ALBERT

Notes: A computer simulation of evolution of random structures of spherical particles has been performed by solving Newton's equations of motion. The forces considered are gravity, Hertz contact force, frictional force, and van der Waals interaction (VDWI). 948 monosized particles were placed without overlapping inside a rigid cylindrical container by generating the coordinates of the centers of the particles with the help of a random number generator. The initial packing density was only 0.363. The particles were allowed to settle under gravity. When the coefficient of friction (μ) among the particles and between the particle and the wall of the container was 0.3, the packing density reached a value of 0.578. If there is no friction, the density reached 0.633 which is comparable to the random close-packed density obtained in the random structures of steel ball bearings. For small particles, VDWI can reduce the packing density by agglomeration of particles into local clusters. For example, with VDWI and μ of 0.3, the random structures of particles with diameter 100 and 50 μm had packing densities of 0.528 and 0.420, respectively.When μ was increased to 0.7, the packing density of 100-μm particles with VDWI was 0.505, compared to 0.528 in the case of μ equal to 0.3. The average velocity of the particles initially increased, and after reaching a maximum, started to decrease due to collisions amongst the particles. During the dynamic simulation, the trajectories of a few selected particles were traced. To study the rearrangement process, the average displacement of a few particles in a direction normal to the direction of gravity and the average angle of rotation of the straight line joining the centers of a few selected pairs of particles were calculated at various instants of time. It is seen that the rearrangement of 50-μm particles was smaller than that of 100-μm particles because of VDWI. The radial distribution functions of the computer-generated close-packed structure is similar to that of Finney's close-packed structure of steel balls. In the case of 50-μm particles with VDWI, there were additional peaks close to the first peak, due to agglomerating effect of VDWI. The cumulative number of contacts was computed as a function of the radial distance and, in the case of 100-μm particles without friction and VDWI, it agreed with Mason's data for steel balls.

Notes: Fatigue crack growth (FCG) behaviour and its characteristics following tensile overloads were investigated for AISI 304 stainless steel in three different atmospheres; namely dry argon, moist air and hydrogen. The FCG tests were performed by MTS 810 servohydraulic machine. CT specimens were used for the tests and crack closure measurements were made using an extensometer. FCG rates of 304 stainless steel at both dry argon and moist air atmospheres have shown almost the same behaviour. In other words, the effect of moisture on FCG of this material is very small. However, in a hydrogen atmosphere, the material showed considerably higher crack growth rate in all regimes. In general, for all environments, the initial effect of overloads was to accelerate the FCG rate for a short distance (less than a mm) after which retardation occurred for a considerable amount of time. The main causes for retardation were found as crack blunting and a long reinitiation period for the fatigue crack. Regarding the environmental effect, the overload retardation was lowest in a hydrogen atmosphere. This low degree of retardation was explained by a hydrogen embrittlement mechanism. In a general sense, hydrogen may cause a different crack closure mechanism and hydrogen induced crack closure has come in to the picture. Scanning electron microscope and light microscope examinations agreed well with the above results.

Notes: The sintering behaviour of powders of two calcium phosphates, namely hydroxyapatite (HA) and dicalcium phosphate (DCP), were studied at various temperatures and in various environments. The density, flexural strength and Knoop hardness of HA sintered in air for 4 h initially increased with the sintering temperature, reaching maxima at around 1150°C, and then decreased due to decomposition of HA into tri- (TCP) and tetracalcium phosphates. Sintering in vacuum caused decomposition of HA at lower temperatures, and consequently the mechanical properties were poorer than those of HA sintered in air. The densification and mechanical properties of DCP sintered in air and vacuum showed similar behaviour to those of HA. In air DCP underwent phase transformation from γ- to β- and to α-phases. In vacuum DCP started to decompose into tricalcium phosphate at 1000°C. To reduce dehydroxylation, HA powder was sintered in moisture at various temperatures up to 1350°C and X-ray diffraction study did not indicate any decomposition at the highest sintering temperature. The density, flexural strength and hardness of HA sintered in moisture increased with the sintering temperature and eventually reached plateaux at about 1300°C, but below 1200°C they were lower than those of HA sintered in air at corresponding temperatures. Thus, it is seen that dehydroxylation did not hinder sintering of HA. On the other hand, decomposition obstructed sintering of both HA and DCP.

Notes: Thick films of hydroxyapatite (HA) were deposited on silicon single crystal wafers placed in close proximity to a plate of apatite- and wollastonite-containing glass and dipped into a simulated physiological solution at 36°C. Amorphous calcium phosphate phase present in the glass leached into the solution, causing supersaturation of Ca2+ and PO4 3- ions. Spherical cap-like islands of calcium phosphate nucleated on Si crystals and grew in size with time. The thickness of the film grown on Si (111) in a solution having a composition similar to that of human blood plasma, and maintained at pH of 7.2, reached 7.1 μm in 336 h, compared with a thickness of 12.7 μm when the ion concentrations of the solution were doubled. HA films grown on Si (111) showed strong (102) texture. In contrast, hardly any HA film could be grown on Si (100). With increasing pH value of the solution the Ca/P ratio of the film increased. At a pH of 7.2 the as-grown and annealed (at 800°C for 3 h in argon) films had Ca/P ratios of 1.10 and 1.72, respectively. The Vickers hardness and the adhesion strength of the film increased upon annealing. Our results suggest that the driving force for formation of apatite films arises from the lowering of free energy of the supersaturated solution by deposition of ions (Ca, P, O, H) in certain crystallographic arrangements on suitable substrates with low interface energies.

Notes: Abstract When a parallelepiped specimen of polycrystalline copper is compressed in thex-direction (primary direction) while holding thez-dimension unchanged by a vice, the specimen is anisotropically hardened as follows: when thez-direction is subsequently compressed while holding thex-dimension unchanged the yield stress (0.2% offset) is higher than the final flow stress of the primary deformation. This is similar to latent hardening in single crystals. On the other hand, if the second compression is in they-direction instead of thez-direction, the yielding (0.2% offset) occurs at a stress less than the final flow stress of the primary deformation. Both effects are reported here together with the results of two successive compressions in two mutually perpendicular directions without any constraints in either compression. These results are compared with the earlier results of high density polyethylene.

Notes: Abstract Hollow spheres of lead zirconate titanate (PZT) [chemical formula Pb(Zr0.52Ti0.48)O3)], with outer diameter of 1–2 mm and a wall thickness of about 100 μm, were fabricated by gellation of a PZT sol inside solid polymer spheres and then burning the polymer out. Monomodally sized polyacrylamide spheres, with diameter 1.40–1.90 mm, were soaked in a PZT sol, prepared by dissolving Pb(NO3)2, zirconiumn-butoxide and titanium isopropoxide inN, N-dimethylformamide. The absorbed sol was then gelled beneath the surface of the polymer sphere by the action of NH3. Upon calcination of the spheres at 850 °C for 4 h in air, hollow spheres of pure PZT perovskite phase (as identified by X-ray diffraction patterns) were obtained. The density of the hollow spheres was 1.13gcm−3, while that of the wall of the spheres was 3.10g cm−3. The scanning electron microscopic examination of the broken spheres showed that the inner surface of the spheres contained rib-like structures, which provided strength to the hollow spheres. The planar coupling factor,k p, of six hollow spheres, placed at a close-packed arrangement in a plane, was 0.22, indicating the possibility of fabrication of low-density transducer arrays.

Notes: Abstract A new sol–gel processing technique has been developed for preparing Nb-doped lead zirconate titanate (PZT) with the composition Pb0.99Nb0.02(Zr0.52Ti0.48)0.98O3. The sol was prepared from Pb(NO3)2, NbCl5, Ti(OC3H7)4 and Zr(C4H9O)4, dissolved in N, N-dimethyl-formamide with stirring at ambient atmosphere. The gel was calcined at 500°C for 30 min and pressed pellets were sintered in air at 1000°C for 2–20 h. The average grain size in the pellets sintered for 4 h was about 1.4 μm, which was smaller than that in undoped PZT pellets. Nb doping decreased angle α in the rhombohedral crystal structure, thereby enhancing electromechanical coupling.

Notes: Abstract Conductive rubber composites were derived from different blends of ethylene–propylene–diene monomer (EPDM) rubber and acrylonitrile butadiene rubber (NBR) containing acetylene black. The electrical and mechanical properties of these composites were measured. The percolation limit for achieving high conductivity of conductive filler depends on the viscosity of the blend. The higher the viscosity, the higher is the percolation limit. The conductivity rises with increasing temperature, and the activation energy of conduction increases with the decrease in the loading of conductive filler and percentage of NBR in the blend. Electrical hysteresis and an electrical resistivity difference during the heating–cooling cycle are observed for these systems, which is mainly due to some kind of irreversible change occurring in the conductive networks during heating. The mechanisms of conduction of these systems were discussed in the light of different theories. It was found that the degree of reinforcement by acetylene black in blends compares with those in the pure components NBR and EPDM. This is due to incompatibility of two elastomers in the blend.

Notes: Abstract Powder of lanthanum-modified lead zirconate titanate (PLZT) with the composition La/Zr/Ti=8/65/35, was synthesized from aqueous nitrate solutions. A single-phase PLZT was obtained at ∼550 °C. The reactivity of the powder during low-temperature heating was determined using X-ray diffraction and various thermal analysis techniques. The dielectric properties of the compound were also studied at 1 and 10 kHz frequency from room temperature to 200 °C. Diffuse phase transition (DPT) in the material was observed around 136 °C at 1 kHz. An increase in peak-permittivity temperature (i.e. the transition temperature) with increasing frequency (a characteristic of relaxor ferroelectrics) was also observed.

Notes: Abstract Powder of lanthanum-modified lead zirconate titanate (PLZT) with the composition La/Zr/Ti=8/65/35, was synthesized from aqueous nitrate solutions. A single-phase PLZT was obtained at ∼550 °C. The reactivity of the powder during low-temperature heating was determined using X-ray diffraction and various thermal analysis techniques. The dielectric properties of the compound were also studied at 1 and 10 kHz frequency from room temperature to 200 °C. Diffuse phase transition (DPT) in the material was observed around 136 °C at 1 kHz. An increase in peak-permittivity temperature (i.e. the transition temperature) with increasing frequency (a characteristic of relaxor ferroelectrics) was also observed.