Preparation of stable superamphiphobic surfaces on X80 pipeline steel substrates

Abstract

This paper provides a simple, high efficiency and low-cost approach for the preparation of superamphiphobic surfacea on X80 pipeline steel substrates. The whole process included three simple steps: first, the metallic copper was electrochemically deposited onto the surface of X80 pipeline steel substrates under hot alkaline conditions to obtain a layer of the bi-material interface structure. Second, the treated surface was further immersed in ammonia solution at a particular temperature in order to fabricate hierarchical structure to increase the surface roughness. Finally, to reduce the surface energy of the fabricated structures, the surfaces were subsequently chemically modified with 1H,1H,2H,2H-perfluoro-decyl-triethoxysilane. After treatment using the optimum parameters, the as-prepared surfaces exhibited repellency toward distilled water, glycerol, ethylene glycol, and olive oil with contact angles at 163°, 157°, 155° and 152°, respectively, the corresponding sliding angles are all within 10°, which can be attributed to the combination of micro/nano rough reentrant and hierarchical laminated structures with low surface energy modification. Furthermore, in an attempt to improve the superamphiphobic properties of the surface, the thermal stability, long-term stability and mechanical stability of the fabricated nanostructure film surface were examined using a UV light test, immersion test, temperature test and abrasion test, which show that the created superamphiphobic surfaces possess excellent stability under harsh conditions. The developed approach presented might provide a facile, low-cost and scalable route toward the preparation of novel films on metal substrates for various industrial applications, and a high potential for large-scale applications of pipeline steel.

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Introduction

Wettability is one of the most common phenomena in nature, also it is one of the important characteristics of a solid surface,^1,2 which is usually characterized by measuring the CA (contact angle) between a liquid droplet and the surface of the substrate. When the contact angle between the solid surface and the liquid is more than 90°, the surface is called a hydrophobic surface. Similarly, a material is considered superhydrophobic, when the contact angle for the contacting liquid droplet is greater than 150° and the surface displays a low SA (sliding angle).^3,4

Compared to superhydrophobic surfaces, however, it is very difficult to prepare superoleophobic surfaces. Many researches have tried various approaches to create superoleophobic surfaces that resist wetting with low-surface-energy oils and achieved some good results.^5–7 By controlling both the surface morphology and surface energy, which are the two key surface process parameters that dominant the wettability of solid surfaces, artificial superoleophobic surfaces can be fabricated within a certain scope optionally.

In consideration of their superior performance and potential applications, superamphiphobic surfaces have aroused a vast concern in various fields of the world. In recent years, many materials with superamphiphobic surfaces have been developed and obtained wide applications in the industrial and agricultural production, such as its self-cleaning properties,⁸ the anti-ice of the outdoor transmission lines,⁹ the prevention of water and dirt on clothes and buildings,^10–12 etc. These functional surfaces are not only applied in the field of basic research, but also in the protection of metallic material such like the oil pipeline steel.

As pipeline steels are very commonly used in modern industry fields including many of the above mentioned applications, especially in the areas of the transportation of natural gas and oil. Oil–gas equipment is easy to be corroded slowly because of harsh service conditions and high pipeline pressure turbulence.¹³ Moreover, an intense fluctuation of the level of liquid in the oil–gas separator at archibathyal downstream positions of the pipeline would undermine the substrate material when a longer liquid slug leaves the end of pipeline.^14–16 As a consequence, it is of large interest to explore novel approaches to optimize the wettability of pipeline steel surfaces.

Solid surface wetting behavior (the CA of liquid droplet on the rough surface θ*, along with the effect of the surface roughness) can be explained by the classical Wenzel and Cassie models,^17,18 which suggest that for a flat with intrinsic contact angle more than 90°, the more severe the roughness is, the more excellent its superhydrophobicity is. However, the internal mechanisms between them are not the same. In the Wenzel model, the contacting liquid droplet completely permeates the surface protrusions, shaping the so called “fully-wetted” interface, which modifies the CA according to the equation:^17,19,20

cos θ* = r cos θ

where r is the roughness factor, which represents the ratio of the real contact area of solid–liquid interface area and the apparent contact area, and θ is the equilibrium CA on the smooth surface of the same material.

In the Cassie state, the fluid drop rests on the rough surface and does not completely wet the surface texture because of the air pockets that remain trapped underneath the hierarchical micro-nanostructures. So that the equation can be modified into the expression:^19–21

cos θ* = ϕ_s(cos θ + 1) − 1

where ϕ_s is the fraction of the solid underneath the droplet. The equation of this model is rewritten as follow:

cos θ* = f₁ cos θ − f₂

where f₁ and f₂ represent the proportion of the contact areas of liquid–solid and liquid–gas to the total actual contact area, (f₁ + f₂) = 1.

It is well-known that, it is necessary to obtain a micro/nano scale structured surface with low surface energy materials in order to achieve a super-liquid-repellent surface.^22,23 It has been reported that the surface energy for some commonly used groups decreased in the order –CH₂– > –CH₃ > –CF₂– > –CF₂H > –CF₃.²⁴ So, fluoropolymer coatings are the best choice for the enhancement of liquid repellency due to their high content of –CF₃ and –CF₂– groups.²⁵ At present, some research groups have developed various methods to make up complex and rough surface structure. These methods included metal anode oxidation,²⁶ chemical etching,²⁷ sol–gel method,²⁸ plasma enhanced chemical vapor deposition,²⁹ phase separation,³⁰ template method,³¹ electrochemical deposition method,³² etc.

However, the researches about the superamphiphobic surfaces layer on the pipeline steel substrate material were rarely reported. It has large significance to optimize the surface superamphiphobicity to advance the corrosion resistance of the pipeline steel. In general case, before, surface treatment with the pipeline substrate is mostly reacted with acidic solution etching.^33,34 However, the disadvantage is that micro-topography is hard to control, neither nor the multilayer micro/nanometer structure. Up to now, there have been different reports on the fabrication of artificial superhydrophobic steel surfaces. Furthermore, the system of alkaline etching was rarely used to study the superamphiphobic surface layer of pipeline steel, while the relevant research about copper substrate surface was relatively mature and achieved very good application.^35–37

In order to better develop the application platform of X pipeline steel. In our current work, inspired by the existing ideas and methods based on referring a lot of excellent research team, we successfully developed a simple, efficient and environment-friendly approach to fabricate a superamphiphobic surface on X80 pipeline steel surface with both micro-nanometer scale binary multi-layered hierarchical structures. It display high contact angle and low sliding angle for different liquids by copper electroplating, alkaline etching and fluoroalkylsilane modification. The entire process consists of three steps: in the first place, the pipeline steel substrate was copper electroplating in the EDTA (Ethylene Diamine Tetraacetic Acid) solution system, and subsequently was chemically etched in different hot alkaline solution concentrations to achieve hierarchical structure composed of micro/nano flowers and arrays. Finally, the surface was chemically modified with HFTTMS (1H,1H,2H,2H-perfluoro-decyl-triethoxysilane) to decrease the surface energy of the as-fabricated structures to obtain an ideal superamphiphobic surface. The whole procedure was easy to carry out. The resulting surface, according to the methods described above, exhibited enhanced excellent superamphiphobic properties toward water and several oil liquids. Our work is expected to create a new avenue for the basic research as well as practical applications.

Furthermore, the fabricated superamphiphobic surface above mentioned also exhibited outstanding thermal stability and long-term stability along with excellent mechanical stability under some specific conditions. Therefore, our work may be able to provide some valuable information for the future related research.

Experimental

Materials and reagents

The substrate material used in this work was X80 pipeline steel, its chemical composition are listed in Table 1. The dimension of the steel specimen was cut in pieces of 20 mm × 10 mm × 3 mm. Copper substrates (purity 99.9 wt%). Ethylene diamine tetraacetic acid, ammonium hydroxide and 1H,1H,2H,2H-perfluoro-decyl-triethoxysilane were obtained from Aladdin, the physical properties of HFTTMS are listed in Table 2, and its molecular structure is shown in Fig. 1. Other chemicals were of analytical grade and used without further purification.

Table 1 Chemical compositions of X80 pipeline steel (wt%)

C	Si	Mn	Mo	Nb	V	P + S	Ni + Ti + Cr	Fe
0.065	0.24	1.85	0.34	0.057	0.005	0.0138	0.426	Balance

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Table 2 Physical properties of 1H,1H,2H,2H-perfluorodecyltriethoxysilane

Molecular formula	Molecular weigh	Relative density	Boiling point (°C)	Molar volume (cm^3 mol^−1)	Surface tension (dyne per cm)
C16H19F17O3Si	610.38	1.4889 g mL^−1	111.6	440.4	18.1

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Fig. 1 The molecular structure of HFTTMS.

Electrochemical plating pretreatment of pipeline steel surface

First, the samples were mechanically ground with 320, 600, 800, 1000, and 2000 grit sandpapers, respectively, and then were immersed in 0.5 wt% NaOH solution to remove the oil and other dirt. After that, the samples were further ultrasonically cleaned with acetone, anhydrous ethanol for 10 min each, respectively, then rinsed three times with deionized water to remove surface impurities and dried with a dryer. In the first place, basic cupric carbonate was used as main salt and EDTA disodium was used as coordination agent in electro-plating solution. Electroplating technological parameters are listed as follows: Cu₂(OH)₂CO₃ 14 g L⁻¹, C₆H₅O₇K₃·H₂O 150 g L⁻¹, KNO₃ 4 g L⁻¹, pH = 12, experimental temperature was at 60 °C water-bath heating, current density was 2.5 A dm⁻², and electroplating time was 45 min. After plating, the treated substrate was rinsed by anhydrous ethanol, deionized water, respectively, to remove the excess of stain and completely dry samples were obtained in the constant temperature of oven under a high vacuum environment. Then transfer the pre-treated specimen to the Petri dish.

Chemical etching treatment of the samples

The substrate plates were immersed in a ground-glass stoppered flask (250 mL) containing a NH₄OH solution and heated in an oven at 70 °C. In order to obtain chemical etching process parameters, a series of experiments were carried out as a comparison. By comparing the subsequent contact angles, respectively, under various processing conditions. The optimum chemical etching process parameters used in this work were determined as follows: 0.04 M NH₄OH solution etching for 40 h. The samples were then rinsed with deionized water and dried at room temperature (25 °C). Finally, the resulting sample was placed in an oven and heated for 2 hours at 160 °C. Then, a uniform, dark black film was deposited on the substrate.

Surface modification

To decrease the surface energy on the fabricated structures, the as-prepared sample was immersed in a mixed solution containing 1% HFTTMS (500 μmL), anhydrous ethanol (50 mL) in a sealed glass bottle, at room temperature for 30 min. After the fluorination treatment, the pre-treated specimen was transfered to the Petri dish and dried in an oven at a constant temperature 100 °C for 10 min, then, substrates were taken out and dried at room temperature (Scheme 1).

Scheme 1 Schematic diagram of steps involved in the fabrication of a superamphiphobic substrate.

Contact angle measurements

Contact angles were measured by a contact angle measuring instrument (JC2000DM) with a computer-based image processing system (produced by Shanghai Zhongchen digital technic apparatus co, ltd company, China). Distilled water, glycerol, ethylene glycol, olive oil were used as the test liquids, respectively, and about 5 μL of the liquid droplet randomly dropped onto the surfaces of the specimens at room temperature. Five different regions on substrate sample surface were measured, and the average value of the contact angle was calculated as the reported result.

Sample characterization

The microscopic morphology of the specimen surface was observed using a type field emission scanning electron microscopy (FE-SEM, Hitachi, S4800), the attached energy-dispersive spectroscopy (EDS, INCA Energy, Oxford135Ins), and the optical images of the droplets were obtained using a digital camera (Nikon eclipse LV150). The crystal structure of the fabricated samples was characterized by an X-ray diffractometer system (XRD, Shimadzu-6000), equipped with a Cu K-radiation source, and EDS and FTIR are performed to investigate corresponding surface the chemical compositions of specimen surfaces. Fourier transform infrared spectrum data were collected on a Fourier transform microscopic infrared spectrometer (FI-IR, NICOLET 8700, Thermo).

Results and discussion

Surface morphology

The morphology of the surface was investigated with an SEM. Fig. 2a shows the surface morphology of the surface after copper electroplating. It is obvious that the surface has a certain roughness. Lumpy protrusions have been formed. It provides good surface in order to obtain micro/nano rough structure.

Fig. 2 SEM images of the specimen at different processing conditions (a) copper plating, (b)–(f) etching with 0.04 M NH₄OH at 70 °C for different time (b) 5 h, (c) 10 h, (d) 15 h, (e) 40 h and (f) 45 h.

Fig. 2b shows the morphology of the formation of copper oxide in the beginning, which chaotically distributed into strips. Next, with the extension of the etching time, the banded structure fractured, knotted mutually (Fig. 2c). Then, composite structure of shaped flower cluster gradually formed (Fig. 2d). From a greater magnification image, novel dandelion-like CuO nanostructures were successfully generated on the sample surfaces with diameters ranging from 1 to 2 μm. Finally, Fig. 2e shows the surface morphology of the as-prepared copper–oxide hierarchical structures on the pre-existing substrate, processed by the optimal process parameters. It is very obvious that after alkaline etching treatment with 0.04 M NH₄OH solution for 40 h at 70 °C, the base material surface was covered with uniformly distributed flower-like clusters with diameters about 1–2 μm. Flower clusters are composed of rich layers structured by stereoscopically covered surface. As the etching time is prolonged, micro/nanoflower structure is destroyed (Fig. 2f), only some single flake structures are on the surface. Fig. 3a shows the morphology after heating with 0.04 M NH₄OH at 70 °C. A greater magnification SEM image exhibits that micro/nano sized texture of flower-like micro-cluster features were generated on the hierarchical with diameters about 1 μm (Fig. 3b). Flower-like micro-cluster was composed of nanosheets with thicknesses about 20–30 nm and the thickness of the nanosheets was between 30–50 nm (Fig. 3c). The overlapping laminar clusters were hanging over each other. Importantly, the superimposition of hierarchical structure contributed to the re-entrant structures, and rich multi-layer laminated micro/nano flowers staggered which can trap a large fraction of air. According to recent study, specific rough structures like the laminated or overlapping structure are indispensable to the formation of a composite solid–liquid–air interface with some liquids, and when the droplets are placed on a certain position of as-prepared structures, the Laplace pressure force is directed upward, which can effectively prevent the liquids such like certain oils from penetrating into the textures.^38,39

Fig. 3 SEM images of micro/nano particles grown on top of as-pretreated specimen surfaces with (a) low and (b and c) high magnification image of CuO microflowers after heating with NH₄OH for 40 h at 70 °C.

Surface chemical composition

To further study the relationship between the crystal structures of the fabricated samples surface and the superamphiphobicity, the crystal structures of specimen surfaces are investigated by XRD. Fig. 4a shows the XRD patterns of the sample surface after copper plating. It can be observed that the substrate simply contains a cubic copper phase. The XRD spectrum of copper oxide surface (Fig. 4b), contains multiple peaks and the diffraction peaks can be indexed to monoclinic phase of CuO, which is consistent with the reported data, clearly indicates that the surface was composed of copper oxide surface. Besides, EDS and FTIR are used to analyse the chemical compositions of specimen surfaces. Before with low surface energy modification, the specimen surface which exhibits hydrophilicity and oleophilicity. However, by the previous two-step process, it becomes superamphiphobic after HFTTMS modification, which can be explained by Wenzel's theory that the micro-nanoscale rough structures have significant potential to improve the hydrophobic properties of the sample surface.

Fig. 4 XRD patterns of (a) copper electroplating substrate and (b) as-prepared copper–oxide surface.

The EDS spectrum of the treated surface is shown in Fig. 5a. After HFTTMS modification, distinct peak of elements C, F and Si appear on the already treated samples surface, which then repel water and oil extremely. It indicates that HFTTMS molecules maybe have been self-assembled or adsorbed onto the pipeline steel substrate surface which was already processed. Fig. 5b shows the FTIR spectrum of the well-prepared surfaces after HFTTMS modification. The peak at 1374 cm⁻¹ was attributed to the vibration of the –CH₃ stretch band. The absorption bands at around 1134 cm⁻¹ and 1207 cm⁻¹ were attributed to the C–F stretching vibration of the –CF₂– and –CF₃ groups of the HFTTMS molecules. The absorption band at 1042 cm⁻¹, 1004 cm⁻¹ was identified as the framework vibration of Si–O–Si bonds. The peak at 870 cm⁻¹ was identified as the framework Si–C stretch band. The FTIR results further confirm that there is a HFTTMS film on the prepared surface with low surface energy modification. The HFTTMS film has a very low surface energy and can effectively reduce the samples surface energy after electroplating, contributing to the surface wettability change from hydrophily to superamphiphobicity. Therefore, we can draw a conclusion that the combination of the preparation of hierarchical micro/nano rough structures and low surface energy material modification are two key integrant factors for the super-amphiphobicity of the pipeline steel substrate surfaces.

Fig. 5 (a) The EDS spectrum and (b) FTIR spectrum of the as-prepared superamphiphobic surface with HFTTMS modification.

Wetting behavior of as-prepared surfaces

The surface wettability of the as-obtained superamphiphobic surfaces were studied by CA measurements of several liquids with different surface tension, showed in Table 3 and Fig. 6. By comparison, after a single alkaline etching in NH₄OH solution, the contact angle of the surface of the material reduced, it becomes more hydrophilic. After a single modification with HFTTMS ethanol solution, the contact angle of the surface increased, the surface becomes hydrophobic from hydrophilic. However, the combination of alkaline etching and modification together, a good performance superamphiphobic surface can be achieved. We studied the effect of alkaline etching by performing a series of experiments at different reaction time with 0.04 M NH₄OH solution. Fig. 7a shows the changes in the CAs as the function of reaction time in NH₄OH solution. It is clear that the specimen surfaces display superhydrophobic and moderate oleophobic to glycerol and olive oil after 15 h processing time. The water and oil repellency increased with the reaction time extending within 40 h attributed to that the micro/nano rough reentrant and hierarchical laminated structures develop gradually on the surface as time goes on. Good superamphiphobicity was obtained on the surface with alkaline etching time at 40 h. As the reaction time goes beyond 40 h, the CAs for water and oil decreased, owing to that the relatively stable hierarchical laminated micro/nano rough structures have been slowly reduced and damaged. Fig. 7b shows the influence of the soaking time in HFTTMS ethanol solution on the contact angle. The contact angle reaches a maximum value when the soaking time is 30 minutes in HFTTMS ethanol solution. With the extension of immersion time, the CAs slightly decreased, owing to the hydrolysis interaction between HFTTMS molecular films. As a consequence, the optimum process parameters are as follow: etching in 0.04 M NH₄OH solution for 40 h and modified with 1% HFTTMS ethanol solution for 30 min. Superamphiphobic surface are obtained and the CAs for water, glycerol and olive oil are 163°, 157° and 152°, respectively, corresponding SAs are all lower than 10°. As shown in Fig. 8, it is clear that the specimen surfaces had a strong, sticky attachment to the oil droplet in the absence of any treatment. The sliding angle for water and oil decreased with the reaction time extending. The value of sliding angle was minimum when alkaline etching time was at 40 h. The SAs for water, glycerol and olive oil are 2°, 5° and 9°, respectively. As the reaction time goes by, the SAs for water and oil increased, owing to that the relatively stable hierarchical laminated micro/nano rough structures have been slowly damaged.

Table 3 The variations of contact angles between specimen at different processing conditions for different liquids

Liquid	Surface tension (mN m^−1) (20 °C)	Copper plating	Chemical etching	Modification	Chemical etching + modification
Water	72.8	70°	45°	120°	163°
Glycerol	63.6	59°	34°	105°	157°
Ethylene glycol	47.7	46°	27°	93°	155°
Olive oil	32.0	29°	19°	80°	152°

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Fig. 6 (a) Optical image and (b) the variation of cross-sectional view of different liquid droplets on the as-prepared superamphiphobic surface.

Fig. 7 (a) Influence of the etching time in NH₄OH solution on the contact angle modified with 1% HFTTMS ethanol solution. (b) Influence of the soaking time in HFTTMS on the contact angle etching in NH₄OH solution for 40 h.

Fig. 8 Relationship of the processing time with the SAs in NH₄OH solution.

Stability analysis

It appears that the fabricated superamphiphobic surfaces have many promising practical applications. However, before introducing these fabricated surfaces to real environmental conditions, we have to deal with some prominent actual problems that await resolution. In order to further investigate the robustness of the superamphiphobic X80 pipeline steel surfaces under harsh conditions, it is of great significance to carry out stability tests.

Fig. 9 shows that the CAs slightly change even though the prepared superamphiphobic surfaces have been irritated with UV light for many days, indicating that both surface morphology and chemistry does not change greatly after UV irradiation.

Fig. 9 CAs of the superamphiphobic surfaces exposed to the UV irradiation for different days.

In this work, the long-term stability of the fabricated superamphiphobic surface we investigated after exposure in air for various time intervals. Fig. 10 shows the relationship between the CAs of different liquids and the exposure time. The CAs on the prepared surface still maintain a high values, only slightly decreased to 150°, 145°, and 141° from 163°, 157°, and 152°, and still has high lyophobic state for water, glycerol, and olive oil, respectively, displaying the long-term stability of the fabricated hierarchical superamphiphobic surface.

Fig. 10 Changes in CAs on the as-prepared surface for different liquids after exposure in air for 200 days.

To better estimate the performance of corrosion resistance of the prepared surfaces above, the as-prepared specimens were immersed into acidic or alkaline solution for 4 weeks to observe their CAs change for water and oil. Fig. 11 shows the variation of CAs with different pH values. Minor changes were evident in the CAs, owing to that the hierarchical laminated micro/nano rough structures were not damaged after the immersion as well as the HFTTMS film layer had a high chemical stability, which suggests that the X pipeline steel surfaces could be able to retain good amphiphobic stability in corrosive solutions treated with the above-described processing parameters.

Fig. 11 CAs of the superamphiphobic surfaces immersed into the solution with different pH values for 4 weeks.

The thermal stability about the prepared hierarchical structures was investigated by heating the substrate fabricated above at different temperatures for 1 h. The results show that there was no obvious change in the CA values for different liquids, just a slight change in CA when heated at 100 °C (the boiling point of HFTTMS is 112 °C) (Fig. 12). The temperature of oil and gas pipelines at the outlet is about 80 °C, which satisfies a need or want in a practical application.

Fig. 12 The relationship between temperature and CAs on the fabricated superamphiphobic surfaces.

The scoured region of high-speed flow test is studied by high velocity impact abrasion test experimental method to check the mechanical stability of the fabricated surface.⁴⁰ The measured CAs result of water and ethylene glycol, before and after applying different fluid velocity indicates that there is slight decrease in the CAs from 163° and 150° to 157° and 141°, respectively, with scour abrasion 10 cycles, implying that the prepared surface composed of micro/nano roughness structure possess a good mechanical stability and still remain high amphiphobicity after being liquid friction scratched repeatedly. We also checked the mechanical property of the fabricated superamphiphobic surface by applying different loads. We applied load from 1 N up to 3 N on the fabricated superamphiphobic surface. The measured CAs result of water and ethylene glycol, before and after applying different loads indicates that there is slight decrease in the CAs after applying different loads (Fig. 13), and still has superhydrophobic state for water and high oleophobic state for olive oil. Thus, we believe these long-term, thermally stable and good mechanical performance superamphiphobic hierarchical surfaces with super-repellency toward a broad range of liquids can be used in practical applications under different environmental conditions.

Fig. 13 Changes in CAs on the as-prepared surface by applying different loads.

Conclusion

Superamphiphobic surfaces on X pipeline steel substrates were fabricated by a simple, efficient, cost-effective and environment-friendly method, which can be applied to a large-area. The hierarchical laminated micro/nano rough structures were generated on the sample surfaces via combining a two-step surface texturing procedure, including electrochemical deposition and alkaline etching followed with HFTTMS molecules modification, the as-prepared surfaces display good superamphiphobicity. The average CAs for water are more than 160°, for ethylene glycol, olive oil are more than 151°, corresponding SAs are all low than 10°. A series of stability tests mentioned in this work further prove that the created superamphiphobic surfaces above possess excellent chemical stability, mechanical stability and long-term stability. Therefore, we believe that our work may be able to provide a new perspective approach to fabricate robust superamphiphobic surfaces on X pipeline steel, including other metallic surfaces for a great number of industrial applications.

Acknowledgements

This work was supported by the Twelfth Five-Year National Science and Technology Program of China under Grant no. 2011ZX05027-004-06. The authors appreciate the anonymous reviewers for their constructive comments and suggestions.

Notes and references

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