Method and device for structuring small aluminum bodies by etching

Abstract

The invention relates to a method for structuring small bodies made of aluminum or aluminum alloy by etching, comprising the steps of: a. providing a hydrochloric acid solution having a pH value less than zero as an etchant in an etching trough; b. heating the etchant to a temperature greater than 70°C and below its boiling point; c. introducing small aluminum bodies into the heated etchant in the etching trough, the mixture of small bodies and etchant having a temperature greater than 80°C for a predefined holding time of at least five seconds; d. cooling the mixture to a predefined working temperature between 35°C and 45°C within a predefined cooling time of less than one minute from the end of the holding time, wherein e. the etching trough is dipped into a temperature-controlled cooling bath and f. a circulation flow is produced in the mixture; g. maintaining the working temperature by controlling the circulation flow in the mixture for a few minutes, subsequently h. cooling the mixture to a temperature below 30°C; i. removing the etched small bodies from the etchant and j. drying the etched small bodies. The invention also relates to a device for structuring small bodies made of aluminum or aluminum alloy by etching.

Description

[0001] Method and device for etching structuring of small aluminum bodies

[0002] The invention relates to a wet-chemical etching process for small bodies made of aluminum or an aluminum alloy. The etching attack on the entire surface of the small bodies aims to dissolve aluminum from the surface, leaving behind a large number of irregularly shaped residual structures—so-called anchoring structures—in a materially bonded connection with the etched small body. The etching out of such sculpture-like anchoring structures is now known in the literature as "nanoscale sculpturing."

[0003] For the purposes of this description, a small body is any object of any shape with a volume of less than 1 cubic centimeter, usually less than 100 cubic millimeters, usually in the range 0.001 to 1 cubic millimeter, with the constraint that the smallest structural width of the object is greater than 50 micrometers, often greater than 100 micrometers, and frequently in the range 0.5 to 5 millimeters. Small bodies are therefore material bodies with a predetermined minimum material thickness along all spatial axes. For example, in the case of a cuboid, the shortest edge length is the smallest structural width, and for a wire body of any shape, the smallest structural width is the wire diameter. For a small body, e.g. in the shape of a torus of revolution, which is uniquely described by two radii r, R with r < R, the diameter of the annular bead, 2 r, is the smallest structural width.A microstructure can also have a more complex shape, such as a planar or bent wire mesh or a paper clip. The shape of the microstructure is generally retained during etching.

[0004] A microparticle is usually visible to the naked eye in isolation. A microparticle is explicitly not a nanoparticle and is usually no longer classified as a microparticle. Common alternative terms for microparticles without a precise shape specification in the literature include flakes, scales, nodules, or even granules for an aggregation of microparticles. The term "fragment" is occasionally used as a general synonym for microparticles.

[0005] In this description, the term “small aluminum bodies” is used as an abbreviation for small bodies made from (almost) pure elemental metal as well as from the technically common alloys of primarily aluminum with manganese, magnesium, copper, silicon, nickel, zinc, and beryllium. These small aluminum bodies are generally made from solid metal, but they can also be closed hollow bodies. Patent application DE 102016 102 379 B3 already shows that small aluminum bodies structured in this way are suitable, among other things, for producing mechanically robust composite materials made from plastics and metal. A composite material is a solid material formed from at least one initially flowable and subsequently hardening material phase, which, in a flowable state, comes into contact with, and is usually mixed with, the structured small aluminum bodies.Curing of the initially flowable material phase can occur, for example, through thermal solidification, polymerization, or chemical conversion, i.e., chemical oxidation or reduction of at least one component of the initially flowable material phase, possibly with solvent expulsion, or through a combination of such processes. After curing, a very robust mechanical bond is observed at the interfaces between the cured material phase and the small aluminum bodies. This bond is due to mechanical interlocking along the structured surfaces, with the anchoring structures acting like barbs against mechanical tension.

[0006] Various electrochemical processes are known for producing anchoring structures on metal surfaces, particularly on aluminum and aluminum alloys. However, to the inventors' knowledge, only DE 102016 102 379 B3 describes a process for creating such structures on small bodies that cannot be individually electrically contacted and are intended to be structured over their entire surface, if possible. As has been shown, the process described therein is only suitable for structuring small quantities (approximately 10-50 g) of small bodies for laboratory tests. Upscaling to the kilogram scale fails due to the extremely unfavorable process time and resource consumption – particularly due to the dilution of the etching agent hydrochloric acid to stop the exothermic process. The production of production-relevant quantities of structured small bodies is not yet economically viable because the product would be orders of magnitude too expensive.

[0007] The invention therefore has the object of providing a more efficient etching process for producing small aluminum bodies with anchoring structures on the surface.

[0008] The object is achieved by a method for etching structuring small bodies made of aluminum or aluminum alloy, comprising the steps of: a. providing a hydrochloric acid solution with a pH value of less than zero as an etchant in an etching trough; b. heating the etchant to a temperature greater than 70°C and below its boiling point; c. adding aluminum small bodies to the heated etchant in the etching trough, wherein the mixture of small bodies and etchant has a temperature greater than 80°C for a predetermined holding time of at least five seconds; d. cooling the mixture within a predetermined cooling time of less than one minute from the end of the holding time to a predetermined working temperature of between 35°C and 45°C, wherein e. the etching trough is immersed in a temperature-controlled cooling bath and f. a circulating flow is generated in the mixture; g. maintaining the working temperature by regulating the circulating flow in the mixture for a few minutes, thereafter h.Cooling the mixture to a temperature below 30°C; i. removing the etched microstructures from the etchant; and j. drying the etched microstructures.

[0009] A secondary claim is directed to a device for carrying out the method. The subclaims specify advantageous embodiments of the method and device.

[0010] After removing the small bodies, the etchant can be filtered and optionally refreshed and then reused.

[0011] The recirculation flow in the mixture can be generated predominantly poloidally directed.

[0012] Furthermore, the circulation flow can be configured as a pulsed flow with an adjustable pulse frequency. In a preferred embodiment, the mass ratio of aluminum microparticles to etchant in the mixture can be predetermined within the range of 1:5 to 1:3, particularly preferably around 1:4.

[0013] The aluminum miniatures can be heated to temperatures above 100°C before being added to the heated etchant.

[0014] The device according to the invention for etching structuring small bodies made of aluminum or aluminum alloy, comprising a temperature-controlled cooling bath, at least one movable holder for an etching trough, wherein the etching trough can be immersed in the cooling bath while the holder is moved, at least one controllable electric drive for generating a controllable circulating flow inside the etching trough, and a measuring device designed for the continuous electronic recording of the temperature inside the etching trough, is characterized in that a. the etching trough is formed from a deformable, acid-resistant polymer film with a film thickness of less than 1 millimeter; b. the electric drive causes a push rod to make a repetitive up and down movement at a predetermined repetition rate, wherein c. the push rod is arranged centrally below the etching trough and deforms the etching trough in the course of its movement.

[0015] Furthermore, in one embodiment, the electric drive can be designed to change the repetition rate of the movement of the push rod depending on electronically recorded measured values ​​of the temperature inside the etching trough.

[0016] The amplitude of the up and down movement of the push rod can be arranged such that the push rod temporarily pierces the surface of the mixture of small particles and etchant.

[0017] Furthermore, in one embodiment, the device can be characterized by a movable gripping device which is designed to remove an acid- and gas-permeable textile bag filled with small bodies from the etching trough and to feed it to a drying process.

[0018] The starting point of the invention is the inventors' finding from experiments with hydrochloric acid solutions with concentrations > 1 mol HCl per liter of water, i.e. pH < 0, that the desired anchoring structures on the microstructures are only formed at high speed in a working temperature range between 35°C and 45°C. Furthermore, the inventors have recognized that it is very expedient to approach this temperature range as quickly as possible from higher temperatures. This is because at temperatures above approximately 50°C a rapid, violently exothermic etching of the microstructures takes place, destroying all structures on the microstructures and dissolving them completely after a relatively short time. The structuring process in the working temperature range 35 - 45°C is also exothermic and threatens to undo the successful structuring due to rapid heating of the mixture.Therefore, efficient cooling of the mixture of etchant and small bodies in a cooling bath is essential, and an approach to a predetermined working temperature at, for example, preferably 40°C - 42°C from above is easier to achieve.

[0019] At the same time, this procedure has the advantage that the etching agent is enriched with intermediate products of the dissolution process that are not long-term stable - i.e. inoculated, as described in DE 10 2016 102 379 B3 - as soon as the microstructures to be structured are placed in the etching agent that is initially heated to at least 70°C, whereby the mixture of microstructures and etching agent initially has a temperature greater than 80°C, preferably greater than 90°C, particularly preferably greater than 95°C, for a predetermined holding time. This is because a violent etching reaction then immediately begins on all surfaces of the microstructures. The native aluminum oxide layer on the microstructures is destroyed and the nucleation of etching pores occurs approximately simultaneously on all surfaces of all microstructures. According to the invention, the holding time should not be less than five seconds. For larger microstructures it can also be longer, for example 10 to 20 seconds.Simple preliminary tests for different sizes and shapes of small bodies to select an advantageous holding time are within the scope of professional skills.

[0020] After the holding time, the initially hot mixture in the etching tank must then be cooled as quickly as possible – during a similarly predetermined cooling time – to the predetermined working temperature. To do this, the etching tank is immersed in a temperature-controlled cooling bath, and a circulating flow is initiated within the etching tank for the mixture. The cooling bath is temperature-controlled in the sense that precautions are taken to ensure that the temperature of the cooling bath does not exceed a predetermined maximum temperature, even when heat is introduced from the etching tank. The temperature-controlled cooling bath can, for example, be a commercially available water bath whose water is largely kept at a preset temperature, for example around room temperature 20 °C, by a flow-through cooler. Likewise, for example, the flow-through cooler can be set so that the maximum temperature of the cooling bath never exceeds 22 °C. Great accuracy is not required when it comes to the cooling bath temperature.Preferably, the cooling bath volume is much larger than the volume of the etching tank, allowing heat to be quickly dissipated from the etching tank. The etching tank is open at the top to allow the hydrogen gas released during etching to escape. Therefore, it cannot be completely submerged in the cooling bath. There is no mixing of the etchant with the cooling liquid of the cooling bath. In particular, the etchant is not deliberately diluted to stop the structuring process, as in DE 102016 102 379 B3.

[0021] Essential to the invention is that the predetermined cooling time until the working temperature is reached is not longer than one minute from the end of the holding time. It is preferably less than 30 seconds for large, small bodies with millimeter dimensions, and particularly preferably less than 10 seconds for small bodies with the smallest feature widths in the range of 50-100 micrometers. In general, excessively long cooling times have a detrimental effect on the desired etching result; therefore, as a rule of thumb, the cooling time should be set as short as technically reasonably practicable.

[0022] The temperature progression of the mixture over time, and thus also the etching process, is controlled by a regulated heat transfer from the etching vat into the cooling bath via the walls of the etching vat. For this purpose, a high thermal conductivity of the etching vat walls is highly desirable. However, the heat is generated in the etching vat not only near the walls, but also precisely in the interior of the mixture, from where it must be carried out as quickly as possible. The circulating flow to be established in the mixture should therefore preferably be poloidal, i.e. the movement of small particles and acid should occur as vertically upwards as possible in the center of the etching vat, so that the raised mass of the mixture must flow simultaneously in all directions - towards the walls - under the effect of gravity.

[0023] The term “poloidal flow direction” is introduced here merely for clarification purposes in analogy to the poloidal magnetic field direction of a plasma flow circulating in a torus.

[0024] It is not necessary for the circulation flow to remain constant over time. Rather, it is advantageously sufficient for the circulation of the mixture to occur in a repetitive or pulsed manner, whereby the repetition rate or pulse frequency can be controlled. Preferably, the temperature in the mixture is continuously measured, and the repetition rate of the circulation flow is controlled depending on the temperature measurement. In particular, this makes it possible to keep the mixture at the predetermined operating temperature during the exothermic etching structuring.

[0025] The etching patterning according to the invention takes only a few minutes, typically 2-5 minutes, to obtain a usable end product. The etching is finally terminated by cooling the mixture to temperatures below 30°C, which can be easily achieved by increasing the repetition rate of the circulation flow in the colder cooling bath. At such a final temperature, etching still takes place, but it proceeds so slowly that the small particles can be removed from the etchant without haste and subjected to a drying procedure. The drying procedure can usually involve centrifuging the small particles, interrupted by one or more rinses with pure water, which is familiar to those skilled in the art.

[0026] After removing the small bodies, the etchant can preferably be filtered and optionally refreshed and then reused. Filtration is intended to remove the aluminum residues detached from the small bodies, which would otherwise continue to dissolve in the acid. Refreshing can optionally be performed after a pH measurement, for example, by adding fresh concentrated acid. The reuse of the etchant, which must be reheated, for the next batch of small bodies is one of the key advantages of the invention.

[0027] The method described here is advantageously suitable for setting a predetermined mass ratio of aluminum microparticles to etchant in the mixture from 1:5 to 1:3, preferably to approximately 1:4. For example, using 2 liters of concentrated hydrochloric acid (e.g., 1.4 mol / l HCl, pH = -0.15) with a mass of approximately 2 kg, approximately 500 g of aluminum microparticles can be structured in less than 10 minutes. It should be noted that the addition of such a large quantity of microparticles to the etchant heated to at least 70°C can result in the temperature of the resulting mixture falling below 80°C or not exceeding 80°C. To avoid this, the microparticles themselves can preferably be heated to temperatures above 100°C - e.g., in an oven - before being added, so that when added to the etchant, they raise the mixture temperature above that of the heated etchant.

[0028] It should be noted here that the user is fundamentally free to decide exactly which initial temperatures they wish to heat the etchant and the microstructures to before starting structuring, as long as they maintain a mixture temperature greater than 80°C, preferably greater than 90°C, and particularly preferably greater than 95°C for the duration of the holding time after mixing. However, excessively high temperatures of the microstructures relative to the etchant are not recommended to avoid local evaporation and delayed boiling caused by sinking very hot microstructures. Preferably, the microstructures should not be heated above 120°C initially, and the hydrochloric acid solution should not be heated below 80°C.

[0029] With the process according to the invention, production quantities of several kilograms per hour are now possible without excessive material consumption, and commercial production is therefore within reach.

[0030] An advantageous embodiment of the method can be seen in the fact that the small bodies are structured inside a textile bag in the etching agent. The size of the textile bag can preferably correspond to the size of the etching trough, i.e. the textile bag can rest against the inner walls of the etching trough at any point. This means that there are no restrictions on the movement of the small bodies when the mixture is circulated. The textile bag can be placed in the etching trough once the etching trough has been completely emptied. As a woven fabric made of acid-resistant textile fibers, the textile bag is permeable to the hydrochloric acid solution and even more so to the hydrogen gas formed. The etching trough can be filled with hydrochloric acid solution and heated throughout before small bodies - possibly heated - are poured into the textile bag, the etching trough and the heated etchant.

[0031] The advantage of the textile bag is the simplified separation of the etched small objects from the etchant after cooling to below 30°C. Separation can be achieved, for example, with a movable gripper device that simply grasps the textile bag and lifts it upwards out of the etching tank. The gripper device can also perform additional work steps, such as transporting the etched small objects from the etching device to a drying device. The gripper device can move and operate independently and simultaneously with the movable holder of the etching tank, thus particularly while the etchant is being filtered, optionally refreshed, and reheated for the next batch.

[0032] However, the simplified handling of the small bodies in textile bags that can be removed from the etching tank has the serious disadvantage that conventional stirring tools for generating a circulating flow in the etching tank, in particular a poloidally directed flow, are not well suited, because such tools would foreseeably either tear the textile bag or come to a standstill due to the textile fabric winding up around the stirring heads.

[0033] As already mentioned, the etching trough should be constructed with walls that conduct heat as well as possible, which suggests the use of a metal rather than glass. However, an alternative according to the invention can also consist of constructing the etching trough from a deformable, acid-resistant polymer film with a film thickness of less than 1 millimeter. Preferably, the film thickness is even less than 0.5 millimeters, particularly preferably around 0.1 millimeters. All known acid-resistant polymers can be used as the polymer material, for example silicones or polyethylene. It is even entirely possible to use conventional boil-and-freeze bags as an etching trough with an open top and suspended in a movable holder. Due to these very thin walls of the etching trough, the heat can be transferred very effectively to the cooling bath.

[0034] A further advantage of a deformable etching trough is the possibility of generating the required circulating flow in the etching trough by mechanically deforming the etching trough from the outside. According to the invention, a push rod arranged centrally below the etching trough can bring about a deformation of the etching trough through a repetitive up and down movement, which leads to the vertical lifting of the mixture of small particles and acid in the center of the etching trough and causes a temporally pulsed, poloidally directed circulating flow in the etching trough. The repetitive up and down movement of the push rod is to be initiated by an electric drive, for example by a rotating eccentric, and its repetition rate (e.g. rotational speed of the eccentric) is to be controllable by current supply. The temperature of the mixture inside the etching trough is to be continuously electronically recorded by a measuring device to monitor the etching process according to the invention.A thermal sensor can be used as a measuring device, for example, on one of the non-deformed walls of the etching tank. Since the etching tank is open at the top, the temperature of the mixture can also be detected contactlessly from above using an optical pyrometer.

[0035] In summary, for carrying out the etching process described above, a device for etching structuring small bodies made of aluminum or aluminum alloy is proposed, said device comprising a temperature-controlled cooling bath, at least one movable holder for an etching trough, the etching trough being immersible in the cooling bath while the holder is moved, at least one controllable electric drive for generating a controllable circulating flow inside the etching trough and a measuring device designed for the continuous electronic recording of the temperature inside the etching trough, characterized in that a. the etching trough is formed from a deformable, acid-resistant polymer film with a film thickness of less than 1 millimeter; b. the electric drive causes a push rod to make a repetitive up and down movement at a predetermined repetition rate, wherein c.the push rod is arranged centrally under the etching trough and deforms the etching trough during its movement.

[0036] It is considered particularly advantageous that the amplitude of the up-and-down movement of the push rod is set such that the push rod temporarily pierces the surface of the mixture of microparticles and etchant. In doing so, the underside of the foil-like etching trough, lying on the push rod, is also moved through the mixture surface. In other words, it is advantageous if the central region of the etching trough is repeatedly and completely emptied, with the mixture being poured towards the walls of the etching trough. Thus, for a short time, the etching trough is deformed to such an extent that the mixture is present only in an annular volume. When the push rod is retracted, the mixture then flows back to the center of the etching trough.

[0037] Such a device is not known to the inventors from the prior art. A person skilled in the art might initially object that a thin polymer film bag containing a nearly boiling and concentrated hydrochloric acid solution appears to be a technically unsafe container, which should not be repeatedly deformed for fear of the film tearing. However, the deformation occurs almost exclusively while the etching trough is immersed in the cooling bath (here containing water). The weight of the mixture of small particles and acid is almost compensated in the bath and does not place any strain on the film bag. If the bag were to tear during deformation, the acid would immediately be greatly diluted and cooled in the cooling bath. And indeed, in the inventors' numerous experiments with the etching trough made of polymer film, not a single film bag has to date torn.

[0038] As already mentioned, it is very advantageous for the automation of the etching device if the electric drive is designed to change the repetition rate of the push rod movement depending on electronically recorded measured values ​​of the temperature inside the etching tank. The device can then be controlled by a microprocessor with implemented software, e.g., a personal computer, such that it realizes, among other things, pre-tabulated temperature profiles in the etching tank. At the same time, the microprocessor control can also carry out the movement of the etching tank holder and / or the movement and activity of the aforementioned gripping tool for textile bags containing small bodies and / or the control of means for heating the etchant and small bodies and / or the control of the cooling bath.

[0039] Finally, it should be noted that the advantageous textile bags with small bodies have proven to be unproblematic and practical in conjunction with the etching trough deformable by a push rod. It is therefore advantageous to provide a movable gripping device that is at least designed to remove the textile bag from the etching trough and feed it into a drying process.

Claims

A N S P R Ü C H E 1. A method for etching structuring small bodies made of aluminum or aluminum alloy, comprising the steps of: a. providing a hydrochloric acid solution with a pH value of less than zero as an etchant in an etching trough; b. heating the etchant to a temperature greater than 70°C and below its boiling point; c. adding aluminum small bodies to the heated etchant in the etching trough, wherein the mixture of small bodies and etchant has a temperature greater than 80°C for a predetermined holding time of at least five seconds; d. cooling the mixture within a predetermined cooling time of less than one minute from the end of the holding time to a predetermined working temperature of between 35°C and 45°C, wherein e. the etching trough is immersed in a temperature-controlled cooling bath and f. a circulating flow is generated in the mixture; g. maintaining the working temperature by regulating the circulating flow in the mixture for a few minutes, thereafter h.Cooling the mixture to a temperature below 30°C; i. removing the etched microstructures from the etchant; and j. drying the etched microstructures.

2. Method according to claim 1, characterized in that the etchant is filtered after the removal of the small bodies and optionally refreshed and then reused.

3. Method according to one of the preceding claims, characterized in that the circulating flow in the mixture is generated in a predominantly poloidal direction.

4. Method according to claim 3, characterized in that the circulating flow is set up as a pulsed flow with an adjustable pulse frequency.

5. Method according to one of the preceding claims, characterized in that in the mixture the mass ratio of small aluminum bodies to etchant is predetermined from the interval 1:5 to 1:3 or around 1:

4.

6. Method according to one of the preceding claims, characterized in that the small aluminum bodies are heated to temperatures above 100°C before being added to the heated etchant.

7. A device for etching structuring small bodies made of aluminum or aluminum alloy, comprising a temperature-controlled cooling bath, at least one movable holder for an etching trough, the etching trough being immersible into the cooling bath while the holder is moved, at least one controllable electric drive for generating a controllable circulating flow inside the etching trough, and a measuring device designed for the continuous electronic recording of the temperature inside the etching trough, characterized in that a. the etching trough is formed from a deformable, acid-resistant polymer film with a film thickness of less than 1 millimeter; b. the electric drive causes a push rod to make a repetitive up and down movement at a predetermined repetition rate, c. the push rod is arranged centrally below the etching trough and deforms the etching trough in the course of its movement.

8. Device according to claim 7, characterized in that the electric drive is designed to change the repetition rate of the movement of the push rod as a function of electronically recorded measured values ​​of the temperature inside the etching trough.

9. Device according to one of claims 7 or 8, characterized in that the amplitude of the up and down movement of the push rod is arranged such that the push rod temporarily pierces the surface of the mixture of small bodies and etchant.

10. Device according to one of claims 7 to 9, characterized by a movable gripping device which is designed to remove an acid- and gas-permeable textile bag filled with small bodies from the etching trough and to feed it to a drying process.