Tis 05 TiNs 335 kJmol 3Si 22 750 kJmol

The third main type of CS is reduction combustion synthesis, described by:

is an oxide that reacts with a reducing metal i (e.g., Al, Mg, Zr, Ti), resulting in the appearance

of another, more stable oxide J , and reduced metal i

. This reaction may be followed by the p(M)

interaction of i with other elemental reactants i to produce desired products 3 . Thus, in general, the reduction combustion synthesis may be considered a two-step process, where the first step is a thermite reaction:

and the second step is the synthesis from elements similar to Eq 1:

with the total heat release Q = Q1 + Q2. An example of this type of CS is:

_l>2 a + 2A1(S) + Ti(s) = Al2 3 + llö2 + 700 kJ/mol where TiB2 is the desired product and Al2O3 can be removed (e.g., by centrifugal separation) and used separately, or a ceramic composite material (Al2O3 + TiB2) can be produced. In some cases, the reducing reactant (Z) is the same as that used for the synthesis (X), for example:

The number of products synthesized by CS increased rapidly during the 1970s and 1980s, and currently there are more than 400 different compounds. Specifically, these materials include carbides (TiC, ZrC, SiC, B4C, etc.), borides (TiB2, ZrB2, MoB2, etc.), silicides (Ti5Si3, TiSi2, MoSi2, etc.), nitrides (TiN, ZrN, Si3N4, BN, AlN), and intermetallics (NiAl, NisAl, TiNi, TiAl, CoAl, etc.).

Throughout this work, a greater emphasis is placed on the SHS mode of synthesis, because more information is available for it than for the VCS mode. Also, the production of powders by gas-phase combustion synthesis processes (Ref 13, 14) is not considered in this article.

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