Revew

セラミックス基盤工学研究センター年報（2011）．Vol．11，15-20

SiC-based Diesel Particulate Filter Developed by New Sintering Process
Seiji Hayama1, 2
1. Guest Professor of Ceramics Research Laboratory, Nagoya Institute of Technology,
10-6-29 Asahigaoka, Tajimi, Gifu 507-0071 JAPAN2. TYK Corporation, 3-1 Ohbata, Tajimi, Gifu 507-8607 JAPANThis paper explains the characteristics and the performance results of evaluation test on the SiC-based Diesel Particulate Filter(DPF) developed by a new sintering process (Partial reaction sintering in nitrogen atmosphere). The new process enables a controlof porosity and pore size distribution of the filter material with comparatively high flexibility, in addition to the cost reduction
compared with the normal sintering process. The developed filter indicated sufficient thermal durability and higher filtration
efficiency on the initial stage which can respond to the severe regulation demanded in the future by controlling the pore size on thewall surface and the microstructure in the wall.

1. INTRODUCTION
In the present exhaust gas purification filter, SiCDPF is widely used especially in European passenger
car owing to excellent robustness and high filtrationefficiency. In recent years, environmental regulation
tends to become severe rapidly by introduction ofparticle number restrictions, and the further highly
performance is demanded for DPF.1) Especially, higher
soot filtration efficiency is required in the initial stage
after regeneration. Normally, highest filtration efficiency
is produced by the soot accumulation (soot cake) on thefilter wall. 2), 3) A considerable amount of soot leakage
before the soot cake formation should be decreased
quickly.
The performance of the SiC-DPF is mainly influenced
by the microstructure of filter materials describedin terms of the porosity and pore size distribution
substantially. 4) Generally, in order to increase thefiltration efficiency, methods of decreasing the pore sizein the filter material or increasing the wall thickness areused. On the other hand, we succeeded in developmentof SiC-DPF having a high soot mass limit (SML)
performance and filtration efficiency by introducing
a new sintering process (Partial reaction sintering in
nitrogen atmosphere) which can control the porosity andthe pore size of SiC material comparatively easily.5)
Material Development
As materials constituting the filter material in the newprocess, the SiC powder of high purity with uniform
particle size, proper amount of carbon powder andsilicon nitride powder as sintering aid material, and theorganic binder were used. After the mixing of those
mixture with proper water, a normal extrusion wasapplied to form a honeycomb body. After the drying
and plugging, the filter elements were debindered andsintered in the high temperature sintering furnace withnitrogen atmosphere.
On this sintering process, the bonding between SiCgrains was accelerated by the next reaction.
Si3N4 + 3C→

3SiC + 2N2Sintering in N2 atmosphere furnace

Si3N4 + 3C → 3SiC + 2N2
SiC grain

Si3N4Graphite

Base SiC grains are connected by synthetic SiC
Connection strength is intensified by re-crystallization− 15 −

SiC-based Diesel Particulate Filter Developed by New Sintering Process

Therefore, since this sintering process is completed
in shorter time and lower temperature as compared withthe usual high temperature sintering, abnormal grain
growth is restricted. Moreover, the control of porosityand pore size is easy, and lead to energy saving by thissintering process. The sintered filter element body showsa characteristic dark green color.

X-ray analysis was performed in order to check thecrystaliyne phase in them, and the X-ray diffraction
chart is shown in Fig. 3. They are different in thesintering process, but diffraction chart shows almost thesame pattern. Since no compounds other than SiC areidentified in each specimen, it turns out that they arehigh purity SiC re-crystallized material.
The engine bench testing and the measurement ofinitial filtration efficiency were carried out in order tocompare the performance of these testing samples.

2. EXPERIMENTS
DPF test samples were prepared as shown in Table 1.
Filter-A is the developed SiC filter which was producedby the new sintering process (Partial reaction sinteringin nitrogen atmosphere). Filter-B is the commercial
product which is a typical filter currently used inEurope. Usually, this filter is produced by higher
temperature sintering in argon atmosphere. Although thespecifications of these samples differ a little, they hardly
influenced comparison testing.
The pore size distributions measured by mercury
porosimetry are given in Fig. 2. Both samples show
the sharp distribution peak which is peculiar to the SiCfilter.

2-1. Engine bench testing
Engine tests were conducted using a facility in Fig. 4
with the sample of size D5.66" xL6".
2-1-1. Soot loading : A set point of 1400rpm/200Nm
was used for soot loading. A theoretical soot loading
charge of 10 g/liter was also used.
2-1-2. Drop to idle test (DTIT) : As shown in Fig.
5, as soon as the DPF temperature reach 680℃ by
3000rpm/190Nm, engine rotation is changed to idlingcondition immediately.

Table 1. Properties of DPF Test Samples
Filter-A(Developed)

Filter-B(Commercial)

Cell Density / cpsi
169
178Wall thickness / mm
0.38
0.36
Porosity / Vol%
4242
Mean Pore Size / μm
Bending Strength / MPa *9.87.6
Initial Pressure Drop / KPa**
4.74.6* Size: 2cell×3 cell×40mm, Span: 30mm, Load speed: 0.5mm/min** Size: D5.66inch×L6inch, Air flow rate: 9.5Nm3/min

Fig. 2 Pore size distribution of DPF test sample.

Fig. 1 Appearance of the test sample

Fig. 3 X-ray diffraction patterns.
− 16 −

Seiji Hayama

DPF

DOC

Dynamometer

EngineEEPS

Fig. 4 Schematic of equipments.

Fig. 5 Drop to idle test

2-1-3. Crack check : After cooling, DPF was cut andthe existence of crack was investigated.
2-2. Initial ﬁltration eﬃciency
In the same conditions as the soot loading, theconcentration of particle which leaks from the outlet sideof DPF during initial 2 minutes was measured by EEPS(Engine Exhaust Particle Sizer : TSI Model 3090).
3. RESULTS AND DISCUSSION
3-1. Engine bench testing
The data of exhaust gas temperature of inlet/outlet
on DPF and engine r.p.m. are shown in Fig. 6 and Fig.
7. Summary of DTIT result is shown in Table 2. The
maximum temperature of Filter-A is slightly lower thanFilter-B although combustion efficiency is higher thanthe latter. Crack was not found in both samples. Filter-A
shows preferable tendency of properties for regeneration
and robustness.
3-2. Initial ﬁltration eﬃciency
The concentration of particle which leaked fromthe outlet side of DPF is shown in Fig. 8, and thedistributions of particle size are shown in Fig. 9 and Fig.
10. for Filter-A and B, respectively. The trapped sootamount was 0.1〜0.2g/L in the initial 2 minutes. Filter-A

showed less amount of soot leakage, and lapsed time
until particle concentration decreases to 5×106/cm3 washalved as compared with Filter-B.
In order to investigate the difference in filtration
behavior, pieces of each filter wall after the initial 2
minutes filtration were cut out carefully, and the SEM
observation was performed. SEM images of the wall
surface are shown in Fig. 11. The SEM images of
the wall sections are shown in Fig. 12. These SEM
observation parts in the test samples are shown inFig. 13.
On the SEM observation, the formation of soot cakewas hardly found on the wall surface of each sample.
This fact means that almost full filtration ability should
function before the visible soot cake was formed on
the wall surface. Therefore it is expected also that
initial filtration efficiency is strongly affected by themicrostructure of the filter material.
Generally, soot filtration on ceramic wall-flow typeis performed by a combination of surface and depth
filtration mechanisms. 6) On the surface of Filter-A, since
the surface pores are distributing uniformly with smallsize, it is expected that particulates deposit efficiently on
the grains which construct the pore channels and divide
gas flow-lines homogeneously.

Table 2. Summary of DTIT result
Max temperature (℃)

Combustion efficiency (%)

Weight (g)

Crack

880

783040

noFilter-B (Commercial)
900※Weight: Initial DPF includes canning weight692818

noFilter-A (Developed)− 17 −

SiC-based Diesel Particulate Filter Developed by New Sintering Process

Fig. 6 DTIT Chart of Filter-A

Fig. 7 DTIT Chart of Filter-B

Fig. 8 Total concentration of particle which leaked from the outlet side of DPF in theinitial 2 minutes.

Fig. 9 Particle concentration and size distribution（Filter-A）

Fig. 10 Particle concentration and size distribution（Filter-B）− 18 −

Seiji Hayama

Filter -A

Filter -B

50μm

Fig. 11 SEM image of wall surface← ← Gas flow direction

Filter-A

Filter-B
Fig. 12 SEM image of wall cross section− 19 −50μm

SiC-based Diesel Particulate Filter Developed by New Sintering Process

Fig. 13 SEM observation parts in the test samples.

Fig. 14 Concept of microstructure in the wall.

On the section of Filter-A, the sizes of bounded SiC
grains are uniform and small. Comparatively large poreare connected with complicated flow-pass distributing
in the wall. This microstructure is brought about fromthe sintering process. It is thought that such structure
is preferable for the diffusive deposition and flow-line
interception of particulates7). (Fig. 14) Therefore, thesuperior initial filtration behavior on the Filter-A would
be realized by the combination of the uniform pore
distribution on the wall surface and complicated flowpass structure inside the wall. On the other hand, theback pressures of Filter-A and Filter-B were almost thesame level through the all range of soot loading.
4. CONCLUSION
The newly developed SiC-based diesel particulate
filter which are brought about from a new sintering
process (Partial reaction sintering in nitrogen
atmosphere) has sharp peak of pore size distribution
with high purity crystalline phase. Developed filter
shows preferable tendency of properties for regenerationand robustness having lower maximum temperatureand higher combustion efficiency. Developed filter
shows superior initial filtration efficiency with normal
back pressure. It was thought that this performance wasderived from its uniform pore distribution on the wall
surface and the characteristic flow-pass structure inside
the wall.

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