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Invited Lectures

IL-1: Mechanochemistry and Chemical Activation: Modern Trends

V.V. Boldyrev

Institute of Solid State Chemistry and Mechanochemistry SO RAN, Russia

1. Introduction. Comminution and mechanical activation. Mechanochemistry and mechanical activation. Mechanical activation as the method of production of metastable solids. Comparison with other methods of the production of metastable states.

2. Mechanical activation of monocomponent systems. The various kinds of metastasble states obtained as a result of mechanical activation. Physical and chemical properties of the mechanically activated solids.

3. Mechanical activation in the polycomponent systems. Processes on contacts between particles as a result of mechanical activation. Soft mechanochemical synthesis. Hydrothermal processes during mechanical treatment of liquid - solid systems. Self-propagating high-temperature synthesis. Initiation of reactions at the contacts between the particles by heat evolution.

4. Application of mechanochemistry and mechanical activation in industry. Selection of apparature for mechanical activation. Mechanochemical technologies as an example of "dry technologies".

5. Problems to be solved. Perspectives of development.

IL-2: Reactive Infiltration Processing

N.Claussen

Advanced Ceramics Group, Technische Universitaet

Hamburg-Harburg,Germany

Reaction forming of ceramics and ceramic composites has proved to be a low-cost, fast and simple manufacturing route to high-performance components. In many cases, near-net shapes are obtainable, a fact which becomes an increasingly important feature. Al-alumina composites made by the DIMOX or RMP (reactive melt penetration) process exhibit improved fracture toughness and thermal shock resistance, which, however, can be more readily be achieved by just infiltrating Al into porous alumina preforms. For a number of applications though, Al-containing composites do not meet the required properties such as refractoriness, stiffness, or errosion resistance. Therefore, other metal-ceramic systems have been studied like Ni₃Al-Al₂O₃ which exhibits high fracture toughness even to temperatures as high as 800°C. However, direct liquid infiltration of such refractory metals into precursors is complicated and expensive, partially because of the high infiltration temperatures and the costs of the prealloyed metals. An acceptable solution seems to be reactive infiltration.

In the present paper, reactive infiltration techniques are reviewed with respect to their special features and advantages. Methods that make use of low-temperature ( < 600°C) processing, which then result in composites having melting points >1000°C, are emphasized, e.g., Al₂O₃-aluminide alloys (i-3A).

IL-3: Self-Organization and Dissipative Structures in Materials Science

Yu.D.Tret’yakov

Higher School of Materials Science, Moscow State University,

119899 Moscow, Russia

Dissipative selforganization is a dynamic process which can occur in open, highly nonequilibrium systems. Conditions of selforganization appearance followed by dissipative structure formation in the open systems can be defined as follows:

- the deviation form the equilibrium state should exceed a critical value, i.e. the system must be in the bifurcation region;

- the volume of the system must be sufficiently large and reach a certain critical value that provides the necessary quantity of undamped fluctuations, whose interaction will cause ordering in the system;

- positive feedback should exist in the system.

There is a great variety of dissipative structures with scaling properties, which more often form in solids and include products of martensite transformation, eutectoid and peritectoid reactions, as well as dendrite structures in metallic and nonmetallic systems. The growth of dendrites from solution and melts is promoted by the instability of the interface at certain critical supersaturations, If, due to fluctuations, the interface becomes curved, the rate of its movement still increases with the decrease of the radius of interface curvature.

At present, perhaps, nobody doubts that such important phenomena as the plasticity and yield of materials cannot be described adequately on the basis of purely equilibrium concepts. We should point out that any mechanical failure of a material under load is necessarily preceeded by ordering of planar defects with the formation of dislocation pile-up in the region of future rupture of the material. Application of periodic loads to a metallic rods leads to the formation of dislocation pile-ups arranged in a certain way.

A whole class of phenomena accompanying so-called explosion crystallization of metastable solid phases can also be classified as self-orgaization. Techniques for achieving highly nonequilibrium conditions at the stages of formation and quenching of a metastable phase have been developed. Crystallization experiments with considerable temperature and pressure gradients yielded crystal modifications characterized by lower enthropy in comparison with samples synthesized under nearly equilbrium conditions. On going beyond a critical rate of energy delivery in the solid-state treatment of boron nitride, its graphite-like polymorph (BN)g was transformed into a new, wurtzite-like modification (BN)w - II, whose enthropy is lower than that of (BN)w - I, which forms at lower rates of energy delivery.

It should be noted that the formation of high-Tc superconductive phases upon melt solidification proved to have a periodic character. The specific character of YBa₂Cu₃O_7-xcrystallization has been confirmed by computer modelling using “cellular automata” software.

IL-4: Ecology and Advanced Technologies

V.V.Alekseev

Moscow State University,119899 Moscow, Russia

There are two importal ecological problems. First problem is the problem of energy. It is well known that use of coal, oil and gas as main sources of fuel causes intensive environmental impact: green-house effect is controled by carbon emission. It is well known from palaeoclimate records and modelling studies that the climate system has more than one equilibrium state, and that perturbations can trigger transitions between them. Anticipated future changes in today’s climate system due to human activities have the potential to weaken the thermohaline circulation of the North Atlantic Ocean and to became cool the climate of Europe[1].

At first nuclear energetics seems to be the carbon problems solution as far as thermonuclear synthesis or full burning of natural uranium with the help of bridders. But in this case the problem of receiving nitrogen less steel must be solved because C¹⁴ appears as the result of nitrogen and neutrons contact. Half-time of C¹⁴ is about 6000 years[2]. The second aspect of the problem is that C¹⁴ has the accumulation coefficient in living organisms compared to environment is about 1000-10000 times. Carbon is the principle constraction element of living organisms including genetic material. That is why radiactive decay effect in many times exeeds that of strontium as well as cezium, which are external for the genetic nuclear materials[3].

The second problem is problem of fresh water. SHS can be to use for decision of there problems. For restoration Si from diatomit we can made SHS. It is necessary for PV-solar. It developed homogeneus and graded SHS filters. They allow to purify drinking water from impurities of light and heavy metals[4].

References:

1.Thomas F.Stocker & Andress Schmittner// Influens of CO₂ tmission rates on the stability of the thermohaline circulation., Nature,1997, v.388, p. 862-865.

2.Ivanov L.I. On the problems of development of low-activation, radiaton-resistant metallic materials for atomicthermonuclear pover and space technology. In:Structural materials for fussion reactors. M., Nauka, 1993, p. 4-13 (in russian).

3.Rublevsky V.P., Golenetsky S.P., Kirdin G.S. Radiocarbon in the biosphere. Moscow, Atomizdat, 1979, p.152 . (in russian).

4.Merzhanov A.G. Theory and practic of SHS: Worldwide state of the art and the nevest results. Int. J.SHS, 1993, v.2, N 2.