Journal of the Ceramic Society of Japan 103 [1] 78-80 (1995)

Note

Preparation of Thick Silica Films by the Electrophoretic Sol-Gel

Deposition on a Stainless Steel Sheet

Hideki NISHIMORI, Masahiro TATSUMISAGO and Tsutomu MINAMI

Department of Applied Materials Science, College of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai-shi, Osaka 593

ゾル ・ゲ ル電 気 泳 動 電 着 法 に よ る ス テ ン レス基 板 上 へ の シ リ力厚 膜 の 作 製

西森秀樹 ・辰巳砂昌弘 ・南 努

大阪府立大学工学部機能物質科学科,593堺 市学園町1-1

[Received September 8,1994; Accepted October 19, 1994]

Thick silica films were prepared on a stainless steel sheet by the electrophoretic sod; gel deposition method in the presence of sodium dodecyl sulfate (SDS). Electrophoretic deposition was carried out for the silica sols prepared from tetraethoxysilane under the basic conditions. SDS was added as a dispersant in sol preparation. While deposited films were peeled off during drying process in the absence of SDS, thick films were

prepared in the presence of SDS; the maximum thickness of the films was about 20pm at an applied voltage of 30V and an SDS content of 0.05 mass%.

Key-words: Electrophoretic sol-gel deposition, Thick films, Silica, Stainless steel sheets, Coating, Sodium dodecyl sulfate (SDS)

1. Introduction

Conventional coating techniques in the sol-gel

method like dip-coating and spin-coating are excel

lent ways for obtaining uniform thin films on various

substrates. It is also known to be difficult to form

thick films with more than an order of um in thick

ness. On the other hand, the electrophoretic deposi

tion of metal oxides is useful to produce thick and

dense films. Although some coating processes by the

electrophoretic deposition in the sols prepared by

the sol-gel method were reported,1)-3) to our knowledge there is no investigation of the elec

trophoretic sol-gel deposition of silica particles on a

stainless steel sheet.

Previously, we have successfully prepared thick

silica films by the electrophoretic deposition of silica

particles on an aluminum substrate in the sols pre

pared by the sol-gel method.4) This technique was very useful to produce thick films on an anodized alu

minum substrate. Maximum thickness of these films

was about 17um. However, the deposition on a stain

less steel sheet was difficult because the films easily

peeled off.Surfactants have widely been used in the elec

trophoretic deposition as effective dispersants. Anionic surfactants accelerate electrophoresis of nega

tively charged particles because they adsorb on

these particles by the hydrophobic interaction and en

hance negative charge of the surf aces.5) Such surfactants are also expected to improve the adhesion between deposited films and stainless steel sheets.

We tried to add an anionic surfactant, sodium dodecyl sulfate (SDS), in the electrophoretic deposition process of the sol-gel method, and found thick silica films successfully adhered to a stainless steel sheet. In this paper reported are the effect of SDS for adhesion of the films, the relation between deposited weights and the content of SDS, and the surface structure of the films.

2. Experimental procedureSilica sols were prepared by hydrolysis of

tetraethoxysilane (TEOS) under the basic conditions. Deionized and distilled water, distilled ethanol (EtOH) and reagent grade chemicals of TEOS, ammonium hydroxide (NH4OH) , and SDS (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used as starting materials. The pH of water was adjusted to be 11.7 by adding NH40H; we reported previously that homogeneous and thick films were prepared on

an aluminum substrate at pH=

11.7.4)The SDS dissolved in H2O (pH=11.7) and TEOS

were diluted separately with the same amount of

EtOH. The two solutions were mixed and stirred at

room temperature for lh. The molar ratio of TEOS/

H2O/EtOH was fixed to be 0.2/10/10. The concen

tration of SDS added was 0-0.2 mass% against the

total weight of the sols (70g).

Figure 1 illustrates the setup used for the elec

trophoretic deposition. A stainless steel sheet

(40•~25•~0.4mm, SUS304) as a coating substrate

and a stainless spiral as a counter electrode (0.9mm

in diameter, SUS304) were cleaned with isopropyl al

cohol in an ultrasonic bath and immersed in the sols

prepared. Since more homogeneous films were ob

tained on both sides of the sheet by using a spiral in

stead of a sheet as a counter electrode, we used the

spiral and settled the coating sheet at the center of

the spiral. The immersed surface area of the stain

less steel sheet as an anode was 14 cm2. A constant

DC voltage was applied across the two electrodes for

10 min by using a power supply (Takasago, Ltd.,

78

Hideki NISHIMORI et al. Journal of the Ceramic Society of Japan 103 [1] 1995 79

GPV 0650-0.5), causing the migration of negatively charged colloidal particles toward the anode. The voltage, the current and the number of coulombs consumed were respectively monitored by using an electronic voltmeter (Toa Dempa, PM-12), an electrometer (Takeda Riken Industry, TR-8651) and a coulometer (Nichia, N-CR646), respectively, throughout the electrophoresis process.

Fig. 1. Schematic drawings of the setup for the electrophoretic

deposition system in the present study.

After the electrophoresis the coated sheet was dried in a desiccator over silica gel for one day before weighing. The weight of coating films on the stainless steel sheet was measured with a microbalance

(Mettler, M5SA). A scanning electron microscope (SEM; JEOL, JSM-5300) was used for the observation of particles deposited on the stainless steel sheet and the estimation of the film thickness.

3. ResultsFigure 2 shows the weight of silica films prepared

by the sol-gel electrophoretic deposition as a function of applied voltage. The contents of SDS added were changed from 0 to 0.2 mass%. In the absence of SDS, the deposited films collapsed in 10 min after withdrawing. On the other hand, the sols containing SDS yielded thick films on stainless steel sheets by the electrophoretic deposition. At [SDS]=0.05 mass%, the weight is increased with increasing voltage up to 30V. In cases of [SDS]=0.1 and 0.2 mass%, the weight of the deposited films is gradually increased with increasing applied voltage in the same way under the applied voltage smaller than 15V. However, the weight is saturated when the voltage is larger than 15V. In these voltages, sedimentation of the particles was observed near the cathode during the electrophoresis of the sols.

Figure 3 shows SEM micrographs of the surface

structure of the silica films deposited on stainless steel sheets under different contents of SDS in the range of 0.05-0.2 mass%. These films consist of close-packed fine particles. At any concentration of SDS, particles which construct the films are monodispersed and the particle size is increased with an increase in the content of SDS added. In the absence of SDS, particles prepared were too fine to be observed by a conventional SEM.

Fig. 2. Weights of silica films prepared by the electrophoretic

sol-gel deposition with different amounts of SDS added against ap

plied voltage.

Fig. 3. SEM micrographs of surface structure of the films

deposited on a stainless steel sheet under different contents of SDS.

When [SDS] = 0.05 mass Yo and the applied voltage was 30V, there was no indication of peeling off of the prepared film and the particles were closely

packed. Thickness of the films was about 20um in this case with some cracks smaller than 1um wide in the films.

4. Discussion

The thick silica films were prepared on a stainless

steel sheet in the presence of SDS, whereas films

could not be prepared without SDS as shown in Fig.

80 Preparation of Thick Silica Films by the Electrophoretic Sol-Gel Deposition on a Stainless Steel Sheet

2. This may be caused by the fact that SDS prevents

the particles from aggregating excessively on the

sheet in the electrophoresis because SDS is a typical

dispersant. SDS also reduces the surface tension of

the sols and prevents the films from cracking caused

by capillary force in the drying process.

The sedimentation of silica particles was observed

during the electrophoresis of the sols with SDS

>0.1 mass%, indicating that the addition of large

amount of SDS probably makes sols unstable. In

such a case the migration of sodium ions toward the

cathode (counter electrode) might induce the double

layer compression near the counter electrode, which

caused the flocculation and sedimentation of the par

ticles. As shown in Fig. 3 larger amount of SDS ad

ded resulted in larger particle size. Larger particle

sizes (at [SDS]•†0.1mass) may lower the stabili

ty of the sols. The silica content of the supernatant

portion was decreased by the sedimentation in the

electrophoresis, which brought about the saturation

of deposited weight at higher applied voltages when

[SDS]•†0.1 mass%. The sols including fine parti

cles (ca. 0.3ƒÊm) were stable and the electrophoresis

of the particles was accelerated at [SDS]=0.05

mass%. In this amount of SDS thick and dense films

were prepared by 30V. When the applied voltage

was larger than 30 V, edges of films were partially

peeled off during the electrophoresis. The peeling off

of the films was chiefly caused by the generation of

large amount of 02 at the anode (coating sheet).

5. Conclusions

In the presence of SDS thick silica films (•…20

ƒÊm) were prepared on a stainless steel sheet by the

electrophoretic sol-gel deposition. These films con

sisted of agglomerates of fine particles (0.3-0.6 am).

In the absence of SDS the deposited films were

peeled off during the drying process.

Although adhesion mechanisms of particle-parti

cle and particle-sheet has not completely been eluci

dated, an addition of a proper amount of SDS

(=0.05 mass%) was suggested to accelerate the

electrophoresis of silica particles and to improve the

adhesion of deposited films to a stainless steel sheet.

Influence of the concentration of TEOS and SDS on

the particle size and dispersibility will be reported el

sewhere in more detail.

Acknowledgment This work was supported in part by the

Grant-in-Aid from the Ministry of Education, Culture and Science

of Japan.

References

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3) H. Schmidt and W. Wolter, J. Non-Cryst. Solids, 121, 428-35 (1990).

4) K. Kishida, M. Tatsumisago and T. Minami, J. Ceram. Soc. Japan, 102, 336-40 (1994).

5) T. Imae, K. Muto and S. Ikeda, Colloid Polym. Sci., 269, 43 48 (1991).