WO2011029031A1

WO2011029031A1 - High temperature, high strength-to-weight-ratio composite material

Info

Publication number: WO2011029031A1
Application number: PCT/US2010/047863
Authority: WO
Inventors: Edward I. Moses; Joseph C Farmer; Joshua D. Kuntz; Scott Groves; Luis Zepeda
Original assignee: Lawrence Livermore National Security LLC
Current assignee: Lawrence Livermore National Security LLC
Priority date: 2009-09-04
Filing date: 2010-09-03
Publication date: 2011-03-10
Anticipated expiration: 2012-03-04

Abstract

A high temperature resistant composite structure preferably for use in fusion and fusion-fission systems includes an inner layer of relatively rigid material and an outer layer of relatively rigid material. Between the inner and outer layers an intervening layer of nano-structural foam is disposed. The foam provides a high level of thermal isolation, yet is resistant to neutron damage from the fusion source. Preferably the inner and outer layers are silicon carbide or silicon carbide fiber wrapped structure to enable the manufacture of complex shapes.

Description

HIGH TEMPERATURE, HIGH STRENGTH-TO-WEIGHT-RATIO

COMPOSITE MATERIAL

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application

No. 61/240,126, filed September 4, 2009, entitled "High Temperature Radiation-Resistant High Strength-to-Weight-Ratio Composite Materials and Chambers for Fusion, Fission, and Fusion-Fission Hybrid Reactors," which is incorporated by reference herein in its entirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC.

BACKGROUND OF THE INVENTION

[0003] This invention relates to composite materials with high strength-to-weight ratios, and capable of withstanding high temperatures and extreme radiation doses. Such structural components have particular utility in high temperature applications where intense neutron bombardment occurs, for example, as structural components in fusion or fusion-fission applications. These materials are also promising structural materials for nuclear reactor applications.

[0004] The development of such materials is, in part, driven by the need for nuclear energy. Projections by the Energy Information Agency and current Intergovernmental Panel on Climate Change (IPCC) expect worldwide electric power demand to double from its current level of about 2 teraWatts electrical power (TWe) to 4 TWe by 2030, possibly reaching 8-10 TWe by 2100. They also expect that for the next 30 to 50 years, the bulk of the demand of electricity production will be provided by fossil fuels, typically coal and natural gas. Coal supplies about 40% of the world's electric energy today, and is expected to supply 45% by 2030. In addition, the most recent report from the IPCC has placed the likelihood that man- made sources of C0₂ emissions into the atmosphere are having a significant effect on the climate of planet earth at 90%. "Business as usual" baseline scenarios show that C0₂ emissions could be more than twice the current level by 2050. More than ever before, new technologies and alternative sources of energy are essential to meet the increasing energy demand in both the developed and the developing worlds, while attempting to stabilize and reduce the concentration of C0₂ in the atmosphere and mitigate the concomitant climate change.

[0005] Nuclear energy, a non-carbon emitting energy source, has been a key component of the world's energy production since the 1950's, now accounting for about 16% of the world's electricity production, a fraction that could - in principle - be increased. Several factors, however, make its long-term sustainability difficult. These concerns include the risk of proliferation of nuclear materials resulting from the nuclear fuel cycle; the handling and disposal of radioactive nuclear waste; the current reliance on the once through open nuclear fuel cycle; and the availability of low cost, low carbon footprint uranium ore. In the United States alone, nuclear reactors have already generated more than 55,000 metric tons (MT) of spent nuclear fuel (SNF). In the near future, the US will have enough spent nuclear fuel to fill the Yucca Mountain geological waste repository to its legislated limit of 70,000 MT.

[0006] Fusion is an attractive energy option for future power generation, with two main approaches to fusion power plants now being developed. In a first approach, Inertial Confinement Fusion (ICF) uses lasers, heavy ion beams, shock ignition, impulse ignition, pulsed power or other techniques to rapidly compress capsules containing a mixture of isotopes of hydrogen, typically, deuterium (D) and tritium (T). As the capsule radius decreases and the DT gas density and temperature increase, DT fusion reactions are initiated. These DT fusion reactions generate both alpha particles and 14.1 MeV neutrons. A second approach, Magnetic Fusion Energy (MFE) uses powerful magnetic fields to confine a DT plasma and to generate the conditions required to sustain a burning plasma and generate energy gain.

[0007] Important technology for ICF is being developed primarily at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in Livermore, California. At LLNL, a laser-based inertial confinement fusion project, designed to achieve

thermonuclear fusion ignition and burn, utilizes laser energies of more than one megajoule (MJ).

[0008] A challenge for any commercial power plant relying upon fusion or fusion-fission reactions is the extreme environment which materials situated near such reactions must withstand. For example, such materials are expected to be required to operate continuously at temperatures over 1000°C, and perhaps higher than 1300°C. In that environment the materials will be subjected to a combination of 14-Mev neutron flux, severe neutron dosage, and highly corrosive environments from molten salts or other material used for cooling. Of course such materials will find uses in other demanding environments.

BRIEF SUMMARY OF THE INVENTION

[0009] New materials are described herein that are capable of withstanding the

environments mentioned above for commercially reasonable periods of time. These materials promise a solution to some of the problems facing future fusion systems. In particular silicon carbide, silicon carbide fiber, and aerogel-based materials are capable of operating continuously at temperatures in excess of 1000°C in environments with intense 14-Mev neutron flux, severe neutron dose, even when exposed to highly corrosive molten salts.

[0010] In one embodiment a structural material is disclosed which consists of an aerogel sandwiched between appropriate outer layers, which can be made from material such as, silicon carbide plates, a silicon carbide fiber wrapped structure, oxide-dispersed ferritic steel, tungsten-, tantalum-, vanadium- and other refractory-metal alloys. Such a structure is not only lightweight and heat resistant, but the aerogel is self healing in the presence of intense neutron bombardment. In a preferred implementation, silicon carbide fiber winding technology is used to enable the manufacture of large scale complicated shapes, with silicon carbide/silicon carbide fiber composite panels and aerogel layers for thermal isolation.

[0011] Preferably the high-temperature composite structure includes an inner layer of relatively rigid material, an outer layer of relatively rigid material, and an intervening layer of nano-structural foam disposed between the inner layer and the outer layer. Preferably the nano-structural foam is an aerogel, such as a silicon carbide aerogel or a zirconium tungstate aerogel. The inner and outer layers may be formed from silicon carbide, oxide-dispersion strengthened steel, or other suitable materials.

[0012] In some embodiments a layer comprising tungsten is disposed on the inner layer adjacent the fusion source to further protect that side of the inner layer. In addition in some embodiments a refractory tantalum-tungsten alloy, known to be resistant to corrosion by high-temperature molten fluoride salts, is used for the fabrication of an embedded cooling channel, disposed within the structure, or is used as one of the inner or outer layers or plated onto those layers. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a cross sectional view of the chamber wall showing the inner layer, the aerogel, and the outer layer;

[0014] Figure IB is a cross sectional view illustrating in more detail the cooling portion of the structure shown in Figure 1.

[0015] Figure 2 is a more detailed view of the structure of Figure 1.

[0016] Figure 3 is a detailed view illustrating the effect of ion energy on aerogel.

[0017] Figure 4 is a diagram illustrating the relationship of yield strength to temperature for a variety of materials.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The capability of lasers to create conditions required for inertial confinement fusion is expected to be demonstrated at NIF in 2010-201 1. Ignition and modest target gains are expected during this period. The fusion yield to laser energy ratio is expected to be about 10, resulting in fusion energy yields of 10 to 15 MJ. The first experiments to demonstrate ignition and gain probably will use 350-nm laser light with a central hot spot (CHS) ignition geometry and laser energy on the order of one megajoule. Such demonstrations can lead the way to the production of fusion power as a means of generating electricity and providing the carbon free energy needed by the ever growing demands of the world's economies.

[0019] To mitigate the challenges of nuclear energy and advance the time scale of the usefulness of fusion sources, one approach is to combine aspects of nuclear fusion and fission by providing power using both fusion and fission. One approach surrounds a relatively modest inertial confinement fusion neutron source with a subcritical fission fuel blanket. In this approach, the point source of fusion neutrons acts as a catalyst to drive the fission blanket, which obviates the need for a critical assembly to sustain the fission chain reaction. Starting from as little as 300 to 500 megawatts of fusion power (MW_f), a single engine will be able to generate 2000 to 3000 megawatts of thermal power (MW_t) in steady state for periods of years to decades, depending on the fuel and engine configuration. Because neutrons are provided by the fusion targets, the fission blanket in a fusion-fission system can be subcritical. This enables the engine to burn any fertile or fissile nuclear material, including un-enriched, natural or depleted uranium and spent nuclear fuel, and to extract virtually 100% of the energy content of its fuel.

[0020] The fusion-fission hybrid approach results in enhanced energy generation per metric ton of nuclear fuel, and reduces the amount of nuclear waste. Even the resulting waste has vastly reduced concentrations of long-lived actinides. Such fusion- fission engines thus can provide substantial amounts of electricity, yet reduce the actinide content of nuclear waste, thereby extending the availability of low cost nuclear fuels for thousands of years. The fusion-fission hybrid also provides a pathway for burning excess weapons grade plutonium.

[0021] In view of these advantages, fusion or fusion-fission engines offer a path toward sustainable and safe power. In either a fusion or fusion-fission implementation, inertial confinement fusion is used to produce 14 MeV neutrons from a fusion reaction of deuterium and tritium. The neutrons, in turn, either produce heat in a blanket of coolant surrounding the engine, or in a fusion-fission hybrid, drive a subcritical blanket of fissile or fertile fuel.

[0022] The inertial confinement fusion reaction can be implemented using various mechanisms. In our initial approach we use central hot spot fusion initiated using indirect drive. Indirect drive uses energy from lasers to heat a hohlraum which contains the fusion fuel, preferably a small pellet containing deuterium and tritium. The hohlraum emits x-rays which compress and heat the fuel, causing fusion, ignition and burn. In alternate approaches direct drive (no hohlraum), or fast ignition (separate compression and ignition lasers) may also be used.

[0023] Of importance to the systems described above is a need to achieve a long service life with high efficiency operation. These conditions become more advantageous as temperatures increase. For example, the maximum thermal dynamic efficiency increases from 55% at 700°C to 80% at 1200°C. Thus, the need for materials that have long service life at such high temperatures and in the presence of an intense neutron environment is especially beneficial.

[0024] Figure 1 is a diagram illustrating a preferred cross-section of a wall for a fusion or fusion-fission implementation. An inner layer 20 (closest to the fusion source) and an outer layer 40 with an nano-structural foam layer 30 disposed between them provides the basic structure for the wall. In some implementations an additional layer of material 10, for example, tungsten, is provided between the inner layer and the fusion source of the neutron bombardment. [0025] The nano-structural foam itself may be any appropriate foam, for example, a silicon-carbide based aerogel. Such materials can be formed using well-known techniques, e.g. chemical vapor deposition of silicon onto a carbon foam, followed by a heating step to produce SiC aerogel in situ. Other suitable nano-structural foams include iron oxide foams. Aerogels are particularly advantageous because of their self-healing property in intense neutron environments. Aerogels have the advantage of providing extraordinarily low thermal conductivity, typically less than 0.2 Watts per meter per degree Kelvin. Of course, aerogels other than SiC based aerogels, may be used for applications in fusion and fusion- fission systems. For example, the aerogel composition zirconium tungstate (ZrW₂0₈) has low thermal conductivity, low coefficient of thermal expansion, chemical compatibility and radiation resistance.

[0026] In addition to the aerogel, other stiffener designs may be employed with the aerogel or separately. For example, composite honeycombs and cubic webbing could be used to provide greater strength and stiffness, with a marginal weight penalty. Furthermore, in the illustration of Figure 1, the structure is shown as a "three-ply" composite, but multiple ply layers also may be used.

[0027] The inner and outer layers supporting the aerogel advantageously are resistant to the thermal and neutron environments. In one embodiment we use silicon carbide for these layers. Silicon carbide is resistant to very high temperatures, and the use of silicon carbide fibers allows complex shapes to be manufactured. The silicon carbide fibers themselves may be composite structures, e.g. SiC wrapped tungsten fibers or SiC wrapped graphite fibers. In manufacturing such structures, after the winding, the material can be infiltrated with another material, for example material which when heat treated forms silicon carbide to fill in the interstitial spaces of the structure. In such structures the cooling channels, fittings and fasteners may be incorporated into the wrapping at the time of formation.

[0028] The inner layer 20 and outer layer 40 containing the aerogel 30 may themselves each be composite structures. For example, the inner layer may consist of a silicon carbide fiber structure in which the fibers nearest the inner wall 10 are SiC wrapped tungsten fibers while those nearer the aerogel are SiC wrapped graphite fibers. Of course the inner and outer layers may comprise materials other than SiC based materials, for example, ODS steel.

[0029] The chamber structure for a fusion or fusion-fission engine is a complex shape in view of its need to provide openings for the incoming laser beams, openings for the injection and ejection of targets, and channels for coolant. One technique for manufacturing such a structure is fiber winding. This technology has been used to manufacture complex shapes, e.g. Boeing 787 aircraft aerodynamic components. Such fiber winding may be performed over a thermally sprayed tungsten mandrel, or over a removable mandrel, for example one which is inflatable or frangible. Such large fiber winding systems are known in the industry and can be used with silicon carbide fiber.

[0030] In addition to fusion applications described above, the materials and structures described herein have applicability to many other applications where lightweight structural materials are desired. For example, in aerospace applications the ply structure can be used to provide a structural component such as an aerodynamic surface having low weight and high strength. In addition, the porosity of the nano-structural material allows fluid to flow through the material, or be stored therein, enabling cooling fluid to be in close proximity to the plates containing the foam. The porosity also permits storage for fuel or other liquids integrally within the structure itself.

[0031] Figure IB illustrates in more detail the coolant/structure portion of Figure 1. As shown in Figure IB, in one embodiment to transfer heat away from the fusion chamber, a molten salt flows in proximity to the outer layer. The molten salt is eventually routed to a heat exchanger, where the heat can be extracted and used for power generation. Depending upon the particular implementation of the chamber, the channels may be formed integrally with the outer layer, or attached to it. In a preferred embodiment to resist the corrosive effects of the molten salt, tantalum tungsten alloys are used to form, or at least plate, the walls of the cooling channels. In another implementation the tantalum tungsten alloys are formed integrally with the structure, for example, by forming the outer layer 40 of tantalum tungsten itself, or by forming a composite structure with tantalum tungsten plating adjacent the cooling channels. In some cases, the coolant can pass within the pores of the micro-porous stiffening material.

[0032] The formation of the inner and outer layers with aerogel between them can be achieved using a variety of techniques. In one implementation, a fixture secures the two layers at the desired spacing, then the aerogel is blown in between the layers. In another implementation bracing, for example, a honeycomb structure, is provided between the two layers to secure them in position. Then aerogel is blown in between the layers, with the bracing remaining within the aerogel.

[0033] The use of tantalum tungsten alloys provides improved corrosion resistance. Iron- chromium alloys such as ODS ferritic steel produce corrosion products such as chromium fluoride (CrF₂) which has a free energy of formation of -75.2 kcal per g-mole at 1000°K. Tantalum allows only slight grain boundary penetration at 900°C, similar to that found for ODS ferritic steel at 600°C. The tantalum tungsten alloys thus better resist corrosion when exposed to a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF₂) [FLiBe] or lead- lithium coolant. Of course, ODS steel, or other composition, fasteners and fixtures may be embedded in the cooling chamber structure material as necessary during its manufacture.

[0034] The materials and structures described above enable a fusion based engine to operate at higher temperature, yet have a high strength-to-weight ratio, and a low coefficient of thermal expansion. Such materials can survive intense neutron flows and highly corrosive environments of molten salt coolant materials. Of course, the materials described herein can also have many other applications, for example in other energy-related technology, in aerospace applications such as re-entry vehicles, as well as other applications where weight, temperature resistance, and strength are important.

[0035] Figure 2 is a diagram illustrating in further detail the composite inner

layer/aerogel/outer layer structure depicted in Figure 1. As shown in Figure 2, the nano- structural foam provides a stiffener between two plates which may be formed of the same or different materials. In the illustration the inner and outer layers comprise ODS ferritic steel, silicon carbide, or a tungsten- or vanadium-based alloy layers.

[0036] Figure 3 illustrates the radiation resistance of the aerogel material used as the structural stiffener. Note that the damage cascade in the materials is comparable to the size of the particular ligaments, making the material relatively radiation resistant. The radiation resistance is believed to be a result of migration of the defects to the surface of the material, where they diffuse into the surface of the "nano-struts," recombine, and therefore heal.

[0037] Figure 4 is a diagram illustrating the relationship between yield strength and temperature for different materials. Note that as temperatures increase, the yield strength for essentially all materials depicted, which include various ferritic steels, decreases

dramatically, especially as temperatures even as hot as 800°C are reached. In contrast, silicon carbide materials have yield strength on the order of twice that of conventional materials at 800°C and above. Thus SiC wound structures with aerogel stiffeners provide sufficient strength at high temperatures for fusion and fusion-fission applications. Another advantage of the SiC based structures for high temperature applications is that they exhibit relatively low swelling at the irradiation temperatures expected in a fusion based engine. [0038] We have examined the impact of thermal gradients on the deflection of refractory composites having nano- structural foam stiffening layer (silica aerogel). The calculations that follow assume the properties of stainless and carbon steels, vanadium, tungsten and silica aerogels for the various layers. The simplest formulae for deflection of heated flat plates can be found in the following reference: Raymond J. Roark, Warren C. Young, "Dynamic and Temperature Stresses," Chapter 15, Formulas for Stress and Strain, 5^th Edition, McGraw- Hill, New York, NY, 1975, pp. 564-587.

[0039] Two cases are considered for the simple heated plate: (1) unconstrained edges, and (2) constrained edges, with uniform edge moments. In the unconstrained case it is assumed that one face of the cold side of the heated plate is kept at temperature T, while the hot side of the plate is kept at temperature Τ+ΔΤ. The homogeneous plate had a coefficient of thermal expansion γ and thickness t. The approximate radius of curvature of the heated plate is then: t

^~

[0040] In practice, edges of plates in structures are frequently constrained. In this situation there is a uniform edge moment that opposes the bending of the plate. The stress opposing the tendency to bend σ is given by:

_ 1 γΔΤΕ

σ^_ 2 (1 -ν) where E is the modulus of elasticity and v is Poisson's ratio.

[0041] In the preferred embodiment where the structure consists of an inner and outer layer separated by aerogel, the direct application of simple deflection formulae to such a laminated composites is inappropriate. We believe, while still an approximation, it is more appropriate to treat the system as three distinct layers.

[0042] In this analysis, the inner layer is assumed to be a 15 mil thick first metallic cladding, adjoining a 0.5 cm thick nano structural (aerogel) foam stiffener, with an outer layer of 15 mil thick second cladding. The first cladding is faces the heat source, and is tungsten or vanadium; the stiffener is a silica aerogel; and the second cladding faces away from the heat source, and is oxide dispersion-strengthened ferritic steel (having properties comparable to either 304 stainless steel, or carbon steel). The assumed properties are summarized below: Material Density Thermal Conductivity Coefficient of Comments

Thermal Expansion

(g/cm³) (J/sec-cm-K = W/cm-

(Mm/cm-K =

K)

ppm/K)

SS 304 8.02 0.3 13.5 Ambient

Carbon Steel 7.86 1.0 9.45 Ambient

Vanadium 6/1 1 8 Ambient

Tungsten 19.25 2.35 4.5 Ambient

Si0₂ Aerogel 0.003-0.35 15 2 Ambient

Vitreous Si0₂ 0.4 Ambient

[0043] The temperature drop across the individual layers is estimated with Fourier's first law. The power density on the wall is estimated, assuming a point thermal source of 3 x 10⁹ (3000 MW_th = 3GW_th), and a approximately spherical chamber diameter of 3.5 meters. The surface area of this chamber is estimated to 38.5 x 10⁶ cm².

[0044] The thickness of the first layer thin metallic cladding in the tri-layer laminate is assumed to be 38.1 x 10^"3 cm. The temperature differential across this layer is then estimated:

_{A T} (3 x l0V)(38.1 x lQ-³ cm)

Al « r : _r = J

(38.5 x 10⁶ cm^l){\ .0Wcm^~lK^~l ) [0045] The corresponding radius of curvature is then:

38.1 x l0^"3cm

r 12.7 x l0³cm = 127m

3^ x l0x l 0^~6^

[0046] Curvature in the first laminate layer is insignificant. Estimates for the second layer in the tri-layer laminate, the 0.5-cm thick layer of silica aerogel, are:

[0047] Thus 0.5 cm of silica aerogel is sufficiently insulating to maintain a temperature difference in excess of 800K across such a composite wall. The corresponding radius of curvature is then: 0.5cm 2 o _ι

r « _r = 3.13 x 10 cm = 3.13m

soo r x ixi o^-6^¹

[0048] The radius of curvature that would be induced in the aerogel stiffener would be approximately 3.13 meters, which is larger than the radius of curvature of the assumed spherical fusion chamber, and would therefore be insignificant in most cases. Estimates for temperature-induced curvature of the backside cladding are no worse than for the front side. In summary, the radius-of-curvature due to thermal distortion of the proposed laminated composite does not appear to be problematic. Furthermore, the bending moment in the constrained case appears insignificant.

[0049] While the preceding has been a description of a preferred embodiment of our invention, it will be apparent to those of skill in the art that numerous modifications may be made without departing from the scope of the invention. For example, the three layer structure depicted in Figure 1 may itself be used a structural support, such as a beam or stiffener, for structures manufactured using other types of materials.

Claims

What is claimed is:

1. A high temperature radiation resistant composite structure comprising:

an inner layer of relatively rigid material;

an outer layer of relatively rigid material; and

an intervening layer of nano-structural foam disposed between the inner layer and the outer layer.

2. A structure as in claim 1 wherein the nano-structural foam comprises an aerogel.

3. A structure as in claim 1 wherein the aerogel comprises a silicon carbide aerogel.

4. A structure as in claim 1 wherein the aerogel comprises zirconium tungstate.

5. A structure as in claim 1 wherein at least one of the inner layer and the outer layer comprises silicon carbide.

6. A structure as in claim 1 wherein at least one of the inner layer and the outer layer comprises an oxide-dispersion strengthened steel.

7. A structure as in claim 1 further including a layer comprising tungsten disposed on the inner layer on a side of the inner layer opposite to the intervening layer.

8. A structure as in claim 1 further comprising a tantalum tungsten enclosed cooling channel in proximity to the outer layer.

9. A structure as in claim 5 wherein the silicon carbide comprises fiber wound silicon carbide fibers impregnated with silicon carbide.

10. A structure as in claim 1 wherein at least one of the inner layer and the outer layer comprises a tantalum tungsten alloy.

11. A structure as in claim 10 wherein at least one of the inner layer and the outer layer comprises a tantalum tungsten alloy plated onto an underlying layer.

12. A method of manufacturing a high temperature radiation resistant composite structure comprising:

disposing and inner layer and an outer layer in a fixed relationship in proximity to each other; and

introducing a nano-structural foam between the inner and the outer layers.

13. A method as in claim 12 wherein the step of introducing further comprises blowing the nano-structural foam between the inner and the outer layers.

14. A method as in claim 12 wherein the nano-structural foam comprises an aerogel.

15. A method as in claim 1 wherein the aerogel comprises a silicon carbide aerogel.

16. A method as in claim 1 wherein the aerogel comprises zirconium tungstate.

17. A method as in claim 1 wherein at least one of the inner layer and the outer layer comprises silicon carbide.

18. A method as in claim 1 wherein at least one of the inner layer and the outer layer comprises an oxide-dispersion strengthened steel.

19. A method as in claim 1 further including a layer comprising providing a tungsten layer disposed on the inner layer on a side of the inner layer opposite to the intervening layer.

20. A method as in claim 1 further comprising a step of forming a cooling channel comprising a tantalum tungsten alloy in proximity to the outer layer.

21. A method as in claim 5 wherein the silicon carbide comprises fiber wound silicon carbide fibers.

22. A method as in claim 1 wherein at least one of the inner layer and the outer layer comprises a tantalum tungsten alloy.

23. A method as in claim 10 wherein at least one of the inner layer and the outer layer comprises a tantalum tungsten alloy plated onto an underlying layer.