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Electrostatic formation of ceramic membranes using CVD ultra-fine particles
Journal of Electrostatics, 25 (1990) 125-133 Elsevier
125
Electrostatic formation of ceramic membranes using CVD ultra-fine particles Hideo Yamamoto, Tsuyoshi N o m u r a Institute of Industrial Science, University of Tokyo, Minato-ku, Tokyo 106, Japan
and Sen-Ichi Masuda Fukui Institute of Technology, Fukui 910, Japan
(Received November 9, 1988; accepted in revised form September 24, 1989)
Summary A new method for forminga ceramic membrane has been devised. Ultrafine particles of silicon nitride synthesizedby thermally activated CVD (ChemicalVapor Deposition) were deposited on the outer wall surface of a porous ceramic tube (substrate) by an electrostatic force and sintered in an inert gas atmosphere. The ceramic-made electrode assembly has been used for charging ultra-fine particles efficientlyby surface discharge. A new ceramic membrane with a three-dimensionalnetwork similar to a fiber filter was obtained by this method. Its effectivepore size was found to be around 0.2-1.0/~m in diameter with high porosity.
1. Introduction CVD ultra-fine particles (particles under submicron size prepared by the chemical vapor deposition method) have generated considerable interest recently as advanced industrial materials because of their superior chemical and physical properties, purity and uniformity of particle size. At present, however, their production cost is very high, and there are m a n y handling difficulties in their use as industrial materials in large quantities. For instance, it is necessary to prevent contamination of the particle surface and to develop new handling techniques for separation, mixing, dispersion, classification, formation etc. Thus it is important to develop special uses for t h e m in small quantities and with high value added. From these viewpoints, a new m e t h o d for forming a fine porous ceramic membrane has been proposed and investigated as one of the advantageous uses of the CVD method. The principle of this m e t h o d is as follows: CVD ultra-fine particles are charged directly after production, deposited onto a surface of a porous ceramic
0304-3886/90/$03.50
© 1990 Elsevier Science Publishers B.V.
126
substrate by an electrostatic force to form a particle layer, and sintered into the membrane. These processes from particle production to sintering are carried out continuously in an inert gas atmosphere. This prevents contamination and agglomeration of the particles during handling and produces a fine porous membrane. We call this method "Electrostatic Formation of a Ceramic Membrane (EFCM)". It is very important to develop this method successfully for application of this membrane to water treatment under high-pressure and high-temperature conditions, to separation or condensation of organic solutions, or to the cleaning of high temperature gas. This paper describes the principle of the electrostatic formation of a ceramic membrane, the structure of the membrane prepared in these experiments and the results of basic performance tests of the membrane made from silicon nitride particles and deposited on the outside of a porous ceramic tube. 2. E x p e r i m e n t a l apparatus and the principle of electrostatic formation of a ceramic membrane
Figure 1 shows a schematic view of the experimental apparatus which comprises a reactor and a membrane-forming section, contained within a 42 mm diameter quartz tube (1). The reaction tube consists of four coaxial cylinders within the quartz tube (1). In the membrane-forming section, an a.c. corona surface discharging electrode (3) and a d.c. electrode (4) are positioned in order to charge the ultra-fine particles travelling directly from the reactor and supplied with high-frequency high voltage (10 kHz, 10-20 kV) and high d.c. voltage (below 5 kV) by two power supplies (5) and (6), respectively. A ceramic cylindrical electrode assembly [ 1 ] is used for the electrode (3) to form a plasma ion source on its inner surface stably even under high temperature conditions and in a complex gas composition. Supplying a d.c. voltage between the electrodes (3) and (4) produces monopolar ions which are emitted from the ion source along the inner surface of the electrode (3) and travel across the charging zone between the electrodes (3) and (4). Ultra-fine particles prepared in the thermal reactor are carried down with the sheath gas (N2) which flows from the inner and the outer inlet like a thin cylinder. When they reach the forming section, they are charged with the same polarity and travel across the d.c. field to deposit on the outer wall of the ceramic substrate (7) fixed on the center axis. The substrate can be moved up or down and rotated in order to acquire uniform deposition. The particle layer formed on the surface of the ceramic substrate is sintered into a membrane at high temperature, without being taken out of the apparatus, by heaters located around the forming section. SIC14 and NH3 gas are used for preparing silicon nitride particles [2]. This reaction proceeds under excess ammonia, ammonium chloride being deposited as the by-product following the reactions:
127
3SIC14 + 4NH3 - - - ~ Si3N4 + 12HC1 12HC1 + 12NH3
' 12NH4C1
The deposition onto the substrate can be prevented by heating the forming section above about 340°C which is the sublimation temperature of ammonium chloride [3]. The porous ceramic substrate utilized in this experiment is a sintered alumina tube with outer and inner diameter of 10 and 7 mm, respectively, and length of 140 mm. Its mean pore size is 10 pm according to catalog data. 3. Membrane formation
3.1 Size and size distribution of the particles prepared by CVD It was expected that the pore size of the membrane formed by sintering of the deposit would depend on the size of the constituent particles and that a smaller pore size would be produced by a smaller particle size. The size of the particles prepared by thermal CVD depends upon the concentration of the N2
f
SiCI4+N 2 NH3+N2
N2
1.quartz tube
2.heater
3.discharging electrode
",+
4.DC elecl
c
__
'+~
5.AC high voltage
l~ I
D f
7.porous substrate (tube)
6.DC high voltage
8.substrate guide
Fig. 1. Schematic diagram of experimental apparatus.
9.trap
V-
128
reactive gases and the reaction temperature and therefore the particle deposition condition must be optimized. The reaction conditions were not studied and kept constant throughout this experiment since the first aim of this investigation was confirmation of the possibility of the formation of a ceramic membrane by electrostatic processes. Figure 2 shows a TEM micro photograph of the particles, and Fig. 3 shows the size distribution measured by the photo extinction centrifugal sedimentation method. For these experiments the reactive conditions were: concentration of SiCl+ was 1.4%, the ratio of NH3 to SIC14 was 6, and reaction tempera-
Fig. 2. T E M photograph of silicon nitride particles prepared by thermally activated CVD.
~1oo
50 ~
80
40 ~
.*4
50 Z 40
2O
~ 20
IO
~
o
0.01
m
o o.1
1.o
particle diamete'r [pm}
Fig. 3. Size distribution of the particles in Fig. 2 measured by the centrigufal sedimentation method.
129 ture was 1200°C. As may be observed in Fig. 2, the particles are of uniform diameter of several tens of nm, and the median diameter is 70 nm according to Fig. 3. The size of the particles prepared by CVD can be changed by varying the reactive conditions, but this will be investigated in later experiments and described in another report. The particles shown in Fig. 2 were used in this work.
3.2 Electrostatic deposition of CVD particles Figure 4 shows a photograph of the particles deposited on the substrate before sintering. The upper sample is the alumina substrate. Both the particles and substrates are white, so they are difficult to distinguish. To avoid this difficulty the middle specimen shows the particles which are deposited on a quartz tube. The lower specimen shows the test pieces prepared for examination with an electron microscope. The white part is the deposit layer in the middle sample. It was observed that the particles deposited uniformly all over the tube surface. The length and thickness of the particle layer deposited on the substrate was controlled by distance and velocity of movement of the substrate and the concentration of the particle produced, respectively. Figure 5 shows a SEM micrograph of the surface of the particle layer deposited by electrophoresis. It shows a structure peculiar to the electrostatic deposit. The structure of the particle layer is supposed to be reflected by the membrane structure as discussed in the following section. The deposit struc-
Fig. 4. Photographof the particlesdepositedon the substrate beforesintering.
130
Fig. 5. SEM photograph of the surface of the particle layer deposited by an electrostatic force {before sintering).
ture is expected to be controlled by the intensity of the d.c. field and the quantity of charge on the particles. 3.3 The structure of the membrane It is necessary to sinter the particle layer to provide as a porous layer as possible, with the largest voids, to form a better membrane by electrostatic deposition. In sintering ceramic materials, the temperature, time of reaction, rate of temperature rise and the atmosphere must be controlled. In these experiments, the deposits were sintered under various trial and error conditions and the membrane structures examined by SEM. The silicon nitride particles prepared in this experiment could be sintered at 1200-1300°C in N2. This is lower than the sintering temperature of Si3N4 when utilized as industrial material. For this reason these particles are considered to be very small and amorphous, or the deposit layer is very thin. As this was a very interesting phenomenon, it was considered necessary to study it in more detail. A cross-sectional photograph of the membrane is shown in Fig. 6. This shows that a dense membrane about 20 ~m thick is formed on the 1.5 m m thick alumina substrate. Figure 7 shows the surface structure. This membrane was sintered at 1200°C for 2 hours in N2. A very peculiar membrane with a threedimensional network is observed. It is assumed that this structure resulted from the deposition of the electrophoresis particles shown in Fig. 5. A mem-
131
Fig. 6. Cross-sectionalphotographof the ceramicmembrane.
Fig. 7. Surfacestructureof the ceramicmembrane. brane with this structure has a large porosity similar to a fiber filter and, therefore, is advantageous in keeping the pressure drop very low when it is utilized form membrane separation. The effective pore size of this membrane is around 0.2-1.0 ~tm according to the SEM photographs and to measurement of latex-particles filtration. Thus this membrane can be placed in the category of micro-filtration. Now, the control of the particles produced and the structure of the particle layer deposited by an electrostatic force will be investigated for much smaller pore size.
132 4. The basic characteristics of the m e m b r a n e Figure 8 shows the comparison of the gas pressure drop between the alumina substrate and the membrane shown in the Figs. 6 and 7. A large difference in pressure drop is not observed, though the pore size of the membrane is 1/20th that of the substrate. This is due to the large porosity of the membrane and because the thickness of the membrane (20 ~m) is much less t h a n that of the substrate (1500 }~m). Figure 9 shows the fluxes of water through the substrate and the membrane as a function of temperature. The decrease of the water flux caused by forming the membrane is about 10%, and its resistance may be seen to be very low. 5OO
1.5
ion e x c h a n g e p = I kPa
water
substrate + membrane
~
1.2100
E
substrate
e4
~ 50
0.9 $ubstrate
0.6 10
~2 ga~
!
at r o o m
;
I
; i IIIII
0
0.5 Gas
temperature
;
I
I ; il;l
1
velocity
5
|
I I0
O,3 0
0.2 h . . . . .
100 ~
'U ~
Dp = 1 . 8
% 0.15 c~
nm
Cb t
= 1000 p p m = I O0 °C
•P
= 1.5
kpa
90 n
80
0.1 0
0:0~
0
30
60 Filtration
90 time
I
I
60
80
120
100
[ °C]
Fig. 9. Water flux of the membrane influenced by varying temperature.
0.30 L
0.20
I 40
Temperature
[cm/s]
Fig. 8. Gas pressure drop of the membrane.
,z
I 20
50
[mini
Fig. 10. Removalof silica particles in boilingwater.
133
Figure 10 shows the experimental results of separation of fine silica particles dispersed in boiling water. The silica is in the form of mono-dispersed particles of 1.8 ~m diameter. The rejection of particles is 100% at the water temperature of 100 ° C and under the filtration pressure of 1.5 kPa. The flux in steady state is around 0.1 m3/m2h. These results are very good for membrane separation. 5. Conclusion
A new method of forming ceramic membranes using ultra-fine particles of silicon nitride produced by a thermally activated CVD method has been investigated. It is possible to use the ultra-fine particles produced for membrane formation directly without taking t h e m out of the apparatus. By this method, the difficulty of handling and processing of ultra-fine particles can be avoided. Also surface contamination and particle agglomeration can be prevented. An effective and advantageous means for the utilization of ultra-fine particles has, therefore, been shown in this paper. The membrane produced is about 20 pm in effective thickness and its pore size is around 0.2-1.0 p m with a three-dimensional network structure. Also the porosity of the membrane is very large. This structure is advantageous for membrane separation and is a special feature which has not been observed in normal ceramic filters up to the present. The main aim of this experiment was the confirmation of the possibility of "Electrostatic Formation of a Ceramic Membrane (EFCM)", and the membrane produced in this experiment can be classified as a micro-filtration filter. In the future, investigations to extend the utilization of this membrane into ultra-filtration by realizing smaller pore sizes will be undertaken.
References 1 S. Masuda and E. Kiss, Proe. Int. Conf. on Industrial Electrostatics, Budapest, Hungary, 1984. 2 S.T. Buljan, U.S. Patent, 4,073,845 (1978). 3 H. Yamamoto, J. Soc. Powder Tech., Japan, 26 (1989) 163.
125
Electrostatic formation of ceramic membranes using CVD ultra-fine particles Hideo Yamamoto, Tsuyoshi N o m u r a Institute of Industrial Science, University of Tokyo, Minato-ku, Tokyo 106, Japan
and Sen-Ichi Masuda Fukui Institute of Technology, Fukui 910, Japan
(Received November 9, 1988; accepted in revised form September 24, 1989)
Summary A new method for forminga ceramic membrane has been devised. Ultrafine particles of silicon nitride synthesizedby thermally activated CVD (ChemicalVapor Deposition) were deposited on the outer wall surface of a porous ceramic tube (substrate) by an electrostatic force and sintered in an inert gas atmosphere. The ceramic-made electrode assembly has been used for charging ultra-fine particles efficientlyby surface discharge. A new ceramic membrane with a three-dimensionalnetwork similar to a fiber filter was obtained by this method. Its effectivepore size was found to be around 0.2-1.0/~m in diameter with high porosity.
1. Introduction CVD ultra-fine particles (particles under submicron size prepared by the chemical vapor deposition method) have generated considerable interest recently as advanced industrial materials because of their superior chemical and physical properties, purity and uniformity of particle size. At present, however, their production cost is very high, and there are m a n y handling difficulties in their use as industrial materials in large quantities. For instance, it is necessary to prevent contamination of the particle surface and to develop new handling techniques for separation, mixing, dispersion, classification, formation etc. Thus it is important to develop special uses for t h e m in small quantities and with high value added. From these viewpoints, a new m e t h o d for forming a fine porous ceramic membrane has been proposed and investigated as one of the advantageous uses of the CVD method. The principle of this m e t h o d is as follows: CVD ultra-fine particles are charged directly after production, deposited onto a surface of a porous ceramic
0304-3886/90/$03.50
© 1990 Elsevier Science Publishers B.V.
126
substrate by an electrostatic force to form a particle layer, and sintered into the membrane. These processes from particle production to sintering are carried out continuously in an inert gas atmosphere. This prevents contamination and agglomeration of the particles during handling and produces a fine porous membrane. We call this method "Electrostatic Formation of a Ceramic Membrane (EFCM)". It is very important to develop this method successfully for application of this membrane to water treatment under high-pressure and high-temperature conditions, to separation or condensation of organic solutions, or to the cleaning of high temperature gas. This paper describes the principle of the electrostatic formation of a ceramic membrane, the structure of the membrane prepared in these experiments and the results of basic performance tests of the membrane made from silicon nitride particles and deposited on the outside of a porous ceramic tube. 2. E x p e r i m e n t a l apparatus and the principle of electrostatic formation of a ceramic membrane
Figure 1 shows a schematic view of the experimental apparatus which comprises a reactor and a membrane-forming section, contained within a 42 mm diameter quartz tube (1). The reaction tube consists of four coaxial cylinders within the quartz tube (1). In the membrane-forming section, an a.c. corona surface discharging electrode (3) and a d.c. electrode (4) are positioned in order to charge the ultra-fine particles travelling directly from the reactor and supplied with high-frequency high voltage (10 kHz, 10-20 kV) and high d.c. voltage (below 5 kV) by two power supplies (5) and (6), respectively. A ceramic cylindrical electrode assembly [ 1 ] is used for the electrode (3) to form a plasma ion source on its inner surface stably even under high temperature conditions and in a complex gas composition. Supplying a d.c. voltage between the electrodes (3) and (4) produces monopolar ions which are emitted from the ion source along the inner surface of the electrode (3) and travel across the charging zone between the electrodes (3) and (4). Ultra-fine particles prepared in the thermal reactor are carried down with the sheath gas (N2) which flows from the inner and the outer inlet like a thin cylinder. When they reach the forming section, they are charged with the same polarity and travel across the d.c. field to deposit on the outer wall of the ceramic substrate (7) fixed on the center axis. The substrate can be moved up or down and rotated in order to acquire uniform deposition. The particle layer formed on the surface of the ceramic substrate is sintered into a membrane at high temperature, without being taken out of the apparatus, by heaters located around the forming section. SIC14 and NH3 gas are used for preparing silicon nitride particles [2]. This reaction proceeds under excess ammonia, ammonium chloride being deposited as the by-product following the reactions:
127
3SIC14 + 4NH3 - - - ~ Si3N4 + 12HC1 12HC1 + 12NH3
' 12NH4C1
The deposition onto the substrate can be prevented by heating the forming section above about 340°C which is the sublimation temperature of ammonium chloride [3]. The porous ceramic substrate utilized in this experiment is a sintered alumina tube with outer and inner diameter of 10 and 7 mm, respectively, and length of 140 mm. Its mean pore size is 10 pm according to catalog data. 3. Membrane formation
3.1 Size and size distribution of the particles prepared by CVD It was expected that the pore size of the membrane formed by sintering of the deposit would depend on the size of the constituent particles and that a smaller pore size would be produced by a smaller particle size. The size of the particles prepared by thermal CVD depends upon the concentration of the N2
f
SiCI4+N 2 NH3+N2
N2
1.quartz tube
2.heater
3.discharging electrode
",+
4.DC elecl
c
__
'+~
5.AC high voltage
l~ I
D f
7.porous substrate (tube)
6.DC high voltage
8.substrate guide
Fig. 1. Schematic diagram of experimental apparatus.
9.trap
V-
128
reactive gases and the reaction temperature and therefore the particle deposition condition must be optimized. The reaction conditions were not studied and kept constant throughout this experiment since the first aim of this investigation was confirmation of the possibility of the formation of a ceramic membrane by electrostatic processes. Figure 2 shows a TEM micro photograph of the particles, and Fig. 3 shows the size distribution measured by the photo extinction centrifugal sedimentation method. For these experiments the reactive conditions were: concentration of SiCl+ was 1.4%, the ratio of NH3 to SIC14 was 6, and reaction tempera-
Fig. 2. T E M photograph of silicon nitride particles prepared by thermally activated CVD.
~1oo
50 ~
80
40 ~
.*4
50 Z 40
2O
~ 20
IO
~
o
0.01
m
o o.1
1.o
particle diamete'r [pm}
Fig. 3. Size distribution of the particles in Fig. 2 measured by the centrigufal sedimentation method.
129 ture was 1200°C. As may be observed in Fig. 2, the particles are of uniform diameter of several tens of nm, and the median diameter is 70 nm according to Fig. 3. The size of the particles prepared by CVD can be changed by varying the reactive conditions, but this will be investigated in later experiments and described in another report. The particles shown in Fig. 2 were used in this work.
3.2 Electrostatic deposition of CVD particles Figure 4 shows a photograph of the particles deposited on the substrate before sintering. The upper sample is the alumina substrate. Both the particles and substrates are white, so they are difficult to distinguish. To avoid this difficulty the middle specimen shows the particles which are deposited on a quartz tube. The lower specimen shows the test pieces prepared for examination with an electron microscope. The white part is the deposit layer in the middle sample. It was observed that the particles deposited uniformly all over the tube surface. The length and thickness of the particle layer deposited on the substrate was controlled by distance and velocity of movement of the substrate and the concentration of the particle produced, respectively. Figure 5 shows a SEM micrograph of the surface of the particle layer deposited by electrophoresis. It shows a structure peculiar to the electrostatic deposit. The structure of the particle layer is supposed to be reflected by the membrane structure as discussed in the following section. The deposit struc-
Fig. 4. Photographof the particlesdepositedon the substrate beforesintering.
130
Fig. 5. SEM photograph of the surface of the particle layer deposited by an electrostatic force {before sintering).
ture is expected to be controlled by the intensity of the d.c. field and the quantity of charge on the particles. 3.3 The structure of the membrane It is necessary to sinter the particle layer to provide as a porous layer as possible, with the largest voids, to form a better membrane by electrostatic deposition. In sintering ceramic materials, the temperature, time of reaction, rate of temperature rise and the atmosphere must be controlled. In these experiments, the deposits were sintered under various trial and error conditions and the membrane structures examined by SEM. The silicon nitride particles prepared in this experiment could be sintered at 1200-1300°C in N2. This is lower than the sintering temperature of Si3N4 when utilized as industrial material. For this reason these particles are considered to be very small and amorphous, or the deposit layer is very thin. As this was a very interesting phenomenon, it was considered necessary to study it in more detail. A cross-sectional photograph of the membrane is shown in Fig. 6. This shows that a dense membrane about 20 ~m thick is formed on the 1.5 m m thick alumina substrate. Figure 7 shows the surface structure. This membrane was sintered at 1200°C for 2 hours in N2. A very peculiar membrane with a threedimensional network is observed. It is assumed that this structure resulted from the deposition of the electrophoresis particles shown in Fig. 5. A mem-
131
Fig. 6. Cross-sectionalphotographof the ceramicmembrane.
Fig. 7. Surfacestructureof the ceramicmembrane. brane with this structure has a large porosity similar to a fiber filter and, therefore, is advantageous in keeping the pressure drop very low when it is utilized form membrane separation. The effective pore size of this membrane is around 0.2-1.0 ~tm according to the SEM photographs and to measurement of latex-particles filtration. Thus this membrane can be placed in the category of micro-filtration. Now, the control of the particles produced and the structure of the particle layer deposited by an electrostatic force will be investigated for much smaller pore size.
132 4. The basic characteristics of the m e m b r a n e Figure 8 shows the comparison of the gas pressure drop between the alumina substrate and the membrane shown in the Figs. 6 and 7. A large difference in pressure drop is not observed, though the pore size of the membrane is 1/20th that of the substrate. This is due to the large porosity of the membrane and because the thickness of the membrane (20 ~m) is much less t h a n that of the substrate (1500 }~m). Figure 9 shows the fluxes of water through the substrate and the membrane as a function of temperature. The decrease of the water flux caused by forming the membrane is about 10%, and its resistance may be seen to be very low. 5OO
1.5
ion e x c h a n g e p = I kPa
water
substrate + membrane
~
1.2100
E
substrate
e4
~ 50
0.9 $ubstrate
0.6 10
~2 ga~
!
at r o o m
;
I
; i IIIII
0
0.5 Gas
temperature
;
I
I ; il;l
1
velocity
5
|
I I0
O,3 0
0.2 h . . . . .
100 ~
'U ~
Dp = 1 . 8
% 0.15 c~
nm
Cb t
= 1000 p p m = I O0 °C
•P
= 1.5
kpa
90 n
80
0.1 0
0:0~
0
30
60 Filtration
90 time
I
I
60
80
120
100
[ °C]
Fig. 9. Water flux of the membrane influenced by varying temperature.
0.30 L
0.20
I 40
Temperature
[cm/s]
Fig. 8. Gas pressure drop of the membrane.
,z
I 20
50
[mini
Fig. 10. Removalof silica particles in boilingwater.
133
Figure 10 shows the experimental results of separation of fine silica particles dispersed in boiling water. The silica is in the form of mono-dispersed particles of 1.8 ~m diameter. The rejection of particles is 100% at the water temperature of 100 ° C and under the filtration pressure of 1.5 kPa. The flux in steady state is around 0.1 m3/m2h. These results are very good for membrane separation. 5. Conclusion
A new method of forming ceramic membranes using ultra-fine particles of silicon nitride produced by a thermally activated CVD method has been investigated. It is possible to use the ultra-fine particles produced for membrane formation directly without taking t h e m out of the apparatus. By this method, the difficulty of handling and processing of ultra-fine particles can be avoided. Also surface contamination and particle agglomeration can be prevented. An effective and advantageous means for the utilization of ultra-fine particles has, therefore, been shown in this paper. The membrane produced is about 20 pm in effective thickness and its pore size is around 0.2-1.0 p m with a three-dimensional network structure. Also the porosity of the membrane is very large. This structure is advantageous for membrane separation and is a special feature which has not been observed in normal ceramic filters up to the present. The main aim of this experiment was the confirmation of the possibility of "Electrostatic Formation of a Ceramic Membrane (EFCM)", and the membrane produced in this experiment can be classified as a micro-filtration filter. In the future, investigations to extend the utilization of this membrane into ultra-filtration by realizing smaller pore sizes will be undertaken.
References 1 S. Masuda and E. Kiss, Proe. Int. Conf. on Industrial Electrostatics, Budapest, Hungary, 1984. 2 S.T. Buljan, U.S. Patent, 4,073,845 (1978). 3 H. Yamamoto, J. Soc. Powder Tech., Japan, 26 (1989) 163.