L. I. Chernyshev

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.

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.

The Porous Structure of a Permeable Nickel Material with a Bimodal Pore-Size Distribution

An SIAMS-600 computer system has been used to analyze the biporous structure of a nickel material formed with the use of a pore-forming agent. The pore size distribution is narrower in a material in which the size of the pore-forming agent particles is smaller, which is due to the better mixing of the metal and pore-forming agent powders. These materials have an anisotropic porous structure, with the pores having a shape elongated perpendicular to the pressing direction.

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.

Alloy formation during the liquid-phase sintering of tungsten-base powder composites

Conclusions1.It was demonstrated that an activator present in a liquid phase intensifies the formation of a tungsten-base alloy much more strongly than it does in solid-phase sintering. This is attributable to a better distribution of the activator on the tungsten surface, resulting from its transport by the liquid phase, which readily wets tungsten.2.In the solid-phase sintering of a mixture of tungsten and chromium powders with a large amount of a nickel activator a redistribution of the refractory alloying element may occur between the activator and tungsten. Such redistribution is largely suppressed in liquid-phase sintering thanks to the formation in tungsten-chromium-copper-nickel pseudoalloys of two binders — a copper-base liquid-phase solution and a nickel-base solid solution.

Principles of the design of highly porous layered composites working in the bending mode

A comparative analysis is made of the spesific mechanical characteristics of a sandwich-type laminated porous composite under bending. The external layers of the composite are compact while the internal layer consists of a highly porous material made by using a pore-forming agent. The specific stiffness, strength, and yield load as a function of the volume fraction of pores Θ and the porous/compact-layer thickness ratio λ are considered. The stiffness, strength, and yield load are shown to be affected by variations of Θ and λ when the weight of the composite is constant.

Structural sensitivity of the ultimate mechanical properties of biporous materials prepared using a pore-former

The structural sensitivity of strength, fracture toughness, and ultimate strain in biporous powder molybdenum, iron, and nickel is studied. The effect of porosity and matrix characteristics of the structure on ultimate mechanical properties is analyzed.

Densification behavior of mixtures of metal powders and a pore-forming agent

ConclusionsIn an investigation into the pressing of mixtures of a metal powder with a pore-forming agent the metallic component was found to experience isostatic densification in a matrix of the powdered poreforming agent. The densification of metal powders mixed with a pore-forming agent obeys the same laws as the densification of single-component materials. Increasing the amount of a pore-forming agent in a mixture decreases, at any given pressure, the porosity of compacts.

Porous Permeable Polymers

Features of extrusion for mixtures of polyvinyl chloride and a pore-forming agent with the aim of preparing very porous materials are studied. It is shown that extrusion rate depends on the particle size of the pore-forming agent with the same volume content. This is explained by the difference in the structure of mixtures with respect to pore-forming agent. Marked anisotropy is established for the porous structure of materials in the extrusion direction and perpendicular to it. The hydraulic and mechanical properties of porous materials are determined and possible fields for their application (aeration in fish breeding, capillary transport for under-root irrigation, etc.) are suggested.