Figure 1.1 - Scheme of production of bulk nanomaterials by evaporation, condensation and compaction

During evaporation and condensation of a metal with a high melting point, typically, smaller particle size. Precipitated condensate special scraper removed from the surface of the cylinder and is collected in a reservoir. After pumping inert gas into the vacuum interlocutory (a pressure of about 1 GPa) and final (at pressures up to 10 GPa) compaction nanopowder. This results in a sample with a diameter of 5 ... 15 mm and a thickness of 0.2 ... 0.3 mm with a density of 70 ... 95% of the theoretical density of the material (up to 95% for nanomaterials and to 85% for nanoceramics). Produced by this method compact nanomaterials, depending on the conditions of evaporation and condensation, are composed of crystals (grains) with an average size from a few to tens of nanometers. It should be emphasized that the creation of dense powders are close to 100% of the theoretical density of nanomaterials - the problem is very complex and has not yet been solved, as nanocrystalline powders of bad press, and traditional methods of static compaction fail.

Another method is to compaction of powders produced by the method of mechanical grinding and mechanical alloying. However, there are also problems of compacting nanopowders produced and manufacture of bulk nanostructured samples and blanks with high density.

In applying the method of heat pressing compacting powders of amorphous metallic systems (AMS) is carried out in the temperature range of T near the glass transition temperature T g, which is a significant reduction in viscosity of the amorphous alloy, a few consecutive values. At T> Tr, where Tr (≈ 0,7 Tg) - temperature of the transition from low-temperature heterogeneous plastic flow to the high-homogeneous flow, the material deforms uniformly, and the level required to achieve this stress can be greatly reduced.

However, the development of methods of compacting AMC faces serious problems - the temperature range of the implementation mechanism of the homogeneous flow of low viscosity is rather narrow and is adjacent to the crystallization temperature. Requirements must be satisfied in the process of compacting to prevent crystallization of the AMC. The successful development of methods for the synthesis of massive powder AMC and their products should be able to predict the optimal parameters (time, temperature, pressure) pressing process. The values of these parameters must be determined from the conditions for achieving the desired density compact particular alloy while maintaining the amorphous state. Establishing optimum conditions experienced by compaction - an extremely complex and time consuming task. Therefore, great importance is the theoretical analysis of heat pressing AMC, including clarification of the conditions for obtaining nanoamorfnyh solids.

The approach of the particles under external loading occurs visco-plastic flow in the surface layers of inhomogeneous stress fields in the contact interactions of the particles. At the same time there are processes of homogenization of the material of the nanoparticles. Depending on the relative size and structural parameters of the particles and their surface layers (layer thickness and the ratio of the particle radius, and the ratio of viscous-plastic properties of the material layer and the bulk of the particles), the formation of different structural states nanoamorfnyh solids.

If the concentration of excess free volume in the surface layer of the particles is higher than in the bulk, while the surface layer is enriched with impurities embrittle AMC, the viscosity of the material of the layer will be lower than the internal volume of the material and the process of sealing a compact will be due to viscoplastic flow of the material the surface layer. This will form the structure shown in Figure 1.2, a. Surface layer thickness (δ) of the particles should be at the same time sufficient to realize the growth of the contact neck and approach of the particles flow through the material of the surface layer (δ> δr). Parameter δr (r) - the distance particles approach to achieve the required density of the compact set (p) under external pressure (P). In this case, the kinetics of the compact packing material will be determined by the viscosity of the surface layers of the particles.

Otherwise (δ <δr) growth process in the contact neck seal compact will be due to the flow as a material of the surface layer of the particle, and the main material of its volume. In this case, the calculation of the kinetics of densification should use an effective viscosity of the material in contact neck, defined by the rule of mixtures by viscosity material layer and the internal volume of the particle. This will form the structure of the type shown in Figure 1.2b. Subject to the more complex processes such as the formation of regions with different parameters and composition of the topological order on the border of contacting particles (Figure 1.2c).

During compaction compact are also homogenization of particle structure.

For the compact materials with low porosity are used by hot pressing, when pressing occurs simultaneously with sintering. In this case, the pressing pressure is reduced tenfold compared to cold-pressing. Temperature of hot pressing, depending on the nature of the sintered material is in the range 50 ... 90% of the melting temperature of the main component. However, the increase in temperature leads to a rapid compaction of grain growth and yield of nanostructured state and the consolidation of nanopowders at low temperatures, even at high applied pressures, leading to a residual porosity. Moreover, the problem remains-contamination of samples in the preparation of powders and especially the increase in their geometric dimensions.

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Figure 1.2 - Structure of solids state nanoamorfnyh