Metal based hierarchical foams with zones of different porosities

Abstract

Method of manufacturing a metal-based foam body (7), comprising steps of: - preparing at least a first metal-based foam precursor slurry; - foaming said precursor slurry to form a first wet foam having a first bubble size distribution; - depositing and shaping said first wet foam; - forming a second wet foam (3) which differs from said first wet foam (1) by at least one of said bubble size distribution or chemical composition; - depositing and shaping said second wet foam (3) on said first wet foam (1); - drying said wet foams to form a greenbody (5); - thermally treating said greenbody (5) to form said metal-based foam body (7).

Description

DescriptionMETAL-BASED HIERARCHICAL FOAMS WITH A ZONES OF DIFFERENT POROSITIESTechnical Field

[0001] The present invention relates to metal-based foams. More particularly, it relates to metal, metal alloy or metal oxide foams with zones of different porosities.State of the Art

[0002] Metal-based foams show much promise for various applications such as thermal management (heat exchangers, cooling systems, heat storage), catalysts (e.g. for electrolysis for green hydrogen production, heterogenous catalysis, carbon dioxide electrolysis etc.), battery materials (e.g. current collectors, electrodes), filtration systems, sound dampeners, wave shielding materials or similar, due to their extremely high surface area to volume ratios. These foams may be made of single-metal metallic, metal alloys, or metal oxides, these latter typically being initially formed as a metal foam or metal alloy foam which is subsequently oxidised in order to convert the surface, the entire volume, or part of the volume to oxide.

[0003] Conventionally, porous metal-based foams are made using one of the following methods:

[0004] - Melt route, which is commonly used, in which liquid metals are foamed either by gas injection, addition of blowing agents, dissolved gas etc. In some cases, the melted metal is used in combination with space holders, e.g., salt or polymer beads, or with templates, typically a polymeric foam, to create the air bubbles, these space holders or templates being subsequently removed.

[0005] - Powder route, in which metallic powders are processed to form the foams through solid state processes such as sintering. Blowing agents, space holders, or gas entrapment are typically used to create open cell metal foams.

[0006] - Metal vapor deposition, typically used on a sacrificial polymeric foam template which is subsequently removed.

[0007] - Electrochemical deposition, also using a sacrificial polymeric foam template which is removed therefrom.

[0008] However, with these approaches, hierarchical porosities are very difficult, or even impossible, to obtain.

[0009] An improvement to these processes is disclosed in document WO2023 / 161285, which describes a process of manufacture of metal foams with an intrinsic hierarchical pore structure, this process comprising the following steps:1 . Preparing a slurry made of a solvent (generally water), metal-based particles, and additives such as surfactants or polymers;2. Foaming the slurry through mechanical frothing, gas foaming or similar;3. Shaping the prepared foam into so-called “greenbodies” (sheets, cylinders, disks, rings, arbitrary shapes etc.) e.g. by moulding, tapecasting, or 3D printing;4. Drying said greenbodies;5. Thermally treating the shaped foam to finalize the structure of the foams, e.g. by thermal reduction and optionally also sintering.6. Post processing to the precise desired final shape, e.g. by milling, drilling holes, lathing or similar, and / or assembly with other parts, e.g. by welding, glueing, soldering, brazing or similar.

[0010] Further details of a particular implementation of this process are described in 3D Printing of Hierarchical Porous Steel and Iron-Based Materials, Adv. Mater. Technol. 2023, 8, 2200971 , Carpenter et al (hereinafter “Carpenter et al, 2023”).

[0011] Various metal foams can thus be produced, e.g. iron, stainless steel, copper, nickel, titanium, vanadium, tungsten, etc. An intrinsic property of this method is that it obtains a hierarchical porosity of the foam, since air bubbles in the foaming step create larger pores, called “cells”, typically with dimensions in the 20 pm to 2000 pm range, then the thermal treatment and / or sintering process creates wall pores in micron range (typically 1-2 pm) in the walls ofthe cells, as well as inter-cell pores that open from one cell into an adjacent cell and have a size range from several microns to 100s of microns, typically in the range of 0.05 to 0.5 times the cell size.

[0012] However, this process is limited to homogeneous pore structures and materials for any given metal-based foam body produced by this method.

[0013] In many applications, different pore structures and / or foam materials are desirable at different positions in the foam body, and the typical solution is to produce substantially homogeneous sub-elements, which are then stacked and unified into the final element by pressing, sintering or welding (e.g. friction, sonic, laser welding etc.). However, welding severely damages, or even destroys, the structure of the foam at the welded interface, and if the interface is only partially welded to minimise this phenomenon, the unwelded contact portions have a material discontinuity, and are hence not in perfectly intimate contact. Also, pressing and sintering also tend to severely damage the porous structure at the interface. This can have negative effects which vary depending on the application, e.g. reducing heat transfer, solvent flow, gas escape etc., as may be the case in question. Furthermore, these conventional processes for creating porosity and / or material gradients are limited to discrete gradients, and by the shapes of the sub-elements that can be combined to make the body, since this must occur with the sub-elements in their finished state.

[0014] KR 2020 0036380 discloses a non-hierarchically-porous metal body with zones of different porosities, manufactured by a method which globally involves forming a first greenbody from a first slurry comprising metal particles which is shaped and dried to form this greenbody, and then depositing the second slurry thereupon, which is then also dried. The entire, combined greenbody is then heat treated. The interface between the two zones is well-defined and not seamless, since the first greenbody was dry before the second slurry is placed thereupon, which is not optimal for mechanical strength or other properties where the nature of the interface has an impact. US 2011 / 055694 uses a similar principle, where a second slurry is applied to a first greenbody tape, giving the same drawbacks and furthermore only being suitable for a continuous tape application.

[0015] US 2013 / 231754 discloses a porous metal body with zones of different porosities, made by forming two green bodies from expandable slurries, which are either sintered together (e.g.

[0070] ), or bonded, e.g. by diffusion bonding (

[0080] ). This results in two clearly-defined zones with a sharp seam or transition between them, which far from optimal for mechanical strength or other properties where the nature of the interface has an impact

[0016] The aim of the invention is hence to at least partially overcome the above- mentioned drawbacks of the prior art.Disclosure of the invention

[0017] More precisely, the invention relates to a method of manufacturing a metalbased foam body, as defined in claim 1. The term “metal-based” is used in the chemical sense, covering not only native metal, but also metal-based compounds such as but not limited to metal oxides, nitrides, carbonitrides or carbides.

[0018] The method of the invention comprises steps of:

[0019] - preparing at least a first metal-based foam precursor slurry, e.g. following the teaching of WO2023 / 161285;

[0020] - foaming said precursor slurry to form a first wet foam having a first bubble size distribution, e.g. by mechanical whipping, gas injection, chemical gas foaming or similar;

[0021] - depositing and shaping said first wet foam, e.g. manually, with a 3D printer, tape caster or similar;

[0022] - forming a second wet foam which differs from said first wet foam by at least one of said bubble size distribution or chemical composition, either by foaming the same precursor slurry to a different consistency or by preparing a different precursor slurry and foaming it;

[0023] - depositing and shaping said second wet foam on said first wet foam, i.e. the two wet foams are brought together in a wet state such that they are in intimate contact with each other;

[0024] - drying said wet foams to form a greenbody, i.e. the two wet foams are dried together to form a single greenbody;

[0025] - thermally treating said greenbody to form said metal-based foam body, this thermal treatment typically involving sintering and / or reduction.

[0026] This process results in a metal-based foam body with hierarchical porosity comprising a major porosity and at least one minor porosity, the body comprising at least two zones of differing major porosity and / or chemical composition, with a smooth, seamless transition between the two zones with no disruption of the porous structure, since the bubbles of the two different wet foams self-organise at the interface by surface tension and hence leave a coherent structural transition without damage, disruption, voids or similar. This transition is within the bulk of the material (i.e. not exclusively at the surface), and is not damaged by welding, pressing or similar, resulting in an open-pore structure that is continuous yet has a change in major porosity and / or chemical composition. This enables complex metal-based foam structures with various zones with different properties (chemical, thermal, electrical etc.) to be created in order to optimise the material for e.g. catalysis, electrolysis, heat exchange, electrical conductivity in battery current collectors, etc., with a strong bond and intimate transition at the interface.

[0027] Advantageously, neither the first nor the second wet foam expands after deposition. This ensures excellent control of the shape of the deposited foam, which is fully foamed prior to deposition.

[0028] In one variant, said second wet foam has the same chemical composition as said first wet foam and a different bubble size distribution. This results in zones of different major porosity for a unitary chemical composition. However, different chemical compositions for the zones are also possible.

[0029] Typically, said one or more metals comprise one or more of copper, iron, nickel, titanium, vanadium, tungsten, silver, palladium, stainless steel, nickeliron, copper-iron. These may be in their native form, or in compound form such as in an oxide, nitride, carbonitride or carbide form. In the case of an oxide, nitride, carbonitride or carbide, this may either be a surface layer of a predetermined depth, or the entire structure of the material, and may result after the thermal treatment step or after a later oxidation step. Preferably, copper and / or nickel are the favoured two metals, used either singly or in combination.

[0030] Advantageously, a step of surface treating said metal-based foam body may be carried out after the thermal treatment step resulting in conversion of said greenbody to said metal-based foam body.

[0031] For instance, this surface treatment may comprise one or more of oxidation, deposition, or providing a coating of a material such as a hydrophobic material, anticorrosion material, thermally-conductive material, electrically- conductive material, catalyst layer, passivation or similar, which may be deposited e.g. via physical vapour deposition, chemical vapour deposition, or a wet deposition process depending on the nature of the coating and as appropriate.

[0032] Furthermore, the invention relates to a metal-based foam porous material comprising at least a first zone and a second zone within the bulk of said material, i.e. considered at a distance from the surface rather than at the surface itself, each zone having a chemical composition and a hierarchical porosity with at least a major porosity (i.e. major pores) and a minor porosity (i.e. one or more minor pores, smaller than said major pores), wherein said first zone and said second zone differ by one or more of:

[0033] - the size distribution of the pores of the major porosity,

[0034] - chemical composition,

[0035] wherein a transition between said first zone and said second zone is situated within the bulk of said material and is seamless. In other words, there is no “hard” transition where the pore structure of the material is damaged or otherwise interfered with by a joining process such as welding, pressing, or similar. This material may be manufacturable, or indeed may be manufactured by, the method described above.

[0036] As a result, the material has an open-pore structure that is continuous yet has a change in major porosity and / or chemical composition. This enables complex metal-based foam structures with various zones having various properties (chemical, thermal, electrical etc.) to be optimises for use in e.g. catalysis, electrolysis, heat exchange, electrical conductivity in battery current collectors, etc.

[0037] In a variant, said second zone has the same chemical composition as said first zone and a different major pore size distribution. However, different chemical compositions for each zone are also possible.

[0038] Typically, said one or more metals comprise one or more of copper, iron, nickel, titanium, vanadium, tungsten, silver, palladium, stainless steel, nickeliron, copper-iron. These may be in their native form, or in compound form such as in an oxide form. In the case of an oxide, this may either be a surface layer of a predetermined depth, or the entire structure of the material. In other words, the metal-based foam may comprise or consist of a metal oxide. Copper and nickel, used individually or in combination, are the preferred metals.

[0039] Advantageously, said metal-based foam may be provided with a surface treatment, such as an oxidation, nitriding, carbonitriding or carburization treatment, a coating of a material such as a hydrophobic material or similar.Brief description of the drawings

[0040] Further details of the invention will appear more clearly upon reading the detailed description, in reference to the appended figures which illustrate:- Figure 1 : a schematic block diagram of a first variant of a method according to the invention;- Figure 2: a schematic block diagram of a second variant of a method according to the invention;- Figure 3: schematic illustrations of variations of metal-based foam bodies which can be produced by the methods of the invention;- Figure 4: scanning electron micrographs of the pore structure of the types of metal-based foams used in the invention, as described in WO2023 / 161285;- Figures 5-7: scanning electron micrographs of broken-open samples of metal-based foams produced by the methods of the invention;- Figure 8: scanning electron micrographs of various regions of the sample of figure 7; and- Figure 9: histograms of pore diameters for each of the parts of the samples identified with A, B and C on Figure 8;- Figure 10: an optical image of a copper-nickel sample of a metalbased foam according to the invention;- Figure 11 : a scanning electron micrograph of a copper-nickel sample of a metal-based foam according to the invention.Embodiments of the invention

[0041] The present invention leverages the method of manufacturing metal-based foams described in WO2023 / 161285 and Carpenter et al, 2023, both herein incorporated by reference in their entireties, in order to create new metalbased foams with have an intrinsic hierarchical porosity, namely at least two, preferably at least three, different porosities incorporated in bulk of the same material structure, and also an easily-manipulable gradient of porosity and / or chemical composition of the metal-based foam along any chosen axis. In other words, the hierarchical porosity comprises a major porosity, i.e. the porosity with the largest average pore size, and at least one minor porosity, with a smaller average pore size than the major porosity.

[0042] This allows optimising the properties of the resulting metal-based foam body to the desired application, for instance one of the applications mentioned in the introduction, or any other application.

[0043] The metal-based foam material can be made of one or more metals (e.g. copper, iron, nickel, titanium, vanadium, tungsten, silver, palladium, etc.), one or more metal alloys (e.g. stainless steel, nickel-iron, copper-iron, etc...), metal oxides, nitrides, carbides, carbonitrides etc., or any combination thereof, and may also be coated post-process with other metals, metal oxides or similar. It should be noted that the term “metal-based” is used in the chemical sense, and covers not only native metals and alloys, but also metalbased oxides, nitrides, carbides, carbonitrides etc..

[0044] Figure 1 illustrates a first variant of a method according to the invention.

[0045] In step 101 , a first precursor slurry is formed as described in WO2023 / 161285 and Carpenter et al, 2023. A non-limiting example of forming this slurry is taken from Carpenter et al, 2023, in which an aqueous suspension of 40-50 wt% metal oxide powder (Magnetite, FesO4, E8840,DOWA; Nickel(ll)oxide green, 99%, abcr GmbH) and 3 pL g-1of hexylamine (99%, ACROS Organics) was homogenized by ball-milling with alumina balls (1-5 mm diameter) in a THINKY mixer (ARE-250) at 2000 rpm for 6 min (in two 3-min increments to avoid overheating). For compositions with iron oxide particles, the powder concentrations of 40-50 wt% correspond to volume fractions of 12.9-19.4 vol%. The standard composition contained 50 wt% magnetite powder. More generally, the slurry is prepared with metal-based particles (metallic particles, metal compounds such as medal oxides etc.), additives, and solvent (e.g. water).

[0046] In step 103, this precursor slurry is foamed by any convenient means, to form a first wet form with a first bubble size distribution. Typically, this is by mechanical foaming, by whipping the slurry with air, nitrogen, argon or any other convenient gas, to reach the desired consistency which corresponds to a desired size of bubbles in the first wet foam. Again, in Carpenter et al, 2023, the slurry was foamed at 800 rpm with an electrical stirrer (Heidolph, RZR 2102) until the desired stiffness was achieved (5-15 min). The resulting foam was left to rest overnight in a sealed container.

[0047] Alternatively, the foaming can take place in a foaming nozzle of a 3D printer, tape casting device or similar immediately prior to deposition and shaping in step 105. In such a case, air or another gas is injected directly into the slurry in or before the nozzle. Further alternatively, gas foaming can be performed by combining the slurry with additives which react and produce bubbles of gas in-situ, whether in a deposition nozzle or in bulk prior to deposition.

[0048] In step 105, the first wet foam 1 is deposited and shaped, e.g. by moulding, 3D printing, extrusion, tape casting, depositing and shaping by hand or similar, on a support such as a flat substrate, a mould, or any other suitable surface. Again, in the example of Carpenter et al, 2003, the first wet foam was printed volumetrically with a modified fused-filament-fabrication (FFF) printer (Ultimaker 2+) using 20 mL syringes and conical nozzles (0.84 mm diameter). Print speed was set at 10 mm s-1and the extrusion rate was adjusted manually (100%— 150%) to achieve maximum print quality. PMMA plates coated with commercial skin cream (NIVEA) to prevent sample adhesion were used as substrates.

[0049] In step 107, a second wet foam 3 is prepared from the first precursor slurry, e.g. by foaming it under different parameters to a different consistency than the first wet foam so as to have a different bubble size distribution, in particular a different average (mean or median) bubble size, which may be larger or smaller than those of the first wet foam. Characterisation of the bubble sizes is usually performed by analogy to the consistency of whipped food products, such as chocolate mousse, whipped egg white, raw meringue or similar.

[0050] In step 109, the second wet foam 3 is deposited and shaped upon the previously deposited and shaped first wet foam 1 , and the same deposition and shaping considerations as discussed for the first wet foam 1 apply equally for the second wet foam 3. In other words, the second wet foam 3 is deposited directly on the first wet foam 1 , in intimate contact therewith, while they are both wet (hence the terminology “wet foam”).

[0051] Further zones of first and / or second and / or further wet foams may also be built up, along any given axis, as desired.

[0052] Subsequently, in step 111 , the wet foams 1 , 3 are dried to form a greenbody 5. This can be carried out either at room temperature for a sufficiently long period of time, or e.g. in an oven at 60°C for 1 hour. This greenbody 5 comprises a first greenbody zone 5a in the bulk of the greenbody 5, resulting from the first wet foam 1 after it has dried, and a second greenbody zone 5b in the bulk of the greenbody 5, resulting from the second wet foam 3 after it has dried. If more layers of wet foam were used, this will result in more greenbody zones.

[0053] In step 113, a thermal treatment is carried out in order to convert the greenbody into a metal-based foam body 7. Depending on the composition of the slurry and the material of the body 7, this may involve thermal reduction and / or sintering. In Carpenter et al, 2023, the greenbody 5 was reduced in a quartz tube oven (Gero, SR-A 100-500 / 12) with 12 L h-1forming gas flow (95% N2 and 5% H2, Pangas), with various heating protocols being used to investigate the effect of reduction and sintering conditions on the microstructure and properties of the resulting porous metals.

[0054] This thermal treatment typically results in shrinkage of the structure.

[0055] This results in a body 7 comprising zones 7a, 7b of different porosities, i.e. different pore sizes, particularly different major pore sizes, as will be discussed more in detail below, each of the zones resulting from a corresponding greenbody zone 5a, 5b again corresponding to each zone of deposited wet foam 1 , 3. The interface between the two zones 7a, 7b is within the bulk of the metal-based foam body 7, and should not be conflated with a slightly different surface porosity due to the drying process or other process parameters.

[0056] In optional step 115, further treatment can be carried out, such as, but not limited to:

[0057] - surface or complete oxidation, nitriding, carbonitriding or carburizing of the body 7;

[0058] - surface treatment and / or functionalisation, e.g. by applying a hydrophobic coating, anticorrosion material, thermally-conductive material, electrically- conductive material, catalyst layer, passivation, or similar.

[0059] Figure 2 illustrates a further method according to the invention, which will be described in terms of how it differs from the method of figure 1 .

[0060] Whereas in the method of figure 1 the second wet foam 3 has the same chemical composition as the first wet foam 1 , in the method of figure 2, second wet foam 3 has a different chemical composition to first wet foam 3, and is hence produced from a second precursor slurry prepared in step 106 which differs chemically from said first precursor slurry. For instance, the difference may be in the type of metal compounds present in the second precursor slurry, such that the resulting metal-based foam body 7 comprises zones of different metals or metal compounds chosen from those discussed above. Furthermore, the second wet foam 3 may also differ from the first wet foam 1 in terms of its major pore size, and hence the zones of the resulting body 7 may also contain zones 7a, 7b of different porosities. In this case, pairs of metals are chosen which can be sintered at the same temperature, which is easy to achieve since the thermal treatment temperature ranges for many suitable metals have significant overlap. For instance, a temperature of around 400-800 °C (more generally 250-1100 °C) is suitable for copper,nickel, iron, tungsten, silver, palladium, nickel-iron and copper-iron, whereas a higher temperature over 1000 °C is typically indicated for stainless steel and titanium. In any case, the range of possible temperature is quite large, particularly in combination with varying hydrogen concentration (with increasing hydrogen concentration permitting lower temperatures), provided that the melting temperature of the metals concerned is not exceeded. The skilled person can determine which metals and / or alloys are compatible at what temperature and hydrogen concentration ranges by routine experimentation.

[0061] In all of the above cases, since the two wet foams 1 , 3 are shaped together in a wet state, the interface between each foam 1 , 3 is intimate, with the wet foams 1 , 3 being in direct contact with each other. The bubbles in each of the wet foam organise themselves (i.e. self-organise) at the interface by surface tension, irrespective of any size difference in the bubbles, such that the transition from one wet foam to the next is seamless, without damage, voids or other similar issues. This seamless transition survives the drying 111 and thermal treatment 113 steps. Once the greenbody 5 is thermally treated in step 113, the resulting metal-based foam body 7 is of fully unitary construction, interfaces between the zones 7a, 7b being fully intimate and without disruption due to a joining process. The pore structure of the material is hence undamaged and undisrupted at these interfaces, which is not possible to achieve with prior art methods where different sub-elements are produced and joined together either at the greenbody stage or after the thermal treatment stage, or by application of a second wet foam onto a greenbody.

[0062] Figure 3 illustrates schematically several non-limiting examples of crosssections of metal-based foam bodies 7 exhibiting zones of various different major pore sizes and / or chemical compositions of the metal-based foam, the various zones being represented by different densities of textures. From left to right, the structures exhibit:

[0063] - a parallelopiped body 7 comprising zones 7a-7f of increasing major pore size from bottom to top;

[0064] - a parallelopiped body 7 comprising a thicker zone 7b of lower major pore size sandwiched between two thinner zones 7c of higher pore size;

[0065] - a parallelopiped body 7 comprising alternating zones 7a-7f of two different major pore sizes and / or chemical compositions (such as different metals, alloys, metal oxides or similar);

[0066] - a cylindrical body 7 comprising non-concentric zones 7a-7c of different major pore sizes and / or chemical compositions (such as different metals, alloys, metal oxides or similar).

[0067] However, the possibilities in terms of the shape of the body, distribution of the various zones etc. is essentially unlimited, within the boundaries of what is possible in manipulating and moulding the wet foams 1 , 2, and introducing them into moulds, onto supports etc.

[0068] The methods according to the invention and various possibilities for the resulting bodies 7 having now been described schematically, certain experimental results will be presented in reference to figures 4-11 .

[0069] Figure 4 is a pair of scanning electron micrographs illustrating the intrinsic hierarchical porous structure of metal-based foams obtained by the process of WO2023 / 161285 and exploited by the present invention, the left image representing the structure within the bulk of the material, the right image representing the outer surface. The horizontal bars represent the measurements of various pores representative of the various associated porosities. The exact material illustrated in figure 4 is an iron foam, produced by the method of Carpenter et al, 2003, the different bubble, and hence pore, sizes being prepared by frothing the slurry at different rotational speeds.

[0070] As can clearly be seen, the major pore structure is the cells, which have a size in function of the size of the gas bubbles in the wet foams, taking into account any shrinkage which occurs during drying and / or thermal treatment steps 111 , 113. The largest dimension of the cells, i.e. the major pore size of the cells, is typically between 20 pm and 2000 pm.

[0071] The second largest pores in the hierarchy, i.e. the first minor pores, are the inter-cell pores, which pass through the walls of certain cells, thereby joining the free space within these adjacent cells. These are typically 0.05 to 0.5 times the major size of the cells.

[0072] The third pores, i.e. the second minor pores, are the smallest in the hierarchy, are the wall pores, which are formed in the surface of the cells and on the outer surface of the body 7. These typically have maximum dimensions in the region of 1 pm to 2 pm.

[0073] The various pore dimensions may, of course, also be affected by any oxidation, nitriding, carbonitriding, carburization or surface treatments carried out in optional step 115.

[0074] In view of this, it can clearly be seen that the metal-based foam has an opencell structure with a very large surface area to volume ratio, typically 0.2 to 10 km2 / kg, more particularly around 0.5 to 2 km2 / kg, typically around 1-2 km2 / kg, though this may of course vary.

[0075] Figure 5 illustrates examples of metal-based foam bodies 7 made of copper foam via the method of figure 1 , that displays zones 7a, 7b with two different average major (i.e. cell) pore sizes. Two foamed layers were stacked immediately after foaming the first precursor slurry, then dried, and sintered. Body 7 was then split open in order to image it with a scanning electron microscope, the clear interface between the two zones 7a, 7b of different major porosities resulting from the different bubble sizes in the first and second wet foams being indicated with white chain lines. As can clearly be seen, the interconnectivity of the two said zones is high, the porosity is completely open, and the transition is seamless with no anomalies in the structure (such as undesired voids) due to the bubbles in each of the two wet foams self-organising together as a result of surface tension. The mechanical, thermal and other properties at the transition are hence excellent. Each zone 7a, 7b has an intrinsic hierarchical porosity, similar to that illustrated in figure 4.

[0076] Figure 7 illustrates a particularly clear example of the interface between zones 7a, 7b of different porosity of a copper foam. Wall pores of a few pmdiameters are present in the walls, as well as inter-cell pores that connect the cells. This hierarchical porosity creates an ultra-high surface area (several millions of m2 / m3) that is useful for catalysis, electrolysis, heat exchange, and electrical conductivity in battery current collectors.

[0077] Figure 8 illustrates SEM micrographs of parts of the structure of figure 7, with figure 9 illustrating histograms showing the measured cell diameter ranges in the zones indicated with A, B and C on figure 8. As can clearly be seen, the different major pore diameters are clearly measurable, illustrating also how the method of the invention leaves measurable features in the finished body 7.

[0078] Figures 10 and 11 illustrate sectioned hierarchically-porous metal foam bodies 7, in which the zones 7a, 7b not only have different porosities, but also have different chemical compositions. In each case, first zone 7a is of copper in its native form, with a smaller porosity, and second zone 7b is of nickel in its native form, with a larger porosity.

[0079] The first zone 7a was formed as follows. An aqueous suspension comprising copper oxide microparticles at 40-70 wt% and surfactants and dispersants at global concentration 1-4 mg.g'1was prepared by mixing all the components. In other embodiments, the microparticles could alternatively have been be nickel and nickel oxide, iron and iron oxide, tungsten and tungsten oxide, zinc and zinc oxide, molybdenum and molybdenum oxide, or a mixture (e.g., iron, chromium, nickel, and molybdenum).

[0080] The second zone 7b was formed as follows. An aqueous suspension comprising nickel oxide microparticles at 40-70 wt% and surfactants and dispersants at global concentration 1-4 mg.g'1was prepared by mixing all the components. Again, in alternative embodiments, the microparticles could have been copper, nickel and nickel oxide, iron and iron oxide, tungsten and tungsten oxide, zinc and zinc oxide, molybdenum and molybdenum oxide, or a mixture (e.g., iron, chromium, nickel, and molybdenum)

[0081] Both slurries were foamed independently by introducing air in the slurry. Typically, this is by mechanical foaming, by whipping the slurry with air, nitrogen, argon or any other convenient gas, to reach the desired consistencywhich corresponds to a desired size of bubbles in the wet foam. The slurries here were foamed at 800-1200 rpm with an electrical stirrer (Heidolph, RZR 2102) until the desired stiffness was achieved (5-15min). The pore size and porosity can be tuned during that process, but pore sizes are typically distributed around 50-150 pm and porosity between 65% and 95%.

[0082] The resulting foams were then placed into a plastic mould, overlayed one on top of the other, and dried in ambient atmosphere to obtain a greenbody comprising two different layers of different composition. They can be moulded, cast, sprayed, 3D printed, blade-coated, bar-coated and other means of obtaining a suitable shape, as discussed above.

[0083] The greenbody was then thermally treated at 600 to 700 °C (but more generally this can be from 250 to 1100°C depending on composition of the two foams and hydrogen concentration), in reducing atmosphere which in this case was forming gas with 2 to 100% H2 in N2, flow rate 2 to 12 L / min, (more generally 2-10%, at a flow rate of 0.5 to 30 L / min) for 3 to 30h.

[0084] As can clearly be seen, the interface between the two zones 7a, 7b is absolutely seamless, despite not only the difference in porosity but also in chemical composition.

[0085] Although the invention has been described in terms of specific embodiments, variations thereto are possible without departing from the scope of the invention as defined by the appended claims.

Claims

Claims1. Method of manufacturing a metal-based foam body (7), comprising steps of:- preparing at least a first metal-based foam precursor slurry;- foaming said precursor slurry to form a first wet foam having a first bubble size distribution;- depositing and shaping said first wet foam;- forming a second wet foam (3) which differs from said first wet foam (1) by at least one of said bubble size distribution or chemical composition;- depositing and shaping said second wet foam (3) on said first wet foam (1);- drying said wet foams to form a greenbody (5);- thermally treating said greenbody (5) to form said metal-based foam body (7).

2. Method according to the preceding claim, wherein said first wet foam and said second wet foam do not expand after deposition.

3. Method according to any preceding claim, wherein said second wet foam (3) has the same chemical composition as said first wet foam and a different bubble size distribution.

4. Method according to any preceding claim, wherein said one or more metals comprise one or more of copper, iron, nickel, titanium, vanadium, tungsten, silver, palladium, stainless steel, nickel-iron, copper-iron.

5. Method according to any preceding claim, wherein said metal-based foam (7) comprises or consists of a metal oxide.

6. Method according to any preceding claim, further comprising a step of surface or complete treating said metal-based foam body (7).

7. Method according to the preceding claim, wherein said step of surface or complete treating said metal-based foam body (7) comprises at least one of:- oxidation;- nitriding;- carbonitriding;- carburizing;- deposition;- coating.

8. Metal-based foam porous material (7) comprising at least a first zone (7a) and a second zone (7b) within the bulk of said material, each zone (7a, 7b) having a chemical composition and a hierarchical porosity with at least a major porosity and a minor porosity, wherein said first zone (7a) and said second zone (7b) differ by one or more of:- the size distribution of the pores of the major porosity,- chemical composition, wherein a transition between said first zone (7a) and said second zone (7b) is situated within the bulk of said material (7) and is seamless.

9. Metal-based foam porous material (7) according to the preceding claim, wherein said second zone (7b) has the same chemical composition as said first zone and a different major pore size distribution.

10. Metal-based foam porous material (7) according to any of claims 8-9, wherein said one or more metals comprise one or more of copper, iron, nickel, titanium, vanadium, tungsten, silver, palladium, stainless steel, nickel-iron, copper-iron.

11. Metal-based foam porous material (7) according to any of claims 8-9, wherein said metal-based foam comprises or consists of a metal oxide, a metal nitride, a metal carbonitride or a metal carbide.

12. Metal-based foam porous material (7) according to any of claims 8-11 , wherein said metal-based foam (7) is provided with a surface or complete treatment.

13. Metal-based foam porous material (7) according to claim 12, wherein said surface treatment comprises at least one of:- an oxide layer;- a nitride layer;- a carbonitride layer;- a carbide layer; - a coating.

14. Metal-based foam porous material according to one of claims 8-12, produced by the method according to one of claims 1-7.

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