CA2703020A1 - Open cell porous material, and a method of, and mixture for, making same - Google Patents

Abstract

The porous material of the present invention is produced toy heating a dry powder mixture, containing mainly an organic solid hinder and inorgnnic particles and containing no foaming agent.
The mixture is heated to melt the organic binder.
The resulting solid structure comprising inorganic particles embedded in an organic binder is then heated to eliminate the organic binder, and finally healed again To melallurgically bond the remaining inorganic tri-dimensional network into a rigid structure having interconnected porosiiy.

Description

OPEN CELL 12O13OUS MATERIAL, AND A METHOD OF, AND MIXTURE FOR, MAKING SAME
CROSS REFERENCE. TO RELATED APPLICATIONS
The present application claims the benefit of priority to International Patent Application No. PCT/CA2007/001874 filed October 19, 2007 with the Canadian Receiving Office entitled "Heat Management Devices [Bing Inorgunir Foam" (hereinafter "the '874 application')- (The '874 application is incorporated by reference herein.) FIELD Of THE INVENTION
The. present invention relates. to porous materials, methods of making porous materials, and mixtures for making porous materials.

BACKGROUND OF TH TNVFNTION
Porous metal or ceramic materials are currently used for the fabrication of devices such as filters, heat exchangers, sound absorbers, electrochemical cathodes, fuel cells, catalyst SUpports, fluid treatment units, lightweight structures and biomatenals. The structures (open/closed porosity, pore size distribution and shape, density) and properties (permeability, thentlal, electrochemical and mechanical properties) required greatly depend on the application.
Closed porosity is generally sought lbr lighiwcight: structure while open porosity is particularly sought when surface exchange is involved or when permeability or pore connectivity is required.
Different approaches have been proposed for the fabrication of such porous materials, Reviews of manufacturing methods and characterization or porous metal and ceramic materials are given in United States Patent No. 6,660,224 (hereinafter "the '224 patent") and the documents referred to (herein. (The '224 patent and all the documents referral to therein are incorporaicd herein by reference.) In particular, the invention described in the '224 patent is a porous material that is produced by heating a dry powder mixture containing mainly an organic solid binder and inorganic particles. The mixture is foamed while the organic'binder is melted.
Foaming comes froin a foaming agent in the powder mixture. The resulting solid foamed structure that comprises the inorganic particles embedded in the organic binder is next heated to cure and then eliminate the organic binder and 11nally to Sinter the remaining inorganic three-dimensional network into it rigid structure having interconnected porosity.
As is more fully described. in the '874 application, some embodiments of the porous material described in the '224 patent are particularly well suited for use in working-liquid-phasc-change heat transfer devices, such as heat pipes and vapor chambers.
Notwithstanding the ativanecrnent presenied by the use of such materials in such devices, there are some executions of working-liquid-phase-change heat transfer devices that require a porous material with an even greater wicking capacity and smaller pore site than is possible to obtain by k llnwing the methods described in the `224 patent No other suitable method exists for producing such materials.

SUMMARY OF THE INVENTION
it is an object of the present invention to ameliorate at least some of the inconveniences present in the prior art.
It is also an object of the present invention to provide an open cell porous material, and a method of, and mixture for, making same, where the material is suitable for use in certain working-liquid-phase-change heat transfer devices.
Thus, in one aspect, as embodied and broadly described herein, the invention provides a method for making an open cell porous structure, comprising: (a) Providing a dry flowable powder mixture including (i) a fu-st predetermined amount of inorganic particles having a first melting temperature, (ii) a second predetermined amount of a binding agent having a decomposition temperature, the decomposition temperature being lower than the first melting temperature, and (iii) an absence of foaming agent. (i.e. no lbnning agent).
(b) Heating the mixture to a temperature lower than the decomposition temperature at least to cure the binding agent to obtain a solid structure (if the hinder is not a liquid or a gel it will first melt and surround the inorganic particles before being cured). (c) Treating the solid structure to the decomposition temperature to decompose the binding agent and obtain a non-metallurgically-bonded open cell porous slruclurc. *(A metallurgically-bnntded material is one that is held together by direct. metal atom to metal atom bonds. In the present step after the binder is heated to the decomposition temperature, the metal is oxidized and it is the oxygen atoms that are interconnecting the metal atoms and bonding the structure together.) It is highly preferred that the binder be cleanly decomposed, i.e. that it leaves no residue behind after its decomposition.
(d) Heating the non-metallurgically-bonded open cell porous structure to a temperature lower than the first melting temperature to metallurgically bond the inorganic particles and obtain a solid metallurgically bonded open cell porous structure. It is preferred that healing the non-metallurgically-bonded open cell porous structure to a temperature lower than the first melting temperature to metallurgically bond the inorganic particles and obtain a solid metallurgically bonded open cell porous structure comprises heating the nun-metallurgically-bonded open cell porous structure to a temperature lower than the first melting temperature to sinter the inorganic particles and obtain a solid sintered open cell porous structure. Sintering is usually accomplished by heating the structure to a temperature between 70% and 90% of the melting temperature of the metal to be sintered- Sintering in most cases will not eliminate the oxygen atoms but will create direct metal atom to metal atom bonds.
The present inventors have realized that in certain situations, contrary to what is taught in the `224 patent, it is possible to carry out a method similar to that described in the `224 patent, without having any foaming agent present in the mixture. It had been thought that if no foaming agent were present in the mixture there would not have been enough space for the gas produced by the decomposition of the binder agent to exit the material being iorrucd.
Thus, the binder agent decomposition gases would build up within the maternal and damage or destroy the material owing to pressure and/or combustion of the gases.
Surprisingly, the present inventors have observed than in some situations, particularly where the material to he made is of reduced thickness (e.g. less than 2 mm), damage or destruction of material does not occur nutwithstarlding the fact that there is no foaming agent present In the mixture from which the material is bcing produced. The absolute limit for any particular material before damage or destruction occurs will vary depending on the shape of the material, the composition of the inorganic particles from which the material is being produced, and the nature and type of binding agent being used. Simple visual inspection of the material will allow one to determine whether the limit has been passed.
Owing to the absence of foaming agent, the voids present in materials of the `224 patent that are created by the lb uning agent are not present in materials of the present invention.
Therefore, the capillary radius of capillaries of materials of the present invention is smaller (i.e.
in the order of between 50 100 microns) versus those in the materials of the `224 patent (i.e.
greater than 100 microns); the permeability of materials of the present invention is lower (i.e. III
the order of between 9.4x 10"12 m2 to 1.3x 10-" m2 than that of the materials of the `224 paten (i.e. higher than 1.3x1('" m2). Materials of the present invention thus have a significantly greater wicking strength and an equivalent or inferior pumping speed than materials of the `224 patent, Materials of the present invention can thus he used in applications (e.K. certain executions of working-liquid-phase-change heat transfer devices) for which materials of the '224 patent are not appropriate.
Furthermore, materials of the present invention are di flcrent than conventional sintered powder materials. The capillary radius of capillaries of materials of' the present invention is higher (i.e. in the order of between 50 100 microns) versus those in conventional sintered powder materials (i.e. lower than 50 microns); the permeability of materials of the present invention is higher (i.e. in the order of between 9.410-12 M2 to 1.3x10 rr ml ikan that of eunveritionat Sintered powder materials (i.e. lower than 9,4x10'12 m).
Materials of the present invention thus have a lower wicking strength and signil,tcantly higher pumping speed than conventional sintered powder materials. Materials of the present invention can thus be used in applications (e.g. certain executions of working-liquid-phase-change heat transfer devices) for which conventional sintered powder materials are not appropriate.
Preferably, the first predetermined amount of inorganic particles is between about 10 wt % to about 90 wt % inclusive of a total weight of the mixture. More preferably, the first predetermined amount is between about 40 wt % to about 90 wt % inclusive of the total weight of the mixhire. Still more preferably, the first predetermined amount is between about 55 wt %
to about 80 wt /n inclusive. Most preferably it is between 60 wt % to about 75 wt. % inclusive.
The first predetermined amount is selectable by persons skilled in the art according to the final use of the material to be made. For instance, for applications where a high thermal conductivity is required, it is likely that first predetermined amount will be at the relatively high end of the ranges disclosed (e.g. about 75 wt % or higher). Forapplications where low density is required, it likely the first predetermined amount will be at the relatively low end of the ranges disclosed (e.g. about 60 wt % or lower).
It is preferred that the second predetermined amount of binder agent he between about 10 wt % to about 90 wt % inclusive of the total weit;bt of the mixture. More preferably, the second predetermined amount is between about 20 wt % to about 35 wt /r inclusive. In mixtures whore only the inorganic particles and the binding agent are present (e.g. metallic particles and a thermoset binding agent), the wt % of the binding agent will he directly related to the wt % of the inorganic particles. In other mixtures (e.g. metallic particles, a thermoplastic resin, and a curing agent), the binding agent will likely be the vast majority of the wt %
of the mixture that is not. inorganic particles, Preferably, the inorganic particles consist essentially of non-rneLallic particles (preferably ceramic particles), metallic particles, or combinations thereof. The selection will depend on the final use to which the material being made is put, and thus hue required characteristics thereof (e.g. thermal conductivity, electrical conductivity, wicking capacity, absorptive capacity, etc.).
Where the mixture contains metallic particles, it is preferred that the particles be at least one of metal particles and metal alloy particles. In some such casts it is preferred that the metallic particles be metallic particles of at least one transition metal, and preferably at least one transition metal selected from the group consisting of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, and gold. More preferably, the metallic particles are al least one selected from the group consisting of copper, nickel, iron, titanium, copper-based alloy particles, nickel-based alloy particles, iron-based alloy particles, and titanium-based alloy particles.
Most preferably the 5 metallic particles areal least one of copper and copper-based alloy particles. These materials are preferred given their ability to be (relatively) easily sintered.
Tt is also preferred that for some applications the inorganic particles consist essentially of coated particles. The particles may he coated by chemical reaction (e.g. an alurinum particle will generally oxidize in an oxidizing environment to produce an alurninurn particle with an aluminum oxide coating (i.e. outer layer)) or by mechanical deposition (e.g. a copper particle mechanically coated with a silver-based brazing agent).
It is also preferred that the binding agent: be a therinuset resin or a thermoplastic polymer.
Suitable resins and polymers are well known in the art. In such cases, it is preferred that the binding agent he blended with the other component(s) of the mixture by dry mixing or milling.
Where the binding agent is a thermoplastic polymer, it is preferred that the binding agent be cured with the aid of a curing agent, or alternatively by an irradiation cross-linking treatment, or x light-exposure cross-linking treatmcnl It is also preferred that the mixture further include at least one additional agent adapted to minimize segregation and dusting and to improve the flowability ot'thc mixture. Such agents are well known in the art. An example would he a fine silica power that is added to the mixture in a very small amount (e.g. less than 0.01 wt %) where the mixture is to be injection molded or extrusion molded.
Ti is preferred that the mixture be subject to successive increases of temperature during execution of (b), (c), and (d) set forth above. For methods carried out in a continuous process, it is preferred that the temperature be increased in a stepwise manner.
It is preferred that (a), (b), (c), and (d) set forth above he carried one of continuously, sequentially, partially continuously and partially sequentially.
It is also preferred that pressure be applied to the mixture at least one of before and during the heating thereof in (h), (c) or (d) as set forth above. Pressure can he used for various purposes depending on at which point in the process the pressure is being applied. For example, pressure in the order of 206 kPa to 278 kPa (30 to 40 psi), applied via an hydraulic press exerting force on the mold containing the mixture, can be used before (b) in order to ensure a smooth finish to the final material to be made. As another example, pressure in the order of 7 kPa (1 psi), applied via the application of a perforated flat plate, can be applied during (c) to ensure that the final material will be flat and not warp. As another example, pressure in the order 890 kPa (129 psi), applied during (d) via the application of force to the mold in which the rrialerial is being made, can be applied to ensure that bonding of the material to a substrate (e.g. a copper plate occurs (see below). As a final example, pressure can also be used during extrusion or injection molding, if ihese are part of'the process- The selection of the amount of pressure, how it is applied, and when it is applied, is within the understanding of those skilled in the art.
It is also preferred that the method further comprise shaping the mixture, preferably before it is heated. In such cases it is preferred that the shaping be carried nut via at least one of molding, deposition, lamination, and extrusion. Each of these processes is well known in the art.
In addition, at any one of various stages in the process, the intermediate or final structure can be machined through the use or, number of conventional machining t.t,ehsriyucs.
In some cases, it is also preferred that the method further comprise providing a substrate, and that the mixture be disposed on the substrate prior to (d). The presence of and selection of a substrate will depend on the application to which the material to he made is being put. For example, the substrate may be a copper plate where the material will be used in a vapor chamber.
The copper plate provides good thermal conductive properties as well as mechanical support for the material, enabling it to het:ter serve its intended function irl the vapor chamber.
It is also preferred that the mixtures further comprise at least one spacing agent. As is well known in the art, a spacing agent is added to mixtures to occupy space during the (urination of the materials, which will create a void in the material when the spacing agent is removed. An example is a salt, that is not affected by the application of heat during the manufacturing process, but that Can be removed from the final material by being dissolved in an appropriate liquid (i.e.
by leaching), usually water. In the context of the present invention, it is preferred that the at least one spacing agent he a scaffold. It is also preferred that the at least one spacing agent be removed by at least one of thermal decomposition and leaching.
In some instances it is preferred that the mixture further comprise at least one brazing agent to metallurgically bond the inorganic particles, as is described in international patent application l'CT/C'A2007/000679 filed April 23, 2007 entitled "Open Coll Porous Material and Method fur Producing . rime" published as WO 2007/121575 Al on November 1, 2007 (which is incorporated herein by reference). Brazing is usually used in place of sintering (as opposed to in addition to). A brazing agent creates a solid solder-like bond between Adjacent particles which results in a material having generally improved mechanical properties.
Typically, brazing is achieved at a tower temperature and in a shorter time, than a conventional sintering step, and can thus lead to reduced manufacturing time and reduced energy costs. Many conventional brazing agents exist Typically brazing agents are silver, copper or cadmium-based powders.
h'urthcr, in additional tsspcets, as embodied and broadly described herein, the invention, provides an open cell porous structure made according to the methods described hereinabove, as well as to a mixture as described above used therein.
Embodiments of the present invention each have at least one of the above-mentioned objects and/or aspects, but do not necessarily have all of them. It should he understood that some aspects of the present invention that have resulted from attempting to attain the ahovc-mentioned objects may not satisfy these objects and/or may satisfy other objects not specifically recited herein.

DETAILED DESCRIPTION
The porous material according to the present invention is produced from a dry flowable powder mixture comprising a base material and a binding agent, each provided in predetermined amounts, and having an absence of (i.e. no) foaming agent. 'l'hc base material includes inorganic particles having a first melting temperature, the binding agent is preferably, but not exclusively, an organic binder having a decomposition temperature lower than the first melting temperature and having clean bum out characteristics. All of these materials are readily available from appropriate commercial suppliers.
As it will be readily understood, the exact amount of each constituent of the mixture is determined, prior to the execution of the method of the present invention, based on the physical and chemical properties of the inorganic particles and of the binding agent.
and based on the desired properties of the finished open cell porous structure. Consequently, the exact composition of the mixture will vary according to the nature of the base material and of the binding agent.
The inorganic particles comprise metallic particles, metallic alloy particles, ceramic particles, coated particles and/or a combination thereof. In the case of metallic and metallic alloy particles, the metal or metals are preferably transition metals (e-g- copper, nickel, iron) as defined by the periodic table of elements. The inorganic particles will have a first melting temperature. The inorganic particle content may be between about 10 to about 90 wt %
inclusive of the total weight of the mixture (preferably between about 40 to about 90 wt %
inclusive, more preferably between about 55 wt % to 80 wt % inclusive, still inure preferably between about 60 wt % to 75 wt "/n inclusive). The exact amount of the inorganic particles and the choice thereof will be determined by the skilled addressee. depending on the requirement-, of the application for which the open cell porous material ix being manufactured.
The hinder used in the mixture is preferably an organic hinder provided in a dry Plowable powdered form and with clean burn out characteristics. The binder can be a thermoplastic polymer, a thermoset resin and/or a combination thereof The binder can also he an inorganic binder, a synthetic binder or a mixture of organic and/or inorganic and/or synthetic binders. The binder may be provided in solid forte (preferably powder particles), in semi-solid fdrnn, in liquid form, in gel form or in semi-liquid form. The binder has a decomposition tcmper2htrc lower than the first melting temperature of the inorganic particles in order to prevent premature melting of the inorganic particles during the decomposition step. Though the binder content in the mixture may vary from about 10 to about 90 wt % of the total weight of the mixture, the exact amount thereof will be determined by the skilled addressee depending on the nature of the inorganic partid;ld:s and on the requirements of the application for which the open cell porous material is being manufactured. Most preferably, the hinder should no decomposition products in the porous structure. However, some residues can be accepted if they do not negatively affect the final properties of the final product or if they improve some of its properties.
Optionally, the mixture may comprise a curing agent (e.g. a cross-linking) agent to induce faster curing of the hinder during or after the curing step and improve the mechanical strength of the cured structure before the decomposition of the binder.
Optionally, the mixture may also comprise other additives such as a lubricant to ease shaping, molding or demolding, or flowing agents to improve the flowability of the powder when all the constituents are in powdered (tour).
The organic binder can be blended with the other Constituent using various techniques such as, but not limited to, mixing, milling, mixing the binder in suspension or in solution in a liquid, blending the binder in molten, liquid, gel or semi-liquid form with the inorganic particles and the other additives. Whichever mixing technique is used, the resulting product should be a curable mixture.
In other variants, a spacing agent may be added to the mixture to provide additional porosity and to improve pore ecrrrncelivity. A spacing agent is removed after curing to leave voids in the structure after decomposition of the binder or after sintering.
The spacing agent can be removed by thermal decomposition after curing or by leaching after curing, decomposition of the binder or sintering. The spacing, agent can be particles or a scaffold.
When particles are used, they are admixed with the rest of the Mixture. In one non limitative example, the spacing agent can he polymeric particles admixed with the mixture. In this case, the spacing agent concentration can vary from about 5 to 50 wt % inclusively, but preferably between 10 and 30 wt % inclusively. When a scaffold is used, its porous structure is filled with the mixture used to produce the porous material. The scaffold is, for example, a polymeric foam, that can be filled with the mixture and removed by thermal decomposition or by leaching.
It is also contemplated to add additional binder in amount varying between 0.05 wt % to 5 wt %, but preferably between 0.05 wt % to I wt %, in the mixture. This additional binder may be generally used to hind different constituents of the mixture together in Such :1 way that the final product is less prone to segregation and/or dusting. The additional binder may he added at different steps of the mixing procedure, either bctbre mixing the inorganic particles with the binder, after the binder addition, after the lubricant addition, after the flowing agent addition or after the addition of any combination of those constituents. Whichever mixing technique is used, the resulting product should be a curable mixture.
The resulting mixture may be shaped using methods such as molding, deposition, lamination or exttuaien_ The product is then heated at a moderate temperature to melt the binder, IS if the latter is not already in liquid, gel or serni-liquid form, and to initiate the curing of the mixture. Optionally, pressure may be applied to the mixture before or during heating the mixture.
The porosity and structure of the resulting open cell porous material will depend on the particle size, shape, density and content of the inorganic particles; the content and viscosity of the hinder, as well as the processing conditions. However, in most cases the material will have two pore groups, namely a first pore group and a second pore group. The first pore group has an average pore site in the range from about 20}tm to about 200 m, preferably in the range from about 40 m to about 150pm and most prefcrahly from about 60 m to about 100itm.
In each case the standard deviation is in the range from about 10 m to about I001im.
The first pore size group constitutes from about 50% to about 80% of the void volume of the metallic porous sttucture. The second pore group has an average pore size in the range from about 250n 1 to about 15 m, preferably in the range from about 500iun to about 15 m and most preferably from about 500ttm to about Wpm. In each case the standard deviation is in the range from about 200tun to about 10 m. The second pore size group constitutes from about 20% to about 50% of the void volume of the metallic porous structure. The capillary radius will he an average of the two pore groups, thus on average, relatively low because of the second pore group, The first pore group will lead to a high permeability. Hence the Structure provides a high permeability and low capillary radius leading to a high pumping speed.

lO
Materials ran be cured in a mold to provide three-dimensional porous structures.
The mixture can be cured on or in a substrate to produce aA coating or to produce eompositt;
structures. Curing can be done for example on a plate, on a rod, in or outside a tube Or cylinder, ter or OIL other porous structure (mesh, beads, foam for example) or any other substrate. The material can be machined after curing, decomposition of the binder or sintering.
I''unctionrtlly graded materials can he produced using mixtures with variable composition.
Graded layered structures can be produced for example by deposing layers of mixtures with different composition. Functionally graded materials can also be produced by controlling the thermal gradient during curing in order to control material curing and pore site distribution.
Optionally, the mechanical strength of the cured structure may be further increased, bei:ore decomposition of the hinder and sintering, by using externally Assisted cross-linking techniques such as irradiation or light exposure.
After curing and optionally cross-linking, the cured mixture is treated at a sufficiently high temperature to decompose the binder. The atmosphere (with or without the presence. of oxygen), duration and temperattue of the thermal treatment should preferably allow a clean decomposiliort ol'1.he hinder. Hinder decomposition should prel:crably not deteriorate the three-dimensional structure of the cured mixture. if gas pressure generated during binder decomposition is too great. cracking in or destruction of the still unmetallurgically-bonded structure may occur. Oxidizing or reducing conditions during the thermal treatments may be chosen to optimize binder decomposition. After decomposition, the cured structure is composed of upon cull metal (usually oxidized metal), and/or rectal alloy (usually oxidized metal alloy), and/or ceramic .material particles.
Sintering (metallurgical bonding) is done after the decomposition of the binder to create bonds between the inorganic particles of the cured mixture. Sintenng conditions (temperature, time and atmosphere) should be such that the inorganic particles do not melt to create the bond between them; conditions should be such that the material particles adhere to each other through a bond mainly created by solid-state dif'ftrsion to form a strong metallurgical joint between them.
Effective solid-state dtftusion occurs between material particles when they are heated, for a certain time, at temperatures slightly under the melting temperature of the material particles.
Sintering is generally done in reducing atmosphere for metal particles to avoid the formation of surface oxides on. the structure and to reduce the oxides that were present prior to sinteruig.
Mechanical strength may be adjusted for the application. The choice, size, nature and/at physical state of the inorganic particles and of the binder Cunteut will have a substantial influence of the physical properties (e.g, mechanical strength) of the produced open cell porous material.
Additional treatment can be done on the porous material produced- The internal surface of the structure can be modified for example by heat treatment, chemical treatment or deposition of coatings using various state of the art deposition techniques. The external surfaces of the structure can he modified for example by a stamping, etching, embossing, or grooving teclaniyuc and by state of the art surface coating techniquues. The structures can be integrated in other products and/or to other structures using different state of the art techniques such as diffusion bonding, press fitting, welding, brazing, sintering or gluing. The invention is not so limited.
Example 1 In a first specific example, a metallic open cell porous structure, with copper (0j) as the base material, was produced from a mixture having the formulation presented the table below.
TABLE I
Inorganic Particles Binding Agent Cu Powder Phenolic Resin -- 70 wt.% 30 wt yin The different constituents were dry-mixed together until the mixture became humug;encvus. A1ler mixing, the mixture was poured into a mould and cured at 110"C in air for 2 hours. After curing, the material was submitted for the decomposition of the binding agent in a furnace at 654 C for 4 hours in a dry air stream. Finally, the material was sintered in a 7S%Arl25%H2 atmosphere for 2 hours al 1000 C'. (The melting temperature of copper is 1080 C) Example 2 In a scuond specific example, a metallic open cell porous structure, with nickel (Ni) as the base material, was produced from a mixture having the firrniutation presented the table below.

TABLE, 2 Inorganic Particles Binding Agent Ni Powder Phenolic Resin 70wt.% 30wt.%

The different constituents were dry-mixed together until the mixture became homogeneous. After mixing, the mixture was poured into a mould and cured at I
[0 C in air for 2 hours. After curing, the material was submitted for the decomposition of the binding agent in a furnace at 650 C: fur 4 hours in a dry air stream. Finally, the material was silitcrcd in a 75%Ar/25%TT,2 atmosphere for 2 hour at 1300 C .

H:xample 3 In a third specific cxamplk, a metallic open cell porous structure, with iron (Fe) as the base material, was produced from a mixture having the formulation presented the table below.

Inorganic Particles Binding Agent Fe Powder Plicoolic Resin 7Owt.% 30wt.%
The different constituent,, were dry-mixed together until the mixture became homogeneous. After mixing, the mixture was poured into a mould and cured at 1 10 C in air for 2 hours. After curing the rrla.t.cnal was subrnillcd for the decomposition of the binding agent in a furnace at 650GC for 4 hour, in a dry air stream. Finally, the material was sintered in a 75%Ar/25%1I atmosphere for 2 hours at 1400 C.

Example 4 In a fourth specific example, a metallic open cell porous structure, with copper (Cu) as the base material, was produced from a mixture, having the formulation presented in the table below.

Inorganic Particles Binding Agent Brazing Agent Cu Powder Phenolic Resin 72% Ag &
28 % Cu 6Owt.% 3Owt.% 10wt.%

The different constituents were dry-mixed together until the mixture became homogeneous. After mixinu, Ilic mixture was poured into a mould and cured at I
IOC in air for 2 hours. After curing, the material was submitted for the decomposition of the binding agent in a furnace at 650'C for 4 hours in a dry air stream. Finally, the material was brazed in a 75 /õ A.r/25%H~ atmosphere for I hour at 785 C.
Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting, the invention. Various modifications will become apparent to those skilled in the art and arc within the scope of this invention, which is defined more particularly by (tic attached Claims.

Claims (40)

1. A method for making an open cell porous structure, comprising:
a) providing a dry flowable powder mixture including i) a first predetermined amount of inorganic particles having a first melting temperature, ii) a second predetermined amount of a binding agent having a decomposition temperature, the decomposition temperature being lower than the first melting temperature, and iii) an absence of foaming agent;
b) heating the mixture to a temperature lower than the decomposition temperature at least to cure the binding agent to obtain a solid structure; and c) heating the solid structure to at least the decomposition temperature to decompose the binding agent and to obtain a non-metallurgically-bonded open cell porous structure, and d) heating the non-metallurgically-bonded open cell porous structure to a temperature lower than the first melting temperature sufficient to metallurgically bond the inorganic particles to obtain a solid metallurgically-bonded open cell porous structure.

2. A method for making an open cell porous structure as claimed in claim 1, wherein the first predetermined amount is between about 10 wt % to about 90 wt % inclusive of a total weight of the mixture.

3. A method for making an open cell porous structure as claimed in claim 2, wherein the first predetermined amount is between about 55 wt % to about 80 wt % inclusive of the total weight of the mixture.

4. A method for making an open cell porous structure as claimed in claim 3, wherein the first predetermined amount is between about 60 wt % to about 75 wt %, inclusive of the total weight of the mixture.

5. A method for making an open cell porous structure as claimed in claim 3, wherein the second predetermined amount is between about 20 wt % to about 35 wt %
inclusive of the total weight of the mixture.

6. A method for making an open cell porous structure as claimed in any one of claims 1 to 5, wherein the inorganic particles consist essentially of ceramic particles.

7. A method for making an open cell porous structure as claimed in any one of claims 1 to 5, wherein the inorganic particles consist essentially of metallic particles.

8. A method for making an open cell porous structure as claimed in claim 7.
wherein the at least one transition metal is al least one selected from the group consisting of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, and gold.

9. A method tor making an open cell porous structure as claimed in claim 7, wherein the metallic particles are at least one selected from the group consisting of copper, nickel, iron, titanium, copper-based alloy particles, nickel-based alloy particles, iron-based alloy particles, titanium-based alloy particles, and copper-based alloy particles.

10. A method for making an open cell porous structure as claimed in claim 7, wherein the metallic particles are at least one of copper and copper-based alloy particles.

11. A method for making an open cell porous structure as claimed in any one of claims 1 to 10, wherein the inorganic particles consistent essentially of coated particles.

12. A method for making an open cell porous structure as claimed in any one of claims 1 to 11, wherein the binding agent is cured with the aid of a curing agent.

13. A method for making an open cell porous structure as claimed in any one of claims 1 to 12, wherein the binding agent is a thermoset resin.

14. A method for making an open cell porous structure as claimed in any one of claims 1 to 12, wherein the binding agent is a thermoplastic polymer.

15. A method for making an open cell porous structure as claimed in claim 14, wherein the thermoplastic polymer is cured with the and of one of a curing agent, an irradiation cross-linking treatment, and a light-exposure cross-linking treatment.

16. A method for making an open cell porous structure as claimed in any one of claims 1 to 15, wherein the mixture further includes at least one additional agent adapted to minimize segregation and dusting and to improve the flowability of the mixture.

17. A method for making an open cell porous structure as claimed in any one of claims 1 to 16, wherein pressure is applied to the mixture at least one of before and during the heating thereof in b., c, or d.

18. A method for making an open cell porous structure as claimed in any one of claims 1 to 17, further comprising shaping the mixture prior to heating.

19. A method for making an open cell porous structure as claimed in any one of claims 1 to 18, further comprising providing a substrate, and wherein the mixture is disposed on the substrate prior to heating.

20. A method for making an open cell porous structure as claimed in any one of claims 1 to 19, wherein the mixture further comprises at least one spacing agent.

21. A method for making an open cell porous structure as claimed in any one of claims 1 to 20, wherein heating the non-metallurgically-bonded open cell porous structure to a temperature lower than the first melting temperature sufficient to metallurgically bond the inorganic particles to obtain a solid open cell porous structure comprises heating the non-metallurgically-bonded open cell porous structure to a temperature lower than the first melting temperature sufficient to sinter the inorganic particles to obtain a solid open cell porous structure.

22. A method for making an open cell porous, structure as claimed in any one of claims 1 to 20, wherein the mixture further includes a brazing agent for metallurgically bonding the inorganic particles.

23. An open cell porous structure made according to the method as claimed in any one of claims 1 to 22.

24. A dry flowable powder mixture for making open cell porous, structures, the mixture comprising:
a first predetermined amount of inorganic particles having a first melting temperature;
a second predetermined amount of a binding agent having a decomposition temperature, the decomposition temperature being lower than the first melting temperature.

25. A mixture as claimed in claim 24, wherein the first predetermined amount is between about 10 wt % to about 90 wt % inclusive of a total weight of the mixture.

26. A mixture as claimed in claim 25, wherein the first predetermined amount is between about 55 wt % to about 80 wt % inclusive of the total weight of the mixture.

27. A mixture as claimed in claim 26, wherein the first predetermined amount is between about 60 wt % to about 75 wt % inclusive of the total weight of the mixture.

28. A mixture as claimed in claim 26, wherein the second predetermined amount varies from about 20 wt % to about .15 wt % of the total weight of the mixture.

29. A mixture as claimed in any one of claims 24 to 28, wherein the inorganic particles consist essentially of ceramic particles.

30. A mixture as claimed in any one of claims 24 to 28, wherein the inorganic particles consist essentially of metallic particles.

31. A mixture as claimed in claim 30, wherein the at least one transition metal is one selected from the group consisting of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, indium, platinum, and gold.

32. A mixture as claimed in claim 30, wherein the metallic particles are at least one selected from the group consisting of copper, nickel, iron, titanium, copper-based alloy particles, nickel-based alloy particles, iron-based alloy particles, and titanium-based alloy particles.

33. A mixture as claimed in claim 30, wherein the metallic particles are one of copper and copper-based alloy particles.

34. A mixture as claimed in any one of claims 24 to 33, wherein the inorganic particles consist essentially of coated particles.

35. A mixture as claimed in any one of claims 24 to 34, further comprising a curing agent for assisting in curing the binding agent

36. A mixture as claimed in any one of claims 24 to 35, wherein the binding agent is a thermoset resin.

37. A mixture as claimed in any one of claims 24 to 35, wherein the binding agent is a thermoplastic polymer.

38. A mixture as claimed in any one of claims 24 to 37, wherein the mixture further comprises at least one additional agent adapted to minimize segregation and dusting and to improve the flowability of the mixture.

39. A mixture as claimed in any one of claims 24 to 38, wherein the mixture further comprises a lubricating agent to assist in at least one of shaping, molding and demolding.

40. A mixture as claimed in any one of claims 24 to 39, wherein the mixture further comprises a brazing agent for metallurgically bonding the inorganic particles.