S.C. Ng

Mechanochemical synthesis of nanocrystalline hydroxyapatite from CaO and CaHPO4.

Ceramic hydroxyapatite phase was triggered to occur by high-energy mechanical activation of a dry powder mixture of calcium oxide (CaO) and anhydrous calcium hydrogen phosphate (CaHPO4). A singe-phase hydroxyapatite of high crystallinity was realised by >20 h of mechanical activation without further thermal treatment at high temperatures. The resulting hydroxyapatite powder exhibits an average particle size of approximately 25 nm and a specific surface area of 76.06 m2/g, as measured by multi-point BET technique. It was sintered to a density of 98.20% theoretical density at 1200 degrees C for 2 h.

Reduced glass transition temperature and glass forming ability of bulk glass forming alloys

Abstract Onset temperature (solidus) T m and offset temperature (liquidus) T l of melting of a series of bulk glass forming alloys based on Zr, La, Mg, Pd and rare-earth elements have been measured by studying systematically the melting behaviour of these alloys using DTA or DSC. Bulk metallic glass formation has been found to be most effective at or near their eutectic points and less effective for off-eutectic alloys. Reduced glass transition temperature T rg given by T g / T l is found to show a stronger correlation with critical cooling rate or critical section thickness for glass formation than T rg given by T g / T m .

Processing of hydroxyapatite via microemulsion and emulsion routes

Hydroxyapatite powders have been prepared by reacting CaCl2 and (NH4)2HPO4 in bicontinuous microemulsion, inverse microemulsion and emulsion, which have the same components as cyclohexane, non-ionic surfactant (NP5 + NP9) and aqueous solution. The characteristics of the resulting hydroxyapatite powders, such as the particle size, particle size distribution, chemical homogeneity and the degree of particle agglomeration, are strongly affected by the structure of the reaction medium. Both bicontinuous and inverse microemulsions led to the formation of much finer hydroxyapatite powders than that prepared from the emulsion composition. The two fine hydroxyapatite powders are sintered to a relative density of >95% theoretical density at 1000 degrees C, compared with a relative density of <73% theoretical density for the emulsion-derived one. The two microemulsion-derived hydroxyapatites also exhibit a higher sintered density and are more refined in grain size than that of the emulsion-derived one when sintered at 1200 degrees C for 2h.

Processing of fine hydroxyapatite powders via an inverse microemulsion route

Abstract A submicrometer-sized hydroxyapatite powder has been synthesized via an inverse microemulsion processing route, which uses cyclohexane as the oil phase, mixed poly(oxyethylene) 5 nonyl phenol ether (NP-5) and poly(oxyethylene) 9 nonyl phenol ether (NP-9) as the surfactant phase, and an aqueous CaCl 2 solution as the water phase. In comparison with the hydroxyapatite powder prepared by directly reacting (NH 4 ) 2 HPO 4 with CaCl 2 in an aqueous solution, the microemulsion processing route led to a significant refinement in the particle size, particle size distribution, and the degree of particle agglomeration. XRD phase analysis indicates that a high-purity crystalline hydroxyapatite powder was obtained when the microemulsion-derived precursor was calcined at 800 °C. In contrast, there existed impurity phases such as β-Ca 3 (PO 4 ) 2 and CaO in the conventionally processed hydroxyapatite powder after calcination at various temperatures up to 1200 °C. This shows that the microemulsion processing route results in the formation of a more homogeneous ultrafine powder precursor than that obtained from the conventional processing route.

Glass forming ability and in-situ composite formation in Pd-based bulk metallic glasses

Abstract Pd 60− x Ni 10 Cu 30 P x alloys with x =10–20 were prepared by chill-casting, Bridgman solidification and melt spinning, giving a cooling rate range from 0.63 to ∼10 6 K/s. The effects of P content on the glass forming ability and in-situ bulk metallic glass (BMG) matrix composite formation in these alloys have been studied. Glass forming ability of the Pd–Ni–Cu–P alloys improves with increasing P content. This is interpreted in terms of the reduced glass transition temperature T rg given by T g / T L and the crystallization rate constant K T . F.c.c α-Pd crystalline phase reinforced BMG matrix composites have been synthesized in a wide range of compositions. The formation of the in-situ BMG matrix composite is explained qualitatively in terms of competition between eutectic growth and amorphous phase formation.

Electrically conductive and fluorescent poly[1,4-bis(3-alkyl-2- thienyl)phenylenes]: syntheses and preliminary characterization aspects

Abstract A series of polymers containing alternating phenylene and symmetrically substituted bithiophene repeat units have been synthesized and characterized using UV-Vis absorption spectroscopy, fluorescence emission spectroscopy, conductivity measurement and thermal analyses. The results reveal that the polymers have a bandgap energy of 2.5 eV and a fluorescence quantum yield of up to 26% with respect to quinine sulfate. The polymers show a maximum conductivity of 0.2 S cm −1 upon doping with I 2 and display good thermal stability in both neutral and doped states. The influence of alkyl chain size on conductivity, spectral behavior and thermal stability is discussed.

Microemulsion processing of manganese zinc ferrites

Nanosized precursor particles of manganese zinc ferrites, Mn0.45Zn0.55Fe2O4, have been synthesized from inverse microemulsions of metal nitrates and aqueous ammonia solution. Two processing routes, namely, a single-microemulsion route and a double-microemulsion route, were studied and compared. The double-microemulsion-derived precursor led to the formation of ultrafine manganese zinc ferrite particles when calcined at 600 °C, although particle agglomeration had occurred.

Novel heteroarylene polyazomethines : their syntheses and characterizations

Novel thiophene or furan-containing polyazomethines were synthesized by chemical oxidative polymerization with ferric(III) chloride. The structures of the undoped and doped polymers were evaluated by FTi.r., elemental analysis, u.v.—vis spectrometry, X-ray photoelectron spectroscopy (XPS) and their thermal stability determined using thermogravimetric analysis (t.g.a.). The conductivities of the undoped polymers were less than 10−8 S cm−1, while those of the doped ones were in the range of 10−7 to 10−6 S cm−1. The low conductivity values were attributed to the low degree of conjugation in the polymers caused by non-planarity of the polymer chains. The thiophene-containing polymers underwent sulfur extrusion during polymerization, resulting in an interruption of conjugation in the polymer backbone with consequent diminished electrical conductivity.

Polymerization of methyl methacrylate in ternary oil-in-water microemulsions

Abstract Polymerization of methyl methacrylate has been studied in ternary o/w microemulsions that were stabilized by stearyltrimethylammonium chloride. The latices produced still remain transparent and stable about one year after preparation. It is calculated that each small latex particle of hydrodynamic radius (Rh) ranging from 20 to 24 nm contained two or three polymer chains of high molecular weight of 5 to 8 million. The rate of polymerization shows only two intervals for most of the systems. This corresponds to the rates of nucleation and growth of particles. But a constant rate of polymerization is also noted for the first time for microemulsions containing a lower concentration of initiator and/or at a lower temperature (T

Separation of glass transition and crystallization in metallic glasses by temperature-modulated differential scanning calorimetry

The glass transition behaviour of the four metallic glasses La55 Al25 Ni20, Zn65Mg35, and Nd60Fe30Al10 and Zr30Y30Al15Ni25 has been studied by temperature-modulated differential scanning calorimetry (MDSC). It is clearly demonstrated that the glass transition can be separated from crystallization by MDSC. Two glass transitions have been observed for Zn65Mg35, Nd60Fe30Al10 and Zr30Y30Al15Ni25. Owing to the clearer observation of the glass transition, calculation of the reduced glass temperatures Trg (glass transition temperature divided by the melting point) can now be made with greater confidence.