Thermal transformations of composites based on hydroxyapatite and zirconia

We have examined the effect of synthesis and heat-treatment conditions on the phase composition of composites based on hydroxyapatite and zirconia. The zirconia present in the composite based on a mechanical powder mixture crystallizes predominantly in tetragonal symmetry at heat-treatment temperatures in the range 400–600°C and in monoclinic symmetry at 800°C. The zirconia present in the composites prepared via the mixing of hydroxyapatite gel with hydrated zirconia sol or coprecipitation crystallizes only in monoclinic symmetry. Annealing at 1300°C leads to the formation of a solid solution with the composition Ca_0.15Zr_0.85O_1.85 in all of the composites as a consequence of complete or partial hydroxyapatite decomposition and the thermal diffusion of Ca²⁺ ions into the ZrO₂ lattice. In the composites prepared by liquid-phase processes, heat treatment in the range 800–1300°C produces the smallest changes in the hydroxyapatite structure, which ensures high biological activity of the material.     

Finite element formulation for thermal stress analysis of thin reactor structures

This paper describes the formulation of a finite-element procedure for the thermal stress analysis of thin wall reactor components. A general temperature-dependent constituent relationship is derived from a Gibbs potential function and a temperature-dependent yield surface. This form of constitutive relationship is applicable to problems of small strain. A similar form of a hypoelastic-plastic type is also developed for large strains. The variation of the yield surface with temperature is based upon a temperature-dependent, work-hardening model. The model uses a temperature-equivalent stress-plastic strain diagram which is generated from isothermal unaxial stress-strain data.The above constitutive relationships are incorporated into two computer codes and a previously developed numerical algorithm is used with minor modifications. A set of problems is presented validating the thermal analysis capability of the computer codes to a variety of problem types.     

Investigation of composite hydroxyapatite powder for plasma spraying bioceramic coatings

Powder synthesis on the basis of a matrix from nanocrystalline titanium dioxide with incorporated HA particles was developed. Using scanning and transmission electron microscopy (SEM and TEM), the shape, size, and relief of TiO₂-HA powder particles are investigated. When calculating for the powder particles using the SEM photos and TEM negative films on a picture analyzer, the granulometrical composition and the average maximum particle diameter (0.7 μm) are determined.     

Convective heat transfer under the influence of body forces in self-propagating high-temperature synthesis

A model of a heterogeneous medium is proposed; it consists of a high-melting component and a low-melting component and can take into account convective heat transfer under the influence of body forces. A system of equations is derived for the steady-state propagation of a combustion front with allowance for convective heat transfer. Expressions for the combustion rate and temperature are obtained in the capillary regime.Vitebsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 2, pp. 54–61, March–April, 1993.     

Composition and structure of products from high-speed steel impact-centrifugal spinning

General evaluation has been applied to the products from impact-centrifugal spinning for high-speed steels of R6M5 type by chemical analysis, microprobe, x-ray diffraction, optical and scanning electron microscopy, and microhardness measurement. These products are mixtures of scaly or flaky particles (2×0.4×0.04 mm), large (2–8 mm) equiaxial fragments, and small (0.5 mm) equiaxial particles, which include spherical ones. The scales have higher contents of carbon and carbide-forming elements than do the initial materials, while the large fragments have lower ones. The elements are unevenly distributed in the scales. There are parts differing in chemical composition. These parts are formed in the scales because of concentrational inhomogeneities in the initial alloy. The thickness of a scale varied monotonically along the length, which leads to differences in cooling rate and consequently to inhomogeneities in phase composition after recrystallization.     

A GA-based solution approach for balancing printed circuit board assembly lines

Printed circuit board (PCB) assembly lines consist of a number of different machines for mounting electronic components onto PCBs. While high-speed placement machines are employed to assemble standard components, so-called fine-pitch placement machines are used to mount complex electronic components with high precision and by use of specific nozzles. In this paper, we investigate a typical mass production environment where a single type of PCB is assembled in a line comprising high-speed as well as high-precision placement machines. The PCB assembly line balancing problem consists of assigning component feeders, each holding a specific electronic component type, and the corresponding placement operations to machines in the line so as to minimize the assembly cycle time. To solve this problem, a two-stage solution procedure based on genetic algorithm (GA) is proposed. In the first stage, component feeders are assigned to the placement machines with the objective of balancing the workload within the assembly line. A number of candidate solutions are then transmitted to the second stage, where specific machine optimization algorithms are applied to determine the feeder-slot assignment in the component magazine of the machines and the placement sequence of the various components. As a result, fine-tuned placement operation times are achieved which reflect the individual operation mode and the actual component setup of the placement machines. Finally, from the candidate solutions the one which minimizes the actual PCB assembly time is selected.     

Influence of the structure on the high-temperature cyclic crack resistance of Ti-8Al-1.4Si-2.2Zr alloy

It is shown that, for various structural modifications of Ti-8Al-1.4Si-2.2Zr alloy, the elevation of temperature leads to the increase in the cyclic fracture toughness ΔK _fc and the decrease in the fatigue threshold ΔK _th. At 20°C, the cast material with coarse-grained lamellar structure exhibits better properties than the fine-grained modifications obtained by thermomechanical treatment under medium and high levels of loading. At 700°C, this is true only for medium amplitudes of the load. The procedure of quenching from the β-region of the alloy thermally deformed by 90% doubles the value of ΔK _th at a temperature of 20°C as compared with the cast alloy. At the same time, the value of ΔK _fc at 700°C becomes 1.6 times higher. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 50–56, May–June, 2008.     

Structural and Phase Transformations in the Thiourea/Zinc Acetate System

Inorganic Materials - Using differential thermal analysis and thermogravimetry, we have studied thermal decomposition of a mechanical mixture of thiourea and zinc acetate, resulting in the...