation properties were investigated by X-ray excited radioluminescence in Ce³⁺:LaAlO₃ ceramics and the emission peak is 428 nm.     

Synthesis of the giant dielectric constant oxide CaCu3Ti4O12 via ethylenediaminetetraacetic acid precursor

CaCu₃Ti₄O₁₂ powders were prepared via EDTA route and single-phase CaCu₃Ti₄O₁₂ was obtained at 800 °C for 2 h. DTA/TG and XRD were used to characterize the precursor and derived oxide powders. The dielectric properties of CaCu₃Ti₄O₁₂ ceramics were presented. Increasing sintering temperature leads to the increase in dielectric constant. CaCu₃Ti₄O₁₂ ceramics sintered at 1090 °C for 3 h exhibited giant dielectric constant of up to 2.1 × 10⁵ at room temperature and 100 Hz, which is significantly higher than those obtained from other chemical methods.