Effect of Substitutional Monovalent and Divalent Metal Ions on Mechanical Properties of β-Tricalcium Phosphate

β-tricalcium phosphate (β-TCP) powder doped with monovalent or divalent metal ions was hot pressed at 1100°C, and the effect of substitutional monovalent and divalent metal ions on mechanical properties of β-TCP was investigated. The sinterability of β-TCP would be enhanced by the substitution of monovalent and divalent metal ions for β-TCP. Sintered β-TCP doped with 7.6 mol% of Mg²⁺ ion showed a bending strength of 160 MPa. It was found that the substituition of Mg²⁺ ion up to 9.6 mol% and a small amount of monovalent metal ions for β-TCP is effective to improve the mechanical strength of β-TCP.     

Densification and Phase Transformation of β-SiAlON–Cubic Boron Nitride Composites Prepared by Spark Plasma Sintering

β-SiAlON–cubic boron nitride (cBN) composites were prepared from β-SiAlON and cBN powders at 1600°–1900°C under a pressure of 100 MPa by spark plasma sintering. The effects of cBN content and sintering temperature on densification and phase transformation of the β-SiAlON–cBN composites were studied. When 10–30 vol% cBN was added to β-SiAlON, the shrinkage rate of the compacts increased. The compacts of β-SiAlON–BN composites originally containing 10–30 vol% cBN ceased to shrink at a temperature lower than that of β-SiAlON and the density of the composites increased. The densification of β-SiAlON–BN composites originally containing >40 vol% cBN was suppressed. The phase transformation of cBN to hexagonal BN in the β-SiAlON–BN composite was inhibited to a greater degree than that in the cBN body.     

Formation of Strontium-Stabilized β-Tricalcium Phosphate from Calcium-Deficient Apatite

A series of strontium-stabilized β-tricalcium phosphate (β-TCP) was prepared via the aqueous precipitation method, and characterized by X-ray diffraction and infrared fourier spectrometer techniques. The general principle of preparing calcium-deficient apatite and thereby heat-treating the resultant apatite above 700°C to form single-phase β-TCP was attempted to create strontium-substituted β-TCP. The results have proved that single phase β-TCP could be formed with the substituted strontium with only an influence on the increase in lattice constants with an increase in the concentration of strontium. Further, it is also stated from the present findings that strontium has a specific role in the crystallinity of the resultant β-TCP.     

Microstructural Development of Silicon Carbide Containing Large Seed Grains

Fine (}0.1μm) β-SiC powders, with 3.3 wt% large (}0.44μm) α-SiC or β-SiC particles (seeds) added, were hot-pressed at 1750°C and then annealed at 1850°C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into β-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable β→α phase transformation occurred. The growth of matrix grains in materials with β-SiC seeds was slower than that in materials with α-SiC seeds. The material with β-SiC seeds, which was annealed at 1850°C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with α-SiC seeds, also annealed at 1850°C for 4 h, had a uniform microstructure consisting of elongated grains. The fracture toughnesses of the annealed materials with α-SiC and β-SiC seeds were 5.5 and 5.4 MPa·^1/2, respectively. Such results suggested that further optimization of microstructure should be possible with β-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure.     

Corrosion Behavior of Sialon Ceramics in Supercritical Water

The corrosion behavior of sialon ceramics was investigated in supercritical water at 450°C under 45 MPa for 2–50 h. α-sialon exhibited better corrosion resistance than β-sialon and α/β-sialon. Pitting corrosion with the formation of corrosion products was observed in the case of β-sialon and α/β-sialon. By contrast, the corrosion behavior of α-sialon was characterized by uniform corrosion with the formation of corrosion products. The degree of strength deterioration was strongly dependent on the corrosion morphology. The bending strength of α-sialon after corrosion for 30 h was about 90% of its initial strength, while the strength of β-sialon decreased to 65% of its original strength.     

Microstructural Evolution in Liquid-Phase-Sintered SiC: Part II, Effects of Planar Defects and Seeds in the Starting Powder

The effects of planar-defect density in a β-SiC starting powder and the addition of α-SiC seeds to that powder on microstructural evolution in liquid-phase-sintered (LPS) SiC have been studied separately. Planar-defect density is altered by appropriate heat treatment of an as-received β-SiC starting powder. It was found that a decrease in the planar-defect density in the powder retards the β→α phase transformation rate. It is proposed that, because nucleation of α-SiC occurs on the planar defects present in the β-SiC starting powders, the nucleation rate and the attendant rate of transformation decrease with a reduction in planar-defect density. Consequently, this reduces the frequency of formation of elongated β/α composite grains, resulting in lower average aspect ratios, as the initial untransformed β-SiC grains coarsen in an equiaxed manner. In contrast, addition of external α-SiC seeds has no effect on the β→α phase transformation rate, although a significant reduction in the average aspect ratio occurs. It is proposed that preferential equiaxed coarsening of the α-SiC seeds over elongated coarsening of β/α composite grains occurs, resulting in a reduction of overall coarsening anisotropy.     

Microstructure of an Extruded β-Silicon Nitride Whisker-Reinforced Silicon Nitride Composite

β-Si₃N₄ whisker-reinforced β'SiAlON composites were fabricated by extrusion and densified, using pressureless sintering. Whisker alignment was observed in both the green and sintered microstructures. SEM analysis of polished, sintered samples showed a microstructure consisting of the original β-Si₃N₄ whiskers in a matrix of fine SiAlON grains. SEM of plasma-etched samples and TEM analysis showed that the whiskers, as a result of grain growth, consisted of two phases, a core and a sheath layer. X-ray mapping and EDS analysis revealed that the core material contained no trace of Al, confirming the presence of original β-Si₃N₄ whiskers. The composition of the sheath was qualitatively identical to that of the fine β' SiAlON grains in the matrix. The sheath was thus formed by the precipitation of the β'SiAlON during liquid-phase sintering and led to substantial growth of the whiskers. Microdiffraction showed that the β'SiAlON grew epitaxially on the β-Si₃N₄ whiskers, resulting in a heavily faulted SiAlON layer.     

Grain Growth Studies of Silicon Nitride Dispersed in an Oxynitride Glass

Isothermal growth of β-Si₃N₄ crystals dispersed in an oxynitride glass (Y-Si-Al-O-N) was studied by electron microscopy after heat treatment at temperatures between 1550° and 1640°C for 1 to 18 h. The β-crystals exhibited growth striations introduced by intermediate coolings and these striations were used for developing a sophisticated technique for analysis of growth. It was determined that α/β-transformation and Ostwald ripening can be treated as different kinetic stages of grain growth, while β-nucleation was found to be insignificant. The mean diameter of the needlelike β-grains was almost constant during phase transformation, indicating negligible growth of the β-prism plane; growth was mainly one-dimensional with the maximum mean length and aspect ratio just at the end of the phase transformation. The growth rate of the β-basal plane was independent of diameter, indicating interface-controlled growth. During Ostwald ripening, the length distribution broadened and the aspect ratio of smaller grains decreased. Dissolution of small grains caused an increase in the mean diameter, while the mean aspect ratio decreased.     

Characterization and Mechanical Performance of the Mg-Stabilized β-Ca3(PO4)2 Prepared from Mg-Substituted Ca-Deficient Apatite

The preparation of Mg-stabilized β-tricalcium phosphate (β-TCP) was carried out by an aqueous precipitation method and the characterization of the powders was performed by powder X-ray diffraction, FT-IR spectra, Raman spectroscopy, and elemental analysis. The transformation of calcium-deficient apatite into β-TCP has occurred in the range of 700°–800°C. The calculated values for lattice parameters confirm the stabilization role played by Mg. The thermal stability of the Mg-stabilized β-TCP powders was evident until 1400°C, thus broadening the sintering temperature range without transformation into the undesirable α-TCP. Accordingly, the mechanical properties of the Mg-stabilized β-TCP were improved in comparison with those of pure β-TCP.     

Recrystallization and Phase Transformation in Reaction-Sintered Sic

Reaction-sintered Sic specimens prepared at ∼1500°C and heat-treated at ∼1850°C in the presence of molten silicon were examined by transmission electron microscopy. The β-Sic grains in as-prepared specimens grew to a large size when heat-treated in the presence of molten Si; the number of growth twins in such °-Sic crystals was very small. The boundary areas of α- and β-Sic crystals were microsyntactic, consisting of thin strips of α- and β-Sic after heat-treating. These changes were due mostly to thin lamellar growth of α-Sic into β-Sic grains along the basal plane. There is also a high density of similar growth of transformation twins in β-Sic crystals. The results provide clear evidence of in situ solid-state transformations in sic.     

Ultrarapid Phase Conversion in β"-Alumina Tubes

β"-Alumina, the electrolyte of choice of sodium/sulfur and sodium/metal chloride batteries and alkali-metal thermal-electric converters, was sintered from precursor phases to high β"-phase purity in less than 15 s from the onset of densification by rapid pass-through rf induction coupled plasma sintering. The maximum instantaneous shrinkage rate was 1.8%/s. The resistivity was measured to be 13.8 ± 1.4 Ω·cm. The rapid conversion found is a significant improvement over conventional processing of β"-alumina, which requires extended postsintering annealing times to obtain high β"-phase purity.     

Lath-like Structure of β-Tricalcium Phosphate in Orthopedic Implants

The lath-like β-tricalcium phosphate (β-TCP) in the sintered porous β-TCP implants was revealed by transmission electron microscope (TEM). Samples of sintered porous β-TCP implants were extracted from rabbit tibia after implanted for 1–6 months. Although the majority of sintered β-TCP particles are in a granular shape, the lath-like structures of implants are observed occasionally. The length of laths is on the order of 1 μm, while the thickness of laths is on the order of 10 nm. The X-ray energy dispersive spectroscopy and the electron diffraction indicate that the lath-like material is β-TCP with its rhombohedral (     ) plane parallel to the longitudinal direction of laths. High resolution TEM imaging also confirms the finding of electron diffraction. This abnormal morphology of β-TCP have raised our attention, even though its formation mechanism and effects on osseointegration is not yet certain.     

Band Gap Modification of Diamond Photonic Crystals by Changing the Volume Fraction of the Dielectric Lattice

Photonic crystals with a diamond structure of epoxy lattices in which TiO₂-based ceramic particles are dispersed were fabricated by stereolithography. The periodicity of the lattice was designed to reflect electromagnetic waves in the gigahertz range. The volume fraction (β) of the dielectric lattice medium was modified from 14% to 33% by changing the rod diameter of the lattice. The photonic band gap was observed along Γ-L 〈111〉, Γ-X 〈100〉, and Γ-K 〈110〉 directions and the complete photonic band gap was formed at over β= 20%. The width of the forbidden gap increased gradually when the β increased over 14%, and reached 2.4 GHz at β= 33%. These results agreed with the band calculation using the plane wave expansion method.     

Plasma Etching of α-Sialon Ceramics

Plasma etching of β-Si₃N₄, α-sialon/β-Si₃N₄ and α-sialon ceramics were performed with hydrogen glow plasma at 600°C for 10 h. The preferential etching of β-Si₃N₄ grains was observed. The etching rate of α-sialon grains and of the grain-boundary glassy phase was distinctly lower than that of β-Si₃N₄ grains. The size, shape, and distribution of β-Si₃N₄ grains in the α-sialon/β-Si₃N₄ composite ceramics were revealed by the present method.     

Highly Texturing β-Sialon Via Strong Magnetic Field Alignment

This paper reports the texturing behavior of β-sialon by strong magnetic field alignment (SMFA) during slip casting, followed by reaction pressureless sintering, using either α or β-Si₃N₄, Al₂O₃, and AlN as the starting materials. It is found that the β-Si₃N₄ crystal exhibits a substantially stronger orientation ability than the α-Si₃N₄ crystal regardless of the Si₃N₄ raw powders in the magnetic field of 12 T. The β-raw powder produces a highly a , b -axis-oriented β-Si₃N₄ green body with a Lotgering orientation factor of up to 0.97. During sintering, the β-raw powder allows the a , b -axis-oriented β-sialon to retain the Lotgering orientation factor similar to and even higher than that of β-Si₃N₄ in the green body. In contrast, the α-raw powder leads to a faster transformation rate of α/β-Si₃N₄ to β-sialon but a substantially lower texture in β-sialon. The results indicate that the use of the β-raw powder is more efficient for producing highly textured β-sialon via SMFA than that of the α-raw powder as well as the prolonged sintering.     

Effect of the HA/β-TCP Ratio on the Biological Performance of Calcium Phosphate Ceramic Coatings Fabricated by a Room-Temperature Powder Spray in Vacuum

Four calcium phosphate ceramic coatings, the less soluble hydroxyapatite (HA) coating, the more soluble β-tricalcium phosphate (β-TCP) coating, and two biphasic calcium phosphate (BCP) coatings with HA/β-TCP ratios of 70/30 and 30/70 were fabricated by spraying each corresponding powder onto a titanium substrate at room temperature (RT) in a vacuum, in order to investigate the effect of the HA/β-TCP ratio on the dissolution behavior and the cellular responses of the coating. No secondary phases, except for HA and β-TCP, were observed for the coatings in the X-ray diffraction results. The coating compositions were almost the same as those of the starting powders because the coating was conducted at RT. Microscopic examination of the coatings revealed crack-free and dense microstructures. The BCP coatings exhibited dissolution rates intermediate between those of the pure HA and β-TCP coatings. The dissolution rate of the coatings was largely dependent on their HA/β-TCP ratio. The cell proliferation and differentiation results indicated that the cellular responses of the coatings were not proportional to their dissolution rates. The 3HA–7TCP (HA/β-TCP ratio of 30/70) coating exhibited an optimal dissolution rate for excellent biological performance.     

Thermomechanical Properties of β-Sialon Synthesized from Kaolin

β-Sialon powder was synthesized by the simultaneous reduction and nitridation of Hadong kaolin at 1350°C in an N₂–H₂ atmosphere, using graphite as a reducing agent. The average particle size of β-sialon powder was about 4.5 μm. The synthesized β-sialon powder was pressureless sintered from 1450° to 1850°C under a N₂ atmosphere. The relative density, modulus of rupture, fracture toughness, and microhardness of β-sialon ceramics sintered at 1800°C for 1 h were 92%, 248 MPa, 2.8 MN/m^3/2, and 13.3 GN/m², respectively. The critical temperature difference (ΔT_c) in water-quench thermal-shock behavior was about 375°C for the synthesized β-sialon ceramics.     

Phase Evolution during the Formation of α-Tricalcium Phosphate

The formation of α-tricalcium phosphate from monobasic ammonium phosphate and calcium carbonate was investigated using a number of complementary techniques. Differential thermal analysis showed five distinct thermal events attributed to melting of the ammonium phosphate, decomposition of acidic calcium orthophosphate into an amorphous calcium metaphosphate, crystallization of β-calcium metaphosphate, crystallization of β-calcium pyrophosphate, and calcination of calcium carbonate. X-ray diffraction analyses as a function of temperature supported the evolution of these events and phases and also showed the formation of other intermediates, including an amorphous phase, apatite, lime, and β-tricalcium phosphate. Thermal gravimetric analysis (TGA) showed a large weight loss occurring between 150° and 250°C due to reaction between the acidic phosphate liquid and the calcium carbonate. This resulted in an amorphous intermediate with a Ca/P ratio between 0.5 and 1.0 from which both β-calcium metaphosphate and β-calcium pyrophosphate crystallized. Mass spectroscopy indicated that the ammonia and carbon dioxide were evolved in four different steps, while water was evolved in at least five steps. These decomposition steps correlated with those observed by TGA. Scanning electron microscopy indicated the formation of an intermediate phase that coated the calcium carbonate by 250°C. The proposed mechanistic reaction path to alpha-tricalcium phosphate involves the formation and consumption of the following sequence of intermediates: an acidic amorphous condensed phosphate; β-calcium metaphosphate and β-calcium pyrophosphate; lime, apatite, and β-tricalcium phosphate.     

Light-Scattering Measurements of Structural Relaxation in Glass by Digital Correlation Spectroscopy

Digital clipped auto correlation spectroscopy was used to investigate the kinetic features of thermodynamic fluctuation processes which relax on time scales from 10⁻⁶ to 10² s. The autocorrelation function associated with structural relaxation in Na₂O· K_2/O·6SiO₂, glass was determined in the annealing region. The results indicate a nonexponential correlation function that can be fitted with the decay function exp - ( t /τ_β)^β, where β and τ_β are adjustable parameters. The parameter β which characterizes the deviation from a simple exponential, was essentially constant (β= 0.57 ± 0.02) from 468 ° to 550 ° C. The characteristic time constant, τ_β ranged from 1.35 × 10⁻² to 6.30 s. The computed average relaxation times were compared with shear viscosity data reported for a similar glass; both the temperature dependence and the magnitude agreed well with the predictions of viscoelastic theory.     

Fabrication of Functionally Graded SiAlON Ceramics by Tape Casting

Functionally graded SiAlON ceramics (FG-SiAlONs) were successfully prepared by the tape casting and lamination approach. Non-aqueous SiAlON slurries with five different α to β-SiAlON ratios (85α:15β, 70α:30β, 55α:45β, 40α:60β, and 25α:75β) were prepared by a 66 methyl ethyl ketone/34 ethanol (vol%) mixture. Phase and microstructure analyses incorporation with hardness measurements clearly show that the FG-SiAlONs, prepared by tape casting, exhibit a continuous and gradual change in composition and hardness. Thus, the tape casting approach is a viable method to produce FG-SiAlONs with a precisely controlled composition, and subsequently properties, as a function of position.