WO2000044959A1

WO2000044959A1 - Coated graphite crucible

Info

Publication number: WO2000044959A1
Application number: PCT/GB2000/000140
Authority: WO
Inventors: Paul Gilchrist; Walter Weaver
Original assignee: British Nuclear Fuels Plc
Priority date: 1999-01-28
Filing date: 2000-01-21
Publication date: 2000-08-03
Also published as: AU3064000A

Abstract

The present invention provides a coating for an article for use with molten metal, the coating obtainable by depositing the coating on the article by magnetron sputtering. Unbalanced magnetron sputtering is preferred. The molten metal may comprise molten uranium and/or molten uranium salt. A coating deposited by magnetron sputtering is particularly effective for improving the performance and lifetime of graphite crucibles for containing molten uranium metal and/or molten uranium salts.

Description

COATED GRAPHITE CRUCIBLE

The present invention relates to coatings for vessels used for the containment of molten material. Particularly, but not exclusively it relates to coatings for vessels used for the containment of molten uranium metal and/or molten uranium salts .

Many problems exist in the handling and containment of molten materials such as molten metals. Particular problems relate to the vessels used to contain the molten metal during processing. Such vessels must be made of material such that the vessel is capable of being manufactured at an acceptable cost on a large scale, resistant to thermal and physical shocks, chemically resistant to the molten material being handled and capable of being cleaned for re-use if required.

The number of materials which fulfil all of the above requirements is relatively few. Graphite vessels are commonly used for handling molten metals including molten uranium. Ceramic vessels are also used to contain molten metal but often suffer from thermal shock problems.

These types of vessels do have problems however. For example, graphite may not be entirely un-reactive with molten metals such as uranium leading to loss of the vessel material into the melt. Thus, not only does the vessel physically deteriorate with time (and eventually become unusable) but the melt becomes contaminated. This latter problem is particularly important where the processed metal must be of very high purity, eg nuclear fuel manufacture. Other problems include adhesion of the process metal to the walls of the vessel after processing, causing both loss of yield and contamination of the vessel which may have to be cleaned before re-use.

Analogous problems exist with condensers and collectors, for example in the form of plates or baffles, for condensing and collecting metal vapour. A particular problem is to find a suitable material for a condenser or collector for use with uranium vapour.

A solution to these problems is to provide the vessels with a thin coating of a resistant material from which it would be impractical to manufacture the whole vessel, eg due to cost and complexity of fabrication.

A number of such coatings have been used previously. Typically the coatings have been applied as a "wash" . For example, in the nuclear industry, graphite crucibles for containing molten uranium have been coated with a Y₂0₃ wash. Whilst the Y₂0₃ coating provides some improved resistance when handling molten uranium metal alone, when salts, eg fluoride, are present in the molten uranium, the yttrium reacts with the fluoride to produce yttrium fluoride thus causing deterioration of the coating. Moreover reaction between the graphite vessel and the Y₂0₃ coating can also occur leading to undesirable effects on the coating and vessel walls. Other examples of protective washes include washes comprising zirconium.

It is an aim of the present invention to provide improved coatings for such vessels, condensers and collectors and a method for their production. According to a first aspect of the present invention there is provided a method of coating an article for use with molten metal, the method comprising depositing a coating by magnetron sputtering .

Preferably unbalanced magnetron sputtering is used.

According to a second aspect of the present invention there is provided a coating for an article for use with molten metal, the coating obtainable by depositing the coating on the article by magnetron sputtering.

The coating may comprise a ceramic material.

The composition of the coating can be varied depending on the nature of the molten metal, e.g. depending on the types of impurities which may be present in the molten metal.

The coating may in particular comprise a material selected from the group consisting of EuS, HfN, HfC, HfB₂, MoB₂, NbB₂, Nb₂C, Ni₃B, ScO, Ta, TaB₂, Ta₂C, Th0₂, TiN, TiC, TiB, V₂C, V₃B₂ W, ZrN, ZrC, ZrB₂ and Zr₂S₃.

The article may be a vessel for containing molten metal or may be a condenser or collector as hereinbefore described.

According to a further feature of the invention we provide a coated article adapted for use with molten metal comprising a coating according to the second aspect of the invention.

The term molten metal includes metal alloys.

The molten metal may comprise molten uranium. The molten metal may substantially comprise molten uranium. The molten metal salt may comprise a molten uranium salt. The molten metal salt may substantially comprise a molten uranium salt. The uranium salt may comprise a uranium fluoride. The uranium fluoride may have a stoichiometry from UF₃ to UF₆- The fluoride may comprise UF₃ or UF₄.

The metal may comprise a uranium alloy, eg uranium-iron alloy.

The vessel may be comprised of graphite or ceramic. Typically the vessel comprises graphite. The vessel may be used for example for melting molten metal or casting into a coated mould.

Surprisingly, it has been found that a coating deposited by magnetron sputtering is particularly effective for improving the performance and lifetime of graphite crucibles for containing molten uranium metal and/or molten uranium salts.

Advantageously, the molten uranium or uranium salt is not reactive with the coating and is not contaminated as a result. This enables higher purity uranium product to be obtained.

Preferably the coating comprises a carbide of tantalum and/or niobium. Such coatings are especially effective for use with molten uranium and/or molten uranium salts.

The coating may be comprised substantially of a tantalum carbide. The tantalum carbide may have a stoichiometry from around TaC to around Ta₂C. A stoichiometry around Ta₂C is preferred.

The coating may be comprised substantially of a niobium carbide . The coating may comprise a mixed carbide of tantalum and niobium.

The coating may comprise a mixture of a tantalum carbide and a niobium carbide.

The coating may comprise a single phase of a carbide material, ie with a fixed stoichiometry throughout the coating. Alternatively, the coating material may comprise a plurality of phases, ie with varying stoichiometries .

Advantageously the coating is substantially un-reactive with a graphite substrate, under processing conditions employed in handling molten uranium. Typically the operating temperatures are at or above the melting point of the metal.

Advantageously the coating has a similar thermal expansion coefficient to graphite.

Advantageously the coating displays high adhesion to graphite.

The coating material is preferably non-wettable by the molten metal composition. Graphite vessels often contain small cracks and fissures at the surface into which the molten metal may leak leading to loss of metal in the cracks and deterioration of the vessel surface due to chemical attack. The coating material is typically applied as only a thin layer and may not fill in these cracks but only coat around the opening of the crack. Thus, if the coating is wettable the molten metal may still leak into the crack but if the coating is non-wettable the molten metal may be held back from entering the crack. Advantageously the coating is substantially unreactive with gases in the atmosphere typically present during molten metal processing such as air or oxygen.

Using magnetron sputtering it has been found that a thin coating can be deposited which exhibits greatly improved adherence to the vessel walls compared with prior art techniques of coating such as washing. A coating deposited in such a way also displays low reactivity with eg molten uranium metal and uranium salt, particularly coatings comprising a carbide of tantalum and/or niobium.

The molten metal and/or salt has less tendency to stick to the coating than to the bare graphite vessel wall. The molten metal and/or salt also has less tendency to stick to the coating of the present invention than to the prior art coatings such as Y₂0₃. Thus, less product is lost and less cleaning of the vessel is required. The vessel coated according to the present invention also has an overall longer life.

Higher purity uranium product is obtainable using the vessel. Processes such as nuclear fuel manufacture and recycling will benefit from using the vessel.

The term magnetron sputtering includes unbalanced magnetron sputtering. Unbalanced magnetron sputtering has been found to produce the most effective coatings and is preferred. The most preferred methods are those which involve high ion current densities, high ion to atom ratios at the substrate and high mean energy per deposited atom. The method of the present invention enables dense adherent coatings to be formed. Reactive magnetron sputtering, in particular reactive unbalanced magnetron sputtering, may be used. These methods enable the stoichiometry of the coatings to be controlled by varying the amount of reactive gas in the coating system. For example, by varying the amount of gaseous hydrocarbon (a carbon source) , a coating may be formed having a stoichiometry from Ta₂C to TaC as desired.

The coating may be from 1 micron to 100 microns thick. Preferably, the thickness is from 1 micron to 20 microns. More preferably, the thickness is from 1 micron to 10 microns. Typically, the thickness is about 5 microns.

The thickness may vary across the surface of the vessel and the mentioned thickness values should be understood as representing average thickness.

The coating may have an amorphous or crystalline microstructure or a mixture thereof. Typically, the coating is substantially amorphous. A coating containing a tantalum carbide, e.g. Ta₂C, typically comprises at least some crystallinity.

Unbalanced magnetron sputtering in particular produces a fully dense coating free of pores, cracks and macroparticles .

The coating is very fine grained.

Advantageously, residual stress in the coating can be controlled by appropriate setting of the bias level in the coating system.

Advantageously, in general, the coating follows the substrate topography and is of consistent thickness. The coating may comprise a multilayer configuration such as a dual layer structure. A dual layer structure is shown schematically in Figure 1. Such multilayer structures may be advantageous where the desired coating material reacts with the vessel material. For example, in such a case an intermediate or "barrier" layer 2 may be disposed between the vessel 1 and the desired coating material 3. The barrier layer need not be thick and typically is of smaller thickness than the outer coating material layer. For example, the layer 2 may be of the order of one micron thick.

An example of a dual layer as illustrated in Figure 1 could be where the desired coating material 3 for a graphite vessel 1 comprises a tantalum carbide, e.g. Ta₂C, or tungsten. However, under given operating conditions, the coating may react with the graphite leading to deterioration of the coating. To prevent the reaction, an intermediate layer 2, e.g. of yttria (Y₂0₃) , may be deposited onto the graphite and the coating 3 deposited on top of the yttria. The coating 3 does not react with the yttria thus avoiding the problem of reaction .

The article may be a condenser or collector, such as a plate or baffle, for condensing or collecting metal vapour. Such condensers and collectors are used in processes to condense or collect metal vapour such that the metal then runs of the condenser or collector as molten metal, optionally to further collection or treatment stage.

The options and features of the coating described above in relation to vessels for containing molten metal also apply in relation to the coating for the condenser or collector. Referring to the method of coating, where the coating comprises a carbide of tantalum and/or niobium as defined above, the material suitable for depositing the coating may comprise a tantalum or niobium element. The element may be in the form of a target.

For coatings comprising a carbide, the material suitable for depositing the coating may comprise one or more carbon containing compounds. The one or more compounds may comprise a gaseous compound, eg hydrocarbon.

The second aspect of the present invention also includes the options and features of the first aspect described above.

An embodiment of the invention will now be described in detail by way of the following example. The embodiment is only illustrative of the invention and does not limit the invention in any way.

Example - Coating a Graphite crucible with Ta₂C for use with molten uranium and UF₄

A coating was applied to a graphite crucible for use in the nuclear industry in containing molten uranium and molten uranium fluorides (particularly UF₃ or UF₄) . The coating material was ditantalum carbide (Ta₂C) , which was determined to be substantially chemically unreactive with both the molten uranium and the uranium fluorides at the maximum temperature envisaged, 1801°C. The Ta₂C also displayed good thermal shock resistance .

The coating was deposited using an unbalanced magnetron sputtering (UBMS) method. The basic UBMS method is shown schematically in Figure 2. The coating was carried out inside a vacuum chamber 2. A tantalum target 4 was situated inside a magnetron 14 which comprised two similarly polarised (eg north) magnets 8 located at the outer edges of the magnetron and an oppositely polarised (eg south) magnet 10 located between the two outer magnets. The object to be coated 6, ie the graphite crucible, was located inside the chamber 2 oppositely facing the tantalum target. Argon and the source of carbon, in this example butane, were admitted into the chamber 2 through inlet 20. In operation the tantalum target 4 was biased to -600V so that argon ions were accelerated at the target thereby sputtering tantalum atoms from the target. Reaction of the carbon source gas with the sputtered tantalum surface formed tantalum carbide which was also sputtered from the target. In this way sputtered tantalum and tantalum carbide was deposited on the graphite crucible opposite which was un-biased (ie earth potential) . Optionally the crucible could be subjected to a bias. Operation of the magnetron in unbalanced mode, where the outer two magnets were of larger strength than the inner magnet produced a better performing coating than operation in balanced mode where all magnets are of substantially equal strength.

The Ta₂C coated graphite crucibles were then tested and compared with non-coated graphite crucibles.

The crucibles were loaded with uranium and UF₄ and heated to 1700°C in an argon purged furnace. The loaded crucibles were held at temperature with the molten metal/salt for various periods of time (eg 3 hours, 1 day, 3 days) before cooling and removal for analysis.

The uranium dropped out of the coated crucibles after one run without appreciable persuasion compared with the non-coated crucibles where substantial amounts of uranium stuck to the crucible walls.

After the second and third runs, the uranium still dropped out of the coated crucibles without persuasion or with only light tapping .

Visually, the Ta₂C coating appeared not to be attacked by the molten uranium and was re-usable.

Claims

Cl aims

1. A method of coating an article for use with molten metal, the method comprising depositing a coating by magnetron sputtering.

2. A method as in claim 1 wherein the magnetron sputtering is unbalanced magnetron sputtering.

3. A method as in claim 1 or 2 wherein the coating comprises a carbide of tantalum and/or niobium.

4. A method as in claim 3 and wherein a tantalum and/or niobium target is used for depositing the coating.

5. A method as in any one of claims 1 to 4 and wherein a carbon containing compound is used for depositing the coating.

6. A coating for an article for use with molten metal, the coating obtainable by depositing the coating on the article by magnetron sputtering.

7. A coating as in claim 6 wherein the coating is obtainable by depositing the coating on the article by unbalanced magnetron sputtering.

8. A coating as in claim 6 or 7 wherein the coating comprises a ceramic material.

9. A coating as in claim 6 or 7 wherein the coating comprises a material selected from the group consisting of EuS, HfN, HfC, HfB₂, MoB₂, NbB₂, Nb₂C, Ni₃B, ScO, Ta, TaB₂, Ta₂C, Th0₂, TiN, TiC, TiB, V₂C, V₃B₂ W, ZrN, ZrC, ZrB₂ and Zr₂S₃.

10. A coating as in claim 6 or 7 wherein the coating comprises a carbide of tantalum and/or niobium.

11. A coating as in claim 10 wherein the coating comprises a mixture of a carbide of tantalum and a carbide of niobium.

12. A coating as in claim 10 or 11 wherein the coating comprises a mixed carbide of tantalum and niobium.

13. A coating as in any one of claims 6 to 12 and wherein the molten metal comprises uranium.

14. A coating as in claim 13 and wherein the molten metal substantially comprises uranium.

15. A coating as in any one of claims 6 to 12 and wherein the molten metal salt comprises a uranium salt.

16. A coating as in claim 15 and wherein the molten metal salt substantially comprises a uranium salt.

17. A coating as in claims 15 or 16 and wherein the uranium salt comprises a uranium fluoride.

18. A coating as in any one of claims 6 to 17 wherein the coating thickness is between about 1 micron and 100 microns thick.

19. A coating as in claim 18 wherein the thickness is between about 1 micron and about 10 microns.

20. A coating as in any of claims 6 to 19 wherein the article is a vessel for containing molten metal.

21. A coating as in claim 20 wherein the vessel comprises graphite .

22. A coating as in any one of claims 6 to 19 wherein the article is a condenser or collector for condensing or collecting metal vapour.

23. A coating as in claim 22 wherein the condenser or collector comprises a plate or baffle.

24. A coated article adapted for use with molten metal comprising a coating according to any one of claims 6 to 23.

25. A coated article according to claim 24 wherein the article is a vessel for containing molten metal.

26. A coated article according to claim 24 wherein the article is a condenser.

27. A coated article according to claim 24 wherein the article is a collector.

28. Use of a material obtainable by magnetron sputtering as a coating for an article for use with molten metal.

29. A method of processing at least one of molten uranium and molten uranium salt using a graphite vessel characterised in that the graphite vessel has a coating according to any one of claims 6 to 23.