What are the Main Properties of Silicon Nitride

(What are the Main Properties of Silicon Nitride)

The crystal structure of silicon nitride

Si3N4 has two crystal structures: α-Si3N4 is a granular crystal, and β-Si3N4
is a needle-shaped crystal. Both are three-dimensional spatial
networks composed of [SN4] tetrahedra sharing vertices, and both belong to the
hexagonal crystal system. Their difference lies in the arrangement order of the
[SiN4] tetrahedral layers. The β phase is composed of almost completely
symmetrical six [SN4] tetrahedrons composed of hexagonal ring layers
superimposed in the c-axis direction; while the α phase is composed of two
deformed and different non-hexagonal ring layers superimposed. The α phase can
dissolve oxygen in the crystal structure, and its internal strain is larger than
that of the β phase, so the free energy is higher than that of the β phase. From
a thermodynamic point of view, the beta phase is more stable at higher
temperatures. The α phase has low symmetry and is easy to form. At about 1500°C,
the α phase undergoes a reconstructive transformation and transforms into the β
phase. This transformation is irreversible, and the existence of certain process
conditions and qualities is more conducive to the transformation of α phase to β
phase. α-Si3N4 is formed when it is lower than 1350℃, and β-Si3N4 can be
directly prepared at a temperature higher than 1500℃.

Basic Properties of Silicon Nitride

The molecular formula of silicon nitride is Si3N4, of which Si accounts for
60.06% and N accounts for 39.94%. There is a strong covalent bond between Si and
N (in which the ionic bond only accounts for 30%), so Si3N4 has high hardness
(Mohs hardness 9), high melting point and stable structure.

Chemical stability of silicon nitride

Si3N4 is a thermodynamically stable compound. Silicon nitride ceramics can be
used up to 1400°C in an oxidizing atmosphere and up to 1850°C in a neutral or
reducing atmosphere.

Silicon nitride is stable to most metal solutions, not corroded or wetted,
such as Al, Sn, Pb, Bi, Ga, Zn, Cd, Au, Ag, etc. But for Cu solution, it is not
corroded only in vacuum or inert atmosphere; Mg can weakly react with Si3N4;
silicon solution can wet Si3N4 and erode it slightly; transition element
solution can strongly wet Si3N4 and form silicide with Si And quickly decompose
silicon nitride, while escaping N2. Si3N4 is very stable to alloy solutions such
as brass, hard aluminum, nickel-silver, etc., and has good corrosion resistance
to cast iron, medium carbon steel, etc., but is not resistant to corrosion of
nickel-chromium alloys and stainless steel.

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(What are the Main Properties of Silicon Nitride)