Method for etching the surface of aluminum fragments, aluminum fragments with an etched surface and material composites containing such fragments

Abstract

A method for etching fragments of aluminum or an aluminum alloy comprising the steps of: a. providing a hydrochloric acid solution in a trough-shaped container; b. inoculating the hydrochloric acid solution by chemically dissolving an amount of aluminum to produce an etching solution; c. adding the fragments to the etching solution immediately after the inoculation; d. etching the fragments for 0.5 to 10 minutes while stirring in such a way that the fragments are entrained by the motion of the etching solution; e. stopping the etching by diluting the etching solution with water; f. removing the etched fragments; g. repeatedly rinsing the fragments with water and h. rinsing the fragments with an organic desiccant. An etched fragment of aluminum or an aluminum alloy and also to a composite material comprising etched fragments.

Claims

1. An aluminum or aluminum alloy fragment etching method comprising the steps of: a. providing a hydrochloric acid solution having a concentration of 1.1 to 4.2 moles HCl per liter of water in a trough- or vat-shaped container; b. inoculating the hydrochloric acid solution by chemically dissolving a quantity of aluminum from the interval 0.5 g to 17.5 g per liter to produce an etching solution; c. adding aluminum or aluminum alloy fragments to the etching solution immediately after the inoculation; d. etching the aluminum or aluminum alloy fragments for 0.5 to 10 minutes while circulating the etching solution with the fragments such that the fragments are entrained by the movement of the etching solution to produce etched aluminum or aluminum alloy fragments; e. stopping the etching within a few seconds by diluting the etching solution with water; f. removing the etched aluminum or aluminum alloy fragments from the etching solution; g. repeatedly rinsing of etched aluminum or aluminum alloy fragments with water immediately after the removal and h. rinsing the etched aluminum or aluminum alloy fragments with an organic desiccant.

2. The etching method according to claim 1, wherein the hydrochloric acid solution has a concentration of 1.7 to 2.5 mol of HCl per liter of water.

3. The etching method according to claim 1, wherein the hydrochloric acid solution has a concentration of 2.1 mol of HCl per liter of water.

4. The etching method according to claim 1, further comprising seeding the hydrochloric acid solution by chemically dissolving 2 to 2.5 g of aluminum per liter.

5. The etching method according to claim 1, wherein the aluminum or aluminum alloy fragments are etched while circulating the etching solution with the aluminum or aluminum alloy fragments for 4 to 5 minutes.

6. An etched fragment made of aluminum or an aluminum alloy produced by the method according to claim 1, wherein the surface of the etched aluminum or aluminum alloy fragment has anchoring structures, wherein the shape of the fragment corresponds to its shape before the etching, wherein the fragment is a wire having a wire diameter of at least 50 micrometers and a length of at least 0.5 millimeter.

7. The etched aluminum or aluminum alloy fragment according to claim 6, wherein the etched aluminum or aluminum alloy fragment is a tetrapod.

Description

(1) In the drawings:

(2) FIG. 1 an etched surface on nearly pure aluminum (Al: about 99.5%, prior art);

(3) FIG. 2 an etched surface on AlMg3 alloy (prior art);

(4) FIG. 3 an etched surface on the alloy AlMgSi0.5 (prior art);

(5) FIG. 4 a schematic sketch of the anchoring structures, along the z-axis perpendicular to the aluminum surface which form by the etching attack (see FIGS. 1 to 3) (prior art);

(6) FIG. 5 a) a braid of modified aluminum wire—here: alloy AlMg5—with a wire diameter of just over 100 microns and

(7) FIG. 5b) an irregularly shaped aluminum fragment—here: Al 99.7%—with etched surface and a smallest structure width of a few 100 microns.

(8) FIG. 6 a crack surface by a composite containing aluminum fragment embedded in a polymer (here: polythiourethane, PTU) after a tensile test (prior art);

(9) FIG. 7 a crack surface by a composite as in FIG. 6, now with etched aluminum fragments, which break with the tensile test;

(10) FIG. 8 the same crack area as in FIG. 7 elsewhere and enlarged, where the enclosing of the anchoring structures by the PTU is visible;

(11) FIG. 9 the tensile test data for the composites of FIGS. 6 to 8 compared to a pure PTU sample;

(12) FIG. 10 pure polydimethylsiloxane (PDMS) tensile strength measurements and PDMS composites containing untreated and etched aluminum fragments;

(13) FIG. 11 a) to c) a prefabricated thermoplastic polyoxymethylene (POM) sheet which has been partially melted by heating the surface in the center and sprinkled with etched particles and tested accordingly.

(14) FIGS. 1 to 4 were initially discussed in the description of the prior art to illustrate the shape and manufacturability of anchoring structures on various aluminum alloys.

(15) From FIG. 5 is now well visible that even the etching attack according to the invention with hydrochloric acid leads to such anchoring structures on the aluminum fragments. In this case, the etching surrounds each individual fragment with a sheath with extreme surface roughness, which comprises densely located pores provided with undercuts and angled portions. The thickness of the sheath corresponds to the depth of the etched pores or the length of the angled residual structures.

(16) The sheath encloses the fragment, but it does not have to completely cover it. In fact, the acid attack will take place on all exposed surfaces of the fragment, but in particular it is also possible to provide a long wire with anchoring structures and to divide this after the etching into a plurality of wire filaments. The individual filaments then have no anchoring structures on their cut surfaces, which, however, can only be detrimental if the non-etched surfaces exceed in magnitude the etched surfaces of the filaments. This will usually be avoided and can be achieved by leaving the filaments long enough.

(17) FIG. 5 a) shows a wire mesh of AlMg5 wires, which have been added to the etching bath in the already braided form. Before etching, the wires had a wire diameter—in this case: structure width—of more than 100 micrometers and, after the etching, had anchoring structures covering every originally free surface of the mesh.

(18) FIG. 5 b) shows a corresponding etching result for a coarse-spherical fragment whose smallest structure width can be identified here with its smallest diameter of a few 100 micrometers. The inlays in FIGS. 5 a) and b) each show detail enlargements.

(19) It will be readily understood that blending anchored-structure aluminum bodies with a first flowable and subsequently hardening material after curing will result in a composite which in any event can not be destroyed or decomposed by a failure of the aluminum to adhere to the material. Rather, it is necessary to accomplish either a cohesive failure of the material, such as a polymer, or the breaking of the fragment or both.

(20) For experimental verification, the inventors have, inter alia, incorporated etched fragments of aluminum granules with several 100 microns grain diameter in flowable polymer precursor—polythiourethane (PTU) and polydimethylsiloxane (PDMS)—and poured several test strips of the same size. The aluminum filling levels of the composites are each about 20% to 30%. After curing of the polymers, the tensile strength of the strips was measured. In all cases, the strips were finally ruptured and also inspected for PTU under the electron microscope.

(21) By way of example, FIG. 6 shows the crack surface of a test strip in which PTU has been mixed with untreated aluminum granules. This corresponds to a variation of the proposal of GB 791,653 when resin is replaced by PTU, and is to be considered here as an example of the prior art. The crack surface shows that the aluminum fragments are undamaged and that the polymer has been cleanly removed from the aluminum surface to a large extent. The adhesion of the PTU to the aluminum has apparently failed, thereby initiating the overall tear of the test strip.

(22) In contrast, the aluminum fragments etched according to the invention have been torn after tearing off the test strip containing them, as shown in FIG. 7. The sharp-edged, rugged surface of the broken aluminum body differs significantly from the rather wavy and rounded surface of the intact granule grain in FIG. 6.

(23) At some points of the crack surface, one can also recognize the mechanically robust positive connection between the PTU and the aluminum fragment with anchoring structures, for example, as shown in FIG. 8.

(24) In FIG. 9, the measurement results for PTU is shown in a tensile stress-strain diagram. The composites show from the beginning a significantly increased tensile strength compared to the pure PTU. While the pure polymer can be stretched more or less elastically up to a tensile stress of about 58 MPa and then irreversibly deformed (cohesive limit), such deformation is not available to the composites—instead they break off abruptly. Although the composite with untreated aluminum granules proves to be more tensile—in accordance with the teaching of GB 791,653—for lower tensile stresses, but fails due to the moderate adhesion of the PTU on aluminum already below 50 MPa. In contrast, in the composite with etched fragments, no adhesion failure occurs; it shows superior tensile strength up to about 60 MPa and is destroyed only by breaking the fragments.

(25) For the more elastic and less tensile polymer PDMS (useful as a medical grade silicone), the effects of the aluminum fragments are on trend identical, but much more pronounced, as can be seen in the tensile stress strain diagram in FIG. 10.

(26) It is noteworthy that the upper limit of the elastic elongation of the pure polymer, especially in the case of PDMS, can very clearly be exceeded. This suggests that the polymer matrix is locally relieved of tensile stress by the fragments. This in turn suggests that the concrete shape of the fragments may have an impact on the measurement result, especially if it contributes to an efficient distribution of force in the environment. A particularly advantageous form for the fragments is therefore seen in the tetrapod shape, also known as “foot fishing” or “crow's feet”. Accordingly, wire bundles formed from aluminum wire fragment, for example, from two equal length wires, are inexpensive to produce and readily equipped according to the invention with anchoring structures.

(27) The use of the etched fragments is not limited to polymers.

(28) For example, the initially flowable and thereafter curing material may be a precursor for a sol-gel method for producing ceramic layers. Such precursors cure by a thermal sintering step at typical temperatures of about 100° C. to drive off the organic solvents. The aluminum fragments with the enclosing sheath with anchoring structures can withstand such temperatures without damage. They will also—soon after the production and removal of the etched fragments from the etching solution to the air—be very quickly covered with a few nanometer thick layer of aluminum oxide and thereby further passivate chemically.

(29) In the following some composite materials according to the invention are presented, which may result from a certain type of use of modified aluminum fragments.

(30) Example of Use “Cold Casting of Electrically Conductive Castings”

(31) Preferably, small balls made of aluminum, whose surfaces are equipped with anchoring structures, can be placed in a largely arbitrary casting mold, where they—possibly after some shaking of the mold—compact into a spherical packing. The intermediate spaces can be impregnated with an initially flowable—preferably low-viscosity—polymer precursor, for example with a polyurethane or polymethyl methacrylate or polyethylene. After curing of the polymer, the modified aluminum spheres with diameters between 100 microns and more than one centimeter, preferably between 0.5 and 5 millimeters, are only separated from each other with rupture of the polymer. At the same time, the molded body is completely electrically conductive, since the balls were in conductive contact from the beginning and thus fixed. It has formed a percolating, electrical network. It is of particular advantage to use balls with a relatively large diameter, for example greater than 0.5 millimeters, since this forms a coherent, open pore space in the spherical packing, which is very easily permeated by the polymer precursor and can be filled up.

(32) Such a molded body can be formed at room temperature—that is, without significant energy input at the place of casting—in any size and shape. It essentially has the mechanical strength of the polymer and the electrical conductivity of aluminum. The degree of aluminum filling of a conductive composite is usually above 50%.

(33) Usage Example “Pegging”

(34) Materials which either intrinsically adhere weakly to one another, e.g. silicone and PTFE, or which, although exhibiting some adhesion, tend to separate from one another as a result of very different thermal expansion during thermal cycling, can be described as incompatible when used as composite materials. Nevertheless, such material systems can be firmly mechanically connected to one another, so to speak “pegged”, by holding bodies along their boundary surface. For this, however, it is necessary that the incompatible materials to be joined in the production of the composite material can both be provided in initially flowable and subsequently curing phases. Holding bodies with anchoring structures are each partially anchored with one of the two materials and then the two materials are held together continuously by their own structure. Suitable holding bodies may be aluminum wire filaments, for example in cylindrical or rectangular form, with the surface modification according to the invention. These can be applied or sprinkled onto a first material which is flowable at least in the area of its surface, whereby a part of the anchoring structures is already penetrated by the flowable phase. Once this hardens, the holding bodies are very firmly connected to the first material. This itself now carries practically anchoring structures for the second material, namely specifically on the still free surfaces of the holding body embedded only partially in the first material.

(35) The fact that the holding bodies are not completely submerged in the flowable phase of the first material can be most easily prevented by precisely predetermining the layer thickness of the flowable phase—for example, as a function of time in a polymer extrusion—and then applying holding bodies at the desired time which are just too big to be completely immersed. This is an important difference from similar anchoring proposals in the prior art in which nano- to microparticles are to be used for anchoring. Their degree of embedding is fundamentally uncertain, and the “amount of anchoring possibilities” provided there for the second material is uncertain. When using aluminum fragments, however, high security can be ensured over the surface with anchoring structures which the second material can use for fastening.

(36) In FIG. 11 a) a prefabricated disc of thermoplastic polyoxymethylene (POM) is shown, which has been partially melted by heating the surface in the middle and sprinkled with etched fragments. The fragments were additionally pressed into the soft POM. After re-solidification of the POM a cylinder of ethylene vinyl acetate (EVA) is thermally softened on a top surface and pressed with the soft side on the partially embedded in POM fragment until the EVA is solidified again.

(37) The composite thus formed, seen in FIG. 11 b), is torn apart in the subsequent tensile test, and it is found that the EVA fails cohesively just above the aluminum fragments.

(38) The holding bodies remain wrapped in polymer on the POM disk, as shown in FIG. 11 c).

(39) Furthermore, composite materials of a brittle ceramic, for example of lead zirconate titanate (PZT), and a soft elastic polymer, such as silicone, can be prepared by, for example, arranging on a PZT workpiece, a sol-gel film of a PZT precursor with predetermined film thickness, wherein aluminum fragments, whose smallest feature width is greater than the sol-gel film thickness, are applied to the sol-gel film and embedded in these. After the heat treatment for pyrolysis of the sol-gel film, the PZT workpiece exhibits on the treated surface tightly bound fragments with anchoring structures towards the outside, onto which silicone can “hold on” in an outstanding manner.

(40) It should be emphasized here that the anchoring structures of the aluminum fragments have a certain flexibility and resilience to force attacks due to their filigree, coral-like structure. Force attacks on the holding body itself are thereby somewhat buffered, i.e. attenuated. Two materials with very different coefficients of thermal expansion can easily remain joined even under high temperature fluctuations if their cohesion is based on common anchoring to the same fragments with such anchoring structures, because they act as retaining bolts or dowels which can not be removed as long as the materials themselves do not fail structurally.