Hydroxyapatite-coated metals: Interfacial reactions during sintering

Electrophoretic deposition (EPD) is a low cost flexible process for producing HA coatings on metal implants. Its main limitation is that it requires heating the coated implant in order to densify the HA. HA typically sinters at a temperature below 1150C, but metal implants are degraded above 1000C. Further, the metal induces the decomposition of the HA coating upon sintering. Recent developments have enabled EPD of metathesis-synthesised uncalcined HA which sinters at 1000C. The effects of temperature on HA-coated Ti, Ti6Al4V, and 316L stainless steel were investigated for dual coatings of metathesis HA sintered at 1000C. The use of dual HA coatings (coat, sinter, coat, sinter) enabled decomposition to be confined to the undercoat (HA layer 1), with the surface coating decomposition free. The tensile strength of the three metals was not significantly affected by the high sintering temperatures (925C < T < 1000C). XRD/SEM/EDS analyses of the interfacial zones revealed that 316L had a negligible HA:metal interfacial zone (1 m) while HA:Ti and HA:Ti6Al4V had large interfacial zones (>10 m) comprising a TiO₂ oxidation zone and a CaTiO₃ reaction zone.     

Hydroxylapatite-zirconia composites: Thermal stability of phases and sinterability as related to the CaO-ZrO<Subscript>2</Subscript>phase diagram

Composites of hydroxylapatite (HA) with pure zirconia, and 3 and 8% Y₂O₃ in zirconia, were pressure-less sintered at temperatures from 900 to 1300C, and hot-pressed at 1200C in argon gas atmosphere for 1 h. The reactions and transformations of phases were monitored with X-ray diffraction and thermal analysis. At sintering temperatures higher than 1000,C, calcium from HA diffused into the zirconia phase, and the HA phase decomposed to tri-calcium phosphate (TCP). Above about 1200,C, CaZrO₃ was formed. These reactions and transformations were interpreted in terms of the ZrO₂-CaO phase diagram. On the other hand, zirconia and hydroxylapatite phases in hot pressed composite remained mainly stable suggesting that air in the sintering environment increased the reactivity between the phases. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ sintered at 1300,C. The densification of the composites decreased at lower sintering temperatures and higher zirconia contents upon air-sintering.     

Sulphonated poly(ether ether ketone): Synthesis and characterisation

The sulphonation of commercially available PEEK in powder form (Gatone, Gharda Chemicals Limited, India) was carried out using conc. H²SO⁴ under different reaction conditions. The duration of reaction was varied from 3–5 h, polymer concentration 4–10% (w/v) and temperature 35–50C. Structural characterisation of sulphonated polymers was done by elemental analysis, FT-IR and ¹H-NMR spectroscopy. The degree of sulphonation as calculated from ¹H-NMR and elemental analysis (S-content) was found to be in the range of 50–80%. Multistep mass loss was observed in thermogravimetric traces (recorded in N² atmosphere). The first step (50–225 ± 25C) was due to loss of moisture (1–10%) and second step (250–425 ± 25C) has been attributed to volatilization of SO³ from the sulphonic group. The backbone degradation takes place above 450C. The mechanical properties and proton conductivities of various sulphonated samples was also evaluated.     

Sintering behavior and thermal property of Mg<Subscript>2</Subscript>SnO<Subscript>4</Subscript>

The sintering behavior of Magnesium Orthostannate (Mg₂SnO₄) by Magnesium oxide (MgO) and tin oxide (SnO₂) was investigated. Mg₂SnO₄ compound was formed by traditional solid state reaction (SSR) at elevated temperatures over a range of 600 C–1300 C. X-ray studies have revealed that the resulting, as-fired, compound is polycrystalline composed of an inverse spinel-structure grains separated partially by porosity. However it failed to detect any phase change as a result of annealing. Scanning electron microscope (SEM) was employed to follow-up any change in the morphology throughout the sintering and reduction processes. Thermal property Differential Scanning Calorimetry (DSC) revealed that reduction takes place at a temperature between 400 and 600 C, depending on the concentration of H₂ in the atmosphere in accordance with the X-ray studies. The technique employed has also demonstrated the stability of reduced species in typical atmospheres and working conditions.     

Precipitation of single crystalline AlN from Cu-Al-Ti solution under nitrogen atmosphere

Precipitation and growth behavior of AlN from Cu-Al-Ti-N solution under the atmospheric pressure of nitrogen has been studied. The solubility of AlN in Cu flux ranges from 5 mol% to 20 mol% at the temperatures between 1700 C and 2000 C. The addition of the proper amount of Ti drastically enhances the solubility of AlN. Acicular crystallites and platelets of AlN precipitants, which were all water white colored, were obtained by super-cooling the Cu-Al-Ti-N solution. The largest pencil type AlN reached more than 3 mm in its length and 0.2 mm in its diameter. The axis of the pencil was c-axis of wurtzite AlN and it contained no dislocation, no diffractive defects. A platelet of AlN of which diameter exceeded 1.5 mm was also obtained by regrowth technique. Thermodynamic calculation reveals the validity of using Cu-Al-Ti-N solution for the growth of AlN.     

Effect of Intense Self-Irradiation on the Phase Composition of Metallic Plutonium

The crystal structure of film samples of "high-level" (based on ²³⁸Pu) and low-level (based on ²³⁹Pu) metallic plutonium during their prolonged (up to 343 days) storage (self-irradiation) at room temperature was studied by X-ray diffraction. In the samples of high-level plutonium, the -Pu and -Pu lattices coexist. In the period of 40-60 days, the other known crystal modifications of plutonium (-Pu, -Pu, -Pu, and -Pu) are also present. Low-level plutonium had only the -Pu lattice. A possible origin of this phenomenon is discussed.     

Thermodynamic investigations of liquid Bi-Na and Sn-Na alloys by coulometric titration using β″-alumina

The following double galvanic cell was assembled and the thermodynamic properties of liquid Bi-Na and Sn-Na alloys, and the ion selectivity of -alumina during coulometric titration, were investigated. Mo, Na(I)¦-alumina¦M-Na(I), Mo [I] M-Na(I)¦-alumina¦Au + Au₂Na, Mo [II] (M = Bi or Sn) where M-Na(1) and Au + Au₂Na were used as the common electrode and reference electrode, respectively. Sodium was coulometrically titrated through the -alumina electrolyte of cell I both ways, and the EMFs were measured. It was found that no ion-exchange reaction occurs between the liquid alloys and the -alumina, and only Na was transferred in the -alumina during coulometric titrations. The thermodynamic properties of liquid Sn-Na and Bi-Na alloys were found to be in agreement with the literature.     

High-accuracy bench mark solutions to natural convection in a square cavity

A fourth-order high-accuracy finite difference method is presented for the bouyancy-driven flow in a square cavity with differentially heated vertical walls. The two bench mark solutions against which other solutions can be compared were obtained. The present solution is seemed to be accurate up to fifth decimal. The proposed scheme is stable and convergent for high Rayleigh number, and will be applicable to general problems involving flow and heat transfer, especially in three dimensions.List of symbols a thermal diffusivity - g gravitational acceleration - h mesh length in the x-direction - k time step - L side length of cavity - Pr Prandtl number - Ra Rayleigh number = gTL ³/a - t time - T _c, T _h surface temperatures (see Fig. 1a) - T temperature - T temperature difference = T _h - T _c - u, v velocity in the x and y directions, respectively - x, y coordinates - x, y mesh lengths in the x- and y-directions, respectively Greek symbols volumetric expansion coefficient - kinematic viscosity - stream function - vorticity     

Two-step growth of SrTiO<Subscript>3</Subscript> films on Sr-modulated Si(001) substrates

Perovskite-type SrTiO₃ (STO) films have been grown on the Sr-modulated Si(001) substrates using two-step growth method with a molecular beam epitaxy technique. About 6 Å Sr was deposited on the surface of chemically formed SiO₂/Si at 800 C, which resulted in the formation of Si(001)-Sr(2 × 1) surface. About 250-Å-thick STO films have been deposited on the Si(001)-Sr(2 × 1) surface at 350 C in the O₂ partial pressure of approximately 3 × 10^–7 Torr (first step), followed by an annealing at 600 C in a pressure of 3 × 10^–9 Torr (second step). Reflection high-energy electron diffraction patterns of grown films become elongated streaky features after the annealing, suggesting the solid-phase epitaxial growth of STO. The presence of strong STO(001) and STO(002) diffraction peaks in X-ray diffraction measurements suggests that the high-quality crystalline STO films grow on the Sr-modulated Si(001) substrates by the two-step growth method. Atomic force microscopy image shows the smooth surface with root mean square roughness of about 18 Å.     

Ordinary size effects and deviations from Matthiessen's rule in the resistance of fine wires

Contrary to previous statements in the literature, large deviations from Matthiessen's rule in fine wiresare to be expected on the basis of a straight-forward solution of the ordinary transport equation, assuming the relaxation-time approximation and imposing the idealized condition of diffuse scattering of electrons at the boundaries. Using Chambers' path-integral method to evaluate the current density in a wire of arbitrary cross-sectional shape, the effects of boundary scattering on the resistivity in the regimed 0.1 have been calculated for two model Fermi surface geometries. For the temperature-dependent part of the resistivity, _d (T) _d (T)– _d (0), two distinct types of behavior are found in the alternative cases: (1) for a spherical Fermi surface, _d(T) increases logarithmically with _d(0); (2) for a cylindrical Fermi surface, _d (T) increases essentially linearly with _d (0). [In each case the qualitative dependence of _d(0) on /d is, for practical purposes, linear. However, the correct value of the product in the cylindrical case is not simply given in the ordinary way by the slope of an empirical plot of _d (0) vs.d ^–1.] A comparison of theoretical results for the two simple models with the published data for indium and gallium shows that the actual temperature-dependent size effects are consistent, both qualitatively and, by a rough estimation, quantitatively, with the expected behavior.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.     

A note on the similarity solutions for free convection on a vertical plate

Summary The similarity solutions for free convection on a vertical plate when the (non-dimensional) plate temperature is x and when the (non-dimensional) surface heat flux is –x are considered. Solutions valid for 1 and 1 are obtained. Further, for the first problem it is shown that there is a value ₀, dependent on the Prandtl number, such that solutions of the similarity equations are possible only for >₀, and for the second problem that solutions are possible only for >–1 (for all Prandtl numbers). In both cases the solutions becomes singular as ₀ and as –1, and the natures of these singularities are discussed.     

Lateral grain growth during reactive isothermal solidification in the Ni-Sn system

In this research lateral growth of Ni₃Sn₄ grains in the Ni-Sn system during isothermal solidification was investigated. Experiments involved multi-layered samples heat-treated for different durations at temperatures in the range 235–600C. During solidification Ni₃Sn₄ was the only intermediate phase found to grow as a layer at the solid/liquid interface. SEM and optical microscopy cross-sectional views of samples heat-treated at 500C and at 600C reveal that lateral grain growth occurs, the mean lateral grain size being proportional to t(t-the duration of the heat treatment). This process was found to be connected with solution reprecipitation process between adjacent grains in order to equate the local curvature of the solid/liquid interface at groove zone. Rotation of the grain boundary groove due to this mass transfer induces a curvature of the grain boundary, which is the controlling mechanism for its migration. The process was numerically formulated, which provides quantitative fitting to the observed kinetics. The formulation includes a contribution due to the good wetting of the grain boundaries by the liquid phase supporting the observed enhancement in growth rate.     

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al_xGa_1–xN (0.1 < x < 0.8) films, 0.4 m in thickness, were prepared on (0001) sapphire substrates at 500,C by alternate supply of Ga and Al alkyls and ammonia (NH₃). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 m in thickness, were also grown at 1000,C on the LT-Al_xGa_1–xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al_xGa_1–xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al_0.43Ga_0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al_xGa_1–xN buffer layer induced mosaic microstructure and bonding defects in the films.     

Shear viscosity measurements of liquid 4He and 3He-4He mixtures near the tricritical point

A torsional oscillator cell is described, by means of which simultaneous precision measurements of () and of the molar volume can be made in liquid ⁴He-⁴He mixtures over the temperature range between 0.5 and 3 K. Here is the mass density, the shear viscosity and in the superfluid phase they become the contributions _n and _n of the normal component. The results of for ⁴He near the superfluid transition are compared with the predictions by Schloms, Pankert and Dohm, and by Ferrell. Measurements of () are reported for mixtures with 0.64X0.74, where X is the ³He mole fraction. Those for X = 0.67 and 0.70 are compared with data by Lai and Kitchens. The viscosity experiments show no evidence of a weak singularity at the tricritical point.     

Investigation of microstructure of molybdenum—copper black electrodeposited coatings with reference to solar selectivity

Optical and microstructural properties of electrodeposited molybdenum-copper (Mo-Cu) black coatings have been studied with reference to their selectivity in absorption of solar radiation. Such coatings were found to have a solar absorptance, , about 0.87 and low thermal emittance, , such that the selectivity, /, was 3.6. Electrodeposited molybdenum-black coatings generally have selectivity /3. The oxidation state of molybdenum in (Mo-Cu) black coatings as determined by X-ray photoelectron spectroscopy is about + 5 (which is fairly close to that of Mo₄O₁₁). Large numbers of irregular particles were found on the surface of molybdenum-copper black coatings. There is evidence that the particles contain copper oxide.     

Temperature dependence of high strain-rate impact fracture behaviour in highly filled polymeric composite and plasticized thermoplastic propellants

The effect of temperature and strain-rate on the fracture behaviour during high strain-rate ( 10³ sec^–1) impact of two highly filled polymeric composite propellants (containing segmented polyurethanes based on hydroxy-term inated polybutadiene (HTPB) or glycidyl azide polymer (GAP) filled with ammonium perchlorate (AP) particles) and a plasticized thermoplastic (cast double base (CDB) nitrocellulose-nitroglycerine) propellant have been examined over a wide temperature range encompassing the ittle-ductile transition. In the elastic region of the loaddisplacement curve, the yield stress and fracture toughness is highest for GAP/AP and lowest for HTPB/AP. In the elastic and post-yield ductile regions CDB is more fracture-resistant than GAP/AP and HTPB/AP over the temperature range –20 to 50° C, but below –40° C, where both CDB and GAP/AP are brittle, GAP/AP is more fracture-resistant than CDB (as observed in the elastic region). Although all the propellants are known to develop small cracks in the elastic and post-yield ductile regions of the load-displacement curve, the overall fracture behaviour is largely governed by viscoelastic properties (because the cracks close up in compression). The good mechanical properties of CDB, above the brittle-ductile transition temperature, can be attributed to the presence of a large-transition loss peak. In the composites, the fracture behaviour is also influenced to a lesser extent by the degree of filler-binder interactions. Dynamic mechanical analysis indicates that GAP/AP has a slightly higher degree of filler-binder interactions than HTPB/AP. A temperature-strain rate reduction has been obtained for the yield stress and the composite curve can be expressed by the equation _y =K ₁ +K ₂ log (ea _T ) whereK ₁ andK ₂ are constants anda _T is a shift factor.K ₂ is a material constant which reflects the temperature and strain-rate sensitivity.     

Microwave properties of the systems Ba1?xSrx(Zn1/3Ta2/3)0.94T{i}0.06 O3 and Ba(Zn1/3Nb2/3)1?xZrxO3

By means of a solid-phase synthesis two types of microwave dielectric materials are obtained as follows: Ba_1–xSr_x(Zn_1/3Ta_2/3)_0.94Ti_0.06O₃, where x = 0.20, 0.25, 0.30, 0.35 and 0.40 at sintering temperature T_S = 1350, 1400 and 1450 C; Ba(Zn_1/3Nb_2/3)_1–xZr_xO₃, where x = 0.04, 0.06, 0.08 and 0.10 at T_S = 1300, 1350, and 1400 C. The microwave characteristics of the materials are investigated at f = 10 GHz. The composition Ba(Zn_1/3Nb_2/3)_1–xZr_xO₃ demonstrates _r = 38, Q = 6100 and _f = +15 ppm C^–1 and the composition Ba_0.80Sr_0.20- (Zn_1/3Ta_2/3)_0.94Ti_0.06O₃ has _r = 42, Q = 8200 and _f = –13 ppm C^–1. The composition Ba_0.75Sr_0.25(Zn_1/3Ta_2/3)_0.94Ti_0.06O₃ has _r = 40, Q = 6500 and low _f = –13 C^–1 ppm. This composition could be used successfully for realisation of dielectric microwave resonators for the satellite television.     

Tensile strength and fracture surface characterisation of sized and unsized glass fibers

The tensile strength of commercial glass fibers is examined by single fiber tensile tests. The fibers are analysed as received from the manufacturer (sized) and after a heat treatment at 500C (unsized). Weibull plots of the two series are used for comparison of the strengths of the sized and unsized fibers. It is shown that large sample sizes (over 60 tests) are required to lead to a reliable two-parameter Weibull distribution. The experimental tests clearly indicated that the unsized fibers were weaker in the low strength range, but had similar strength in the high strength range. An investigation of the fracture surfaces in the SEM showed distinct differences in the fracture patterns for high and low strength fibers. Fracture mechanics were applied to estimate the original flaw size and relate the observed fracture mirror surface to the fiber strength. Based on the observation of surface flaws, a healing mechanism by the sizing is considered likely for this type of fiber and sizing, thereby effectively increasing the strength of the fiber in the presence of larger surface flaws.     

Single-Mode As–S Glass Fibers

Single-mode As–S glass fibers with a core diameter from 3 to 20 m and a clad diameter of 125 m are prepared by the double-crucible method. The cutoff wavelength of the fibers is 0.9–6 m. The lowest transmission losses in the fibers at 2.2–2.3 m are 100 dB/km, and their mean bending strength is 800–1000 MPa.