GB1572180A - Thermally stable sintered porous metal articles - Google Patents

Description

(54) THERMALLY STABLE SINThRED POROUS METAL ARTICLES (71) We, GOULD INC., a corporation organized and existing under the laws of the State of Delaware, of 10 Gould Center, Rolling Meadows, Illinois 60008, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns porous sintered metal articles, such as nickel articles, which are characterized by their thermal stability when subsequently heated to elevated temperatures, that is, temperatures approaching that at which they were originally sintered.

In addition, the invention also concerns a method of producing the before-described type of article.

The formation of porous metal articles typically involves the steps of shaping metal powders into a green compact, for example, by loose packing, compaction, extrusion, rolling, or moulding, and further consolidation of theh green compact into the desired article by sintering.

In this general purpose, a quantity of loose starting material, usually irregularly shaped metal or metal alloy particles ranging in size from 0.1 microns to 200 microns, is used. The particles of the desired size are typically obtained by means of a sieve of predetermined mesh opening. The classified powders are then shaped by loose packing or under pressure into a green compact wherein the metal particles contact each other at many points and areas of their surfaces. In most cases, the interparticle voids left between the particles remain open to form inter-connected pore channels penetrating the body of the compact. These openings are generally of irregular crosssection with jagged, sharp-edged walls. However, the green preform is mechanically weak due to insufficient bonding between the particles.

In order to enhance body strength, the green compact is sintered, that is, heated for a specific length of time at a temperature at which diffusion of metal is activated at the points of contact between the particles so that they become bonded to each other. As sintering progresses, the particle contacts grow to form neck-like joints and the pore channels assume a rounded, cylindrical shape due to surface tension forces acting on their surfaces. These forces also enhance diffusion flow of metal into the empty channels, decreasing their cross-sectional area to the point where the channels become unstable and reduce into spherical voids separated from each other by the body of densified metal.

To obtain a framework or structure with interconnecting pore channels, it is accordingly necessary to terminate sintering prior to pore channel breakdown. However, it is recognized that for irregularly shaped powder particles having a size distribution around an average value, the several stages of sintering occur at different times within hte body of the compact. Closure of pore channels takes place sooner at some locations in the body than in others, causing gross inhomogeneities in the structure. It is therefore practically impossible by this technique to control pore size, strength and thermal stability of the finished structure.

The minimum pore size within the wide range of pore sizes found in such a framework is generally limited by the size of metal particles. This restriction on minimum pore size is due to the surface tension forces causing pore closure which are inversely proportional to the diameter of the pore. The stability of the open channels decreases sharply with diameter. This results in inherent shrinkage in the framework throughout further heat treatment.

Several modifications of the conventional sintering technique have been utilized by the art in an attempt to more closely control the structure of the formed framework. Some of these modified processes suffer disadvantages and, in common with the basic technique, do not inhibit shrinkage in further heat treatment applications of the sintered structure.

By one such conventional technique, carefully sized, spherical powders are utilized to form the porous body with the choice of particle size determining the pore diameter of the interconnected channels. For pore diameters larger than about 5 microns, sintering can be terminated prior to breakdown of the pore channels since particles uniform in size and shape sinter uniformly.

Growth of interpartide joints, formation, and shrinkage of cylindrical channels, and their eventual breakdown into separated voids accordingly occur in the same sequence throughout the whole compact. A major disadvantage of this technique, in addition to the limitation on pore size, is the cost and availability of spherical particles. Illustrative processes are disclosed in German Patent No. 918,357 (1954) and Japanese Patent No. 203,580 (1953) dealing with self-lubrication bearings, and U.S.

Patent No. 2,863,562 (1958) dealing with porous filters. By another conventional technique, pore-forming materials which volatalize during sintering are blended with the initial powder mixture of the more conventional non-uniform particles. Upon completion of the sintering operation, the resultant body will contain pore channels everywhere the pore former initially resided.

By virtue of the non-uniformity of the initial metal particles, the resultant framework contains a large size distribution of interconnected channels. Illustrative processes are disclosed in U.S. Patent Nos. 2,721,378 (1955); 2,792,302 (1957); and 2,877,114 (1957).

Both of the above techniques require precise control of sintering conditions to ensure termination thereof prior to pore channel breakdown.

One recently developed technique, where a dispersed phase of critical amounts of inert dispersoid particles of specified size are incorporated in sintered metal or metal alloy, approximates controlling the pore size, its distribution, and shrinkage inhibition. To be effective, the inert particles must form with the sintered particles a wetting angle of at least 900 as measured from the sintered metal dispersoid particle interface to the sintered metal atmosphere interface. The resulting materials exhibit a network of stabilized interconnected pore channels of narrow size distribution.

Particles of any shape can be used in the matrix so long as the voids remaining between them after they have been loosely packed or pressed together form interconnected pore channels penetrating the body of the compact. This process, as it is described in U.S. Patent No. 3,397,968, has certain critical disadvantages, specifically, sintered metal articles produced by the concerned technique exhibit limited electrical conductivity. Accordingly, when it is desired to provide a thermally stable sintered porous metal article which is conductive, the technique of U.S. Patent No. 3,397,968 cannot be used.

The present invention provides a means of overcoming various limitations found in prior art methods of producing sintered porous metal articles. Specifically the instant invention provides sintered porous metal articles which are both thermally stable and characterized by their degree of electrical conductivity.

More particularly, the present invention provides a method of producing a porous sintered metal article, comprising: preparing a mixture consisting of only a) 95 to 70 weight percent base metal particles selected from the group consisting of nickel, cobalt, iron and mixture thereof with said particles having a particle size ranging from 0.1 microns to 200 microns and b) 5 to 30 weight percent conductive dispersoid particles selected from the group consisting of chromium, tungsten, molybdenum and mixtures thereof with said particles having a particle size ranging from 0.01 microns to 50 microns; forming said mixture into an article of the desired shape; and heating said formed article at sintering temperatures to form a sintered article by solid state sintering; the method being so performed as to provide a sintered article which is porous, electrically conductive and which retains its physical porosity characteristics at a given temperature less than the sintering temperature after thermal cycling in which the temperature of the article approaches the sintering temperature.

The invention includes porous sintered metal articles made by this method. Such articles have a myriad of uses, such as, for example, high temperature heating elements, conductive metallic grids which are to be utilized at high temperature, electrodes for fuel cells (especially high temperature fuel cells), and as conductive elements for high temperature electrostatic precipitators. These uses are not exclusive, but are merely set forth herein as typical examples.

In the preferred practice of the invention, after the porous metal article has been sintered it is sometimes subjected to a compacting and then an annealing treatment, depending on contemplated applications.

In order that the invention may be more fully understood examples thereof will now be described by way of illustration.

Base metal particles used in the practice of the invention are selected only from tbe group consisting of nickel, iron, cobalt, and mixtures thereof. In this regard, it is to be noted that more than one of the base metals the base metal particles have a particle size ranging from 0.1 to 200 microns.

The active or conductive dispersoid particles used in the practice of the invention are both refractory and conductive selected only from the group consisting of chromium, molybdenum, tungsten, and mixtures thereof. Like the base metal particles, dispersoid particles of different metals or alloys from the aforesaid group can be utiliized in the practice of the invention.

Essentially, all that is required is that the dispersoid particles are (1) conductive, refractory, and capable of interlocking with adjacent base metal particles to form a unitary porous mass and (2) do not form a liquid phase during sintering.

The dispersoid particles have a particle size di strbuti on ranging from 0.01 to 50 microns.

The dispersoid particles in the starting mixture should be as small as possible to prevent their interference with the pore channel network with the only limitation on minimum size being the practical limit of the availability of sizes below 0.01 microns.

It has been noted that dispersoid particles are desirably less than one-third the size of the pore channels. For larger sizes, the particles tend to block and close many of the pores to the detriment of usable pore volume. To ensure that the dispersoid particles in the materials of the invention are within these limits, there must be taken into account the tendency of many particles to grow during sintering. When such growth occurs, the dispersoid particles in the initial mixture prior to processing must naturally be smaller than the aforementioned sizes.

Such growth can readily be compensated for by workers skilled in the art.

Base metal particles and dispersoid particles can be mixed together by any convenient means. In practice, it is preferred to use a V-twin shell-type blender. The exact duration of blending is not critical. All that is required is that the materials are uniformly mixed.

The mixture of base metal and dispersoid particles can range from 95 to 70 weight percent base metal and from 5 to 30 weight percent dispersoid particles.

The mixture of base metal particles and dispersoid particles can be formed into any desired shape by conventional techniques which are known to the art and will not be discussed herein in detail. The initial powder mixture should be such as to ensure uniform distribution of the dispersoid particles at the surfaces of the metal particles during formation of a uniform mixture wherein the dispersoid particles are located at or on the surfaces of the metal particles.

No significant separation of the phases should occur causing some metal particles to be devoid of or have less dispersoid on their surfaces than others. Agglomeration of the dispersoid should also be avoided in the mixture. The relative concentration and sizes of the metal and dispersoid powders must naturally be such as to produce, upon further processing, the desired microstructure in the finished body.

In the preferred practice of the invention articles are formed by filling a mould with the desired amount of material. Obviously, there are many ways in which the desired article can be fabricated.

The pore size of channels in the finished product is influenced not only by the concentrations and sizes of metal and dispersoid particles and sintering conditions but also by the degree of compaction experienced in forming the green compact. The compact may be formed by any of the various wellknown techniques, including uni- or multidirectional die pressing, isostatic pressing, powder rolling, extruding, and roll-bar moulding. Various degrees of compaction are achieved in these processes, resulting in a variety of pore size changes in the green compact.

The sintering of the formed powder metal articles is preferably accomplished in a sintering furnace having an inert or reducing atmosphere, usually hydrogen. The sintering temperature depends on the type of metal particles utilized for both the base metal and the dispersoid particles. Sintering is usually carried out at a temperature which is approximately 75% of the melting point of the base metal. It being preferred that when nickel is employed as the base metal and chromium as the dispersoid the sintering temperature should range from 19000F to 20500F. The sintered article is usually cooled to about room temperature before it is removed from the furnace.

In the preferred practice of the invention the sintered article is then compacted, if required, by any conventional means to form an article having the desired degree of porosity. In the preferred practice of the invention, it is desirable to have a porosity in the final article of about 55 to about 85%. This article is then subjected to an annealing treatment if required. The exact temperature and duration of annealing depends on the materials used to form the porous metal article.

In addition to the foregoing, another method for obtaining the desired mixture of base metal particles and dispersoid particles is realized by depositing the dis persoid on the surface of the base metal particles by chemical means. The process described by N. J. Grant in "Powder Metallurgy", volume 10, pp. 1 through 12, and also in U.S. Patent 3,175,904, issued March 30, 1965, are particularly effective in forming initial mixtures by chemical means.

Specific examples of procedures used in making materials and articles of manufac ture of the invention are given below. These examples are to be considered as illustrative only and not as limiting in any manner the scope of the invention as defined by the appended claims.

Example I 90 grams of nickel having a particle size distribution ranging from 3 to 7 microns was mixed with about 10 grams of chromium having a particle size distribution ranging from 3 microns to 5 microns in a V-twin shell-type blender for about 10 minutes. The resultant mixture was essentially homogene ous and contained, in weight percent, about 90 nickel and about 10 chromium. This mix ture was then screened through a 100-mesh screen and about 100 grams of the through 100-mesh material was placed into a 6" by 6" by .070" rectangular mould to form a porous compacted green metal body having an apparent density of about 1.3 g/cc. The moulded green metal body was then placed in a sintering oven and sintered in a hydro gen atmosphere by heating first to a tem perature of 1400"F for a period of 15 minutes and then to a temperature of 19500F for a period of 15 minutes. The sintered metal article was then cooled to room temperture and subsequently compacted by mechanical means to a porosity of about 70%. It was then subjected to an annealing treatment by heating it in a hydrogen atmos phere to a temperature of 19500F for a period of 15 minutes.

The article, produced as above described, was subjected to certain physical tests and found to be electrically conductive and to exhibit a porosity of about 70%, with a mean pore size of 5 microns.

The so-produced sintered porous metal article was then placed in an oven having a reducing atmosphere and heated at a temperature of 14000F for a period of 3,000 hours. Thereafter, the physical properties of the metal article were again measured with -ffld result being that essentially no deterioration in such properties was observed. That is, the article was still highly electrically conductive and evidenced the same general physical properties set forth above. There was no further sintering due to this high temperature testing.

Due to its extreme thermal stability, together with its electrical conductivity, the foregoing article finds utility as an electrode which is especially adapted for use in a high temperature fuel cell.

Example 2 90 grams of nickel having a particle size distribution ranging from 3 to 7 microns was mixed with about 10 grams of alloyed chromium and tungsten having a particle size distribution ranging from 9 to 11 microns in a V-twin shell-type blender for 10 minutes. The resultant mixture was essentially homogeneous and contained, in weight percent, 90% nickel and 10% chromium and tungsten. This mixture was then screened through a 100-mesh screen and 100 grams of the through-100-mesh material was placed into a 6" by 6" by 0.70" rectangular mould to form a porous compacted green metal body having an apparent density of about 1.3 g/cc. The moulded green metal body was then placed in a sintering oven and sintered in a hydrogen atmosphere by heating first to a temperature of 14000F for a period of 15 minutes and then to a temperature of about 1950"F for a period of 15 minutes. The sintered metal article was then cooled to room temperature and subsequently compacted by mechanical means to a porosity of about 70%. It was then subjected to an annealing treatment by heating it in a hydrogen atmosphere to a temperature of 19500F for a period of 15 minutes.

The article, produced as above described, was subjected to certain physical tests and found to be electrically conductive and to exhibit a porosity of 70%, with a mean pore size ranging from about 7 microns.

The test procedure used to measure stability was similar to that used in Example 1.

Example 3 90 grams of cobalt having a particle size distribution ranging from 9 to 15 microns was mixed with 10 grams of chromium having a particle size distribution ranging from 3 to 5 microns in a V-twin shell-type of blender for 10 minutes. The resultant mixture was essentially homogeneous and contained, in weight percent, 90% cobalt and 10% chromium This mixture was then screened through a 100-mesh screen and 100 grams of the through-100-mesh material was placed into a 6" by 6" by 0.07" rectangular mould to form a porous compacted green metal body having an apparent density of about 1.3 g/cc. The moulded green metal body was then placed in a sintering oven aad smtered in a hydrogen atmosphere by heating first to a temperature of 1400"F for a period of 15 minutes and then to a temperature of 1950"F for a period of 15 minutes. The sintered metal article was then cooled to room temperature and subsequently compacted by mechanical means to a porosity of about 70%. It was then subjected to an annealing treatment by heating it in a hydrogen atmosphere to a temperature of 19500F for a period of 15 minutes.

The article, produced as above described, was subjected to certain physical tests and found to be electrically conductive and to exhibit a porosity of about 70%, with a mean pore size ranging from about 6 microns.

The stability tests were similar to those of previous examples.

While there have been described herein what are, at present, considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as hereinafter claimed.

WHAT WE CLAIM IS: 1. A method of producing a porous sintered metal article, comprising: preparing a mixture consisting of only a) 95 to 70 weight percent base metal particles selected from the group consisting of nickel, cobalt, iron and mixtures thereof with said particles having a particle size ranging from 0.1 microns to 200 microns and b) 5 to 30 weight percent conductive disp ersoid particles selected from the group consisting of chromium, tungsten, molybdenum and mixtures thereof with said particles having a particle size ranging from 0.01 microns to 50 microns; forming said mixture into an article of the described shape; and heating said formed article at sintering temperatures to form a sintered article by solid state sintering: the method being so performed as to provide a sintered article which is porous, electrically conductive and which retains its physical porosity characteristics at a given temperature less than the sintering temperature after thermal cycling in which the temperature of the article approaches the sintering temperature.

2. The method of claim 1 wherein after sintering the formed article is further compacted.

3. The method of claim 2 wherein said article is compacted in such a manner so as to exhibit a porosity ranging from 55 to 85 percent.

4. The methohd of claim 2 or 3 wherein said compacted article is subsequently subjected to an annealing treatment.

5. The method of claim 1 wherein said mixture of base metal particles and conductive dispersoid particles is obtained by depositing the dispersoid material on the surface of the base metal particles.

6. A method substantially as herein described with reference to any of the examples, of producing a porous sintered metal article.

7. A sintered porous metal articles produced by any method hereinbefore claimed.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. of about 1.3 g/cc. The moulded green metal body was then placed in a sintering oven aad smtered in a hydrogen atmosphere by heating first to a temperature of 1400"F for a period of 15 minutes and then to a temperature of 1950"F for a period of 15 minutes. The sintered metal article was then cooled to room temperature and subsequently compacted by mechanical means to a porosity of about 70%. It was then subjected to an annealing treatment by heating it in a hydrogen atmosphere to a temperature of 19500F for a period of 15 minutes. The article, produced as above described, was subjected to certain physical tests and found to be electrically conductive and to exhibit a porosity of about 70%, with a mean pore size ranging from about 6 microns. The stability tests were similar to those of previous examples. While there have been described herein what are, at present, considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as hereinafter claimed. WHAT WE CLAIM IS:

1. A method of producing a porous sintered metal article, comprising: preparing a mixture consisting of only a) 95 to 70 weight percent base metal particles selected from the group consisting of nickel, cobalt, iron and mixtures thereof with said particles having a particle size ranging from 0.1 microns to 200 microns and b) 5 to 30 weight percent conductive disp ersoid particles selected from the group consisting of chromium, tungsten, molybdenum and mixtures thereof with said particles having a particle size ranging from 0.01 microns to 50 microns; forming said mixture into an article of the described shape; and heating said formed article at sintering temperatures to form a sintered article by solid state sintering: the method being so performed as to provide a sintered article which is porous, electrically conductive and which retains its physical porosity characteristics at a given temperature less than the sintering temperature after thermal cycling in which the temperature of the article approaches the sintering temperature.

2. The method of claim 1 wherein after sintering the formed article is further compacted.

3. The method of claim 2 wherein said article is compacted in such a manner so as to exhibit a porosity ranging from 55 to 85 percent.

4. The methohd of claim 2 or 3 wherein said compacted article is subsequently subjected to an annealing treatment.

5. The method of claim 1 wherein said mixture of base metal particles and conductive dispersoid particles is obtained by depositing the dispersoid material on the surface of the base metal particles.

6. A method substantially as herein described with reference to any of the examples, of producing a porous sintered metal article.

7. A sintered porous metal articles produced by any method hereinbefore claimed.