1-2 SYNTHESIS OF NON-OXIDES
1-2.1 Powders
1-2-.1.1 Carbides
There are three main families:
– Alkaline metal or alkaline-earth carbides;
– Interstitial carbides with transition metals Ti, Zr, Hf, V, Nb, Ta, etc.
– Covalent macromolecular carbides with B, Al, Si.
Only compounds of the two last families are used for preparing ceramics. These
products are synthesized industrially according to various methods, the choice of
the process depending on certain criteria: purity, crystalline state, morphology of
the powder.
●Solid-Solid Reaction
Carbides can be obtained by direct synthesis at high temperature by mixing
powders compacted beforehand:
After the carburization stage, the products are ground. This process, which is the
oldest, does not yield fine powders. The carboreduction of oxides is the most
developed production method:
Grinding is also necessary, followed by a chemical treatment to eliminate oxide
traces that very often play a harmful role during sintering. Thus, for the second
example, after washing with hydrofluoric acid, the powder contains 98-99% of
SiC, with iron as the main impurity.
Sol-gel processes developed recently, in particular in the synthesis of
carbosiloxanes, yield ultrafine amorphous powders. The distribution on a
nanometric scale of the Si, C, and O elements increases their reactivity.
Amorphous silicon carbide can be synthesized between 1,000 and 1,300°C. It
then crystallizes into β-SiC at about 1,450°C.
●Gas-Gas Reaction
A large number of gas phase reactions can be developed to manufacture
carbides:
During these last few years, synthesis methods “assisted” by the use of plasma
or a lazer beam have been developed. They offer several advantages compared
to conventional methods; in particular, they lead to ultrafine multi-element
powders (mixed carbides) of uniform size (a few dozen nanometers).
1-2.1.2 Nitrides
Nitrides can, like carbides, be classified under three main families. Actually,
only interstitial (TiN, ZrN, HfN, TaN) and macromolecular covalent (Si3N4,
AlN, BN) nitrides have been developed industrially.
●Solid-Gas Reaction
The oldest production method consists of nitriding an element by nitrogen,
ammonia or N2/H2 mixtures:
Extensive research has shown that the nitriding of ultrapure silicon is very slow
because of the formation of the protective nitride coating on the grains.
“Catalysts” are necessary, generally containing iron, to achieve a complete
reaction. As starting powders have high grain size in order to avoid excessive
sintering and a blockage of the reaction, the products obtained are ground and
then purified chemically. The thermal carboreduction of oxides in the presence
of nitrogen is also used. The overall reactions are expressed below:
The yield and rate of the reaction can be enhanced by the use of ammonia or
nitrogen/ammonia mixtures. The powders manufactured are finer than with the
previous process, but with a purity that varies with the conversion rate. In
particular, the excess carbon necessary for the reaction to be complete must be
eliminated by combustion in air at temperatures lower than 600-700°C to avoid
a parasitic oxidation.
● Gas-Gas Reaction
This refers to high temperature reactions between gas precursors, followed by
the formation of solid particles by germination-growth:
As for carbides, this process which leads to ultra fine powders has not yet
reached a stage of industrial development.
●Liquid-Liquid Reaction
Liquid medium reactions can synthesize, from mineral or organ metallic
compounds, precursors which by Pyrolysis lead to amorphous nitrides:
If this remains a laboratory process for aluminum or boron nitride, for silicon
nitride it has reached the stage of industrial production. Silicon tetrachloride
dissolved beforehand in a cyclohexane-benzene mixture reacts at –40°C with
liquid ammonia to form solid polymeric silicidimide:
Ammonium chloride is extracted by liquid ammonia washing and [Si (NH 2)]n
recovered by filtration. Pyrolysis under nitrogen up to 1,200°C leads to nitride
Si3N4, whose grain diameter is submicronic:
Lastly, we must mention the precursor-organometallic method, which is similar
to the sol-gel method for oxides. According to the degree of polymerization of
the precursors, we can manufacture by pyrolysis the following: powders,
coatings or long fibers. As regards powders, the advantage of this procedure lies
in the possibility of obtaining multi-element powders: SiC, SiCO, SiCNO,
SiBCN, etc. These alloys, which cannot be prepared by traditional methods,
exhibit a variable state of crystallization according to the experimental
conditions.