Valerii V. Skorokhod

Computer Simulation of Spherical Particle Sintering

A mathematical description is given of the sintering kinetics for an ensemble of particles on the basis of a unified phenomenological approach, which has been used in a computer model for spherical particle sintering. A series of computer experiments has been performed to check the fit of the model to the physical process. The sintered spherical particle packing has a cluster organization. It is confirmed that the mesostructure is inherited when monodispersed spherical particles are sintered. The computer experiment showed that the phase boundaries initiate macropore nucleation and growth. The sintering rate and the contact area are critical factors determining macropore size and position.

Layered Composites: Structural Classification, Thermophysical and Mechanical Properties

The most important properties of multilayer composites that are promising as structural and functional materials in view of their geometric structural characteristics are considered. A structural-geometrical classification of unidimensional composites is provided. Thermal conductivity and expansion of layered ceramic-metal composites taking account of the inherent thermal stresses are considered in detail, and conditions for loss of integrity at interlayer interfaces are analyzed. Special attention is devoted to the anisotropy of layered systems. Methods are described for calculating the effective values of thermophysical and elastic properties for unidimensional composites along and across layers, the upper and lower limits of properties are evaluated, and cases of deviation from additivity of these properties are analyzed. Some features are considered for failure of ceramic-metal composites with static and impact loading, and also the promising nature of their application as shock-resistant and heat-resistant high-temperature materials in structural elements that operate under extreme heat and mechanical conditions.

Mechanism of the sintering of high-porosity materials in the presence of a volatile pore-forming agent

Conclusions1.During the sintering of compacts with a filler, their volume shrinkage is a result of the densification of the regions with the fine natural pores, but their macroporosity remains unchanged (except in the case of compacts of very high starting porosity).2.A matrix mixture of metal and voids predominates in the structure of porous solids produced with a coarse filler, while a statistical mixture is characteristic of the structure of solids produced with a fine filler. A formula is proposed for calculating the statistical weight of the matrix mixture in the structure.3.The differences in the structures of porous materials associated with the particle size of the filler lead to differences in the structural weakening of these materials (the volume viscosity is less for porous solids with a mainly statistical distribution of the large pores), and this results in a decrease in macroporosity during the sintering of compacts with a fine filler.4.The growth of interparticle contacts during the sintering of compacts with a filler is analogous to the growth of contacts in unpressed powders being sintered and is directly determined by the kinetics of the volume shrinkage of the porous solids.

Reversible Hydriding of LaNi5 − xAlx ― Pd Composites in the Presence of Carbon Monoxide

Carbon monoxide is one of the most dangerous impurities in gas mixtures involved in the hydriding of intermetallic compounds, and up to the present time the concentration limit 0.03 vol.% CO has not been surmounted. The objective of the present work was to investigate the possibility of reversibly hydriding LaNi5 − xAlx ― Pd composites in gas mixtures containing up to 5 vol.% CO. The experimental specimens were mixtures of palladium black and LaNi5, LaNi4.7Al0.3, LaNi4.5Al0.5, LaNi4.2Al0.8, and LaNi4Al powders cold pressed at 300 MPa. The intermetallics were preliminarily dispersed by hydriding and mechanical grinding. The concentration of palladium black in the powder mixtures was varied from 0 to 1.5 mass%. It was shown that it is possible to reversibly hydride LaNi5 − xAlx ― Pd composites in mixtures of hydrogen and carbon monoxide containing up to 5 vol.% CO at the temperatures 423 and 473 K. It was established that alloying LaNi5 with aluminum in amounts which raised the stoichiometric concentration above 0.5 stabilized the absorption properties of LaNi5 − xAlx ― Pd in the presence of CO. The composition LaNi4Al was optimal. Palladium additions to LaNi5 − xAlx ― Pd composites also increased the stability of hydrogen absorption by the intermetallic. It is noted that the effect of palladium was more positive than that of aluminum.

Permeable tungsten-copper materials

Conclusions1.A porous permeable tungsten-copper composite material can be produced by partially infiltrating with copper a high-porosity tungsten skeleton sintered with a volatile pore-forming agent.2.It has been established that during partial infiltration of a tungsten skeleton possessing a biporous structure the fine pores become completely filled with copper, while the large pores remain unfilled. As a consequence of this, at any given porosity a tungsten-copper material has larger pores and greater gas permeability than porous tungsten.3.At any given porosity the transverse rupture and impact strengths of a tungsten-copper material are much higher than those of porous tungsten.4.The surface of a porous tungsten-copper material becomes coated with a cupric oxide film which protects the material against high-temperature oxidation wear.

Effects of Porous Structure on the Electrical Conductivity of Highly Porous Metal-Matrix Materials

It is shown that high-porosity materials based on tungsten and nickel that have been made with the use of pore-forming agents show an increase in specific conductivity as the ratio between the dimensions of the metal particles and those of the pore-forming ones increases. This result is explained from the viewpoint of the fractal dimensions and the percolation threshold.

Reaction of Tungsten with Liquid Co ― Sn Alloys

The solubility of tungsten in Co ― Sn melts and the growth kinetics of a W6Co7 phase layer at the tungsten ― melt interface were studied at 1200°C. The liquid alloys composition in the three-phase equilibrium W ― W6Co7 ― melt was established as (at. fraction) 0.51 Co, 0.49 Sn, 2.3·10−3 W. The solubility of tungsten in the investigated range of melt compositions is well represented by the equation lgxW = −0.964-3.420xSn, where xW and xSn are atomic fractions of the elements in the melt. The calculated thermodynamic properties can be used for the analysis of other systems which include cobalt and tungsten.

Synthesis and Sintering of Nanocrystalline Barium Titanate Powder under Non-Isothermal Conditions. Part 6. Structure, Grain Boundaries, and Dielectric Properties of Barium Titanate Obtained by Various Sintering Methods

The grain boundary structure of barium titanate obtained by controlled-rate sintering and high-pressure sintering (HPS), and the dependence of dielectric properties on grain size and consolidation method were studied. It was shown that sintering without the application of pressure leads to a diffusion-controlled formation of equilibrium grain boundaries with minimal impedance factor, which minimally decrease the dielectric constant of the ceramics. HPS results in the formation of non-equilibrium grain boundaries which have a large free volume, and which substantially decrease the dielectric constant. The Curie-Weiss constant was analyzed from the viewpoint of a matrix structural model, and a «brick-wall» model.

High-porosity tungsten-copper materials produced by liquid-phase sintering

Conclusions1.Use of a coarse pore-forming agent for the stabilization of a porous structure ensures the retention of a high final porosity in the liquid-phase sintering of tungsten-copper materials.2.The volume changes occurring during the liquid-phase sintering of high-porosity tungsten-copper materials can be virtually eliminated by combining the sintering operation with the infiltration of the fine pores in biporous tungsten compacts with copper from external or internal sources.3.During the infiltration of a biporous tungsten compact from internal sources the copper component becomes redistributed, as a result of which a coarse porous structure forms in the material.

Deformation Features for Highly Porous Metallic Materials

Ultralight metallic structural materials based on foamed nichrome and cellular sandwiches made from stainless steel are prepared with a porosity of 90-95% and high specific strength indices are obtained. Effects of loss of stability and nonuniform deformation over the height are detected with compression of foamed nichrome and nickel, and they are duplicated with radial deformation of model honeycomb structures. This makes it possible to establish that on the basis of these phenomena there are local structural arches in the material. A model is suggested for an arched structural element and the elastic problem is resolved for its deformation that adequately describes the effect of loss of stability.