US2476222A - Production of powdered metal mixtures for sintering - Google Patents

Description

Patented July 12, 1949 UNITED STAT ENT OFFICE PRODUCTION OF POWDERED METAL MIXTURES FOR SINTERING ware No Drawing. Application February '7, 1945', Se-

rial No. 576,709. In Great Britain March 10,

The present invention relates to the production of alloys containing one or more metals of the platinum group and more particularly to the production of such alloys of precise predetermined composition in a form such as a sponge which may readily be reduced to a fine powder form by attrition oroth'er means.

It has already been proposed to produce homogeneity in a powder mixture by preparing a solution of a mixture of mutually non-reactive compounds of the metals'to be formed into an alloy, such as the metals n-lckel and iron for example, and to precipitate the metals together from such solutions in the form of oxides or hydroxides and to then reduce the precipitated mixture to a metallic state with subsequent reduction to a iowdered form. Powder mixtures containing metals of the platinum group cannot be prepared in this manner, however, since the platinum group metals do not conveniently precipitate in the form of oxides or hydroxides. The use of alloys of the platinum group metals has become of increasing importance in recent years, particularly as electrodes in spark plugs for high efficiency aeroplane engines. It has also been found that such electrodes for use inhigh efilciency internal combustion engines are considerably more reslstant to failure during suchuse when they are prepared from a sintered compact of mixtures of metal powders in the desired proportions to form the alloy. The copending application of Arthur Beresford Middleton, Serial No. 560,487 filed October 26, 1944, for example, discloses a method for producin a spark plug electrode from an alloy containing a metal of the platinum group in which the platinum group metal, in finely powdered form, is mixed with the alloying metal, also in finely powdered form, and the mixture is compacted to a briquette or other easily handled shape and the shape is sint'ered and hot worked with subsequent cold working and wire drawing to the final form desired. Alloys produced according to the present invention are particularly adaptable for use'in' producing the electrodes of Middleton.

When producing alloys from mixtures of metal powders by ordinary methods, certain difiiculties are encountered'which must be avoided if the article which is eventually produced from the alloy is to have the characteristics which are desired. During the sinteringstep there is a diffusion of the metals into one another and the smaller the particle size, the more readily will this diffusion take place. The particles should be intimately mixed since this will permitsubstantial reduction 1 Claim. (Cl. 75-172) in both the temperature and duration of the sintering. However, very small particles of metal are liable to surface oxidation under ordinary conditions of handling. Moreover, if the metals concerned have a considerable difference in density, it is very difiicultto obtain a homogenous mixture by mechanical means and, if one or more of the metals preponderate, this difiiculty is considerably increased.

I have discovered a method of producing by a sintering process an alloy of metals of the platinum group or at least one metal of the platinum group and at least one other metal which may be a base metal, with or without an inert material as an added ingredient in the alloy, whereby all of the aforesaid difficulties are eliminated or reduced to a minimum, the alloy produced being in a form which may be readily reduced to a fine powder.

It is an object ofthe present invention to prepare a precipitate from a solution or a suspension of salts or compounds of metals at least one of which is a complex salt of a metal of the platinum group, the said precipitate being in a form which may be readily reduced tothe metallic state.

It is a furtherobject of my invention to produce powdered metal alloys of precise predetermined composition.

The invention further contemplates the production of a finely divided r'ne'tal'of the platinum group in a finely divided 'alloy'of such a metal with an inert material uniformly distributed throughout the finely divided metal 01' alloy.

Other objects and advantages of the invention will become apparent from the following description:

Generally speaking the invention contemplates the production of an alloy of at least one metal of the platinum group and at least one other metal in finely powdered form suitable for sintering and subsequent hot and/or cold working.

Broadly speaking, the present invention contemplates the production of an alloy suitable for subsequent sintering and containing at least one metal of the platinum group and at least one other metal, by precipitating the constituents from a liquid vehicle in which the constituents are in solution or suspended; by the addition of a base to said solution or by evaporating or otherwise mechanically removing the liquid Vehicle if the constituents are in suspension. The platinum group metal may be present or precipitated in the form of an insoluble salt which will readily decompose on ignition in air or in a reducing atmosphere to the metallic form, the other metal or metals present in the solution being present or precipitated at the same time in the form of insoluble salts or in the form of oxides or hydroxides which will similarly decompose on ignition, the precipitate being thereafter ignited until all of the metal salts or metal compounds contained in the precipitate or precipitates are reduced to metallic form, the ignited precipitate being subsequently reduced to finely divided powder form by comminution or any other convenient means, the powdered alloy thus formed being adapted to subsequent sintering, forging and working. If the alloy is to contain inert particles, these inert particles are included in the solution in such form as will also precipitate as salts which will decompose to inert particle form on ignition.

Binary, ternary, or more complex alloys may be made according to the present invention.

The alloy may contain more than one metal of the platinum group and it may contain one or more metals other than those of the platinum group such as base metals and gold and silver which may likewise be precipitated as salts or as oxides or hydroxides which may decompose on ignition in the manner described. When one or more of the metals is precipitated as an oxide or hydroxide, the ignition is efiected in a reducin atmosphere such as in a hydrogen atmosphere.

Precipitation from a solution may be efiected by the addition of one or more appropriate reagents added together or in sequence, with heating if necessary. It is preferred to use ammonium hydroxide as the reagent inducing precipitation, but other bases which will react to produce the insoluble salts may be used, such as pyridine, methylamine and ethylamine. It is necessary, of course, that when solution are used, the solutions be made from compounds which will precipitate the desired salts on the addition of one of the aforesaid reagents. When the compounds or insoluble salts are in suspension in a liquid vehicle, the aforesaid agents are not required, precipitation from the liquid vehicle being attained by heat, cold, or other force. or by mechanical means, such as filtering, centrifuging, and the like.

In a modification of the present invention, I may precipitate one of the compounds that is to be ignited from a solution in which another one of the compounds is suspended in an insoluble form. The insoluble particles then appear to form nuclei on which the precipitate forms.

In a further modification of my invention, a

mixture suitable for sintering may be prepared by intimately mixing together the compounds of the metals with or without the addition of an inert ingredient, in the form of a paste or slurry, the mixture being made in an appropriate liquid vehicle. In this case all of the constituents of the mixture may be insoluble in the liquid vehicle, homogeneity being produced, for example, by wet grinding. One or more of the compounds may be soluble in the vehicle, however, in which case the vehicle is removed by evaporation, preferably with agitation.

The most stable salts of the metals of the platinum group are generally complex in character. Platinum, palladium and rhodium are the three most common metals of this group and examples of typical insoluble salts of those metals that may be precipitated are ammonium chloroplatinate, palladosammine chloride and chloropentammino rhodium chloride, respectively. All these compounds will leave only the metal as a residue when .4 ignited under reducing conditions. There are many other salts of platinum group metals that are both insoluble in water or aqueous solutions and, therefore, can be precipitated, the other con stituents besides the parent metal being the volatile elements sulphur, nitrogen, oxygen, carbon and the halogens, present in the compound as the radicals of sulphide, nitrite, nitrate, carbonate, amino, and similar groups.

In order to give those skilled in the art a better understanding of the invention and of the benefits and advantages that may be obtained by practicing the same, the following illustrative ex amples are given:

The following is an example of the preparation of a binary alloy in powder form, the alloy being formed of two metals of the platinum group with inert material in the form of thoria uniformly distributed throughout the alloy:

Example 1 A solution containing 188.6 parts of chloro-- platinic acid, 16.6 parts of palladium chloride, and 0.5 part of thorium nitrate in water was formed. The solution was treated by the addition of ammonium hydroxide until faintly ammoniacal and the compounds were precipitated in accordance with the following reactions:

Composition Per Cent Platinum About 89.75 Palladium About 10.00 Thoma About 0.25

The thorium nitrate may be omitted in the foregoing example in which case a platinum-palladium alloy is formed.

Example 2 A binary alloy containing a metal of the platinum group and a base metal was prepared by forming a solution of 202 parts of chloroplatinic acid and 6 parts of ammonium meta tungstate. Ammonium hydroxide was added until the solution was slightly ammoniacal and insoluble salts V were precipitated in accordance with the following reactions:

Composition Per Cent Platinum About 96 Tungsten About 4 cases Example" '3 A ternary alloy of metals of the platinum group was formed by preparing a solution containing the chlorides of platinum, palladium and rhodium. The solution was heated to boiling and was then treated with a slight excess of a concentrated ammonium hydroxide-solution. '-'The platinum was almost completely precipitated and the rhodium-was partially precipitated inaccordance with the following reactions:

Only a soluble salt of palladium was formed according to the reaction:

This last precipitation appeared to have taken place with the earlier formed precipitate serving as a nuclei. This final precipitate was ignited and a sponge containing platinum, palladium and rhodium in the metallic form was obtained. The

sponge was reduced to a fine powder which was sintered to form a platinum-palladium-rhodium alloy.

Example 4 Insoluble rhodium trichloride was suspended in a solution of chloroplatinic acid under agitation and while agitation was continued the platinum was precipitated in the form of ammonium chloroplatinate entraining the insoluble rhodium trichloride in simultaneous precipitation. The precipitated compounds were ignited in an atmosphere of hydrogen and a platinum-rhodium sponge resulted. The sponge was reduced to a! fine powder and the powder was compacted and sintered to form a platinum-rhodium alloy. It

will be noted in the foregoing example that only one of the metallic compounds was in solution, the other compounds being suspended in that solution.

In the foregoing examples, at least one of the compounds has been in solution in the liquid ve-; hicle and has been precipitated by the addition of a precipitating agent. As has been stated here-' tofore, my invention may be also put into practice by forming a paste or slurry of compounds of the metals in a liquid vehicle, the compounds being suspended in and insoluble in that vehicle. An example of a paste or slurry formed in accordance with this modification of my invention follows hereinafter:

Example 5 328 parts of finely crystalline ammonium chloroplatinate and 8 parts of tungstic oxide were moistened with water to form a paste and the mixture was ground by hand in a mortar until an intimate mixture of the salts was obtained. The mixture was dried and was ignited at a temperaturev :offiOO? v(Into 700 C. The :resmtingrsoft sponge was powdered by attrition and was: passed through a 100-mesh sieve. The powderzwasithen reducedin an atmosphere of hydrogen at a.-;temperature of 700 C. to 800 C. The powderthus obtained was suitable for subsequent .sintering, forging,v hot and cold working'in accordance with the :methods described in i'the.;application of Middleton to which reference is made; herein.

wIn'ertr-particles, such as thoriagforvexample, may be incorporated inthe alloy in the foregoing exam'pleby including an appropriate amountwof oxide-in the initial salt mixture or preferably it may be precipitated onto the-mixed salt particles by introducing the. required amount ofusoluble thorium nitrate in the manner shown in Example 1. When the wet grinding is complete, the mixture is made slightly alkaline with ammonium hydroxide and is reground. By this means the thorium is precipitated as insoluble thorium hydroxide intimately mixed with the other two salts. On ignition, the thorium hydroxide is decomposed to thoria.

It will be apparent from the foregoing example that extremely intimate mixtures of metal powders are obtained by the methods of the present invention and that the intimacy of admixture is not adversely affected by marked differences in' the density of the alloying metals nor by extreme variations in the proportions of the metals forming the alloy.

Powders produced in each of the foregoing examples were compressed to a compact which was sintered and forged with subsequent cold working and wire drawing to form a severely cold drawn wire electrode by the method such as is described in detail in the co-pending application of Arthur Beresford Middleton, Serial No. 560,437, filed October 26, 1944.

The present invention provides a method for producing extraordinarily intimate mixtures of metal powders which will remain homogeneous throughout any steps to which it may subsequently be subjected. It is also to be observed that these extremely intimate mixtures are obtained by the method of the present invention even in alloys where one of the metals or an inert ingredient is an extremely minor proportion of the whole and a uniform distribution of this minor proportioned ingredient throughout the mass is obtained. The alloys obtained when mixtures of powdered metals produced according to the present invention are sintered are, therefore, alloys of very uniform characteristics because of this intimate and uniform distribution of the alloy ingredients.

In my use of the term igniting in the foregoing specification and the appended claims, I intend the term to be understood as meaning subjecting to heat sufiiciently high to reduce insoluble compounds of the metals to the metallic state whether or not bonding and/or diffusion of the adjacent particles occurs.

Although the present invention has been described in conjunction with preferred embodiments thereof, it is to be understood that variations and modifications may be made as those skilled in the art will readily understand. Thus, other insoluble salts than those given in the examples may be produced by precipitation from a solution or they may be suspended in a liquid vehicle in which they are insoluble and from which they are later removed as an intimate mixture by any convenient means. Such variations and modifications are to be considered within the droxide, pyridine, methylamine and ethylamine to 10 said solution whereby insoluble salts of platinum and tungsten are co-precipitated, drying said precipitates and igniting said precipitates in an atmosphere of hydrogen whereby a friable platinum-tungsten alloy is obtained.

EDWIN CLEMENTS RHODES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 612,614 Porter Oct. 18, 1898 2,177,412 Scott et a1. Oct, 24, 1939 2,254,976 Powell Sept. 2, 1941 2,315,876 Sivil et a1 Apr. 6, 1943 2,406,172 Smithells Aug 20, 1946 Certificate of Correction Patent No. 2,476,222 July 12, 1949 EDWIN CLEMENTS RHODES It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 7, line 8, for the word metal read meta;

and that the said Letters Patent should be read with this correction therein that the some may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 16th day of May, A. D. 1950.

THOMAS F. MURPHY,

Assistant Oommissz'oner of Patents.