Heinrich J.G., Aldinger F. (Eds.) Ceramic Materials and Components for Engines
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after 5OOtimes tests, the thickness was approximately
same as before use and it showed a normal measured
value. According to the SEM observation and the
EDS
analysis, it was found that the erosion of silicon nitride
protective pipe by molten copper was prevented in
the
tens-of-
p
m magnesia layer which exists
in
the surface.
Results
on
molten aluminum
The operation temperature of molten Aluminum is less
than 973K and
this
temperature is much lower than that
of molten copper and the cast iron. When doing
temperature measurement repeatedly, it proved that it
endured repeat use with equal to or more than
4OOO
times
in
molten aluminum. And the response was
almost l0seconds. As for thermocouple for molten
aluminum
,
conventional stainless sheath type would
become a competition with the developed one, but
newly developed thermocouple is excellent about the
response and durability. Moreover, the there is a merit
that process of the powder spray before use becomes
unnecessary because molten aluminum
are
hard to
adhere to silicon nitride pipe.
CONCLUSION
Tungsten-rhenium alloy wire which
has
high
thermo-
electro motive force, good linearity and
high
melting
point, was used as sensor for temperature measuring and
it was sealed by ceramics in silicon nitride pipe with
film on the surface, and outer sleeve having layer
structure was arranged around the silicon nitride pipe.
On thus obtained thermocouple assembly, its
performance was investigated.Ql For molten cast iron
:
it shows
6
seconds of response,
400
times of durability.
@
For the molten copper, it shows
8
seconds of
response, and more than
500
times of durability.
0
For
the molten aluminum, it shows 10 seconds of
response, and more than 4000 times of durability.
REFERENCES
1) K.Ogawa,
S.
Suzuki,
Japanese Patent N0.05-034033
2)H.Kita, Dimensional change chracteristics of ceramics
produced by simultaneous nitriding of Si and Ti
powders and its application to adiabatic engine
parts,lO0,4,PP.488-493( 1992) Japan journal of ceramic
society
72
ADVANCES IN HOT GAS FILTRATION TECHNIQUE
R.
Westerheide*,
J.
Adler**
(*)Fraunhofer-Institut fur Werkstoffmechanik,
D-79108
Freiburg im Breisgau, Germany
(**)Fraunhofer-Institut
fur
Keramische Technologien und Sinterwerkstoffe,
D-01277
Dresden, Germany
ABSTRACT
The efficiency of coal fired power plants is mainly a
function of the firing process. However the process
parameters have to meet the requirements of the raw gas
filtration unit, when a gas turbine is used in combined
cycle plants. Combined cycle plants are offering an
increase of efficiency of about
4%
by using new tech-
nologies compared to conventional plants. These tech-
nologies requiring at least higher operating tempera-
tures. Therefore new materials for higher filtration tem-
peratures in both oxidizing and reducing atmospheres
are necessary. Further the lifetime of the filter elements
regarding the increased temperature and regarding the
element design has to be guaranteed up to
16OOO
h
operating time.
Usually the filter elements are hollow candles made
of siliceous bonded silicon carbide with an overall
length of
1500
mm.
However, the binder is often sub-
jected to creep due to the softening of the binder at high
temperatures and the candle may
be
damaged due to
vibrations in the filter unit.
The paper gives an overview about the work in de-
veloping new ceramic filter materials based on silicon
carbide and alumina. The goal of the development was
to increase the potential application temperature. It was
found that the application temperature can be increased
up to
900
"C due to an optimization of binder and sin-
tering process. The analysis of stress distribution in the
filter element has shown that the material damage can
be reduced by slight optimization of the geometry.
INTRODUCTION
The combined cycle technology offers the prospect
of higher generation efficiency than the only steam
cycle based technology. One of the central components
in the combined cycle technology is the filtration unit in
which the removal of particles from the feed of the gas
turbine takes place.
For
a long term stable operation of the filter ele-
ments, their resistance against thermal and mechanical
loading and aganist chemical attack has to
be
very good.
Additionally, the filter media must ensure a minimum
pressure drop in order to reach a maximum operation
efficiency. These requirements are met by ceramic ma-
terials. Rigid ceramic filter elements are a leading tech-
nology in the removal of particles from gases, however
their application has been seen to be highly dependent
on
the properties of the dust being separated from the
raw gas
[
13.
The structure of ceramic filter elements consists of-
ten of a highly porous support which ensures the me-
chanical strength and a layer which operates as the
functional part for the particle removal (Fig. 1). The
durability of the ceramic filter elements is limited in
principal by chemical reactions between gas and ce-
ramic material, the mechanical loading
or
thermal shock
during pulse cleaning. Thereby the lifetime of the whole
element is mainly determined by the lifetime of the
support material and not by the functional layer.
Fig.
1:
Structure
of
a ceramic surface filter element, Dia Schumalith"
made
by
USF-Schumacher.
In
this work different ceramic support materials with
an increased resistance to high temperature creep de-
formation and increased strength were developed and
investigated. The materials are oxide and non-oxide
based ceramics.
On
the one hand they were produced as
a conventional filter medium made of bonded silicon
carbide
or
sintered oxides and
on
the other hand they
were produced
as
self bonded silicon carbide by re-
crystallization
or
granulation of fine grained silicon
carbide powder.
As
an alternative route for the produc-
tion of rigid filter media, the formation of multilayered
ceramic foam was investigated.
73
MATERIALS DEVELOPMENT
0.15
SILICON CARBIDE FILTER ELEMENTS
WITH
A
SILICEOUS BINDER
Most interest in the past
was
focused on silicon car-
bide filter elements bonded with siliceous material. The
production of these filter elements
is
relatively cheap
compared to self bonded silicon carbide due to the
lower sintering temperatures and therefore the use of
cheaper furnaces. An effective method is to add a clay-
based binder phase, e.g. alumindmullite which form
glassy phases on firing and act
as
the binder. However,
during long term exposure the
so
called clay bonded
ceramics are subject to bending and creep, in which the
candles have shown irreversible elongation. Due to the
elongation the structure can be damaged and the ele-
ment can fail during operation.
L
Creep strain in &point-bending.
All materials contain the
same
SiCpowder and bioder.
-
First
optimisation
of
sintering. Test at
m.
During the last years new, high-temperature' SiC-
based candles with improved resistance to creep were
developed and extensively tested in demonstration
plants
[2]
and in the laboratory scale
[3]
up to 870°C.
The resistance against creep was found to
be
good; no
element failed during operation. However, to meet the
requirements of further increase of filtration tempera-
ture, the development of new materials was necessary.
The work was focused on the optimization of the
binder, the silicon carbide powder and the whole pro-
duction process, i.e. the powder granulation, the powder
compaction and the sintering.
As the influence of the silicon carbide powder
was
found to be negligible in the range of the considered
Sic-powders, the mechanical properties are strongly
influenced by the kind and amount of binder. Fig.
2
shows the change of strength due to the variation of
binder without changing the sintering process. The
strength was measured at 4-point bending specimens at
room temperature and at 900°C. All specimen have
nearly the same porosity. An increase of the amount of
binder leads to an increase of strength as it could be
measured for an increase amount of
flux.
However a
higher amount of binder has no positive effect on the
deformation resistance at high temperatures, as it is
shown for a constant load of 5MPa and temperatures
from 850°C up to
1OOO"C
(Fig.
3).
Therefore compro-
mises between strength and high temperature deforma-
tion behavior have to be found.
After the optimization
of
the binder the variations of
powder processing, powder compaction and sintering
have shown that a further potential for the improvement
of
properties exists. Especially the creep resistance
could be improved due to controlled crystallization of
the binder during the sintering process (Fig. 4).
Figure 4 shows the deformation behavior of the
op
timized clay bonded silicon carbide filter material which
is the basis for the newly developed Dia Schumalith"-N
(USF-Schumacher) material. This material has a high
strength at a similar porosity in comparison to the com-
mercial available material Dia Schuma1ith"-T. The
high strength is mainly a result of
an
optimal transition
between silicon carbide grains and binder.
-0-
Strenglh at
20°C
p*
-
&Strength at
900°C
z:
-
Binder:
30
-
increase
of
binder
Binder:
Alumina+clay Alumina4ay
constant flux constant binder
20-
increase
of
binder increase
of
flux
*
Fig.
2:
measured
by 4-point bending at mom temperature and at
900°C.
Strength
of different Sic materials with siliceous binder
150
lo00
900
125
E,
c
'09s
-
f
700
75
L
p
800
.-
Q)
-
0
n
s
8600
504
500
400
25
0
0
5
10
15
20
tie
in
h
Fig.
3:
Deformation behavior of silicon carbide filter materials
with
two
binder
systems
and different amounts of binder.
Consequently the optimization of clay bonded sili-
con carbide filter elements resulted in a further im-
provement of mechanical strength, high temperature
creep resistance maintaining the strong adherence of the
ceramic membrane coating, the good cleanability and
the high filtration efficiency as it is known for the ear-
lier developed materials Dia Schumalith" F-40, Dia
Schumalith"
10-20
and Dia Schumalith@-T (details are
described in [4,5]).
74
SELF BONDED SILICON CARBIDE FILTER
ELEMENTS
The disadvantages of clay bonded silicon carbide
filter elements due to the softening of the binder and
resulting high temperature deformation can be avoided
by self bonding of silicon carbide.
An alternative route to produce a silicon carbide
material without significant amount of siliceous secon-
dary phases is the reaction bonding of silicon carbide
(RSiC). This method is well known for less porous
structures
(15%)
used for the setup in furnaces for
sanitary and tableware.
Also
there were attempts in the
US
for using structures with higher porosity for the hot
gas filtration technique. The results of intensive materi-
als development and prototype filter tests has shown
promising results [6]. The RSiC-route was applied in
this project by the AnnaWerk GmbH. The bonding of
silicon carbide results from the formation of &SIC dur-
ing high temperature firing at 2000°C. Also
for
this
material
the
drawback is the necessity of higher sinter-
ing temperatures and therefore a more expensive pro-
duction route, however here the resistance against cor-
rosive attacks seems to be very good and there is hardly
any creep deformation measurable. The development of
RSiC-filter elements resulted
in
the production of 1.5 m
candles based
on
AnnaNox@ CK225. The strength of
the RSiC with a porosity between 25% and
30%
is in
the range of the strength measured for Dia Schu-
malith@-T.
For
this kind of silicon carbide filter ele-
ments a new membrane coating was developed
on
the
basis of liquid phase sintered silicon carbide. At the
moment the RSiC-candles undergo tests in demonstra-
tion plants to study the performance of this material.
In
the laboratory scale
no
material degradation due to
oxidation, corrosion
or
any thermal
or
mechanical fa-
tigue could
be
detected.
Fig.
5:
Necks
between granulated sub-micron silicon carbide forming
the open porosity for the filter support.
Another possibility of self bonding is the conven-
tional sintering process of granulated sub-micron silicon
carbide powder. This production method resulted in a
very homogeneous structure (Fig. 5) with a similar
porosity as it was measured for the clay bonded silicon
carbide filter elements. The drawbacks of this produc-
tion route are the cost of powder, the effort for granula-
tion and the higher sintering temperatures. However
it
is
expected that the creep resistance and the resistance
against corrosive attack is improved,
so
that the much
higher cost for production will in some cases be war-
ranted.
OXIDE BASED FILTER ELEMENTS
Oxide based filter elements should retain their
sta-
bility in both oxidizing and reducing gases, providing
that the free silica content is low,
or
not available for
reaction. They are already in their most stable state of
oxidation, and less likely undergo further phase transi-
tions [7].
The use of filter materials based
on
spinel and co-
rundum can led to a better adaption to specific filtration
atmospheres in different power plant processes. The
development of oxide based porous materials has led to
strength values which are similar to those of the opti-
mized clay bonded silicon carbide filter elements.
In
contrast to the clay bonded Sic the resistance against
creep deformation was found to be excellent up to
1000°C.
For
the corundum material a pressure drop of
27mbar at 7cmls was measured. The successful devel-
opment of oxide based filter materials resulted
in
tests
at a hot gas filtration demonstration plant. After 1250h
at 760°C under combustion conditions a corundum
material has shown nearly
no
strength
loss
from a level
of 24 MPa (measured by O-ring testing). However, the
spinel materials have shown high strength
loss.
Details
to the material production and to
the
differences bet-
ween spinel and corundum are described in [6].
ALTERNATIVE CONCEPTS FOR CERAMIC
FILTERS
Most of the filter elements for the hot gas filtration
have
been
formulated from rigid granular ceramic
structures. Other concepts were focused
on
fibrous
media
or
in whisker reinforcement
[8].
The main ad-
vantage of such elements is their low density, however,
their mechanical strength is generally lower as it can be
observed for porous metallic structures. High tempera-
ture experience with fibrous candles is limited. Tests
performed in the Tidd demonstration facility have
shown an embrittlement and oxidation of the fibers [2].
Fig. 6: Silicon carbide foam
structure
with a 6Oppi support and
a
6OOppi layer.
75
Another possibility for the formation of low density
filter materials is the production of ceramic net like
structures made from polymeric foams. These foams are
limited in the variability and controllability of the cell
geometry
as
well
as
the strut shape, but it is possible to
produce either isotropic
or
anisotropic structures with a
wide variation of pore sizes. For the application
as
hot
gas filter element layered structures of sintered silicon
carbide as shown in Figure
6
were produced in the
shape of plates. The structure is similar to those
of
coated granular silicon carbide candles consisting of a
support material with
60
pores per inch (ppi) and a layer
with 6OOppi. However, the production of the common
long hollow cylindrical candles is not reasonable at the
moment. Rather it is more suggestive to think about
other geometrical structures for the application of
foam
structures
as
filter elements.
ANALYSIS
OF
GEOMETRY
The geometry of a small hollow filter element with a
wall thickness of about
lOmm, an
outer diameter of
60mm
and an overall length of about
15oOmm
with a
porous support structure is not typical for ceramic mate-
rials. Therefore the risk of mechanical fatigue has to be
investigated.
A
mechanical fatigue can
be
at first originated by the
dead weight of the element and the additional thermal
loading. This can
be
investigated by creep experiments
to get the values for the high temperature deformation
resistance. At second a mechanical fatigue can
be
origi-
nated by vibrations of the filter systems
or
vibrations of
plant components which are connected to the filter
sys-
tems. This
has
to be investigated by an analysis of the
natural frequencies of the filter elements and the fol-
lowing local stresses. Also the results
of
the analysis
have to be compared to results from experimental in-
vestigations at the filter element.
It can
be
assumed that the vibrations in the filter
systems are in the region of
1
to
200Hz
as
the maxi-
mum value. The calculation for the above defined
stan-
dard element has resulted in a
first
natural frequency
of
19
Hz
and a second natural frequency of
1
19
Hz
in
consideration of basic values
for
the density
(1.9
g/cm3).
the porosity
(45
%),
the Young$ modulus
(34
GPa) and
the possions ratio
(0.2).
In regard to the order of natural
frequencies shown in Figure
7
the
local stresses are
maximum at the top of the elements where the element
is fixed at the mounting plate (see Figure
8).
.
Thus it can
be
assumed that either during start-stop
cycles of the plant
or
during operation the vibrations in
the filter system are of the magnitude
of
the natural
frequencies. Therefore high local
stresses
will
be
gener-
ated.
3:30
Fig.
7:
Natural
frequencies
of
a
,standard"
filter element.
Different possibilities to reduce the stress and to
modify the stress distribution are existing. First of all
one have to think
of
alternations of the material. Taking
into account that a changing in materials properties are
counterproductive for the resistance against high tem-
perature deformation
or
the filtration efficiency one has
to think only of changes in the geometry
of
the ele-
ments.
Fig.
8:
Stress
distribution
at
the
top
of
a
filter element.
76
The simulation of other geometries has shown that a
reduction of length leads to a significant shifting of the
natural frequencies but the stress level is sometimes
higher than for standard elements. A variation of the
inner and outer diameter to acceptable values leads only
to slight changes of natural frequencies but resulted in
significant reductions of stresses. The results of further
calculations for different geometries were that a change
to conical elements or an adaptation
of
additional
weights are promising favorable stress levels and stress
distribution. This can lead to a reduction of the risk of
mechanical fatigue. The calculations have to
be
verified
by experiments. This has
started
in the middle of 2000
and will be published later.
CONCLUSION
It has been shown that other structures than the
common clay bonded hollow cylindrical candles are
applicable for the particle removal from hot gas
streams. For example oxide based candles, RSiC can-
dles and materials with granulated structures made from
sub-micron silicon carbide are offering
new
potentials
due to the findings of high strength, high creep resis-
tance and good oxidation resistance obtained in the
laboratory scale. These materials are still under investi-
gation.
For clay bonded silicon carbide candles significant
improvements in strength and creep resistance main-
taining the good properties of Dia Schumalith@ T, i.e. a
strong adherence of the ceramic membrane coating, a
good cleanability and a high filtration efficiency, were
achieved. The new material Dia Schumalith@
N
will
be
extensively characterized in different demonstration
plants soon.
The analysis of geometry
has
shown that different
possibilities to reduce the risk of mechanical fatigue due
to vibrations
in
the filter systems are existing. The
variations are possible without changes of materials
properties and filtration efficiency. The calculated re-
sults have to
be
verified in experimental work in the
near future.
ACKNOWLEDGEMENT
The authors gratefully acknowledges the Bundes-
ministerium fiir Bildung und Forschung (BMBF) for
financial support under contract
No.
0326832 and under
contract
No.
0326866F. In addition
we
would like to
thank the companies AnnaWerk
I
Rdental, USF-
Schumacher
I
Crailsheim, ESK-SIC GmbH
I
Grefrath
and H.C. Starck
I
Laufenburg for their technical and
financial support.
REFERENCES
S.K.
Grannell et al.: Investigation into the Behav-
iour of Particle Compacts and Comparison with In-
dustrial Experience. High Temperature Gas Clean-
ing, ed. by E. Schmidt et al., Karlsruhe 1996, 145-
156.
M.A. Alvin: Performance and Stability of Porous
Ceramic Candle Filter During PFBC Operation,
Materials at High Temperature, Vol. 14, 3 1997, p.
355-364.
P.J. Pastila et al.: Effect of Crystallization on Creep
of Clay Bonded Sic-Filters. 22" Annual Cocoa
Beach Conference on Composites, Advanced Ce-
ramics, Materials and Structures, Jan. 1998, Fla,
USA. To
be
published in Cer. Eng. and Sc. Proc.
of
the
Am.
Cer.
Soc..
K.
Schulz et al.: Improved Ceramic Filter Candles
for Hot Gas Cleaning. High Temperature Gas
Cleaning, ed. by E. Schmidt et al., Karlsruhe 1996,
R. Westerheide et al.: Filter aus Hochleistung-
skeramik
fiir
die Energietechnik. Final Report,
Fraunhofer Institut
fiir
Werkstoffmechanik, W2199,
Freiburg 1999.
P. Eggerstedt, J. Zievers: High Temperature Par-
ticualte Removal Using Recrystallized Silicon Car-
bide Candle Filters. High Temperature Gas Clean-
ing Vol.
II,
ed. by A. Dittler et al., Karlsruhe 1999,
J.P.K. Seville et al.: Recent Advances in Particulate
Removal from Hot Process Gases. High Tempera-
ture Gas Cleaning Vol.
I,
ed. by
E.
Schmidt et al.,
Karlsruhe 1996.3-25.
Y.
Sawada et al.: Evaluation on Fundamental Prop-
erties of Filter Materials at High Temperature. High
Temperature Gas Cleaning Vol.
11,
ed. by A. Dittler
et al., Karlsruhe 1999,393-404.
843-845.
375-383.
77
This Page Intentionally Left Blank
NANO-SCALED CERAMIC MEMBRANES
FOR THE FILTRATION OF FINE AND STICKY DUST
B. Zimmerlin*, I. Miillner,
H.
Leibold, H. Seifert
Forschungszentrum Karlsruhe GmbH, Institut fir Technische Chemie,
Bereich Thermische Abfallbehandlung,
D-76021
Karlsruhe, Germany
ABSTRACT
Hot gas filtration by using ceramic filters at tem-
peratures up to
1000
"C is of crucial importance as key
technology for numerous high-temperature processes.
However, filter clogging by he and sticky dusts
as
well
as material degradation still remain problems, limiting
the long term stable operation. In order to avoid filter
clogging, multilayer grain ceramics were coated by a
homogeneous and highly porous filter membrane con-
sisting of nano-scaled A1203-particles. Filtration and
recleaning tests with a fine and sticky model-dust re-
vealed an excellent filtration and recleaning perform-
ance
of
the produced membranes, even at a temperature
of
780
"C.
INTRODUCTION
In advanced power generation systems fine dust
separation at high temperatures is essential to protect
down-stream components as
gas
turbines or heat ex-
changers. The integration of a hot gas filtration unit also
offers an immensely optimization capacity for product
recovery as well as for various thermal waste treatment
processes. In the latter, thus enables among others the
fiuther utilization of the product gases
of
pyrolysis or
gasification of bio mass and waste. Furthermore, it
im-
proves heat recovery methods and reduces corrosion
damages. Due to the high temperatures and chemical
aggressive conditions, only ceramic filter media are
suitable for this purpose. In their long-term stable per-
formance, however, problems arise from material deg-
radation
-
e.g. by corrosion, dynamic or static overload,
etc.
-
and above all from filter clogging, based on fine
and sticky dusts. Those dusts again are typical for ther-
mal waste treatment processes, whereas softening of
salts and
tarry
components and subsequently sticky or
baking dust behaviour already occurs at temperatures
below
500
"C.
The performance of ceramic filter media is not only
determined by the filtration conditions and dust charac-
teristics. It is also determined by their microscopic
structure. The structure of grain ceramic filters
-
mostly
designed as filter candles
-
consists
of
a coarser grained
supporting layer which ensures a high mechanical
strength
of
the filter element and one or more coating
layers with reduced grain size. Preferentially, filtration
takes place on the top layer. For both, filtration and
recleaning of fine and sticky dusts, a top layer is
required that minimises the penetration of the particles
into the filter media and reduces the adhesion between
filter and filter cake. Thus
can
be obtained by a homo-
geneous nano-scaled plus highly porous ceramic filter
membrane. In general, the adhesion forces between
particles are the lower the smaller one or both of the
contacting particles are. As a consequence of the si-
multaneous high porosity not only the adhesion forces,
but also the number of contact points are reduced by
such a membrane. Investigations concerning the influ-
ence of the microstructure on filter cake formation re-
vealed
[l],
that a highly porous filter structure
or
re-
duced contact points result
in
an enhanced porosity
of
the filter cake near the filter surhce. Corresponding to
the porosity, the strength of the filter cake will
be
re-
duced in these areas and a complete detachment of the
filter cake is promoted. In case of
soft
or sticky particu-
lates it
can
also
be
expected, that wetting of the mem-
brane will be hindered by the strong curvature of the
spherical nano-particles and the high porosity.
EXPERIMENTAL SET-UP
The experimental set-up of the filter device for the
coating of the multilayer grain ceramics as well as for
the filtration and recleaning studies is shown in
figure
1.
The main modules are the raw gas duct and the clean
gas duct with the filter section, the aerosol generation,
particle analysis and the recleaning installations.
Aerosol generation is designed to produce polydis-
perse aerosols in two different ways. For the coating
of
the filters a spray type generator is used, with a maxi-
mum
particle concentration of about
lo7
particles/cm3.
The filtration and recleaning studies are performed with
a brush type dust feeder at a dosage of
2
g/m3. Coupling
of other aerosol generators for monodisperse or narrow
distributed aerosols is optional.
Particle analysis, i.e. size distribution and number
concentration,
above
0.15
pm
is
performed at room
temperature by a laser particle counter (LPC) with a
dilution section during raw gas measurement at concen-
trations
>
1
o5
particles/cm3.
AII
electrostatic classifier
(EMS)
combined with a condensation particle counter
(CNC) enables the particle analysis fiom
10
nm
to
1
pm.
During a filter test
run
the aerosol or dust is fed into
the filter section after the determination of the size and
number concentration. The filtration velocity is con-
trolled via mass flow controller (FC) and pressure drop
79
A
Particle Analysis
air
control unit
EMS
1
-1
EMS
I
dilution
raw
gas
control unit
Recleaning
L/dust feederv
Aerosol Generation
as well as the clean gas particle concentration are
monitored. The recleaning modus is started via maxi-
mum pressure drop and interval time, respectively.
Therefore the filter section is set off-line via remote
valves and the pressure in this section is increased by a
loading pressure up to pL
=
150
kPa. The recleaning
itself is induced by the fast opening
of
a valve or a
membrane against surrounding atmosphere, resulting in
a fast pressure decrease on the raw gas side of the filter.
After recleaning, the filter section is set on-line again
and the next filtration cycle starts.
COATING
As supporting structures for the nano-scaled A12@-
membranes served a commercial standard material, a
mullite-coated SIC filter ceramic
(3-2SPK).
Structural
data
of
this ceramic are
shown
in
table
1,
whereas the
samples were used in
form
of
discs (diameter
=
45
mm).
The nanometer-sized powder used for the production
of
the nano-scaled Alz03-membranes was obtained by
means
of
electrical explosion of wires
[2,3].
A
thin
aluminium wire of about
0.7
mm is heated and evapo-
rated in air by a short current pulse. Oxidation and con-
densation of the evaporated aluminium results in hest
Alz03-particles. Due to an automatic wire feeding
and
a
continuos air flow rate through the explosion chamber,
this method enables production rates of some
kg/h.
The
generated particles, however, consist mainly of
y-
and
6-
A1203 and exhibit a wide particle size distribution
be-
tween about
10
and
1000
nm.
The relevant finer
parti-
cles are captured in an electrostatic filter at a yield
of
about
10
%,
corresponding to a production rate of ap-
proximately
100
g/h.
For the coating only these particles
were used.
Fig.
1:
Experimental set-up for
tration and recleaning tests.
fil-
Table
l:
Structural
data
obtained by image analysis
methods of mullite-coated Sic filter ceramic
(3-2SPK).
1
3*2SPK
I
I
I
std.deviation
%
I
2.4
I
I
I
I
Dore
diamete?
I
1- 1
1
meanvalue
ym
I
54.9
I
std.deviation
ym
50.2
maximum
vm
653
'
*
maximum Feret's diameter [minimum 32 measurements per
grain (angle
V)]
pore channel diameter by measuring the chord length
(intercept lengths)
of
lines intersecting the pores perpen-
dicular
to
the flowing direction. These lines are
set
every
fourth pixel line
of
the micrographs (resolution: 1364 x 1024
pixels).
80
A suspension of 1 ml A12Q-powder
/
1000
ml 2-
propanol
was
treated
in
a ultra-sonic bath for disaggre-
gation of the powder, sprayed,
dried,
and then analyzed
in
the raw gas duct (fig.
1).
Particle size distribution of
the 2-propanol and the nano-aerosol is represented
in
figure
2.
10000
8000
ii
n
5
6000
z
0
*Q
4000
-
?i
[L
Particle Diameter
d,
,
nrn
Fig.
2:
Size distribution of particles generated fiom 2-
propanol and of the A1203 nano-aerosol.
The particles resulting fiom the 2-propanol ranged
between about 20 and
60
nm
with a mean diameter of
47
nm.
The particle size distribution of the A12Q*ano-
particles was
in
a range of approximately 15 to 450 nm,
at a mean diameter
of
78
nm. Here, about 10
%
of the
particles were related to the 2-propanol.
Coating of the multilayer Sic-discs
(Fig.
3)
was per-
formed at a filtration velocity of uF
=
2.5
ads.
Their
filtration behaviour with respect to the A1203-
nanoparticles is shown
in
figure
4
as function of the
additional pressure drop caused by the applied mem-
branes.
In
case of a low additional pressure drop of
Apd.
=
91
Pa the surhce was only partly covered by a
relatively thin nano-scaled membrane. Areas closed by
glassy binder and larger mullite grains of the first coat-
ing remained on top (Fig. 4a). At higher additional pres-
sure
drop
of Apadd.
=
415
Pa
the
nano-scaled membrane
was significantly thicker and covered the most part of
the surface. But some larger areas closed by glassy
binder and a few larger mullite grains of the first coating
were also not completely bridged by the growing mem-
brane (Fig. 4b). These uncovered areas are considered to
reduce the beneficial effect of the nano-coated samples.
To
achieve uniform membranes the samples were
coated up to an additional pressure drop of
700
Pa. Thus
resulted
in
a multilayer filter ceramic (Nano-3-2SPK)
with a homogeneous nano-scaled filter membrane on
the top. The thickness of the membranes was in a range
between approximately 2 and
7
pm with a porosity
of
about
80
to
90%.
Maximum pore channel diameters
were between 500 nm and 2 pm. Sintering studies on
the
so
obtained Nano-3-2SPK filters were carried out
in
an electrical hace by
varying
the sintering tempera-
ture between
Ts
=
1000
"C
and
Ts
=
1350 "C as well as
the dwell time between 0.5 and
4
hours.
To
avoid
sin-
tering stresses, heating and cooling rates of 3 Wmin
Fig.
3:
SEM-micrograph of the uncoated mullite top-
layer of the supporting structure (3-2SPK).
Fig.
4
SEM-micrographs
of
the nano-scaled top layer
after filtration of A1203-nanoparticles as function of the
additional pressure drop caused by the applied mem-
brane, (a) Apad.
=
9
1
Pa and
(b)
Apdd.
=
4
15 Pa.
were chosen. Independent of the sintering parameters,
the appearance
of
the sintered membranes
(Fig.
5)
showed no or only slightly differences to the unsintered
membranes.
A
pronounced sintering neck formation
was not visible. However, at sintering temperatures
of
Ts
>
1300
"C
diffusion of the clay-binder out of the
supporting structure (Sic and mullite) led to a bubble
formation on
the
top of the filters. Consequently, sin-
tering temperature
of
the samples was limited to less
than 1300 "C.
81