INFLUENCIA DE LAS VARIABLES METEOROLÓGICAS EN LA PROPAGACIÓN DEL SONIDO

Solid state metathesis (SSM) is an associated technique to SHS, that has been used recently in the synthesis o f materials with particles in the nanometre range. It was initially reported by R. B. Kaner and co-workers^^. SSM has been extensively used and developed by Professor I. P. Parkin^^ and numerous materials have been produced in this manner. These include nitrides, phosphides, selenides, sulphides, teleurides, borides and sUicides. Metathetical reactions involve the exchange (metathesis) o f two precursors.

TiCl3(s) + Li3N(s) —> 3LiCl(s) + TiN(s) (Eqn. 1.12)

It is usual in SSM to combine a metal hahde with an alkah metal chalcogenide to obtain various products^"^. Chalcogenides, pnictides^^ (also metal mixed pnictides and mixed-metal pnictides), silicides and nitrides have been produced using SSM. This section describes the synthesis o f ceramic materials (ferrites, oxides) using SHS and SSM techniques.

SHS and SSM are very similar. They have features in common. Both processes derive their self-propagation from the AH o f reaction. The driving force o f SSM reactions is the formation o f the co-produced salt, this accounts for most o f the reaction enthalpy (up to 90%)^^. The melting temperature o f the co-produced salt is often the temperature limiter. This therefore determines which phases o f

material are obtained. Kaner has proposed that for a propagation to take place in SSM the reaction enthalpy must be large enough to melt the co-produced salt^^. In order to break down the diffiision barrier the co-produced salt must melt. In fact, it has been noticed that if the calculated adiabatic temperature is not high enough to melt at least a fraction the co-produced salt, no propagation reaction will take place.

Essentially, SSM reactions rely on the fact that an unstable combustion must be achieved in order for a propagation wave to be observed. This limits the maximum temperature that the SSM can reach, which is usually lower than in SHS. Co-produced salts formed in SSM are often easily removed by trituration with a suitable solvent^^. Similar to SHS, SSM phase products can be altered by the addition o f a diluent, sometimes termed inert heat-sink. For example, addition o f LiCl to a reaction mixture o f TiCE and LigN reduces the crystallite size o f the product^^’^*. The addition o f diluent absorbs part o f the reaction enthalpy. It also increases the path length for the diflusion o f reacting particles, thus decreasing the reaction temperature. One example o f how a product phase can be changed is for the reaction between WCU and LigN. In the absence o f diluent only elemental tungsten was produced. However, if a diluent was added WN was produced^^.

There are two proposed mechanistic pathways for solid-state metathesis; direct ionic metathesis and reductive combination metathesis^^. In the direct ionic pathway intermediates which are ions are thought to be created that permeate in the melted co-produced salt (Eqn. 1.13). In the reductive combination reagents are reduced to their elemental form and recombined due to the high reaction temperatures (Eqn. 1.14).

MCln + M’E ^ M*^ + E”- + nM’Cl ME + nM’Cl (Eqn. 1.13)

MCln + M’E ^ M + E + nM’Cl ^ ME + nM’Cl (Eqn. 1.14)

two proposed pathways. Due to the nature o f these reactions, i.e. being very fast, it has been impossible to isolate intermediates. An example^*^’^^ o f ionic metathesis is shown in Eqn 1.15 and reductive recombination in Eqn. 1.16.

3HÆ:i4+4Li3N-^3Hf^^+4N^‘+12L iC l->H f3N 4+12L iC l-^3H fN +12L iC l+l/2N 2

(Eqn. 1.15)

3HfCl4 + 4Li3N -> 3Hf + 4N + 12LiCl ^ 3HfN + 12LiCl+ I/2N2 (Eqn. 1.16)

The derivation o f these mechanistic pathways has been deduced from end product analysis i.e. if the elemental pathway was to be preferred by the reaction some elemental material would be detected in the final product. The presence o f a strong reducing agent corroborates an ionic pathway. For example, the reaction o f lanthanide halides with magnesium nitride produces after heating for one hour (900 °C) an intermediate^^ o f Ln2Cl3N. This suggests an ionic mechanism. In essence the detection o f elemental components points to a reductive combination pathway, while the detection o f (amalgamated) intermediates such as Lu2Cl3N suggests a direct ionic metathesis. It has also been observed that lanthanide halides prefer the ionic metathesis route while transition metal halides have no preference^^.

It has also been observed that certain reactions are thermally driven since no thermoflash is observed in the bulk mode. For example^^'^^ the reaction o f M0CI5

with Na2S and Na2Se for 12 hours at 800 °C to produce Mo(SxSey).

SSM reactions have also been used in solution. Wells^^ et a l have performed SSM reactions in glyme solvents. They have found that alkali metal pnictides and gallium or indium trichloride react in a solvent to produce gallium and indium phosphides and arsenides. Some o f the starting materials were soluble in solvent while others were not. Halides were soluble while pnictides were partially soluble. However reactions required reftuxing between 2 and 12 hours. Some o f the products such as GaP, InP and GaA showed particle sizes in the nanometre range.

SSM reactions have the advantage o f having a temperature regulator in the form o f the enthalpy o f fusion o f the co-produced salt. This allows for the isolation o f specific phases which cannot be accessed by SHS (e.g. TagNg or cubic TaN)^"^. SSM reactions can also be initiated by point source propagation. Unusual micro structures can also be obtained by SSM.