Patent Application
20250162028
The invention relates to aqueous suspensions for producing porous metallic structures, to methods for producing a porous metallic material, and to uses of such metallic materials.
Porous metals have long been used as filters in chemical processes and electrodes in functional devices. Steel and iron-based structures with highly tunable porosity have been explored as sustainable, lightweight materials for a variety of structural, biomedical, and energy-related applications.
Porous metals have been used as jewelry, filters and battery components [1-2] and have also been considered for lightweight construction, catalysis, thermal management, and tissue regeneration [3-9]. This broad range of established and prospective applications relies on the unique set of properties offered by the percolating metallic network combined with the low weight and the open channels provided by the porous phase.
Control over the porosity and pore size of the metallic structure allows for tuning of the properties of the porous metal to meet the requirements of specific applications. Manufacturing technologies directly affect the porosity and pore size of the final structure and therefore have been investigated to control the properties and functionalities of porous materials [10-13].
The advent of 3D printing has opened new opportunities for the design and fabrication of novel, bio-inspired hierarchical porous materials with antagonistic properties that are not accessible by conventional manufacturing [14]. This can potentially lead to bone-inspired structures that are simultaneously lightweight and mechanically efficient, as well as or filters inspired by marine sponges that combine high efficiency with a low pressure drop [15].
3D printing technologies utilized for the fabrication of porous and cellular metals include powder bed fusion [16-18], electron beam melting [19-20], various replication techniques [3, 7, 21], and direct ink writing [22-24]. Among the techniques that can generate large centimeter-scale objects, direct ink writing (DIW) is especially attractive due to the possibility to print complex structures at room temperature using readily available and low-cost desktop printers.
For printing cellular and porous iron by DIW, metal or metal oxide precursor particles are suspended in a binder-containing liquid phase to generate a viscoelastic ink that later is extruded through a sub-millimeter nozzle [23, 25]. The printed cellular structures are then dried, reduced, and sintered to densify the deposited filaments [26]. The resulting metallic structures exhibit open cells, the size and relative fraction of which are the result of the tool path of the printing nozzle. Despite the biocompatibility, the mechanical properties, and the electrochemical response demonstrated so far [23, 25], the potential of direct ink writing as a tool to manufacture metals with tunable hierarchical porous architectures could not yet be fully exploited.
A strategy to control the porosity of materials across multiple length scales is to print inks or resins containing building blocks that generate porosity through predeterminable self-assembly processes [27-29]. In this approach, the large pores created through the tool path are combined with the small pores arising from the self-assembling ink to create structural features below the printer's resolution. Air bubbles, liquid droplets, or organic particles have been used as self-assembling building blocks to print ceramics and polymers with programmable hierarchical porosity [27, 30-31]. Porous silica-based glasses with tunable multiscale pores have also been created by Digital Light Processing (DLP) of phase-separating resins containing self-assembling monomers and metal alkoxide precursors [29].
GB 339645 discloses a process in which metal oxide powder is suspended in water, more than 1 wt % foam producing agents are added, the resulting foam is cast, the casted foam is dried, and the dried foam is reduced to a porous metall. A maximum porosity of 80% is reached.
US 2009/0325780 A1 discloses the preparation of wet foams exhibiting long-term stability, wherein colloidal particles are used to stabilize the gas-liquid interface, said particles being initially inherently partially lyophobic particles or partially lyophobized particles having mean particle sizes from 1 nm to 20 μm.
US 2021/0130244 A1 discloses another method for producing particle-stabilized foams.
WO 2016/187097 A1 discloses a particle-stabilized foam ink composition that can be used for 3D printing hierarchical porous structures by continuous filament deposition of particle-stabilized ink. To achieve partial open porosity, fugitive particles comprising a polymer or carbon must be added to the foam ink composition. If a metal foam is desired, the foam ink composition can include metal oxide or metal hydride particles, and a reducing or oxidizing step follows the deposition and drying of the continuous filament.
C. Kenel et al., Addit. Manuf. 2021, 37, 101637 (Ref. discloses the production of hierarchically porous metallic scaffolds via 3D extrusion and reduction of oxide particle inks with salt space-holders. Metal structures with three-level hierarchical porosity can be printed using an ink containing iron oxide particles as metal precursors and dissolvable salt space holders as sacrificial templates [32]. This process leads to relatively low porosity (<38% for iron).
A. Verdooren et al., J. Mat. Sc. 2005, 40, 4333 disclose a method for producing a ceramic foam using a precursor containing iron (III) oxide, calcium oxide, aluminum, and carbon black in an acidic solution comprising 50 wt % of aluminum orthophosphate. The foaming is the result of generated hydrogen gas. A similar method using a precursor containing iron (III) oxide, magnesium oxide, aluminum, and carbon black of is disclosed in A. Verdooren et al., J. Am. Ceram. Soc. 2006, 89, 3101.
C. Minas et al., Adv. Mat. 2016, 28, 9993 (Ref. 27) discloses a method for producing particle-stabilized foams and emulsions of metal oxide particles with closed and open porosity.
US 2015/0035209 A1 discloses a method for forming metal in which a paste comprising metal oxide particles, a polymeric binder and an organic solvent is extruded through a tip to deposit sequential layers of the metal oxide paste on a substrate to form a three-dimensional object. The object is exposed to a reducing gas at a temperature and for a time period sufficient to reduce and to sinter the metal oxide particles to form a three-dimensional metallic object.
WO 2015/175880 A1 discloses 3D printable ink compositions including metal oxide particles and an elastic polymer binder.
WO 2020/243428 A1 discloses wet foam compositions for producing sintered open-cell ceramic foams.
US 2006/0211802 A1 discloses a process for manufacturing a porous metal-containing material. Particles are dispersed in at least one solvent, the particles comprising at least one polymer material and at least one metal-based compound. The solvent is removed from the composition, and the polymer material decomposed, thereby converting the solvent-free particles into a porous metal-containing material.
There is a general need for improvements in this field.
It is the overall objective of the present invention to provide improvements in the manufacture of metal foams and porous metallic structures.
In the context of this specification, the terms metal foam and porous metallic structure are used synonymously.
One object of the invention is to provide advantageous aqueous suspension for producing porous metallic structures that allow the additive manufacturing of three-dimensional objects that in a subsequent step can be converted to porous metallic structures.
Another object of the invention is to provide methods for producing porous metallic materials.
Yet another object of the invention is to provide advantageous uses and applications of such porous metallic materials.
These and other objects are substantially achieved through the features of the independent claims. Advantageous embodiments and variants are set forth in the dependent claims.
Further aspects of the present invention become evident as this description proceeds.
The present invention may comprise one or more of the features recited in the attached claims, and/or one or more of the following features and combinations thereof.
A first aspect of the invention concerns an aqueous suspension for producing porous metallic structures, in the form of a foam or an emulsion.
An aqueous suspension according to the invention comprises 2-49 vol % of a mixture of at least one chemical compound comprising a metal atom, wherein said at least one compound is solid at room temperature, has the form of a powder, and is suspended in water, and comprises 109 to 0.1 mol a surfactant per mol of said chemical compound comprising a metal atom. The aqueous suspension is part of a foam, and/or is part of an oil-in-water emulsion, comprising 30-90 vol % of a lipophilic phase, said lipophilic phase not comprising a polymerizable compound.
Such polymerizable compounds may for example be monomeric compounds or oligomeric compounds that are prone to polymerization under the conditions of a subsequent processing step.
The amount of surfactant in relation to the amount of powderous, suspended chemical compound comprising the metal atom that needed for obtaining stable foam depends among other factors on the chain length of the surfactant. Long-chain surfactants can lead to particle-stabilized foams using very low concentrations. As explained in A. R. Studart et al., “Unifying Model for the Electrokinetic and Phase Behavior of Aqueous Suspensions Containing Short and Long Amphiphiles”, Langmuir 2011, 27, 11835-11844, the amphiphile concentration needed to halve the zeta potential is a good indication of the minimum concentration needed for foaming.
Such an aqueous suspension according to the invention can be used as a stable wet foam or a stable emulsion usable as an ink for additive manufacturing, particularly for Direct Ink Writing. Such a wet body can then be dried to a green body with a pore structure, which can subsequently be further processed.
The foamed aqueous suspension according to the invention can be produced by foaming the aqueous suspension with a gas, for example air or advantageously an inert gas, such as nitrogen gas.
Such a foamed aqueous suspension according to the invention allows to produce mechanically stable metallic structures with hierarchical porosity over several orders of magnitude. On a first level in the macroscopic scale, porosity is controllably produced within the additive manufacturing process. On a second, smaller level, porosity results from the cells of the foamed aqueous suspension. In the micrometer range, a third level of porosity can be found on the walls of the cells of the second levels, where holes in the cell walls connect the cells. The result is an open pore structure.
Without wishing to be bound to any specific theory, it is assumed that this third level of porosity is the result of rearranging processes of the metal atoms at elevated temperatures, for example during a sintering step.
The emulsified aqueous suspension according to the invention can be produced by emulsifying the aqueous suspension with the lipophilic phase.
Advantageously, the lipophilic phase of the oil-in-water emulsion is a lipophilic organic solvent, for example n-octane.
The droplets of such an oil-in-water emulsion provide a scaffold for the formation of macropores, similar to foam bubbles. The lipophilic organic solvent is advantageously chosen such that the boiling point (n-octane: 125-127° C., at normal pressure) is above the boiling point of water, but in a similar range. The printed emulsion-based ink will remain mechanically stable when the printed object is dried, and the organic solvent can be removed in the same step as the water.
Since the droplets of such an oil-in-water emulsion do not affect the mechanical stability of the dried green body, there is no need to mechanically stabilize them, for example by polymerizing the droplets. Such a polymerization would actually be disadvantageous since the polymer then cannot be completely removed during the drying step. Thus, advantageously, the lipophilic phase of the emulsion does not comprise a polymerizable compound, such as for example a monomeric compound or an oligomeric compound that is prone to polymerization during the drying step.
The particle size of the powdery chemical compound comprising a metal atom advantageously lies between 0.03 and 5 μm.
Advantageously, an aqueous composition according to the invention is a foam or an emulsion.
More advantageously, an aqueous composition according to the invention is a foam or an oil-in-water emulsion with a solid loading of 2-49 vol %.
Alternatively, or in addition, the bubble size of an advantageous foamed aqueous composition according to the invention lies between 20 μm and 2 mm. Correspondingly, in an emulsion based on the aqueous suspension according to the invention, the diameter of the emulsion droplets lies between 5 μm and 2 mm.
Advantageously, in an aqueous suspension according to the invention, one or more of said chemical compounds comprising a metal atom is a compound that is reducible to metal in a gaseous atmosphere.
In addition, or alternatively, in an aqueous suspension according to the invention one or more of said chemical compounds comprising a metal atom can be a metal oxide, a metal hydroxide, a metal chloride, or a metal fluoride.
For example, oxides or hydroxides of Fe, Cr, Zn, Al, Mn, Sn, Pb, Co, Ni, Cu, Mo, Ta, W can be used as metal atom comprising chemical compounds. Such compounds can be reduced in a hydrogen atmosphere at a temperature between 20° and 800° C.
Iron oxide, e.g. magnetite, optionally in combination with NiO, and/or Cr2O3, is particularly advantageous, since it allows to produce steel foams, the necessary carbon originating from organic compounds in the aqueous suspension.
AgCl or PdCl2 can be as the metal atom comprising chemical compound and can be reduced under hydrogen atmosphere at 200-900° C. (for AgCl) or 500-800° C. (for PdCl2). Porous palladium structures are particularly useful for heterogeneous catalysis.
Also, MgF2 can be used as metal atom comprising chemical compound. Porous magnesium structures can be used a biocompatible bone scaffold.
Metallic structures can be obtained with such a suspension by reducing, and optionally sintering, dried structures previously additively manufactured with such a suspension.
The reduction parameters, particularly the temperature, depend on the particle size. On one hand, the kinetics depend on the surface area and the thickness of the particles, and on the other hand, the sintering process may also take place in parallel to the reduction process.
Advantageously, in such an aqueous suspension according to the invention the metal oxide powder is Fe3O4, or NiO, or a mixture thereof. Particularly, such a variant of the invention allows to produce ferrous metallic foams and steel foams.
Even more advantageously, in such an aqueous suspension according to the invention, the particles of the metal oxide powder have a prolate shape. This improves the rheological characteristics and the stability of the aqueous suspension after foaming.
In another advantageous variant of the invention, one or more of said chemical compounds comprising a metal atom in the aqueous suspension comprises a metal hydride.
Particularly advantageous, said metal hydride is TiHx, PdHx, ZrHx, or MgH2.
For the metallic hydrides MHx, such as said TiHx, PdHx, ZrHx, the parameter x is the absorption ratio between H and M. In the case of Ti and Zr, 0<x<2; and in the case of Pd, 0<x<1.
Such a variant of the invention allows to obtain metallic structures by subjecting dried structures previously additively manufactured with such a suspension to elevated temperatures at which the molecular hydrogen leaves the crystal structure of the metal.
For TiHx, a suitable temperature range lies between 35° and 600° C. For ZrHx, a suitable temperature range lies between 60° and 1000° C. For MgH2, a suitable temperature range lies between 20° and 500° C.
In yet another advantageous variant of the invention, one or more of said metal atom comprising chemical compounds is a metal carbonyl compound.
Such a variant of the invention allows to obtain metallic structures by subjecting dried structures previously additively manufactured with such a suspension to elevated temperatures at which the metal carbonyl decomposes into the metal and carbon monoxide gas.
Particularly advantageous, said metal carbonyl compound is Rh2(CO)8, or Ru(CO)5.
For Rh2(CO)8, a suitable temperature range lies between 20° and 600° C. For Ru(CO)5, a suitable temperature range lies between 30° and 700° C.
Advantageously, cationic surfactants are used as a surfactant in aqueous suspensions according to the invention, for example hexylamine.
Non-ionic surfactant can also be advantageously used as a surfactant in aqueous suspensions according to the invention, for example octyl gallate.
For hexylamine, an advantageous surfactant concentration lies between 0.01 and 0.05 mmol/g. The amount of surfactant is advantageously adjusted to the length of the hydrocarbon chain of the surfactant.
An aqueous suspension according to the invention preferably comprises 0.01-20 g/l of a binder, depending on the density and the chain length.
Particularly advantageous, said binder is methylcellulose, PVA (poly vinyl alcohol), or PVP (poly vinyl pyrrolidone), or a mixture thereof.
The use of such a binder allows to stabilize the foam or emulsion, and also temporarily stabilizes the dry structure after drying the printed object.
A second aspect of the invention concerns methods for producing porous metallic materials. In a
A method according to the invention for producing a porous metallic material comprises the steps:
In a method according to the invention, the metallic structure is advantageously sintered.
The method according to the invention has the advantage that small particles (e.g. 0.03-5 μm) of metal containing compounds can be produced more cost effectively than metal particles of similar size. Furthermore, such pulverulent compounds are inflammable or less flammable of corresponding pulverulent metals and are not prone to oxidation of the particle surface.
The foaming step can be carried out mechanically, for example by stirring. Alternatively, the foaming can be achieved in situ by decomposition of a suitable chemical compound, for example hydrogen peroxide.
In an advantageous variant of the method according to the invention, one or more of said chemical compounds comprising a metal atom is a compound that is reducible to metal in a gaseous atmosphere; and the dry structure is reduced by subjecting it to a reducing atmosphere, resulting in a metallic structure.
More advantageously, the reduction is carried out in an atmosphere of 0.5-100 vol % H2 in an inert gas.
The temperature at which the reduction process is carried out depends on the metal ions to be reduced. For Fe3O4 as the metal containing compound, said temperature advantageously lies between 60° and 1000° C.
The resulting iron foam can have an air content of 75-98 vol %. The density of the resulting iron foam can lie in the range of 0.4-2 g/cm2. The relative density can be 0.03 to 0.5, compared to massive iron.
In another advantageous variant of a method according to the method, one or more of said chemical compounds comprising a metal atom is an interstitial metal hydride; and the dry structure is subjected to a temperature under which the hydrogen is released as hydrogen gas, resulting in a metallic structure.
Advantageously, in the discussed methods according to the invention, the processing step resulting in the metallic structure and the sintering step are carried out in one process step.
It is also advantageous if in a method according to the invention, the three-dimensional structure is formed by additive manufacturing, for example by three-dimensional printing.
In an advantageous variant of the method according to the invention, the surface of the metallic structure is hydrophobized or lipophilized. This allows for the use of the produced metallic structures for efficiently absorbing lipophilic compounds, such as for example spilled oil floating on water surfaces.
In another advantageous variant of the method according to the invention, after the reduction step, the metallic structure is at least partially oxidized.
A third aspect of the invention concerns advantageous uses and applications of porous metallic structures according to the invention.
A metallic material according to the invention is produced according to a method of the invention.
In an advantageous use according to the invention, such a metallic material according to the invention is used as a resistive heating element.
When an electric current is lead through the material, thereby producing heat in the metal matrix of the material, the large surface of the porous metallic material and its open cell structure allows for an efficient heating of gases or liquids in the pores of the material. For example, a metallic material according to the invention can be used for vaporizing liquids by restive heating.
In another advantageous use according to the invention, a metallic material according to the invention is used as a cooling element.
Such an advantageous use can be realized by heat conductively contacting said material with a Peltier element.
In a further advantageous use according to the invention, a metallic material according to the invention is used for absorbing lipophilic substances floating on water.
For this use, the metallic material is hydrophobized before corresponding metal foam bodies are brought into contact with a lipophilic substance, such as for example oil spilled on water. The hydrophobized surface of the porous metallic structure increases the surface tension between the surface and water, which slows down the intrusion of water into the air-filled of the metal foam. The metal foam bodies, having an overall density lower than water, float on the surface. Oil that comes into contact with the metal foam body will be drawn into the pores of the metal foam by the capillary effect of the open cell structure. This allows to efficiently absorb the lipophilic substance on the extended surface and the pores and capillaries of the metal foam body.
A metal foam body that has its pores completely filled with oil has then finally an overall density higher than water. The metal foam body will no longer float and will sink. Such hydrophobized metal foam bodies thus allow cleaning of oil spills on water bodies where collecting the oil from the surface is difficult or impossible, for example offshore. The metal foam bodies will sink to the bottom together with the absorbed oil, where they will eventually be covered by sediment.
For such applications, preferably iron is used as the metal, due to its comparably low price and absent toxicity.
Alternatively, floating metal foam bodies partially filled with oil can be collected from the surface by using magnets or sieves.
In yet another advantageous use according to the invention, a metallic material according to the invention is used as a heterogeneous catalyst or as a carrier structure for a heterogenous catalyst.
In a further advantageous use according to the invention, a metallic material according to the invention is used as an electrode. For this purpose, said metallic materials can be used with and without functional coatings, as both cathodes and anodes in batteries, including lithium iron phosphate batteries and iron air batteries.
In another advantageous use according to the invention, a metallic material according to the invention is used as a component in alkaline electrolysis. Components that can be manufactured from said metallic material are the anion and proton exchange membranes, the proton transport layer, and the electrodes.
In another advantageous use according to the invention, a metallic material according to the invention is used as a storage for gases, in particular molecular hydrogen. For example, a metallic material according to the invention, due to its porosity and thus large accessible active surface, can be used for the adsorptive storage of such molecular hydrogen or carbon dioxide. Another approach is the use of a metallic material according to the invention for chemically storing molecular hydrogen in an iron/iron oxide based storage cycle,
3 Fe+4H2OFe3O4+4 H2
In a further advantageous use according to the invention, a metallic material according to the invention is used as a filtration element.
In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings and figures. These references should not be construed as limiting the present invention and are intended to be exemplary only.
Components that are identical, or that are identical at least in terms of their function, are designated below by identical or similar reference numbers.
The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the present invention, in addition to those described herein, will be apparent to those skilled in the art from the foregoing description and accompanying drawings. Thus, such modifications are intended to fall within the scope of the appended claims. Additionally, various references are cited throughout the specification, the disclosures of which are each incorporated herein by reference in their entirety.
An aqueous suspension of metal atom containing precursor particles according to the invention allows to realize an extrusion-based 3D printing process for manufacturing metal foams, for example steel or iron-based porous structures with ultra-high porosity distributed over three hierarchical levels. The precursor particles also stabilize the foamed ink.
Using particle-stabilized foams of aqueous suspensions according to the invention as 3D printing inks, pores can by generated at multiple length scales. On a first length scale, porosity can be directly controlled by the printing pattern of the foam filament. On a second, smaller length scale, gas bubbles of the foamed inks serve as templates for the formation of a large fraction of mesopores within the printed filaments. The structure can be controlled via the concentration and distribution of the bubbles in the ink. On a third, even smaller length scale, porosity results from the thermal process leading to conversion of the suspended precursor particles (for example reduction with hydrogen gas in the case of iron oxide particles) and an optional sintering process.
This enables for example the manufacturing of iron and iron-based alloys with an elastic modulus above 300 MPa and a density below 1 g/cm3, which is in keeping with the mechanical efficiency expected for porous structures.
The advantageous manufacturing process for such iron-based metal foams is schematically shown in
The stabilization of wet foams with particles can be achieved by decreasing the wettability of the particles in a liquid and thus promoting their adsorption to the air/water interface. Particle adsorption reduces the total interfacial area and decreases the free energy of the system. This energy gain can easily be orders of magnitude higher than thermal energy, implying that the adsorption is an irreversible process [33]. The irreversible particle adsorption renders the foams nearly immune to common foam destabilization processes, such as liquid drainage, collapse by lamellar breakage, and Ostwald ripening [34].
Oblong magnetite micro particles 11 are functionalized with a short-chain amphiphilic molecule 12,
It was experimentally found that the functionalization of magnetite particles with hexylamine (3 μl/g particles) provides the ideal wettability to promote adsorption at the air/water interface and thus generate strong, stable foams upon air incorporation. To prepare such stable foams, magnetite particles are first suspended and de-agglomerated in water containing the short-chain amphiphile hexylamine. The positively charged polar head group of the molecule absorbs to the negatively charged particle surface due to electrostatic interactions. This makes the hydrophobic tail of the amphiphile protrude outwards and render the entire particle slightly hydrophobic. Mechanical frothing of the suspension of modified particles eventually leads to the desired ultra-stable foams. A particle volume fraction of 50 wt % and an optimized foaming procedure (10 min at 800 rpm) provide the additional rheological requirements for printing.
This stabilized viscoelastic ink 1 can then be used in a Direct Ink Writing (DIW) process for additively forming a three-dimensional structure 3a,
The DIW approach is especially suitable to fabricate open cell structures, such as grid-like and open cellular designs.
There are three rheological requirements for most direct ink writing inks: First, the ink must be shear thinning, which allows the material to flow through the nozzle at low pressures. Second, it must have a high yield stress, below which the ink does not flow, to ensure that the material remains in place after printing. Third, the ink must also show a high storage modulus to prevent the sagging of spanning filaments in grid-like structures. For compressible foams, the printing conditions also need to be adjusted to ensure continuous deposition rather than stop-and-go extrusion [35].
After drying of the printed wet foam body 3a, the resulting green body 3b (
To quantify the rheological behavior and the viscoelastic properties of the particle-stabilized wet foams, strain-controlled oscillatory rheology measurements were performed (cf.
DIW printing of the viscoelastic wet foam using a desktop extrusion-based printer equipped with a syringe and nozzle size of 0.84 mm leads to a three-dimensional object 3a (cf.
The ability to print distortion-free grids confirms the suitable rheological properties of the ink and indicates that the wet foams are resistant to the high shear forces applied during extrusion-based printing, which could otherwise cause phase separation and bubble collapse. Moreover, the experiments revealed that the high stability and viscoelastic nature of the particle-stabilized wet foams mitigate any compressibility issues under typical printing conditions.
After printing, the wet foam object 3a is dried at 60° C. for 1 h, leaving in the in the grid-like structure of the resulting green body 3b (cf.
The size of these mesopores can be adjusted by the volume fraction of particles, amphiphile concentration, and foaming speed. Generally, increasing the particle and the amphiphile content will increase the suspension viscosity, and thereby reduce the size of the bubbles due to the higher shear stresses generated during mechanical frothing. The increased shear resulting from higher foaming speeds can also reduce the bubble size.
The green body 3b can be converted into a hierarchically porous metal body 3c by thermal treatment under a reducing atmosphere at temperatures ranging from 690 to 1000° C. The green body 3b of
The combined reduction and sintering process leads to a linear shrinkage of approx. 50%, as can be seen when comparing the metal foam body 3c shown in
To demonstrate the possibility of adjusting the bubble size, and thus of the mesopores of the material according to the invention, the particle concentration of the foam was reduced from 50 wt %, as in the example discussed above, to 40 wt %, while keeping the amphiphile concentration and the foaming procedure constant. Foams with such lower particle concentration were found to be still sufficiently stiff enough for 3D-printing grid-like structures. Foams with 40 wt % particles lead to larger mesopores with an average diameter of 125.1 μm.
The reduction process is affected by several factors, including sample volume, gas flow, H2 and H2O content of the reducing gas, oxide particle size, and temperature. Depending on the temperature and particle size, the reduction is also accompanied by sintering processes of the metal particles and metal oxide particles. To explore part of this ample processing parameter space, the effect of different reduction protocols on the microstructure, relative density, and mechanical properties of the resulting hierarchical porous materials was investigated. To this end, specimens produced with the 50 wt % magnetite ink discussed above were systematically heat-treated for 30 hours at 750° C., 850° C., and 1000° C., and for optimized conditions (40 hours at 690° C.). SEM images of the green body prior to reduction and the metal foam after reduction are shown in
A marked change can be observed in the microstructure of samples heat-treated between 750° C. and 1000° C. (
Along with the microstructural changes, the determined carbon content decreases from 0.20±0.08% at 850° C. to only 0.05±0.01% C at 1000° C. Such a reduction is likely caused by the long exposure of the high-surface area structure to high temperatures, which allows for carbon diffusion out of the material. From this microstructural analysis, it is concluded that high temperatures should be avoided in order to retain a high carbon content and a three-level hierarchical porosity.
Particularly advantageous hierarchical steel structures with 3-level porosity were eventually obtained by employing a two-step procedure, in which samples are first fully reduced at 690° C. for 40 h and afterward sintered for 2 h at 850° C. This provides the microstructure of samples reduced at low temperature, but with the thicker struts resulting from sintering.
The mechanical characterization reveals that the elastic modulus and the compressive yield strength of the porous metal structures increase with the heat-treatment temperature (
To compare these hierarchical porous structures with other iron-based porous materials, the mechanical data are shown in an Ashby-type plot depicting the effect of the relative density on the elastic modulus and compressive strength of previously reported structures (
The proposed hierarchical pore design can be extended to a range of other iron oxides and iron-based alloys. To illustrate this, hierarchical porous Ni—Fe alloys using NiO and Fe3O4 particles as inorganic precursors in the initial printable foam. Nickel is a common alloying material for steel, as it ensures that the steel retains its tough high-temperature crystal structure, austenite.
The nickel content in the alloy can be controlled precisely by tuning the relative fraction of the oxide powders used in the foam formulation. This was validated by elemental analysis (Proton Induced X-Ray Emission) of an alloy designed to contain 28 wt % (calculated) nickel. After heat treatment of the dried precursor foam, the alloy was found to contain 27.3 wt % Ni atoms. X-ray diffraction analysis confirmed the formation of austenite in the heat-treated Ni—Fe alloy (
In terms of mechanical properties, the measurements show that the addition of nickel improves both the elastic modulus and the compressive strength of the porous Fe-based alloys considerably. Similar to the pure iron compositions, the presence of three hierarchical porosity levels results in much lower relative densities compared to previously reported iron-based porous materials. Because the measured mechanical properties still follow the trend expected from theoretical predictions, this reduction in relative density is achieved without compromising the mechanical efficiency of the porous structure. Compared to the pure iron counterparts with similar densities and processing conditions, the presence of nickel in the iron-based alloy was found to increase the elastic modulus of the porous structures by a factor of 7.5, and the compressive strength by a factor of 5.
Iron-based porous structures can be used as a pure metal foams, as metal oxide foams or as foams of mixtures of metal and metal oxide, in a wide range of applications. Iron oxide foams with tailored chemical compositions can be obtained by heat-treating the 3D-printed green body objects at intermediate temperatures in the range of 550-650° C. A two-hour heat treatment at 550° C. air atmosphere leads to the partial oxidation of magnetite to brown-colored maghemite. Red-colored hematite structures can be obtained by heat-treating a 3D-printed magnetite foam at 650° C. for 2 hours in air.
The porous metal structures themselves show the intrinsic magnetic properties and electrical resistivity of iron (0.00125 Ωm).
Mixed iron/iron oxide porous structures can be obtained through partial oxidation or iron oxide green bodies. The composition of crystalline phases present in an iron porous structure (850° C. for 30 h) after oxidative corrosion in phosphate buffer (37° C.) for 1-90 days, determined by Rietveld fitted XRD spectra, is shown in
The (a) relative and absolute compressive modulus and the (b) relative and absolute yield strength, and (b) the relative strength of an iron-nickel based metallic foam according to the invention are shown in
The determined surface area of printed structures prepared from mixed foams containing different fractions of iron oxide particles and heat-treated at distinct reduction/sintering conditions is shown
SEM images of metal foams containing different fractions of iron oxide particles (40, 45, and 50 wt % iron oxide compared to iron) and heat-treated at distinct reduction/sintering conditions are depicted in 730° C., 30 h; (f) 730° C., 15 h; (g) 650° C., 70 h; (f) before reduction.
Because of the higher electrical resistivity of mixed iron/iron oxide foams compared to pure iron counterparts (0.7 Ωm for 35 wt % iron at room temperature), these porous materials can be used for resistive heating, for example as low-power resistive evaporators. It was experimentally observed that Joule heating with only 3 W power is enough to heat up an 8 mm wide cubic porous structure to a steady temperature of 180° C. Steady state is reached due to a balance between the resistive heating and the convective cooling enabled by the high surface area of the structure (1 m2/g).
Moreover, the very low density (<1 g/cm3) of the metal foam structures according to the invention makes them sufficiently light to swim on water, which is remarkable in view of the density of pure iron (approx. 8 g/cm3). The hierarchical open-celled structure induced by the reduction procedure acts like a sponge to liquids that can wet the metal surface. Under favorable wetting conditions, capillary forces cause spontaneous absorption of at least 0.4 ml/g of liquid into the porous structure. The capillary forces were observed to be strong enough to suck liquid not only into the micro-pores but also into the 3D printed channels.
Because such metal foam bodies can float on water, they can be used as environmentally friendly oil absorbers. For this application, the surface of the metallic structure is hydrophobized with fumed silica via dip-coating. This hydrophobization procedure makes the surface wettable by oils and other apolar solvents, ensuring the spontaneous absorption of these liquids into the porous structure. As can be seen in
A particular advantage of such a use of porous structures as oil-absorbers comes from their ability to swim on water, which enables absorption of the oil right where it resides. As the oil replaces the air in the pores and channels, the density of the structures increases until they start to sink. This allows for crude oil-spill clean-up where the oil is contained in the metal bodies and then deposited down to the ocean floor.
Another method for producing metal foams according to the invention is using an emulsion-based ink system instead of a wet-foam based ink system. Compared to a wet-foam composition, the surfactant concentration is increased by a factor of 5-50 (examples: 1 mmol/g of hexylamine, 0.1 mmol/g of octyl gallate). 30-80 vol % of an hydrophobic organic solvent (such as n-Octane) are added dropwise while mixing. The pH can be adjusted to the pKa of the surfactant (with NaOH, or HCl, respectively) for improving emulsification. Binders like methylcellulose, PVA (poly vinyl alcohol), and PVP (poly vinyl pyrrolidone) can be used to stabilize the resulting emulsion (1-7 g/L). The emulsion based ink can be processed (printed, dried, sintered, reduced) in the same way as the foams. The macropores in this case result from the organic solvent droplets in the emulsion, similar to the air bubbles in the foam. A metal foam produced by 3D printing of such an emulsion based ink system is shown in
Furthermore, it was surprisingly found that the porous metal structures according to the invention show a special strain stiffening behavior.
Iron foams and Nickel-iron alloy foams were subjected to strain-stress measurements, and the elasticity modulus E1 was determined in the proportional range. The measurement was then repeated. Unexpectedly, the elasticity modulus E2 found in this second run was considerably higher. For a nickel-iron alloy foam with 7 wt % Ni, reduced at 850° C. for 30 h, the elasticity modulus measured in a first run was E1=565 MPa, and the second measured elasticity modulus was E2=2724 MPa (cf.
The compressive modulus will improve after compression along the compression direction by a factor of 2-20 with a Poisson's ratio <0.01. This uniaxial stiffening will not be accompanied by a change in the density exceeding 2.5% and is permanent, as long as only uniaxial stress is applied. (
The effect of the increased elasticity modulus is permanent, as can be seen in
The strain stiffening effect is uniaxial.
Without wishing to be bound to a certain theory, it is assumed that the strain stiffening effect occurs due to the superposition of stress responses at two different length scales, as is illustrated in
It has been known for nanoporous gold structures obtained by dealloying Ag97Au3 to show on the nanoscale and large porosities a similar effect [40, 41]. However, the effect has so far neither been known to exist for microstructures in the length scale range of micrometers and with lower porosities, as they can be obtained with metal foams according to the invention, nor at such low strains (<5%).
Ink preparation: An aqueous suspension of 40-50 wt % metal oxide powder (Magnetite, Fe3O4, E8840, DOWA; Nickel (II) oxide green, 99%, abcr GmbH) and 3 μl/g of 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). The standard composition contained 50 wt % magnetite powder. The slurry was then foamed at 800 rpm with an electronic 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 before printing 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 and 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. The final samples were either dried at room temperature overnight or in the oven at 60° C. for 1 h.
Reduction of printed structures: The samples were reduced in a quartz tube oven (Gero, SR-A 100-500/12) with 12 l/min forming gas flow (95% N2 and 5% H2, Pangas). Various heating protocols (Table 2) were used to investigate the effect of reduction and sintering conditions on the microstructure and properties of the resulting porous metals (cf.
Oxidation of printed structures: As-printed magnetite foams were oxidized into different oxide phases (cf.
Surface hydrophobization: Hierarchical porous structures were hydrophobized (cf.
Microstructural characterization: High-resolution SEM images of the foams before and after reduction were taken with a scanning electron microscope (LEO 1530 instrument, Zeiss GmbH, Germany).
Compositional characterization: X-ray diffraction spectra were collected with a powder diffractometer (PANalytical Empyrean) equipped with a Cu Kα X-ray tube (15 kV, 40 mA) and a monochromator. Samples were prepared by either crushing them into powder, if brittle, or compressing them into flat sheets, if tough. Particle induced X-Ray emission (PIXE) measurements were performed by Max Dobeli at the Laboratory of Ion Beam Physics, ETH Zürich. The carbon content of the iron samples was measured) with a carbon/sulfur analyzer (LECO CS230) using four 1 g samples per composition.
Mechanical characterization: Compressive strength and modulus were measured with a universal mechanical testing machine (AGS-X, Shimadzu) equipped with a 1000 N load cell. To ensure the opposing faces were parallel, the samples were ground into shape (Struers, LaboPol-25, 1000 grit SiC paper) using ethanol as a lubricant to prevent sample oxidation. Tests were performed by applying a constant displacement rate of 12 mm/min. The raw data were corrected to take into account the mechanical compliance of the testing machine. The machine compliance was measured with a 9×9×8 mm aluminum cube as a placeholder. The data was evaluated using Matlab®. The reported compressive yield strength was determined with the 0.2% offset method.
Surface area characterization: N2 gas sorption measurements were performed with a pore size analyzer (Quantachrome Autosorb iQ) at 77 K. Prior to the measurement the samples were outgassed in vacuum at 80° C. for 24 h. The surface area was determined by the Brunauer-Emmett-Teller method.
Absorption experiments: The oil absorption capability of the hydrophobized porous iron structure (cf.
Conductivity and resistive heating experiments: The resistive heating capabilities of partially oxidized porous iron structures (cf.
The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the present invention, in addition to those described herein, will be apparent to those skilled in the art from the foregoing description and accompanying drawings. Thus, such modifications are intended to fall within the scope of the appended claims. Additionally, various references are cited throughout the specification, the disclosures of which are each incorporated herein by reference in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22158308.1 | Feb 2022 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/054440 | 2/22/2023 | WO |
