Introduction
Due to its high thermal conductivity (up to 320 W/(m·K) in theory), low
thermal expansion coefficient matched with semiconductor materials such
as Si and GaAs (4.2 × 10-6 /K, Si and GaAs: 3-4 ×
10-6 /K, 25-200 °C), low dielectric constant (1 MHz,
8.0) and dielectric loss, as well as good electrical insulation
properties, aluminum nitride (AlN) has become one of the most ideal
ceramic substrates and electronic packaging materials [1-3].
High-property AlN ceramic substrates cannot be fabricated without
high-quality AlN powder raw materials of high N concentration and low O
and C concentrations. Currently, the carbothermal reduction nitridation
process (CRN) is the mainstream technology used around the world
[4-6] for synthesizing AlN powder in batch quantities. Other
techniques such as direct nitridation, self-propagating high temperature
synthesis and sol-gel method have been also widely used [7-10].
The CRN process in AlN powder synthesis is carried out in terms of a
gas-solid reaction, including two sub-processes of the reduction
reaction of Al2O3 by C and the reaction
of the reduction products with N2 to form AlN [11].
In detail, as the reaction temperature increases above 1500 °C, the
reaction between the carbon black and the
Al2O3 powder takes place to produce
vapors of Al(g) and/or low-valent oxides of Al, such as
Al2O(g). The reduction products of Al(g) and/or
Al2O(g) have high reactivity and react rapidly with
N2 to synthesize AlN [12]. The concentrations of the
C and O impurities are the main factors affecting the quality of the
synthesized AlN powder. Generally, the C impurity content in the AlN
powder comes from residual carbon black [13], while the O
impurity content is derived from
the residual Al2O3 and O atoms dissolved
in the AlN crystal lattice in the CRN process [14]. The O atoms
dissolved in the AlN crystal lattice occupy the N atom positions, which
induce Al vacancies and generates a strong phonon scattering effect. The
thermal conductivity of the AlN ceramics is therefore sharply reduced
[15]. Almost no other sources of C and O impurities have been
reported in the CRN process.
Regarding the solid-state reaction process, if the thermodynamic and
kinetic driving forces of the reaction are inadequate, one or more
mesophases will be formed. Using
the reduction reaction of Al2O3 with C
as an example, Al4C3 is first formed,
and then it reacts with Al2O3 to form
Al2OC and/or Al4O4C
[16]. AlN powder is synthesized by the CRN process employing C and
Al2O3 as the main raw materials. It is
unknown which of the following compounds, Al2OC,
Al4O4C, or
Al4C3, acts as the mesophase in the CRN
process, so far. If these mesophases form in the CRN process and exist
in the AlN powder, the O and C concentrations of the AlN powder are
highly increased and the N concentration of the AlN powder is decreased,
accordingly. The quality of the AlN powder is therefore highly degraded.
It is of great value to study the formation and characteristics of the
mesophase in the AlN powder synthesized by the CRN process both
theoretically and industrially.
In the current work, high-purity and ultrafine
Al2O3 powder and nanoscale carbon black
are used as raw materials to prepare AlN powder in batch quantities via
the CRN process. Taking the AlN powder of low N content and high C and O
contents as samples, its composition, morphology, crystal structure, and
phases were investigated to identify
the mesophase in the AlN powder.
Furthermore, the formation mechanism and condition of the mesophase was
determined via thermodynamic calculations. Accordingly, an improved
process plan was proposed to eliminate the mesophase in the AlN powder
in the CRN process. Finally, the AlN powders of different C and O
contents synthesized in this work were used as raw materials to prepare
bulk AlN ceramics via a commonly-used process of tape casting and
subsequently pressureless sintering [17, 18]. The thermal
conductivities and bending strengths of the AlN ceramics were mainly
concerned to verify the significant improvement of the AlN ceramic
performance by reducing the C and O contents through eliminating the
mesophase in the AlN powder. This study is believed to provide important
background for the production of high-quality AlN powder in the CRN
process, both theoretically and experimentally.
Material and methods
Synthesis of the AlN powder
TABLE 1 Characteristics of ultrafine
Al2O3 and nanoscale carbon black powders