Structure formation and gas evolution in the vacuum liquid-phase sintering of ferrites

Conclusions In the ferrite investigated chains of bubbles and channels constitute the main gas transport routes both between pores and between pores and the surface of a part. They are of complex shape and structure, and are linked together with grain boundaries in a single transport network. Together with gas, the liquid phase is discharged on the surface of the part. Vacuum in the cooling stage once again intensifies the gas and liquid phase discharge process.Translated from Poroshkovaya Metallurgiya, No. 8(284), pp. 30–34, August, 1986.     

Effect of the sintering temperature on the crystalline and magnetic structure of nickel-zinc ferrites

Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 39–42, May, 1988.     

Kinetics of neck growth during loose stack sintering

The kinetics of loose stack sintering of spherical iron powder were monitored by measuring some global microstructural attributes associated with the interparticle contact necks. The neck size and the number of interparticle contacts per unit volume were calculated from these data. A direct comparison of the experimental and theoretically predicted neck sizes has been carried out. The neck sizes predicted by the model for surface diffusion controlled sintering are muchhigher than the corre-sponding experimental values. The number of interparticle contacts per particle does not change with isothermal sintering time or temperature, and it is predominantly determined by the initial stacking of particles in the powder mass. The change of atmosphere from dry hydrogen to argon does not affect the neck growth kinetics significantly.     

Kinetics of heating of constructional parts of ferromagnetic powders in induction sintering

Translated from Poroshkovaya Metallurgiya, No. 7(307), pp. 49–53, July, 1988.     

Kinetics of interparticle contact formation during the laser sintering of two-component powders

The kinetics of interparticle contacts formation during the liquid-phase sintering of unlike materials under the action of laser irradiation was investigated. Trios of particles arranged in a row according to the schemes glass—polymer glass, metal—polymer—metal, and metal—glass—metal were sintered. The processes of particle approach and liquid neck formation between particles were studied. Possibilities for the manufacture of powder components with polymer and glass binders by the use of selective laser sintering followed by heat treatment are considered]. Institute of Technical Acoustics, Academy of Sciences of Belarus', Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 37–41, May–June, 1999.     

Kinetics of pore size increase in the activated sintering of powdered tungsten

Conclusions An increase in the size of pores determined by the Barus-Bechgold method in porous specimens from fine tungsten and tungsten-nickel powders takes place during heating to the isothermal sintering temperature. The addition of nickel to tungsten activates the pore size growth process. The size of the increased pore channels in porous solids from W and W-0.46% Ni powders in the temperature range 1000–1300°C depends on the particle size and sintering temperature. A correlation has been found between the integral shrinkage during isothermal sintering and the capillary stresses acting on the attainment of the isothermal sintering temperature in compacts from W-0.46% Ni powders of various particle sizes. The rates of isothermal shrinkage are the same, being independent of the previous history of the powders.Translated from Poroshkovaya Metallurgiya, No. 9(249), pp. 18–23, September, 1983.     

Kinetics of the dissolution of zinc-magnesium ferrites in sulphuric acid solutions related to zinc leach processes

The presence of sparingly soluble zinc and magnesium ferrites in roasted zinc concentrates can cause problems during leaching. This study aimed at the determination of the dissolution kinetics of zinc ferrite-magnesium ferrite solid solutions. The investigations have been performed for the (Zn_1?xMg_x)Fe₂O₄ powder specimens (x = 0, 0.25, 0.50, 0.75 and 1) in solutions of 100–200 g/l H₂SO₄ at 323–363 K. Dissolution of all specimens has been found to be chemically controlled and homothetic, being described by the “unreacted-core shrinking” model. The dissolution rate constant of zinc ferrite was about seven times higher than that of magnesium ferrite (e.g. 3.83·10^?6 against 0.53·10^?6 mol/m²s in 100 g/l H₂SO₄ at 363 K). The relation between rate constant and the x value was not linear. The activation energy was not dependent on the x value and amounted to ca. 75 kJ/mol. The results have important relevance to the “magnesium problem” often experienced in roach-leach processes in hydrometallurgy.     

Mechanics of sintering materials with bimodal pore distribution. III. Kinetics of sintering combined with various forms of loading and adhesion conditions

A study has been made on the effects of loading scheme and conditions restricting macroscopic strain on the work-hardening kinetics and strain accumulation in the solid state in sintering materials with bimodal pore size distributions. Active loading intensifies the reduction in the small pores. The greatest effect comes from combining sintering with hydrostatic compression. At the same time, kinematic constraints (partial or complete adhesion in surfaces) substantially retards the shrinkage of large pores, which means that the porous structure can be controlled. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 10–15, May–June, 2006.     

Kinetics of grain coarsening during sintering of Co-Cu and Fe-Cu Alloys with low liquid contents

The coarsening of grains immersed in varying amount of liquid matrix is investigated in Fe-Cu and Co-Cu alloys at the liquid phase sintering temperatures. Specimens containing 20, 30, and 50 wt pct Cu have been prepared by compacting and sintering mixtures of fine powders. With 50 wt pct of Cu, spherical grains are dispersed in the liquid matrix. With 20 wt pct of Cu, anhedral grains are in contact with the neighbors across grain boundaries or thin liquid films, and the liquid matrix forms continuous prisms along the three grain contacts. The form of the rate law for grain coarsening at all compositions agrees with predictions of the diffusion controlled Ostwald ripening theories of Lifshitz, Slyozov, Wagner, and others. The coarsening rate also increases with decreasing matrix content. The activation energy for grain coarsening does not vary with specimen composition. Therefore, the rate controlling mechanism for coarsening of the anhedral grains in contact with each other appears to be the solution and reprecipitation of solute atoms by diffusion through the liquid matrix. SU SOK KANG, formerly a student in the Department of Materials Science at the Korea Advanced Institute of Science and Technology     

Reaction synthesis processes: mechanisms and characteristics

Mechanisms of the self-propagating high-temperature synthesis (SHS) are described. The determination of activation energies from wave velocity measurements and temperature profile analyses is discussed. The application of these methods to the respective cases of titanium nitride and zirconium boride is provided. The utilization of the SHS method for the preparation of ceramic-ceramic and ceramic-metallic composites is described with a focus on the mechanism. The concept of “SHS diagrams” is proposed, and its utility is demonstrated with the synthesis of hafnium boride as an example. The influence of the activation energy on the establishment of SHS/non-SHS boundaries in these diagrams is described. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.