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The conductivity of aqueous Al2O3 slips for electrophoretic deposition
October
ELSEVIER
1996
Materials Letters 28 (1996) 353-355
The conductivity of aqueous Al,O, slips for electrophoretic deposition B. Ferrari, R. Moreno
*
Institute de Cercimica y Vidrio (C.S.I.C.), Ctra de Valencia Km. 24 300, 28500 Arganda de1 Rey, Madrid, Spain Received 7 December
1995; revised 11 March 1996; accepted
14 March 1996
Abstract In electrophoretic deposition (EPD) experiments, the electrical conditions applied to the slip determine the deposit homogeneity, but the dispersing conditions (electrochemical and rheological) determine the reliability of this process. In this work, after a careful study, a critical slip parameter such as the conductivity is proposed as a key factor to take into account in EPD experiments. Keyw0rd.r: Conductivity;
Al,O,
slips; Electrophoretic;
Deposition
1. Introduction Electrophoretic deposition (EPD) has received a great deal of attention due to its advantages for coatings and film applications [ 1,2], as well as laminar ceramic composites [3,4]. The mechanism of EPD involves charged particles in a suspension being deposited onto an electrode under the influence of an electric field. Two groups of parameters determine the characteristics of this process, those related to the suspension and those related to the process, including the physical parameters such as the electrical nature of the electrodes, the electrical conditions (voltage/intensity relationship, deposition time, etc.>. Regarding the suspension properties, many parameters must be considered, such as the physicochemical nature of both suspended particles and liquid medium, the surface properties of the powder
* Corresponding
author.
OOl67-577X/96/$12.00 Copyright PfI SO167-577X(96)00075-4
and the influence of the type and concentration of the slip additives, mainly dispersants. Most of the work has been done in non-aqueous media because of the higher density and the lower electrical conductivity. However, the use of aqueous systems has important advantages since they need much lower voltages to be applied and the environmental problems associated with organics are avoided. Some researchers have studied the influence of slip properties such as pH, deflocculant type and concentration, solid loading and zeta potential [5,6]. However, the slip conductivity has not been considered for EPD experiments, though it determines the formation of the deposit, as proposed in this work.
2. Experimental Aqueous cx-Al,O, (Alcoa A16SG) slips were prepared to a solid content of 5 wt’%. Deflocculant concentrations and types were tested using rheologi-
0 1996 Elsevier Science B.V. All rights reserved.
354
B. Ferrari, R. Moreno /Materials
cal measurements on concentrated slips. The one selected was a commercial carbonic acid based polyelectrolyte (Dolapix CE-64, Zschimmer-Schwarz, Germany). The slips were prepared using a high shear mixer (Silverson L2R, UK) for 3 min. The conductivity was measured using a WTW conductimeter. The temperature was controlled with a heating instrument with stirring function (IKAMAG RCT, IKA, Germany), completed with an electronic contact thermometer with Pt 1000 measuring sensors (IKATRON ETSD3, IKA, Germany). EPD experiments were performed by applying constant current intensities ranging from 10 to 80 mA for deposition times up to 10 min. A LABCONCO 433-3250 power source was used. The electrolytic cell was a glass beaker containing the counterelectrode (square foil of Pt> and the working electrode (square foil of graphite). The distance between electrodes was maintained at 2 cm.
3. Results and discussion Rheological measurements on concentrated slips give us a good idea about the optimum dispersing state when adding Dolapix CE-64, which adsorbs onto the particle surface providing an electrosteric stabilizing mechanism. For concentrated slips a minimum viscosity is obtained for a deflocculant concentration of 0.3-0.4 wt%. Lower or higher concentrations produce higher viscosities. However, for solid loadings as low as those required for EPD (5 wt%), the viscosity of tested slips (with different deflocculated concentration, from 0 to 1 wt%) does not vary significantly. On the other hand, the resulting pH is = 9.5 and only a little decrease of pH with increasing additions of deflocculant (up to 9.2 for 1 wt%) has been observed. Hence, a slip parameter, other than viscosity, must be considered in EPD slips. Fig. 1 shows the conductivity of Al,O, slips containing different concentrations of deflocculant as a function of the slip temperature. The conductivity increases linearly with temperature and with very little variation of polyelectrolyte concentration. Moreover, not all the conductivity values are useful for the formation of a deposit.
Letters 28 (1996) 353-35.5
100
i
I
I
23
25
27
/-
31
29
Temperature
33
(“C)
Fig. 1. Variation of the slip conductivity different deflocculant concentrations.
with temperature
at
Many different slips have been prepared changing their conductivity (that is, deflocculant content) in order to see if they form deposit or not. Fig. 2 shows the conductivity and temperature of slips subjected to EPD experiments, at different applied intensities. Empty points represent the slips which do not produce any deposit, while the full points give place to a deposit. An approximation for low temperatures of the linear conductivity/temperature ratio shown in Fig. 1 for deflocculant contents of 0.2, 0.3 and 0.4 wt%, has also been plotted in this figure. As observed, there is a margin in which the deposit is formed. Conductivity values out of this region are not suitable for EPD, limiting the forming possibilities. In this region of conductivity, the slip gives place to deposit for all the current intensities considered in this work (20-80 mA). When a deposit is forming the voltage increases. So, the voltage evolution is directly related to the
100
._-..a/
23
25
27 Temperahlre
29
31
33
(“C)
Fig. 2. Conductivity of the slips employed for EPD experiments at different current intensities. (0) no deposit formation and (W) deposit is formed.
B. Ferrari,
R. Moreno/Marrrials
700
400
I
B > z
355
4. Conclusions
500 600 I s x
Letters 28 (1996) 353-355
300 1 200 + 100 + I
01
0
_i_~~~
--~ 2
4 Deposition
6
,
~,
6
10
time (mln)
Fig. 3. Polarization curves of slips with different conductivities (as shown in Fig. 2) obtained for EPD experiments at a constant current intensity of 40 mA.
Summarizing the previously reported data, the deposit is only obtained when the slip conductivity value is in a precise region. Higher and lower values of conductivity impede the deposition. Once the conditions of conductivity required for the formation of a deposit are satisfied, its characteristics and properties can be optimized by changing the electrical parameters involved in the process (voltage/intensity, deposition time, etc.).
Acknowledgements deposit growing during the deposition. Obviously, the electrical conditions imposed onto the system (current intensity and time) will determine the characteristics of the deposit, as well as the formation kinetics. However, if a constant current intensity of 40 mA is applied on differently dispersed slips, the voltage evolution can also be different, as drawn in Fig. 3 for the slip conductivity conditions 1, 2, 3 and 4 plotted in Fig. 2. When the slip conductivity is nearly the same as that corresponding to the slips deflocculated with 0.4 wt% dispersant, the voltage increases rapidly and a homogeneous deposit over the work-electrode is obtained. It has been observed that both the deposit rate (that is, the thickness) and the green density increase with the applied intensity up to a maximum, located for this system at 50 mA.
This work has been supported by CICYT (Spain) under contract No. MAT94-0741.
References 111R. Chaim, G. Stark, L. Gal-Or and H. Bestgen, J. Mater. Sci. 29 (1994) 6241.
121M. Nagai, K. Yamashita, T. Umegaki and Y. Takuma, J. Am. &ram.
Sot. 76 (1993) 253.
[31 P. Sarkar, X. Haung and P.S. Nicholson,
J. Am. Ceram. Sot. 75 (1992) 2907. 141 P. Sarkar, 0. Prakash, G. Wang and P.S. Nicholson, Ceram. Eng. Sci. Proc. 15 (1994) 1019. El J.Y. Choudhary, H.S. Ray, K.N. Rai, Trans. J. Br. Ceram. Sot. 81 (1982) 189. [61R. Fisher, E. Fisher, G. De Portu and E. Roncari, J. Mater. Sci. Len. 14 (1995) 25.
ELSEVIER
1996
Materials Letters 28 (1996) 353-355
The conductivity of aqueous Al,O, slips for electrophoretic deposition B. Ferrari, R. Moreno
*
Institute de Cercimica y Vidrio (C.S.I.C.), Ctra de Valencia Km. 24 300, 28500 Arganda de1 Rey, Madrid, Spain Received 7 December
1995; revised 11 March 1996; accepted
14 March 1996
Abstract In electrophoretic deposition (EPD) experiments, the electrical conditions applied to the slip determine the deposit homogeneity, but the dispersing conditions (electrochemical and rheological) determine the reliability of this process. In this work, after a careful study, a critical slip parameter such as the conductivity is proposed as a key factor to take into account in EPD experiments. Keyw0rd.r: Conductivity;
Al,O,
slips; Electrophoretic;
Deposition
1. Introduction Electrophoretic deposition (EPD) has received a great deal of attention due to its advantages for coatings and film applications [ 1,2], as well as laminar ceramic composites [3,4]. The mechanism of EPD involves charged particles in a suspension being deposited onto an electrode under the influence of an electric field. Two groups of parameters determine the characteristics of this process, those related to the suspension and those related to the process, including the physical parameters such as the electrical nature of the electrodes, the electrical conditions (voltage/intensity relationship, deposition time, etc.>. Regarding the suspension properties, many parameters must be considered, such as the physicochemical nature of both suspended particles and liquid medium, the surface properties of the powder
* Corresponding
author.
OOl67-577X/96/$12.00 Copyright PfI SO167-577X(96)00075-4
and the influence of the type and concentration of the slip additives, mainly dispersants. Most of the work has been done in non-aqueous media because of the higher density and the lower electrical conductivity. However, the use of aqueous systems has important advantages since they need much lower voltages to be applied and the environmental problems associated with organics are avoided. Some researchers have studied the influence of slip properties such as pH, deflocculant type and concentration, solid loading and zeta potential [5,6]. However, the slip conductivity has not been considered for EPD experiments, though it determines the formation of the deposit, as proposed in this work.
2. Experimental Aqueous cx-Al,O, (Alcoa A16SG) slips were prepared to a solid content of 5 wt’%. Deflocculant concentrations and types were tested using rheologi-
0 1996 Elsevier Science B.V. All rights reserved.
354
B. Ferrari, R. Moreno /Materials
cal measurements on concentrated slips. The one selected was a commercial carbonic acid based polyelectrolyte (Dolapix CE-64, Zschimmer-Schwarz, Germany). The slips were prepared using a high shear mixer (Silverson L2R, UK) for 3 min. The conductivity was measured using a WTW conductimeter. The temperature was controlled with a heating instrument with stirring function (IKAMAG RCT, IKA, Germany), completed with an electronic contact thermometer with Pt 1000 measuring sensors (IKATRON ETSD3, IKA, Germany). EPD experiments were performed by applying constant current intensities ranging from 10 to 80 mA for deposition times up to 10 min. A LABCONCO 433-3250 power source was used. The electrolytic cell was a glass beaker containing the counterelectrode (square foil of Pt> and the working electrode (square foil of graphite). The distance between electrodes was maintained at 2 cm.
3. Results and discussion Rheological measurements on concentrated slips give us a good idea about the optimum dispersing state when adding Dolapix CE-64, which adsorbs onto the particle surface providing an electrosteric stabilizing mechanism. For concentrated slips a minimum viscosity is obtained for a deflocculant concentration of 0.3-0.4 wt%. Lower or higher concentrations produce higher viscosities. However, for solid loadings as low as those required for EPD (5 wt%), the viscosity of tested slips (with different deflocculated concentration, from 0 to 1 wt%) does not vary significantly. On the other hand, the resulting pH is = 9.5 and only a little decrease of pH with increasing additions of deflocculant (up to 9.2 for 1 wt%) has been observed. Hence, a slip parameter, other than viscosity, must be considered in EPD slips. Fig. 1 shows the conductivity of Al,O, slips containing different concentrations of deflocculant as a function of the slip temperature. The conductivity increases linearly with temperature and with very little variation of polyelectrolyte concentration. Moreover, not all the conductivity values are useful for the formation of a deposit.
Letters 28 (1996) 353-35.5
100
i
I
I
23
25
27
/-
31
29
Temperature
33
(“C)
Fig. 1. Variation of the slip conductivity different deflocculant concentrations.
with temperature
at
Many different slips have been prepared changing their conductivity (that is, deflocculant content) in order to see if they form deposit or not. Fig. 2 shows the conductivity and temperature of slips subjected to EPD experiments, at different applied intensities. Empty points represent the slips which do not produce any deposit, while the full points give place to a deposit. An approximation for low temperatures of the linear conductivity/temperature ratio shown in Fig. 1 for deflocculant contents of 0.2, 0.3 and 0.4 wt%, has also been plotted in this figure. As observed, there is a margin in which the deposit is formed. Conductivity values out of this region are not suitable for EPD, limiting the forming possibilities. In this region of conductivity, the slip gives place to deposit for all the current intensities considered in this work (20-80 mA). When a deposit is forming the voltage increases. So, the voltage evolution is directly related to the
100
._-..a/
23
25
27 Temperahlre
29
31
33
(“C)
Fig. 2. Conductivity of the slips employed for EPD experiments at different current intensities. (0) no deposit formation and (W) deposit is formed.
B. Ferrari,
R. Moreno/Marrrials
700
400
I
B > z
355
4. Conclusions
500 600 I s x
Letters 28 (1996) 353-355
300 1 200 + 100 + I
01
0
_i_~~~
--~ 2
4 Deposition
6
,
~,
6
10
time (mln)
Fig. 3. Polarization curves of slips with different conductivities (as shown in Fig. 2) obtained for EPD experiments at a constant current intensity of 40 mA.
Summarizing the previously reported data, the deposit is only obtained when the slip conductivity value is in a precise region. Higher and lower values of conductivity impede the deposition. Once the conditions of conductivity required for the formation of a deposit are satisfied, its characteristics and properties can be optimized by changing the electrical parameters involved in the process (voltage/intensity, deposition time, etc.).
Acknowledgements deposit growing during the deposition. Obviously, the electrical conditions imposed onto the system (current intensity and time) will determine the characteristics of the deposit, as well as the formation kinetics. However, if a constant current intensity of 40 mA is applied on differently dispersed slips, the voltage evolution can also be different, as drawn in Fig. 3 for the slip conductivity conditions 1, 2, 3 and 4 plotted in Fig. 2. When the slip conductivity is nearly the same as that corresponding to the slips deflocculated with 0.4 wt% dispersant, the voltage increases rapidly and a homogeneous deposit over the work-electrode is obtained. It has been observed that both the deposit rate (that is, the thickness) and the green density increase with the applied intensity up to a maximum, located for this system at 50 mA.
This work has been supported by CICYT (Spain) under contract No. MAT94-0741.
References 111R. Chaim, G. Stark, L. Gal-Or and H. Bestgen, J. Mater. Sci. 29 (1994) 6241.
121M. Nagai, K. Yamashita, T. Umegaki and Y. Takuma, J. Am. &ram.
Sot. 76 (1993) 253.
[31 P. Sarkar, X. Haung and P.S. Nicholson,
J. Am. Ceram. Sot. 75 (1992) 2907. 141 P. Sarkar, 0. Prakash, G. Wang and P.S. Nicholson, Ceram. Eng. Sci. Proc. 15 (1994) 1019. El J.Y. Choudhary, H.S. Ray, K.N. Rai, Trans. J. Br. Ceram. Sot. 81 (1982) 189. [61R. Fisher, E. Fisher, G. De Portu and E. Roncari, J. Mater. Sci. Len. 14 (1995) 25.