shock wave synthesis (rus. синтез, ударно-волновой otherwise детонационный синтез ) — a method of mechanical shock-wave effects which represents a rapidly proceeding process creating a dynamic environment for the synthesis of the final product and its dispersion into a powder with nanometre-sized particles.

Description

Detonation synthesis is used to produce different morphological forms of carbon, mainly nanocrystalline diamond powder (nanodiamond), and nanopowder oxides of different metals: Al, Mg, Ti, Zr, Zn, etc.

When producing diamond nanopowders from mixtures of graphite with metals, shock impact lasts not longer than 10-20 microseconds at pressures as high as 20-40 GPa. A more efficient method to produce diamond powders consists in explosion of organic substances with high carbon content and relatively low oxygen content, i.e. detonation of condensed explosives with negative oxygen balance; in this case, the explosion released free carbon which forms into the diamond phase. There are two options to arrange detonation synthesis of diamond powders from fused carbon-containing explosives with negative oxygen balance: in "dry" synthesis of diamond nanoparticles explosion products expand into an inert atmosphere and are cooled in the gas phase; in "water" fusion, diamond particles are cooled down with a water cooler.

The pressures of up to hundreds of thousands of atmospheres and temperatures up to several thousand degrees characteristic for the detonation process correspond to the diamond phase thermodynamic stability region on the P-T diagram of possible states of carbon. At the same time, in the detonation synthesis, where the duration of high pressures and temperatures required to form a diamond is very short, an important role is played by the kinetics of formation and growth of diamond phase's nucleation centres. Usually, diamond nanopowders are produced with the use of a mixture of TNT and RDX in a weight ratio of 1:1 or 3:2. For such mixtures, the pressure and temperature in the detonation wave reach P > 15 GPa and T > 3000 K. In the "dry" detonation synthesis the process is carried out in special explosion chambers filled with an inert gas or carbon dioxide, which prevents the oxidation of formed diamond particles and their transformation into graphite. The formation of diamond nanoparticles takes about 0.2-0.5 ms because in the detonation synthesis, at a very short time required to form the particles, their growth rate is by several orders of magnitude higher than for static conditions. After the explosion, condensed synthesis products are collected and processed in hot mineral acids under pressure to remove soot and other impurities, repeatedly washed in water and dried. In this method, the yield of diamond powder is 8-9% of the initial mass of explosives. A characteristic feature of diamond nanopowders produced by detonation synthesis is an extremely small dispersion of nanoparticles' sizes: the bulk of the particles have a size of 4-5 nm.

When source materials for detonation synthesis are metals or chemicals, the process requires the use of a gas or liquid environment chemically neutral to the final product, an environment which is also intended to ensure rapid cooling of produced materials and stabilise its high-temperature and metastable crystalline modifications. In this case, a layer of source substance (highly porous metal medium, chemical compound, sol or gel of metal hydroxide) is exposed to the shock-wave impact of the explosive. The shock wave causes compression and heating of the highly porous metal or decomposition of the parent compound to oxide with subsequent stabilisation of oxide phases. After the shock wave reaches the free surface of the source material, the latter scatters into the explosion chamber's gas atmosphere or the liquid coolant.

In the detonation synthesis of metal oxide powders an oxygen-active medium (e.g., O2 + N2) is used. Burning of metal with the formation of oxide occurs at the stage of expansion. In carbon dioxide atmosphere carbon nanotubes and spherical carbon nanoparticles can be synthesised.

Illustrations

Phase 
Phase p–T-diagram of carbon states indicating diamond synthesis regions: 1 — detonation synthesis involving graphite; 2 — static transformation with a catalyst; 3 — static transformation without a catalyst; 4 — detonation synthesis involving TNT and hexogen.

Author

  • Gusev Alexander I.

Sources

  1. Gusev A. I. Nanomaterials, Nanostructures, and Nanotechnologies (in Russian) // Fizmatlit, Moscow (2007) - 416 pp.
  2. Gusev A. I., Rempel A. A. Nanocrystalline Materials. — Cambridge: Cambridge International Science Publishing, 2004. — 351 p.

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