sol-gel derived antimony-doped tin oxide coatings on ceramic cloths

February 1995

Materials Letters 22 (1995) 175-180

Sol-gel derived antimony-doped tin oxide coatings on ceramic cloths

Sung-Soon Park, Haixing Zheng, J.D. Mackenzie Department of Materials Science and Engineering, University of California-Las Angeles, Los Angeles, CA 90024, USA

Received 10 August 1994; in final form 7 November 1994; accepted 9 November 1994

Abstract

Antimony-doped tin oxide (ATO) coatings were deposited on the Nextel and E-glass cloths by the sol-gel process. The influence of dip coating parameters, such as solution concentration, the number of coating applications, firing temperature, and firing atmospheres, on the sheet resistance of the AT0 coated cloths was investigated. The study demonstrates that the AT0 coated Nextel cloths and E-glass cloths of 12”X 12” have sheet resistance as low as 20 O/O and 120 n/Cl respectively. The lack of the thermal stability of the AT0 coatings seems to be due to the potential barriers between particles, which are developed by the chemisorption of oxygens from the atmosphere on the surface of the particles.

1. Introduction

Transparent conducting tin oxide-based coatings, such as indium-doped tin oxide (ITO) and antimony- doped tin oxide (ATO), have been attracting increas- ing interest since they have many important applications such as liquid display, photo detectors, solar cell, gas sensors, and protective coatings [l-5]. For the applications of tin oxide-based coatings as transparent conducting electrodes a low sheet resis- tance is required. Tin oxide-based coatings have been fabricated by a number of techniques, including spray pyrolysis [ 61, sputtering [ 71, CVD [ 81, and evapo- ration [ 91. The evaporation techniques or sputtering techniques have some advantages, e.g. low temperature processing (the coatings have been deposited on flat polymer substrates such as plastics). However, these techniques cannot be applied to deposit coatings on the large and complex shapes such as cloths. It is well known that the sol-gel technique has several advan- tages such as excellent homogeneity, easy control of

coating thickness, ability to coat large and complex shapes, and simple and low cost processing [ lo], but the most significant advantage of the sol-gel technique is the ability to coat large and complex shapes.

In this study we investigated the influence of anti- mony on the sheet resistance of the prepared coatings, which were deposited on the Nextel and E-glass cloths. Also we investigated dip coating parameters such as solution concentration, the number of coating applica- tions, firing temperature, and firing atmospheres and their influences on the sheet resistance.

The present study shows that the sheet resistance is successfully controlled by controlling the dip coating parameters, and that there exist optimum dip coating conditions which give low sheet resistance of the AT0 coatings.

2. Experimental procedures

2.1. Deposition of AT0 coatings on ceramic cloths

Tin (IV) isoproxide (Sn( OC&),)-isopropanol

0167-577x/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO167-577x(94)00241-X

176 S.-S. Park et al. /Materials Letters 22 (1995) 175-180

WOC,H,),

Stir for 12 hours

7

AT0 solution

-

Dip coat

Repeat Dry at 100°C for lhour

Fire at 55OoC for 5 minutes

Fire at 75O’C for 6 hours

1

AT0 coated Next4 cloths

Fig. 1. Diagram of the preparation of AT0 coatings on the Nextel cloths.

solution ( 10 g/100 ml) from Chemat Technology, Inc., and antimony-butoxide (Sb( OC,H,) 3) from Alfa Chemicals were used as starting chemicals. Coating solutions were prepared by mixing tin (IV) isopropox- ide solution with antimony-butoxide in isopropanol (Fig. 1).

Pre-cleaned ceramic cloths were dipped into the AT0 solutions and impregnated in the solutions for 2 min. The coated cloths were dried in air at 100°C for 1 h. The coated Nextel cloths and E-glass cloths were fired at 550 and 500°C respectively for 5 min in air (heating rate: 5Wmin). The above procedures were repeated to obtain desired thickness of coatings. The coated Nextel cloths were then fired at desired temper- atures in air. The coated E-glass cloths were fired at 550°C for 15 h in vacuum.

Table 1 The solubility of metal akoxides

2.2. Measurement and characterization

X-ray diffraction studies were carried out with a Noreco X-ray diffractometer, where Cu Ko (A = 1.5405 A) radiation was used. SEM was used to analyze the quality of the coatings and measure the thickness of the coatings. The sheet resistance of the AT0 coated cloths was measured by two-point probe method. To investigate the thermal stability of the coat- ings the sheet resistance of the coated cloths was meas- ured on heating and cooling in air. Both the heating and cooling rate were regulated at 2Wmin.

3. Results and discussion

3.1. Determination of coating solutions

A large number of oxides have been suggested for transparent conducting coatings, e.g. indium-doped tin oxide (ITO), antimony-doped tin oxide (ATO), indium-doped zinc oxide (IZO) , and cadmium stan- nate (CTO) [ 11,121. Depending on the impurities present, the resistivity of the oxide coatings can have the difference of 2-3 orders. Various metal alkoxides were considered for the oxide coatings. Since the dep- osition of oxides via the sol-gel process requires some degree of solubility of metal alkoxides in normal organic solvents, the consideration was based on the solubilities of the corresponding metal alkoxides. Table 1 shows the solubilities of various metal alkoxides and indicates that tin and antimony alkoxides have higher solubility. Logically, antimony-doped tin oxide (ATO) will be one we should work on.

3.2. Characterization of coatings

The AT0 gel powders, which were formed by the hydrolysis of AT0 solution with water, were fired at

Metal alkoxides Solvent Solubility (g/ 100 ml)

antimony butoxide Sb(OGW3 isopropanol completely soluble tin (IV) isopropoxide Sn(OGQ& isopropanol > 10 zinc tert-butoxide zn(OGWZ ether < 10

cadmium tert-butoxide Cd(OC&), hexane <5

indium isopropoxide In(OC&)3 isopropanol <I

S.-S. Park et al. /Materials Letters 22 (1995) 175-180 177

20 30 40 50 60

2 8 (degree) Fig. 2. X-ray diffraction patterens of the sol-gel derived AT0 pow- ders fired at different temperatures for 1 h in air.

different temperatures for 1 h. X-ray diffraction studies were performed on the powders and the results are shown in Fig. 2. It was noted that the powders were amorphous when the fimg temperature was below 350°C. The AT0 powders started to crystallize at 400°C and showed characteristic X-ray diffraction peaks of typical SnO,, powders. The X-ray diffraction peaks became sharper with increasing firing tempera- ture, which indicated that the crystallite size of the powders increased with increasing firing temperature. Fig. 3 shows SEM micrographs of the AT0 coated Nextel cloths, which were coated five times and fired at 750°C for 6 h in air. The thickness of the coatings was about 1 pm. The coatings were not so smooth.

3.3. Effects of &pant

As is well known, the high conductivity of pure tin oxide coatings is due to high electron carrier concen- tration [ 1 l-l 31. The high electron carrier concentra- tion is attributed to deviation from stoichiometry (oxygen deficiency). The stoichiometric deviation is cussed by oxygen vacancies or interstitial tin atoms. The dependence of the sheet resistance of the coated Nextel cloths on antimony concentration was investi- gated (Fig. 4). The cloths were coated five times and fired at 650°C for 5 h in air. The tin isopropoxide solu-

Fig. 3. SEM micrographs of (a) the cross section and (b) the surface for the AT0 coated Nextel cloths.

tions (0.3 M) with different antimony concentration (up to 10 mol%) were used to coat the cloths. The sheet resistance decreased with increasing antimony concentration up to 7 mol% and gradually increased at higher concentration. At 7 mol%, the cloths had the lowest sheet resistance of 18 LKI.

It was illustrated that doping of antimony into tin oxide could enhance conductivity due to the increase in electron carrier concentration [ 141. The sharp decrease of the sheet resistance at low antimony con- centration seems to be due to the increase in electron carrier concentration. The increase in the sheet resis- tance at higher antimony concentration is suggested to be attributed to impurity scattering or precipitation of SbzOs.

S.-S. Park et al. /Materials Letters 22 (19951175-180 178

i Y

II

J

0 2 4 6 8 IO 12

Sb concentration (mole 90)

Fig. 4. The dependence of the sheet resistance of the AT0 coated Nextel cloths on antimony concentration.

3.4. Effects of dip coating parameters

Sol-gel processing of coatings involves the coating of substrates with hydrolyzable metal-organic solution followed by hydrolysis and polycondensation of the solution layers, and the evaporation of solvents in the coatings and densification of the coatings [ 101. During coating, part of the metal alkoxides (e.g. Sn( OC,H,),),) is hydrolyzed by water vapor in the atmosphere and polycondense within the coatings and on the substrate surface. Hydrolysis:

Sn(OR),+H,O + Sn(OR),(OH) +ROH. (1)

Polycondensation:

Sn(OR)3(0H) +Sn(OR),(OH)

+ (OR)$n-0-Sn(OR),+HzO,

substrate - OH + (OR) Sn( OR) 3

+ substrate-O-Sn(OR)3+ROH. (2)

In the coating layer, the Sri-OH groups condense and produce water. On the other hand the Sri-OR groups react with the substrate-OH groups and this results in good adhesion of tin oxide coatings to the substrate. During evaporation of solvents, which is embedded in the coating layer, water vapor diffuses from the atmos- phere into the coating layer to maintain the hydrolysis. At the same time the polycondensation process contin-

ues. Finally, during firing the tin alkoxide gelled coat- ing layer first transforms into amorphous SnO, and then crystallizes. The important dip coating parameters that influence the sheet resistance of the coatings are solu- tion concentration, the number of coating applications, firing temperature, and firing atmospheres [ lo].

Fig. 5 shows the dependence of the sheet resistance of the AT0 coated Nextel and E-glass cloths on the number of coating applications. The AT0 solution of 0.15 M Sn(OC&) with 3 mol% Sb was used to coat both cloths for the measurement. The coated Nextel cloths were fired at 650°C for 5 h in air. Since the E- glass cloths could not withstand the temperatures above 550°C and oxygen deficiency may be created in the AT0 coatings by heating them in a slightly reducing atmosphere, the coated E-glass cloths were fired at 550°C for 15 h in vacuum. The sheet resistance of both coated cloths had an initial rapid drop which tapered off increasing number of coating applications. It was expected that thicker coatings would have lower sheet resistance.

It was found that resultant coatings were thicker and more conductive by using the solution with higher con- centration. This seemed to be due to the enhanced poly- condensation reaction rate with increasing solution concentration. The highest solution concentration, which could be used without forming large cracks, was 0.3 mol. The dependence of the sheet resistance of the coated cloths on firing temperature and firing time was

IOh

IO5

IOJ

ld

102

0 2 4 6 8 10 12

Number of coating

Fig. 5. The dependence of the sheet resistance of the AT0 coated ceramic cloths on the number of coating applications: (a) the Nextel cloths; (b) the E-glass cloths.

S.-S. Park et al. /Materials Letters 22 (1995) 175-180 179

35

30

2s

20

15

IO

500 600 700 800 900 1000

Firing temperature (“C)

Fig. 6. The dependence of the sheet resistance of the AT0 coated Nextel cloths on firing temperatures.

investigated. The AT0 solution of 0.3 M Sn( OC,H\) with 7 mol% Sb was used to coat the Nextel cloths for the measurement. The ceramic cloths were coated five times and fired at 550°C for 5 min for each coat. Finally, the coated cloths were fired at each desired temperature for 1 h in air. Fig. 6 shows the dependence of the sheet resistance of the coated Nextel cloths on firing temper- ature. It was found that the sheet resistance had a sharp drop in the firing temperature range of 550 to 650°C and continuously decreased with increasing firing tem- perature. This tendency could be explained by the fact that a higher firing temperature would produce more dense and crystalline coatings, which should have higher conductivity. The effect of firing time on the sheet resistance of the AT0 coated Nextel cloths is illustrated in Fig. 7. The firing temperature was fixed at 750°C. Longer firing time led to lower the sheet resistance. No large difference in the sheet resistance was observed when firing time was extended past 6 h.

Based on the above experimental results, the follow- ing dip coating parameters were applied to coat the Nextel cloths of 12” X 12”:

solution concentration: 0.3 M Sn( O&H:)

drying temperature: 100°C for 1 h number of coating applications: 5 first firing temperature: 550°C for 5 min in air final firing temperature: 750°C for 6 h in air

The AT0 coated Ne.xtel cloths of 12” X 12” with the sheet resistance of 200/D were obtained.

0 I 2 3 4 5 6 7 8

Firing time (hour)

Fig. 7. The dependence of the sheet resistance of the AT0 coated Nextel cloths on firing time.

Since the E-glass cloths could not withstand the tem- peratures above SSO’C, the dip coating parameters were adjusted to meet the special requirement:

solution concentration: 0.3 M Sn( OC,I$)

drying temperature: 100°C for 1 h number of coating applications: 5 first firing temperature: 500°C for 5 min in air final firing temperature: 550°C for 15 h in

vacuum The AT0 coated E-glass cloths of 12” X 12” with the sheet resistance of 120 JKI were obtained.

3.5. Thermal stability of the AT0 coatings

The sheet resistance of the AT0 coatings was meas- ured on heating and cooling in air to evaluate the ther- mal stability of them (Fig. 8). The sheet resistance showed a hysteresis curve for the increasing and decreasing temperatures. However, the sheet resistance decreased with increasing temperature up to 325”C, then the sheet resistance increased with increasing tem- peratures up to 400°C. Keeping at 400°C for 4 h the sheet resistance increased from 60 to 350 fKl. During the cooling, the sheet resistance slightly decreased and remarkedly increased to 1500 RIO. But when the coated Nextel cloths were heat treated in vacuum at 550°C for 5 h, the sheet resistance decreased from 1500 to 350 RIO.

S.-S. Park et al. /Materials L.etters 22 (1995) 175-180 180

0-

Fig. 8. The variations in the sheet resistance of the. AT0 coated Nextel cloths on heating and cooling in air.

Polycrystalline coatings usually have fine particles. The properties of polystalline coatings may be close to those of powders. The particles are conneted to neigh- bors either by grain boundaries or by necks. It is well known that above approximately 200°C oxygens from the atmosphere adsorb on the surface of Sn02 particles, binding electrons out of the bulk donors below the surface of the SnO, particles, developing depletion lay- ers below the surface of the particles [ 15,161. Finally, the chemisorption of oxygens induces a potential bar- rier at each grain boundary. Conduction electrons should move across the potential barriers. This results in decreased conductivity. The tendency of decreasing sheet resistance with increasing temperature up to 325°C seems to be due to the thermal exitation of elec- trons, but above 325°C the chemisorption of oxygens seems to predominate and results in irreversible increase in sheet resistance. However, the potential bar- riers, that is, the chemisrbed oxygens, may be reversi- bly removed by heat treating the cloths in vacuum.

4. Conclusion

Conducting antimony-doped tin oxide (ATO) coat- ings were successfully deposited on the Nextel and E- glass cloths of 12” X 12”. The influence of dip coating

parameters on the sheet resistance of the coated cloths was investigated. The sheet resistance decreased sharply with increasing antimony concentration up to 7 mol% and gradually increased at higher concentra- tion. With increasing the number of coating applica- tions, firing temperature, and firing time the sheet resistance increased. The optimum conditions to achieve the highest conductivity of AT0 coatings ( 18 LOCI) have been determined. The lack of the thermal stability of the AT0 coatings seems to be due to the potential barriers between particles, which are devel- oped by the chemisorption of oxygens from the atmos- phere on the surface of the particles.

Acknowledgements

The authors are grateful to the Northrop corporation for supporting this research.

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