Electrophoretic Deposition of ZnO
A. Dogan, E. Suvaci, G. Gunkaya and E. Uzgur
Anadolu University, Department of Materials Science and Engineering, Eskisehir/Turkey
Keywords: Electrophoretic deposition, ZnO, Thin film
Abstract. Electrophoretic phonemenon is applied in different parts of science. Electrophoretic
Deposition (EPD) is effective, fast and controllable process for obtaining various thin or thick film
layers. In this study application of EPD to functional ceramics especially for sensors is aimed. Sub-
micron ZnO powder was synthesized by using homogeneous precipitation method. A 5 wt% ZnO
aqueous suspension prepared for EPD and an anionic dispersant was used as stabilizer. ZnO powder
is deposited on alumina substrate at various voltages which is made conductive. Coated samples
were sintered at 1200°C for two hours. Characterization of the sintered ZnO deposited substrates
were examined by SEM and XRD techniques.
Introduction
Electrophoretic deposition is a coating technique which is searched increasingly demand. By EPD
thin or thick, porous or nonporous films can be obtained on conductive layers. In this study by using
EPD ZnO coatings on alumina substrates were applied. The advantage of EPD when it is compared
by other techniques like CVD, PVD, spray pyrolysis, sputtering, laser ablation, sol-gel method,
MOCVD, PECVD and LPE is being low cost process, simple, rapid, having a high potential for
depositing on to curved or cylindrical shaped bodies in different scales and thicknesses from 10nm
to 1m [1,2].
ZnO is one of the first and most widely used ceramic material for sensor applications. Some of the
ceramic semiconductors such as tin oxide and zinc oxide with controlled porous bodies show an
electrical conductivity as a response to trace amount of hydrocarbon based gases, hydrogen, and
ammonia etc. at elevated temperature in air [3,4]. Gas sensing in n-type semiconductor gas sensors
like ZnO is achieved by the decrease in resistivity as a response to increasing amount of reducing
gases in the air. The reason for decrease in resistivity is explained by desorption of oxygen absorbed
on the surface and grain boundaries of metal-oxides at high temperatures in air [4]. By using EPD
technique small thin film layered sensors can be produced so low power consumption and easy
integration to other devices is provided. Because of its simplicity and versatility as mentioned above
electrophoretic deposition might have potential for being preferable technique for ceramic sensor
applications. Accordingly there are some studies reporting the use of deposited ZnO films for sensor
application. In this study EPD is used for the formation of ZnO thin films on alumina substrates for
sensor applications.
Experimental Procedure
Sub-micron sized ZnO particles were synthesized by using homogenous precipitation method [5].
The synthesized powders were characterized by using a particle size analyzer. It was observed that
the particles were highly agglomerated even in 4-µm size. To disperse the agglomerated particles
grinding process was employed. In this study, two types of milling, conventional and chemical
added milling (CAM) was used to see the effect of dispersants on particle de-agglomeration during
Key Engineering Materials Vols. 264-268 (2004) pp 269-272Online available since 2004/May/15 at www.scientific.net© (2004) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.264-268.269
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 134.99.128.41, Universitaets- und Landesbibliothek Duesseldorf, Duesseldorf, Germany-26/09/13,23:07:05)
Degree (2θ)
(002)
the milling of ZnO particles. The powder was milled for 22 hours after milling particle size was like
in Fig. 1.
Figure 1. Laser particle size analysis of ZnO powder which is synthesized by using homogeneous
precipitation method.
An anionic dispersant was used for the chemical aided milling to stabilize the 5 wt% ZnO aqueous
suspension prepared for Electrophoretic Deposition (EPD). After milling sedimentation experiments
were performed to test the stability of suspensions from each milling process.
After obtaining well dispersed medium, EPD experiments were conducted under constant DC
voltage. High-grade alumina (Coors Ceramics) with 1 in.2
dimension was used as substrate. The
surfaces of the substrates were electroded with gold-palladium by sputtering process. A Pyrex glass
made deposition chamber due to chemical and electrical inertness was used for EPD studies.
Coating was done altering voltage and time by using stable suspension of ZnO. Coating was carried
out in the range of 5-240 V. Coated samples were dried at room temperature to eliminate surface
tension originated cracks [6]. Samples were sintered at 1200°C for two hours. Characteristics of the
sintered ZnO coated substrates were investigated by scanning electron microscope (SEM) and X-ray
difractometer (XRD).
Results and Discussion
XRD pattern of the synthesized powder shows that the formed phase is pure ZnO Fig.2.
0
50
100
150
200
250
25 30 35 40 45 50 55 60 65 70
Figure 2. XRD chart of the synthesized ZnO powder.
(100)
(101)
(102)
(110)
(103) (112)
Intensity
ZnO
270 Euro Ceramics VIII
Stabilization of ZnO is obtained by anionic polyelectrolyte, well dispersion is achieved by charging
surface of the powder negatively. The anionic polyelectrolyte was chosen because ZnO dissolves in
acid.
While the conventional milled particles settle with in about 10 minutes; the particles in CAM
system deflocculates after 24 hours. Prepared suspensions conductivity was 0.87 mS/cm and
pH=8.4 so the stability was enough for EPD. This study shows that chemical milling is required to
achieve stable suspension of ZnO particles. Coating layers were formed with EPD. However, for
sensor applications rather thin porous layers are preferable. For that reason we focused on coating
layers around 10 µm Fig.3. Fig.4 shows SEM photographs of sintered coatings for 2 hours at
1200°C. As it is seen on the figure it is possible to create porous structure. However the pore
distribution was not homogenous, but for sensor application uniform porosity required. As a next
step of this research, homogeneous distribution of nanopores in EPD coated films will be
investigated.
Figure 3. SEM image of Al2O3 substrate and ZnO coating from a different point of view.
Al2O3
ZnO
20 µm
20 µm 10 µm
Fig. Figure 4. SEM images of porous body by EPD after sintering 1200ºC for 2 hours.
Key Engineering Materials Vols. 264-268 271
Density of the sintered body should be changed by using different powders, which have different
grain sizes and sintering temperatures. In this experiment nearly 5 µm powders and 2 µm pores were
get after sintering as seen on Fig.4.
Summary
It has been demonstrated that the EPD technique can be utilized to form ZnO films with ~10 µm
thickness for sensor applications. A stable suspension is necessary for a successful EPD process.
The stable suspension of submicron ZnO particles can be achieved by chemically-aided milling
approach.
Acknowledgements
The authors gratefully thank to C. A. Randall and J. Van Tassel of The Pennsylvania State
University, MRI for their support. They also would like to thank Cem Caglar for the ZnO synthesis.
This study is sponsored by Turkish State Planning Organization DPT (2003K120170).
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