Preparation of super-hydrophilic amorphous titanium dioxide thin film via PECVD process and its application to dehumidifying heat exchangers

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Abstract

The super-hydrophilic amorphous titanium dioxide (TiO2) thin film was prepared by plasma-enhanced chemical vapor deposition (PECVD) process for an application to dehumidifying finned-tube heat exchangers. The chemical components and surface structure were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM). The wettability and long-term durability were investigated by measuring the water contact angle and by performing wet/dry cycles. The samples were subjected to 1000 times of wet/dry cycles to establish long-term durability. The water contact angle of the amorphous TiO2 thin film was about 8° at as-deposited film with O2 plasma treatment and was about 15° after 1000 wet/dry cycles. The amorphous TiO2 thin film had excellent wettability and long-term durability under full wetting conditions.

Introduction

Recently, the super-hydrophilic surface of titanium dioxide (TiO2) thin film has emerged as new and very attractive applications in the field of surface modifications [1], [2], [3], [4], [5] since the thin film can provide excellent surface properties to enhance the wettability of the surface. For the preparation of such thin films, the variety of deposition techniques have been developed including sol–gel [6], [7], thermal evaporation and sputtering [8], [9], and chemical vapor deposition (CVD) process [10], [11]. It was reported that the hydrophilic property of TiO2 surface originated from the water adsorption on oxygen vacancies created by ultraviolet (UV) light irradiation [7] and that the wettability of the TiO2 surface was converted into super-hydrophilic by the irradiation of UV light [1], [7], [11].

Dehumidifying heat exchangers in residential air conditioners use aluminum fins and copper tubes. When the surface temperature of an evaporator having poorly wetting fins is below the dew point, moisture condenses on the fins and the water adheres as droplets. The water droplets may cause bridging between fins of heat exchangers, leading to an even greater increase in the air-side pressure drop. These droplets may also cause carry-over that they may be blown off the fins and entrained in the air stream [13], [14], [15], [16]. Condensate drainage will be improved by the good wettability on the fin surface. This allows higher air velocities for the same air pressure drop and will result in reduced fan power and lower aerodynamic noise level at fixed air velocity, because the fan can operate at reduced discharge pressure [14].

Most of hydrophilic coatings which have been applied to the fins of the finned-tube heat exchanger are polymer coatings. However, the wetting ability gradually deteriorates with increasing numbers of wet/dry cycles because of the water-soluble property of polymer coating [17]. The long-term durability of fin coating is important to keep the performance of heat exchanger. It is reported that long-term durability of surface coating was achieved by ion assisted reaction (IAR) process [18] and plasma treatment method [14]. In case of TiO2 thin film, UV irradiation can be applied to the fin coating of dehumidifying heat exchanger and UV irradiation on fin surface could not produce a highly hydrophilic surface because of the complicated shape of heat exchanger. However, condensate is formed on the surface of fins used in dehumidifying heat exchanger such as evaporators and cooling coils. These coils undergo numerous wet/dry cycles. TiO2 thin films have not investigated the wettability and the long-term durability at wet/dry cycles.

This paper presents the wettability and the long-term durability of coating having a super-hydrophilic amorphous TiO2 thin film prepared by plasma-enhanced chemical vapor deposition (PECVD) process. The results for the amorphous TiO2 thin film were compared to the uncoated aluminum and commercial polymer coating. The coating materials were subjected to wet/dry cycling for 1000 cycles to establish long-term durability.

Section snippets

Experimental apparatus

The schematic of PECVD system employed in this study is shown in Fig. 1. The experimental system was composed of a power unit, a discharging unit, an exhaust unit, a vacuum detection and a gas supply unit. The substrate fixture and plasma-discharging electrode were installed in the plasma chamber. The fixed substrate and electrode were apart about 100 mm. The plasma-generating electrode was connected to a DC power supply operated at 0.8 kV, and the substrate for depositing TiO2 thin film was

Preparation of the amorphous TiO2 thin film

The amorphous TiO2 thin films were prepared by PECVD process using the Ti precursor with the flow rate of 1.0 sccm. Plasma-assisted deposition was performed for 90 s. Then all samples were treated by O2 plasma for 60 s at 0.3 Torr. The water contact angle on the surface of the amorphous TiO2 thin films before and after O2 plasma treatment is shown in Fig. 5. The water contact angle decreased from about 32 to 8° by O2 plasma treatment, which indicates that the wettability of the prepared thin film

Conclusion

The super-hydrophilic properties of the amorphous TiO2 thin film prepared by PECVD process for dehumidifying finned-tube heat exchangers have been investigated in this work. The water contact angle of the amorphous TiO2 thin film depends on the UV light irradiation time and the film was converted to super-hydrophilic after about 120 min exposure to the UV light. The surface was gradually converted to the hydrophilic state after for wet/dry cycles, as well.

The water contact angle on the polymer

Acknowledgements

This work was supported by LG Electronics. The authors gratefully acknowledge the support of Su-Won Lee, Jung-Geun Oh and Chang-Il Jung.

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