Patent Grant
10907258
The present invention is related to etching of metal surfaces to modify their surface topology at micro- and nano-length scales, thereby altering their wetting properties.
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Copper has numerous practical and industrial applications such as thermal and fluid transport, and altering the wetting characteristics of copper may positively impact numerous practical applications. Finely polished copper typically has a contact angle (CA) between 70° and 80° for water droplets, and the value changes for different liquids. Improving the wetting characteristics of copper, with its excellent heat conducting properties, with durable, non-toxic hydrophilic (or even better, omniphilic i.e., wettability to almost all liquids) or hydrophobic surface treatments will positively and significantly impacts numerous applications in thermal management, energy, water, automotive, nuclear, electrical, electronics, air-conditioning, machining and electronics packaging industries among many others. Omniphilic copper with extreme wetting characteristics for most liquids has a variety of applications involving enhanced phase change heat transfer, for example in boiling and wicking surfaces in heat pipes, vapor chambers, heat exchangers including micro-channels and thermal spreaders. Alternatively, hydrophobic copper surfaces, could be very useful as anticorrosion, anti-bio fouling, or drag reduction and anti-icing surfaces.
It is known that both -philicity and -phobicity of a surface are functions of its roughness and the surface tension of the liquid. Increasing the roughness and the exposed surface area makes the hydrophilic behavior of a naturally wetting material more pronounced by increasing the contact area, i.e., the useful heat transfer area between liquids and copper. A typical example of this approach is found in industrial heat exchangers. Roughness is typically increased at micro- and/or nanoscales by surface patterning using clean room techniques, by depositing naturally hydrophilic coatings and/or particles, by etching techniques, or by employing innovative assembly approaches. Microfabrication, which can provide an extremely precise control over the roughness features, is relatively expensive and not conducive to implementation on large surface areas. Hydrophilic surfaces of metals have been generated by etching to generate nanostructures and sintering. Hydrophilic coatings have also been pursued for rendering wetting characteristics to copper surfaces. However, the contact angle (CA) of water droplets obtained using most of the current hydrophilic treatments on pure copper surfaces was only as low as 25-30°. With some techniques, such as using surface oxide formation, surface protrusion formation, hydrophilic coatings or sintering of particles on the surface, the CA was found to be less than 10°, but the durability, chemical compatibility, operating temperature range, poor heat transfer properties, poor abrasion characteristics, and poor adhesion of coatings, which also form a barrier to efficient heat transfer from the substrate to the bulk liquid in heat transfer applications, are detrimental to practical use.
The present invention is a hydrophilic surface of a metal or alloy, the surface comprising a plurality of grooves, a plurality of micro-cavities, and a plurality of nano-cavities. The surface preferably does not comprise protrusions, a coating, or an oxide. The surface is preferably superhydrophilic, polyphilic, omniphilic or ultra-omniphilic and preferably has the same composition as the bulk metal or alloy. The grooves preferably each comprise a width of between 1 micron and 1000 microns. The micro-cavities preferably comprise a diameter of between 1 micron and 500 microns. The nano-cavities preferably comprise a diameter of less than 1 micron. The surface preferably comprises a contact angle of zero.
The present invention is also a a hydrophobic surface of a metal or alloy, the surface comprising a plurality of grooves, a plurality of micro-cavities, a plurality of nano-cavities, and an adsorbed ester layer. The ester preferably comprises a stearate.
The present invention is also a method for roughening a surface of a metal or alloy, the method comprising polishing the surface, thermo-catalytically etching the surface, and temperature gradient etching the surface. The method preferably increases a property of the surface, the property selected from the group consisting of hydrophilicity, superhydrophilicity, polyphilicity, omniphilicity, and ultra-omniphilicity. The method preferably does not comprise depositing a coating on the surface or oxidizing the surface. The method preferably produces grooves, micro-cavities, and nano-cavities in the surface. Either etching step is preferably performed using an etching mixture comprising a catalyst, a diluent, and an etching reagent. The concentration of etching reagent is preferably sufficient to etch the surface but not enough to cause surface passivation. Both etching steps are preferably performed using the same etching mixture. The method is preferably not performed in a clean room. The polishing step preferably comprises mechanically polishing the surface using silicon carbide abrasive papers. The step of temperature gradient etching the surface preferably comprises exposing the surface to an etching mixture while continuously decreasing a temperature of the etching mixture. The method preferably further comprises immersing the surface in a solution of stearic acid and ethanol, the surface thereby adsorbing a layer of an ester, thereby making the surface hydrophobic or superhydrophobic.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating certain embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
The present invention is a facile, low-cost, scalable approach to fabricating durable, non-toxic ultra-omniphilic and hydrophobic copper surfaces. The approach, based on controlled etching on artificially created metallurgical surface defects, can be realized on very large surfaces to generate robust ultra-omniphilic copper surfaces with liquid spreading behavior akin to a paper towel with CA of zero. The present invention utilizes tuning of the etching process to produce a surface with desired, multi-scale cavities (instead of protrusions) that increase roughness and improve wetting. The surface has substantially the same composition as the substrate material, unlike other technologies that rely on coatings or formation of different compounds on the surface.
In embodiments of the present invention, wetting characteristics of copper surfaces were significantly altered to be either ultra-omniphilic or super-hydrophobic using a facile, scalable surface treatment approach. This safe and cost-effective fabrication technique involving a simple three-step procedure consisting of mechanical polishing and controlled metallurgical etching resulted in generation of robust copper surfaces with either a contact angle of zero (with liquid spreading akin to a paper towel) without employing any coatings, sintering, electrochemical deposition or cleanroom fabrication techniques. Surface characterization showed that hierarchical micro-/nanoporous structure with embedded nano-cavities in the micro-cavities or embryos thereof was primarily responsible for the observed extreme homogeneous wetting characteristics. As with many other materials, increasing the roughness of copper was found to improve its wetting behavior to liquids. While wetting characteristics depend on surface roughness and surface tension of liquids in general, the modified copper surfaces were observed to exhibit remarkable wetting for numerous liquids similar to a paper towel suggesting that the wetting phenomenon is independent of surface tension of the liquid on these surfaces (as in the Wenzel model) and is only a function of surface roughness. It was also found that rough copper surfaces with an adsorbed hydrophobic monolayer exhibited robust super-hydrophobic characteristics (CA up to) 152°. The present invention has great potential to radically improve heat dissipation performance in devices such as microchannels and heat pipes, which often rely on efficient fluid flow and phase change on copper surfaces.
Embodiments of the omniphilic surface preparation approach of the present invention preferably comprise a three-step procedure (with an optional additional step for hydrophobic surface preparation) involving surface polishing followed by temperature dependent controlled metallurgical etching. In one embodiment, the present invention is a three-step process to produce a certain surface topology in metals and alloys, as shown for example in
Step 2, preferably comprising thermo-catalytic etching, generates nano-cavities 20, typically less than one micron in size, which provide an additional capillary wicking effect and improve the liquid holding capability of the surface. Step 3, preferably comprising temperature gradation etching, generates micro-cavities 30, preferably by expanding the nano-cavities obtained in Step 2; these micro-cavities provide the primary capillary wicking effect. The specific surface topology shown in the SEM images herein results in a very rapid spreading for many liquids, implying the spreading ability on these surfaces is independent of the liquid type and is only a function of the surface roughness features (hence, the paper towel effect). Visual examination of the roughened surfaces revealed that hierarchical micro- and nano-cavities, including nano-cavities 40 within or inside micro-cavities 30, was primarily responsible for the observed ultra-omniphilic behavior akin to a paper towel (CA of zero for multiple liquids). With an adsorbed coating of ester, the same ultra-omniphilic copper surfaces were found to exhibit robust super-hydrophobicity (CA 152° for water). Previously, it was not possible to produce a ultra-omniphilic or hydrophobic surface with hierarchical micro-/nano-scale roughness by directly using any known etching reagents. Typically the microcavities range in size from about 1 micron to 100 microns, but they can be as large as 500 microns.
The physics-backed tuning of the approach of embodiments of the present invention results in a surface with specific desirable roughness features for promoting wetting. Micro- and nano-cavities hold liquids onto the surface through very strong capillary forces, while micro-grooves enable rapid spreading of the liquids on the surface through capillary forces. The extreme wetting ability is applicable to multiple liquids (i.e. ultra-omniphilicity), preferably due to interconnected sub-surface micro- and nano-roughness architecture, including nano-cavities within the micro-cavities, connected by a network of micro-grooves. Although in some embodiments the sequence of polishing and etching steps may be different, in the above embodiment the sequence of steps is important to creating the desired surface structure. For example, if Step 1 is carried out after Steps 2 and 3, the free metal particles created by polishing would block at least some or most of the cavities, reducing omniphilicity. If Step 3 is implemented before Step 2, or if Step 2 is skipped, few if any micro-cavities would form, since the nano-cavities, which act as a nucleus to form micro-cavities, have to be formed first. If Step 3 is skipped, it will be very difficult to form micro-cavities using Step 2 alone, since very strong etching solutions are typically used to directly form micro-cavities, but such strong etching solutions frequently cause surface passivation, corrosion, and/or oxidation, which makes the surface non-reactive to further etching and/or decreases the sample surface purity.
A similar three-step procedure can be used for altering the wetting properties of metals and alloys in general other than copper. Depending on the type of the metal, only the composition of the etching reagent in Step 2 is preferably changed. For example, for making ultra-omniphilic aluminum, a mixture of methanol:water:nitricacid (as catalyst:diluent:etchingreagent) is preferably used as the etching solution instead of ethanol:water:hydrogen peroxide used for copper. The concentrations of the chemicals in Step 2 can be varied; i.e., the ratio of the components in the etching solution can be 3:3:1, 2:3:1, 2:2:1, 1:1:4 etc. depending on the condition of the original sample. The ratio is preferably any combination in the range (1-5):(1-5):(1-5). The concentration of the etching reagent itself is preferably such that the etching solution is sufficiently powerful (or potent) to etch the metal or the alloy surface but not so powerful as to cause surface passivation (which makes the surface non-reactive). Table 1 shows some example chemical combinations for various metals and alloys.
Advantages of the present invention include low cost, rapid processing, scalability (can be produced on large or small surfaces in a same time frame, which is not possible with surfaces prepared in a clean room), highly robust surfaces that are resistant to mechanical and fluidic pressures (which is not possible with coated surfaces and surfaces prepared in a clean room), no barrier to heat transfer in thermal applications (unlike coatings), no contamination to liquids flowing over the metal surface (unlike coatings), can be produced on internal and/or curved surfaces without opening the device (e.g. inside pipes), and no release of harmful chemical gases during implementation of the approach in many cases as well as during the application.
Step 1: Polishing
Copper samples were first mechanically polished to remove surface impurities, including the oxide layer, and create artificial surface defects and micro-grooves. Mechanical polishing can provide a high degree of control over the length scale of the roughness features, for example, when silicon carbide (SiC) abrasive papers of known grits are employed. SiC has a hexagonal-rhombohedral crystal structure that was found to be excellent at imparting the desired three-dimensional features with a high degree of repeatability and consistency. A force of ˜25 N per sample was employed in this study, and grits 60, 100, 150, 220, 320, 400, 600, and 1200 were used, for which the median particle diameters varied from 250 μm (60 grit) to 2.5 μm (1200 grit). The purity of copper used in this study was 99.99% (UNS#C10100, i.e. Alloy 101 Oxygen-free Copper). De-ionized water was continuously sprayed while polishing the samples to wash off the free copper particles that would otherwise fill the generated micro-grooves. The grooves typically have a diameter of that of the grit, from approximately 1 micron to approximately 500 microns. The mechanically polished copper pieces were thoroughly washed using 99.5% pure solutions of ethanol, acetone and isopropyl alcohol in a sequence followed by rinsing in running de-ionized water. The samples were dried using a clean paper towel subsequent to washing with each of the chemicals to remove the remaining copper particles, if any, on the surface.
Step 2: Thermo-Catalytic Etching
A suitable etching mixture was selected in this step by considering its ability to etch copper using the standard half-cell reduction potentials (E), and the chemical equilibrium constant (Keq).
H2O2+2 H++2 e−→2 H2O E=1.770 V
Cu2++2e−→Cu E=0.337 V
Cu+H2O2+2H+=Cu2++2H2O Eo=(1.770-0.337)V=1.433 V
The equilibrium constant, Keq for this redox reaction (obtained using the Nernst equation) can be calculated as Keq=10(n·Eo/0.059). With n=2 (transfer of two electrons) and Eo=1.433 V, Keq=1048.58 indicating the strong ability of the hydrogen peroxide solution to etch copper. (For Keq>103, the chemical reaction strongly favors the formation of products).
A solution of 3:3:2 by volume of ethanol (99.5%), de-ionized water and hydrogen peroxide (30% wt. in water) was used to etch the copper samples prepared in Step 1. The samples in the solution were heated in an oven for 90 minutes at 100° C. Since copper is usually non-reactive in dry air at room temperature, a high temperature environment was employed for promoting and catalyzing the etching reaction.
Step 3: Temperature Gradation Etching
The samples taken out of the oven were retained in the same etching solution for 12 hours to cause etching under a continuously decreasing temperature environment. All the samples were thoroughly washed with de-ionized water and dried in an oven for 15 minutes at temperatures above the boiling point of water at 1 atm. (a temperature of −110° C. was mostly used). The drying time was chosen so as to be sufficient to evaporate all the water, but not so long as to result in surface oxidation.
Results
Of the many different methods to calculate contact angle, the circle fitting method is one of the most widely used methods due to its simplicity and high accuracy. The method uses the complete drop shape for measurement of the contact angle. It assumes the shape of the droplet formed on a solid surface as a part of a sphere (or circle in a two-dimensional viewing plane). The method is prescribed for droplets with volume between 1 μL and 5 μL; accordingly, the effect of body forces such as gravity can be neglected in comparison to the surface tension of the droplet. In the present experiments a high resolution image of the droplet was captured using a 16 megapixel camera with the horizontal planes of the lens and the copper surfaces aligned in a straight line using a laser. The drop shape profile and the base line were realized using edge detection and image segmentation. A circle was curve-fitted to the drop shape profile which enabled finding the equation of the circle. The contact angle was then calculated based on the fitted circle equation and the detected base line.
Table 2 shows the effect of sand paper roughness on the measured CA for 5 μL water droplets on the treated copper samples after step 2. It was found that the CA decreases as the grit (i.e., the smoothness of the sand paper) increases until a grit value of 400, after which an opposite trend is exhibited. In general, the contact angle values were found to arbitrarily depend on the sand paper roughness; however, all of the surfaces were found to have low CAs (less than 20° in most of the cases). In addition to the relatively safe nature of the approach, the employed mechanical polishing approach in Step 1 was found to provide a reasonably high degree of control and repeatability. Further, it was found to provide tremendous scope for promoting preferential etching along the grain boundaries by increasing the size and number of crystal imperfections.
Remarkably, as shown in
The wettability of the surfaces was quantified based on the liquid retention capability. For these tests, water was employed as the liquid. A 5 μL droplet (weighing ˜0.005 g) was placed on the surfaces which were then subjected to repeated tilting (
The ability of the ultra-omniphilic surfaces to strongly hold the wetting liquid was also tested under bulk liquid flow conditions. For these experiments, a channel of size 2.2 mm wide, 10 mm high and 50 mm long was used. Ultra-omniphilic copper walls of the channel were wetted with water mixed with Safranin O (basic red 2) at a concentration of 0.1 mg/mL. Mineral oil was then pumped as bulk liquid using a syringe pump at a flow rate of 140 mL/min for more than ten times. Leica M165 fluorescent microscope was used to observe the robust wetting characteristics of the ultra-omniphilic surfaces.
The surface features of the etched copper samples after Step 2 were observed under a scanning electron microscope (SEM) and the images are shown in
The size of the micro-cavities formed on the surface was found to depend on the orientation of the copper samples in Steps 2 and 3. Horizontal orientation of the surfaces to be etched was observed to provide slightly larger cavities (i.e., with more material removal) when compared to other orientations. This can be attributed to the buoyancy-dependent bubble departure mechanism during etching with the peroxide solution that favors horizontal orientation.
From the surface analysis of the ultra-omniphilic surfaces, the Wenzel model can be used to analyze the extreme spreading behavior of liquid droplets. The Wenzel model describes the homogeneous wetting regime using the equation cos θ*=r·cos θ, where θ* is the apparent contact angle on a roughened surface corresponding to the minimum free energy state for the system, r is the roughness ratio (which is the ratio of total area of a rough surface to the apparent or projected area), and θ is the contact angle made by a liquid droplet as measured on the smooth solid surface. If the present etching approach is assumed to be isotropic, the value of r for any hemispherical embryo will be 2. But with a θ* of zero for water on ultra-omniphilic surfaces and a θ of 70-80° for water on smooth copper, it can be obtained from the Wenzel equation that r for the ultra-omniphilic surfaces is at least 2.92 and possibly larger than 5.76. These r values show a substantial increase in the surface area at micro/nanoscale, which could be primarily attributed to the presence of numerous nano-cavities within the micro-pores and the massively parallel connectivity of the cavities through micro/nano-grooves obtained by the mechanical polishing of Step 1. Such grooves can be seen in
The surface analysis can also be used to discuss the droplet spreading dynamics. The balance of viscous force and surface tension force on a droplet can be used to analytically determine the spreading radius at any instant, Rsp, on a smooth surface. From the analytical solution, Rsp∝(1/Ca)1/12, where Ca is the capillary number, which is the ratio of viscous force to the surface tension. For Ca<<1, interfacial forces dominate viscous force (favors spreading) while for Ca>>1, viscous force dominates interfacial forces. For ultra-omniphilic surfaces, with an r value larger than 2.92, capillary forces dominate the viscous forces more than on a smooth surface. This decreased Ca explains the reason for the spreading of droplets to a larger radius on an ultra-omniphilic copper surface compared to a smooth copper surface.
According to the measured SEM spectral elemental analysis of the surface, the surface was found prone to oxidation in open environments as expected. As shown in
The samples were tested for omniphilicity after surface oxidation; i.e., after exposing them to ambient for 192 hours. It was found that an oxide layer forms inside the cavities, thus blocking them and reducing the omniphilic property of the surface.
Newly prepared samples were also placed in a liquid bath for 16 weeks. After removing the samples from the bath and drying them in an oven, the surfaces were found to exhibit their ultra-omniphilic characteristics without any performance degradation, showing the robustness and suitability of these surfaces for use in closed environments (such as in channel and pipe flows). In applications requiring surface exposure, thin anti-oxidative coatings could be selectively deposited on the surface without blocking the micro/nano-cavities.
Hydrophobic Copper Surfaces
For preparing super-hydrophobic copper surfaces, an additional processing step was employed, in which the same samples obtained after processing Step 3 were immersed in a solution of 0.5% wt. stearic acid and ethanol, and vigorously shaken in an ultrasonic machine for 40 minutes. This ensured a homogeneous distribution of the non-polar solute on the surface, and hence a thin uniform coating of the ester on the samples. The samples were then dried in an oven at 50° C. for 60 minutes.
After carrying out Step 4 the surfaces were found to be hydrophobic, with a measured CA between 127° and 152° depending on the roughness of the omniphilic surface.
Although the invention has been described in detail with particular reference to the disclosed embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover all such modifications and equivalents. The entire disclosures of all patents and publications cited above are hereby incorporated by reference.
This application claims priority to and the benefit of the filing of U.S. Provisional Patent Ser. No. 62/379,702, filed Aug. 25, 2016, entitled “Surface Modification of Metals and Alloys to Alter Wetting Properties”, and the specification and claims thereof are incorporated herein by reference.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. 1449621 awarded by the National Science Foundation.
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| Number | Date | Country | |
|---|---|---|---|
| 62379702 | Aug 2016 | US |
