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United States Patent 3,753,666
Carroll August 21, 1973

NOBLE METALS HAVING A HIGH EMITTANCE COATING OF IRON TITANATE

Abstract

A method of applying a high-emittance coating to the noble metals and intra-alloys thereof, the coating being applied by plasma arc spraying iron titanate powder on the surface of the noble metal. A substrate of a noble metal or intra-alloy thereof having a high emittance coating of iron titanate bonded thereon.


Inventors: Carroll; David F. (Hermosa Beach, CA)
Assignee: TRW Inc. (Redondo Beach, CA)
Appl. No.: 04/687,594
Filed: December 4, 1967


Current U.S. Class: 428/633 ; 165/133; 427/328; 427/453; 428/655; 428/661; 428/670
Current International Class: C23C 4/10 (20060101); C23c 005/00 (); C23d 005/10 ()
Field of Search: 117/93.1,201,217,221,227 29/195,198,199,194 106/299,304,39 219/345,354,553 165/133 252/514,519,520 161/225

References Cited

U.S. Patent Documents
3183337 May 1965 Winzeler et al.
3246114 April 1966 Matuay
3347698 October 1967 Ingham
Foreign Patent Documents
1,003,118 Sep., 1965 GB
Primary Examiner: Leavitt; Alfred L.
Assistant Examiner: Newsome; J. H.

Claims



I claim:

1. A substrate of a platinum-group metal having a high emittance coating, comprising:

a. said metal being one selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum and intra-alloys thereof; and

b. an iron titanate coating on at least one surface of said metal.

2. The invention according to claim 1 in which:

said iron titanate coating has been plasma arc sprayed on said metal.

3. The invention according to claim 1 in which:

said coating has a thickness of at least approximately 1.0 mil.

4. The invention according to claim 1 in which:

said coated metal has a hemispherical emittance range of approximately 0.82 to 0.86 in the temperature range of from 1,600.degree. F to 2,400.degree. F.

5. A refractory metal having a high emittance cladding comprising:

A. a refractory metal selected from the group consisting of vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and rhenium on which is clad

B. a noble metal selected from the group consisting of ruthenium, rhodium,palladium, osmium, iridium, and platinum and intraalloys thereof, said noble metal having a coating of

C. iron titanate.
Description



BACKGROUND OF THE INVENTION

Controlling the surface emittance of radioisotope containment vessels for aerospace applications has been a difficult problem in the prior art. This problem has been particularly severe in view of the safety requirements which are that no release of the radioactive content be allowed under any conceivable situation, that is, both normal operation and all abnormal modes of operation, including abort modes. The most probable abort situation involved is exposure of a fuel vessel to terrestial gases, such as air or water vapor and carbon dioxide, at elevated temperatures up to 2,500.degree. F for an extended period of time, measured in years. The survivability of the vessel in this situation depends strongly upon the temperature of the surface of the vessel, and which can be lowered to an acceptable level by the use of a high emittance coating according to the invention.

It has been determined that containment material for radioisotope vessels should include a refractory metal strength member clad with a noble metal. The refractory metals usually include vanadium, niobium (columbium), tantalum, chromium, molybdenum, tungsten and rhenium. A refractory metal provides high temperature (1,600.degree. F) creep strength and the noble metal provides protection from the rapid, high temperature oxidation that a bare refractory metal container would incur. A bare noble metal cladding has a disadvantage and that is its low hemispherical emissivity of about 0.2. Hemispherical total emissivity, E.sub.ht, is the ratio of the radiancy (rate of emission of radiant energy from a unit area of a surface) of a polished surface to that of a black body at the same temperature.

The steady state temperature of a radioisotope capsule in a ground abort situation, for example, is determined principally by the emissivity of its surface: the higher the emissivity, the lower the capsule temperature. When noble metal claddings are used for oxidation protection of refractory metals, they degrade at a rate dependent strongly upon temperature. Thus, any reduction in capsule temperture achieved by increasing the emissivity of a noble metal cladding will enhance its ground abort survivability.

SUMMARY OF THE INVENTION

The invention relates generally to a means to increase the total emittance of noble metals. It has been surprisingly found that the application of a plasma arc sprayed coating of iron titanate on the surface of a noble metal increases the hemispherical emittance approximately 300 percent. The invention is comprised of a method to plasma arc spray the noble metals or intra-noble alloys thereof with a coating of iron titanate powder.

Accordingly, an object of the invention is to provide a method for coating noble metals and intra-alloys thereof to produce a noble metal and coating having a high emittance.

Another object of the invention is to provide a high emittance cladding formed of the noble metals and intra-alloys thereof.

Still another object of the invention is to provide a high emittance coating for noble metals that is stable in air at up to 2,500.degree. F. A further object of the invention is to provide a high emittance coating, as described in the previous paragraphs, which is ductile.

Still further object of the invention is to provide a coating, as described in the previous paragraphs, and with which no deleterious chemical interactions occur between the coating and the noble metals.

Further objects and advantages of the invention may be brought out in the following part of the specification wherein small details have been described for the competence of disclosure, without intending to limit the scope of the invention which is set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the accompanying drawings, which are for illustrative purposes:

FIG. 1 is an isometric view of a noble metal substrate having a high emittance coating thereon in accordance with the present invention;

FIG. 2 is a magnified end view of a substrate of a platinum-rhodium alloy having an iron titanate coating bonded on opposite sides thereof in accordance with the invention; and

FIG. 3 is an illustration of a cladding for a refractory metal and formed of a noble metal tube with inner and outer surfaces coated with iron titanate in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventive process is comprised of plasma arc spraying iron titanate powder, having a particle size range of 44 to 74 microns, on a surface of one of the metals selected from the group containing ruthenium, rhodium, palladium, osmium, iridium and platinum and intralloys thereof. Either the natural iron titanate, the mineral ilmenite or synthetic iron titanate, Fe.sub.2 O.sub.3.sup.. TiO.sub.2, can be used. The thickness of the iron titanate coating may vary from approximately 1.0 mil to several mils. If required, the surface of the coating can be ground to specified dimensions. When the coated noble metal is used as a cladding for a refractory metal, such as vanadium, niobium (columbium), tantalum, chromium, molybdenum, tungsten and rhenium, it is generally formed as a thin substrate sheet having a thickness of between 0.020 and 0.050 inch.

Prior to coating a substrate of one of the noble metals, it may be desirable to anneal metal to relieve any mechanical stresses. The surface of the noble metal is then cleaned to remove organic contaminants, preferably with ethyl alcohol because it leaves no film. Next, the surface of the metal to be coated is activated by, for example, grit blasting with glass beads or alumina. The activation increases the adherence of the iron titante to the surface. After the surface has been activated, it is again cleaned with ethyl alcohol, preferably in an ultrasonic cleaner to shake out most of the particles embedded during the grit blasting. The substrate is then ready for plasma arc spraying, preferably using an argon gas plasma.

The process described above unexpectedly resulted in the production of a ductile, stable, high emittance coating for nobel metals. A strip of platinum-10 weight percent rhodium, 1/4 inch wide, 0.1 inch thick and 8 inch long, coated with plasma arc sprayed iron titanate, can be bent around a 2 inch radius with no cracking.

Referring again to the drawings, there is shown in FIG. 1 a substrate of a noble metal in the form of a plate 10 having bonded (mechanical-chemical bond) on one side 11 with a thin layer of plasma arc sprayed iron titanate 12. The bonding is shown by the irregular surfaces as at 13 where the iron titanate is joined to the noble metal. This can generally be seen only through high magnifications.

In FIG. 2, there is shown a substrate 17 with platinum-10 weight percent rhodium alloy, having bonded on two opposite sides a layer of iron titanate 18 which was plasma arc sprayed onto the substrate. The drawing in FIG. 2 was made in exact size from a copy of a 200X photomicrograph.

The platinum-rhodium alloy has its normal whitish color, and the iron titanate is a medium gray having black spots 19 which are shadows formed in cavities. There are rough surface bonding lines 20 and 21 between the iron titanate and the platinum-rhodium alloys, illustrating the adherence of the iron titanate coating to the noble metal. After heating the substrate shown for 980 hours in air at 2,000.degree. F, no chemical interactions occurred between the coating and the noble metal. Microhardness measurements across the noble metal substrate also indicated no changes in the metal because of the iron titanate coating. The thickness of the two coating layers and the substrate, as may be used for cladding on radioisotope containment vessels, is in the range of 22 mils and 70 mils, and the thickness of the individual iron titanate coating is in a range of from 1 mil to 10 mils.

In FIG. 3, there is shown a tube 24 having an inner and outer cylindrical coating 25 and 26, respectively, of iron titanate on a tube portion 27 of a noble metal in the relationship as shown in FIG. 2. The thicknesses of the coating and the noble metal are exaggerated and are shown to illustrate a tubular cladding for use on a refractory metal for containing radioisotopes.

The following examples illustrate a way of carrying out the invention as otherwise described above:

1. The noble metal or intra-alloy of a noble metal is cleaned with ethyl alcohol to remove to organic contaminants.

2. The surface of the noble metal to be coated is grit blasted with glass beads to activate its surface to increase the adherence of the coating to the surface.

3. The grit-blasted surface is cleaned with ethyl alcohol, preferably in an ultra-sonic cleaner to shake the particles off the surface.

4. The surface is plasma arc sprayed, using argon gas plasma, with -200 mesh, +325 mesh iron titanate powder, which freezes on the surface.

The measured emittance values and stability of the iron titanate coating on the noble metals can be seen from the data in the table below, in which a comparison is made with the uncoated platinum-10 weight percent rhodium alloy: ##SPC1##

As can be seen from the table, the total emittance of platinum-10 weight percent rhodium is increased by the iron titanate coating approximately 300 precent from 0.2 to 0.8. The stability in air at 2,000.degree. F is shown to be good, that is, exposure to air at high temperatures does not appreciably change the emittance values.

The invention and its attendant advantages will be undetstood from the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the parts of the invention without departing from the spirit and scope thereof or sacrificing its material advantages, the arrangement hereinbefore described as being merely by way of example. I do not wish to be restricted to the specific forms shown or uses mentioned, except as defined by the accompanying claims, wherein various portions have been separated for clarity of reading and not for emphasis.

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