Chinese Journal of Polar Since  2018, Vol. 31 Issue (2): 216-222

The article information

Hai-yun Jin, Shi-chao Nie, Zhi-wei Li, Cheng Tong, Ke-jing Wang
金海云, 聂诗超, 李志伟, 仝程, 王科镜
Investigation on Preparation and Anti-icing Performance of Superhydrophobic Surface on Aluminum Conductor
超疏水铝导线的制备及其防覆冰性能研究
Chinese Journal of Polar Since, 2018, 31(2): 216-222
化学物理学报, 2018, 31(2): 216-222
http://dx.doi.org/10.1063/1674-0068/31/cjcp1707152

Article history

Received on: July 28, 2017
Accepted on: September 11, 2017
Investigation on Preparation and Anti-icing Performance of Superhydrophobic Surface on Aluminum Conductor
Hai-yun Jin, Shi-chao Nie, Zhi-wei Li, Cheng Tong, Ke-jing Wang     
Dated: Received on July 28, 2017; Accepted on September 11, 2017
State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an, 710049, China
*Author to whom correspondence should be addressed. Hai-yun Jin, E-mail:hyjin@mail.xjtu.edu.cn
Abstract: Aluminum is widely used in transmission lines, and the accumulation of ice on aluminum conductor may inflict serious damage such as tower collapse and power failure.In this study, super-hydrophobic surface (SHS) on aluminum conductor with micro-nanostructure was fabricated using the preferential etching principle of crystal defects.The surface microstructure and wettability were investigated by scanning electron microscope and contact angle measurement, respectively.The icing progress was observed with a self-made icing experiment platform at different environment temperature.The results showed that, due to jumping and rolling down of coalesced droplets from SHS of aluminum conductor at low temperature, the formation of icing on SHS could be delayed.Dynamic icing experiment indicated that SHS on aluminum conductor could restrain the formation of icing in certain temperature range, but could not exert influence on the accumulation of icing.This study offers new insight into understanding the anti-icing performance of actual aluminum conductor.
Key words: Super-hydrophobic     Aluminum conductor     Crystal defects     Self-propelled jumping     Anti-icing    
Ⅰ. INTRODUCTION

Icing is a natural phenomenon occurring on the surfaces of objects in the extreme weather of low temperature and freezing rain. Undesired ice accumulation leads to severe economic issues and, in some cases, loss of lives [1]. Icing on the road results in slippery surfaces and often leads to traffic accidents, besides, icing on the wings and surfaces of aircrafts may cause crash accidents [2, 3]. Ice disaster is also a great threat for power system [4-8], and the frequency of ice disaster has increased in recent years due to abnormal weather.

The icing problem has attracted much attention, and a series of anti-icing/de-icing methods have been put forward [9-14]. But some conventional methods, such as electrothermal method, chemical method and mechanical deicing method were not effective enough. Therefore, surface materials, a potential anti-icing method, become one of the hottest spots [15-26]. It is fortunate that heavy ice accretion problems have been mitigated by using polymeric coatings. Guan et al. [18, 19] prepared a semiconducting RTV anti-icing coating for insulator by adding conducting particles (carbon fibers or graphite), and the coating showed good anti-icing performance. Inspired by ice skating, a series of aqueous lubricating layers on the original anti-icing coatings ware fabricated, which will reduce the adhesion between ice and solid substrates, and ice formed atop of a solid surface slides away under its gravity or an action of natural wind [24-26]. Super-hydrophobic surface (SHS), with a contact angle (CA) lager than 150° and sliding angle (SA) less than 10° [27], is also a potential surface for anti-icing. There are lots of methods to fabricate SHS [28-31]. For example, Jin et al. [30] used a hot-pressing template method and Li et al. [31] used a vapor deposition method to fabricate SHS, respectively. Aluminum is the main material of transmission lines, so the fabrication of effective SHS on aluminum is crucial for power system. Bouchama et al. produced the anodic alumina by two types of anodization process, namely single-step and two-step anodizing [32]. Rezayi et al. proposed a simple immersion method accompanied with ultrasound to fabricate desirable roughness on Al through ZnO particle deposition, and subsequently modified the surface with STA [33]. Peng et al. prepared anti-corrosive and mechanically durable superomniphobic aluminium surfaces by a three-step approach involving acid bath (microstructure formation), boiling water bath (nanostructures) and finally immersion in a fluorosilane containing bath [34, 35]. However, most of these methods involve limiting conditions, such as low efficiency, expensive devices, and complex control [32-39]. Saleema et al. [40] developed a simple method by immersing the aluminum alloy substrates in a solution containing NaOH and FAS17 molecules, but the coatings only have low surface energy without micro-nanostructure, therefore, it was very difficult to obtain the super-hydrophobic effect. Yin et al. [41] prepared a super-hydrophobic coating with excellent corrosion resistance property and good stability, but the anti-icing behavior of this coating is unknown.

In this study, the preferential etching principle of crystal defects was used to fabricate micro-nanoscale aluminum surface and stearic acid was used to construct a low surface energy coating. The SHS was prepared on actual aluminum conductor. The anti-icing mechanism of SHS on aluminum conductor was mainly focused on. A self-made icing experiment platform was developed to study the icing progress, and the self-propelled jumping phenomenon of coalesced water droplets was analyzed. In addition, at different environment temperature, the anti-icing performance of SHS on aluminum conductor was also investigated. The results could provide a theoretical basis for the practical application of the SHS on aluminum conductor for transmission line.

Ⅱ. EXPERIMENTS A. Icing experiment platform

The icing experiment platform mainly consists of three parts: the temperature control system, the spray system and the Plexiglas box (as shown in FIG. 1).

FIG. 1 The sketch map of icing experiment platform.

The principle of compressor refrigerating was applied for the temperature control system. Firstly, the refrigerate compressor (GVY66AA, Zanussi, Tianjin) inhaled refrigerant from evaporator, and compressed it from low temperature and low pressure to high temperature and high pressure. Then the refrigerant would be liquefied to low temperature and high pressure by condenser. Then after being subjected to the resistance in the capillary it will become low-temperature and low-pressure liquid. Lastly the refrigerant absorbed heat from surrounding medium in the evaporator. Then low-temperature environment for the icing experiment could be obtained. As for temperature control, temperature sensor was used to collect real-time data about temperature in the icing platform. The start and stop of the refrigerate compressor were controlled by the temperature controller (SF-203, Shang fang, china) according to the temperature data.

The spray system consisted of the switch timer, the micro water pump (maximum pumping amount is 3 L/min), the sprayer and so on. Pumped from the tank by micro pump, water passed water pipes and spurted out from the sprayer, which could be adjusted to get spray containing drops of very small diameter. Splash method was applied to analyze these drops, to satisfy the requirement of the sleet diameter for glaze-icing. The switch timer was programmed to control the micro pump and to achieve intelligent controlling on the condition of spraying. Because of outstanding heat insulation performance and higher transparency, Plexiglas was used as the main body of the icing platform. In order to satisfy the circumstance temperature at which dynamic icing experiment required, the temperature of the whole icing platform was controlled within range of -40 ℃ to 15 ℃, and the range of fluctuation was between $\pm$0.5 ℃.

B. Sample preparation

Firstly, the aluminum conductor should be pre-treated. 1000 mesh sandpaper was used to remove the compact oxidation layers on surface, and 10 min ultrasonic cleaning processes were used in water and absolute alcohol respectively to remove the filth and oil on surface. Put the pre-treated aluminum conductor into 20 wt% hydrochloride for 1 min to etch. Add stearic acid reagent to the alcohol solution and mix well to form 1 wt% stearic alcohol solution, then put the dried aluminum wire into it for 15 min, and after drying it in 90 ℃ oven, the aluminum sample with SHS could be obtained at last.

C. Experiment method

The wettability of SHS on aluminum conductor was measured by contact angle measurement (OCA20, DATAPHYSICS, Germany), and the size of measured droplet was 10 ${\rm{\mu }}$L. The surface microstructure was observed by scanning electron microscope (VE-9800S, KEYENCE, Japan). The icing process for SHS on aluminum conductor in the icing experiment platform was observed by a high-speed camera, the anti-icing performance and the rolling down mechanism of the water droplet were also investigated.

Ⅲ. RESULTS AND DISCUSSION A. Microstructure

FIG. 2 is the SEM images of the aluminum conductor surface etched by hydrochloride for 1 min. In FIG. 2 (b) and (c), many micron-sized pits can be seen on the aluminum surface. FIG. 2(b) presents pits of different size and different depth. Some isolated "island-like" humps, which are circled in red in FIG. 2(b), are also distributed on the aluminum surface. FIG. 2(d) is a further enlargement of FIG. 2(b). A higher magnification SEM image of red frame in FIG. 2(d) is shown in FIG. 2(e). It can be seen that there are micro-nanostructures on surface. This special complex micro-nanostructure etched by hydrochloride can absorb more air and provide the necessary geometric condition for the formation of super-hydrophobicity. A higher magnification SEM image of blue circle in FIG. 2(d) is shown in FIG. 2(f). "Step-like" pits, a fundamental structure in etching process of aluminum, can be observed. The corrosion of aluminum usually originates from pitting corrosion such as dislocations, grain boundaries and mechanical scratches. These crystal defects, due to possessing relatively higher energy, are prone to destroy, and thus when attacked by chemical etchants, they would be dissolved first [42-45]. Then the pits appeared on the surface of crystal. The chemical reaction between aluminum and hydrochloric acid will occur as follows (1) :

$ \begin{eqnarray} 2\rm{Al}+6\rm{HCl}\rightarrow 2\rm{AlCl}_3+3 \rm{H}_2\uparrow \end{eqnarray} $ (1)
FIG. 2 SEM images of aluminum surface at 500$\times$ (a), different size of pits and humps at 1000$\times$ (b), micron pit at 2000$\times$ (c), magnification of (b) at 5000$\times$ (d), nanoscale pits at 20000$\times$ (e), "step like" pit at 30000$\times$ (f).

"Unit pits" preferentially formed in the crystal defects. And with the extension of time, the "unit pit" continuously formed in the crystal, and the "unit step" expanded along the direction of the crystal surface, which worked together to form macroscopic pits. The relative rates of these two processes determine the shape of the etch pits, and further determine the surface microstructure. The greater ratio of the formation rate of the "unit pit" and expansion rate of the "unit step" along the crystal surface, the deeper the pits will be formed. After etched by hydrochloride, a great quantity of hydroxyl exists in surface of aluminum conductor. And this hydroxyl reacted with carboxyl in stearic acid, thus forming a compact, thinner coating with low free energy.

B. Wettability

As shown in FIG. 3, the wettability of 10 ${\rm{\mu }}$L water droplet on the SHS of aluminum conductor is measured by CA measurement at ambient temperature (18$\pm$1) ℃. FIG. 3(a) shows the contact state of SHS taken by CA measurement and the CA is 159°$\pm$0.5°. When the contact form is surface-droplet-air, it can be got from the transformation of Cassie-Baxter equation [46]:

$ \begin{eqnarray} \rm{cos}\theta=f_1 \rm{cos}\theta_1-f_2 \end{eqnarray} $ (2)
FIG. 3 Wettability of 10 ${\rm{\mu }}$L droplets on super-hydrophobic aluminum conductor surface.

where $f_1$ is the fraction between droplet bottom and coating surface, $f_2$ is the fraction between droplet bottom and air in the composite surface, $\theta$ is the actual CA of droplet on SHS of aluminum, $\theta_1$ is the intrinsic CA of droplet on coating surface, the sum of $f_1$ and $f_2$ is equal to 1. The static contact angle of aluminum conductor modified only by stearic acid is 109°. Given $\theta_1$ and $\theta$ equal to 109° and 159°, respectively, $f_2$ is calculated to be 0.901. The very large $f_2 $ indicates that the super-hydrophobicity of the obtained surface is mainly achieved by the air trapped in the micro-and nanoscale pits and humps.

Meanwhile, super-hydrophobic aluminum surface has smaller SA, FIG. 3(a) shows the sliding moment of droplet, its SA is 6°. The contact state is simulated in FIG. 3(b), the water droplet and the SHS of aluminum is contacted only at some micro-nanoscale humps. Due to the cooperation of micro-nanostructure surface and low free energy coating, the SHS on aluminum conductor has larger CA and smaller SA.

C. Dynamic anti-icing performance

FIG. 4(a) shows the contact state of cooled water on the SHS of aluminum conductor, at the temperature of -5°. When the experiment time of icing extends to 50 min, there are no icing and water droplets on the surface of conductor except for local adhered water. In addition, adhered water droplets exist as dispersed and isolated small water ball, which indicates that the water droplet can maintain a large CA on its surface at low temperature. The phenomenon above proves that SHS on aluminum conductor still maintains excellent hydrophobic properties in the low temperature and high humidity environment. The sliding progress of water droplets from SHS is recorded by a high-speed camera. FIG. 4(b) shows the direct sliding progress of droplets with large volume from both sides of cylindrical conductor under the impact of its gravity and wind. FIG. 4(c) shows the coalescence progress of two very close droplets and then the coalesced droplet slipped from the conductor. Therefore the formation of icing is delayed as a result of super-hydrophobicity.

FIG. 4 Static and dynamic process of water droplet on super-hydrophobic aluminum conductor at -5℃.

The research shows that when the two or more condensed water droplets are close to each other, in order to decrease interface free energy (IFE), they will coalesce and have an opportunity to jump [47-53]. The coalesced droplet initially is usually in an unstable state with its IFE greater than the corresponding equilibrium value because of the existence of excess IFE. The excess IFE will turn to kinetic energy if it is large enough to overcome the resistance on three-phase contact line (TPCL) and potential energy change in the progress of deformation of coalesced droplet. Then the droplet jumps and rolls down from conductor as shown in FIG. 4(c). FIG. 5 is a sketch diagram of the coalescence process of two condensed droplets, and the coalescence process is divided into two stages. In the first stage (FIG. 5 (a-d)), the center line of droplets close to each other, mutual contact area is bigger and bigger, droplet exists in an unstable state. In the second stage (FIG. 5 (d-f)), unstable droplet tends to transform itself toward its equilibrium state under the influence of excess IFE. One of the resistances of deformation is the adhesion of TPCL during the base area reduction. The gravity of the drop is another resistance because the drop gravity center will rise up during the transformation process. The unstable droplet will jump if the sum of adhesion energy, gravity potential energy and other energy is less than excess IFE.

FIG. 5 Sketch diagram of the merging process of water droplets on super-hydrophobic aluminum surface.

Assuming that the IFE of a single droplet system is $E_{\rm{surf}}$, then:

$ \begin{eqnarray} E_{\rm{surf}}=\gamma_{\rm{sg}} A_{\rm{sg}}+\gamma_{\rm{sl}} A_{\rm{sl}}+\gamma_{\rm{lg}} A_{\rm{\rm{lg}}} \end{eqnarray} $ (3)

Where $\gamma_{\rm{sg}}$ and $A_{\rm{sg}}$ are the interface tension and contact area between solid and gas, respectively, $\gamma_{\rm{sl}}$ and $A_{\rm{sl}}$ are the interface tension and contact area between solid and liquid, respectively, $\gamma_{\rm{lg}}$ and $A_{\rm{lg}}$ are the interface tension and contact area between liquid and gas, respectively.

The IFE of two separated water droplets is:

$ \begin{eqnarray} E_{\rm{surf1}}=\gamma_{\rm{sg}} \sum\limits_1^2 A_{\rm{sgi}} +\gamma_{\rm{sl}} \sum\limits_1^2A_{\rm{sli}} +\gamma_{\rm{lg}} \sum\limits_1^2A_{\rm{lgi}} \end{eqnarray} $ (4)

The IFE of the condensed water droplets is:

$ \begin{eqnarray} E_{\rm{surf2}}=\gamma_{\rm{sg}} A_{\rm{sg}}^*+\gamma_{\rm{sl}}A_{\rm{sl}}^*+\gamma_{\rm{lg}} A_{\rm{lg}}^* \end{eqnarray} $ (5)

where $A_{\rm{sg}}^*$, $A_{\rm{sl}}^*$, and $A_{\rm{lg}}^*$ are the contact area after coalescence. The excess IFE is:

$ \begin{eqnarray} \Delta E=E_{\rm{surf1}}-E_{\rm{surf2}} \end{eqnarray} $ (6)

From FIG. 5(d, e), the change of the potential energy of the system is:

$ {E_g} = \rho vg\Delta h $ (7)

where $\rho$ is the density of water droplets, $v$ is the volume of droplets, $g$ is the gravity acceleration, $\Delta h$ is the height variation of gravity center. The energy loss of overcoming adhesion is [50, 51]:

$ \begin{eqnarray} W=\gamma_{\rm{sl}} f(1+\rm{cos}\theta)A_{\rm{sl}}^* \end{eqnarray} $ (8)

where $f$ is the contact fraction between solid and liquid. If we regard water droplet (FIG. 5(d)) as a spherical segment, then the contact area between spherical segment and super-hydrophobic conductor is:

$ \begin{eqnarray} A_{\rm{sl}}^*=\pi R^2 \rm{sin}\theta \end{eqnarray} $ (9)

where $R$ is the radius of spherical segment. The volume of spherical segment is:

$ \begin{eqnarray} V=\frac{\pi R^3 (2-3\rm{cos}\theta+\rm{cos}^3 \theta)}{3} \end{eqnarray} $ (10)

Bring (9) to (8):

$ \begin{eqnarray} W=\pi \gamma_{\rm{sl}} f(1+\rm{cos}\theta)R^2 \rm{sin}\theta \end{eqnarray} $ (11)

Bring (10) to (7):

$ {E_g} = \frac{{\pi \rho g\Delta h{R^3}(2 - 3{\rm{cos}}\theta {\rm{ + co}}{{\rm{s}}^{\rm{3}}}\theta )}}{3} $ (12)

There is some other energy loss ($E_{\rm{loss}}$) in the progress of droplet deformation. According to the law of conservation of energy, if $\Delta E$$>$$E_\rm{g}$+$W$+$E_{\rm{loss}}$, water droplets may obtain kinetic energy and jump, otherwise the droplet gets equilibrium state. The super-hydrophobic aluminum conductor, with micro-nanostructure and low free energy, has small radius and large inclination angle on both sides. The self-propelled jumping of coalesced droplets and the gravity of coalesced droplets make it easy to roll down from the both sides of conductor, which contributes to delay the formation of icing.

In order to observe the dynamic icing progress directly and test the anti-icing performance of SHS, common aluminum conductor with the diameter of 3.3 mm is pre-treated. One half is fabricated with SHS and the other half is only polished. Put this special aluminum conductor into the icing experiment platform and observe the icing progress with a high-speed camera. FIG. 6 is a comparison of the icing process of the common polished aluminum conductor and the aluminum conductor with SHS at the same ice time when the temperature is -5 ℃ and the relative humidity is 85%. It can be seen from the figure that the polished aluminum is quickly covered with water film. When the icing test time extends to 5 min, ice appears on the surface of polished aluminum conductor, since then the amount of ice has increased rapidly over time. But for aluminum conductor with SHS, there is no ice but some isolated small water balls on the surface. These small water balls are not immediately frozen into ice and then rolls down from the surface. When the icing test time extends to 60 min, the surface of the polished aluminum conductor is covered with ice layer, and the length of the ice cone also increases rapidly, while the aluminum conductor with SHS is still not covered with ice. When the icing test time extends to 110 min, there is only a little ice on the SHS, which indicates that the SHS delays the formation of icing. When the icing test time extends to 180 min, polished conductor is covered with a thick layer of ice and a large number of long ice cones. There is only partial ice on the SHS and the ice length is also limited, most of the areas are not covered by ice. The results show that aluminum with SHS had an obvious effect on resisting the formation of ice.

FIG. 6 The icing process of aluminum conductor surface (the left part is the polished surface and the right part is the SHS).

FIG. 7 shows the statistics of average ice cone length of the polished aluminum conductor and aluminum conductor with SHS, indicating the anti-icing capacity of surfaces at various temperatures and icing time. At the environment temperature of -5 ℃, for polished aluminum conductor, the average ice cone length are 4, 35.2, 42.2, and 60.3 mm when the icing time are 5, 110, 120, and 150 min respectively. For aluminum conductor with SHS, 110 min is a critical point. Before this point, there is no ice on the conductor's surface. After this point, there is one ice cone, and the lengths of the cone are 5.8 and 10 mm at 110 and 120 min, respectively. And at 150 min, there are two ice cones and the average length of cones was 18.1 mm (the length of other small one is only 3 mm). The number and the average length of the ice cone on aluminum conductor with SHS are significantly less than the polished aluminum conductor. The analysis shows that the cooperation of micro-nanostructure surface and low free energy coating on SHS increase the contact area between water droplet and air film, resulting in the low adhesion and easy slipping characteristics of water droplet. Simultaneously, the special self-propelled jumping phenomenon of coalesced droplet also delay the formation of icing. So the super-hydrophobic surface shows excellent anti-icing performance. At the environment temperature of -25 ℃ and the relative humidity of 85%, the number of ice cone and the average length of the ice cone of aluminum conductor with SHS are still less than the polished aluminum conductor. However, the growth trend of ice cones for SHS at the temperature of -25 ℃ is similar to the polished aluminum at the temperature of -5 ℃. At very low temperature, the energy exchange rate of water droplets and air is accelerated, which reduces the ability of SHS against the formation of ice and the ice forms rapidly. On the other hand, the excess IFE of the coalesced droplets is not large enough at such extremely low temperature, the water droplets are difficult to roll down before icing. According to the information implied in FIG. 7, each curve has almost the same growth trend. It means that the SHS on materials can restrain the formation of ice coating at a certain temperature, and the effect of inhibition is closely related to temperature. However, the super-hydrophobicity cannot exert influence on the accumulation of icing because the icing coatings break the super-hydrophobicity of the material.

FIG. 7 Comparison of the length of ice cones between superhydrophobic and common aluminum conductor at different temperature and time.
Ⅳ. CONCLUSION

According to the preferential etching theory of crystal defects, an SHS on aluminum conductor, which had a CA of 159° and a SA of 6°, was fabricated. The free rolling processes of one water droplet and two coalesced droplet were recorded by a high-speed camera. When the excess IFE is bigger than the energy change (gravity potential energy, adhesion energy and other energy loss) in the process of water deformation, self-propelled jumping phenomenon of coalesced droplet will happen on the SHS of aluminum conductor and the formation of icing at the environment temperature of -5 ℃ was delayed by this phenomenon. Dynamic icing experiment showed that the super-hydrophobic surface possesses excellent anti-icing performance at low temperature. The number of ice cone and the average length of the ice cone of aluminum conductor with SHS were less than the polished aluminum conductor at the temperature of -5 ℃. The growth trend of ice cones for two kinds of aluminum conductor at different temperature was similar at different temperature. The formation of ice coating could be restrained on aluminum conductor with SHS at a certain temperature, and the inhibitory effect was closely related to the temperature.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.51272208).

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超疏水铝导线的制备及其防覆冰性能研究
金海云, 聂诗超, 李志伟, 仝程, 王科镜     
西安交通大学电力设备电气绝缘国家重点实验室, 西安 710049
摘要: 本文利用晶体缺陷的优先刻蚀原理在铝导线上制备出具有微-纳复合结构的超疏水表面,并分别使用扫描电镜和接触角测量仪对超疏水铝导线的微观结构和浸润性进行了研究.随后利用自制的覆冰实验平台对不同环境温度下超疏水铝导线表面的结冰过程进行了研究,结果表明低温条件下合并水滴在超疏水铝导线表面的跳跃滚落能有效阻止覆冰的形成,动态覆冰实验结果表明超疏水铝导线只能抑制覆冰的形成但不能抑制覆冰的积累.对进一步研究超疏水铝导线的防覆冰性能提供了新思路.
关键词: 超疏水     铝导线     晶体缺陷     自跳跃     防覆冰