Manufacturing of multilayer structures with internal voids

Abstract

The invention relates to a method for manufacturing a multilayer structure with internal voids, such as monolithic stacks for solid-state electrochemical devices such as batteries, SOFC / SOEC stacks, gas separation devices, etc. The method comprises providing green layers comprising a first binder and enclosing a mold material that fills out an interstice between the layers. The layers are adhered, such as by application of elevated pressure and / or temperature, to form a multilayered structure having a mold-filled interstice with no fluid connections to an outside of the multilayer structure. Thereafter, the multilayer structure is machined to provide a fluid connection between the at least one mold-filled interstice and the outside of the multilayer structure. A viscosity of at least part of the mold material is decreased to allow easy removal of the mold material from the at least one interstice via the fluid connection, thereby forming an internal void in the multilayer structure. The viscosity decrease is induced without initiating a phase change to gas phase in and / or combustion of the first binder, i.e., without initiating de-binding or binder burnout. After removal of the mold material, there are free access to the binder-containing materials inside the structure via the voids and fluid connections, and the multilayer structure can be heat treated to perform binder burnout and sintering without the risk of structural damage.

Description

[0001] MANUFACTURING OF MULTILAYER STRUCTURES WITH INTERNAL VOIDS

[0002] FIELD OF THE INVENTION

[0003] The invention relates to methods of manufacturing multilayer structures using green layers, and more particularly methods of manufacturing monolithic stacks for electrochemical devices and the resulting monolithic stacks.

[0004] BACKGROUND OF THE INVENTION

[0005] There is a continued effort of developing and improving the manufacturing monolithic stacks for electrochemical devices such as solid oxide fuel / electrolysis cells (SOFC / SOEC), solid-state monolithic batteries and electrochemical gas separators. Compared to conventional stacking of cells, with interconnects / separating plates in between, a monolithic architecture significantly reduces materials and costs and provides a high power to volume ratio. The single repeating units (SRUs) of a monolith stack are manufactured by fabrication techniques generally involving the three main steps: 1 ) producing and shaping the different layers, 2) adhering the layers to form a multilayer structure, also referred to as lamination or compaction, and 3) sintering the structure to form a solid mass of material. The fuel / electrolysis cells incorporate flow channels with inputs / outputs to bring in gases to the fuel / oxygen electrodes and exhaust out of the cell. SOFC / SOEC formed as monolithic structures must integrate these channels internally in layers bordering the electrodes. Examples of such cells and their manufacturing are described in e.g. US 2011 / 272081 , JP 2004 / 296382 A, US 4,913,982 A1 , or US 2010 / 065189 A1.

[0006] The most complex step in the manufacture of monolithic fuel / electrolysis cells with integrated flow channels, and other monolithic structures with hierarchical microstructures, is typically the removal of organic additives from the green matrix, which provide needed voids, pores, and channels, as there is a high risk of damaging the structure during this process.

[0007] SUMMARY

[0008] Accordingly, there is a need for methods for manufacturing a multilayer structure with internal voids, which may mitigate, alleviate or address the shortcomings of existing methods.

[0009] A method is disclosed, for manufacturing a multilayer structure with internal voids. The method comprises providing layers of at least a first and a second material encompassing a mold material, wherein at least a first material layer is a first green layer with a material composition comprising a first powder and a first binder. The first and second material layers are adhered to form a multilayered structure having at least one mold-filled interstice with no fluid connections to an outside of the multilayer structure. The multilayer structure is machined to provide a fluid connection between the at least one mold-filled interstice and the outside of the multilayer structure. A viscosity of at least part of the mold material is decreased without initiating a phase change to gas phase in and / or combustion of the first binder, and the mold material is then removed from the at least one interstice via the fluid connection to form at least one internal void in the multilayer structure. The multilayer structure can then be heat treated, such as including binder burnout and sintering.

[0010] The mold material is a material composition used to fill an indentation, extrusion, interstice, or another space between the first and second material layers during manufacturing of a structure, which material composition is also removed again during manufacturing to form a void in the structure. The mold material may also be referred to as a sacrificial material or layer, a filler or filling material, or an internal isostatic pressure medium, The invention utilizes changes in viscosity of the mold material at different steps of the manufacturing.

[0011] It is an advantage of the present disclosure that the mold material is removed from the structure after adhesion but before a heat treatment including binder burn-out and sintering, since it allows gases created during heat treatment to easily escape via the voids without damaging or degrading the multilayer structure.

[0012] It is an advantage of the present disclosure that adhering is performed on a multilayered structure having at least one mold-filled interstice with no fluid connections to an outside of the multilayer structure. These integrated, mold-filled interstices provide several advantageous effects in the manufacturing process as well as in the final structure.

[0013] It is an advantage of the present disclosure that a viscosity of at least part of the mold material is decreased without initiating a phase change to gas phase in and / or combustion of the binder(s), since this allows for easy removal of the mold material from the multilayered structure after adhering but before heat treatment (debinding and sintering), without deforming the multilayered structure.

[0014] In an exemplary embodiment, the step of adhering the first and second material layers comprises decreasing a viscosity of at least part of the mold material prior to or simultaneously with adhering and without initiating a phase change to gas phase in and / or combustion of the binder(s). Preferably, the mold material is liquid or quasi-solid during the adhering. This is advantageous since it provides smooth edges in the interstice and reduce stress concentration at the corners resulting on fewer distortions and improved shape maintenance during heat treatment and later use.

[0015] In addition, when the mold is liquid, it acts as internal isostatic fluid that adjusts the interstices shapes to keep the multilayer structure final volume minimized, i.e. distortions of the layers are eliminated and the resulting cross section of the multilayer structure more uniform.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other features and advantages of the present disclosure will become readily apparent to those skilled in the art by the following detailed description of examples thereof with reference to the attached drawings, in which: Fig. 1 is a flow-chart illustrating an example method for manufacturing a multilayer structure with internal voids according to this disclosure.

[0018] Figs. 2A-E illustrate an exemplary embodiment for manufacturing of a multilayer structure with internal voids according to this disclosure

[0019] Fig. 3 is a cross-section diagram illustrating an exemplary process of adhering the first and second material layers according to this disclosure,

[0020] Figs 4A-C are diagrams illustrating an example method for manufacturing a multilayer structure with internal voids (4A and 4B in sequence) according to this disclosure,

[0021] Figs. 5A-3B are diagrams illustrating an example method for manufacturing a multilayer structure with internal voids according to this disclosure,

[0022] Fig. 6 is a diagram illustrating example assembly of a multilayer structure according to this disclosure,

[0023] Figs. 7A-B are diagrams illustrating an example method for manufacturing a multilayer structure with internal voids according to this disclosure,

[0024] Fig. 8 is a diagram illustrating an example multilayer structure with internal voids according to this disclosure,

[0025] Figs. 9A and B are SEM Cross section microscopies of a monolithic stack manufactured in accordance with the disclosed method, and

[0026] Figs. 10A-B are diagrams illustrating layers of an example multilayer structure with internal voids that are connected between the layers according to this disclosure.

[0027] DETAILED DESCRIPTION

[0028] Various examples and details are described hereinafter, with reference to the figures when relevant. It should be noted that the figures may or may not be drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the figures. It should also be noted that the figures are only intended to facilitate the description of the examples. They are not intended as an exhaustive description of the disclosure or as a limitation on the scope of the disclosure. In addition, an illustrated example needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular example is not necessarily limited to that example and can be practiced in any other examples even if not so illustrated, or if not so explicitly described.

[0029] The figures are schematic and simplified for clarity, and they merely show details which aid understanding the disclosure, while other details have been left out. Throughout, the same reference numerals are used for identical or corresponding parts. A method for manufacturing a multilayer structure with internal voids is disclosed herein. The multilayer structure is a structure formed by at least two layers with voids formed there between. Voids may also be referred to as channels, flow paths, indents, etc., i.e. spaces that are not occupied by solid material but can be used to provide fluid access to the interior of the structure. In an exemplary embodiment, the method for manufacturing is a powder-based ceramic manufacturing process.

[0030] The method for manufacturing comprises providing layers of at least a first and a second material encompassing a mold material, wherein at least a first material layer is a first green layer with a material composition comprising a first powder and a first binder. In this respect, a green layer is a layer produced from a mixture of powder (e.g. ceramic or metallic) and organic additives (such as binder, solvent, plasticizer, dispersant, etc.). In ceramic processing, the mixture before shaping is commonly referred to as a slurry and “green” refers to that the material has been shaped but has not yet been sintered.

[0031] The green layer can be prepared and shaped using any technique (such as tape casting, indentation or extrusion, slip casting, injection molding or other techniques that imply the production of a green body or multilayer structure such as binder jet 3d printing, direct inkjet deposition, ceramic stereolithography, fused filament fabrication, and robocasting). In an exemplary embodiment, the green layer is a green tape resulting from doctor blading or calendaring a slurry.

[0032] The part of the method where the first and second (and further) material layers are provided - such as applied or laid or formed - on top of each other prior to adhering is herein generally referred to as ‘assembly’. The assembly may include shaping of the layers, provision of the mold material, and other preparatory steps.

[0033] In an exemplary embodiment, both the first and second material layers are green layers in that the first material layer is a first green layer with a material composition comprising a first powder and a first binder, and the second material layer is a second green layer with a material composition comprising a second powder and a second binder. The material compositions may of course also contain other components commonly used in ceramic processing. In an exemplary embodiment related to electrochemical devices such as fuel cells, the first powder is a ceramic powder, and the second powder is a metallic powder, wherein the mold material fills at least one indentation in the first green layer so that the first green layer and the mold material form a substantially planar surface whereupon the second green layer can be provided. Either or both first and second material layers may comprise multiple layers, such as an electrode-separator-electrode layer formed by thin, possible green layers. In such embodiments, the first and second green layers can form an interconnect with flow channels and a single repeating unit (SRU) comprising electrodes and electrolyte of a monolith fuel cell / electrolysis stack.

[0034] The first and second material layer encompasses the mold material, meaning that they include or surround the mold material completely, such as enclosing or encapsulating the mold material. In one or more exemplary embodiments, the mold material is encompassed, such as completely encompassed or completely enclosed, by the first and second material layer - without any fluid connection to an outside so that it cannot escape the multilayer structure.

[0035] The method for manufacturing comprises adhering the first and second material layers to form a multilayered structure having at least one mold-filled interstice with no fluid connections to an outside of the multilayer structure. The mold material is encompassed with no fluid connections to an outside so that it cannot escape the multilayer structure during the step of adhering. The integrated, mold- filled interstice provide several advantageous effects in the adhering process. Since the mold material is completely encompassed, it cannot escape regardless of its phase or viscosity and of the temperature and pressure of the adhering process.

[0036] The method for manufacturing comprises machining the multilayer structure to provide a fluid connection between the at least one mold-filled interstice and the outside of the multilayer structure. In this context, machining or green machining - is the process of cutting, shaping, sanding, polishing or otherwise removing material from the green structure using a machine tool, including non-contact tools such as in laser cutting. Preferably, one or more sides perpendicular to the layers of the multilayer structure are machined until part of the mold material is exposed, thereby providing fluid connection between the at least one mold-filled interstice and the outside of the multilayer structure.

[0037] The method for manufacturing comprises decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the first (and any second or further) binder and removing the mold material from the at least one interstice via the fluid connection to form at least one internal void in the multilayer structure. The mold material is to be removed while the at least part of the mold material has a decreased viscosity, such as a viscosity in the liquid range of at least parts of the mold material. Preferably, the parts of the mold material with decreased viscosity are parts of the mold material that is in contact with surfaces of the interstices in the multiplayer structure. Thereby, removal of the mold material is made easier and without introducing friction and stresses to the shaped green structure.

[0038] As mentioned previously, the binder burnout (also referred to as debinding) of green structures with internal microstructures is difficult since the structure is at risk of being damaged by gases created in this process. It is an advantage that the mold material is removed before heat treatment (debinding and sintering), since it allows gases created during later debinding to more easily escape via the voids formed when the mold material is removed from the opened interstices. Since the mold material is to be removed before debinding, the viscosity of the mold material is decreased without initiating binder burnout, i.e. without initiating a phase change to gas phase (such as by sublimation and / or evaporation) in and / or combustion of the first binder.

[0039] The method for manufacturing comprises heat treating the multilayer structure. The heat treatment typically comprises exposing the structure to a thermal debinding and sintering profile or cycle. Here, the green multilayer structure is first heated in a furnace to a temperature where the binder evolves or evaporates and escapes from the structure. The furnace then ramps up to a sintering temperature where the powder particles are bonded together through different mechanisms. As described above, it is an advantage that the mold is removed before the binder burnout since gases can escape via the void in the structure, making it possible to scale up the dimensions of the products more easily.

[0040] A multilayer structure with internal voids produced by the above method for manufacturing is disclosed.

[0041] A method for manufacturing a monolithic stack with internal voids is disclosed. This method comprises manufacturing at least two multilayer structures with internal voids according to the above method for manufacturing, wherein the at least two multilayer structures are adhered together to form a multilayer structure-stack prior to the step of heat treatment, and wherein heat treating the multilayer structurestack forms a monolithic stack with internal voids.

[0042] The monolithic stack may be used in a solid-state electrochemical device such as a battery, a SOFC / SOEC stack, electrolysis devices such as for electrolysis of water (steam), CO2, methane etc., other components of electrolysis technologies, or a gas separation device.

[0043] The invention utilizes changes in a state and / or viscosity of the mold material at different steps of the manufacturing. Whereas pure substances have mostly well-defined gaseous, liquid, and solid states, many mixtures have less well-defined forms. As an example, many oils, greases, and waxes are technically liquids within the temperature and pressure ranges where they are typically used. While they behave like solids at low temperatures and liquids at high temperatures in the range, there is a range of intermediate temperatures - at ambient pressure - in between where they behave like a high-viscosity liquid or a quasi-solid, with gradual transitions to the solid and liquid ranges. While the classical phase transitions (e.g. water <-> ice) is typically associated with large changes in volume, there are typically little to no volume changes over the high-viscosity liquid or quasi-solid range of mixtures described herein.

[0044] In an exemplary embodiment, the mold material comprises one or more of the following mixtures: polymers / hydrocarbons, alkanes, triglycerides, greases, waxes, oils, soaps, artificial or natural resins, mixtures of waxes and oils, emulsions of waxes and oils, emulsions of water, waxes and oils, emulsions and / or mixtures of binder and wax and / or oils, emulsions of hard resin in grease, e.g. a resin blended with an oil and a wax.

[0045] In the present description, “quasi-solid” is understood as solid materials under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force, also commonly referred to as “soft solids”.

[0046] In the present disclosure, the state or viscosity of the mold material is described as changing between solid <-> quasi solid <-> liquid. This change may or may not be related to a phase transition of substances comprised in the mold material. In exemplary embodiments, the composition of the mold material is a mixture of substances with no applicable pressure-temperature phase diagram. Two directions of change in viscosity are:

[0047] Decreasing the viscosity (p) of the mold material to go in the direction towards “more fluid”. This could be from solid -> quasi solid and from quasi solid -> liquid (and from solid / quasi-solid / liquid - > gas).

[0048] Increasing the viscosity of the mold material to go in the direction towards harder or more solid. This could be from liquid -> quasi solid and from quasi solid -> solid.

[0049] Generally, the terms “solid”, “quasi solid” and “liquid” as used herein apply to the different states of viscosity utilized in the invention. For the present description, at atmospheric pressure and room temperature (20-25° C), these states are defined as: o liquid: piiquid = [ 10-4Pa-s ; 1 Pa-s [ o quasi-solid: jq-soiid = [ 1 Pa-s ; 105Pa-s ] o solid: j soiid > 105Pa-s

[0050] Here, it is understood that the states are used for ease of nomenclature when designating a preferred viscosity range of the mold material and that a transition from one state to another is a continuous transition with no sudden changes in viscosity. Hence, the viscosity intervals provided above, in particular the viscosity values used to define the quasi-solid, may be changed with e.g. an order of magnitude without substantial implications in the manufacturing. For example, if it is indicated that part of the mold material is in a liquid state to ease removal, what is meant is that the mold material may be removed with little force such as gravity or a light tapping without risk of damaging the structure. Since the required force also depends on many other parameters, such as surface roughness, shape and dimensions of the void, a viscosity of 1 ,1 Pa s - i.e. just outside the liquid state define above - may still be sufficient.

[0051] For a given mold material, a change in viscosity is preferably induced by adjusting the temperature, pressure, or both. However, other methods may be applied such as vibration or the addition of a solvent.

[0052] For a given mold material and at atmospheric pressure, the different viscosity values may correspond to different temperatures ranges: o tiiquid = temperatures where the mold material has viscosity value in the liquid state range. A melting point in typically the lower temperature in this range. o tq-soiid = temperatures where the mold material has viscosity value in the quasi-solid-state range. o tsoiid = temperatures where the mold material has viscosity value in the solid-state range.

[0053] As is well known to the skilled person, these temperature ranges depend on the pressure and should be adjusted accordingly where the ambient pressure is above or below atmospheric pressure.

[0054] In an exemplary embodiment of providing a first and a second material layer encompassing a mold material, the mold material forms a precursor of an internal void and does not extend to an edge or a rim of the first or the second material layer. This ensures that the mold material is encompassed.

[0055] In an exemplary embodiment, providing the first and second material layers comprises: providing the first material layer; applying mold material to the first material layer; and applying the second material layer on the first material layer so that the mold material is encompassed by the first and second material layers. In an exemplary embodiment, applying mold material to the first material layer comprises forming an indent in the first material layer and applying the mold material to the indent. The indent in the first material layer does preferably not extend to an edge or a rim of the first material layer. The indent may be formed by any of the green machining processes described previously herein.

[0056] In an exemplary embodiment, providing the first and second material layers comprises providing or applying the mold material with the mold material being in a solid or quasi-solid state, preferably by controlling the ambient pressure and temperature. This may comprise controlling the ambient temperature to a temperature lower than tq-soiid such as lower than tsoiid for the mold material. Having the mold material in a solid or quasi-solid state makes the handling easier during the assembly of the first and second material layers.

[0057] It is preferred that the mold material is non-soluble by solvents in the first and second materials layers, such as in the green layer, and that the first and second materials layers are non-soluble by the mold material in a liquid or low-viscosity state.

[0058] Fig. 1 is a flow-chart 100 illustrating an exemplary embodiment of a method for manufacturing a multilayer structure with internal voids according to this disclosure. In a first assembly step S102 at least a first and a second material layers and a mold material are provided and assembled for the first and a second material layers to encompass the mold material. In step S104, the at least first and second material layers are adhered to form a multilayered structure. The encompassed mold material is now enclosed in mold-filled interstices inside the multilayered structure, i.e. there are no fluid connections between the mold-filled interstices and an outside of the multilayer structure. The multilayer structure is machined in step S106 to provide at least one fluid connection between the mold-filled interstice and the outside of the multilayer structure. Then, in step S108, the viscosity of at least part of the mold material is decreased to allow removal of the mold material via the fluid connection. The decrease in viscosity is effectuated without initiating binder burn-out. Removing S109 of the mold material from the mold-filled interstice forms an internal void with fluid connection to the outside of the multilayer structure. Now, with the internal void(s) ensuring sufficient pathways for the gaseous binder to escape without damaging the multilayer structure, the heat treatment S110 can be performed.

[0059] Figs. 2A-E illustrate an exemplary embodiment for manufacturing of a multilayer structure with internal voids according to this disclosure. These figures include the method steps from the example method 100 described with reference to Fig. 1. Fig. 2A shows a cross-sectional view of a first material layer 25 and a second material layer 28 provided S102 to encompass a mold material 27. One or both of the first and second material layers 25, 28 is a green layer comprising a first binder that will be removed in a later binder burnout. As illustrated by the curved arrows, the first and second material layers 25,

[0060] 28 - which may be identical - are brought into contact along their edges to completely enclose or encapsulate the mold material 27, resulting in the structure illustrated in Fig. 2B. Fig. 2B also illustrates the adhering S104 of the first and second material layers 25 and 28, here performed by applying an isostatic pressure as illustrated by the block arrows. This forms a multilayered structure

[0061] 29 illustrated in Fig. 2C, having a mold-filled interstice 30 with no fluid connection to an outside of the multilayer structure 29.

[0062] The complete encompassing, enclosing, or encapsulating of the mold material 27 by the first and second material layers 25, 28, and thus the closing of any fluid connection to an outside of the multilayer structure 29, may be effectuated either in the step of providing S102 or at the beginning of the step of adhering S104. As the skilled person will understand, material layers of different sorts may be provided to completely cover the mold material 27, but depending on their material properties and how they are provided, there may still be small gaps or slits between the first and second layers, so that the mold material is not yet completely enclosed. Then, as soon as the structure is pressed, i.e. at the beginning of the step of adhering S104, such small gaps or slits will be closed off whereby the mold material is completely enclosed in the interstice 30. This is of importance for the rest of the adhering, since the mold-filled interstice 30 acts as an internal isostatic fluid that shapes and prevents collapse of the void precursor.

[0063] Fig. 2C shows a cross-sectional view of the multilayer structure 29 and illustrates the machining S106 to provide a fluid connection between the at least one mold-filled interstice 30 and the outside of the multilayer structure 29. Here, the machining is performed by cutting of the sides of the multilayer structure 29, resulting in the multilayer structure 29 of Fig. 2D. Fig. 2D also illustrates the decreasing S108 a viscosity of the mold material 27 without initiating removal of the first binder. Here, as illustrated by the few wavy arrows, the viscosity is decreased by carefully heating the structure to moderate temperatures where the mold material becomes fluid (low viscosity) but where a phase change to gas phase in and / or combustion of the first binder is not initiated. Fig. 2D also illustrates removing S109 the now fluid mold material from the interstice 30 via the fluid connection to form an internal void 31 as illustrated in Fig. 2E. The removal S109 may be assisted by e.g. tilting the structure or blowing compressed air in one of the fluid connections. Fig. 2E also illustrates heat treating S 110 the multilayer structure 29 as illustrated by the many wavy arrows, which preferably comprises binder burnout and sintering. Now that the voids 31 are free of the mold material, the organic additives of the first binder are free to escape the structure as illustrated by the small particles and the dotted curved arrows.

[0064] In one or more exemplary embodiments, the method 100 comprises one or more intermediate steps prior to the heat treatment S110, such as during adhering S104, where the viscosity of the mold material is manipulated to provide low viscosity (fluid) at conditions where binder-burn-out is not initiated. During adhering, where the mold material acts as an internal isostatic fluid that shapes the void precursor, it may be preferred that the mold material is fluid so that the first and second material layers and the mold material reshape to a low energy state geometry with for instance smooth angles and edges.

[0065] In one or more exemplary embodiments, the method 100 comprises one or more intermediate steps prior to the heat treatment S110, such as during assembly S102 and / or machining S106, where the viscosity of the mold material is manipulated to provide a high viscosity (quasi solid or solid) at conditions where binder-burn-out is not initiated. This is advantageous since the mold material is typically easier to control, manipulate, machine and cut when it has a high viscosity since it will not flow or smear other materials, tools and instruments.

[0066] In one or more exemplary embodiments, the method 100 comprises one or more intermediate steps prior to the heat treatment S 110, where additional material layers, mold materials, and / or multilayer structures are provided and assembled and adhered to the multilayer structure 29, so that these additional parts will be heat treated together with the multilayer structure 29.

[0067] The following provides some optional or preferred details to the various method steps according to exemplary embodiments.

[0068] Adhering to form multilayer structure:

[0069] In an exemplary embodiment, adhering the first and second material layers comprises applying pressure and / or temperature to achieve adhesion between the first and second material layers. In this description, the adhered layers are referred to as a multilayer structure.

[0070] Preferably, the adhering comprises an isostatic pressing of the provided layers. Isostatic pressing can be performed at elevated temperatures, known as hot isostatic pressing (HIP), or at ambient temperatures, known as cold isostatic pressing (CIP). In the powder-based manufacturing processes, the isostatic pressing has the effect of densifying green parts.

[0071] In an exemplary embodiment the step of adhering the first and second material layers comprises decreasing a viscosity of at least part of the mold material prior to or simultaneously with adhering and without initiating a phase change to gas phase in and / or combustion of the binder(s). In an exemplary embodiment the step of adhering the first and second material layers comprises adhering the first and second material layers with the mold material being in a liquid state or a quasi-solid state. Binder burnout should be avoided during adhering for the same reasons as described in relation to the step of decreasing viscosity of and removing mold material. Details on how to decrease the viscosity of mold material without initiating binder burnout - i.e., a phase change to gas phase in and / or combustion of the binder(s) - will be described in later embodiments. For both these embodiments, the effect of decreasing the viscosity of the mold material is that the mold material will behave as an internal isostatic fluid during adhering. Having the at least parts of the mold material in a liquid state or a quasi-solid state, preferably the parts in contact with surfaces of the interstice, is advantageous since it allows the readjustment of the 3D solid matrix during adhering according to its “natural fit”, i.e. reshaping to a low energy state geometry (for instance smoothing angles and edges). This reshaping due to the mold material providing an internal isostatic fluid is illustrated in Fig. 3. This improves the mechanical stability of the product during pressing and sintering and helps to reduce stress in the structure and changes in geometry with respect to the design.

[0072] The steps of providing and adhering the first and second material layers may comprise providing one or more further layers above and / or below the first and second material layers so that all layers are adhered together to form the multilayer structure. The one or more further layers may comprise a layer providing an interconnect layer, a seal layer, an electrode layer, an electrolyte layer, or any layer being a combination of one or more such layers.

[0073] Machining

[0074] After adhering the first and second material layers, plus possible one or more further layers, the multilayer structure can be machined to expose the mold material so that the mold material can be removed. Several exemplary embodiments of the adhering involved decreasing the viscosity of the mold material to make it more fluid during adhesion. In these embodiments, the mold material would have a low viscosity immediately after adhesion, which would be disadvantageous since the liquid or quasi-solid mold material may spill out and / or not give enough structural support to the structure during machining resulting in unwanted deformations. This again could affect the quality of the machined structure, damage machining equipment, and result in unnecessary contamination of the structure and equipment which would then have to be cleaned.

[0075] In an exemplary embodiment, machining the multilayer structure comprises increasing a viscosity of the mold material prior to machining. In an exemplary embodiment, increasing the viscosity is achieved by cooling the mold material such as cooling the multilayer structure. In an exemplary embodiment machining the multilayer structure comprises machining the multilayer structure with the mold material being in a solid-state. Machining the multilayer structure while the mold material is high viscosity such as solid is advantageous since it reduces spillage of the mold material during machining of the multilayer structure, and it conserves it final shape.

[0076] Decreasing viscosity

[0077] The machining results in fluid connection between the at least one mold-filled interstice and the outside of the multilayer structure, allowing the mold material to be removed from the multilayer structure. In an exemplary embodiment, machining the multilayer structure comprises providing at least two separate fluid connections between each mold-filled interstice and the outside of the multilayer structure. This is advantageous since it eases the removal of mold material.

[0078] Since exemplary embodiments related to machining the multilayer structure comprises increasing the viscosity of the mold material prior to machining to improve the machining process, the mold material will typically have a high viscosity after machining. Prior to or simultaneous with removing the mold material, a viscosity of at least part of the mold material is decreased to aid, facilitate, or expedite the removal. Again, the decrease in viscosity is to be induced without initiating a phase change to gas phase in and / or combustion of the binder(s).

[0079] In the following, exemplary embodiments relating to decreasing the viscosity of the mold material in the multilayer structure without initiating a phase change to gas phase in and / or combustion of the binder(s) (i.e. binder burnout) are described. More fluid, i.e. low viscosity, mold material is preferred both in the steps of when adhering and removing. The following exemplary embodiments can thus be used in combination with embodiments for adhering the first and second material layers to form the green multilayer structure and for removing the mold material from the green multilayer structure.

[0080] In an exemplary embodiment, decreasing a viscosity of at least part of the mold material comprises inducing a quasi-solid or liquid state in at least parts of the mold material in contact with the multilayer structure.

[0081] In an exemplary embodiment, decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder(s) comprises one or more of heating, applying pressure, adding a solvent capable of dissolving the mold material, and liquefaction.

[0082] In an exemplary embodiment, decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder(s) comprises adding a solvent to the mold-filled interstices to dissolve at least part of the mold material, thereby decreasing a viscosity of the dissolved parts of the mold material. Adding a solvent is considered a change of the material composition of the mold material, and the new material composition has a lower viscosity that the original mold material.

[0083] In an exemplary embodiment, decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder(s) comprises heating the multilayer structure to a temperature smaller than a thermal debinding temperature of the multilayer structure.

[0084] In an exemplary embodiment, decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder(s) comprises heating the multilayer structure to a temperature smaller than a temperature where organic additives in the first and second binder undergo sublimation and / or evaporation and / or combustion.

[0085] In an exemplary embodiment, decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder(s) comprises heating the multilayer structure to a temperature between 10 - 400 °C, such as between 10 - 100 °C or 10 - 150 °C.

[0086] In exemplary embodiments, the temperature to which the multilayer structure is heated to decrease the viscosity is adjusted to correspond to a temperature, tiiquid, where the mold material has viscosity value in the liquid state range, tiiquid is specific to the mold material and may be determined empirically or provided by the fabricant of the mold material.

[0087] In an exemplary embodiment, the viscosity of at least parts of the mold material in contact with the surface of the at least one interstice is decreased to be equal to or lower than 10 Pa s to aid, facilitate, or expedite the removal.

[0088] In an exemplary embodiment, decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder(s) comprises heating and / or vacuuming the multilayer structure to a pressure below 105Pa to facilitate the mold material extraction.

[0089] Mold material removal or extraction

[0090] The dimensions of the interstice are often designed to create high throughout flow channels, and walls between adjacent interstices / channels are typically made thin to maximize the flow channel cross-section relative to the size of the structure, without limiting current collection in a SOFC / SOEC stack application or another solid-state electrochemical device such as a battery, a gas separation device, or an electrolyser / electrolysis device. Since the structure is not yet sintered when the mold material is removed, the green layer(s) are a soft system where the powder is held in place by the binder to achieve the final structure with desired macroscopic and microscopic features in the not yet solidified green layer. Therefore, removal or extraction of the mold material from a mold-filled interstice without damaging or alternating the multilayer structure is often a delicate task. In accordance with this disclosure, a viscosity of at least part of the mold material is therefore decreased to facilitate easy removal of the mold. The decrease in viscosity serves to reduce the friction between the mold material and the multiplayer structure, i.e. the inner surfaces of the interstices. To facilitate a low friction or frictionless extraction of the mold material, the decreased viscosity state of the mold material is preferably a good lubricant.

[0091] Removal of the now low-viscosity mold material from a mold-filled interstice may be effectuated by applying a force to the mold material. In an exemplary embodiment, the multiplayer structure is tilted for gravity to facilitate the removal. In an exemplary embodiment, compressed air is directed into the fluid connection to facilitate the removal. In an exemplary embodiment, vacuum is applied to the fluid connection to facilitate the removal. In an exemplary embodiment, a pressurized liquid is directed into the fluid connection to facilitate the removal, the pressurized liquid may comprise a solvent capable of dissolving the mold material. Where the machining produced at least two separate fluid connections to a mold-filled interstice, such compressed air or pressurized fluid is preferably directed into one fluid connection for the mold material to escape via a different fluid connection.

[0092] Heat treatment

[0093] After removal of the mold material from the multilayer structure, the structure is heat treated. The heat treatment typically comprises exposing the structure to a thermal debinding and sintering profile or cycle, typically taking place in a furnace. In exemplary embodiments, a first step in the heat treatment cycle is to remove the binder from the multilayer structure. Depending on the specific binder being used, this is done via degradation and / or evaporation or sublimation. In degradation, a chemical reaction between the gas used in the furnace and the binder that degrades the binder into new components that can then be evaporated. In evaporation, binder is brought to an elevated temperature where it becomes vapor. In sublimation the binder changes directly from solid to gas state. Many times, the binder will break into small molecules before evaporating. The vapors can then leave the multilayer structure through surface-connected pores that will later be removed during sintering. The larger the open access to the binder- and organic additive-containing parts of the multilayer structure, the easier the gas / vapors can leave and the smaller the risk of damages to the structure. It is therefore an advantage that the mold material is removed prior to the heat treatment since the now mold-free voids dramatically increases the access to the binder-containing parts and thus ease its thermal removal. Gas flows over the multilayer structure and through the voids helps to sweep the binder away and out of the furnace chamber.

[0094] The temperature at which thermal debinding occurs depends on both the binder material and metal and / or ceramic powders being used, but generally happens at temperatures between 150°C and 550°C. Furnaces typically ramp up slowly to ensure the binder isn’t converted to gas too quickly, potentially damaging multilayer structure.

[0095] After binder burnout, the cycle continues to the sintering. There are several mechanisms of sintering, depending on the material being joined, i.e., on the metal and / or ceramic powder in the green layer(s). Sintering generally happens at temperatures between 800°C and 1.700°C

[0096] A multilayer structure with internal voids is disclosed, being manufactured using the method of manufacturing a multilayer structure with internal voids also disclosed herein.

[0097] A method for manufacturing a monolithic stack with internal voids is disclosed herein. This method comprises manufacturing at least two multilayer structures with internal voids by application of the method of manufacturing a multilayer structure also disclosed herein. The at least two multilayer structures form a multilayer structure-stack with mold material being removed prior to the step of heat treatment, and wherein heat treating the multilayer structure-stack forms a monolithic stack with internal voids. Additional layers may be provided on or between the multilayer structures making upon the stack.

[0098] The different steps of manufacturing the at least two multilayer structures may be carried out in different order. The order to be used depends on multiple factors such as the materials of the layers and mold, the size of the layers, the total number of layers, the available machinery involved in the manufacturing, the type and application of the monolithic stack, etc. A few exemplary embodiments are described below.

[0099] In an exemplary embodiment, all layers to form part of the monolithic stack are assembled and then adhered in in one step. Thereafter the stack is machined, and the mold is removed before heat treatment of the entire stack.

[0100] In another exemplary embodiment, several multilayer structures are assembled and adhered separately. Thereafter, the separate multilayer structures can be assembled before being adhered to form a multilayer stack. The separate multilayer structures can be machined, and the mold be extracted in each multilayer structures separately, or it can be done for the stack after adhering.

[0101] In an exemplary embodiment, the multilayer structure-stack comprises interconnects with flow channels, electrode layers (with or without integrated sealings), and electrolyte layers to form a monolith SOEC / SOFC stack or another solid-state electrochemical device such as a battery, an electrolyser / electrolysis device, or a gas separation device. In particular for monolith SOEC / SOFC, each multilayer structure could be an SRU including an interconnect with flow channels, a fuel electrode, an electrolyte, and an oxygen electrode. Here, the interconnect may include flow channels in two vertically separated planes with one plane being adjacent to an electrode of the SRU and the other plane being adjacent to an electrode of the next SRU. After assembly, adhering, machining and removal of mold material of all SRUs (the order of these steps being elaborated above), the entire multilayer stack is heat treated in one step to form a monolithic stack with internal flow channels.

[0102] Prior art manufacturing of monolithic SOEC / SOFC stacks, or conventional single SOEC / SOFC cells stacks, typically involves heat treating each cell individually prior to assembly into a stack. In such manufacturing, more than one heat treatment is required. Moreover, stacks formed in the conventional way are less compact (lower volume density) as the cells need a support layer (usually 150-500 pm thick) to provide the mechanical strength to the cells before being assembled with interconnects into a stack. In the monolithic structure, the interconnect is co-sintered with the rest of the cell (electrodes and electrolyte) thus providing the mechanical strength and preventing the need of an additional support layer.

[0103] A monolithic stack with internal voids is disclosed, being manufactured using the method of manufacturing a monolithic stack with internal voids also disclosed herein. Fig. 3 is a cross-section diagram illustrating an exemplary process of adhering the first and second material layers according to this disclosure.

[0104] Fig. 3 shows a first material layer 25 and a second material layer 28 encompassing a mold material 27.

[0105] Fig. 3 shows adhering the first material layer and the second material layer, for example by hot isostatic pressing. Fig. 3 shows a pre-adhesion boundary 16 between the first material layer and the second material layer and a post-adhesion boundary 18 between the first material layer and the second material layer. The pre-adhesion boundary 16 can be seen as the boundary between the first and second material layers prior to adhering the first and second material layers. For example, the first material layer 25 has been applied to, such as laid on, the second material layer 28. The pre- adhesion boundary 16 can be seen as the contact patch between the first and second material layer, e.g., the point of contact where the first material layer 25 has been applied to the second material layer 28. In some exemplary embodiments, more than two separate material layers are used to encompass the mold material, in which case a second pre-adhesion boundary 17 will arise. This may be a result of one of the first or second material layers being applied in two separate steps, or of a third material layer (not shown) being applied to encompass the mold material. The second material layer 28 may be a second green layer with a material composition comprising a second powder and a second binder. In an exemplary embodiment related to electrochemical devices where the voids are located next to an electrode, the first powder may be a ceramic powder and the second powder may be a metallic powder.

[0106] The post-adhesion boundary 18 can be seen as the boundary between the first and second material layers after adhering (e.g., pressing) the first and second material layers to from a multilayered structure 29. Arrow 20 is for example indicative of carrying out adhering, such as by hot or cold isostatic pressing, of the first and second material layers. After adhering (indicated by arrow 20), the first and second material layers can be seen as forming the multilayered structure 29 having at least one mold filled interstice 30.

[0107] Fig. 3 shows an isostatic pressure vessel 21 for application of pressure to the mold material and the first and second material layers. The pressure (e.g., greater than atmospheric pressure) within pressure vessel 21 may exert force directly on the first and second material layers. Fig. 3 shows forces, represented by arrows 22, and 23, between the mold material and the first and second material layers. Arrows 22 indicate force applied to the mold material 27 via the first and / or second material layers. Arrows 23 indicate force applied to the first and / or second material via the mold material.

[0108] In an exemplary embodiment, the step of adhering the first and second material layers comprises decreasing a viscosity of at least part of the mold material 27 prior to or simultaneously with adhering and without initiating a phase change to gas phase in and / or combustion of the first binder. Hence, during the adhering of the first and second material layers, the mold material 27 may be in a liquid state or a quasi-solid state. As shown in Fig. 3, this enables the mold-filled interstice 30 to, such as after adherence (e.g., pressing), to have smoother edges and angles. As previously mentioned, this may advantageously result in a reduced geometric energy state of the mold filled interstice 30. For example, the edges of the mold-filled interstice 30 may have a lower stress concentration such that the multilayered structure may be less prone to crack formation and / or crack propagation.

[0109] The following figures describe an exemplary embodiment of manufacturing of a solid oxide fuel / electrolysis cell (SOFC / SOEC). For manufacturing of other exemplary electrochemical devices, such as batteries or solid oxide electrolyzers such as SRUs or stacks for alkaline electrolysis (AEL) and proton exchange membrane electrolysis (PEMEL), the basic manufacturing of the multilayer structure is analogue, but dimensions, channel or void layouts, and materials may differ.

[0110] Figs 4A-C are diagrams illustrating an example method for manufacturing a multilayer structure 29 with internal voids according to this disclosure.

[0111] Fig. 4A shows example assembly of elements of an example multilayer structure prior to adhering the first and second material layers. Fig. 4A shows a first material layer 25. Fig. 4A shows indents 26 in the first material layer, such as formed by one or more green machining processes previously described herein. Fig. 4A shows a mold material 27 which has been applied to the indents 26 of the first material layer 25. Fig. 4A shows a second material layer 28 being applied, such as laid on, the first material layer 25. Indents 26 can for example be seen as mold filled interstices once the second material layer is applied to the first material layer. One of or both the first and second material layers 25, 28 can be seen as a first green layer comprising a first powder and a first binder.

[0112] Fig. 4B shows an example multilayer structure 29 after adhering the first and second material layers but prior to machining so that the mold material is still completely encompassed inside the structure with no possibility to leak out during the adhesion, even if it is in a liquid or quasi-solid state. Fig. 4B also shows the multilayer structure 29 after fluid connections (e.g., openings) between the mold filled interstices 30 and the outside of the multilayer structure 29 have been formed. For example, the fluid connections are formed by machining (e.g., cutting) the multilayer structure 29. In an exemplary embodiment, machining the multilayer structure 29 comprises increasing a viscosity of the mold material 27 prior to machining, such as by cooling the mold material 27 or the multilayer structure 29. The multilayer structure is preferably machined with the mold material being in a solid-state to avoid smearing of the mold material 27. The face of the fluid connections can be seen as perpendicular to the planes of the first and second material layers (25 and 28 respectively) of which the multilayered structure 29 is comprised (as shown in Fig. 4A). Fig. 4B shows the multilayer structure 29 comprising internal voids 31 . The internal voids 31 can be seen as the mold filled interstices where the mold material 27 has been removed. The internal voids 31 are formed by removing the mold material 27 from the mold filled interstices 30 via the fluid connections.

[0113] Fig. 4C shows a multilayer structure 29 where the multilayer structure 29 has been machined to provide fluid connections with a greater surface area than shown in Fig. 4B. In other words, multilayer structure 29 has been cut such that the fluid connections comprise faces both perpendicular and parallel to the planes of the first and second material layers (25 and 28 respectively) of which the multilayered structure 29 is comprised. Fig. 4C shows the multilayer structure 29 comprising internal voids 31. The internal voids 31 can be seen as the mold filled interstices where the mold material 27 has been removed. The internal voids 31 are formed by removing the mold material 27 from the mold filled interstices 30 via the fluid connections.

[0114] Removal of the mold material 27 comprises decreasing a viscosity of at least part of the mold material 27 without initiating a phase change to gas phase in and / or combustion of the first binder and removing the mold material 27 from the at least one interstice via the fluid connection to form at least one internal void 31 in the multilayer structure 29. In an exemplary embodiment, this comprises heating the multilayer structure 27 to a temperature so high that the mold material becomes liquid but still smaller than a temperature where organic additives in the first and second binder undergo sublimation and / or evaporation and / or combustion. The temperature where the mold material becomes liquid depends on the used mold material. In exemplary embodiments, it is a temperature between 40 - 400 °C, such as between 40 - 100 °C or 40 - 150 °C.

[0115] Figs. 5A-5B are diagrams illustrating an example method for manufacturing a multilayer structure with internal voids according to this disclosure. Fig. 5A shows assembly of a multilayered structure comprising a first material layer 25 and a second material layer 34. Fig. 5A shows mold material 27. For example, the first material layer 25 and the mold material 27 is the same as shown in Fig. 3 and Fig. 4A. Here, the second material layer 34 may be a multi-layer comprising multiple layers, such as an electrode-separator-electrode layer formed by thin, possible green layers. The multi-layer may be a prefabricated green layer which may be applied in one step so save time over applying three separate layers on the structure.

[0116] Fig. 5B shows a multilayer structure comprising a first material layer 25, a second material multi-layer 34 and internal voids 31 . The multilayered structure shown in Fig. 5B can for example be seen as single solid oxide cell with integrated gas channels. For example, the multilayered structure can be seen as the assembled multilayer structure shown in Fig. 5A after adhering the first and second material layers (25 and 34 respectively), after machining to provide fluid connections, and after removing the mold material 27 from the mold filled interstices to form internal voids 31 in the multilayer structure. The multilayer structure shown in Fig. 5B may be heat treated (e.g., debinded and / or sintered).

[0117] Fig. 6 show diagrams illustrating an exemplary assembly of a multilayer structure according to this disclosure, where two multilayer structures are assembled to form a monolithic single repeating unit (SRU) 44 as shown in Fig. 7B. Fig. 6 shows step by step an example method for manufacturing a multilayer structure with internal voids. The diagrams shown in Fig. 6 are exploded diagrams. The order of the steps shown in Fig. 6 is top diagrams left to right, then bottom diagrams left to right. Fig. 6 shows a first material layer 25 and indents 26 formed in first material layer 25 by one or more green machining processes previously described herein. Fig. 6 shows a mold material 27 which has been applied to the indents 26 of the first material layer 25. Before or after filling in the mold material, the first material layer 25 is applied to a first material layer part 40 of the same or similar composition to close the underside of indents 26. A second material layer 34 is applied to the first material layer 25 to close the upper side of indents 26, the second material layer 34 is here a here a multi-layer comprising electrodes (such as fuel electrode and oxygen electrode) sandwiching a separator (such as an electrolyte) layer. The electrodes and separator layers of layer 34 may be provided and applied as single, prefabricated layer or be applied separately on top of each other to form layer 34 on layer 25. The applicable materials and dimensions for this fuel electrode - electrolyte - oxygen electrode sandwich may be specific to SOFC / SOEC’s and well-known to the skilled person. For other electrochemical devices such as batteries, a cathode - solid separator - anode sandwich will be used instead with applicable materials and dimensions well-known to the skilled person.

[0118] A first material layer 41 comprising indents 26 is applied on the second material layer 34 (preferably so that the indents 26 of the first material layer 41 are perpendicular, in the horizontal plane, to the indents 26 of the first material layer 25) to the second material layer 34. A first material layer part 42 of the same or similar composition is applied to the first material layer 41 to close the upper side of indents 26 therein. Once all layers shown in Fig. 6 have been applied (in other words, once the structure has been assembled), the layers can be adhered to form multilayer structure 43 shown in Fig. 7 A, with adhered layers 25 and 40 hereafter referred to as layer 25 and adhered layers 41 and 42 hereafter referred to as layer 41. After adhering, the mold-filled indentations 26 are referred to as mold-filed interstices 30, see in Fig. 7A.

[0119] Figs. 7A-B are diagrams illustrating exemplary manufacturing steps of a monolithic SRU. Following the adhering and machining of the layers in the last diagram of Fig. 6, Fig. 7A shows the resulting multilayer structure 43 with the fluid connections to the mold-filled interstices 30 visible. Fig. 7B shows the multilayer structure 43 of Fig. 7A after the mold material has been removed (e.g., by decreasing a viscosity of at least one part of the mold material 27) from the mold filled interstices 30 to form voids 31 . The multilayer structure of Fig. 7B can be heat treated (e.g., debinded and / or sintered) to form a monolithic SRU 44, e.g., of a monolithic fuel cell I electrolysis stack.

[0120] Fig. 8 is a diagram illustrating an example monolithic stack 45 with internal voids 31 , comprising three monolithic SRUs 44 according to this disclosure. The monolithic stack 45 of Fig. 8 can be seen as a monolithic fuel cell I electrolysis stack. The monolithic stack 45 comprises three SRUs 44 shown in Fig. 7B. The monolithic stack 45 may be assembled by applying the material layer 25 to the material layer 41 of another structure multiple times. This assembly may take place either after adhering, machining or removal of mold material, but should take place prior to the heat treating. The resulting multilayer structure may be heat treated (e.g., debinded and / or sintered) to form monolithic stack 45.

[0121] Figs. 9A and B are a SEM Cross section microscopy images of a monolithic solid oxide cell (SOC) stack manufactured in accordance with the disclosed method. All the layers from Fig. 8 (first layers 25, 41 , second layers 34, and empty channels 31) can be seen. In this structure, the channels were machined by cutting through the first material layer, then the second layer material was added to act as a base for the filling with the mold material. The procedure was repeated for all the layers following the process illustrated in Figure 6, leaving no ways out to the channel forming mold before cutting the edges. Figs. 9A and B show cross-sections in different planes normal to the channels in layer 25, where empty cannels in layer 41 are visible in Fig. 9B.

[0122] Figs. 10A-B are diagrams illustrating layers of an example monolithic multilayer structure 50 with internal voids according to this disclosure. Fig. 10A shows exemplary first material layers 51 being shaped to have circular shape and have indentations 53 that may be filled with mold material (not shown). The first material layers 51 can be stacked on top of each other so that indentations 53 align to form channels. Second material layers 52 are provided to close the channels so that layers 51 and 52 encompasses the mold material in indentations 53. After adhering, the resulting multilayer structure can be machined to remove second material layers 52 and form fluid connections to the mold-filled interstices, whereafter the mold-material can be removed as described herein. Fig. 10B shows the resulting monolithic structure 50 with internal voids 54.

[0123] The selection of suitable mold material depends primarily on the material composition of the green layer(s), since the rheological behavior of the mold material is to be adjusted without the green layer(s) and the mold material reacting chemically and / or deform the multilayer structure and without initiating binder burn-out in the green layer. Referring to the previous discussion of viscosity change and ‘states’, the following provides a discussion of the related mold material characteristics.

[0124] To prevent chemical reaction with the green layer(s), the mold material preferably has a low affinity with the used green matrix solvent (such as EtOH, MekEt, water ...), and a low solubility toward the binder (such as polyvinyl butyral (PVB) and polyvinyl pyrrolidone (PVP)) and other organics present in the binder mixture like plasticizers (such as poly-ethylene glycols) and surfactants and lubricants (such as fish oil or Additol).

[0125] To facilitate a low friction or frictionless extraction of the mold material, the decreased viscosity state of the mold material is preferably a good lubricant. Some liquids are better lubricants than others of the same viscosity. Lubricity is a value characterizing the boundary lubrication between the lubricant and a solid (here the first and second material layers) and is due to chemical action between the lubricant and the solid and the physical condition of the rubbing surfaces (here solid or quasi-solid mold material and first / second material layer). Hence, a preferred mold material has a low lubricity in a liquid state, such as a low coefficient of friction, where the coefficient of friction is measured as the ratio of the frictional force to the normal force between a surface of the multilayer structure in contact with a surface of the mold material. It may be preferred that the mold material is selected to have, for example in the decreased viscosity state, a coefficient of friction with a surface of the multilayer structure that is lower than 0,1 , such as lower than 0,05.

[0126] If the viscosity change is induced by heating / cooling the mold material, a melting point (MP) of the mold material with respect to the temperature at which the binder mixture approaches the glass transition temperature in the green layer(s) is determining for the selection. Here, two approaches are provided:

[0127] In exemplary embodiments, the mold removal is preceded by liquefaction of only outer parts of the mold material, i.e., where a viscosity of the mold material is decreased to induce a liquid state in parts of the mold material in contact with the multilayer structure. These partial liquefaction embodiments are particularly applicable when the mold material is one or more of waxes and soaps. Since only the parts in contact with the multilayer structure (the inner surfaces of the interstices) are to be liquefied, a shorter heating time at higher temperature can be used. Thereby, mold materials with melting points up to 10 C above the glass transition temperature of the binder(s) may be used without damaging the green layer.

[0128] In exemplary embodiments, the mold removal is preceded by a complete liquefaction of the mold material, i.e. where a viscosity of the mold material is decreased to induce a liquid state in at least a major part of the mold material. These complete liquefaction embodiments are particularly applicable when the mold material is one or more of oils, emulsion of oils, waxes, soaps, emulsion of water and oil and / or waxes and emulsion of water and / or oil and / or waxes and soap. Since a major part of the mold is liquefied, the required heating time will typically be longer than when only parts in contact with the multilayer structure are liquefied. It is preferred that the melting point of the mold material is lower than a glass transition temperature of the binder(s), such as 10-15°C lower. In an exemplary embodiment where a binder is polyvinyl butyral (PVB), It is preferred that the melting point of the mold material is lower than 70 °C.

[0129] Where viscosity change is induced by heating / cooling, there are different approaches to heat / cool the mold material. In exemplary embodiments, the green multilayer structure can be placed in an oven or a refrigerator / freezer. In exemplary embodiments, the green multilayer structure can be cooled / heated by while a thin heating / cooling element is applied to inner parts of the mold to melt / solidify the mold material from within. This may only be possible after machining the multilayer structure and thus not applicable prior to or during the step of adhering. In exemplary embodiments, the mold can be melted by a laser or microwave radiation. Using wavelengths with poor absorption in the green layer(s) and proper focusing on the mold material, this approach may even allow melting the mold through the green layer.

[0130] In exemplary embodiments, two general rules for the selection of mold material may be formulated:

[0131] 1. The mold material may be any compound that liquefies or partially liquefies at a temperature below a glass transition temperature and / or 100°C below the decomposition I evaporation I combustion temperature of the binder in the green layer(s); and

[0132] 2. the mold material may be any compound with a solubility of less than 2 g / ml at ambient temperature and pressure in a solvent used in the green layer(s). As described previously, the viscosity reduction may also be induced by adding a solvent to the mold- filled interstices to dissolve at least part of the mold material. Here, it is important that the solvent for the mold material is not also a solvent for the green layer(s). For example, where the mold material is a soap, water is a solvent for the soap whereas ethanol is a solvent for the green layer(s) binder. Water and ethanol are miscible, so water can also interact with the binder. However, given the higher affinity with soap, the interaction of water with the assembled multilayer structure will allow to remove the mold, without compromising the integrity of the structure.

[0133] Exemplary oil- and wax-based mold materials for ethanol based binder systems, their characteristics, and details of relevant method steps are described in the following.

[0134] The general composition of an oil and wax emulsion that may be used as mold material comprises three agents: a wax with higher melting point in higher mass quantity that provides low solubility with the binder mixture; a structuring wax with the highest melting point to allow the quasi solid behavior at ambient temperature, a fluidifying agent that with low melting point (close to ambient temperature) that facilitates the liquefaction of the emulsion. The emulsion can also be made adding a low percentage of salt (up to 2% weight) or soap (such as sodium stearate) to substitute the structuring wax (up to 15% weight) to increase the lubricity. The emulsion can also be made adding water up to 15% in weight as additional fluidifying agent. The weight content can be tuned in each emulsion to adapt the solid I quasi solid I liquid transitions as desired. Examples of emulsions are:

[0135] • Synthetic recipe of stearin emulsion: o Stearin (insolubility) [66 wt%]: o palmytic acid (structure) [1-10 wt%] o oleic acid (fluidifier) [24%-33 wt%]

[0136] • Synthetic recipe of stearin emulsion with enhanced lubricity: o Stearin (insolubility) [66-56 wt%]: o palmytic acid (structure) [5-0 wt%] o sodium stearate (structure + lubricity) [9-15 wt%] o oleic acid (fluidifier) [20%-29 wt%]

Claims

CLAIMS1. A method (100) for manufacturing a multilayer structure (29, 43, 44) with internal voids (31) for a monolithic stack in a solid-state electrochemical device, the method comprising: providing (S102) at least a first material layer (25) and a second material layer (28) encompassing a mold material (27), wherein at least a first material layer is a first green layer with a material composition comprising a first powder and a first binder; adhering (S104) the first and second material layers to form a multilayered structure (29) having at least one mold-filled interstice (30) forming a precursor of an internal void and having no fluid connection to an outside of the multilayer structure; machining (S106) the multilayer structure to provide a fluid connection between the at least one mold-filled interstice and the outside of the multilayer structure; decreasing (S108) a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the first binder, and removing (S109) the mold material from the at least one interstice via the fluid connection to form at least one internal void in the multilayer structure; and heat treating (S110) the multilayer structure.

2. The method according to claim 1 , wherein the second material layer is a second green layer with a different material composition than the first green layer comprising a second powder and a second binder.

3. The method according to any of the preceding claims, wherein providing layers comprises forming at least one indent (26) in the first material layer which does not extend to an edge of the first material layer and applying the mold material to the at least one indent, and wherein the mold material fills the at least one indent so that the first green layer and the mold material form a substantially planar surface whereupon the second material layer can be applied.

4. The method according to any of the preceding claims, wherein the mold material is non-soluble or partly soluble by solvents in the first and second material layers and that the first and second materials layers are non-soluble by the mold material in a liquid or low-viscosity state.

5. The method according to any of the preceding claims, wherein the step of adhering the first and second material layers comprises adhering the first and second material layers with the mold material being in a quasi-solid or liquid state.

6. The method according to any of the preceding claims, wherein the method comprises increasing a viscosity of the mold material prior to machining the multilayer structure.

7. The method according to any of the preceding claims, wherein decreasing a viscosity of at least part of the mold material comprises inducing a liquid state in at least parts of the mold material in contact with the multilayer structure.

8. The method according to any of the preceding claims, wherein decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder comprises one or more of heating, applying pressure, adding a solvent capable of dissolving the mold material, and liquefaction.

9. The method according to any of the preceding claims, wherein decreasing a viscosity of at least part of the mold material without initiating a phase change to gas phase in and / or combustion of the binder comprises heating the multilayer structure to a temperature smaller than a temperature where organic additives in the first binder and a second binder undergo sublimation and / or evaporation and / or combustion.

10. The method according to any of the preceding claims, wherein the mold material comprises one or more of the following: polymers / hydrocarbons; alkanes; triglycerides; greases; waxes; oils; soaps; artificial or natural resins; mixtures of waxes and oils; emulsions of waxes and oils; emulsions of water, waxes, and oils; emulsions and / or mixtures of binder and wax and / or oils; emulsions of hard resin in grease.

11. A multilayer structure (29, 43, 44) with internal voids (31 ) manufactured by the method (100) according to any of claims 1-10.

12. A method for manufacturing a monolithic stack (45) with internal voids (31), the method comprising manufacturing at least two multilayer structures (29, 43, 44) with internal voids according to any of claims 1-10, wherein the at least two multilayer structures form a multilayer structure-stack with mold material being removed prior to the step of heat treatment, and wherein heat treating the multilayer structure-stack forms a monolithic stack (45) with internal voids.

13. A method according to claim 12, wherein the multilayer structure-stack comprises interconnects with flow channels, electrode layers, and electrolyte or separator layers to form a monolith solid-state electrochemical device such as a battery, a SOFC / SOEC stack, a gas separation device, an electrolyser.

14. A monolithic stack (45) with internal voids (31) manufactured by the method (100) according to any of claims 12-13.

Images