Mechanism and kinetics of sintering of nonstoichiometric Y-Ba-Cu-O powders

Using high-resolution optical and scanning electron microscopy with x-ray microanalysis at temperatures of 880 to 920°C, we studied the kinetics and mechanism of the formation of intergranular contacts during sintering of powder-substrate models and also studied the mechanism of shrinkage during sintering of Y-Ba-Cu-O compacts with a stoichiometric composition and two nonstoichiometric compositions, Y_0.95Ba₂Cu₃O_y and Y_1.05Ba₂Cu₃O_y. The formation and the shrinkage of powders of different compositions were found to be described by equations of different degrees, which points to different diffusion sintering mechanisms. Powders with stoichiometric and hyperstoichiometric compositions are sintered by a volume diffusion mechanism, which is controlled by the slowest ion, Y³⁺. Powders with a substoichiometric composition (Y-0.95) are sintered like a liquid phase, i.e., is controlled by diffusion through liquid or solid layers where new phases with higher diffusion coefficients are formed as a result of segregation effects.     

Theory and technology of sintering processes,thermal and thermochemical treatment reaction kinetics for interaction of tungsten with copper-zirconium melts

We investigated the growth kinetics of layers of W₂Zr phase on tungsten interacting with copper-silicon melts at 1150°C. We determined the limiting value of the zirconium content in the copper melt (40.1 at. %). Below this value, we do not observe the indicated layer. We discuss the mechanism of reactive diffusion in the system and the effect of zirconium additives on the liquid phase sintering of the copper-tungsten materials. We have discovered well-defined differences between the growth rates of the layers on polycrystalline and single-crystal specimens of tungsten.Institute of Problems in Materials Science, National Academy of Sciences of Ukraine. Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 3–4(384). pp. 25–29. March–April 1996. Original article submitted October 25, 1994.     

Plastic deformation,recrystallization, and sintering of nickel and tungsten powders subjected to vibratory milling

Conclusions Prior plastic working of powder particles may either activate or deactivate the recrystallization and sintering processes, depending on the nature of the mechanical action to which they are subjected, the physical properties of the material of the powder, the starting condition of the particles, and the type of the dislocation structure forming during the plastic working. In the nickel powder investigated, which in the starting condition is characterized by a comparatively high density of dislocations and is a ductile material, the level of stress and temperature generated during vibratory milling leads to the formation of a dislocation structure impeding the recrystallization and sintering processes. With the tungsten powder investigated, which has a comparatively low starting density of dislocations and a high shear stress, vibratory milling results in the formation of a dislocation structure which activates both the recrystallization and sintering processes. Prolonged vibratory milling of tungsten gives rise also to the formation of a polygonized structure hindering the thermally activated recrystallization and sintering processes.Translated from Poroshkovaya Metallurgiya, No. 9 (189), pp. 7–15, September, 1978.     

Mechanism and sintering kinetics of moistened Y—Ba—Cu—O powders

The influence of preliminary moistening of YBa₂Cu₃O_7−x powder on the kinetics of sintering and recrystallization, and the evolution of element and phase compositions in the bulk and on grain boundaries, was studied by the methods of scanning electron microscopy, x-ray microprobe analysis, and x-ray diffractometry. It was established that moistening the powder leads to a change of the sintering mechanism from bulk diffusion (for unmoistened powder) to diffusion through solid layers in the grain boundary and subsurface regions of moistened powder, formed from the interaction products of YBa₂Cu₃O_7−x with the moist atmosphere. The volume diffusion coefficients in YBa₂Cu₃O_7−x, and the diffusion coefficients in the solid layers, were calculated from the sintering kinetics data. The diffusion coefficients in the solid layers were 2–3 orders of magnitude hither than the volume diffusion coefficients. This results in more rapid sintering of the moistened than the unmoistened powders. Kharkov University. Institute of Monocrystals, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 1–2, pp. 50–54, January–February, 1997.