Page 508 - Book Hosokawa Nanoparticle Technology Handbook

APPLICATIONS 15 CERAMIC FILTER FOR TRAPPING DIESEL PARTICLES
deposited, the pressure loss is reduced more effec- efforts to this end. We next come to the question of
tively by increasing the filter cell density and increas- structures with only pores, in other words, porous
ing the filter surface area in order to thin the deposit matter in which the defects can be considered to be
layer. Thus, it is important to skillfully combine the distributed more or less evenly. As in the case of
methods most effective for each case and eventually dense matter, it is essential, with porous matter, to
to design a system that performs optimally and eliminate the processes that produce critical defects at
reduces the loss of fuel for regeneration without every stage of manufacture, from material procure-
imposing a burden on the engine. ment up to commercialization of the final product.
The theoretical strength of porous matter is lower than
4. Features of porous silicon carbide that of dense matter as a matter of course. It must be
asked, however, whether small variation in strength
Silicon carbide is a covalently bound substance. The can be safely predicted if the defects are thought to
strength of covalent bonding of a substance increases have been distributed evenly from the outset.
with an increase in the melting temperature and Thinking this way, this point becomes advantageous,
decomposition temperature of the substance. For sili- as porous matter is easier to work with than dense
con carbide, the decomposition temperature is as high matter in the design stage. Moreover, recrystallized
as 2,400 C or even 2,500 C. It can also be seen from porous silicon carbide has a very small shrinkage of
this that silicon carbide is a material that shows very 1% or less in the burning process. This makes it much
strong covalent bonding and is also excellent in chem- easier to control the dimensions during manufacture.
ical stability. As a result of these properties, silicon
carbide also provides high hardness and excellent 5. Functions and characteristics of DPF
resistance to heat, corrosion, oxidation, and wear.
Silicon carbide also has a high modulus of elasticity, The function of DPF, as its name indicates, is to purify
and with the light weights of Si and C, the constituent gas emissions by trapping PM through filtration. In this
atoms of silicon carbide, and the small difference in case, the DPF must not allow the PM deposited on the
atomic weights, the lattice vibrations tend to become filter through trapping to impede the flow of gas emis-
harmonic vibrations. Hence, silicon carbide provides sions, as this can prevent an engine from running.
a high coefficient of thermal conductivity among the Further, deposited PM is removed by methods such as
ceramics [6]. If pores exist inside a ceramic, the combustion [12]. Of the removal methods, the regener-
mechanical properties of the ceramic are generally ation method by combustion works by igniting a part of
thought to deteriorate. In addition, ceramics at low the trapped PM forcibly in various ways and dispersing
temperature tend to fracture with brittleness, and combustion to ensure that all of the PM is burned. This
ceramics with little or no plastic deformation tend to method regenerates the filters. In this case, the temper-
fracture rapidly under critical stress. This can be easily ature of DPF reaches high values of 900 C or above. A
understood by contemplating the fracture phenome- rapid increase in temperature occurs during the regener-
non in the context of the conditional equation for ation of DPF and a temperature gradient develops dur-
brittle fracture proposed by Griffith [7] in 1920 based ing propagation of heat from the ignition area. Thermal
on the assumption that microcracks exist inside brittle shock resistance, heat resistance, and low reactiveness
materials and by contemplating the crack growth from at high temperatures are therefore required. The honey-
the viewpoint of energy balance. From this approach, comb-type recrystallized silicon carbide DPF has a
it is seen that the strength will be determined by the higher rate of trapping compared with that of other
size of the cracks inside the material. The strengths of types, such as foam type and fiber type. It is also capa-
ceramics that manifest this type of behavior are fre- ble of trapping PM on the nano-order with low pressure
quently treated in the Weibull distribution presented loss. The requirements for this PM trapping efficiency
in 1950 [8] based on the hypothesis (weakest link and low pressure loss are fulfilled by a pore structure
hypothesis) that the chain strength is determined not obtainable by virtue of the properties of recrystallized
by the average strength of the individual rings running silicon carbide, namely, because silicon carbide is diffi-
in a line as a chain, but by the weakest ring [9–11]. cult to sinter and has little or no sintering shrinkage.
This is because the strength of a dense ceramic is gov- Because of these properties, the pore structure can be
erned by the size of the defects present in the material controlled by the particle diameter of the coarse grain
independently of the average characteristics of the and particle size distribution, or by adding a pore-
material. The strength is determined, for example, by making agent. Further, recrystallized silicon carbide is
cracks or portions with air gaps occurring during for- poorly reactive during regeneration up to the tempera-
mation. This means that ceramics can fracture at tures of 900 C or above, and thus it fully meets the char-
strengths much lower than the strengths at which acteristics requirements for a DPF. A high coefficient of
cracking is theoretically predicted. It is therefore thermal conductivity can also be listed as a feature. This
extremely important, in the manufacture of ceramics, high coefficient of thermal conductivity is a very impor-
to remove the causes of defects from the processes. tant characteristic for DPF. When trapped PM is burned,
Ceramics manufacturers have made considerable the heat is efficiently diffused radially, and from the

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