EP1991713A2 - Composite metal-aerogel material - Google Patents

Abstract

The invention relates to a porous composite material that is made of a metal matrix with embedded nanostructured materials.

Description

 Metal composite Aeroqel

The present application relates to a composite of a metal matrix with embedded nanostructured materials having macroscopic dimensions (micro to millimeter).

Most of the processes developed in recent decades to produce porous metals provide closed or open-celled foams and sponges. A foam-like morphology is necessary for high mechanical properties (rigidity) and thus for structural applications such as lightweight construction elements in vehicle construction. Functional applications such as filters, silencers, or heat exchangers require an open cell structure to allow a fluid medium to penetrate or penetrate the foam or sponge. Open-celled foams or sponges have so far been generated via the process step of precision casting. However, this method is very expensive and therefore expensive. A long-known alternative process is the casting over of fillers with metallic melts. After removal of the fillers, there is a sponge-like open-cell body with interconnected pores. Metallic foams are usually produced by gassing a melt or by thermal decomposition of, for example, hydrides. The foam production is in principle a transient, unstable and difficult to control process. The previously known methods are described in detail in the Literature documents (J. Banhart, J. Baumeister, M.Weber, Metallschaum, Aluminum, 70, 209-211 (1994);

J. Banhart, J. Baumeister, M. Weber: Foamed Metals as New Lightweight Materials, VDI Berichte 1021, 277-284, (1993); H. Cohrt, F. Baumgärtner, D. Brungs, H. Gers: Principles of the Production of PM-based Aluminum Foam, Proceedings of the First German-Speaking Symposium Metal Foams; (Proceedings of the First German Symposium on Metal Foams); Bremen (Germany), 6.-7. March; 91-102, (1997); JJ. Bikerman: Foams: Theory and Industrial Applications, Reinhold, New York, Ch. 4, (1953); M. Weber: Production of metal foams and description of material properties, dissertation, TU Clausthal, (1995)).

Foams according to the invention are largely equated with sponges and are as colloidal systems systems of gas-filled, spherical or polyhedron-shaped cells, which are bounded by solid cell webs. The cell barriers, connected by so-called nodal points, form a coherent framework. The foam lamellae (closed-cell foam) stretch between the cell bridges. If the foam lamellae are destroyed or flow back into the cell ridges at the end of foaming, an open-celled foam is obtained. Foams are thermodynamically unstable because surface energy can be obtained by reducing the surface area. The stability and thus the existence of a foam is therefore dependent on how far it succeeds in preventing its self-destruction. DE 40 18 360 C1 describes the foaming of aluminum alloys with the aid of titanium hydride powder. DE 41 Ol 630 C2 describes the foaming of other metals and alloys such as bronze also with the aid of titanium hydride powder.

WO 96/19314 A1 describes a composite material as a solder material with high mechanical stability, consisting of a high-melting and a low-melting metal component and a filler component. After brazing, intermetallic phases are formed having a melting point above the processing temperature, which has internal surfaces on the filler components. These internal surfaces improve the mechanical stability of the solder joint.

The German translation DE 603 01 737 T2 emerges from EP 1 333 222 B1 describes a method for producing a super insulating composite plate which is enclosed as insulating soul a porous superinsulating material with micro or nano cell structure and with a dense barrier jacket under vacuum.

Many of the aforementioned methods, in particular the foaming of metals by the use of hydride powders, have in common that these metallic foams are often not reproducible in their properties and have an uneven distribution of the pores. Many of these methods result in metal foams having a porosity of more than 85%, making these metal foams unsuitable for applications requiring high mechanical strength and, in particular, high compressive strength. - A -

If the metal foams obtained by encapsulation of fillers, the fillers must be removed consuming in an additional step.

Object of the present invention is therefore the simplest possible production of metal foams, which have high mechanical stability despite low weight.

This object of the invention is achieved in a first embodiment by a pore-containing composite material made of a metal matrix with embedded nanostructured materials.

Pores in the sense of the invention are those volume ranges of the composite which are not filled with metal and have a density in a range of 0.001 g / cm ³ to 0.1 g / cm ³ . Advantageously, the pores may be partially or completely filled by the embedded nanostructured materials. The term pores according to the invention, which are classically filled with gas, thus deviates deliberately from the previous understanding, since the pores according to the invention can also be filled, for example, with solids such as airgel.

Nanostructured materials in the sense of the invention include those which have elevations on their surface, of which at least 80% of the elevations are spaced from adjacent elevations in the range of 5 nm to 500 nm, the elevations themselves having a height in the range of 5 nm have up to 500 nm. In addition are including materials whose inner structure consists of nanoparticles, that is, particles with a diameter in a range of 2 to 100 nm, which are crosslinked. If the nanostructured materials are present as particles, the particle size is advantageously in a range of 0.1 to 5 mm.

Advantageously, the porosity of the composite material according to the invention is in a range of 20 to 80%, particularly preferably in a range of 30 to 70%. The porosity according to the invention is the ratio of the weight of a certain predetermined volume of the composite material according to the invention to the weight of a corresponding non-porous metal body of the same volume. If the porosity is too large, this composite material has too low a mechanical strength for many applications. If the porosity is too low, the weight of this composite is too high for many applications. Because the pores may advantageously also be filled by the nanostructured materials, in this case the porosity essentially corresponds to the volume content of the nanostructured materials, in the event that the nanostructured materials are negligibly light.

The volume of each filled pore is preferably adjusted so that the volume of at least 80% of the pores is at most 500 mm ³ each. If the volume of more than 80% of the pores is more than 500 mm ^{3 in} each case, then these composite materials are not sufficiently mechanically strong. The pore size of the composite according to the invention can be determined, for example, according to ASTM 3576-77. Advantageously, the nanostructured materials are chemically inert. Chemically inert in the sense of the invention means that the nanostructured materials do not undergo a chemical reaction with molten metal. This is particularly advantageous since such a degradation, for example oxidation, of the metal matrix can be avoided.

The nanostructured materials are preferably aerogels or expanded phyllosilicates. Due to the low density of these materials, during production, particles of these materials can be encapsulated with metallic melts to form the pores of the composite of the present invention without the need to remove these materials from the composite. This applies in particular to airgel, since the density of the airgel used according to the invention is advantageously in a range of 0.005 to 0.025 g / cm ³ . Airgel is particularly advantageous because it is open-cell, has a high specific surface area and therefore can be used in both open-cell and closed-cell materials. In contrast, closed-cell nanostructured materials could not result in open-cell composites.

In the event that the nanostructured materials include phyllosilicates, these are advantageously selected from vermiculites, biotites, or zeolites and mixtures thereof (for example, expanded mica). If the nanostructured materials according to the invention are aerogels, they advantageously comprise silica aerogels. Although the composite materials according to the invention can be obtained with hydrophilic aerogels, hydrophobic aerogels are nevertheless preferred, since they are particularly easy to wet with molten metal. The pore diameter of the airgel itself is advantageously in a range from 5 to 50 nm. The specific surface of the aerogels used according to the invention is advantageously in a range from 200 to 1500 m ² / g. Advantageously, the thermal conductivity of the aerogels is in a range from 0.005 to 0 , 03 W / mK at 25 ^° C. The airgel is preferably present as granules, in particular as granules, in which the particle size distribution is such that at least 80% by volume of the airgel granules have a particle size in the range from 0.1 to 5 mm having. The shape of the grains of the airgel is advantageously selected from spherical, polyhedral, cylindrical or platelet-shaped.

The metal of the matrix is advantageously selected from aluminum, zinc, tin, copper, magnesium, silicon or an alloy of at least two of these metals. The metal matrix is particularly preferably made of aluminum or an aluminum alloy. Further preferred alloys are, in particular, AISi, AISiMg, AICu, bronze or brass. The melting point of the metal matrix according to the invention is advantageously in a range from 600 to 900 ^° C., in particular in a range from 600 to 750 ^° C.

Although airgel has hitherto been regarded as very unstable mechanically, in the present invention it is surprising for the first time managed to process airgel to a composite material while retaining the structure with a molten metal. By choosing the aerogels, a cell morphology with defined pore sizes in metal foam can be set for the first time. The airgel as a filler due to its light weight, unlike the conventional production of a metallic foam must not be removed.

The composites of the invention advantageously have a compressive strength or compressive strength at a compression of 20% of at least 8 MPa (according to DIN 53577 / ISO 3386). The density of the composite materials according to the invention is advantageously in a range of 0.3 to 2 g / cm ³ , in particular in a range of 1 to 2 g / cm ³ . If the density of the composite is too high, then the composite is unsuitable for many applications where lightweight materials are required. However, if the density is too low, the resulting composites are not sufficiently mechanically stable.

In a further embodiment, the object on which the invention is based is achieved by a method for producing the composite material according to the invention which is characterized in that the following steps are carried out: a) external mixing of the nanostructured material with a molten metal and transfer into a casting mold or a ') Mixing the nanostructured material with a molten metal in the mold, b) allowing it to solidify, and c) removing it from the mold. Alternatively, it is also possible to mix the nanostructured materials with a metal powder and then to melt the metal.

The object is thus achieved, for example, by stirring polyhedral or spherical nanostructured silica airgel particles into an optionally thixotropic molten metal. Since the airgel is advantageously chemically inert, no reaction takes place between the metal and the melt. During stirring, the metal solidifies and encloses the airgel particles. Even in the soft state of the metal composite can be pressed advantageously, so that a desired shape can be done. Thixotropic in the context of the invention, the molten metal is always when the temperature is between the liquidus and solidus.

The method may also be advantageously based on backfilling an aggregate of airgel granules with a molten metal. The advantageously pressurized melt penetrates into the interstices and fills the gussets. After solidification, the airgel no longer needs to be removed, since at a density of, for example, about 0.015 g / cm ^{3 it is} only a fraction of the total weight. The pressurization can be advantageously realized in smaller components by the centrifugal force in the centrifugal casting and in larger components in die casting.

In a further embodiment, the object underlying the invention is achieved by the use of composites according to the invention in structural lightweight construction, in particular in applications in motor vehicles or in portable electronic devices.

EXEMPLARY EMBODIMENTS:

Example 1 :

Silica airgel granules were recovered from airgel monoliths by grinding. The thus-obtained hydrophilic polyhedral silica airgel (Airglas ^®, Staffanstorp, Sweden) was baked as granules initially at 600 ⁰ C. An AISi alloy (aluminum containing 7% by weight of silicon) was melted and subsequently brought into the thixotropic (semi-solid) state by slow stirring while simultaneously lowering the temperature to the interval between liquidus and solidus temperatures. Airgel granules (grain size 0.1 mm to 5 mm) were added to 40% by volume of the metal by stirring. The mixing was done by hand. The semi-solid metal prevented floating of the extremely lightweight silica airgel granules. As soon as stirring was no longer possible due to the advanced solidification, the still soft connection was pressurized and could thus be brought into any desired shape. The porosity was 40% for pore diameters in a range of 0.1 to 5 mm. Fig. 1 shows the metallic composite material according to Example 1.

Example 2: Airgel according to Example 1 was mixed with a 720 ⁰ C hot AlSiMg alloy (aluminum containing 7 wt.% Silicon and 0.6 wt.% Magnesium) backfilled. For this purpose, a mold was filled with a loose bed of airgel granules. The casting took place from below, so that the melt with a slight pressure completely filled the spaces between the particles. In this case, a slight overpressure of 1 atm sufficed. After completion of the casting, a metallic composite of airgel granules and metal was obtained.

Example 3:

The processes mentioned in Examples 1 and 2 were also carried out with spherical airgel granules, so-called airgel beads from Cabot Corp. When choosing this filler, the later cell morphology was clearly specified.

Example 4:

The thermally expanded phyllosilicates vermiculite, biotite and muscovite (3 g) were each added to a 73 O ⁰ C hot AICu melt (300 g, aluminum containing 9 wt.% Copper) and carefully stirred to solidification. After solidification, a composite of inorganic silicates and a metallic alloy was present. The porosity was 30% for pore diameters in a range of 0.1 to 7 mm. Fig. 2 shows the metallic composite according to Example 4 with coarse particles of expanded biotite.

Example 5: The airgel granules as in Example 1 were filled in a heat-resistant mold until complete filling and used in a centrifugal casting plant. The crucible of the centrifugal casting plant (AuTϊ 2.0, Linn High-Term, Eschfelden) was filled with an alloy of aluminum containing 7 wt.% Silicon (about 100 g). Through the normal process of centrifugal casting, the voids between the airgel particles were completely filled with metal. The volume content of pores, which are completely filled with airgel, could be varied by the particle size distribution of the filler particles between 50 and 80%.

Claims

claims: 1. Pore-containing composite of a metal matrix with embedded nanostructured materials. 2. Composite material according to claim 1, characterized in that the metal matrix comprises aluminum or an aluminum alloy. 3. Composite material according to one of claims 1 to 2, characterized in that the nanostructured materials are chemically inert. 4. Composite material according to one of the preceding claims 1 to 3, characterized in that the nanostructured materials are aerogels or phyllosilicates. 5. Composite material according to claim 4, characterized in that the airgel is a silica airgel. 6. The composite according to claim 4, characterized in that the layered silicates are selected from expanded vermiculites or biotites. 7. Composite material according to claim 1, characterized in that the porosity is in a range of 20 to 80%. 8. A method for producing a composite material according to any one of claims 1 to 7, characterized by carrying out the following steps: a) external mixing of the nanostructured material with a molten metal and into a casting mold or a ') mixing of the nanostructured material with a molten metal in the mold, b) solidify, and c) removal from the mold. 10. Use of the composite materials according to one of claims 1 to 7 in structural lightweight construction, in particular in applications in motor vehicles or in portable electronic devices.