Abstract:
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.

Description:
BACKGROUND 
       [0001]    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. 
         [0002]    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. 
         [0003]    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 INVENTION 
       [0004]    In 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. 
         [0005]    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. 
         [0006]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows kinking of a cable. 
           [0008]      FIG. 2  shows an example of fatigue failure. 
           [0009]      FIG. 3  shows typical stress strain curves for metals that are commonly used in conducting wire,  90 , and for a superelastic shape memory alloy,  91 . The strains indicate that the conventional elastic material,  90 , can achieve 0.1-0.9% strain before it accumulates any plastic permanent deformation. Shape memory alloy,  91 , can achieve roughly 10 times as much strain, stretching superelastically to 1-10% strain that is fully recoverable. 
           [0010]      FIG. 4  shows a schematic phase diagram for a generic shape memory alloy. Austenite is stable to the right of the four parallel lines, and martensite is stable to the left of the lines. 
           [0011]      FIG. 5  shows different form factors for a bare superelastic conductor. In one configuration,  71 , the wire has a circular cross-section. In one configuration,  72 , the wire has a flat cross-section, much like a ribbon. In one configuration,  73 , the wire has an elliptical cross section. 
           [0012]      FIG. 6  shows a bundle of several individual wires,  75 , where at least one of the wires is a superelastic conductor, stranded together to form a wire rope or a stranded cable,  74 . 
           [0013]      FIG. 7  shows an example of a cross-section of a cable consisting of a superelastic conducting core,  29 , and an insulating layer,  28 . In the top part of the figure the wire is straight; in the middle part of the figure the wire is severely deformed; and in the bottom part of the figure the wire is locally deformed and kinked. If the conducting cable has been programmed to assume a straight shape in the austenite phase, the top figure is free of force while the other two wires exert forces to return to the straight shape if the phase of the conductor is austenitic. If the conductor is martensitic, the conductor is flexible and assumes shape changes through martensite plate redistribution. The conductor will return to the straight shape only if subjected to heat or electricity sufficient to bring the metal into the austenite phase. 
           [0014]      FIG. 8  shows ear buds in two configurations. In the left part of the drawing the ear bud cable does not see any external force and assumes a compact shape, such as a tangle-free roll. In the right part of the drawing the cable is being used and is therefore stretched out in order to connect the jack,  37 , to an electronic apparatus, and the ear buds,  35 , to a person&#39;s ears. In the drawing on the right the ear bud cable,  36 , exerts a force that is small compared to the stretching force imposed by a human being when using the cable. When the cable to the right is devoid of any external force it will assume the shape of the cable to the left again. 
           [0015]      FIG. 9  shows an example of a composite cable where  29  is a superelastic conductor as described herein and the other cables,  30 , are superelastic conductors or other wires serving different purposes. The cable assembly may be kept together by insulation,  31 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    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. 
         [0017]    A large angle deformation can lead to a permanent set or kink in the material as shown in  FIG. 1 . Here the local strain exceeds the yield point of the metal conducting element and therefore there is permanent deformation. If the cable is either forcibly straightened or is subjected to a revere strain or reverse bending due to environmental or loading conditions, then a fatigue scenario could arise. In such a scenario, the alternating plastic deformation of the cable could result in tearing and rupture of the outer insulation or compound insulating sheath as well as fatigue failure of the metal conductor thereby resulting is catastrophic failure of the entire cable assembly. This is illustrated in  FIG. 2 . 
         [0018]    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  FIG. 3 . In a conventional metal, the small elastic region is followed by a larger plastic region. This elastic strain is schematically indicated in  FIG. 3  by 0.X %. The plastic region, however, represents permanent, irreversible deformation. This contributes to cable kinking and eventual fatigue. For a superelastic material as shown in  FIG. 3  the initial elastic region is then followed by a large superelastic region. This superelastic region allows the conductor to locally undergo significant strain without any permanent set or permanent plastic deformation in the material, thereby avoiding kinking. Upon reversal of the bending or localized strain, the cable is again able to resist permanent deformation up to and until the local strain exceeds the maximum superelastic strain achievable in the material. As this can be as high as 10 times greater, or x % as shown in  FIG. 3 , there is a significant extension of the ability of the conductor to accommodate local strains without undergoing permanent plastic deformation and thereby to resist to a significant degree kinking and localized fatigue failure. 
         [0019]    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. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Composition of Shape Memory 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 
               
               
                   
                   
               
             
          
         
       
     
         [0020]    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. 
         [0021]      FIG. 4  shows a schematic phase diagram that applies to all SMAs and describes characteristic temperatures and stresses. Austenite is the stable phase to the right of the four parallel lines in  FIG. 4  while martensite is the stable phase to the left of the four parallel lines. In other words this diagram shows that austenite is stable at relatively higher temperatures and lower stresses while martensite is the stable phase at relatively lower temperatures and higher stresses. The absolute position of the transformation temperatures A f , A s , M s  and M f  (austenite finish, austenite start, martensite start, martensite finish, respectively) depends on several factors including alloy composition. The crystal structures shown in  FIG. 4  are schematic; they show how a martensite variant can transform into a second martensite variant when subjected to stress. The arrows indicate whether the change in crystal structure will be reversed when the transformation path is reversed; 
         [0022]    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. 
         [0023]    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  FIG. 4 . When the external stress is removed martensite reverts to austenite and the macroscopic deformation is recovered. It is to be noted that if the stress to induce martensite exceeds the critical stress for slip, traditional plastic deformation occurs. Superelasticity is therefore only possible in a relatively small temperature range above A f . Similarly to thermal transformation both the forward and reverse transformation occur gradually: the forward transformation starts at the martensite start stress, σ Ms , and comes to a completion at the higher martensite finish stress, σ Mf ; the reverse transformation starts at the austenite start stress, σ As , and finishes at the austenite finish stress, σ Af . 
         [0024]    Furthermore, we note that σ As &lt;σ Mf  and σ Af &lt;σ Ms  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. 
         [0025]    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. 
         [0026]    Martensitic material refers to a SMA that is in the martensitic regime depicted in  FIG. 4  (to the left of the four parallel lines). When subjected to deformation a martensitic material will deform through a redistribution of martensitic plates. Because there are several symmetry related martensite variants, these can simply transform between one another to accommodate a shape chance. Unlike superelasticity, however, a martensitic material does not return to its original shape when the external mechanical stimulus is removed. Like superelasticity, a martensitic SMA also avoids conventional plastic deformation and damage accumulation. 
         [0027]    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.  FIG. 4 ) the spooled circular shape is recovered. 
         [0028]    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  FIG. 5  where  71  shows a circular cross section,  72  shows a flat, ribbon-like cross section and  73  shows wire with an elliptical cross section. The wire may be a single bare or insulated wire or may be part of a cable or wire rope.  FIG. 6  illustrates an example of how this may done; several individual wires,  75 , where at least one is a superelastic conductor and which are insulated or bare, are stranded together to form a cable,  74 . This may further enhance mechanical properties, such as fatigue and strength, and it may add functionality, power or bandwidth, to the cable. 
         [0029]    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. 
         [0030]    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.  FIG. 7  show some configurations of a wire with insulation; the wire is delivered with one preferred shape and any local or global deformation will result in an opposing force by the wire; the wire exerts a force to return to the preferred shape. 
         [0031]    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.  FIG. 6  shows some configurations of a wire with insulation. The wire is delivered with one preferred shape that is only attained at temperatures above the intended operating range. The wires may undergo deformation locally or globally through redistribution of martensite variants, which does not result in a restoring force and does not involve conventional plasticity. A restoring force is exerted by the wire only when its temperature is raised above the transformation temperature. 
         [0032]    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. 
         [0033]    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. 
         [0034]    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  FIG. 8  where under no external mechanical force the wire curls up into a predetermined shape. Instead of tangling up into a difficult shape that is hard to straighten, the wire returns to a preferred shape when not used. When used (e.g. stretched to position one end in the pocket and the other in the ears) the wire is straight and without the tendency to tangle. This is illustrated in the drawing at the right side of  FIG. 8 , where the wire has been rolled out, the ear buds inserted into a person&#39;s ears and the plug or connector inserted into a jack or connector of an electronic device. When left without external forces, such as on a table or other surface, the wire returns to the curled-up, tangle-free configuration. 
         [0035]    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.  FIG. 9  shows one example of a wire that is part of a greater assembly of wires with similar, complementary or separate functions. This assembly may be bare or insulated or embedded in a composite or polymeric matrix. The superelastic conductor may furthermore be woven or braided into 2D or 3D structures or it may be stranded or twisted into a helix and thus forming a wire rope. 
         [0036]    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.