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Amorphous, nano- and microcrystalline materials: production, structure and possible practical application.
The unique properties of submicro- and nanoscale particles have long been known and widely used, for example, in catalysts, additives to motor oils, microelectronics, etc. It is known that a decrease in crystallite sizes in metals and alloys below the corresponding threshold value leads to a significant change in their properties. Such effects occur when the average size of the crystal grains does not exceed 100 nm, and is especially noticeable when the size of the grain or particles is less than 10 nm.
Synthesis methods. The existing methods for producing of nanoparticles can be divided into chemical and physical methods. Among the chemical methods, the sol-gel method, chemical vapor deposition and chemical precipitation in liquids and salt reduction, thermal decomposition (pyrolysis), and plasma-chemical synthesis are often used. As for the physical methods of obtaining amorphous, nano- and microcrystalline materials, the promising methods are the electric-spark treatment of metal granules in aqueous and organic media, the sputtering of electrode materials in a plasma discharge, the method of gas and water spraying of liquid metals (alloys), and the casting of liquid metals (alloys) on a water-cooled copper disk. For the production of compact nano- and microcrystalline materials, the use of bulk amorphous alloys is promising.
Types of materials and possible practical application. Due to their active surface, nanoparticles have an enhanced ability to interact intensively with the environment. Uncoated metal nanoparticles are oxidized and agglomerated. Coating of nanoparticles with inert materials can preserve their special properties and prevent oxidation. At the same time, uncoated particles with the active surface can be effective in modification the structure of metals and alloys. Electric plasma discharge in an organic liquid under ultrasonic irradiation produced nano-, microcrystalline, and amorphous particles from different metallic materials (WC1-x i W2C, Fe3C, χ-Fe2.5C, Co3C, CoCx, Fe38Pt62), which were encapsulated in graphite shells (so called carbon nanocapsules). Hollow carbon shells and graphite nanosheets were produced by the same method of plasma discharge in an organic liquid. The range of application of carbon nanocapsules with magnetic cores, hollow carbon shells, and graphite nanosheets are as follows:
- for information recording media;
- as anode material in lithium-ion batteries;
- as a contrast agent for magnetic resonance imaging;
- in ferromagnetic fluids;
- in magnetic hyperthermia;
- for targeted delivery of drugs;
- transparent graphite nanosheets can be used for the manufacture of electrodes in various optoelectronic devices and for the manufacture of conductive polymer nanocomposites.
It is shown that it is possible to use the obtained metal nano- and microparticles to modify casting alloys, on condition that the parameters of the crystal lattices of the nanoparticle and the modifying alloy coincide. In addition, carbon nanocapsules with tungsten carbide cores can be used as highly rigid reinforcing phases in composite antifriction materials. High specific reactive surface of tungsten carbide nanoparticles can be used in catalysis, as a substitute for noble metals (Pt, Pd, Ir). Electrochemical measurements of the obtained tungsten carbide nanoparticles distributed in an amorphous carbon matrix showed a rectangular shape of cyclic voltammograms with specific capacitance of 92.5 F/g. Hence the prepared material has potential application as an electrode material for electrochemical supercapacitors.
Microcrystalline modifiers. The microstructure and mechanical properties of cast alloys can be improved by disperse hardening and modification. It is known that the maximum disperse hardening of alloys is observed when the strengthening phase has nano-dimension, and the maximum modification effect is possible in the case when the modifier and the base alloy are structurally coherent (have the same crystal structure and lattice parameters). Methods for obtaining of highly efficient modifiers:
- the heat treatment of amorphous alloys in order to produce nanostructured intermetallic compounds;
- the decomposition of saturated solid solutions obtained from rapidly quenched alloys in the form of powders and fibers, that leads to the formation of highly dispersed inclusions.
The alloy systems of Al-Zr, Al-Cu-Zr, Al-Cu-Ni-Zr were chosen for the development of promising modifier compositions in amorphous and fine-grained states. The examples of successful modification of selected wrought and cast aluminum alloys by the developed metastable compound Al3Zr are shown.