2. Spark plasma sintering of SiC powders produced by
different combustion synthesis routes
Silicon carbide (SiC) is an important ceramic material with properties
such as low density and high hardness, high strength at elevated
temperatures ,excellent thermal conductivity, and good wear and
corrosion resistance make SiC a highly attractive material for a variety
of applications.
it is not an easy task to manufacture fully dense bulk SiC specimens
by conventional sintering, which requires high temperatures (>2000 K)
and long duration of the consolidation process.
3. Spark plasma sintering(SPS)
The pulsed DC current directly passes through the graphite die, as well as the
powder compact, in case of conductive samples. Joule heating has been found
to play a dominant role in the densification of powder compacts, which results
in achieving near theoretical density at lower sintering temperature compared
to conventional sintering techniques.
The heat generation is internal, in contrast to the conventional hot pressing, where
the heat is provided by external heating elements. This facilitates a very high heating
or cooling rate (up to 1000 K/min), hence the sintering process generally is very
fast (within a few minutes).
The general speed of the process ensures it has the potential of densifying powders
with nanosize or nanostructure while avoiding coarsening which accompanies standard
densification routes.
4. In this work silicon carbide (SiC) powders were synthesized by four different
combustion-based routes:
chemical activation of reaction by adding of polytetrafluoroethylene
(Teflon),
mechanical activation of Si–C powder mixture,
in ternary Si–C–N2 system, and
reduction reaction
with the four as-synthesized SiC powders being abbreviated as SiCT,
SiCMA, SiCN, and SiCR, respectively
5. Route 1: Si/C/Teflon (SiCT)
Milling of t Si powder (−325 mesh) to reach
the average particle size of ∼5m with 90% of less than 10m is necessary
The Si/C/Teflon powder mixing procedure involved two steps
first step was to mix as-milled Si powder with carbon lampblack powder
for 6 h in a rotary mixer
second step included 12 h mixing of the Si +C powder mixture with Teflon
(∼18 wt.%)
keep the mixture in SHS reactor in presence of argon atmpsphere.
After combustion synthesis the reactor was allowed to cool down to room
temperature before collecting the as-synthesized powder. The as-synthesized
SiC sample pellet was then pulverized by agate mortar and pestle for 5 min
and subjected to analyses, as described below.
6. Route 2: Si/C (mechanical activation)
(SiCMA)
• stoichiometric ratio (1:1) of silicon and carbon lampblack powders was
used as the reactants.
• After the milling jar was sealed, it was outgassed and purged with argon
gasto ∼0.2 MPa.
The milling ball (2 mm) to powder ratio was 40:1 and the rotation speed
(planetary ball mill Retsch PM100, Germany) was 650 rpm and the process
duration was 1 h.
After ball milling the jar was allowed to cool down to room temperature
and then the prepared powders were collected.
Combustion synthesis of the mechanically activated mixture was carried
out by using the same procedure, as described for route
7. Route 3: Si/C/N (SiCN)
• Similar experimental procedures, as described for route 1,
were performed for this route.
• the following mixing conditions were used: milling ball to
powder ratio 5:1, rotation speed 300 rpm and process
duration 30 min.
• Also, instead of argon, nitrogen (99.998%; Praxair Distribution
Inc., USA) gas was used and the reactor was pressurized up to
3 MPa before reaction initiation
8. Route 4: SiO2/C/Mg (SiCR)
The desired ratio (1:2:1) of SiO2, Mg and carbon lampblack (SiO2 +
2Mg +C→2MgO + SiC) powders were mixed in an ethanol bath
(∼100 ml) for 6 h and then filtered and collected.
The loose powder mixtures were then placed in a quartz tube in
the SHS reactor, followed by reaction initiation in an argon
atmosphere (2 MPa).
Synthesized product was then subjected to a leaching process in
diluted hydrochloric acid (5%) for 12 h to completely eliminate the
undesired MgO phase, followed by filtering and drying in an oven at
373 K.
9. The sample was wrapped in graphite tape
(0.2 mm in thickness).
A uni-axial pressure of 50 MPa was then
applied through the top and bottom
plungers.
Each plunger has a diameter of 15 mm and
a length of 20 mm. The samples were
sintered using a spark plasma sintering
system to 2073 K (1800 ◦C) with a heating
rate of 200 K/min in an argon atmosphere
11. • For SiCma and SiCt powders, only one phase (i.e., -SiC) was
detected.
• In the SiCn case, XRD showed -SiC and some amount (∼5
wt.%) of -Si3N4. The presence of Si3N4 is attributed to the
reaction of Si and nitrogen gas, which produces Si3N4 and
leadsto the preheating of the system, followed by the carbide
formation (i.e., Si +C→SiC).
• In the SiCr system, two phases (-SiC and -SiC) can be observed,
with -SiC being the primary phase.
12. SEM images of the as-synthesized and commercial SiC
powders
13.
14. gas phase reaction (4) and condensed phase reaction (5) lead to different
morphologies of the SiC product, i.e., agglomerates of extremely fine spherical-type
particles with high specific surface area and agglomerates of plate-like species .
This process allows one step without high energy ball milling treatment, posses the
highest combustion temperature (2300 K), which leads to the relatively low (∼4
m2/g) specific surface area of the as-synthesized powder
15. Advantage of using high energy ball milling to activate a combustion reaction
in the Si–C system is that the morphology of the produced SiC powder is
essentially the same as that for initial Si/C composite particles.
This route, having a relatively low combustion temperature (∼1900 K), permits
synthesis of SiC sub-micron particles of roughly spherical morphology and a
relatively narrow particle size distribution, which leads to a high specific surface area
(∼21 m2/g).
(SiCMA), whiskers represent SiC phase that is probably formed due to a small amount of
Fe impurities present in the composite particles, which make a low temperature eutectic
with silicon, providing different reaction mechanism
the Si–C–N2 combustion wave consists of two stages.
The first is the reaction of nitrogen with silicon to produce some silicon nitride, which
leads to preheating of the
This is followed by the carbidization reaction with maximum temperature around 2100
K. Formation of 5 wt.% of Si3N4 cannot sufficiently preheat the reaction media, which
suggests that one additional reaction, i.e. silicon nitride reduction by carbon.
16. • reduction type reaction allows synthesis of powder with extremely high
SSA (∼140 m2/g), which involves both -SiC and -SiC phases.
• Conducting experiments at a high inert gas pressure allows one to prevent
evaporation of magnesium and leads to the full reduction of silicon
dioxide in the first stage of the combustion process.
17. conclusions
• In this work SiC powders were synthesized by different combustion
routes, i.e., Si/C/Teflon (chemical activation, SiCT); Si/C (mechanical
activation, SiCMA); Si/C/nitrogen (SiCN) and SiO2/Mg/C (reduction
reaction, SiCR).
• Being all sub-micron size, these powders possess different
characteristics in terms of microstructures, SSA and thus
consolidation ability.
• During the SPS process all four SiC revealed different densification
rates during both preheating and isothermal stages.
• SPS results suggest that powder with the highest SSA, shows the
highest sintering ability.
• It is important to point out that no experimental parameter
optimizations were performed and no sintering additives (i.e., B, C)
were utilized
