Consultant on Analytical Investigation. G. Starck ...... consulting with experts of Wright-Patterson Laboratories. Tables 1 and 2 ...... erent csi.-ates. fti.uther on.
AEOSR.R3. 93
AD-A265 141
0
44
I
Electrophoretic and Electrolytic Deposition of Ceramic Particles on Porous Substrates
L. Gal-Or, S. Haber, S. Liubovich •
5, --
Israel Institute of Metals
1
--
Technion Research and Development Foundation
74
Final Scientific Report 3
March 1, 1988 - November 30, 1992
DTIC ELECTE
SBD
MAY2 7.1993
Prepared for
EOARD - England, London AFOSR-Bolling, Washington, D.C.,20332, U.S.A.
93-11951 • II lN Iii 11vr1111 fOP P1111 distri but ton Unlalsited.
93 -5 26 072
Electrophoretic and Electrolytic Deposition of Ceramic Particles on Porous Substrates
L. Gal-Or, S. Haber, S. Liubovich
Israel Institute of Metals Technion Research and Development Foundation
Final Scientific Report March 1, 1988 - November 30, 1992 AFOSR Grant 89-0474
Prepared for EOARD - England, London AFOSR - Boiling, Washington, D.C.,20332, U.S.A.
Approved for ni?.I e, relcaso; distributiouj uu1imited.
Electrophoretic and Electrolytic Deposition of Ceramic Particles on Porous Substrates
L. Gal-Or, S. Haber, S. Liubovich
Israel Institute of Metals Technion Research and Development Foundation
Final Scientific Report March 1, 1988 - November 30, 1992 AFOSR Grant 89-0474
Prepared for EOARD - England, London AFOSR - Bolling, Washington, D.C.,20332, U.S.A.
Approvedt fory• di~tribuLt ia
i ! release; Luilimi ted.
I
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13. REPORT TYPE AND DATES COVERED Finai
March 1, 1988 - Sept. 30, 1992
4. TITLE AND SUBTITLE
Electrophoretic and Electrolytic Deposition of Ceramic Particles on Porous Substrates
Report
S. FUNDING NUMBERS
AFOSR89-0474
6. AUTHOR(S)
L. Gal-Or, S. Liubovich, S. 7. PERFORMINt,
1Haber
ORGANIZATION NAME(S) AND ADORESS(ES)
..
8. PERFORMING ORGANIZATION REPORT NUMBER
Technion R&D Foundation Technion City, Haifa, 32000, Israel 9. SPONSORING; MONITORING AGENCY NAME(S) AND ADDRESS(ES)
10. SPONSORING/ MONITORING
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AGENCY REPORT NUMBER
11. SUPPLEMENTARY NOTES
12a. DISTRIBUTION /AVAILABILITY STATEMENT
12b. DISTRIBUTION CODE
Unlimited Approved for publ 10 release distribution unlimitsd, '13. ABSTRACT (Maximum 200 word•)
Electtopnoretic deposicion of ceramic particles and their penetration into tne pores •f graphite and a 2D C-C composite were demonstrated and studied tneoreticaiiv andc experiimental v for colloidal SiO,, SiC, SiN 4 , AI 2 TiOs and HfTIO,. The effect of deposition parameters (field intensity, particle concentration, ratio of dielectric constant/viscositv of fluid) on the amount of induced material was experimentallv studied. Penetration depth of particles as function of above parameters was analyzed theoreticaily. Penetration is enhanced by large Peclet numbers and is therefore higher in aqueous suspensions, while coatina morphology is better in propanoi suspensions. OiF-imal conditions for coverage and penetration in a two-stage process *;ere tperelore aetermined. Subsequent deposition of two layers ý.as demonstrated and studitd for Si 3 N4 on AI 2 TiO5 and SiC on a glass ceramic. Single (Si 3 XN, AI 2 TiO5 , HfTiO,) and multilaver ,Si,%,N on Ai 2 TiO,, and Si:N4 on fused SiO0) coatings were deposited, sintered in a N2 atmosmnere and their phase composition, and Inorphology studied. Oxidation resistance of coated specirvens was studied at 83U0C in continuous and intermittent exposures. Grapnite exnaust nozzles were coated and tested in a9 air breatning enaine simulation. Very efficient protection was achieved. Electrolvtic deposition of ZrO2, A1lO3 and Al.O-ZrO2 was achieved from aqueous solutions of ZrO(NO3 ), and AII(O 3 )3 . The deposition kinetics and mrcrostructure of the deposits were studied. The initial amorphous deposits transformed into crystalline pnases with nonocrsztaiiLre dimensions after heat treatment.
'14. SUBJECT TERMS
...
Electrophoresis,
Ceramics,
Impregnation,
Coating,
Electrolytic Deposition 17. SECURITY CLASSIFICATION OF REPORT
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NSN 7540-01-280-5500
Ia. SECURITY CLASSIFICATION OF THIS PAGE
Unclassified
1.. 15.
NUMBER OF PAGES
16. PRICE CODE 19.
SECURITY CLASSIFICATION OF ABSTRACT
20. LIMITATION OF ABSTRACT
Unclassified
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4.
5ummary and Conclusions The work presented in this report concentrated on the final steps of the
development
of electrophoretic
carbon-carbon composites.
protective
coatings
for
graphite
and
The emphasis was therefore on the sintering of
these coatings and the testing of their protective properties. The deposition conditions were determined in the previous stages of this project. The coating systems studied were: Al 2TiO 5 , HfTiO 4, Si3N 4 , SisN4 on AI 2TiO 5 , Si 3N 4 on fused SiO 2 and Si3N4 on BN. The Al 2TiO 5 and HiTiO 4 coatings were selected due to their compatibility with graphite and C-C in terms of thermal expansion.
However, it was expected that due to the
anisotropicity of the thermal expansion coefficient microcracking will take place and therefore additional overcoating will be needed for oxidation protection. The HfTi0
4
disintegrated after sintering and no viable undercoating
could be obtained.
Al 2TiO 5 was impregnated and uniformly deposited.
Sintering at 1650"C in N2 resulted in adherent coatings which however decomposed partially into A12 0 3 and TiO 2 as shown by the X-ray diffraction pattern. Si 3N4 coatings were obtained both in single and multiple deposition stages. Multiple deposition was done primarily in order to change the holder position to minimize defects in the coatings. After sintering at 1650'C in N 2 uniform, adherent coatings were obtained with exte,,tive anchoring in the pores and voids of the substrate. Interaction with the substrate took place as indicated by diffraction peaks of SiC and Si. Si3N 4 coatings with underlayers of A12TiO 5 were obtained.
Uniform
layers of Si 3N 4 with penetration into the Al 2TiO5 underlayer formed in the "green" state.
Afte: sintering cross-sections showed dense layers of Si 3N4
65
anchored in the substrate with a discontinuous layer of a Ti containing component below the Si3N 4 and an even distribution of the element Al in the entire coating. Very extensive anchoring of the Si3N 4 coating is seen both in graphite and C-C specimens.
Overall coating thicknesses were up to 88A.
Adherent coatings of Si 3 N4 on fused SiO 2 were obtained with anchoring in the substrate. Horizontal and vertical microcracks are seen in the coatings after sintering. Impregnation of BN into the pores prior to Si3N4 coating was attempted. Since both B and N are not visible in the EDS-SEM no evidence of penetration was seen except for weight gain after impregnation. Shaped objects, exhaust nozzles and casting crucibles, were coated with Si 3N 4 + A12TiOs and Si 3N 4 + Si0 2. Uniform internal and external coatings were obtained using shaped electrodes. Oxidation experiments were carried out by recording the weight loss during continuous and repeated exposures of 830" C.
Best results with
graphite specimens were obtained with the Si3N4 + A12TiO 5 system where in some cases nil weight loss was recorded after 7 repeated 1-hr exposures, a regime that exposed the specimens to repeated thermal shock. No beneficial effect of an underlayer of fused SiO 2 or BN was found in laboratory specimens when compared with Si3N4 only. However, very good results were obtained with an underlayer of Si0
2
on exhaust graphite nozzles under
specific testing conditions. On C-C specimens low weight losses were obtained with SiSN 4 and Si 3N4 on A1 2TiO 5 in continuous exposure experiments. In most repeated exposure tests high weight losses were recorded. However, in some experiments Si 3N4 coatings gave low weight loss. The irreproducibility of the results in both
66
materials and in particular in the case of C-C may stem from a major defect in the coatir5 formed on the location of the holder during deposition. Although repeated depositions were made this problem may not have been solved completely and should be elaborated in further experiments.
REPORT DOCUMENTATION PAGE ¶set-main 4:00v, cO.tbon ot imiar'maita P.J~d.Cta~r~$q h,*m. 1n.ntarnn the 43ta nnl,•t a.Carl, cOm ttflglfga r r,.toucfing :thil d luqqesItOMS ma" Or,,to. .~ctudclnq n'CCtl 3Z02.and to u- 0.': H•,lwaV. SWtte 1204. Aretq0tn. VA •
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3. REPORT TYPE AND DATES COVERED
2. REPORT DATE I August 30, 1991
Annual
report
1 July 90-30 June 91 5. FUNDING NUMBERS
4. TITLE AND SUBTITLE
Electrophoretic and electrolytic deposition of ceramic particles on porous substrates
AFOSR-89-0474
6. AUTHOR(S)
L.
Gal-Or,
S.
Haber, S.
Liubovich 8. PERFORMING ORGANIZATION
7. PERFORMING ORGANIZATION NAME(S) AND AODRESS(ES)
REPORT NUMBER
Technion Research and Development Foundation Technion City, Haifa 32000,
Israel
10. SPONSORINGIMONITORING
9. SPONSORING/ MONITORING AGENCY NAME(S) AND ADDRESS(ES)
= EOARD 223/231 Old Marylebone Rd.,
AFOSR/NE, L:.Col. L. Burggraf, DC 20332-6448
London NWI STH UK
AGENCY REPORT NUMBER
Bolling AFB
11. SUPPLEMENTARY NOTES
1 20. 0t5TIEuT!CN ::DE
STATEMENT
12a. OISTRISUTION/AVA)LABWLITY
Unlimited.
13. ABSTRACT (Maximum 200worts)
The electrophoretic deposition of low expansion ceramic materials (AlTiO5 and HfTi04) on porous graphite and their penetration into the pores was studied. Penetration is
1
enhanced by large Pe numbers and is
therefore higher in aqueous
suspensions while coating morphology is improved in ipropanol suspensions. Optimal conditions for coverage and penetration in a two-stage process were determined. The feasibility of subsequent electrophoretic deposition of two different layers of materials with zeta potentials of the same polarity was demonstrated for Si 3 N4 on Al 2 TiO5 (cathodic deposits) and SiC on glass ceramic (anonic deposits). Layer thicknesses in
14.
these experiments were in
the range of 100-300 ;j .
Electrolytic Deposition, Zirconia, Alumina 17.
In
parallel studies on electrolytic deposition from ionic aqueous solutionsthe deposition of A12 03 and co-deposition of A120 3 +ZrO2 from solutions of AI(NO 3 ) 2 It was shown that the atomic ratio and ZrO(N0 3 ) 2 were demonstrated and studied. of A!/Zr in the deposit can be varied by varying this ratio in the solution. The as-denosited coatinas are amorphous and became crystalline after heat treatment. ¶5, NUMBER OF PAGES SUBJECT TERMS Electrophoresis, Ceramics, Penetration,
SECURITY CLASSIFICATION OF REPORT
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C "SS, 0 ,narard ,.40
298 'Rev L11S
'-89)
2
ELECTROPHORETIC AND ELECTROLYTIC DEPOSITION OF CERAMIC PARTICLES ON POROUS SUBSTRATES L.
S.
Haber,
S.
Gal-Or,
Liubovich
Summary of Annual Report
1 July 1990 - 30 June 1991
INTRODUCTION The present
phase of the research project concentrated on the
following aspects: -
Surface
deposition
ceramic materials -
such
as
2
TiO5
ceramic and SiC. Electrolytic deposition
of
co-deposition of -
penetration
graphite. Multilayer electrophoretic ceramic materials:
-
and
Al
two
Al
2
TiO5
a
S 3 N4 single
oxides
low-expansion
and HfTi0 4 on porous
deposition and
of
of
two
as
well
oxide
(Al 2 0 3 +ZrO 2 )
ionic soltions. Theoretical analysis of particle penetration
The interest in
the depositAon of
impregnation
appropriate ceramic at elevated
of
materials
temperatures.
as glass
(A1
2
from
03)
and
aqueous
into pores
low-expansion ceramics stems
from the intention to apply electrophoretic coverage and
different
deposition for the
carbon-carbon
composites with
so as to prevent A1
2
TiOs
its oxidation
and
HfTi0
4
are
characterized by low thermal expansion coefficients
similar to
that of
as coating
carbon-carbon
materials may
provide
.
the
Therefore,
their
use
neccessary compatibility with the
3
the
substrate from
of
view
of
expansion.
thermal
known that the expansion of these materials is
is
it
However,
point
anisotropic and
therefore
has been realized for some time that
It
place upon sintering.
microcracking of the coating takes
the complicated problem of carbon-carbon protection will have to be solved by the application of several different coatings. It
was thus considered of great importance
electrophoretic Si3N4 on
to study sequential
deposition of another ceramic material
such as
the Al 2 TiOs as a remedy of the detrimental effect of
microcracking. The above mentioned studies were performed on the basis of the conclusions from theoretical
the
was
now
deposition extended
experimental
Thus
studied and
then
the
The
effect
demonstrated.
the
morphology
of ZrO2 , studied in
to
oxide-Al 2 03.
kinetics,
basic
studies of the deposition of colloidal
The electrolytic year,
previous
its
codeposition
deposition
of
and
of
of
deposition
composition
of
S10 2 .
the previous with
another
only was first
A1 2 0 3
feasibility
and
co-deposition parameters the
was
on the
coatings
was
studied. The penetration
of particles
it
pores
which
analyzed theoretically
(an
consists
of
into a porous substrate assuming are
closcd
open
at the far end was
interconnected
porous
structure was considered previously).
EXPERIMENTAL The electrophoretic out on
porous
and
electrolytic
graphite substrates
porosity average
pore
size
60 it).
depoqition was carried
(UCAR
grade 45,
Initial
48% volume
experiments with
C-C were performed on a 2-D composite supplied by the USAF. AI 2 TiO 5 , prepared BaCO3
addition
HfTiO4 was
at
and
acquired
average size of 70 u.
the Israel an
average
from
Cerae
Ceramics Institute,
had a 5%
particle size of 1.4 u. Corp.
The
and had originally an
4
Si 3 N 4 particles
SiC>40 u.
were
the
in
range of
0.5-1.0
u and that
of
Suspensions of the ceramic particles were prepared
in isopropanol,
water
and
mixtures
Particle concentrations were 30,
of
the
two
100 and 250 g/l.
solvents.
the electric
field was in the range of 5-300 V/cm and deposition t.mes were 30 sec-6
0
min.
The amount
of
impregnated
material was determined by weight
change after removal of the external deposit. The multilayer substrate,
deposition was carried out by immersion of the
coated
with
the
first
the second material while still
layer in
the suspension of
wet.
The composition and morphology of the multilayer deposits were studied on crossections obtained by cutting coated specimens that were
infiltrated
with
an
epoxy resin and cured for 10
hrs. These studies were performed with a JEOL 840 SEM equipped with an EDS unit. The electrolytic
deposition was carried out from solutions of
0.02-0.2M AI(NO,),
with and without 0.01-0.05M ZrO(NO 3 ) 2 .
The deposition was done at constant current with c.d.'s range of 15-60 mA/cm2
the
and deposition times of 20 sec - 40 min.
Some deposits were heat-treated at 1200"C for 1 hr. and microanalysis
in
were
studied in
Morphology
the JEOL 840 SEM and phase
content with an x-ray diffractometer Phillips Model PW-1730.
SUMMARY OF RESULTS In contrast studies it the cathode
to was both
positive charge point since
the
SiO
2
particles
deposited
in
previous
found that both Al 2 TiOs and HfTiO 4 .deposit on from
water and ipropanol
suspensions.
The
on the particles indicates a high isoelectric
the pH of these suspensions was determined as
8.1
in
water
SiO2
and
6.2
deposition,
the porous However,
in
it
propanol.
As in the previous study of
was found that
substrate
is
penetration of AITiO 5
higher
in
water than in
the addition of x20% ipropanol
maximal penetration
probably
due
to
ipropanol.
to the water lead to a better
wetting of the
graphite as demonstrated in simple wetting experiments. 6% of
substrate weight of Al
at 100
V/cm,
however,
is
due to
30
g/l
2 TiO5
after
lower
About
were induced into the pores
60 min.
The surface deposition,
both higher and morphologically
a
into
better
fraction of penetrated material
in
propanol
and lower H,
formation on the cathode. It
was
thus
penetration
concluded
a
two
impregnation from relatively
low
ipropanol min).
at
It
that
stage an
duration
was
process
aqueous
shown
that
coverage
would
be
75-100 V/cm)
(200 V/cm)
deposition
optimal
and
employed:
solvent with 20% ipropanol
(60 min,
higher field
intensity and
for
and coating in
and shorter duration
impregnation time.
The
for
(x2
increases with field
morphology
of
surface
deposits and impregnated crossections were studied. The feasibility
of
subsequent
deposition
of
two different
layers of
materials with zeta potentials of the same polarity
was shown
for Si
glass ceramic out
by
3
transferring
first
in
However, layer
these experiments
The co-deposition
Al(NOj)
3
the
from
is
carried
the
specimens with an Epoxy
analysis
and
first of the
observed. were in
of
two
of
the
The layer
resin and
Si and Ti x-ray imaqes
the SEM and the existence of
solutions
÷ZrO(NO3 ),
deposits.
of
penetration
was
from aqueous
substrate
and SiC on
second to continue the deposition
Elemental
were performed shown.
coated
The deposition
Crossections of the two layers were prepared
impregnation curing.
(cathodic deposits)
deposits). the
the
second material.
its
on AITiQ5
(anodic
suspension into
after
N4
Si
3 N.
the two layers
and SiC into the
thicknesses obtained
in
the range of 150-300 u.
oxides containing
was demonstrated
by an electrolytic salts
cf
process both
and studied for Al
Prior to that the deposition of AIO
3
cations 2
O3'ZrO,
was studied.
6
The effect of field intensity on the penetration in the optimal solvent is seen in Fig. 4. The penetration increases with field intensity in the range tested (further increase in field intensity caused excessive beating of the suspension and was therefore not studied). A deccelerating effect of deposition time on penetration is seen in Fig. 5. Figs. 6 & 7 describe the morphology of the coating and penetration. The surface deposit of AI 2 TiOs is seen in Fig. 6. The coverage is uniform on a macroscale with a high degree of roughness reflecting the porous surface of the substrate. It was found that a denser and more uniform deposit is obtained in ipropanol at high field intensities and short deposition times. A crossection of a penetrated specimen obtained at optimal penetration conditions compared with a reference untreated specimen is seen in Fig. 7.
b) Multilayer Deposition
A schematic description of the multilayer deposition is given in Fig. 8. A deposit of particles A is formed in suspension A, the coated specimen is removed from the suspension and while still wet introduced in suspension B. A deposit of particles B is formed. A more thorough study of this two-stage deposition process should be done in the future. So far, two systems were deposited: Si 3 N4 on top of Al.TiO, and SiC on a glass ceramic deposit. In each system the zeta potential of the different materials had the same sign, Si 2 N4 and A12 TiOs deposited both on the cathode, while SiC and the glass ceramic migrated to the anode. When couples of materials with opposite zeta potential were studied, the first deposit migrated away from the substrate when its polarity was changed so as to deposit the next layer. Yet conditions can be envisaged where such two-layer deposition can still be performed and these will be studied in a future work. Crossections of the two layer deposits are seen in Figs. 9-12. Fig. 9 shows an optical micrograph of the layer of AlaTiO and Si2NA as well as an SEM x-ray image of Si and Ti at a higher magnification. The overall layer thickness is '350 um. The element Si is seen in both layers with a
much higher existence in
Its in the outer layer (the Si 3 N4). density the inner layer (the A12TiO5 ) is explained by
into the initial porous Si3 N, some penetration of The element Ti, however, exists layer. ("green") Al 2 TiOQ An interesting the inner layer. as expected only in the "pegging" of the inner layer in the pores feature is The thickness of the SixN4 is &200 um of the substrate. Composition graphs of the and that of Al 2 TiO5 t150 um. 11 showing again the the two layers are seen in Fig. penetration of Si into the underlying Al 2 TiOs layer. An inner Two anodic deposits are shown in Figs. 11, 12. glass ceramic layer (a300 u) with an elemental composition Fig. 11a and an x-ray image characterized by the seen in An outer layer of SiC (f150 v) with traces of element K. The x-ray image (Fig. 12b) shows Fe (from the anode). again the penetration of SiC (characterized by the element The Si) into the underlying layer of the glass-ceramic. amount of Si 3 N4 deposited on AI 2 TiOs as function of deposition time was determined with the intention to study layer on the of the underlying AI 2 TiO, the effect It is seen that the (see Fig. 13). deposition of Si3N4 existence of the underlying layer does not affect the deposition kinetics probably due to the high porosity of "green the in still is which layer the first state"
CONCLUSIONS A low expansion ceramic material-aluminium titanate (A12TiOT) deposited and penetrated into porous was electrophoretically The Al 2 TiOs which is positively charged deposits graphite. on a cathodic substrate at a pH=8.1 indicating a relatively amount of material that The point. high isoelectric in an aqueous solution of 20% maximal penetrated was an outcome of the balance This composition is ipropanol. between the high E/7 ratio of water and the improved wetting of the substrate in the presence of ipropanol. Penetration increases with electric field intensity in the For optimal coverage range studied and with deposition time. and penetration a two stage process should be applied: a 20% ipropanol solution in water.(at 100 impregnation in 6% of substrate weight of AI 2 TiOs is about g/l V/cm, 30 penetrated after 60 min.).
8
-
Surface coating in ipropanol at a high field strength (200 V/cm) and short duration ý2 min.).
At optimal penetration conditions the whole crossection of a 7 mm thick substrate of porous graphite was penetrated. Multilayer deposition of different ceramic materials is possible by subsequent deposition from different suspensions of materials with zeta potential of the same polarity. This was demonstrated by forming an inner layer of AI 2 TiOs and an outer layer of Si 3 N. (cathodic deposits) as well as an inner layer of a glass -eramic and an outer layer of SiC (anodic deposits). The individual layer thicknesses were in the range of 150-300 u.
In the range of thickness discussed, the existence of the underlying "green" layer did not affect the kinetics of deposition of the external layer.
ACKNOWLEDGEMENT
Research sponsored Research,
Air
Force
the
by
Air
Force
Systems Command,
Office U.S.A.,
of Scientific under Grant No.
89-0474. The U.S.
authorized to reproduce and distribute Governmental _ purposes not withstanding any
Goevernment is
reprints for
copyright notation thereon.
9
LIST OF FIGURES
Fig.
1:
Ceramic thermal expansion characteristics 14).
Fig.
2:
Amounts of Al 2 TiO5 deposit.d and porous graphite as function concentration in water.
Fig.
3:
Model for a two-stage deposition process.
Fig.
4:
Amount of AlTiO5 penetrated as function of field intensity (20% ipropanol in H2 0, 60 min, 30 g/l).
Fig.
5:
Amount of Al 2 TiO5 penetrated as function of deposition time (20% ipropanol, 50 V/cm, 30 g/l).
Fig.
6:
Surface deposit on graphite obtained in at 200 V/cm, 2 min, 30 g/l.
ipropanol
Fig.
7:
Crossections of treated (20% ipropanol, 60 min) and untreated specimens.
100 V/cm,
Fig.
8:
Schematic description of multilayer formation.
Pig.
9:
Crossection of a double layer deposit of Si 3 N4 (xl0) (100 g/l, ipropanol, 50 V/cm, 2 min) and AI 2 TiOs (30 g/l, ipropanol, 200 V/cm, 90 sec) and x-ray images (xl00).
penetrated in of ipropanol
Fig. 10:
Elemental composition of the two layers a) Si 3 N4; b) Al 2 TiOn
Fig. 11:
Crossection of a double layer deposit of SiC on glass ceramic (xl0). SiC (100 g/l, ipropanol, 50 V/cm, 15 sec), glass ceramic (250 g/l, ipropanol, 200 V/cm, 10 sec).
Fig. 12:
Elemental composition of the two layers b) glass ceramic (i100).
Pig. 13:
Amount of SiN, d..positep function of deposition time.
nn
A)-TiO,
a)SiC
170 0) as
10
BIBLIOGRAPHY
1.
L. Gal-Or, S. Liubovich, S. Haber, "Deep Electrophoretic Penetration and Deposition of Ceramic Particles Inside Porous Substrates - Experimental", J. Elc'trochem. Soc., in press.
2.
L. Gal-Or, S. Liubovich, S. Haber, "Deep Electrophoretic Penetration and Deposition of Ceramic Particles Inside Porous Sunstrates An Analytical Model", J. Electrochem. Soc., in press.
3.
D.W.
4.
I.
5.
K.L.
6.
J.R. Strife, (1988).
7.
R.W. Lynch, (1972).
8.
G.A.
McKee,
Jawed,
Carbon,
24,
D.C. Nagle,
Luthra, Carbon,
Parks,
J.E.
B.
Chem.
737(1986).
Mat.
Res.
Bull.,
21,1391(1986).
26,217(1988).
Sheenan,
Morosin,
Rev.,
J.
Ceramic
Am.
65,177(1985).
Bulletin,
67,
369
Ceram. Soc.,
55,
409
TEMPERATURE (*0
Soo
200
2000
1500
1000 STABIUZED HfO2
1.5
Alz03
T602 STABIUZEO0T0 rz r
1.6
Y203 1-4
"'B2 12
I
!2 1.0
T
0.8
06
12SO
0.4
0.2
rig.
80
C.D C.)
0 00
L0Lo
_
CVto0
CCto
Iý
I
00
TWO
STAGE
_
PROCESS:
_
I
,
I
I)OPTIMAL Pe>>
CONDITIONS FOR IMPREGNATION: Pe = (U )-r U' =
•H0= 81 b.,H2 0 = Ilcp
•prop=
18.3
I•prop= 2 cp
20% propanol in H2 0 II)OPTIMAL CONDITIONS FOR COATING: 100%
propanol
. less penetration of particles
Fig.
3
{N I-
-\
E
---
o0( C)
E oi
I
9 O!IL•IV
%
oo• 00
0
00
,\
to
LO
LOO
i o
-,-
--
-
LOO!JCi
%
9OLL
toV %
If
0C
Fig 6
Fig 7
A
C
-.
,~4b
AC
A
A
.K
Substrate Coated with A
Fig 8
A
Substrate Coated with A and B
Si 3N4
on
,AI2Ti05
X-ray
Si
X-ray Ti
Fig 9
:Ps
,
.-
0.Q'.
R
,
b
~7
if
5.
>
iK
'.
,I
•ii
iX-RAY: jL ive:
-
1100: P r se t:
I.,
EMi :•SI34
Fig 10
o'
R e ma i n i n:
Os'
Fig I
L::,-R t,-: :Live-:.
I0 - ,Fre k).t.:. 1,cr',f-.
loft-
R'~na ir n :
i.i
I I
SI..
X-ray of Si Iit
: •.
i
1 00
-..
FS= 4-K tIEM1 :'SIC:
,L
1.~t.
.•:
z F'" -,. r.:
11 .2
V
,h
31 5=
" LC"
-"
110CI
-C•
-F
'ItJ
It
X-ray of K
':"
. .
,.:|,
'A u
1 12
Fig 12
>;
z
EE
ES' >()
CE,
Cl
'nVon
CDJ
Ul)
m~
0 cn
1
0
04J
04
0
LO
SINGLE AND MULTILAYER ELECTROPHORETIC DEPOSITION AND PENETRATION OF LOW-EXPANSION CERAMICS ON POROUS SUBSTRATES
L.
Gal-Or,
S.
Liubovich
Institute of Metals - Technion, of Technology
Israel Institute
ABSTRACT
Previous electrophoretic deposition and penetration studies were further extended to the deposition of a low expansion ceramic material
-
A1 2 Ti0 5
multilayer deposition.
AI 2 TiOs into However,
the
maximum
propanol in
on
graphite surface.
porous
graphite
and to
It
was found that the penetration of
pores is
higher in water than in propanol.
penetration is
water
efficient and
-
probably The
due
surface
obtained in
a 20% mixture of
to enhanced wetting of the deposition
morphologically better in
is
both
propanol.
Thus,
more for
optimal average and penetration a two stage process should be employed. The feasibility
of
subsequent
deposition
of two different
layers of materials with zeta potentials of the same polarity was shown for deposition of SiaN4 on AI 2 TiOs (cathodic deposits) and deposits). hundreds of
of
SiC
on
Crossecttions
a of
glass
ceramic
layer
(anodic
the two layers in the order of
microns were prepared and elemental analysis and
x-ray images are shown.
INTRODUCTION Basic principles and parameters of electrophoretic deposition of ceramic particles in the pores of a conductive porous substrate, were described in two pievious publications (1,2]. These reports summarized studies of deposition of colloidal Si0 2 from aqueous and nonaqueous suspensions on porous graphite. It was shown, experimentally 111, that penetration is enhanced by a high ratio of dielectric constant to viscosity of the solvent as well as by increased particle concentration and electric field. At optimal conditions the whole crossection of the porous graphite substrate was penetrated and a skeleton of SiOa with the same shape and dimensions as the original graphite specimen was obtained after burning off of the graphite. A theoretical analysis [2] of the penetration enables to predict the penetration depth as function of two non-dimensional parameters, the Peclet number (Pe) and the deposition rate (A). Large Pe and low X enhance the penetration. In addition to the above mentioned results the feasibility of deposition of a variety of other oxide and non oxide ceramic materials (fused SiO2 , SiC, SiN) was demonstrated. The present report is an extension of concetrating on the following two aspects: Surface deposition ceramic material -
and
penetration
of
these a
studies
low expansion
- A1 2 TiOs - aluminium titanate on porous
graphite. Multilayer electrophoretic deposition which includes deposition of two consecutive layers of different composition: AI2 TiO 5 and Si 3 N4 as well as a glass ceramic and SiC.
The interest in the low expansion ceramic and the multilayer deposits stems from the intention to apply electrophoretic deposition for the coverage and impregnation of carbon-carbon composites with ceramic materials so as to prevent oxidation at elevated temperatures. Carbon-carbon (C-C) composites are of great interest for aerospace applications since they exhibit excellent high-temperature mechanical stability and low weight. However, they are characterized by a residual porosity and low oxidation resistance in air at temperatures above 500"C [3-5]. Another feature of these composites is their low thermal expansion coefficient (1-3 x 10-/*C). The ceramic protective coating should therefore be characterized
3
by a low expansion coefficient so as to prevent its cracking during use. (Al 2 TiOS) has one of the lowest thermal expansion coefficients as seen in Fig. 1 (61. Therefore, it is a plausible candidate for the application mentioned above. However, the expansion of this material is anisotropic and therefore microcracking will occur when applied as a coating (7]. The detrimental effect of the microcracking on the oxidation protection can be remedied by another coating deposited on the Al 2 TiOs such as Si 3 N4 . A1 2 0 3 -TiO 2
In addition to the practical interest described above the study of sequential electrophoretic deposition of different ceramic materials is of interest for understanding the process and widening its scope. Thus another system (glass ceramic and SiC) was also deposited.
EXPERIMENTAL PROCEDURE The deposition and impregnation were performed on porous graphite - UCAR grade 45 with a 48% volume porosity and an average pore size of 60u. (The porous graphite serves as a model material for the C-C composite). The specimens were 20x20x7 mm. rhe ceramic materials deposited were: -
A1 2 TiO5 prepared at the Israel Ceramics Institute with 5% BaCO3 added for sintering enhancement. After attrition milling the average particle size was 1.42 u.
-
Si 3 N4 -0.5 V. SiC - 600 mesh ( N02-
+ 20H-
Dissociation of the salts: - AI(NO)
A A>
-
3
- ZrO(NO,) 2
3)
40H-
+ 4e ->
+ 2H 2 0
02
- NO"
2)
expected to be the following:
->
+ 3NO+
ZrO2
-
+ 2N03-
ZrO2° + H2 0
Interaction of hydroxyls with the cations: A1
3
-
+
Zr(OH)
2
30H-
->
2
20H-
'
+
AI(OH)3
->
Zr(OH) 4
--
Zr(OH)
22
3
4)
Dehydration of the hydroxides: 2AI(OH)
A
3 -
Zr(OH)4
2
+ 2H 2 0
Zr0 2
->
The hydroxides
+ 3H2 0
03
form
thus
electrochemical generation
of
in
two
OH-
and
between the cations and the hydroxyls. description of the co-deposition is
successive a
chemical
steps: reaction
A schematic
given in
Fig.
1.
The yield of the deposit depends on the faradeic efficiency of OH- formation (affected by reduction reactions that do not result in
OH-
the fraction substrate.
formation such as H- + e -)
of
hydroxide
The
latter
formed
parameter
diffusion coefficients of OHZr(OH) 2 2 ÷ on deposit will A13 - and
H2 and
at sites removed from the depends
on
the
relative
on one hand and that of A13 - and
other.
The ratio of A12 0 3 and ZrOa in the depend on the relative diffusion coefficients of
Zr(OH)22-
generated at of the
the
H H + H ->
the
cations
which
compete
for the OH-
ions
cathode,
and the solubility constants (K.,) two hydroxides formed. A quantitative analysis of the
kinetics of deposit formation and its take into account diffusion phenomena hydroxide formation.
all these factors assuming that are the rate determing steps in The
the the
present paper describes the effect
of electrochemical morphology and
composition will have to
deposition composition of
parameters the
A1 2 03
on
the
and Al2 03
rate, +
ZrO2
deposits. EXPERIMENTAL Deposits were obtained on graphite specimens (UCAR45) 2 x 10 x 20 mm. They were polished with a 1000 grit SiC abrasive paper, then rinsed with ethanol in an ultrasonic bath, washed with distilled water and dried in 0.02M - 0.2M Al(NO0).
air.
Electrolytes were:
with and without
0.01M - 0.05M ZrO(NO3 ) 2
4
Deposition was
carried out at constant current with a Horizon
Model SR-365 d.c. 14 x
28
mm
were
measured with experimental the
AVO
Cell
meters.
set-up is
voltages A
seen in
of
Fig.
2.
curves were
stirrer
obtained
Deposits were an accuracy 1200"C for
with
dried in of
1
0.05 hr in
during a
air
currents were
drawing
of
the
Current densities were
and deposition times were
20 sec - 40 min.
magnetic
and
schematic
the range of 15-60 mA/cm
range
with a
used.
Two counter anodes made of Pt
2
varied in in
power supply.
Mild stirring
was applied
deposition.
Polarization
PAR
Model
270
potentiostat.
and their weight determined within
mg.
Some deposits were heat treated at
argon.
Morphology and microanalysis were
studied with a JEOL SEM Model JSM 840.
X-ray diffraction was
performed with a diffractometer Model PW-1730 operated at 40 KV and
40 mA using Cu k-alpha radiation and scanning speed of
1.2 deg/min.
RESULTS AND DISCUSSION Cathodic polarization solution of Al(NO)) first
and
-1000
region is
mv SCE in
A limiting
current
of
setting in
at
same
the
similarity in
by the
seen (down to -800
place
Fig.
3a,b.
mV SCE in
a In
the
the second solution respectively). mA/cm2
is
seen
potential
of
-2000
u10
the two curves is
reaction taking current being
an AI(N0 3 ) 3 solution and in
and ZrO(NO 3 ) 2 are given in
3
both curves a Tafel
curves in
in
in
both curves
mV
SCE.
The
a result of the same cathodic
both
systems
with
the
limiting
determined by the hydroxide deposit rather than
concentration
of
the
NO3-
which
is
higher in
the
combined solution. Fig.
4
presents the variation in
time for two current densities.
cell voltage with deposition
At the higher c.d.
(25 mA/cm2 )
the voltage increases steeply for the first 3.5 min as a result of the build-up of the hydroxide deposit and then levels off. weight of
Yet,
as
seen
in
the deposit continues
Fig.
5 for the same c.d.,
the
to rise slowly after 3.5 min.
5
The cell
voltage
local breakdowns
stability in
deposition
significant
increase
2
mg/cm /10sec) change in
form on
then
breakdown.
The
lower c.d.
(20
voltage takes
.
A
its
As
different,
the
the
of the A1 2 0,
stage
due
to
the
a monotonic
as-deposited, in
on the deposit
increase in
is
seen in
deposit
Fig.
(1G min). Figs.
consisting
6a.
A
seen in
very Fig.
6a of
dense
6b.
Two
microcracks that formed during
non-uniform contraction of the wet
bubbles attributed to H2 formation as a result of
H, reduction.
In
preliminary studies
aim of controlling
the density
of
drying rate is An attempt
cracks
to thick
current.
Pulses
overcome
the
formation of defects and yet to
deposit
was
made
of
voltage
reached when
was shown that
can be reduced significantly when the
by
the
use
2 min duration at 25 cA/cm'
with intermissions of 5 sec (see Fig. increase in
(to be published later)
the drying rate it
lowered.
obtain a
obtained.
by
entire time interval
seen
ions
voltage/time curve at the
close-packed
is
depends
created
the individual clusters is
coating and with the
build-up
sites
is
for
The
the nonconductive deposit
further
kinds of defects are also seen: the drying
conductive and the OH-
mA/cm2)
relatively
structure of
20 secs (:0.3
explained by the fact that at the
of
individual clusters
The slope of the
the first
behaviour place
without further
slows down to about 1/10 of that.
reduction
The morphology
continue
for
surface.
substrate
of reduction which
cell Voltage.
high
rate is
entire
availability of
& b
to
whole susbstrate is
the
covers the
the
very
deposits
beginning the
process in
is
and
explained by the formation of
the film creating sites
enable the
deposition curve
is
up
applying
to
a
7).
were applied
A relatively smooth
much higher voltage than that
a continuous current
Accordingly,
of a pulsed
(25V
vs.
08V)
was
the deposit was thicker and smoother.
This effect can be explained by the possibility of creation of new nucleation
sites
for deposit formation during the current
intermission. X-ray diffraction deposits fired
patterns at
1200"C
of
dry as-deposited coatings and for
1
hr,
according
to
the
6
recommendation of (dried in
(5),
are seen in
Fig. 8.
The as-deposited
air)
coating does not show any diffraction pattern except that of the graphite. Crystallization of
the
deposit
takes
place during the heat
treatment with the formation of a aA1 2 03 . The crystallite size, as calculated with the Scherrer equation (6) from the line broadening effects exist
of
the
due
to
110 the
reflection, substrate,
assuming no strain
was found to be about
1oam. Deposition from solutions ZrO(NO 3 ) 2 was carried out 14"C. The dependence seen in Fig. 9. A place in
spite
deposition of
of
containing at two c.d.
both AI(NO 3 ) 3 and (30 and 60 mA/cm2 at
of cell voltage on deposition time is more gradual rise in the voltage takes the
A1 2 0 3
higher only
c.d.s when compared with the
(see
Fig,.
4).
A
possible
explanation is the difference in deposit properties as a result of the presence of Zr(OH) 4 . From our previous work (3) it can be inferred that the voltage reached prior to breakdown onset is much higher for Zr(OH) 4 , at the same c.d. and so is the deposit weight per unit area. The AI(OH)a forms probably a denser Zr(OH)
4
film is
The phase
at lower thicknesses and thus is
modified when
codeposited. structure
of
a combined deposit was studied after
forming at 1200"C for 1 hr. The x-ray diffraction spectrum shows the presence -of two phases: tetragonal ZrO2 and %A12 0 3 (Fig. 10). The were studied in
morphology of the deposit and microanalysis the SEM by x-ray imaging of Al and Zr and
local elemental
semiquantitative
the x-ray
analyses.
Fig. 11-12 shows
image of Al and Zr in a combined deposit along with
the spectrum
and
semiquantitative
results
of the Al and Zr
content. The relative solution is
of the A1 3 - and ZrO 2 - ions in the to affect the deposit composition. The
concentration expected
latter was therefore studied (in terms of AI/Zr atomic ratios as function of the same ratio in the solution). Fig. 12 shows
7
this
dependence.
concentrations solution, can be of the
It in
is
the
seen
deposit
hence the Al(OH)3
is
that
is
two
ions
diffusion rate
A1
and,
hydroxide formation.
higher
preferentially
explained by the differences 3
much
the
in
Al
than in
the
deposited.
This
diffusion coefficients
and ZrO2 with the Al- 3 therefore,
relative
having a higher
being more available for the
LIST OF FIGURES
Fig.
1:
Schematic description of the co-deposition of ZrO2 +AI 2 0
3
.
Fig.
2:
Experimental set-up for deposition.
Fig.
3:
Cathodic polarization curves in a.
0.05M ZrO(NOa)
2
b.0.O5M Al(NO3 )3+0.05M ZrO(N0 25"C, 3 mV/sec, vs. SCE. Fig.
4:
Cell voltage
as
function
densities (0.05M AI(NO3 )3, Fig.
5:
6:
),
of
time for two current
10 min,
25"C).
Deposit weight as function of time (0.05M AI(N0
Fig.
3
3
)j,
25 mA/cmr,
25"C).
A1 2 0 3 deposit from 0.05M AI(NO3 )3, 25 mA/cm2,10 min.
at
a) X75
b) X750 Fig. 7:
Voltage/time curves for deposition of A1 2 0 3 with pulsed current: 0.05M AI(NO)) 3 , 25 mA/cm 2 , t..=2 min toff
Fig.
8:
- 5 sec,
25'C.
X-ray diffraction spectra from: a) as-deposited A1 2 0 3 b) after firing at 1200'C for I hr.
Fig. 9:
Cell voltage
as
function
of
time for two current
densities for deposit formed from 0.05M Al(NO3 ) 3 +0.05M ZrO(NO 3 ) 2 at 14"C, 60 and 30 mA/cm 2 .
t=5 min c.d.
9
Fig.
10:
X-ray diffraction pattern of co-deposit.
Fig. II:
X-ray image and local analysis of a deposit formed from a 0.05M AI(NO3 ) 3 +O.OSM AI(NO0) 2 solution.
Fig. 12:
Atomic ratio of AI/Zr in deposit as functior of this ratio in solution.
10
ACKNOWT.FDGEMENT
by
Research sponsored Research,
Air
the
Force
Air
under Grant No.
U.S.A.,
Systems-Command,
Force
of Scientific
Office
89-0474. The U.S.
Goevernment
authorized to reproduce and distribute
is
withstanding
not
purposes
Governmental
reprints for
any
copyright notation thereon.
BIBLIOGRAPHY
1.
J.A.
2.
J.A.
Am.
Switzer, Switzer.
Ceram. R.J.
Soc.
Bull.
66,
MRS
Phillips,
1521(1987). Proc.
Symp.
121,
111(1988).
3.
L.
I.
Gal-Or,
Soc.
In
R.
Silberman,
Chaim,
Jour.
Electrochem.
press. Silberman,
4.
R.
Chaim,
I.
5.
A.
Ayral,
J.C.
6.
B.D.
Cullity,
Droguet, "Elements
Addison-Wesley Pub.
Co.,
L.
ibid.
Gal-Or,
Jour. of
Res.
Mat. X-ray
Reading,
4,
970,
1984.
Diffraction',
Mass.,
1978.
p.
254
NO 3 - Electrolyte e Substrate
NO 2 -
OH"
Zr(OH) 2++ = Zr(OH) 4 SJ4
(Cathode)
---- o-e
e
OH"
e
H20
H2 e.. 01-1F
Fig I
AI+3
AI(OH) 3
POWER SUPPLY - -(.'*)
-
23 45-
Power supply Ammeter Voltmeter Solution Holder
67 -
Graphite specimen Electrodes - Platinum
89-
Stirrer Plate
5
(-)
"
-
"
••4
-
-
-
-_
Fig 2: Experimental Set-up For Electrolytic Deposition
-
|
a 5 ' " . ..
.......
,J 10
I
,i , S,"'1
S ii " i...1
"'
*
y I SJI""l ...
,
..
z
-2400
b
Zii: 0 10
I%,a
0
'
1.0 C0 t( UA/CM^2)
. Is '
*I
..
I
"
'4 10
10
..
K,
0
iE
5-40
, ,,.... a
-. 0
10
Fig 3
10
I
, , , ,,,,I
1
2 10..
... , , , ...5! I0
, , , , .. S.I, '4 10
,. . 10
(Njj
ci
L'1
30VI -10A
00
Ln
CI00 (Z3/AlN)i
H
dci
a
b Fig.
6
Ln
jcLLJ
-9l
L
I
I
30V~lA i
CIO
IDO -j
A jb&) P)~ 7
(11i 047t LO I~7b
Co
1D LL)
ci I. cx r':
Lr") Cl% cr)
'O
a
Ia
c
Kz>e Go
LL.
Ein
U,
z Lii
AL
14
(A)
ilO
t
(vzLT0) 1V.J
(L~~
(ZOZ) I z
100). Cases in which X = 0 would obviously result in the deepest penetration. It rmght be achieved by introducing a repelling force between the particles and the pore walls, stronger than van der Waals forces. The asymptotic value of the cumulative probability for large values of z is always unity, since after a long time the probability that a particle has been deposited anywhere over the pore walls is unity (the particle cannot disappear). Figure 7 summarizes the foregoing results. We define the penetration depth as the zlb value for which P, - 0.9. In other words, 10% of the particles would pebetrate deeper than the value shown in the figure for a given set of parameters, the Damk6hler number X,and the Peclet number P,. Obviously, the 0.9 numerical value chosen for P, is quite arbitrary and a lower or a higher percentage could have been chosen. The basic interpretation of the results, however. .,ould remain unaltered. It is obvious from Fig. 7 that the penetration depth for a given value of X grows almost linearly with the Peclet number (which is not at all surprusing). The slope of the curves depends strongly on X,;the smaller the 4, value the deeper the particle penetration. Conclusions The following general conclusions can be drawn from the foregoing analysis. Deep penetration and coating is feasible by electrophoretic processes. Enhanced penetration is obtained for large electrophoretic Peclet numbers and small Damk6hler numbers Electroosmosis (directly dependent on the electroosmotic permeability -f and thus on the sign of the wall zeta potential) increases the Peclet number and thus penetration only if the electroosmotic
-"
4
2
6
8
10 1
Pe Fog 7. P"s deo (deflee Wthe do•irm "Ave ef 11b fo whid the is 90% piabildhy tht the pwmete ho dwmowd ow a Y le" time) v. t tro Pelet au. (t Dankair namber is use as a Parameter). and electrophoretic velocities coincide in direction. Le. 4w and 4, possess the same signs. The diffusion coefficient of the colloidal particles has a dual effect. Decreasing diffusion coefficient causes increasing Peclet number which has a favorable effect on penetration depth. Decreasing diffusion coefficient causes increasing deposition rate which has an uniavorable effect on penetration depth. The process can be controlled by a judicious selection of part. dce size and concentration, solution properties, electric field intensity and its time protocol Quantitative comparison with experimental results that we presented in Part 11 is not yet possible since only global results of total mass deposition inside the pores was measured and not how mass is distributed inside the porous slab (which is the main result we presented in this paper). Further experimental work is required to analyze this very important feature of deep electrophoretic deposition. which will determine its advantage over other coating processes. On the other hand. the total of mass deposited L side the porous slab depends on the rauo between the flux of particles depositing over the exterior surface of the slab and flux of particles penetrating it. Thus. further theoretical work is required to determine this ratio dependence on the process parameters before comparison can be made with the experiments conducted in Part IM Notwithstanding, several recommrnendations based on the theoretical results were made and found in qualitative agreement with experiment, e.g. the electroosmotic permeability depends Linearly on the ratio rv• which depends solely on solvent properties. Enhancement of penetration is predicted to occur if this ratio is high. Several liquids were examined (water, isopropanol, ethanol and pentanol) and water nossessing the highest ratio has shown the best results (Fig. 9 in Part ID. A rather limited quantitative comparison with Fig. 9 in Part II can be made. We calcu. late the ratio of SiO2 percentile of particles deposited inside porous graphite and migrating in a certain solvent to SiO%percentile of particles migrating in a given solvent, say, water. solvent
y(theory)
y(expenment)
0.052 0.112 0.283
0.078 0.29 0.42
Pentanol Propanol Ethanol where
%SiOsolventl " %SiO~water) The foregoing table reveals that the trend predicted in this paper is defirutely correct and there is a fixed bias which can be attributed to inaccuracies in measurements (difficulty in removing the deposit over the extehor sir-
J. Electrochem. Soc., Vol. 139, No. 4. April 1992 C The Electrocrhemical Society. Inc. face of the graphite slab), variations in zeta potentials and mathematical model simplifications. Acknowledgment Research sponsored by the Air Force Office of Scientific Research Air Force Systems Command U.S.A.F. under Grant No. AFSOR 89-0474. The U.S. Government is Authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. Manuscript submitted Feb. 15, 1991; revised manuscript received Oct. 17, 1991. APPENDIX The flow through porous media generated by an electric potential gradient is discussed in (28) and is briefly recapitulated. The governing field equations for the flow through a long circular tube induced by an electric potential gradient are based on Stokes' equations for low Reynolds number flows. iLe. V = Vp, V-v=0 [A-1] where v and p are the velocity and pressure fields and -n stands for the viscosity of the fluid. The boundary conditions which the velocity field satisfies over the cylinder walls are (17) f4. a6 , i,, = 0 at r = b
4"
-
[A-2]
where f is the dielectric constant of the fluid, ;,, is the zeta potential at the cylinder wall, d stands for the electric potential, b is the radius of the tube and (r, z) are polar coordinates. the z coordinate coinciding with the cylinder axis. It should be noted that the no-slip condition can no longer be applied and in its place we use condition [A-21. The solution for v is easily derived for a fully developed flow, namely, avpaz = 0. It is a simple superposition of the effects of the pressure gradient and the electric potential gradient. Integrating over the tube cross section. the mean interstitial velocity is obtained b, ap V,=
- -
-
8-q az
q. -
a'
--
am
[A-3]
Integrating over the entire cross section of the porous medium the superficial velocity is v=
81
az
where ov stands for the medium porosity. Since a homogeneous porous medium contains pores in all possible directions, a simular procedure can be applied for pores oriented along the x and t, directions. Expressions similar to [A-4] can be obtained for v and ý,, and fur, ther generalized to derive the following expression for v in vectorial notation [A-51
where
r=,
= FPj
such
as
the
silica: 2SiO-
b) adsorption mode,
+ H2 0
(negatively charged particles)
such as the adsorption of H- from water or
from weak acids: n AI(OH)
3
+HNO 3->[AI(OH)
3
1-,xH'+xNO-3
(positively charged particles). The surface
charge
opposite charge layer is
once formed is
derived
from
the
then balanced by ions of an solution and thus a double
formed.
Usually the
polarity
of
experimentally because spontaneously when
mixed
the the with
particles particles the
has to be determined acquire
solvent.
the
Further,
charge this
charge may be reversed upon addition of ionic compounds. According to constitutes of
the a
model
proposed
by
Stern
the double layer
rigid part with a linear potential gradient,
4
and a
diffuse part with a non-linear potential gradient termed "Zeta pqtential".
CHARGED PARTICLE ++ +
+
+
IONIC CLOUD
+
+ + +
+ +
+
+
+
0
+
+
++
POT ENTIAL
DISTANCE Fig. 1:
The double layer and potential gradient surrounding a charged particle.
The Zeta
potential
can
be
determined
by
two main methods:
microscopic determination of the velocity of individual particles using the Smoluchovski equation or in the Burton cell where a pure filtrate of solvent is poured on the suspension, a field is
applied
and
the
movement
of the sharp boundary is
monitored [2]. When an electric field is
applied to a charged particle it
will
5
move to
the
electrode with the opposite charge.
However,
the
diffuse double movement. The to which
it
layer around the particle interferes with its diffuse charge tends to move with the particle is attached but on the other hand it is influenced
by the electric field which pulls it The particle
apparently wins.
It
in the opposite direction.
moves through the liquid with
a diffuse egg-shaped double layer surrounding it (although it is not actually carry'ing the oppositely charged ionic atmosphere but rebuilding it ions in the the movement produces a
is in
rather
leaving
front as it
part
of
it
behind
moves along).
The tendency of the diffuse d.l. to move in the direction opposite to of the particle has an effect on the velocity. It "drag"
which
slows down its movement
(see Fig.
[3]:
DRIVING FORCE
RELAXATION FORCE
RETARDING
SFORCES
(ELECTRIC)
VISCOUS FORCE
L64-SHAPAe IONIJC CLOUD
Fig:.
and
Egg-shaped ionic cloud and forces acting on a moving particle.
2)
6
The mobility of the particle is derived by equating electric force with the frictional resistance and relaxation force. Thus the mobility U is given by: )=
EEý/4nX[l+f(kr)I
the the
where:
U
-
mobility
E
-
field
E
-
dielectric
-
zeta potential of particles
-
viscosity of fluid
K
-
1/double layer thickness
r
-
radius of particle
strength constant of fluid
f(kr) varies
between
0 and 1 for small and large values of kr
respectively. f(kr) represents the relaxation phenomenon, The Smoluchowski equation is
ii=(E-E-
Values of
ý
u
k [Al(OH)
3
1. XH'+XN0
3
placed in water an electric charge develops on it from the transfer of ions between the surface and the liquid balanced by ions of an The surface charge layer is phase. opposite charge in a diffuse layer on the solution side of the The two layers constitute the electric double interface.
When silica
layer.
is
The
potential
just
potential outside
constitute the charge of sign of the surface charge.
is the
a
measure
layer
of
the surface;
of
the
adsorbed the sign of
electrical that
ions •
is
the
10
can be measured also by passing a solution under pressure P through a packed bed of particles and measuring E across the bed.
=4w
-
rIX E/EP
viscosity.
X - specific cond. E - dielectric constant Most silicas (:Si-OH)
have
a
certain
population
of
silanol
groups
which can dissociate:
ESiOH+OH- -)>
i(SiO')+HzO
Thus a negative charge forms on the surface. Certain ions
such
as A13- can be adsorbed and change the sign of the 4 potential. On top of them citrate ions can be adsorbed to change again to-(91. Consecutive transformation of charge is following events [101: Silica (neg.
charged)
+ A13 - ->
possible such as in the
positive charge citrate ions ->
+
neg.
charge
Various degrees of hydration are possible. Thus silica particles prepared by precipitation in water or long ageing in it have surfaces entirely covered with ESiOH groups. If heated at >400"C the following happens: £Si
2ESi-OH
->
=
O+H2 0(siloxane groups)
O/ 2 Si At 1200"C only siloxane remains and hydration is
slow.
11.
on the
solvent.
Therefore,
the
coated
electrode
must
be
determined experimentally for each material since the particles acquire the charge spontaneously when mixed with the solvent. Further this charge may be reversed upon addition of ions. 2.2
ELECTROPHORESIS IN POROUS STRUCTURES
previous sections the assumption was made that the particle is suspended in an infinite fluid. When electrophoresis in porous structures is considered the presence of rigid boundaries will affect both the electric field and the In the
velocity of the particles. According to
J.
Anderson
(11,12,131
the
pore wall produces
three effects on the particle velocity: -
-
-
The applied electric field exerts a force on the d.l. of the An electrosmotic the wall is charged ( I .' pore wall if flow of the fluid is thus proc ted which either augments or of the particle velocity electrophoretic opposes the . depending on the polarity of ,. vs. The pore wall distorts the electric flux (current lines) around the particle thereby intensifying the local electric field so that the particle velocity is enhanced. The pore wall creates additional viscous stresses in the
fluid which retard the particle velocity. Anderson analyzed the effect of the presence of a pore when the located in the centerline of a long pore. Particle particle is Two geometries are considered: a neglected. interaction is cylindrical one and a slit
X=a/R
x=a/B (B
a
- particle radius,
(R - pore radius)
- half width).
The basic equations which apply to this system are:
nVIV-Vp=O
(conservation of charge) (Stokes eq., velocity dominated by
7°V=0
viscous stre-ses). (conservation of mass)
17-E=O
12
Solving these .quations one obtains: n UL=[(-1.2899X+l•8963X5 -l.0278X0 +O(Xs)x• for a cylinderical configuration, U,=[l-0.
and:
2 6 7 7X3+0. 3 3 8 3X-0,0402Xe÷O(Xa)$E(,pDZw)Ev 47M
for a slit
configuration
It
be
should
noted
that the pore size effect enters first
at
Therefore X has a weak effect. This is because of a fast decay of electrical and velocity disturbances from a particle A,3
moving in an unbounded field. The disturbance to the electric field is Ep=.L (
) 3[1_I -
V =_1a
3[
1
given by:
-3Ec
T7 3 UO
u
(ýV - w) Ea
Both fall off with X3 . The pore
wall
intensifies
the
electric
field
m..ving
the
particle faster but the effect on the hydrodynamics leads to a larger retardation, thus overall the velocity is reduced as the pore size decreases. In a
closed
system
the
average flow of solution through the porous medium is zero and a macroscopic pressure gradient develops to oppose the electroosmotic flow produced by the electric field. The cross-section available for the particle 1 is ( -X)l for a cylinder and (1-X) for a slit. cylinder: U=-•' E.[ 1(
(2 X_72)-_(i-r)X,289X3]
"p• slit: U
•--A p 47M E.[I÷(_-.w) (),-
2 IpI -_...
"0.p2677,'] I
13
Here the. electrophoretic effect comes in at 3 and is weak while the pressure flow has a contribution dependent on and has therefore a stronger contribution. It
is
to be stressed that in
the above analysis the pore walls were considered to be non-conductive. 2.3 It
ELECTROCHEMICAL DEPOSITION OF CERAMIC FILMS has
recently
[141
been described in literature that it is possible to synthesize ceramic films from water soluble ceramic precursors by an electrochemical method. The method is based on cathodic reduction of water or another anion as N0 3 -, to generate OH-.
A
secondary
appropriate metal formation of the Switzer (141 has catholyte was contained 1.0M
1.0M
ion
reaction
present
between
the
OH- and an
in
the solution can result in hydroxide and oxide of that metal. Thus, synthesized CeOm in a system where the cerous
ammonium
nitrate
and the anolyte
NaNO 3 .
The cathodic reactions which took place on a platinum cathode at c.d. of 50mA/cm2 were: 2H 2 O+2e ->
H2+20H-E,--0.83V
N03-+H2 0+2e ->
N0 2 -+20H-
Eo = 0.01V
The mechanism of the secondary reaction is may be: Ce 4 *÷4OH-
->
not clear yet but it
Ce(OH) 4
Ce(OH) 4 -2H 2 0 ->
CeO 2
The powder deposited on the cathode was identified by X-ray diffraction to be ceric oxide with a cubic lattice constant of 0.5409nm and a crystallite size of 7 .0nm. After ultrasonic dispersion the material was found to have a narrow particle size distribution with an average particle size of l.8U. The
14
morphology of
the
ceramic
film
depend
will
mainly
on the
Thus highly electric-conductivity of the oxide formed. conductive materials can form thick dense films or powders. It
is
possible
This is
which a
substrates.
By
by an anodic reaction.
0.5M
hydrolysis
to
an hydroxide or
has deposited thallic oxide films from a
1141
acetate in
thallous by
1.OM NaOH on silicon
reactions oxides of Ni,
similar
deposited
Mn were
undergoes
it
Switzer
solution of
oxides
synthesize
on the oxidation of a metal ion to an oxidation
based
state at oxide.
to
Cu,
Co,Fe and
Tend and Warren 1151 while Sakai et al.
f16] have deposited a mixed Pb-Ti oxide film. The methods
described
above
differ
from the electrophoretic
deposition of colloidal ceramic particles. that they
can
become
powerful
However,
we believe
for coating flat and
methods
complex shapes as well as for impregnation of porous structures with ceramic materials. 2.4
ELECTROCHEMICAL REACTIONS IN POROUS ELECTRODES
in Electrochemical reactions studied in two systems mainly: in fuel
electrodes
cell
electrodes
porous
have
been
and in electrochemical reactors used
for processes such as metal ions removal from waste water. Two groups
of
diffusion porous kinetics in researchers One of
such
exist:
electrodes
forced
flow
through
and
Theoretical analysis of reaction have been performed by numerous
electrodes. systems
(17,18,19,20].
the models used in
the analysis of diffusion electrodes
is the "straight pore" model
(see Fig.
5).
15
Z=O' TI=
Z=i n=TJ
0
r2"
a b b_
PLdz
Za
Fig.
This model
The straight pore model for diffusion electrodes and the current distribution.
assumes
that the electrode consists of a number of identical, straight, nonintersecting, cylindrical pores running
through the entire length of a conductive matrix. The basic
assumptions
filled with
in
this
model
are that the pores are
electrolyte, no potential drop exists in the solid
electrode so that potential gradients are in the solution only, the concentration of reactants is high so that no mass transfer limits exist. The potential distribution in such a system is:
TI
4RT z - qO = -F
16
The current,
up to point Z-I., and the total pore current
-
is:
I=tanf.-3 (XC)
1 k= os-'-h Yo
2io.Z2 F K=.......
---
2RT
22
where io exchange current S-
overpotentiai.
The current distribution will depend mainly on conductivity of the solution. For i2