Method for fabricating metal foams having ligament diameters below one micron

A method for fabricating a metal foam component from an aerogel containing a polymer and nanoparticles is disclosed. The method may comprise: 1) exposing the aerogel to a reducing condition at an elevated temperature for a reaction time to provide a metal foam; and 2) using the metal foam to fabricate the metal foam component. At least one of the elevated temperature and the reaction time may be selected so that at least some ligaments of the metal foam have a desired ligament diameter or at least some pores of the metal foam have a desired pore size. The desired ligament diameter may be less than about one micron and the component may be a component of a gas turbine engine.

FIELD OF DISCLOSURE

The present disclosure relates to lightweight and high-strength metal foams, and more specifically, relates to a method for producing nanocellular metal foams having ligament diameters below one micron as materials for aerospace components.

BACKGROUND

Metal foams, which are lightweight and high-strength porous metallic structures, are gaining increasing interest in numerous industries, such as the aerospace and automotive industries. In particular, the introduction of components formed from metal foam materials into aerospace or automotive structures may lead to improvements in fuel efficiency, while providing other beneficial properties such as vibration dampening, erosion resistance, and enhanced mechanical strength and overall performance. Moreover, metal foams may have high temperatures resistances and, therefore, may provide thermal protection properties for a range of applications as well.

Nanocellular metal foams are a sub-class of metal foams which have pore sizes in the nanoscale or submicron range. Open-celled nanocellular metal foams, which have open and gas-filled pores, may appear as a network of interconnected ligaments that form the solid, metallic portion of the metal foam. The diameters of the ligaments (as measured by the width of the ligament at its narrowest part) may be correlated with the strength-to-weight ratio of the metal foam. In particular, it has been predicted that the strength of a nanocellular metal foam may approach the strength of an identically-sized solid metal part as its ligament diameters decrease, while at only a fraction of the weight of the solid metal part. For at least this reason, nanocellular metal foams having high integrity ligaments with diameters on the nanoscale or submicron scale may be a desirable target for many engineers. Despite the benefits that such lightweight and high-strength materials may provide for numerous applications, it currently remains a challenge to fabricate metal foams with ligament diameters below one micron.

Current methods for producing stochastic metal foams may use powder metallurgy in which a metal powder may be mixed with a foaming agent and compacted to a dense structure. The metal and foaming agent mixture may then be heated to cause the foaming agent to release gas and expand the metal material, causing it to form a porous structure. Such methods for producing metal foams have been described, for example, in U.S. Pat. Nos. 6,444,007 and in 2,751,289. In addition, electroplating may also be used to produce metal foams. While effective, the existing metal foam fabrication methods may fail to provide metal foams having ligament diameters below one micron. Furthermore, these fabrication methods may offer limited control over the ligament diameters of the metal foams and their corresponding mechanical properties. Clearly, there is a need for fabrication methods capable of producing metal foams with ligament diameters on the submicron scale.

SUMMARY

In accordance with one aspect of the present disclosure, a method for fabricating a metal foam component from and aerogel containing a polymer and nanoparticles is disclosed. The method may comprise: 1) exposing the aerogel to a reducing condition at an elevated temperature for a reaction time to provide a metal foam, wherein at least one of the elevated temperature and the reaction time may be selected so that at least some ligaments of the metal foam have a desired ligament diameter or at least some pores of the metal foam have a desired pore size; and 2) using the metal foam to fabricate the metal foam component.

In another refinement, the desired ligament diameter may be less than about one micron.

In another refinement, the metal foam component may be a component of a gas turbine engine.

In another refinement, exposing the aerogel to the reducing condition at the elevated temperature for the reaction time may both pyrolyze the polymer and at least partially reduce the nanoparticles to the metal foam.

In another refinement, the reducing condition may be an atmosphere of hydrogen gas in an inert gas.

In another refinement, the elevated temperature may be in the range of about 400° C. to about 1000° C.

In another refinement, the method may further comprise preparing the aerogel from a mold prior to exposing the aerogel to the reducing condition.

In another refinement, preparing the aerogel from the mold may comprise: 1) polymerizing a polymer precursor in a solvent containing a metal salt to form a gel comprising the polymer and the nanoparticles; and 2) evaporating the solvent by a supercritical drying process to provide the aerogel.

In another refinement, the polymer precursor may be propylene oxide and the polymer may be polypropylene oxide.

In another refinement, the metal salt may be a hydrate of a nickel (II) salt and the nanoparticles may be nickel (II) oxide nanoparticles.

In accordance with another aspect of the present disclosure, a metal foam component having ligament diameters below one micron is disclosed. The metal foam component may be produced from an aerogel containing a polymer and nanoparticles by a method comprising: 1) exposing the aerogel to a reducing condition at an elevated temperature for a reaction time to provide a metal foam, wherein at least one of the elevated temperature and the reaction time may be selected so that at least some ligaments of the metal foam have a desired ligament diameter or at least some pores of the metal foam have a desired pore size; and 2) using the metal foam to fabricate the metal foam component.

In another refinement, the metal foam component may be a component of a gas turbine engine.

In another refinement, exposing the aerogel to the reducing condition may both pyrolyze the polymer and at least partially reduce the nanoparticles to the metal foam.

In another refinement, the reducing condition may be an atmosphere containing hydrogen gas in an inert gas.

In another refinement, the elevated temperature may be in the range of about 400° C. to about 1000° C.

In another refinement, the method may further comprise preparing the aerogel prior to exposing the aerogel to the reducing condition.

In another refinement, preparing the aerogel may comprise: 1) polymerizing a polymer precursor in a solvent containing a metal salt to form a gel comprising the polymer and the nanoparticles; and 2) evaporating the solvent by a supercritical drying process to provide the aerogel.

In another refinement, the polymer precursor may be propylene oxide and the polymer may be polypropylene oxide.

In accordance with another aspect of the present disclosure, a method for producing a metal foam from an aerogel comprising a polymer and nanoparticles is disclosed. The method may comprise: 1) heating the aerogel at an elevated temperature to pyrolyze the polymer; and 2) exposing the aerogel to a reducing condition to at least partially reduce the nanoparticles to the metal foam.

In another refinement, heating the aerogel and exposing the aerogel to a reducing condition may be carried out simultaneously for a reaction time.

In another refinement, a ligament diameter or a pore size of the metal foam may be controllable by at least one of the elevated temperature and the reaction time.

These and other aspects and features of the present disclosure will be more readily understood when read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

Referring now to the drawings, and with specific reference toFIG. 1, a gas turbine engine10in accordance with the present disclosure is depicted. In an upstream to downstream direction, the gas turbine engine10may consist of a fan section12, a compressor section14(which may include both a low-pressure compressor and a high-pressure compressor), an annular combustor16(although circumferentially-spaced “can” combustors may also be used), and a turbine section17(which may include a high-pressure turbine18and a low-pressure turbine20), all of which may be encased in an engine case21, as shown. A nacelle22may surround the engine case21and define a bypass duct23, as shown.

In normal operation, air24may be drawn into the engine10and accelerated by the fan section12. After passing the fan section12, a part of the air24may be routed through the compressor section14, the combustor(s)16, and the turbine section17. More specifically, the air24may first be compressed and pressurized in the compressor section14and it may then be mixed with fuel and combusted in the combustor(s)16to generate hot combustion gases. The hot combustion gases may then expand through and drive the turbines18and20which may, in turn, drive the compressor section14and the fan section12by driving the rotation of an interconnecting shaft26. After passing through the turbines18and20, the gases may be exhausted through an exhaust nozzle28to provide some of the propulsive thrust to an associated aircraft or to provide power if used in land-based operations. The remaining propulsive thrust may be provided by the air24passing through the bypass duct23and exiting the engine10through a nozzle30, as shown.

Each component in the gas turbine engine10and the nacelle22may contribute to the overall weight of an associated aircraft. Where there is a desire to reduce the weight and/or increase the mechanical strength (elastic modulus, etc.) of components or regions of the gas turbine engine10, one or more gas turbine engine components may be at least partially formed from a metal foam32(seeFIG. 2). The metal foam32may be lightweight and have a high mechanical strength. Accordingly, the fabrication of gas turbine engine components from the metal foam32may lead to advantageous reductions in fuel efficiency of the gas turbine engine10as a whole, as well as improvements in the performance features of the gas turbine engine components made from the metal foam. These performance features may include, but are not limited to, increased mechanical strength, improved vibration dampening and erosion resistance, and increased temperature resistance. In particular, the fan section12, the compressor section14, the combustor(s)16, the turbine section17, and/or the nacelle22may include one or more components formed, at least in part, from the metal foam32. As one non-limiting possibility, one or more blade outer air seals may be fabricated, at least in part, from the metal foam32. As will be understood by those with ordinary skill in the art, a blade outer air seal may be located between the rotating blades of a rotor stage (located in the compressor section14or the turbine section17) and the engine case21. Furthermore, although the incorporation of the metal foam32into gas turbine engine components is disclosed as a non-limiting example, it is to be understood that the concepts described herein are not limited to gas turbine engines and may be applicable to other applications as well.

The metal foam32may have any structure suitable for its intended use and, therefore, may deviate from the exemplary box-like structure shown in practice. The metal foam32may be formed from a monolithic metal, a multi-metal, a metal alloy, a monolithic ceramic material, a ceramic-containing material, or combinations thereof. The metal or metals may be various metals such as, but not limited to, manganese, titanium, tungsten, vanadium, niobium, hafnium, tatalum, rhenium, ruthenium, iridium, palladium, platinum, zirconium, cobalt, yttrium, copper, molybdenum, aluminum, chromium, iron, nickel, or combinations thereof. In addition, other elements may be synthesized into the metal foam32such as silicon and carbon. Moreover, additional elements may be present in the metal foam to enhance specific properties. For example, if the metal foam32is formed from nickel aluminide (Ni3Al), boron may be added to enhance desired properties. It is also noted that the metal foam32may contain fractions of metal oxide in some cases (see further details below).

As best shown inFIG. 3, the metal foam32may have a plurality of pores33formed between a network of interconnected ligaments34. Each of the ligaments34of the metal foam32may extend between two joints36, wherein each of the joints36may form a branching point between two or more ligaments34. Notably, at least some, if not all, of the ligaments34of the metal foam32may have a diameter (d) of less than about one micron. As best shown inFIG. 4, the diameter (d) of each ligament34may be measured by the width of the ligament at its narrowest point. The pores33of the metal foam32may be open and gas-filled, and they may have non-uniform or random sizes and geometries. Accordingly, the metal foam32may be a stochastic, open-celled foam. However, in some circumstances, the pores33may be filled (closed-cell foam) and/or the geometries of the pores33may be uniform throughout the metal foam32. In any event, the sizes of the pores33may range from less than about 50 nanometers up to about 1.5 micron (1500 nanometers). Given the nanoscale size of its pores, the metal foam32may be a nanocellular foam.

Due to its submicron ligament dimensions, the metal foam34may have a substantially increased strength-to-weight ratio compared with similarly-sized metal foams of the prior art, which typically have ligament diameters greater than one micron. In addition, the mechanical properties (strength-to-weight ratio, elastic modulus, etc.) of the metal foam32may be correlated with the diameter (d) of the ligaments34. In general, the mechanical strength and the strength-to-weight ratio of the metal foam32may increase as the diameter (d) of the ligaments34decrease. Furthermore, a desired submicron diameter (d) of the ligaments34and/or the pore sizes of the metal foam32may be selected in order to impart the metal foam32with specific mechanical properties such as, but not limited to, a desired elastic modulus or a desired strength-to-weight ratio. Once the desired ligament diameter (or pore size) is selected, the metal foam32may be prepared by selecting reaction conditions that provide the desired ligament diameters and/or pore sizes (see further details below).

Turning now toFIGS. 5 and 6, a method for producing the metal foam32and a component38at least partially made from the metal foam32is depicted. As explained above, the component38may be a component of the gas turbine engine10, or it may be a component for use in another suitable application. Beginning with a first block40, a desired ligament diameter (d) and/or pore size for the metal foam32may be selected in order to impart the metal foam32with specific mechanical properties such as, but not limited to, a desired strength-to-weight ratio and/or a desired elastic modulus. In general, the desired ligament diameter may be a diameter less than about one micron.

Once the desired ligament diameter and/or pore size is selected, the metal foam32may be prepared from an aerogel42according to a next block45(also seeFIG. 6). As will be apparent to those with ordinary skill in the art, the aerogel42may be a lightweight and low-density porous structure that is formed by replacing solvent molecules with gas molecules. The pores of the aerogel42may be filled with gas molecules, while the solid portions of the aerogel42may consist of a polymer and nanoparticles. Notably, the nanoparticles in the aerogel42may be the molecular precursors to the metal foam product. In this regard, the identity of the nanoparticles in the aerogel42may be dependent on the desired material composition of the metal foam32. For example, if a nickel foam is the desired product, the nanoparticles in the aerogel42may be nickel (II) oxide nanoparticles. However, if the desired product is a ceramic foam or a ceramic-containing foam, the nanoparticles may be ceramic oxide nanoparticles or ceramic-containing metal oxide nanoparticles.

The aerogel42may be prepared by a sol-gel process (see further details below) or it may be obtained from a commercial supplier. In any event, the block45may involve exposing the aerogel42to a reducing condition at an elevated temperature for a reaction time to produce the metal foam32having at least some ligaments34with the desired diameter and/or at least some pores with the desired pore size. During the block45, the polymer may be pyrolyzed and removed (burned-off) from the aerogel42, while the nanoparticles may be simultaneously converted to the metal foam32. The reducing condition may be a reducing atmosphere, such as an atmosphere of hydrogen gas in an inert gas, although other reducing conditions and/or reducing agents may also be used. As a non-limiting possibility, the reducing condition may be an atmosphere of 4% (v/v) hydrogen gas in argon. In addition, suitable elevated temperatures for carrying out the block45may be in the range of about 400° C. to about 1000° C., although other temperatures may be used.

Importantly, at least one or both of the elevated temperature and the reaction time used for the block45may be used to control the ligament diameters (or pore sizes) of the resulting metal foam32. In particular, Table 1 shows the influence of the elevated temperature on the pore sizes of a nickel metal foam (at a constant reaction time of 480 minutes) and Table 2 shows the influence of the reaction time on the pore sizes of a nickel metal foam (at a constant elevated temperature of 1173K).

TABLE 1Influence of the Elevated Temperatureon Pore Sizes of Nickel Foams.a,bSintering Temperature (K)Pore Size (nm)2731900773900873700973500aProcess time held constant at 480 minutes.bPore sizes measured using porosimetry and neutron scattering.

TABLE 2Influence of Reaction Time on Pore Sizes of Nickel Foams.a,bSintering Time (minutes)Grain Size (nm)Pore Size (nm)031900112180090100800480200500aTemperature held constant at 1173 K.bPore sizes measured by x-ray diffraction and neutron scattering.

As can be seen in Tables 1 and 2, there is a correlation between the elevated temperature and/or the reaction time and the pore size of the resulting metal foam32. In general, shorter reaction times and/or lower elevated temperatures provide metal foams with larger pores, while longer reaction times and/or higher elevated temperatures provide metal foams with smaller pores. However, it is noted that the correlation between the reaction time and/or the elevated temperature may be dependent on the type of metal or ceramic forming nanoparticles of the aerogel42. In addition, the ligament diameters of the resulting metal foam may or may not exhibit the same temperature and reaction time dependence as those shown in Tables 1 and 2. Based on known correlations between the elevated temperature and/or the reaction time and the pore sizes (or ligament diameters) of the resulting metal foam32such as those shown in Tables 1 and 2 above, at least one of the elevated temperature and the reaction time used for carrying out the block45may be selected so that at least some of the pores33have a desired pore size and/or at least some of the ligaments34of the metal foam product have a desired diameter.

It is also noted that, in some circumstances, less than complete reduction of the nanoparticles in the aerogel42and/or less than complete pyrolysis of the polymer in the aerogel42may occur during the block45, leaving behind a fraction of polymer and/or nanoparticles in the metal foam32. However, the pyrolysis/reduction of the aerogel42may driven to at least near completion by varying the reaction conditions, such as the elevated temperature, the reaction time, and/or the concentration of hydrogen gas. Furthermore, while the block45may be carried out as a single manipulation as described above, in some circumstances it may be possible to instead perform the pyrolysis of the polymer and the reduction of the metal oxide nanoparticles by separate manipulative steps. For example, the aerogel42may first be heated to an elevated temperature sufficient to pyrolyze the polymer, and it may be subsequently exposed to a reducing condition to reduce the nanoparticles to the metal foam32.

Following the block45, the metal foam32may be used to fabricate the component38according to a next block50, as shown. The metal foam32may be formed in the shape of the desired component38during the block45with an appropriate mold or other tooling, or it may be shaped following the block45using appropriate tooling or other shaping techniques. Alternatively, the aerogel42may already be formed in the shape of the desired component38prior to the block45. The metal foam32may form the entire body of the component38, or it may be applied as a coating or surface layer of a desired thickness to selected surfaces of the component38. In other circumstances, the metal foam32may form the core of the component38, and the metal foam core may be surrounded by a solid shell which protects the metal foam32from mechanical damage, such as abrasion, and/or environmental damage. In the latter case, the solid shell may be formed from one or more metals or metal alloys. In any event, the mechanical properties of the component38may be tailored as desired by tuning the diameters (d) of the ligaments34(or pore sizes) of the metal foam32. As explained above, tuning of the diameters (d) (or pore sizes) of the metal foam32may be achieved by varying the elevated temperature and/or the reaction time that is used during the block45.

Referring now toFIGS. 7 and 8, a method for producing the aerogel42is shown. According to a first block55, one or more polymer precursors may be polymerized in a liquid solvent containing one or more metal salts. In particular, the block55may involve preparing a solution containing the liquid solvent and the metal salt(s) in a reaction vessel56, adding the polymer precursor to the solution to form a mixture57, and allowing the mixture57to form a biphasic gel58(seeFIG. 8). Although shown as a beaker, the reaction vessel56may be any type of reaction vessel, or even a mold to control the shape of the resulting aerogel42. In general, the block55may be carried out at room temperature and under atmospheric pressure, although other conditions may also be used. During the block55, the polymer precursor may be converted to a polymer and the metal cations of the metal salt may be converted to a plurality of metal oxide nanoparticles by a sol-gel process, as will be understood by those skilled in the art. Depending on the identity of the polymer precursor(s), the polymer generated by the block55may be a homopolymer (i.e., consisting of one type of monomer subunit) or a copolymer (i.e., consisting of two or more types of monomer subunits), in which case it may be a block copolymer (i.e., consisting of ‘blocks’ of one type of monomer subunit alternating in series with ‘blocks’ of another type of monomer subunit) or another type of copolymer. The biphasic gel58produced by the block55may consist of at least two distinct phases, including a liquid solvent phase and an insoluble solid phase, with the polymer forming the insoluble solid phase. The metal oxide nanoparticles may be soluble in both the liquid solvent phase and the insoluble polymer phase of the gel58.

Suitable solvents for use in the sol-gel process (block55) may include, but are not limited to, ethanol, ethanol-water mixtures, other alcohols or polar protic solvents, or other suitable solvents. The polymer precursor may be an epoxide such as propylene oxide or another suitable polymer precursor. Accordingly, if propylene oxide is used as the polymer precursor, the polymer produced by the block55may be polypropylene oxide. In addition, suitable metal salts for the sol-gel step (block55) may depend on the composition of the desired metal foam32. For example, if a nickel foam is the desired product, the metal salt may be a nickel (II) salt or a hydrate of a nickel (II) salt such as, but not limited to, nickel (II) chloride, nickel (II) nitrate, a hydrate of nickel (II) chloride, a hydrate of nickel (II) nitrate, or combinations thereof. If a nickel salt is used, then the metal oxide nanoparticles produced during the block55may be nickel oxide nanoparticles. However, as will be understood by those skilled in the art, other types of metal salts may also be employed depending on the desired composition of the metal foam product. In addition, if a ceramic foam is the desired product, the metal salts may instead be a ceramic precursor such as, but not limited to, molybdenum chloride and tetraethyl orthosilicate.

Following the block55, the solvent (or the liquid phase portion) of the gel58may be evaporated to convert the gel58to the aerogel42according to a next block60. The block60may be performed by a supercritical drying process or another suitable solvent evaporation process. If supercritical drying is used to evaporate the solvent of the gel58, the gel58may be subjected to temperatures and pressures above the critical point (including a critical temperature (Tc) and a critical pressure (Pc)) of the solvent where a distinction between the liquid phase and the gas phase of the solvent does not exist, followed by depressurization of the system and cooling to atmospheric conditions. As will be appreciated, the conditions used for the supercritical drying process (block60) may be dependent on the critical point of the solvent used. As one possibility, the block60may be carried out by first exchanging the liquid solvent in the gel58with liquid carbon dioxide (CO2), followed by supercritical drying of the liquid CO2at its critical point (for CO2: Tc=31° C. and Pc=7.4 MPa). The resulting aerogel42may then be subjected to pyrolysis/reduction according to the block45as described in detail above (seeFIGS. 5-6).

It has been found that treatment of a solution of propylene oxide and nickel (II) chloride hexahydrate in ethanol according to the method described above and shown inFIGS. 7-8provides an aerogel42consisting of polypropylene oxide and nickel oxide nanoparticles having an average diameter of about5nanometers. Treatment of the resulting aerogel42to a reducing atmosphere of hydrogen gas in argon at an elevated temperature as described above (seeFIGS. 5-6) produces a nickel foam having ligament diameters below one micron as measured by scanning electron microscopy (SEM) (seeFIG. 9). Furthermore, the method was found to successfully provide other types of metal foams with ligament diameters below one micron when using other metal salts (or ceramic precursors) as starting materials.

The following non-limiting example protocol for producing a nickel foam further illustrates the method of the present disclosure.

Formation of a Nickel Foam Having Ligament Diameters Below One Micron

I. Gel Preparation: 0.37 g (1.56 mmol) of nickel (II) chloride hexahydrate (NiCl2.6H2O) was dissolved in 2.5 mL of 200-proof ethanol to provide a clear, light-green solution. Propylene oxide (1.0 g; 17 mmol) was added to the solution and the resulting mixture was incubated at room temperature under ambient conditions for 30 minutes to allow an opaque, light-green gel to form. The gel was aged for at least 24 hours under ambient conditions and the 200-proof ethanol was exchanged at least four times over the course of several days to a week.

II. Nickel Oxide Aerogel Preparation: The gel produced by step (I) was placed in a supercritical dryer and the ethanol was exchanged with liquid CO2over the course of 2-3 days. The liquid CO2was then dried under supercritical conditions by heating the gel to about 45° C. while maintaining a pressure of about 100 bar (10 MPa) to produce a light-green nickel (II) oxide aerogel. The resulting aerogel was then depressurized at a rate of about 7 bar/hr.

III. Nickel Foam Preparation: The light-green aerogel produced by step (II) was placed in a tube furnace under flowing 4% (v/v) hydrogen gas in argon and the tube furnace was heated to about 600° C. for 8 hours. The tube furnace was then cooled to about 60° C. under argon and perfluoroalkyl carboxylic acid was flowed into the tube furnace to control the degree of oxidation of the resulting nickel foam. The tube furnace was then purged with argon for three hours and the resulting nickel foam was removed.

INDUSTRIAL APPLICABILITY

In general, it can therefore be seen that the technology disclosed herein may have industrial applicability in a variety of settings including, but not limited to, industrial applications which may benefit from lightweight and high-strength materials. In particular, the method of the present disclosure provides a route for producing nanocellular metal foams having high-integrity ligaments with diameters on the submicron scale by exposing an aerogel containing a polymer and metal oxide nanoparticles to an elevated temperature under reducing conditions. As metal foams prepared by current fabrication methods have ligament diameters above one micron, the method disclosed herein may provide metal foams with significantly higher strength-to-weight ratios than existing metal foams. Notably, desired mechanical properties (e.g., mechanical strength, strength-to-weight ratio, elastic modulus, etc.) for the metal foam (and the component that is fabricated from the metal foam) may be engineered by tuning the ligament diameter of the metal foam. As disclosed herein, the diameters of the ligaments of the metal foam may be controlled by varying the reaction conditions (e.g., temperature, reaction time) that are used during reduction/pyrolysis of the aerogel. Furthermore, the method of the present disclosure may be amenable to bulk processing and scale up to quantities appropriate for industrial materials and components, such as gas turbine engine components. It is expected that the technology disclosed herein may find wide industrial applicability in numerous areas such as, but not limited to, aerospace and automotive applications.