AND METHODS OF MANUFACTURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to, and the benefit of, U.S. provisional patent application serial number 62/261 ,21 1 filed on November 30, 2015, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under

BRCALL08-Per3-C-2-0006, awarded by the Defense Threat Reduction Agency; under DMR-1008791 , awarded by the National Science

Foundation; and under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The Government has certain rights in the invention.

INCORPORATION-BY-REFERENCE OF

COMPUTER PROGRAM APPENDIX

[0003] Not Applicable

BACKGROUND

[0004] 1 . Technical Field

[0005] The technology of this disclosure pertains generally to nanoscale structure fabrication methods, and more particularly to an ultralow-density, nanostructured, monolithic pure metal foams and fabrication methods.

2. Background Discussion

[0006] Nanoporous metal foams have a host of fascinating electrical,

magnetic, mechanical, optical, thermal and chemical properties due to their extremely high surface areas, nanoscale constricted geometries, and high porosity. They have critical potential applications in such fields as high energy density laser targets, lightweight materials, coatings, photovoltaics, heat exchangers and regenerators, trapping and targeted drug delivery, etc.

[0007] Although a number of methods for preparing pure metal

nanoparticles exist, there are relatively few synthetic processes for producing bulk, monolithic forms of nanostructured metals that have been developed. Conventional methodologies for fabricating monolithic metallic nanoporous materials or foams have limited practical applications and are difficult to adapt to large scale production because of their complicated procedures and use of expensive materials.

[0008] One approach to fabricating nanoporous metal foams is by the

selective dealloying of a binary alloy which involves selectively etching a less-noble metal from a bimetallic alloy. The metallic alloy starting material contains two or more elements, one of which is readily susceptible to selective chemical or electrochemical etching that preferentially removes the element. The dealloying process involves the removal of a substantial portion of the starting material to create porosity. For example, aluminum based alloys are often used with conventional chemical dealloying approaches because aluminum can be removed from the alloy with either a strong acid or a strong base.

[0009] Although selective dealloying approaches have been effective for fabricating some nanoporous metallic structures, the diffusion-controlled processes of electrochemical or acid etching limit the practical dimensions of the structure that can be formed.

[0010] In addition, the dealloying process also requires a starting alloy with a percolating network of pores or the formation of a network of pores to complete the etching process throughout the structure. Immersion times in the acid or base are typically 2 to 5 days and times increase with the size of the starting structure.

[0011] In some settings, full removal of the etched metal is necessary for the functionality of the final monolith structure. For example, residues of the selected etched metal of the starting alloy can greatly affect the catalytic properties of the nanoporous foam. This further limits the size and morphology of the starting structure and final metal foam. [0012] Additionally, many metals are readily oxidized in air at room temperature resulting in the formation of oxides. Oxides may form on the surfaces of the metal structure that may interfere with the functionality of the final foam.

[0013] Another approach to the production of nanoporous metal monoliths involves certain forms of combustion synthesis such as the thermal decomposition of transition-metal complexes containing high nitrogen energetic ligands. For example, nanostructured metal monolithic foams can be formed with a self-propagating combustion synthesis process utilizing metal complexes of the energetic high-nitrogen ligand, bistetrazole amine

(BTA) that are ignited in inert environments. Generally, the BTA metal complexes are prepared by the reaction of monohydrated

bi(tetrazolato)amine or ammonium bi(tetrazolo)amine and a selected metal salt. The product is collected and dried as a fine powder.

[0014] This thermal decomposition process results in the formation of a bulk material with nanoscale features in a matter of seconds. However, the microstructural features of the resulting metal foams can vary significantly depending on the composition and processing conditions.

[0015] Other approaches have used sacrificial templates of carbon or

organic aerogels such as polysaccharide templates. However, these approaches are usually better suited for use with metal oxides.

[0016] Therefore, there is a need for fabrication methods for producing

mechanically stable, ultralow-density, nanostructured, monolithic, metal foams, that are facile, inexpensive, environmentally benign, and amenable to scale-up processing. The present technology satisfies these needs and is generally an improvement in the art.

BRIEF SUMMARY

[0017] The present technology provides methods for fabricating low cost, ultralow density pure metal foams, with tunable densities between 50% and

0.05% by volume of the bulk density, using interconnected metallic nanowires. The highly porous foam structures have scalable and macroscopic overall sizes, in the range of several millimeters or more. Such materials will provide an unprecedented platform for exploration of lightweight materials, coatings, photovoltaics, thermoelectrics, heat exchangers, hydrogen storage and catalysts, and could have

transformative impacts in advanced materials and energy research.

[0018] The fabrication methods begin with the selection of the type of metal nanowires that are preferably made from pure metals such as Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one or more of these metals. Combinations of compatible metal nanowires can also be formed in some embodiments. The term nanowire is used in a general sense for a nanoscale element and is intended to include a variety of structures such as nanotubes, nanorods, nanowires and nanoribbons etc., that are either solid or hollow.

[0019] In one embodiment, the nanowires are formed with a template and electrodeposition. The characteristics of the nanowires are determined by the configuration of the template. The electrodeposition process allows the synthesis of a wide variety of metallic nanowires as the building blocks for the foams. This is easily scalable for mass production and is cost-effective, which are highly advantageous features particularly compared to current lithography approaches in the art.

[0020] The selected metal nanowires are dispersed in a fluid that is suitable for lyophilization such as deionized water. The concentration or density of the nanowires within the freezable fluid can be controlled to tune the density of the final foam. The dispersion may be prepared with the density of nanoscale metal wires tuned to a given application over a continuous range from 0.05 % to 50% by volume.

[0021] The outer surfaces of some metal nanowires may oxidize or have otherwise reacted with the environment during formation or in storage. Such contamination can be removed before the foam formation. For example, nanowires may be placed into a solution of an acid such an L- ascorbic acid to treat the wire surfaces and remove oxides and other contaminants before being placed in the lyophilization fluid.

[0022] The tuned dispersion of treated nanowires is then used to fill shaped molds for freezing. The molds can be essentially any shape and size. In another embodiment, the dispersion is dropped or injected directly into a cryogenic liquid such as liquid nitrogen to form spheres, cylinders, discs, cubes, rectangular prisms, sheets, or other forms.

[0023] The frozen molded forms are then placed into a vacuum chamber for lyophilization. The frozen liquid sublimates under controlled conditions leaving a structure of interlocked nanowires.

[0024] This structure of loosely interlocked nanowires is processed further to bond the points of contact between the nanowires to form the final foam without significant increase in density. The technology utilizes a sintering step, the oxidation and reduction or sintering of the nanowires to transform the foam from an interlocked structure (where the nanowires are touching) into an interconnected structure (where the nanowires are further bonded).

This greatly improves the strength of the material, allowing the formation of low and ultralow density metal foams at 0.05% of the bulk density (e.g., 5 mg/cm ³ Copper foams), which are useful for a far wider range of

applications than previously possible.

[0025] According to one aspect of the technology, interconnected pure

metallic foams with a wide range of tunable densities and characteristics is provided. Aspects of the final metal foam can be tuned by the selection of the length and the cross-sectional dimensions of the nanowire elements as well as the selection of the concentration or density of the nanowires in the nanowire dispersion.

[0026] Another aspect of the technology is to provide nanowire surface

treatments using L-ascorbic acid to help ensure that the nanowires remain sufficiently metallic, which facilitates the subsequent bonding processes during sintering.

[0027] An aspect of the technology is to provide a method for the creation of three dimensional, low and ultralow density foam structures through electrodeposition and a freeze-drying process [0028] A further aspect of the technology is the creation of interconnected pure metallic foams that have low density (down to 0.05% of its bulk density) and are still mechanically stable.

[0029] Another aspect of the technology is to provide a pure metal foam that is particularly suited for producing x-ray emissions from laser illumination. For example, metallic foam structures with densities from about 20 g/cm ³ to about 1 mg/cm ³ are particularly useful as targets for high energy density lasers to generate bright x-rays.

[0030] Further aspects of the technology described herein will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the technology without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0031] The technology described herein will be more fully understood by reference to the following drawings which are for illustrative purposes only:

[0032] FIG. 1 is a schematic flow diagram of a method of fabricating low density interconnected pure metal foams according to one embodiment of the technology.

[0033] FIG. 2 is a schematic flow diagram of an alternative embodiment of the method of fabrication of pure metal foams of interconnected nanowires according to the technology.

FIG. 3 is an indentation curve with loading and unloading portions for a 9 mg/cm ³ strengthened Cu cylindrical foam that was 2 mm in height and diameter. By measuring the initial slope of the unloading curve the modulus of the material can be extracted.

FIG. 4 is a relative modulus vs. relative density plot for various low density materials as compared to the foams of the present technology.

DETAILED DESCRIPTION

Referring more specifically to the drawings, for illustrative purposes, embodiments of systems and methods for producing low-density

interconnected metal foams are generally shown. Several embodiments of the technology are described generally in FIG. 1 through FIG. 4 to illustrate the fabrication system and methods. It will be appreciated that the methods may vary as to the specific steps and sequence and the systems and apparatus may vary as to structural details without departing from the basic concepts as disclosed herein. The method steps are merely exemplary of the order that these steps may occur. The steps may occur in any order that is desired, such that it still performs the goals of the claimed

technology.

[0035] Turning now to FIG. 1 , a flow diagram of one embodiment of a

method 10 for fabricating low density metal foams with controllable characteristics is shown. In the step at block 20, the fabrication process generally begins with the preparation of a dispersion of metal nanowires in a freezable liquid.

[0036] A variety of fluids can be used to prepare the dispersion at block 20 of FIG. 1 . The fluid that is used is preferably one that does not react with the type of metal forming the metal nanowires that have been selected for the foam. The preferred fluid will also freeze and subsequently sublimate under controlled conditions and otherwise be suitable for lyophilization processes. Preferred fluids also will exert limited stresses on the dispersed wires upon freezing. Deionized water is particularly preferred but other solvents or pure substances that have a triple point may also be suitable freezable fluids for wire dispersions, for example, liquid C0 ₂ , iodine, arsenic, naphthalene and ammonium chloride.

[0037] The metal nanowires that are provided at block 20 are preferably made from pure metals or metal alloys. Suitable metals for metal nanowire foams generally include Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one or more of these metals.

[0038] As used herein, the term "nanowire" generally refers to nanoscale structures that have at least one cross-sectional dimension that is less than 1 m, (i.e. in the nanoscale range), and preferably about 500 nm or less, and more preferably, 200 nm or less. Such nanoscale structures will typically have an aspect ratio of the length to the smallest cross-sectional dimension of greater than 1 , preferably, greater than 10, and in many cases, greater than 100. However, in some embodiments, the nanoscale structures are nanorods with length and diameter dimensions that produce aspect ratios of between 2 and 10. Accordingly, both the length and the cross-sectional dimensions of the nanowire element are variable.

[0039] The cross-section of the nanowire element may also have any

arbitrary shape, including circular, square, rectangular, tubular, or elliptical, and may have a regular or an irregular shape. The nanoscale wire element may be solid or hollow. The term nanowire is intended to include a variety of structure such as nanotubes, nanorods, nanowires and nanoribbons etc.

[0040] The dispersion that is prepared at block 20 also has a nanowire

density that can be selected. Wire density refers to number of nanowires per unit volume (e.g. pm ³ ). The wire density of the final foam is determined, in part, by the concentration of nanowires in the nanowire dispersion volume that is initially prepared.

[0041] The dispersion may be prepared with essentially any density of

nanoscale wires. However, the preferred volume ratio of the fluid in the dispersion vs. the nanowire volume, for example, ranges from 1 to 10,000.

[0042] Therefore, some important aspects of the final metal foam can be tuned by the selection of the length and the cross-sectional dimensions of the nanowire elements as well as the selection of the concentration or density of the nanowires in the dispersion that is provided at block 20 of the method of FIG. 1 .

[0043] The surfaces of the nanowires in the dispersion that is provided may be treated to ensure that the nanowires remain sufficiently metallic at block 30. This process is used to reduce any oxides that may be present in on the surfaces of the nanowires. Wires are then moved back into deionized water through fluid exchanges immediately prior to freezing at block 40 of FIG. 1 . [0044] For example, the nanowires may be placed into a solution of an acid such an L-ascorbic acid to treat the wire surfaces at block 30. For copper nanowires, the wires may be moved into 100g/L L-ascorbic acid, for example, where they are soaked for 48 hours in order to reduce any oxide in the copper to ensure the wires are pure copper during the foam formation process and to strengthen the van der Waals attractions between the wires during the foam formation step.

[0045] However, this L-ascorbic acid treatment may not be necessary for silver, palladium, gold or platinum wires since they do not readily oxidize. Instead, the nanowire surface treatment at block 30 may be the immersion of the nanowires into a volume of 0.25g/10mL polyvinyl-pyrrolidone (PVP) surfactant to separate the wires. The treated wires are then moved back to water through a fluid exchange process. Alternative chemicals for such surface treatment may include oxalic acid for oxide removal and surfactants are Cetrimonium bromide (CTAB), Sodium dodecyl sulfate (SDS), and polyvinyl alcohol (PVA) for nanowire separation.

[0046] Lyophilization techniques may be employed at block 40 and block 50 to turn the resulting ice-wire structure into a freestanding, low-density foam composed of interlocked nanowires. In order to create a nanowire structure from the resulting wire-water solution at block 30, the wires must undergo a freeze-drying process to avoid collapse from the surface tension of the water. Once the wire-water solution has been created, the density of the final foam structure can be adjusted by changing the water to wire ratio.

[0047] In one embodiment, the dispersion of treated nanowires and

freezable fluid may be placed into various shaped molds and frozen at block 40 of the method of FIG. 1. To make foams of selected shapes, thin copper molds (for high thermal conductivity) may be used to freeze nanowire solutions into the shapes of the molds.

[0048] To create spherical ice-wire structures and foams, wire-water

solutions can also be dropped directly into liquid nitrogen or other cryogenic liquid in the form of droplets and then frozen as spheres at block 40.

Pipettes of varying diameters can be used to adjust the diameters of the spherical ice-wire structures.

[0049] Lyophilization techniques that are employed at block 50 normally work by reducing the surrounding pressure of wire-liquid material frozen at block 40, for example in a vacuum, to allow the frozen water in the material to sublimate (i.e. move directly from the solid phase to the gas phase).

[0050] Commercial lyophilization devices generally use a four-step process:

1 ) pretreatment; 2) freezing; 3) primary drying; and 4) secondary drying. In the freezing step, the material is cooled to a temperature that is below the lowest temperature where the solid phase and liquid phase can coexist, called the triple point. This cooling ensures that the material will sublimate rather than melt during the drying steps.

[0051] In the primary drying phase, the pressure is lowered and the material may be heated to an optimal point where the frozen material sublimates efficiently. Typically approximately 95% of the water sublimates away in the primary drying phase.

[0052] During the secondary drying phase, the pressure may be lowered further and the material temperature may also be increased to facilitate desorption of remaining water molecules from the material.

[0053] The freeze-drying steps of block 50 of FIG. 1 leave a free-standing structure of interlocked nanowires. This interlocked structure is processed further at block 60 of FIG. 1 . The process changes the structure from a loosely interlocked lattice of wires into a single interconnected pure-metal foam without a substantial increase in density. At block 60, the ice-wire structure preferably undergoes several oxidation-reduction cycles to physically coalesce the wires together at the intersections to form the final foam.

[0054] Referring now to FIG. 2, a flow diagram of an alternative

embodiment of a method 70 for fabricating low density metal foams with controllable characteristics is shown. The method 70 described in FIG. 2 begins with the formation of a nanoporous membrane template at block 80. This membrane template has pores or channels with controllable

dimensions that provide a template for the creation of nanowires. Nanowires can be grown by electrodeposition into nanoporous templates of various sizes and types. The preferred template materials include anodized aluminum oxide (AAO) membranes, polycarbonate membranes, porous mica membranes or nanochannel glass templates, to lengths of between approximately 1 pm to 100 pm and diameters from approximately 10 nm to

1 ,000 nm.

[0055] At block 90, a working electrode is coated onto one side of the

starting membrane that was fabricated or provided at block 80. For example, a metal layer may be coated onto one side of the membrane by a variety of procedures such as by magnetron sputtering to cover the template pores and to be used as a working electrode during

electrodeposition.

[0056] Nanowires are then formed by electrodeposition in the membrane at block 100. One preferred electrodeposition process is carried out in a three-electrode cell with electrolytes tailored to the particular metal that is to be deposited in the pores and channels of the template.

[0057] After electrodeposition at block 100, the working electrode on the membrane template deposited at block 90 is removed at block 1 10. The working electrode may be removed by conventional etching procedures at block 1 10 that will dissolve the electrode.

[0058] The nanowires are then released from the membrane template by disintegration of the template at block 120. The nanowires can be separated from template material by both chemical and mechanical methods such as by etching and sonication.

[0059] At block 130 of FIG. 2, the released nanowires may be optionally agitated to separate and randomize the nanowires. It was found that during this template removal process the wires tended to clump together (long axes parallel to one another) due to van der Waals attractions between the wires. To disrupt this clumping, the containers could be shaken

aggressively by hand or sonicated.

[0060] The surfaces of the nanowires are then treated at block 140 to

eliminate the presence of any oxides or other materials that may appear on the surface of the nanowires during the fabrication steps. One surface treatment is with a mild acid such as ascorbic acid that will remove oxides formed on the surface. The nanowire surface treatments at block 140 may also be in the form of exposure to a surfactant.

[0061] The treated nanowires are then washed and dispersed in a suitable fluid for freeze drying (lyophilization) at block 150. Washing removes the unused surface treatments and other contaminants. The washing and dispersion step at block 150 can also include concentrating or diluting the nanowires to a desired density, which will influence the characteristics of the final foam that is produced.

[0062] At block 160 of the method of FIG. 2, the prepared dispersion of treated nanowires is then frozen. Optionally, the dispersion can be placed in shaped molds to produce foams with a selected shape. Spherical shaped foams can also be produced by placing drops of nanowire-fluid mixture into a cryogenic liquid such as liquid nitrogen.

[0063] Lyophilization of the frozen wire-fluid mixture at block 160 will

produce an interlocked nanowire structure at block 170. This Lyophilization procedure can also be performed in commercially available freeze-drying machines.

[0064] The interlocked wire structure that remains after the removal of the frozen fluid at block 170 is processed further to form an interconnected pure-metal foam at block 180 of the method of FIG. 2. The points of contact between nanowires of the interlocked nanowire structure are preferably sintered by heating the interlocked nanowire structure without melting the wires to the point of liquefaction or by exposing the structure to several oxidation-reduction cycles.

[0065] The technology described herein may be better understood with reference to the accompanying examples, which are intended for purposes of illustration only and should not be construed as in any sense limiting the scope of the technology described herein as defined in the claims appended hereto.

[0066] Example 1 [0067] In order to demonstrate the operational principles of the fabrication methods, several metal foams were prepared using the fabrication method shown schematically in FIG. 2. In this example, pure metal nanowire foams of Cu, Pd, Co and Ag were fabricated with a wide range of tunable densities.

[0068] Nanowires were grown by electrodeposition into nanoporous

templates of various sizes and types, including anodized aluminum oxide (AAO) membranes and polycarbonate membranes, to lengths of between 5 pm and 40 pm and diameters from about 50 nm to 200 nm.

[0069] Magnetron sputtering was used to coat thick conductive layers (200 nm to 500 nm) on the backsides of the membranes for use as working electrodes. For copper and cobalt nanowires, a 500 nm thick copper layer was coated onto the backside of the membrane to cover the template pores and to be used as a working electrode during electrodeposition. For silver, palladium, gold and platinum nanowires, a 500 nm gold working electrode was used instead.

[0070] These coated membranes were then placed into an

electrodeposition cell where metals were deposited potentiostatically into the pores to grow nanowires. The electrodeposition of wires was carried out in a three-electrode cell with suitable electrolytes. In all cases, a platinum counter electrode was used along with an Ag ⁺ /AgCI reference electrode.

[0071] Potential was pulsed to help ensure uniform deposition (for example, from 0 mV to -200 mV at one second intervals for copper deposition). The deposition current was monitored and growth was halted when an increase in current corresponding to the onset of over-deposition was detected.

Deposition potential was pulsed and electrolytes containing large amounts of metal ions were used to ensure deposition uniformity. For copper nanowires, the deposition was carried out with an electrolyte composed of 238 g/L copper sulfate and 21 g/L sulfuric acid. For silver nanowires the electrolyte was composed of 15.6 g/L silver sulfate and 224 g/L potassium thiocyanate. For palladium nanowires the deposition electrolyte was composed of 5 g/L PdCI ₂ and 10 g/L HCI. Finally, for cobalt nanowires the electrolyte was composed of 50 g/L cobalt sulfate and 40 g/L boric acid.

[0072] Following deposition, the working electrodes of the templates were removed by floating the wire-filled AAO (working electrode side down) in an etchant solution. For a copper working electrode, 7.9M nitric acid was used for 15 seconds. For a gold working electrode, a solution containing 10 ml_ of water, 4 grams of potassium iodide and 2 grams of iodine was used.

[0073] The wire-filled membrane was then thoroughly rinsed in deionized water. The template was then immersed in 6M NaOH and sonicated for 15 minutes to dissolve the AAO template and remove the nanowires. It was found that during this template removal process the wires tend to clump together (long axes parallel to one another) due to van der Waals

attractions between the nanowires. To prevent this clumping it was necessary to aggressively shake by hand the NaOH-nanowire solution periodically (e.g., at 5 minutes, 10 minutes and directly at the end of the sonication).

[0074] Following this process, multiple fluid exchanges were used to move the nanowires into deionized water. Average wire diameter of 150 nm and an average length was 20 pm were found using SEM.

[0075] For copper nanowires, the same exchange process was then used to transfer the wires into 100 g/L L-ascorbic acid where they were soaked for 48 hours in order to reduce any oxide in the copper to ensure the wires are pure copper to strengthen van der Waals attractions between the wires during the foam formation step. Wires are transferred back into deionized water through fluid exchanges immediately prior to foam formation. The Co, Ag and Pd wires were instead immersed in 250 g/L PVP to separate the wires, which were then transferred back to water through the fluid exchange process.

[0076] In order to create a nanowire foam from the resulting wire-water solution, the wires must undergo a freeze-drying process to avoid collapse from the surface tension of the water. Once the wire-water solution has been created, the density of the final foam can be adjusted by changing the wire-water ratio. Densities from 200 mg/cm ³ to 8 mg/cm ³ were achieved using this process. To create spherical foams, droplets of this solution were placed directly into liquid nitrogen and frozen as spheres. Pipettes of varying diameters were used to adjust the diameters of foams from 2 mm to 5 mm. Ice-wire spheres were then moved into small baskets composed of a single loop of thin (127pm diameter) wire to support the structure during freeze-drying. This wire was always made of the same material as the foam. To make foams of other shapes, thin copper molds (for high thermal conductivity) were used to freeze nanowire solutions into the shapes of the rectangular and cylindrical molds. Frozen foams were placed in rough vacuum (~10mTorr) in order to sublimate out the ice from the sample, leaving a free-standing nanowire structures of interlocked nanowires.

[0077] An additional process was used to strengthen the metal nanowire foams. Spherical and cylindrical foams were transported into a tube furnace and underwent oxidation-reduction cycles to physically coalesce wires together at intersections, changing the structure from a loosely interlocked lattice of wires into a single interconnected pure-metal foam without a significant increase in density. For copper nanowires, the foams were placed first in air at 300 °C for 20 minutes in order to oxidize them completely. Next they were reduced back to pure copper by putting them in a forming gas (5% hydrogen in nitrogen) for 20 minutes at 300 °C. Oxygen kinetics and copper atom mobility during these processes allow the foams to form into interconnected pure-metal structures.

[0078] Example 2

[0079] Nanoindentation experiments were carried out on the foams

produced with the methods shown in Example 1 to quantify the enhanced strength of the interconnected Cu nanowire foams. In order to perform nanoindentation measurements, cylindrical Cu foams with a 2 mm height and diameter were fabricated and strengthened using the

oxidation/reduction process. The final foams ranged in densities from 8 mg/cm ³ to 70 mg/cm ³ The indenter tip was a 2 mm ruby sphere and measurements were conducted at room temperature and ambient laboratory humidity. The loading and unloading rate was kept constant at 100 m/m. A sample loading/unloading curve is shown in FIG. 3.

[0080] Loading/unloading curves were derived for a 9 mg/cm ³ strengthened Cu cylindrical foam with 2 mm in height and diameter. By measuring the initial slope of the unloading curve it was possible to extract the modulus of the subject material. The elastic modulus of each sample was extracted from the initial slope of the unloading curve using the Oliver-Pharr multipoint unloading method. For a spherical indenter tip contacting a flat surface the Elastic Modulus, E ^r can be calculated as:

dP

where S is the unloading stiffness of the sample,— taken here as the slope of the first 15% of the unloading curve of each sample; A(h _contact is the area of contact between the indenter tip and the sample at the depth of penetration below the plane of contact:

A(h _C ontact) ⁼ 2nRh _contact ^{~ π} (.^-contact) ² and h _contact was calculated as:

3 P

— h

" ^■ contact — a- ^ ^ where P is the load on the sample and h is the depth of penetration.

[0081] FIG. 4 is a relative modulus vs. relative density plot for various low density materials. The graph of FIG. 4 includes the results of the Cu measurements as well as results from previous measurements of low and ultralow density materials.

[0082] Plotted here are the relative density (defined as the final density of the material normalized by the bulk density of the material from which they are constructed) and the relative modulus (the elastic modulus of the sample normalized by the bulk modulus of its constituent material) of such samples.

[0083] The results shown as open circles in FIG. 4 represent interlocked copper wire foams from previous results in the art for comparison. At 9 mg/cm ³ , for example, the prior work shows a modulus of about 5 Pa. At the same density, the interconnected foams structure made from the same material produced by the current methods was found to have a modulus of 1200 Pa, an improvement in elastic modulus of three orders of magnitude at the same density.

[0084] Also depicted in FIG. 4 are several other low and ultralow density materials whose modulus has been measured through nanoindentation for comparison. Using the methods of the present technology it was possible to create a pure metallic foam whose relative modulus is comparable to strong materials such as C and Si aerogels at the same relative densities. Note that at similar relative densities Ni microlattices have a much greater relative modulus than the interconnected Cu foams, but require an elaborate nanolithography process. The nanowire foams produced by the methods described herein can be fabricated over large areas and volumes much more easily and cost-effectively as the methods rely only on electrodeposition and the samples are fabricated inside of macroscopic molds.

[0085] Example 3

[0086] One potential use of ultralow density metal foams is for use as high energy density laser targets due to their ability to be heated volumetrically that allows targets composed of high-Z elements to uniformly reach the extreme temperatures that are required for X-ray emission. Gas targets, oxides, metal-doped aerogels and metal-lined cavities have been employed previously for reaching the necessary effective density. However, the pure metal targets allow higher X-ray conversion efficiencies if the targets can be fabricated at densities low enough to allow volumetric heating while still maintaining mechanical stability.

[0087] To demonstrate the functionality of pure metal foam targets

produced by the methods, spherical Cu targets (2 mm and 4 mm

diameters) and cylindrical Ag targets (4 mm diameter and length) were produced to be used as targets for testing at the OMEGA and the National Ignition Facility (NIF) laser facilities. The enhanced mechanical strength was critical to the use of such metal foams for use as x-ray emission materials.

[0088] At the OMEGA facility each of 40 beams delivered 475 J of 351 nm laser light to the spherical foam sample. Of these beams, 20 beams were incident on each hemisphere of the spherical Cu targets and were focused 600 pm inside the sample.

[0089] In the OMEGA demonstration, the volumetric heating of the Cu

foams allowed for substantially higher K-shell energy conversion efficiency than the solid copper reference, reaching a maximum of 2.59% for the Cu foam with a 22 mg/cm ³ density.

[0090] At the NIF facility all 192 beams were used, delivering a total of 500 kJ of energy to the sample with 351 nm laser light, and 96 beams were focused 550 m inside each hemisphere. Spherical samples were mounted on the end of small sticks with a support ring also made of AWG 36 copper (99.9%).

[0091] In the NIF demonstration, time-resolved X-ray emission data for energy integrated in the Cu K-shell was obtained from three different foam density samples. The results obtained from Cu foams with densities of 10 mg/cm ³ to 18 mg/cm ³ showed increasing conversion efficiency as the density was increased.

[0092] Accordingly, the pure Cu metal foams with density around 20

mg/cm ³ and below have been successfully shot at OMEGA and NIF facilities, where the laser energy has allowed the foams to be heated volumetrically and to produce high X-ray conversion efficiencies. This opens up an uncharted territory to explore high-Z plasma physics.

[0093] In summary, ultralow density Cu, Ag, Co and Pd pure metal foams have been fabricated using nanowire starting material (made by

electrodeposition or other methods) and a freeze-drying process. The densities of the foams can be tuned from as low as 8 mg/cm ³ , or 0.09% of the bulk density, to as high as 200 mg/cm ³ and beyond.

[0094] Furthermore, it was shown that the Cu nanowire foams can be greatly strengthened through an oxidation and reduction process by transforming a loosely interlocked structure into a truly interconnected nanowire monolith. The elastic modulus of such foams was found to increase by three orders of magnitude compared to that of an interlocked structure. Such mechanically stable pure metal foams open up a wide range of applications, including high energy density laser targets.

[0095] Embodiments of the present technology may be described herein with reference to flowchart illustrations of methods and systems according to embodiments of the technology, and/or procedures, algorithms, steps, operations, formulae, or other computational depictions, which may also be implemented as computer program products. In this regard, each block or step of a flowchart, and combinations of blocks (and/or steps) in a flowchart, as well as any procedure, algorithm, step, operation, formula, or computational depiction can be implemented by various means, such as hardware, firmware, and/or software including one or more computer program instructions embodied in computer-readable program code.

[0096] Accordingly, blocks of the flowcharts, and procedures, algorithms, steps, operations, formulae, or computational depictions described herein support combinations of means for performing the specified function(s), combinations of steps for performing the specified function(s), and computer program instructions, such as embodied in computer-readable program code logic means, for performing the specified function(s). It will also be understood that each block of the flowchart illustrations, as well as any procedures, algorithms, steps, operations, formulae, or computational depictions and combinations thereof described herein, can be implemented by special purpose hardware-based computer systems which perform the specified function(s) or step(s), or combinations of special purpose hardware and computer-readable program code.

[0097] From the description herein, it will be appreciated that that the

present disclosure encompasses multiple embodiments which include, but are not limited to, the following:

[0098] 1 . A method for fabricating low density and ultralow density nanostructured metal foams, the method comprising: (a) forming a dispersion of metal nanowires in a freezable fluid; (b) freezing the liquid dispersion to form an ice-nanowire structure; (c) sublimating the frozen liquid to expose a free standing nanowire foam; and (d) binding the nanowire foam structure at points of contact to form an interconnected metal foam monolith.

[0099] 2. The method of any preceding embodiment, further comprising: treating nanowire surfaces with an acid to remove oxide contaminants; and dispersing treated nanowires in a freezable liquid.

[00100] 3. The method of any preceding embodiment, further comprising: treating nanowire surfaces with a surfactant to separate nanowires from each other; and dispersing treated nanowires in a freezable liquid.

[00101] 4. The method of any preceding embodiment, wherein the metal nanowires are formed from a metal selected from the group of metals consisting of Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one or more of these metals.

[00102] 5. The method of any preceding embodiment, wherein the metal nanowires have an aspect ratio of length vs. diameter within the range of 2 to 1 ,000,000.

[00103] 6. The method of any preceding embodiment, wherein the

dispersion contains wires diluted in volume by a factor of 2 to 10,000.

[00104] 7. The method of any preceding embodiment, further comprising: depositing the liquid dispersion in one or more molds; and freezing the liquid dispersion in the molds.

[00105] 8. The method of any preceding embodiment, wherein the binding of points of contact of the ice-nanowire structure is performed by sintering.

[00106] 9. The method of any preceding embodiment, wherein the binding of points of contact of the ice-nanowire structure is performed by one or more oxidation and reduction cycle(s) performed at elevated temperatures.

[00107] 10. A method for fabricating low density and ultralow density

nanostructured metal foams, the method comprising: (a) preparing a nanoporous membrane template; (b) applying an electrode to one side of the nanoporous membrane template: (c) forming nanowires within the nanoporous membrane template by electrodeposition; (d) releasing the formed nanowires from the membrane template; (e) forming a dispersion of metal nanowires in a freezable fluid; (f) freezing the liquid dispersion; (g) sublimating the frozen liquid to expose an interlocked nanowire structure; and (h) binding the interlocked nanowire structure at points of contact between nanowires to form an interconnected metal foam.

[00108] 1 1 . The method of any preceding embodiment, wherein the

releasing of formed nanowires comprises: etching the membrane template to remove the electrode; disintegrating the membrane template to release nanowires; dispersing the released nanowires in a freezable fluid; and agitating released nanowires to separate and randomize nanowires in the freezable fluid.

[00109] 12. The method of any preceding embodiment, wherein the

nanoporous membrane template comprises an anodized aluminum oxide

(AAO) membrane, a polycarbonate membrane, a porous mica membrane or a nanochannel glass membrane.

[00110] 13. The method of any preceding embodiment, further comprising: treating nanowire surfaces with an acid to remove oxide contaminants; and dispersing treated nanowires in a freezable liquid.

[00111] 14. The method of any preceding embodiment, further comprising: treating nanowire surfaces with a surfactant to separate nanowires from each other; and dispersing treated nanowires in a freezable liquid.

[00112] 15. The method of any preceding embodiment, further comprising: depositing the liquid dispersion in one or more molds; and freezing the liquid dispersion in the molds.

[00113] 16. The method of any preceding embodiment, wherein the metal nanowires are formed from a metal selected from the group of metals consisting of Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh,

Pd, Ag, Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one or more of these metals. [00114] 17. The method of any preceding embodiment, wherein the metal nanowires have an aspect ratio within the range of 2 to 1 ,000,000.

[00115] 18. The method of any preceding embodiment, wherein the

dispersion wherein the dispersion contains wires diluted in volume by a factor of 2 to 10,000.

[00116] 19. The method of any preceding embodiment, wherein the binding of points of contact of the ice-nanowire structure is performed by sintering.

[00117] 20. The method of any preceding embodiment, wherein the binding of points of contact of the ice-nanowire structure is performed by one or more oxidation and reduction cycle(s) performed at elevated temperatures.

[00118] 21 . A metallic foam structure comprising; (a) an interconnected

metallic nanowire network; (b) wherein the network structure has a density of about 0.1 % of the bulk density.

[00119] 22. The metallic foam structure of any preceding embodiment,

wherein the network structure has a density from about 20 g/cm ³ to about 1 mg/cm ³ .

[00120] 23. The method of any preceding embodiment, wherein the metal nanowire network is formed from one or more metals selected from the group of metals consisting of Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one or more of these metals.

[00121] Although the description herein contains many details, these should not be construed as limiting the scope of the disclosure but as merely providing illustrations of some of the presently preferred embodiments. Therefore, it will be appreciated that the scope of the disclosure fully encompasses other embodiments which may become obvious to those skilled in the art.

[00122] In the claims, reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural, chemical, and functional equivalents to the elements of the disclosed embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed as a "means plus function" element unless the element is expressly recited using the phrase "means for". No claim element herein is to be construed as a "step plus function" element unless the element is expressly recited using the phrase "step for".