Shape Memory Alloy Conductor That Resists Plastic Deformation
A conductor that resists plastic deformation is provided for an electronic signal-carrying or electric power-carrying cable, cable assembly, or device. The conducting element itself has favorable mechanical properties and therefore combines plastic deformation resistance with conductance. In one embodiment, the superelastic conductor is fabricated using a shape memory alloy such that the transformation temperature of the superelastic conductor is set outside the useful operating range of the conductor. In another embodiment, the conductor is fabricated using a shape memory alloy that is nominally in a martensitic phase under stress free conditions. In both embodiments, the conductor microstructures are able to accommodate externally applied strain, bending, deformation, or other external displacement through mechanisms which do not involve plastic deformation.
Conductors and the cables that consist of conductor are often subjected to external mechanical loading and vibrations. These vibrations or other mechanically or environmentally applied loads can often result in strain, deformation, bending, or other displacement occurring globally across the length of the conductor or cable assembly or locally at specific regions. These large elastic or plastic deformations in turn can compromise the mechanical integrity of the conductor or cable assembly comprised of conductors. Specific examples of large elastic or plastic deformations in conductors and cables and the problems they can cause include: kinking, tangling, strain localization resulting in severe sharp radius permanent bends, repetitive straining resulting in work hardening, loss of conductor or cable fatigue life, or eventual cable or conductor continuity through breakage and failure.
Shape memory alloys are a class of materials commonly used for their ability to change shape (e.g. actuators) or for their superelasticity (eyeglass frames, orthodontic bridge wire, cellular phone antennas). Because SMAs can accommodate large deformations or strains through a reversible phase transformation they are excellent materials in applications where great flexibility is needed. SMAs have therefore been used as reinforcing members in electric cables (U.S. Pat. No. 8,399,769, U.S. Pat. No. 6,717,056, U.S. Pat. No. 5,275,885, U.S. Pat. No. 7,093,416), where the role of the SMA element is to keep the cable or wire straight or to improve fatigue performance or other mechanical properties of the wire or cable assembly. Furthermore, SMA wires have been used extensively as superelastic wires and cables, where the wire or cable serves a structural or functional purpose only and no element in the assembly is conducting electric signal, data signals, electromagnetic signals or power. Such wires have greater flexibility than wires made of common materials such as for example Fe, Cu and Al and can therefore provide better fatigue and resist local plastic deformation.
Up to now, SMAs have been used as structural reinforcements to electric cables or wires. In such applications the SMA element is improving some mechanical property of the cable assembly as a whole (e.g. fatigue, resistance to kinking) and is not the conductor of electromagnetic signals, power, data signals or other types of electricity which is meant to be transported from one location of the cable to another. SMAs also have been used as electrically activated actuators. In this role, current is passed through an SMA wire for the sole purpose of generating heat to trigger the phase transformation from martensite to austenite; the electrical signal is not transported from one location to another, but is used as an indirect means of generating heat through Joule heating.
SUMMARY OF THE INVENTIONIn this invention, one or more SMA elements (either austenitic or martensitic) are used as electric conductors of data signals, electromagnetic waves or electrical power. SMA wires are good electrical conductors and also have superior mechanical properties compared to conventional conducting wires and cables, and therefore overcome the limitations of previous conducting structures.
It is an object of this invention to provide for a conductor and cable or cable assembly comprising or consisting of conductors which are resistant to the deleterious effects of large elastic or plastic deformation. It is a further object of this invention to provide for a conductor material which specifically combines the attributes of good conductivity with mechanical properties which resist the adverse effects of large elastic deformation or permanent plastic deformation.
The practical implications of this invention are the development and deployment of a conductor and cable or cable system comprising or consisting of such conductors that is damage tolerant, that resists cable fatigue and premature failure, that resists the inconvenience of cable kinking and tangling for consumer electronics applications, and that provides additional safety margin for highly performance critical or safety critical applications.
Cable failures occur in a variety of ways including failure of any one or more of the various elements that make up a cable, including the conductor, the insulation, the insulation shield if any, the metal outer shield if any and the outermost cable jacket if distinct form the insulation. In some cables, there is simply the conductor or conductors surrounded directly by insulation that also serves as the outer protective sheath. In other cables, there can be the various layers listed previously. Each portion of the cable can have its own specific failure mechanisms caused by a combination of environmental conditions and loading conditions, but the focus of this invention will be on preventing permanent plastic deformation of the conducting element.
A large angle deformation can lead to a permanent set or kink in the material as shown in
It is an object of this invention to help avoid such strain localization through the use of a superelastic conducting element. The superelastic element has a stress strain curve as shown in
Shape memory alloys (SMAS) are a class of materials that exhibit a martensitic transformation, which is a first-order, solid-state lattice-distortive diffusionless structural change having a shape change such that strain energy dominates the kinetics and morphology. Since their discovery in the 1960s they have been extensively researched and are now commercially available in the form of wire, sheets, tubes and more complex form factors. Some common alloys exhibiting superelasticity and/or the shape memory effect are Ni—Ti, Cu—Al—Ni, Cu—Zn—Al, Cu—Al—Be, Cu—Mn—Al, Fe—Mn—Si Ni—Mn—Ga, but many others exist and more alloy systems are also expected to be discovered in the future. Among the most commercially successful applications of these materials are spectacle frames, orthodontic bridge wire, various actuators, bra underwire, cellular phone antennas, medical stents, endodontic files, drug delivery micropumps, and others. The majority of commercial applications use either binary Ni—Ti or Ni—Ti with alloying additions from elements such as Cu, Pd, Pt, Hf or others. The current invention is not limited to a specific alloy system. Rather it applies to any SMA system with an electric resistivity that is lower than about 500 n-ohm-m. Table 1 provides a non-limiting list of different alloy systems that can be used as flexible conductors according to the invention.
SMAs are characterized by a solid-to-solid reversible phase transformation between a higher temperature phase, called austenite, and a lower temperature phase, called martensite. In most SMAs austenite exhibits a superlattice structure with the sublattices being body-centered cubic (bcc). Because the lattice of the higher temperature austenite has higher crystallographic symmetry than that of the lower temperature martensite there are multiple symmetry-related variants of martensite. Using Cu—Zn—-Al as an example, austenite may transform into twelve different variants of martensite. Often, the entire crystal does not transform from the austenite to a single variant of martensite, but rather to a complex arrangement of different variants. Under some conditions martensite is formed by nucleation and growth when a crystal of austenite is cooled into a temperature range where martensite is the stable phase; martensite is thermally induced. At isothermal conditions a mechanical force may also trigger the phase transformation, in which case martensite is said to be stress-induced. The applications of thermal or mechanical stimuli to provoke shape change are closely related to the shape memory effect (SME) and superelasticity (SE), respectively.
after applying stress to a martensite plate to transform it to a different plate the initial plate is not generally recovered when the stress is relaxed.
Superelasticity is the term used to describe stress-induced phase transformation. At a constant temperature where austenite is the stable phase, the martensitic transformation is now triggered by stress. This transformation path is shown by the vertical double arrow in
Furthermore, we note that σAs<σMf and σAf<σMs so it is clear that a complete transformation cycle always shows hysteresis. Hysteresis is related to thermodynamic irreversibility and reflects energy dissipated as heat due to frictional work spent on moving the austenite/martensite interface. Transformation strains may vary from about 2 to 10% and the transformation stresses vary from about 20 to 500 MPa depending on alloy, temperature, microstructure, strain-rate, orientation and others.
It is worth noting that the deformation, which can be as high as 10%, is accommodated purely by the phase transformation and that essentially no conventional plasticity through dislocation generation and movement is expected to occur. Therefore SMAs can accommodate large strains without failure and can also do so repeatedly; their fatigue performance at high strains is much greater than regular metals; the material resists fatigue failure by avoiding local plastic deformation. Furthermore, the superelastic material exerts an opposing force when subjected to deformation and this opposing force tries to bring the material back to the undeformed shape; the material resists kinking. For example, when a kink is introduced in a straight superelastic wire the material transforms locally from austenite to martensite at the kink, but returns to a straight shape when stress free.
Martensitic material refers to a SMA that is in the martensitic regime depicted in
Both martensitic and austenitic SMAs can be trained to assume some predetermined shape. For example a martensitic material can be given a shape; it will behave like described above, with its ability to assume different shapes through reorientation of martensite variants; when subjected to heat or electricity it will assume a predetermined shape by transforming to austenite. For example the material is manufactured and heat-treated so that the austenite shape is in the form of a circular spool of wire and that the transformation temperatures are above the operating temperature. When in use the material is martensitic; the material may be deformed and stretched to any particular shape such as a straight shape; if it is desirable to recover the spooled configuration one can subject the wire to a mild heat or to electricity; when the temperature brings the wire into the austenite regime (cf.
The superelastic conductor may be have a circular cross section but may also be flat or elliptical, or have another cross-sectional shape, as desired. Three cross sections are shown in
In this aspect of the invention the intended operational temperature range is such that the conducting wire is austenitic. The wire may or may not be delivered in a specified shape (e.g. straight, round, kinked). In a normal mode of operation the wire may or may not be subjected to severe mechanical stimulus such as tension, vibration, kinking, bending or others and the austenitic wire may or may not simply conduct current. When subjected to a mechanical input, such as vibrations, bending, tension, compression or other, the wire may accommodate this strain by transforming locally to martensite. In a vibrating assembly for example, a larger part of the wire may strain and transform; if bent of kinked, the transformation may be local. In both cases conventional plastic deformation does not occur and the wire is not damaged. When the mechanical stimulus stops (e.g. the vibration stops or the force resulting in a kink is removed) the wire will recover its original predetermined shape and the martensite transforms back to austenite. This prevents tangling, breaking and fatigue failure at large strains.
The strains that can be achieved are on the order of 2-10% locally. The recovery forces, or in other words the resistance to kinking or deformation, can be tuned within the range 10 MPa to 800 MPa depending on composition and temperature and transformation temperatures.
In this aspect of the invention the material is manufactured to be in the martensite phase under normal operating conditions. The material is therefore highly flexible and can accommodate large shape changes, with local strains as high as 10%. The advantage of this over conventional signal or electric power wire is that it can be deformed to much higher strains without accumulating microstructural damage; moreover, it can do so repeatedly. Furthermore, a predetermined shape may be recovered in the following way: the martensitic wire is heated though direct heating or through Joule heating or through other mechanisms so that the martensite transforms to austenite; the wire has been manufactured in such a way that the austenite shape takes some known form. The wire, which is now in the desired form cools back to martensite while maintaining the desired form (i.e. the thermal transformation from austenite to martensite does not result in a macroscopic shape change due to a phenomenon known as self-accommodation of martensite variants). The material is again martensitic and may be used accordingly.
The following example illustrates how the invention may be used to create both as a conductor of electromagnetic signals, as a flexible and kink free cable and as a functional wire with a memory of some particular shape. An electric conductor that transports electromagnetic signals from one end to the other is made of austenitic superelastic alloy. The signal may be transported from an electronic device such as a cell phone, a digital music player, a tablet computer, a personal computer, a stereo music system, an amplifier, an mp3 player or others. The signal may be transported to speakers, ear buds, headphones or another electronic device. The cable length may be short (e.g. a few centimeters) or long (hundreds of meters) or of intermediate length (e.g., approximately 1 to 2 meters, about 1 meter, about 2 meters, or about 3 meters). The cable may be bare or insulated. The conductor may be braided or not and may be placed alongside other cables or ropes with other functions.
The electrical conductor is austenitic and therefore has a preferred shape at the operating temperature; this shape may be straight or circular, the cable may be rolled up into some preferred shape that is compact or serves some other function. Because the cable is austenitic it will resist deformation; this resistance force may be programmed to be small or large. In one embodiment of the invention the recovery force may be small so that deformation imposed by a person is easy and the wire may be stretched to assume different shapes with small forces. When the electric conductor is left without any external force it will return back to its predetermined shape.
In one embodiment of the invention the wire may be connected to ear buds or headphones, and the preferred shape may be a compact shape such as a circular roll that fits well in the pocket or in a dispenser device, and is tangle free. This is illustrated in
The wire may be bare or insulated and it may be single-wire or part of a cable containing multiple conducting wires or other wires with complementary or separate functions that can be electrical, functional, structural or other.
A shape memory alloy material of the present invention can be either polycrystalline, single crystalline, or oligocrystalline as defined by U.S. Pat. No. 8,282,746.
Claims
1. A superelastic conductor comprising a shape memory alloy material that has an electrical resistivity less than 500 nano-ohm-meters and is configured to carry a data signal or to supply electrical power, or both.
2. The conductor of claim 1 that carries a direct current or alternating current waveform having a voltage in the range from about 0V to about 10V and a current in the range from 0 to about 20 milliamps.
3. The conductor of claim 2 that is capable of transmitting a signal having a frequency in the range from DC to about 10 GHz.
4. The conductor of claim 1 that is capable of carrying a digitally encoded signal.
5. The conductor of claim 1 that is capable of carrying a voltage of 100V or more.
6. The conductor of claim 1 that has a cross-sectional dimension in the range from about 10 micrometers to about 10,000 micrometers.
7. The conductor of claim 1, wherein the material is substantially in an austenitic phase when stress free.
8. The conductor of claim 1, wherein the material is substantially in an austenitic phase when stress free but transforms to a substantially martensitic phase when subjected to gross or localized bending, strain, or other externally applied deformation and accommodates such externally applied deformation through said phase transformation.
9. The conductor of claim 1, wherein the shape memory alloy has a transformation temperature that is at or below an intended operating temperature of the conductor.
10. The conductor of claim 1, configured for use as a single conductor.
11. The conductor of claim 1 configured as a bare wire.
12. The conductor of claim 1 configured as a wire with an electrically insulating coating substantially or completely covering its length.
13. The conductor of claim 1 configured as a pair of conductors for use in an audio application.
14. The conductor of claim 1 configured as part of a multi-conductor cable with or without shielding, wherein each individual conductor is a solid conductor.
15. The conductor of claim 1 configured as part of a stranded cable.
16. The conductor of claim 1 configured as part of a coaxial cable.
17. The conductor of claim 1 configured as a flat ribbon.
18. The conductor of claim 1 where the shape memory alloy is selected from the group consisting of the following alloys: ALLOY COMPOSITION (atomic %) Ag—Cd 44-49 Cd Au—Cd 46.5-48.0 Cd Au—Cd 49-50 Cd Cu—Zn 38.5-41.5 Zn Cu—Sn 14-16 Sn Cu—Zn—X 0-40 Zn 0-20 X X = Si, Sn, Al, Ga Cu—Al—Ni 10-35 Al, 2-5 Ni Cu—Al—Mn 0-30 Al, 0-30 Mn Cu—Au—Zn 23-28 Au, 45-47 Zn Cu—Al—Be 22-25 Al, 0.5-8 Be In—Tl 18-23 Tl In—Cd 4-5 Cd Mn—Cd 5-35-Cd Fe—Pt 25 Pt Fe—Ni—Co—Ti 23 Ni, 10 Co, 10 Ti Fe—Ni—Co—Ti 33 Ni, 10 Co, 4 Ti Fe—Ni—Co—Ti 31 Ni, 10 Co, 3 Ti Fe—Ni—C 31 Ni,.4 C Fe—Ni—Nb 31 Ni, 7 Nb Fe—Mn—Si 30 Mn, 1 Si Fe—Mn—Si 28-33 Mn, 4-6 Si Fe—Cr—Ni—Mn—Si 9 Cr, 5 Ni, 14 Mn, 6 Si Fe—Cr—Ni—Mn—Si 13 Cr, 6 Ni, 8 Mn, 6 Si Fe—Cr—Ni—Mn—Si 8 Cr, 5 Ni, 20 Mn, 5 Si Fe—Cr—Ni—Mn—Si 12 Cr, 5 Ni, 16 Mn, 5 Si Fe—Mn—Si—C 17 Mn, 6 Si, 0.3 C Fe—Pd 30 Pd Fe—Pt 25 Pt
19. The conductor of claim 18, wherein the shape memory alloy is polycrystalline.
20. The conductor of claim 18, wherein the shape memory alloy is single crystalline.
21. The conductor of claim 18, wherein the material is oligocrystalline.
22. A shape memory alloy conductor comprising a shape memory alloy that is substantially in a martensitic phase under stress free conditions and that has an electrical resistivity less than 500 nano-ohm-meters and is configured to carry a data signal, to supply electrical power, or both
23. The conductor of claim 22 that carries a direct current or alternating current waveform having a voltage in the range from about 0V to about 10V and a current in the range from 0 to about 20 milliamps.
24. The conductor of claim 23 that is capable of transmitting a signal having a frequency in the range from DC to about 10 GHz.
25. The conductor of claim 22 that is capable of carrying a digitally encoded signal.
26. The conductor of claim 22 that is capable of carrying a voltage of 100V or more.
27. The conductor of claim 22 that has a cross-sectional dimension in the range from about 10 micrometers to about 10,000 micrometers.
28. The conductor of claim 22, wherein the shape memory alloy has a transformation temperature that is at or above the intended operating temperature of the conductor.
29. The conductor of claim 22 configured for use as a single conductor.
30. The conductor of claim 22 configured as a bare wire.
31. The conductor of claim 22 configured as a wire with an electrically insulating coating substantially or completely covering its length.
32. The conductor of claim 22 configured as a pair of conductors for use in an audio application.
33. The conductor of claim 22 configured as part of a multi-conductor cable with or without shielding, wherein each individual conductor is a solid conductor.
34. The conductor of claim 22 configured as part of a stranded cable.
35. The conductor of claim 22 configured as part of a coaxial cable.
36. The conductor of claim 22 configured as a flat ribbon.
37. The conductor of claim 22 where the shape memory alloy is selected from the group consisting of the following alloys: ALLOY COMPOSITION (atomic %) Ag—Cd 44-49 Cd Au—Cd 46.5-48.0 Cd Au—Cd 49-50 Cd Cu—Zn 38.5-41.5 Zn Cu—Sn 14-16 Sn Cu—Zn—X 0-40 Zn 0-20 X X = Si, Sn, Al, Ga Cu—Al—Ni 10-35 Al, 2-5 Ni Cu—Al—Mn 0-30 Al, 0-30 Mn Cu—Au—Zn 23-28 Au, 45-47 Zn Cu—Al—Be 22-25 Al, 0.5-8 Be In—Tl 18-23 Tl In—Cd 4-5 Cd Mn—Cd 5-35-Cd Fe—Pt 25 Pt Fe—Ni—Co—Ti 23 Ni, 10 Co, 10 Ti Fe—Ni—Co—Ti 33 Ni, 10 Co, 4 Ti Fe—Ni—Co—Ti 31 Ni, 10 Co, 3 Ti Fe—Ni—C 31 Ni,.4 C Fe—Ni—Nb 31 Ni, 7 Nb Fe—Mn—Si 30 Mn, 1 Si Fe—Mn—Si 28-33 Mn, 4-6 Si Fe—Cr—Ni—Mn—Si 9 Cr, 5 Ni, 14 Mn, 6 Si Fe—Cr—Ni—Mn—Si 13 Cr, 6 Ni, 8 Mn, 6 Si Fe—Cr—Ni—Mn—Si 8 Cr, 5 Ni, 20 Mn, 5 Si Fe—Cr—Ni—Mn—Si 12 Cr, 5 Ni, 16 Mn, 5 Si Fe—Mn—Si—C 17 Mn, 6 Si, 0.3 C Fe—Pd 30 Pd Fe—Pt 25 Pt
38. The conductor of claim 37, wherein the shape memory alloy is polycrystalline.
39. The conductor of claim 37, wherein the shape memory alloy is single crystalline.
40. The conductor of claim 37, wherein the material is oligocrystalline.
41. A signal carrying or power distribution cable assembly comprising a conductor that resists plastic deformation; wherein the conductor is in a superelastic state.
42. The cable assembly of claim 41 that carries a direct current or alternating current waveform having a voltage in the range from about 0V to about 10V and a current in the range from 0 to about 20 milliamps.
43. The cable assembly of claim 41 that is capable of transmitting a signal having a frequency in the range from DC to about 10 GHz.
44. The cable assembly of claim 41 that is capable of carrying a digitally encoded signal.
45. The cable assembly of claim 41 that is capable of carrying a voltage of 100V or more.
46. The cable assembly of claim 41, wherein the conductor resists plastic deformation through superelasticity (the transformation between austenitic and martensitic phases).
47. The cable assembly of claim 41, wherein the conductor resists plastic deformation through movement of martensitic plates.
48. The cable assembly of claim 41 configured for use with audio headphones, ear buds, or other electro-magnetic devices carrying electrically encoded audio information to the human ear.
49. The cable assembly of claim 41 configured for use as speaker wire or acable that transmits electrically encoded audio information to an electromagnetic system that converts said encoded audio information back into sound.
50. The cable assembly of claim 41 configured for use in a high vibrational loading environment.
51. The cable assembly of claim 41 configured for use as aerospace applications including data signal transmission and power distribution.
52. The cable assembly of claim 41 configured for use in a weapon system subjected to ballistic recoil or other intermittent, high amplitude shock loading, for either data signal transmission or power distribution.
53. The cable assembly of claim 41 configured for use in automotive wiring harness assemblies, either for data signal transmission or power distribution.
54. The cable assembly of claim 41 configured for use in wires or cable assemblies for downhole instrumentation used in oil and gas assemblies, either for data signal transmission or power distribution.
55. The cable assembly of claim 41 configured for use in wires and cable assemblies used in computers or computer boards, either internal to the computer or as external cables providing power or data to the computer, either for data signal transmission or power distribution.
56. The cable assembly of claim 41 configured for use in wiring or power distribution for railroad, commuter rail, or other rail applications, either for data signal transmission or power distribution.
57. The cable assembly of claim 41 configured for applications wherein the shape memory alloy is trained using thermal processing to return to a specific shape or configuration when the cable assembly is not in use or is in a stress free condition.
58. The cable assembly of claim 41 configured for applications wherein the shape memory alloy is able to recover a shape or revert to a shape when subjected to a change in temperature that is different form the intended use temperature.
59. The cable assembly of claim 58, wherein the change in temperature is electrically actuated through resistive heating of the superelastic conductor.
60. The cable assembly of claim 58, wherein the change in temperature is brought about by a change in environmental temperature.
Type: Application
Filed: Nov 3, 2014
Publication Date: Sep 15, 2016
Inventors: Stian M. Ueland (Cambridge, MA), Vivek R. DAVE (Concord, NH)
Application Number: 15/033,223